SURFACE COATING OF GRANULAR MATERIALS

摘要

Uniform surface-coating of individual grains of flowable materials of approximately uniform grain-size, having smooth, rough, or fissured surfaces is affected using a coating agent in suspension or solution. A container has an upper part defined by a peripheral wall which diverges upwardly in the shape of a funnel and having a shorter, flatter, funnel-shaped lower part. A predetermined quantity of the material to be coated is added and a flow of air is introduced at the bottom of the container to flow from bottom to top at a specific flow velocity, so that the air flow entrains said material in a vortex-fountain of limited diameter and height in the container. There is a high volume concentration of the material in the core area of the container. The suspension or solution is sprayed into the core area of the vortex-fountain in the same direction as the hot air flow, through nozzles located at the lower end of the container, the nozzles being directed upwardly and having an initial spraying angle of between 30 and 60.degree., the spraying being done at a predetermined flow rate per unit of time. The suspension is absorbed completely and uniformly on the surface of the material, drying occuring simultaneously as the treated material ascends in the core area, moves laterally outwardly from the core area into an edge-area of reduced flow-pressure, descends along the container wall into the funnel-shaped lower part which returns it towards the lower end of the vortex-fountain. The spraying step is repeated intermittently with uniform drying, until the predetermined amount of coating has been applied, and the coated material has been dried to a predetermined final humidity.