**3. Results**

A total of 21 women with FM from a local association participated in this study. The main characteristics of the participants are shown in Table 1.

**Table 1.** Descriptive characteristics of the participants.

Abbreviations: N, sample; SD, standard deviation; BMI, body mass index, FIQR, Fibromyalgia Impact Questionnaire Revised.

Table 2 shows the relative reliability (ICC) and absolute reliability (SEM and MDC) of the performance obtained in the 3MBWT, under the single and dual-task conditions in both test and retest with the different devices. Following the recommendations by Koo et al. [41], the 95% confidence intervals of the ICC were used to interpret the reliability values. Regarding the single condition, "poor" to "good" and "moderate" to "excellent" reliability values were obtained for the stopwatch and Chronopic, respectively. On the other hand, in the dual-task condition, the reliability values for the stopwatch were "moderate" to "excellent" and for the Chronopic were "good" to "excellent".

Abbreviations: 3MBWT, 3 m backward test; s, seconds; SD, standard deviation; ICC, intraclass correlation coefficient; CI, confidence interval; SEM, standard error of measurement; MDC, minimal detectable change; DT, dual-task.

Reliability values obtained by comparing the different devices in both the single and dual-task conditions in the test and retest are shown in Table 3. Taking into account the 95% confidence intervals of the ICC, a reliability value of "good" to "excellent" was obtained for the test, and an "excellent" value was obtained for the retest in the single and dual-task condition.

**Table 3.** Reliability of the 3MBWT under single and dual-task conditions using stopwatch and Chronopic in the test and retest.

Abbreviations: 3MBWT, 3 m backward test; s, seconds; SD, standard deviation; ICC, intraclass correlation coefficient; CI, confidence interval; DT, dual-task.

Figure 1 shows the Bland–Altman plots of the times obtained by stopwatch and Chronopic in test and retest in the single and dual-task conditions, and the times obtained between the two devices in both the single and dual-task conditions in the test and retest, respectively.

Table 4 shows the concurrent validity analysis results of 3MBWT and TUG test. All correlation values obtained were classified as strong [43] in the test and retest and the different devices.

**Table 4.** Concurrent validity between 3MBWT and TUG under single and dual-task conditions, in test and retests, with stopwatch and Chronopic.

Abbreviations: 3MBWT, 3-m backward test; TUG, timed up and go. \*\*\* *p*-value < 0.001.

Finally, the relationship between the performance of the 3MBWT in test and retest conditions under single and dual-task conditions measured with different devices and the impact of the disease were strong, excepting the 3MBWT in test condition under single condition measured with a stopwatch (r: 0.488), which was moderate. In analyzing the dimensions that make up the FIQR questionnaire, strong correlations were obtained between the "symptoms" dimension and all the conditions and devices. Similarly, strong correlations were also obtained between the dimension "overall impact" except for the single and dual-task conditions in the test period, measured with the stopwatch and the Chronopic, respectively, where moderate correlations were reported. As for the dimension "function," only moderate correlations were found in the single condition in the retest and test periods, measured with a stopwatch and Chronopic, respectively. Results are shown in Table 5.

**Figure 1.** (**a**) differences between test and retest vs. the mean of the two measurements under the single condition using a stopwatch; (**b**) differences between test and retest vs. the mean of the two measurements

under the single condition using a Chronopic; (**c**) differences between test and retest vs. the mean of the two measurements under the dual-task condition using a stopwatch; and (**d**) differences between test and retest vs. the mean of the two measurements under the dual-task condition using a Chronopic; (**e**) differences between stopwatch and Chronopic vs. the mean of the two measurements under the single condition in test; (**f**) differences between stopwatch and Chronopic vs. the mean of the two measurements under the single condition in retest; (**g**) differences between stopwatch and Chronopic vs. the mean of the two measurements under the dual-task condition in test; and (**h**) differences between stopwatch and Chronopic vs. the mean of the two measurements under the dual-task condition in retest.

**Table 5.** Correlation between 3MBWT and FIQR under single and dual-task conditions, in test and retest, with stopwatch and Chronopic.

Figure 1 Bland–Altman plots of the times obtained by stopwatch and Chronopic in test and retest under the single and dual-task conditions and Bland–Altman plots of the times obtained between the two devices under the single and dual-task conditions in the test and retest.

#### **4. Discussion**

This study aimed to investigate the test–retest reliability and concurrent validity of the 3MBWT in women with FM under single and dual-task conditions. We also aimed to investigate the agreement between a manual stopwatch and a Chronopic automatic stopwatch. In order to provide clinical and objective directions, the SEM, MDC, and Bland–Altman plots were reported. Generally, good reliability values were obtained for the 3MBWT test when measured with a stopwatch and Chronopic in both single and dual-task conditions. Similarly, the 3MBWT and the TUG achieved good concurrent validity. Lastly, performance between 3MBWT and the impact of the disease were analyzed, and consistent relationships were found between both.

In the present study, walking backward has been analyzed because it is a complex task that requires high neuromuscular and proprioceptive control [22]. Walking backward usually occurs in activities of daily living, such as opening a door, avoiding an obstacle, or backing up to a chair [23]. Therefore, it is a sensitive measure for assessing mobility and balance deficits [24,25]. In people with FM, performing a functional assessment of mobility and balance is essential to manage the disease. However, one of the main tests used to evaluate these characteristics in this population is the TUG [9,11,17]. However, this test does not include backward gait. In this sense, we consider it relevant to analyze the psychometric properties of the 3MBWT in people with FM since previous studies have focused on other populations obtaining good reliability and validity values [26–29]. Therefore, the 3MBWT is a tool that can be used elsewhere in the clinical and research context. Moreover, due to the fact that more than one task is performed at the same time in activities of daily living, and the impairment in dual-task ability detected on people with FM [32,44,45], we decided to incorporate the dual-task paradigm in this cross-sectional study.

