HIGH-RESOLUTION THERMAL GRAVIMETRIC ANALYSIS (HI-RES TGA) OF BAUXITES AND MUDS

Tolentino, E.N., Scanlan, C.J. and Aboagye, A.R.

High-Resolution Thermal Gravimetric Analysis was applied to a suite of bauxites and their muds to quantify gibbsite, boehmite, and goethite. A dynamic heating rate is applied to improve resolution of overlapping decompositions within a reasonable measurement time. The weight losses measured as LOI’s/LOM’s are related to the amounts of phases decomposing/transforming at specific temperature ranges.

The LOI’s at three temperature ranges were used for phase quantification: 180-275°C is associated with gibbsite decomposition/dehydroxylation to boehmite and chi-alumina; 275-380°C is associated with goethite decomposition to hematite; and 410-580°C with dehydration of boehmite. The method performance was validated against certified reference bauxites and two other independent techniques, Differential Scanning Calorimetry and Powder-X-ray Diffractometry (XRD) with Rietveld refinement.

Investigation on decomposition reactions of gibbsite by TGA-XRD showed that at 275°C gibbsite is not completely dehydoxylated and decomposes to boehmite and chi-alumina. Therefore, 3 moles of H2O cannot be used to relate the weight loss to gibbsite. A weighted average of 2.5 moles of H2O loss for the two transitions was calculated.

Accuracy of Hi-Res TGA for gibbsite quantification was validated against reference bauxites within 1% error. Results for bauxite suite correlate well with Rietveld-generated values with an average difference of 1.1% gibbsitic alumina. An average difference of 0.3% boehmite was observed between Hi-Res TGA and DSC results. An average difference of 1% goethite was observed between Hi-Res TGA and XRD-Rietveld-generated results.