Method for calcining moist gypsum

A method for thermally treating moist hydrates comprising the steps of:

• feeding moist hydrates into a first fluidized bed apparatus having a static heat exchanger including heat exchange surfaces disposed in the apparatus;
• vaporizing the surface water of the hydrates in the first fluidized bed apparatus to form surface-dried hydrate by a complete homogenous fluidization of the hydrates with hot gas and by heating the heat exchange surfaces and contacting the fluidized hydrates with the heat exchange surfaces, the majority of the heat introduced into the apparatus via the hot gas and heat exchange surfaces being from the heat exchange surfaces;
• conveying surface-dried hydrate from the first fluidized bed apparatus to a second, separate fluidized bed apparatus having a static heat exchanger including heat exchange surfaces disposed in the apparatus;
• and
• withdrawing the water of crystallization from the surface-dried hydrate in the second fluidized bed apparatus by a complete homogenous fluidization of the surface-dried hydrate with hot gas and by heating the heat exchange surfaces in the second apparatus and contacting the fluidized surface-dried hydrate with the heat exchange surfaces in the second apparatus, the majority of the heat introduced into the second fluidized bed apparatus via the hot gas and heat exchange surfaces being from the heat exchange surfaces.