The investigation of scale formation in the Bayer process

This thesis reports an investigation of the formation of zeolite scales in the industrial extraction of alumina from bauxite ore. These materials have been characterised by powder x-ray and neutron diffraction, infra-red spectroscopy, scanning electron microscopy and thermogravimetric analysis. Samples of zeolite scale were removed from both low temperature (c.a. 135°C) and high temperature (c.a. 220°C) Bayer plants and analysed. At low temperature the scale crystallises predominantly as natrodavyne with minority phases of sodalite and cancrinite whilst at high temperature the cancrinite phase is exclusively formed. Although a variety of different anions are present in both the scale and the liquor in small amounts (e.g sulphate, hydroxide, vanadate, and oxalate) the majority anions, with apparently controlling influence, are the carbonate and hydroxide anions. Laboratory experiments using the representative anions in Bayer liquor (hydroxide, carbonate, oxalate, sulphate and chloride) as a function of temperature (90° - 225°C) and anion concentration (10-3 - lO-IM) were performed using hydrothermal methods. Sulphate formed cancrinite at all temperatures and concentrations and sodalite was consistently formed for both oxalate and chloride anions. However, both temperature and concentration were critical in the case of carbonate where sodalite and intermediate phase dominated at low temperatures and concentrations but at high temperature and concentration, the cancrinite structure was exclusively formed. Experiments with sodium hydroxide liquors of different concentrations and standing times have shown that the cancrinite structure only forms after standing, where carbon dioxide is readily absorbed from the air, indicating a pure hydroxycancrinite is unlikely to form. In the absence of standing time, hydroxysodalite is formed at all temperatures and all concentrations. The addition of concentrations of lime used extensively to remove carbonate from Bayer liquor between 0.013 M and O.13M was also investigated. At high concentrations the addition oflime results in no zeolite scale being formed.