**10. Sperm motility and progression**

124 Enhancing Success of Assisted Reproduction

**8. Injection with immature sperm**

practice [71].

Other tests of sperm nuclear DNA integrity include *in situ* nick translation and the comet assay. The toluidine blue and sperm chromatin dispersion test are potential new assays [69]. At present, there are two major strategies that may be considered for the treatment of men exhibiting high levels of DNA damage in their sperm: (i) selective isolation of relatively undamaged sperm and (ii) antioxidant treatment [70]. The lack of consensus in defining a clinically relevant standard DNA fragmentation test with a meaningful cut-off level brings challenges in implementing the routine use of sperm DNA integrity assessment in daily

Round spermatid nucleus injection (ROSNI) or round spermatid injection (ROSI) is a method in which precursors of mature sperm obtained from ejaculated specimens or testicular sperm extraction (TESE) are injected directly into oocytes. ROSNI is proposed as a treatment for men in whom other more mature sperm forms (elongating spermatids or sperm) cannot be identified for ICSI [72]. It is not widely performed, not as successful as ICSI and is still an experimental procedure. It should be applied only in the setting of a clinical trial approved and overseen by a properly constituted institutional review board. Accurate identification of round spermatid is a technical challenge of ROSNI. It is difficult to distinguish haploid round spermatids from diploid spermatogenic precursors and somatic cells using the standard optics present in most clinical IVF laboratories. Mouse round spermatids have increased levels of DNA fragmentation [73] that may interfere with fertilization [63]. Increased DNA damage may occur because of deficient sperm nuclear protamine to histone replacement and decreased nuclear condensation in these immature sperm allowing increased susceptibility to reactive oxygen species and other damaging agents in culture. Another major concern is genetic risk. Any genetic abnormality sufficiently severe to result in meiotic arrest during spermatogenesis may also have adverse effects on other normal cellular processes or other systemic manifestations. Occurrence of significant congenital anomalies in ROSNI-conceived pregnancies raises serious concerns [74]. ROSNI should not be performed when more mature sperm forms (elongating spermatids or sperm) can be identified and used for ICSI. Patients who may be candidates for ROSNI should receive careful and thorough pre-treatment counselling to ensure they are

clearly informed of the limitations and potential risks of the procedure [75].

When a cell, with chromosomes in MII, fuses with an inter-phase cell, the nuclear membrane of the cell in the inter-phase dissolves and its chromatin condenses. This phenomenon is called premature chromosomal condensation (PCC) [76]. Following penetration of sperm into an oocyte; oocyte activation is triggered, resulting in completion of meiosis and formation of both male and female pronuclei. Under some circumstances although the sperm is within the oocyte, fertilization fails to occur, the oocyte remains in the MII stage and the sperm head transforms into PCC, separate from the oocyte chromosomes [77, 78]. Chromatin analysis of human oocytes has revealed that sperm PCC is one of the prevalent

**9. Premature chromosomal condensation**

causes of fertilization failure in both IVF and ICSI [77].

Defective sperm tail is the principal cause of sperm motility disorders. There are two main forms of tail disorders with different phenotypic characteristics and consequences for male fertility: non-specific tail anomalies and various genetic disorders including primary ciliary diskinesia and the dysplasia of the fibrous sheath [89]. In non-specific tail anomalies, ICSI has good prognosis and does not pose additional risks in view of the lack of recognized genetic components in this Disorder. Significant sperm abnormalities of proven or suspected genetic origin are rare conditions responsible for extreme asthenozoospermia or total sperm immotility. Affected patients complain of male infertility and chronic respiratory disease, alterations caused by abnormal function of sperm flagella and respiratory cilia. In patients with tail genetic disorders, ICSI results in normal rates of fertilization and implantation, and

many births of healthy babies have been reported. The main concern that remains is the potential transmission to the offspring [89].

Intracytoplasmic Sperm Injection – Factors Affecting Fertilization 127

Obtaining viable spermatozoa from testicular biopsies using pentoxifylline is more effective in terms of fertilization and pregnancies than obtaining it through an HOS test [97]. The clinical use of pentoxifylline for activation of immotile ejaculatory sperm before ICSI in patients with Kartagener's syndrome improves the outcome of the treatment and reduces the need of invasive intervention such as TESE in these patients. The immotile sperm are treated for 30 min with pentoxifylline (1.76 mM) before ICSI. Some spermatozoa show minimal motion and can be used for ICSI. Fertilization rate after ICSI is about 75% [105].

A new era in the field of assisted reproduction opened after the achievement of pregnancies and births after ICSI of human oocytes [106]. In special cases of long-standing male infertility, only a few functional sperm are available. By means of ICSI, most sub-fertile men and even men previously considered sterile (those with azoospermia, extreme

Azoospermia, is the most severe form of male factor infertility. The condition is currently classified as 'obstructive' or 'non-obstructive'. Obstructive azoospermia is the result of obstruction in either the upper or lower male reproductive tract. Sperm production may be normal but the obstruction prevents the sperm from being ejaculated. Non-obstructive azoospermia is the result of testicular failure where sperm production is either severely impaired or nonexistent, although in many cases sperm may be found and surgically

Conflicting results for fertilization and pregnancy rates are available in the literature after use of ejaculated or surgically retrieved sperm. After ICSI, ejaculated or surgically extracted sperm, when motile and morphologically normal, result in similar fertilization, implantation [108, 109] and clinical pregnancy rates ([109]. The incidence of early or late spontaneous abortion and ectopic pregnancy, or malformations is also similar [108]. However, after conventional IVF, even testicular or epididymal aspirates with very good sperm concentration and motility, generally achieve low fertilization and pregnancy rates [110].

