Fluorine, phosphate rock

Production Technology. Processes for extraction of P2O3 from phosphate rock by sulfuric acid vary widely, but all produce a phosphoric acid—calcium sulfate slurry that requires soHds-Hquid separation (usually by filtration (qv)), countercurrent washing of the soHds to improve P2O3 recovery, and concentration of the acid. Volatilized fluorine compounds are scmbbed and calcium sulfate is disposed of in a variety of ways. 

The ores of most importance are fluorspar, CaF2 fluorapatite, Ca (P0 2Fj cryoHte [15096-52-3], Na AlF. Fluorspar is the primary commercial source of fluoiine. Twenty-six percent of the world s high quaHty deposits of fluorspar are ia North America. Most of that is ia Mexico. United States production ia 1987—1991 was 314,500 metric tons, most of which occurred ia the Illinois-Kentucky area. Imported fluorspar ia 1990—1991 represented about 82% of U.S. consumption 31% of U.S. fluorspar imports were from Mexico and 29% from China compared to 66% from Mexico ia the 1973—1978 period. The majority of the fluorine ia the earth s cmst is ia phosphate rock ia the form of fluorapatite which has an average fluorine concentration of 3.5%. Recovery of these fluorine values as by-product fluorosiHcic acid from phosphate production has grown steadily, partially because of environmental requirements (see Phosphoric acid and THE phosphates). 

In the geochemistry of fluorine, the close match in the ionic radii of fluoride (0.136 nm), hydroxide (0.140 nm), and oxide ion (0.140 nm) allows a sequential replacement of oxygen by fluorine in a wide variety of minerals. This accounts for the wide dissemination of the element in nature. The ready formation of volatile silicon tetrafluoride, the pyrohydrolysis of fluorides to hydrogen fluoride, and the low solubility of calcium fluoride and of calcium fluorophosphates, have provided a geochemical cycle in which fluorine may be stripped from solution by limestone and by apatite to form the deposits of fluorspar and of phosphate rock (fluoroapatite [1306-01 -0]) approximately CaF2 3Ca2(P0 2 which ate the world s main resources of fluorine (1). 

The large amount of fluorine values released from phosphate rock in the manufacture of fertilisers (qv) gives a strong impetus to develop fluorine chemicals production from this source (see Phosphoric acid and the phosphates). Additional incentive comes from the need to control the emission of fluorine-containing gases. Most of the fluorine values are scmbbed out as fluorosiUcic acid, H2SiPg, which has limited useflilness. A procedure to convert fluorosihcic acid to calcium fluoride is available (61). 

The majority of the fluorine ia the earth s cmst is present in the form of the phosphoms fluoride fluoroapatite [1306-05 ] Ca (P0 2F- Phosphate rock deposits contain an average concentration of 3.5 wt % fluorine. During phosphate processing these fluorine values are partially recovered as by-product fluorosihcic acid. The amount of fluorosiUcic acid recovered has grown steadily, in part because of environmental requirements (see Phosphoric acid and THE phosphates). 

Synthetic cryolite solved the supply problem, but synthetic cryolite requires fluorine which is actually more abundant in the Earth s crust than chlorine, but dispersed in small concentrations in rocks. Until the 1960s, fluorspar (CaFj) a mineral long known and used as a flux in various metallurgical operations was the source. A source is phosphate rock that contains fluorine i.s 3% quantity,... 

Wet Process Phosphoric Acid. A production process flow diagram is shown in Figure 8. Insoluble phosphate rock is changed to water-soluble phosphoric acid by solubilizing the phosphate rock with an acid, generally sulfuric or nitric. The phosphoric acid produced from the nitric acid process is blended with other ingredients to produce a fertilizer, whereas the phosphoric acid produced from the sulfuric acid process must be concentrated before further use. Minor quantities of fluorine, iron, aluminum, sUica, and uranium are usually the most serious waste effluent problems. 

In the specific case of wastewater generated from the condenser water bleedoff in the production of elemental phosphorus from phosphate rock in an electric furnace, Yapijakis [33] reported that the flow varies from 10 to 100 gpm (2.3-23 m /hour), depending on the particular installation. The most important contaminants in this waste are elemental phosphoms, which is colloidally dispersed and may ignite if allowed to dry out, and fluorine, which is also present in the furnace gases. The general characteristics of this type of wastewater (if no soda ash or ammonia were added to the condenser water) are given in Table 9. 

The major ones are fluorapatite (general formula Ca5 [ F/OH/CI j (P04)3 - also called phosphate rock) and fluorspar (CaF2 - sometimes called fluorite) see Finger, G.C. Adr. Fluorine Chem. 1961, 2, 35... 

Developments of recent years include plants designed to precipitate the calcium sulfate in the form of the hemihydrate instead of gypsum, hi special cases, hydrochloric acid is used instead of sulfuric acid for rock digestion, the phosphoric acid being recovered in quite pure form by solvent extraction. Solvent-extraction methods have also been developed for the purification of merchant-grade acid, which normally contains impurities amounting to 12 18% of the phosphoric acid content. Processes for recovering part of the fluorine in the phosphate rock are in commercial use. 

