Bauxite residue is a by-product of the Bayer Process.
Bauxite residue is primarily composed of the insoluble fraction of the bauxite ore that remains after extraction of the aluminium-containing components. Iron oxides (10 – 30%), titanium dioxide (2 – 15%), silicon oxide (5 – 20%) and undissolved alumina (0 – 20%) make up the residue, together with a wide range of other oxides which will vary according to the initial bauxite source. The high concentration of iron compounds in the bauxite gives the by-product its characteristic red colour, and hence its common name “Red Mud”.
Initially, the residue is washed, to extract as much valuable caustic soda and dissolved alumina as possible. The caustic soda is recycled back into the digestion process, reducing production costs and in turn lowering the alkalinity of the residue. The pH level of the residue is generally up to 13 or higher in some cases, due to the presence of alkaline sodium compounds, such as sodium carbonate and sodium hydroxide.
Like most ores and soils, bauxite can contain trace quantities of metals such as arsenic, beryllium, cadmium, chromium, lead, manganese, mercury, nickel and naturally-occurring radioactive materials, such as thorium and uranium. Most of these trace elements remain with the residue after extraction of the alumina.
After washing, the residue is contained in special facilities known as Bauxite Residue Disposal Areas (BRDA) or Residue Storage Areas (RSA).
The IAI’S dynamic material flow model indicates that, by 2050, there could be a bauxite residue global inventory of 10 billion tonnes.
Demand for aluminium is expected to grow, with supply coming from both primary and recycled sources. This means bauxite residue will continue to be generated and available for industrial symbiosis. Bauxite residue can be used as an alternative raw material in industrial processes especially where traditional materials may become scarcer.
The type of disposal employed by alumina refineries varies across the world, depending on factors such as land availability, technology availability, climatic & geographic conditions, logistics and regulatory requirements.
Companies are required to ensure that BRDAs comply with the respective environmental standards. Modern BRDA guidelines will include both general and location-specific design criteria such as soil conditions, earthquake risk, long term stability and management of storm events. Careful monitoring ensures structural integrity is maintained.
After being washed, the residue is filtered to produce a dry cake (> 65% solids). Drum filters have been used since the 1930s but there is now increasing use of press filters capable of achieving 70 to 75% solids. The dry residue material is carried by truck or conveyor to the storage site and stored without further treatment.
This method minimises the land area required for storage and the risk of leakage to groundwater. Rehabilitation and closure costs are greatly reduced and the material is in a more readily usable form. For sites with a constrained space this approach can often present the best option.
The residue is thickened to a high density slurry (48 – 55% solids or higher), deposited and allowed to consolidate and dry before successive layers are deposited. This forms a slope on the deposit, allowing rainwater to run off and minimising liquid stored in the disposal area; lowering risk of leakage and improving structural integrity.
The water reclaimed from the surface is pumped back to the plant to recover and recycle the soluble sodium salts. Dry stacked residue is often “under-drained” to improve the consolidation of the residue and recover further water for re-use in the refinery. The combination of dry stacking and a well drained deposits leads to a very stable deposit of residue.
The residue is pumped into land based ponds where naturally impervious layers or sealants minimise seepage. The residue is typically deposited as a dilute slurry, with the solids settling and consolidating over time and the surface water collected for return to the refining process. The design, construction and operation of these storage dams follow guidelines as set out in individual countries and undergo regular maintenance checks.
In some countries an early method of bauxite residue disposal was to transfer the material via pipeline to deep sea locations following treatment to reduce caustic soda levels. No new refineries have been built using this method since 1970.
The most important barrier to remediation, use and long term sustainability of bauxite residue management is its high alkalinity. Bauxite residues and the runoff from storage areas can be treated to reduce their pH, further reducing the risk of environmental impacts and facilitating rehabilitation of the disposal area after closure. There are a number of methods currently employed, including treatment with seawater or carbonation with CO2.
Bio-remediation, which aims to convert bauxite residue into a well structured soil, through chemical and physical treatment, is currently the focus of projects in Ireland, Australia, India and Jamaica. Bio-remediation is largely dependent on the characteristics of the bauxite residue and local climatic conditions. Treatments include reducing pH by carbonation, washing with large quantities of seawater, adding gypsum and other amendments such as bitterns (all of which replace sodium with more favourable elements such as calcium and magnesium) and increasing organic matter content by mixing in organic waste or establishing grass pasture to initiate the nutrient cycle. Critically, leaching of salts resulting from pH reduction must occur and drainage and disposal of saline leachate is essential. Supplementation of natural rainfall with irrigation may be needed to accelerate the process along with tillage and other physical treatments to improve structure and porosity.
Hundreds of patents have been issued and thousands of trials have been undertaken on different uses for bauxite residue, some of these applications have been commercialised but matching the tonnage arising annually (around 150 million tonnes) with possible commercial applications has been, and continues to be, a major challenge. The majority of patents filed have involved bauxite residue being used in the construction, building or agricultural industries. It is estimated that some two million tonnes is recycled annually as an input to cement production, refractories, soil amelioration and landfill covering.
The industry is constantly working on new residue treatment methods to increase the removal of alkaline fluids and salts.