Selective Leaching of Arsenic from Copper Concentrates in Hypochlorite Medium

Abstract

The selective leaching of arsenic using sodium hypochlorite was evaluated in order to reduce its concentration in a copper concentrate. The best conditions achieved in the synthetic concentrate were applied to the industrial concentrate. First, the individual behaviors of pure samples of enargite and chalcopyrite were evaluated under a hypochlorite medium. The enargite reaction is significantly faster than chalcopyrite, allowing for greater selectivity to ClO⁻ (0.1–0.3 M), pH 12–12.5; 20 to 40 °C, reaction time <60 min. Under these conditions, the reagent consumption for pure compounds approaches the stoichiometric consumption and presents a selectivity factor of 5/1. Furthermore, concentrate leaching in a sodium hypochlorite medium, containing enargite, releases arsenic ions into the solution, while copper and iron remain in the solid phase, as CuO and Fe(OH)₃, respectively. A novel copper concentrate cleaning process by selective leaching is proposed, which transforms unacceptable copper concentrate for smelters (>0.5%As) into clean concentrates (<0.2%As) or low penalty concentrates (0.2 < %As < 0.5). The estimated consumption for the cleaning process is in the order of 0.4–0.6 kg Cl₂ equivalent per kg of concentrate.

1. Introduction

The main sources of copper are sulfide ores. The principal constituents of the concentrates are chalcopyrite, chalcocite/digenite, bornite, and covellite, depending on the mineralogy of the ore deposits. In some deposits, the copper sulfides are associated with sulfoarsenides such as enargite (Cu₃AsS₄) and tennantite (Cu₁₂As₄S₁₅), where copper and arsenic are directly bound. These kinds of concentrates are becoming more common and represent a process challenge for arsenic compound separation and safe disposal in a sustainable way. The traditional treatment process of these minerals is through flotation, smelting/conversion, and electrorefining. Arsenic is considered a “penalty element” [1,2], and its removal in copper metallurgy presents a scientific and technological challenge; smelters impose penalties on the price of the concentrate (<0.5%As) or reject the concentrate if it exceeds a certain percentage of arsenic.

An industrial method to remove arsenic from copper concentrates containing enargite/tenantite is fluidized bed roasting [3]. This process partially oxidizes the concentrate, keeping the copper in a solid phase or calcine, while the arsenic passes to a gaseous phase as arsenic oxide, which is subsequently stabilized and disposed of as ferric arsenate. Calcine follows the traditional smelting and electrorefining process.

For efficient and sustainable arsenic extraction, several hydrometallurgical processes have been studied and proposed, such as partial and total leaching of the enargite. In general, it is possible to increase metal extraction by incorporating leaching technologies or through a combination of traditional and new technologies with low waste generation or effective waste control [4]. Hydrometallurgical treatment options in acid media have been extensively studied, determining that enargite in sulfuric acid shows a refractory behavior due to the formation of an elemental sulfur layer [5,6]. This reaction is improved with the incorporation of oxygen at high pressure and temperatures, and the incorporation of catalysts [7]. The same system has been studied using chloride, and better results in terms of arsenic and copper dissolution have been achieved with the addition of pyrite as a catalyst under atmospheric conditions and using ferric as the main oxidant [8,9,10,11,12]. In addition, in a chlorine–chloride acid medium at ambient pressure, a kinetic reaction and the generation of a refractory layer have been determined [13]. Recent studies of interest, related to arsenic leaching processes in an SO₂/O₂ medium, applied to arsenic-containing coal, show in their experimental results that under conditions of darkness and UV irradiation, both citrates and radicals generated during UV irradiation could effectively enhance the leaching of coal pyrite and arsenic [14].

High levels of arsenic dissolution have been reported in alkaline leaching systems. This is the case of the alkaline sodium sulfide leach, which reaches removals above 95%, strongly depending on the Na₂S/NaOH concentrations and temperature. The reaction mechanism is chemically controlled with an activation energy in the region of 55–74 kJ mol⁻¹. However, corrosion issues produced by caustic conditions and the high price of sodium sulfide have hindered its industrial use [15]. Leaching enargite using ammonia (NH₄OH) and sodium persulfate (Na₂S₂O₈) as an oxidant reached 98% dissolution [16]. Industrial technologies such as nitrogen species catalyzed pressure leaching (NSC) and alkaline sulfide leaching (ASL) have been used for the treatment of copper and gold ores with the presence of enargite. Both technologies have been shown to be effective in the recovery of these metals, as well as in the dissolution of As, reaching values of 72.1% and 93.5%As, respectively [17]. In general, alkaline leaching systems show a high degree of dissolution of arsenic and a high degree of selectivity with respect to copper, which remains in the solid phase. The challenge for the industrial application of these processes is related to cost and reagent consumption.

