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Phosphate rock: analysis of the phosphate rock situation in the United States: 1990-2040.



Availability of phosphate rock

Geology: Magmatic or igneous rocks are the primary source of phosphorus on Earth. In igneous rocks, phosphorus is found in the microcrystalline apatite form. Granite contains an average O.1% phosphorus (0.23% [P.sub.2O.sub.5]); however, other igneous rocks are sufficiently higher in phosphorus to justify commercial mining and concentration.

All of the phosphorus found in sedimentary formations or deposits is derived from the weathering of igneous rock during geologic time. Sedimentary rocks also are weathered and leached and in the presence of limestone (calcium) are precipitated to form deposits rich in calcium phosphates throughout the Earth. The deposits are called phosphorites and are the principal supply source for the phosphate industry. Phosphorus also occurs in iron-oxide deposits as an impurity, and some calcium and aluminum phosphates are mined for agricultural purposes.

In igneous rocks, phosphorus occurs as apatite, usually a fluorapatite, [Ca.sub.5([PO.sub.4).sub.3]F. In sedimentary rocks, the form is usually hydroxyapatite [Ca.sub.5(PO.sub.4).sub.3]OH) or carboxyapatite [Ca.sub.5PO.sub.4CO.sub.3OH.sub.3)F. The solubility of apatites is relatively low, but on a geologic time scale they are degradable.

The lithosphere contains an average 0.12% P (0.27% [P.sub.2O.sub.5). The pedosphere contains only 0.04% P (0.09% [P.sub.2O.sub.5]). The hydrosphere contains only 0.01 mg/l P (0.023 mg/l [P.sub.2O.sub.5]) for North Atlantic surface water.

All living things contain phosphorus in either the soluble or insoluble form, organic or mineral. In the dry matter of plants, P content averages 0.4% (0.92% [P.sub.2.O.sub.5]). Phosphorus averages 0.9% (2.07% [P.sub.2O.sub.5]) in animal liveweight. In vertebrates, 75% of the phosphorus is located in the skeleton and teeth as hydroxyapatite microcrystals.

All U.S. production of phosphate rock comes from marine sedimentary rocks. Most of the current U.S. production is from the Atlantic Coastal Plain of Florida and North Carolina. Idaho, Montana, and Utah contribute smaller quantities. Only marine sedimentary deposits are expected to be mined in the United States during the next 50 years. There are no known U.S. deposits of igneous phosphate-rock.

The phosphate-ore deposits in the Atlantic Coastal Plain are flat-lying beds covered by overburden ranging from less than 15 m (50 ft) in Florida to about 30 m (100 ft) in North Carolina. The marine sedimentary deposits in the Western States, although of great lateral extent, are found in complex structural configurations. It is probable that future mining operations in the Atlantic Coastal Plain and Western States will be similar to present day operations. The principal gangue materials in all marine phosphorites are quartz, carbonate minerals (calcite and dolomite), alkali feldspars, clay minerals (montmorillonite, attapulgite, illite, sepiolite, chlorite, and kaolinite), iron oxide minerals (geothite and hematite), pyrite, and organic material.

The Florida and North Carolina phosphorite deposits contain potentially recoverable quantities of heavy minerals including rutile, zircon, ilmenite, and monazite. Phosphorites in the Western States contain zircon.

Marine phosphorites have an average uranium oxide ([U.sub.3O.sub.8]) content of about 0.01%. From five U.S. phosphoric acid plants that have a combined capacity of about 3.8 million mt/yr [P.sub.2O.sub.5], the [U.sub.3O.sub.8] recovery potential is about 1.7 million kg/yr [U.sub.3O.sub.8]. This potential has not been realized because of unfavorable economics and less demand for [U.sub.3O.sub.8] than was forecast.

The fluorine analysis of phosphate ore concentrates ranges from 3-4%. The industry controls fluorine emissions, and the amount lost to the environment is small. In the dihydrate phosphoric acid process, fluorine is evolved as a silicon tetrafluoride gas. Concentration of 30-54% phosphoric acid releases large volumes of silicon tetrafluoride and in most processes is recovered as fluosilicic acid.

Phosphate ores have very little potential as a source of rare-earth elements and some minor elements. Lanthanides and yttrium are found in apatites in trace amounts. They are not recovered from the phosphoric acid solution. Ferrophosphorus, a byproduct of electric-furnace reduction of phosphate ores, contains vanadium and chromium. Some vanadium is recovered.

