Phosphogypsum refers to the calcium sulfate hydrate formed as a by-product of the production of fertilizer from phosphate rock. It is mainly composed of gypsum (CaSO4·2H2O). Although gypsum is a widely used material in the construction industry, phosphogypsum is usually not used, but is stored indefinitely because of its weak radioactivity. The long-range storage is controversial. Somewhere between 100,000,000 and 280,000,000 tons are estimated to be produced annually as a consequence of the processing of phosphate rock for the production of phosphate fertilizers.
- Ca5(PO4)3X + 5 H2SO4 + 10 H2O → 3 H3PO4 + 5 (CaSO4 · 2 H2O) + HX
- where X may include OH, F, Cl, or Br
Phosphogypsum is radioactive due to the presence of naturally occurring uranium and thorium, and their daughter isotopes radium, radon, polonium, etc. Marine-deposited phosphate typically has a higher level of radioactivity than igneous phosphate deposits, because uranium is present in seawater. Other components of PG are Cd (5-28 ppm), fluoride (ca 1%), and silica.
In the United States
The United States Environmental Protection Agency has banned most applications of phosphogypsum having a 226Ra concentration of greater than 10 picocurie/gram (0.4 Bq/g). As a result, phosphogypsum which exceeds this limit is stored in large stacks.
Central Florida has a large quantity of phosphate deposits, particularly in the Bone Valley region. However, the marine-deposited phosphate ore from central Florida is weakly radioactive, and as such, the phosphogypsum by-product (in which the radionuclides are somewhat concentrated) is too radioactive to be used for most applications. As a result, there are about 1 billion tons of phosphogypsum stacked in 25 stacks in Florida (22 are in central Florida) and about 30 million new tons are generated each year.
Various applications have been proposed for using phosphogypsum, including using it as material for:
- Artificial reefs and oyster beds
- Cover for landfills
- Road pavement
- Roof tiles
- Soil conditioner
- Red mud, a comparable waste product, but from aluminium processing
- Ayres, R. U., Holmberg, J., Andersson, B., "Materials and the Global environment: Waste Mining in the 21st Century", MRS Bull. 2001, 26, 477. doi:10.1557/mrs2001.119
- Tayibi, Hanan; Choura, Mohamed; López, Félix A.; Alguacil, Francisco J.; López-Delgado, Aurora (2009). "Environmental Impact and Management of Phosphogypsum". Journal of Environmental Management. 90: 2377–2386. doi:10.1016/j.jenvman.2009.03.007. PMID 19406560.
- Florida Institute of Phosphate Research. "Phosphogypsum and the EPA Ban" Last accessed June 19, 2007.