Some authors have studied the ClO⁻/OH⁻ effects of the leaching system on sulfide minerals, specifically on enargite, transforming it into crystalline CuO and releasing arsenic in solution, according to Reaction (1) [18].

Cu₃AsS₄ + 11OH⁻ + 35/2 ClO⁻ → 3CuO + AsO₄³⁻ + 4SO₄²⁻ + 11/2H₂O + 35/2 Cl⁻

(1)

The reaction is valid until 0.05 M OH⁻, reaching complete dissolution of arsenic at 50 °C in 60 min. The characteristics of Reaction (1), in terms of separating arsenic and copper into two different phases, suggest this process is suitable for cleaning copper sulfide concentrates. The reactivity of different sulfide ores to ClO⁻/OH⁻ leaching was compared, with the reference standard being the condition of 100% enargite attack. Under this condition (pH = 12.5; 25 °C; 0.34 M ClO⁻, 60 min), only 4% chalcopyrite, 2% bornite, and less than 1% chalcocite reaction were observed. Furthermore, covellite had a similar behavior to enargite, reacting easily with sodium hypochlorite [19,20]. Studies using X-ray photoelectron spectroscopy, XPS, allowed us to determine the solid products of the chalcopyrite reaction in a sodium hypochlorite medium. They are mainly tenorite and amorphous iron hydroxides, leaving the sulfur in the form of sulfate in the solution according to Reaction (2) [21].

CuFeS₂ + 17/2 ClO⁻ + 4OH⁻ = CuO + Fe(OH)₃ + 2 SO₄²⁻ + 1/2 H₂O + 17/2 Cl⁻

(2)

Recent studies have shown the behavior of chalcopyrite concentrates on alkaline leaching media (60 °C in 300 min when the chlorine sparging rate was 0.69 mmol min⁻¹, the pulp density was 10 g/L, and the pH of the solution was 11−12.7), which transform 93.8% of chalcopyrite to tenorite [22]. A recent review of new reagents for chalcopyrite leaching includes a description of leaching with ionic liquids, a class of solvents primarily based on imidazolium (C₃H₅N₂) with extraction rates of up to 98%. Additionally, it presents the amino acid glycine (CH₂NH₂COOH) as a more economical alternative, showing diffusion control in most studies using H₂O₂, Fe₂(SO₄)₃, or dissolved O₂ as oxidants. Another cost-effective option mentioned in the review is methanesulfonic acid, HCH₃SO₃, which achieves up to 100% copper extraction from chalcopyrite at high concentrations of H₂O₂, Fe₂(SO₄)₃, or FeCl₃ and elevated temperatures. Additionally, sodium hypochlorite, NaClO, leaching is highlighted as an alternative with rapid solutions reaching up to 92.5%. The review notes that although these lixiviants have not been implemented on an industrial scale, they are novel proposals with valuable insights into the dissolution behavior of chalcopyrite in various chemical systems [23].

Given the rise in enargite content in Chilean copper concentrates and the challenges associated with treating this sulfarsenide using traditional pyrometallurgical methods, a study was conducted on a selective arsenic cleaning process. The research included a comparative evaluation of the behavior of pure samples of enargite and chalcopyrite in a ClO⁻/OH⁻ medium. The conditions resulting in the highest selectivity were then applied to both a synthetic concentrate and an industrial concentrate. The findings indicated that the ClO⁻/OH⁻ medium is an effective method for removing arsenic from copper concentrates containing enargite.

2. Materials and Methods

In this comparative study, natural crystalline samples of enargite and chalcopyrite were used, which were reduced in size by grinding and were classified under 100 μm.

Table 1. Chemical composition of feed materials (wt%).

