690,400. Pneumatic separating. HARVENGT E. Feb. 9. 1951 [Feb. 10, 1950], No. 3274/51. Class 117 In a method of freeing dry coals and like materials from dust. the maior portion of dust from fine particles is initiallv removed bv a gaseous fluid current and the residual coarse particles are subjected to preliminary and final treatments in separate fluid currents to . comnlete the dust removal. the fine particles being separated out from the said major portion of dust and fine narticles resulting from the initial removal and delivered, preferably in controlled quantities, into the fluid current leaving the zone of final treatment of the coarse particles to ensure complete removal of dust from the fine particles. As shown in Fig. 1 material is fed from a hopper 1 via a valve 2 into a vertical conduit 3 to follow adjustably spaced and sloping blades 9 forming an apertured wall of the conduit 3 opening into an expansion chamber 10. The initial removal of the major portion of dust and fine particles is effected by an air current flowing up through the conduit 3 and carrying the said dust and fine particles between the blades 9 into the chamber 10 where the fine particles settle out, the dust being carried on via a conduit 5 and a spiral expansion chamber 22 to a cyclone separator 6 connected to a main suction fan. The residual coarse particles fall through the conduit 3 to pass the air inlet 4 near the bottom of the conduit, the air current entering at this point effecting the preliminary stage of dust removal from the coarse particles. From the base of conduit 3 the coarse particles are delivered via a valve 14 into a second vertical conduit 15, at a point intermediate its height, to meet a second upwardly flowing air current entering the conduit 15 through a bottom inlet 16, and, where desired. through additional inlets 17. The final treatment of the coarse particles is effected in the lower portion of the conduit 15 and the dust free particles are discharged via a valve 18. The dust entraining air leaving the final treatment zone passes up through the conduit 15 to meet and act on the fine particles which are delivered into the conduit 15 from the expansion chamber 10 via a hopper 11 and valve 12. The treated dust free particles are subsequently discharged with the coarse particles and the dust laden air passes from the conduit 15 into the conduit 5 leading to the cyclone 6 via the expansion chamber 22. Any fine particles which settle out in chamber 22 are delivered via a valve 25 and a conduit 21 to a valve controlled discharge opening 26, an opening 27 being arranged between conduits 15 and 24 so that any dust in the conduit 24 is sucked back into conduit 15. At the junction of the conduits 3 and 15 with the exhaust conduit 5, an adjustable flap valve 21 is provided to vary the relative degrees of suction in the conduits 3 and 15. An auxiliary blower 29 may be provided to boost the air current in the conduit 15 when required on excessive load conditions. Various alternative arrangements are described wherein the hopper 11 is formed with one or more compartments each with a separate outlet into the conduit 15 provided with spaced adjustable pivoted feeder blades. Fig. 5 shows two such sets of blades 30a, 30b respectively adjustable simultaneously in opposite directions, such adjustment being effected through levers 31a, 31b, 35-38, operated by a relay means according to the flow of air at the inlet 16 of the conduit 15. In further arrangements the spiral expansion chamber may be augmented or replaced by an extension of conduit 15 formed as an expansion chamber with an adjustable wall to vary its cross-section, or the conduits 3 and 15 may lead to separate cyclone separators via separate conduits each with an expansion chamber. The blades 9 in the wall of conduit 3 may be replaced bv a fixed or vibrating sieve to assist in the initial removal of dust and fine particles, and the entry of air into conduit 3 mav be controlled by a rotary device causing a pulsating flow in the conduit 3. Specification 690,401 is referred to.