Red mud

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Red mud near Stade (Germany)
Bauxite, an aluminum ore (Hérault department, France). The reddish color is due to iron oxides that make up the main part of the red mud.

Red mud is a highly alkaline waste product composed mainly of iron oxide that is generated in the industrial production of alumina (aluminium oxide, the principal raw material used in the manufacture of aluminium metal and also widely used in the manufacture of ceramics, abrasives and refractories). Annually, about 77 million tons of the red special waste are produced, causing serious disposal problem in the mining industry,[1] the scale of production makes the waste product an important one, and issues with its storage are reviewed and every opportunity is explored to find uses for it.

Over 95% of the alumina produced globally is through the Bayer process; for every tonne of alumina produced, approximately 1 to 1.5 tonnes of bauxite tailings/residue are also produced. Annual production of alumina in 2015 was approximately 115 million tonnes resulting in the generation of about 150 million tonnes of bauxite tailings/residue.[2]

Red mud is also known as bauxite tailings, red sludge, bauxite residue and alumina refinery residues (ARR).

Production[edit]

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]

Red mud is composed of a mixture of solid and metallic oxides, the red colour arises from iron oxides, which 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 alumina, and titanium oxide.[1]

There are over 60 manufacturing operations across the world using 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. To extract the alumina, the soluble part of the bauxite ore is dissolved 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 and allowed to cool and aluminium hydroxide precipitates from the solution. Though some of the aluminium hydroxide is then returned and used to seed the next batch, the remainder is calcined (heated) at over 1000 °C in rotary kilns or fluid flash calciners to produce aluminium oxide (alumina). The alumina content of the bauxite used is normally about 50% but ores with a much wider range of alumina contents can be used; the aluminium compound may be present as gibbsite (Al(OH)3), boehmite (AlOOH) or diaspore (AlOOH). The tailings/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 tailings, causing the material to have a high pH/alkalinity, normally >12. Various stages in the solid/liquid separation process are introduced to recycle as much sodium hydroxide as possible from the residue back into the Bayer Process in order to make the process as efficient as possible and reduce production costs, this also lowers the final alkalinity of the tailings making it easier to handle.

Composition[edit]

The main constituents of the residue after the extraction of the aluminium component are unreacted 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 0.3–15%
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%
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 2–15%
Muscovite K2O⋅3Al2O3⋅6SiO2⋅2H2O 0–15%
Calcium carbonate CaCO3 2–10%
Gibbsite Al(OH)3 0–5%
Kaolinite Al2O3⋅2SiO2⋅2H2O 0–5%

The objective is to remove as much of the aluminium containing component as economically possible; 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.

Environmental hazards[edit]

Discharge of red mud is hazardous environmentally because of its alkalinity.

In October 2010, approximately one million cubic meters of red mud 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.[6] 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.[7]

Residue storage areas (RSA)[edit]

Tailings storage methods have changed substantially since the original plants were built, the practice in early years was to pump the tailings 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, whilst for some operations valleys were dammed and the tailings deposited in these holding areas.[8]

It was also common practice for the tailings 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. All disposal in the sea, estuaries and rivers has now stopped,[9] as residue storage space ran out and concern increased over wet storage, since the mid-1980s dry stacking has been increasingly adopted.[10] [11] [12] [13] In this method, tailings are thickened to a high density slurry (48-55% solids or higher), and then deposited in a way that it consolidates and dries.[14]

An increasing popular storage method is filtration whereby a filter cake (typically <30% solids) 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.[15] 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.

Use[edit]

Since the Bayer process was first adopted industrially in 1894, the value of the remaining oxides has been recognised. Attempts have been made to recover the principal components – especially iron, since mining began, an enormous amount of research effort has been devoted to seeking uses for the residue.

