The Enhancement of Enargite Dissolution by Sodium Hypochlorite in Ammoniacal Solutions

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Article The Enhancement of Enargite Dissolution by Sodium Hypochlorite in Ammoniacal Solutions

Lilian Velásquez-Yévenes 1,* , Hans Álvarez 2,Víctor Quezada 2 and Antonio García 2

1 Escuela de Ingeniería Civil de Minas, Facultad de Ingeniería, Universidad de Talca, Curicó 3340000, Chile 2 Laboratorio de Investigación de Minerales Sulfurados, Departamento de Ingeniería Metalúrgica y Minas, Universidad Católica del Norte, Avenida Angamos 0610, Antofagasta 1270709, Chile; [email protected] (H.Á.); [email protected] (V.Q.); [email protected] (A.G.) * Correspondence: [email protected]

Abstract: The dissolution of both copper and arsenic from a copper concentrate was investigated in oxidative ammonia/ammonium solutions at moderate temperatures and atmospheric pressure. The main parameters studied were temperature, pH, concentrations of different ammonia salts, the presence of sodium hypochlorite, pretreatment with sodium chloride, and curing period. In all ammoniacal solutions studied, increasing the temperature enhanced the dissolution of copper, but the dissolution of arsenic remained marginal. Mixing the copper concentrate with sodium chloride and leaving it to rest for 72 h before leaching in ammoniacal solutions signiﬁcantly increased the dissolution of copper and slightly increased the dissolution of arsenic from the concentrate. A maximum of 35% of Cu and 3.3% of As were extracted when ammonium carbonate was used as the lixiviant. The results show relatively rapid dissolution of the concentrate with the addition of

sodium hypochlorite in ammonium carbonate solution, achieving a dissolution of up to 50% and 25% of copper and arsenic, respectively. A copper dissolution with a non-linear regression model ◦ Citation: Velásquez-Yévenes, L.; was proposed, considering the effect of NaClO and NH4Cl at 25 C. These ﬁndings highlight the Álvarez, H.; Quezada, V.; García, A. importance of using the correct anionic ligands for the ammonium ions and temperature to obtain a The Enhancement of Enargite high dissolution of copper or arsenic. The results also showed that the curing time of the packed Dissolution by Sodium Hypochlorite bed before the commencement of leaching appeared to be an important parameter to enhance the in Ammoniacal Solutions. Materials dissolution of copper and leave the arsenic in the residues. 2021, 14, 4529. https://doi.org/ 10.3390/ma14164529 Keywords: enargite dissolution; ammonia salts; sodium hypochlorite; pretreatment with NaCl; curing time Academic Editor: Peter Baláž

Received: 6 July 2021 Accepted: 9 August 2021 1. Introduction Published: 12 August 2021 According to [1], more than 300 arsenic (As) minerals are known to occur in nature. Of

Publisher’s Note: MDPI stays neutral these, approximately 60% are arsenates, 20% are sulfides and sulfosalts, 10% are oxides, and with regard to jurisdictional claims in the rest are arsenite, arsenide, native elements, and metal alloys. The most important pri- published maps and institutional affil- mary As-bearing minerals are those where the arsenic occurs as either the anion (arsenide) iations. or di-anion (diarsenide), or as the sulfarsenide-anion; these anions are bonded to metals such as Fe (arsenopyrite), Co (cobaltite), Ni (gersdorffite), and Cu (enargite, tennantite). Arsenic is one of nature’s most toxic elements, and excessive arsenic exposure has been linked to skin lesions, increased bladder, and lung cancer mortality in Northern Chile [2]. One of the most dreaded environmental impacts is related to the release of arsenic into Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. the environment. Ores and concentrates from flotation containing high concentrations of This article is an open access article arsenic are considered a potential hazard, requiring special precautions in the beneficiation distributed under the terms and and smelting processes. According to [3], the annual copper output from ore smelting conditions of the Creative Commons reaches more than 7 million tons in China. Generally, the contents of As and Cu in copper Attribution (CC BY) license (https:// concentrate are 0.2% and 25%, respectively. However, the cumulative total arsenic brought creativecommons.org/licenses/by/ into copper smelting systems is estimated to be more than 56,000 tons every year. According 4.0/). to [4], 10 tons of arsenic associated with copper concentrates are emitted each day from the

Materials 2021, 14, 4529. https://doi.org/10.3390/ma14164529 https://www.mdpi.com/journal/materials Materials 2021, 14, 4529 2 of 11

