Effect of integration of mechanical ball milling and flue gas desulfurization gypsum on dealkalization of bauxite residue

The synergistic impact of mechanical ball milling and flue gas desulfurization(FGD)gypsum on the dealkalization of bauxite residue was investigated through integrated analyses of solution chemistry,mineralogy,and microtopography.The results showed a significant decrease in Na_(2)O content(>30 wt.%)of FGD gypsum-treated bauxite residue after 30 min of mechanical ball milling.Mechanical ball milling resulted in differentiation of the elemental distribution,modification of the minerals in crystalline structure,and promotion in the dissolution of alkaline minerals,thus enhancing the acid neutralization capacity of bauxite residue.5 wt.%FGD gypsum combined with 30 min mechanical ball milling was optimal for the dealkalization of bauxite residue.

Enhanced dealkalization of bauxite residue through calcium-activated desulfurization gypsum

A novel integrated approach to remove the free alkalis and stabilize solid-phase alkalinity by controlling the release of Ca from desulfurization gypsum was developed.The combination of recycled FeCl_(3)solution and EDTA activated desulfurization gypsum lowered the bauxite residue pH to 7.20.Moreover,it also improved the residual Ca state,with its contribution to the total exchangeable cations increased(68%-92%).Notably,the slow release of exchangeable Ca introduced through modified desulfurization gypsum induced a phase transition of the alkaline minerals.This treatment stabilized the dealkalization effect of bauxite residue via reducing its overall acid neutralization capacity in abating pH rebound.Hence,this approach can provide guidance for effectively utilizing desulfurization gypsum to achieve stable regulation of alkalinity in bauxite residue.

Construction of a Gravel Road Using Bauxite Residue

Results from research efforts over many years by the Highway Laboratory of Civil Engineering Department of Aristotle University of Thessaloniki(AUTH)and other academic or research institutions on behalf of"Aluminium of Greece"have demonstrated the potential use of bauxite residue in the construction industry as bricks,in the ceramic industry,for restoring abandoned surface mines and road construction.This paper details the construction of a gravel road using bauxite residue on the road"St.Nikolas-Kiriaki"in the site of the"Aluminium of Greece".The final layer included the addition of bauxite aggregates to provide an additional road carrying capacity.The aim of the project was to construct a trafficable surface for trucks and other vehicles with adequate resistance to heavy traffic loads and external factors as changing climatic conditions or soil erosion due to the rain.The construction is the right balance between technically excellent result and economically feasible.The prospect of using bauxite residue for similar road construction,or in general road construction,remains open.

Industrial wastes applications for alkalinity regulation in bauxite residue: A comprehensive review

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.

Variation of alkaline characteristics in bauxite residue under phosphogypsum amendment

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 CO3^2–>OH^–>AlO2^–>HCO3^–.In addition,alkaline phase decreased from 52.81%to 48.58%and gypsum stably presented in bauxite residue which continuously provided Ca^2+to inhibit dissolution of combined alkali.Furthermore,phosphogypsum promoted formation of macroaggregate structure,increased Ca^2+,decreased Na+and Al^3+on the surface of bauxite residue significantly,ultimately promoting soil formation in bauxite residue.

Effect of ammonium chloride on leaching behavior of alkaline anion and sodium ion in bauxite residue

This study focused on leaching behavior of alkaline anion and sodium in bauxite residue through ammonium chloride treatment.The results showed that the pH of bauxite residue decreased from 10.49 to 8.93,total alkaline anion(HCO3^-,CO3^2-,OH^-,AlO2^-)concentration reduced from 38.89 to 25.50 mmol/L,leaching rate of soluble sodium was 80.86%with ammonium chloride addition of 0.75%,liquid/solid(L/S)ratio of 3(mL/g),temperature of 30°C and reaction time of 18 h;L/S ratio was the main factor affecting the removal of alkaline anion and the leaching of sodium.Furthermore,ammonium chloride promoted the dissolution of diaspore and changed the micro/morphological characteristics with the increase of massive structure.The findings of this work will contribute to achieve soil-formation of bauxite residue.

