Chemistry of bauxite residue neutralisation and aspects to implementation

Bauxite residue neutralisation is intended to reduce the environmental impact of this highly alkaline material. Also, it can potentially reduce the need for on-going management of disposal impoundments post-closure, as well as open opportunities for re-use of the residue material. In this study bauxite residue neutralisation has been examined using a range of neutralising agents measuring the change in pH, in addition to the quality of leachate produced, specifically alkalinity and solution aluminium concentration. Solution alkalinity is removed by neutralisation and precipitation for mineral acid addition and through precipitation by soluble calcium or magnesium salt addition. Carbonation removes alkalinity through neutralisation and transformation to bicarbonate. An equilibrium pH of less than 10 is achieved only if the solid phase alkalinity is suppressed (Ca/Mg) or dissolved (acid). The kinetics of this process is inhibited by mass transfer limitations (acid processes) and a sustainable presence of a soluble excess (Ca/Mg processes). The solution alkalinity after neutralisation to pH 10 is reduced to less than 1% of the initial value using mineral acid or a soluble calcium or magnesium source. Using carbon dioxide the solution alkalinity is only reduced by about 40%. Comparing acid to Ca/Mg treated bauxite residue there is a difference in the mechanism in which the soluble aluminium is precipitated from the solution alkalinity, such that at equivalent pH values, the aluminium solution concentration is higher for acid treated bauxite residue compared to Ca/Mg treated bauxite residue. Aspects of implementing bauxite residue neutralisation within the context of a dry stacking operation are discussed.