Structure of amorphous aluminosilicates obtained from mineral transformation: Potential path for partial remediation of alkaline bauxite residues

Abstract

Highly alkaline wastes known as bauxite residue are the abundant byproducts of refining bauxite into alumina through the Bayer process, and worldwide estimates of these bauxite residue stockpiles exceed 3 billion tons, with hundreds of millions of tons produced annually. Bauxite residue utilization and remediation are significantly complicated by the presence of recalcitrant minerals that buffer pH, such as sodalite and cancrinite, commonly referred to as desilication products (DSP) in the alumina industry. In this work, a mixture of sodalite (∼52 wt%) and cancrinite (∼30 wt%) was produced using similar conditions to those encountered in some alumina refineries using the Bayer process. These synthetic minerals were then transformed to hydrated aluminosilicates using a simple, low temperature aqueous process. The transformed products were then characterized with X-ray diffraction, infrared spectroscopy, nuclear magnetic resonance, and electron microscopy. Results suggest that a hypothesized conversion to 2:1-type clay minerals was not achieved, but that the materials were ∼79 wt% X-ray amorphous, resembling protoclay-like materials, with silicate network structures and primarily 4-coordinated Al atoms. This was confirmed by independent synthesis and structural comparison with amorphous aluminosilicates based on allophane and imogolite. Water vapor sorption behavior was similar for the synthesized allophane and the converted DSP minerals; ∼0.3 g/g sorbed water content. The current process produces highly amorphous aluminosilicate gels with hydraulic properties similar to silica-rich allophane, which have the potential to aid in the remediation of bauxite residue storage facilities.