Many studies have been conducted to develop uses of red mud.[16] An estimated 2 to 3 million tonnes are used annually in the production of cement,[17] road construction[18] and as a source for iron.[3][4][5][5] Potential applications include the production of low cost concrete,[19] application to sandy soils to improve phosphorus cycling, amelioration of soil acidity, landfill capping and carbon sequestration.[20][21]

  • Cement manufacture, use in concrete as a supplementary cementious material. From 500,000 to 1,500,000 tonnes.[22][23]
  • Raw material recovery of specific components present in the tailings/residue: iron, titanium, steel and REE(rare-earth element)s production. From 400,000 to 1,500,000 tonnes;
  • Landfill capping/roads/soil amelioration – 200,000 to 500,000 tonnes;[24]
  • Use of the bauxite residue 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.[25] * use in building panels, bricks, foamed insulating bricks, tiles, gravel/railway ballast, calcium and silicon fertiliser, 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.[26]

Vedanta Aluminium Ltd has commissioned a red mud powder-producing unit at Lanjigarh refinery in Odisha, India, describing it as first of its kind in alumina industry tackling major environmental hazards.[27]

In 2015 a major initiative was launched in Europe with funds from the European Union to address the valorisation of bauxite residue, some 15 Ph.D students have been recruited as part the European Training Network for Zero-Waste Valorisation of Bauxite Residue,[28] the key focus will be the recovery of iron, aluminium, titanium and rare-earth elements (including scandium) while valorising the residue into building materials.

See also[edit]

References[edit]

  1. ^ a b Ayres, R. U., Holmberg, J., Andersson, B., "Materials and the global environment: Waste mining in the 21st century", MRS Bull. 2001, 26, 477. doi:10.1557/mrs2001.119
  2. ^ Annual statistics collected and published by World Aluminium.
  3. ^ a b c Schmitz, Christoph (2006). "Red Mud Disposal". Handbook of aluminium recycling. p. 18. ISBN 978-3-8027-2936-2. 
  4. ^ a b c Chandra, Satish (1996-12-31). "Red Mud Utilization". Waste materials used in concrete manufacturing. pp. 292–295. ISBN 978-0-8155-1393-3. 
  5. ^ a b c Mining, Society for; Metallurgy,; , Exploration U.S (2006-03-05). "Bauxite". Industrial minerals & rocks: commodities, markets, and uses. pp. 258–259. ISBN 978-0-87335-233-8. 
  6. ^ "Toxic Red Sludge Spill From Hungarian Aluminum Plant 'An Ecological Disaster'", David Gura, NPR, October 5, 2010
  7. ^ "Hungarian chemical sludge spill reaches Danube". BBC. 7 October 2010. 
  8. ^ K. Evans, E. Nordheim and K. Tsesmelis, "Bauxite Residue Management", Light Metals, 63–66 (2012).
  9. ^ G. Power, M. Graefe and C. Klauber, "Bauxite residue issues: Current Management, Disposal and Storage Practices", Hydrometallurgy, 108, 33–45 (2011).
  10. ^ B. G. Purnell, “Mud Disposal at the Burntisland Alumina Plant”. Light Metals, 157–159. (1986).
  11. ^ 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).
  12. ^ E. I. Robinsky, “Current Status of the Sloped Thickened Tailings Disposal System”, Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).
  13. ^ J. L. Chandler, “The Stacking and Solar Drying Process for disposal of bauxite tailings in Jamaica”, Proc. Int. Conf. Bauxite Tailings, Kingston, Jamaica (1986).
  14. ^ “Bauxite Residue Management: Best Practice”, published by World Aluminium the European Aluminium available from the International Aluminium Institute, 10 King Charles II Street, London, SW1Y 4AA, UK and on line from http://bauxite.world-aluminium.org/refining/bauxite-residue-management.html.
  15. ^ K. S. Sutherland, "Solid/Liquid Separation Equipment", Wiley-VCH, Weinheim (2005).
  16. ^ 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. doi:10.1016/j.resconrec.2006.03.003. 
  17. ^ Y. Pontikes and G. N. Angelopoulos "Bauxite residue in Cement and cementious materials", Resourc. Conserv. Recyl. 73, 53-63 (2013).
  18. ^ W.K.Biswas and D. J. Cooling, “Sustainability Assessment of Red SandTM as a substitute for Virgin Sand and Crushed Limestone”, J. of Ind. Ecology, 17(5) 756-762 (2013).
  19. ^ Liu, W., Yang, J., Xiao, B., "Review on treatment and utilization of bauxite residues in China", Int. J. Miner. Process. 2009, 93, 220. doi:10.1016/j.minpro.2009.08.005
  20. ^ "Bauxite Residue Management". bauxite.world-aluminium.org. The International Aluminium Institute. Retrieved 9 August 2016. 
  21. ^ Si, Chunhua; Ma, Yingqun; Lin, Chuxia (2013). "Red mud as a carbon sink: Variability, affecting factors and environmental significance". Journal of Hazardous Materials. 244-245: 54–59. doi:10.1016/j.jhazmat.2012.11.024. Retrieved 9 August 2016. 
  22. ^ Y. Pontikes and G. N. Angelopoulos "Bauxite residue in Cement and cementious materials", Resourc. Conserv. Recyl. 73, 53–63 (2013).
  23. ^ 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).
  24. ^ W. K. Biswas and D. J. Cooling, “Sustainability Assessment of Red SandTM as a substitute for Virgin Sand and Crushed Limestone”, J. of Ind. Ecology, 17(5) 756–762 (2013).
  25. ^ 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.
  26. ^ B. K. Parekh and W. M. Goldberger, “An assessment of technology for the possible utilization of Bayer process muds”, published by the U. S. Environmental Protection Agency, EPA 600/2-76-301.
  27. ^ "Vedanta commissions red mud powder plant in Odisha". Business Line. 19 November 2013. 
  28. ^ http://etn.redmud.org/project/