The effect of cryopreservation of sperm on ICSI outcome has been thoroughly studied. Current studies suggest that the use of fresh or frozen-thawed sperm does not appear to affect ICSI outcomes [111]. Testicular tissue and epididymal sperm can be cryopreserved successfully without markedly reducing subsequent fertilization and implantation rates and repeated

The origin of the sperm used in ICSI does not have a major influence on the early life outcomes for the offspring, but transgenerational and epigenetic effects remain unknown. From the limited information available, it appears that there is no increased risk of congenital malformations in children born from ICSI. There is, however, a small increase in both de novo and inherited chromosome abnormalities. In terms of growth and neurodevelopment, there are very few studies, and so far, no adverse outcomes have been

testicular biopsy can be avoided without the risk of any decrease in the outcome [112].

found in young children whose fathers have a sperm defect [113].

oligozoospermia or cryptozoospermia) can now father a child.

extracted directly from the testicles [107].

**11. Sperm origin**

Whether sperm movement is slow or rapid generally has no influence on ICSI results. However, injection of immotile sperm usually results in impaired fertilization. In particular, where a non-viable immotile sperm is injected into an oocyte, normal fertilization and pregnancy rarely occurs [90, 91]. In case of immotile sperm, it is possible that the sperm may be dead. The most common practice to select viable non-motile sperm for ICSI involves the hypo-osmotic swelling (HOS) test. However, preliminary results in animal experiments (mouse and rabbit) indicate that viability of injected sperm is not an absolute pre-requisite for fertilization. Embryos derived after injecting mouse oocytes with freeze-dried and thawed sperm developed normally [92]. It appears that provided the DNA integrity of the sperm is maintained, embryos can be generated, at least in animal model, from severely damaged sperm that are no longer capable of normal physiological activity.

The identification of a viable spermatozoon amongst immotile spermatozoa for ICSI often is difficult. However, selection of birefringent spermatozoa under Polscope shows promising results in asthenozoospermic men and men undergoing testicular sperm aspiration or extraction before ICSI [93]. The other tests employed are hypo-osmotic swelling test, the stimulation of motility with pentoxyfilline and non-contact diode laser [94, 95, 96, 97, 98].

In patients with 100% immotile sperm, HOS test is a useful method to examine sperm viability. It measures the functional integrity of the sperm membrane [99]. Upon exposure of the sperm to hypo-osmotic conditions, the intact semi-permeable barrier formed by the sperm membrane allows an influx of water and results in swelling of the cytoplasmic space and curling of the sperm tail fibers. Only viable sperm react to the HOS solution since dead sperm are unable to maintain the osmotic gradient.

Sperm HOS test based on fructose and sodium citrate dihydrate is applied for identification of immotile sperm for ICSI [100]. A significantly greater fertilization and cleavage rate after injection of sperm selected using the HOS test is achieved in contrast to injection of randomly selected sperm. A modified HOS test based on NaCl solution further improves fertilization rate in patients with 100% immotile sperm [101]. In these procedures, approximately 200,000 sperm are exposed to the HOS solution for 1 hour at 37°C. A modified HOS test has been used for samples with a low sperm count such as testicular samples [102]. In this technique, individual morphologically normal sperm is aspirated by microinjection pipette and is exposed to HOS solution for a brief period to minimize the sperm membrane damage.

A mixture of 50% culture medium and 50% deionized grade water has the least delayed harmful effects on sperm vitality [103]. This mixture achieves similar implantation, pregnancy and ongoing pregnancy rates in the ejaculated and testicular non-motile sperm groups [104]. It is a simple and practical procedure and achieves acceptable and comparable pregnancy rates.

Obtaining viable spermatozoa from testicular biopsies using pentoxifylline is more effective in terms of fertilization and pregnancies than obtaining it through an HOS test [97]. The clinical use of pentoxifylline for activation of immotile ejaculatory sperm before ICSI in patients with Kartagener's syndrome improves the outcome of the treatment and reduces the need of invasive intervention such as TESE in these patients. The immotile sperm are treated for 30 min with pentoxifylline (1.76 mM) before ICSI. Some spermatozoa show minimal motion and can be used for ICSI. Fertilization rate after ICSI is about 75% [105].


2.3.3. Test Dataset

In test set-1, the test subject (skier 4) performs only one of the techniques of one of the XC-skiing styles repeatedly on the flat course. It has 8 files (one file for each technique of the classical and skating styles) and resembles the training data. Thus, its preprocessing is performed analogously to the training data. Similarly, the test set-2 resembles the validation data and its preprocessing is performed analogously to the validation data. Thus, 8 tensors are obtained for test set-1 and 1 for the test set-2. Table 6 contains more information about the 2 types of test sets.

**Table 6.** Number of cycles of the classical (DS, P-Off, KDP, DP) and skating (V2, V2A, V1, FS) techniques performed by skier 4 in the test set-1 and test set-2.


**As transfer from soil to shoot and grain**

**Overview of total shoot, soil and grain As for ESR, SSR and WSR**

100

80

60

Cumulative distribution (%)

40

20

0

Cumulative distribution (%)

(solid), WSR (dash) and SSR (square dot).

Cumulative distribution (%)

ESR, WSR and SSR.

864 Environmental Risk Assessment of Soil Contamination

A one-way analysis of variance (ANOVA) was performed on each of the three regions studied. The ANOVA showed that that the mean As levels for grain between sites were not significantly different (*P*=0.662 ), but that those for soil and shoot were (soil: *P*<0.001; shoot: *P*<0.002). Fig.9 shows a comparison of the distribution of As concentrations in the shoot, soil and grain from

0 5 10 15 20

Soil A s (mg/kg)

0.0 0.1 0.2 0.3 0.4

Shoot As (mg/g)

13

0.0 0.1 0.2 0.3 0.4 0.5

Grain As (mg/kg)

**Figure 8 Cumulative ranked distribution of As concentrations in Libyan agricultural soil, shoot and grain from ESR (solid), WSR (dash) and SSR (square dot). Figure 9.** Cumulative ranked distribution of As concentrations in Libyan agricultural soil, shoot and grain from ESR The range of As shoot/soil transfer factors (shoot/soil TFs) was considerable. Minimum and maximum shoot/soil TFs were 0.06 for ESR, 0.04–0.29 for SSR and 0.04 for WSR, while means and medians were 0.1 and 0.02, respectively, for ESR; 0.13 and 0.12 for SSR and 0.06 and 0.06 for WSR. In SSR, the median shoot/soil TF was nearly twice as high as that in ESR and WSR. In this study, transfer of As was an order of magnitude greater from shoot to grain, despite lower rates of soil to grain transfer. The differences in these transfer ratios may be due to differences in As speciation. [18],reported that, in general, higher shoot As levels were consistent with low grain/shoot transfer factors.

