Phosphoric acid production

The estimated world production of wet-process phosphoric acid was 24,001,000 metric tons of P20 in 1993. Capacity was 34,710,000 metric tons. Over 90% of phosphoric acid production is wet-process (agricultural-grade) acid the remainder is industrial-grades (technical, food, pharmaceutical, etc) made by the thermal route or by the purification of wet-process acid. Table 11 fists U.S. production of wet-process and industrial-grade acids. 

Aluminum fluoride is also made by the reaction of fluosiUcic acid [16961 -83-4] H2SiFg, a by-product from phosphoric acid production (see Phosphoric ACID AND THE PHOSPHATES), and aluminum hydroxide from the Bayer process. 

Phosphate fertilizer complexes often have sulfuric and phosphoric acid production facilities. Sulfuric acid is produced by burning molten sulfur in air to produce sulfur dioxide, which is then catalytically converted to sulfur trioxide for absorption in oleum. Sulfur dioxide can also be produced by roasting pyrite ore. Phosphoric acid is manufactured by adding sulfuric acid to phosphate rock. The... 

Ca5 (P04)3 F( ) + 5 H2 S04(cz - 3 H3 P04(t2 ) + 5 CaS04( ) + HF(ts The dilute phosphoric acid obtained from this process is concentrated by evaporation. It is usually dark green or brown because of the presence of many metal ion impurities in the phosphate rock. However, this impure acid is suitable for the manufacture of phosphate fertilizers, which consumes almost 90% of phosphoric acid production. 

No compound containing a halogen other than chlorine appears among the top 50 industrial chemicals, but fluorine nevertheless has considerable commercial value. Fluorine occurs as the mineral fluorite (CaF2) and is prevalent in phosphate-bearing rock. As already mentioned, HF produced from sulfiaric acid treatment of fluorite supplies some 70% of industrial HF the remainder comes as a by-product of phosphoric acid production. 

This subcategory involves phosphoric acid (dry process), phosphoms pentoxide, phosphoms pentasulfide, phosphoms trichloride, and phosphoms oxychloride. In the standard dry process for phosphoric acid production, liquid phosphoms is burned in the air, the resulting gaseous phosphoms pentaoxide is absorbed and hydrated in a water spray, and the mist is collected with an electrostatic precipitator. Regardless of the process variation, phosphoric acid is made with the consumption of water and no aqueous wastes are generated by the process. 

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. 

In a recent document [25] presenting techniques adopted by the French for pollution prevention, a new process modification for steam segregation and recycle in phosphoric acid production is described. As shown in Figure 9, raw water from the sludge/fluorine separation system is recycled to the heat-exchange system of the sulfuric acid dilution unit and the wastewater used in plaster manufacture. Furthermore, decanted supernatant from the phosphogypsum deposit pond is recycled for treatment in the water filtration unit. The claim was that this process modification permits an important reduction in pollution by... 

About 60 percent of the sulfuric acid in turn is used for agricultural purposes, mainly in phosphoric acid production, and intermediate in fertilizer manufacture. Another important use of sulfuric acid is in petroleum refining. Chemical uses, including plastics, paper and paint, account for 8 percent of sulfur production, sulfuric acid again being a major outlet. Ferrous and non-ferrous metal production consumes 8 percent and miscellaneous uses add up to 18 percent. 

Kislik V and Eyal A. Heavy metals removal from wastewaters of phosphoric acid production. A comparison of hybrid Uquid membrane (HLM) and aqueous hybrid liquid membrane (AHLM) technologies. In Proceedings of the Conference on Membrane in Drinking and Industrial Water Production, 2000 1 503-514. 

In phosphoric acid production plants, both red shale and fireclay brick have excellent resistance to all concentrations of phosphoric acid at temperatures up to 250°F, provided the acid contains no HF. If HF is present in the phosphoric acid, carbon brick construction must be used. As a rough rule-of-thumb, HF levels above 50 ppm in phosphoric acid require tank linings of carbon brick bonded and jointed with a carbon (or barytes) filled furan mortar over a suitable membrane to match the steel or concrete substrate structure. 

The volatile fluorides formed by this process are captured by water scrubbers (Section 10.4.3). However, about 58% of agricultural rock, or 50% of the total, ultimately goes into wet process phosphoric acid production (Section 10.4). Nonagricultural uses consume 13% of the total for elemental phosphorus production, from which both high-purity phosphoric acid and other phosphorus derivatives are made (Table 10.2). Nearly half of this, about 6% of the total, goes into the manufacture of phosphate builders for detergents, about a tenth into food and beverage additives, and the remainder into a multitude of small-scale applications. 

Phosphoric acid production by phosphorus combustion is usually accomplished in a stepwise manner as outlined, with intermediate isolation of the phosphorus. However, it may also be made by direct contact of phosphorus vapor from the furnace of a phosphorus plant with an air stream, and then passing the phosphorus pentoxide produced directly into a hydrator, without collection of the intermediate phosphorus as a liquid. Direct conversion to... 

The fluoride ion usually present in the phosphate rock is of no commercial value, and adds substantial difficulties to phosphoric acid production by this method since strong acid acidulation releases virtually all of the fluoride originally present in the rock. Considering just the phosphoric acid-forming part of the overall acidulation reaction, this is a moderately exothermic process [1] (Eq. 10.22). 

FIGURE 10.3 Process outline for phosphoric acid production by sulfuric acid acidulation of phosphate rock. Fume control and slurry evaporative cooling systems are shown, together with a countercurrent filtration and washing system which serves to maximize product acid concentrations accessible without evaporation. (From Shreve and Brink [30 a] by permission, McGraw-Hill.)... 

Incorporation of methanol at the rate of 2 kg/kg phosphorus pentoxide in the dried acidulate significantly improves the purity of the phosphoric acid product in a two-stage process. This modification is especially advantageous with a low-grade phosphate rock [42]. Acetone has also been found of value in an experimental variant which enables much less expensive sulfurous acid, rather than sulfuric acid, to be used as the acidulant [43]. In this case, the solvent forms an a-hydroxysulfonic acid with the sulfurous acid (Eq. 10.27) which is sufficiently strong to attack phosphate rock at a reasonable rate. 

TABLE 10.8 Phosphoric Acid Production by Selected Countries"... 

Most of the phosphoric acid production in the U.S., about 85%, is consumed by fertilizer manufacturers, mostly for preparation of ammonium phosphates and triple superphosphate (Chap. 11). This consumption picture may be slightly distorted since the U.S. is also a substantial exporter of phosphoric acid. 

H. El-Shall, E.A. Abdel-Aal, and B.M. Moudgil, Effect of Surfactants on Phosphogypsum Crystallization and Filtration... During Wet-process Phosphoric Acid Production, Sep. Set. Technol. 35(3), 395 10 (2000). 

Phosphogypsum is the major byproduct of wet-process phosphoric acid production. Phosphate rock, which is composed of apatite minerals (4], (calcium phosphates containing varying amounts of carbonate and fluoride), is digested with sulfuric acid and water to produce phosphoric acid, phosphogypsum, and minor quantities of hydrofluoric acid. 

If the reaction is carried out in water enriched with 0, the oxygen-18 isotope is found in the phosphoric acid product but not in the methanol. What does this tell us about the mechanism of the reaction ... 

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