Bauxite tailings, also known as bauxite residue, red mud, red sludge, or alumina refinery residues (ARR), 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). It is important to differentiate between the alkaline waste generated in the Bayer process and the tailings generated during the mining of bauxite. This article specifically covers the alkaline waste generated when operating the Bayer process. 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 2018 was approximately 126 million tonnes resulting in the generation of over 160 million tonnes of bauxite tailings/residue.
Red mud, or bauxite residue, 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.
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. 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 1000 °C in rotary kilns or fluid flash calciners to produce aluminium oxide (alumina).
The alumina content of the bauxite used is normally between 45 - 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 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 and safer to handle and store.
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. In addition to iron, the other dominant components include silica, unleached residual alumina, and titanium oxide.
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:
Mineralogically expressed the components present are:
|Chemical name||Chemical formula||Percentage composition|
|Aluminous-goethite (aluminous iron oxide)||α-(Fe,Al)OOH||10–30%|
|Hematite (iron oxide)||Fe2O3||10–30%|
|Silica (crystalline & amorphous)||SiO2||5–20%|
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.
Discharge of red mud is hazardous environmentally because of its alkalinity, though it should be stressed that all disposal to rivers or sea has now stopped.
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. 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. However, the long-term environmental effects of the spill have been very minor.
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, while for some operations valleys were dammed and the tailings deposited in these holding areas.
It was once common practice for the bauxite residue/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.
As residue storage space ran out and concern increased over wet storage, since the mid-1980s dry stacking has been increasingly adopted. 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.
An increasingly popular treatment process is filtration whereby a filter cake (typically resulting in 26 - 29 % 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. 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.
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.
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. 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. An estimated 2 to 3 million tonnes are used annually in the production of cement, road construction and as a source for iron. Potential applications include the production of low cost concrete, 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 sulfo-aluminate cements have been extensively research and well documented.
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 were recruited as part the European Training Network (ETN) for Zero-Waste Valorisation of Bauxite Residue. The key focus will be the recovery of iron, aluminium, titanium and rare-earth elements (including scandium) while valorising the residue into building materials.
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