FIELD: chemical engineering. SUBSTANCE: invention places special emphasis on low- temperature heat treatment of wet hydrates, mainly gypsum, and includes removing free water in the first apparatus with fluidized bed supplied with heat coming with fluidizing gas and performing either direct heat and mass exchange with gas flow, or indirect one with heat carrier followed by removing crystallization water in the second apparatus with fluidized bed whereto heat is supplied both with fluidizing gas and through built-in heat exchangers. Heat treatment is carried out in continuous mode. Prior to be introduced into the first apparatus, wet hydrate may be subjected to mechanical dehydration followed by mixing with a part of hydrate subjected heat treatment in the first apparatus. Wet hydrate is fed into the first apparatus from above through several inlet holes. The both apparatuses are rectangularly shaped in plane. Thermally treated hydrate may be cooled in the second apparatus comprising heat exchangers on outlet inside fluidized zone thereby forming a cooling zone. EFFECT: enhanced efficiency of process. 5 cl, 1 dwg
