0.05 Concentration of inorganic contaminants at various final pH of solution and CFA:AMD ratios for treatment of Brugspruit AMD with CFA-B (concentrations are

12.64

0.14

12.62

0.12

6–9

88 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

720

22.3

385.28

19.5

285.52

6.8

276.15

20.3

449.73

15.8

326.12

19.2

6165

10

6137

7

5668.03

47.53

4601.77

3.37

7182.96

7.46

3709.80

58.99

0.178

0.02

0.015

0.002

0.017

0.002

0.018

0.003

0.011

0.005

0.013

0.001

**AMD**

**1:3.5**

**1:5**

**1:8**

**1:2**

**1:3**

**Figure 2.** pH and EC trends for the treatment process for various CFA-B:Brugspruit AMD ratios. Values reported as mean ± SD (*n* = 4), error bars reflect 1 sample SD above and below the mean.

is a strong indication that AMD chemistry has a significant role to play in the final product water chemistry.

**Figure 3** shows the trends of pH and electrical conductivity (EC) for the treatment process between CFA-B and Navigation AMD at various CFA:AMD ratios.

The pH and EC trends for the treatment process of Navigation AMD with CFA-B are distinctively different from those of the treatment process for Brugspruit AMD (**Figures 2** and **3**). For the Navigation AMD treatment process, there is a stepwise increase in pH as the treatment progresses. Buffer zones are also observed at pH 3.5–4 and pH 5.5–6.5.

Despite the high CFA:AMD ratios employed in the treatment as compared to Brugspruit treatment, the breakthrough to pH > 10 only occurs after 210 min (**Figure 3**) and for lower CFA:AMD ratio of 1:3, a breakthrough was not achieved after 360 min. It should be noted that the stepwise increase in pH with time is lacking for the Brugspruit AMD. Stepwise and gradual decrease in EC is noted for the Navigation AMD treatment process (**Figure 3**). The initial decrease in EC for the Brugspruit treatment is not sustained and stabilizes at 8.67–9.56 mS/cm. Uhlmann et al. [34] observed that the buffering observed at pH 3.5–4 is due to hydrolysis of Fe3+ while oxidation and hydrolysis of Fe2+ contributes greatly to buffering at pH 5.5–6.5 [25] (Eqs. (2) and (3)). At pH ≥ 4.5, Al3+ undergoes hydrolysis forming insoluble hydroxides and will buffer pH in the region of 3.5–4 (Eq. (2)) [34, 35]. It should be noted that Brugspruit AMD had very low concentration of Fe3+/Fe2+ and Al3+ as compared to Navigation AMD (**Table 2**). This could explain the lack of stepwise decrease in pH and buffering during the treatment process. There are two

**Figure 3.** pH and EC for various FA:AMD ratios for reaction between CFA-B and Navigation AMD. Values reported as mean ± SD (*n* = 4), error bars reflect 1 SD above and below the mean.

opposing processes that finally control the final pH of the product water in this treatment process. The dissolution and hydrolysis of soluble alkaline oxides such as CaO and MgO (**Table 1**) from coal fly ash will contribute to increase in pH (Eqs. (4) and (5)). This is also confirmed by the fact that the time taken for breakthrough of the buffer zone 5.5–6.5 reduced with increase of CFA in the treatment mixture. CFA:AMD ratio of 1:1.5 took the shortest time to breakthrough this buffer zone (**Figure 3**).

$$\text{CaO} \,\text{+H}\_2\text{O} \Rightarrow \text{Ca}^{2+} + 2\text{OH}^- \,\text{pH} \,\text{increase} \tag{4}$$

$$\text{MgO} \star \text{H}\_2\text{O} \Rightarrow \text{Mg}^{2+} + 2\text{OH}^- \text{pH} \text{ increase} \tag{5}$$

Hydrolysis of AMD constituents such as Fe3+, Al3+ and Fe2+ releases protons (acidity increase) (Eqs. (2) and (3)) and offsets the pH increase attributed to the dissolution of the alkaline oxide from coal fly ash. The net result is a pH buffer zone, this buffer zone is only overstepped when the components leading to release of acidity are completely hydrolyzed. These hydrolysable components will finally determine if the final pH of the product water will be acidic or alkaline at any given treatment time. It will also be observed that the final pH of the product water also depended on the CFA:AMD ratio applied. Higher CFA:AMD led to higher pH of product water regardless of treatment time. It can therefore be concluded that factors that will control the product water chemistry and quality would be, CFA:AMD ratio, the treatment time and the chemistry of the AMD.

#### **3.5. Inorganic contaminants removal: product water quality compared to South African water quality guidelines**

Acid mine waters are highly reactive solutions that can dissolve most primary minerals when reacted with an alkaline solid waste material such as coal fly ash with subsequent formation of a variety of secondary minerals that can potentially adsorb trace chemical species. Dissolution of coal fly ash by AMD triggers several processes such as hydrolysis of soluble alkaline oxides leading to increase in pH. Several authors observe that pH is the most important parameter in coal fly ash solutions and accounts for significant inorganic toxic elements removal through precipitation, ion exchange and adsorption processes [13, 30].

**Tables 3** and **4** present results of inorganic contaminants removal as a function of final solution pH and CFA:AMD ratios for Brugspruit and Navigation AMD treatment with CFA-B. The results are discussed with respect to pH of precipitation of the various metal species as determined from thermodynamic calculations and experimental observations from titration of solutions containing the stated species as reported by Britton [36].

#### **3.6. Brugspruit AMD treatment with CFA-B: inorganic contaminants removal**

opposing processes that finally control the final pH of the product water in this treatment process. The dissolution and hydrolysis of soluble alkaline oxides such as CaO and MgO (**Table 1**) from coal fly ash will contribute to increase in pH (Eqs. (4) and (5)). This is also confirmed by the fact that the time taken for breakthrough of the buffer zone 5.5–6.5 reduced with increase of CFA in the treatment mixture. CFA:AMD ratio of 1:1.5 took the shortest time to breakthrough

**Figure 3.** pH and EC for various FA:AMD ratios for reaction between CFA-B and Navigation AMD. Values reported as

Hydrolysis of AMD constituents such as Fe3+, Al3+ and Fe2+ releases protons (acidity increase) (Eqs. (2) and (3)) and offsets the pH increase attributed to the dissolution of the alkaline oxide from coal fly ash. The net result is a pH buffer zone, this buffer zone is only overstepped when the components leading to release of acidity are completely hydrolyzed. These hydrolysable components will finally determine if the final pH of the product water will be acidic or alkaline at any given treatment time. It will also be observed that the final pH of the product water also depended on the CFA:AMD ratio applied. Higher CFA:AMD led to higher pH of product water regardless of treatment time. It can therefore be concluded that factors that will control

O ⇒ Ca2+ + 2OH− pH increase (4)

**EC (mS/cm)**

O ⇒ Mg2+ + 2OH− pH increase (5)

this buffer zone (**Figure 3**).

**0**

**0**

**1**

**5**

mean ± SD (*n* = 4), error bars reflect 1 SD above and below the mean.

**10**

90 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

**15**

**30**

**40**

**50**

**60**

**Reaction time (mins) pH:1:3: CFA-B & Navigation AMD pH:1:2: CFA-B & Navigation AMD pH: 1:1.5: CFA-B & Navigation AMD EC:1:3: CFA-B & Navigation AMD EC: 1:2: CFA-B & Navigation AMD EC:1:1.5: CFA-B & Navigation AMD**

**90**

**120**

**150**

**180**

**210**

**240**

**300**

**360**

**2**

**4**

**6**

**pH**

**8**

**10**

**12**

CaO +H2

MgO + H2

Brugspruit AMD had different chemistry compared to that of Navigation AMD, it had low concentration of the major hydrolysable chemical species Al3+, Fe3+, Fe2+ and Mn2+. Major inorganic contaminants Fe, Mn, Al and Mg were reduced significantly in all the ratios investigated (**Table 3**). Fe and Al were reduced by >95% at pH ≥ 9.16 attained in the final process waters for CFA:AMD ratios, 1:3, 1:5, 1:8, 1:2 and 1:3. It should be noted that final pH also depended on the treatment time employed. Mn removal was ≥90% for all CFA:AMD ratios evaluated except for 1:3 ratio at 80%. Jenke et al. [25] observed that at the pH of minimum solubility of the hydroxides of Fe3+ (pH 3.0), of Fe2+ (pH 6.0–8.0), of Mn2+ (pH 8.41–9.0) and of Zn2+ (pH 6.0–6.5), a significant proportion of the initial concentration should be precipitated out of solution. Gitari et al. [19] observed that at pH ≥ 4.5, most of the mineral phases bearing these species were at saturation or oversaturation (SI ≥ 1). At pH ≥ 12, a slight increase in Al, Fe and Mn was observed. At pH 12.0–12.5 attained for ratios 1:8, 1:2 and 1:3, the formation of hydroxy complexes was attributed to the decreased removal. Drever [37] observes that Al3+ hydroxide exhibits minimum solubility at pH 6–6.5, therefore at pH > 6.5, the Al-hydroxy complexes become more important [38].

Among the minor and trace species, Ni, Cu, Pb, Mg, Cr, Ni,Co, Cu, As and Cd were all observed to decrease as the pH increased. The Cu, Cr, Zn and Pb removal efficiency was observed to decrease at CFA:AMD ratio of 1:8 which attained a pH of 12.04. The decrease in removal of these species could be attributed to soluble hydroxo species formed at pH > 12.0. Another reason for the increase could be reduced adsorption due to reduced formation of amorphous Fe(OH)3 , MnOOH and Al(OH)3 due to the low concentration of Fe3+, Mn and Al3+ in Brugspruit AMD. Mg2+ removal efficiency approached 100% at pH 9.16 with a decrease observed as the pH increased to 12.62. Decreased removal could be due to the formation of

**FA:AMD**

**Species**

B Na Mg

Al

Si K Ca Cr Mn

Fe Ni Co Cu Zn As Se Sr Mo Cd Ba Pb

0.314

0.107

0.019

0.002

0.041

0.004

0.0301

0.005

0.0154

0.0021

0.209

0.002

0.369

0.003

0.347

0.032

0.336

0.034

0.319

0.013

0.032

0.001

0.003

0.0015

0.004

0.001

0.003

0.0015

0.002

0.0004

0.04

0.002

0.025

0.002

0.622

0.002

0.665

0.002

0.77

0.04

0.01

0–0.05

7.69

0.226

15.71

1.34

17.18

2.543

14.48

2.87

17.72

1.48

0.032

0.002

0.037

0.002

0.026

0.003

0.052

0.001

0.112

0.003

0.193

0.012

0.003

0.0005

0.005

0.0015

0.003

0.0005

0.004

0.001

48.99

30.624

1.3

0.012

1.2

0.05

1.26

0.02

0.736

0.012

1

3

0.355

0.007

0.045

0.003

0.055

0.011

0.073

0.005

0.034

0.006

0.2

1

4.3

0.105

0.312

0.012

0.014

0.001

0.019

0.003

0.002

0.0015

0.05

0–0.05

6.95

0.018

0.58

0.04

0.134

0.002

0.088

0.004

0.051

0.001

0.2

0–0.02

5599.92

80.862

293.3

19.57

52.25

6.78

43.23

4.578

4.7

0.67

226.2.5

4.742

56.65

5.68

5.15

0.456

1.11

0.005

0.133

0.005

1.11

0.009

0.069

0.003

0.163

0.013

0.141

0.001

0.083

0.015

653.33

10.626

368.19

23.8

495.11

20.45

448.53

19.67

477.92

35.63

23.03

2.856

16.27

2.45

19.45

1.56

14.87

2.35

10.33

2.45

50

92 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

32

82.01

1.238

4.39

0.52

2.05

0.003

2.3

0.16

1.09

0.03

1068.00

11.279

2.85

0.05

3.26

0.35

2.35

0.56

9.41

1.97

2661.67

35.008

636.85

80.45

618.06

70.8

200.04

34.5

1.5

0.02

358.72

2.946

71.76

10.23

72.53

9.87

68.21

10.12

62.02

21.8

70

100

30

1.37

0.163

23.44

3.44

24.38

2.56

17.57

2.356

17.86

3.456

0.5

0–0.5

**ave**

**SD**

**ave**

**SD**

**ave**

**SD**

**ave**

**SD**

**ave**

**SD**

**Irrigation**

**Domestic**

**use**

**AMD**

**1:3**

**1:2.5**

**1:2**

**1:1.5**

**DWAF limits**

**Table 4.** Concentration of inorganic contaminants at various final pH of solution and CFA:AMD ratios for treatment of Navigation AMD with CFA-B (concentrations are in mg/L except for pH). soluble hydroxo species at the high pH. Na and K were removed by 50% as the pH increased. Gitari et al. [19] observed that as pH increases and in presence of SO4 2-, several Na-K bearing mineral phases such as jarosite are precipitated and would account for this decrease. Sulfate did not register significant removal and in some CFA:AMD ratios (1:2) was observed to increase, this increase is attributed to release from the coal fly ash.

