Dynamic and controlled rate thermal analysis of halotrichite

Three halotrichites namely halotrichite Fe²⁺SO₄·Al₂(SO₄)₃·22H₂O, apjohnite Mn²⁺SO₄·Al₂(SO₄)₃·22H₂O and dietrichite ZnSO₄·Al₂(SO₄)₃·22H₂O, 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 °C and (b) from 430 to 980 °C. For halotrichite in the dynamic experiment mass losses due to dehydration were observed at 80, 102, 319 and 343 °C. Three higher temperature mass losses occurred at 621, 750 and 805 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 82 and 97 °C followed by a non-isothermal dehydration step at 328 °C. For apjohnite in the dynamic experiment mass losses due to dehydration were observed at 99, 116, 256, 271 and 304 °C. Two higher temperature mass losses occurred at 781 and 922 °C. In the controlled rate thermal analysis experiment three isothermal dehydration steps are observed at 57, 77 and 183 °C followed by a non-isothermal dehydration step at 294 °C. For dietrichite in the dynamic experiment mass losses due to dehydration were observed at 115, 173, 251, 276 and 342 °C. One higher temperature mass loss occurred at 746 °C. In the controlled rate thermal analysis experiment two isothermal dehydration steps are observed at 78 and 102 °C followed by three non-isothermal dehydration steps at 228, 243 and 323 °C. In the CRTA experiment a long isothermal step at 636 °C attributed to de-sulphation is observed.