About 30 million tons of phosphogypsum are being placed on piles or gyp stacks each year. It has been proposed that this phosphogypsum and tailings sand, also associated with the phosphate mining operations, be used to produce high value glass or glass-ceramics products while returning the sulfuric acid back to the mining operations. Phase one successfully demonstrated feasibility of making glass from these materials. The second phase researched the marketing of a product made from these materials and completed an initial risk assessment. Glass-ceramics made with high concentrations of calcia or gypsum appear attractive for making a variety of products that are strong, abrasion and corrosion resistant. Wall, floor and roof tile were selected as the target products. The method for identifying and selecting the product was reported. A preliminary risk assessment indicated that radon exposure attributable to these tile are much lower than for other construction materials. Annual gamma dose from maximal use of tile, all floors and roof, and conservatively exposure times was less than 50 mrem/yr. With a more realistic tile use, limited to the kitchen, bathroom and entry way, the estimated exposures were less than 10 mrem/yr. This compares favorably with the average United States annual dose from all sources of about 360 mrem/yr.
This final phase completed a conceptual design and cost estimate after selecting the preferred unit operations for a specific plant site. Laboratory scale confirmation of the product and general process was used to confirm the feasibility of the preferred approach and to make representative samples. A significant effort was expended on determining the preferred process. Many alternatives were considered. The decision made was made to transform the phosphogypsum into calcium carbonate using an aqueous process and produce sodium sulfate. Sodium sulfate could then be used as a marketable product or be used as a feed material to produce other products. The calcium carbonate resulting from the aqueous process together with the other insolubles found in phosphogypsum was to be used as a feedstock to a ceramic tile manufacturing process.
Laboratory tests were used to assess the feasibility of the aqueous process and the ceramic tile process. Both sets of laboratory experiments yielded positive results. Subsequent to these laboratory confirmations, capital and operating cost estimates for the respective facilities were completed and are reported.
Chris Chapman, Bogdan Wojak, and Richard Peters, with P. K. Bhattacharjee, Vitrification International Technologies, Inc., Richland, Washington. April 2006.