Study of the removal of Cd(II) from phosphoric acid solutions by precipitation of CdS with Na2S

The removal of Cd(II) from phosphoric acid solutions by precipitation of CdS with Na2S is investigated. The precipitate obtained was yellow and identified by X-ray powder diffraction as being mainly cubic CdS. The precipitation efficiency decreases both with the increase of the acidity level (i.e., with the increase of H3PO4 concentration and/or addition of H2SO4) and the increase of temperature. The efficiency of cadmium precipitation increases when the amount of Na2S added is increased (i.e., when the Na2S/Cd(II) molar ratio is increased). Thus, the removal of cadmium from H3PO4 solutions by precipitation of CdS is achievable, but the driving force for precipitation is not very strong and a large excess of S2− ions (or H2S) is needed. Furthermore, at high H3PO4 concentrations and/or moderate excess of S2− ions, the kinetics of CdS precipitation is slow as a result of the supersaturation phenomenon and/or the existence of colloidal forms of CdS.