HEMATITE FLOCCULATION UNDER BAYER CONDITIONS
Farrow, J. Jones, F. van Bronswijk, B.
Despite its importance to the Bayer process, little is known about the mechanism of residue solids flocculation in the primary settlers and how flocculation is influenced by liquor impurities. Towards this end, an investigation has been made of the flocculation of pure hematite by a commercial polyacrylate flocculant in synthetic Bayer liquors.
The effect that the major liquor impurities, namely carbonate, sulphate, chloride, phosphate, silicate and calcium, have on flocculation performance was determined from standard cylinder settling measurements. These results have been correlated to changes in the extent of flocculant adsorption determined from analysis of the residual flocculant concentration in the synthetic Bayer liquor under different conditions.
Contrary to a widespread belief, the polyacrylate flocculant was found to be effective in flocculating the hematite in the absence of calcium. The flocculant adsorption density on the hematite surface remained unchanged when calcium was added. Flocculation performance was unaffected by the presence of sulphate, chloride and phosphate. Both carbonate and silicate had a major influence on flocculation performance, with the flocculant adsorption density on the hematite surface decreasing significantly with increasing concentration of these impurities in the liquor. It appears that although the flocculant has a higher affinity for the hematite surface than carbonate, the rate at which flocculant can displace carbonate pre-adsorbed on the hematite surface limits its performance.