Red mud, now more frequently termed bauxite residue, is an industrial waste generated during the processing of bauxite into alumina using the Bayer process. It is composed of various oxide compounds, including the iron oxides which give its red colour. Over 95% of the alumina produced globally is through the Bayer process; for every tonne (2,200lb) of alumina produced, approximately 1 to 1.5 tonnes (2,200 to 3,300lb) of red mud are also produced. Annual production of alumina in 2020 was over 133million tonnes (290billion pounds) resulting in the generation of over 175million tonnes (390billion pounds) of red mud.[1]
Due to this high level of production and the material's high alkalinity, if not stored properly, it can pose a significant environmental hazard. As a result, significant effort is being invested in finding better methods for safe storage and dealing with it such as waste valorization in order to create useful materials for cement and concrete.[2]
Less commonly, this material is also known as bauxite tailings, red sludge, or alumina refinery residues.
Production
Red mud is a side-product of the Bayer process, the principal means of refining bauxite en route to alumina. The resulting alumina is the raw material for producing aluminium by the Hall–Héroult process.[3] A typical bauxite plant produces one to two times as much red mud as alumina. This ratio is dependent on the type of bauxite used in the refining process and the extraction conditions.[4]
More than 60 manufacturing operations across the world use the Bayer process to make alumina from bauxite ore.[citation needed] Bauxite ore is mined, normally in open cast mines, and transferred to an alumina refinery for processing. The alumina is extracted using sodium hydroxide under conditions of high temperature and pressure. The insoluble part of the bauxite (the residue) is removed, giving rise to a solution of sodium aluminate, which is then seeded with an aluminium hydroxide crystal and allowed to cool which causes the remaining aluminium hydroxide to precipitate from the solution. Some of the aluminium hydroxide is used to seed the next batch, while the remainder is calcined (heated) at over 1,000°C (1,830°F) in rotary kilns or fluid flash calciners to produce aluminium oxide (alumina).
The alumina content of the bauxite used is normally between 42 and 50%, but ores with a wide range of alumina contents can be used. The aluminium compound may be present as gibbsite (Al(OH)3), boehmite (γ-AlO(OH)) or diaspore (α-AlO(OH)). The residue invariably has a high concentration of iron oxide which gives the product a characteristic red colour. A small residual amount of the sodium hydroxide used in the process remains with the residue, causing the material to have a high pH/alkalinity, normally above 12. Various stages of solid/liquid separation processes recycle as much sodium hydroxide as possible from the residue back into the Bayer Process, in order to reduce production costs and make the process as efficient as possible. This also lowers the final alkalinity of the residue, making it easier and safer to handle and store.
Composition
Red mud is composed of a mixture of solid and metallic oxides. The red colour arises from iron oxides, which can comprise up to 60% of the mass. The mud is highly basic with a pH ranging from 10 to 13.[3][4][5] In addition to iron, the other dominant components include silica, unleached residual aluminium compounds, and titanium oxide.[6]
The main constituents of the residue after the extraction of the aluminium component are insoluble metallic oxides. The percentage of these oxides produced by a particular alumina refinery will depend on the quality and nature of the bauxite ore and the extraction conditions. The table below shows the composition ranges for common chemical constituents, but the values vary widely:
Chemical
Percentage composition
Fe2O3
5–60%
Al2O3
5–30%
TiO2
0–15%
CaO
2–14%
SiO2
3–50%
Na2O
1–10%
Mineralogically expressed the components present are:
In general, the composition of the residue reflects that of the non-aluminium components, with the exception of part of the silicon component: crystalline silica (quartz) will not react but some of the silica present, often termed, reactive silica, will react under the extraction conditions and form sodium aluminium silicate as well as other related compounds.
Environmental hazards
Discharge of red mud can be hazardous environmentally because of its alkalinity and species components.
