The Hungarian caustic sludge spill last week has gathered international attention. While we wonder if 2010 should be remembered as the year of the spill, let’s just see how red mud can be handled to reduce reliance on stockpiling and storage, which has unfortunately shown its limits.
Red mud is produced during the transformation process of bauxite into alumina during the Bayer process. About 1.5 to 2 tonnes of red mud is generated for every tonne of alumina produced. This residue is highly alkaline (high pH, that is) and contains traces of various metal substances and other minor or trace amounts of heavy metals and radioactive isotopes.
It has been estimated that in 2000, the global inventory of bauxite residue stood at about 2 billion tonnes and is likely to reach 4 billion tonnes by 2015 unless improved means of storage, rehabilitation and re-use options are developed in large scale.
Current Means of Disposal
For now, red mud is often stored on land for future rehabilitation or uses (with or without neutralising its toxicity). This is done through either lagooning or dry stacking and accounts for 90% of alumina refineries surveyed by this study. Red mud can also be discharged into mid-sea. This accounts for less than 10% of cases surveyed by the study. The practice has steadily declined steadily since the 60s. The move towards dry stacking has been economically motivated by the need to reduce the land required for storage, minimise risks of caustic liquor release while maximising the recovery of liquor for the refinery.
The next big thing has a fancy name. Hyperbaric steam filtration is an emerging technology which discharges residue as a dry, granular material of low soda content. This gives red mud properties that allow long term storage, remediation and re-use. This method is not economically viable at the moment but is undergoing trials in refineries in Germany, Japan and Venezuela.
Re-using red mud into new applications would solve the problems arising from its disposal and would create economic incentives to treat its toxicity. Since the 60s, several hundreds of researches have sought new end uses for red mud but emissions of radon coming from the mud as well as the low value of materials that red mud could be substituting have hindered its economic viability. Red mud can be used in metallurgy (for the recovery of iron, vanadium, chromium, rare earths and titanium dioxide), catalysis, and soil remediation.
The most promising uses would be in construction materials such as cement, bricks, roofing tiles and glass ceramics. A few examples include the Jawaharlal Nehru Aluminium Research Design and Development Centre’s work on ceramics and Jamaica’s National Work’s Agency’s reliance on geopolymers in order to use red mud in road construction. Other uses can be found here.