#### **3.7. Navigation AMD treatment with CFA-B: inorganic contaminants removal**

The pH increase with increase of CFA:AMD ratios was observed to be gradual for Navigation AMD treatment as compared to that of Brugspruit AMD (**Tables 3** and **4**). Several inorganic contaminants removal trends are observed as the pH increases. At final pH 6.33 achieved for the CFA:AMD ratio of 1:3, a sharp decrease in concentration is observed for Fe, Al and Mn. These are the major inorganic contaminants that characterize Navigation AMD. With subsequent CFA:AMD ratios, the removal efficiency of ≥90% are observed. Gitari et al. [19] observed that as pH increases, mineral phases bearing these chemical species are precipitated and account for their increased removal in solution. Minor and trace inorganic species such as Si, Cr, Ni, Co, Cu, Zn, As, Se, Cd and Pb were all observed to decrease as the pH increased with subsequent CFA:AMD ratios. Their removal could be attributed to several mechanisms such as precipitation of mineral phases and adsorption to high surface area Fe and Al oxyhydroxy precipitates. Gitari et al. [19] observed that co-precipitation of Fe, Al-oxyhydroxides, oxyhydroxy sulfates and adsorption could be responsible for attenuation of Cu2+, Zn2+ species.

Britton [36] observes that the pH of minimum solubility of the hydroxides/oxyhydroxides of Fe3+, Fe2+, Al3+, Zn2+, Cu2+ and Ni2+ are 3.0, 6.0–8.0, 4–4.5, 6.0–6.5 and 6.66, respectively. The pH attained for CFA:AMD ratio of 1:3–1:2 is within this range. This would explain the high removal of these species within this pH range. However, Cu and Pb registered an increase in concentration in final process waters for CFA:AMD 1:2 and 1:2.5 ratios whose final pH ranged from 8.7 to 9.5 (**Table 4**). This would be attributed to the formation of soluble hydroxo complexes for both species at this pH range.

Mg, Na, K and Ca removal increased with increasing pH of the final process water. These chemical species are highly soluble in the pH range generated in process waters and likely removed through incorporation in precipitating mineral phases. Gitari et al. [19] observed that mineral phases such as alunite, basaluminite, jurbanite, brucite and jarosite are likely to precipitate out in CFA:AMD reactions leading to removal of these species. Sulfate was likely removed through formation of gypsum, incorporation into Al, Fe-oxyhydroxysulfates. A direct relationship between percent sulfate removal and FA content in the reaction mixture suggests that dissolution of CaO and subsequent formation of gypsum accounts for the sulfate removal in both AMD treatments. XRD analysis identified gypsum in the solid residues of the CFA:AMD reactions [3, 39].

#### **3.8. Leaching of chemical species from coal fly ash in the treatment process**

Coal fly ash is a reactive material and on contact with aqueous solutions such as AMD undergoes dissolution with subsequent release of chemical species. An observation of results in **Tables 3** and **4** show that interaction of coal fly ash and AMD leads to the release of B, Sr, Mo and Ba. The fact that these chemical species increased with increase in the ratio of CFA in the treatment mixture attests to the fact they were being released from the coal fly ash. Mattigod et al. [17] and Eary et al. [20] reported that the glassy phase in coal fly ash is enriched with As, Na, Mg, K, Sr, B, Mo and Ba as soluble salts which are released into solution when the CFA contacts aqueous solutions.

#### **3.9. Comparison to South Africa water quality guidelines**

The product water for various CFA:AMD ratios was compared to the Department of Water Affairs and Forestry [32] water quality limits for irrigation and domestic use. Treatments of Navigation AMD with CFA-B resulted in much cleaner water with Cu, K, Mo, Na and Zn being within the domestic water use limits for 1:3 CFA:AMD ratio and Co, Cu, K, Na and Zn being within the domestic water limits for 1:2.5 CFA:AMD ratio. Treatment of Brugspruit AMD with CFA-B produced less clean water although breakthrough to alkaline pH was established within less than an hour. Only Cu and K were within the domestic limits for 1:3 FA:AMD ratio. This comparison includes only the FA:AMD ratios that resulted in process water in the pH range 6–9. The alkali and alkaline earth metal species (Na, B, Mg, Ca, Sr and Ba) remained largely in the process water in concentrations beyond the guideline limits. B, Mg, Sr, Mo and Ba were largely released from dissolution of coal fly ash and additional treatment options such as reverse osmosis will be required to further clean the water to required standards.

#### **4. Conclusions**

soluble hydroxo species at the high pH. Na and K were removed by 50% as the pH increased.

ing mineral phases such as jarosite are precipitated and would account for this decrease. Sulfate did not register significant removal and in some CFA:AMD ratios (1:2) was observed

The pH increase with increase of CFA:AMD ratios was observed to be gradual for Navigation AMD treatment as compared to that of Brugspruit AMD (**Tables 3** and **4**). Several inorganic contaminants removal trends are observed as the pH increases. At final pH 6.33 achieved for the CFA:AMD ratio of 1:3, a sharp decrease in concentration is observed for Fe, Al and Mn. These are the major inorganic contaminants that characterize Navigation AMD. With subsequent CFA:AMD ratios, the removal efficiency of ≥90% are observed. Gitari et al. [19] observed that as pH increases, mineral phases bearing these chemical species are precipitated and account for their increased removal in solution. Minor and trace inorganic species such as Si, Cr, Ni, Co, Cu, Zn, As, Se, Cd and Pb were all observed to decrease as the pH increased with subsequent CFA:AMD ratios. Their removal could be attributed to several mechanisms such as precipitation of mineral phases and adsorption to high surface area Fe and Al oxyhydroxy precipitates. Gitari et al. [19] observed that co-precipitation of Fe, Al-oxyhydroxides, oxyhydroxy sulfates and adsorption could be responsible for attenuation of Cu2+, Zn2+ species. Britton [36] observes that the pH of minimum solubility of the hydroxides/oxyhydroxides of Fe3+, Fe2+, Al3+, Zn2+, Cu2+ and Ni2+ are 3.0, 6.0–8.0, 4–4.5, 6.0–6.5 and 6.66, respectively. The pH attained for CFA:AMD ratio of 1:3–1:2 is within this range. This would explain the high removal of these species within this pH range. However, Cu and Pb registered an increase in concentration in final process waters for CFA:AMD 1:2 and 1:2.5 ratios whose final pH ranged from 8.7 to 9.5 (**Table 4**). This would be attributed to the formation of soluble hydroxo com-

Mg, Na, K and Ca removal increased with increasing pH of the final process water. These chemical species are highly soluble in the pH range generated in process waters and likely removed through incorporation in precipitating mineral phases. Gitari et al. [19] observed that mineral phases such as alunite, basaluminite, jurbanite, brucite and jarosite are likely to precipitate out in CFA:AMD reactions leading to removal of these species. Sulfate was likely removed through formation of gypsum, incorporation into Al, Fe-oxyhydroxysulfates. A direct relationship between percent sulfate removal and FA content in the reaction mixture suggests that dissolution of CaO and subsequent formation of gypsum accounts for the sulfate removal in both AMD treatments.

XRD analysis identified gypsum in the solid residues of the CFA:AMD reactions [3, 39].

Coal fly ash is a reactive material and on contact with aqueous solutions such as AMD undergoes dissolution with subsequent release of chemical species. An observation of results in **Tables 3** and **4** show that interaction of coal fly ash and AMD leads to the release of B, Sr, Mo

**3.8. Leaching of chemical species from coal fly ash in the treatment process**

2-, several Na-K bear-

Gitari et al. [19] observed that as pH increases and in presence of SO4

94 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

to increase, this increase is attributed to release from the coal fly ash.

plexes for both species at this pH range.

**3.7. Navigation AMD treatment with CFA-B: inorganic contaminants removal**

The dissolution and hydrolysis of basic oxides such as MgO and CaO from coal fly ash on contacting AMD led to an increase in pH. On the other hand, hydrolysis of AMD chemical species such as Al3+, Fe3+, Fe2+ and Mn2+ led to release of acidity that counteracted the pH increase leading to stepwise increase in pH of process water and buffer zones as the treatment processes progressed. This was clearly observed for Navigation AMD as opposed to Brugspruit AMD that had low hydrolysable species. These two processes initiated several processes such as precipitation and adsorption that led to attenuation of the major and minor chemical species leading to much cleaner product water. Efficiency of the inorganic contaminants removal was directly linked to amount of FA in the reaction mixture and to the final pH in product water. Three processes were identified that would finally control the quality of the product water, these were (1) CFA:AMD ratios, (2) treatment time and (3) chemistry of the AMD. Most of the major contaminants were reduced to below South African water quality guidelines; however, the coal fly ash was observed to release other contaminants in solution and this could be a drawback for the proposed treatment process. Other factors that would improve the sustainability of the treatment process are beneficial application of the treatment residues as backfill material. Economic viability of this process would also depend on closeness of the AMD generating mines and the coal power utilities generating the coal fly ash.

## **Acknowledgements**

The authors would like to acknowledge and thank Water Research Commission (WRC), National Research Foundation (NRF) and Coaltech 2020 for funding this research work.

## **Author details**

Wilson Mugera Gitari1 \*, Leslie F. Petrik2 and Segun A. Akinyemi3

\*Address all correspondence to<EMAIL_ADDRESS>
1 Environmental Remediation and Water Pollution Chemistry Group, School of Environmental Sciences, University of Venda, Thohoyandou, South Africa

2 Environmental and Nano Science Research Group, Department of Chemistry, University of the Western Cape, South Africa

3 Department of Geology, Ekiti State University, Ado Ekiti, Nigeria

## **References**

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The authors would like to acknowledge and thank Water Research Commission (WRC), National Research Foundation (NRF) and Coaltech 2020 for funding this research work.

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\*, Leslie F. Petrik2

3 Department of Geology, Ekiti State University, Ado Ekiti, Nigeria

1 Environmental Remediation and Water Pollution Chemistry Group, School of Environmental Sciences, University of Venda, Thohoyandou, South Africa

2 Environmental and Nano Science Research Group, Department of Chemistry, University

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EC. Comparison of CO2

DOI: 10.1021/es00110a007

org/10.1016/S0956-053X(01)00053-8

DOI: 10.2134/jeq2001.3041420x

jeq1979.00472425000800020007x

013-0439-1

2008;**87**:2450-2462. DOI: 10.1016/j.fuel.2008.03.018

98 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

**Provisional chapter**

## **Phytoreclamation of Abandoned Acid Mine Drainage Site After Treatment with Fly Ash Site After Treatment with Fly Ash**

**Phytoreclamation of Abandoned Acid Mine Drainage**

DOI: 10.5772/intechopen.69527

Madhumita Roy, Roopali Roychowdhury, Pritam Mukherjee, Atanu Roy, Bulti Nayak and Satarupa Roy Pritam Mukherjee, Atanu Roy, Bulti Nayak and Satarupa Roy Additional information is available at the end of the chapter

Madhumita Roy, Roopali Roychowdhury,

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69527

#### **Abstract**

Acid mine drainage and coal fly ash both are the sibling products from man's increasing demand for power. Mining of coal from coal mines generates acid mine drainage (AMD), and burning of coal for thermal power generation produces fly ash (FA). Although both are hazardous to the ecosystem and human health, reunion of them into one would reduce their toxic effect on nature. Mining causes exposure of hidden rock materials containing an abundance of sulphide to the atmosphere. Oxidation of the metal sulphides (pyrite, as iron sulphide) within the surrounding rock and overburden generates acidity. Subsurface mining that pumps out water to prevent flooding releases acidic water to nearby areas, known as acid mine drainage. Tailings piles, mine waste rock dumps, and coal spoils contribute in AMD. Improper disposal of the fly ash contaminates the soil, water and air by leaching of the pollutants or air borne particulate matters. However filler properties and presence of macronutrients makes fly ash an excellent filler material for mine sites, and its soil-like properties help in plant growth if provided with organic carbon and nitrogen. This chapter proposes development of a green cover into AMD site after treatment of the AMD site with FA.

