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 04 wastes from physical and chemical processing of non-metalliferous minerals
- 01 04 10 dusty and powdery wastes other than those mentioned in 01 04 07
The red mud is the by-product of the Bayer process which extracts alumina by caustic digestion (with NaOH) of crushed bauxite at high temperature and pressure in an autoclave, followed by clarification, precipitation, washing and finally calcination to produce pure anhydrous alumina. The red mud is the solid wet residue after separation. Its red colour comes from its iron-oxide content. In spite of separation, the red mud still contains NaOH for which reason the pH of the wet (fresh) red mud is highly alkaline. After the Ajka red mud spill on October 4, 2010 an area of 1000 hectars land was flooded. Six months following the spill in the area between Devecser and Somlóvásárhely the red mud layers thicker than 5 cm were excavated/removed together with 30 cm soil from underneath and were piled in a temporary storage area. In case of a red mud layer thinner than 5 cm, the red mud was mixed into the flooded soil.
The composition of the red mud varies function of the mined bauxite residue. The concentration of the chemical substances shown in the datasheet were determined by the members of the MTA ATK TAKI (Institute for Soil Sciences and Agricultural Chemistry of the Hungarian Acadamy of Science).
- 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 of red mud: 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. Mixed with soil it is a soil-like material. Its Al, Fe, Si content influences soil weathering. Its alkaline pH increases the pH of acidic soils.
Arany number: 51
Red mud and soil mixture (RMSM) may be used as surface layer of landfill cover systems at landfill sites, as demosntrated by Ujaczki et al, 2016 study.
To characterize and evaluate the applicability of the RMSM as additive to the surface layer of the landfill cover system at the municipal solid waste deposit in Gyál (A.S.A.Hungary) Ujaczki et al, 2016 conducted a field study in two steps: in lysimeters and in field plots.
Ujaczki et al. (2015) in previous microcosm studies on the potential utilization of the Ajka red mud (main components: Fe, Ti and Al oxides and hydroxides, pH = 10–12) as soil ameliorant found that the Ajka red mud may be mixed into soil at up to 5% w/w without any mid-term adverse effects on the natural habitat of the soil. Based on this finding, it was assumed that the RMSM might be applicable also as landfill surface cover.
Ujaczki et al (2016) mixed RMSM at ratios ranging between 0 and 50% w/w with low quality subsoil (LQS) originally used as surface layer of an interimlandfill cover. The characteristics of the LQS + RMSM mixtures compared to the subsoil (LQS) and the RMSM were determined by physical–chemical, biological and ecotoxicological methods. The addition of RMSM to the subsoil (LQS) at up to 20% did not result any ecotoxic effect, but it increased the water holding capacity. In addition, the microbial substrate utilization became about triple of subsoil (LQS) after 10 months. According to the results the RMSM mixed into subsoil (LQS) at 20% w/w dose may be applied as surface layer of landfill cover systems.
1) Ujaczki, É., Klebercz, O., Feigl, V., Molnár, M., Magyar, Á., Uzinger, N., et al., 2015. Environmental toxicity assessment of the spilled Ajka red mud in soil microcosms for its potential utilisation as soil ameliorant. Period. Polytech., Chem. Eng. http://dx.doi.org/10.3311/PPch.7839.
2) Ujaczki, É., Feigl V., Molnár, M., Vaszita E., Uzinger, N., Erdélyi A., Gruiz, K. (2016) The potential application of red mud and soil mixture as additive to the surface layer of a landfill cover system: field-study, Journal of Environmental Sciences, 44, 189–196. doi: 10.1016/j.jes.2015.12.014
The Environmental Microbiology and Biotechnology research group of BME-ABÉT have conducted biological-ecotoxicity tests with red-mud flooded soils of various pH. The tests include aerobic heterotrophic cell counts, substrate utilization with BIOLOG Ecoplate, Aliivibrio fischeri bioluminescence inhibition tests, Sinapis alba seed germination test.
10-20% red mud mixed into the soil had stimulating effect on fungi and bacteria after 3 months. The red mud proved to be toxic in the Aliivibrio fischeri bioluminescence inhibition test only at 100% red mud proportion after 5 months. According to the Sinapis alba test seed germination inhibition decreased by the 5th months and the inhibition exceeded 50% only in the microcoms containing 100% red mud.
The ecotoxicity tests performed by Ujaczki et al, 2016 on leachate and soil showed that 20% w/w RMSM had no adverse effect on the following testorganisms: S. alba, T. aestivum, A. fischeri and F. candida during the experiments (10 months).
Assessing all the environmental toxicity test results Ujaczki et al, 2016 concluded that 20% RMSM application did not have any harmful effect on the subsoil (LQS).
According to the Bayer technology, applied also by the Ajka plant, the aluminium content of the bauxite was separated by NaOH, for which reason the remaining red mud residue was highly alkaline. The spilled alkaline red mud (pH: 12) removed from the soil surface together with the soil, resulted a red mud-soil mixture with a lower pH (Ujaczki et al, 2016). However due to its NAOH content, the Na ion concentration was still high, therefore the suspected harmful effect is soil salinisation. The As, Cr, Hg and Ni content of the studied Ajka red mud-soil mixture exceeds the limit values set by the Hungarian regulation (Government Decree No. 50/2001 IV. 3. on the rules and handling of wastes and sludge in agriculture). However, when adding this red mud-soil mixture to soil the toxic effect and the limit value exeedance does not occur (Ujaczki et al, 2016).