Kinetics and Mechanisms of Chalcopyrite Dissolution at Controlled Redox Potential of 750 mV in Sulfuric Acid Solution
To better understand chalcopyrite leach mechanisms and kinetics, for improved Cu extraction during hydrometallurgical processing, chalcopyrite leaching has been conducted at solution redox potential 750 mV, 35-75˝C, and pH 1.0 with and without aqueous iron addition, and pH 1.5 and 2.0 without aqueous iron addition. The activation energy (E a ) values derived indicate chalcopyrite dissolution is initially surface chemical reaction controlled, which is associated with the activities of Fe 3+ and
... + with reaction orders of 0.12 and´0.28, respectively. A surface diffusion controlled mechanism is proposed for the later leaching stage with correspondingly low E a values. Surface analyses indicate surface products (predominantly S n 2´a nd S 0 ) did not inhibit chalcopyrite dissolution, consistent with the increased surface area normalised leach rate during the later stage. The addition of aqueous iron plays an important role in accelerating Cu leaching rates, especially at lower temperature, primarily by reducing the length of time of the initial surface chemical reaction controlled stage. Minerals 2016, 6, 83 2 of 18 diffusion controlled leaching e.g., elemental sulfur (S 0 ) [27-33], polysulfide (S n 2´) [31,32,34,35] and Fe oxyhydroxide [36,37]. Saxena and Mandre  reported that chalcopyrite dissolution was initially surface chemical reaction controlled, followed by surface diffusion control (E a of 16 kJ¨mol´1) through the S 0 layer formed. Sokic, Markovic and Zivkovic  reported a high value of E a (83 kJ¨mol´1) for chalcopyrite leaching and ascribed this to surface chemical reaction control when the fraction extent leached was below 0.6; surface diffusion control through the S 0 layer formed was proposed to occur subsequently. Elevated temperatures, 65-75˝C, can be achieved for industrial Cu extraction in the absence of external heating by application of thermophilic microbial heap leaching via approaches such as Geocoat™ [38, 39] . Li, et al.  investigated the effect of solution redox potential from 650 to 850 mV (all solution redox potentials, E h , are reported relative to the standard hydrogen electrode, SHE) and observed maximum dissolution rates at 750 mV, the E h condition adopted here. Li, et al.  found that in the absence of added aqueous iron, Fe 3+ and H + activities played positive and negative roles, respectively, in chalcopyrite leaching at 750 mV and 75˝C. This trend also held for selected examples with added aqueous iron but leach data were not compared directly across the cases with and without added iron. Although there has been considerable focus on understanding chalcopyrite leaching, the resulting surface chemistry is still not well understood. No universal agreement has been achieved as to the mechanisms resulting in the variable surface layers observed and the role of the new surface species. E h , pH, temperature and pulp concentration have all been reported to play a role in chalcopyrite dissolution [5,    . Hence, the aim of this study was to observe chalcopyrite leaching at controlled potential of 750 mV with both solution and surface speciation being examined to develop a comprehensive rate law and mechanism. To achieve this aim, investigations of the leaching kinetics and evolution of surface species with and without the addition of 4 mmol iron have been conducted at fixed E h (750 mV), pH 1.0-2.0 and 35-75˝C. Methodology Chalcopyrite Sample The chalcopyrite used was from Sonora, Mexico. A chunk of the chalcopyrite was crushed and rod milled to obtain a size fraction of 38-75 µm via wet sieving. The resulting sample was then sonicated, to remove clinging fines, and dried in an oven purged with N 2 , at 70˝C. Brunauer, Emmett and Teller (BET) analysis gave a surface area of 0.24˘0.04 m 2¨g´1 . Each 4 g sub-sample was placed into a plastic tube which was subsequently sealed after being filled with N 2 gas to minimise surface oxidation by air. All the samples were stored in a freezer prior to use. X-ray powder diffraction Rietveld analysis indicated that this final sample contained 92˘5 wt % chalcopyrite, 2˘1 wt % quartz, 1˘1 wt % pyrite, 0.8˘0.9 wt % sphalerite with another 4˘2 wt % of unknown component(s) (uncertainty estimates to the 95% level are based on χ 0.35 , where χ is the wt %, as per Geelhoed, et al.  ). Inductively coupled plasma atomic emission spectroscopy (ICP-AES, carried out by Rio Tinto Technology and Innovation, Melbourne, Australia, Table 1 ) resulted in a commensurate mineralogical assessment giving rise to a stoichiometry of 95.9 wt % Cu 0.96 Fe 0.99 S 2.00 .