Radionuclide flow during the conversion of phosphogypsum to ammonium sulfate

Approximately 30 million tons of the by-product phosphogypsum are currently produced annually by the phosphate fertilizer industry in Florida. Nearly all of this material is stockpiled because radioactive impurities prevent utilization of what could otherwise be a useful agricultural amendment or construction material. Long-term storage and maintenance of this material presents economic as well as potential environmental concerns. One partial solution to this problem may be conversion of phosphogypsum to ammonium sulfate by the so-called Merseberg ammonocarbonation process. Ammonium sulfate is an excellent fertilizer which supplies sulfur as well as nitrogen to soils. We have assessed the flow of the natural decayseries radionuclides 238U, 226Ra, 210Pb and 210Po through the Merseberg process by the analysis of starting materials and products from overseas industrial-scale plants. Results indicate that the radionuclides associated with phosphogypsum do not report to the ammonium sulfate product but are found instead almost exclusively in the by-product calcium carbonate. Thus, the radiochemical results are encouraging in terms of using this process as an option for partial removal of waste phosphogypsum. Although there is a clear and recognized need for increased sulfur addition to many crops, the price of sulfur has been so low and alternative supplies of ammonium sulfate so common that investment in this process has been discouraged. Recent price increases and demand for ammonium sulfate may make the Merseberg process more attractive.