Calcium carbonate crystallization process from the mineralization of calcium chloride waste

The distillation waste stream from the ammonia-alkali process contains 90–120 g/L of calcium chloride. Using calcium chloride as raw material to mineralize carbon dioxide can achieve efficient utilization of calcium chloride waste and CO2 emission reduction. This article is based on a two-step synthesis method, using ammonium carbonate and calcium chloride as raw materials to synthesize calcium carbonate. The changes in pH of the mineralized solution with time were investigated under factors such as temperature, ammonium carbonate concentration, feeding method, and retention time. The crystallization mechanism of carbonation process shows that the pH of solution will affect the degree of supersaturation of calcium carbonate, and high supersaturation is conducive to the precipitation of metastable aragonite and vaterite phases. The experimental results showed that when ammonium carbonate was passed into calcium chloride at a rate of 10 mL/min at different temperatures and different ammonium carbonate concentrations, the pH change of the mineralized solution could be divided into three stages: a rapid pH decrease stage (9.5–7), a pH stabilization stage (∼7) and a pH rebound stage (7–8.5). Increasing the temperature is conducive to the formation of submicron calcite type calcium carbonate, changing the concentration of ammonium carbonate, and the crystal phase and size of the product are almost unaffected. If calcium chloride is added to ammonium carbonate, under low calcium ion concentration conditions, calcium carbonate tends to form aragonite phase.