Preparation of refined phosphoric acid with recycling of raffinate acid by the extraction of metal impurities

Solvent extraction is the most effective method to remove impurities from wet-process phosphoric acid (WPA). However, the raffinate acid produced by this method cannot be effectively utilized because of a high impurity content and high viscosity, resulting in the waste of phosphorus resources. In this paper, a new process is proposed for extracting metal impurities and purifying the raffinate acid, and utilizing purified raffinate acid to prepare refined phosphoric acid. The effects of the extraction temperature, volumetric phase ratio (R), extraction time, and agitation speed on the metal-ion extraction were investigated. The MER (mass ratio of the sesquioxide content to the P2O5 content, MER = w(Fe2O3 + Al2O3 + MgO)/w(P2O5)) of the raffinate acid was reduced from 0.1516 to 0.1001 under the optimum conditions: temperature = 60 °C, R = 6, reaction time = 30 min, and agitation speed = 500 rpm. More than that, the MER value was reduced to 0.0357 through five-stage cross-current extraction. The theoretical number of stages to reduce the MER of the raffinate to 0.086 was obtained by the McCabe-Thiele method and found to be two stages. Finally, the purified raffinate acid obtained in the pilot experiment was used to prepare refined phosphoric acid, of which the product quality reached food-grade. In addition, the phosphorus yield of the whole process was 95.5%. The application of this new process will significantly improve the recovery ratio of phosphorus in refined phosphoric acid industry.