Experimental Study on the Dissolution Behavior of Calcium Fluoride

The presence of halogens has an adverse effect on the zinc extraction process through electrowinning, the last phase of the RLE (Roasting, Leaching and Electrowinning) zinc extraction route. Fluoride (F⁻) may be present as calcium fluoride (CaF₂) and this is, for example, the case in double leached Waelz oxide (DLWO). Efficient removal of F⁻ from primary and secondary raw materials for zinc extraction results in a simplified process and increases flexibility in the selection of raw materials. Understanding of the solubility behavior of pure CaF₂ can give valuable information on treatment for maximized halogen removal. Dissolution of CaF₂ was studied with the addition of sodium carbonate (Na₂CO₃) and sodium bicarbonate (NaHCO₃). Dissolution studies were combined with thermodynamic calculations to understand the solubility behavior of CaF₂ under different conditions. Results from the experiments and the thermodynamic calculations show that Na₂CO₃ and NaHCO₃ have similar behavior if the pH is controlled at the same value. The available carbonate (CO₃²⁻) ion in the system limits the concentration of calcium (Ca²⁺) ion by precipitation of CaCO₃, which enhances the dissolution of CaF₂. At higher temperatures and pH, calcite, vaterite, and aragonite were formed and co-precipitation of CaF₂ along with calcium carbonate (CaCO₃) was observed. At lower temperatures and lower pH levels, only calcite and vaterite were formed and a coating by CaCO₃ on CaF₂ was found to hinder complete dissolution reaction. The results of this study indicate that the temperature along with the reagents used for the dissolution tests have a significant impact on the CaCO₃ polymorph mixture (calcite, vaterite and aragonite) formation.