Bauxite processing residue : a critical review of its formation, properties, storage, and revegetation

Bauxite is processed in alumina refineries by the Bayer process in which Al-containing minerals are dissolved in hot NaOH. The insoluble solids (bauxite processing residue mud and sand) are washed, sometimes partially neutralized (using CO2 or seawater treatment), and deposited in impoundments surrounding the refinery using either wet (15-30% solids) or dry (50-65% solids) disposal techniques. Revegetation strategies for impoundments include constructing a soil cap over the mud deposit, amending the mud with residue sand and other materials or revegetating the sand fraction that surrounds and covers the mud deposit. Major limitations to plant growth in residues include salinity, sodicity, alkalinity, Al toxicity, and deficiencies of macro- and micronutrients. Physical properties are also problematic since residue mud consolidates to form a solid mass that waterlogs easily or dries to form a massive structure, while sand has a very low water holding capacity. Gypsum amendment reduces pH, displaces Na with Ca and tends to improve porosity. Organic amendments supply nutrients, increase CEC, and improve physical conditions while inorganic fertilizers supply essential nutrients. An integrated approach using all of the previously mentioned amendments as well as saline sodic-tolerant plant species is likely to increase the success of revegetation compared to use of only 1 or 2 techniques. There are very few reports on microbial activity in residue but since it is heat and chemically treated, activity is likely to be initially very low. Organic residue addition provides a substrate for heterotrophic microbes and provide the beginnings for a below-ground living ecosystem. Inoculation of plants with appropriate mycorrhiza and legumes with Rhizobium might also increase plant establishment and growth. These aspects deserve future examination. Although much research has concentrated on the establishment phase of revegetation, little consideration has been given to the subsequent growth and self-propagation of the introduced species, changes in residue properties during weathering and leaching of alkalinity from residue over time. These facets need careful study before definitive statements can be made regarding the sustainability of various revegetation strategies.