IMPURITY EFFECT OF CHROMIUM(III) ON THE GROWTH AND DISSOLUTION RATES OF POTASSIUM-SULFATE CRYSTALS

Growth and dissolution rates of a potassium sulfate crystal were measured under controlled pHs in potassium sulfate solutions contaminated by traces of chromium(III): up to 5 ppm for growth and 15 ppm for dissolution. Four kinds of chromium(III) salts, Cr-2(SO4)(3) . 4H(2)O, CrCl3 . 6H(2)O, CrF3 . 3H(2)O and Cr(CH3COO)(3) . H2O, were examined as the chromium(III) impurity source. The former two suppressed both rate processes, while the latter two were inactive. The suppression effect on growth rate was increased with increasing the solution pH when CrCl3 . 6H(2)O was added, but the effect was unchanged with the pH for the case of Cr-2(SO4)(3) . 4H(2)O. However, on the dissolution rate, the suppression effect was increased with increasing the solution pH for both impurities. With increasing the impurity concentration, the effects on the growth and dissolution rates were increased. These results are explained by a mechanism in which the adsorbed hydrolysis product (an assumed active species) of a hydrated chromium(III) retards the rate processes. The hydrolysis product (hydroxo complex), which is in equilibrium with the inactive hydrated complex in the solution, is assumed to adsorb on the surface of the potassium sulfate crystal according to the Langmuir adsorption mechanism.