Kinetic modelling of cadmium removal from wet phosphoric acid by precipitation method

Phosphate fertilizers are most commonly obtained from wet phosphoric acid, which contains a majority of impurities that were present in raw materials used during the production process. It is essential to limit heavy metal contents, including cadmium, in manufactured phosphoric acid for environmental protection purposes. This work investigates kinetics of cadmium removal from wet phosphoric acid by precipitation method. Precipitating agents used in this study are: zinc ethylphenyldithiocarbamate (ZnEPDTC), sodium ethylphenyldithiocarbamate (NaEPDTC), sodium cellulose xanthate (SCX), sodium dibutyldithiocarbamate (NaDBDTC) and sodium sulfide (Na2S). For each agent, a proper cadmium reaction model is fitted, the simplified kinetic mechanism is proposed and pseudo-kinetic parameters are derived. Analysed precipitating agents were found to perform following three different mechanisms - ZnEPDTC and NaEPDTC did not react with cadmium ions in investigated conditions, SCX and NaDBDTC were decreasing the concentration of cadmium ions over time and Na2S initially decreased cadmium concentration to almost zero and then created Cd(HS)(x)(2-x) complexes.