The behaviour of cadmium during jarosite precipitation

The behaviour of cadmium during the precipitation of ammonium jarosite has been investigated as a function of the cadmium concentration, the ZnSO4 concentration and the pH of the synthesis solutions. The Cd content of the jarosite seems to increase in a linear manner with increasing Cd concentrations in the solution, but the extent of Cd incorporation is always low. Less than 100 ppm Cd is present in the precipitates made from solutions containing up to 3,000 mg/l Cd, and similar Cd contents were realized in the presence of 75 g/l Zn and for all pH values from 0.9 to 2.0. Mass balance calculations showed that < 1% of the Cd reported to the ammonium jarosite precipitates under all conditions studied; virtually 100% of the Cd remained in the final solution. Ammonium jarosite precipitates made in the presence of 0.1-40 g/l each of Cd and Zn incorporated 5-10 times more Zn than Cd. Low concentrations of Cd have a negligible effect on the morphology of the ammonium jarosite precipitates, but high concentrations of CdSO4 and/or ZnSO4 appear to promote jarosite crystallization. Finally, the laboratory results were used to rationalize the behaviour of cadmium during jarosite precipitation in commercial zinc operations.