Comparative analysis of industrial processes for cadmium removal from phosphoric acid: A review

Abstract

The manufacture of phosphoric acid by wet process, where the phosphates are solubilized by mineral acids, frequently gives a product which inevitably contains several impurities. Some of these elements are detrimental to the quality of the acid for its end uses in the fertilizers or the food industries. Among these impurities, one can find cadmium whose final content in fertilizers depends both on the type of raw materials and the chemical synthesis pathway. Hence, the wet phosphoric acid (WPA) must be purified.The present review compares and analyzes the different industrial processes for cadmium removal from WPA, starting with solvent extraction that has always been the most widely used technique in this field, but considering also precipitation, ion exchange, adsorption, flotation, and even the more recent membrane processes that can become a relevant alternative. The efficiency of cadmium removal techniques is compared and their advantages and limitations are discussed. This review also provides insights into the thermodynamic modeling of the H3PO4/H2O system, and compares the ability of current models to predict thermodynamic properties, including osmotic coefficient and speciation, in a wide range of phosphoric acid concentrations. Besides, the cost and the environmental impact of WPA production have also been evaluated using Life Cycle Assessment and available cost data which reveals that thermal phosphoric acid is still economically and environmentally more onerous than purified WPA.