Factors Affecting the Precipitation of Potassium Jarosite in Sulfate and Chloride Media

The factors affecting the precipitation of potassium jarosite in both sulfate and chloride media were systematically investigated for the range of conditions likely to be encountered in hydrometallurgical practice. In sulfate solutions at 97 °C, the amount of precipitate increases with increasing retention times up to 10 hours, with increasing temperatures to approximately 100 °C, and with increasing ferric-ion concentrations. Despite the variations in the amount of precipitate, the composition remains nearly constant and is characteristic of that of potassium jarosite. The presence of potassium jarosite seed accelerates the rate of precipitation and results in near-maximum amounts of precipitate after just a few hours of reaction. The presence of ferrous sulfate has a negligible effect on both the product yield and composition. The amount of precipitate decreases with increasing acid concentrations, but the composition of the precipitates remains nearly constant, even for acid concentrations as high as 1.0 M H₂SO₄. Both the amount and composition of the precipitates vary as the K₂SO₄ concentration increases; in the presence of excess ferric ions, nearly complete K precipitation occurs. Small amounts of Cu²⁺, Zn²⁺, and Pb²⁺ are incorporated in the structure of potassium jarosite, and the amount increases with increasing concentrations of the divalent metal ions. Potassium jarosite is readily formed in chloride media at 140 °C, provided that an independent source of sulfate ions is available. A minimum K concentration is required in order to avoid the formation of hematite; at higher K concentrations, however, the product composition becomes nearly constant and reflects that of potassium jarosite. Both the product yield and composition are independent of the chloride concentration, added as LiCl, and the ferrous ion concentration, added as FeCl₂. Although increasing concentrations of HCl result in a near-linear decrease in the amount of precipitate, the composition is independent of the HCl concentration and is characteristic of that of potassium jarosite. Increasing temperatures to 200 °C result in an increase in the product yield, but have no significant effect on the composition of the potassium jarosite precipitates.