Cation exchange behavior of bauxite refining residues from Western Australia

Over 60 million tonnes per year of bauxite refining residue (red mud) is produced worldwide. Its high Na concentration inhibits plant growth and hence reclamation. The cation-exchange capacity (CEC) of red mud and the cation exchange equilibria between Na⁺ and several other cations were measured to elucidate the mechanism of Na release from red mud. The CECs obtained by using K⁺ and NH₄⁺ were significantly higher than those obtained using Ca²⁺ and Ba²⁺. This unusual cation exchange phenomenon can be attributed to the presence of zeolitic minerals in red mud. Cation exchange equilibria show that Na⁺ originally present in red mud was preferentially adsorbed by the mud over other cations. At high cation fractions in red mud (>0.1), the mud selectively sorbed monovalent over divalent cations, with the following order of selectivity: K⁺>Li⁺>NH ₄⁺>Ba²⁺≥Ca²⁺>Mg²⁺. The exchange of Na⁺ has been found to have significant negative correlations with the radius of hydration and Debye-Huckel parameter. Divalent cations have little ability to exchange Na⁺ from zeolitic exchange sites. Incremental extraction of Na⁺ in red mud shows that K⁺ and NH₄⁺ displaced 99 (63%) and 57 (44%) cmol_c kg^-1 red mud whereas Ca²⁺ and Mg²⁺ could only displace 33 and 29 cmol_c kg^-1 red mud, respectively, out of a total of 99 cmol_c Na kg^-1 red mud. In a reverse process 95% of K⁺ adsorbed on red mud was readily replaced by Na⁺. The experimental evidence suggests that the release of Na⁺ from red mud is due to cation exchange.