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

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 AlO and SiO from high-silica bauxite without residue discharge, that is, sintering bauxite with NaCO 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 NaCO with mole ratio of NaO/(AlO + SiO) of 1 was sintered at 950 °C for 30 min, diaspore and kaolinite were primarily converted into NaAlSiO and an amorphous phase, respectively. In the water leaching process, NaAlSiO was dissolved while the amorphous phase underwent some transformations to form the water leaching residue, resulting in ∼84% of AlO being extracted for alumina production. In the sulfuric acid leaching process, the amorphous phase in the water leaching residue dissolved, resulting in ∼13% of AlO and ∼86% of SiO 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 SiO after drying.