Results showed that the 3MBWT could be considered reliable under single and dualtask conditions when measured with both a stopwatch and a Chronopic. However, it is necessary to consider the ICC fluctuation range [41]. Nevertheless, the data presented in the Bland–Altman plots (Figure 1) showed that most of the 3MBWT values were close to the mean of the test–retest differences in both single and dual-task conditions, having a bias close to zero and a reduced variability seeing the limits of agreement. Therefore, there is a high level of agreement between the test–retest measures evaluated in the 3MBWT since the values obtained provide consistent results. The reported ICC in the single condition with a stopwatch (0.71, 95% CI 0.283–0.882) is lower than those reported in previous studies that investigated test–retest reliability in stroke [26] (0.985, 95% CI 0.973–0.992), community-dwelling older adults [27] (0.940, 95% CI 0.90–0.96), and primary total knee arthroplasty [46] (0.942).

Our findings might be due to the symptomatology of FM [47], characterized by widespread pain and fatigue, which can fluctuate in intensity and severity over time [48,49]. For this reason, a person's symptoms and level of functioning can vary daily and affect the physical fitness performance. In this regard, previous studies highlighted what can affect performance in physical fitness tests [50,51]. The same rationale can also affect cognitive function [52], including attention, memory, and information processing. These factors may have contributed to the variability of results between the test and retest. Nevertheless, the ICC values obtained in the present study are acceptable, so the test analyzed seems to be stable enough to be used in the characterization of people with FM.

The optimal time interval between tests may vary depending on the construct being measured, the stability of the construct over time, and the target population [53]. For this study, a seven-day period was selected, the same as previous studies conducted in people with FM [11,18,38,39], to minimize the impact of potential confounding variables, such as recovery or learning effects [54]. However, other studies have used shorter test–retest times to assess test–retest reliability on the 3MBWT [26–29]. In addition, the tests (TUG and 3MBWT), as well as the conditions (single and dual-task), were randomized to ensure that the order of administration may not bias the results. In this way, obtaining more accurate and reliable results is possible by eliminating biases and reducing the influence of external factors that may affect the results.

As expected, the ICC values were slightly higher when using Chronopic versus stopwatch, since the use of a manual stopwatch adds human variability to the measurement by the evaluator [9,55]. Therefore, using an automatic timer can be a cost-effective alternative to assess performance in the 3MBWT in both single and dual conditions. However, although the ICC values obtained by the stopwatch are slightly lower than those obtained by the Chronopic, our data suggest that the use of a manual stopwatch could be also very useful for this test, since it yields relatively good reliability values in both single (0.71, 95% CI 0.283–0.882) and dual-task conditions (0.89, 95% CI 0.718–0.954) (see Table 2). Similarly, the lower ICC obtained in the single and dual-task conditions in the test performance was probably due to human error experienced when using a manual stopwatch. This may be suggested since the scores between the stopwatch and the Chronopic differed slightly, unlike the values obtained in the new test. Nevertheless, the ICC values obtained are classified as good to excellent (see Table 3).

Our study also provided the SEM and MDC values of the 3MBWT under all conditions and devices. These values are important for interpreting the results of the 3MBWT, because they can help clinicians and researchers to determine if there are meaningful changes in the performance of this test. Furthermore, the estimate of random variation in the data (SEM) and the minimum detectable change (MDC) are both lower when the Chronopic is used. Previous research has also observed this trend assessing test–retest reliability under the single condition in TUG and 30 s chair stand test [9,55]. Bland–Altman plots were also reported for a more comprehensive analysis of the results, showing bias and limits of agreement.

In our study, the TUG test was used to test the validity of the 3MBWT since it is a tool frequently used in clinical practice [56,57], showing high reliability in FM in both single [9] and dual-task conditions [11]. Although the TUG test does not contemplate walking backward, as previous studies did [26–29], in the present study we have used the TUG test to conduct the concurrent validity. Moreover, this test has obtained the highest relationship values in most cases [26,27,29]. Significant correlations between 3MBWT and TUG tests were obtained, which can be considered relevant since the TUG test comprises movements that can occur in activities of daily living, including walking, turning, and sitting [37]. In addition, correlation analyses showed a strong correlation in the single condition between the 3MBWT and TUG test measured with a stopwatch, as in previous studies in stroke [26], community-dwelling older adults [27], and adults [23]. In our study, the correlation in single conditions between 3MBWT and TUG measured through the automatic timer also showed strong correlation values. Similarly, the correlation in the dual-task conditions also obtained strong values in the test and retest measured by a stopwatch and a Chronopic.

Our results also found a positive correlation between the 3MBWT performance and the impact of FM obtained through the FIQR questionnaire. Strong to moderate correlations were found between FIQR total score and the performance obtained in single and dual-task conditions using manual stopwatch and Chronopic. Additionally, correlations between FIQR dimensions and 3MBWT have been reported. In this line, the strongest correlations were found in the symptoms dimension, followed by overall impact, where moderate correlations were also found. However, function dimension did not show significant correlation with 3MBWT performance in most cases. These findings could indicate that performance on the 3MBWT does not correlate well with the actions included in the function dimension by including activities that are not highly associated with walking backward (i.e., combing hair, preparing food, shopping) but do correlate with symptomatology and overall impact. Nevertheless, future studies should corroborate this hypothesis.

Previous studies have highlighted the importance of assessing backward gait in older adults [24,25], showing a reduction in performance in different parameters when comparing to young and middle-aged adults. In this regard, during backward gait, greater neuromuscular and proprioceptive control is required, in addition to faster and more frequent balance corrections due to the elimination of visual feedback [22,58]. Additionally, it has been shown that walking backward shows greater or equal sensibility than walking forward, and it is strongly related to the risk of falling [23]. In this sense, assessing backward gait, via the 3MBWT, in FM may be of interest to clinicians to observe changes in mobility and balance due to the balance impairment manifested in this population [59]. Thus, a previous study [60] conducted a physical exercise intervention based on walking backward in community-dwelling older adults. Given these reasons and the strong relationships between the impact of the disease and the performance obtained in the 3MBWT, this test can be used as a useful tool in FM populations to assess exercise-based interventions.

The present study has some limitations. In this regard, the relatively small sample size did not allow us to generalize results to all women with FM. Moreover, only women were included in this cross-sectional study. Thus, results cannot be extrapolated to men with this disease. Therefore, it would be interesting to extend the sample with different age ranges, which allow us to establish cut-off points.