	Cu	Fe	As	S	Sb	Insoluble
enargite	46.2		17.0	31.1	1.80	4.20
chalcopyrite	35.5	30.5		34.5

shows the chemical analysis of the ore, using inductively coupled plasma mass spectrometry (ICP-MS).

The samples were analyzed using optical and electronic microscopy and through X-ray diffraction (XRD), verifying the crystallinity of the samples and the majority presence of enargite and chalcopyrite phases, respectively, in addition to isolated quartz phases.

2.1. Experimental Procedure—Leaching of Pure Compounds

The leaching tests were carried out in a 1 L glass reactor, stirred at 700 rpm. The pH and temperature of the system were continuously measured and controlled. The pH was evaluated in the range of 11.5–13.0 by the addition of NaOH. The leaching solution was prepared using 160 g/L industrial sodium hypochlorite. The concentration of ClO⁻ is determined by measuring the amount of active chlorine present. The hypochlorite solution is acidified with acetic acid (3), and in the presence of an excess of iodide (KI), it forms iodine (I2), according to Reaction (4).

ClO⁻ + H⁺ → Cl₂ + H₂O

(3)

ClO⁻ + 2H⁺ + 2I⁻→I₂ + H₂O + Cl⁻

(4)

Subsequently, the iodine is titrated using a standard solution of sodium thiosulfate 0.1 N with starch as an indicator. The particle sizes used in the leaching experiments, both enargite and chalcopyrite, were obtained by grinding and classification in the 15–25 µm range. To quantify the progress of the reaction, 1 mL samples of the leach solution were taken at different time intervals and then diluted to 20 mL with deionized water for the analysis of arsenic concentrations using ICP-MS and sulfur by XRF. The leaching residues were examined through optical microscopy and scanning electron microscopy (SEM) to identify changes in composition, morphology, and the presence of solid products.

2.2. Synthetic Concentrate Leaching in a ClO⁻/OH⁻ Medium

Synthetic concentrates were prepared by grinding and blending chalcopyrite, enargite, and quartz ore. A mass balance was performed from the chemical analysis of each of the ores. This resulted in a mixture of 2.0%As and 0.6%As in synthetic concentrate 1 and synthetic concentrate 2, respectively. The mixtures were classified into granulometries under 15 µm, 25–53 µm, and P₈₀ = 55 µm (80% under 55 µm). The composition of the concentrates is shown in

Table 2. Composition of synthetic copper concentrates (wt%).

%wt	Cu	Fe	As	S
Concent. 1	34.0	25.9	2.00	33.1
Concent. 2	33.0	28.1	0.60	33.3

. Concentrate N.1 is a concentrate deemed “not admissible” for smelting due to its elevated arsenic content. Concentrate N.2 is an example of a concentrate considered “permissible”, but subject to sanctions.

The degree of transformation of the mineral species that are part of the concentrate was determined by analyzing As and S by ICP-MS. The dissolution of those species was calculated according to the atomic ratio in the chemical formula of enargite and chalcopyrite. The percentage of chalcopyrite attack was also calculated based on the consumption of sodium hypochlorite, using the stoichiometry of chemical Equations (1) and (2). The experiments were carried out by contacting the synthetic concentrate with sodium hypochlorite solutions containing from one to four times the stoichiometric hypochlorite necessary for the elimination of the arsenic present. The leaching experiments were carried out in a 1 L glass reactor, magnetically stirred, with an initial pH of 12.5, at room temperature. The experiments were stopped at the moment when the pH decrease stabilized; that is, when the oxidation reactions of enargite and chalcopyrite stopped as a consequence of the depletion of the leaching reagent. The final solutions were filtered, and a sample was taken to be analyzed for As and S by ICP-MS. Solid residues were analyzed using light microscopy and SEM/EDS.

To assess copper recovery from synthetic concentrate leaching residues, the residue from the experiment with the highest selectivity of reaction between chalcopyrite and pyrite underwent an acid wash. This particular experiment achieved an 80% reaction yield for enargite and 17.2% for chalcopyrite. The residue was treated with an agitated solution of 0.5 M sulfuric acid for 5 min; copper and arsenic in the solution were analyzed using ICP-MS. Subsequently, the acid-washed residues were rinsed with deionized water, filtered, and analyzed using SEM.

