Dynamic and controlled rate thermal analysis of halotrichite

Three halotrichites namely halotrichite Fe(2+) SO(4)center dot Al center dot(SO(4))(3)center dot 22H(2)O, apjohnite Mn(2+) SO(4)center dot Al(2)(SO(4))(3)center dot 22H(2)O and dietrichite ZnSO(4)center dot Al(2)(SO(4))(3)center dot 22H(2)O, were analysed by both dynamic, controlled rate thermogravimetric and differential thermogravimetric analysis. Because of the time limitation in the controlled rate experiment of 900 min, two experiments were undertaken (a) from ambient to 430 degrees C and (b) from 430 to 980 degrees C. For halotrichite in the dynamic experiment mass losses due to dehydration were observed at 80, 102, 319 and 343 degrees C. Three higher temperature mass losses occurred at 621, 750 and 805 degrees C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 82 and 97 degrees C followed by a non-isothermal dehydration step at 328 degrees C. For apjohnite in the dynamic experiment mass losses due to dehydration were observed at 99, 116, 256, 271 and 304 degrees C. Two higher temperature mass losses occurred at 781 and 922 degrees C. In the controlled rate thermal analysis experiment three isothermal dehydration steps are observed at 57, 77 and 183 degrees C followed by a non-isothermal dehydration step at 294 degrees C. For dietrichite in the dynamic experiment mass losses due to dehydration were observed at 115, 173, 251, 276 and 342 degrees C. One higher temperature mass loss occurred at 746 degrees C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 78 and 102 degrees C followed by three non-isothermal dehydration steps at 228, 243 and 323 degrees C. In the CRTA experiment a long isothermal step at 636 degrees C attributed to de-sulphation is observed.