**David Torres 1,2, Luís Ayala 3, Ricardo I. Jeldres 4, Eduardo Cerecedo-Sáenz 5, Eleazar Salinas-Rodríguez 5, Pedro Robles <sup>6</sup> and Norman Toro 1,2,\***


Received: 5 December 2019; Accepted: 4 January 2020; Published: 9 January 2020

**Abstract:** Most copper minerals are found as sulfides, with chalcopyrite being the most abundant. However; this ore is refractory to conventional hydrometallurgical methods, so it has been historically exploited through froth flotation, followed by smelting operations. This implies that the processing involves polluting activities, either by the formation of tailings dams and the emission of large amounts of SO2 into the atmosphere. Given the increasing environmental restrictions, it is necessary to consider new processing strategies, which are compatible with the environment, and, if feasible, combine the reuse of industrial waste. In the present research, the dissolution of pure chalcopyrite was studied considering the use of MnO2 and wastewater with a high chloride content. Fine particles (−20 μm) generated an increase in extraction of copper from the mineral. Besides, it was discovered that working at high temperatures (80 ◦C); the large concentrations of MnO2 become irrelevant. The biggest copper extractions of this work (71%) were achieved when operating at 80 ◦C; particle size of −47 + 38 μm, MnO2/CuFeS2 ratio of 5/1, and 1 mol/L of H2SO4.

**Keywords:** dissolution; CuFeS2; chloride media; manganese nodules

#### **1. Introduction**

The most abundant type of copper mineral is chalcopyrite [1–5]. Chalcopyrite has traditionally been treated by conventional pyrometallurgical techniques [6], which consist of flotation, smelting and refining, and electrorefining [7]. These techniques yield approximately 19 million tonnes per annum [8]. Despite the high level of copper production, there is concern about the environmental contamination resulting from the application of these techniques owing to SO2 atmospheric emissions [9,10]. Because of this, it is necessary to study more environmentally friendly hydrometallurgical alternatives [11]. The slow copper extraction rate of conventional leaching from chalcopyrite in sulfur media makes commercial scale leaching economically unfeasible [12]. This may be due to the formation of a passive layer that forms on the surface of the mineral [13–15]. There have been numerous studies on dissolving

copper from chalcopyrite [16–18]. However, none of these studies have obtained positive results working at ambient temperature and atmospheric pressure [19].

The polymetallic nodules are rock concretions formed by concentric layers of hydroxides [20]. Their high content of base, critical and rare metals makes them commercially interesting [21–23]. Their metal content includes high concentrations of Co, Ni, Te, Ti, and Pt, as well rare earth elements [24, 25].

There have been few studies on acid leaching of chalcopyrite using marine nodules (MnO2) as an oxidizing agent [26–29]. These studies showed that good copper dissolution rates of chalcopyrite can be obtained at room temperature, provided that the MnO2/CuFeS2 rate is high. Devi et al. [26,27] indicated that this is due to the galvanic interaction between chalcopyrite and MnO2, the action of Fe3+/Fe2<sup>+</sup> ratio, and the formation of chlorine gas through the reaction between MnO2 and HCl. Havlik et al. [28] showed that 4 mol/L of HCl and a 4/1 de MnO2/CuFeS2 ratio is optimal conditions to obtain good results at ambient temperature (54% of copper in 90 min).

The proposed reaction for chalcopyrite leaching with magnesium nodules is expressed as follows [29]:

$$\begin{aligned} \text{CuFeS}\_{2(s)} + 2\text{MnO}\_{2(s)} + 8\text{H}^{+}\_{(aq)} + 5\text{Cl}^{-}\_{(aq)} &= 2\text{Mn}^{2+}\_{(aq)} + 4\text{H}\_{2}\text{O}\_{(aq)} + \text{CuCl}^{-}\_{2(aq)} + \\ \text{FeCl}\_{3(s)} + 2\text{S}^{0}\_{(s)}\text{ }\Delta\text{G}^{0} &= -202.6\text{ kJ} \\ \text{\_2-2} &= -2 \qquad \text{\_2-2} \qquad \text{\_2-2} \qquad \dots \qquad \text{\_2-2} \qquad \dots \qquad \dots \end{aligned} \tag{1}$$

$$\text{Ca}^{2+}\_{(aq)} + \text{SO}^{2-}\_{4(aq)} = \text{CaSO}\_{4(s)}\text{ }\Delta G^0 = -28.0\text{ kJ} \tag{2}$$

Equation (1) represents dissolving copper in a sulfur-chloride medium, owing to the use of sulfuric acid and the high presence of chloride (wastewater) in the system. Among the advantages of leaching in a chlorinated rather than sulphated environment is increased leaching kinetics, the generation of elemental sulfur and cupric and/or cuprous ions are stable in the form of chloride complexes. The Gibbs free energy of Equation (1), which negative, is spontaneous under normal conditions and forms a stable copper product and a non-polluting elemental sulfur residue. While the calcium in wastewater and the manganese nodules reacts with the sulfate in the system, forming Equation (2) which is spontaneous and more likely to occur under normal conditions with the elements present (higher affinity of sulfate for calcium than magnesium and manganese in solution), the calcium sulfate formed is insoluble because calcium precipitates when it comes in contact with sulfate, nitrates and other elements. Equation (1) shows a 2/1 MnO2/CuFeS2 ratio for leaching copper using manganese nodules as an oxidizing agent, which was initially proposed by Toro et al. [29], but the best conditions to leach copper is at a 4/1 MnO2/CuFeS2 ratio. The values of the Gibbs free energy were calculated using the software HSC 5.1.

