Advancements in Anode Slime Treatment: Effects of pH, Temperature, and Concentration of ClO⁻/OH⁻ on Selenium Dissolution from Decopperized Anode Slimes

Abstract

Selenium has been classified as a strategic element as it is required for the technology and energy industry. It is not found in abundance in the Earth’s crust, which is why about 90% of selenium is obtained from the treatment of anode slimes, which are a by-product of copper mining. In recent years, several hydrometallurgical treatments have been investigated; as a result, this article presents an alternative proposal using an alkaline-oxidizing medium (ClO⁻/OH⁻). The Taguchi method was used to design an experiment to evaluate the changes in the conditions and interactions found in previous studies with regard to the ClO⁻ concentration, temperature, and pH. The best combination of conditions was a ClO⁻ concentration between 0.53 and 0.68 M, pH between 11.0 and 11.5, and temperature of 55 °C, with which selenium dissolution values between 91.8 and 94.2% were achieved. According to the SEM/EDS analysis, it is evident that an increase in temperature allowed an increase in the selenium reaction, and the selenium was not trapped in the AgCl layer formed by the same selenium dissolution reaction; the slowness of the selenium dissolution mainly depends on the low availability of sodium hypochlorite over time.

1. Introduction

Selenium is considered a strategic element [1,2,3,4] due to its importance in the technology and energy industries [3], particularly in semiconductor and solar panel manufacturing, which play a key role in the transition to sustainable technologies and renewable energy [5]. Selenium is not abundantly found in the Earth’s crust and is rarely found in its elemental form, but it is found in trace amounts, mainly in copper sulfide ores, and can be recovered as a by-product of copper metallurgy [2,4,6]. During electrorefining, copper from the anode is deposited on the cathode, and impurities such as selenium, tellurium, bismuth, arsenic, gold, silver, and PGMs are not deposited on the cathode but fall to the bottom of the electrolytic cell, forming anodic copper slimes [7,8,9]. These slimes have proven to be a strategic secondary source for the recovery of critical and high-economic-value elements [3,9,10].

The composition of anode slimes varies from one refinery to another depending on the composition of the anode used. The chemical composition of anode slimes is generally determined by metallic impurities and other elements that are not dissolved anodically such as gold, silver, selenium, and platinum group metals, and metallic compounds (sulfides, oxides, selenides, and tellurides), such as silver selenide (Ag₂Se), chalcocite (Cu₂S), copper selenide (Cu₂Se), copper telluride (Cu₂Te), and, in some refineries, nickel oxide (NiO). Several authors [3,4,7,11,12,13,14,15,16] have carried out several mineralogical analysis studies on copper anodes and anode slimes from refineries located in Canada, Mexico, and Chile, finding that, in copper anodes, selenium is commonly present as copper selenide (Cu₂Se), but in anode slimes, it is present in different phases of Ag-Cu selenides. Selenide is usually one of the main silver carriers in anodic slimes. According to the studies indicated above, silver is found in the solid copper matrix, and during electrorefining, the dissolved silver reacts with Cu₂Se inclusions to form different Ag-Cu selenides. As an approximation, it can be predicted that the main selenide compound in anode slimes depends on the Ag/(Se + Te) molar ratio in the copper anodes. Silver can sometimes occur as silver chloride (AgCl), depending on the Cl content in the electrorefining process, which is responsible for chloride precipitation. Significant amounts of barium sulfate (BaSO₄) [6,17,18], lead sulfate and oxides (PbSO₄), and Pb₅(AsO₄)₃(OH, Cl) can also be detected in anode slimes. Traditionally, anode slimes have been treated through pyrometallurgical processes, sulfate roasting, and oxidizing roasting [19]; recently, alternative processes that partially or totally replace pyrometallurgical processes have been developed [20,21,22]. The treatment of the slimes mainly depends on their chemical composition, starting with a decopperized process, which traditionally involves leaching with sulfuric acid in the presence of oxygen [3,14,19,23,24]. The decopperized process allows for the total or partial elimination of the copper, tellurium, and arsenic content, producing slime with a lower amount of copper, and the presence of selenium has been detected in CuSe, Ag₂Se, and AgCuSe phases. Selenium generally requires strong oxidizing agents for dissolution from anodic slimes, which is why several alternatives have been explored based on hydrometallurgical processes in oxidizing media, such as nitric acid (HNO₃), chlorination processes, hydrogen peroxide (H₂O₂), manganese dioxide (MnO₂), sodium chlorate with ultrasound, and sodium hypochlorite, among other oxidants [10,17,25,26,27], as well as a combination of hydrometallurgical and pyrometallurgical processes [5]. These oxidants convert selenium into soluble species, such as selenate (SeO₄²⁻) or selenite (SeO₃²⁻). MnO₂ has a strong oxidative capacity; however, it introduces Mn²⁺ ions into the system, which makes the system non-selective for selenium [25]. The same is true for HNO₃; it is not a selective reagent, and it also presents environmental challenges, limiting its industrial application [26]. H₂O₂ is a powerful oxidizing agent and has been recognized for being efficient for copper and selenium leaching, resulting in efficiencies of 96 and 90%, respectively [10,26]. In the case of sodium chlorate, it was demonstrated that conventional leaching yields a selenium dissolution efficiency of 57.5%, and this increases considerably when incorporating the ultrasound process, reaching an efficiency of 98.2% [17].