**Keywords:** acid mine drainage, fly ash, phytoreclamation, green capping, biofuel

#### **1. Introduction**

Acid mine drainage (AMD) and fly ash (FA) from coal-based thermal power plants have been recognized worldwide as major environmental problems caused by mining industries across the globe. But FA although a hazardous waste product has great beneficiation property for

Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons

reclamation of other degraded landscapes like AMD sites. Mining activities accelerate the acid mine drainage by exposing sulfide-containing rocks to the weathering process. Mine tailings (byproducts leftover from mining), mine waste rock dumps, and coal spoils/overburdens all are contributing factors to acid mine drainage. Certain minerals like sulfides of iron (pyrite, FeS2 , and pyrrhotite, Fe1-XS) present in the rocks remain stable and insoluble until they come in contact with atmospheric oxygen and water. But upon exposure to water and air, they react to form sulphuric acid. This acid further causes dissolution of harmful heavy metals from the rocks. The heavy metal-loaded acid is carried off by rainwater or surface drainage to the nearby streams, rivers, or lakes, creating environmental risks. As the flowing water mixes with mine wastes and other mining leftovers, it picks up metals and other substances that pollute the water posing serious threats to ecosystems. Iron is an important metal in the AMD process. Combined with sulfate and/or sulfide, a yellow-orange solid colloquially known as yellow boy is produced which if deposited into streams can disrupt stream ecosystem. Acid mine drainage is a big problem in areas where abandoned mines are common. Processes that affect the release and transport of trace metals have to be known in order to achieve effective management and amendment of AMD and its consecutive impacts. FA from burnt coal contains certain heavy metals and nonmetals, which are otherwise hazardous if not properly disposed. The deleterious effects of AMD and FA have been studied earlier by many researchers. Tons of these wastes generated each day have certain characteristic features which can help in treating AMD. It has been reported that the acidic AMD has a pH of 2–4, along with a high sulfate concentrations (1–20 g/l). The mineralization and extremely acidic pH levels cause toxicity of these wastes, which are not only environmentally damaging but also require expensive storage, remediation, and disposal techniques. Fly ash on the other hand is alkaline in nature due to its constituent lime fraction along with other metal components. Many studies have found fly ash a very good neutralizing agent compared to chemical-neutralizing agents such as lime, slaked lime, calcium carbonate, sodium carbonate, sodium hydroxide, and magnesium oxide and hydroxide whose cost and effectiveness remain questionable [1]. Fly ash generated by coal-based thermal power plants provide ample amount of cheap resource which on successful implementation on AMD will address two main issues:

As compared to the conventional processes of AMD neutralization, which have their own pros and cons, the cost involved in FA application is less. FA is freely available in close adjacency to coal mines generating AMD which even reduces the cost of transfer when mines are placed in close proximity. Millions of tons of FA generation are a matter of concern regarding proper disposal, as it poses environmental threats.  It can be effectively used to fill mine spoils and remediate acidic soils. From a pH range of 2.5–2.7, AMD can be neutralized to a pH of 7.0 with fly ash.

Increasing the pH and immobilizing the metal contaminant elements as amorphous hydroxides or oxyhydroxides, fly ash is an excellent option which contains substantial amounts of silicon dioxide (SiO2 ) (both amorphous and crystalline), aluminum oxide (Al2 O3 ), and calcium oxide (CaO). Liming process requires further treatments with flocculants and biological procedures for sulfate reduction, whereas soluble residues are produced when AMD is treated with FA which needs disposal. If FA is utilized in acid mine water neutralization, it can curb the additional costs of conventional chemical treatments. This will not only prove to be cost-effective but also treat and dispose both AMD and FA in an easy way. Treatment of AMD with FA not only controls acidification process but also helps in proper utilization of FA generation. Experimental procedures with FA in AMD neutralization have revealed that a predetermined mixture of acid mine water and fly ash when continuously stirred at room temperature and pressure at a specific fly ash to acid mine water ratio (AMD-FA) of 3:1 for 30 min resulted in pH increase toward making it alkaline. From a pH range of 2.5–2.7, AMD can be neutralized to a pH of 7.0 with fly ash [2]. FA has been used successfully in controlling mine drainage water through construction of a constructed wetland [3], mine backfilling [4, 5], and reclamation of abandoned open cast mine sites [6] and abandoned, underground coal mines [7]. However, the present chapter would discuss application of FA for phytoreclamation of abandoned mine sites. Mining is a temporary use of land, with the life span of an active mine ranging from few years to several decades. Before the 1970s, it was a common practice for mining companies to leave mine sites in its present state. However, now it is mandatory that mine closure would require the reclamation or rehabilitation of decommissioned mine sites, which involves returning the land and watercourses to an acceptable standard. Reclamation is the process by which degraded lands are returned to productivity and by which some measures of biotic function and productivity are restored. Various types of reclamation activities which are in use includes removing or remediating hazardous materials, reshaping the land, restoring topsoil, and planting native grasses, trees, or ground cover. Sometimes the methods used remain inadequate for the complete prevention of environmental contamination, and the mines are therefore considered to be improperly reclaimed by today's standards. Some abandoned mines are announced as "orphaned"—as the owners cannot be traced or in cases where the owner rejects to clean the mine site. In those cases, governments decide to close and rehabilitate orphaned and abandoned mines. Phytoremediation (phyto stands for plant and remediation stands for removal or correction of an evil) is a technology that uses various plants to extract, contain, degrade, or immobilize contaminants from soil and water. This technology has been receiving increasing attention for remediation of hazardous waste sites as an innovative, cost-effective, environment-friendly alternative to other treatment methods. Although phytorestoration has been successfully used for reclamation of FA ponds, phytorestoration of acid mine drainage has not been tried. The present chapter discusses the problems associated with direct phytoremediation of AMD soils. The extreme acidic pH and high sulfate concentration would not be able to provide environment for plant growth. Although few native species of plants have been found in different countries that grows in acidic soils [8], most plants studied suitable for phytoremediation would not grow in AMD sites. But it may be noteworthy to say that the AMD treated or filled with FA would present a suitable matrix for plant growth. In addition to the high concentration of plant essential nutrients such as Fe, Zn, Cu, Al, Mn, etc., which are already present in coal burned fly ash, amendment with biofertilizers and mycorrhizas can accelerate phytocover development. Soil productivity can be increased by amending soil with many types of natural materials such as animal manures, saw dust, sewage sludge, wood residues, etc. These materials stimulate the microbial activity and provides N, P, and organic carbon to the soil. The phytocover would

reclamation of other degraded landscapes like AMD sites. Mining activities accelerate the acid mine drainage by exposing sulfide-containing rocks to the weathering process. Mine tailings (byproducts leftover from mining), mine waste rock dumps, and coal spoils/overburdens all are contributing factors to acid mine drainage. Certain minerals like sulfides of iron (pyrite,

102 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

, and pyrrhotite, Fe1-XS) present in the rocks remain stable and insoluble until they come in contact with atmospheric oxygen and water. But upon exposure to water and air, they react to form sulphuric acid. This acid further causes dissolution of harmful heavy metals from the rocks. The heavy metal-loaded acid is carried off by rainwater or surface drainage to the nearby streams, rivers, or lakes, creating environmental risks. As the flowing water mixes with mine wastes and other mining leftovers, it picks up metals and other substances that pollute the water posing serious threats to ecosystems. Iron is an important metal in the AMD process. Combined with sulfate and/or sulfide, a yellow-orange solid colloquially known as yellow boy is produced which if deposited into streams can disrupt stream ecosystem. Acid mine drainage is a big problem in areas where abandoned mines are common. Processes that affect the release and transport of trace metals have to be known in order to achieve effective management and amendment of AMD and its consecutive impacts. FA from burnt coal contains certain heavy metals and nonmetals, which are otherwise hazardous if not properly disposed. The deleterious effects of AMD and FA have been studied earlier by many researchers. Tons of these wastes generated each day have certain characteristic features which can help in treating AMD. It has been reported that the acidic AMD has a pH of 2–4, along with a high sulfate concentrations (1–20 g/l). The mineralization and extremely acidic pH levels cause toxicity of these wastes, which are not only environmentally damaging but also require expensive storage, remediation, and disposal techniques. Fly ash on the other hand is alkaline in nature due to its constituent lime fraction along with other metal components. Many studies have found fly ash a very good neutralizing agent compared to chemical-neutralizing agents such as lime, slaked lime, calcium carbonate, sodium carbonate, sodium hydroxide, and magnesium oxide and hydroxide whose cost and effectiveness remain questionable [1]. Fly ash generated by coal-based thermal power plants provide ample amount of cheap

resource which on successful implementation on AMD will address two main issues:

soils. From a pH range of 2.5–2.7, AMD can be neutralized to a pH of 7.0 with fly ash.

Increasing the pH and immobilizing the metal contaminant elements as amorphous hydroxides or oxyhydroxides, fly ash is an excellent option which contains substantial amounts of

cium oxide (CaO). Liming process requires further treatments with flocculants and biological

) (both amorphous and crystalline), aluminum oxide (Al2

O3

), and cal-

As compared to the conventional processes of AMD neutralization, which have their own pros and cons, the cost involved in FA application is less. FA is freely available in close adjacency to coal mines generating AMD which even reduces the cost of transfer when mines are placed in close proximity. Millions of tons of FA generation are a matter of concern regarding proper disposal, as it poses environmental threats.  It can be effectively used to fill mine spoils and remediate acidic

• The reduction of the AMD treatment cost.

• The effective management of FA.

silicon dioxide (SiO2

FeS2

not only have esthetic value but would also be economically profitable in long terms through cultivation of bioenergy/biofuel crops. The role of certain microorganisms (bioremediation) also has been reported which can inhabit in acidic and alkaline environment and can participate in the reclamation process [9]. They have heavy metal-accumulating properties and are capable of biotransforming them into a less toxic form. This property of heavy metal reduction by microorganisms has been widely studied and accepted by many researchers. Certain rhizobacteria (bacteria inhabiting root system of plants) are there which are heavy metal tolerant and have various plant growth-promoting features [10]. Bioaugmenting those bacteria in the site would serve additional benefits. Exclusive properties of FA as a niche for certain heavy metal-resistant bacteria would prove it to be an excellent stride towards an ecofriendly effort of better AMD beneficiation.

## **2. Hazards from the two by-products of mining industries**

Power generation in most countries is largely dependent on thermal power plants, and they are dependent on coal, a product from mining industry. On the other hand, coal mines and other types of mines are source of acid mine drainage (AMD). Both types of byproducts from the mining industry are deleterious to environment if not managed properly. Following is a discussion on the hazardous nature of coal fly ash (ash derived from coal burning) and AMD.

#### **2.1. Ecosystem and health hazards from coal fly ash**

Coal ash is the leftover waste after combustion of coal. It includes fly ash (fine powder-like particles that are driven out from the boiler of thermal power plants with the flue gases and are captured by pollution control devices) and other coarser materials that drop to the bottom of the furnace. Coal ash typically contains toxic heavy metals including arsenic, lead, mercury, cadmium, chromium, and selenium and sometimes radioactive wastes as well. The Environmental Protection Agency (EPA) has warned that living next to a coal ash and particularly unlined wet ash pond (surface impoundment) disposal site can increase risk of cancer or other diseases. Leaching of heavy metals from dump grounds to the underground water reservoir may contaminate drinking water supply with arsenic and other heavy metals. If eaten, drunk, or inhaled, the air-borne toxicants released from thermal power plants or landfills or ash ponds can cause cancer and several other diseases effecting the nervous system (cognitive deficits, developmental delays, and behavioral problems), respiratory damage of the heart and lungs, kidney disease, reproductive problems, gastrointestinal illness, and birth defects. The EPA estimates that 140 million tons of coal ashes are generated annually. Arsenic is one of the most common, and most dangerous, pollutants from coal ash. The EPA also found that living near ash ponds increases the risk of damage from cadmium, lead, and other toxic metals.