Until 1972, Italian company Montedison was discharging red mud off the coast of Corsica.[7] The case is important in international law governing the Mediterranean sea.[8]
In October 2010, approximately onemillion cubic metres (35million cubic feet) of red mud slurry from an alumina plant near Kolontár in Hungary was accidentally released into the surrounding countryside in the Ajka alumina plant accident, killing ten people and contaminating a large area.[9] All life in the Marcal river was said to have been "extinguished" by the red mud, and within days the mud had reached the Danube.[10] The long-term environmental effects of the spill have been minor after a €127 million remediation effort by the Hungarian government.[11]
Residue storage areas
Residue storage methods have changed substantially since the original plants were built. The practice in early years was to pump the slurry, at a concentration of about 20% solids, into lagoons or ponds sometimes created in former bauxite mines or depleted quarries. In other cases, impoundments were constructed with dams or levees, while for some operations valleys were dammed and the residue deposited in these holding areas.[12]
It was once common practice for the red mud to be discharged into rivers, estuaries, or the sea via pipelines or barges; in other instances the residue was shipped out to sea and disposed of in deep ocean trenches many kilometres offshore. From 2016, all disposal into the sea, estuaries and rivers was stopped.[13]
As residue storage space ran out and concern increased over wet storage, since the mid-1980s dry stacking has been increasingly adopted.[14][15][16][17] In this method, residues are thickened to a high density slurry (48–55% solids or higher), and then deposited in a way that it consolidates and dries.[18]
An increasingly popular treatment process is filtration whereby a filter cake (typically resulting in 23–27% moisture) is produced. This cake can be washed with either water or steam to reduce alkalinity before being transported and stored as a semi-dried material.[19] Residue produced in this form is ideal for reuse as it has lower alkalinity, is cheaper to transport, and is easier to handle and process. Another option for ensuring safe storage is to use amphirols to dewater the material once deposited and then 'conditioned' using farming equipment such as harrows to accelerate carbonation and thereby reduce the alkalinity. Bauxite residue produced after press filtration and 'conditioning as described above are classified as non-hazardous under the EU Waste Framework Directive.
In 2013 Vedanta Aluminium, Ltd. commissioned a red mud powder-producing unit at its Lanjigarh refinery in Odisha, India, describing it as the first of its kind in the alumina industry, tackling major environmental hazards.[20]
Use
Since the Bayer process was first adopted industrially in 1894, the value of the remaining oxides has been recognized. Attempts have been made to recover the principal components–especially the iron oxides. Since bauxite mining began, a large amount of research effort has been devoted to seeking uses for the residue. Many studies are now being financed by the European Union under the Horizon Europe programme.[citation needed] Several studies have been conducted to develop uses of red mud.[21] An estimated 3 to 4million tonnes (6.6 to 8.8billion pounds) are used annually in the production of cement,[22] road construction[23] and as a source for iron.[3][4][5] Potential applications include the production of low cost concrete,[24] application to sandy soils to improve phosphorus cycling, amelioration of soil acidity, landfill capping and carbon sequestration.[25][26]
Reviews describing the current use of bauxite residue in Portland cementclinker, supplementary cementious materials/blended cements and special calcium aluminate cements (CAC) and calcium sulfo-aluminate (CSA) cements have been extensively researched and documented.[27]
Cement manufacture, use in concrete as a supplementary cementitious material. From 500,000 to 1,500,000 tonnes (1.1 to 3.3billion pounds).[28][29]
Raw material recovery of specific components present in the residue: iron, titanium, steel and REE (rare-earth elements) production. From 400,000 to 1,500,000 tonnes;
Landfill capping/roads/soil amelioration – 200,000 to 500,000 tonnes;[23]
Use as a component in building or construction materials (bricks, tiles, ceramics etc.) – 100,000 to 300,000 tonnes;
Use in building panels, bricks, foamed insulating bricks, tiles, gravel/railway ballast, calcium and silicon fertilizer, refuse tip capping/site restoration, lanthanides (rare earths) recovery, scandium recovery, gallium recovery, yttrium recovery, treatment of acid mine drainage, adsorbent of heavy metals, dyes, phosphates, fluoride, water treatment chemical, glass ceramics, ceramics, foamed glass, pigments, oil drilling or gas extraction, filler for PVC, wood substitute, geopolymers, catalysts, plasma spray coating of aluminium and copper, manufacture of aluminium titanate-mullite composites for high temperature resistant coatings, desulfurisation of flue gas, arsenic removal, chromium removal.[31]