In previous studies, research was carried out with respect to sodium hypochlorite, which is an agent with high oxidizing power that allows for the 90% dissolution of selenium under conditions of pH 11.5, 45 °C, and 0.54 M ClO⁻/OH⁻; greater dissolution is not achieved due to a layer of AgCl forming around the selenium particles that hinders the effective diffusion of the reagent [8]. Given this difficulty, and with the aim of continuing this research, the objective of this study was to evaluate the effect of increasing the leaching time, ClO⁻/OH⁻ concentration, and temperature on selenium dissolution.

In contrast to previous work, this study evaluates the effect of changing these empirical variables in a combined manner using a factorial design, which allows for the identification of interactions that exist between these variables in selenium dissolution. In addition, new experimental strategies are discussed that could help overcome the limitations observed in previous research.

2. Materials and Methods

The methodology used in this study was based on the experimental approach described in previous studies [8], with some modifications to deepen our understanding of selenium dissolution and the formation of the AgCl layer in the residual product of anode slime leaching. These modifications include the characterization of decopperized anode slimes from a Chilean national refinery, as well as the evaluation of the effect of leaching time and increases in temperature and ClO⁻/OH⁻ concentration on the efficiency of the process using the Taguchi method.

2.1. Characterization of Decopperized Anode Slimes

The decopperized anode slime sample was subjected to a three-step washing procedure with distilled water and subsequently dried at 343 K (70 °C). A representative sample was obtained for the chemical analysis of selenium using atomic absorption spectrometry (AAS) (Perkin Elmer, model PinAAcle 500, Singapore). Additionally, an X-ray diffraction (XRD) analysis (Siemens model D5600, Bruker, Billerica, MA, USA) was performed with an analysis time of one hour, using the ICDD (International Center for Diffraction Data, PDF-2 Version, Bruker, Billerica, MA, USA) database to identify the phases present in the sample. The TOPAS program (Total Pattern Analysis Software, Version 2.1, Bruker, Billerica, MA, USA) was used to quantify the present phases. To determine the distribution of selenium according to particle size, the quantitative evaluation of materials by scanning electron microscopy (QEMSCAN) analysis was performed with equipment (Model E430, with an intellection X275HR detector, Cambridge, UK). To determine the phases present and their morphology, analysis was performed through scanning electron microscopy (SEM) (JEOL USA Inc., Peabody, MA, USA) with energy-dispersive X-ray spectroscopy (EDX) (Zeiss Ultra Plus, Zeiss, Jena, Germany). This analysis was performed in two ways; initially, the sample was arranged in a specimen in a dispersed form on a carbon ribbon, and in the second, it was pretreated with epoxy resin, polished, and coated with carbon. To determine the particle size, the samples were analyzed in Microtrac S3500 (Montgomeryville, PA, USA), with water as the dispersion medium, and ultrasound was applied for 60 s to prevent particle agglomeration, considering the hygroscopic nature of the anode slime.

