Gypsum (calcium sulphate dihydrate) is extracted from quarries,
and subsequently undergoes several preparatory and production phases including
calcination to produce plaster, a dehydrated form of gypsum.
By directly controlling and finely tuning the production
process, the structure and properties of the obtained plaster formulation are
closely matched with the needs of a variety of highly technical industries.
Extraction
Gypsum is extracted from open air or underground mines, using
specific drilling machinery and non-polluting explosives. Rock size may reach
up to 50 cm in diameter.
Crushing
Primary crushing aimed at reducing rocks to a size of less than
10cm, subsequently easier to handle, is carried out in the quarry or at the
entrance to the plaster manufacturing station.
Storage
Rock that has undergone primary crushing is stored to ensure
production continuity, and optimal homogeneity between rock extraction batches.
Sifting
It is necessary to separate and control gypsum particle size in
order to obtain the exact product properties required for the plaster being
manufactured.
Calcination
Calcium sulphate hemihydrate (CaSO4.½H2O)
or plaster is obtained through the partial or total dehydration of gypsum at a
temperature ranging from 120° to 400° C The structure and properties of the
final product are directly dependent on the chosen calcination conditions
(temperature, pressure, rapidity)
Several types of calcination processes exist:
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Alpha Process
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Alpha type plaster is used mainly in industrial plaster
formulations for its high mechanical strength. This plaster type is a compact
crystal with a low specific surface and low water demands to produce hard, low
porosity casts . Alpha plaster can be formed through 2 different production
procedures:
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Dry process that involves injection of steam vapour
during calcination. The plaster is dried and then treated in the regular
manner.
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Wet process that involves calcination of a gypsum slurry
under pressure. The plaster is then spun and dried.
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Beta Process
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During the calcination process, under regular environmental
pressure, dehydration water evaporates and a micro-porous structure is formed.
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Beta plaster crystals have a high specific surface and high
water demands. Beta plasters casts have high porosity, but low mechanical
properties and are therefore used for example in lightweight building
applications or moulds in ceramic applications for their absorbent properties.
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Often a mixture of both Alpha & Beta type plasters will be
used to combine the properties of both and optimise product solutions to suit
market requirements.
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Grinding
Following the calcination process, the plaster is ground to
obtain a powder. Particle size distribution is an important factor in the
product properties
Mixing
With the plaster now in finely ground form, the final mixing
stage is possible. A choice of additives will finely tune the products
properties to match the customer’s needs, in terms of setting time, viscosity,
porosity, colour, and mechanical strength….
Testing
Laboratory testing is carried out at several production phases,
to ensure all products meet the strict product specifications before being
bagged and shipped.
Packaging
Feasibility studies are carried out to ensure the packaging item
chosen for each product gives optimal protection and guarantees the product
quality all the way to the end-user.
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