Other investigations have reported the positive effect of the chloride concentration on chalcopyrite leaching [18,30–32]. Velasquez et al. [33] indicated that chloride ions play an important role in oxidizing copper and iron. The copper dissolution is improved with high chloride concentrations.

The level of energy consumed in industrial scale operations related to comminution processes, reactor design, and leaching residence time largely depend on the particle size of the working material [19]. Studies have found a positive effect of smaller particle size on chalcopyrite leaching owing to the large area of contact for leaching [34,35]. Skrobian et al. [36] conducted chalcopyrite leaching tests in agitating reactors, with the addition of 300 g/L of NaCl to all the reactors, but with different particle sizes (−40 μm, −80 + 60 μm and −200 + 100 μm) and a temperature of 100 ◦C. Their results indicate that particle size has a negligible effect on the copper dissolution rate from chalcopyrite.

Different researchers agree on the positive effects of higher temperature on copper dissolution from chalcopyrite in terms of substantially increasing dissolution velocity [37]. Ruiz et al. [17] used sulfate–chloride media for dissolve chalcopyrite of a particle size 12.3 μm, 20 g/L of acid, 35.5 g/L of chloride, a stirring rate of 1000 rpm and 0.3 L/min O2 and obtained a copper dissolution rate of 90% in 180 min, with. Other studies of chalcopyrite leaching in chloride media and using oxidizing agents like cupric ions [30] and nitrates [38] have also reported good results in copper extraction at higher temperatures.

The scarcity of fresh water in arid zones is an economic, environmental and social problem [39,40]. The availability of water resources and the quality of potable water have decreased significantly owing to human activity, whose effects at the small-scale are significant for the entire basin [41]. Because of this situation the mining industry is driven to conserve the water it uses and minimize water discharges [41,42]. As well, conventional water resources that mining companies and communities compete for are limited [43]. Seawater has been shown to be a good alternative for mining, not only because of its positive effects on leaching owing to its chloride content, but also as a strategic and indispensable resource [40]. Another attractive alternative is using wastewater from desalination plants, not only because of the economic benefits, but also to avoid contaminating ocean waters [44].

There are few studies for the dissolution of chalcopyrite incorporating MnO2 and chloride in the system [26–29], achieving positive results in the extraction of Cu at room temperature, mainly evaluating the concentration of MnO2 in the system. Previously, Toro et al. [29] conducted an investigation in which they evaluated the use of wastewater with high chloride, seawater and manganese nodules contents, for the dissolution of chalcopyrite in an acidic medium. In this investigation, the effect on the concentration of MnO2, chloride and agitation speed in the system was evaluated. The authors found that high levels of MnO2 (4/1 and 5/1) allow potential values to be between 580 and 650 mV, favouring the dissolution of CuFeS2, and preventing the formation of a passivating layer. However, no other fundamental variables have been evaluated to favor the dissolution of CuFeS2. In the present research, we evaluated the use of wastewater with high chloride content, and MnO2 present in manganese nodules as an oxidizing agent in leaching chalcopyrite. Also, wastewater with high chloride levels from a desalination plant was reused. The particle size and temperature were optimized.

#### **2. Methodology**

#### *2.1. Chalcopyrite Sample*

The chalcopyrite sample used in this study was the same as that used in the first part, published in Toro et al. [29]. The sample was taken selectively from a copper deposit (800 g) and then crushed in a porcelain mortar to avoid contamination. We removed the impurities by hand (with the help of a microscope). The homogenization of the material was done by sampling techniques, selecting a representative fraction of 40 g (20 g for chemical analysis and 20 g for mineralogical analysis). Through a mineralogical analysis using a Bruker brand X-ray diffractometer (Bruker, Billerica, MA, USA), automatic and computerized model of D8 determined that the sample has a purity of 99.9% as can be seen in Figure 1. Finally, a chemical analysis performed by means of an atomic emission spectrometry via induction-coupled plasma (ICP-AES) (AMETEK, SPECTRO, Boschstraße, Germany) determined 33.89% of Cu, 30.62% of Fe and 35.49% of S (See Table 1).

**Table 1.** Chemical analysis of chalcopyrite.


In addition, the sample was analyzed mineralogically using a Bruker brand X-ray diffractometer, automatic and computerized model of D8. In Figure 1, you can see the results of the analysis, from which it was obtained that the chalcopyrite mineral has a purity of 99.90%.

**Figure 1.** X-ray diffractogram for the chalcopyrite.