2.2. Leaching Experiments

The leaching experiments were carried out in a 500 mL four-hole glass reactor at atmospheric pressure, using 10 g of decopperized anode slimes, with a selenium concentration of 12%, in a 500 mL solution. Mechanical stirring was performed using equipment with a digital display (JOANLAB, model OS-30Pro, Huzhou City, Zhejiang Province, China). The temperature, pH, and oxidation–reduction potential (Eh) were monitored using a pH-ORP meter (Hanna Instrument, Model HI5522, Padua, Italy). To maintain the desired temperature, the system was placed in a thermostatic bath (Lauda, model Alpha A24, Lauda-Königshofen, Germany). The pH was controlled using a 25 mL (±0.03) burette, adding sodium hydroxide (NaOH, 3 M) or sulfuric acid (H₂SO₄, 1 M).

Compared to the previous study [8], the leaching time was extended by collecting 5 mL samples at 5, 15, 30, 60, 120, and 180 min of leaching to evaluate the sodium hypochlorite consumption and selenium dissolution. The selenium concentration was measured using an AAS coupled with a flow injection system (FIAS) for hydride generation. The consumption of hypochlorite was measured by iodometric titration. Compared to the previous study [8], the ClO⁻/OH⁻ concentration was increased, and the pH and temperature were modified. The experimental modifications from the previous study included an increase in the ClO⁻/OH⁻ concentration (0.4 to 0.68 M); varying the temperature to 35, 45, and 55 °C; adjusting the pH to 11.0, 11.5, and 12.0; and increasing the leaching time to 180 min. All the experiments were conducted at an agitation rate of 400 min⁻¹. At the end of the leaching, the product solution and residue were filtered. The leaching residue was washed three times with distilled water and then dried in a drying oven at 70 °C for 24 h. The experiments were performed in duplicate, and the results presented here are the averages of the data obtained (±2.0% in selenium dilution). The experimental conditions are presented in

Table 1. Leaching conditions with sodium hypochlorite.

Factor/Level	(1)	(2)	(3)
Temperature, °C	35	45	55
ClO−, M	0.4	0.53	0.68
pH	11	11.5	12

. In order to determine the optimal combination of variables, the Taguchi optimization method was used to evaluate the three factors and three levels, obtaining the L₉ model.

3. Results

3.1. Mineralogical Characterization

XRD analysis showed the significant presence of PbSO₄ (13.9 pct by weight), BaSO₄ (7.3 pct by weight), Ag₂Se (22.0 pct by weight), and AgCl (56.9 pct by weight) (Figure 1); these compounds have been detected by other authors [4,6,7,17,18]. Due to the superposition of peaks, the identification of specific phases was complex, showing 41.1 pct by weight of amorphous compounds. The presence of AgCl depended on the conditions of the copper electrorefining process, as reported in previous studies [11,18], which demonstrated that Ag₂Se and AgCl are the predominant phases in anode slimes. These results suggest that part of the selenium is in an amorphous phase or in compounds not detectable using XRD, indicating the importance of complementing this method with other characterization techniques.

The Qemscan analysis made it possible to determine the distribution of selenium as a function of particle size and its association with other elements. Figure 2 shows that selenium was distributed throughout the sample in different ranges of particle sizes. Furthermore, the results confirm the coexistence of Ag-Se associations, which is in agreement with the XRD analysis. It was determined that more than 70% of the selenium was released in particles smaller than 12 μm. This implies that the particle size was suitable for agitation leaching processes, favoring the contact of the reagent and leachable material due to a high surface area-to-volume ratio. The particle size analysis performed using Microtrac (Montgomeryville, PA, USA) indicated that the P₈₀ of the material was 18 μm, which is within the range reported for anode slimes [6,7,10,17].