#### **Estimation of bioavailable As concentrations by nitric acid extraction**

Nitric acid extraction is a widely used method of estimating the bioavailable concentration of elements. The precision of the results in the present work was controlled by analysis of CRM, blank and spike samples. The correlation factors showed good precision, with the exception of two analytical bioavailable and total results (Table 3). A summary of the bioavailable As in soils sampled from ESR, WSR and SSR are shown in Table 3. A one-way analysis of variance (ANOVA) showed that the differences in As levels between location means are highly significant (*P*<0.001, Table 3). SSR recorded the highest mean levels of bioavailable soil As (0.05 ± 0.01 mg/kg) (Fig.10). In contrast, the highest total mean As level was recorded in ESR (8.10 ± 0.48 mg/kg;.Table 3 and Fig.11b). The mean bioavailable As concentrations in the three regions studied decreased from SSR > ESR > WSR (Table 3) and was in general low, ranging from 0.02–0.07 mg/g, 0.02–0.03 mg/g and 0.01–0.14 mg/g for SSR, ESR and WSR, respectively. Bioavailable concentrations of As are 1–5 % of the total concentrations. In this study, the highest median bioavailable / median total (B/T) As was recorded in SSR (Table 3, Fig.10 and 3.11b).

a **\*** Correlation is significant at the 0.05 level

bB/T= Median bioavailable/ Median total %

**Table 3.** The descriptive statistics for total (T) and bioavailable (B) trace element concentrations in Libyan soils (mg/kg).

There was no correlation between the mean total and the bioavailable As concentrations in ESR and WSR. However, there was a negative correlation between the mean total and the bioavailable As in SSR. [16]. reported that appreciable As can move with leaching water, especially in coarse-textured soils. The mean and median bioavailable As concentration values for SSR were close to the those for WSR and ESR (Table 3). The correlation factors between the bioavailable and the total element concentrations were: 0.073, –0.419 and –0.097 (As); 0.371, – 0.349 and 0.892 (Co); 0.318, –0.124 and 0.614 (Cu); 0.24, –0.161 and –0.545 (Pb); and 0.466, 0.659 and –0.309 (Mn) for ESR, SSR and WSR, respectively (Table 3, Fig.10). Adriano (1986) reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements were derived from P fertilization ) [12]. The low correlation value for total Cd is because the concentrations were lower than the ICP-MS detection limit. Weathering will increase bioavailability of trace elements in SSR. [1],found that at P:As ratios of 4:1 or greater, phytotoxicity on wheat was markedly reduced and deduced phytotoxicity to be a function of P concentration.

There was no correlation between the mean total and the bioavailable As concentrations in ESR and WSR. However, there was a negative correlation between the mean total and the bioavailable As in SSR. [16]. reported that appreciable As can move with leaching water, especially in coarse-textured soils. The mean and median bioavailable As concentration values for SSR were close to the those for WSR and ESR (Table 3). The correlation factors between the bioavailable and the total element concentrations were: 0.073, –0.419 and –0.097 (As); 0.371, – 0.349 and 0.892 (Co); 0.318, –0.124 and 0.614 (Cu); 0.24, –0.161 and –0.545 (Pb); and 0.466, 0.659 and –0.309 (Mn) for ESR, SSR and WSR, respectively (Table 3, Fig.10). Adriano (1986) reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements were derived from P

**Table 3.** The descriptive statistics for total (T) and bioavailable (B) trace element concentrations in Libyan soils (mg/kg).

 Scatter diagram of total vs. bioavailable trace element concentrations in agricultural soils from ESR, SSR and WSR.

**Figure 11.** Comparison of (b) soil extractable of As with (a) total soil concentrations in Libya

#### **4. Discussion**

Total As mean concentrations in ESR are nearly 4.8 and 4.1 times higher than in SSR and WSR, respectively. Total As loads in ESR may be related to the soil parent material, which is comprised of subsequently covered Lower Palaeozoic rocks. This may be the reason why the clay agricultural soils of ESR have the highest background levels of As. This is in agreement with [8], who reported higher concentrations of trace elements in Eastern Libyan soil than in other areas. In the present study, the trace elements present in Eastern Libyan soil (Co, Zn, Cu, As, Cd, Pb and Mn) were nearly five times higher than in WSR and SSR. Trace element concentrations in agricultural soils showed positive linear correlations (Table 2).

In this study, transfer of As was an order of magnitude greater from shoot to grain, despite lower rates of soil to grain transfer. In addition, this study considered samples of soil, shoot and wheat grains from arable land with a sampling design that allowed for the consideration of intra-field variations. The differences in these transfer ratios may be due to differences in As speciation. [18]reported that, in general, higher shoot As levels were consistent with low grain/shoot transfer factors. Although this similarity in trends for plants from varying geographical locations strongly suggests the involvement of plant physiological regulation, the actual mechanisms involved are still far from clear. However, the findings from the present study indicate that As transfer to grain isgoverned by multiple factors (Fig. 7.1). The bioavail‐ ability of trace elements in soils is influenced by a wide array of biophysical factors (Fig. 7.1), including pH, redox potential, organic matter content and soil texture. These factors were, however, not considered in the experimental/sampling design since the main thrust of the field studies was to establish the contribution of elevated soil As to grain As levels.

