Pentlandite being the most abundant nickel mineral, chalcopyrite is the most abundant copper mineral in nature (Córdoba et al., 2008). Hydrometallurgical processes for the extraction of chalcopyrite from low-grade ores has become of greater interest during past years because of for example easier control of waste
and other benefits to the environment. At an industrial level chalcopyrite concentrates are still mostly treated by pyrometallurgical processes, but investigations on hydrometallurgical processes are already made on laboratory scale and in pilot plant tests.
Antonijević et al. (2008) investigated the leaching of flotation tailings with sulfuric acid in the presence and absence of an oxidizing agent. The content of sulfidic minerals in the flotation tailings sample used in the investigation was 21
% of which 20.81 % were pyrite minerals. The copper sulfide minerals found in the sample were composed of 0.081 % covellite (CuS) 0.037 % enargite (Cu3AsS4) 0.049 % chalcopyrite and 0.018 % chalcocite. Sulfuric acid and tap water were used to form the leaching solution whereas iron(III) sulfate was used as an oxidant. Weathering can cause pyrite from the flotation tailings to oxidize and Fe2(SO4)3 and H2SO4 to be formed and hence these were chosen to study. The dissolution of chalcopyrite is shown in equation 19 (Antonijević et al., 2008;
Smalley and Davis, 2000). As a result of the experiment 60-90 % of copper was recovered. The amount of recovered copper in the absence and presence of an oxidizing agent were 60-68 % and 68-88 % respectively.
CuFeS2 + 2Fe2(SO4)3 CuSO4 + 2S° + 5 FeSO4 (19)
Hansen et al. (2005) investigated the leaching of copper from mine tailings in sulfuric acid. The primary ore minerals of the mine tailing investigated were pyrite, chalcopyrite, bornite, molybdenite (MoS2), galena (PbS), tennantite (Cu12As4S13), magnetite (Fe3O4) and hematite (Fe2O3). The experiments were conducted at room temperature by adding 2 grams of the tailings sample into 20 ml of leaching agent after which this mixture was shaken in an orbital shaker for 24 hours. The dry mine tailing was divided into different particle size fragments after which leaching experiments were done for different size fragments. As leaching agents HNO3, H2SO4, HCl and citric acid were used in concentrations of 0.001, 0.005, 0.01, 0.1 and 1 mol/dm3. Only small differences in the initial copper content of the tailings sample between different size segments was noticed. For all leaching agents dissolving copper from the larger particle segments was difficult.
This has been explained by the sulfide content in different size fragments. The larger the particles, the more copper is present in sulfides instead of oxides, phosphates or carbonates. The main results of the investigation were that more copper was dissolved at smaller particle size fractions and that sulfuric acid and hydrochloric acid were the best leaching agents.
Leaching of copper from a chalcopyrite concentrate (CuFeS2–PbS–ZnS) divided into different size fragments was also studied by Aydogan et al. (2006). The experiments were conducted in 1 liter glass reactors at temperatures between 50 and 90 °C. As leaching agent H2SO4 was used and K2Cr2O7 was added as an oxidizing agent. The effect of stirring speed, acid concentration, oxidizing agent concentration, temperature and particle size was investigated. As a result, the highest recovery for copper was obtained at following process conditions: pulp density 10 g/dm3, acid concentration 0.4 mol/dm3 H2SO4, temperature 97 °C, stirring speed 400 rpm, oxidant concentration 0.1 mol/dm3 K2Cr2O7. At these process conditions, about 80 % of copper was dissolved whereas at a temperature of 50 °C about 50 % of copper was dissolved.
4 KINETICS OF SULFIDE LEACHING
The leaching mechanism of sulfide minerals is described by different models of which the classic is the shrinking core model. By describing the leaching process by fitting a mathematical equation to the experimental data the rate determining step can be found out and the process can be controlled and optimized.
The rate of leaching is mostly affected by the transport of the leaching agent to the reaction site and the transport of the leached components away from the reaction site (Moore, 1981). These reactions are diffusion controlled. If reactions are not diffusion controlled, they are reaction controlled in which the rate determining step is the actual chemical reaction at the reaction site. When the rate determining step is the diffusion to and from the solid surface, the rate of reaction can be increased by liquid agitation whereas mixing does not have any significant effect
on the rate of the chemical reaction (Rosenqvist, 1974). The mixing speed should always be taken into consideration when determining the rate determining step.
The leaching of chalcopyrite can be illustrated by three kinetic models: diffusion, surface reaction and a mixed model containing factors of both diffusion and surface reaction (Li et al., 2013). Several investigations have indicated that the rate determining step would be the diffusion through a passivating layer (Aydogan et al., 2006; Hackl et al., 1995; Harmer et al., 2006; Muñoz et al., 1979). The nature of this layer is somewhat unclear since the mechanisms of dissolution are not completely understood. Nicol et al. (2010) and Hackl et al. (1995) have suggested that leaching is inhibited by a thin copper-rich polysulfide layer whereas Aydogan et al. (2006) and Muñoz et al. (1979) suggest that the layer would be formed of elemental sulfur. According to the study made by Lu et al.
(2000b) the leaching of pentlandite is chemically controlled at low temperatures whereas at high temperatures the process is diffusion controlled.