compositional series having as end members fluorapatite (Ca10[PO4]6F2) and francolite (Ca10-a-bNaaMgb[PO4]6-x[CO3]xF0.4xF2 where a, b and x denote moles of Na, Mg and CO3- respectively). In particular in sedimentary phosphate rock, a large variety of minerals make up the so-called gangue (= non ore minerals). Generally speaking, the presence of these minerals means that the ore is not commercially viable. The most important are Ca-Fe-Al-phosphates, quartz, carbonates and evaporites such as halite (rock salt) and gypsum. A large number of ions may substitute for Ca, P, F and O in the crystal lattice, thus causing impurities mostly unwanted not only for technical, but also for environmental reasons. For example, iron, magnesium and carbonate are considered disadvantageous in the wet processing of the ore, whereas cadmium, uranium and fluorine may cause environmental problems (see Chapter 4). Nevertheless, some of these elements, when occurring in sufficient concentration, may be recovered from the ore as a useful by-product. 13 2.1.2 Beneficiation and processing of phosphate ore The term phosphate rock generally refers to the marketable, mostly beneficiated, product from the mining and concentration of phosphate ore. In general, phosphate rock contains 30 to 40 wt% P2O5 (e.g. Schipper et al., 2001). In some cases, the grade and/or composition of phosphate ore is such that it can be put to use without further treatment, e.g. as a fertiliser. In most cases, some form of beneficiation or processing is necessary to render the material useful. Beneficiation refers to some type of simple mechanical purification and concentration to increase the phosphate content of the ore. In a first processing step, much of the phosphate rock is nowadays processed using sulphuric acid to produce phosphoric acid. The largest portion of this phosphoric acid is used for the acidulation of phosphate rock in the manufacture of fertilisers. An alternative way of producing phosphoric acid is by thermal treatment. In the thermal treatment of phosphate rock to produce elemental phosphorus, the ore is pelletized with clay and sintered in a first process step at a maximum temperature of 1000 °C. In a second step, the sinter is mixed with flux, usually gravel, and cokes and molten in electric furnaces in a reducing environment. The phosphorus is liberated from the melt in gaseous form in accordance with the following generalised reaction: Ca10(PO4)6F2 + 8SiO2 + 15C 6CaSiO3 + 15CO + 1½P4 + Ca4Si2O7F2 Elemental phosphorus, which is recovered from the gas mixture by condensation, may then be burned to diphosphoruspentoxide for the production of phosphoric acid. The carbon monoxide is further burned to carbon dioxide. The remaining reaction products end up in Pagina 22

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