#### **5. Conclusions**

The results obtained from this study show that the 3MBWT is a reliable tool under the single and dual-task conditions in women with FM. It shows higher reliability values when time is taken using a Chronopic. This test also shows high concurrent validity with the TUG test, and its performance is related to the impact of the disease. These results may help clinicians and researchers in the assessment of balance and functional mobility and to interpret the effect of interventions in this population.

**Author Contributions:** Conceptualization, J.L.L.-L., F.J.D.-M. and N.G.; Data curation, A.M.-G. and F.J.D.-M.; Formal analysis, J.L.L.-L., A.M.-G. and F.J.D.-M.; Investigation, S.V.; Methodology, S.V. and N.G.; Project administration, N.G.; Supervision, S.V.; Validation, A.M.-G. and N.G.; Visualization, J.L.L.-L.; Writing—original draft, J.L.L.-L. and S.V.; Writing—review and editing, A.M.-G., F.J.D.-M. and N.G. All authors have read and agreed to the published version of the manuscript.

**Funding:** The author J.L.L.-L. was supported by a grant from the Spanish Ministry of Education, Culture, and Sport (FPU18/05655). The author S.V. was supported by a grant from the Universities Ministry of Spain and the European Union (NextGenerationUE) "Ayuda del Programa de Recualificación del Sistema Universitario Español, Modalidad de ayudas Margarita Salas para la formación de jóvenes doctores" (MS-03). The author A.M.-G. was supported by a grant from the Spanish Ministry of Education, Culture and Sport (FPU17/03130). In the framework of the Spanish National R+D+i Plan, the current study was co-funded by the Spanish Ministry of Sciences and Innovation (reference PID2019-107191RB-I00/AEI/10.13039/501100011033). This study was also funded by the Research Grant for Groups (GR21176) funded by the Junta de Extremadura and the European Regional Development Fund (ERDF/FEDER) "a way of doing Europe". This study was supported by the Biomedical Research Networking Center on Frailty and Healthy Aging (CIBERFES) and FEDER funds from the European Union (CB16/10/00477) "a way of doing Europe".

**Institutional Review Board Statement:** The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the Research Ethics Committee of the University of Extremadura (approval reference: 51/2021).

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study.

**Data Availability Statement:** Data are available under reasonable request to the corresponding author.

**Acknowledgments:** The authors are very grateful to the Extremadura Association of Fibromyalgia (AFIBROEX) in Cáceres for helping recruit the participants for this study.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

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**4. Management**

#### **4.1 Conservative management**

Prevention of prolapse is important and measures should be taken to avoid conditions that increase intra-abdominal pressure. Constipation and chest conditions such as chronic obstructive pulmonary disease (COPD) should be treated adequately. Weight loss is key and hormone replacement therapy may help reduce the incidence. Antenatal exercises, proper intrapartum care and timely caesarean section are also important in reducing the incidence of prolapse.

Initial management should involve appropriate counselling and initiation of pelvic floor muscle training. The best outcomes have been seen when a trained physiotherapist is involved. A perineometer and biofeedback device can be used. Vaginal cones and electrical stimulation have been shown to be effective in incontinence management but no data is available regarding efficacy on urogenital prolapse.

There is a role for devices including vaginal pessaries. They are either silicone or polythene and come in different sizes. They are inserted into the upper vagina and support pelvic organs. They can be cleaned and replaced every 6 months which gives the practitioner a chance to check for any complications. Special attention and counselling should be done with regards to regular changing, sexual function and small possibility of side effects including vaginal discharge, infection and rarely fistula formation.

#### **4.2 Surgical management**

The definitive management of post hysterectomy vaginal prolapse is surgery. This should be done by an appropriately qualified surgeon following thorough assessment and counselling of a patient. Different routes of surgery should be considered and discussed with patient. The choice of surgery depends on type of prolapse, age of patient, previous surgery, comorbidities, surgeon's skills and level of sexual and physical activity.

The aim of surgery is to restore normal vaginal anatomy and restore sexual, bowel and bladder functions. In studies, restoration of apical vagina (Point C on POPQ ) to 0 or I is used as the measure of prolapse treatment [8].

The type of surgery chosen should be individualised to the patient. This would depend on concomitant prolapse in other compartments, previous prolapse surgery, previous abdominal surgery, sexual activity, presence/absence of bowel or urinary symptoms, total vaginal length, presence of comorbidities and patient's preference.

Abdominal sacrocolpopexy can be undertaken via open or laparoscopic route. Patient selection is vitally important. It is the route of choice for women with a short vaginal length, those who require concomitant abdominal surgery and those with history of dyspareunia. The vaginal vault is fixed to the longitudinal ligament on the anterior part of the sacrum using a permanent mesh. Abdominal sacrocolpopexy is associated with lower rates of recurrence, dyspareunia and post-operative stress urinary incontinence compared to sacrospinous ligament fixation [8]. Common complications include infection, bleeding from presacral veins and mesh erosion. Laparoscopy may not be available in all centres and the learning curve is long.

Robotic sacrocolpopexy is available in limited centres around the world owing to the huge cost associated with setting up.

Sacrospinous ligament fixation entails fixing the vaginal vault to the sacrospinous ligament on one side using absorbable or non-absorbable materials. For right handed surgeons this tends to be fixed to the right sacrospinous ligament. No benefit has been shown for bilateral compared to unilateral fixation. It is associated with low recurrence, high satisfaction and takes a short time to perform and a short recovery time. Common complications include buttock pain, pudendal nerve injury, high recurrence of 8–30% of anterior compartment and ureteral obstruction.

High uterosacral ligament suspension (HUSLS) is also an acceptable procedure for vault prolapse but should not be offered as a first choice and should be undertaken by a well-trained pelvic floor surgeon owing to the risk of complications. An RTC comparing high uterosacral ligament suspension and sacrospinous ligament fixation and found the two similar in terms of anatomical, functional and adverse effects [9]. Complications of high uterosacral ligament suspension include bladder injury, ureteric injury, urinary tract infection, blood transfusion and bowel injury.

