Fertilizer and Fertilizer Production Wastes
Due to its chemical properties, phosphate rock may contain significant quantities of naturally occurring radioactive materials (NORM):
Uranium concentrations in phosphate ores found in the U.S. range from 20 to 300 parts per million(ppm) (or 7 to 100 pCi/g), while thorium occurs at essentially background levels, between 1 to 5 ppm (or about 0.1 to 0.6 pCi/g).
The most important use of phosphate rock is in the production of phosphate fertilizers. It accounted for over 90 percent of the phosphate rock demand in the United States in 2006.
Before phosphate ore is turned into fertilizer or other products, it is transformed into either phosphoric acid (through the wet process), or elemental phosphorus (through the thermal process). This processing concentrates NORM in the waste products, transforming them into TENORM (Technologically-Enhanced Naturally-Occurring Radioactive Materials).
On this page:
- Phosphate Fertilizer
- Waste Generation
- Disposal and Reuse
- What is being done about these wastes?
The yearly consumption of phosphate based fertilizers in the U.S. averaged close to 5.8 million metric tons (MT) between 1960 and 2007 and had increased to over 8.5 million MT by 2007. While phosphate fertilizers are not assumed to be waste, they do contain some of the naturally-occurring radium (Ra-226) found in phosphate ores.
The concentration of Ra-226 varies from 5 to 33 picocuries per gram (pCi/g), depending upon the type of fertilizer blend and the origin of the phosphate rock.
Fertilizer application rates are known to vary depending upon the type of crops and soils. A typical phosphate fertilizer application rate is about 40 kg per hectare. Fertilizers are available in over 100 different blends with varying concentrations of nitrogen, phosphorus, and potassium. Further data on fertilizer consumption in the U.S. by type of fertilizer, but without any data on radionuclide content, can be found at the Department of Agriculture's website: http://www.ers.usda.gov/data-products/fertilizer-use-and-price.aspx
Phosphate rock (phosphorite) mining is the fifth largest mining industry in the United States in terms of quantity of material mined. In 1989 the total production of phosphate rock in the U.S. was estimated at 38 million metric tons (MT). Most of the phosphate production goes into the making of fertilizers.
The phosphate industry is concentrated in the southeastern U.S. with Florida, North Carolina, and Tennessee having about 90 percent of the domestic production capacity. Florida alone accounts for approximately 80 percent of the current capacity making it the worlds largest phosphate producing area. The western U.S. also has a measurable phosphate industry particularly in Idaho.
The wastes of most concern are the byproducts created during the wet and thermal processing steps of fertilizer production.
Phosphogypsum is the primary waste byproduct of the wet-acid process for producing phosphoric acid. During this process sulfuric acid dissolves phosphate rock creating a solid/liquid mixture (slurry) of phosphoric acid and calcium sulfate (phosphogypsum). The desired phosphoric acid component is separated from the mixture by filtration leaving phosphogypsum as the waste product.
Phosphate production generates huge amounts of phosphogypsum wastes, nearly 48 million MTs in 1988 alone. Phosphogypsum has little market value and is hauled off as a slurry to waste piles called phosphogypsum stacks.
Radiation in Phosphogypsum
Phosphogypsum contains appreciable quantities of uranium and its decay products, such as radium-226, due to their high concentrations in phosphate ores. Uranium in phosphate ores found in the U.S. ranges in concentration from 7 to 100 pCi/g.
During the wet process, radionuclides present in the phosphate ore are selectively separated and concentrated. Around 80 percent of the radium-226 becomes concentrated in the phosphogypsum. Radium concentrations at phosphogypsum stacks range from 11 to 35 pCi/g.
Radon (Rn-222) can be found emanating from the surface of phosphogypsum stacks. Average radon fluxes range from 1.7 to 12 pCi/m2-sec and can be as high as 340 pCi/m2-sec, with a mean value of 6.8 pCi/m2-sec.
Radiation levels in phosphogypsum vary considerably from stack to stack and from different locations in a single stack due to a number of factors:
- radium concentration in the phosphate rock
- emanation rate
- vegetation cover
- moisture content
- presence of standing water
- temperature/barometric pressure.