To check the feasibility of abatement and disposal of the arsenic compounds obtained in the alkaline leaching solution, the arsenic was precipitated from the solution in the form of ferric arsenate. The solution was placed in a glass reactor and stirred, measuring for pH. The high content of sulfate in the solution, which decreases arsenic precipitation, was controlled via the addition of calcium chloride (CaCl₂) in a molar ratio of Ca⁺²/SO₄²⁻ equal to 1, for the formation of CaSO₄ gypsum. Subsequently, ferric chloride FeCl₃ was added to the solution in a Fe/As molar ratio equal to 6. The precipitation pH was set to 5 by adding calcium carbonate CaCO₃, maintaining this condition for 1 h, after which the solid was allowed to settle in suspension, and a sample of the solution was taken. The washed and dried precipitate was analyzed using microscopy and XRD. The precipitation of ferric arsenates was carried out based on the work done by references [24,25,26,27,28].

2.3. Industrial Concentrate Leaching

The industrial concentrate was supplied by CODELCO, Chile. The material was analyzed, with a quantitative evaluation of minerals, using scanning electron microscopy, QEMSCAN.

Table 3. Mineralogical species contained in the sample of industrial concentrate.

	Enarg/Ten/Tet	Pyrite	Chalcct/Dign	Bornite	Chalcpy.	Covellite	Sphaler.	Quartz
wt%	35.9	22.3	16.6	6.01	4.66	3.98	2.27	2.11

Ten: tennantite; Tet: tetrahedrite.

shows the main species determined. A reactor system and the measurement of temperature variables, pH, and agitation similar to the experiments of pure species and a synthetic concentrate were used.

Table 4. Chemical analysis by ICP.

wt%	Cu	Fe	As	S	Sb	Soluble
Concent.	34.8	12.5	5.99	29.1	0.160	1.62

S was identified using X-ray fluorescence.

shows the chemical analysis of the concentrate obtained through ICP. Figure 1, obtained using optical microscopy (200X), allows verification of the extensive mineralogical diversity of the sulfide species found in the concentrate. Enargite can be easily distinguished from tetrahedrite-tennantite. Furthermore, the mineralogical associations of the ore fragments can be observed.

Table 5. Experimental leaching conditions.

	Leaching of Pure Mineral Samples	Synthetic Concentrate		Industrial Concentrate
		C1 (2%As)	C2 (0.6%As)
mass, g	0.5	10.0	20.0	1.0
V, L	0.3			0.4
rpm	700	700	700	700
pH	11.5–12.0–12.5–13.0	12.5	12.5	12.5
T, °C	25–40–60	21	21	30–40–50
ClO−, M	0.13–0.33–0.67	0.67	0.67	0.2–0.3–0.5
Particle size, µm	25–53 15–25	15–53 <15	15–53	25–53
Pulp density, %p/p		7.2

summarizes the experimental conditions for the three types of materials leached in ClO⁻/OH⁻ media.

3. Results and Discussion

3.1. Leaching Pure Species

To determine the conditions of greater selectivity between the pure compounds of enargite and chalcopyrite, leaching was carried out for each mineral sample separately, focusing on the variable of hypochlorite concentration. The evaluated hypochlorite concentrations were 0.13, 0.33, and 0.67 M, while maintaining a constant particle size of 15–25 µm, pH 12.5, and 25 °C. The reaction percentage of both minerals is assessed through the analysis of dissolved sulfur in the solution. The reaction percentages are directly influenced by the concentration of hypochlorite present in the leaching solution, resulting in an increase in both reaction rates.

In general, the reaction of chalcopyrite is observed to be slower compared to enargite, especially during the first 60 min. Figure 2 illustrates the ratio of the percentage of enargite reaction to chalcopyrite reaction over time for the three evaluated hypochlorite concentrations. Selective dissolution is evident between the two minerals when low hypochlorite concentrations (0.13 M) and short contact times are employed. In the first 30 min, the percentage of enargite reaction is nearly 5.8 times higher than that of chalcopyrite, with this selectivity decreasing to 3.7 times after 120 min. At higher concentrations, reacted fractions tend to be approximately equal.