Transvaginal mesh involves use of permanent mesh to support the vaginal vault to the uterosacral ligament bilaterally in order to restore level I supports. It has been withdrawn in most centres around the world owing to safety concerns and complications. An RTC compared transvaginal mesh and laparoscopic sacrocolpopexy and found that laparoscopic sacrocolpopexy had longer operating time but better success rate and patient satisfaction at 2 year follow up and women in the transvaginal mesh group had shorter vaginal length and risk of erosion [10].

 It can be used to treat vaginal vault prolapse after hysterectomy following careful assessment and counselling. It has the advantage of being minimally invasive, can be done under regional anaesthesia and the technique is easy to learn. Several techniques are known including purse string closure [11], vaginectomy [12], colpocleisis after performing standard anterior and posterior vaginal wall repair [13], purse-string closure of enterocele followed by approximation of perivesical and rectovaginal fascia and high levator plication [14] and le Forte's colpocleisis [15].

It is recommended that one inserts a mid-urethral tape for cure of stress urinary incontinence at the time of surgery if the vaginal route is chosen. Colposuspension has not been shown to be effective in these patients. In case of recurrent vault

prolapse post hysterectomy, the case should ideally be discussed at a multi-disciplinary team before an appropriate plan is made regarding the type of surgery and the person to undertake it.


3. Antioxidant protection systems

The process of oxidative modification of proteins, carbohydrates, DNA, and lipids is a universal mechanism of damage to the cell, especially at the membrane level. On the other hand, the numerous roles of free radicals in physiological processes make their creation a mandatory precondition of life, which is why a protective system has been established during evolutionary development. The basic role of this protection system is to reduce the amount and uncontrolled creation of free radicals and their precursors in the cell.

From a functional point of view, the antioxidant protection of the organism includes three levels of action:

	- a. Enzymatic (superoxide-dismutase, catalase, xanthine oxidase, peroxidase, glutathione peroxidase, glutathione reductase, glutathione-S-transferase). These enzymes make the so-called primary line of antioxidant protection.
	- b.Nonenzymatic or the so-called secondary line of defense.


*Article* **E**ff**ect of Preliminary Alkali Desilication on Ammonia Pressure Leaching of Low-Grade Copper–Silver Concentrate**

**Kirill Karimov 1, Andrei Shoppert 1,\*, Denis Rogozhnikov 1, Evgeniy Kuzas 1, Semen Zakhar'yan 2 and Stanislav Naboichenko 1**

Received: 19 May 2020; Accepted: 15 June 2020; Published: 18 June 2020

**Abstract:** Ammonia leaching is a promising method for processing low-grade copper ores, especially those containing large amounts of oxidized copper. In this paper, we study the effect of Si-containing minerals on the kinetics of Cu and Ag leaching from low-grade copper concentrates. The results of experiments on the pressure leaching of the initial copper concentrate in an ammonium/ammonium-carbonate solution with oxygen as an oxidizing agent are in good agreement with the shrinking core model in the intra-diffusion mode: in this case, the activation energies were 53.50 kJ/mol for Cu and 90.35 kJ/mol for Ag. Energy-dispersive X-ray spectroscopy analysis (EDX) analysis showed that reagent diffusion to Cu-bearing minerals can be limited by aluminosilicate minerals of the gangue. The recovery rate for copper and silver increases significantly after a preliminary alkaline desilication of the concentrate, and the new shrinking core model is the most adequate, showing that the process is limited by diffusion through the product layer and interfacial diffusion. The activation energy of the process increases to 86.76 kJ/mol for Cu and 92.15 kJ/mol for Ag. Using the time-to-a-given-fraction method, it has been shown that a high activation energy is required in the later stages of the process, when the most resistant sulfide minerals of copper and silver apparently remain.

**Keywords:** ammonia leaching; high-pressure leaching; copper extraction; silver extraction; desilication; kinetics

## **1. Introduction**

The depletion of reserves of high-quality raw materials of non-ferrous metals is an increasingly significant problem around the globe today. As a result, low-grade ores and concentrates are involved in processing [1,2], and enterprises have to sacrifice recovery and quality to maintain the required levels of productivity.

The deterioration of technological parameters and indicators is due to a decrease in the content of valuable components and an increase in harmful impurities, along with the fact that non-ferrous and noble metals often exist in minerals as fine impregnations in gangue with mutual intergrowth and the presence of nanodispersed, colloidal grains, etc. [3,4].

Traditional methods of processing and the beneficiation of such raw materials are characterized by significant losses, since such finely disseminated components cannot be separated and recovered to an acceptable degree to obtain monometallic concentrates and intermediates, which leads to significant losses of valuable metals and the stockpiling of various industrial products and waste.

At the same time, the use of such off-balance materials in processing can increase the term of exhaustion of natural resources by several times, which is greatly important for sustainable economic development and environmental safety. Therefore, science and industry are in a constant search for new effective ways of processing such troublesome raw materials.

Currently, the world has developed a large number of diverse technologies for processing such complicated raw materials. The most common technologies involve the ultrafine grinding of raw materials [5–7]; various versions of hydrochemical oxidation with different oxidizing agents, including microorganisms [8–11], nitric acid and its decomposition products [12–15]; processes at elevated temperatures and pressures [16,17]; and combined technologies with various types of preliminary roasting, which are less and less popular due to environmental problems and low technical and economic indicators [18,19].

One of the most widespread technologies in the industry involves sulfuric acid and neutral pressure leaching for the quantitative oxidation of sulfides containing valuable metals (pyrite, arsenopyrite, chalcopyrite, etc.), followed by the cyanidation of the obtained cakes to recover noble metals and non-ferrous metals from solutions [20]. The recovery of silver by cyanidation from sulfuric acid autoclave leachates is difficult due to intergrowths with quartzite and the formation of argentojarosite, inert with respect to the cyanide ion [21]. Therefore, leaching in ammonia media can be used to avoid the formation of argentojarosite and to increase the degree of silver recovery directly at the pressure-leaching stage [22].