seven copper smelting plants in Chile, the largest producer of copper in the world. Along with the smelting process of copper, arsenic is successively removed and transformed into various byproducts such as slags, flue dust, black copper, anode slime, etc. [3]. These Arsenic compounds contain byproducts that could be processed to either recover the valuable metals or be discarded as waste in dumps. Due to the small market capacity of arsenic-related products, arsenic has little commercial value to be further exploited. Hence, seeking efficient and safe ways for the disposal of arsenic is always of great significance. The presence of copper deposits with arsenic content in the Antofagasta Region of Chile is widely known [5–8]. The arsenic-bearing copper minerals such as enargite (Cu3AsS4) and tennantite (Cu12AsS13) attract considerable interests because of their high copper content [9]. However, the presence of arsenic in the copper concentrate is a challenge due to its subsequent content in the metallic copper. Because of the toxicity of arsenic, the smelting industry is forced to be selective in the type of copper concentrates used. By regulation, concentrates can have a maximum content of 0.5% arsenic [10], but this is expected to be reduced shortly. Hence, there is a great need for finding alternative, cleaner, and effective treatments for arsenide copper minerals. A roasting technique was commonly used to eliminate most of the arsenic, but nowadays, this process requires alternatives due to the stringent environmental regulations [11]. The hydrometallurgical process seems to be a cleaner route to process complex ores such as enargite. However, there was a crucial problem with developing a leach process for copper sulfide minerals as the leaching is inhibited at ambient temperature. Several studies reported the formation of a layer on the surface of the sulfide minerals during dissolution. Some researchers claim that this layer could be composed of elemental sulfur or a copper-rich polysulfide layer. However, the nature of this “passivating layer” has not yet been established with certainty. The layer inhibits contact between the mineral and the oxidizing agents, which reduces the dissolution rate [12–18]. Over the years, research has demonstrated some success in adding chloride [14–22] and nitrate to the acid leaching solution [23]. However, in the treatment of ores with high consumption of acid, these processes become costly. Therefore, the new option requires the use of an alkaline leaching solution. The alkaline solutions that can be used to dissolve copper from enargite are cyanide and ammonia. However, the use of cyanide is not preferable due to its instability, toxicity, and complications in the recovery of copper from cyanide solutions. In ammonia media, Cu2+ ions form stable ammine complexes that could be extracted by the conventional solvent extraction process (copper leaching in acid media). The essential advantage of leaching in an alkaline media such as ammonium salts is the ability to recover the dissolved metal by direct electrodeposition or by precipitation with sulfur compounds. As ammonia can only exist in a media of moderate alkalinity (pH = 8.7–9.8), the obtained PLS contains very few impurities, which facilitates its subsequent purification, making it more straightforward [24]. The first report using ammoniacal leaching on an industrial scale was in Kennecott, Alaska, where copper ore was present in a matrix containing carbonates (limestone- dolomite) [25,26]. Another process described as one of the first industrial-scale processes corresponds to the method developed by Sherritt-Gordon in a plant in Fort Saskatchewan, Canada, in 1953. In this process, the ammoniacal leaching of sulphate ores containing Cu, Ni and Co was carried out in autoclaves at high pressures and temperatures using oxygen as an oxidizing agent [27]. Other processes were the Arbiter process for recovering copper from chalcopyrite concentrates in the presence of oxygen [28] and the Escondida process to produce high-grade copper concentrates [29]. On the other hand, hypochlorite ion (OCl−), a strong oxidizing agent, has been used as an alternative method to remove arsenic from copper concentrates [30,31]. The topochemical reaction occurs according to Reaction (1). Arsenic dissolves into the solution, and copper stays as a CuO in the residue. In the second stage, the copper residue is leached.

− − 3− 2− − Cu2AsS4 + 11OH + 17.5ClO → 3CuO + AsO4 + 4SO4 + 5.5H2O + 17.5Cl (R1) Materials 2021, 14, 4529 3 of 11

In order to minimize the problems associated with the arsenic content in copper concentrate, this paper presents results from a study on the leaching of enargite in ammoniacal solutions at various alkaline pH, anionics ligands, chloride ions, using a strong oxidizing (OCl−), variation of temperature and curing time.

2. Experimental 2.1. Copper Concentrate Sample The copper concentrate sample was obtained from a flotation plant. The sample was prepared to a size fraction of −38 µm. The chemical composition of the screened sample was determined using inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Optima 2000 DV, PerkinElmer, Überlinge, Germany). The chemical composition of the sample was 36.2% of Cu, 14.9% of Fe and 4.3% of As (Table1). Mineralogical data were obtained by QEMSCAN, using a Model Zeiss EVO 50 (Zeiss, Oberkochen, Germany), with Bruker AXS XFlash 4010 detectors (Bruker, Billerica, MA, USA) and Software iDiscover 5.3.2.501 (FEI Company, Brisbane, Australia). For the QEMSCAN analysis performed, the Bulk Mineral Analysis (BMA) technique was used. In a BMA, the analysis points are distributed over the entire surface of the briquette, according to a previously established collection grid. In the “X” coordinates a “pixel spacing” is established, while in the “Y” coordinate the term “line spacing” is used. BMA reported a robust quantification of modal mineralogy (total ore), allowing at the same time to obtain the contribution (main element) that each mineral species present. The major minerals identified were pyrite, enargite, chalcocite, chalcopyrite and covellite (Table2). According to Table2, enargite is the principal copper mineral in the sample.

Table 1. Chemical composition of copper concentrates determined by XPS analysis.

Cu (%) Cusoluble (%) Fe (%) Mo (ppm) Mosoluble (ppm) As (%) S (%) 36.2 1.55 14.9 230 12.5 4.26 31.5

Table 2. Mineralogical composition of copper concentrate determined by Qemscan analysis.