The silica is an essential raw material that serves as an acid and a flux. About 20 percent of the fluorine present in the phosphate rock is converted to SiF4 and volatilized. In the presence of water vapor this reacts to give silica (Si02) and fluorosilicic acid (H2SiF6). 

Crude phosphoric acid obtained from the wet process (action of dilute sulfuric acid on phosphate rock) contains many impurities such as fluorine, metals, and... 

The neutralization reaction of the above equation is conducted in one or more strongly agitated reaction vessels, whether in a gypsum or in a hemihydrate mode. The system is highly exothermic and the slurry is maintained at 80-85°C for dihydrate processing, 95-100°C for hemihydrate, by evaporative or air cooling. During the reaction of phosphate rock with sulfuric acid, fluorine is evolved and must be scrubbed from the vent gas. 

Phosphate rock contains about 3.5 percent fluorine, some of which is recovered as a byproduct in manufacturing wet process phosphoric acid. During acidulation, the fluorine is released as hydrofluoric acid, HF, which reacts with the silica present as an impurity in the rock to form fluosilicic acid, H2SiF6. Some of the fluorine is lost with the gypsum as sodium or potassium fluosilicates, and some remains dissolved in the filter acid. When the acid is concentrated, much of the fluorine in the feed is boiled off, appearing as HF and silicon tetrafluoride, SiF4, in the vapors. 

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. 

This mineral is also known as a phosphate rock. It contains variable amounts of fluorine (3-4%). This rock also contains silica. When heated with sulfuric acid to produce phosphoric acid, it also gives fluorosilicic acid, H2SiF6, as a by-product (commercially known as fluosilicic acid). 

Fluorine is widely distributed in nature [1] and it is estimated that, among the elements, fluorine is about thirteenth in abundance. Phosphate rock, which is processed on a multimillion-ton scale as raw material for the fertiliser industry, contains as much as 3.8% of fluorine and is a very rich source of the element. However, the fluorine recovered from this process as fluorosilicic acid is still not a commercially competitive source of fluorine compared with fluorspar (CaF2), although reserves of the latter are said to be limited and it is expected that use of fluoride in phosphate rocks will eventually be increased. 

Phosphate rock preparation—Domestic phosphate rocks are essentially fluorapatite admixed with various proportions of other compounds of calcium, fluorine, iron, aluminum, and silicon. Phosphate rock preparation involves beneficiation to remove impurities, drying to remove moisture, and grinding to improve reactivity. Phosphate rock, when very finely pulverized, has limited direct use as a fertilizer. However, it is mainly used as a raw material for the manufacture of phosphate acid, superphosphate, phosphorus, and phosphorus compounds. 

Sedimentary phosphate rocks that are obtained from. insular and cave deposits often contain carbonate apatites that have a lower F content than that of stoichiometric fluorapatite and, according to calculations, contain significant amounts of hydroxyl in their structures. Although some of these carbonate apatites may meet the francolite definition, they have crystallographic, chemical, and other physical properties that differ substantially from those of francolites that contain excess fluorine [7]. These carbonate apatites form a series with end members that contain almost no fluorine (carbonate-hydroxylapatite) (Table 5.4) and end members that are very close in composition to pure fluorapatite and francolites that have almost no carbonate substitution. Members of this series are referred to as hydroxyl-fluor-carbonate apatites in this section. Table 5.5 shows the a-values of some phosphate rocks containing hydroxyl-fluor-carbonate apatites in this series. 

As mentioned in Chapter 5, phosphate rock contains many impurities both in the apatite itself and in accessory minerals. These impurities partidpate in numerous side reactions. Most phosphate rocks have a higher Ca0 P205 ratio than pure fluorapatite. The additional CaO consumes more sulfuric add and forms more cal-dum sulfete. The HF formed by the reaction reads with silica and other impurities (Na, K, Mg, and Al) to form fluosilicates and other more complex compounds. A variable amount of the fluorine is volatilized as Sip4, HF, or both. The amount volatilized and the form depend m phosphate rock composition and process conditions. 

All commercial phosphate rocks contain fluorine (F) no special effect has been noted due to variations in fluorine content within the range of experience, Effects of fluorine on scaling, corrosion, and postprecipitation are related to other elements that combine with fluorine, including Na, K, AI, Mg, and Si. 

Most phosphate rocks of the fluoroapatite - type contain a significant quantity of fluorine, usually 3%-4% F by weight. In some cases up to 60% of this can be evolved during the manufacture of wet-process phosphoric acid. The remainder of the fluorine is retained in the gypsum (depending mainly on the rock composition), and most of remainder is in the filter acid 130], The fluorine is evolved from various stages of phosphoric acid processes from the reactor slurry surface, in the flash cooler, and in the concentration plant. In a dihydrate process the proportions are ------ --------------... 

Depending on the type of phosphate rock, formation of add gases containing, for example, nitrogen oxides and compounds of fluorine takes place during the digestion. In the second reactor the overflow from the first reactor is ammoniated, and scrubber liquor is added. 

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