Phosphogypsum
Phosphogypsum refers to the gypsum 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.[1]
Production
Phosphogypsum is a side-product from the production of phosphoric acid by treating phosphate ore (apatite) with sulfuric acid according to the following reaction:
- 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. Marine-deposited phosphate typically has a higher level of radioactivity than igneous phosphate deposits, because uranium is present in seawater. Phosphogypsum also can contain high levels of cadmium.[1]
In the United States
The United States Environmental Protection Agency has banned most applications of phosphogypsum with a radium-226 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.[2]
Various applications have been proposed for using phosphogypsum, including using it as material for:[1]
See also
References
- ↑ 1.0 1.1 1.2 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
- ↑ Florida Institute of Phosphate Research. "Phosphogypsum and the EPA Ban." Last accessed June 19, 2007.
