Extraction of cadmium from phosphoric acid solutions with amines. Part III. A thermodynamic extraction model

A thermodynamic model for cadmium extraction from phosphoric-hydrochloric acid mixtures has been developed. The model enables calculations of cadmium extraction as well as coextraction of phosphoric and hydrochloric acids to be made. In the aqueous phase Bromley's model for the activity coefficients is adopted. In the organic phase simple expressions for the non-ideal behaviour of phosphoric acid are used. The model is capable of calculating cadmium extraction from 0.02 M up to 6.7 M H3PO4 and chloride concentrations between 0.005 and 0.10 M HCl. For high amine concentrations the prediction for cadmium extraction is too high, indicating that high concentrations of phosphoric acid in the organic phase affect the activity coefficient for the extracted cadmium complex significantly. The thermodynamic constant for the extraction equilibrium has been determined as log K11 = 10.23