Calcite in the mud washing circuit of the Bayer process: Experimental solubility and adsorption

In the mud circuit of the Bayer process, alumina losses may be reduced via the inhibitive action of calcium ions on gibbsite precipitation. Here, we present the first stage of this investigation: An experimental study of calcite solubility, and calcium adsorption under mud circuit conditions. Solubilities were measured, in deionised water, caustic, and both synthetic and refinery Bayer liquors, with respect to time, temperature, and caustic and aluminate concentrations, Solubility was observed to increase with caustic concentrations up to 1M, but decrease thereafter. This may be attributed to the overlapping solubility domains of calcite and the less-soluble Ca(OH) ₂. Increasing the aluminate concentration was found to have a negligible effect on solubility, as the higher level of carbonate present in liquors, restricted calcite dissolution. Calcium ions were found to adsorb on mud particles, but showed a considerably stronger affinity, for gibbsite particle surfaces, regardless of their larger size and lower surface area.