GB996633A - Improvements in and relating to granular compositions - Google Patents

Abstract

<PICT:0996633/C1/1> <PICT:0996633/C1/2> Granular fertilizers are coated with ammonium salts by feeding them, if desired with another solid fertilizer component, to a rotating cylinder through which a current of gas is passed and which is provided with lifters, in which is maintained in agitation a bed of discrete granules, continuously introducing into the zone gaseous or aqueous ammonia at points along the length of the zone by means of a longitudinal pipe or pipes within the bed, and at least one aqueous acid at points along the length of the zone by means of a longitudinal pipe or pipes either above or within the bed, showering the granules through the gas stream by means of the lifters as the cylinder rotates, whereby the granules become coated with an ammonium salt or salts, the proportion of water fed at each point per unit of time, the weight of the reactants added at each point per unit of time and the solids recirculation rate per unit of time through the zone being selected so that throughout the zone the water is being continuously evaporated by the heat of the chemical reaction and the bed of granules is maintained at a moisture content and temperature at which little, preferably no, agglomeration of the granules occurs. There may be a short longitudinal interval between each cooperating group of acid and ammonia spray points and the next pair or group, or the ammonia spray points may be pitched at intervals different from those of the acid spray points to give a substantially continuous cycle throughout the length of the zone. Solid material in an amount only sufficient to act as nuclei for granule formation may be fed to the zone, or undersize and crushed oversize material may be recycled to the zone, the "recycle ratio" (defined in the Specification) being between 20:1 and 1:1, and preferably between 5:1 and 2:1 by weight. Nitric acid, preferably of concentration between 61% and 69% by weight, and phosphoric acid, preferably of concentration between 40% and 71% by weight, may be used to produce ammonium nitrate or phosphate granules. Granules of ammonium nitrate/ammonium phosphate may be produced by spraying in both phosphoric and nitric acids. A mixed fertilizer containing potassium may be produced by feeding potassium chloride to the reaction zone to serve as nuclei. Where the product contains appreciable proportions of di-ammonium phosphate, the partial pressure of ammonia in the gas stream passing through the bed is preferably kept high, excess ammonia in the exit gas being recovered by scrubbing. The acid or acids may be partially ammoniated before entering the reaction zone. Figures 1 and 2 show apparatus for this process in which a rotatable cylindrical drum 1 with large openings 3 at each end is provided with an inlet 4 and outlet 5 for the gas stream, circumferential lifters 2 extending for substantially the whole length of the drum, longitudinal pipe or pipes 8 and 9 for acid provided along their lenght with sprays 14 and 15, longitudinal pipe or pipes 10 for ammonia provided along their length with sprays 16 which may comprise bent pipes with a splayed outlet as shown, an inlet 6 for feed and recycle material, and a product outlet 7. The pipes, which are detached from the drum and not rotatable with it, are supported at intervals on triangular frame members 11 and small brackets 81 and 91 such that the acid pipes are normally above, and the ammonia pipe, shown as integral with the frame, is normally within the bed of granules. The whole framework is supported by two end box members 12 and 13. Alternatively, the acid and ammonia feed pipes may have a number of perforations constituting the sprays and may be located within the bed of granules.