Hydrothermal Treatment of Arsenopyrite Particles with CuSO₄ Solution

The nature of the hydrothermal reaction between arsenopyrite particles (FeAsS) and copper sulfate solution (CuSO₄) was investigated in this study. The effects of temperature (443-523 K), CuSO₄ (0.08-0.96 mol/L) and H₂SO₄ (0.05-0.6 mol/L) concentrations, reaction time (1-120 min), stirring speed (40-100 rpm) and particle size (10-100 μm) on the FeAsS conversion were studied. The FeAsS conversion was significant at >503 K, and it is suggested that the reaction is characterized by the formation of a thin layer of metallic copper (Cu⁰) and elemental sulfur (S⁰) around the unreacted FeAsS core. The shrinking core model (SCM) was applied for describing the process kinetics, and the rate of the overall reaction was found to be controlled by product layer diffusion, while the overall process was divided into two stages: (Stage 1: mixed chemical reaction/product layer diffusion-controlled) interaction of FeAsS with CuSO₄ on the mineral's surface with the formation of Cu¹⁺ and Fe²⁺ sulfates, arsenous acid, S⁰, and subsequent diffusion of the reagent (Cu²⁺) and products (As³⁺ and Fe²⁺) through the gradually forming layer of Cu⁰ and molten S⁰; (Stage 2: product layer diffusion-controlled) the subsequent interaction of CuSO₄ with FeAsS resulted in the formation of a denser and less porous Cu⁰ and S⁰ layer, which complicates the countercurrent diffusion of Cu²⁺, Cu¹⁺, and Fe²⁺ across the layer to the unreacted FeAsS core. The reaction orders with respect to CuSO₄ and H₂SO₄ were calculated as 0.41 and -0.45 for Stage 1 and 0.35 and -0.5 for Stage 2. The apparent activation energies of 91.67 and 56.69 kJ/mol were obtained for Stages 1 and 2, respectively.