Chemistry of Seawater Neutralization of Bauxite Refinery Residues (Red Mud)

Abstract

Bayer process red muds are highly alkaline, and their disposal and potential for reuse are complicated by high concentrations of sodium, high pH, and high alkalinity. Several methods of neutralization of red muds have been reported including: infiltration of rainwater and atmospheric CO₂, treatment with strong acids, gypsum addition, and seawater neutralization. Using seawater to neutralize red mud at the Queensland Alumina Ltd. (QAL) refinery at Gladstone, QLD, began as a fresh water conservation measure but led to many benefits, including reduced freshwater use at the refinery, increased settling rates at the pond and reduced alkalinity and sodicity in the solid wastes and entrained liquor. The study reported here describes and quantifies the geochemical consequences of seawater neutralization of bauxite refinery residues. Methods and results presented allow description of the chemical, mineralogical, and physical characteristics of seawater-neutralized red mud. Experiments then identify and quantify the chemical consequences of seawater neutralization. Scanning electron micrographs show how the analytically characterized chemical changes are manifested physically. Seawater neutralization of bauxite refinery residues causes chemical changes and improves physical characteristics of the material. At the refinery, faster settling due to agglomerate consolidation and increased ease of handling after neutralization are positive impacts for management. In postdisposal reuse, seawater neutralization results in an increased acid neutralization capacity, improved soil properties, and increased phosphate adsorption capacity.