A giant spill occurred in Kingston, TN, just before Christmas 2008. More than a billion gallons of water and coal ash spilled into the neighboring river valley. EPA found arsenic at 149 times the allowable standard for drinking water in the water, as well as elevated levels of other toxic metals including lead, thallium, barium, cadmium, chromium, mercury, and nickel. Another case study of leaching hazards from FA dump occurred in Niagara (NY) Mohawk Power Corporation on Lake Erie. The drinking water wells were severely contaminated with leached lead which is a very potent neurotoxin that can harm the developing nervous system, even at low levels of exposure. Another case of contamination from use as "fill" occurred at a 216-acre golf course in Chesapeake, VA, where 1.5 million cubic yards of fly ash were recycled to give contour to the course. When groundwater at the golf course was tested, arsenic, boron, chromium, copper, lead, and vanadium were detected.

#### **2.2. Ecosystem and health hazards from open cast, underground, abandoned, and other types of mines**

#### *2.2.1. Open cast mining*

not only have esthetic value but would also be economically profitable in long terms through cultivation of bioenergy/biofuel crops. The role of certain microorganisms (bioremediation) also has been reported which can inhabit in acidic and alkaline environment and can participate in the reclamation process [9]. They have heavy metal-accumulating properties and are capable of biotransforming them into a less toxic form. This property of heavy metal reduction by microorganisms has been widely studied and accepted by many researchers. Certain rhizobacteria (bacteria inhabiting root system of plants) are there which are heavy metal tolerant and have various plant growth-promoting features [10]. Bioaugmenting those bacteria in the site would serve additional benefits. Exclusive properties of FA as a niche for certain heavy metal-resistant bacteria would prove it to be an excellent stride towards an eco-

Power generation in most countries is largely dependent on thermal power plants, and they are dependent on coal, a product from mining industry. On the other hand, coal mines and other types of mines are source of acid mine drainage (AMD). Both types of byproducts from the mining industry are deleterious to environment if not managed properly. Following is a discussion on the hazardous nature of coal fly ash (ash derived from coal

Coal ash is the leftover waste after combustion of coal. It includes fly ash (fine powder-like particles that are driven out from the boiler of thermal power plants with the flue gases and are captured by pollution control devices) and other coarser materials that drop to the bottom of the furnace. Coal ash typically contains toxic heavy metals including arsenic, lead, mercury, cadmium, chromium, and selenium and sometimes radioactive wastes as well. The Environmental Protection Agency (EPA) has warned that living next to a coal ash and particularly unlined wet ash pond (surface impoundment) disposal site can increase risk of cancer or other diseases. Leaching of heavy metals from dump grounds to the underground water reservoir may contaminate drinking water supply with arsenic and other heavy metals. If eaten, drunk, or inhaled, the air-borne toxicants released from thermal power plants or landfills or ash ponds can cause cancer and several other diseases effecting the nervous system (cognitive deficits, developmental delays, and behavioral problems), respiratory damage of the heart and lungs, kidney disease, reproductive problems, gastrointestinal illness, and birth defects. The EPA estimates that 140 million tons of coal ashes are generated annually. Arsenic is one of the most common, and most dangerous, pollutants from coal ash. The EPA also found that living near ash ponds increases the risk of damage from cadmium, lead, and other toxic metals. A giant spill occurred in Kingston, TN, just before Christmas 2008. More than a billion gallons of water and coal ash spilled into the neighboring river valley. EPA found arsenic at 149 times the allowable standard for drinking water in the water, as well as elevated levels of other toxic metals including lead, thallium, barium, cadmium, chromium, mercury, and nickel. Another

friendly effort of better AMD beneficiation.

104 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

**2.1. Ecosystem and health hazards from coal fly ash**

burning) and AMD.

**2. Hazards from the two by-products of mining industries**

In open cast mining, material to be mined is excavated from an open pit. This type of mining is the most common type of mining for obtaining strategic minerals. This type of mining is very environmentally hazardous as strategic minerals are normally available in small amounts, which increases the amount of ore needed to be mined. Hard-rock mining causes rock exposure. When these rocks are crushed, radioactive elements, asbestos-like minerals, metallic dust, and other toxic elements get exposed. During separation, residual rock slurries, which are mixtures of pulverized rock and liquid, are produced as tailings, toxic and radioactive elements from these liquids can leak into bedrock if not properly contained.

#### *2.2.2. Underground mining*

Underground mining is very dangerous as it has the potential for tunnel collapses and land subsidence. It involves large-scale movements of waste rock and vegetation, similar to open pit mining in addition to releasing a variety of toxic compounds into the environment. As water takes on harmful concentrations of minerals and heavy metals, it becomes a contaminant. This contaminated water can pollute the region surrounding the mine and beyond. Most underground mining operations increase sedimentation in nearby rivers through their use of hydraulic pumps and suction dredges; blasting with hydraulic pumps removes ecologically valuable topsoil containing seed banks, making it difficult for vegetation to recover. Underground mine also causes deforestation leading to land erosion.

#### *2.2.3. In situ leach (ISL) mining*

ISL mining in the best type of mining over conventional mining in that the ore body is dissolved and then pumped out, leaving minimal surface disturbance and no tailings or waste rocks (World Nuclear Association, 2012). There is no formation of ore dust, and a lower water consumption is required [11]. But the use of strong acids for dissolving the ore body also dissolves metals in the host rock body. The fluids retained after the leaching process is also dangerous to the nearby ground and surface water sources as it contains elevated concentrations of heavy metals and radioactive isotopes [11]. Additionally, the acid-containing ISL mining wastewater can result in significant acidification of the surrounding environment if not neutralized by neutralizing agents.

#### *2.2.4. Abandoned mines*

Abandoned mines are areas where mining activities once occurred, but proper mine closure and reclamation did not take place or were incomplete. Ineffective enforcement on mine reclamation policy and minimal penalties for noncompliance were the main contributing factors for the large numbers of abandoned mines present today. Mines were used to be abandoned for various reasons such as reduction in the price of mining minerals, unforeseen disaster that caused bankruptcy of the mining company, national security issues in times of conflict leading to the rapid development of a number of mines without proper operations or closure requirements, political unrest causing the unscheduled halt of mining operations and small-scale illegal mining, etc. Although abandoned they still continue to release toxic products from the leftover mining residues, underground workings, open pit mine faces, waste rock piles, and tailings storage areas to the nearby lands and water bodies for a long time. The majority of the AMD actually comes from inactive or abandoned mines [12]. Until the 1970s, there was a lack of mine reclamation policies. However, nowadays most countries have adopted strong regulations according to which mining companies have to provide specific mine closure plan before their mining project is approved.

## **3. Physical and chemical methods of AMD treatments with their inherent advantages and disadvantages**

Physical and chemical procedures of AMD treatment include prevention of AMD drainage by the following processes involved:

• Surface water diversion

In this technique, the water supply to certain acid-generating materials is reduced by the use of safeguard channel pipes, slope changes, and water proof channels.

This technique should be applied after a thorough study of hydrological and hydrogeological area, generally lack of which results in an inappropriate application of this procedure. Sometimes due to surface water diversion, certain less acid-producing areas receive less water, leading to extreme acid production. However, being a cheap technique and easily applicable, it works well in collaboration with other techniques.

• Dry covers

Dry cover application aims at achieving two different objectives:

Different types of materials are used for dry cover. Advantages and disadvantages of different materials used for dry covers are displayed in **Table 1**.

Sludges combined with certain inorganic covers improve the sealing properties and physical conditions of the soil. In promoting vegetation cover, it prevents erosion improving the appearance of the area.

• Sealing with clay

*2.2.4. Abandoned mines*

before their mining project is approved.

the following processes involved:

• Surface water diversion

• Dry covers

**inherent advantages and disadvantages**

106 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

it works well in collaboration with other techniques.

Abandoned mines are areas where mining activities once occurred, but proper mine closure and reclamation did not take place or were incomplete. Ineffective enforcement on mine reclamation policy and minimal penalties for noncompliance were the main contributing factors for the large numbers of abandoned mines present today. Mines were used to be abandoned for various reasons such as reduction in the price of mining minerals, unforeseen disaster that caused bankruptcy of the mining company, national security issues in times of conflict leading to the rapid development of a number of mines without proper operations or closure requirements, political unrest causing the unscheduled halt of mining operations and small-scale illegal mining, etc. Although abandoned they still continue to release toxic products from the leftover mining residues, underground workings, open pit mine faces, waste rock piles, and tailings storage areas to the nearby lands and water bodies for a long time. The majority of the AMD actually comes from inactive or abandoned mines [12]. Until the 1970s, there was a lack of mine reclamation policies. However, nowadays most countries have adopted strong regulations according to which mining companies have to provide specific mine closure plan

**3. Physical and chemical methods of AMD treatments with their**

of safeguard channel pipes, slope changes, and water proof channels.

Dry cover application aims at achieving two different objectives:

(1) Prevention of erosion by wind and water, by stabilization of mine wastes.

Physical and chemical procedures of AMD treatment include prevention of AMD drainage by

In this technique, the water supply to certain acid-generating materials is reduced by the use

This technique should be applied after a thorough study of hydrological and hydrogeological area, generally lack of which results in an inappropriate application of this procedure. Sometimes due to surface water diversion, certain less acid-producing areas receive less water, leading to extreme acid production. However, being a cheap technique and easily applicable,

(2) Pollutant releases inhibition and prevention which can be achieved by preventing oxygen and water penetration in wastes. Oxygen deficiency decreases microbial oxidation of sulfides. Different covering materials such as soil, rock, or any cement-like materials form a crust. The cover must be capable of providing prolonged protection. **Figure 1** shows a

layout of a dry cover generation (reproduced from Johnson and Hallberg [13]).

A convenient coating in the mine holes with clay avoids acid transfer through groundwater discharge. This reduces the water input in mining wastes. Sometimes inland water pits are made in the mines which contain the acidic water acting as a reservoir for the same. This scenario is quite common in abandoned mines, which needs remedial techniques; hence sealing is a desirable option.

#### **3.1. Chemical procedures**

Alternatively, neutralization of AMD is carried out by application of chemicals. Abatement of these unfriendly wastes is possible according to studies. Today there are many methods that are in practice on active mine sites to prevent this from happening. This includes capping of waste piles, having on-site treatment with constructs like limestone trenches [14].

The most widespread method used to mitigate acidic effluents is an active treatment process involving addition of a chemical-neutralizing agent [15, 16]. Addition of an alkaline material to AMD will raise its pH and accelerate the rate of chemical oxidation of ferrous iron.

Earlier, various chemicals including lime, slaked lime, calcium carbonate, sodium carbonate, sodium hydroxide, and magnesium oxide and hydroxide have been used to overcome the acidic fractions as a procedure. However, their cost and effectiveness remain questionable. As reported earlier by scientists, sodium hydroxide is 1.5 times effective in neutralization as compared to lime but remains a dearer option. Diminishing lime reserves and costing systems remain as issues for treatment of AMD along with the formation of ferric oxyhydroxide mantle, thereby compelling researchers to explore different options.

**Figure 1.** Layout of a dry cover for reduction of mine spoil effluents (source: Johnson and Hallberg [13]).

**Table 1.** A comparative analysis of the advantages and disadvantages associated with different materials used for dry cover.

The studies regarding interaction of certain chemicals with tailings, and reduction in acid release and iron and other heavy metals' solubility decrease with the increase in pH, have been reported. Certain inexpensive alkaline chemicals can be added to these mining wastes, which would be beneficial in neutralization of acid pH in mining wastes. But the recurrence of acidification remains an issue.

Limestone shallow pits known as leach beds are filled with limestone chips and stone, adding alkalinity to the passing water. However, the water for treatment must be diverted toward it and should have a passage out after a waiting time. Fairly easy to construct, it seems a wetland with stone bed which is not deep. **Figure 2** shows the use of limestone for reducing the acidity of an AMD site.

Liming process involves application of a lime layer to produce an alkaline environment inhibiting bacterial growth and partial neutralization of the acid. Being a cheap alternative, the practice is highly applicable; however, the neutralizing potential becomes challenging and interrupted on deposition of iron hydroxide layer on the surface, in extreme acidic conditions. Deposition of iron on the limestone reduces its ability to neutralize the acid. Removal of this iron is necessary as its role in leaching is customary. Iron removal from solutions is achieved by precipitation with siliceous materials like furnace slag, portland cement, and fly ash. Oxidation inhibition lessens the amount of ferric ion reacting with pyrites. Various types of alkaline chemicals can be added in the mine wastes to decrease the release of acid, iron, and other metals from the waste deposit. They will neutralize the acid, and as many metals have a lower solubility at higher pH, the release of heavy metals from the mine waste would also decrease. **Figure 3** shows addition of chemicals over mine waste.

