Synthesis and characterisation of hydrotalcite from bauxite residue liquor and magnesium nitrate

Introduction The extraction of alumina from bauxite by the Bayer process produces a slurry by-product referred to as ‘bauxite residue’ or ‘red mud’. The liquid component of the slurry is highly alkaline and requires neutralisation to render it environmentally benign. A common practice is neutralisation using Mg rich seawater, which results in the formation of the layered double hydroxide (LDH) mineral hydrotalcite Mg6Al2(OH)16CO3.4H2O (Johnston et al. 2010). Hydrotalcite consists of brucite-like sheets containing magnesium and aluminium co-ordinated with hydroxyl groups. The presence of aluminium imparts a positive charge which allows anions such as carbonate to be incorporated into the interlayer regions. Gillman and Noble (2001) and Tamagawa (2003) have explored the use of hydrotalcite as a slow release fertiliser by incorporating plant macronutrients P and N as anions PO4 3−