Aluminium is the third largest element in the earth's crust and its ores are abundantly available. Aluminium is commercially produced from bauxite ore through two main process steps. In the first, alumina (Al₂O₃ ) is extracted  from the bauxite ore by the Bayer process (invented by Karl Bayer in 1887) and in the second, the alumina is electrolysed in a Hall –Heroult cell to get the pure metal, aluminium. Over 90% of the world's alumina is produced by the Bayer process and the solid waste generated during the processing of bauxite by this process is known as Red mud. It is also known as 'bauxite tailings' or 'Bauxite Residue'. Bauxite residue (red mud—RM), a byproduct of the Bayer process for refining bauxite to alumina, is highly alkaline and its treatment and storage poses unique waste management challenges. The present world wide generation of red mud is ~75 million tonne of which ~ 4 million tonne of red mud is generated annually in India. By the year 2010, red mud generation in India is estimated to be ~ 13 million tonne due to the several green field alumina plants coming up in the country. In India, red mud is disposed off in land-based impoundments. Stored bauxite residue represents a long term environmental liability for aluminium manufacturers. Safe storage of these materials requires engineered impoundments with leachate collection and treatment to prevent contamination of soil and groundwater. The storage impoundments typically occupy hundreds of acres of land at processing sites. The dust (predominantly Na₂ CO₃ )  formed on the dry residue surface can pose a health risk to wildlife or humans when it is carried away from the site. Hence, stored bauxite residue is often covered in some manner, typically with a layer of soil. The physical and chemical properties of the residues, especially the high pH and large fraction of fine, silt-sized particles, are the major constraints limiting reclamation efforts on residue storage areas. The high pH, exchangeable sodium, and fine-grained characteristics of red mud result in a low hydraulic conductivity and a high impedance to the penetration of roots. These properties also limit the availability of essential macro- and micronutrients for plant growth and adversely affect plant growth.

Red mud disposal methods include traditional Closed Cycle Disposal (CCD) methods and Modified Closed Cycle Disposal (MCCD). A new class of Dry Stacking (DS) technology has also emerged which requires much less land.

Soda in red mud is present mainly in two forms in red mud, free soda as ionized sodium aluminate and bound soda as DSP (desilicated product). It is disposed as dry or semi dry material in red mud pond or abandoned bauxite mines. It is also disposed as slurry having a high solid concentration of 30-60% and with a high ionic strength. But alkaline nature and large quantity of the waste have steered up researchers world wide to think of its alternative disposal / utilization. An enormous quantity of red mud is generated world wide every year posing a very serious and alarming environmental problem. Voluminous research and development work for the utilization, storage and disposal is being carried out all over the world. Up to 2 tons of liquor with a significant alkalinity of 5- 20 gpl caustic (as Na₂CO₃ ) accompany every ton of dry  mud. Incidentally, neutralisation of red mud seems to be a viable option which will reduce the reactivity of red mud and will certainly help in plummeting its environmental hazards thus assuring its safe disposal. In this paper, utilization of red mud and its neutralisation for safe disposal have been discussed.

The chemical and mineralogical compositions of red mud vary widely depending on the bauxite source and the process parameters. It is a highly complex material containing six major oxides viz. Fe₂O₃ (35-55 wt %), Al₂O₃(16-22 wt %), SiO₂ (4-16 wt %), Na₂O (3-6 wt %), TiO₂ (2-19 wt 2 2 2 %), CaO (0.8-4.5 wt %). It also contains several minor / trace amount of numerous other elements as oxides of V, P, Ga, Cr, Zr, Mg, Sr, Ba, Li, K, Pb, Cu, Mn, Ni, Zn, Co, rare earth etc. Red mud is composed of as many as 15-21 mineral phases. In addition to all those mineral phases present in the bauxite processed, red mud also contains a number of other phases which are formed during the digestion process. Mineral phases such as gibbsite, bohemite, diaspore, hematite, magnetite, goethite, alumogoethite, rutile and anatase are present in the bauxite to be processed. The phases formed in the process include sodalites (sodium aluminosilicates). calcium aluminosilicates, cancrinite, sodium titanate, calcium titanate etc. It is also disposed as slurry having a high solid concentration of 30-60% with a pulp density  ranging between 1.1-1.6 gm/cm³ . It is extremely fine sized (over 60% less than 10 microns) and have inherently poor settling properties [1]. In addition, red muds are also thixotropic and possess poor soil- mechanical properties and creates environmental problems like caustic seepage in ground water, deterioration of associated ground fertility, disposal space scarcity and many more.

