Apparatus for coating small solids

Apparatus for fluidizing small particulate solids having a diameter of less than about 50 microns in average particle size, and at least partially enveloping these small solids with a coating material or a precursor thereto, said apparatus comprising in combination:

• a cross-current multi-stage fluid bed reactor having N fluid beds, separated by main baffles, said beds being in flow communication with one another, wherein N.gtoreq.2;
• a hopper for introducing small solid particles having a diameter of less than about 50 microns in average particle size, to a first bed of said N beds of the reactor, said hopper having an upper section and a lower section having an inclined sidewall for promoting the flow of phosphor into said first bed, at least one duct connected to said lower section for feeding a mixture of gas and small particulate solids into said hopper, means for the exit of gas from the upper section, said gas exit means including a filter for separating gas to be discharged from said small particulate solids;
• means for introducing a fluidizing gas to the N beds of the reactor, thereby promoting a flow of the small solid particles across the tops of the beds, from the first bed through bed N of the multi-stage fluid bed reactor;
• means for introducing the coating material or a precursor thereto, to one or more of the N beds of the reactor;
• means for distributing the coating material or precursor thereto throughout the cross-section of the reactor beds containing the same;
• means for controlling the residence or contact time of the small particulate solids in the beds containing the coating material or the precursor thereto, including means for maintaining a gas flow across the inner surface of the roof of the apparatus, keeping said roof substantially free of deposited solids, such that the small particulate solids therein are at least partially enveloped by the coating material or precursor thereto;
• and
• means for removal of the fluidizing gas and the small solid particles at least partially enveloped by coating material or precursor thereto.