In this study, the highest median bioavailable / median total (B/T) As was recorded in SSR (Table 3, Fig.10 and 3.11b).[15]reported that appreciable amounts of As can move with leaching water, especially in coarse-textured soils. This is in agreement with the [9], which reported that As mobility and phytotoxicity is usually greater in sandy than in clayey soils. The SSR samples had high B/T bioavailable As, Co, Pb and Mn contents, compared to B/T total elements (Table 3 &Fig.10). In these samples, the levels of bioavailable elements derived from P fertilization (Hurd-Karrer 1939) and the high pH affects the availability of many crop nutrients in the southern Libyan soil [12]. Weathering will increase bioavailability of trace elements in SSR. [10]found that at P:As ratios of 4:1 or greater, phytotoxicity on wheat was markedly reduced and deduced phytotoxicity to be a function of P concentration.

The present study agrees with [16], who reported that the available soil moisture holding capacity for silty clay loams is three times higher than for loamy sands. Therefore, in the present study, total As mean concentrations in ESR are nearly 4.8 times higher than in SSR. Total As loads in Libyan agricultural soil may be related to the soil moisture holding capacity, with different types of soil having varying capacities and also different levels of As.


**1. Introduction**

Rio Tinto Kennecott Copper (RTKC) is a primary copper producer based in Utah, U.S. Like many primary copper producers, RTKC utilizes a flash smelting stage in their process [1]. During the flash smelting process, a portion of the input material is captured, as a dust, by the gases generated by the smelting reactions [2–4]. These gases must be processed prior to release in order to meet regulatory emissions requirements. One of the unit operations necessary for gas treatment is to separate the solid dust from the gases. This is accomplished using a waste heat boiler and an electrostatic precipitator (ESP) [4]. The separated material from the ESP is referred to as the flash furnace electrostatic precipitator dust (FF-ESP dust).

The copper content of the FF-ESP dust is high, over 20% by mass. Because of this, it is necessary to reprocess this material for its copper. Commonly, this is done by simply recycling the FF-ESP dust back into the flash furnace. However, in the case of RTKC, the dust contains high concentrations of both bismuth and arsenic, both of which cause problems further downstream in the copper production process and the final copper product [5].

In metallic copper, arsenic introduces a number of detrimental qualities. It reduces conductivity; increases recrystallization temperature, which may result in increased processing costs if annealing is necessary; and causes grain boundary cracking [6]. In the case of bismuth impurity in copper, bismuth causes embrittlement and increases the work hardening rate [6]. This is because bismuth has no practical solubility in copper and therefore

**Citation:** Caplan, M.; Trouba, J.; Anderson, C.; Wang, S. Hydrometallurgical Leaching of Copper Flash Furnace Electrostatic Precipitator Dust for the Separation of Copper from Bismuth and Arsenic. *Metals* **2021**, *11*, 371. https:// doi.org/10.3390/met11020371

Academic Editor: Srecko Stopic

Received: 13 January 2021 Accepted: 19 February 2021 Published: 23 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

segregates to the copper grain boundaries. In some conditions this can result in liquid metal embrittlement and/or grain boundary wetting. Because of these detriments, the chemical specification for most high-quality cathode coppers, including LME Grade A Cathode Copper, follows ASTM B115-10 Grade 1, which limits arsenic content to 5 ppm and bismuth content to 1.0 ppm [7].

Regarding processing, bismuth and arsenic cause anode passivation and reductions in current efficiency [8]. This is especially true of bismuth. Because of this, the FF-ESP dust cannot always be recycled, resulting in otherwise unnecessary copper losses. Additionally, FF-ESP dust is considered a hazardous waste by U.S. regulatory agencies (in large part due to arsenic), and therefore, the inability to process it results in increased waste stream costs [9]. Alternative methods for FF-ESP dust processing, especially methods that separate copper from bismuth and arsenic, are necessary. That said, any process that deals with the RTKC FF-ESP dust must deal with the arsenic present in it. Generally, this is done via fixation and is well understood in the industry [9,10].

Hydrometallurgical leaching is a commonly used method for the selective separation of one or more elements or compounds. A number of papers have reported on the efficacy of leaching copper flue dusts using sulfuric acid in order to recover copper and other metals; however, separations between copper and bismuth or arsenic are not common [11,12]. Copper flue dusts commonly contain water-soluble copper sulfates; because of this, water is also used to recover copper from copper flue dusts. [4,12] Ha et al. have shown that bismuth can be leached from copper flue dusts using sulfuric acid and sodium hydroxide, but copper is also leached during this process [13].

The potential for hydrometallurgical selectivity and separations can be exhibited by Pourbaix diagrams. These diagrams depict electro-potential and pH on the axes and show the regions of thermodynamic stability for known species of a given system.

Figure 1 shows overlaid copper and bismuth Pourbaix diagrams. This provides regions where separation between copper and bismuth is theoretically possible. The same is shown in Figure 2 but for copper and arsenic instead. All Pourbaix diagrams were generated using StabCal software at 25 ◦C, and molarities are as stated in the captions.

**Figure 1.** Copper (0.173 M) and bismuth (0.00178 M) Pourbaix diagram overlay.

**Figure 2.** Copper (0.173 M) and arsenic (0.0320 M) Pourbaix diagram overlay.

As shown in this paper, the majority of the RTKC dust is composed of mixed amorphous oxide phases containing the copper, bismuth, and arsenic. Ideally, Pourbaix diagrams are generated using the actual phases present in the system. That said, the phase chemistry and population of the dust is complex and not easily simulated. Because of this, the Pourbaix diagrams used are for pure, ideal phases, and therefore they cannot be used to predict exactly what conditions will provide the thermodynamic possibility for separation, but they can be used as a viable starting point.

From Figure 1, it can be seen that it is thermodynamically possible to separate copper and bismuth in the regions of overlap between Bi(OH)+ 2 and CuO or Cu2O. From Figure 2, it can be seen that it is thermodynamically possible to separate copper and arsenic in the regions of overlap between HAsO2− 4 or AsO3− 4 and CuO or Cu2O. By contrasting both figures, it can be seen that it is not thermodynamically likely to separate copper from both arsenic and bismuth in a single leaching stage.

