Thermogravimetric study of the dehydration and reduction of red mud

Abstract

The processes of drying and reduction of red mud in the pure state and with coal additions in vacuum or in gaseous media (helium, hydrogen) have been experimentally studied by thermogravimetry using a Setaram TAG24 thermogravimetric analyzer. The minimum total weight loss (∼20%) is observed for red mud samples without additives in forevacuum, and the maximum loss (∼38%) is detected in samples with coal. It is demonstrated that, for this type of red mud with iron oxide Fe₂O₃, water molecules are bonded in the form of iron hydroxide Fe₂O₃ · 3H₂O rather than goethite FeOOH. The peak of magnetite formation is observed in differential thermogravimetry (DTG) curve in the range 270–400°C. The simulation of the magnetite dehydration and formation rates under experimental conditions in the relevant temperature ranges agrees with the experimental data. A peak of wustite formation in hydrogen above ∼600°C is recorded in a DTG curve, and the removal of one-third of sodium oxide, which is likely not to be fixed into strong sodium alumosilicate, is observed in the range 800–1000°C. The peak detected in the DTG curve of the mud with charcoal in helium in the range 350–450°C is similar to the peak of hematite reduction in magnetite in a hydrogen atmosphere. The most probable source of hydrogen-containing gases in this temperature range consists of the residual hydrocarbons of charcoal. The reduction reactions of disperse iron oxides with coal proceed only at temperatures above 600°C. These processes occur in the same temperature range (600–900°C) both in forevacuum and in a helium atmosphere. It is experimentally demonstrated that sintering process occurs in the mud in the temperature range 450–850°C.