GB702860A - Improvements in and relating to phosphate fertilizers - Google Patents

Abstract

<PICT:0702860/III/1> Granular compound phosphate fertiliser is produced by reacting ground mineral rock phosphate with insufficient nitric acid for complete conversion to monocalcium phosphate, the reaction being carried out in two stages in one of which ammonium sulphate is added, the first stage, during which all the acid is added, being not longer than 2 minutes with rapid mixing and the second being for 12 to 25 minutes with moderate agitation; then adding with continued mixing over a period of 10 minutes, aqueous ammonia sufficient to convert all the monocalcium phosphate to dicalcium phosphate while maintaining the temperature not higher than 70 DEG C. adding with continued mixing, dried final product sufficient to give the mixture a water content of 6 to 12 per cent., and passing the mixture to a rotary drier where granulation is completed and the granules are dried in countercurrent so that their temperature leaving the drier is not higher than 100 DEG C. The granules, preferably after cooling to room temperature, are screened and the fines, together with the crushed oversize granules, are recycled. In the drawing, rock phosphate, ground to 295m or less, is added to a paddle mixer 2 together with nitric acid of 45 to 65 per cent. strength sufficient to give a 55 to 96 per cent. conversion added from pipe 3. Ammonium sulphate which may be dissolved in the nitric acid, can be added in this stage but it is preferred to add it in the second stage of the acid reaction through line 5 in the next mixer 4 which is preferably a screw mixer. The amount of ammonium sulphate added is not less than that required to convert the calcium nitrate which would otherwise remain after ammonification, into calcium sulphate and ammonium nitrate. The mixture then passes to a mixer 6 where aqueous ammonia of 20 to 40 per cent. strength is added from pipe 7 and the reaction mass is maintained at 50 to 60 DEG C., and thence to a mixer 8 where dried product constituting 1 to 4 times the weight of the material fed to the process is added at 9. The partially granulated mixture then passes to the rotary granulator and drier 10. Muriate of potash may be added after ammoniation for example in the mixer 8 before adding the recycled dried product. Specification 697,019 is referred to.