Fresh Water Leaching of Alkaline Bauxite Residue after Sea Water Neutralization

Abstract

Processing of bauxite to extract alumina produces a strongly alkaline waste, bauxite refining residue, which is commonly stored in engineered structures. Once full, these waste dumps must be revegetated. In many alumina refineries, the waste is separated into fine-textured red mud and coarse-textured residue sand (RS). The sand component has physical characteristics that make it a suitable plant growth medium, provided the adverse chemical characteristics can be addressed. Neutralization of the highly saline-sodic RS with sea water lowers pH, reduces Na saturation, and adds plant nutrients. However, sea water-neutralized RS remains saline sodic and needs fresh water leaching before use as a plant growth medium. Columns containing sea water-neutralized RS were leached with 30 m depth-equivalent of fresh water to evaluate the effects of rainfall on the RS and its leachate. Entrained cations were rapidly displaced by the fresh water, lowering salinity to non-plant-limiting levels (< or =0.3 dS m(-1)). The percentage of the effective cation exchange capacity (ECEC) saturated by Na decreased from 71 to 62% due to a reduction in soil solution ionic strength (causing a decrease in the ECEC) and the preferential displacement of Na(+) (and K(+)) from the exchange. Fresh water leaching increased pH (leachate pH increased from 8.0 to 10.1). This pH increase is attributed to the slow dissolution of the Na-containing mineral sodalite. Under the current experimental conditions, the application of 30 m depth-equivalent of leaching reduced the total RS sodalite content by <10%.

Proposal for management and alkalinity transformation of bauxite residue in China

“…The residue can be disposed of more safely or be reused in other applications. Calcium chloride and magnesium chloride neutralization (seawater neutralization) of bauxite residue is used by some coastal refineries in Australia (Menzies et al 2009;Santini et al 2011;Couperthwaite et al 2014). Though an excess of NH 4 Cl is required by ammonium chloride neutralization, this technology produces no secondary pollutants.…”

Microbially-driven strategies for bioremediation of bauxite residue

“…Major minerals present in bauxite residue include a mixture of residual minerals from the parent bauxite (hematite, goethite, quartz, kaolinite, anatase, rutile, undigested gibbsite, boehmite, or diaspore) as well as precipitates formed during the Bayer process (perovskite, calcite, tricalcium aluminate, and zeolitic desilication product minerals such as sodalite and cancrinite) [4]. With the exception of perovskite, Bayer process precipitate minerals dissolve slowly during rainfall leaching and weathering of bauxite residue and release salts (Na + , Ca 2+ , various anions depending on mineral composition) and alkalinity (in the form of CO 3 2and OH -) to pore water solutions, maintaining the high pH and salinity of bauxite residue over time [4,6]. The chemical and physical properties of bauxite residue pose significant challenges for remediation, the aim of which is to establish and maintain a vegetation cover after closure of tailings storage facilities, and convert the land area occupied by tailings to an alternative use.…”

“…[Suggested location for Figure 2] Applications of various chemical and physical amendments to accelerate remediation and soil development in bauxite residue have been studied for nearly four decades [6,[8][9][10][11][12][13][14][15][16] ( Figure 2). The focus of these studies was to lower salinity, sodicity, and alkalinity, and to encourage structure development.…”

Microbial Community Composition in a Rehabilitated Bauxite Residue Disposal Area: A Case Study for Improving Microbial Community Composition

“…Practices to ameliorate the high alkalinity of bauxite residue include gypsum amendment (e.g. Jones et al ) residue carbonation, seawater neutralization (Menzies et al ) and bacterial amelioration (Hamdy & Williams ). Gypsum, combined with organic matter (composts) and/or fertilizers, are commonly applied to lower pH and sodicity and improve nutrient status.…”