Effective utilization of red mud is undoubtedly the best way to fight the situation. But high alkalinity of red mud is the main hurdle in its utilization path. Utilization of red mud in building materials production has got a promising chance due to presence of alumina, silica and aluminosilicate compounds which play important role in building materials property development. Though neutralization / soda fixation is achieved in case of sintered / fired bricks, presence of soda (high pH) creates problem in the case of stabilized blocks and unfired bricks as in those cases soda leaches out and contaminates surrounding water bodies. Soil amendment of red mud to neutralize it and reduce other harmful ions present in it to make it suitable for vegetation or vegetation support cover is also a good approach. But special treatment of such a huge volume of solid waste is very slow process. Direct neutralization of red mud followed by disposal is also a way which has been studied by many people around the world. Some bio-remediation routes have also been studied in Australia in which some microbial species which can survive in high pH environment were allowed to grow by consuming some organic substrate (e.g. source of glucose, sucrose, lactose etc; provided externally). During this growth period they generate some weak organic acid and sometimes CO₂ also which in turn  neutralizes red mud. In some specific cases where Al₂O₃ or  TiO₂ quantity is appreciable in red mud, recovery of the  same has been tried. Specially treated red mud has also been tried as adsorbent in water pollution control purposes. Appreciable quantity of iron (35% - 55%) present in red mud makes a sense that successful removal / recovery of iron from red mud will lead to substantial reduction in red mud volume i.e. less disposal problem. Several methods have been tried so far e.g. magnetic separation for iron beneficiation and reducing red mud volume to be disposed off, acid leaching and liquid-liquid extraction etc. But partial or complete reduction of iron by different reductant has improved the quantity and quality of iron present in treated red mud which in turn made iron recovery from red mud much more feasible. In next sections, different scopes of red mud utilization such as construction and building materials, metal value recovery, soil amendment and vegetation, pollution control have been reviewed.

Making red mud bricks may provide a partial answer to the alumina industry's waste disposal problem. Red mud has been used to make bricks for inexpensive housing by researchers from the Jamaica Bauxite Institute and the University of Toronto [1]. Recovery of iron from red mud was studied with direct reduction roasting process followed by magnetic separation, and then building materials were prepared from alumosilicate residues [3]. Brick specimens were prepared with alumosilicate residues and hydrated lime. Mean compressive strength of specimens was 24.10 MPa. It was indicated that main mineral phase transformed from nepheline (NaAlSiO₄ ) in alumosilicate residues to gehlenite (Ca₂Al₂SiO₇ ) in brick specimens  through X-ray diffraction (XRD) technology. Also, presence of alumina, iron oxide and silica makes red mud a suitable ingredient for the preparation of suitable cements [4].

Different types of cements were made using admixtures of red mud, fly ash, bauxite, gypsum and lime. The strengths were found to be comparable to Ordinary Portland Cement (OPC). Bricks were made from red mud using clay and water fixing agent [5].

A method have been invented for making bricks, particularly light-weight construction bricks, from red mud in which strongly water-fixing organic or inorganic substances are added to the filter-wet red mud before processing in order to influence its consistency [6]. Lime stabilised red mud bricks have also been prepared.

Red mud can also be used in ceramic industry. Red mud was investigated for use in making of ceramic glazes such as porcelain, vitreous (sanitary ware glazes), tile and electroporcelain glazes in the ceramic industry [7]. Specific mixtures of red mud, fly ash and spent pot liner were used to prepare glass – ceramic products which showed excellent properties and aesthetic appearance for possible applications as decorative tiles in the building industry [8]. Use of red mud as plasma- spray coatings on aluminium has also been reported [9].

Metals such as iron, titania and alumina can be recovered from red mud depending upon their quantity present in the red mud. An extensive study on the possibility of magnetic separation of red mud was carried out on red mud generated by Bayer process from Fria Deposit (Guinea) and reported that ~ 85 % of the iron present in red mud was recovered at 0.06 Tesla magnetic intensity [10]. In another work Turkish red mud was mixed with dolomite and coke to make pellet and sintered  (1100^o C) followed by smelting (1500^o C) to produce pig iron [11]. Studies were conducted on Jamaican red mud to recover undigested alumina by ash-sintering method followed by carbothermonic reduction of iron for metallization. Scope of magnetic separation and smelting of reduced material for pig iron production is also reported. If smelted, acid leaching of titania from smelter slag is also studied [12].

