Page 195 - Chemical and process design handbook

Speight_Part II_C 11/7/01 3:08 PM Page 2.136

CALCIUM SULFATE

Calcium sulfate occurs in large deposits throughout the world as its
hydrate (gypsum; CaSO •2H O). When heated at moderate temperatures,
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gypsum loses part of the water to form a semihydrate.
CaSO •2H O → CaSO • /2H O + 1 /2H O
1
1
4 2 4 2 2
At higher temperatures, gypsum loses all its water and becomes anhy-
drous calcium sulfate, or anhydrite.
Calcined gypsum (the half-water salt) can be made into wall plaster by
the addition of a filler material such as asbestos, wood pulp, or sand.
Without additions, it is plaster of Paris and is used for making casts and
for plaster.
The usual method of calcination of gypsum consists in grinding the
mineral and placing it in large calciners holding up to 20 or more tons. The
o
temperature is raised to about 120 to 150 C, with constant agitation to
maintain a uniform temperature. The material in the kettle, plaster of Paris
or first-settle plaster, may be withdrawn at this point, or it may be heated
o
further to 190 C to give a material known as second-settle plaster. First-
settle plaster is approximately the half-hydrate (CaSO • /2H O), and the
1
4 2
second form is anhydrous (CaSO ). Gypsum may also be calcined in rotary
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kilns similar to those used for limestone.

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