Ashes to Ashes, Dust to Building Material
In honor of construction season, I offer an odd fact about a common building material. Plasterboard (also known as wallboard, gypsum board, and Sheetrock---the brand name of the best-known kind) may seem pretty solid, but nowadays some of it is made out of thin air.
Well, not quite. As a politician might say, I misspoke. Actually, it's generated out of thick air-polluted air, to be precise.
The chief ingredient of wallboard is gypsum, a mineral also known as calcium sulfate dihydrate. Pure gypsum can take remarkably different forms. Alabaster is a fine-grained rock gypsum, appreciated by sculptors since prehistory because it is beautiful but soft enough to carve. Satinspar is gypsum in fibrous, translucent form; selenite is gypsum in sheets so transparent that it is sometimes mistaken for mica.
These and other less chemically pure but more common forms of gypsum are mined or quarried virtually all over the world. Gypsum is bulky stuff, so it is expensive to transport any distance. Manufacturers of wallboard, plaster of Paris, portland cement, and agricultural gypsum for soil conditioning thus usually set up shop near deposits of the mineral.
However, gypsum deposits aren't to be found everywhere. And existing deposits can be exhausted. So, although it is fairly common and fairly inexpensive, natural gypsum is not quite adequate for the needs of modern industry. For at least the past couple of decades, synthetic gypsum has been looking more and more interesting to manufacturers. Recent changes in environmental law have given that interest a sharp nudge.
The Federal Clean Air Act of 1990 mandates a reduction of 10 million tons in the emissions of sulfur dioxide by the year 2000. The law is only the most stringent in a line of cleaner air laws, however. Operators of coal-burning power plants have been worrying about pollutant output for years. One of the technologies employed to help coal burners clean up their act is flue gas desulfurization. When the power company talks of installing scrubbers, they usually mean flue gas desulfurizers.
In a typical scrubber, the flue gases are forced through a slurry of powdered limestone and water. The result of this process is cleaner air and a sludge of calcium sulfite, a chemically inert material that can be safely disposed of in a landfill.
But---environmental law in operation again---dumping things in landfills is getting to be an expensive matter. Landfill operators charge tipping fees; hauling can be costly, especially as local landfills close. Finding a better way to deal with waste makes economic sense.
The United States Gypsum Company brags, with understandable pride, that it is doing just that through an arrangement with Northern Indiana Public Service Company---NIPSCO to its friends and consumers. The advanced scrubbers at NIPSCO's new power plant in Bailly, Indiana, will carry the cleansing process one step further. At that plant, the waste sludge will be transported to a reaction tank where air is introduced under just the right conditions to change the calcium sulfite into calcium sulfate. The result is a thin gypsum mud, 25 percent raw gypsum and 75 percent water.
The stack emissions from the Bailly plant will have up to 95 percent of the sulfur dioxide removed and started on its way toward being something useful. U.S. Gypsum will finish that transition at its nearby East Chicago factory, where the drying process begun in NIPSCO's centrifuges will be completed before the synthetic gypsum is turned into wallboard and joint compound.
The new use for scrubber sludge is the latest refinement in a long process; synthetic gypsum is a by-product of other chemical production techniques and even of other flue-gas cleaning methods. The NIPSCO collaboration promises gypsum of uniformly high quality, an improvement over other methods.
