GB954423A - Fertilizers - Google Patents

Abstract

In a continuous process for phosphate fertilizer production by attack on phosphate rock with nitric acid with or without other acids, potassium sulphate is added at a rate such that the soluble sulphate content of the liquid phase is maintained from 0.5-8% and the reaction temperature is maintained at above 90 DEG to 140 DEG C. The calcium sulphate is produced as anhydrite and the reaction product comprising potassium phosphate and nitrate is continuously removed from the system and if desired neutralized for example with ammonia, potassium carbonate or potassium hydroxide to form ammonium phosphate and potassium nitrate, preferably after the removal of the solid anhydrite. The reaction product may be granulated. The potassium sulphate addition should be not less of 0.5 moles per mole of P2O5 in the rock but the sulphate ion concentratio may be increased by the use of sulphuric acid in admixture with the nitric acid. Phosphoric acid may also be employed but the nitric acid content of the mixed acid must not be less than 3 moles per mole of P2O5. If sulphuric acid is employed it is preferred to add it before the filtration stage where the anhydrite is removed and phosphoric acid if employed is preferably added after the filtration step. Nitric and sulphuric acid may be used sequentially and in any order and the acid addition may be incremental. Nitric acid of 40-68% concentration, sulphuric acid of 50-100%, and phosphoric acid of 50-70% are preferably employed. According to Example (I) Morocco phosphate was attacked with a mixture of nitric and sulphuric acid and potassium sulphate was added, the reaction temperature being maintained at 120 DEG C. Anhydrite was removed from the continuously withdrawn reactants on a belt filter and nitric acid was added to the filtrate; thereafter <PICT:0954423/C1/1> anhydrous ammonia was added to yield after drying and granulation a fertilizer containing monoammonium phosphate, ammonium nitrate and potassium nitrate (3) in a continuously stirred three tank reaction system, phosphate rock and sulphuric acid were added to the first tank together with nitric acid and potassium sulphate; potassium sulphate was added to the second tank and the slurry was drawn from the third tank to a filter to remove anhydrite after which nitric acid was added, followed by neutralizing with ammonia. (4) Phosphate rock was attacked using the cycle schematically indicated in the Figure, the reaction temperature being maintained at 130 DEG C. and the sulphate concentration being maintained at 2%. Calcium sulphate in the form of anhydrite was withdrawn from the belt filter and the filtrate after treatment with nitric acid and ammonia yielded a fertilizer having an NPK ratio of 2 :2 : 3. Comparative examples are given wherein reaction conditions are similar save that the temperature in one case is 55 DEG C. whereupon the calcium sulphate is precipitated in the form of the pentasalt (5CaSO4.K2SO4.H2O) and syngenite and in another case the sulphate concentration was maintained at 10% whereupon the calcium sulphate precipitated as penta salt. Specification 954,424 is referred to.