Superphosphate Fertilisers

The now long-established superphosphate is still an important fertiliser, although its use is declining. It is made by the direct action of sulphuric acid on phosphate rock and the active constituent is monocalcium phosphate Ca(H2P04)2 H2O. 

A little less sulphuric acid is used than indicated by Equation 12.1. This is to ensure that the product contains no free sulphuric or phosphoric acids which would render it hygroscopic and lead to caking. In practice not all the fluorine is removed from the rock and some hydrofluoric acid can be produced by reaction with silica impurities (12.2). The efficiency of reaction (12.1) depends on the grade of phosphorite rock which is used, and is limited by the crystallisation behavionr of the by-product, calcium sulphate. The latter is complex and is influenced, not only by the silica and fluorine, but by the level of the other impurities which are present. The phosphate rock (phosphorite) should contain not more than about 1% MgO and 5% AI2O3 + Fe203, and the content of other impurity (potentially toxic) elements such as Cd, Se and Ni must be very low. 

A commercially satisfactory large-scale use for the by-product gypsum has not yet been developed (as for wet process acid Chapter 5.2). 

A higher grade of fertiliser known as triple superphosphate is made by the action of wet process phosphoric acid on ground phosphate rock. The reaction may be summarised by Equation 12.3. The product contains a higher proportion of available phosphate than ordinary superphosphate, and its manufacture and use has become more economic than the latter, over the past two decades. On the industrial scale, reactions such as (12.1) and (12.3) do not go to completion immediately, and reaction periods up to 30 days may be required under some conditions. 

After application to the soil, the monocalcium phosphate in the fertiliser is hydrolysed to dicalcium phosphate according to Equation 12.4. 

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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 . 

Parrish, P. and OGILVIE, H. Calcium Superphosphates and Compound Fertilisers. Their Chemistry and Manufacture (Hutchison, 1939). 

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). 

The phosphates of lime are the most valuable natural fertilisers and raw material of the fertiliser and phosphorus industry, which uses also to a less extent phosphates of aluminium and the apatites. Over 80 per cent, of the easily decomposible phosphorites mined are used in the preparation of superphosphate or other fertilisers (1980). 

Basic slag is a slow fertiliser the phosphate is not immediately available as is that of calcium superphosphate. It is particularly valuable for fruit trees, and for heavy grass-land, on which it develops the growth of white clover and hence increases the amount of combined nitrogen. It neutralises acid soils, and its beneficial effects extend over many years.1... 

The System Lime and Phosphoric Acid —Solubility of Calcium Phosphates— Conditions of Formation of Basic and Acid Calcium Phosphates—Composition of Solutions Saturated with Calcium Hydrogen Phosphates—Equilibria between Solid Phases and Solutions at Various Temperatures—Changes during Neutralisation—The Acid Phosphates—Manufacture of Superphosphate—Retrogression—Treatment of Special Ores—Phosphoric Acid— Commercial Preparation and Extraction from Rook—The History and Technology of Superphosphate Manufacture—Mixed and Concentrated Phosphorio Fertilisers—Potassium Phosphates—Ammonium Phosphates. 

Materials not allowed include highly soluble mineral fertilisers, whether natural or synthetic, such as potassium chloride, urea, Chilean nitrate, single and triple superphosphate and synthetic insecticides, fungicides and all herbicides. 

The development of synthetic fertilisers in Spain during the first third of the 20th century had two completely different aspects. Whereas the production of superphosphates rose spectacularly (from 76,000 metric tons in 1905 to 1,076,000 metric tons in 1935) satisfying domestic demand, nitrogen fertilisers had to be imported on a massive scale. Nitrogen fertiliser production rose from practically nothing by the turn of the century to 3,800 metric tons in 1935. ... 

In 1842, British patents for the mannfacture of superphosphate by the action of sulphuric acid on bones were taken out independently by J.B. Lowes and J. Murray [28-31]. This led to the world s first fertiliser factory at Deptford, Kent, England. A few years later superphosphate manufacture conunenced in the United States, but it was not until about 1855, however, that the work of Lowes... 

Solid fertilisers are usually produced in granular rather than in powder form. Bulk blending of these by mechanical mixing is often practised to produce compound or mixed fertilisers with specific N P K ratios. The principal materials used in bulk blending are ammonium nitrate, ammonium sulphate, urea, superphosphate, ammonium phosphates and potassium chloride. Under EEC regulations, mixed fertilisers are required to have a minimum of 3% N, 5% P2O5 and 5% K2O with the sum of these not less than 20%. 

Liquid micronutrient compositions are made by dissolving metal salts such as CUSO4 and MnCl2 in phosphoric acid, and then neutralising with anunonia. A small amount of a phosphonate of type (12.13a) or (12.13b) may be added to complex and prevent any precipitation of the metallic salts. Commercial superphosphoric acid, when used for superphosphate manufacture, functions as a source of micronutrient elements. The add itself can be used for liquid fertilisers, since the small quantities of polyphosphates which are present will sequester the impurity metal ions present. 

This ore, about 200.10 tonne/annum at maximum output, is approximately 10% apatite, which is taken south by train and converted into superphosphate with sulphuric acid. This is an essential fertiliser for the wheatfields of central Russia. 

Sulphur (S) Part of many proteins and some oils Usually sufficient in the soil Atmospheric sulphur absorbed by the soil and plant. Added in some fertilisers (e.g. sulphate of ammonia and superphosphate)... 

The cost of plant food per kilogram can be calculated from the cost of fertilisers which contain only one plant food such as nitrogen in ammonium nitrate, phosphate in triple superphosphate, and potash in muriate of potash. 

Fluorine is another potentially-toxic trace element which is dispersed by atmospheric pollution and it has long been recognised that damage to plants occurs and that there is a hazard to man and farm stock, in the vicinity of industrial plants processing fluoride-containing minerals. Such plants include factories for the production of aluminium, superphosphates and compound fertilisers based on the liberation of phosphoric acid from rock phosphate. In October 1976, ten cows had to be destroyed on two farms in the vicinity of the British Aluminium Company s aluminium smelter at Invergorden in the north of Scotland, and problems of fluoride toxicity have been commonly associated elsewhere with the production of aluminium. 

The most commonly used liquids are solutions of nitrate fertilisers (containing between 36 and 40 kg nitrogen per hectolitre), but solutions of superphosphate 14-48-0 (14 kg nitrogen and 48 kg phosphoros per hectolitre) are also used. These solutions are generally slightly acidic (pH between 6 and 7). 

Sohd fertilisers are more or less hygroscopic and deliquescent, depending on their composition. In the presence of humidity, the prolonged contact with the granules or fertiliser dust can lead to a pitting attack on aluminium. This attack is slight with nitrate fertilisers, and the more severe the attack is, the richer the fertiliser is in phosphoros. Superphosphates are very aggressive. 

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