FACTORS AFFECTING TAA DETERMINATIONS FOR BAUXITE CHARACTERISATION

Total available alumina (TAA) is usually determined by digestions of bauxite in caustic solutions. We have investigated two aspects of this digestion that can influence the result. For a gibbsite-boehmite mixture we have found that boehmite does not dissolve in pure caustic provided there is sufficient gibbsite dissolution to exceed boehmite solubility. This simplifies the TAA digests by removing the requirement for the digest liquor to contain pre-dissolved alumina. Additionally we have found that desilication product, formed in TAA digests using pure caustic contain higher levels of included alumina than would be expected in a Bayer liquor. The extra alumina causes an under-reporting of TAA. The under-reporting is a linear function of the reactive silica content of the bauxite and is ~0.125 g _Al2O3 per 1% reactive silica in the bauxite.