Nitric Acid Dissolution of Tennantite, Chalcopyrite and Sphalerite in the Presence of Fe (III) Ions and FeS2

Synthetic Sulfide Concentrate Dissolution Kinetics in HNO3 Media

The nature of tennantite (Cu12As4S13), chalcopyrite (CuFeS2) and sphalerite (ZnS) particles’ mixture dissolution in nitric acid (HNO3) media was investigated in this study. The effects of temperature (323−368 K), HNO3 (1−8 mol/L) and Fe3+ (0.009−0.036 mol/L) concentrations, reaction time (0−60 min) and pyrite (FeS2) additive (0.5/1−2/1; FeS2/sulf.conc.) on the conversion of the minerals were evaluated. It has been experimentally shown that the dissolution of the mixture under optimal conditions (>353 K; 6 mol/L HNO3; FeS2/synt. conc = 1/1) allows Cu12As4S13, CuFeS2 and ZnS conversion to exceed 90%. The shrinking core model (SCM) was applied for describing the kinetics of the conversion processes. The values of Ea were calculated as 28.8, 33.7 and 53.7 kJ/mol, respectively, for Cu12As4S13, CuFeS2 and ZnS. Orders of the reactions with respect to each reactant were calculated and the kinetic equations were derived to describe the dissolution rate of the minerals. It was found that the interaction between HNO3 solution and Cu12As4S13, CuFeS2 and ZnS under the conditions investigated in this are of a diffusion-controlled nature. Additionally, the roles of Fe(III) in the initial solution and FeS2 in the initial pulp as catalysts were studied. The results indicated that the increase in Fe3+ concentration significantly accelerates the dissolution of the mixture, while the addition of FeS2 forms a galvanic coupling between FeS2, and Cu12As4S13 and CuFeS2, which also accelerates the reaction rate. The results of the study are considered useful in developing a hydrometallurgical process for polymetallic sulfide raw materials treatment.

Application of Mixed Potential Theory to Leaching of Mineral Phases

Leaching is a central unit operation in the hydrometallurgical processing of minerals, which often occurs by means of electrochemical reactions. Application of mixed potential theory to explain the kinetics of oxidative and reductive leaching processes is a useful concept in explaining observed results. Native metals, selected oxides, and most base metal sulfides are electron-conducting phases. For these minerals, leaching may take place by normal corrosion, passivation or galvanic couple mechanisms, which provide individual electrode kinetics enabling the calculation of mixed potentials and overall reaction kinetics. Examples of the electrochemical nature of selected leaching processes are presented and include the effect of mixed potentials, geometry, and associated kinetic reactions.

The Kinetics of Pyrite Dissolution in Nitric Acid Solution

Refractory sulphidic ore with gold captured in pyrite has motivated researchers to find efficient means to break down pyrite to make gold accessible and, ultimately, improve gold extraction. Thus, the dissolution of pyrite was investigated to understand the mechanism and find the corresponding kinetics in a nitric acid solution. To carry this out, the temperature (25 to 85 °C), nitric acid concentration (1 to 4 M), the particle size of pyrite from 53 to 212 µm, and different stirring speeds were examined to observe their effect on pyrite dissolution. An increase in temperature and nitric acid concentration were influential parameters to obtaining a substantial improvement in pyrite dissolution (95% Fe extraction achieved). The new shrinking core equation (1/3ln (1 − X) + [(1 − X)^−1/3 − 1)]) = kt) fit the measured rates of dissolution well. Thus, the mixed–controlled kinetics model describing the interfacial transfer and diffusion governed the reaction kinetics of pyrite. The activation energies (E_a) were 145.2 kJ/mol at 25–45 °C and 44.3 kJ/mol at higher temperatures (55–85 °C). A semiempirical expression describing the reaction of pyrite dissolution under the conditions studied was proposed: 1/3ln(1 − X) + [(1 − X)^−1/3 − 1)] = 88.3 [HNO₃]^2.6 r₀^−1.3 e^−44280/RT t. The solid residue was analysed using SEM, XRD, and Raman spectrometry, which all identified sulphur formation as the pyrite dissolved. Interestingly, two sulphur species, i.e., S₈ and S₆, formed during the dissolution process, which were detected using XRD Rietveld refinement.