Leaching optimization and dissolution behavior of alkaline anions in bauxite residue

Bauxite residue is a highly alkaline waste containing soluble alkaline anions, which can cause environmental concerns. The optimal leaching conditions, distribution of alkaline anions, types of pivotal alkaline anions and their dissolution behaviors were investigated based on the combination of single factors-orthogonal experiments and leaching stage experiment. Using a two-stage leaching, 86% of the soluble alkaline anions（CO3^2-, HCO4^-,Al（OH）4^-, OH^-） were leached with a L/S ratio of 2 mL/g, at 30 ℃, over 23 h. During the first stage of leaching, approximately 88% of alkaline anions were leached from the dissolution of free alkali（Na OH, carbonate, bicarbonate, NaAl（OH）4） with the rest originating from the dissolution of alkaline minerals（calcite, cancrinite and hydrogarnet）. Supernatant alkalinity was 69.78 mmol/L with CO3^2- accounting for 75%. Furthermore, carbonate leaching was controlled by solid film diffusion using the Stumm Model with an apparent activation energy of 10.24 kJ/mol.

Effect of substrate amendment on alkaline minerals and aggregate stability in bauxite residue

Bauxite residue is an alkaline waste material in the process of alumina production due to its characteristics of higher salinity and alkalinity,which results in environmental issues and extremely restricts the sustainable development of alumina industries.In this work,we conduct a column experiment to study the effects of two amendments on aggregate stability and variations in alkaline minerals of bauxite residue.The two amendments are phosphogypsum(PG)and phosphogypsum and vermicompost(PVC).The dominant fraction in aggregate is 1–0.25 mm in diameter on the surface,which takes up 39.34%,39.38%,and 44.51%for CK,PG,and PVC,respectively.Additions of PG and PVC decreased pH,EC,ESP,exchangeable Na^+concentration and the percentage of alkaline minerals,and then increased exchangeable Ca^2+concentration in bauxite residue.There was significant positive correlation between pH and exchangeable Na^+concentration,the percentage of cancrinite,tricalcium aluminate and calcite;while negative correlation was found in pH value versus exchangeable Ca^2+concentration.Theses findings confirmed that additions of phosphogypsum and vermicompost have a stimulative effect on aggregate stability in bauxite residue.In particular,amendment neutralization(phosphogypsum+vermicompost)in column represents an advantage for large-scale simulation of vegetation rehabilitate in bauxite residue disposal areas.

Salt ions accumulation and distribution characteristics of pioneer plant species at a bauxite residue disposal area, China

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 Ca^2+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.

Effect of phosphogypsum on saline-alkalinity and aggregate stability of bauxite residue

A column experiment was conducted to investigate the effect of phosphogypsum(PG)on the salinealkalinity,and aggregate stability of bauxite residue.Results showed that:with increasing leaching time,the concentrations of saline−alkali ions decreased while the SO_(4)^(2-)concentration increased in bauxite residue leachate;compared with CK(control group)treatment,pH,electric conductivity(EC),exchangeable sodium percentage(ESP),sodium absorption ratio(SAR),and exchangeable Na+content of bauxite residue were reduced following PG treatment;average particle sizes in aggregates following CK and PG treatments were determined to be 155 and 193 nm,respectively.SR-μCT test results also confirmed that bauxite residue following PG treatment acquired larger aggregates and larger pore diameter.These results indicate that the PG treatment could significantly modulate the saline-alkalinity,and simultaneously enhance aggregate stability of bauxite residue,which provides a facile approach to reclaim bauxite residue disposal areas.

Migration and distribution of saline ions in bauxite residue during water leaching

Bauxite residue,a highly saline solid waste produced from digestion of bauxite for alumina production,is hazardous to the environment and restricts vegetation establishment in bauxite residue disposal areas.A novel water leaching process proposed here was used to investigate the dynamic migration and vertical distribution of saline ions in bauxite residue.The results show that water leaching significantly reduced the salinity of bauxite residue,leaching both saline cations Na+,K+,Ca2+and anions CO32-,SO42-,HCO3-.Na+and K+migrated from 40-50 to 20-30 cm of the column,presenting a high migration capacity.The migration capacity of Ca2+was lower and accumulated at 30-40 cm of the column.CO32-initially distributed at 20-30 cm of the column,subsequently transported to 30-40 cm of the column,and finally returned to 20-30 cm of the column along with evaporation.SO42-was originally distributed at 40-50 cm,but finally migrated to 20-30 cm of the column.Nevertheless,HCO3-remained at the bottom of the column,and its migratory was less affected by evaporation.

