Manufacture of Superphosphate

The finely ground rock is mixed with sulphuric acid in the proportions required by the equations

CaCO₃ + H₂SO₄ + H₂O = CaSO₄.2H₂O + CO₂
Ca₃P₂O₈ +2H₂SO₄ + 5H₂O =CaH₄P₂O₈.H₂O +2[CaSO₄.2H₂O]

The evolution of carbon dioxide plays an important part in keeping the mass porous; if a sufficient proportion of carbonate is not present in the rock it may be supplied by blending. The heat evolved by the reaction is used to evaporate the surplus water. This heat depends 011 the composition of the rocks—those which contain much carbonate evolve more heat and may be treated with cold acid, while those which contain little may require hot acid. Chamber acid of density 1.53 to 1.61 is used; this is also the chief source of the water required. The hydrates retain their water when dried at 100° C., or even to a great extent up to 150° C. If artificial drying is used the temperature should not rise over 150° C. or else an undue proportion becomes insoluble. The loss in weight is 10 to 12½ per cent. The product hardens on cooling and is cut out and powdered by a mechanical disintegrator. It contains, when freshly made, 30 to 45 per cent, of phosphate (calculated as Ca₃P₂O₈) soluble in water, according to the composition of the rock used. A more concentrated form ("double superphosphate") is made by adding sulphuric acid sufficient in amount to set free all the phosphoric acid, which, after filtration, is concentrated to a syrup and used to decompose more phosphate according to the equation

Ca₃P₂O₈ + 4H₃PO₄ + 3H₂O = 3(CaH₄P₂O₈ +H₂O)

The manufacture has been of great value as an outlet for surplus sulphuric acid, of which 11 cwt. (69 per cent, acid) is required for every ton of (ordinary) superphosphate.

"Retrogression"

Superphosphate may require to be stored for several months, and during this time insoluble CaHPO₄ is formed according from the small amount of undecomposed Ca₃P₂O₈. Even a week after manufacture the soluble phosphate may have diminished by about 2 to 4 per cent. This " retrogression " is particularly marked when the phosphatic material contains more than 2 per cent, of iron plus alumina. The excess of these bases reacts with the CaH₄P₂O₈ to give insoluble phosphates of iron and aluminium according to the equation

CaH₄P₂O₈.H₂O + Fe₂(SO₄)₃ + 5H₂O = 2[FePO₄.2H₂O] + CaSO₄.2H₂O + 2H₂SO₄

The phosphates of iron and aluminium form gelatinous precipitates which are insoluble in weak acids, or in hydrolysed acid phosphates or sulphates. Ferric phosphate may be decomposed, using up more sulphuric acid, as in the equation

3FePO₄ + 3H₂SO₄⇔ FePO₄.2H₃PO₄ + Fe₂(SO₄)₃

or it may easily lose its water, becoming insoluble, thus:

FePO₄.2H₂O + CaSO₄ = CaSO₄.2H₂O + FePO₄

If the original rock contains up to 2 per cent, of iron oxide the resulting phosphate of iron is soluble, but with more than 4 per cent, of iron oxide the phosphate is insoluble—hence such a rock is considered unsuitable for the manufacture of superphosphate. The "regression" of the phosphate by the iron salt just described may be avoided if the rock is dissolved in ammonium sulphate solution and then treated with sulphur dioxide; the iron is then converted into (NH₄)₂SO₄.FeSO₄.6H₂O.

The Treatment of Special Ores

Alumina does not appear to induce "retrogression." It may be removed by caustic alkalies or hot alkali carbonate solutions. Redonda phosphate (AlPO₄) may be made soluble by fusion with ammonium bisulphate, giving a dry powder which is a mixture of ammonium alum, ammonium bisulphate and biphosphate.

Rocks which contain calcium chloride or fluoride (apatites) are decomposed according to the equations

CaCl₂ + H₂SO₄ = 2HCl + CaSO₄
CaF₂ + H₂SO₄ = 2HF + CaSO₄

The corrosive gases which are liberated are absorbed in towers containing water and furnish solutions of hydrochloric or hydrofluosilicic acid by reaction with the silica of the phosphate rock. Thus

4HF + SiO₂ = SiF₄ + 2H₂O
SiF₄ + 2HF = H₂SiF₆

By addition of common salt silicofluoride may be precipitated and the filtrate may be worked up for hydrochloric acid. Thus

H₂SiF₆ + 2NaCl = Na₂SiF₆ + 2HCl

Apatite may be treated by the following process (Palmer):— Perchloric or chloric acid made by electrolysis of the sodium salts is mixed with the coarsely ground rock. The liquid, containing H₃PO₄, is precipitated by the alkaline kathode liquors so as to give a slightly acid precipitate of the composition CaHPO₄.2H₂O, which is soluble to the extent of 95 per cent, in ammonium citrate.

