Sifting gypsum | Sustainable and efficient fractionation
Chemically gypsum is calcium sulphate (CaSO4 • 2H2O). In nature gypsum occurs as compact material as well as fine powder with different colours. Furthermore there are deposits containing fibrous gypsum or satin spar. Crystalline gypsum is termed selenite. Gypsum crystals are ductile and often very large. Gypsum can also be found as complex structure, the so called Gipsrose resp. Wuestenrose. Gypsum containing potassium sulphate or magnesium sulphate is termed polyhalite. The material impurified by bitumen is termed bituminous gypsum or Stinkgips.
The extraction of gypsum
In former times gypsum was mainly mined, in the meantime gypsum gained from flue gas desulphurisation becomes more and more. The calcinations process of the material makes use of a special characteristic of this material: in the mixing process gypsum is able to re-absorb the water of cristallisation released during calcination and to harden. During the exothermal hardening gypsum needles are formed which lead to stability. Calcinated gypsum is termed as hemhydrate, stucco or annaline.
In building industry gypsum from flue gas desulphurisation is used for interior walls in form of gypsum plasterboard wall as well as filling material. Gypsum plasterboard walls ensures a proper regulation of humidity in housing spaces.
In a first step gypsum is sifted directly after mining, it is humid and tends to stick together. After calcinations the material often becomes mixed together with other building materials. The particle size distribution has to be set respectively by crushing and screening. Coarse grain is unwanted since it impacts the processability negatively. Regarding to this gypsum is fine screened at high feed rates and finest separations with the demand for preferably small machines at the same time. Due to this requirements RHEWUM sifting machines are working for well known gypsum producers in Europe.
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