MEIER, Patrick (Kirchgasse 17, Wettswil, CH-8907, CH)
KELLER, Robert (Pilgerstrasse 12, Baden-Daettwil, CH-5405, CH)
MEIER, Patrick (Kirchgasse 17, Wettswil, CH-8907, CH)
| CLAIMS 1. A method for detaching a frozen charge (14) from an inner wall (12) of a grinding pipe (10), the method comprising the steps of: controlling a driving device of the grinding pipe (10) to detach a frozen charge from an inner wall (12) of the grinding pipe (10), which driving device is operable to apply a driving torque to the grinding pipe (10), wherein controlling the driving device comprises varying the driving torque applied to the grinding pipe (10) about a predetermined and increasing torque reference level (20, 30). 2. A method as claimed in claim 1, wherein the driving torque is always kept in the same direction during such control. 3. A method as claimed in claim 1 or 2, wherein varying the driving torque comprises varying the driving torque sinusoidally about the predetermined reference level. 4. A method as claimed in claim 1 or 2, wherein varying the driving torque comprises varying the driving torque in a stepwise manner about the predetermined reference level. 5. A method as claimed in claim 1 or 2, wherein varying the driving torque comprises varying the driving torque in any pattern about the predetermined reference level and comprised within a torque range proportional to the torque reference level. 6. Apparatus for detaching a frozen charge (14) from an inner wall (12) of a grinding pipe (10), the apparatus comprising a controller operable to control a drive device of a grinding pipe (10) such that a driving torque applied by the drive device varies about a predetermined and increasing torque reference level (20, 30). |
The present invention relates to a method and associated apparatus for detaching a frozen charge from an inner wall of a grinding pipe of a tube or pipe mill or similar.
Tube mills are most commonly used for grinding material such as ore. Large particulate material, such as gold ore, is delivered into the grinding pipe of the tube mill where it can be mixed with grinding media charges such as balls. Water can also be added to this mixture. It is not uncommon for the use of a tube mill to be intermittent, either due to an intermittent supply of material to be ground, due to maintenance of the mill itself or due to an emergency stop of the system. During the periods when the tube mill is not is use, the charge within the grinding pipe can consolidate and become firmly stuck to the inner wall of the grinding pipe, this is referred to as "frozen charge". When the tube mill is set in motion again after a period of non-use during which frozen charge has occurred, there is a high likelihood that the frozen charge may become detached at the highest point of rotation of the grinding pipe. This will result in the frozen charge dropping onto the inner surface of the grinding pipe at the lowest point of rotation which, given the potential height of drop and the materials involved, could result in substantial damage occurring to the tube mill.
In view of this, it is common for a grinding pipe to be checked for frozen charge, and when detected, rotation of the tube mill is ceased immediately. Until recently the removal of frozen charge was a laborious manual process involving the use of air compressed hammers upon the charge which may, or may not, have first been softened by spraying with water.
Patent application publication DE 3528409 Al describes an arrangement which detects the presence of frozen charges and stops rotation of the drum in the affirmative. If a tube mill is driven with constant angular speed, the corresponding torque increases to an absolute maximum indicative of loose charges starting to tumble towards the lower parts of the rotating tube. Under the presence of frozen charges however, such maximum is not observed at moderate angles of rotation.
Recently, a more efficient method and associated devices for removing such frozen charge has been disclosed in US Patent Application No. 2008/0169368 (Becker et al.). This method involves controlling a gearless drive of a ring motor surrounding the grinding pipe to effect targeted detachment of frozen charge. The grinding pipe drive is operated to rotate the grinding pipe in an angular range and at an appropriated speed such that falling material does not cause damage to the grinding pipe or other components of the tube mill. An angle of rotation is set to oscillate about a predetermined angle of rotation, with a corresponding torque reference, or mean, value decreasing proportionally to the fraction of frozen charge.
A system with a driving torque applied to the grinding mill that can be both positive and negative is not suitable in mills having a geared drive as this can create a backlash of force on the gear teeth which in time will cause damage to the gears and will subsequently decrease the lifetime of the drive train.
In view of the above, an object of the present invention is to obviate or mitigate at least one of the aforementioned problems.
According to a first aspect of the present invention there is provided a method for detaching a frozen charge from an inner wall of a grinding pipe, the method comprising the steps of controlling a driving device of the grinding pipe to detach a frozen charge from an inner wall of the grinding pipe, which driving device is operable to apply a driving torque to the grinding pipe, wherein controlling the driving device comprises varying, or oscillating, the driving torque applied to the grinding pipe about a predetermined reference level which is steadily, or continuously, increasing during the variation.
By varying the driving torque applied to the grinding pipe the torque acting upon the frozen charge is also varied which facilitates the dislodging of the frozen charge.
Preferably, the driving torque is always kept in the same direction during such control.
Application of such driving torque prevents back-lash which causes mechanical stress on gear teeth associated with the grinding pipe thus increasing the lifespan of the grinding pipe machinery.
Conveniently, varying the driving torque comprises varying the driving torque sinusoidally about the predetermined reference level.
A sinusoidal varying of the driving torque results in a smoother pattern of movement being applied to the grinding pipe machinery resulting in less strain, particularly on the drive train mechanism of the grinding pipe.
Alternatively, varying the driving torque comprises varying the driving torque in a stepwise manner about the predetermined reference level. A stepwise varying of the driving torque results in a greater effect of inertia acting upon the frozen charge providing an efficient dislodging process.