In 2015, a major initiative was launched in Europe with funds from the European Union to address the valorization of red mud. Some 15 PhD students were recruited as part the European Training Network (ETN) for Zero-Waste Valorisation of Bauxite Residue.[32] The key focus will be the recovery of iron, aluminium, titanium and rare-earth elements (including scandium) while valorising the residue into building materials. A European Innovation Partnership has been formed to explore options for using by-products from the aluminium industry, BRAVO (Bauxite Residue and Aluminium Valorisation Operations). This sought to bring together industry with researchers and stakeholders to explore the best available technologies to recover critical raw materials but has not proceeded. Additionally, EU funding of approximately €11.5million has been allocated to a four-year programme starting in May 2018 looking at uses of bauxite residue with other wastes, RemovAL. A particular focus of this project is the installation of pilot plants to evaluate some of the interesting technologies from previous laboratory studies. As part of the H2020 project RemovAl, it is planned to erect a house in the Aspra Spitia area of Greece that will be made entirely out of materials from bauxite residue.
Other EU funded projects that have involved bauxite residue and waste recovery have been ENEXAL (ENergy-EXergy of ALuminium industry) [2010–2014], EURARE (European Rare earth resources) [2013–2017] and three more recent projects are ENSUREAL (ENsuring SUstainable ALumina production) [2017–2021], SIDEREWIN (Sustainable Electro-winning of Iron) [2017–2022] and SCALE (SCandium – ALuminium in Europe) [2016–2020] a €7million project to look at the recovery of scandium from bauxite residue.
In 2020, the International Aluminium Institute, launched a roadmap for maximising the use of bauxite residue in cement and concrete.[33][34]
In November 2020, The ReActiv: Industrial Residue Activation for Sustainable Cement Production research project was launched, this is being funded by the EU. One of the world's largest cement companies, Holcim, in cooperation with 20 partners across 12 European countries, launched the ambitious 4-year ReActiv project (reactivproject.eu). The ReActiv project will create a novel sustainable symbiotic value chain, linking the by-product of the alumina production industry and the cement production industry. In ReActiv modification will be made to both the alumina production and the cement production side of the chain, in order to link them through the new ReActiv technologies. The latter will modify the properties of the industrial residue, transforming it into a reactive material (with pozzolanic or hydraulic activity) suitable for new, low CO2 footprint, cement products. In this manner ReActiv proposes a win-win scenario for both industrial sectors (reducing wastes and CO2 emissions respectively).
Fluorchemie GmbH have developed a new flame-retardant additive from bauxite residue, the product is termed MKRS (modified re-carbonised red mud) with the trademark ALFERROCK(R) and has potential applicability in a wide range of polymers (PCT WO2014/000014). One of its particular benefits is the ability to operate over a much broader temperature range, 220–350°C (428–662°F), that alternative zero halogen inorganic flame retardants such as aluminium hydroxide, boehmite or magnesium hydroxide. In addition to polymer systems where aluminium hydroxide or magnesium hydroxide can be used, it has also found to be effective in foamed polymers such as EPS and PUR foams at loadings up to 60%.
In a suitable compact solid form, with a density of approximately 3.93 grams per cubic centimetre (0.142lb/cuin), ALFERROCK produced by the calcination of bauxite residues, has been found to be very effective as a thermal energy storage medium (WO2017/157664). The material can repeatedly be heated and cooled without deterioration and has a specific thermal capacity in the range of 0.6 – 0.8 kJ/(kg·K) at 20°C (68°F) and 0.9 – 1.3 kJ/(kg·K) at 726°C (1,339°F); this enables the material to work effectively in energy storage device to maximise the benefits of solar power, wind turbines and hydro-electric systems. High strength geopolymers have been developed from red mud.[35]
Sustainable Approach to Low-Grade Bauxite Processing
The IB2 process is a French technology developed to enhance the extraction of alumina from bauxite, especially low-grade bauxite. This method aims to boost alumina production efficiency while decreasing the environmental impacts typically linked with this process, notably the generation of red mud and carbon dioxide emissions.
