Gypsum Is A Multifaceted Industrial Mineral
Industrial mineral mining by Harold Hough
For such a lowly industrial mineral, gypsum is extremely versatile. What else was used in the pyramids and toothpaste, helps peanuts and mushrooms grow, and is the key ingredient in movie snow and the walls of your house.
The ancient Assyrians called the rock alabaster, and used it for sculpturing. Five thousand years ago the Egyptians learned to make plaster from gypsum and lined the walls of their palaces, and their tombs with it (some of this earliest gypsum plaster has been found inside the great Pyramids). It was the ancient Greeks who named this mineral “Gypsos,” from which we have derived the name Gypsum. There is another form of gypsum which is clear and transparent, in layers like mica. This was used, many centuries before glass was invented, as windows in the old temples. The Greeks named it after their Moon Goddess, "Selene", and today we still call it “Selenite.”
Over the centuries, gypsum has found many uses. It is used to control hardening in Portland Cement. Car windows are made of plate glass which is held in a bed of gypsum during the polishing process. Small metal gears are cast in gypsum molds by a process that was largely perfected during World War II. In medicine, a dentist uses a gypsum dental plaster mold in which to cast a bridge and doctor uses gypsum to build splints for broken limbs from a gauze impregnated with orthopedic plaster. Even Benjamin Franklin used ground raw gypsum as a soil conditioner, called land plaster, on his farm.
One of gypsum’s greatest features is that it is relatively fireproof. Chemically, gypsum rock is calcium sulfate with two molecules of water which is released when exposed to intense heat. When a wall or ceiling coated with gypsum lath or plaster is exposed fire, the heat soon exceeds the boiling temperature of water - 212 F. However, at that temperature the water in the gypsum starts to vaporize and becomes steam. This absorbs the surrounding heat energy and keeps the drywall no hotter than 212 F. Until all the crystallized water has been driven off, the gypsum will protect the wooden framework of the structure, or whatever flammable materials the gypsum may be covering.
Of course this protection lasts for only a limited time, depending upon the thickness of the gypsum, which is being destroyed as the fire continues. However, the fire-retarding action of the gypsum may cause the fire to burn out with minimal damage to the building or provide time for firefighters or people in the building.
The largest North American producer of gypsum is National Gypsum, which operates several mines on the continent. The company mines and quarries gypsum in Arizona, Texas, Kansas, Iowa, Michigan, Indiana, and Nova Scotia. Its Nova Scotia quarry is the largest in the world, producing rock for the company’s plants located along the eastern seaboard. A single shipload of rock will produce enough gypsum board to complete approximately 6000 homes.
National Gypsum mines and quarries gypsum rock, crushes and grinds it, and calcines it to remove chemically bound water. It adds starch and other additives and water to form a stucco slurry. The stucco is sandwiched between two sheets of paper and hardens, forming gypsum board. Since water content is critical to its ability to retard fires, quality control is critical in terms of measuring water content and even kiln temperatures.
Ironically, it may be another mined mineral, coal, which may be the future source of gypsum. The newest gypsum plants use solid waste byproduct gypsum produced when coal-fired power plants scrub their emissions to remove sulfur dioxide. The byproduct of this operation is calcium sulfate. National Gypsum uses this byproduct, which would otherwise go to a landfill, to make gypsum board. The company’s Shippingport Plant (near Pittsburgh), Apollo Beach Plant (near Tampa), Westwego Plant (near New Orleans) and Mt. Holly Plant (near Charlotte), are all byproduct operation plants.