Recovery of desilication product in alumina industry

Abstract

Bayer process is used for the production of alumina from bauxite which contains siliceous minerals known as reactive silica. Reactive silica is also digested during the Bayer process, forming desilication product (DSP) which traps significant amount of sodium from the caustic soda used. DSP containing red mud is discarded to the bauxite residue storage areas causing alarming economic and environmental concerns to the alumina industry. Separation of DSP from red mud is a critical step to subsequent sodium recovery. Therefore, methods of separating DSP from red mud originated in a Western Australian alumina refinery were investigated in the present study. Red mud classified into five size classes was characterised using particle size measurement, material density, X-ray diffraction, X-ray photon spectroscopy, electron microscopy fitted with energy dispersive spectroscopy. Size separation alone resulted in an increase of DSP from ~7% to 12-14% in finer size classes of red mud. Since red mud contains more than 50% iron oxides, dissolution of these oxides would invariably result in an increase of DSP, therefore, several dissolution mechanisms of these oxides were investigated. Sodium Citrate-Bicarbonate-Dithionite (CBD) method resulted in an increase of DSP content from ~12 to 22%, while a comparative study was also carried out using deferroxiamine B (DFO-B) as the complexing agent. The use of ultrasonically assisted-CBD resulted in a significant improvement of iron dissolution from red mud. Floatation behaviour of three key minerals (DSP, goethite and haematite) in red mud was also investigated through the use of ionic surfactants (CTAB and potassium Oleate) in neutral and basic pH value ranges (7.5-11.5). Electrostatic attraction was recognised as the mechanism of ionic collector attachment during floatation, which was explained through zeta potential and PZC measurements. Actual red mud was also used for the floatation studies with the ionic surfactants as collectors. CTAB provided superior performance compared with potassium oleate during floatation of red mud in terms of DSP recovery after ultrasonic pre-treatment.