Phosphate rock ground

In the Prayon process, commonly used in Florida, the phosphate rock, ground to pass a 150-p.m (100-mesh) sieve, is treated with 30 to 46% phosphoric acid and 55 to 60% sulfuric acid. The rock and acid is circulated through reaction tanks to maintain the optimum time and temperature for the reaction and growth of phosphogypsum crystals. The phosphogypsum is filtered, washed with water, and pumped as a slurry to ponds from which the phosphogypsum settles to form the phosphogypsum stacks [5]. 

Direct Application Rock. Finely ground phosphate rock has had limited use as a direct-appHcation fertilizer for many years. There have been widely varying results. Direct appHcation of phosphate rock worldwide amounts to about 8% of total fertilizer phosphate used, primarily in the former Soviet Union, France, Brazil, Sri Lanka, Malaysia, and Indonesia. The agronomic effectiveness of an apatitic rock depends not only on the fineness of the grind but also strongly on the innate reactivity of the rock and the acidity of the sod performance is better on more acid sods. Probably more than half of the potentially productive tropical sods are acidic, some with pH as low as 3.5—4.5. Certain phosphate rocks may thus become increasingly important as fertilizer in those areas. The International Fertilizer Development Center at Muscle Shoals, Alabama is active in researching this field (30). 

Phosphate Rock Grinding. Phosphate rock is mined and mechanically ground to provide the optimum particle size required for phosphoric acid production. There are no hquid waste effluents. 

Normal Superphosphate. Normal superphosphate is produced by the reaction between ground phosphate rock and sulfuric acid, followed by three to eight weeks of curing time. Obnoxious gases are generated by this process. 

Rock Grinding. Until 1973, most phosphate was ground dry in roller or ball mills. In that year, Agrico, at South Pierce, Florida, converted one of its dry mills to wet slurry grinding and proved that the plant water balance could manage the rock at a 65-68 percent solids slurry. Since that time, most U.S. installations have converted to wet grinding. 

Defluorinated Phosphate Rock. There is substantial production of defluorinated phosphate rock for fertilizer use in Japan (about 100,000 mt/year). Ground, high-grade rock is mixed with small proportions of sodium carbonate or sulfate and wet-process acid. The mixture is calcined at a temperature of 1350°C in an oil-fired rotary kiln 45.0 m in length and 2.7 m in diameter. The product contains 38-42 percent P205 of which more than 90 percent is soluble in neutral ammonium nitrate solution and is an effective fertilizer on acid soils. During the production of defluorinated phosphate rock, substantially all fluorine is driven off. Sodium bifluoride (NaHF2) is recovered as a byproduct. A similar product is made in the United States, but it is mainly used for animal feed supplement. 

Partially Acidulated Phosphate Rock. Not all phosphate rock is suitable for direct application and may require the addition of a more soluble form of phosphate. An alternative is partial acidulation to render its P205 more available. The partially acidulated phosphate rock (PAPR) process depends on treating ground phosphate rock with only a portion of the stoichiometric value of acid, for example 50 percent PAPR.19 The amount of water-soluble phosphate in PAPR varies according to the degree of acidulation. The crop response to the product is often quite similar to fully acidulated products such as SSP or TSR... 

Controlled-Release Phosphate Fertilizers. Several controlled-release phosphates have been produced commercially. Important among them are ground phosphate rock, basic slag, and fused calcium magnesium phosphate. These products have been discussed earlier. 

The mixture of bone-ash or ground phosphate rock, sand and coal dust or wood charcoal is introduced continuously into a furnace heated by the electric current, passing between carbon electrodes (see p. 8). The reaction begins at about 1100° C., but a much higher temperature is required to melt the charge and slag of calcium silicate which is drawn off continuously, while the phosphorus distils at about 1300° C. The yield is said to be about 92 per cent, of the theoretical. 

Phosphorus (0.11% of the lithosphere) is found mainly in minerals based on calcium phosphate, collophanite, the monohydrate, Ca3(P04)2.H20, and apatite, CagF(P04)3. About 90% of the phosphate rock mined is converted into fertilisers, the rest is used for making elementary phosphorus, phosphorus compounds and such alloys as phosphor bronze. For fertilisers rock phosphate is finely ground and treated with sufficient concentrated sulphuric acid to convert it to the soluble dihydrogen phosphate ... 

Some heavy discharges of F into the atmosphere and waters have occurred in connection with the manufacture of elemental phosphorus, phosphate fertilizer, and aluminum. In the manufacture of elemental phosphorus, ground phosphate rock, whose main component is Ca10F2(PO4)6, is mixed with silica and coke and then heated in a carbon-lined furnace with carbon electrodes. Equation (8.1) shows the chemical reaction involved in this process ... 

The oldest, and still the lowest cost route to phosphoric acid (see Section 10.2) is via the addition of high concentrations of sulfuric acid to finely ground phosphate rock. This acidulation step releases phosphoric acid from the calcium phosphate salts present and produces insoluble gypsum (calcium sulfate dihydrate) which can be removed by filtration (Eq. 10.21). 

Finely ground phosphate rock (Ca3(P04)2) is occasionally added to fertilizer formulations as a diluent or filler. However, because phosphate rock has only a very limited water solubility its action as a phosphate nutrient is small and very slow. Hence, this ingredient is not allowed to be included in the % P2O5 analysis appearing on fertilizer packaging. Before use as a filler, phosphate rock is normally defluorinated by heating with silica and steam to decrease the risk of soil contamination by fluoride (Chap. 10). 

Phosphate rock, crude (ground or not ground) Sulfur (brimstone)... 

Supeiphosphate" is now made by the (highly exixhermic) addition of HjSOa to fine-ground phosphate rock ... 

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