Rehabilitation of bauxite residue to support soil development and grassland establishment

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.

Regional-scale investigation of salt ions distribution characteristics in bauxite residue: A case study in a disposal area

Revegetation on bauxite residue disposal areas is the most promising strategy to reduce its potential ecological risk during stacking or disposing.Migration of salt ions in bauxite residue is one of the major issues to stimulate soil formation to support plant growth.21 residue samples were collected and the related parameters including exchangeable cations,soluble ions,total salt,pH,electrical conductivity(EC)and exchangeable sodium percentage(ESP)were selected to evaluate alkalization and salinization of bauxite residue.High levels of ions,cation exchange capacity(TOC),total salt,exchangeable sodium percentage(ESP)and cation exchange capacity(CEC)in bauxite residue were detected with greater coefficient of variation(CV),which indicated that distribution characteristics of salt ions varied significantly.The percentage of sulfate-chloride-soda type in the residues accounted for 71.43%.The mean value of pH was 10.10,whilst mean value of ESP was 52.05%.It indicated that the residues in this case study belonged to sulfate-chloride-soda saline and alkaline soil.The research results could provide theoretical basis for soil formation in bauxite residue.

Alkalinity stabilization behavior of bauxite residue:Ca-driving regulation characteristics of gypsum

Alkaline anions,include CO3^2–,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(CO3^2–,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 Ca^2+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.

Dynamic change and diagnosis of physical, chemical and biological properties in bauxite residue disposal areas

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.

Natural ripening with subsequent additions of gypsum and organic matter is key to successful bauxite residue revegetation

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.

Isolation of an acid producing Bacillus sp. EEEL02: Potential for bauxite residue neutralization

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℃;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.

Physical, chemical, and surface charge properties of bauxite residue derived from a combined process

A detailed understanding of the composition,buffering capacity,surface charge property,and metals leaching behavior of bauxite residue is the key to improved management,both in reducing the environmental impact and using the material as an industrial by-product for other applications.In this study,physical,chemical,and surface charge properties of bauxite residue derived from a combined process were investigated.Results indicated that the main alkaline solids in bauxite residue were katoite,sodalite,and calcite.These minerals also lead to a higher acid neutralizing capacity of bauxite residue.Acid neutralizing capacity(ANC)to pH 7.0 of this residue is about 0.9 mol H^+/kg solid.Meanwhile,the Fe-,Al-,and Si-containing minerals in bauxite residue resulted in an active surface;The isoelectric point(IEP)and point of zero charge(PZC)were 7.88 and 7.65,respectively.This also leads to a fact that most of the metals in bauxite residue were adsorbed by these surface charged solids,which makes the metals not readily move under natural or even moderately acidic conditions.The leaching behavior of metals as a function of pH indicated that the metals in bauxite residue present low release concentrations(pH>3).

Synergistic coagulation of bauxite residue-based polyaluminum ferric chloride for dyeing wastewater treatment

Considering that Fe,Al elements in bauxite residue are active components for water purification,an effective polyaluminum ferric chloride(PAFC)coagulant derived from bauxite residue,with Fe2O3 content>5.1%,Al2O3%>6.5%,basicity>65%,was successfully prepared.The effect of as-prepared PAFC on the zeta potential for printing and dyeing wastewater was investigated.Comparing with polyferric chloride(PFC)and polyferric sulfate(PFS)for printing and dyeing wastewater treatment,prepared bauxite residue-based PAFC exhibited the optimal performance in the aspects of chromaticity and chemical oxygen demand(COD)removal rate.Furthermore,the combination of bauxite residue-based PAFC and PFS for synergy coagulation of such wastewater demonstrated an obvious positive effect.With the proportion between as-prepared PAFC and PFS to be 2.5:1,the COD of treated wastewater could be further reduced to meet the national level A standard of China,providing a promising route to solve the problem of substandard printing and dyeing sewage outfall by a simple coagulation strategy.

Colonization of Penicillium oxalicum enhanced neutralization effects of microbial decomposition of organic matter in bauxite residue

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.