A fundamental study of the reductive leaching of chalcopyrite using metallic iron
Creators: name: Abed, Nedam
Publisher: name: University of British Columbia
Year: 2009

description: A fundamental study of the reductive leaching (decomposition) of chalcopyrite was performed in both sulfate and chloride media. This was done to understand the physical chemistry of the leaching reactions and the possibility of developing a process flowsheet. The main objective of reductive leaching is to achieve the enrichment of chalcopyrite by rejection of iron and sulfur. A chalcopyrite concentrate containing around 60% chalcopyrite and analyzing around 28% copper was leached under reducing conditions using metallic iron. Various parameters were studied to understand their effect on leaching kinetics, including : temperature, particle size, agitation, acid concentration, molar ratios, and others. The leaching data were analyzed to determine the leaching mechanism and develop a kinetic model. The leaching reaction was found to be rapid on fresh surfaces of the concentrate, but slows markedly in one hour, as a film of products, mainly chalcocite, forms on particle surfaces. Iron was also found to enter the leach solution as soluble ferrous ions and sulfur is released as hydrogen sulfide. The general leaching reaction may be written as : 2CuFeS₂ (s) + Fe(s) + 6H⁺ (aq) -&gt; Cu2S(s) + 3Fe²⁺ (aq) + 3H2S(g) The proposed mechanism is a series of reactions. It is envisaged to be composed of two parts : a corrosion mechanism, which is iron dissolution, and galvanic mechanism, which is chalcopyrite reduction. The kinetic analysis indicated that the leaching reaction, which is electrochemical in nature, follows the shrinking core model under product layer diffusion control, and the rate determining step is the transport of one or more of reaction species, through the product layer. The reaction was dependent on the initial acid concentration, chalcopyrite particle size and molar ratio of iron to chalcopyrite. Moreover, the reaction was independent of the rate of agitation beyond that required to provide a well-mixed reaction mixture. Under stoichiometric conditions, room temperature and atmospheric pressure, the conversion (decomposition of chalcopyrite to simpler copper sulfides) was always below 60%, unless the initial amount of the reductant was increased or very fine chalcopyrite particles were used. For the studied experimental conditions, the developed parabolic leaching model for sulfate media takes the form : [chemical formula1] and for chloride media, [chemical formula2] The parabolic leaching behavior was confirmed from the successful estimation of the related thermodynamic and kinetic properties of the leaching systems. The analysis of temperature dependence indicated that leaching increases with increasing temperature up to 65 °C. The apparent activation energy for leaching in sulfate media was estimated to be ~33 kJ/mol, and for chloride media, ~26 kJ/mol under stoichiometric conditions, in the temperature range 25-65 °C. Chemical analysis was extensively used based on wet chemistry methods, which were capable of demonstrating the general reaction stoichiometry including the composition of the new solid phase. Further, qualitative analysis by SEM confirmed the findings of the kinetic and chemical analysis. The findings from the fundamental study show that conversion is preferred under high solid pulp density (SPD), and back reaction kinetics have essentially little or no adverse effect. In an attempt to improve the conversion and utilize the results for a possible flowsheet, the concentrate was leached in the presence of excess chloride content, at -35% SPD. Based on material balance calculations, at room temperature and under near stoichiometric conditions, greater than 80% of the added chalcopyrite can be decomposed to yield copper sulfides (chalcocite) by rejection of iron and sulfur, using size fractions smaller than 74 jam. As a result of these findings, a process flowsheet was developed and further investigation is required to demonstrate the viability of the proposed process.