The IB2 technology, patented in 2019,[36] is the outcome of a decade of research and development efforts by Yves Occello, a former Pechiney chemist. This process provides an alternative to the Bayer process, which has been utilized for more than a century to extract alumina from bauxite. It presents a significant decrease in caustic soda consumption and a notable reduction in red mud output, thereby minimizing hazardous waste and environmental risks.
In addition to reducing red mud production, the IB2 process aids in lowering CO2 emissions, primarily through the optimized treatment of low-grade bauxite. By limiting the necessity to import high-grade bauxite, this process reduces the carbon footprint associated with ore transportation. Furthermore, the process yields a byproduct that can be utilized in the production of eco-friendly cements, promoting the concept of a circular economy.[37]
Aluminium is a chemical element; it has symbol Al and atomic number 13. Aluminium has a density lower than that of other common metals, about one-third that of steel. It has a great affinity towards oxygen, forming a protective layer of oxide on the surface when exposed to air. Aluminium visually resembles silver, both in its color and in its great ability to reflect light. It is soft, nonmagnetic, and ductile. It has one stable isotope, 27Al, which is highly abundant, making aluminium the twelfth-most common element in the universe. The radioactivity of 26Al, a more unstable isotope, leads to it being used in radiometric dating.
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite, boehmite and diaspore, mixed with the two iron oxides goethite and haematite, the aluminium clay mineral kaolinite and small amounts of anatase and ilmenite . Bauxite appears dull in luster and is reddish-brown, white, or tan.
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations Na+ and hydroxide anions OH−.
Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula Al2O3. It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly called alumina and may also be called aloxide, aloxite, or alundum in various forms and applications. It occurs naturally in its crystalline polymorphic phase α-Al2O3 as the mineral corundum, varieties of which form the precious gemstones ruby and sapphire. Al2O3 is used to produce aluminium metal, as an abrasive owing to its hardness, and as a refractory material owing to its high melting point.
Calcium oxide, commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term lime connotes calcium-containing inorganic compounds, in which carbonates, oxides, and hydroxides of calcium, silicon, magnesium, aluminium, and iron predominate. By contrast, quicklime specifically applies to the single compound calcium oxide. Calcium oxide that survives processing without reacting in building products, such as cement, is called free lime.
Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric, i.e., it has both basic and acidic properties. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide or alumina, the latter of which is also amphoteric. These compounds together are the major components of the aluminium ore bauxite. Aluminium hydroxide also forms a gelatinous precipitate in water.
Slag is a by-product of smelting (pyrometallurgical) ores and recycled metals. Slag is mainly a mixture of metal oxides and silicon dioxide. Broadly, it can be classified as ferrous, ferroalloy or non-ferrous/base metals. Within these general categories, slags can be further categorized by their precursor and processing conditions. "Slag generated from the EAF process can contain toxic metals, which can be hazardous to human and environmental health".
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the rest being a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be further purified before it can be refined into aluminium.
In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.
The Deville process was the first industrial process used to produce alumina from bauxite.
Carl Josef Bayer was an chemist from Austria-Hungary who invented the Bayer process of extracting alumina from bauxite, essential to this day to the economical production of aluminium.
Aluminium recycling is the process in which secondary commercial aluminium is created from scrap or other forms of end-of-life or otherwise unusable aluminium. It involves re-melting the metal, which is cheaper and more energy-efficient than the production of virgin aluminium by electrolysis of alumina (Al2O3) refined from raw bauxite by use of the Bayer and Hall–Héroult processes.
