CLAY – IT’S NOT JUST FOR POTTERY CLASSES
Industrial Mineral Article by Harold Hough
Mention clay and most people think of pottery and low tech products. However, clay is a critical part of our life and is found in everything from paper to electrical insulators to ceramics. It has also become one of our latest medical innovations to save soldiers suffering from gunshot wounds. In April 2008, the US Naval Medical Research Institute announced the successful use of a clay-derived infusion in traditional gauze, known commercially as QuikClot Combat Gauze. It can cause clotting in serious wounds that once were life threatening.
Kaolin is a soft, earthy, usually white mineral produced by the weathering of minerals like feldspar. In the US the main kaolin deposits are found in central Georgia, on a geological formation, called the Fall Line, between Augusta and Macon. US production was 5.5 million tons in 2011. Almost 80 percent of the kaolin produced in Georgia is used in filling and coating: fine publication papers, light-weight catalog papers, and consumer product packaging. Over the last three decades, kaolin products have been tailored for paper applications ranging from low-cost pulp extenders to high-opacity fillers and high-gloss and high-brightness coatings.
Good quality clay (which is determined by whiteness and particle size) is actually quite rare and most of it is mined in Britain or the Eastern United States. The word 'clay' is derived from the Old English 'claeg', meaning sticky. The oldest deposits are in England and clay from these deposits have been used by manufacturers of white-bodied pottery since the days of the famous 18th century potters such as Josiah Wedgwood, Astbury and Spode.
Clay can be mined with open pit methods or hydraulically with high pressure hoses that wash it off a bench. Processing includes separation to remove the quartz, mica and unaltered feldspar. The clay is improved through bleaching, grinding, magnet separation, and blending before being dried (and in some cases, calcined).
Ceramics is the second largest use of clay and it usually requires the finest particles. This clay often includes a special clay called ball clay.
Ball clays consist of ultra fine clay particles. This helps to make them sticky (or 'plastic') and easily shaped when damp. Some also have fluid properties that are valuable in the casting of large ceramic pieces such as toilet bowls. Many ceramics contain both ball clay and china clay. The ball clay helps to shape the piece, adding 'green' strength to the body before firing, while china clay provides extra whiteness. It got its name because in earlier days, it was cut by workmen into cubes. However, as they were handled, the corners wore off and they soon turned into balls.
Today most kaolin in the United States is mined with small earth moving equipment like backhoes or smaller excavators, which allow the mine to better differentiate the clay grades. Deposits range from 18 to 40 feet in depth and there is usually 25 to 80 feet of overburden on top of the deposit. Since the characteristics of the deposit vary from place to place, the excavators work several benches and the clays are carefully blended to provide a consistency of color, shrinkage, and composition. Usually the top part of the deposit has a clay with the largest amount of organic material in it. The middle part of the deposit is the purest and the lower part of the deposit has more silicious clay in it.
The key to clay production is constantly monitoring what is being mined. Mines regularly monitor the clay quality as it is mined so they don’t inadvertently break into a different layer of clay and contaminate the product and make processing more expensive. The mined clay is first sent to a primary drying facility.
There it goes through several processes of shredding, drying, air floating, and slurrying. Then each type of clay is separately stored until it is needed. These end point clays are then used to blend the final product.
Kaolin mining is the largest mineral resource in Georgia and its mining and processing employees over 4,000 workers. The export of kaolin is one of the major revenue sources for the Port of Savannah, GA.