Sea water neutralisation of Bayer process residue has been used as a method for alkali and dissolved aluminium removal by direct disposal in the ocean for many years. The reaction mechanisms for such a process are quite complex. The neutralisation process does not eliminate hydroxide but converts the soluble, strongly caustic into less soluble, weakly alkaline solids with active participation of the magnesium content in seawater.
In order to achieve a status of ‘Zero Harm’ towards the environment, Queensland Alumina has sought to improve residue disposal by commissioning a novel neutralisation process. This process permits the controlled reaction between 800 m3/h of Bayer residue, containing 10 to 15 g/l liquor phase soda, with seawater in an appropriately sized reactor. The neutralised solids and magnesium deficient seawater are then separated via conventional clarifier, fitted with Vane Feedwell technology which provides a uniform rise profile, resulting in overflow solids of approximately 30 mg/l. The neutralised and de-watered mud is then pumped and prepared for semi-dry stacking.