A study of scale formed in the Bayer process

This thesis describes an investigation into the problem of aluminosilicate scales formed as a by-product of the industrial extraction of alumina from bauxite ore. Three distinct zeolite scales with different morphologies have been identified; the type of scale depending on the temperature, caustic concentration and type of plant. These scales have been characterised using powder X-ray diffraction, scanning electron microscopyand infra-red spectroscopy. Synthetic scale was formed at three different temperatures 90, 145 and 220°C and treated with acid to mimic the behaviour of the three different phases found in the plant during acid cleaning. Mineral acids (sulfuric, nitric and hydrochloric) were added to the three individual scales at 30 and 60°C for 30 minutes and the amount of scale dissolved was ascertained by back-titration against 2M sodium hydroxide. The causticity of Bayer liquor varies from plant to plant depending on the composition of the bauxite ore. The effect of the variation from high caustic 300 gll, through medium causticity 250 gll to low causticity 210 gl1 was investigated, in terms of yield, type of phase formed and rate of phase formation at different temperatures. The behaviour of the three zeolitic scales varies under dry conditions due to the differing water content of the zeolites. The removal of the water from plant scale resulted in the sample faulting along well-defined planes within the scale, due to incomplete removal of scale and build-up of acid dissolution product. The thermal behaviour of the scales have been investigated by mimicking drying conditions using the HK1200 furnace attached to the Bruker D8 diffractometer.