Ammonia has been widely used as an effective leaching agent in many hydrometallurgical processes over many years. Ammonia can be used to leach base metals (Cu, Ni, Co and Zn), as well as precious metals (Ag and Au), owing to the formation of soluble and very strong ammonia complexes. Various ammonium salts [23] can act as complexing agents. For example, for sulfide copper minerals and silver, the formation of complexes can be represented as follows (Equations (1) and (2)):

$$\text{CuS} + 2\text{NH}\_3 + 2\text{O}\_2 \rightarrow \text{Cu(NH}\_3\text{)}\_2{}^{2+} + \text{SO}\_4{}^{2-},\tag{1}$$

Cu(NH3)2 2+ + 2NH3 → Cu(NH3)4 2+, (2)

$$\text{Ag}\_2\text{O} + 4\text{NH}\_3 + \text{H}\_2\text{O} \to 2\text{[Ag(NH}\_3\text{)}\_2\text{]}^+ + 2\text{OH}^-,\tag{3}$$

$$\text{2Ag} + 2\text{NH}\_3 + 0.5\text{O}\_2 + \text{H}\_2\text{O} \to 2\text{[Ag(NH}\_3\text{)}\_2\text{]}^+ + 2\text{OH}^-.\tag{4}$$

Ammonia is also considered an attractive leaching agent because of its low toxicity, low cost and ease of regeneration by evaporation from alkaline solutions [24]. Selectivity is an important advantage of the ammonia medium in hydrometallurgy, meaning that the desired metals can be dissolved and the undesired iron segregated in the same unit.

For many years, Alexander Mining (has plants using the "AmmLeach" technology in the Democratic Republic of the Congo and Australia) led the implementation and development of ammonia leaching for the recovery of copper from sulfide, oxidized ores and tailings, as well as the recovery of cobalt [25]. According to Alexander Mining, the key advantages of this technology include the ability to leach in atmospheric conditions, the low consumption of reagents to process acid-intensive materials, the ability to use standard equipment, high selectivity, and a low environmental impact.

The first industrial application of an ammonia leaching process related to the hydrometallurgical recovery of copper from oxidized ores was put into operation in 1916 [26]. Since that time, a large number of studies have been carried out to study the kinetics of leaching various copper ores and copper-containing wastes in ammonia media at atmospheric [27–31] and high pressure [32,33]. For example, Liu et al. [30] studied the leaching of calcareous bornite ore in a solution containing ammonia and ammonium persulfate. The study showed that copper recovery rises from 82.7 to 90.3% as the temperature increases from 303.15 to 333.15 K. It was also revealed that copper recovery reaches 77.8% and 88.1% at ammonia concentrations of 1.0 mol/dm3 and 2.5 mol/dm3, respectively, and that an increase in the concentration of ammonium persulfate from 0.5 mol/dm3 to 2.0 mol/dm3 effects

an increase in copper recovery from 80.9% to 90.3%. Baba et al. [34] investigated the kinetics of leaching with ammonium sulfate solutions of complex covellite ore mined in Nigeria. It was revealed that the process was controlled by the diffusion kinetics, with an activation energy of 37.37 kJ/mol. In this case, copper recovery was 86.2% under optimal conditions.

Ammonia leaching with oxygen as an oxidizing agent was used commercially in 1954 to extract copper, nickel and cobalt from sulfide ores and concentrates by Sherritt Gordon [23] in Fort Saskatchewan. In the 1970s, Anaconda developed the Arbiter ammonia leaching process [24]. In one study [35], it was shown that for a concentrate containing 25% copper in the form of chalcopyrite in a solution of ammonium hydroxide and ammonium carbonate with the use of oxygen as an oxidizing agent under optimal conditions (НН4OH concentration of 5 mol/dm3; (NH4)2CO3, 0.3 mol/dm3; L/S, 20 to 1; stirring speed, 1000 rpm; temperature, 333 K; and oxygen flow rate, 1 dm3/min), only 70% of copper could be recovered into solution. A kinetic analysis of this process shows that the leaching rate was controlled by diffusion through the product layer, and the activation energy was 25 kJ/mol.

Ammonia leaching can also be used to extract precious metals. The ammonia–cyanide process was used commercially by Hunt to process tails from Comstock Lode in Nevada and copper–iron gold ores in Dale, California [36]. The industrial application of the Hunt process started in 2014, when ammonia leaching was introduced at the Gadabay plant [26].

Few studies, however, have focused on the kinetics of simultaneous copper and silver recovery by ammonia leaching from low-grade concentrates or ores under pressure [22,24,37]. Therefore, the aim of this work is to investigate the kinetics and mechanisms of the ammonia pressure leaching of low-grade sulfide raw materials containing copper and silver, in order to study the effects of Si-containing minerals on the kinetics of Cu and Ag leaching.

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

#### *2.1. Materials*

All the reagents used in this study were of analytical purity. A 25% ammonia solution was provided by JSC Azot (Kemerovo, Russia), and caustic soda was provided by JSC Bashkir Soda Company (Sterlitamak, Russia).

To achieve the required concentrations of solutions, ammonia and caustic alkali solutions were diluted with distilled water obtained on the apparatus manufactured by GFL GmbH (Burgwedel, Germany).

The original high-siliceous copper concentrate was provided by an ore processing plant at a Zhezkazgan mine, Kazakhstan [38]. The concentrate was obtained by a two-stage flotation enrichment (rougher and scavenger) of the initial low-grade ore containing 0.53% of copper and 12.53 ppm of Ag. The yield of the concentrate was 7.09%, while the copper recovery 93.7%. The average particle size of the concentrate was 30 μm; 90% of the particles were smaller than 61 μm.

Table 1 shows the chemical composition of the obtained concentrate. Figure 1 shows an X-ray diffraction pattern of this concentrate. As can be seen from Table 1 and Figure 1, the initial concentrate contains a large amount of aluminosilicates and quartz, while the content of sulfur and copper does not exceed 5%. It is obvious that the iron is partly present in oxidized form (chlorites).

**Table 1.** Chemical composition of initial low-grade copper–silver concentrate, wt. %.

**Figure 1.** XRD pattern of initial low-grade copper–silver concentrate.

Table 2 shows the distribution of copper-containing minerals in the concentrate. The amount of oxidized copper was determined by leaching the initial concentrate in 10% sulfuric acid solution for 2 h at 95 ◦C without an oxidizing agent. The distribution of primary and secondary copper sulfides was determined by the x-ray analysis of the concentrate after the complete removal of aluminosilicates [39]. It can be seen that the largest part of the copper is represented by primary and secondary sulfides, but oxidized minerals are also present.