The consumption of hypochlorite is linked to its concentration in the solution. At high concentrations, hypochlorite experiences degradation, catalyzed by transition metals such as Ni (II), Cu (II), and Fe (III) and by intermediate species such as cuprates (III) and ferrates (VI) [29]. This degradation is visible in reactions with chalcopyrite.

Figure 3 summarizes the consumption of sodium hypochlorite, in moles of enargite reacted by mole of sodium hypochlorite consumed. It was observed that the consumption of hypochlorite increases when its concentration is increased in the solution. For a 0.13 M sodium hypochlorite solution, consumption was 0.051 moles reacted by mole of hypochlorite consumed, a value very close to the stoichiometry consumption of 0.057.

In the case of chalcopyrite attacked by a mole of sodium hypochlorite, it is observed that, at low hypochlorite concentrations (0.13 M), the amount of reagent consumed is close to the stoichiometric situation, which changes at higher concentrations of hypochlorite, probably due to decomposition generated in the reagent by the presence of rusty iron compounds.

3.1.1. pH Effect

The pH was evaluated between 11.5 and 13 at intervals of 0.5 in the leaching of enargite and chalcopyrite. The concentration of sodium hypochlorite was 0.13 M, at room temperature and particle sizes of 15–25 μm. The results obtained, summarized in Figure 4, show the percentage of mineral reaction depending on the pH of three different reaction times—60, 120, and 180 min. The enargite reaction is favored at a pH between 12.0 and 12.5, a situation that is reversed at a pH of 13. In the case of chalcopyrite, its leaching reaction is favored as the pH increases in the evaluated range, especially above a pH of 12.5.

The percentage of reaction of the enargite shows a maximum located in the range of pH from 12 to 12.5, a situation that was previously reported by Viñals [18]. The best conditions for a selective attack between chalcopyrite and enargite are located in the range between a pH of 12 and 12.5; at higher pH values, the decomposition of minerals is similar, decreasing the selectivity.

3.1.2. Temperature Effect

The effect of temperature in the leaching of chalcopyrite and enargite was evaluated in experiments performed at 0.13 M of sodium hypochlorite, pH 12.5, and particle sizes of 15–25 μm. Figure 5 summarizes the results of the ratio between the reacted fraction of enargite and the reacted fraction of chalcopyrite over time and at three different temperatures. The system is highly sensitive to temperature and reaction time. There is a general consensus among several authors that the influence of temperature on the dissolution of copper sulfide minerals is significant; increasing the temperature improves leaching kinetics [30,31]. On the other hand, the kinetic study of enargite in ClO⁻/OH⁻ media shows a chemical control model, sensitive to temperature variations, and this difference, which was considered suitable for a selective process, diminishes over time due to the advancement of the chalcopyrite reaction. At 60 °C, the chalcopyrite reaction is favored, and therefore, the advantage of enargite over chalcopyrite in terms of reaction rate disappears. At 60 min, both reactions exceeded 90% reaction. These results align with a study that demonstrates the positive effect of temperature on chalcopyrite leaching within a range of 35 to 68 °C in acidic ferric sulfate leaching solutions [32].

This difference, suitable for a selective process, decreases over time due to the advancement of the chalcopyrite reaction. At 60 °C, the reaction of chalcopyrite is favored and therefore the advantage of enargite over chalcopyrite in terms of the speed of the reaction disappears. At 60 min, both reactions exceeded 90% reaction.

3.2. Synthetic Concentrate Leaching

The leaching of 10 g of concentrate N1, with a pulp density of 7.2% weight/weight, in contact with a leaching solution at a pH of 12.5, hypochlorite 0.67 M, at room temperature, produces a rapid reaction that generates a temperature increase of up to 20 °C. The system shows a rapid decrease in the hypochlorite concentration: in just 15 min of leaching, 98.5% of the reagent was consumed, thus stopping the reactions of the system. The temperature did not exceed 45 °C, which favors the process from the point of view of selectivity. Arsenic removal reached 83%, which would represent the arsenic content from a non-admissible characteristic concentrate (2%As) for smelting to an admissible one, subject to penalization (0.3%As). The consumption of reagents to achieve this extraction was 9.13 moles of hypochlorite and 7 moles of sodium hydroxide per kg of concentrate.

