α‐calcium sulfate hemihydrate formation using natural anhydrite as raw material

This study investigates the potential of α-HH formation from gypsum originating from activated and hydrated natural anhydrite. For this, gypsum was prepared from natural anhydrite and treated in an autoclave process under hydrothermal condi-tions (150 mbar and 130 °C). For comparison, fgd-gypsum was treated accordingly. Succinic acid, maleic acid and additional α-HH seeds were used as crystallization aids. QXRD results show the transformation of gypsum into hemihydrate after a residence time of 8 h and 4 h. SEM images distinguish the α-HH and β-HH modifi-cations. For fgd-gypsum pellets, the β-HH modification predominates. Gypsum from natural anhydrite shows α-HH crystals with succinic acid and without crystallization aid. When maleic acid is present, crystals appear as intermediates between α-HH and β-HH. To isolate the effect of the crystallization aids, α-HH and succinic acid were used separately and simultaneously. SEM images indicate the formation of α-HH from β-HH. The shapesof the original β-HH conglomerates are still visible, yet acicular crystals with smooth surfaces (α-HH) are visible at a high magnification. Longest crystals appear when α-HH and succinic acid are used simultaneously indicating a synergetic effect of both crystallization aids.     «

This study investigates the potential of α-HH formation from gypsum originating from activated and hydrated natural anhydrite. For this, gypsum was prepared from natural anhydrite and treated in an autoclave process under hydrothermal condi-tions (150 mbar and 130 °C). For comparison, fgd-gypsum was treated accordingly. Succinic acid, maleic acid and additional α-HH seeds were used as crystallization aids. QXRD results show the transformation of gypsum into hemihydrate after...     »