The table below shows the range of activity in fertilizer production wastes:
|Source||Radiation Level [pCi/g]|
|Phosphate Ore (Florida)||7||17.3-39.5||6.2-53.5|
The Radiation in TENORM Summary Table provides a range of reported concentrations, and average concentration measurements of NORM associated with various waste types and materials.
Phosphate slag is the principle waste produced from the thermal process for the conversion of phosphate rock to elemental phosphorus. Production of elemental phosphorus, primarily in Idaho, has steadily due to decreasing demand and increasing energy costs associated with its production.
Phosphate slag is a glassy substance created during furnace processing. Because of its physical properties and its high carbonate content, slag is less susceptible to radionuclide leaching than phosphogypsum. However, concentrations of uranium, thorium, and radium in phosphate slag have been measured as high as 50 pCi/g in some instances.
The average radon measurements emanating from slag piles is just 0.5 pCi/m2-sec. This is relatively low when compared to the radon measurements from native soil samples which range from 1.7 to 17 pCi/m2-sec.
Disposal and Reuse
Phosphogypsum has little market value and is hauled off as a slurry to huge waste piles called phosphogypsum stacks. The solid portion of the slurry consolidates while the water pools on the stack's surface. Eventually gypsum is dredged from the pools to build up the edges around the stack forming a reservoir for storing process water.
Stacks are generally constructed on unused land or on mined out areas at production sites with little or no prior preparation of the land. They are not covered with soil or any other material.
There are over 70 identified stacks in the U.S. with the highest percentage found in Florida. The stacks are of considerable size ranging from 2 to 324 hectares (800 acres) in surface area and 3 to 60 meters in height.
Since there are large quantities of phosphogypsum waste, the industry encourages research into potential uses in order to minimize the disposal problem. The greatest use of phosphogypsum is in agricultural applications. Researchers proposing new uses must file an application with EPA.
Phosphogypsum has been used in agriculture as a source of calcium and sulfur for soils that are deficient in these elements. When the phosphogypsum is used as a fertilizer, it is simply spread on the top of the soil. When used for pH adjustment or sediment control, it is tilled into the soil.
The activity of phosphogypsum used for agricultural purposes may not exceed 10 pCi/g. An estimated 221,000 metric tons of phosphogypsum are taken from the phosphogypsum stacks and used in agriculture each year. There is no limitation on the amount of material that can be applied and farmers do not have to maintain certificates or application records.
In the past phosphogypsum was incorporated into a Portland cement mixture for use in road construction. Currently, the use of phosphogypsum for such purposes is banned under the EPA final rule issued on June 3, 1992, which amends 40 CFR 61 Subpart R.
In response to the need for new ways to make use of phosphogypsum, EPA has provided a process by which researchers may apply for approval from EPA for new uses. In addition, the state of Florida has created an independent state research agency, Florida Institute of Phosphate Research (FIPR) charged with investigating ways to minimize adverse environmental impacts of the phosphate industry.
Slag is generally stored in waste piles on site at production facilities. It is reused for a variety of applications:
- Highway construction aggregate (crushed base and crushed aggregate for asphalt)
- Portland cement and concrete (banned by the state of Idaho in 1977 for use in habitable construction)
- Railroad ballast and general construction
What is being done about these wastes?
Because of concerns over elevated radionuclide concentrations in phosphogypsum, the U.S. Environmental Protection Agency issued a final rule on June 3, 1992 amending 40 CFR 61 Subpart R. It states : "Phosphogypsum intended for agricultural use must have a certified average concentration of radium-226 no greater than 10 pCi/g. There is no limitation on the amount of material that can be applied and farmers do not have to maintain certificates or application records."
Due to concerns over radiation exposure, the State of Idaho has prohibited the use of phosphate slag in the construction of habitable structures since 1977, although slag is still used as an aggregate in road construction.
Subpart R: Radon from Phosphogypsum Stacks
This site provides information on EPA's National Emission Standards for Hazardous Air Pollutants: Radionuclides that apply to radon emissions from phosphogypsum stacks. It also provides information on the formation of phosphogypsum, the stacks, and potential uses for the material.
Phosphogypsum Fact Sheet
This fact sheet provides an overview of the phosphogypsum issue.