This invention concerns a method and a device for controlling the movement of a pneumatic cylinder operated combined alumina feed and crust breaking chisel in an aluminum production cell.
Whether for pre-baked, Sxc3x6derberg or other ovens, a chisel is moved downwards through a crust, which covers a melt of molten aluminium, in order to feed the cell with alumina, and upwards to a rest position. The chisel, which is also referred to as a point feeder, in its reciprocating movement brings along the powder-shaped alumina through the crust into the melt.
During this movement it is important that the chisel effectively breaks through the crust so as to safely penetrate into the melt in order to fulfil the feeding function. This is sometimes a problem since the resistant afforded by the crust varies over time to sometimes be very great. Also the level of the surface of the melt and thus the crust varies over time.
U.S. Pat. No. 4,347,452 concerns an apparatus using the Hall-Heroult process, wherein the movement of a reciprocating plunger, which is electrically insulated, is monitored so as to establish if it has penetrated into the melt or not. If negative, the electrical power supply to the cell is increased in order to re-melt solidified electrolyte. It is also discussed to adjust the travel of the plunger in dependence on the level of the surface of the melt.
U.S. Pat. No. 4,563,255 discloses that energy applied to the crust breaking device is increased by raising the lowering force if necessary to penetrate the crust.
U.S. Pat. No. 4,606,257 addresses the problem of compressed air consumption in connection with the movement of the feeding and crust breaking chisels, which are moved by pneumatic cylinders. In order to reduce the consumption, initially in the downward movement it is arranged to feed with reduced pressure. If the chisel fails to reach the desired position, the pressure acting on the working cylinder, and thus the force on the chisel, is increased. This document also discloses re-use of air from the side of the cylinder, which is inactive.
Reference may also be made to U.S. Pat. No. 5,163,353.
The first two mentioned prior art solutions suffer from potential high compressed air consumption and poor point feeder control, whereas the solutions according to the two latter documents potentially suffer from reduced working life of the associated working cylinder, increased maintenance and replacement costs.
It is an aim of this invention to provide a method and a device according to the above, wherein compressed air consumption is optimised while providing possibility of prolonged working life of the working cylinder and thus reduced costs.
This aim is obtained according to the invention by, during the downward movement, feeding the first side of the cylinder with air at a first, low, pressure no longer than a predetermined time period, monitoring whether said electrical contact has been detected within said time period, and if not, feeding air at a second, high, pressure to said first side until electrical contact has been detected, and feeding air at high pressure to the second side of the cylinder after said electrical contact has been detected so as to quickly withdraw the chisel from the melt in order to minimize heat transfer from the melt to the cylinder.
Hereby harmful heating of the cylinder is reduced to a minimum, resulting in lower costs for maintenance and replacements, and in total, fewer production stops. In many instances it has been noticed that the chisel movement away from the crust is obstructed because it has been stuck to the crust. This would result in prolonged dwell time in an environment where harmful heating results. According to the invention, the chisel is almost instantaneously pulled away from this environment, this way resulting in the desired reduced heating.
By setting the predetermine time period so as to minimise the time of contact between the chisel and the crust, the heat transfer also during the downward movement is reduced.
According to a preferred embodiment of the invention, the cylinder is fed with air with a high pressure at the initial part of the upward movement and fed with air at a low pressure in the remaining path upwards. This arrangements ensures minimal heat transfer, since the chisel is quickly pulled out from the melt and the crust, while ensuring low compressed air consumption. This effect is enhanced when the initial part is the part where any substantial heat transfer to the chisel occurs.
By having the chisel moved further into the melt after the electrical contact has been detected, effective alumina feed is guaranteed.
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