About 5 mt phosphogypsum is produced for each ton of 100% [P.sub.2O.sub.5] phosphoric acid. Except for a small quantity used as a soil conditioner, it is stockpiled. It is not economically attractive to use purified phosphogypsum as a construction material. The sulphur cannot be extracted profitably until the price of sulphur reaches a much higher level than at present. Demonstrated Resources: Geological Survey Circular 882, "Sedimentary Phosphate Resource Classification System of the Bureau of Mines and the U.S. Geological Survey," was published in 1982 to develop a standardized, definitive, broadly applicable classification system to develop uniform, coordinated resource estimates. The principle of this system, published in Circulars 831 and 882, presents the criteria for classification of phosphate resources.

The phosphate-rock reserves and inferred reserves on federally managed lands are assessed and evaluated by the U.S. Department of the Interior. Some of this land is made available to the public for development through federally managed leasing programs. National phosphate-rock resource assessment requires an estimate of the reserve base and inferred reserve base that includes the physically determined deposits from which the reserves and the inferred reserves have been calculated.

Resource-reserve definitions are listed in Geological Survey Circular 882 with criteria for each resource class and diagrams showing the criteria for the southeast and northwest areas of the United States. Identified resources of phosphate rock are further classified by the terms demonstrated and inferred.

Measured resources of phosphate rock are computed from dimensions established by outcrops, trenches, workings, or drill holes. The criteria are set by industry and usually, consist of a sampling density of more than 64 boreholes/259 hectares (1 [mi.sup.2]).

Inferred resources of phosphate rock are estimates based on an assumed continuity beyond measured and/or indicated resources for which there is geologic evidence. Inferred resources of phosphate rock may or may not be supplied by samples or measurements.

Table 8, prepared by the Bureau's Branch of Minerals Availability, lists estimated production costs in January 1989 $/mt phosphate rock for various market economy countries (MECs). The costs of mining, beneficiation, incountry transportation, recovery of capital investment, taxation, and potential return on investment are included in the production cost table. Costs for processing phosphate rock into end products are not included. The costs include both surface- and underground-mining operations and in some countries, where a potential deposit has not been developed, the costs are included for comparative purposes. The costs are in constant dollars and are based on current technology. Since the costs are expressed in U.S. dollars, they will vary with the value of the dollar. In Brazil and Israel, where the rate of inflation is high and the currency is frequently devalued, the exchange rate to U.S. dollars causes costs to appear unrealistically high or low. [Tabular Data Omitted]

The table shows that the costs for producing mines in the southeastern United States are some of the lowest in the MECs. This reflects lower stripping ratios, higher pebble to concentrate ratios, and the economy of scale of large mining operations. It is probable that some of these advantages will be lost if new mines are developed in the southeastern United States. The higher costs are shown for the nonproducers in this region and for relatively small mines.

Table 9, prepared by the Bureau's Branch of Minerals Availability, shows the estimate of the U.S. phosphate-rock reserve base with recoverable product classified by costs. [Tabular Data Omitted]

Table 10 shows the estimate of world phosphate rock and reserve base. From 195 deposits, the reserve was estimated to be 12.6 billion mt and the reserve base was estimated to be 34 billion mt.

                                                      Reserve base
                     Number of        Reserves       (cost $100/mt
                      deposits   (cost<$40/mt fob)        fob)

North America
  Canada                  1                50                 50
  Mexico                  2                10                110
  United States          94             1,230              4,440
Total                    97             1,290              4,600

South America
  Brazil                 11               330                370
  Colombia                1                --                100
  Preu                    1               310                310
  Venezuela               1                --                 10
Total                    14               640                790

  Finland                 1                --                 70
  Turkey                  1                30                 30
  Soviet Union           11             1,330              1,330
Total                    13             1,360              1,430

  Algeria                 1               240                240
  Egypt                   5                --                760
  Morocco                10             4,950             20,490
  Western Sahara          1               950                950
  Senegal                 2                --                160
  South Africa            1             2,530              2,530
  Togo                   12                --                 60
  Tunisia                11                --                270
Total                    43             8,670             25,480

  China                   6               210                210
  Christmas Island        1                10                 10
  Israel                  4                --                180
  Jordan                  3                90                480
  Syria                   2               190                190
  Other                   6                30                330
Total                    22               530              1,400