#### **3.2. The use of bioreactors**

Utilization of microorganisms for active metal precipitation is the key concept of certain bioreactors, which do not require water diversion.

Phytoreclamation of Abandoned Acid Mine Drainage Site After Treatment with Fly Ash http://dx.doi.org/10.5772/intechopen.69527 109

**Figure 2.** Schematic layout of a reducing and alkalinity producing system (RAPS), redrawn after Younger et al. [17] and Johnson and Hallberg [13].

The studies regarding interaction of certain chemicals with tailings, and reduction in acid release and iron and other heavy metals' solubility decrease with the increase in pH, have been reported. Certain inexpensive alkaline chemicals can be added to these mining wastes, which would be beneficial in neutralization of acid pH in mining wastes. But the recurrence

**Table 1.** A comparative analysis of the advantages and disadvantages associated with different materials used for

**Material Advantages Disadvantages**

Low installation costs

Low installation costs

Cheap and available

Asphalt Fully impermeable Expensive material

Capable of boring Relatively expensive

rate

Undefined

application

Susceptible to weathering

compaction/permeability rate Dry conditions required for

High installation costs Liable to deformation

Undefined compaction/permeability

PVC sheet Completely impermeable

108 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

Grout Lower price

Clays (bentonite) Durable

Limestone shallow pits known as leach beds are filled with limestone chips and stone, adding alkalinity to the passing water. However, the water for treatment must be diverted toward it and should have a passage out after a waiting time. Fairly easy to construct, it seems a wetland with stone bed which is not deep. **Figure 2** shows the use of limestone for reducing the

Liming process involves application of a lime layer to produce an alkaline environment inhibiting bacterial growth and partial neutralization of the acid. Being a cheap alternative, the practice is highly applicable; however, the neutralizing potential becomes challenging and interrupted on deposition of iron hydroxide layer on the surface, in extreme acidic conditions. Deposition of iron on the limestone reduces its ability to neutralize the acid. Removal of this iron is necessary as its role in leaching is customary. Iron removal from solutions is achieved by precipitation with siliceous materials like furnace slag, portland cement, and fly ash. Oxidation inhibition lessens the amount of ferric ion reacting with pyrites. Various types of alkaline chemicals can be added in the mine wastes to decrease the release of acid, iron, and other metals from the waste deposit. They will neutralize the acid, and as many metals have a lower solubility at higher pH, the release of heavy metals from the mine waste would

Utilization of microorganisms for active metal precipitation is the key concept of certain

also decrease. **Figure 3** shows addition of chemicals over mine waste.

bioreactors, which do not require water diversion.

of acidification remains an issue.

acidity of an AMD site.

dry cover.

**3.2. The use of bioreactors**

**Figure 3.** Treatment of solid mine waste with chemicals (redrawn from Ledin and Pedersen [9]). Application of chemicals on mine waste reduces leaching of heavy metals and decreases acid production from pyrite oxidation (source: Ledin and Pedersen [9]).

## **4. Treatment of acid mine drainage with fly ash: killing two birds with a single stone**

Fly ash for reclamation of abandoned coal mine is an environmental approach, which is the most viable option for maximum utilization of disposed ash. Saving land requirements for ash dumping, reclamation also proves to be a better choice for gainful use of reclaimed mines.

Bulk quantity of fly ash can be utilized in lying of underground mines and open cast mine voids with preference, compared to sand [18, 19]. Backfilling of mines provides support to pillars and walls and helps reduction of void volume in underground coal mines. Studies suggest greater stability of backfilled mines than non-backfilled ones. FA works as a backfilling material effortlessly due to its pozzolanic nature which has low permeability with respect to soil or other mine wastes, thereby reducing the chance of chemical leaching. AMD generation and sinking in are reduced in FA backfilled mines, and the substituent solid residues generated remain strongly alkaline and possess pozzolanic properties. Fly ash can be an inherent candidate for mine reclamation due to its unique properties of alkaline nature, easy flowing capable as filler, good water holding capacity initiating afforestation, and a good soil ameliorant [20–22].

However, coal fly ash beneficiation in the case of AMD requires the below-listed criteria, studies of which have been acclaimed toward utilization of coal ash on active mine site. Before ash placement, certain procedural sequential steps need to be taken which would suggest successful ash placement in acid mine reclamation projects. Fly ash contains:

Alkaline quality generates alkaline leachates and is further used to counteract the ability of onsite materials to generate AMD. However, it is recommended that a neutralization potential and pH ranging between 7.5 and 12 are available. For proper utilization of fly ash having pozzolanic properties and low permeability, it is important that the permeability under lab conditions be equal to or less than 1.0 × 10−6 cm/s. The alkaline nature makes fly ash a good soil cover for the soil or soil substitute where it acts as a liming agent. Coal fly ash acts as an ameliorant and improves the characteristics of soil when the soil ash mixture contains a pH of 6.5–8.0. This pH is ideally the most acceptable to attain growth medium. Backfilling of pits and recontouring refused piles make utilization of FA known as ash placement. In short coal ash is beneficial in many ways, acting as a capping agent, for placements and a soil additive or enhancer.

The application and use of fly ash on-site follow certain guidelines before approval which involve detailed application for utilization of fly ash, purpose and amount of requirement, and above all details of handling and incorporation at the project site. A clear outline and indication stating the ash is physically and chemically fit for utilization are mandatory. Collection of background data and written specifications regarding hydrogeological study of the ash utilization area is required to monitor the effect of ash on surface and groundwater quality.

Before application of ash on-site, a chemical analysis on dry weight basis has to be performed (testing of metals and pH). Similarly, leachate tests by SPLP synthetic precipitation leach procedure has to be conducted for pH, sulfate, chloride, and other metals. As long as the ash is used on the mine site, periodic recertification or monitoring is required to ensure ash quality. Sometimes a mixture of bottom ash and fly ash is considered for use on-site; in such cases both the ash quality tests should be provided before utilization. A detail soil analysis report which provides data on pH, PCB, and different heavy metals and nonmetals and trace elements (micro and macro) including arsenic, cadmium, copper, chromium, lead, etc. is required when the proposed utilization of ash is in the form of a soil additive. This leads to the possibilities known for efficient plant take-up process [23].

Ash application on all site areas requires groundwater monitoring, except those where the motive of ash application is just as a soil additive, because the amount of ash addition is comparatively less. However, in all other mines, groundwater monitoring is required before and at the time of application and after ash placement. Monitoring of groundwater includes monitoring wells where there are insufficient groundwater discharge points.

Ash monitoring points should be sampled time to time so as to meet the standard mine drainage parameters along with certain metals like aluminum, cadmium, calcium, selenium, etc. A time interval for monitoring on quarterly basis is required for mine drainage parameters and annually for chlorides and additional existing metals. Moreover, some sites might require additional monitoring. On completion of the site, monitoring continues until all parameters are assessed as stable. Ash placement is done not less than and within 8 feet on top of the groundwater table of that region.

Coal ash utilization for mine reclamation takes into consideration the following special coal ash conditions:

• Grading the disposal area for a stable base.

candidate for mine reclamation due to its unique properties of alkaline nature, easy flowing capable as filler, good water holding capacity initiating afforestation, and a good soil amelio-

However, coal fly ash beneficiation in the case of AMD requires the below-listed criteria, studies of which have been acclaimed toward utilization of coal ash on active mine site. Before ash placement, certain procedural sequential steps need to be taken which would suggest suc-

Alkaline quality generates alkaline leachates and is further used to counteract the ability of onsite materials to generate AMD. However, it is recommended that a neutralization potential and pH ranging between 7.5 and 12 are available. For proper utilization of fly ash having pozzolanic properties and low permeability, it is important that the permeability under lab conditions be equal to or less than 1.0 × 10−6 cm/s. The alkaline nature makes fly ash a good soil cover for the soil or soil substitute where it acts as a liming agent. Coal fly ash acts as an ameliorant and improves the characteristics of soil when the soil ash mixture contains a pH of 6.5–8.0. This pH is ideally the most acceptable to attain growth medium. Backfilling of pits and recontouring refused piles make utilization of FA known as ash placement. In short coal ash is beneficial in many ways, acting as a capping agent, for placements and a soil additive

The application and use of fly ash on-site follow certain guidelines before approval which involve detailed application for utilization of fly ash, purpose and amount of requirement, and above all details of handling and incorporation at the project site. A clear outline and indication stating the ash is physically and chemically fit for utilization are mandatory. Collection of background data and written specifications regarding hydrogeological study of the ash utilization area is required to monitor the effect of ash on surface and groundwater

Before application of ash on-site, a chemical analysis on dry weight basis has to be performed (testing of metals and pH). Similarly, leachate tests by SPLP synthetic precipitation leach procedure has to be conducted for pH, sulfate, chloride, and other metals. As long as the ash is used on the mine site, periodic recertification or monitoring is required to ensure ash quality. Sometimes a mixture of bottom ash and fly ash is considered for use on-site; in such cases both the ash quality tests should be provided before utilization. A detail soil analysis report which provides data on pH, PCB, and different heavy metals and nonmetals and trace elements (micro and macro) including arsenic, cadmium, copper, chromium, lead, etc. is required when the proposed utilization of ash is in the form of a soil additive. This leads to the pos-

sibilities known for efficient plant take-up process [23].

cessful ash placement in acid mine reclamation projects. Fly ash contains:

110 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

rant [20–22].

or enhancer.

quality.

• Alkaline condition • Low permeability

• A placement alternative

• Soil substitute or additive qualities

The abovementioned measures not only ensure mine reclamation but also indulge in wise disposal and utilization of a waste product into a proposed, eco-restored site at no extra cost. This wise adaptation of acid mine filling also helps in afforestation of the otherwise deserted land.

In summary, FA gives following benefits for reclamation of AMD sites:

(1) The alkaline nature of FA is ideal for seal or fill material of mining sites and can prevent the generation of acid mine drainage.

Example of a successful application of using FA for treating abandoned mine was shown by Maryland Department of Natural Resources Power Plant Research Program and the Maryland Department of Environment Bureau of Mines in 1995. They jointly launched an effort to prevent acid formation from abandoned, underground coal mines. They used large volume of coal combustion products (CCPs) from power plants to create flowable grouts. They injected 100% CCP grout into the Frazee Mine, located near Friendsville, Garrett County, MD, and showed that CCP grout mixture can be beneficially used for abandoned underground coal mines to reduce acid formation as well to fill mine voids. The CCP grout mixture appears to be a high-strength, low-permeability material that can easily control mine subsidence and has no negative impact on water quality [24].

## **5. Posttreatment phytoreclamation of the abandoned mine**

After filling the AMD site with fly ash, the next strategy would be preparing it for development of a plant cover. Although FA has many plant growth-promoting macronutrients and micronutrients, it lacks organic carbon, nitrogen, and phosphorous. So prior to plantation development, addition of various supplementations such as organic manure, biofertilizer, vermicompost, etc., is necessary [25].

#### **5.1. Phytoremediation or phytoreclamation**

Phytoreclamation or phytoremediation can be defined by three basic processes [26]:

compounds that plants uptake from soil or water. Sometimes plant enzymes degrade them totally or transform (incomplete degradation) them to a less toxic state.

• Phytostabilization: Phytostabilization is the in situ immobilization or containment of the contaminants in the plant rhizosphere zone. Plants with extensive root system can reduce soil erosion to a large extent and in this procedure immobilize the heavy metals and other compounds present in soil. It is an intermittent way to prevent leaching of harmful chemicals or heavy metals from entering groundwater.

Phytoremediation is a newly evolving field of science and technology that uses plants (grasses/ herbs/woody species) to clean up polluted soil, groundwater, and wastewater and through this procedure removes, contains, or renders contaminants such as heavy metals, metalloids, trace elements, organic compounds, and radioactive compounds. Phytoremediation is dependent on the extraordinary uptake capabilities or sequestration ability of plant roots. Following uptake plants can translocate and bioaccumulate them to shoots or leaves. Some pollutants also undergo phytodegradation, phytotransformation, and phytovolatilization too. Phytoremediation is environment friendly and inexpensive as it does not need expensive excavation and transport of polluted media, and this also reduces the risk of spreading the contamination and has the potential to treat sites polluted with more than one type of pollutant. Phytoreclamation of FA disposal grounds and ash ponds has been applied with documented successes [27]. It is fast becoming acceptable to the public and in most cases is less expensive than traditional treatment technologies such as incineration, bioslurry composting, etc.

