Bauxitmaradvány (gyakran nevezik „vörösiszapnak”)

Bauxite residue (often referred to as “red mud”)

Bauxite residue is a by-product of the Alumina refining process, a chemical process for producing alumina (aluminium oxide), the raw material for primary aluminium production as well as for chemical applications. The Bayer process is the common process used, however, other processes such as Sintering are in use in some Refineries particularly in China.

Bauxite ore is mined and then transferred to an alumina refinery for processing.  To extract the alumina, the soluble part of the bauxite ore is dissolved using caustic soda.  The insoluble part of the bauxite (the residue) is removed and the alumina component is then precipitated.  The product is then calcined (heated) to produce aluminium oxide (alumina).  The high concentration of iron compounds in the residue gives the product its characteristic red colour.  A residual amount of the caustic soda used in the process remains with the bauxite residue causing the material to have a high pH/alkalinity.

Maximising the separation of the caustic soda from the bauxite residue maximizes the recovery of alumina and allows the caustic soda to be recycled back into the Bayer Process as part of an efficient closed loop system, reducing production costs and lowering the alkalinity of the residue.

After washing of the residue to extract as much caustic soda and dissolved alumina as possible, the residue is stored in special facilities known as bauxite residue disposal areas (BRDA) or residue storage areas (RSA). 

The main constituents of bauxite residue
Bauxite residue composition varies by region (depending on the qualities of the ore), but typically contains the following components, in roughly the proportions listed:

Components	CAS Registry No.	Percent
Sodalite 3Na2O.3Al2O3.6SiO2.Na2SO4)		4 - 40
Aluminous-goethite                        (aluminous iron oxide)		10 - 30
Haematite (Iron oxide)	1317-60-8	10 - 30
Silica, crystalline & amorphous	14808-60-7;            7631-86-9	5 - 20
Tricalcium aluminate (3CaO.Al2O3.6H2O)		2 - 20
Boehmite [Al(OH)03]	1318-23-6	0 - 20
Titanium Dioxide	13463-67-7	2 - 15
Muscovite (K2O.3Al2O3. 6SiO2. 2H2O)	1318-94-1	0 - 15
Calcium carbonate	1317-65-3	2 - 10
Gibbsite [Al(OH)3]	21645-51-2	0 - 5
Kaolinite (Al2O3. 2SiO2.2H2O)	1318-74-7	0 - 5

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 stay with the residue.   The pH level 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. 

Bauxite Residue Disposal Areas (BRDA)
The type of BRDA employed by alumina refineries varies across the world, depending on factors such as land availability, technology availability, climatic & geographic conditions, logistics and regulatory requirements.

Until the 1970s bauxite residue was mainly disposed to deep sea or to wet (lagoon or pond) containment on land.  As environmental requirements and technology have developed there has been a significant trend away from these forms of disposal to the situation today, where dry stacking at land-based facilities is the preferred method.

Companies are required to ensure that BRDAs comply with the respective environmental standards.  Modern BRDA guidelines will include both general and locational specific design criteria such as soil conditions, earthquake risk, long term stability and management of storm events.  Careful monitoring ensures structural integrity is maintained.

Types of Bauxite Residue Disposal

Seawater Discharge
In some countries an early method of bauxite residue disposal was to transfer the material via pipeline to deep sea locations.  The residue is generally treated to reduce caustic soda levels prior to disposal.  No new refineries have been built using this method since 1970.

Lagooning/Ponding
Bauxite 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.

Dry Stacking or ‘Sloped Deposition’ 
Bauxite residue is thickened to a high density slurry (48-55% solids or higher) by advanced thickener and flocculation technologies at the alumina plant.
In these processes, the residue slurry is deposited and allowed to consolidate and dry before successive layers are deposited. This forms a slope on the deposit, allowing rainwater to run off, minimising liquid stored in the disposal area. The water reclaimed from the surface is pumped back to the plant to recover 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 in a well drained deposits leads to a very stable deposit of residue.

Dry Disposal
Bauxite residue is filtered to form a dry cake (>65% solids) and can be washed with either water or steam to reduce alkalinity before being transported and stored as a semi dried or dried material. 

Remediation/Neutralisation

Bauxite residues and the runoff from storage areas can be treated to reduce their alkalinity.  This further reduces the potential for any environmental impacts associated with the stored residue and makes  it easier for the disposal area to be rehabilitated  once closed.  There are a number of neutralisation methods currently employed:

Seawater
Seawater neutralises the pH levels of the residue lowering environmental risks.  After neutralisation the reduced pH of the bauxite residue helps with the subsequent re-vegetation of the BRDA.  The runoff or “decant liquor” can also be neutralised with seawater. 

CO₂
Bauxite residue can be treated with the addition of CO₂, lowering the pH of the residue and at the same time “sequestering” the greenhouse gas.  The pH of both the residue and decant liquor can be significantly reduced.  Drying time is reduced allowing stacking to be undertaken more quickly thus reducing the land area required.  The mechanical strength of the residue is increased and the potential for dust emissions is reduced.

Emerging Technology

The industry is constantly working on new bauxite residue treatment methods to increase the removal of alkaline fluids and salts.  For example, hyperbaric steam filtration has been successfully tested in a number of plants.  Many companies now have R&D teams specialising in bauxite residue storage. Research into neutralisation using new methods (e.g. bioremediation) is currently being undertaken with funding from individual companies and the International Aluminium Institute.

 

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