Activation of low-grade diasporic bauxite via sodium nitrate roasting coupled with alumina extraction in low-alkaline solution

With the rapid expansion of China's alumina industry and the depletion of high-grade bauxite, efficient utilization of low-grade diasporic bauxite has become a critical challenge. This study proposes an innovative method combining sodium nitrate activation roasting with low-alkali leaching to enhance alumina extraction. Activation roasting at moderate to high temperatures significantly improves bauxite solubility, and subsequent leaching with a low-concentration caustic soda solution (N_k = 160 g/L) enhances alumina dissolution. XRD analysis reveals that sodium nitrate promotes phase transformations, converting diaspore and kaolinite into Na_2-xAl_2-xSi_xO₄ at 700–1000 °C. Increasing the roasting temperature within the 700–1000 °C range lower the x value in Na_2-xAl_2-xSi_xO₄, but also inhibits its decomposition of Na_2-xAl_2-xSi_xO₄ into NaAlO₂ and Na₂SiO₃. At an aluminum-to-sodium nitrate molar ratio of 1 (N/A = 1), NaAlO₂, a highly soluble phase, forms predominantly at 750 °C. However, at 750 °C excessive roasting time or sodium nitrate leads to Na_2-xAl_2-xSi_xO₄ with increased x, reducing solubility. Through systematic optimization, we determined the ideal conditions: roasting at 750 °C for 2 h, followed by leaching at 270 °C for 60 min with a solid–liquid ratio of 300 g/L. This optimized process achieved an exceptional alumina leaching rate exceeding 75 % from bauxite with an initial alumina-to-silica ratio (A/S) ratio of 3.57, while maintaining silica dissolution at a minimal 0.59 %. The resulting red mud had an A/S ratio of 0.88, significantly lower than that of the Bayer process. This methodology not only enhances the leaching rate of alumina from low-grade bauxite but also provides a sustainable solution for the comprehensive utilization of low-grade bauxite resources.