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 conditions (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 modifications. 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 shapes of 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.