Fluid bed calcining apparatus

A calcining system for phosphate rock of the "hot rock" variety containing excessive amounts of organic carbon comprising a preheat fluidized bed compartment and a calcining fluidized bed compartment, a hot windbox serving said preheat fluidized bed, transfer conduit means for transporting preheated phosphate rock from said preheat fluidized bed compartment to said calcining fluidized bed compartment, said transfer conduit means depositing preheated phosphate rock into said calcining compartment at a level well above the calcining fluidized bed so that phosphate rock discharged from said transfer conduit means must fall a substantial distance through the freeboard of said calcining compartment thereby accomplishing pyrolysis of a portion of said phosphate rock, means for removing the calcined product from said calcining bed compartment to a cooling zone, a hot cyclone communicating with said freeboard of said calcining compartment for receiving the gases therefrom and separating the dust and fines entrained in said gases, an afterburner connected to said hot cyclone for receiving said gases therefrom, means for introducing air into said afterburner to support combustion therein, conduit means for conveying the combustion gases from said afterburner to said hot windbox for fluidizing said preheat fluidized bed, a fluidized bed dust oxidation chamber connected to said hot cyclone to receive dust and fines therefrom, conduit means for introducing fluidizing air into said dust oxidation chamber, said fluidizing air also serving as combustion air in said dust oxidation chamber, a recycle conduit for conveying the off-gases of said dust oxidation chamber to said calcining fluidized bed to recycle fines entrained in said off-gases and means for removing reacted solids from the fluidized bed of said dust oxidation chamber to a cooling zone.