A guest post by C&EN’s European correspondent, Sarah Everts.
The toxic red sludge from alumina refining that is causing what might be Hungary’s worst ever environmental catastrophe turns out to have a potpourri of unusual applications currently under development. In particular, because some 84 factories around the world produce 70 million tons of the stuff ANNUALLY (approximately 2 tons for every ton of alumina), researchers are trying to find alternate usages for the voluminous waste.
For example, Justin Hargreaves and his collaborators at the University of Glasgow, in Scotland, are floating methane, another industrial byproduct, over red mud to produce magnetic aluminum carbide and iron carbide. They are hoping that the stuff can help clean up drinking water (removing chromate or arsenic, for example) and then be removed from solution using a magnet. The project is of course contingent on making sure the heavy metals already in the red mud are contained in the aluminum carbide and iron carbide material. The idea is also contingent on not having radioactive elements in the red mud—something that is entirely dependent on where the stuff was mined in the first place.
Others such as Marcel Schlaf at the University of Guelph, in Canada, are trying to use red mud as a catalyst “for the upgrading of bio-oil.”
There’s also talk of using the red mud to make ceramics, such as roofing tiles and bricks, says George Angelopoulos at the University of Patras in Greece. “Red mud is already used as raw material by one cement factory in Greece,” Angelopoulos says.
Angelopoulos supplied a lot of facts about red mud that didn’t end up in my news story due to space restrictions, some of which I will post verbatim below for those wanting to get nerdy on the chemical processing:
Red mud is the waste produced during the digestion of the bauxite with sodium hydroxide (NaOH) for the production of alumina (Al2O3), which is primary used as a feedstock for the aluminium industry. In the case of Hungary, bauxite is mined at the open pits of Bicske, Obarok and Halimba III underground mines. Bauxite is processed in the alumina plant of Ajka.
Bayer (Karl Bayer 1888) is the main process for the production of alumina. The digestion of bauxite with the addition of sodium hydroxide is performed at temperatures from 140 to 260 °C under high pressure around 35 atm. The process generally follows five steps: bauxite preparation (beneficiation), digestion, clarification/settling, precipitation and calcination. Red mud is produced in the third step of clarification/settling.
Red mud is a high alkaline, ionic slurry containing around 300 to 500 g/L solid. Its pH varies from 9.5 to 12.5. Its alkalinity is imposed from the production process as described roughly above. It is a complex material whose chemical and mineralogical composition varies widely, depending upon the source of bauxite and the technological process parameters. It contains mainly Fe2O3 (20 to 50%), Al2O3 (20 to 30%), SiO2 (10 to 20%), CaO (10 to 30%), Na2O (10 to 20%) and TiO2 (3 to 10%) and small quantities of numerous minor/trace elements as oxides mainly, such as V, Ga, Cr, P, Mn, Cu, Cd, Ni, Zn, Pb, Mg, Zr, Hf, Nb, U, Th, K, Ba, Sr, rare earth elements, etc. Moreover it may contain about 7 radionuclides such as U, Ra, Th, K, Cs, etc and about 5 anions, fluoride, phosphate, chloride, nitrate and sulphate.
Update: Sarah expanded on this post a bit at Recycling Red Mud.