**Table 2.** Copper distribution in initial low-grade copper–silver concentrate, by minerals.

### *2.2. Analytical Methods*

The chemical analysis of the original material and the resulting solid products of the studied processes was performed using an Axios MAX X-ray fluorescence spectrometer (XRF) (Malvern Panalytical Ltd., Almelo, The Netherlands). The phase analysis was performed on an XRD 7000 Maxima diffractometer (Shimadzu Corp., Tokyo, Japan).

Scanning electron microscopy (SEM) was performed using a JSM-6390LV microscope (JEOL Ltd., Tokyo, Japan) equipped with a module for energy-dispersive X-ray spectroscopy analysis (EDX).

Samples from each experiment were taken at the selected time intervals, and the obtained solutions were analyzed using inductively coupled plasma mass spectrometry (ICP-MS—NexION 300S quadrupole mass spectrometer, PerkinElmer Inc., Waltham, MA, USA).

#### *2.3. Experiments*

Laboratory experiments for NaOH and NH4OH leaching were carried out using a 0.6 dm3 autoclave reactor (Parr Instrument, Moline, IL, USA), equipped with sample collection vessel. The reactor was thermostated. The materials were stirred using an overhead mixer at 700 rpm, which ensured a uniform density of the pulp and eliminated diffusion limitations. A portion of the raw material weighing 100 g was added to a prepared solution of NaOH (300 g/dm3) or NH4OH and

(NH4)2SO4 (with solution concentrations of 0.5 mol/dm3 (NH4)2SO4 and 7 mol/dm3 NH3); the L/S ratio in all the experiments was 5 to 1. Then, the resulting pulp was heated in an autoclave with constant stirring. After the required temperature was reached, the oxygen supply and time countdown began. At the predetermined time intervals, a portion of the leaching pulp was taken with help of excessive pressure in reactor and cooled in a sealed vessel to atmospheric temperature. The obtained pulp was filtered in a Buchner funnel (ECROSKHIM Co., Ltd., St. Petersburg, Russia); the solutions were sent for ICP-MS analysis. At the end of experiment, the leaching cake was washed with distilled water, dried at 100 ◦C to a constant weight, weighed and sent for XRF analysis. All the experiments were performed twice, and the mean values are presented here.

#### **3. Results and Discussion**

#### *3.1. Leaching Kinetics of the Initial Concentrate*

The pressure leaching of sulfide copper concentrate in an ammonia environment is possible only in the presence of an additional oxidizing agent, necessary for the conversion of copper and silver into oxidized forms. However, even with a strong oxidizing agent, the efficiency of copper and silver recovery will strongly depend on the process temperature. In order to determine the limiting stage and to better understand the mechanisms of the reactions, we studied the kinetics of the ammonia leaching of a sample of the original concentrate at various temperatures and otherwise-equal conditions. The oxygen pressure in reactor was 0.6 MPa, and the total pressure was 0.9 MPa at 120 ◦C and 1.8 MPa at 160 ◦C. The obtained leaching kinetic curves at various temperatures ranging from 120 to 160 ◦C are shown in Figure 2.

**Figure 2.** Effect of temperature and ammonia leaching duration on the degree of Cu (**a**) and Ag (**b**) recovery into solution.

Figure 2a shows that as temperature the increased from 120 to 160 ◦C, the degree of copper recovery into solution increased significantly. After 3 h of leaching at 120 ◦C, the degree of copper recovery into the solution is 36.0% and reaches 81.3% at 160 ◦C within the same amount of time. The degree of silver recovery also increases (Figure 2b). However, unlike copper, silver can be effectively recovered into solution only at temperatures above 160 ◦C. After 3 h of leaching at 160 ◦C, the recovery of silver in the solution was 51.1%, while at below 130 ◦C, it did not exceed 15–16%. According to published data [28], oxidized copper leaches quite efficiently into an ammonia solution, even at atmospheric pressure. The oxidation rate for copper sulfides in an ammonia solution in autoclave conditions is also quite high, even at temperatures below 140 ◦C [22,35], which means that the process under study has certain limitations.

As shown in Figure 3, the corresponding experimental data for the leaching of silver and copper from the initial concentrate were linearized using various shrinking core models (SCM) [40–42] (Table 3), and the most adequate one proved to be the internal diffusion model, 1–2/3α − (1 − α) 2/3 = Kt, where α

is the degree of component recovery into the solution, K is the reaction rate constant (min−1) and t is the duration (min). This result indicates that the leaching of copper and silver is limited by the rate of the supply of oxygen or a leaching reagent to the reaction surface through a layer of the product or unreacted substance.

**Figure 3.** Dependence of 1–2/3α − (1 − α) 2/3 on time for Cu (**a**) and Ag (**b**) recovery into solution.

sp—spherical particles; α—the degree of Cu recovery into the solution.

Figure 4 shows a plot of lnk versus the inverse of temperature, 1/T (K−1). In accordance with the Arrhenius equation, the slope of the straight line in Figure 4 was used to determine the apparent activation energy of the leaching of copper and silver as 53.50 kJ/mol and 90.35 kJ/mol, respectively. It follows that the silver leaching reaction requires higher activation energies than copper leaching. This is probably due to the fact that the copper in the initial material is represented by various minerals, including oxides, some easily soluble at lower temperatures. By contrast, silver and a certain part of sulfide copper are more refractory. It is also possible that the copper and silver minerals are finely impregnated in the aluminosilicate rock, resulting in diffusion limitations. As shown before [43], a high value of the activation energy is not always representative of a kinetically controlled reaction.

**Figure 4.** Dependence of lnk-1000/T for the recovery of copper (**a**) and silver (**b**) into solution.

To reveal the effect of aluminosilicates on the mechanism of leaching, electron micrograph and EDX analyses of the initial concentrate particles (Figure 5) were performed. According to Figure 5, there are two types of grain with different morphologies in the initial concentrate: grains with a smooth surface with ultrafine particles scattered on them and grains with a rough surface. The result of the EDX analysis (Table 4) shows that the first type of grain is most likely a sulfide mineral. The EDX analysis also indicates that sulfides are bound to aluminosilicates by mutual germination, since silicon is distributed, although unevenly, over the entire surface, including at accumulations of copper and silver. At the same time, silver is inextricably linked with copper accumulation areas.