#### **5.2. Production of a phytocover**

(2) FA is a very good substrate for agricultural application to produce artificial soil on

(3) The acid-producing materials in the spoil can be neutralized by this alkaline amendment. (4) A flowable fill seals and stabilizes abandoned underground mines to prevent subsid-

(5) Cost of mine site reclamation can be reduced significantly with the use of adequately available FA. This also solves storage-related issues of FA as power plants always face problems on the storage of huge quantities of FA produced as a by-product of coal

Example of a successful application of using FA for treating abandoned mine was shown by Maryland Department of Natural Resources Power Plant Research Program and the Maryland Department of Environment Bureau of Mines in 1995. They jointly launched an effort to prevent acid formation from abandoned, underground coal mines. They used large volume of coal combustion products (CCPs) from power plants to create flowable grouts. They injected 100% CCP grout into the Frazee Mine, located near Friendsville, Garrett County, MD, and showed that CCP grout mixture can be beneficially used for abandoned underground coal mines to reduce acid formation as well to fill mine voids. The CCP grout mixture appears to be a high-strength, low-permeability material that can easily control mine subsidence and has

After filling the AMD site with fly ash, the next strategy would be preparing it for development of a plant cover. Although FA has many plant growth-promoting macronutrients and micronutrients, it lacks organic carbon, nitrogen, and phosphorous. So prior to plantation development, addition of various supplementations such as organic manure, biofertilizer,

Phytoreclamation or phytoremediation can be defined by three basic processes [26]:

• Plant extraction or phytoextraction: Phytoextraction is the removal of contaminants from soil or water through plant uptake and accumulation within plant tissue (either underground or aboveground biomass or both). One subtype is rhizofiltration by which hydrophytes or hydroponically grown plants use their roots in water to uptake or absorb the pollutants from water and bioconcentrate or precipitate them in the root or

• Phytodegradation: Phytodegradation is the degradation or metabolism of the xenobiotic compounds by plant processes or plant-associated enzymes, bacteria, and other microflora. The endophytes present within plant tissues occasionally degrade PAH

abandoned mine lands where native soils are not available.

**5. Posttreatment phytoreclamation of the abandoned mine**

ence and the production of acid mine drainage.

112 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

burning.

no negative impact on water quality [24].

vermicompost, etc., is necessary [25].

**5.1. Phytoremediation or phytoreclamation**

translocate and store them in shoots or leaves.

The initial phytocover development is generally made by growing a mat of grasses and legumes [28, 29]. Grass-legume cover by the native species (and sometimes invasive species) has become the most efficient method of choice as they can readily colonize the area and develop a thick vegetation mat within a short period of time. In the meantime, they enhance fertility of the soil (FA amended with nutrients), prevent erosion and emission of air pollutants, and also phytosequester the heavy metals. This paves way for long-term management and gradual restoration of the site. However, prior to consideration of phytoremediation, factors like studying characteristics of the substrate, properties of the plants and residual microbes, and effect of the phytocover that it would have on the nearby ecosystem should be studied. Natural succession on FA basins has been studied in different countries. Grasses are the primary colonizers in degraded soils as they are better tolerant to drought and other environmental stresses (e.g., xenobiotic compound load) and require fewer nutrients and proliferate rapidly. The fibrous nature of the grass roots is effective in slowing down erosion and preserving soil moisture. Their soil-forming characters eventually produce a layer of organic soil upon which other leguminous crops grow and add nitrogen. Grasses and legumes play great roles for formation of a nutrient-rich top soil. Native species of grasses have ready adaptability with the prevailing environmental conditions, increase soil fertility, have faster establishment, and provide a sustainable microclimate for establishment of commercially useful species. This gives economical return from the rehabilitation programs. The initial cover must allow the development of diverse self-sustaining plant communities. Recently, Pandey et al. [29, 30] and Żołnierz et al. [31] have enumerated various naturally growing species on FA deposits during initial colonization and 11 years after vegetation establishment. Some examples of naturally growing species are *Saccharum munja* [32], nonnodulated species *Cassia siamea Lamk*, nodulated species of chickpea [33], and *Pteris vittata*, a fern [34]. *S. munja* was termed as an "ecological engineer" due to properties such as firm ash-soil binding capability and stabilizes the ash dump surface. According to Pandey [35], *Ricinus communis* L., naturally growing grass species on FA land fill sites, is a suitable plant species for revegetation of degraded lands like mine sites or ash dump sites in tropical and subtropical climates. In addition to acting as carbon sequestrator, it has great commercial value as it is an industrially valuable castor oil yielding crop. It provides ecosystem benefits like carbon sequestration, heavy metal uptake and accumulation, phytodegradation of organochlorine pesticides, substrate quality enhancement, biodiversity conservation, and feedstock for biofuel production [36]. Ferns like *P. vittata* L., *Ampelopteris prolifera* (Retz.) Copel., *Diplazium esculentum* (Retz.) Sw. and *Thelypteris dentata* (Forsk.), etc. also grow well on FA without showing heavy metal-related toxicity symptom. Kumari et al. [37] reported that the fern *T. dentata* is also suitable for phytoremediation and can be used efficiently to revegetate FA landfills. Some woody tree species like *Leucaena leucocephala, Dendrocalamus strictus*, and *Eucalyptus sp*. have the potential power for revegetation of FA disposal sites. These tree species have huge economic importance. Some timber and plywood-yielding plants that have been used successfully in studies are *Shorea robusta, Tectona grandis, D. sissoo, Bombax ceiba, Populus euphratica*, and *Eucalyptus tereticornis* [38]. Examples of some fuel wood tree species recommended by some workers recommended for phytorestoration job are *Acacia auriculiformis, Albizia lebbeck, Acacia nilotica, Casuarina equisetifolia, Cassia siamea, Dalbergia sissoo*, *and Prosopis juliflora.* In addition to their economic importance, they have nitrogen-fixing properties and excellent growth characteristics in nutrient-limited conditions. Examples of plywood-producing tree species which also can be used to act as fuels are *Eucalyptus hybrid, Eucalyptus globules, Melia azedarach, Populus deltoids, Tamarindus indica,* and *Syzygium cumini* [39]. In a study by Carlson and Adriano [40], it was depicted that a new ecosystem can been created on alkaline and acidic FA dump sites with *Platanus occidentalis* (sycamore) and *Liquidambar styraciflua* (sweetgum), which are important timber trees.

Trees contribute greatly to the formation of self-sustaining nutrient-rich top soil formation through biological nitrogen fixation, maintenance of soil organic matter, and increasing water infiltration and storage and reduce loss of nutrients by erosion and leaching. They also improve soil physical structure through reduction of soil acidity and improvement of soil biological activity. Through plant litters and root exudates, nutrient cycle between soil and plants is established. Nitrogen is a major limiting factor of mine spoils, and regular addition of nitrogen-rich fertilizer may be required to maintain vegetation on mine sites. Introduction of legumes and other nitrogen-fixing species may solve this problem. Formation of a productive forest ecosystem from primary or secondary plant community serves as the habitat for biotic communities, establishes its own food-chain trophic levels, and allows biogeochemical cycles. Most tedious process of fly ash dump reclamation or mine site reclamation is the establishment of the initial colonizing grass species. After the initial establishment of grass cover, strategies such as additions of organic amendments and spreading of grass legume fodder seeds, forage legumes, tuft, or hardy grasses are an efficient choice for development of a forest community. The grass legume cover helps in the development of a vegetation mat in a small period of time when compared to the tree species. A reclaimed site can then be used for different purposes like agricultural use or agro-forestry or silviculture. Finally, evaluation must be carried out to determine the success of the reclamation and monitor the groundwater quality. It is to also necessary to determine how closely the reclaimed site functions, in comparison to similar undisturbed sites, as an ecosystem. Reclamation of abandoned mine land is a very complex process. Most researchers agree that reclamation success must be measured by several parameters since no individual parameter provides sufficient information for ecosystem reclamation [41].

## **6. Conclusion**

growing species on FA deposits during initial colonization and 11 years after vegetation establishment. Some examples of naturally growing species are *Saccharum munja* [32], nonnodulated species *Cassia siamea Lamk*, nodulated species of chickpea [33], and *Pteris vittata*, a fern [34]. *S. munja* was termed as an "ecological engineer" due to properties such as firm ash-soil binding capability and stabilizes the ash dump surface. According to Pandey [35], *Ricinus communis* L., naturally growing grass species on FA land fill sites, is a suitable plant species for revegetation of degraded lands like mine sites or ash dump sites in tropical and subtropical climates. In addition to acting as carbon sequestrator, it has great commercial value as it is an industrially valuable castor oil yielding crop. It provides ecosystem benefits like carbon sequestration, heavy metal uptake and accumulation, phytodegradation of organochlorine pesticides, substrate quality enhancement, biodiversity conservation, and feedstock for biofuel production [36]. Ferns like *P. vittata* L., *Ampelopteris prolifera* (Retz.) Copel., *Diplazium esculentum* (Retz.) Sw. and *Thelypteris dentata* (Forsk.), etc. also grow well on FA without showing heavy metal-related toxicity symptom. Kumari et al. [37] reported that the fern *T. dentata* is also suitable for phytoremediation and can be used efficiently to revegetate FA landfills. Some woody tree species like *Leucaena leucocephala, Dendrocalamus strictus*, and *Eucalyptus sp*. have the potential power for revegetation of FA disposal sites. These tree species have huge economic importance. Some timber and plywood-yielding plants that have been used successfully in studies are *Shorea robusta, Tectona grandis, D. sissoo, Bombax ceiba, Populus euphratica*, and *Eucalyptus tereticornis* [38]. Examples of some fuel wood tree species recommended by some workers recommended for phytorestoration job are *Acacia auriculiformis, Albizia lebbeck, Acacia nilotica, Casuarina equisetifolia, Cassia siamea, Dalbergia sissoo*, *and Prosopis juliflora.* In addition to their economic importance, they have nitrogen-fixing properties and excellent growth characteristics in nutrient-limited conditions. Examples of plywood-producing tree species which also can be used to act as fuels are *Eucalyptus hybrid, Eucalyptus globules, Melia azedarach, Populus deltoids, Tamarindus indica,* and *Syzygium cumini* [39]. In a study by Carlson and Adriano [40], it was depicted that a new ecosystem can been created on alkaline and acidic FA dump sites with *Platanus occidentalis* (sycamore) and *Liquidambar styraciflua* (sweetgum), which are important timber trees.

114 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

Trees contribute greatly to the formation of self-sustaining nutrient-rich top soil formation through biological nitrogen fixation, maintenance of soil organic matter, and increasing water infiltration and storage and reduce loss of nutrients by erosion and leaching. They also improve soil physical structure through reduction of soil acidity and improvement of soil biological activity. Through plant litters and root exudates, nutrient cycle between soil and plants is established. Nitrogen is a major limiting factor of mine spoils, and regular addition of nitrogen-rich fertilizer may be required to maintain vegetation on mine sites. Introduction of legumes and other nitrogen-fixing species may solve this problem. Formation of a productive forest ecosystem from primary or secondary plant community serves as the habitat for biotic communities, establishes its own food-chain trophic levels, and allows biogeochemical cycles. Most tedious process of fly ash dump reclamation or mine site reclamation is the establishment of the initial colonizing grass species. After the initial establishment of grass cover, strategies such as additions of organic amendments and spreading of grass legume fodder seeds, forage legumes, tuft, or hardy grasses are an efficient choice for development of a forest community. Ecological restoration of abandoned mine spoil sites has become an important part of the sustainable development strategy in many countries. Long-term mine spoil reclamation requires the establishment of a stable nutrient cycle from plant growth and microbial processes. Application of FA would help formation of soil over mine spoil by neutralizing the acidity and adding other important properties of soil. Growth of grasses and legumes over this would further help in the formation of top soil and by stable nutrient cycling would return the land to its pristine condition that can survive as a self-sustaining ecosystem. A successful green cap on the mine site would control pollution, stabilize the site, and give visual and esthetic pleasure.