Mineral Mass (%) Enargite 21.0 Pyrite 29.8 Chalcocite 17.9 Other Cu-minerals 7.79 Chalcopyrite 6.30 Covellite 4.58 Gangue 3.14 Bornite 2.92 Quartz 2.69 Tenantite 1.84 Sphalerite 1.42 Plagioclase 0.32 Feldspars 0.22 Mn-minerals 0.07 Molybdenite 0.05 As-minerals 0.01

2.2. Leaching in Shake Flasks Leaching experiments of enargite concentrate were conducted in 250 mL shake ﬂasks at 25 ◦Cand 35 ◦C, with stirring speed of 140 rev/min for 8 h. 10 g of copper concentrate sample was used for all leaching tests. Leach solutions were prepared in a volume of 500 mL using distilled water. Three different salts were investigated, NH4Cl, (NH4)2SO4 and (NH4)2CO3, in a concentration of 1.5 M. To study the effect of pretreatment in the dissolution of Cu and As, selected tests were performed by mixing the copper concentrate Materials 2021, 14, 4529 4 of 11

with 15 g/L of NaCl and left to rest for 24 or 72 h before leaching. All leach solutions contained 0.5 g/L of cupric ions, which is the typical concentration in a rafﬁnate in any Chilean plant. The effect of the addition of NaClO complexing and oxidizing agent was also investigated by adding 10 g/L to the leaching solution in ammonia media. Table3 shows a summary of the shake ﬂasks leaching tests conditions. Finally, the modelling of copper extraction with non-linear regression was studied using Minitab 18 computational tool (Minitab LLC, State College, PA, USA), considering the effect of NaClO and NH4Cl at 25 ◦C.

Table 3. Shake ﬂask leaching conditions of enargite concentrate.

◦ − Test Temp. C Curing (h) NH4Cl (M) (NH4)2CO3 (M) (NH4)2SO4 (M) NaCl (g/L) OCl (g/L) 1 25 - 1.5 - - - - 2 35 - 1.5 - - - - 3 25 - - 1.5 - - - 4 35 - - 1.5 - - - 5 25 - - - 1.5 - - 6 35 - - - 1.5 - - 7 25 24.0 1.5 - - 15 - 8 25 24.0 - 1.5 - 15 - 9 25 24.0 - - 1.5 15 - 10 25 72.0 1.5 - - 15 - 11 25 72.0 - 1.5 - 15 - 12 25 72.0 - - 1.5 15 - 13 25 - 1.5 - - - 10 14 25 - - 1.5 - - 10 15 25 - - - 1.5 - 10

3. Results and Discussions Based on the study by [32], species distribution diagrams were constructed for the different ammonium salts and working conditions used in this research. Figure1 shows the pKa values for ammonium chloride, ammonium carbonate and ammonium sulfate. It can be seen that as the pH increases, the ammonia concentrations increase, while the ammonium concentrations decrease. This is due to the transformation that occurs from ammonium to ammonia. However, a point was reached where both curves converge; this is an equilibrium point where the concentrations of ammonium and ammonia are equal. This so-called pKa must be the desired and optimal concentration to obtain the best dissolutions. The pKa of ammonium chloride was reported to be 9.50 [33], which is consistent with Materials 2021, 14, x the results shown on Figure1. According to Figure1, pKa values of 9.50, 9.43 and 9.55 5 of 11

were obtained in solutions contained NH4Cl, (NH4)2CO3 and (NH4)2SO4, respectively. Therefore, the study will be conducted by using 1.5 M of each ammonium salts.

+ + Figure 1. Distribution diagram of NH3/NH4 from varying the NH3–NH4 concentration of the solution in the presence of Figure 1. Distribution diagram of NH3/NH4+ from varying the NH3–NH4+ concentration of the solution in the presence of different ammonium salts, NH4Cl (A), (NH4)2SO4 (B) and (NH4)2CO3 (C). different ammonium salts, NH4Cl (A), (NH4)2SO4 (B) and (NH4)2CO3 (C).

3.1. Effect of Temperature on the Dissolution of Copper and Arsenic Figure 2 shows the results of Cu and As extraction from copper concentrate in shake flasks at varying temperature and ammonia salts. Leaching tests were performed with 1.5 M of three different ammonia salts (NH4Cl, (NH4)2SO4 and (NH4)2CO3) at 25 °C and 35 °C. Increasing the temperature resulted in an increase in both copper and arsenic extraction. The highest dissolution of copper of up to 40% was achieved when (NH4)2CO3 was used, followed by (NH4)2SO4 (30%) and NH4Cl (20%), all of them at 35 °C. At 25 °C, the three different tests achieved no more than 10% of copper dissolution. It is interesting to note that there was a limited dissolution in arsenic, less than 1% for all condition and temperature. A slight increase in arsenic dissolution was obtained when (NH4)2CO3 was added at 35 °C. It could be inferred that enargite dissolved at a lower rate than the other copper minerals present in the concentrate (Table 2).

Figure 2. Copper (A) and arsenic (B) extraction in different ammonia solution at 25 °C and 35 °C.

3.2. Effect of Pretreatment and Curing Time From Figure 3, it is evident that the copper (Figure 3A) and arsenic (Figure 3B) dissolutions were enhanced by the pretreatment with NaCl and the rest period of 24 h before leaching with NH4Cl solution. The possible chemical reaction is shown in Reaction 2. Still, also it is suggested that during the curing time, enargite can be decomposed to Cu2S or CuS and then dissolved with the irrigation of ammonia solution. The dissolution of copper achieved was 23% with pretreatment versus 10% without pretreatment; and the dissolution of arsenic increased slightly from 0.5% to 1%. According to [34], curing time generates a homogeneous distribution of the mineral bed’s leaching solution, and hence increasing the Cu dissolution. It was observed that the longer the curing time, the higher the copper and arsenic extraction. Curing time for 72 h resulted in 30% final copper extraction, which is 6.6% increase compared to 24 h. The use of chloride/ammonia has an additional advantage. On the one hand, it fulfils the objective, just as other chlorides do, of providing the solution with the appropriate concentration of chloride ion. Ammonia dissolved in water forms a strongly alkaline solution of ammonium hydroxide. The high solubility of ammonia in water is the result of the tendency of the two to interact with each other through a hydrogen bond (Reaction 3).

