Acid dissolution of jarosite-type compounds: Effect of the incorporation of divalent cations into the structure on the reaction rate

The present study analyzes the effect of the incorporation of divalent cations (Cu²⁺ and Zn²⁺) in the structure of a synthetic sodium jarosite on the dissolution rate in acid medium (H₂SO₄). Four syntheses of sodium jarosite with different amounts of Zn and Cu were performed using the hydrothermal method. The precipitates were characterized in detail using different analytical techniques. The feasibility of substituting Cu²⁺ and Zn²⁺ in the synthetized jarosites and in other previously reported jarosites was also analyzed by considering the Hume-Rothery rules for substitutional solid solutions. The variables investigated to determine the dissolution kinetics were temperature, pH and particle size. The rate-controlling step was the chemical reaction on the jarosite particle surface, which occurred according to the shrinking core kinetic model for spherical particles without the formation of a solid byproduct layer. In addition, the decomposition curves showed that the concentrations of Cu, Zn and Fe in the leach liquor increased as the reaction progressed. The reaction rate was affected by the presence of Cu in the structure of the sodium jarosite; the rate constant of dissolution for the Zn-incorporated jarosite (0.0035 min⁻¹) was very similar to that obtained for the nonsubstituted jarosite (0.0034 min⁻¹), but these values were higher than that determined for the Cu-incorporated jarosite (0.0026 min⁻¹). This difference might be related to the difference between the atomic radii of Cu and Fe, which may cause a distortion of the jarosite crystal lattice due to the variation in the a parameter. Finally, a global kinetic equation was proposed that considers the kinetic parameters determined from the induction and progressive periods; this model was able to satisfactorily describe the leaching behavior of Cu-incorporated sodium jarosite under acidic conditions.