Bauxite tailings

"Red mud" storage facility in Stade, Germany. The aluminium industry generates over a hundred million tons of this waste annually.

Bauxite tailings, bauxite residue or alumina refinery residues (ARR) is a by-product in the production of alumina (aluminium oxide). Alumina is the principal raw material used in the manufacture of aluminium metal and also widely used in the manufacture of ceramics, abrasives and refractories. 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.^[1]

Production

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^oC 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

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 compositional range can vary widely but is typically: Fe₂O₃ 5 - 60%, Al₂O₃ 5 - 30%, TiO₂ 0.3 - 15%, CaO 2 - 14%, SiO₂ 3 - 50% and Na₂O 1 - 10%.

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.

Mineralogically, the components present are: Sodalite 3Na₂O.3Al₂O₃.6SiO₂.Na₂SO₄) 4 - 40%; Aluminous-goethite (aluminous iron oxide) 10 - 30%; Haematite (iron oxide) 10 - 30%; Silica, crystalline & amorphous 5 - 20%; Tricalcium aluminate (3CaO.Al₂O₃.6H₂O) 2 - 20%; Boehmite (AlO(OH)) 0 - 20%; Titanium Dioxide 2 - 15%; Muscovite (K₂O.3Al₂O₃. 6SiO₂.2H₂O) 0 - 15%; Calcium carbonate 2 - 10%; Gibbsite (Al(OH)₃) 0 - 5%; Kaolinite (Al₂O₃. 2SiO₂.2H₂O) 0 - 5%.

Residue Storage Areas (RSA)

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.^[2]

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.^[3] As residue storage space ran out and concern increased over wet storage, since the mid-1980s dry stacking has been increasingly adopted.^{[4] [5] [6] [7]} 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.^[8]

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.^[9] 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.

Uses

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. Possible applications can be broadly broken down into various categories: recovery of specific components present in the tailings/residue, e.g. iron, titanium, rare earth elements REEs; use as a major component in manufacture of another product, e.g. cement; use of the bauxite residue as a component in building or construction material, e.g. concrete, tiles, bricks; soil amelioration or capping; and conversion of the residue to a useful compound, such as through the Virotec process.

The wide compositional range of the residue has resulted in an enormous number of technically feasible applications including: cement manufacture, use in concrete as a supplementary cementious material, iron recovery, titanium recovery, use in building panels, bricks, foamed insulating bricks, tiles, gravel/railway ballast, soil amelioration, 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, soil amelioration.^[10]

It is estimated that some 2 to 3.5 million tonnes of the bauxite residue produced annually is used in some way:

• Cement – 500,000 to 1,500,000 tonnes;^[11][12]
• Raw material in iron and steel production – 400,000 to 1,500,000 tonnes;
• Landfill capping/roads/soil amelioration – 200,000 to 500,000 tonnes;^[13]
• Construction materials (bricks, tiles, ceramics etc.) – 100,000 to 300,000 tonnes;
• Other (refractory, adsorbent, acid mine drainage (Virotec), catalyst etc.) – 100,000 tonnes.^[14]

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.^[15] The key focus will be the recovery of iron, aluminium, titanium and rare earth elements (including scandium) while valorising the residue into building materials.

References

Additional References

• “Bauxite Residue Management: Best Practice”, 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

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/