磷酸盐沉淀法除铬热力学研究

除铬是含铬电镀污泥湿法冶金过程重要步骤.针对磷酸盐沉淀法从溶液中净化除铬过程进行热力学分析，绘制了25 ℃时Me-P-H₂O(Me: Cr(III), Zn(II), Cu(II), Fe(II), Fe(III), Ni(II))系组浓度对数-pH图，利用热力学平衡图对磷酸盐沉淀法从含铁等金属元素中净化除铁和磷酸铬碱分解过程进行热力学分析.结果表明，pH值为1.0 ~ 5.0磷酸盐形成由易至难依次为Cr(III)>Fe(III) >Fe(II)>Ni(II)>Cu(II)>Zn(II); 磷酸盐沉淀法难以有效地将Cr(III)与Fe(III)分离，而可分离Cr(III)和Fe(II)，且较优pH约为2;整个pH值范围Me-P-H₂O系可以分为难溶磷酸盐稳定区、Me(OH)_n稳定区; 高pH区磷酸盐中的Me转变为稳定的Me(OH)_n，实现磷酸盐碱分解.验证实验表明，加入1.1倍理论量的磷酸钠，控制沉淀pH值为2.0，铬、铁、锌、铜、镍沉淀率分别为94.12 %、5.51 %、0.33 %、0.22 %、0.34 %; 氢氧化钠分解磷酸铬时，磷、铬浸出率分别为90.63 %、5.10 %，实现磷铬有效分离.实验与理论基本相符. 

Thermodynamic analysis on removing Cr(III) by phosphate precipitation

Cr (III) removal is an important step in the hydrometallurgical process of chromium electroplating sludge. After the thermodynamics analysis of chromium removal through phosphate precipitation, the logarithm concentration-pH diagram of Me-P-H₂O (Me: Cr(III), Zn(II), Cu(II), Fe(II), Fe(III), Ni(II)) at 25 ℃was drawn. Based on this thermodynamic equilibrium diagram, thermodynamic analysis was carried out to discuss the removal of iron from metal elements and the decomposition process of chromium phosphate sodium hydroxide. The results show that the formation of phosphate with pH value from 1.0 to 5.0, from easy to difficult, is in the following order: Cr(III) > Fe(III) > Fe(II) > Ni(II) > Cu(II) > Zn(II). It is difficult to separate Cr (III) from Fe (III) effectively by phosphate precipitation method, but Cr(III) and Fe(II) can be separated by it, and the best pH value is 2. The scope of the whole pH value of Me-P-H₂O can be divided into the stable zone of sparingly soluble phosphate, and that of Me(OH)_n. With the transformation of Me in the high pH zone of phosphate into stable Me(OH)_n, the decomposition of phosphate base was achieved. The verification experiment results show that when added 110 % theoretical amounts of sodium phosphate and the pH is 2.0, the precipitation rate of Cr, Fe, Zn, Cu, Ni is 94.12 %, 5.51 %, 0.33 %, 0.22 % and 0.34 % respectively. The leaching rate of phosphorus and chromium is 90.63 % and 5.10 % respectively when the chromium phosphate is decomposed by sodium hydroxide, which is consistent with the thermodynamic analysis.