To complement the findings obtained using Qemscan and XRD, a detailed morphological and semiquantitative analysis was carried out using SEM/EDS in order to detect the phases and associations of selenium with other elements. The first analysis was carried out on a sample of decopperized anode slime arranged in a test tube in order to observe its morphology, particle size, and composition according to the semiquantitative analysis.

In Figure 3a, at 500× magnification, heterogeneity in particle size is evident, with the presence of porous structures and flat surfaces. Some particles exceed the average size detected through Microtrac analysis, which is attributed to the hygroscopic characteristic of the anode slime [11]. Upon increasing the magnification to a particle size of 18.6 μm (Figure 3b), amorphous particles cemented on its surface become evident.

Secondary electron analysis revealed variations in shades between black and white, where the lighter phases corresponded to elements of higher atomic number. EDS analysis detected the presence of O, Te, Ba, Sb, Ag, Pb, Se, and Cu (see Figure 4). The presence of tellurium is due to its partial dissolution (~70%) during the decopperized process, while residual copper comes from this treatment [8]. The presence of oxygen is associated with the formation of compounds such as lead sulfate (PbSO₄), barite (BaSO₄), and SbO₄, which have been previously reported by other authors [4,7,11,13,15,17].

The elemental distribution mapping (Figure 5a,b) reveals that selenium (blue color) is mostly found in smaller particles, which is in agreement with the results obtained through Qemscan analysis. In addition, an association between the Se-Cu-Ag (Figure 5a–c) phases, which has been detected by other authors [7], is observed.

In particular, in Figure 5e, the coexistence of selenium and copper is observed, suggesting the possible presence of copper selenide (Cu₂Se). Likewise, the mapping in Figure 5f shows evidence of Se-Ag associations compatible with the Ag₂Se phase, as reported in other studies [6,7,11,13,14,15,16,17,25,26,28]. To confirm the presence of the detected phases, anode slime briquette was analyzed. The results are consistent with those obtained in the analysis of anode slime dispersed in a test tube, showing agglomerated particles with a similar elemental composition (Figure 6).

The semiquantitative Cu-Se analysis (Figure 5b,c) determined an atomic ratio of 3.35% Cu and 96.65% Se. This copper deficit suggests that selenium is not only present as Cu₂Se but could form Cu-Ag-Se phases; this is due to the partial or total dissolution of copper in the decopperization process as Cu-Ag selenides are largely dissolved and replaced by silver selenides during decopperization [7]. To verify this, the composition of Ag-Se-rich particles was analyzed, indicating a deficit of silver, which could indicate the existence of Ag_(2−x)Se phases, as shown in

Table 2. Semiquantitative analysis of the Ag-Se phase.

Element	%Weight	%Atomic
Ag	65.48	58.13
Se	34.52	41.87

.

Mineralogical characterization through XRD, Qemscan, and SEM/EDS allowed for the identification and quantification of the main phases present, selenium associations, and selenium release. These findings are key for the metallurgical evaluation of selenium dissolution by ClO⁻/OH⁻. In addition, it can be asserted that selenium compounds did not undergo significant transformations over time compared to previous studies [8,11]. This suggests that there is no direct relationship between changes in the mineralogy of copper sulfides and selenium compounds.

3.2. Selenium Leaching

Selenium dilution was evaluated using Minitab 2025 software (Minitab LLC, State College, PA, USA) according to the Taguchi experimental design (L₉).

Table 3. Response to the Taguchi L₉ experimental model.