Bulk utilisation of red mud can be thought of by making its use in landfill and land reclamation. Bauxite mines or other quarries can be filled up by red mud after neutralisation. For neutralisation, lime, gypsum, seawater or other materials having similar neutralisation properties are mixed with red mud. Establishment of vegetation on the residues is an attractive desirable method of suppressing dust generation. However, there are constraints in achieving vegetation growth on the residues due to the inherent high pH and sodium levels. Use of gypsum and thermally dried sewage sludge as amendments for establishing the clover species Trifolium pratense on a red mud/process sand mix have been studied [13]. Gypsum amendment improved chemical conditions of the substrate and increased plant yield. Potential application of bauxite residue in soil/ sediment remediation, soil/ sediment stabilisation has studied. Wherever the agricultural land soil is acidic, red mud mixed with manures/ composts to neutralise the soil. It was seen that under certain conditions the properties of bauxite residue potentially can allow its use for improving soil conditions [14].

The most interesting applications of red mud are however in the environmental field. After adequate neutralisation red mud can be used for the remediation of contaminated sites and treatment of contaminated liquid waste. With this principal aim, several studies were carried out on the metal trapping and acid neutralisation capacities of red mud. The possibility to reuse treated red mud for contaminated waters and soils have been evaluated [20]. Acid mine drainage (AMD) is an environmental problem produced when sulphides come in contact with an oxidant (bacteria) and water, producing acid generation and metals leaching. One solution proposed is to use red mud which is very alkaline, to neutralize oxidized acidic tailings [15]. Plain soil was treated with red mud neutralized with either waste gypsum from the phosphate industry or ferrous sulfate from the titanium dioxide industry to investigate the reduction in the leaching of phosphorus (P) applied as superphosphate fertilizer from a very sandy Swan Coastal region [16].

In recent years, a great deal of concern has been expressed with regard to global climate change and its link to growing atmospheric concentrations of carbon dioxide. In order to decrease the impact of anthropogenic CO₂ on global climate, several strategies  are under development that will potentially remove CO₂ from the atmosphere or decrease CO₂ emission. Addition  of CO₂ to this caustic material will not only result in  carbonate mineral formation but will also serve to decrease the pH of the residue, making disposal more environmentally sound. Moreover, bauxite residue (pH 13.5) can serve as an effective source of caustic material to treat oil and gas field wastewater brines (pH 3 to 5) for mineral carbonation. The use of bauxite residue/brine to capture and store CO₂ will serve not only to help mitigate  the impact of anthropogenic CO₂ on global warming but  will also help neutralize caustic industrial waste for safe storage and reuse [17]. Also the Department of Agriculture, Western Australia has been working with Alcoa World Alumina Australia Ltd. for investigating the potential to use bauxite residues as soil amendments for the poor, acidic, sandy soils of the Swan Coastal plain [18].

Studies have also been carried out to use red mud as adsorbent for cleaning of industrial gases and as synthetic coagulants.

Residue neutralisation will diminish the potential for environmental impacts from residue storage activities, and will also lessen the need for significant levels of ongoing management of the deposits after closure. Neutralisation will also open opportunities for re-use of the residue which to date have been prevented because of the high pH.

A pH-reduction processing step is incorporated to ameliorate the red mud. Several methods of neutralization of red muds have been reported including: infiltration of rainwater and atmospheric CO₂ , treatment  with strong acids, gypsum addition, and seawater neutralization and sintering. Work on neutralization techniques have been primarily done in Australia [21] as it is the largest producer of red mud.

Various aqueous acidic solutions have been considered for this application, including acidic industrial wastewater. The use of carbonic acid has also been considered. A number of studies have been done for the feasibility of treating bauxite residue with acid on Kwinana red mud slurry. Large volumes of reagent are required to fully neutralize the residue at a relatively high cost, even if spent (waste) acid could be used. The use of acid also introduces large volumes of impurities to the process water stream (sulphate in the case of sulfuric acid, chloride in the case of hydrochloric acid. It is therefore likely that the return of any water from the residue deposits will be unacceptable to process without further treatment to remove these added impurities.

Mechanisms of neutralization of red mud by carbon dioxide gas have been studied [26]. Gaseous CO₂ or CO_2- containing flue gas has been bubbled through aqueous slurries to form carbonic acid in the aqueous phase. The carbonic acid reacts with basic components of the red mud, lowering its pH. However, the pH of water exposed to gaseous CO₂ is not likely to drop below 5.5  (approximately), and hence the rate of neutralization of the solids in the aqueous slurry is typically not fast enough to satisfy industrial needs. At the short contact times which industrial process rates demand, only a fraction of the alkaline material in red mud is neutralized using gaseous CO₂ . Hence although the pH of the aqueous phase drops  rapidly upon exposure to CO₂ gas, it soon rises again to  unacceptable levels as additional alkaline material leaches from the mud. CO₂ from the flue gases can be  utilized for treating the soda in red mud adding to the greenhouse benefit. A calculation of the amount of carbon dioxide that could be removed annually at aluminium refineries in Australia is potentially 15 million tonnes, and suggests that further studies are necessary to maximize this carbon removal process [24]. Furthermore, carbonation produces a product, which can potentially be used in other industrial and agricultural activities to remove toxic metals and nutrients

