Leaching of scheelite by hydrochloric acid in the presence of phosphate

Abstract

The reaction kinetics for the leaching of scheelite CaWO 4 by hydrochloric acid in the presence of a complexing agent, phosphate ion, have been studied in a stirred reactor to determine the effect of the important process variables, such as particle size, temperature, stirring speed, acid concentration and phosphate addition. In the presence of sufficient PO 4 3− corresponding to P:W ⩾ 1:10, only soluble 12-tungstatophosphoric acid H 3PW 12O 40 instead of insoluble tungstic acid H 2WO 4 is formed, and the rate of leaching is greatly accelerated without fine grinding of the ore. The conclusion, that the reaction at the solid-liquid interface governs the major portion of the leaching process, is supported by an apparent activation energy of 18.9 kcal/mol (79 kJ/mol) and by the insignificant influence of increasing the stirring speed from 800 to 1000 min −1. The shrinking core model is considered to describe the whole process and an empirical rate equation was found to be k ̄ = k ̄ 0 C HCl 1.47 C PO 4 0.31 S 0.53 e −( 18,900 RT ) (eqns. 15 and 16).