No. Test	Temperature, °C	ClO−, M	pH	Selenium Solution (%)
1	35	0.40	11.0	78.0
2	35	0.53	11.5	85.9
3	35	0.68	12.0	83.4
4	45	0.40	11.5	81.7
5	45	0.53	12.0	77.1
6	45	0.68	11.0	84.7
7	55	0.40	12.0	79.6
8	55	0.53	11.0	94.2
9	55	0.68	11.5	91.8

shows the average dilution values for each level of the evaluated variables.

Table 4. Average responses to the variables evaluated.

% Selenium Dissolution
Level	ClO−, M	pH	Temperature, °C
Low (1)	79.70	85.0	82.40
Medium (2)	85.70	86.50	81.20
High (3)	88.53	86.63	80.03

and Figure 7 present the main effects of the evaluated variables (ClO⁻, pH, and T °C) on selenium dissolution. Each point in Figure 7 represents the average selenium dissolution corresponding to each level of the evaluated variable. For example, for the effect of the sodium hypochlorite concentration, the first data point represents the average selenium dissolution for all the tests in which the ClO⁻ concentration was classified as level 1 (0.4 M) (Table 1). The second data point for the ClO⁻ concentration effect represents the average selenium dilution for tests at level 2 (Table 1). The third data point represents the average at level 3 (Table 1).

To evaluate the significance of the main effects of the independent variables on selenium dissolution, an ANOVA test was performed. The results are presented in

Table 5. The significance of the main effects of the variables for selenium dissolution.

Factor	F	p-Value
ClO−	17.74	0.05
pH	6.80	0.12
Temperature	14.72	0.06

.

According to the statistical analysis, the p-values for the temperature and pH variables were greater than 0.05, which indicates that they do not have a statistically significant effect on selenium dissolution; however, when reviewing the results and comparing the effect of an increase in temperature from 35 to 45 °C, the effect increases by approximately five points. In addition, its p-value is close to the threshold; i.e., it is relatively close with a significance of 95%, but if the significance level is decreased to 90%, i.e., p < 0.10, the temperature can be considered statistically significant. The p-value for pH presents a significance close to the threshold for selenium dissolution if a significance level of 90% is considered. This effect is clearly seen in an increase from 11.0 to 12.0 because it decreases the dilution by five points. The concentration of sodium hypochlorite has the highest F value (F = 17.74), which indicates that it is the factor with the greatest influence on the variability of the response.

The maximum mean selenium dissolution (%) was obtained with 0.53 and 0.68 M CIO⁻, a pH between 11.0 and 11.5, and a temperature of 55 °C, reaching values between 91.8 and 94.2, which represents an increase of 1.6 to 4.0% with respect to the previous study. In contrast, a pH of 12.0 reduced the dissolution significantly. This was also reflected in the redox potential value, which, for pH 12.0, starts with values of 700 mV vs. SHE, decreasing to values of 530 mV vs. SHE. The pH had the greatest effect on selenium dissolution (pH 11.0 and 11.5), and the potential value changed from 800 to 787 mV vs. SHE, indicating that the system was highly oxidizing and favorable for selenium dissolution [8,29]. According to the Pourbaix diagram, the system was in the selenite and selenate zone.

Figure 8 shows the interactions between the variables evaluated. It can be observed that the combination of high temperature and low pH improved selenium dissolution. In addition, the interaction between pH and sodium hypochlorite concentration suggests that, at medium values of hypochlorite, pH has a more pronounced effect on dissolution.

The results obtained extend the previous study [8], where different combinations of pH, temperature, and ClO⁻ were evaluated. In that investigation, it was observed that selenium dissolution was lower under high pH conditions (pH = 12), which is confirmed in this work. However, here, it was identified that hypochlorite has a significant effect, but this depends on its interaction with other variables.

3.3. Characterization of Leaching Residue Using SEM/EDS

In order to visualize the products formed, specifically the AgCl passivating layer observed in previous studies [8], three residues obtained under the best leaching conditions were characterized. For this purpose, the residues were vacuum-filtered and washed three times with distilled water after each experiment. Subsequently, the residue was analyzed as a dispersed powder and arranged in a test tube with carbon tape.