By using three mols of sulphuric acid instead of two, the whole of the lime is converted into sulphate and the whole of the phosphoric acid set free according to the equation

Ca₃P₂O₈ + 3H₂SO₄ = 2H₃PO₄ + 3CaSO₄

The raw material should contain at least 50 per cent, of Ca₃P₂O₈ and be as free as possible from sesquioxides. It may be ignited if high in organic matter, reduced to a fine powder, and fed continuously into tanks lined with wood or hard lead alloy, where it meets on the counter current principle hot sulphuric acid of about 5 per cent, concentration. The reaction is quickly completed and the precipitated calcium sulphate is allowed to settle and filtered off continuously through filter presses. This sulphate is " phosphatic gypsum " and contains 3 to 4 per cent, of phosphoric acid of which 1 per cent, is soluble in water. The solution is evaporated in wrought-iron pans up to a concentration of 50 per cent, phosphoric acid, which may be further refined for use in pharmaceutical products or foods.

The crude phosphoric acid is also used in the manufacture of " high analysis " or " triple superphosphate." The solution containing about 45 per cent, of H₃PO₄ is mixed with more of the ground rock and evaporated. The product, distinguished from ordinary superphosphate by freedom from gypsum, sets to a tough mass. It is broken up while comparatively fresh and dried at about 200° C. It contains CaH₄P₂O₈ mixed with sandy crystals, is non-hygroscopic and may have the following composition:—

Total P2O5	48.0-49.0
Water-solubleP2O5	41.0-42.0
Citrate-soluble P2O5	4.0-5.0
Citrate-insoluble P2O5	2.0-3.0
CaO	20.80
Fe2O3 + Al2O3	2.25
Na2O + K2O	2.0
SiO2	1.4

together with fluoride, sulphate, other bases (Cu, etc.) and about 2 per cent, of water.

If apatite or other phosphatic material is placed round the anode in a solution of sodium chloride which is being electrolysed, a citrate-soluble calcium phosphate is precipitated. Or perchloric acid may be made separately in the anode compartment and mixed with the alkali produced at the kathode, giving a precipitate of Ca₂H₂P₂O₈.

Phosphate rock may also be made soluble by heating the powdered material with soda ash, carbon and silica, thus:

Ca₃(PO₄)₂ + 3SiO₂ + 3Na₂CO₃ = 3CaSiO₃ + 2Na₃PO₄ + 3CO₂

The History and Technology of Superphosphate Manufacture

Allusion has already been made to the suggestion of Liebig that bones could be made more available for agriculture by fine grinding and treatment with acid. This treatment of bones and other phos- phoritic materials was patented by J. B. Lawes in 1842, but the claim was modified later to cover only minerals such as apatite, phosphorite, etc. A paper published by the Rev. Henslow called attention to the use of crag coprolites from Suffolk, which with guano and bones were used by Lawes in his factory at Deptford. From 1842 to 1854 the manufacture was essentially a British industry and in the early period, until about 1870, the plant was of the simplest description. The right quantity of acid, determined by trial and error, was run on to a heap of ground phosphatic material in a" den " made of tarred pitch pine or tarred bricks secured by cast-iron plates, and mixed by means of rakes and shovels. The mass soon set and dried itself by the heat of the reaction, and after storing for a month or so was broken up, screened and bagged. The only machinery used consisted of stone mills for grinding the rock. Rotary stirrers operated by hand were sometimes installed and were in service into the twentieth century. The production of 1 ton of superphosphate requires about 11 cwt. of chamber acid, 69 per cent. H₂SO₄, and the weight of superphosphate from a good grade of rock, containing about 32 per cent. P₂O₅, is rather less than twice the weight of the rock. This industry helped to absorb some of the large excess of sulphuric acid which became available after the decline of the Leblanc process.

The necessity of mixing in a closed room by external power became urgent with the introduction of rocks which evolved hydrochloric or hydrofluoric acid when treated with sulphuric acid. Machinery suited to these operations has now been devised. The stone grinders were replaced by ball mills and later by rotary crushers and roll-jaw crushers which will reduce 90 per cent, of the material to a fine powder which will pass through a sieve having 10,000 meshes to a square inch. Hand labour was employed at first for the mixing. Charges of sulphuric acid and phosphates were weighed into a closed " den " and, after the reaction was completed, were dug out with the aid of gravity. These reaction chambers were replaced by mechanically operated " dens," some of which could be rotated. Various types of mechanical excavators are used. In one the block of superphosphate is forced by a ram against tearing and cutting wheels, and the broken material after falling through a grid is elevated to the storage rooms. Or the fixed chambers, each holding 150 to 200 tons, are emptied by grab-buckets which are let down into the mass. The reaction in modern plant, aided by fineness of grinding and good mixing, is complete in a few minutes.