Revealing the alkaline characteristic evolution of bauxite residue under biomass fermentation

Abstract

Biomass fermentation has been proposed as a simple and economical strategy to alleviate the high alkalinity of bauxite residue. This study investigates the neutralization of bauxite residue following the application of biomass as an alkali modifier by natural fermentation. Fresh bauxite residue samples were collected from Pingguo refinery (Aluminum Corporation of China). Samples were treated with straw mulching (SC), straw mixing (SM), bagasse mulching (BC), and bagasse mixing (BM), respectively. Treatments were analyzed for pH, EC, metal cations, and soluble alkali (OH−, Al(OH)4− and CO32−). The mineral phase and Na speciation were analyzed by X-ray diffraction (XRD) and near-edge X-ray absorption fine structure (Na-XANES). Optimum application rate for either straw or bagasse was 20% (w/w), reducing leachate pH from 10.26 to 8.56. During biomass transformation, the alkaline mineral grossular was completely dissolved, while calcite and cancrinite were dissolved to a lesser degree. No treatment changed the spatial distribution of Na+, but the basic anions (OH−, CO32−, and Al(OH)4−) were significantly reduced. Following treatment application, soluble alkali in the residues was significantly reduced while the alkaline minerals were slightly dissolved. This was determined as the main cause for the decrease in residue pH.