Investigation of electrodeless conductivity as a technique for monitoring sulfate levels during production of wet-process phosphoric acid

Abstract

In the production of wet-process phosphoric acid (WPA), sulfate concentrations influence both the crystal form and growth rate of calcium sulfate and hence the filtration rate for separating calcium sulfate from the product WPA. The filtration rate must be rapid to maintain high rates for the production of WPA. One approach for continuously monitoring sulfate concentrations during production of WPA is to measure the electrical conductivity, temperature, and specific gravity of WPA, and then relate these data to the sulfate concentration using regression equations. An electrodeless conductivity probe was used to provide reproducible conductivity measurements in return acids (12–32% P2O5) used for the production of WPA (24–36% P2O5). The effect of iron, aluminum, and magnesium impurities on the conductivity measurements was also evaluated. Regression equations were developed from the data which can be used to accurately calculate P2O5 and sulfate concentrations in return acids within a specified temperature range.