Gypsum is calcium sulfate dihydrate (CaSO4 2H2O), a white or gray
naturally occurring mineral. Raw gypsum ore is processed into a
variety of products such as a portland cement additive, soil
conditioner, industrial and building plasters, and gypsum
wallboard. To produce plasters or wallboard, gypsum must be
partially dehydrated or calcined to produce calcium sulfate
hemihydrate (CaSO4 ½H2O), commonly called stucco. A flow diagram
for a typical gypsum process producing both crude and finished
gypsum.
In this process gypsum is crushed, dried, ground, and calcined. Not
all of the operations are performed at all gypsum plants. Some
plants produce only wallboard, and many plants do not produce soil
conditioner. Gypsum ore, from quarries and underground mines, is
crushed and stockpiled near a plant. As needed, the stockpiled ore
is further crushed and screened to about 50 millimeters (2 inches)
in diameter.
If the moisture content of the mined ore is greater than about 0.5
weight percent, the ore must be dried in a rotary dryer or a heated
roller mill. Ore dried in a rotary dryer is conveyed to a roller
mill, where it is ground to the extent that 90 percent of it is
less 149 micrometers (µm) (100 mesh). The ground gypsum exits the
mill in a gas stream and is collected in a product cyclone. Ore is
sometimes dried in the roller mill by heating the gas stream, so
that drying and grinding are accomplished simultaneously and no
rotary dryer is needed. The finely ground gypsum ore is known as
landplaster, which may be used as a soil conditioner. In most
plants, landplaster is fed to kettle calciners or flash calciners,
where it is heated to remove three-quarters of the chemically bound
water to form stucco.
Calcination occurs at approximately 120 to 150°C (250 to 300°F),
and 0.908 megagrams (Mg) (1 ton) of gypsum calcines to about 0.77
Mg (0.85 ton) of stucco. In kettle calciners, the gypsum is
indirectly heated by hot combustion gas passed through flues in the
kettle, and the stucco product is discharged into a "hot pit"
located below the kettle. Kettle calciners may be operated in
either batch or continuous mode. In flash calciners, the gypsum is
directly contacted with hot gases, and the stucco product is
collected at the bottom of the calciner. At some gypsum plants,
drying, grinding, and calcining are performed in heated impact
mills. In these mills hot gas contacts gypsum as it is ground.
The gas dries and calcines the ore and then conveys the stucco to a
product cyclone for collection. The use of heated impact mills
eliminates the need for rotary dryers, calciners, and roller mills.
Gypsum and stucco are usually transferred from one process to
another by means of screw conveyors or bucket elevators. Storage
bins or silos are normally located downstream of roller mills and
calciners but may also be used elsewhere.
