砷酸钙渣形成类型的热力学分析

Thermodynamics Analysis on Formation of Calcium Arsenate Slag

Ca-As盐的形成发生于含砷溶液的钙盐沉淀及砷在地下水迁移的过程中。通过热力学计算,绘制了25℃时Ca^2+-AsO4^3-H2O系热力学平衡图,并考察了Ca/As摩尔比对沉淀的物相组成及沉淀效果的影响。结果表明,在0<pH<14范围内,Ca^2+-AsO4^3-H2O系可形成4种固体物相,分别为CaHAsO4·H2O(s)、Ca3(AsO4)2·4H2O(s)、Ca5(AsO4)3OH和Ca4(OH)2(AsO4)2·4H2O(s)。当Ca/As<2时,CaHAsO4·H2O(s)和Ca3(AsO4)2·4H2O(s)的稳定区随着pH的增大而缩小,Ca5(AsO4)3OH的稳定区随pH增大而扩大。当Ca/As>2时,Ca4(OH)2(AsO4)2·4H2O(s)才能形成。结合已有研究,认为要达到理想的脱砷效果,Ca/As应该大于2,且溶液中应该保持较高的游离钙浓度。

Calcium arsenate(Ca-As)forms during arsenic precipitation with calcium salt or migration of groundwater.According to thermodynamic calculation,equilibrium diagrams of Ca^2+-AsO4^3-H2O system were established at 25℃.Effect of Ca/As mole ratio on phase composition of precipitate and precipitation was investigated.The results indicate that four solid phases,CaHAsO4·H2O(s),Ca3(AsO4)2·4H2O(s),Ca5(AsO4)3OH and Ca4(OH)2(AsO4)2·4H2O(s),are generated at 0<pH<14.With increase of pH value,stable areas of CaHAsO4·H2O(s)and Ca3(AsO4)2·4H2O(s)shrink,and stable area of Ca5(AsO4)3OH enlarge with Ca/As<2.Combined with the existing work,Ca/As ratio should be higher than 2 and free calcium concentration should be kept on a high level to obtain a high arsenic removal rate.