‘Red Mud’ in alumina production and how is it handled

Alumina production employs the Bayer’s process which leaves behind a residue known as Red Mud. If the alumina extraction is done through any other process then the residue left behind is known as White Mud or Bellite. Over 95% of the alumina manufactured globally is derived from bauxite by the Bayer process.

For more on the Bayer’s Process: http://redmud.org/red-mud/production/

Red mud near Stade (Germany)

Over 90% of the alumina produced in the world is used to manufacture aluminium metal. This is termed as metallurgical or smelter grade alumina (SGA). The rest is termed non-metallurgical grade alumina (NMGA). Red mud is composed of a mixture of metallic oxides. The bauxite filtrate primarily consists of iron oxides, silicon oxide, titanium oxide and un-dissolved alumina together with a wide range of other oxides. This depends upon the place of extracted bauxite. The iron oxides, which comprise up to 60% of the mass of the sludge impart the red colour. The mud is highly basic with a pH ranging from 10 to 13.

As caustic soda is the major compound consumed in alumina production, a key focus of alumina refineries is to maximize the recovery of the valuable caustic from the residues in order to reuse it during the extraction process.

There are generally a few techniques to handle this effluent through:

• Seawater Discharge – This was one of the primitive methods of disposal in some costal countries. The residue was transferred via pipeline to deep sea locations following treatment to reduce caustic soda levels. This method has been abandoned since the ‘70s.
• Dry Disposal - After being washed, the residue is filtered to produce a dry cake and passed through press filters capable of achieving around 75% solid mass. The dry residue material is carried to the storage site and stored without further treatment. This method reduces the land area required for storage and the seepage to groundwater. Closure costs are greatly reduced and the material is in a more readily usable form.
• Lagooning/Ponding - The residue is usually dumped as a thinned slurry, with the solids settling and consolidating over time. The surface water is then collected and returned to the refining process. The residue is pumped into land based ponds where naturally impervious layers or sealants curtail seepage.
• Mud/Dry Stacking or “Sloped Deposition” – This process involves the thickening of the sludge to high density (> 48 - 55% solids), deposited and then allowed to consolidate and dry before successive layers are deposited. This forms a slope on the deposit, allowing rainwater to run off and minimizing liquid stored in the disposal area. It drops the risk of leakage and improves the 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. This combination leads to a very stable deposit of residue.

Utilization of Bauxite Residue:

he majority of patents filed for re-usability of the Red Mud have involved bauxite residue being used in the construction, building or agricultural industries. It is estimated that approx. 2 million tonnes is recycled annually as an input to cement production, refractories, soil amelioration and landfill covering.

Vedanta Aluminium Ltd. has commissioned a red mud powder unit at Lanjigarh refinery in Odisha, describing it as first of its kind in alumina industry tackling major environmental hazards. This will have advantages like savings in caustic consumption by 10-15 kg per tonne of alumina, minimizing land requirement by 50 to 60 per cent, and doing away with wet red mud storage thereby eliminating environmental hazards.

Vedanta Aluminum has adopted the highest international standards for its red mud disposal by adopting techniques similar to those developed by Toronto University for tailing disposal from other site in the metals and mining industry. This technique eliminates the risk of dyke wall failures due to hydraulic pressure or any overflow due to non retention of water inside the dyke. The red mud pond in Lanjigarh has been designed and certified by the Indian Institute of Science, Benguluru, the most reputed institute in India with a world-class experience in designing disposal sites around the globe.

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