Clark, M.W., Howe, P., Reichelt-Brushett, A. and Johnston, M.

Neutralisation of Bayer liquors provides a means of reducing the caustic nature and alkalinity of the red mud discharge so that muds maybe dewatered. However, how the liquor is neutralised provides different results and different residual geochemistry, which then results in liquors with different ecotoxicology. Five neutralisation techniques of Bayer liquor are investigated, and the geochemistry and ecotoxicology assessed. Results show that raw liquor (the do nothing approach) possesses not only a high basicity, and alkalinity, but has a very high ecotoxicological response, whereas seawater neutralised liquor, and a hybrid liquor (CO2, followed by seawater additions) have significantly lower basicity and alkalinity, and is the liquor most tolerated by the two test species. LC50 values for the small, freshwater planktonic crustacean Ceriodaphnia dubia (daphnia) a standard ectoxicology test species, are recorded for raw liquor (0.5 mL/L) < carbon dioxide neutralized liquor (16 mL/L) ≤ acid neutralised liquor (26 mL/L) < seawater neutralised liquor (74 mL/L) ≤ hybrid neutralised liquor (98 mL/L). For C. dubia, toxicity is mostly through osmotic shock for seawater and hybrid materials. However, for the marine species Paracalliope australis, an amphipod crustacean and non-standard ectoxicology test species, an LC50 of 400-570 mL/L are recorded for seawater and hybrid discharges, because of the tolerance to the Electrical Conductivity (EC), and soluble salts in these Bayer treatment liquors.