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Alkaline anions, include CO32-, HCO3-, Al(OH)4-, OH-, continuously released from bauxite residue (BR), will cause a potential disastrous impact on surrounding environment. The composition variation of alkaline anions, alkaline phase transformation pathway, and micro-morphological transition characteristics during the gypsum addition were investigated in an attempt to understand alkalinity stabilization behavior. Results demonstrated that alkaline anions stabilization degree in leachates can reach approximately 96.29%, whilst pH and alkalinity were reduced from 10.47 to 8.15, 47.39 mmol/L to 2 mmol/L, respectively. During the alkalinity stabilization, chemical regulation behavior plays significant role in driving the co-precipitation reaction among the critical alkaline anions (CO32-, HCO3-, Al(OH)4-, OH-), with calcium carbonate (CaCO3)) being the most prevalent among the transformed alkaline phases. In addition, XRD and SEM-EDX analyses of the solid phase revealed that physical immobilization behavior would also influence the stability of soluble alkali and chemical bonded alkali due to released Ca2+ from gypsum which aggregated the clay particles and stabilized them into coarse particles with a blocky structure. These findings will be beneficial for effectively regulating strong alkalinity of BR. [ABSTRACT FROM AUTHOR]

A feasible approach to selectively recover iron and rare earth elements (REEs) from red mud through acid leaching-coordination-solvent extraction was proposed. The leaching efficiencies of Fe, Al, Ti, Sc, La, Ce, Nd and Y can reach up to 95.9%, 82.1%, 68.3%, 93.3%, 82.3%, 96.9%, 98.3% and 95.6%, respectively, under the optimal condition in the leaching process. Aliquat 336 showed excellent extraction performance of iron in chloride-rich solution, and the maximum extraction efficiency can reach over 96% in one time extraction while the loss of other metals was less than 10%, under the condition of Aliquat 336 concentration (v/v) of 30%, aqueous-organic ratio of 1.0 and extraction time of 20 min. Furthermore, P204 can effectively extract the scandium while Al and most other REEs remain in the aqueous phase. This approach may provide a new insight for the recovery of valuable resources from red mud. [ABSTRACT FROM AUTHOR]

The absorbent composing of Bayer red mud and water was prepared and applied to removing SO2 from flue gas. Effects of the ratio of liquid to solid (L/S), the absorption temperature, the inlet SO2 concentration, the O2 concentration, SO42- and other different components of Bayer red mud on desulfurization were conducted. The mechanism of flue gas desulfurization was also established. The results indicated that L/S was the prominent factor, followed by the inlet SO2 concentration and the temperature was the least among them. The optimum condition was as follows: L/S, the temperature and the SO2 concentration were 20:1, 25 °C and 1000 mg/m3, respectively, under the gas flow of 1.5 L/min. The desulfurization efficiency was not significantly influenced when O2 concentration was above 7%. The accumulation of SO42- inhibited the desulfurization efficiency. The alkali absorption and metal ions liquid catalytic oxidation were involved in the process, which accounted for 98.61%. [ABSTRACT FROM AUTHOR]

Vegetation encroachment occurred in bauxite residue disposal area (BRDA) following natural weathering processes, whilst the typical indicators of soil formation are still uncertain. Residue samples were collected from the BRDA in Central China, and related physical, chemical and biological indicators of bauxite residue with different storage years were determined. The indicators of soil formation in bauxite residue were selected using principal component analysis, factor analysis, and comprehensive evaluation to establish soil quality diagnostic index model on disposal areas. Following natural weathering processes, the texture of bauxite residue changed from silty loam to sandy loam. The pH and EC decreased, whilst porosity, nutrient element content and microbial biomass increased. The identified minimum data set (MDS) included available phosphorus (AP), moisture content (MC), C/N, sand content, total nitrogen (TN), microbial biomass carbon (MBC), and pH. The soil quality index of bauxite residue increased, and the relative soil quality index decreased from 1.89 to 0.15, which indicated that natural weathering had a significant effect on improveing the quality of bauxite residue and forming a new soil-like matrix. The diagnostic model of bauxite residue was established to provide data support for the regeneration on disposal area. [ABSTRACT FROM AUTHOR]