A general observation of the sample at an approximate magnification of 500x was made for the three residues (see Figure 9), showing the formation of fine particle agglomerates (<5 μm). This suggests that there was chemical crushing compared to the initial raw material (decopperized anode slimes). When compared with residues from previous studies [8], it was observed that the particle size in this study was smaller, which indicates a greater attack of the anode slimes.

To perform a semiquantitative analysis, the magnification of the samples was increased. Figure 10 shows different morphologies.

From a morphological point of view, fine agglomerated particles with an amorphous appearance are observed in Figure 11a. In contrast, in Figure 11b, amorphous particles are observed together with others that have defined faces, on which fine cemented particles are found.

According to the EDS analysis, the presence of Ag and Cl was identified at a majority, together with traces of selenium, which confirmed the efficiency of the process. To verify this composition, a semiquantitative analysis was performed on a general plane, in addition to elemental mapping, to determine the association between these elements.

Table 6. Overall semiquantitative analysis of leach residues.

Element	Weight %	Atomic %
Cl	16.50	21.55
O	14.17	40.97
Pb	4.80	1.08
Na	4.12	8.32
Sb	3.73	1.43
Cu	2.28	1.66
Ag	50.04	21.48
Si	0.96	1.59
S	0.38	0.55
Ba	1.47	0.51
Te	0.83	0.30
Se	0.34	0.21
As	0.28	0.18
Al	0.09	0.18

presents the elements detected the most in the leach residue. It is observed that the composition is mainly constituted by Ag and Cl, together with oxidized species that form compounds with Ba, Sb, Pb, and As, as supported by previous studies [11]. It is evident that the weight and atomic percentages are lower than those of the raw material, which confirms the efficiency of the selenium dissolution process through sodium hypochlorite, which is a highly oxidizing reagent.

Figure 12 shows the formation of AgCl in the residue; no association with selenium was identified. To corroborate these results, an XRD analysis was performed, which detected that the majority of compounds present correspond to AgCl, with a smaller proportion of residual Ag₂Se. This suggests that the increase in temperature accelerates the dissolution of selenium, preventing its entrapment in the AgCl layer. This hypothesis was corroborated by leaching tests measuring the selenium concentration over time. Figure 13 shows that, in the first 5 min, selenium dissolution exceeded 60%, a value significantly higher than that reported in previous studies [8], where this dissolution was reached after 30 min.

It is observed that an increase in temperature helps the release of selenium so that it is not trapped in the AgCl layer; however, it is not possible to obtain a dissolution greater than 94% if a comparison is made with the consumption of sodium hypochlorite. Figure 14 shows that there is a high consumption of hypochlorite in the first 60 min, at which point the selenium dissolution stops (Figure 13). This study indicates that an increase in temperature has a significant effect but has a negative impact on the stability and concentration of sodium hypochlorite; the solution to this problem may be to incorporate a fresh reagent into the system. This was mentioned in the study of another author [30], in which it was mentioned that the fractional addition of ClO⁻ maintained the fixed concentration of the reagent over time and achieved an increase in the dissolution up to six times.

Future studies on the fractional addition of ClO-/OH- could be a strategy to reduce the concentration of sodium hypochlorite, which could increase the efficiency of the process technically and economically.

