Mill (River Rouge, Michigan)

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The Mill Plant operates about two days per week and produces stucco, land plaster, and granular. Gypsum rock from USG's gypsum mine in Alabaster, Michigan, is delivered via truck and is unloaded and stored outside in piles until it is moved inside using front-end loader, where it is stored in open "silos" (which are basically segregated bins) prior to crushing. The aggregate is moved by crane loaders to the rock crusher, where it is crushed down to 2 inches or smaller, then sent to the rock storage bin.

The rock crusher can crush up to 100 1 tons per hour. The crushed aggregate in the storage bin is conveyed to a natural gas-fired rotary rock dryer to dry off free moisture from the aggregate. After drying, the crushed aggregate is elevated via a screw conveyor to a cyclonic air separator and screening system; courser materials (greater than 15 mesh) go to the granular system while finer materials (smaller than 15 mesh) go to the land plaster system. The granular system consists of a crusher and granular feed bin, which feeds the material to a bagger after further screening -granular material is approximately the size of sand. The bagged material is then put on pallets, weighed, and labeled and sent to the warehouse for shipping for use in agricultural feedstock, soil conditioner, glass making, and pharmaceutical production.

For the land plaster system, material must be 100 mesh or smaller to be land plaster grade (about talcum powder consistency). Screened material from the dryer which is already smaller than 100 mesh is sent directly to the land plaster bins, but material sized 15 to 100 mesh is sent to either of two 50-ton Raymond Mill feed bins and then fed into the Raymond Mill, which uses rollers to further crush the material to 100 mesh or less before sending to the land plaster bins. Land plaster is considered both a final product which can be loaded into pneumatic trucks or into 12,000-pound bags and sold for use in other industries (primarily agriculture and cement industries) or a material used on site in other production.

Land plaster used at this facility is sent from the land plaster bins to either the calcining kettles to produce stucco in the Mill Plant or sent to the HRA bin prior to use in the Cement Board Plant. There are three natural gas-fired calcining kettles. Using indirect heat, the kettles heat the land plaster to 250 F for approximately 90 minutes to remove 75% of the water molecules to produce stucco; this process is known as calcining. As the land plaster turns to stucco, it becomes lighter and overflows in the kettles and goes to the warehouse bins. There are two warehouse bins-No. 1 Warehouse Bin has a capacity of 180 tons and No.2 Warehouse Bin has a capacity of 300 tons. Stucco in the warehouse bins can be loaded directly into bulk tankers and is sold as a material for use in other industries, including the production of fire suppressants and drywall.