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Federal Highway Administration Research and Technology
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Publication Number: FHWA-RD-97-148
User Guidelines for Waste and Byproduct Materials in Pavement Construction
Fluorogypsum and phosphogypsum are sulfate-rich by-products generated during the production of hydrofluoric and phosphoric acid, respectively.
Fluorogypsum is generated during the production of hydrofluoric acid from fluorspar (a mineral composed of calcium fluoride) and sulfuric acid. Fluorogypsum is discharged in slurry form and gradually solidifies into a dry residue after the liquid has been allowed to evaporate in holding ponds. When removed from the holding ponds (if it is to be used), the dried material must be crushed and screened. This produces a sulfate-rich, well-graded sandy silt material with some gravel-size particles. Approximately 90,000 metric tons (100,000 tons) of fluorogypsum are generated annually in the United States, mostly in Delaware, New Jersey, Louisiana, and Texas.
Phosphogypsum is a solid by-product of phosphoric acid production. The most frequently used process for the production of phosphoric acid is the "wet process," in which finely ground phosphate rock is dissolved in phosphoric acid to form a monocalcium phosphate slurry. Sulfuric acid is added to the slurry to produce phosphoric acid (H3PO4) and a phosphogypsum (hydrated calcium sulfate) by-product.
Phosphogypsum is generated as a filter cake in the "wet process" and is typically pumped in slurry form to large holding ponds, where the phosphogypsum particles are allowed to settle. The resulting product is a moist gray, powdery material that is predominantly silt sized and has little or no plasticity.
Approximately 32 million metric tons (35 million tons) of phosphogypsum are produced annually, mostly in central Florida, but also in Louisiana and southeastern Texas. Total accumulations of phosphogypsum are well in excess of 720 million metric tons (800 million tons) and are expected to approach 900 million metric tons (1 billion tons) by the year 2000.(1) As a general rule, 4 to 5 metric tons (4.5 to 5.5 tons) of phosphogypsum are generated for every ton of phosphoric acid produced.
CURRENT MANAGEMENT OPTIONS
Fluorogypsum is not being used in any commercial applications; however, fluorogypsum has been evaluated for use as a road base material.(2) It has also been proposed for use in the production of impure plasterboard.(3)
All fluorogypsum is currently landfilled or disposed of in holding ponds.
Phosphogypsum is a calcium sulfate hydrate that is pumped into ponds, eventually dewatered, and ultimately disposed of in large stockpiles called stacks. Phosphogypsum has been recovered and reused with some success in stabilized road bases,(4) unbound road bases,(5) and roller-compacted concrete.(6) Phosphogypsum can be used for agricultural purposes, if the radium-226 concentration of the source material is less than 10 pCi/g.(7) At the present time, a petition to the EPA is required if phosphogypsum is planned for use in highway applications.(7)
At the present time all phosphogypsum is stockpiled in large stacks, some of which may occupy several hundred hectares of land.
Fluorogypsum may be obtained by contacting the chemical companies that produce hydrofluoric acid in a number of industrialized states, including Delaware, New Jersey, Louisiana, and Texas.(8)
Fluorogypsum that has solidified in sedimentation ponds is removed by blasting, crushing, and screening, much like rock is obtained from a quarry. The recovery and processing of solidified fluorogypsum results in a coarse or "crusher run" type of aggregate with a 38 mm (1-1/2 in) top size and fine calcium sulfate or "natural fines," which is predominantly a sand- and silt-sized material.(2) The recovery and processing of solidified fluorogypsum may be accomplished by commercial aggregate producers who are under contract to the chemical companies.
Phosphogypsum may be obtained by directly contacting phosphate producers located mainly in Florida, Louisiana, or Texas, since the companies that mine phosphate rock and produce fertilizers from it also have ownership rights to the phosphogypsum stacks.
There are, however, environmental concerns regarding radon emanation from phosphogypsum stacks. Special petition requirements, which are defined by the EPA for any commercial use or research activity, must be followed before phosphogypsum can be used.(7)
HIGHWAY USES AND PROCESSING REQUIREMENTS
Embankment, Fill, and Road Base Material
Limited local use has thus far been made of reclaimed dried fluorogypsum. This material has been previously used in West Virginia as a fill material, as a subbase material, and as aggregate in a lime-fly ash stabilized base. The solidified fluorogypsum was blasted, removed, crushed and screened prior to being used as a coarse and fine aggregate material in base course applications.(1)
Stabilized Base Filler
To date, phosphogypsum has been successfully demonstrated as a road base material in stabilized and unbound base course installations and in roller-compacted concrete mixes. The only processing required for the phosphogypsum is the use of a vibrating power screen to break up lumps prior to mixing with a binder.
