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CAUSTIC SODA ADSORPTION ON BAYER RESIDUES

Thornber, M. Binet, D.

The adsorption data for sodium, caustic, total alkalinity and aluminate were measured for various Bayer residues by a precise washing procedure. The results were expressed as functions of the of the associated liquor TA and fitted by both logarithmic and Langmuir functions within the experimental error. For a residue rich in goethite the Na adsorption was consistent with known adsorption on goethite at lower pH.

The changes in adsorption on residue solids with added DSP gave data on the contribution from the DSP and similarly adsorption data on the effect of lime additions was obtained. The contributions to the adsorption from the various components of the residue solids were identified as follows:

  • DSP on a weight basis was the most active adsorber and there was evidence that its adsorption behaviour could be influenced by its particle size and morphology.
  • The residue minerals that contain iron, aluminium and titanium all made a significant contribution to the adsorption and those that have hydroxyls in the structure such as goethite were especially adsorbent. The degree of adsorption appeared to relate to the amount of a mineral present and not its surface area, as though a bulk exchange reaction was occurring.
  • Lime addition had a profound effect on the adsorption characteristics causing an increase far beyond what would be expected from the small amount added.
  • Quartz did not show significant adsorption.

The results support the hypothesis that mineral surfaces, in a concentrated Bayer liquor have Na+ adsorbed at an inner layer and the negative counter ions such as OH-, CO32-, Al(OH)4- and organic and inorganic anions that are regarded as contaminants to the Bayer process are adsorbed as an outer or double layer. This could also apply to gibbsite surfaces involved in crystallisation and dissolution processes.