钼酸铵溶液镁盐沉淀法除砷的热力学分析

摘要

The coarse ammonium molybdate solution obtained by hydrometallurgical treatment of Ni-Mo ore contains high arsenic. Thcrmodynamic analysis of removing arsenic from the ammonium molybdate solution with addition of MgCl2 was studied. According to the principle of simultaneous equilibrium and the law of conservation of mass, thermodynamic equilibrium diagrams of Mg-NH4-As-H2O system were established at 25 ℃. The effect of technical conditions on removal of arsenic was investigated. The results indicate that the concentration of arsenic decreases with the increase of total ammonium concentration and dosage of MgCl2. There are four stable areas of various solid phases in the whole pH of the system, when total ammonium is 5 mol/L, total arsenic is 0.1mol/L and total Mg is 0.12 mol/L in the system. The first one is MgHAsO4 (4.0H<6.0), the second one is MgNH4AsO4 (6.0H<9.5), the third one is MgNH4AsO4 and Mg(OH)2 (9.5H<11.7), and the forth one is Mg(OH)2 (pH>11.7). Adjusting the pH to 9, the arsenic concentration can be thermodynamically decreased to 2 × 10-6 g/L and residual magnesium concentration is 0.02 mol/L in the solution.%湿法处理镍钼矿所得钼酸铵溶液砷含量较高.针对镁盐沉淀法除砷的工艺,进行热力学分析.根据同时平衡原理和质量守恒定律,进而绘制在25℃时Mg-NH4-As-H2O系热力学平衡图,并考察工艺参数对除砷的影响.研究结果表明:溶液中总氨浓度的升高及镁盐用量增大,平衡时砷含量降低.当钼酸铵溶液中[N]r=5mol/L,[As]r=0.1mol/L,[Mg]T=0.12mol/L时,整个pH范围存在4个平衡固相的稳定区即MgHAsO4稳定区(4.0＜pH＜6.0),MgNH4AsO4稳定区(6.0＜pH＜9.5),MgNH4AsO4和Mg(OH)2稳定区(9.5＜pH＜11.7)及Mg(OH)2稳定区(pH＞11.7):当溶液pH为9时,砷质量浓度可除至2×10-6g/L,溶液中残留镁浓度为0.02mol/L.