Additional References[edit]

Data on global production of aluminium and aluminium oxide. http://www.world-aluminium.org

B. K. Parekh and W. M. Goldberger, “An assessment of technology for the possible utilization of Bayer process muds”, published by the U. S. Environmental Protection Agency, EPA 600/2-76-301.

Wanchao Liu, Jiakuan Yang, Bo Xiao, “Review on treatment and utilization of bauxite residues in China”, in Int. J. of Mineral Processing, 93, 220–231 (2009).

M. B. Cooper, “Naturally Occurring Radioactive Material (NORM) in Australian Industries”, EnviroRad report ERS-006 prepared for the Australian Radiation Health and Safety Advisory Council (2005).

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).

W. K. Biswas and D. J. Cooling, “Sustainability Assessment of Red SandTM as a substitute for Virgin Sand and Crushed Limestone”, J. of Ind. Ecology, 17(5) 756–762 (2013).

Agrawal, K. K. Sahu, B. D. Pandey, "Solid waste management in non-ferrous industries in India", Resources, Conservation and Recycling 42 (2004), 99–120.

Jongyeong Hyuna, Shigehisa Endoha, Kaoru Masudaa, Heeyoung Shinb, Hitoshi Ohyaa, "Reduction of chlorine in bauxite residue by fine particle separation", Int. J. Miner. Process., 76, 1-2, (2005), 13–20.

Claudia Brunori, Carlo Cremisini, Paolo Massanisso, Valentina Pinto, Leonardo Torricelli, "Reuse of a treated red mud bauxite waste: studies on environmental compatibility", Journal of Hazardous Materials, 117(1), (2005), 55–63.

H. Genc¸-Fuhrman, J. C. Tjell, D. McConchie, "Increasing the arsenate adsorption capacity of neutralized red mud (Bauxsol™)", J. Colloid Interface Sci. 271 (2004) 313–320.

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.

H. Genc¸-Fuhrman, J. C. Tjell, D. McConchie, O. Schuiling, "Adsorption of arsenate from water using neutralized red mud", J. Colloid Interface Sci. 264 (2003) 327–334.

http://etn.redmud.org/project/

External links and further reading[edit]