Pollution Control Engineering and Gypsum
Gypsum is a widely occurring mineral, also known as plaster of Paris or calcium sulfate dihydrate (CaSO4 • 2H2O). Some common uses of gypsum are the following:
• Drywall
• Ingredient of plaster
• Component of Portland cement
• Soil conditioner
• Molds and splints
• Model making
To produce plaster or wallboard, the raw gypsum must be calcined, or heated to remove 75% of the chemically bonded water. Soil conditioner (known as landplaster) uses raw gypsum that does not need to be calcined. It is produced by crushing the raw product to less than 2″, then grinding and drying in a roller mill so that at least 90% is < 100 mesh. The ground gypsum leaves the mill and is conveyed to a dust collector.
Calcining
Plaster or stucco is made by calcining the raw gypsum to convert the dihydrate into a hemihydrate, and is known as calcined gypsum. In some plants, landplaster is fed to kettle or flash calciners to produce plaster. Calcining occurs at about 250°-300°. In a kettle, the gypsum is heated indirectly by combustion gases passed through flues in the kettle and discharged into a pit below. In a flash calciner, it contacts hot gases directly and the stucco is collected at the bottom of the calciner. Gases from the calciner are vented to a fabric filter.
Another method of producing stucco involves the following process, which can substitute for the use of kettles or flash calciners, or be used if no landplaster needs to be produced:
• Crushing the raw gypsum to smaller sizes
• Transporting to a feed bin, normally using a screw conveyor
• Feeding the gypsum into a flash-calcining impact mill
The impact mill will accept chunks up to 1″, grind the material to sizes of 5-1000 microns and calcine the material from a source of heated air. The air leaving the mill is conveyed to a dust collector.
Dust Collection
The air leaving the calciner will be at 300°-350° and will have a high water vapor content. In addition, stucco has notoriously poor flow characteristics. The dust collector should be well insulated and account for a high dust angle of repose by using a 70° pyramid hopper or a straight-end trough hopper with a screw conveyor. Bags should be singed to promote cake release. Collectors handling gypsum from other processes may be at ambient temperatures where no insulation will be needed, but the same precautions about providing for the flow properties will apply.