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 97% of the alumina produced globally is through the Bayer process; for every tonne () of alumina produced, approximately of red mud are also produced; the global average is 1.23. Annual production of alumina in 2023 was over resulting in the generation of approximately of red mud.Annual statistics collected and published by World Aluminium .
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.Evans, K., "The History, Challenges and new developments in the management and use of Bauxite Residue", J. Sustain Metall. May 2016. .
Less commonly, this material is also known as bauxite tailings, red sludge, or alumina refinery residues. Increasingly, the name processed bauxite is being adopted, especially when used in cement applications.
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. 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.
More than 60 manufacturing operations across the world use the Bayer process to make alumina from bauxite ore. 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 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. In addition to iron, the other dominant components include silica, unleached residual aluminium compounds, and titanium oxide.Ayres, R. U., Holmberg, J., Andersson, B., "Materials and the global environment: Waste mining in the 21st century", MRS Bull. 2001, 26, 477.
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:
Chemical name Chemical formula Percentage composition Sodalite 3Na2O⋅3Al2O3⋅6SiO2⋅Na2SO4 4–40% Cancrinite Na3⋅CaAl3⋅Si3⋅O12CO3 0–20% Aluminous-goethite (aluminous iron oxide) α-(Fe,Al)OOH 10–30% Hematite (iron oxide) Fe2O3 10–30% Silica (crystalline & amorphous) SiO2 5–20% Tricalcium aluminate 3CaO⋅Al2O3⋅6H2O 2–20% Boehmite AlO(OH) 0–20% Titanium dioxide TiO2 0–10% Perovskite CaTiO3 0–15% Muscovite K2O⋅3Al2O3⋅6SiO2⋅2H2O 0–15% Calcium carbonate CaCO3 2–10% Gibbsite Al(OH)3 0–5% Kaolinite Al2O3⋅2SiO2⋅2H2O 0–5%
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. The case is important in international law governing the Mediterranean sea.
In October 2010, approximately of red mud slurry from an alumina plant near Kolontár in Hungary was accidentally released into the surrounding countryside, killing ten people and contaminating a large area. 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. The long-term environmental effects of the spill have been minor after a remediation effort by the Hungarian government.
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.
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.
As residue storage space ran out and concern increased over wet storage, since the mid-1980s dry stacking has been increasingly adopted.B. G. Purnell, "Mud Disposal at the Burntisland Alumina Plant". Light Metals, 157–159. (1986).H. H. Pohland and A. J. Tielens, "Design and Operation on Non-decanted Red Mud Ponds in Ludwigshafen", Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).E. I. Robinsky, "Current Status of the Sloped Thickened Tailings Disposal System", Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).J. L. Chandler, "The Stacking and Solar Drying Process for disposal of bauxite tailings in Jamaica", Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986). 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.
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.K. S. Sutherland, "Solid/Liquid Separation Equipment", Wiley-VCH, Weinheim (2005). 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.
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 componentsespecially 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. Several studies have been conducted to develop uses of red mud. An estimated are used annually in the production of cement,Y. Pontikes and G. N. Angelopoulos "Bauxite residue in Cement and cementious materials", Resourc. Conserv. Recyl. 73, 53-63 (2013). road constructionW.K.Biswas and D. J. Cooling, "Sustainability Assessment of Red Sand as a substitute for Virgin Sand and Crushed Limestone", J. of Ind. Ecology, 17(5) 756-762 (2013). and as a source for iron. Potential applications include the production of low cost concrete,Liu, W., Yang, J., Xiao, B., "Review on treatment and utilization of bauxite residues in China", Int. J. Miner. Process. 2009, 93, 220. application to sandy soils to improve phosphorus cycling, amelioration of soil acidity, landfill capping and carbon sequestration.
Reviews describing the current use of bauxite residue in Portland cement clinker, supplementary cementious materials/blended cements and special calcium aluminate cements (CAC) and calcium sulfo-aluminate (CSA) cements have been extensively researched and documented.
Cement manufacture, use in concrete as a supplementary cementitious material. From .Y. Pontikes and G. N. Angelopoulos "Bauxite residue in Cement and cementious materials", Resourc. Conserv. Recyl. 73, 53–63 (2013).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). 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; Use as a component in building or construction materials (bricks, tiles, ceramics etc.) – 100,000 to 300,000 tonnes; Other (refractory, adsorbent, acid mine drainage (Virotec), catalyst etc.) – 100,000 tonnes.H. Genc¸-Fuhrman, J. C. Tjell, D. McConchie, "Adsorption of arsenic from water using activated neutralized red mud", Environ. Sci. Technol. 38 (2004) 2428–2434.