Hydrothermal dissolution reactions of magnesium-aluminium-spinel in alkaline solutions

Abstract

Single crystal spheres of spinel of 4 mm diameter having a molar ratio of MgO/Al 2O 3 1:5, were dissolved in 0.1 M solutions of K 2CO 3, NaF, KF, Na 3PO 4 and K 3PO 4 under a pressure of 2 kbar and at temperatures between 390 δC and 630 δC. The final shape of the dissolved spheres are cubes. The reaction layers formed during dissolution varied in porosity and consisted of Mg(OH) 2 in alkali carbonate solutions, MgF 2 in alkali flouride and Mg 3(PO 4) 2 in alkali phosphate solutions. The porous Mg(OH) 2 layer hardly affected the material transport, so that the relationship between log v s and 1/ T is a linear function, which was not found for the less porous layers of MgF 2 and Mg 3(PO 4) 2. In the case of alkali flouride the rate of reaction is limited by the diffusion of flouride ions.