Ammonium Phosphates

Ordinary superphosphate may be treated with aqueous ammonia up to 3 per cent, without any marked increase in the citrate-insoluble proportion. The calcium salt is present as CaH₄P₂O₈ and CaHPO₄ up to 2 per cent, of ammonia, but is wholly converted into Ca₃P₂O₈ at 6 per cent, ammonia.

The ground rock may be treated directly with 2 mols of sulphuric acid and 1 mol of ammonium sulphate, which react according to the equation

Ca₃P₂O₈ + (NH₄)₂SO₄ + 2H₂SO₄ = 3CaSO₄ + 2(NH₄)H₂PO₄

The solution is filtered from calcium sulphate and more ammonia is added until the phosphates of aluminium and iron settle. On concentrating the filtrate phosphates and sulphates of ammonia may be crystallised.

In another process crude calcium acid phosphate is mixed with ammonium sulphate solution below 80° C., and the mixture concentrated and filtered, when (NH₄)H₂PO₄ crystallises. (NH₄)₂HPO₄ is made from ammonia, fumes of phosphoric acid and water. Or calcium phosphate is just dissolved in sulphuric acid, the calcium sulphate filtered off and the acid solution treated with ammonia and carbon dioxide. The ammonium sulphate and phosphate form a good mixed fertiliser.

Very soluble fertilisers are prepared by reactions between phosphoric acid and ammonia or its salts. Thus, if ammoniacal gas liquor is mixed with crude phosphoric acid, diammonium phosphate, (NH₄)₂HPO₄, may be crystallised in the anhydrous state. The composition of such products may range from 45 per cent. P₂O₅ and 14 per cent, nitrogen to 18 per cent. P₂O₅ and 18 per cent, nitrogen. By varying the proportions, monoammonium phosphate, (NH₄)H₂PO₄, may be obtained as a white granular solid, which is non-hygroscopic and stable under ordinary conditions. The product prepared by evaporation down to 2.3 per cent, of water and grinding contains about 53 per cent. P₂O₅ (of which 48 per cent, is soluble in water), with 13.3 per cent, of ammonia and up to 3 or 4 per cent, of a mixture of iron and aluminium, magnesium and the alkali metals.

Various "sulphophosphates" are made by mixing ammonium (also alkali) sulphate solution with 55 per cent. H₃PO₄ at 80° C., thus:

(NH₄)₂SO₄ + H₃PO₄ = (NH₄)HSO₄ + (NH₄)H₂PO₄
K₂SO₄ + H₃PO₄ = KHSO₄ + KH₂PO₄

The products may be obtained in a dry form and are easily soluble in water. On account of their high proportion of free acid they may be mixed with basic slag or phosphatic chalk and still retain a large proportion of soluble phosphate.

By starting with a purer phosphoric acid, ammonium salts may be obtained in a purer state. If ammonia gas is passed into 75 per cent, phosphoric acid a reaction takes place with great heat evolution, and on cooling acid ammonium phosphate, (NH₄)H₂PO₄, crystallises in the anhydrous state. Further saturation with ammonia yields a mixture of the mono- and di-ammonium salts, and on further addition of concentrated ammonia solution, or by carrying out the whole reaction in more dilute solution, the salt (NH₄)₂HPO₄ may be obtained as white non-hygroscopic crystals containing 53.8 per cent. P₂O₅ and 25.8 per cent. NH₃.

By suitable combinations of the methods just described, mixtures of potassium and ammonium phosphates may be prepared. The preparation of very concentrated fertilisers containing potassium and ammonium phosphates has been described by Ross and Merz, 1916, Ross, Jones and Mehring, 1926.

Mixed fertilisers containing ammonium phosphate with other salts have been made in the form of cylindrical granules like smokeless powder. The slower operations of solution, neutralisation, evaporation and crystallisation are avoided, the bases, acids and neutral components being combined, ground, mixed and kneaded in one operation. Salts of potassium or other base are passed through valve-locked mains into a mixing pan which contains scrapers and muller mechanisms enclosed in a gas-tight hood. Liquid phosphoric acid and gaseous ammonia are admitted simultaneously with the salts and combine under pressure in about ten minutes to form diammonium hydrogen phosphate. The sticky mass is removed by means of a vertical screw and passes into an extruding machine which had to be designed specially to deal with non-plastic masses. The extruded sections are dried, first with cold air, then at a temperature below 70° C. They are then cut into pieces about 1½ diameters or 1/8 inch long, screened and polished. The product is uniform, non-hygroscopic and is easily drilled into the land.