With The Energy and Resources Institute (TERI) in New Delhi, the Scottish researchers discovered that red mud, which is waste from an intermediate stage in the processing of bauxite into aluminium, could be carbonized to make it safer. That carbonized red mud could then be used to remove heavy metals from water. The red color is due to a high concentration of iron oxide in the residue.
Dr. Justin Hargreaves, senior lecturer in chemistry at the University of Glasgow, said:
We’ve been working with our partners in India since 2008 to examine ways in which red mud could be treated to make it safer. We’ve also found that this dangerous waste material could be reconfigured to be used to remove metals such as lead and copper from water.
He explains that they are using samples of red mud as a catalyst to crack methane, which liberates hydrogen from the substance. When the hydrogen is freed from iron oxide in the mud, it transforms to iron metal or iron carbide, both of which are coated with carbon. Because the material is magnetic, when it is added to water contaminated with metals, it can attract those contaminants and remove them from the water.
Red mud is the waste that occurs after raw bauxite ores are subjected to the Bayer process, a chemical transformation that turns it into alumina. (Alumina is then smelted to make aluminum.) Every ton of alumina produced generates between one and two tons of red mud, according to “Light Metals 2011” by The Minerals, Metals & Materials Society (TMS).
Some firms use proprietary names to refer to the residue. The slurry left behind contains silica, aluminum, iron, calcium, titanium, and various other chemicals. It is also extremely alkaline with a typical pH greater than 13, which means is it classified as hazardous waste according to the Basel Convention.
The most common disposal method, according to the Red Mud Project, is to pump the slurry into a dammed pit area where it sits in a clay-lined holding pond until it has dried. The option is much improved from early disposal, which involved dumping the residue into nearby rivers or the seam, yet concerns remain about the extent to which these holding ponds might affect the groundwater and environment as there is a possibility of heavy metals leaching into the soil. This did prompt some changes in slurry pit or reservoir construction; more recently constructed ponds use a polymeric membrane as well as clay lining.
Still, this means of disposal is not ideal. In 2010, a red mud reservoir at the Ajka Alumina Plant in western Hungary failed, spilling a million cubic meters of red mud into the area.
Ultimately, 10 people were killed and more than 150 people were injured. The damage in the surrounding communities — the village of Kolontar and the towns of Devecser and Somlovasarhely — has reportedly cost the government $166 million for cleanup and reconstruction, according to the Pulitzer Center on Crisis Reporting. Although the plant now uses a different disposal method, it has increased dust pollution and releases 35,000 extra tons of carbon dioxide into the air per year.
The disaster underscored the urgency of finding alternatives for disposing of red mud. The problem is that many solutions discovered aren’t sufficiently economically viable to interest industry. And yet, continuing to muck about with red mud isn’t economically feasible either. In Australia alone, an estimated AUD80 million is spent annually on managing red mud.
In addition to this latest work from the University of Glasgow, research into red mud reuse has included investigations into its use to remove sulfur compounds from kerosene oil, as a pigment in anticorrosive marine paints, and it has been used as a constituent in cement and brick making in India, China and Japan, according to the Red Mud Project. It is also being considered as a component in roofing tiles in Greece.
The material we’ve been working with has only been a few grams at a time, and we don’t know whether this process can be reproduced on a large scale. The chemical composition of red mud can vary greatly from source to source, which means it’s hard to make any definitive statements at this early stage. We’re still some way away from scaling the process up to industrial levels but we’re keen to further explore the possibilities we’ve uncovered.
The research, funded by the UK-India Education and Research Initiative (UKIERI), has been published in the Journal of Environmental Management.
Image of Weipa bauxite mine by Cape-york-australia, used under its Creative Commons license.
Image of alumina plant accident, Devecser, Hungary, by Közigazgatási és Igazságügyi Minisztérium, Kormányzati Kommunikációért Felelős Államtitkárság/Ministry of Public Administration and Justice, Ministry for Government Communication, is used with permission.