Thermal decomposition of bauxite minerals: infrared emission spectroscopy of gibbsite, boehmite and diaspore

Infrared emission spectroscopy has been used to study the dehydroxylation behavior over the temperature range from 200 to 750degreesC of three major Al-minerals in bauxite: gibbsite (synthetic and natural), boehmite (synthetic and natural) and diaspore. A good agreement is found with the thermal analysis and differential thermal analysis curves of these minerals. Loss in intensity of especially the hydroxyl-stretching modes of gibbsite, boehmite and diaspore as function of temperature correspond well with the observed changes in the TGA/DTA patterns. The DTA pattern of gibbsite clearly indicates the formation of boehmite as an intermediate shown by a endotherm around 500degreesC. Dehydroxylation of gibbsite is followed by a loss of intensity of the 3620 and 3351 cm(-1) OH-stretching bands and the corresponding deformation band around 1024 cm(-1). Dehydroxylation starts around 220degreesC and is complete around 350degreesC. Similar observations were made for boehmite and diaspore. For boehmite dehydroxylation was observed to commence around 250degreesC and could be followed by especially the loss in intensity of the bands around 3319 and 3129 cm(-1). The DTA pattern of diaspore is more complex with overlapping endotherms around 622 and 650degreesC. The dehydroxylation can be followed by the decrease in intensity of the OH-stretching bands around 3667, 3215 and 2972 cm(-1). Above 550degreesC only a single band is observed that disappears after heating above 600degreesC corresponding to the two endotherms around 622 and 650degreesC in the DTA. (C) 2002 Kluwer Academic Publishers.