Shown in Figure 3 is a copper Pourbaix diagram with concentration on the vertical axis. Soluble regions are "filled in". It can be seen that as copper concentration increases, so too does the portion of regions that are insoluble. Figure 4 shows the same diagram but for bismuth. The bismuth diagram exhibits the same pattern as copper. Shown in Figure 5 is the 3D arsenic Pourbaix diagram. Unlike copper and bismuth, arsenic solubility increases with arsenic concentration.

Dust processing in general is well suited to hydrometallurgical processing for two primary reasons: the innate ability for hydrometallurgy to be selective, and the environmental benefit of eliminating the emission of toxic metals in gaseous form [14]. Historically, hydrometallurgical developments have enabled high separability and selectivity, such as in the Sherritt Gordon process for the separation and recovery of cobalt, nickel, and copper [15], as well as in the Merrill Crowe cyanidation process for the recovery of gold [14].

In the specific case of copper smelter dusts, a number of studies have been published involving efforts to separate copper from arsenic and bismuth. Ichimura et al., Kovyazin et al., Yang et al., Shanazi et al., and Chen et al. have published studies showing separation of copper from arsenic and/or bismuth via first leaching all three metals and then separating copper from bismuth and arsenic via precipitation methodology [16–19].

**Figure 3.** 3D Cu Pourbaix diagram.

**Figure 4.** 3D Bi Pourbaix diagram.

This work aims to determine the efficacy of utilizing various hydrometallurgical leaching lixiviants for separating copper from bismuth and arsenic in RTKC FF-ESP dust without a precipitation stage. The lixiviants investigated were sulfuric acid, sulfurous acid, sodium hydroxide, and water. Sulfuric acid is commonly used to recover copper from copper smelter flue dusts and is known to leach arsenic and bismuth as well [19–21]. In this study, it was used to provide a baseline of sorts for recovery. Sulfurous acid was used as it provides similar, but more reductive, conditions compared to sulfuric acid. Li et al. and Guo et al. have shown that alkaline leaching methodologies can be used to recovery arsenic from arsenic-bearing metallurgical dusts [22,23]. Zhang et al. has shown that arsenic can be selectively separated from copper using alkali leaching [21]. Sodium hydroxide was used to confirm the efficacy of alkaline leaching for the separation of arsenic

in the unique chemistry of the RTKC dust. Morales et al. has shown that water can provide at least a partial separation between arsenic and copper; because of this, water was used to determine the efficacy of leaching in neutral environments [12].

**Figure 5.** 3D As Pourbaix diagram.

An initial satisfactory separation would occur if copper was transferred to the leachate, with bismuth and arsenic remaining in the residue, or the opposite, with copper in the residue and both bismuth and arsenic in the leachate.

#### **2. Materials and Methods**

FF-ESP dust was acquired from RTKC and analyzed for copper, bismuth, and arsenic content using inductively coupled plasma mass spectroscopy (ICP-MS). The ICP-MS results are shown in Table 1. By mass, over 20% of the dust is composed of copper, approximately 5% of the dust is arsenic, and the bismuth concentration is over 7000 ppm. As stated, the concentrations of the arsenic and bismuth are unacceptably high and therefore must be separated from the copper in order for non-problematic electrorefining.

In addition to the ICP-MS analysis above, AMICS automated mineralogy was utilized to determine the overall phase composition of the FF-ESP dust. The AMICS work was performed by Eagle Engineering. Table 2 lists the identified phases as well as their fraction of the dust by mass. Figure 6 is an AMICS image, and Table 3 provides a color key for Figure 6. From these, it can be seen that the majority of the dust is composed of mixed amorphous phases containing copper, iron, silicon, and arsenic. Because arsenic and copper are consistently present in the same phases, it is impossible to separate them with physical methods; chemical methods, such as hydrometallurgical leaching, are necessary.

**Figure 6.** AMICS image of FF-ESP dust.

**Table 3.** AMICS color key.

Table 4 shows the AMICS elemental composition for the FF-ESP dust. When compared to the ICP-MS composition data, there is good agreement between the two for copper and arsenic. The bismuth concentration shown in Table 1 is below the detection limit of AMICS and would not show up in the scan. Given that AMICS composition data are considered "semi-quantitative", only ICP-MS composition data are used for calculations.

**Table 4.** AMICS elemental composition data for FF-ESP dust.

A 1 L solution containing the requisite concentration of the appropriate lixiviant (ACS grade or higher) was created using deionized water and the lixiviant. This solution was then placed into a baffled flask, heated using a hot plate, and agitated using a magnetic stir bar. The temperature of this solution was measured and controlled via a glass-coated thermocouple. Once the solution reached the appropriate temperature, 50 grams of the FF-ESP dust were added. Upon completion of the leaching time, the leach liquor was separated from the residue using filtration.

The testing conditions for each sulfuric acid, sulfurous acid, caustic, and water design matrix are shown in Table 5, Table 6, Table 7, and Table 8, respectively. The trial conditions for each matrix were generated using Design-Expert 12 software.

**Table 5.** Test conditions for sulfuric acid trials.

**Table 6.** Test conditions for sulfurous acid trials.

**Table 7.** Test conditions for caustic trials.

**Table 8.** Test conditions for water trials.

Analysis of the leach liquors was done via atomic absorption spectroscopy (AAS) and ICP-MS. A statistical analysis of the data was completed using Design-Expert 12 software.