Materials 2021, 14, x 5 of 11

Materials 2021, 14, 4529 5 of 11 Figure 1. Distribution diagram of NH3/NH4+ from varying the NH3–NH4+ concentration of the solution in the presence of different ammonium salts, NH4Cl (A), (NH4)2SO4 (B) and (NH4)2CO3 (C).

3.1. Effect of Temperature3.1. Effect onof Temperature the Dissolution on ofthe Copper Dissolution and Arsenic of Copper and Arsenic Figure2 showsFigure the results 2 shows of Cu the and results As extraction of Cu and from As extraction copper concentrate from copper in shakeconcentrate in shake ﬂasks at varyingflasks temperature at varying andtemperature ammonia and salts. ammonia Leaching salts. tests Leaching were performed tests were withperformed with 1.5 M of three different ammonia salts (NH4Cl, (NH4)2SO4 and (NH4)2CO3) at◦ 25 °C and 35 °C. 1.5 M of three different ammonia salts (NH4Cl, (NH4)2SO4 and (NH4)2CO3) at 25 C and 35 ◦C. IncreasingIncreasing the the temperature temperature resulted resulted in anin increasean increase inboth in both copper copper and and arsenic arsenic extraction. The highest dissolution of copper of up to 40% was achieved when (NH4)2CO3 was used, extraction. The highest dissolution of copper of up to 40% was achieved when (NH4)2CO3 followed by (NH4)2SO4 (30%) and NH4Cl (20%), all of them at◦ 35 °C. At◦ 25 °C, the three was used, followed by (NH4)2SO4 (30%) and NH4Cl (20%), all of them at 35 C. At 25 C, the three differentdifferent tests achievedtests achieved no more no more than 10%than of 10% copper of copper dissolution. dissolution. It is interesting It is interesting to note to note that therethat was there a limitedwas a limited dissolution dissolution in arsenic, in arsenic, less than less 1% than for 1% all for condition all condition and and temper- temperature. Aature. slight A increase slight increase in arsenic in dissolutionarsenic dissolution was obtained was obtained when (NH when4)2CO (NH3 was4)2CO3 was added added at 35 ◦C.at It35 could °C. It be could inferred be inferred that enargite that enargite dissolved dissolved at a lower at a ratelower than rate the than other the other copper copper mineralsminerals present present in the concentratein the concentrate (Table2 (Table). 2).

FigureFigure 2.2. CopperCopper ((AA)) andand arsenicarsenic ((BB)) extractionextraction inin differentdifferent ammoniaammonia solutionsolution atat 2525◦ °CC andand 3535◦ °C.C.

3.2. Effect of Pretreatment3.2. Effect of and Pretreatment Curing Time and Curing Time From Figure3,From it is Figure evident 3, thatit is theevident copper that (Figure the copper3A) and(Figure arsenic 3A) and (Figure arsenic3B) (Figure 3B) dis- dissolutions weresolutions enhanced were byenhanced the pretreatment by the pretreatment with NaCl wi andth NaCl therest and periodthe rest of period 24 h of 24 h before before leaching with NH4Cl solution. The possible chemical reaction is shown in Reaction 2. leaching with NH4Cl solution. The possible chemical reaction is shown in Reaction 2. Still, Still, also it is suggested that during the curing time, enargite can be decomposed to Cu2S or also it is suggested that during the curing time, enargite can be decomposed to Cu2S or CuS and then dissolved with the irrigation of ammonia solution. The dissolution of copper CuS and then dissolved with the irrigation of ammonia solution. The dissolution of copper achieved was 23% with pretreatment versus 10% without pretreatment; and the dissolution achieved was 23% with pretreatment versus 10% without pretreatment; and the dissolu- of arsenic increased slightly from 0.5% to 1%. According to [34], curing time generates a tion of arsenic increased slightly from 0.5% to 1%. According to [34], curing time generates homogeneous distribution of the mineral bed’s leaching solution, and hence increasing the a homogeneous distribution of the mineral bed’s leaching solution, and hence increasing Cu dissolution. It was observed that the longer the curing time, the higher the copper and the Cu dissolution. It was observed that the longer the curing time, the higher the copper arsenic extraction. Curing time for 72 h resulted in 30% ﬁnal copper extraction, which is and arsenic extraction. Curing time for 72 h resulted in 30% final copper extraction, which 6.6% increase compared to 24 h. The use of chloride/ammonia has an additional advantage. is 6.6% increase compared to 24 h. The use of chloride/ammonia has an additional ad- On the one hand, it fulﬁls the objective, just as other chlorides do, of providing the solution vantage. On the one hand, it fulfils the objective, just as other chlorides do, of providing with the appropriate concentration of chloride ion. Ammonia dissolved in water forms the solution with the appropriate concentration of chloride ion. Ammonia dissolved in a strongly alkaline solution of ammonium hydroxide. The high solubility of ammonia in Materials 2021, 14, x 6 of 11 water is the resultwater of theforms tendency a strongly of the alkaline two to interactsolution with of ammonium each other throughhydroxide. a hydrogen The high solubility of bond (Reactionammonia 3). in water is the result of the tendency of the two to interact with each other through a hydrogen bond (Reaction 3).

