Budapest University of Technology and Economics, Department of Applied Biotechnology and Food Science, Environmental Microbiology and Biotechnology Group
- 01 WASTE RESULTING FROM EXPLORATION, MINING, QUARRYING, AND PHYSICAL AND CHEMICAL TREATMENT OF MINERALS
- 01 03 wastes from physical and chemical processing of metalliferous minerals
- 01 03 09 red mud from alumina production other than the wastes mentioned in 01 03 07
The Ajka red mud resulted from the Bayer process. The Bayer process is the primary method by which alumina (Al2O3)is produced from bauxite ore. In this hydrometallurgical process, caustic soda digestion under elevated temperature and pressure is used to leach soluble alumina minerals from the bauxite ore and subsequently precipitate technically pure aluminum hydroxide. From the pregnant leach solution, the residual mineral matrix is removed as a byproduct, commonly termed as bauxite residue or “red mud”. (Gräfe, M., Klauber, C., 2011. Bauxite residue issues: IV. Old obstacles and new pathways for in situ residue bioremediation. Hydrometallurgy 108, 46–59. http://dx.doi.org/10.1016/j.hydromet.2011.02.005.)
The major steps in alumina refining are as follows:
• Wet grinding of the bauxite ore in rod mills, ball mills, or semiautogenous grinding mills to produce fine slurry.
• Digestion of the slurry by caustic soda (NaOH) at strengths exceeding 170 g/L in vessels under pressure at temperatures ranging from 145 to 265°C depending on the type of bauxite being processed.
• Separation and washing of insoluble residues (termed “sand and mud”) from the process solution (termed “green liquor”). Residues are stored in residue drying areas.
• Crystallization (precipitation) of aluminum trihydrate from the sodium aluminate (NaAlO2) of green liquor, leaving “spent liquor” containing primarily caustic soda to be returned to digestion.
• Calcination of the aluminum trihydrate crystals at approximately 1000°C, driving off water molecules and leaving the final product of anhydrous alumina.
The red mud is the insoluble, wet, solid residue after separation by fitration of the process solution. Due to a novel technology the high water content of this residue is reduced from 80% to 30 % by filtration at high pressure resulting a muddy, solid material.
After separation the filtered residue contains also NaOH, for which reason the wet (fresh) red mud has a highly alkaline pH. Gypsum (3%) was added to the dry red mud in order that the exchangable Na ions get exchanged by Ca ions, thus reducing the risk of salinisation. The concentration of the chemical substances shown in the datasheet were determined by the members of the Environmental Microbiology and Biotechnology Research group of BME-ABÉT and MTA ATK TAKI (Institute for Soil Sciences and Agricultural Chemistry of the Hungarian Acadamy of Science).
The soil limit values are regulated by Government Decree No. 50/2001 (IV. 3.) on the rules and handling of wastes and sludge in agriculture
- Metals, semi-metals and their compounds
- aluminium
- Metals, semi-metals and their compounds
- arsenic
- Metals, semi-metals and their compounds
- boron
- Other inorganic chemical compounds
- barium
- Other inorganic chemical compounds
- calcium
- Metals, semi-metals and their compounds
- cadmium
- Metals, semi-metals and their compounds
- cobalt
- Metals, semi-metals and their compounds
- chromium
- Metals, semi-metals and their compounds
- copper
- Metals, semi-metals and their compounds
- iron
- Metals, semi-metals and their compounds
- mercury
- Other inorganic chemical compounds
- magnesium
- Metals, semi-metals and their compounds
- manganese
- Metals, semi-metals and their compounds
- molybdenum
- Other inorganic chemical compounds
- sodium
Main components: iron-, aluminium-, silica-, titanium-, sodium- and calcium-oxides, and heavy- and light metals: Na, K, Cr, V, Ni, Ba, Cu, Mn, Pb, Zn, etc, radioactive daughter elements . Its composition depends on the bauxite it originates from and on the processing technology.
Red mud is a paste-like, wet, red-coloured material made up of colloid particles stronly retaining water, having speciel rheological properties. After drying it becomes powder and mixed with water it will not regain its plasticity. Originally the rud mud is watertight, but saturated with water, its watertightness is lost. Its volume is changing depending on the temperture and rain conditions.
Arany-féle kötöttség K(A)=40
BME's Environmental Microbiology and Biotechnology research group has studied the biological and ecotoxicity effects of the Ajka red-mud from dry stacking as soil additive. The aim of the test was to determine the red mud dose without adverse effects on soil biota.
The effects were monitored by an integrated methodology including physicochemical methods (pH, metal content after Aqua-Regia extract, moisture content, lime content, water-holding capacity), biological methods (aerobic heterotrophic cell counts, Biolog Eco-plate test), ecotoxicity tests (Sinapis alba germination inhibition test, Collembola (Folsomia candida) lethality test, Aiivibrio fischeri bioluminescence inhibition test).
According to the tests, the studied red mud sample did not exceed the limit values according to Government Decree No. 50/2001 (IV. 3.). The lime content of the soil increased with red mud addition to the soil. Soil water holding capacity increased due to the fine particles of the red mud. According to the biological tests 5% red mud increased the activity of soil microflora and it is not toxic to soil biota according to the ecotoxicty tests.
Hazardousness of red mud may be associated with its: high toxic metal content, radioactivity, alkaline pH, Na-ion content (salinization) and grainsize influencing soil microbiological activity. In case of red mud from dry stacking the alkaline pH and the Na-ion content (salinization) may pose risk, while its metal content and radioactivity may not.