Bauxite residue deposit area (BRDA) is a typical abandoned mining wasteland representing extreme hostile environment with increased alkalinity. Microbially-driven neutralization of bauxite residue, based on the microbial acid producing metabolisms, is a novel strategy for achieving rapid pH neutralization and thus improving its environmental outcomes. The hypothesis was that these extreme conditions promote microbial communities which are capable of novel ecologically relevant functions. Several alkaliphilic acid producing bacteria were isolated in this study. One strain was selected for its superior growth pattern and acid metabolism (termed EEEL02). Based on the phylogenetic analysis, this strain was identified as Bacillus thuringiensis. The optimized fermentation conditions were as follows: pH 10; NaCl concentration 5%; temperature 25 °C; EEEL02 preferred glucose and peptone as carbon and nitrogen sources, respectively. Based on optimal fermentation conditions, EEEL02 induced a significant pH reduction from 10.26 to 5.62 in 5-day incubation test. Acetic acid, propionic acid and CO2 (g) were the major acid metabolites of fermentation, suggesting that the pH reduction in bauxite residue may be caused by acid neutralization derived from microbial metabolism. This finding provided the basis of a novel strategy for achieving rapid pH neutralization of bauxite residue. [ABSTRACT FROM AUTHOR]

Rehabilitation (amendment and vegetation establishment) on bauxite residue is viewed as a promising strategy to stabilize the surface and initiate soil development. However, such approaches are inhibited by high pH, high exchangeable sodium (ESP) and poor nutrient status. Amendment with gypsum is effective in improving residue physical and chemical properties and promoting seed establishment and growth. Application of organics (e.g. compost) can address nutrient deficiencies but supplemental fertilizer additions may be required. A series of germination bioassays were performed on residue to determine candidate species and optimum rehabilitation application rates. Subsequent field trials assessed establishment of grassland species Holcus lanatus and Trifolium pratense as well as physical and chemical properties of amended residue. Follow up monitoring over five years assessed elemental content in grassland and species dynamics. With co-application of the amendments several grassland species can grow on the residue. Over time other plant species can invade the restored area and fast growing nutrient demanding grasses are replaced. Scrub species can establish within a 5 Yr period and there is evidence of nutrient cycling. High pH, sodicity and nutrient deficiencies are the major limiting factors to establishing grassland on residue. Following restoration several plant species can grow on amended residue. [ABSTRACT FROM AUTHOR]

Bauxite residue is a highly alkaline waste product from refining bauxite ore. Bioremediation driven by microbial activities has been evidently effective in lowering the alkalinity of bauxite residues, which is critical to the initiation of pedogenesis under engineered conditions. The present study investigated the changes of alkalinity and aggregation of bauxite residue at different depth in response to the colonization of Penicillium oxalicum in columns. The results demonstrated that the inoculation of P. oxalicum decreased the residue's pH to about 7 after 30 d only at the surface layer, which was exposed to aerobic conditions. The formation of aggregates was improved overall in the organic matter treated bauxite residue. However, the EC of bauxite residue increased with time under the incubation condition, probably due to accelerated hydrolysis of sodium-rich minerals. The inoculation of P. oxalicum had no effects on urease activity, but increased cellulose enzyme activity at surface layer only. [ABSTRACT FROM AUTHOR]

Aiming at alkaline problem of bauxite residue, this work focused variation of alkaline characteristics in bauxite residue through phosphogypsum treatment. The results demonstrated that the pH of bauxite residue reduced from initial 10.83 to 8.70 when 1.50 wt% phosphogypsum was added for 91 d. The removal rates of free alkali and exchangeable sodium were 97.94% and 75.87%, respectively. Meanwhile, significant positive correlations (P<0.05) existed between pH and free alkali, exchangeable sodium. The effect of free alkali composition was CO32->OH-> AlO2->HCO3-. In addition, alkaline phase decreased from 52.81% to 48.58% and gypsum stably presented in bauxite residue which continuously provided Ca2+ to inhibit dissolution of combined alkali. Furthermore, phosphogypsum promoted formation of macroaggregate structure, increased Ca2+, decreased Na+ and Al3+ on the surface of bauxite residue significantly, ultimately promoting soil formation in bauxite residue. [ABSTRACT FROM AUTHOR]