Figure 3. Copper ( A)) and and arsenic arsenic ( (B)) extraction extraction at ambition ambition condition using 1.5 M of NH 4ClCl without without and and with with pretreatment pretreatment (15 g/L of of NaCl NaCl and and curing curing time time of of 24 24 h h and and 72 72 h). h).

2Cu3AsS4 + 6H2SO4 + 5.5O2 → 6CuSO4 + 2H3AsO4 + 8S° + 3H2O (R2)

+ NH3(aq) + H → NH (R3) Similar results were obtained when the copper concentrate was mixed with NaCl and left to rest for 24 h or 72 h before leaching with (NH4)2SO4. The dissolution of copper and arsenic increased with the pretreatment but at a lower rate (Figure 4). Increasing the curing period to 72 h resulted in an increase in the dissolution of copper (Figure 4A) and arsenic (Figure 4B), reaching an extraction of 30% and 1.5% after 8 h of leaching, respectively.

Figure 4. Copper (A) and arsenic (B) extraction at ambient condition using 1.5 M of (NH4)2SO4 without and with pretreatment (15 g/L of NaCl and curing time of 24 h and 72 h).

Tests performed with the (NH4)2CO3 media are shown in Figure 5. As expected, the highest dissolution was obtained in this media, 27% of copper and 3.0% of arsenic, when the concentrate was mixed with NaCl and left to rest for 24 h. When the curing period was increased to 72 h, an increase was observed in the extraction of copper around 35%, however, the arsenic extraction remained the same as the 24 h curing time pretreatment. It is inferred that the other copper minerals contained in the concentrate might dissolve rather than the enargite. A long curing period before the leaching stage was identified as favourable for the dissolution of sulphide ores such as chalcopyrite [21,35] or chalcocite [36]. The advantage of using this media is the buffering effect of the alkaline ammonium carbonate solution, which makes the pH remain practically constant over time without the need for any adjustment.

Materials 2021, 14, x 6 of 11

MaterialsFigure2021 ,3.14 Copper, 4529 (A) and arsenic (B) extraction at ambition condition using 1.5 M of NH4Cl without and with pretreatment6 of 11 (15 g/L of NaCl and curing time of 24 h and 72 h).

2Cu3AsS4 + 6H2SO4 + 5.5O2 → 6CuSO4 + 2H3AsO4 + 8S° + 3H2O (R2) ◦ 2Cu3AsS4 + 6H2SO4 + 5.5O2 → 6CuSO+ 4 + 2H 3AsO4 + 8S + 3H2O (R2) NH3(aq) + H → NH (R3) + + → + Similar results were obtainedNH3 when(aq) theH copper concentrateNH4 was mixed with NaCl (R3)and left toSimilar rest for results 24 h or were 72 h obtained before leaching when the with copper (NH concentrate4)2SO4. The wasdissolution mixedwith of copper NaCl and arsenicleft to rest increased for 24 h with or 72 the h beforepretreatment leaching but with at a (NH lower4)2SO rate4. The(Figure dissolution 4). Increasing of copper the cur- and ingarsenic period increased to 72 h with resulted the pretreatment in an increase but atin athe lower dissolution rate (Figure of 4copper). Increasing (Figure the 4A) curing and arsenicperiod to(Figure 72 h resulted 4B), reaching in an increasean extraction in the of dissolution 30% and 1.5% of copper after (Figure8 h of leaching,4A) and arsenicrespec- tively.(Figure 4B), reaching an extraction of 30% and 1.5% after 8 h of leaching, respectively.

Figure 4. CopperCopper (A ()A and) and arsenic arsenic (B) (extractionB) extraction at ambient at ambient condition condition using using 1.5 M 1.5of (NH M of4)2SO (NH4 without4)2SO4 withoutand with and pretreat- with pretreatmentment (15 g/L of (15 NaCl g/L ofand NaCl curing and time curing of 24 time h and of 24 72 h h). and 72 h).

Tests performed withwith thethe (NH(NH44)2CO33 mediamedia are are shown shown in Figure 55.. AsAs expected,expected, thethe highest dissolution was obtained in this media, 27% of copper and 3.0% of arsenic, when the concentrate was mixed with NaCl andand leftleft to rest for 24 h. When the curing period was increased toto 7272 h,h, an an increase increase was was observed observed in in the the extraction extraction of copperof copper around around 35%, 35%, however, however,the arsenic the arsenic extraction extraction remained remained the same the as same the 24as hthe curing 24 h timecuring pretreatment. time pretreatment. It is inferred It is inferredthat the that other the copper other mineralscopper minerals contained cont inained the concentratein the concentrate might might dissolve dissolve rather rather than thanthe enargite. the enargite. A long A long curing curing period period before before the leachingthe leaching stage stage was was identiﬁed identified as favourable as favour- ablefor the for dissolutionthe dissolution of sulphide of sulphide ores ores such such as as chalcopyrite chalcopyrite [21 [21,35],35] or or chalcocite chalcocite [ 36[36].]. The advantage of using this media is the buffering effect of the alkaline ammonium carbonate Materials 2021, 14, x 7 of 11 solution, which makes the pH remain practicall practicallyy constant over time without the need for any adjustment.

