GB783113A - Method of reducing materials - Google Patents

Abstract

783,113. Grinding. WESTON, D. March 3, 1955 [May 25, 1954], No. 6363/55. Class 59 A method of operating a combined dry crushing and grinding mill of the kind in which a drum 10, Fig. 2, having a diameter at least twice as great as its length and provided with a series of inwardly-projecting crusher bars uniformlyspaced around its inner periphery is rotated about its horizontal axis consists on pre-determining a charge volume dependant upon the desired particle size distribution of the product, maintaining this charge constant throughout the operation and maintaining a sufficient flow of air through the mill to entrain and carry away the product having the desired size. The mill is of ball type and constructed as described in Specifications 696,137, 696,164 and 696,181, material being charged into the drum through the opening 13 and an air current produced by a fan 15 withdrawing the product through an opening 14 to a separator 18. The solid material is discharged from the separator through an air lock valve 19 and the air stream after passing through the fan is recirculated to the inlet side of the drum through a duct 20; a certain amount of the air is withdrawn from the duct by a fan 21 and atmospheric air is admitted to the inlet side of the drum at the point 23. It is explained that since the grinding factor increases with charge volume and finally drops off the crushing factor decreases after a critical charge volume is reached, there is a definite relationship between the charge volume maintained during a milling operation and the particle size of the product, and in carrying out the invention use is made of the graph shown in Fig. 3A, which shows the capacity of the mill in tons per hour plotted against the charge volume (itself a function of the HP drawn by the mill motor) and has been prepared by operating the mill for the particular material at different charge levels and recording the results. Points on the curve at one side of the point X representing the charge volume for maximum capacity produce coarser particles than those on the other side, so that, upon determination of the grade of product desired, the necessary charge volume can be read from the curve and therefore the HP required to operate at the selected point determined. The volume charge in the mill is maintained constant by controlling the feed to the mill by means of a system employing, as described in Specifications 727.925 [Group XXXV] and 782,523 [Group XXXV] a difference signal derived by comparing a control signal functional to the operation condition of the mill and reference signal corresponding to the desired operating condition as determined from the graph; in Fig. 4, the control and reference signals are compared in a comparator 42 and the difference signal is amplified in an amplifier 43 for the purpose of controlling a feeder 44 delivering material to the mill 45. The source of the control signal may be sound or other vibrations emanating from the mill and of which only a selected band of frequencies need be used; the signal may, for example, be functional to sound of a frequency of more than 200 cycles per second. Alternatively, the signal may be derived from the power drawn by the motor by means of an electronic watt meter, and, if desired, a sound signal may then be used as a monitor to keep the power on the desired side of the curve. Generally, power is used as the source of the signal for low charge volumes and sound or other vibrations for higher charge volumes. In some cases, and particularly of the desired operational point occurs in the region of maximum capacity, a composite control signal may be used or a signal based on one of the above sources may be monitored by a signal based on another of the sources. The air current passing through the mill must be of sufficient volume and velocity to carry away the desired product and generally it is operated at a velocity sufficient to entrain some oversize material which is separated from the air stream at the outlet elbow 16, Fig. 2, and returns to the mill.