Cation Exchange Behavior of Bauxite Refining Residues from Western Australia
- J. W. C. Wong * and
- G. E. Ho
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 NH+4 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-Huckei parameter. Divalent cations have little ability to exchange Na+ from zeolitic exchange sites. Incremental extraction of Na+ in red mud shows that K+ and NH+4 displaced 99 (63%) and 57 (44%) cmolc kg−1 red mud whereas Ca2+ and Mg2+ could only displace 33 and 29 cmolc kg−1 red mud, respectively, out of a total of 99 cmolc 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.
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