Bauxite residue disposal areas (BRDAs) are physically degraded and hostile to plant growth. Nevertheless, natural plant colonization was observed in an abandoned BRDA in Central China. The pioneer plant species at the disposal area were identified, whilst distribution characteristics of salt ions such as Na+, K+, and Ca2+ in plant tissues and rhizosphere residues were investigated. The mean concentration of exchangeable Na+ in the rhizosphere soils was 19.5 cmol/kg, which suggested that these pioneer plants had relatively high salinity resistance. Sodium content varied from 0.84 cmol/kg (Digitaria sanguinalis) to 39.7 cmol/kg (Kochia scoparia), whilst K to Na ratio varied from 0.71 (Myricaria bracteata) to 32.39 (Digitaria sanguinalis) in the shoots, which demonstrated that the salinity tolerance mechanisms of these pioneer species differed significantly. Accumulation factors of Na+ in local plant species ranged from 0.04 (D. sanguinalis) to 3.29 (M. bracteata), whilst the translocation factor varied from 0.13 (D. sanguinalis) to 2.92 (M. bracteata). The results suggested that four pioneer plant species including K. scoparia, M. bracteate, Cynodon dactylon and D. sanguinalis could be suitable for revegetation at other disposal areas. [ABSTRACT FROM AUTHOR]

Excess lithium in alumina is significantly bad for aluminum reduction. In this study, the concentration variation of lithium ion in sodium aluminate solution with addition of synthetic lithium aluminate was investigated. Elevating temperature, increasing caustic soda concentration, reducing alumina concentration or raising molar ratio α k improved equilibrium concentration of lithium ion in sodium aluminate solution. Agitation speed had a minimal effect on lithium ion concentration. Over 0.65 g/L lithium ion equilibrium concentration was observed in digestion process, whereas 35 mg/L lithium ion concentration remained in solution after precipitation time of 9 h. Moreover, equilibrium concentration decreased sharply from digestion of boehmite or diaspore to seed precipitation, about 95% lithium was precipitated into red mud (bauxite residue) and aluminum hydroxide. This study provides a valuable perspective in removal or extraction of lithium from sodium aluminate solution in alumina refineries. [ABSTRACT FROM AUTHOR]

The processes involved in the major steps of successful revegetation of bauxite residues are examined. The first phase is the natural physical, chemical and microbial ripening of the profile. This involves allowing the profile to drain, dry, shrink and crack to depth, leaching of soluble salts, alkalinity and Na down out of the surface layers, acidification by direct carbonation and natural seeding of tolerant vegetation with an accumulation of organic matter near the surface and an attendant development of an active microbial community. Following ripening, the surface layer can be tilled and gypsum and organic matter (e.g. manures, composts, biosolids) incorporated. These amendments result in a further decrease in pH, increase in Ca and other exchangeable cations, increased leaching of Na (with a reduction in exchangeable Na and ESP), improved physical properties, particularly aggregation, and a large increase in microbial activity. Other important considerations include the choice of suitable plant species tolerant to salinity/sodicity and local environmental conditions and the addition of balanced fertilizer applications. [ABSTRACT FROM AUTHOR]

Bauxite residue is a highly alkaline material generated from the production of alumina in which bauxite is dissolved in caustic soda. Approximately 4.4 billion tons of bauxite residues are either stockpiled or landfilled, creating environmental risks either from the generation of dust or migration of filtrates. High alkalinity is the critical factor restricting complete utilization of bauxite residues, whilst the application of alkaline regulation agents is costly and difficult to apply widely. For now, current industrial wastes, such as waste acid, ammonia nitrogen wastewater, waste gypsum and biomass, have become major problems restricting the development of the social economy. Regulation of bauxite residues alkalinity by industrial waste was proposed to achieve 'waste control by waste' with good economic and ecological benefits. This review will focus on the origin and transformation of alkalinity in bauxite residues using typical industrial waste. It will propose key research directions with an emphasis on alkaline regulation by industrial waste, whilst also providing a scientific reference point for their potential use as amendments to enhance soil formation and establish vegetation on bauxite residue disposal areas (BRDAs) following large-scale disposal. [ABSTRACT FROM AUTHOR]