Using the same leaching conditions mentioned above, the particle size effect was evaluated: 25–53, P₈₀ = 55 μm, and particles under 15 µm for concentrate N1. The experimental tests were conducted in a final time of 30 min. With the results of As and S analysis, the ratio of the reacted fraction of enargite/reacted fraction of chalcopyrite was calculated for the three granulometries used. The summary of the results is shown in Figure 6.

As expected in this case, the decrease in particle size increases the reaction percentages of enargite and chalcopyrite contained in the concentrate. Regarding the selectivity of the species, leaching also influences it. For particle sizes under 15 μm, the reaction of enargite is approximately 5 times greater than that of chalcopyrite, while, in turn, for particle sizes of 25–53 µm and P₈₀ = 55 μm, the difference in reaction percentages is reduced, and enargite reacts about 2.5 times more than chalcopyrite.

3.2.1. Effect of Semi-Continuous Addition of Hypochlorite

The highest reaction selectivity between enargite and chalcopyrite is obtained at low concentrations of hypochlorite (<0.33 M). The effect of the semi-continuous addition of ClO⁻ was evaluated, adding stoichiometric amounts of the reagent to ensure a low and constant value.

Figure 7 shows the leaching behavior of 20 g of concentrate N2 (0.6%As) at a pH of 12.5 at room temperature.

The effect of the five additions of reagent ClO⁻ is observed in the leaching system, as well as the effect on the temperature that reaches a maximum of 40 °C and descends due to the addition of fresh reagent at room temperature. The pH is regulated by the addition of NaOH (primary y-axis), and to maintain its concentration relatively constant in the leaching solution, five aliquots of ClO⁻ are added every 10 min. The decrease in ClO⁻ concentration is observed, followed by an increase after the addition of a new aliquot, reflected in the distinctive shape of the curve (secondary y-axis). Each variation in concentration also has an impact on temperature control. Initially, at room temperature, the interaction of the solution with the mineral tends to elevate it, returning to lower levels with the subsequent addition of ClO⁻. It is observed that the reaction exhibits greater aggressiveness in the initial stages, manifesting in a higher temperature and increased consumption of ClO⁻ and NaOH. This behavior is associated with the progression of the reaction and the consequent consumption of the mineral in the leaching process.

The fractionated addition of reagent achieved the reaction of 80% enargite and 17% chalcopyrite in 135 min (Figure 8). During the addition of ClO⁻, the enargite reaction is six times greater than the chalcopyrite reaction (average %reaction Enarg/%reaction Cpy = 6.1). The concentrate, initially with 0.6%As, decreases its content to 0.2%As. This was achieved by adding five times the stoichiometric amount necessary for the dissolution of the enargite contained in the concentrate, which represents a consumption of 5.6 moles of ClO⁻ per kg of concentrate.

3.2.2. Acid Washing of Solid Leachate Products

The reaction products obtained in the leaching of concentrates with hypochlorite are essentially copper oxides and oxy-iron hydroxide, which form a layer that coats mineral particles. The use of sulfuric acid solutions is a typical method used in the industry to recover copper from oxides. To evaluate the dissolution of alkaline leaching residues in an acid medium, 16.7 g of residues was contacted with 25 mL of a 0.5 M sulfuric acid solution. It easily and quickly dissolved the oxide layers, as observed in the SEM images (Figure 9).

The copper and arsenic layers were dissolved in the acid solution. The presence of arsenic is a consequence of the adsorption of arsenic in the product layer, as seen in the EDS spectra (Figure 10). This amount of arsenic represents 8% of the initial content. As a consequence, an acid solution with dissolved copper was obtained, which can be treated by solvent extraction (SX) and electro-winning (EW) in order to obtain cathodic copper. Additionally, an improvement in arsenic extraction was obtained, from 80% to 86%.

Table 6. Distribution % of concentrate elements when subjected to alkaline and acidic leaching. Chemical analysis using ICP-MS.

	Solid	Alkaline Leaching		Acid Leaching
	Initial	Solid	Solution	Solid	Solution
wt, g	20.0	19.4		16.7
Cu	33.0	34.0	<0.10 ppm	32.3	6.05
As	0.60	0.12	0.48	0.08	0.04
Fe	28.0	28.8	<0.10 ppm	28.6	4.13
S	33.3	28.5	5.65	33.1	5.65
Sb	0.07	0.04	0.03	0.04	0.03
O	--	3.40	0.00	--	3.30
gangue	5.00	5.10	--	6.0	--

shows the distribution of elements when the evaluated concentrate is leached with hypochlorite followed by acid leaching.

