Phosphogypsum is a the primary byproduct of the wet-acid process for producing phosphoric acid from phosphate rock. It is largely calcium sulfate and has been given the name phosphogypsum. (Gypsum is the common name for hydrated calcium sulfate, a common building material.)Phosphate production generates very large volumes of phosphogypsum, which is stored in huge piles called "stacks" that cover hundreds of acres in Florida and other phosphate-processing states.
On this page:
- General Information
- Processing Phosphate Rock
- Radioactivity in Phosphogypsum
- Other Phosphogypsum Constituents of Concern
How much phosphogypsum is being produced?
Since the mid-eighties, the annual production rate of phosphogypsum has been in the range of 40 to 47 million metric tons per year. The total amount generated in the United States from 1910 to 1981 was about 7.7 billion metric tons.
In Central Florida, one of the major phosphoric acid producing areas, the industry generates about 32 million tons of phosphogypsum each year. They have a current stockpile in stacks of nearly 1 billion metric tons.
Why is so much phosphogypsum produced?
The agriculture industry uses large amounts of chemical fertilizers to replenish and supplement the nutrients that growing plants take up from the soil. The demand for fertilizers and animal feed additives accounts for about 95% of the 8-10 million metric tons of phosphoric acid that is made each year. The production of each ton of phosphoric acid is accompanied by the production of 4½ tons of the by-product calcium sulfate, also known as phosphogypsum.
Phosphate rock, which is processed to make phosphoric acid, contains concentrations of naturally occurring radioactive elements (radionuclides). Even high grade ores, which contain about 70% calcium phosphate, also contain a large number of impurities, such as calcium fluoride, chlorides, chromium, rare earths, and radionuclides. At the end of the production process, the radionuclides end up in the phosphogypsum.
Processing Phosphate Rock
Where does the phosphate rock come from?
In the United States, main deposits of phosphate rock are in Florida, Tennessee, and North Carolina. There are also deposits that can be mined in Idaho. The phosphate rock, which eventually yields the phosphogypsum by-product, is recovered by open pit mining. The rock is transported to a washing facility, where it is separated from accompanying soil, stones, etc. and processed. The desired phosphorus content of the phosphate rock is in a form (calcium phosphate) that will not dissolve in water and so cannot be taken up by crops. As a result, phosphate processors must solve the problem of getting it into a water-soluble form.
The most common solution to the problem is converting the calcium phosphate to phosphoric acid. There are wet and dry processes for doing the conversion. U.S. production facilities utilize a wet process in which the prepared rock is treated with sulfuric acid to produce the phosphoric acid. Phosphoric acid is water soluble so it can be taken up by crops. It can also be concentrated, as desired, by evaporating water from the mixture.
The by-product remaining after the acid conversion is largely calcium sulfate and has been given the name phosphogypsum. (Gypsum is the common trade name for hydrated calcium sulfate, a common building material.)
How many facilities are producing phosphoric acid and phosphogypsum?
As of September 1989, the phosphoric acid production industry consisted of 21 active facilities that use the wet-acid production process. The majority of the 21 facilities are located in the southeast, with 12 in Florida, three in Louisiana, and one in North Carolina.
The phosphogypsum, separated from the phosphoric acid, is in the form of a solid/water mixture (slurry) which is stored in open-air storage areas known as stacks. The stacks form as the slurry containing the by-product phosphogypsum is pumped onto a disposal site. Over time the solids in the slurry build up and a stack forms. The stacks are generally built on unused or mined out land on the processing site.
As the stack grows, the phosphogypsum slurry begins to form a small pond (gypsum pond) on top of the stack. Workers dredge gypsum from the pond to build up the dike around it and the pond gradually becomes a reservoir for storing and supplying process water. A total of 63 phosphogypsum stacks were identified nationwide in 1989. They were in 12 different states, but the majority, two-thirds, were in Florida, Texas, Illinois, and Louisiana.
The surface area covered by stacks ranges from about 5 to 740 acres. The height ranges from about 10 to 200 feet. In 1989, the total surface area covered by stacks was about 8,500 acres. More than half that acreage is in Florida.
The tops of operating phosphogypsum stacks (ones that are still receiving phosphogypsum) are covered by ponds and ditches containing process water. "Beaches," saturated land masses, protrude into the ponds. These surface features may cover up to 75 percent of the top of the stack. Other surface features include areas of loose, dry materials; access roads; and thinly crusted stack sides. (The crust thickens and hardens when the stacks become inactive and no longer receive process slurry.)
Radioactivity in Phosphogypsum
How much radioactivity is in the phosphogypsum?
The concentrations of uranium and radium-226 in phosphogypsum samples taken in central Florida were about 10 times the background levels in soil for uranium and 60 times the background levels in soil for radium-226.
The radium-226 concentration in phosphogypsum varies significantly at different sampling locations on a single stack and also in phosphogypsum from different stacks within the same geographical area.
How are people exposed to the radiation from phosphogypsum stacks if they dont go near them?
Radionuclides that are small particles (i.e., radionuclide dust) can become airborne as wind-blown dust or as dust thrown up into the air by cars and trucks. The radionuclides, uranium and radium-226, are present in the phosphogypsum and can become airborne. Once these radionuclides are in the air, people and animals can breathe them and they can settle out onto ponds and agricultural areas. Radon-222, a decay product of radium-226, is a gas and so may become airborne by diffusing into the air. EPA has determined, however, that the risks associated with stacking phosphogypsum are in line with acceptable risk practices.
Other Phosphogypsum Constituents of Concern
In addition to the radiation health hazards covered by Subpart R, phosphogypsum contains some trace metals in concentrations that EPA believes may pose a chemical hazard to human health and the environment. Analysis of samples from various facilities contained arsenic, lead, cadmium, chromium, fluoride, zinc, antimony, and copper at concentrations that may pose significant health risks. The concentrations of these contaminants vary by more than three orders of magnitude among samples taken from various locations. These trace metals may also be leached from phosphogypsum and migrate to nearby surface and groundwater resources.
The presence of these trace metals in phosphogypsum is mentioned here in order to provide a more complete description of phosphogypsum, but they are not to be addressed in the risk assessment.