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.