#### **3. Results**

The recoveries of each element to the leachates, as well as the starting pH, ending pH, and ending Eh values, are shown in Table 9 (the sulfuric acid trials), Table 10 (the sulfurous acid trials), Table 11 (the caustic trials), and Table 12 (the water trials). The starting pH values were calculated using first principles, and random trials were confirmed using titration. The ending pH values were determined via titration. The ending Eh values were determined via Eh probe with a standard hydrogen electrode as reference. The recovery (R) of each element (i) was calculated using the concentration of i in the leachate and FF-ESP dust, the volume (V) of the leachate, and the mass (M) of the dust sample used, as shown in Equation (1).

$$\text{\textbullet \text{\textbullet Reoverry for element i: } \text{\textbullet R}\_{\text{i}} = 100 \times (\text{[i}\_{\text{leachate}}] \times \text{V}) / (\text{[i}\_{\text{FF-ESP}}] \times \text{M}) \tag{1}$$

**Table 9.** Elemental recoveries (%) to leachates of sulfuric acid trials, pH data, and Eh data.

**Table 10.** Elemental recoveries (%) to leachates of sulfurous acid trials, pH data, and Eh data.

**Table 11.** Elemental recoveries (%) to leachates of caustic trials, pH data, and Eh data.

**Table 12.** Elemental recoveries (%) to leachates of water trials, pH data, and Eh data.

## **4. Discussion**

Given that copper is the primary product of RTKC, either a high or a low copper recovery to the leachate is desired. This allows for the majority of the copper to be processed from either the leachate or residue. By comparing the recovery values shown above, it can be seen that sulfuric acid provides the highest copper recovery to the leachate. Unfortunately, sulfuric acid did not provide a separation between copper and either bismuth or arsenic.

Sodium hydroxide provided the lowest copper recovery to the leachate, leaving over 99% of the copper in the residue in all but the low alkalinity trials. Like sulfuric acid, caustic leaching did not provide a separation between copper and bismuth. That said, trial 4 of the caustic leach matrix did provide a moderate separation between copper and arsenic. It provided essentially no copper recovery and a 63% arsenic recovery. That said, higher arsenic recoveries were expected based on existing studies on alkaline leaching of arsenic in metallurgical dusts, which have shown arsenic recoveries of over 90% at the highest as well as many trials with recoveries over 80% [22,23]. The difference in arsenic recovery may be attributed to the exact type of alkaline leaching used, as the referenced studies utilized either pressure NaOH leaching or NaOH–Na2S leaching, or may simply be due to the difference in the materials leached.

By comparing trial 4 to the other alkaline trials, it becomes apparent that higher alkalinities, higher temperature, and longer leach times provide higher arsenic recovery without increases in recovery of copper or bismuth. This is partially supported by the response surfaces for caustic leaching (below) generated using Design-Expert 12.

It should be noted that caustic trials 2 and 3, which were only slightly alkaline relative to the other caustic trials, were the only caustic trials to leach a non-near-zero amount of copper. It is hypothesized that the sulfur in the FF-ESP dust reacts with water when leached and generates sulfuric acid. The acid then dissolves the copper present in the dust, resulting in the higher copper recoveries. This is supported by the pH data associated with caustic trials 2 and 3, which show that the leach solution ends substantially more acidic than it starts, and the pH data associated with the water leach trials, which show the same pattern. It is believed that the other caustic trials generated sulfuric acid, but that the generated acid was reacted with the sodium hydroxide before it was able to dissolve copper, and thus the lack of a large shift in pH.

Figure 7 shows copper recovery response surfaces from the caustic leaching design matrix at the minimum and maximum sodium hydroxide concentrations tested. Both surfaces indicate that time and temperature have no meaningful effect on copper recovery, but that sodium hydroxide concentration does. The surfaces indicate that at low sodium hydroxide concentrations (approximately 5 g/L) over 30% of the copper can be recovered, and that at higher sodium hydroxide concentrations, copper recovery can be reduced to almost 0%.

**Figure 7.** Caustic leaching copper recovery response surfaces: low alkalinity (**left**) and high alkalinity (**right**).

Figure 8 shows bismuth recovery response surfaces from the caustic leaching design matrix at the minimum and maximum temperatures tested. It can be seen that the surfaces are identical, meaning temperature has little to no effect on bismuth recovery. Additionally, it is evident that time has no meaningful effect on bismuth recovery. Only sodium hydroxide concentration has a measurable effect on bismuth recovery; higher sodium hydroxide concentrations are associated with higher bismuth recoveries. That said, it should be noted that increasing sodium hydroxide did not increase bismuth recovery by a meaningful degree within the ranges tested.

**Figure 8.** Caustic leaching bismuth recovery response surfaces: low temperature (**left**) and high temperature (**right**).

Figure 9 shows the arsenic recovery response surfaces from the caustic leaching design matrix at the minimum and maximum temperatures tested. Like bismuth recovery, temperature and time had no meaningful effect on arsenic recovery. Only sodium hydroxide concentration had an effect on recovery. Unlike bismuth, however, the effect of sodium hydroxide concentration was substantial; increasing sodium hydroxide concentration from 5 g/L to 50 g/L resulted in an increase of over 40% recovery.

**Figure 9.** Caustic leaching arsenic recovery response surfaces: low temperature (**left**) and high temperature (**right**).

By comparing the caustic leaching response surfaces to the recovery data shown in Table 11, there is some discrepancy. Some trials exhibited recoveries substantially above or below the generated surfaces (see trial 4's arsenic recovery value, for example). On the surfaces themselves, this can be seen by the dots, which represent actual recovery values at the conditions shown. These discrepancies indicate that there may be some effect of time and temperature on arsenic recovery that is not statistically significant given the dataset. Additional data are necessary to confirm or refute this, however. It should be noted that other studies which have examined alkaline leaching of arsenic in metallurgical dusts have shown that increases in time and/or temperature improve arsenic recovery [22,23].

Both the surfaces and actual recovery values show that increasing sodium hydroxide concentration results in meaningful increases to arsenic recovery without meaningful increases to copper and bismuth recoveries, which remain near zero. Because of this, caustic leaching is a good candidate for separation of arsenic.

As shown in Table 10, sulfurous acid provided the second-highest copper recoveries to the leachate. Additionally, sulfurous acid provided a good separation between copper and bismuth. Sulfurous acid recovered approximately 70–80% of the copper to the leachate while only recovering, at most, 4.65% of the bismuth. A separation between copper and arsenic did occur, but only somewhat, given that between 25% and 30% of the arsenic was recovered for most trials. Lower arsenic recovery is necessary for a satisfactory copper– arsenic separation.

