Selective separation of chromium from sulphuric acid leaching solutions of mixed electroplating sludge using phosphate precipitation

The selective separation of chromium (Cr) from sulphuric acid leaching solutions is necessary for the harmless disposal and resource recycling of electroplating sludge. A method using phosphate precipitation was investigated. First, the thermodynamic analysis of a Cr3+ - Fe3+ - PO43- - H2O system and a Cr3+ - Fe2+ - PO43- - H2O system was analysed to evaluate the feasibility of Cr recovery, and the optimal operating windows were identified. The corresponding experiments were carried out based on the thermodynamic calculations, and the results showed that Cr and Fe can be selectively separated using phosphate in the presence of excess sulfite. The major factors, including the PO43-/Cr3+ ratio, solution temperature, pH and impurity concentration, were evaluated. Under the best conditions, over 97.04% of Cr can be separated and recovered as phosphate precipitation, while the precipitation efficiency of Fe was only 14.75%. Thermogravimetric and infrared analyses showed that the finial precipitation was a xCr(OH)(3)center dot gamma CrPO4 center dot zH(2)O complex. The surface morphology of the precipitation presented in the form of an agglomeration when Fe existed in the solution. The impurities in the precipitation could be removed through a dilute sulphuric acid cleaning process. The consequence of phosphate precipitation could provide a practical and yet easy-to-adopt perspective for selectively separating Cr from sulphuric acid leaching solutions.