## **Author details**

Madhumita Roy\*, Roopali Roychowdhury, Pritam Mukherjee, Atanu Roy, Bulti Nayak and Satarupa Roy

\*Address all correspondence to<EMAIL_ADDRESS>
Department of Biotechnology, Techno India University, Salt Lake, Kolkata, West Bengal, India

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#### **Challenges in Recovery of Valuable and Hazardous Elements from Bulk Fly Ash and Options for Increasing Fly Ash Utilization** Challenges in Recovery of Valuable and Hazardous Elements from Bulk Fly Ash and Options for Increasing Fly Ash Utilization

DOI: 10.5772/intechopen.69469

Ajit Behera and Soumya Sanjeeb Mohapatra Ajit Behera and Soumya Sanjeeb Mohapatra

Additional information is available at the end of the chapter Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/intechopen.69469

#### Abstract

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Beneficiation of fly ash should require for ensuring the removal of reactive elements to reduce the effect of hazardous impact on our atmosphere and can fill the demand for resources such as metals and rare earths. In this chapter, we concentrate to describe the responsible factors involve in fly ash beneficiation that has a great contribution to our environment. The purpose of the current study is to know the recovery of different minerals; maximum removal of the contaminant, reactivity and neutralization capacity of acid mine drainage (AMD) with fly ash and development of the cost-effective method of disposal of fly ash are achieved. Different beneficiation techniques of fly ash and utilization of fly ash are explained.

Keywords: fly ash, separation technique, fly ash utilization, acid mine drainage

## 1. Introduction

Fly ash is the finest form of the residue generate from coal combustion in different power plant and industries. The main cause of formation of residue is the conversion of mineral matter present in coal particles during combustion. As a consequence of coal-based power generation industries, a large amount of fly ash produced in thermal power plants create various disposalrelated problems. Fly ash continues to be treated as a waste product, which contributes nearly 85% of the fly ash generated in India. It is found that in Odisha specific ash generation is 7–9 MT/day/MW and annual ash generation is about 25 MTPA in 2014–2015 (Figure 1), and various areas in which fly ash can be utilized in Odisha are presented in Figure 2. In India, ash generation likely to increase to about 180 MTPA after commissioning of proposed power plants, which required 120,000 hectares of land for disposal [1–3]. A large number of power

© The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited.

Figure 1. Fly ash generation and utilization in Odisha, India.

Figure 2. Status of utilization of fly ash in Odisha (2014–2015).

plants in India utilize the wet fly ash that results in degradation of the pozzolanic characteristics of the ash, an essential ingredient for several ash-based products. In India, Fly Ash Mission was set up in 1994 for bulk utilization of ash. Fly ash notification came out in 1999 and amended in 2009 for enhancing fly ash utilization in various sectors [4]. Fly ashes can be broadly classified on the basis of cementitious properties (Class C: SiO2 þ Al2O3 þ Fe2O3 minimum 50 wt%) and on the basis of pozzolanic activity (Class F: the sum of the aforementioned oxides exceeds 70 wt%) [5]. Fly ash contains heavy metals (such as Fe, Mn, Al), different elements (such as Zn, Co, Pb, Cr, Cu), toxic/reactive elements (anions like SO4), radioactive elements and hazardous elements, which give an increasing trend of negative impact to our environment year after year. Thus, in the coming years, fly ash recovery would be a major challenge. Recovering coal ash from the hazardous materials extends the life of available landfill space and reduces the negative impact on the environment. Recovered ash can fulfil the demand for resources such as metals and rare earths. Ash recovery and processing requires very less energy and produce significantly lower emissions than that of traditional mining and processing of natural mineral resources. Processing of fly ash by different techniques to make it more suitable for a specific utilization is known as beneficiation of fly ash.

Fly ash consists of inorganic, incombustible matter present in the coal that has been fused during combustion into a glassy, amorphous structure. Coal ash consists mainly of aluminosilicate glass matrix in addition to crystalline mullite and quartz. The glass phase in coal ash has been attributed to the formation of soluble salts of As, Na, Mg, K, Sr and B on the surface of coal ash spheres, which aids in the reduction in concentration of these elements. Fly ash must meet required ASTM specification before use in concrete. The loss on ignition (LOI) of fly ash is an important tool to measure the presence of unburned carbon. ASTM C618-89a gives the standard specifications for the use of fly ash in Portland cement with LOI limit of 6% [6].

Disposal of fly ash has several problems, including (1) huge requirement of water (about 36 million litres of water each day for a 1000-MW coal-fired power plant using coal with 40% ash) for draining towards the ponds, (2) creation of large wasteland and (3) health and environmental concerns associated with the likelihood of leaching of heavy metals as well as air-borne ash.

## 2. Environmental impact of fly ash

plants in India utilize the wet fly ash that results in degradation of the pozzolanic characteristics of the ash, an essential ingredient for several ash-based products. In India, Fly Ash Mission was set up in 1994 for bulk utilization of ash. Fly ash notification came out in 1999 and

Figure 1. Fly ash generation and utilization in Odisha, India.

120 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

Figure 2. Status of utilization of fly ash in Odisha (2014–2015).

Fly ash utilization generates the benefit of resource conservation, reduction of waste, avoided emissions with more durable infrastructure and restoration of industrial sites. Major areas of fly ash utilization are as follows: (1) making different types of bricks/blocks for building construction, (2) manufacture of cement and additives in concrete, (3) road construction and (4) embankment, backfill, land development, etc. Land-filled ash also used in same beneficial applications as that of direct ash from the power utility. In other sides, the fly ash contains traces of heavy metals and toxic metals, which are generally below the level that can cause any impact on the environment or public during its normal use. Some elements of fly ash are radioactive. The radioactivity level of fly ash is 1/10th to 1/20th that of the level that causes any harm to environment. This has been tested and certified by Department of Atomic Energy, Government of India. Fly ash particles are generally spherical in shape and range in size from 0.5 to 300 µm, which can cause pulmonary disease. Fly ash able to accumulate toxic elements (Pb, Cr, B, Mo, As and Se) at higher temperatures and here it is considered as an environmental hazard Also due to some elements present in fly ash cause acid drainage from fly ash mine when in a larger period of contact with rain water or moisture. Therefore, the increased acidity has a range of negative effects calculated from the severity of the pH change. Acidic flow towards river system causes inhospitable to aquatic life that results in periodic fish killing, leading to ecological destruction in watersheds. The site requires expensive active maintenance to prevent the acidic drainage from contaminating the drinking water of nearby communities. Therefore, ash recovery is necessary for social, economic and environmental benefits. The recovery can fill demand for natural resources such as metals, rare earths, lime, sand and aggregate and can increase the life of available landfill space. Nowadays, several types of innovations implement for transforming today's coal ash landfills into accessible, resource-rich stores of needed materials.

#### 3. Composition of fly ash

Fly ash constituents depend upon the source industries specific coal bed but may include substantial amounts of oxide elements such as SiO2 both in amorphous and crystalline, Al2O3 and CaO and trace concentrations of elements (up to hundreds ppm) such as arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, vanadium, dioxins and Polycyclic aromatic hydrocarbon (PAH) compounds (Figure 3). Besides the chemical composition, loss on ignition (LOI) reduction will be a requirement and that mentioned for fly ash to meet the specifications most notably the carbon content. Fly ash is a potential source for mineral carbonation by proper chemical co-ordination to capture atmospheric CO2. The carbonated material can find application as a substrate and a soil conditioner for agriculture. In other cases, when some other types of fuel used instead of fly ash to

Figure 3. Entrapments of minerals in fly ash during combustion.

produce electricity in an incinerator, this kind of ash may contain higher levels of contaminants than coal ash that is often classified as hazardous waste. Petrologic analyses of fly ash from different industries were conducted using a high-resolution optical microscope. It helps to detect the percentage and aggregation of materials in fly ash. It is found that the unburned carbon is not uniform and there are three carbon types identified: (i) inertinite particles (semifusinite/ maceral), (ii) isotropic coke and (iii) anisotropic coke.

## 4. Extraction techniques

Physical beneficiation processes enhance the utility of fly ash and are based on differences in size, density, magnetic properties, electrostatic properties and other physical properties of the particles. Chemical separation processes are also achieving a success route for beneficiation of the fly ash. Various types of physical and chemical beneficiation techniques of fly ash are given in the next section.

#### 4.1. Sieving

when in a larger period of contact with rain water or moisture. Therefore, the increased acidity has a range of negative effects calculated from the severity of the pH change. Acidic flow towards river system causes inhospitable to aquatic life that results in periodic fish killing, leading to ecological destruction in watersheds. The site requires expensive active maintenance to prevent the acidic drainage from contaminating the drinking water of nearby communities. Therefore, ash recovery is necessary for social, economic and environmental benefits. The recovery can fill demand for natural resources such as metals, rare earths, lime, sand and aggregate and can increase the life of available landfill space. Nowadays, several types of innovations implement for transforming today's coal ash landfills into accessible, resource-rich

122 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

Fly ash constituents depend upon the source industries specific coal bed but may include substantial amounts of oxide elements such as SiO2 both in amorphous and crystalline, Al2O3 and CaO and trace concentrations of elements (up to hundreds ppm) such as arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, vanadium, dioxins and Polycyclic aromatic hydrocarbon (PAH) compounds (Figure 3). Besides the chemical composition, loss on ignition (LOI) reduction will be a requirement and that mentioned for fly ash to meet the specifications most notably the carbon content. Fly ash is a potential source for mineral carbonation by proper chemical co-ordination to capture atmospheric CO2. The carbonated material can find application as a substrate and a soil conditioner for agriculture. In other cases, when some other types of fuel used instead of fly ash to

stores of needed materials.

3. Composition of fly ash

Figure 3. Entrapments of minerals in fly ash during combustion.

Sieve analysis involves differentiating particle size distributions of fly ash as well as the carbon and minerals in fly ash. In sieve analysis, a stack of sieve with wire mesh screen attached with a column. The mesh size gradually reduces from top to bottom. Generally, a series of four sieve stacks (sizes 90, 75, 45 and 25 µ) and a bottom pan (receiver) were used for sieve analysis. A 100 g sample was put into the top sieve. The column was placed in a mechanical shaker. After the shaking, the material on each sieve is weighed. The weight of the sample of each sieve was then divided by the total weight to give a percentage retained on each sieve [7].

#### 4.2. Air classifier

In this practice, the fly ash feed in the classifier with the main air flow and aerodynamically classifies the material (Figure 4). In this process, air pollution material having less in size can be recovered. There is a classifying wheel, in which, a frequency converter equipped to control the wheel speed. The size of the particle can be adjusted during operation by controlling the speed of classifying wheel. Fine particles are transported by the rotor blades along with the classifying air. These fine sizes are then discharged and collected in a suitable filter, whereas the coarse particles are rejected. The coarse material is rinsed intensively in a spiral flow taken from the secondary air stream to remove any last fine powder and are discharged via the coarse material discharge. The ultrafine fly ash produced can utilize in high-strength concrete [8].

#### 4.3. Density separation

Different minerals in the fly ash have different density. If a mixture of minerals with different densities can be put in a liquid with an intermediate density, the grains with densities less than that of the liquid will float and grains with densities greater than the liquid will sink. The fly ash by flotation on water produces cenospheres and fly ash particles can be considered to be cenospheres. Cenospheres are the hollow glass spheres having relative density <1. The fly ash

Figure 4. Arrangement of air classifier.

floaters are generally larger (40–250 μm) than the heavier fraction. With unique properties, cenospheres establish fly ash utilization in the refractory and polymer industries. Sphericity of cenospheres, controlled particle size and low density allow the manufacture of polymers with improved properties and/or reduced cost [9]. The applications of cenospheres in thermoplastics have been limited by the high-shear conditions in extrusion and injection moulding. In comparison with other irregularly shaped fly ash fillers, higher volume loadings can be achieved without significantly affecting the rheology and are used in Polyvinyl chloride (PVC), under-body sealants, shoe soles, body fillers, oil well cements, polyurethane and polyester-simulated wooden furniture and fibreglass. Cenospheres are applied in oil well cements that improve the pumpability (sphericity) and maintain the consistency of the mix by maintaining the other components in suspension and limiting the segregation (Figure 5).

