The production of potassium sulfate from phosphogypsum and potassium chloride in the presence of ammonia and isopropanol has been investigated. The solubility of phosphogypsum in aqueous solutions of KCl, HCl, and mixtures of both has been first measured at various temperatures and concentrations. The factors affecting the conversion of potassium chloride to potassium sulfate, such as aqueous ammonia to phosphogypsum ratio, weight percent of isopropanol in the solution, weight percent excess phosphogypsum, and reaction temperature and pressure have also been investigated. The optimum reaction conditions obtained at 25 degrees C and atmospheric pressure have been 50% excess phosphogypsum 4.32 ammonia solution to phosphogypsum ratio, and aqueous solutions containing initial concentrations of 33 wt.% ammonia and 7 wt.% isopropanol. The maximum conversion of KCl to K2SO4 obtained under these conditions was 97.2%. (C) 2000 Elsevier Science S.A. All rights reserved.