Foundry is among the most important current uses of bentonite. Engine blocks, brake drums, transmission housings, manhole covers, street drains, brass water fittings, cast-iron pans, and more are produced from foundry molds containing bentonite.

In most ferrous and many nonferrous foundries around the world, bentonite is used as a bonding agent in a process known as green sand molding. Here, it is mixed with sand, coal, and a small amount of water to form a mixture that is packed around a pattern made in two halves. Once the shape is formed, the patterns are removed, the two halves of the mold are fitted together, and molten metal is poured into the open mold cavity.

After the metal has solidified and partially cooled, the sand is shaken off and the metal cast is further cleaned and machined. The sand and clay mixture is then recycled to form another mold. New sand, clay, coal, and water are continually added to the recycled mixture as old materials are destroyed and removed. Both Na- and Ca-bentonites are used in this process, either individually or as blends. The type and amount of bentonite used depends on the metal being cast, the size and complexity of the casting, and the type of molding equipment.

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