#### 4.4. Magnetic separation

Fly ash contains both magnetic and nonmagnetic mineralogical composition and distribution of different minerals in the particle. It is very easy to separate the magnetic and nonmagnetic particles using magnetic belt separator shown in Figure 6. Powdered minerals put on the belt by a feeder and the belt rotates along with the powder. When the powder reached at the magnetic roll, ferromagnetic and strongly paramagnetic particles attract towards the roll and fall on separate pan. There is a use of aerodynamic forces in the configuration to augment the magnetic force of separation. Ferromagnetic and strongly paramagnetic can be separated by adjusting the induced magnetization in the ash. For a certain set of separation conditions in the magnetic separation process, the yields of magnetic fractions were directly proportional to the saturation magnetization of the ferromagnetic components of the ash [10]. Generally, more than 50% recovery of concentrated ferrous materials can be achieved using a magnetic separator.

#### 4.5. Electrostatic separation

Elements that chemically and mineralogically differ present in the fly ash can be recovered using electrostatic precipitator [11]. In the electrostatic separation process, particle size is Challenges in Recovery of Valuable and Hazardous Elements from Bulk Fly Ash and Options for Increasing Fly… http://dx.doi.org/10.5772/intechopen.69469 125

Figure 5. Density separation chamber.

floaters are generally larger (40–250 μm) than the heavier fraction. With unique properties, cenospheres establish fly ash utilization in the refractory and polymer industries. Sphericity of cenospheres, controlled particle size and low density allow the manufacture of polymers with improved properties and/or reduced cost [9]. The applications of cenospheres in thermoplastics have been limited by the high-shear conditions in extrusion and injection moulding. In comparison with other irregularly shaped fly ash fillers, higher volume loadings can be achieved without significantly affecting the rheology and are used in Polyvinyl chloride (PVC), under-body sealants, shoe soles, body fillers, oil well cements, polyurethane and polyester-simulated wooden furniture and fibreglass. Cenospheres are applied in oil well cements that improve the pumpability (sphericity) and maintain the consistency of the mix by maintaining the other components in suspension and limiting the segregation (Figure 5).

Fly ash contains both magnetic and nonmagnetic mineralogical composition and distribution of different minerals in the particle. It is very easy to separate the magnetic and nonmagnetic particles using magnetic belt separator shown in Figure 6. Powdered minerals put on the belt by a feeder and the belt rotates along with the powder. When the powder reached at the magnetic roll, ferromagnetic and strongly paramagnetic particles attract towards the roll and fall on separate pan. There is a use of aerodynamic forces in the configuration to augment the magnetic force of separation. Ferromagnetic and strongly paramagnetic can be separated by adjusting the induced magnetization in the ash. For a certain set of separation conditions in the magnetic separation process, the yields of magnetic fractions were directly proportional to the saturation magnetization of the ferromagnetic components of the ash [10]. Generally, more than 50% recovery of concentrated ferrous materials can be achieved using a magnetic separator.

Elements that chemically and mineralogically differ present in the fly ash can be recovered using electrostatic precipitator [11]. In the electrostatic separation process, particle size is

4.4. Magnetic separation

Figure 4. Arrangement of air classifier.

124 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

4.5. Electrostatic separation

Figure 6. Magnetic separator.

approximately less than 45 μm fraction. Electrostatic parametric selection and careful blending of the various fractions recovered in different holding tank (Figure 7). Electrostatic charge induced on the surface plays role when selection is based on electrical susceptibility. During separation of Class F ash, it can separate the two types of glass such as alumina silicate and calcium alumina silicate.

#### 4.6. Triboelectrostatic separation

The triboelectrostatic separation process used to separate unburned carbon and mineral particles. In this process, the organic and mineral particles are charged with opposite polarity (Figure 8). When the two different materials are in contact, electrons move until the energy level of the electrons in the two materials at the interface is equalized. The material with a higher affinity for electrons can able to gain electrons and become negatively charge, whereas the material with the lower affinity loses electrons and become positively charge. On contact with a tribocharger, unburned carbons (organic particles) become positively charge and the mineral (inorganic particles) become negatively charge. The differential charging of different particles in the triboelectrostatic method makes it possible to use a static high voltage separator. Carbon particles are attracted to the negative plate and mineral particles are attracted to the positive plate [12].

#### 4.7. Column flotation

Column flotation cells shown in Figure 9 do not use any mechanical agitation/impellers. Mixing can be done by the turbulence of rising bubbles. In this method, the ground ore is mixed with water to form slurry. The desired mineral becomes hydrophobic in contact with surfactant or collector chemical. But some mineral surfaces are naturally hydrophobic, requiring a trace amount of collector. The collector depends on the nature of the mineral to be recovered. As an example, the collector sodium-ethyl-xanthate (CH3CH2OCS2Na) added in the flotation of PbS to separate it from ZnS. Produced slurry/pulp of hydrophilic and

Figure 7. Electrostatic separator.

Challenges in Recovery of Valuable and Hazardous Elements from Bulk Fly Ash and Options for Increasing Fly… http://dx.doi.org/10.5772/intechopen.69469 127

Figure 8. Principles of triboelectrostatic separation.

approximately less than 45 μm fraction. Electrostatic parametric selection and careful blending of the various fractions recovered in different holding tank (Figure 7). Electrostatic charge induced on the surface plays role when selection is based on electrical susceptibility. During separation of Class F ash, it can separate the two types of glass such as alumina silicate and

The triboelectrostatic separation process used to separate unburned carbon and mineral particles. In this process, the organic and mineral particles are charged with opposite polarity (Figure 8). When the two different materials are in contact, electrons move until the energy level of the electrons in the two materials at the interface is equalized. The material with a higher affinity for electrons can able to gain electrons and become negatively charge, whereas the material with the lower affinity loses electrons and become positively charge. On contact with a tribocharger, unburned carbons (organic particles) become positively charge and the mineral (inorganic particles) become negatively charge. The differential charging of different particles in the triboelectrostatic method makes it possible to use a static high voltage separator. Carbon particles are attracted to the negative plate and mineral particles are attracted to

Column flotation cells shown in Figure 9 do not use any mechanical agitation/impellers. Mixing can be done by the turbulence of rising bubbles. In this method, the ground ore is mixed with water to form slurry. The desired mineral becomes hydrophobic in contact with surfactant or collector chemical. But some mineral surfaces are naturally hydrophobic, requiring a trace amount of collector. The collector depends on the nature of the mineral to be recovered. As an example, the collector sodium-ethyl-xanthate (CH3CH2OCS2Na) added in the flotation of PbS to separate it from ZnS. Produced slurry/pulp of hydrophilic and

calcium alumina silicate.

the positive plate [12].

4.7. Column flotation

Figure 7. Electrostatic separator.

4.6. Triboelectrostatic separation

126 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

Figure 9. Column flotation cells.

hydrophobic particles introduced in flotation tank that is equipped with bubble riser. The hydrophobic particles attached with the carrier air bubbles rises to the surface and form froth. The froth is collected from the cell that producing a concentrate of the target mineral [13].

#### 4.8. Ultrasonic column agglomeration

Ultrasonic column agglomeration consists of a column, variable speed electrical motor, a solvent recovery tank and stainless steel screen (Figure 10). This set-up consists of pre-conditioning treatment in the mix tank and treatment in the column [14]. Ultrasonic wave applied to the slurry mixture in the mix tank and column using a transducer. Ultrasonic wave produced in a frequency range of 0–1 MHz. Slurry of cyclohexane and fly ash was prepared with 5:1 weight ratio. This pre-conditioned slurry exposed to a ultrasonic wave of 0.5 MHz. Then the slurry pumped into the column with the column ultrasonic wave frequency. The agitation speed was controlled. The carbon concentrate was collected on the screen (60-mesh) and air dried. All the carbon recoveries were calculated on a total weight carbon basis.

#### 4.9. Fluidized bed separation (FBA)

Fluidized bed ash formed by fluidized bed combustion in a holding vessel. The quantity of solid particulate placed under appropriate conditions to form a fluid (mixture of solid/fluid) (Figure 11). Generally, this solid/fluid mixture is introduced by the pressurized fluid through the particulate medium or by pumping of mixture using fluid type technologies. Now the solid/fluid mixture acts as a normal fluid and able to flow freely under gravity. The fluidized bed is one of the best-known contacting methods. The coal burn with dolomite or limestone in the fluidized bed burning the unburned carbon and releasing SO2 from exhaust gases. Fly ash collected from the air-controlled equipment and dumped with bottom ash. This blend of this mixture is called FBA [15]. The calcium percentage in FBA is more than in fly ash. FBA has

Figure 10. Ultrasonic column agglomeration cell.

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Figure 11. Fluidized bed separation chamber.

4.8. Ultrasonic column agglomeration

128 Coal Fly Ash Beneficiation - Treatment of Acid Mine Drainage with Coal Fly Ash

4.9. Fluidized bed separation (FBA)

Figure 10. Ultrasonic column agglomeration cell.

Ultrasonic column agglomeration consists of a column, variable speed electrical motor, a solvent recovery tank and stainless steel screen (Figure 10). This set-up consists of pre-conditioning treatment in the mix tank and treatment in the column [14]. Ultrasonic wave applied to the slurry mixture in the mix tank and column using a transducer. Ultrasonic wave produced in a frequency range of 0–1 MHz. Slurry of cyclohexane and fly ash was prepared with 5:1 weight ratio. This pre-conditioned slurry exposed to a ultrasonic wave of 0.5 MHz. Then the slurry pumped into the column with the column ultrasonic wave frequency. The agitation speed was controlled. The carbon concentrate was collected on the screen (60-mesh) and air

Fluidized bed ash formed by fluidized bed combustion in a holding vessel. The quantity of solid particulate placed under appropriate conditions to form a fluid (mixture of solid/fluid) (Figure 11). Generally, this solid/fluid mixture is introduced by the pressurized fluid through the particulate medium or by pumping of mixture using fluid type technologies. Now the solid/fluid mixture acts as a normal fluid and able to flow freely under gravity. The fluidized bed is one of the best-known contacting methods. The coal burn with dolomite or limestone in the fluidized bed burning the unburned carbon and releasing SO2 from exhaust gases. Fly ash collected from the air-controlled equipment and dumped with bottom ash. This blend of this mixture is called FBA [15]. The calcium percentage in FBA is more than in fly ash. FBA has

dried. All the carbon recoveries were calculated on a total weight carbon basis.

been applied to agricultural and brine contaminated lands as a source of micronutrients and as a neutralizer for low pH soils.

#### 4.10. Leaching technique w.r.t pH variation

Different metal present in the fly ash has specific pH, which is able to change according to different reaction. pH has strong effect on their potential release from the fly ash. Hence, each metal had a specific pattern of leaching from ash relative to pH. In high pH (basic) solution, calcium and sodium can release easily, whereas at low pH (acidic) solution, iron and chromium released from fly ash. The release of different elements influenced by the solvent volumes, higher temperatures, longer contact times and lower solid-to-liquid ratios. Varying the condition of leaching, it is easy to concentrate Ni, Cu, Cr and Fe from fly ash. It is observed that as the pH decreases, the order of concentration of different metals is as follows [16]:

$$\text{Ca} - \text{Na} - \text{Ni} - \text{Mg} - \text{Pb} - \text{Mn} - \text{Cu} - \text{Zn} - \text{Al} - \text{Fe} - \text{Cr}$$

(most released at high pH) (most released at low pH)

Anions (Cl, SO4 2), oxy-anions of Se, As, Mo, B and Cr and cations (Al, Fe, Na, K, Ca, Sr, Ba, Zn, Cu, Cd and Mg) are leached from the fly ash heaps by the wastewater derived from the ash slurry which is to be collected for further concentration [17].

#### 4.11. Froth flotation

In this technique, a mixture of fly ash and a collector was prepared and put in a mixture to ensure the uniformity. The slurry was added into the froth floatation chamber with air pump agitator. Carbon particles were coated and bound with the collector and rose to the surface after attaching with the air bubbles. Figure 12 shows the top surface of the flotation cell and the collecting beaker when the collector was added to the fly ash sample. Carbon-rich fraction at the top was collected in a pan and the ash became residue at the bottom. Fly ash LOI reduction analysed in this process that depends on the doses of collector/frother [18].

Figure 12. (a) Froth flotation separation with collector dose and (b) bottom fraction from froth flotation.

Figure 13. Flowchart of oil agglomeration process.

#### 4.12. Oil agglomeration process

Anions (Cl, SO4

4.11. Froth flotation

2), oxy-anions of Se, As, Mo, B and Cr and cations (Al, Fe, Na, K, Ca, Sr, Ba,

Zn, Cu, Cd and Mg) are leached from the fly ash heaps by the wastewater derived from the ash