Figure 5. 5. CopperCopper (A ()A and) and arsenic arsenic (B) (extractionB) extraction at ambient at ambient condition condition using using 1.5 M 1.5of (NH M of4)2CO (NH3 without4)2CO3 withoutand with and pretreat- with pretreatmentment (15 g/L of (15 NaCl g/L ofand NaCl curing and time curing of 24 time h and of 24 72 h h). and 72 h).

3.3. Effect Effect of of NaClO NaClO The results indicate that copper extraction increased by around 35% (Figure 66).). Still,Still, thisthis result result is is comparable comparable with with the the result result obtained obtained when when the the leaching leaching was was carried carried out out in NH4Cl after the pretreatment with NaCl and 72 h of curing period. Nevertheless, a con- in NH4Cl after the pretreatment with NaCl and 72 h of curing period. Nevertheless, a siderableconsiderable increase increase in arsenic in arsenic dissolution dissolution wa wass observed observed by by almost almost 20%, 20%, agreed agreed with with the result of [31] (Figure 6B). The passivation of enargite in this media was evident after 2 h of leaching. The same effect was observed when (NH4)2SO4 was used with NaClO (Figure 7). Due to acid generations, monitoring of oxidizing pH conditions and potential reduction is essential to continue the effectiveness of the leaching test [37]. As it was expected, the leaching of the concentrate with (NH4)2CO3 and NaClO achieved the highest dissolution of copper and arsenic, around 50% and 25% of copper and arsenic, respectively (Fig- ure 8). These results highlight the importance of using an oxidant such as NaClO if the objective is to remove arsenic from the concentrate [38].

Figure 6. Copper (A) and arsenic (B) extraction at ambient condition using 1.5 M of (NH4)Cl with and without the addition of 10 g/L NaClO.

Figure 7. Copper (A) and arsenic (B) extraction at ambient condition using 1.5 M of (NH4)2SO4 with and without the addition of 10 g/L NaClO.

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4 2 3 Figure 5. Copper (A)) andand arsenicarsenic ((B)) extractionextraction atat ambientambient conditioncondition usingusing 1.51.5 MM ofof (NH(NH4))2CO3 withoutwithout andand withwith pretreat-pretreatment (15 g/L of NaCl and curing time of 24 h and 72 h).

3.3. Effect of NaClO The results indicate that copper extraction increased by around 35% (Figure 6). Still, Materials 2021, 14, 4529 7 of 11 thisthis resultresult isis comparablecomparable withwith thethe resultresult obtainedobtained whenwhen thethe leachingleaching waswas carriedcarried outout inin 4 NH4Cl after the pretreatment with NaCl and 72 hh ofof curingcuring period.period. Nevertheless,Nevertheless, aa con-considerable increase in arsenic dissolution was observed by almost 20%, agreed with the result ofof [[31]31] (Figure(Figure6 B).6B). The The passivation passivation of of enargite enargite in in this this media media was was evident evident after after 2 h 2 of h of leaching. The same effect was observed when (NH4)2SO4 was used with NaClO (Figure ofleaching. leaching. The The same same effect effect was was observed observed when when (NH (NH4)2SO4)2SO4 was4 was used used with with NaClO NaClO (Figure (Figure7). 7).Due Due to acidto acid generations, generations, monitoring monitoring of oxidizingof oxidizing pH pH conditions conditions and and potential potential reduction reduc- tionistion essential isis essentialessential to continue toto continuecontinue the effectiveness thethe effectivenesseffectiveness of the of leachingthe leaching test test [37]. [37]. As it As was it was expected, expected, the 4 2 3 theleachingthe leachingleaching of the ofof concentratethethe concentrateconcentrate with withwith (NH (NH(NH4)2CO4))2CO3 and3 andand NaClO NaClONaClO achieved achievedachieved the thethe highest highesthighest dissolution dissolu-dissolu- tionoftion copper ofof coppercopper and arsenic,andand arsenic,arsenic, around aroundaround 50% and50%50% 25% andand of25%25% copper ofof coppercopper and arsenic, andand arsenic,arsenic, respectively respectivelyrespectively (Figure (Fig-(Fig-8). ureThese 8). results These highlightresults highlight the importance the importance of using of an using oxidant an suchoxidant as NaClO such as if NaClO the objective if the objectiveis to remove is to arsenic remove from arsenic the concentratefrom the concentrate [38]. [38].

4 FigureFigure 6. Copper ( A)) and and arsenic arsenic ( (B)) extraction extraction at at ambient ambient conditioncondition usingusing 1.51.51.5 MMM ofof (NH(NH44)Cl)Cl with with and and without without the the addition addition ofof 1010 g/Lg/L NaClO. NaClO.

MaterialsFigure 2021 , 7.14 ,Copper x (A) and arsenic (B) extraction at ambient condition using 1.5 M of (NH4)2SO4 with and without the8 of 11 Figure 7.7. Copper (A) andand arsenicarsenic ((BB)) extractionextraction atat ambientambient conditioncondition usingusing 1.51.5 MM ofof (NH(NH44))22SO4 with and without the addition of 10 g/L NaClO. addition of 10 g/L NaClO. NaClO.

Figure 8. Copper (A) and arsenicarsenic ((B)) extractionextraction atat ambientambient conditioncondition usingusing 1.51.5 MM ofof (NH(NH44))22CO3 with and without the addition of 10 g/L NaClO. NaClO.

