Abstract

Bauxite residue (red mud) is produced in large amounts in alumina refineries as a waste product of the Bayer process. This material has the potential to be used as a soil liming agent due to its high pH and high acid neutralisation capacity. Soil incubation experiments compared red mud from several Australian and overseas refineries to estimate its liming effect relative to lime (CaCO₃). Two acid (pH 4.3, 4.6) sandy soils were mixed with 8 rates of 17 red muds, 3 limes, and NaOH. Values of EC and pH were measured at 1, 4, and 16 weeks incubation. Each red mud produced a different buffering curve when added to soil, although the shapes of curves could be classified into 5 behavioural groups. The liming equivalent of red mud decreased with increasing target soil pH. For a target pH of 6 the lime equivalent of the red muds was 11–42% for soil 1 and 13–50% for soil 2. No single characteristic of red mud could be used to accurately predict the liming equivalent, as its liming effect is due to several, variable constituents (e.g. free caustic, sodalite, calcite). The pH reached for a certain rate of addition of red mud to soil can be estimated from a comparison of the buffering (titration) curves of both the soil and the red mud. In every case, CaCO₃ had a much larger liming capacity than red mud and red mud may not be an economical alternative to lime except where other benefits are associated with its use (e.g. lower cost, decreased P leaching, reduced water repellence).