The effect of chemical additives on the electro-assisted reductive pretreatment of chalcopyrite
Introduction
Sulfide minerals are the main source of base metals. Approximately 70% of world's copper reserves are in the form of chalcopyrite (CuFeS2), making it the most profitable copper mineral (Davenport et al., 2001). Currently, copper sulfides are processed mainly by pyrometallurgy and only 15 to 20% of copper production, mostly oxides, is extracted by hydrometallurgical methods. Although there are two types of hydrometallurgical processes, oxidative and reductive, practically all of the processes developed to extract copper from chalcopyrite use the oxidative route, in the leaching stage (Dreisinger and Abed, 2002). However, these last methods require high pressure and temperature or non-aqueous solvents (Muir and Senanayake, 1985) to achieve quantitative chalcopyrite dissolution. The reason for this behavior is that chalcopyrite is passivated by intermediate sulfides, which impede the oxidant from reaching the chalcopyrite surface, thus preventing further reaction (Parker and Muir, 1981, Parker and Power, 1981).
Many research teams in the last 50 years have contributed to the explanation of this passivation phenomenon with the aim of finding ways to diminish its effects. Since the reaction rate depends on the hydrogen ion concentration [H+] (Hiskey and Wadsworth, 1975), the chalcopyrite is usually leached at highly acidic conditions, employing solutions containing concentrated hydrochloric or sulfuric acid. Ferric chloride, copper chloride and hydrochloric acid, as part of the chloride leaching media, lessen the problem, but have the disadvantage of high corrosivity. High pressure leaching and ferric sulfate leaching also offer limited improvement. However, none have been sufficiently successful to lower the leaching temperature to near ambient values, which could permit a more selective copper extraction. Finally, bacterial leaching has the disadvantage of being a slow process.
One option to avoid these problems is to employ the reductive route, specifically electro-assisted reductive pretreatments in which chalcopyrite is transformed into less refractory sulfides (chalcocite and djurleite) or copper metal. Generally, these processes do not show the same degree of passivation as the oxidative ones. However, there is very little information regarding the reduction mechanism.
The nature of the chalcopyrite reduction reaction products is determined by the pH and composition of the electrolytic solution. The thermodynamic data in the form of Eh–pH diagrams provide a guide to obtain information about the reactions taking place. In particular for chalcopyrite, the products of the reduction reaction under acidic conditions are expected to be hydrogen sulfide (H2S) and ferrous ion (Fe2 +), as well as bornite, chalcocite, and copper metal as solid phases, in that order as the potential becomes more negative (Fuentes-Aceituno et al., 2008). In the reduction processes, iron and sulfur are extracted from the mineral, leaving a copper rich sulfide (Biegler and Swift, 1976).
The electrons needed for this reaction may come from a reducing agent or from direct electrolytic reduction (because copper sulfides have semiconducting properties). Metals, such as aluminum, iron, lead or zinc have been used as reducing agents in acidic media (Lapidus and Doyle, 2006). However, the problems of high reagent consumption and toxic H2S production limit this approach. The electro-assisted reduction process helps minimize these problems, but has not previously been investigated in detail.
Electro-assisted reduction of chalcopyrite is possible at ambient temperature and pressure, with an aluminum cathode and a lead-silver alloy anode (Martínez-Jiménez and Lapidus, 2012). According to these authors, the reduction reactions are as follows:
where H ⋅ is the monoatomic hydrogen produced on the cathode surface. The chalcopyrite to chalcocite reduction (3) is proposed as the rate-limiting step. However, at the reaction boundary, high concentrations of ferrous and sulfide ions can lead to the formation of pyrrhotite (Fes(s)), which probably causes passivation.
In order to favor iron extraction and, consequently, lessen the passivation problem, the addition of carboxylic acids to the reduction pretreatment solution, as ligands for the ferrous ion, has been proposed, (Martínez-Jiménez and Lapidus, 2012). The complexation reactions with citrate, acetate and tartrate ions are as follows (NIST Standard Reference Database 46, 2004):
The same authors (Martínez-Jiménez and Lapidus, 2012) reported metallic copper as the main reduction product, which spontaneously reacts to copper oxide when the solids contact air. However the reaction is still quite slow, possibly because of the difficult solid-state transformation.
On the other hand, the copper in chalcopyrite occurs as Cu(I), but the cuprous ion is not stable in sulfate solution, requiring the potential to be high enough to oxidize it to Cu(II) state or, in the reduction route, to stabilize it in the cuprous form, and to avoid disproportionation. Nelson et al. (1961) showed that the cuprous ion solubilizes in polar organic solvents. It has been demonstrated that polar organic solvent solutions increase chalcopyrite dissolution under ambient conditions (Muir and Senanayake, 1985, Solís and Lapidus, 2014) by stabilizing the cuprous ion, thereby enhancing copper extraction.
Regarding the chalcopyritic sulfide ion, aqueous solutions of triethanolamine (TEA) are widely employed in the petroleum industry to remove H2S from gaseous streams and extract it into the solution, according to the following reaction (Kumar, 1987):
As mentioned earlier, a high H2S concentration probably causes the formation of pyrrhotite on the surface of the chalcopyrite, which passivates the mineral surface. By diminishing the hydrogen sulfide concentration, the formation of pyrrhotite may be avoided, thus enhancing the chalcopyrite conversion to chalcocite and metallic copper.
In this investigation, the nature of the reactions in the reduction of chalcopyrite is studied, employing the following complexing media: ethylene glycol for Cu(I) stabilization, triethanolamine to sequester H2S and carboxylic acids to complex the ferrous ion.
Section snippets
Experimental materials and methods
Cyclic voltammetry and chronoamperometry were used to determine the nature of the reactions that take place at the chalcopyrite surface in the different leaching media and to characterize the solid products formed on the surface of the mineral.
Because it has been reported that H2S plays an important role in the reaction mechanism of the reduction step (Martínez-Jiménez and Lapidus, 2012), electro-assisted reductive pretreatment experiments were carried out with the same mineral to validate the
Single-cycle voltammetry
To determine the reduction and oxidation behavior of the carbon–chalcopyrite paste in each of the electrolytic media, the scans were initiated in the negative (reduction) direction (from the OCP to − 490 mV vs NHE [normal hydrogen electrode], reversed and scanned to 1010 mV, where the direction was again reversed and returned to the OCP), as shown in Fig. 1 in 1 M H2SO4. The voltammograms (inset) show the chalcopyrite reduction to bornite (peak I, 100 mV), chalcocite (peak II, − 290 mV) and finally
Conclusions
The use of complexing agents, such as TEA and tartaric acid, together with sulfuric acid was shown to increase the amount of metallic copper formed in the electro-assisted pretreatment of chalcopyrite; hence, the amount of copper extracted in the oxidative leach was enhanced by up to 90% when compared with that obtained from the residue pretreatment with H2SO4 alone (38%). The improvement in the copper extraction is probably due to the complexation of iron by the carboxylic acids and sulfur by
Acknowledgments
The authors wish to thank the Consejo Nacional de Ciencia y Tecnologia (CONACyT) for the postgraduate scholarship granted to the first author (Gerardo Emerson Barrera-Mendoza, Grant number: 215530).
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