Assuming that the main selenium species is Ag₂Se, the possible reactions that may occur are represented by Equations (1) and (2) [8]. This means that the 94% selenium dilution was achieved by adding six times the amount of stoichiometry.

$${\mathrm{A}\mathrm{g}}_2\mathrm{S}\mathrm{e}+4{\mathrm{C}\mathrm{l}\mathrm{O}}^{-}\to 2\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}+{\mathrm{S}\mathrm{e}\mathrm{O}}_4^{2-}+2{\mathrm{C}\mathrm{l}}^{-}∆G^{°}=-174.228 \mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}$$

(1)

$${\mathrm{A}\mathrm{g}}_2\mathrm{S}\mathrm{e}+3{\mathrm{C}\mathrm{l}\mathrm{O}}^{-}\to 2\mathrm{A}\mathrm{g}\mathrm{C}\mathrm{l}+{\mathrm{S}\mathrm{e}\mathrm{O}}_3^{2-}+{\mathrm{C}\mathrm{l}}^{-}∆G^{°}=-134.143 \mathrm{k}\mathrm{c}\mathrm{a}\mathrm{l}$$

(2)

4. Discussion

Sodium hypochlorite is a good oxidizing reagent for dissolving selenium from anode slimes. According to previous studies [8], the difficulty with this reagent is the generation of a passivating layer of AgCl that surrounds the selenium particles and halts the reaction. For this reason, tests were carried out when the concentration of hypochlorite, temperature, and leaching time increased. According to the results obtained, an increase in temperature has a significant effect on the dissolution and the non-formation of the passivating layer; this also occurs with an increase in the concentration of sodium hypochlorite. However, a disadvantage of increasing the temperature is that the concentration of sodium hypochlorite decreases, so after 60 min of leaching, there is no longer any reagent available to continue dissolving selenium.

From an industrial perspective, sodium hypochlorite is a widely available and easy-to-handle reagent. Its use allows for better control of selenium in solution compared to the conventional pyrometallurgical route typically employed in the industry. Therefore, the hydrometallurgical approach proposed in this study could partially replace current pyrometallurgical processes, offering a more selective alternative with lower energy consumption for selenium recovery. Additionally, avoiding high-temperature treatments can help reduce greenhouse gas emissions and contribute to a more environmentally sustainable management of anode slimes.

5. Conclusions

According to the mineralogical analysis, it is possible to identify that the main phases present in the anode slimes correspond to PbSO₄, BaSO₄, and AgCl, that the main phase associated with selenium corresponds to Ag₂Se, and that associations with copper originate from decopperization. This shows that, although the mineralogy of the deposits varied over time, the composition of the anode slime remained without any major variation. With respect to the selenium release in the decopperized anode slime, Qemcan analysis showed that over 70% of the selenium was released in particles smaller than 12 μm, which is a suitable size for a stirred leaching process. The results indicate that selenium leaching is primarily affected by sodium hypochlorite concentration and temperature and, to a lesser extent, by pH levels between 11.0 and 11.5. The importance of pH is reflected between 11.0 and 12.0, where approximately 80% selenium dissolution is achieved, compared to 91% at lower pH values. The best combination of conditions is a ClO⁻ concentration between 0.53 and 0.68 M, pH between 11.0 and 11.5, and temperature of 55 °C, achieving selenium dissolution values between 91.8 and 94.2%. These results correspond to an increase of between 1.6 and 4.0% compared to previous studies. According to the interaction analysis of temperature and pH parameters, the combination of a high temperature and low pH improves selenium dissolution. The mineralogical analysis of the leaching residue through SEM/EDS shows the chemical attack of the decopperized anode slime because agglomerated particles less than 5 μm in size were observed. The semiquantitative EDS analysis indicates a concentration of selenium lower than that of the feed material; in addition, AgCl was formed, but this was not associated with selenium. These results suggest that a higher temperature accelerates the dissolution, preventing the trapping of selenium in the AgCl layer, which has been observed in previous studies by this research team. According to the analysis of selenium dissolution over time and sodium hypochlorite consumption, it has been observed that the reaction is fast in the first few minutes, and its speed remains constant after 60 min. This coincides with the consumption of sodium hypochlorite because the concentration of sodium hypochlorite decreases by 60% compared to the initial concentration. It is suggested that future research evaluate the fractional addition of hypochlorite, with the goal of maintaining a stable concentration throughout the process. This will reduce the reagent concentration and have a technical and economic benefit.