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ABSTRACT
Reaction of bauxite residue with seawater results in neutralization of alkalinity through precipitation of Mg-, Ca-, and Al-hydroxide and carbonate minerals. In batch studies, the initial pH neutralization reaction was rapid (<5 min), with further reaction continuing to reduce pH for several weeks. Reaction with seawater produced a residue pH of S to 8.5. Laboratory leaching column studies were undertaken to provide information on seawater neutralization of the coarse-textured fraction of the waste, residue sand (RS), under conditions comparable with those that might be applied in the field. An 0.80-m-deep column of RS was neutralized by the application of the equivalent of 2-m depth of seawater. In addition to lowering the pH and Na content of the residue, seawater neutralization resulted in the addition of substantial amounts of the plant nutrients Ca, Mg, and K to the profile. Similar results were also obtained from a field-scale assessment of neutralization. However, the accumulation of precipitate, consisting of hydrotalcite, aragonite, and pyroaurite, in the drainage system may preclude the use of in situ seawater neutralization as a routine rehabilitation practice. Following seawater neutralization, RS remains too saline to support plant growth and would require fresh water leaching before revegetation.
Abbreviations: EC, electrical conductivity; ICP-AES, inductively coupled plasma atomic emission spectroscopy; RM, red mud; RS, residue sand.
WORLDWIDE, MORE THAN 100 million Mg of bauxite is mined each year (Oeberg and Steinlechner, 1996). Alumina is extracted from the bauxite using the Bayer process, in which the bauxite is mixed with hot concentrated sodium hydroxide to dissolve the aluminum. The remaining solids, known as bauxite residue, are separated from the process liquor in a series of large thickeners and pumped to disposal areas. The production of residue ranges from 0.6 to 2.0 Mg per Mg of alumina produced, depending on the quality of the bauxite. Current world production of this waste is estimated at 30 million Mg per year (dry basis) (Oeberg and Steinlechner, 1996). This enormous production of saline and alkaline waste poses a major disposal problem for the industry.
Seawater neutralization of alkaline bauxite residue, which can act to lower the salinity and alkalinity of the waste, is used in a number of settings. The simplest scenario is the disposal of residue into a marine...