(**c**)

**Figure 5.** SEM images of typical grains of the initial concentrate (**a**, **b**) and SEM image with the position of an EDX spectrum (**c**).

Thus, we can assume the inhibitory effect of aluminosilicates, as their layer on the particle surface prevents the diffusion of oxygen and ammonia. Aluminosilicates can also adsorb non-ferrous metals from solution [44,45]. Therefore, the direct pressure ammonia leaching of the concentrate without pre-treatment does not allow copper and silver recovery from a low-grade concentrate into the solution at temperatures below 160 ◦C, which is explained by the diffusion limitations created by a layer of aluminosilicates on the surface of sulfides.

In order to remove the passivating effect of aluminosilicates, one can use physical (mechanical activation, fusion) and chemical methods (preliminary alkaline desilication). Therefore, to further study the effect of aluminosilicates on copper and silver recovery into an ammonia solution, we studied the process kinetics after extracting silica from the original material by pressure alkaline leaching.

#### *3.2. E*ff*ect of Preliminary Desilication on Ammonia Leaching of Low-Grade Copper–Silver Concentrate*

To exclude the possible effects of silica-containing minerals, we investigated the preliminary removal of silicon from the concentrate by pressure alkaline leaching under the following conditions: NaOH = 300 g/dm3, L/S = 5:1, t = 200 ◦C, total pressure = 1.5 MPa, and duration = 2h[39]. The method allowed the recovery of 63% of the silicon into the solution, while the cake had the following composition (Table 5):

**Table 5.** Chemical composition of desilicated copper concentrate, wt. %.

#### 3.2.1. Kinetics of Copper and Silver Leaching from a Concentrate after Preliminary Desilication

The effect of temperature and the duration of leaching on copper and silver recovery from a desilicated concentrate is shown in Figure 6.

**Figure 6.** Dependence of the recovery of copper (**a**) and silver (**b**) into the solution on the temperature and duration of leaching of desilicated concentrate.

Figure 6 shows that, as in the case of the initial concentrate, the temperature has a significant effect on the speed of the process; after 60 min of leaching, copper recovery increases from 63% to 94%. A significant influence of the duration in the range 0–60 min is observed at all studied temperatures; after 60 min, the rate of Cu and Ag recovery in the solution decreases significantly, which is most likely due to diffusion limitations. Nevertheless, the rate of Cu and Ag recovery into solution from a desilicated concentrate is much higher, especially clearly seen in case of silver and apparently associated with a decrease in the passivating or sorption effect of aluminosilicates after desilication and with an increase in the surface area of sulfide particles that are in contact with solvent.

As shown in Figure 7, the corresponding experimental data on Cu and Ag leaching from a desilicated concentrate were also linearized using various shrinking core models. In this case, the most adequate was a new model proposed by Dickinson and Heal [40], 1/3ln(1 – α+((1 − α) -1/3 − 1) = kt, where α is the degree of a component's recovery into solution, k is the reaction rate constant (min−1), and t is the duration (min). This result indicates that the leaching of copper and silver is limited by the rate of the supply of oxygen or leaching agent to the reaction surface through the interfacial transfer

and diffusion across the product layer. Thus, we can assume that in this case, the process mode changes: interfacial transfer is added to the internal diffusion.

**Figure 7.** Dependence of 1/3ln(1 – α + ((1 − α) −1/3 − 1) on time for Cu (**a**) and Ag (**b**) recovery into solution.

Based on the slope of the straight line in Figure 8, the apparent activation energy of the leaching is 86.76 kJ/mol for Cu and 92.15 kJ/mol for Ag, respectively. This indicates an increase in the effect of temperature on the leaching of copper.

**Figure 8.** Dependence of lnk−1000/T for the recovery of copper (**a**) and silver (**b**) into solution from a desilicated concentrate.

The obtained high value of the activation energy along with the high degree of description of the kinetic curves by the new SCM equation, which is applicable for the processes restricted by the supply of reagents through the diffusion layer, indicates a complex mechanism of the autoclave leaching of copper and silver in an ammonia environment.

The SCM can only be used to determine the average values of the activation energy, which may lead to the omission of some reaction features. The time-to-a-given-fraction method [46] was used to calculate the apparent activation energy at different points in the leaching process. The time required to achieve a certain degree of leaching and the apparent activation energy, Ea, are related according to Equation (5).

$$\text{Int}\_{\text{x}} = \text{const} - \text{lnA} + \text{E}\_{\text{u}} \text{/RT},\tag{5}$$

The slope of the graph plotted with lntx and 1000/T coordinates allows one to calculate the apparent activation energy.

The calculation of activation energy by the graphical method was performed as shown in Figure 9.

**Figure 9.** Dependence of lntx on 1000/T for various degrees of recovery of copper.

According to Figure 9, the calculated values of the apparent activation energy according to the time-to-a-given-fraction method change from 39.77 to 60.95 kJ/mol as the degree of copper recovery into the solution increases from 10% to 50%. These values are lower than the activation values obtained earlier in the SCM model. The low activation energies in the initial period of the ammonia pressure leaching of copper from desilicated concentrate is explained by the increased dissolution rates for less refractory oxidized compounds and secondary copper sulfides. Additionally, there seems to be a change in the slope of the curves at 140 ◦C (2.42 1/K), suggesting a change in the rate-determining mechanism at higher temperatures. It can be assumed that the effect of kinetic difficulties at low temperatures may be higher. Chalcocite is dissolved according to the following reactions (Equations (6) and (7)):

$$2\text{Cu}\_2\text{S} + 4\text{NH}\_4\text{OH} + 2(\text{NH}\_4)\_2\text{SO}\_4 + \text{O}\_2 = 2\text{Cu(NH}\_3)\_4\text{SO}\_4 + 2\text{CuS} + 6\text{H}\_2\text{O}\tag{6}$$

$$\rm CuS + 4NH\_4OH + 2O\_2 = Cu(NH\_3)\_4SO\_4 + 4H\_2O,\tag{7}$$

The dissolution of covellite and other refractory sulfides (chalcopyrite, bornite) at subsequent stages of leaching occurs at lower rates due to lower solubility, which explains the increase in activation energy [47].