3.2.3. Arsenic Precipitation from Leaching Solutions with Sodium Hypochlorite

Enargite was leached with sodium hypochlorite in an amount equal to two times the stoichiometric requirement of Equation (1) at pH 12.5 and 25 °C. The liquor was filtered and subjected to precipitation with ferric ions in a Fe/As ratio equal to 6, the precipitation pH was set at 5, according to the literature, and controlled by the addition of CaCO₃ [19,20,21,22,23]. An amount of 1.84 g of FeCl₃ *6H₂O at 25 °C was added to 100 mL of residual solution whose arsenic concentration was 0.48 g/L. The pH decreases from 11.93 to 2.29 and is adjusted to pH = 5.0 by adding 0.44 g of CaCO₃. The system considered an agitation of 700 min⁻¹. In one hour, a light brown precipitate with a homogeneous texture was obtained, composed mainly of iron, arsenic, and a small amount of sulfur and calcium. The precipitate is completely amorphous and does not present crystalline species that can be observed by XRD analysis.

Table 7. Characterization of solutions before and after precipitation with iron.

Concentration in Solution, g/L	As	SO42−	Ca	Fe
Real before precipitation	0.48	2.65	--	--
After pp	0.001	1.91	1.51	n.d.
% extraction efficiency of As	99.9

n.d. = not detected.

shows the theoretical concentrations of As and S of the solution that were obtained as a product of the leaching of 80% of the enargite with a solution of ClO⁻, and the concentrations after the precipitation. The effectiveness of arsenic removal is observed by the addition of ferric ions, leaving the solution with less than 1 ppm of arsenic, thereby obtaining an extraction of 99%.

3.2.4. Leaching of Industrial Concentrate in a Sodium Hypochlorite Medium

Leaching in hypochlorite medium was applied to an industrial concentrate (34.8% Cu; 5.99%As). Copper is present in chalcocite/digenite, chalcopyrite, and bornite minerals, and arsenic is found mainly in enargite/tennantite. The particle size of the concentrate was 25–53 μm. The results summarized in Figure 11 show the evolution of extraction of arsenic and sulfur as a function of temperature, with a pH of 12.5, and a concentration of ClO⁻ of 0.2 M. It was seen that in 2 h more than 96% of As was dissolved. The increase in temperature favors the dissolution of sulfur associated with sulphurated species that do not contain arsenic (chalcopyrite, bornite, chalcocite, and digenite). Unlike the arsenic curves that show complete dissolution, in the case of the sulfur curves, they show a slow reacted fraction for the time of the experiments, not reaching 50% dissolution. Hypochlorite consumption increases with temperature and is used in the first 30 min mainly in the reaction of the enargite. After the first 60 min of the reaction, a change in the slope of the reagent consumption curves is observed, which is related to the reagent consumption of other ClO⁻-consuming mineral species (Figure 12).

The effect of the concentration of the reagent was evaluated in the industrial concentrate, similarly to that for pure species and synthetic concentrates. The results maintain the tendency to favor arsenic extraction to the extent that the reagent increases its concentration. Figure 13 shows the hypochlorite consumption curve vs. time, similar to what was seen with the effect of temperature being a sufficient reagent for oxidation reactions. Furthermore, arsenic species are favored in the first 30 min of reaction in the experiment [18,29].

The results of evaluating the effect of the initial pH in the leaching of industrial concentrate for pH values of 11.5, 12.0, and 12.5 at 40 °C and hypochlorite consumption, showed that at 60 min, the reaction in all cases of arsenic extraction is greater than 78%. The best behavior is presented at a pH of 12.0, reaching 99% extraction. This behavior, which shows an extraction maximum between a pH of 11.5 and 12.5, had already been observed in pure mineral samples. The extraction of sulfur from species that do not contain arsenic is favored at a pH of 12.5 with respect to the other pHs evaluated. Additionally, the consumption of hypochlorite is greater at a pH of 12.5 due to an increase in the reaction of sulfides that do not contain arsenic.

In

Table 8. Comparison of results with bibliographic references.

