(190a) Predicting the Citrate Soluble Loss of the Dihydrate Process

A thermodynamic model was developed to predict the limits of distribution of phosphates between the liquid and the solid phases in a reactor used for extracting phosphoric acid from phosphate rock by the dihydrate process. A computer code based on the model was generated to carry out different simulations of the process using several inputs of temperatures and liquid phase content of sulfates and phosphates.

Experimental data of equilibrium constants were regressed and included in the model obtaining a more accurate representation of the thermodynamic equilibrium. In addition, the Edwards-Maurer-Newman-Prausnitz Pitzer based model was incorporated into the model to write the activity coefficients of all species, while published lime solubility data was used to find an expression for the self interaction parameter of phosphoric acid. The model was validated by comparing its predictions to experimental citrate soluble loss data yielding very compatible results. Simulation results for ionic strength, solution acidity, lime solubility, and citrate soluble loss were used to analyze temperature plus solution sulfate and phosphate content effects on the dihydrate process.

Decreasing temperature and increasing sulfate levels was found to raise the acidity and the ionic strength of the solution as well as minimize the citrate soluble loss.