In a further embodiment, varying the driving torque comprises varying the driving torque in any pattern about the predetermined torque reference level, yet comprised between a maximum torque level and a minimum torque level that define an increasing torque range proportional to the increasing torque reference level.
According to a second aspect of the invention there is provided apparatus for detaching a frozen charge from an inner wall of a grinding pipe, the apparatus comprising a controller operable, or adapted, to control a drive device of a grinding pipe such that a driving torque applied by the drive device varies about a predetermined and steadily increasing torque reference level.
By varying, about a predetermined reference level, the driving torque applied to the grinding pipe, the torque acting upon the frozen charge is also varied which facilitates the dislodging of the frozen charge such that upon dislodgement damage to the inner wall is minimised.
These and other aspects of the invention will become apparent from the following descriptions when taken in combination with the accompanying drawings in which:
Figure 1 is a cross sectional view of a grinding pipe inner wall according to the present invention;
Figure 2 is a graphical representation of torque steps applied to the grinding pipe in accordance with one embodiment of the present invention and
Figure 3 is a graphical representation of a sinusoidal variation in torque applied to the grinding pipe in accordance with a second embodiment of the present invention.
With reference to Figure 1 there is shown a grinding pipe 10 including an inner wall 12 to which a mass of frozen charge 14 has become adhered. In accordance with the present invention, the frozen charge 14 can be detached from the inner wall 12 by agitating the arrangement 10. The method of agitation is implemented by operating a controller (not shown) which controls a driving device (not shown) of the grinding pipe 10 by applying a driving torque which in turn applies a driving torque to the grinding pipe 10. By varying the torque applied to the grinding pipe 10, the speed of rotation of the grinding pipe 10 and consequently the angle of rotation of the grinding pipe 10 is varied. During the frozen charge removal operation, the grinding pipe 10 is driven through an angle 16, which is a maximum of 75° to prevent the frozen charge from dropping due to gravity. This angle 16 can also be less than 75° depending on the type of ore. This "shaking" of the grinding pipe 10 by applying a varying driving torque results in the loosening of the frozen charge 14 from the inner wall 12 within a controlled range of angle of rotation, thus limiting the likelihood of damaged caused by the dislodging of the frozen charge 14 at an inappropriate point of rotation.
Figure 2 represents graphically the torque applied to the grinding pipe 10 plotted against time. The torque applied to the grinding pipe 10 is varied around a given reference torque T Reference 20. The pulsed torque steps applied have a given period 26 and vary around the reference torque T Reference between minimum torque T Reference - torque T Reference * Torque factor 22 and maximum torque T Reference + torque T Reference * Torque factor 24. The Torque factor is chosen in such a way that the actual applied torque does not become negative. In other words, the Torque factor is smaller than 1.
The angle of rotation through which the grinding pipe 10 is moved during the process of dislodging frozen charge 14 is limited to a maximum of 75° to ensure that the frozen charge 14 does not dislodge at a height which will cause substantial damage to the inner wall 12 of the grinding pipe 10. The angle 16 is monitored in order to ensure a proper stop before the angle reaches 75°.
After each set of torque pulses is applied and before the angle 16 reaches 75°, the grinding pipe 10 is stopped and brought back to equilibrium position (i.e. where the angle 16 is 0°). The grinding pipe 10 can then be started in the same direction or alternatively in the opposite direction and torque pulses are again applied. This process is repeated until the frozen charge is removed.
The variation around reference torque T Reference 20 oscillating within a torque range of width 2 * torque T Reference * Torque factor between 22 and 24 is such that the torque applied is always positive. The application of positive torque is important, particularly for geared mills, as it prevents back-lash which causes mechanical stress on the gear teeth thus increasing the lifespan of the machinery.
The effect of the pulsed application of torque is that the frozen charge 14 is dislodged due to variation of the acceleration. Furthermore, as the torque T Reference 20 increases, the oscillation amplitude can also increase as there is more room until a negative torque would be reached. In Figure 3 there is illustrated graphically an alternative torque variation that can be applied to the grinding pipe 10 in accordance with the present invention and this is plotted against time. In this embodiment, the torque variations are applied around a positive torque T Reference 30 and are sinusoidal in pattern. The sinusoidal pattern is a "soft" torque variation such that the smoothness of the sinusoidal pattern of movement puts less stress on the drive train mechanism of the grinding pipe 10. This smooth application of torque results in the prevention of unnecessary damage to the drive train mechanism of the grinding mill. The smooth application of torque such as the sinusoidal pattern followed is less efficient in the loosening of frozen charge 14, due to the fact that the acceleration on the charge is not as high as with torque steps. As in the embodiment of Fig.2, a second derivative of the driving torque with respect to time repeatedly becomes negative, while a first derivative of the driving torque may remain positive at all times.
Other patterns of torque pulses or variation around a positive reference torque may be applied to the grinding pipe 10 in accordance with the present invention. It will be understood that the embodiments detailed above can be applied to gearless mill drives and ring-geared mill drives, with particular benefit to geared mill drives.
Various modifications may be made to the embodiments hereinbefore described without departing from the scope of the invention. For example, it will be understood that water may be applied to the frozen charge 14 before or during the torque being applied to the grinding pipe 10 to facilitate the dislodgement of the frozen charge 14 from the inner wall 12.