As previously noted, fluorogypsum solidifies in holding ponds and must be removed, crushed, and graded, if it is to be used as an aggregate substitute material. In the process of size reduction, coarse 38 mm (1-1/2 in) top size material and fine, minus 2.0 mm (No. 10 sieve), sulfate-rich material is produced. The coarse sulfate is a well-graded sand and gravel size material, while the fine sulfate is a silty-clay type material.
Table 19-1 presents some typical fluorogypsum particle size ranges, moisture content and specific gravity values. The average moisture content of the coarse sulfate material reportedly ranges from 6 to 9 percent, while the average moisture content of the fine sulfate material ranges from 6 to 20 percent. The average specific gravity of the coarse and fine sulfate is approximately 2.5, indicating that fluorogypsum is slightly lighter in weight than naturally occurring aggregates, such as crushed limestone or sand and gravel.(1)
Table 19-1. Typical physical properties of fluorogypsum.
Table 19-2 presents an average chemical analysis of samples of coarse and fine fluorogypsum.(1) Fluorogypsum is primarily calcium sulfate with approximately 1 to 3 percent fluoride present. It exhibits slightly acidic properties.
Fluorogypsum particles, although solidified in a holding pond, are relatively soft. The results of Los Angeles Abrasion tests performed on a composite sample of coarse fluorogypsum indicate a relatively high abrasion loss of 84 percent.(1)
Table 19-2. Typical chemical composition of fluorogypsum
As previously noted, phosphogypsum is a damp, powdery, silt or silty-sand material with a maximum size range between approximately 0.5 mm (No. 40 sieve) and 1.0 mm (No. 20 sieve) and between 50 and 75 percent passing a 0.075 mm (No. 200) sieve size. The majority of the particles are finer than .075 mm (No. 200 sieve), and the moisture content usually ranges from 8 to 20 percent. The silty size range of phosphogypsum would classify it as an A-4 soil in the AASHTO soil classification system.(10)
There are two predominant forms of phosphogypsum: dihydrate phosphogypsum (CaSO4 × 2H2O) and hemihydrate phosphogypsum (CaSO4 ×½H2O). Dihydrate phosphogypsum is generally more finely graded than hemihydrate phosphogypsum.
Table 19-3 presents some typical physical properties of phosphogypsum.
Table 19-3. Typical physical properties of phosphogypsum.(5)
The specific gravity of phosphogypsum ranges from 2.3 to 2.6. The optimum moisture content of either type of phosphogypsum can normally be expected to fall within the range of 15 to 20 percent. The maximum dry density is likely to range from 1470 to 1670 kg/m3 (92 to 104 lb/ft3), based on standard Proctor compaction.(5)
These are typical values for phosphogypsum produced in Florida. Values for phosphogypsum produced in other states (such as Texas or Louisiana) may vary somewhat. The addition of fly ash or Portland cement to phosphogypsum yields slightly higher maximum dry density and optimum moisture content values for stabilized phosphogypsum mixtures, in comparison with unstabilized phosphogypsum blends.(11)
The major constituent in phosphogypsum is calcium sulfate and, as a result, phosphogypsum exhibits acidic properties. Table 19-4 presents a listing of some typical chemical analyses of phosphogypsum samples from different production areas.(11) Phosphogypsum often contains small residual amounts of phosphoric acid and sulfuric acid and also contains some trace concentrations of uranium and radium, which result in low levels of radiation.
Table 19-5 presents some typical mechanical property values of phosphogypsum. The shear strength of unconsolidated-undrained specimens of unstabilized phosphogypsum has exhibited average internal friction angles of 32 degrees and cohesion values of 125 kN/m2 (18 lb/in2). Cement-stabilized specimens have exhibited internal friction angle values ranging from 28 to 47 degrees, and cohesion values from 76 to 179 kN/m2 (11 to 26 lb/in2).(12) Coefficient of permeability values for unstabilized phosphogypsum have been found to range from 1.3 ´10-4 cm/sec down to 2.1 ´ 10-5 cm/sec.
Table 19-4. Typical chemical composition of phosphogypsum (percent by weight).(11)
Table 19-5. Typical mechanical properties of phosphogypsum.(12)