Study on the effect of humate and its removal on the precipitation of aluminium trihydroxide from the Bayer process

A process to remove humate from Bayer spent liquor using polymers of diallyldimethyl ammonium chloride (polyDADMAC) has been developed. Using a UV–visible spectrometric technique, the concentration of humate in spent liquors obtained from an alumina refinery was found to be 5 ± 1 g/l. The humate removal process is based on its selective reaction with polyDADMAC, which can be added at 1:1 weight ratio (with respect to the humate content) to effectively remove humate from the spent liquors. The process involves, first the reaction between polyDADMAC and humate. Next, acidification to convert humate to humic acid in carried out, which follows by an esterification step using tertiary butanol. The ester formed is then causticized at pH 9 to be recovered to humate and butanol. Results from precipitation test showed that an increase in humate concentration does not adversely affect the yield of the precipitated aluminium trihydroxide (Al2O3 · 3H2O) product. However the presence of humate during precipitation causes a decrease in the average particle size of the final precipitate product. The particle size range most affected was found to be 20–80 μm, after 48 h precipitation using seeds of average size of 73.6 μm. By removing humate from the spent liquors, the whiteness of the aluminium trihydroxide for specialty alumina could also be improved.