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Phosphogypsum
Phosphogypsum (PG) is the calcium sulfate hydrate formed as a by-product of the production of fertilizer, particularly phosphoric acid, from phosphate rock. It is mainly composed of gypsum (CaSO4 · 2 H2O). 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 caused by the presence of naturally occurring uranium (U) and thorium (Th), and their daughter isotopes radium (Ra), radon (Rn) and polonium (Po). On the other hand, it includes several valuable components—calcium sulphates and elements such as silicon, iron, titanium, magnesium, aluminum, and manganese. However, the long-term storage of phosphogypsum is controversial. About five tons of phosphogypsum are generated per ton of phosphoric acid production. Annually, the estimated generation of phosphogypsum worldwide is 100 to 280 million metric tons.
Phosphogypsum is a by-product from the production of phosphoric acid by treating phosphate ore (apatite) with sulfuric acid according to the following reaction:
It is radioactive due to the presence of naturally occurring uranium (5–10 ppm) and thorium, and their daughter nuclides radium, radon, polonium, etc. Marine-deposited phosphate typically has a higher level of radioactivity than igneous phosphate deposits, because uranium is present in seawater at about 3 ppb (roughly 85 ppb of total dissolved solids). Uranium is concentrated during the formation of evaporite deposits as dissolved solids precipitate in order of solubility with easily dissolved materials such as sodium chloride remaining in solution longer than less soluble materials like uranium or sulfates. Other components of phosphogypsum include silica (5–10%), fluoride (F, ~1%), phosphorus (P, ~0.5%), iron (Fe, ~0.1%), aluminum (Al, ~0.1%), barium (Ba, 50 ppm), lead (Pb, ~5 ppm), chromium (Cr, ~3 ppm), selenium (Se, ~1 ppm), and cadmium (Cd, ~0.3 ppm). About 90% of Po and Ra from raw ore is retained into Phosphogypsum. Thus it can be considered technologically enhanced naturally occurring radioactive material (TENORM).
Various applications have been proposed for using phosphogypsum, including using it as material for:
According to Taylor (2009), "up to 15% of world PG production is used to make building materials, as a soil amendment and as a set controller in the manufacture of Portland cement". The rest remains in stack.
The United States Environmental Protection Agency (EPA) has banned most applications of phosphogypsum having a 226Ra concentration of greater than 10 picocurie/gram (0.4 Bq/g) in 1990. As a result, phosphogypsum which exceeds this limit is stored in large stacks since extracting such low concentrations of radium is either not possible or not economical with current technology for either the use of the gypsum or the radium [citation needed]. Given the traditional definition of the Curie via the specific activity of 226
Ra, this limit is equivalent to 0.01 milligrams (0.00015 gr) of radium per metric ton or a concentration of 10 parts per trillion. (See § Gyp stacks below.)
EPA approved the use of phosphogypsum for road construction during the Trump Administration in 2020, saying that the approval came at the request of The Fertilizer Institute, which advocates for the fertilizer industry. Environmentalists opposed the decision, saying that using the radioactive material in this way can pose health risks. In 2021, the EPA withdrew the rule authorizing the use of phosphogypsum in road construction.
The state of Florida has approximately 80% of the world's phosphogypsum production capacity. In May 2023, the Florida legislature passed a bill requiring the Florida Department of Transportation to study the use of phosphogypsum in road construction, including demonstration projects, though this would require federal approval. The law, which requires the department to complete a study and make a recommendation by April 1, 2024, was signed into law by Governor Ron DeSantis on June 29, 2023.
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Phosphogypsum
Phosphogypsum (PG) is the calcium sulfate hydrate formed as a by-product of the production of fertilizer, particularly phosphoric acid, from phosphate rock. It is mainly composed of gypsum (CaSO4 · 2 H2O). 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 caused by the presence of naturally occurring uranium (U) and thorium (Th), and their daughter isotopes radium (Ra), radon (Rn) and polonium (Po). On the other hand, it includes several valuable components—calcium sulphates and elements such as silicon, iron, titanium, magnesium, aluminum, and manganese. However, the long-term storage of phosphogypsum is controversial. About five tons of phosphogypsum are generated per ton of phosphoric acid production. Annually, the estimated generation of phosphogypsum worldwide is 100 to 280 million metric tons.
Phosphogypsum is a by-product from the production of phosphoric acid by treating phosphate ore (apatite) with sulfuric acid according to the following reaction:
It is radioactive due to the presence of naturally occurring uranium (5–10 ppm) and thorium, and their daughter nuclides radium, radon, polonium, etc. Marine-deposited phosphate typically has a higher level of radioactivity than igneous phosphate deposits, because uranium is present in seawater at about 3 ppb (roughly 85 ppb of total dissolved solids). Uranium is concentrated during the formation of evaporite deposits as dissolved solids precipitate in order of solubility with easily dissolved materials such as sodium chloride remaining in solution longer than less soluble materials like uranium or sulfates. Other components of phosphogypsum include silica (5–10%), fluoride (F, ~1%), phosphorus (P, ~0.5%), iron (Fe, ~0.1%), aluminum (Al, ~0.1%), barium (Ba, 50 ppm), lead (Pb, ~5 ppm), chromium (Cr, ~3 ppm), selenium (Se, ~1 ppm), and cadmium (Cd, ~0.3 ppm). About 90% of Po and Ra from raw ore is retained into Phosphogypsum. Thus it can be considered technologically enhanced naturally occurring radioactive material (TENORM).
Various applications have been proposed for using phosphogypsum, including using it as material for:
According to Taylor (2009), "up to 15% of world PG production is used to make building materials, as a soil amendment and as a set controller in the manufacture of Portland cement". The rest remains in stack.
The United States Environmental Protection Agency (EPA) has banned most applications of phosphogypsum having a 226Ra concentration of greater than 10 picocurie/gram (0.4 Bq/g) in 1990. As a result, phosphogypsum which exceeds this limit is stored in large stacks since extracting such low concentrations of radium is either not possible or not economical with current technology for either the use of the gypsum or the radium [citation needed]. Given the traditional definition of the Curie via the specific activity of 226
Ra, this limit is equivalent to 0.01 milligrams (0.00015 gr) of radium per metric ton or a concentration of 10 parts per trillion. (See § Gyp stacks below.)
EPA approved the use of phosphogypsum for road construction during the Trump Administration in 2020, saying that the approval came at the request of The Fertilizer Institute, which advocates for the fertilizer industry. Environmentalists opposed the decision, saying that using the radioactive material in this way can pose health risks. In 2021, the EPA withdrew the rule authorizing the use of phosphogypsum in road construction.
The state of Florida has approximately 80% of the world's phosphogypsum production capacity. In May 2023, the Florida legislature passed a bill requiring the Florida Department of Transportation to study the use of phosphogypsum in road construction, including demonstration projects, though this would require federal approval. The law, which requires the department to complete a study and make a recommendation by April 1, 2024, was signed into law by Governor Ron DeSantis on June 29, 2023.