Phosphate fertilisers

Industrially. phosphoric(V) acid is manufactured by two processes. In one process phosphorus is burned in air and the phos-phorus(V) oxide produced is dissolved in water. It is also manufactured by the action of dilute sulphuric acid on bone-ash or phosphorite, i.e. calcium tetraoxophosphate(V). Ca3(P04)2 the insoluble calcium sulphate is filtered off and the remaining solution concentrated. In this reaction, the calcium phosphate may be treated to convert it to the more soluble dihydrogenphosphatc. CafHjPOjj. When mixed with the calcium sulphate this is used as a fertiliser under the name "superphosphate . 

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). 

Ligno sulfonate—metal complexes are weaker complexes than those formed from amine-based complexing agents such as ethylenediaminetetracetic acid (EDTA). They are compatible with most pesticides /herbicides, but thek use in phosphate fertilisers is not recommended. 

Phosphorus [7723-14-0] is a nonmetaUic element having widespread occurrence in nature as phosphate compounds (see Phosphoric acid and phosphates). Fluorapatite [1306-03-4], Ca F(P0 2> is the primary mineral in phosphate rock ores from which useful phosphoms compounds (qv) ate produced. The recovery from the ore into commercial chemicals is accompHshed by two routes the electric furnace process, which yields elemental phosphoms and the wet acid process, which generates phosphoric acid. The former is discussed herein (see Furnaces, electric). Less than 10% of the phosphate rock mined in the world is processed in electric furnaces. Over 90% is processed by the wet process, used primarily to make fertilisers (qv). 

Most of the phosphoms produced as the element is later converted to high purity phosphoric acid and phosphate compounds the remainder is used in direct chemical synthesis to produce high purity products. In contrast, phosphoric acid produced by the wet process is used in lower purity apphcations, especially in fertiliser and to a lesser degree in animal feed (see Feeds AND FEED ADDITIVES). More recendy, a small portion of wet acid is purified in a second process and then also used in high purity acid and phosphate compound apphcations. 

Markets. Industrial use of ammonia varies according to region. Eor example, industrial usage represents 20% of the ammonia production in the United States and Western Europe, 10% in the USSR, 1—10% in Asia, and 5% in Latin America and North Africa (79). Fertiliser ammonia consumed domestically in most countries is converted to straight or compound fertilisers such as urea, ammonium nitrate, diammonium phosphate, and various grades of mixed fertilisers. However, almost 29% of ammonia nitrogen in the United States is consumed as direct appHcation material. The use of nitrogen solution such as urea and ammonium nitrate (UAN) has also become popular in the United States and the USSR. 

Sodium nitrate is used as a fertiliser and in a number of industrial processes. In the period from 1880—1910 it accounted for 60% of the world fertiliser nitrogen production. In the 1990s sodium nitrate accounts for 0.1% of the world fertiliser nitrogen production, and is used for some specific crops and soil conditions. This decline has resulted from an enormous growth in fertiliser manufacture and an increased use of less expensive nitrogen fertilisers (qv) produced from synthetic ammonia (qv), such as urea (qv), ammonium nitrate, ammonium phosphates, ammonium sulfate, and ammonia itself (see Ammonium compounds). The commercial production of synthetic ammonia began in 1921, soon after the end of World War I. The main industrial market for sodium nitrate was at first the manufacture of nitric acid (qv) and explosives (see Explosives and propellants). As of the mid-1990s sodium nitrate was used in the production of some explosives and in a number of industrial areas. 

Fertilisers For nitrates, non-molybdenum austenitic steels are satisfactory, but in the manufacture of ammonium sulphate some free acid is often present, so that evaporators and centrifugal dryer baskets in this case are generally made from molybdenum-bearing steels. For super-phosphates this has limited application. 

In nitrogenous fertiliser solutions of the NH4NOJ—NHj —HjO type corrosion of steel can be prevented by 500 p.p.m. of sulphur-containing inhibitors, e.g. mercaptobenzothiazole, thiourea and ammonium thiocyanate. However, these inhibitors are not so effective where most of the NHj is replaced by urea. For these solutions phosphate inhibitors such as (NH4>2HP04 and polyphosphates were more effective... 

The method can be applied to the determination of phosphorus in a wide variety of materials, e.g. phosphate rock, phosphatic fertilisers and metals, and is suitable for use in conjunction with the oxygen-flask procedure (Section 3.31). In all cases it is essential to ensure that the material is so treated that the phosphorus is converted to orthophosphate this may usually be done by dissolution in an oxidising medium such as concentrated nitric acid or in 60 per cent perchloric acid. 

An unusual example of a process that produces a lot of waste is the intensive rearing of pigs. They need additional phosphorus in their feed for healthy growth, and this is usually added as inorganic phosphorus in the form of monocalcium phosphate (calcium dihydroxy-oxido-oxo-phosphorane). Unabsorbed phosphorus passes through into the manure, and if spread onto fields as a fertiliser can lead to excess phosphorus run-off into rivers and lakes leading to eutrophication. 

Basic slag used to be a popular fertiliser for supplying phosphate, but it also had liming value and contained some trace elements. However, the older steel-making processes are now out-of-date and basic slag is much less freely available. The consequence of this is that soils in Britain are becoming more acidic. 