The differences in recovery in bismuth and arsenic between the sulfurous trials and the sulfuric trials may be partially explained by the differences in the ending pH and Eh values for the two lixiviants. The sulfuric trials were more acidic and more oxidizing. In the case of bismuth, the data indicates that the sulfuric trials may have resulted in Bi3+ being the dominant phase rather than Bi(OH)+ 2 , which was the dominant phase in the sulfurous trials based on Figure 1. With arsenic, Figure 2 indicates that H3AsO4 or H3AsO3 may be the dominant phase for the sulfuric trials, while only H3AsO3 may be the dominant phase for the sulfurous trials. That said, neither difference in dominant phase would necessitate a strong separation beyond solubility differences; also, as stated, the Pourbaix diagrams do not utilize the phases present in the RTKC dust.

Figure 10 shows copper recovery response surfaces for the sulfurous acid trials at the maximum and minimum sulfurous acid concentrations tested. The low-acid condition indicates that at low temperatures, time has a negligible effect on copper recovery. The same can be said of temperature for short leaching times. That said, at higher values for either variable, the effect of increasing one of them becomes much more pronounced; increasing either results in increased copper recovery. By comparing the two surfaces, it

can be seen that increasing the concentration of sulfurous acid serves to exaggerate this effect.

**Figure 10.** Sulfurous acid copper recovery response surfaces: low acid (**left**) and high acid (**right**).

Figure 11 shows bismuth recovery response surfaces for the sulfurous acid matrix at the minimum and maximum sulfurous acid concentrations tested. The low-acid surface shows that time does not have a substantial impact on recovery, and that higher temperatures are associated with higher bismuth recoveries. When compared with the low-acid surface, the high-acid surface indicates that increased sulfurous acid concentrations serve to reduce the effect of temperature, resulting in a "flatter" surface.

**Figure 11.** Sulfurous acid bismuth recovery response surfaces: low acid (**left**) and high acid (**right**).

Figure 12 shows arsenic recovery response surfaces for the sulfurous acid matrix at the minimum and maximum sulfurous acid concentrations tested. The low-acid surface shows that time and temperature, individually, have little effect on recovery. That said, increasing both results in increased arsenic recovery. By comparing the two surfaces, it can be seen that increased acid concentration serves to increase this effect so that it is more substantial.

**Figure 12.** Sulfurous acid arsenic recovery response surfaces: low acid (**left**) and high acid (**right**).

By comparing the sulfurous acid response surfaces and the recovery data in Table 10, it can be said that an improved copper–bismuth separation is possible by increasing the sulfurous acid concentration. Bismuth sees reduced recoveries from increased acid, while copper sees increased recovery. Unfortunately, arsenic also sees improved recoveries from increased sulfuric acid, so sulfurous acid is only a good candidate for separation of bismuth.

In Table 11, it can be seen that water only recovered between 64% and 69% of the copper to the leachate. That said, water did provide an excellent separation between copper and bismuth, recovering essentially none of the bismuth. Additionally, water provided a good separation between copper and arsenic, recovering 7.86% arsenic on the high end. Given that bismuth is known to be more problematic than arsenic, the recovery values for bismuth and arsenic are very promising. Additionally, the recovery data indicate that neither time nor temperature had a substantial impact on the recoveries of copper, bismuth, or arsenic. This is confirmed by the water leach response surfaces below, and (in regard to copper and arsenic) agrees with Morales et al [12].

Figure 13 shows the copper recovery response surface for the water leach matrix. The surface indicates that at high conditions of either time or temperature, increasing the other has little effect on copper recovery. That said, when both are low, copper recovery is reduced, but by less than 5% recovery.

**Figure 13.** Water leach copper recovery response surface.

Figure 14 shows the bismuth recovery response surface for the water leach matrix. The surface indicates that time has no measurable effect on bismuth recovery and that temperature does. Increases in temperature are associated with reduced bismuth recoveries. It should be noted that the overall range of bismuth recovery is less than 0.2% recovery. Therefore, temperature cannot be said to have a meaningful impact.

**Figure 14.** Water leach bismuth recovery response surface.

Figure 15 shows the arsenic recovery response surface for the water leach matrix. Like the bismuth surface, the arsenic surface indicates that time has no measurable effect and that higher temperatures are associated with reduced recoveries. Unlike bismuth, the range of arsenic recoveries is meaningful but still low, at approximately 3%.

**Figure 15.** Water leach arsenic recovery response surface.

Generally, the recovery data for the leaching matrices agree with what should occur thermodynamically based on the Pourbaix diagrams. The sulfuric acid tests recovered all three elements, providing no separations between them. This makes sense when the pH and Eh data for sulfuric acid in Table 9 are considered, as the conditions provided by sulfuric acid cause soluble phases of all three elements to form based on the Pourbaix diagrams.

Of the various lixiviants, only water provided a separation between copper and both bismuth and arsenic. When the data in Table 12 and the Pourbaix diagrams are considered, this makes sense, as water provided the thermodynamically correct (that is, fairly neutral and oxidative) conditions to separate copper from both bismuth and arsenic.

As shown in Table 11, the more alkaline trials of the caustic leach matrix were in the region of separation between copper and arsenic based on Figure 2, but not for copper and bismuth based on Figure 1; this agrees with the recovery data for the caustic trials, as only a copper–arsenic separation was achieved.

Interestingly, the conditions shown in Table 10 do not indicate that sulfurous acid should be able to generate a copper–bismuth separation. Based on Figure 1, the stable bismuth phase in the conditions generated by sulfurous acid is Bi(OH)+ 2 , which is a soluble phase. However, based on the recovery data, little bismuth was actually dissolved by the sulfurous acid. This may be due to the leach time being too short to allow for equilibrium.

