PICT:1032613/C1/1 A granulated phosphate fertilizer having a base of superphosphate containing 20 to 40% by weight of P2O5, is prepared by mixing together (a) an ordinary superphosphate in substantially pulverulent form, formed by heating phosphate mineral with sulphuric acid containing 65 to 75% by weight of H2SO4, and (b) a fluid superphosphat mass formed by heating phosphate mineral with dilute phosphoric acid containing 25 to 33% by weight of P2O5, and then granulating and drying the mixture, followed by recycling part of the granulated and dried mixture by mixing it with (a) and (b). The recycled part is preferably that part left after removing the granules of the desired size, e.g. by a screening operation. The two coarse granules are preferably ground before being recycled. The fluid superphosphate mass preferably forms at least 15%, preferably 20%, by weight of the dry granulated phosphate fertilizer produced. Phosphoric acid may be mixed with the sulphuric acid in order to increase the P2O5 content of the fertilizer. A supplementary fertilizer, e.g. a potassium salt, may be added to the mixture prior to and during the granulation operation. The process may be carried out continuously by controlling the rate of flow of the reactants to produce a mixture of products (a) and (b) in the desired proportions. In relation to the drawing, previously ground phosphate mineral is fed into mixer 4 from hopper 1 by conveyer 2. Sulphuric acid, or a mixture of sulphuric and phosphoric acids, at a controlled rate, is also fed into mixer 4, by meter 3. This mixture passes to rotary drum 5, containing scraper 6, and solidifies therein, and is converted into powder form and passes into granulating drum 7. Phosphate mineral and dilute phosphoric acid are introduced into vat 11 and the thus-produced fluid superphosphate mass overflows into drum 7, where it mixes with the powdered ordinary superphosphate from drum 5. This mixture is then dried in rotary drier 13, and is screened at 16. The granulated product of desired size is removed through 18, and the remaining granules are recycled to drum 7. A supplementary fertilizer may be introduced into drum 7, from hopper 21 by conveyer 22.
展开▼