One of the problems is that the phosphate fertiliser allowed by the organic regulations is rock phosphate, and research in Austria has shown (Lindenthal, et al., 2000) that the application of rock phosphate had virtually no effect on subsequent crop yields. 

Sulphuric acid is used to manufacture other nitrogen-containing fertilisers such as ammonium sulphate, (NH4)2S04. Phosphorus-containing fertilisers, derived from rock phosphates such as Ca3(P04)2, are also manufactured using sulphuric acid. 

The practical significance of such competition evolves from the experience that silicate and (anionic) humics can increase the efficiency of phosphate fertiliser because these compounds occupy sites suitable for phosphate adsorption (Kingston et ak, 1968 Schwertmann, 1995). Hydroxyl is another anion that competes effectively with adsorbing anions, owing to its location in the inner Helmholz layer. The release of adsorbed phosphate after liming a soil or after inflow of acidic surface soil into weakly alkaline surface waters due to erosion, can be considered as the result of competition between OH and phosphate ions. 

Lim, H.H., Gilkes, R.J. and McCormick, P.G. 2003. Benefication of rock phosphate fertilisers by mechano-milling. Nutrient Cycling in Agroecosystems 67 177-186. 

The element has a definite fertilising action 2 which is exerted in two ways (1) It supplies sulphuric acid by bacterial oxidation, the presence of the acid increasing the availability of certain mineral constituents in the soil, such as alkalis, ferric oxide, alumina and phosphates. (2) It facilitates the work of the ammonia and nitrifying bacteria, thus placing larger supplies of nitrogen at the disposal of the plants. But although such action may be beneficial in some soils it is equally harmful in others, and sulphur should not be applied to a soil already acid.3... 

NH4N03), ammonium phosphate ((NH4)3P04) and potassium chloride (KC1) in varying proportions (Figure 11.19). Fertilisers have an important role in the nitrogen cycle (see below). 

Water is very good at dissolving substances. Thus, it is very unusual to find really pure water on this planet. As water falls through the atmosphere, on to and then through the surface of the Earth, it dissolves a tremendous variety of substances. Chemical fertilisers washed off surrounding land will add nitrate ions (N03 ) and phosphate ions (PC)43 ) to the water, owing to the use of artificial fertilisers such as ammonium nitrate and ammonium phosphate. 

Vanadium leaches soil from a large number of diverse sources, including waste effluents from the iron and steel industries and chemical industries. Phosphate industries are also a major source of vanadium pollution because vanadium becomes soluble along with phosphoric acids when rock phosphates are leached with sulfuric acid. Vanadium is present in all subsequent phosphoric acid preparations, including ammonium phosphate fertilisers, and is released into the environment along with them. Other sources of vanadium pollution are fossil fuels, such as crude petroleum, coal and lignite. Burning these fuels releases vanadium into the air, which then settles in the soils. 

The presence of elements known to have adverse health effects in humans such as lead and arsenic is obviously undesirable in food. Environmental sources are the main contributors to contamination of food with most metals and other elements. Some elements (e.g. arsenic) are present naturally but the major sources of other elements (e.g. lead) in the environment are from pollution from industrial and other human activities. The presence of metals and other elements in food can also be the result of contamination from certain agricultural practices (e.g. cadmium from phosphate fertilisers) or manufacturing processes (e.g. tin in canned foods). 

Phosphate fertilisers also contain a number of other elements found in the parent phosphate rock (Bowen, 1979 Bockman et al., 1990 Jackson and Alloway, 1992). Cadmium originating from sedimentary rock is particularly undesirable, and processes for the removal of Cd from such fertilisers are being developed (Bockman et al., 1990). Fertilised soils have shown increases in Cd content after a number of years, but there appears to be little evidence for long-term Cd-increase in crop plants, except possibly for wheat (Jones and Johnston, 1989). Mortvedt (1984) determined the uptake of Cd and Zn by several vegetable crops heavily fertilised with triple superphosphate over a ten year period. Cd levels were found to be similar in fertilised and unfertilised snap bean seed, beet blades and roots, and in sweet corn leaves and grain. However, Zn concentrations were found to decrease with P application in all tissues except cabbage heads and cores. Claims that fertilisers promote the uptake of Al by plants have been refuted (Akerstrand et al., 1988). 

Phosphorus in the form of phosphate is the principal source of P for plants. Phosphorus is commonly applied in fertilisers although its uptake by plants is much less than that of N and K and its availability is one of the most common nutritional limitations to crop yield (Bould etal., 1983). Some of the P in soils and soil solutions may be present in organic compounds. However, it is unlikely to be available to plants in its organic form and decomposition of organic matter releases P in its available... 

The most important use of ammonia is in fertiliser production. Fertilisers are added to the soil to improve crop yields. A farmer has the choice of two fertilisers, ammonium nitrate, NH4NOj. or diammonium hydrogen phosphate, (NH4),HP04. 

However, the fact that fluorides are (i) associated with the application of some phosphate fertilisers that many leach into surface waters and shallow ground water and (ii) by-product of the phosphate fertiliser industry which are the primary sources of fluoride pollution call upon the need of receptors that are able to interact with both anions. Calix[4]pyrrole and most of the derivatives have... 

---