钙碳比对CaSO_4-CaCO_3-(NH_4)_2CO_3-H_2O体系反应-结晶过程的影响

Influence of calcium/carbonate ratio on reaction-crystallization of CaSO_4-CaCO_3-(NH_4)_2CO_3-H_2O system

在CaSO_4-CaCO_3-(NH_4)_2CO_3-H_2O反应体系中,生成碳酸钙与硫酸铵的控速步骤是石膏溶解。溶解表面被新生的碳酸钙结晶覆盖是重要影响因素。基于反应面模型,通过调整钙离子溶出速率与碳酸根加入速率之比(钙碳比),将反应-结晶面控制在溶解表面的固液扩散边界层临界距离之外从而避免覆盖。实验采用表面积一定的天然纯净块状石膏试样,并向溶解体系滴加碳酸铵溶液,使钙碳比分别为0.125、0.25、0.5、1、2、4和8,观察碳酸钙在石膏表面的包覆现象,并预测反应-结晶面与溶解表面之间的微距离。结果显示,当钙碳比为0.125、0.25、0.5和1时,试样表面出现碳酸钙包覆但包覆量随λ增大而减小;当钙碳比为2、4和8时,试样表面不出现包覆,碳酸钙结晶生成于液相主体。将钙碳比为1视为临界值,该条件下反应-结晶面与溶解表面的微距离大于18μm。

In reaction system of CaSO4-CaCO3-（NH4）2CO3-H2O, gypsum dissolution was the rate-controlling step to produce calcium carbonate and ammonium sulfate. Coverage of dissolution surface by newly formed Ca CO3 crystal was a significant factor. Based on reaction plane model, adjusting ratio of gypsum dissolution rate to carbonate addition rate could control the reaction-crystallization plane beyond critical distance of solid-liquid diffusion boundary layer of the dissolution surface and avoid CaCO3 coverage on the surface. Pure natural gypsum blocks with constant surface area was used to study CaCO3 coverage on gypsum surface and to predict micro-distance between reaction-crystallization plane and dissolution surface. Ammonium carbonate solution was added into the dissolution system with ratio of gypsum dissolution rate to carbonate addition rate equal to 0.125, 0.25, 0.5, 1, 2, 4 and 8. The results showed that CaCO3 coating on surface was increased as the ratio of gypsum dissolution rate to carbonate addition rate decreased in the range of 0.125, 0.25, 0.5 and 1, whereas CaCO3 particles were formed in the liquid phase without surface coating with the ratio of gypsum dissolution rate to carbonate addition rate in the range of 2, 4 and 8. At the ratio of gypsum dissolution rate to carbonate addition rate equal to the critical value 1, the micro-distance was more than 18 μm.