Effect of Carbonate on Desilication of Sodium Aluminate Solution at High Temperature

Abstract

Carbonate is the main anionic impurity in alumina produced by the Bayer process. The phase composition, morphology, and precipitation activity of desilication products (DSPs) precipitated at 245°C in sodium aluminate solution with different carbonate concentrations have been investigated by x-ray diffraction (XRD) and scanning electron microscopy–energy-dispersive x-ray spectroscopy (SEM–EDS) methods. The crystalline DSPs without carbonate are composed of zeolite, sodalite, and hydroxy-sodalite, but only zeolite and cancrinite precipitate in the presence of carbonate. Increasing the carbonate concentration promotes the precipitation of cancrinite but reduces the crystallinity of various DSPs. The cell parameters and the Na2O/Al2O3 ratios of DSPs are significantly increased because of the entry of sodium carbonate during crystallization, especially for the cancrinite. The morphology of various DSPs is greatly changed by the carbonate. The amorphous phase content and precipitation activity of DSPs increase with increasing carbonate concentration. This contributes to the effective removal of carbonate from sodium aluminate solution by controlling the desilication process.