It was studied by researchers [19] that liquid carbon dioxide was more effective for pH reduction of red mud than vapor phase CO₂ because carbonic acid pH values  less than 3 can be realized with high-pressure carbon dioxide (6.7 - 10 MPa). The initial red mud pH of 12-13 was lowered to 7-8 immediately after treatment, and then slowly rise to values of 9.0-9.5 due to different reactions of the treated red mud. The processing time was reduced to 5-15 minutes using vigorous mixing of the liquid carbon dioxide and aqueous red mud slurry. Approximately 2 gm of carbon dioxide per 100 gm of red mud were bound in carbonate products when the process was conducted at 297 K.

When seawater or other Ca- and Mg- rich brines (eg. Salt lake brines) are added to caustic red mud, the pH of the mixture is reduced causing hydroxide, carbonate or hydroxycarbonate minerals to be precipitated [28]. The neutralizing effect of the calcium and the magnesium ions is initially large but decrease rapidly as complete neutralization is approached. Neutralisation is considered to be complete when the liquid that can be separated from the treated red mud has a pH less than 9.0 and a total alkalinity less than 200 mg/lit (as calcium carbonate equivalent alkalinity) such water can be safely discharged to the marine environment [29]. Studies on geochemical consequences of seawater neutralization of red mud was studied to show that physical characteristics of red mud improves which lead to many benefits such as reduced freshwater use at the refinery, increased settling rates at the pond and reduced alkalinity and sodicity in the solid wastes and entrained liquor, increased phosphate adsorption capacity, improved soil properties [23]. In Germany and France, many aluminum plants pump red mud directly into the sea. Also in countries such as England and Japan where there is availability of land and sea for dumping nearby, the practice is to discharge the residue into the sea.

The Virotec International Ltd. (Australia) announced a treatment process for RM that renders the material safe for a variety of applications. The method is based on the use of seawater. The pH of red mud is also reduced and can be decreased down to pH < 9. This technology is patented and several products with the name Bauxsol™ are available [20]. Environmental compatibility of the treated red mud was also studied. Treated red mud showed a general high metal trapping ability. An environmental friendly method for soda removal have also been reported by using Hungarian bauxite. A pH of approximately 8 can be achieved by irrigation with seawater and is sufficiently low to permit plant growth in the neutralized residue. Seawater irrigation also replaces Na with Ca, Mg, and K, thus improving the medium's plant fertility.

Sintering of red mud at a very high temperature causes the sodium aluminosilicate (DSP) and free soda in the red mud, non-reactive lowering down the pH substantially. Investigations on sintering of residue was done by Alcoa World Alumina Australia and found that sintering process resulted in fixation of all leachable alkali, however the cost was found to be very high, primarily due to the high energy consumption, making the process far more commercially viable.

Bioremediation of bauxite residue in Western Australia by Alcoa of Australia [22] has been carried out by adding some organic substrate to the red mud for growth of microorganisms which generate different organic acids and CO₂ (in some cases) which in turn neutalise the red  mud.

The potential of flyash to neutralize red mud slurry has also been studied [30]. Also for reduction of bound soda, a process [25] have been developed wherein the ferric sulphate can be used as a solid phase sulfatizing agent to destroy sodium aluminium silicates in red mud within 2 hrs at 200-500°C.

In India, studies on treatment of red mud for its complete utilization as soil conditioner/ fertilizer [27] have been carried out. Red mud is separated into light and heavy part. Light material is a source of magnesium ferrite. Coal dust, pyrite, superphosphate and gypsum were used to neutralize alkalinity of red mud. HCl was also used for acid neutralization.

As can be seen, there are several applications of red mud based on the physical and chemical characteristics. A considerable research has been carried out on the utilisation aspect of red mud. But, presently there is no large scale utilisation of red mud reported anywhere in the world. Many of the process developed are not commercially implemented because of the high cost involved in the processing of red mud. In all the alumina plants present in India, red mud is disposed off in red mud lakes or ponds and hence a cost effective process is to be developed where bulk and effective utilisation of red mud can be carried out to reduce its environmental hazards.

As in India, the bauxite residue is disposed off in red mud ponds/ lakes as its without neutralization, the future work should be focused to study the different neutralization techniques on red muds for its safe disposal and suitable utilization.