Alkaline precipitation occurs due to natural and anthropogenic causes. It happens when minerals, such as calcium, aluminum, or magnesium combine with other minerals to form alkaline residues that are emitted into the atmosphere, absorbed by water droplets in clouds, and eventually fall as rain. Aquatic environments are especially impacted by alkaline precipitation. Because alkaline precipitation can be harmful to the environment, it is important to utilize various methods such as air pollution control, solidification and stabilization, and remediation to manage it.
Calcium aluminate cements are cements consisting predominantly of hydraulic calcium aluminates. Alternative names are "aluminous cement", "high-alumina cement", and "Ciment fondu" in French. They are used in a number of small-scale, specialized applications.
An industrial accident at a caustic waste reservoir chain took place at the Ajkai Timföldgyár alumina plant in Ajka, Veszprém County, in western Hungary. On 4 October 2010, at 12:25 CEST (10:25 UTC), the northwestern corner of the dam of reservoir number 10 collapsed, freeing approximately one million cubic metres of liquid waste from red mud lakes. The mud was released as a 1–2 m (3–7 ft) wave, flooding several nearby localities, including the village of Kolontár and the town of Devecser. Ten people died, and 150 people were injured. About 40 square kilometres (15 sq mi) of land were initially affected. The spill reached the Danube on 7 October 2010.
MAL Hungarian Aluminium was a Hungarian company that was specializing in the production of aluminium and related products. It was established in 1995 during the privatization of the Hungarian aluminium industry.
Orbite Technologies Inc. was a Canadian cleantech company based in Montreal, Canada. It specialized in extracting processes for mining industry, especially alumina extraction. Its main asset is the Orbite process which can be used as a cheaper and pollution-free replacement of the Bayer process as well as a way to treat red mud. In April 2017, the company declared bankruptcy, due to ongoing delays and issues relating to its construction of a new plant in Cap-Chat, Quebec. As of January 2018, the bankruptcy process was still ongoing. As of 2022 the company has rebranded itself as Advanced Energy Minerals, and claims to be manufacturing high purity alumina at its new Cap-Chat plant.
Spent Potlining (SPL) is a waste material generated in the primary aluminium smelting industry. Spent Potlining is also known as Spent Potliner and Spent Cell Liner.
China Hongqiao Group Limited is a company founded in 1994 that specializes in the production of aluminium. Hongqiao is currently the second largest aluminium producer in the world after Chinalco. It is listed on the Hong Kong Stock Exchange with stock code 1378, and is incorporated in George Town, Cayman Islands.
The Pederson process is a process of refining aluminum that first separates iron by reducing it to metal, and reacting alumina with lime to produce calcium aluminate, which is then leached with sodium hydroxide. It is more environmentally friendly than the more well-known Bayer process. This is because instead of producing alumina slag, also known as red mud, it produces pig iron as a byproduct. Red mud is considered both an economic and environmental challenge in the aluminum industry because it is considered a waste, with little benefit. It destroys the environment with its high pH, and is costly to maintain, even when in a landfill. Iron, however, is used in the manufacture of steel, and has structural uses in civil engineering and chemical uses as a catalyst.
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↑ Kumar, Sanjay; Kumar, Rakesh; Bandopadhyay, Amitava (2006-10-01). "Innovative methodologies for the utilisation of wastes from metallurgical and allied industries". Resources, Conservation and Recycling. 48 (4): 301–314. Bibcode:2006RCR....48..301K. doi:10.1016/j.resconrec.2006.03.003.
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↑ "Bauxite Residue Management". bauxite.world-aluminium.org. The International Aluminium Institute. Retrieved 9 August 2016.
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↑ Y. Pontikes, G. N. Angelopoulos, B. Blanpain, "Radioactive elements in Bayer’s process bauxite residue and their impact in valorization options", Transportation of NORM, NORM Measurements and Strategies, Building Materials, Advances in Sci. and Tech, 45, 2176–2181 (2006).
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↑ B. K. Parekh and W. M. Goldberger, "An assessment of technology for the possible utilisation of Bayer process muds", published by the U. S. Environmental Protection Agency, EPA 600/2-76-301.
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