Rijkeboer, A. and van der Meer, A.P.
Darling Range bauxites contain some 0.3 wt% of total organic carbon (TOC) of which some 60% is extracted into the Bayer process cycle. The accumulation of TOC in the liquor is the cause of a severe constraint in production capacity and operational flexibility with respect to alumina quality. Roasting of bauxite has been investigated as a potential method to reduce, and in the long run eliminate, the TOC content in Bayer liquors.
Bench scale roasting tests were carried out with Mt. Saddleback bauxite using a quartz tube of 40 mm diameter and 1 m length. Pre-heated air was used at gas velocities in the range 0.01 - 0.20 m/s. A typical heating rate of 100oC/minute was applied. Alumina extractions were obtained through digestion of the treated bauxite in a pure caustic solution and plant liquors at 1750oC.
We found that when roasting is carried out in the temperature range 400 - 6000oC the TOC removal is more than 90% efficient. A typical mass reduction of the bauxite of some 20% resulted, mainly due to the thermal decomposition of gibbsite and goethite. In order to maintain the alumina extraction efficiency at a level similar to that of unroasted bauxite it is preferred to operate a two stage roasting process. The first stage is operated at a temperature of 3500oC, with the subsequent stage (aiming at a lower partial water pressure) in the range 400 - 6000oC. In fact we found that under carefully selected process conditions a slight improvement in alumina extraction is obtained. Bench scale results were confirmed in a continuous fluidised bed roasting operation.
Evaluation of a number of other bauxites (Aurukun, Boké, Gove, Moengo, Pinjarra and Trombetas) showed that our roasting process does not or only marginally affects the amount of available alumina in these bauxites.
Due to the conversion of goethite to hematite a significant improvement of the red-mud settling rate was obtained. Moreover the filterability of the red-mud improved and a reduction in mud load is obtained.
Roasting of the bauxite results in a slight reduction in chemical soda consumption since reaction of soda with organic matter is prohibited. The bauxites evaluated contained 3 to 11% silica of which 1.2 to 3% is of the reactive type. We found that the selected roasting conditions do not additionally affect the chemical soda consumption. Lime consumption is reduced since conversion of TOC to carbonate is eliminated. Considerations on effects of TOC reduction on yield increase and alumina quality are included.