Overcoming the salinity and sodicity of red mud for rehabilitation and reuse
Ho, G.E. (1988) Overcoming the salinity and sodicity of red mud for rehabilitation and reuse. In: Proceedings of the 43rd Purdue Industrial Waste Conference. Lewis Publishers, Michigan, USA, pp. 641-649.
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Western Australia produces about 20% of the world’s annual alumina, and combined with the relatively low grade of bauxite (one third alumina) means that close to 11 million dry tons of residue needs to be disposed requiring annually 40 ha of land stacked 20 m deep. In the past the coarse fraction of the residue (> 150 microns) was combined with fine fraction for disposal in slurry form into dyked impoundments. Current practice is to separate the coarse and fine fractions since the coarse fraction is easier to wash free of caustic and can be used for construction purposes, while the fine (red mud) fraction is disposed in semi-solid form by “stacking” in dyked impoundments).
The difficulty of rehabilitating residue disposal areas has been well documented. The difficulty stems from the high alkalinity, salinity and sodicity of residue. The high pH (> 10) causes direct detrimental effects to plants due to hydroxide ion toxicity, reduces uptake of nutrients such as copper, zinc, iron, manganese and phosphorus and causes aluminate go reach toxic concentrations; the high salinity (> 16 mS/cm) restricts growth of all but a few tolerant species; the high sodium exchangeable percentage (greater than 50%) causes nutritional disturbances and adverse physical properties (deflocculation of clay size particles) leading to poor aeration and low permeability.
Many possible uses of red mud have been suggested, but because of the quantity that is produced the most promising is incorporation into sandy soils. Red mud amended sands have higher water holding capacity, higher cation exchange capacity and are less water repellent thus are better agricultural soils and better at attenuating pollution. Treatment of the mud is necessary, however, before its rehabilitation or application lo sands.
This chapter describes experimental work that has been carried out to overcome the salinity and sodicity of red mud particularly chemical neutralization with gypsum and copperas, and efforts to understand and the exploit properties of alumino-silicates binding sodium cations in the mud.
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