Coprecipitation of CaCO₃ and CaSO₄

Coprecipitation characteristics of CaCO 3 and CaSO 4 were studied under various operating conditions. The investigations were carried out in temperature‐controlled batch tests. Coprecipitation results were compared with our experimental results of a single salt crystallization. It was found that the induction period and kinetics of coprecipitation of these two salts follow that of pure CaCO 3 . However, thermodynamic concentrations of Ca in coprecipitation followed that of pure CaSO 4 at all times. The relationship between the thermodynamic concentrations of Ca for pure CaCO 3 and CaSO 4 solutions depended on the pH of the CaCO 3 solution. CaSO 4 precipitated in form of gypsum and had a needle shape structure; CaCO 3 had a spiral growth and precipitated in form of calcite. The precipitate structure was affected by the co‐existence of salts; the co‐precipitation resulted in CaCO 3 crystals interwoven by CaSO 4 crystals. This tends to result in a co‐precipitate that is stronger than pure CaSO 4 and weaker than pure CaCO 3 precipitate.