3.4. Modelling of Copper Extraction in Function of NH4Cl, NaClO and Leaching Time 3.4. Modelling of Copper Extraction in Function of NH4Cl, NaClO and Leaching Time The addition of NaClO to the alkaline leac leachinghing solution enhanced the dissolution of copper at 25 °C.◦ However, the major increase was observed when NH4Cl was used as the copper at 25 C. However, the major increase was observed when NH4Cl was used as the lixiviant.lixiviant. The The dissolution dissolution of of copper copper increased increased from 10% to 35% when NaClO was added. For the modelling ofof thethe dependentdependent variables variables on on the the copper copper dissolution, dissolution, the the support support of ofstatistical statistical software software was was necessary necessary since since the experimentalthe experimental curves curves show show strongly strongly non-linear non- linearbehaviour. behaviour. In this investigation,In this investigation, the Minitab the Minitab 18 computational 18 computational tool was usedtool andwas theused results and the results with copper extraction using NH4Cl and NaClO considered. After extensive model testing, the following (Equation (1)) was chosen due to its better fit: % Cu dissolution = (a0 + a1 × [NaClO]) × (1 − b0 × e (−b1×leaching time)) (1) With coefficients a0, a1, b0 and b1, NaClO concentration and leaching time are measured in g/L and hours, respectively. The Levenberg-Marquardt algorithm was applied to the model for calculating its parameters or coefficients. Table 4 shows estimate, standard error (SE) and confidence interval (CI) of each coefficient of the model. The resulting equation is exposed in Equation (2).

Table 4. Parameters calculations associated with Cu dissolution model.

Parameter Estimate SE of Estimate CI of 95% a0 9.96826 0.585156 (7.46087;12.5206) a1 2.48720 0.085869 (2.12372;2.8583) b0 1.00000 0.019377 (0.91634;1.0837) b1 1.21448 0.082207 (0.91619;1.6699)

% Cu dissolution = (9.96826 + 2.48720 × [NaClO]) × (1 − b0 × e(−1.21448×leaching time)) (2) There is a 95% confidence interval containing the value of the parameter for the population. The parameter was statistically significant if the range excludes the value of the null hypothesis (the term containing the parameter without effect). For reference, in the case of linear regression, the null hypothesis value for each parameter was 0, so there is no effect. Table 5 exhibits a summary of the statistical parameters of the model fit.

Table 5. Statistical summary of the fit model.

Parameter Value Meaning SSE final 0.986153 Sum of squared residual DFE 2.000000 Degrees of freedom for error, are equal to the sample size, plus 1 MSE 0.493077 Means square of the error, is the variance around the fitted values. MSE = SSE/DFE S 0.702194 Distance (of error) between the data values and the fitted values. S = MSE 1/2

Materials 2021, 14, 4529 8 of 11

with copper extraction using NH4Cl and NaClO considered. After extensive model testing, the following (Equation (1)) was chosen due to its better ﬁt:

% Cu dissolution = (a0 + a1 × [NaClO]) × (1 − b0 × e (−b1×leaching time)) (1)

With coefficients a0, a1, b0 and b1, NaClO concentration and leaching time are measured in g/L and hours, respectively. The Levenberg-Marquardt algorithm was applied to the model for calculating its parameters or coefficients. Table4 shows estimate, standard error (SE) and confidence interval (CI) of each coefficient of the model. The resulting equation is exposed in Equation (2).

% Cu dissolution = (9.96826 + 2.48720 × [NaClO]) × (1 − b0 × e(−1.21448×leaching time)) (2)

Table 4. Parameters calculations associated with Cu dissolution model.

Parameter Estimate SE of Estimate CI of 95% a0 9.96826 0.585156 (7.46087;12.5206) a1 2.48720 0.085869 (2.12372;2.8583) b0 1.00000 0.019377 (0.91634;1.0837) b1 1.21448 0.082207 (0.91619;1.6699)

There is a 95% confidence interval containing the value of the parameter for the population. The parameter was statistically significant if the range excludes the value of the null hypothesis (the term containing the parameter without effect). For reference, in the case of linear regression, the null hypothesis value for each parameter was 0, so there is no effect. Table5 exhibits a summary of the statistical parameters of the model fit.

Table 5. Statistical summary of the ﬁt model.

Parameter Value Meaning SSE final 0.986153 Sum of squared residual DFE 2.000000 Degrees of freedom for error, are equal to the sample size, plus 1 MSE 0.493077 Means square of the error, is the variance around the fitted values. MSE = SSE/DFE S 0.702194 Distance (of error) between the data values and the fitted values. S = MSE 1/2

Figure9 shows that in the plot of residuals vs. fits, it is verified that the residuals are randomly distributed and have a limited variance. The points were located randomly on both sides of 0. The plot of residuals vs. order shows that the residuals were independent of each other. The residuals show no trends or patterns when displayed in chronological order. From the normal probability plot of the residuals, it is verified that the residuals are normally distributed. The normal probability plot of the residuals follows approximately a straight line. For the maximum measured value of copper extraction, 34.68%, the relative error was 2.02%. Therefore, modelled curves of the copper dissolution reproduce well the behaviour of the experimental. The mathematical model fits well with the experimental data, as seen in Figure 10. Materials 2021, 14, x 9 of 11 Materials 2021, 14, x 9 of 11 Materials 2021, 14, x 9 of 11