Leaching Media	Material	Conditions	%As Reaction	Reference
Oxidizing media
sulfuric acid oxygen system H2SO4-O2	Enargite	220 °C; 689 kPa; 64 µm; 120 min	100	[9]
FeSO4-H2SO4-O2	Enargite			[8]
Alkaline media
NaOH-Na2S	Enargite	95 °C; 1 M Na2S; 3.5 M NaOH; 30 min; P80 30 µm	100	[14]
NaOH-Na2S	Enargite concentrate	100 °C; 25 g/L NaOH; ${\mathrm{S}}_2^{-}$ 40 g/L; 360 min	93.9	[16]
NH4+/ClO−	Enargite in copper concentrate	25 °C; 10 g/L ClO−; 1.5 M ${\mathrm{NH}}_4^{-}$	50	[31]
NaClO-NaOH	Enargite	25 °C; 0.3 M NaClO; 0.03 M NaOH; 25–53 µm; 120 min	94	[17]
NaClO-NaOH	Enargite in copper concentrate	30 °C; 0.3 M NaClO; 0.03 M NaOH; 25–53 µm	90.0	This study
		40 °C; 0.5 M NaClO; 0.03 M NaOH;25–53 µm; 60 min.	99.3

, some data and conditions of certain processes applied to enargite minerals are presented and compared with those obtained in this study. Oxidic acid media (sulfuric) are applicable to copper sulfide minerals, and enargite, in particular, proves to be even more refractory than chalcopyrite. High temperature (220 °C) and high pressure (689 kPa) are required to achieve arsenic dissolution. Leaching proposals in alkaline media are less aggressive in terms of temperature and pressure. Reagent concentrations have values to consider, but in the case of the ClO⁻/OH⁻ combination, they allow selective attack of arsenic against copper, with values exceeding 90%, at ambient temperature and pressure. In this study, it was confirmed that the behavior of a complex copper concentrate, with a diverse chemical and mineralogical composition, maintains trends and apparent benefits when subjected to ClO⁻/OH⁻ media, supporting the proposal of this medium as a pre-beneficiation washing process for the concentrate.

4. Conclusions

The behavior of enargite and chalcopyrite was investigated in ClO/OH⁻ leaching with the aim of establishing favorable conditions for the selective leaching of these minerals. The impact of ClO⁻ concentration, pH, temperature, and particle size was quantified in pure mineral samples, mineral mixtures, and concentrate from a mining company. Both chalcopyrite and enargite react in the evaluated media, and the highest selectivity between these mineral species is achieved in short times (<30 min), with low ClO⁻ concentrations (0.1–0.3 M), pH of 12–12.5, temperature of 25 °C, and fine particle sizes (less than 15 µm). The selectivity of enargite can be up to five times greater than that of chalcopyrite. The leaching of the concentrate or mineral mixtures in a sodium hypochlorite media containing enargite dissolves arsenic into the solution, while copper and iron remain in the solid phase as CuO and Fe(OH)₃, respectively.

Tests conducted with a mineral mixture or synthetic concentrates yield similar results in terms of selectivity, even when increasing the solid/liquid ratio. The fractionated addition of ClO⁻ (0.3 M) in five increments maintains the ClO⁻ concentration relatively constant, achieving selectivity of up to six times in a 135 min period. The evaluation of industrial concentrate samples validates previous results, extracting 96% of the arsenic present in the concentrate in 120 min, with 0.2 M ClO⁻, pH 12.5, and particle sizes of 25–53 µm. This demonstrates the possibility of transforming, through the studied leaching process, a concentrate not accepted by smelters (6% arsenic) into a processable concentrate using traditional methods. The results are consistent across all stages, highlighting the viability of a selective approach to the leaching process. These findings bear significant implications for arsenic removal in copper concentrates, opening avenues for future research and industrial applications.

A copper concentrate with low arsenic content and a stable precipitated arsenic compound was achieved, enabling safe removal. The process should capitalize on the rapid leaching of enargite at low concentrations of 0.13–0.2 M ClO⁻, at moderately low temperatures of 25–40 °C, and a pH of 11.5–12.0. These conditions demonstrate a low leaching rate for chalcopyrite and bornite species compared to sulfided Cu-As species.