Based on the data shown, there are two potential leaching methods for separation of bismuth and arsenic from copper in RTKC FF-ESP dust. One method is to use water, which transfers the majority of the copper to the aqueous phase and leaves behind the vast majority of the bismuth and arsenic in the residue.

Another method is to, first, leach with approximately 50 g/L sodium hydroxide solution. This will leave the almost all of the copper and bismuth in the residue, while transferring potentially over 60% of the arsenic to the leachate. Given that arsenic is not nearly as problematic in electrorefining as bismuth, removing this amount may be satisfactory. Second, the caustic leach residue is leached with sulfurous acid. This should transfer the majority of the copper to the leachate while leaving the vast majority of the bismuth in the residue. A similar two-stage methodology has been proposed by Zhang et al. and shown to be effective [21]. A flowsheet for this process is shown in Figure 16. It should be noted that this was not tested, and therefore does not account for the necessary acid to react with any remaining sodium hydroxide or the possibility of unanticipated insoluble metastable phases during caustic leaching.

**Figure 16.** Proposed flowsheet for separation of copper from bismuth and arsenic. Approximate expected recoveries are shown in %.

The benefits of the first method are lower arsenic and bismuth values and a lower operating cost, as virtually no expensive reagents are used. The benefit of the second method is higher copper recovery. Given that copper is the primary product of RTKC, this is an important consideration. It should be noted that whatever copper is not recovered may be leached again some number of times by the same or similar means.

#### **5. Conclusions**

Characterization of FF-ESP dust, coupled with thermodynamic analysis and design of experiments for several leaching methods, resulted in the successful separation of copper from bismuth and arsenic in RTKC FF-ESP dust. Water leaching successfully separates both bismuth and arsenic. Caustic leaching successfully separates arsenic via leaching only arsenic under highly alkaline conditions. Sulfurous acid leaching successfully separates bismuth to a higher level than water leaching. It may be possible to get higher separations that what is possible with water leaching by combining caustic leaching and sulfurous acid leaching.

Differences in the expected separations based on the Pourbaix diagrams and the actual separations shown can be likely be attributed to deviations from ideality and interactions between the substances present in the diagrams and the actual materials. Ideally, thermodynamic data for the actual substances present would be generated, as this should allow for a better agreement between the implied separations based on Pourbaix diagrams and experimentation; such thermodynamic data would require either extensive modeling or determination via experimentation.

Continuation of this line of research should include testing of the process shown in Figure 16. Additionally, optimization studies using water and sulfurous acid as lixiviants with wider ranges among the tested variables, and testing for the effect of solid/liquid ratio during leaching, should be conducted with some focus on kinetic modeling. Other methods of alkaline leaching, such as pressurized alkaline leaching, should be pursued to determine if higher arsenic recoveries can be achieved. Moreover, further work regarding the upgrading of copper once in solution should be pursued. Such work should include the separation of copper and iron. This may be done via iron cementation with copper powder, but must be tested.

From an industrial standpoint, once in solution, the copper may be upgraded using a variety of ion-exchange or solvent-extraction methods, and then fed into an electrorefining circuit for production of metallic copper. Bismuth remaining in the leach residue is generally not considered hazardous and should not pose problems regarding disposal, though other elements in the residue, such as arsenic, may. Remaining arsenic could be dealt with via fixation with iron, which is fairly well established in the industry.

**Author Contributions:** Conceptualization, M.C., J.T., C.A. and S.W.; methodology, M.C and J.T.; investigation, M.C. and J.T.; resources, C.A. and S.W.; writing—original draft preparation, M.C. and J.T.; writing—review and editing, M.C. and C.A.; visualization, M.C.; supervision, C.A. and S.W.; project administration, C.A. and S.W.; funding acquisition, C.A. and S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by DOE CMI and Rio Tinto Kennecott.

**Acknowledgments:** The authors acknowledge Rio Tinto Kennecott Copper, the Colorado School of Mines Kroll Institute for Extractive Metallurgy, and Eagle Engineering. This research was supported by the Critical Materials Institute, an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, and the Advanced Manufacturing Office.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


6.3.3. Limited Scalability

Scalability can somehow contradict with the degree of decentralization defined in the current setting of blockchain implementations. The decentralized solutions generally take more computation time for the same set of operations performed with centralized solutions, such as NAS, and so it is expected that decentralized systems are generally less scalable than its centralized counterparts owing to the fact that consensus is needed for every state changed on the data of processed product records.

The multi-layered validation and creation mechanism of product records processed in the decentralized solutions could in all likelihood imply decreased scalability as they could now involve more steps to store or even update representations of any product record, given these state transitions on product records involved are required to be processed in the decentralized system components as well. In case the number and size of product records grow with more products circulating in the supply chain, longer computation time is definitely needed for these product record operations with more computation resources committed on data processing of product records stored in the off-chain backend database structure. Data stored and processed on decentralized system components, such as the blockchain network, should be kept minimum, given the computation resources needed for these decentralized processes could be exponential compared with its centralized counterparts. The requests sent to the existing microservices and blockchain interface could be handled sequentially for which a new request could be processed only if the previous one is completed. The single-threaded handling of these microservices, involved in any end-to-end product record operations, could possibly hinder the scalability of the decentralized solutions as a whole.

Given the extra decentralized processes required in the decentralized solutions from the point of invoking endpoints made available in blockchain interface all the way to the blockchain network. The size of the blockchain will grow over time with more transactions validated and packed in blocks mined, not to mention a new block could be created in an interval of block time defined when initiating the blockchain network. It could take fairly long period of time for any new blockchain node to synchronize with other blockchain nodes to get to the latest global states of the network. The long synchronization time would hinder the user experience for participants using the decentralized solutions when the size of blockchain is too bulk. Though there are potential scalability concerns on any proposed decentralized solutions, it is still worth decentralizing the supply chain anticounterfeiting and traceability, if comparing with the benefits brought by decentralization, such as the strengthened data integrity and improved system security with distributed instances enabling individual nodes along the supply chain collaborating to combat product counterfeits.