Figure 9 shows that in the plot of residuals vs. fits, it is verified that the residuals are Figure 9 shows that in the plot of residuals vs. fits, it is verified that the residuals are randomlyFigure distributed 9 shows thatand inhave the a plot limited of residuals variance. vs The. fits, points it is verified were located that the randomly residuals on are randomly distributed and have a limited variance. The points were located randomly on bothrandomly sides of distributed 0. The plot and of residuals have a limited vs. order variance. shows Thethat pointsthe residuals were located were independent randomly on both sides of 0. The plot of residuals vs. order shows that the residuals were independent ofboth each sides other. of The 0. The residuals plot of showresiduals no trends vs. order or patternsshows that when the displayedresiduals werein chronological independent of each other. The residuals show no trends or patterns when displayed in chronological order.of each From other. the The normal residuals probability show noplot trends of the or residuals, patterns whenit is verified displayed that in the chronological residuals order. From the normal probability plot of the residuals, it is verified that the residuals areorder. normally From distributed.the normal Theprobability normal plotprobability of the residuals, plot of the it isresiduals verified follows that the approxi residuals‐ are normally distributed. The normal probability plot of the residuals follows approxi- matelyare normally a straight distributed. line. The normal probability plot of the residuals follows approximately a straight line. matelyFor athe straight maximum line. measured value of copper extraction, 34.68%, the relative error For the maximum measured value of copper extraction, 34.68%, the relative error was 2.02%.For the Therefore, maximum modelled measured curves value of theof copper copper extraction, dissolution 34.68%, reproduce the wellrelative the errorbe‐ was 2.02%. Therefore, modelled curves of the copper dissolution reproduce well the be- Materials 2021, 14, 4529 haviourwas 2.02%. of the Therefore, experimental. modelled The mathematical curves of the model copper fits dissolution well with the reproduce experimental well thedata,9 of be- 11 haviour of the experimental. The mathematical model fits well with the experimental data, ashaviour seen in ofFigure the experimental. 10. The mathematical model fits well with the experimental data, as asseen seen in Figurein Figure 10. 10.

Figure 9. Residual plots for copper dissolution. Figure 9. Residual plots for copper dissolution. FigureFigure 9.9. ResidualResidual plotsplots forfor coppercopper dissolution.dissolution.

FigureFigure 10. 10. ExperimentalExperimental and and modeled modeled (• (• ▬ •)•) coppercopper dissolutiondissolution v/s v/s time.time. ExperimentalExperimental test test without with‐ Figure 10. Experimental and modeled (• ▬ •) copper dissolution v/s time. Experimental test with- outNaClOFigure NaClO and10. and Experimental 1.5 1.5 M M NH NH4Cl4 Cland ( (),■ modeled and), and experimental experimental (• ▬ •) copper test test with withdissolution 10 10 g/L g/L NaClO NaClOv/s time. and and Experimental 1.5 1.5 M M NH NH4Cl4Cl test ( N(▲). with-). out NaClO and 1.5 M NH4Cl (■), and experimental test with 10 g/L NaClO and 1.5 M NH4Cl (▲). out NaClO and 1.5 M NH4Cl (■), and experimental test with 10 g/L NaClO and 1.5 M NH4Cl (▲). 4. Conclusions

A comparative study of copper and arsenic dissolution from a copper concentrate using different ammonia salts under the ambient condition with the addition of chloride ions as pretreatment and hypochlorite as an oxidizing source was presented. Based on the results obtained, it can be concluded that:

• In all ammoniacal solutions studied, increased temperature enhanced the dissolution of copper, but no signiﬁcant increase in the dissolution of arsenic was observed. Am- monium carbonate solutions at 35 ◦C dissolved the highest copper and arsenic amount. • Mixing the copper concentrate with NaCl and leaving it to rest for 72 h before the leaching with ammoniacal solutions signiﬁcantly increases the dissolution of both copper and arsenic from the concentrate. A maximum of 35% of Cu and 3.3% of As were extracted when ammonium carbonate was used as the lixiviant. • The addition of an oxidizing agent such as hypochlorite ion, OCl− to the alkaline leaching solution dramatically enhanced the dissolution of enargite contained in the concentrate obtained more than 50% of copper and 25% of As. Materials 2021, 14, 4529 10 of 11

• With an error of 2.02%, Table4 shows that the experimental results were in good agreement with the modelled equation (Equation (2)). Therefore, the mathematical model ﬁts well with the experimental data. • Although the results described above indicate that the leaching of enargite using ammonia solutions could be attractive from a kinetic point of view, many important practical considerations must be considered. As the cost of the lixiviants, a neutralizing reagent to maintain the pH at a level appropriate for efﬁcient leaching.

Author Contributions: Conceptualization, L.V.-Y. and H.Á.; methodology, L.V.-Y.; software, A.G.; val- idation, L.V.-Y. and V.Q.; formal analysis, L.V.-Y. and A.G.; investigation, L.V.-Y. and H.Á.; resources, L.V.-Y.; data curation, L.V.-Y.; writing—original draft preparation, L.V.-Y. and V.Q.; writing—review and editing, L.V.-Y.; visualization, V.Q.; supervision, L.V.-Y.; project administration, L.V.-Y.; funding acquisition, L.V.-Y. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data sharing is not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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