Phosphogypsum

Phosphogypsum stack located near Kėdainiai, Lithuania 55°14′47″N 24°01′44″E / 55.24639°N 24.02889°E / 55.24639; 24.02889.

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

Phosphogypsum stack located near Fort Meade, Florida. These contain the waste byproducts of the phosphate fertilizer industry.

The United States Environmental Protection Agency has banned most applications of phosphogypsum with 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.[2]

Various applications have been proposed for using phosphogypsum, including using it as material for:[1]

See also

References

  1. 1 2 3 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
  2. ↑ Florida Institute of Phosphate Research. "Phosphogypsum and the EPA Ban." Last accessed June 19, 2007.
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