The efficiency of copper and silver recovery will strongly depend on the oxygen pressure [24]. The effect of oxygen pressure in the range of 0.2 to 0.8 MPa on copper recovery at a leaching temperature of 140 ◦C is shown in Figure 10.

**Figure 10.** Dependence of Cu (**a**) and Ag (**b**) recovery on oxygen pressure and duration (temperature: 140 ◦C).

The results in Figure 10 show the positive effect of an increase in oxygen pressure on the recovery of Cu and Ag into solution. As the oxygen pressure increases from 0.2 to 0.8 MPa, copper recovery increases from 77.2% to 93.6% over 180 min of leaching, and silver recovery increases from 53.6% to 73.7%. An increase in the process duration for the entire studied range also positively affects the transition of Cu and Ag into solution. The final concentration of copper in the solution at an oxygen pressure of 0.8 MPa was 20.3 g/dm3, and the concentration of Ag was 0.4 mg/dm3. This solution can be used for the extraction of valuable metals. The final concentration of iron in the solution in all the experiments was 0.6–1 mg/dm3.

Using the slopes of the straight lines obtained by substituting data for Cu and Ag recovery at oxygen pressures of 0.2–0.8 MPa and a temperature of 140 ◦C into the new SCM (Figure 11a,b), we plotted a graph with the lnkc-lnPo2 coordinates to determine the partial order with respect to oxygen pressure (Figure 11c,d) [47]. As a result, the empirical orders with respect to oxygen pressure were 0.959 for Cu and 0.932 for Ag, both close to 1, which may also indicate external diffusion limitations [48].

**Figure 11.** Dependence of 1/3ln(1 − α)+((1 − α) −1/3 − 1) on oxygen pressure for Cu (**a**) and Ag (**b**) recovery into solution, and dependence of lnkc on lnPo2 to determine the oxygen order for Cu (**c**) and Ag (**d**).

#### 3.2.2. Characteristics of Solid Residue

To study the effect of preliminary desilication on the mechanism of leaching, electron micrographs and maps of the surface (SEM-EDX) of solid residue particles obtained at oxygen pressures of 0.8 MPa and a temperature of 140 ◦C after 3 h of leaching were examined. The SEM image and map-scans for the desilicated concentrate are presented in Figure 12.

**Figure 12.** SEM image of desilicated concentrate (**a**), and map-scans (**b**–**e**) for Si, Cu, Fe and S, respectively.

In contrast to the initial material, the cake after pressure desilication contains much less silica (Table 5), although it is also distributed over the entire surface, and has a heterogeneous surface with many cavities and voids (Figure 12a,b), which facilitates the access of reagents to sulfide minerals. The presence of pores is explained by the dissolution of Si-bearing minerals in an alkaline solution. Additionally, spherical particles are formed on the surface, which is associated with the precipitation of sodium hydroaluminosilicate [49]. However, sodium hydroaluminosilicate can also inhibit the process [50] but apparently to a lesser extent due to its high porosity. The existence of amorphous inclusions in the cake is confirmed by the presence of noise in the XRD pattern of the cake after desilication (Figure 13). In addition, the XRD pattern in Figure 13 shows that the main phases after desilication are cancrinite, quartz and pyrite, as well as a small amount of copper minerals.

**Figure 13.** XRD pattern of desilicated concentrate (**a**), and the product of its leaching in ammonia solution (**b**).

The cakes after desilication have a relatively uniform distribution of the above elements, especially Si, which is explained by the formation of sodium hydroaluminosilicates on the surface of copper and iron minerals (Figure 12b).

The cakes after the pressure ammonia leaching of desilicated concentrate were also analyzed by scanning electron microscopy to assess morphological changes. The SEM and map-scanning images obtained for the ammonia pressure leaching cake of the desilicated concentrate are presented in Figure 14.

**Figure 14.** SEM image of ammonia leaching residue (**a**), and map-scans (**b**–**d**) for Si, Cu and Ag, respectively.

According to the SEM images, the residue after ammonia pressure leaching is represented by particles larger than 30 μm with a rough surface (Figure 14a). The diffraction pattern of this cake (Figure 13b) shows that cancrinite and hematite are the main minerals in the material.

The copper content decreases significantly after ammonia pressure leaching (Figure 14c). It follows that preliminary alkaline desilication makes it possible to reduce the diffusion limitations caused by aluminosilicates and quartz (into which sulfide minerals are apparently finely impregnated) due to the formation of a porous surface during leaching. A schematic diagram of the process is presented in Figure 15.

**Figure 15.** Schematic diagram of the effect of pre-desilication on recovery of Cu and Ag during ammonia leaching.

Thus, a new environmentally friendly method of processing such a low-grade copper–silver concentrate is proposed, eliminating the need for ultra-fine grinding, which leads to the formation of a large amount of fine and toxic waste. Silica that is converted to an alkaline solution, in turn, can be used to obtain additional valuable products. Ammonia can also be easily regenerated by distillation [31].

## **4. Conclusions**

The effect of aluminosilicate minerals and their preliminary alkaline leaching on the ammonia pressure leaching of low-grade copper concentrates has been investigated. Using new methods for studying the kinetics of the processes, it has been shown that the limiting stage of the process changes over the course of the leaching. The following conclusions have been reached:

**Author Contributions:** Conceptualization, S.N.; methodology, K.K. and A.S.; validation, S.Z. and E.K.; formal analysis, K.K. and D.R.; investigation, A.S. and E.K.; resources, S.Z.; data curation, D.R.; writing—original draft preparation, K.K.; writing—review and editing, A.S. and S.N.; visualization, A.S. and E.K.; supervision, D.R.; project administration, S.Z.; funding acquisition, S.N. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the Russian Science Foundation Project No. 18-19-00186. The SEM-EDX analyses were funded by State Assignment, grant number 0836-2020-0020.

**Acknowledgments:** Technicians at the Ural Branch of Russian Academy of Sciences are acknowledged for their assistance with the XRD, XRF, SEM, EDS and ICP-MS analyses.

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