Extraction of alumina and silica from high-silica bauxite by sintering with sodium carbonate followed by two-step leaching with water and sulfuric acid

Abstract

Efficient utilization of high-silica bauxite and minimization of bauxite residue are of great significance for the sustainable development of the alumina industry. In this paper, a novel process is proposed to extract Al₂O₃ and SiO₂ from high-silica bauxite without residue discharge, that is, sintering bauxite with Na₂CO₃ followed by two-step leaching with water and sulfuric acid. The effects of the sintering parameters on the process were investigated, and the phase transformations during sintering and leaching were revealed by using phase diagram, thermogravimetric analysis (TGA) and differential scanning calorimetric (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) methods. When the mixture of the high-silica bauxite and Na₂CO₃ with mole ratio of Na₂O/(Al₂O₃ + SiO₂) of 1 was sintered at 950 °C for 30 min, diaspore and kaolinite were primarily converted into Na_1.95Al_1.95Si_0.05O₄ and an amorphous phase, respectively. In the water leaching process, Na_1.95Al_1.95Si_0.05O₄ was dissolved while the amorphous phase underwent some transformations to form the water leaching residue, resulting in ∼84% of Al₂O₃ being extracted for alumina production. In the sulfuric acid leaching process, the amorphous phase in the water leaching residue dissolved, resulting in ∼13% of Al₂O₃ and ∼86% of SiO₂ being extracted for the production of polyaluminium ferric sulfate (PAFS) and silica gel, respectively. The silica gel had a high purity, containing more than 88% of SiO₂ after drying.