Patent GB-867228-A (GB)
Title: Air system for dry material reduction mills and controls therefor
IPC: B02C25/00
Filed: 1957-08-27
Abstract: 867,228. Grinding mills. WESTON, D. Aug. 27, 1957 [Sept. 10, 1956], No. 27021/57. Class 59 [Also in Group XXXV] A dry crushing or grinding mill has the air flows in the mill and its outlet duct separately controlled so that particles below a predetermined size are extracted from the mill and transported in a controlled fluidized mass. Further controls may be provided for the temperature and humidity of air in the system. Fig. 1 shows the elements of a grinding mill system to which these controls may be applied. A mill 11 driven by motor 12 is fed with material by belt conveyer 35 from bin 37; the conveying rate may be adjusted in accordance with the noise in the mill or the power delivered by motor 12. The mill product leaves by duct 13 and over-size material is then removed by classifier 14 for return to the mill by conveyer 33, the remainder passing to a cyclone collector 16. Air is then re-cycled by fan 18 partly to the mill outlet via duct 21, and partly to classifier 14, while some leaves the system via a Venturi scrubber 24. Fig. 2 shows air flow and humidity controls for a similar mill; a control unit 51 adjusts a proportion damper 32 to vary the air velocity in the mill so that a constant proportion of coarse product (measured by weigh-belt 33) to input (measured by weigh-belt 35) is obtained. A unit 54 controls a damper 31 and hence the carrying power of the air duct 13 to maintain a fixed pressure drop across cyclone collector 16. A unit 59 controls the humidity of air in the system; if this rises it opens outlet damper 27 so that more external air is admitted. If the humidity cannot be kept down in this way, a unit 55 senses the fall in temperature of the circulating air due to the influx of outside air and evaporation from input feed (normally the circulating air is 60-80 degrees above ambient temperature) and controls a burner 56 to maintain this temperature differential and thus acts as a further humidity control. Fig. 3 shows an embodiment suitable for dry climates where low humidity may cause a falling-off in the efficiency of cyclone dust extractor 16; a control unit 62 senses the ambient humidity (or it may use a photo cell and light source to measure the quantity of dust in the apparatus) and increases the air bleed-off at 23 when necessary. Alternatively it may inject water at 64. Fig. 3A (not shown) shows a modified apparatus in which temperatures and humidity are controlled independently, a proportion of the saturated air from a Venturi scrubber used in obtaining the "wetbulb" temperature being recycled to maintain the humidity. An "open" system (i.e. in which there is no re-cycling of air) is illustrated in Fig. 8 (not shown). Fig. 5 shows the damper control system of Fig. 2; signals from the weigh belts are fed to coils 96, 97 of a differential relay with contacts E, F in the circuits of two time-delay relays TDR1, 2. The contacts of the latter energize the two halves of split-field motor M which adjusts the damper 32. Specifications 632,532, 696,181, 727,925, 782,523, 783,113 and 788,924 are referred to.