Reactive silica in bauxite is a key challenge for the alumina industry due to the associated losses of sodium hydroxide and alumina associated with processing this impurity. The bauxite residue sinter leach process (BRSLP) is a potential method of recovering sodium hydroxide and alumina from bauxite residue. In this research, bauxite residue lime sinters were characterised using a combination of quantitative X-ray diffraction in conjunction with scanning electron microscopy as well as for leaching performance. It is shown that sintering reactions proceed at a significantly faster rate than previously reported which has a positive implication for industrial application. A new set of reactions and reaction pathways are proposed which describe bauxite residue lime sintering in greater detail than previously possible.

Spodumene concentrate from the Pilbara region in Western Australia was characterized by X-ray diffraction (XRD), Scanning Electron Microscope Energy Dispersive Spectroscopy (SEM-EDS) and Mineral Liberation Analysis (MLA) to identify and quantify major minerals in the concentrate. Particle diameters ranged from 10 to 200 microns and the degree of liberation of major minerals was found to be more than 90%. The thermal behavior of spodumene and the concentration of its polymorphs were studied by heat treatments in the range of 900 to 1050 °C. All three polymorphs of the mineral (α, γ and β) were identified. Full transformation of the α-phase was achieved at 975 °C and 1000 °C after 240 and 60 min treatments, respectively. SEM images of thermally treated concentrate revealed fracturing of spodumene grains, producing minor cracks initially which became more prominent with increasing temperature. Material disintegration, melting and agglomeration with gangue minerals were also observed at higher temperatures. The metastable γ-phase achieved a peak concentration of 23% after 120 min at 975 °C. We suggest 1050 °C to be the threshold temperature for the process where even a short residence time causes appreciable transformation, however, 1000 °C may be the ideal temperature for processing the concentrate due to the degree of material disintegration and α-phase transformation observed. The application of a first-order kinetic model yields kinetic parameters which fit the experimental data well. The resultant apparent activation energies of 655 and 731 kJ mol−1 obtained for α- and γ-decay, respectively, confirm the strong temperature dependence for the spodumene polymorph transformations.

Reactive silica is the most detrimental impurity in bauxite. When bauxite is contacted with Bayer liquor, reactive silica dissolves and then precipitates as a sodium aluminum silicate known as the desilication product (DSP), which is discarded along with the bauxite residue and maintained in long-term storage facilities. DSP recycling using bauxite residue lime sinter-leach is prohibitively expensive due to the high energy requirement for heating as well as high investment costs. In the sandy desilication product process, DSP is coarsened by controlling the desilication conditions, which enables the DSP fraction of the residue to be concentrated by physical separations. The DSP concentrate can then be recycled by sinter-leaching, whereby sodium and aluminum are returned to the Bayer process, reducing the burden associated with bauxite residue and facilitating total residue treatment options. Results are presented herein demonstrating DSP particle enlargement by seeding the reaction and controlling the supersaturation.

The combination Bayer sinter-leach process has been shown to be a technically viable method for reprocessing bauxite residue to recover lost caustic and alumina. However, this method suffers from high energy requirements and reagent consumption associated with processing the large volume of bauxite residue, the majority of which is gangue. To address this issue, processing a desilication product (DSP) concentrate derived from the novel Sandy Desilication Product process is studied. In the present study, critical processing parameters surrounding the sintering stage were investigated including the grade and particle size of the DSP concentrate, the limestone addition rate, using calcium oxalate as an alternative to limestone and reductive sintering. The DSP concentrate sinter mineral phases were specified using quantitative XRD and the sinter products were assessed for leachability. Maximising the silicate reaction extent during sintering was found to be important to ensure high leachability.

No abstract