Talc is a metamorphic rock and the softest mineral on Earth. It is so soft that a fingernail will scratch its surface, and geologists can recognize it by its greasy feel. Talc is a hydrous silicate of magnesium, Mg3Si4O10(OH)2, and usually contains small quantities of nickel, iron and aluminum as impurities. It occurs commonly in thin layers known as foliafolia, but is also found in coarsely granular, finely granular crystalline masses. Talc appears transparent to opaque, is rarely colorless, and can be found in hues ranging from white to purple. One form of this mineral is soapstone, or steatite, which is a larger granular form of talc that is also rich in magnesium.
Manufacturers often use talc when making electronics and insulation, due to its high resistance to heat and electricity. It can also be used as a filler in asphalt roofing, paint and paper. Its softness means it is easily cut and carved into jewelry, ceramics, linoleum, lubricants and paint, and it is the main reason for its use as talcum powder. As soapstone, the material has been used for carvings, sculpture, bowls, kitchen countertops and even tombstones. The color, hardness, size, shape and composition of talc ore helps determine the end products it can become.
Talc ore comes from Germany, France, Italy, Norway, Brazil and Spain. According to the Environmental Protection Agency, 95% of the talc in the United States is extracted from open pit mines that use conventional mining and blasting techniques, and some deposits are in crystal form. Mining companies extract a rare green talc at the Vermont Verde Antique Quarry in Vermont, and there is an area known as the Serpentine Belt along the border of Pennsylvania and Maryland produces a characteristic green and yellow form. The Green Mountain Mine in North Carolina also features green talc, while the Cyprus Mine in Montana has stark white deposits.
Talc processing takes place after trucks haul the extracted ore to a specialized facility. At the plant, a jaw-crusher machine crushes the mineral and then the smaller pieces pass through a screen. Larger pieces caught by the screening process go back to the crusher, and dryers remove moisture from the material.
Next, pebble mills or roller mills grind the material again. After it has been reduced in size, hammermills or steam mills grind the product even further. Other mineral processing machines classify the material by size into coarse, coarse plus fine, and fine. The fine talc is often processed further to extract impurities and concentrate it.
Talc ore in the Southwest U.S. contains organic materials in addition to the talc. A process called calcining removes these contaminants. The raw material goes through a crusher that grinds it down to a specific size, then calcination takes place in a rotary kiln like the ones manufactured by Heyl & Patterson Inc.
The indirect-heated rotary calciner is a continuous-process device for medium- to high-temperature applications. Simply designed, the material processed is heated indirectly in a rotating shell. The rotary shell is enclosed and heated from the exterior in a stationary furnace. The heated shell provides the hot surface for heat transfer to the material stream on the shell interior through a combined radiative and through-wall conductive/convective mode of heat transfer. The rotation and slope of the shell motivate the material through the shell from feed to discharge points, which are located at stationary breechings which enclose the ends of the rotary shell.
After calcining, the material goes into a rotary cooler for storage or further processing according to a customer's specifications. Sometimes, the calcined talc is mixed with other talc to allow it to pass through a roller mill prior to shipping.