Remediation of soil polluted with phosphorus-containing wastes

Details Remediation of soil polluted with phosphorus-containing wastes Remediation of soil polluted with phosphorus-containing wastes

1999-06-14

2001-01-02

Griffin, Steven P. (Department: 1754)

Hazardous or toxic waste destruction or containment

Containment

C588S249000, C588S249000, C423S299000, C423S322000, C423S323000

Utility Patent

active

06169222

ABSTRACT:

BACKGROUND OF THE INVENTION
The Tennessee Valley Authority (TVA) produced about 1.1 million tons of elemental phosphorus at the agency's facility at Muscle Shoals, Ala. The chemical was produced over a 40-year period beginning in 1936. TVA had to content with a hazardous waste problem because some of the phosphorus vapor condensed as phosphorus sludge which was comprised of water, inorganic solids and particles of elemental phosphorus. When the sludge contained as much as 60 percent elemental phosphorus, or more, it could be burned to produce impure phosphoric acid and this acid could be combined with phosphate ore to produce concentrated superphosphate fertilizer. However, some of the sludge contained less than 60 percent elemental phosphorus and it could not be used to produce the fertilizer. Elemental phosphorus in phosphorus sludge is not completely oxidized when the sludge contains less than 60 percent elemental phosphorus. The lower oxides, such as P
2
O
3
, are phytotoxic and can not be used for fertilizer. Phosphorus sludge accumulated at TVA's phosphorus production facility awaiting technology to recover the elemental phosphorus.
Concentrated superphosphate fertilizer was phased out by TVA during the latter part of the 1950's and the agency developed processes to produce liquid fertilizers which were produced in demonstration-scale units operated by TVA. The impure phosphoric acid obtained by burning sludge was unsuited for production of clear liquid fertilizers, furthermore, highly concentrated phosphoric acid needed for liquid fertilizers could not be made by burning phosphorus sludge.
Elemental phosphorus is normally produced by smelting the mineral fluroapatite in electric furnaces. Temperatures in the electric furnaces are in the range of 2700 to 2800° F., and these high temperatures caused silicon tetrafluoride (SiF
4
) to be volatilized. The furnace gases are cooled by contacting them with water and silica precipitates as indicated by the following equation.
3SiF
4
+2H
2
O═2H
2
SiF
6
+SiO
2
.
The precipitated SiO
2
is a gel-like material and it contributes to the formation of phosphorus sludge.
Some inorganic solids volatilize from the electric furnace because of the high smelting temperatures. The volatilized solids condense in the furnace gas forming small particles which are dispersed as colloids in the gas stream by a phenomenon similar to cooling water vapor in the atmosphere to form fog. Some phosphorus vapor condenses as small particles of elemental phosphorus. The dispersion of inorganic materials and small particles of elemental phosphorus in water, an immiscible liquid, together with precipitated SiO
2
, gives rise to the formation of the emulsion called phosphorus sludge.
Furnace gas is cooled by contacting it with water in a condenser. Since some of the phosphorus vapor condenses as small particles the water will contain colloidal particles of elemental phosphorus and a small quantity of the chemical is dissolved in the water. Furthermore, paticles of elemental phosphorus may be large enough for separation by settling. Water containing either dissolved phosphorus, colloidal particles of phosphorus, or settleable particles is called phossy water.
In 1939 TVA was operating Nos. 1, 2, and 3 phosphorus furnaces when a newly constructed furnace—No. 4—was started. The No. 4 furnace was equipped with an electrostatic precipitator to collect inorganic particles in the furnace gas prior to the condensation of the phosphorus vapor by contacting the furnace gas with water. Nos. 1, 2, and 3 furnaces were later retrofitted with electrostatic precipitators, and furnace Nos. 5, 6, and 7 were constructed later and they were equipped with precipitators. The proportion of elemental phosphorus that was recovered as phosphorus sludge was reduced by about half when electrostatic precipitators were used to clean the furnace gas.
Particulates collected by the electrostatic precipitators was called precipitator dust. The dust was gray, extremely fine-grained material mixed with fragments (about 1 mm in size) of coke, phosphate and slag. The bulk of the dust, however, consisted of inorganic materials that had volatilized from the furnace or had been entrained in the gas stream. These particles were approximately 2 microns or smaller in size, but inside the precipitator discrete particles aggregated into loose clusters up to 30 microns in size. Further aggregation into large lumps occurred when the dust was exposed to air.
Phosphorus vapor and carbon monoxide adsorbed on the surfaces of the inorganic materials and precipitator dust burned upon exposure to air. Elemental phosphorus content varied from not detectable to nearly 1.5 percent depending on the temperature of the dust. With the installation of precipitators, TVA had to contend with three phosphorus-containing wastes; they are phosphorus sludge, precipitator dust, and phossy water.
Dry precipitator dust was readily suspended in air and ingestion of the material by personnel was a serious industrial hygiene problem. To avoid the health hazard, precipitator dust was slurried with water inside the precipitator housing. However, contacting phosphorus-containing solids with water generates phossy water which is highly toxic to marine life. A few parts of elemental phosphorus per billion parts of water is sufficient to kill some species of marine life. Accordingly, precipitator dust slurry had to be dewatered before the solids could be stockpiled. Without dewatering phossy water would drain from the stockpiles and would be discharged into the Tennessee River causing fishkills.
Precipitator dust slurry was dewatered by pumping the slurry to one of two areas where the phossy water was allowed to drain from the wet solids. Dikes were constructed using granulated calcium silicate slag to prevent the discharge of phossy water. Phossy water that separated from the solids was pumped to a large mile of granulated slag where elemental phosphorus was removed prior to discharging the liquid as an aqueous waste. The dewatered precipitator dust was loaded into railroad cars and transported to an area nearby for stockpiling.
Soil in the dewatering areas was polluted with precipitator dust. The crane bucket used to load wet precipitator dust into railroad cars leaked because of the small particles and the soil around the dewatering areas became polluted. Soil under the precipitator dust piles is obviously polluted because no impermeable barrier was used to prevent contact between the dust and soil. Phosphorus sludge was stored in tanks, railroad cars, sumps and in a landfill. Soil under the landfill is obviously polluted with phosphorus sludge, A 14-acre settling pond was constructed to remove suspended particles of elemental phosphorus in phossy water. Particles of yellow phosphorus and particles of phosphorus sludge that were in phossy water settled out and have accumulated in the settling pond. Soil under the accumulations in the settling pond is polluted with elemental phosphorus and phosphorus sludge. The present patent application discloses a process to remediate the polluted soil and thereby restore the land to full usefulness.
DISCLOSURES IN THE INVENTION
TVA investigated the smelting of phosphatic materials in the laboratory, in pilot plants, and engineering tests and studies were carried out on production units. The results of TVA's research and development has been compiled and reported in publications such as, “Production of Elemental Phosphorus by the Electric Furnace Method”, Tennessee Valley Authority, Chemical Engineering Report No. 3, 1952, Burt, R. B. and Barber, J. C. This report and similar publications provide a compendium of technical information to draw on for the development of processes to remediate phosphorus-containing wastes and to remediate polluted soil.
Electric furnace smelting of phosphatic materials to produce elemental phosphorus began with British patent No. 14,962 which was issued Oct. 18, 1888. The inventor was James Burgess Readman. A 60-kw electric furnace was c

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