A thermogravimetric and infrared emission spectroscopic study of alunite

Thermogravimetric and differential thermogravimetric analysis has been used to characterise alunite of formula [K2(Al 3+)6(SO4)4(OH)12 ]. Thermal decomposition occurs in a series of steps (a) dehydration up to 225 °C (b) well defined dehydroxylation at 520 degrees Celsius and desulphation which takes place as a series of steps at 649, 685 and 744 degrees Celsius. The alunite minerals were further characterised by infrared emission spectroscopy (IES). Well defined hydroxyl stretching bands at around 3463 and 3449 cm-1 are observed. At 550 degrees Celsius all intensity in these bands is lost in harmony with the thermal analysis results. OH stretching bands give calculated hydrogen bond distances of 2.90 and 2.84-7 Å. These hydrogen bond distances increase with increasing temperature. Characteristic (SO4)2- stretching modes are observed at 1029.5, 1086 and 1170 cm-1. These bands shift to lower wavenumbers on thermal treatment. The intensity in these bands is lost by 550 degrees Celsius