  Australia               5                90                590
  Nauru                   1                 5                  5
Total                     6                95                595
World total             195            12,585             34,275

Mining and processing technology

Mining: Phosphate mining in Florida and North Carolina is accomplished by stripping overburden with a dragline, bucket wheel excavator, or dredge and depositing the material in mined-out cuts or areas. The same equipment or an alternative was used to dig the matrix (ore-bearing material), after which it was deposited in a slurry well. It is then pumped to the beneficiation plant. Since the materials handled are usually in the range of 15-25% moisture, these procedures do not generate airborne particulate emissions. During the dry season, mine roads can create minor quantities of airborne particulates. Radon is not considered a problem in the mining area. The leach zone, near the contact with the underlying matrix, contains the most highly active radioactive material in the mining operation. The material is wasted with the overburden in the stripping process and then covered with sufficient overburden so that it is not exposed when the land is reclaimed. Uranium, other radionuclides, and fluorine in the matrix are all in a water-insoluble state and do not combine with either water or air to create a potential hazard. The long-term decay characteristics of radium-226 does create some airborne radioactivity; however, the concentrations are not sufficiently high to cause working condition hazards. The half-life of radon is very brief.

The overall recovery of [P.sub.2O.sub.5] in the product is rarely greater than 75% of the value in the matrix. [P.sub.2O.sub.5] recovery from mining is of the order of 85-90% and washing and flotation processes lose additional [P.sub.2O.sub.5] in waste streams.

In Tennessee, phosphate ore is mined, after the overburden is stripped, with small 1.5-2.3 [m.sup.3] draglines that can extract the ore between the cutters (limestone pinnacles). The last phosphate-rock mine in Tennessee was closed on April 29, 1991.

A variety of earth-moving machinery is used in the Western United States to strip overburden and mine phosphate deposits. The machinery that is used includes power shovels, scrapers, bulldozers, backhoes, and front-end loaders. Beneficiation is practiced to prepare the ore for WPPA manufacture, and the lower grades are agglomerated prior to charging to electric furnaces.

In beneficiating plants preparing concentrates for WPPA plants, processing is conducted in the wet state and no hazardous chemicals are used. Only small quantities of flotation reagents are used, and they finally report in the clay wastes where they are absorbed and neutralized. Since no chemical alteration occurs in the washing and flotation processes, the minerals remain water-insoluble and are not distributed in the environment. Essentially, only phosphate rock destined for export is dried before shipping. Most phosphate rock is wet-ground prior to manufacturing WPPA.

In southeastern U.S. phosphate operations, the solid wastes from beneficiating the matrix are clay slimes and flotation sand tailings. The colloidal clays are impounded in slime ponds that are reclaimed after dewatering.

The amount of radioactive materials in the wastes appears to vary with the grade of the matrix. Most of the wastes are eventually covered with overburden to minimize the release of radon gas to the atmosphere.

Phosphate-rock-mining wastes, processing wastes such as phosphogypsum from WPPA plants, electric-furnace slag, and phossy water (containing phosphorus) are all under review by the Environmental Protection Agency. The wastes are exempt from hazardous waste regulations. Recently, action has removed some of these wastes from the Bevill amendment that excluded mineral processing waste from regulatory control such as furnace off-gas solids from elemental-phosphorus production. The exemption was retained for phosphogypsum from WPPA production and slag from elemental-phosphorus production.

The phosphate mining industry, particularly in Florida and North Carolina, is restricted from disturbing wetlands unless a feasible reclamation program is demonstrated.

National Forest lands

The Bureau of Land Management has issued leases to mine phosphate rock from the Caribou National Forest. Leases were withheld to mine a phosphate-rock deposit on Pine Mountain in the Los Padres National Forest. Pine Mountain was considered part of the critical habitat of the California Condor, which is endangered.

Forty-one lease applications for phosphate-rock-mine development on the Osceola National Forest were denied by the Secretary of the Interior. The Secretary's decision was based on the findings of several technical studies spanning 15 years on hydrology and endangered species, as well as an Environmental Impact Statement and an updated environmental assessment.
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Title Annotation:Part 2
Author:Stowasser, W.F.
Publication:E&MJ - Engineering & Mining Journal
Date:Oct 1, 1991
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