CA2675697C

CA2675697C - Crust breaker for piercing the crust formed on a metal melting bath

Info

Publication number: CA2675697C
Application number: CA2675697A
Authority: CA
Inventors: Dieter Waldmann; Ulrich Hirling
Original assignee: Festo SE and Co KG
Current assignee: Festo SE and Co KG
Priority date: 2007-02-07
Filing date: 2007-02-07
Publication date: 2013-11-05
Anticipated expiration: 2027-02-07
Also published as: CN101605927B; NO20092861L; AU2007346492A1; CN101605927A; AU2007346492B2; WO2008095510A1; CA2675697A1

Abstract

A crust breaker for breaking through the crust (17) formed on a metal melting bath (18) is proposed, with a fluidic double-acting cylinder (10) with a piston rod (14) provided with an impact element (16), and which has two operating chambers (12, 13) separated by a piston (11) and able to be supplied with either a higher (P2) or a lower (P1) fluid pressure by means of a valve assembly (33, 36). The cylinder (10) is provided with a position measuring unit (24). Monitoring means are provided for monitoring a presettable distance-time curve or a presettable speed curve during the forward movement of the impact element (16) towards the crust (17). A control unit (30) is provided to switch over from the lower to the higher fluid pressure after a detectable deviation from the prescribed course, and is connected to the valve assembly (33, 36) for this purpose.

Description

Crust breaker for piercing the crust formed on a metal melting bath The invention relates to a crust breaker for breaking through the crust formed on a metal melting bath, with a fluidic double-acting cylinder with a piston rod provided with an impact element, and which has two operating chambers separated by a piston and able to be supplied optionally with a higher and a lower fluid pressure by means of a valve assembly.
Crust breakers or crust-breaking cylinders of this kind are used for example in melting furnaces for obtaining crude aluminium, for the purpose of locally breaking up an oxide layer forming over the melt. At these points the raw material aluminium oxide is added, after dressed crude aluminium has been removed. This operation takes place approximately every two minutes. Actually, however, a crust which needs to be penetrated has not yet formed in this period of time. In such cases the cylinder need not be applied with maximum force, and can be operated with a lower pressure.
An impact element or impact lance, fixed to the piston rod of the cylinder, dips into the melt each time the crust is broken. As a result, residues and aluminium oxide stick to the lance. When these residues reach a certain thickness they must be scraped off, otherwise the impact lance will become increasingly heavier. This is usually done by means of a fixed steel ring or a tube which encompass the impact lance. This also requires a higher cylinder force, although this process takes place only sporadically, i.e. the greater force is not needed regularly.
In both cases, i.e. when breaking through the crust and/or scraping off the deposits, the position along the stroke of the cylinder at which the higher force is required can not be predicted accurately. The position of the crust depends on the level of the melting bath and may in practice vary by 30 cm. In just the same way, the point at which deposits form on the impact lance depends on the level of the melting bath, since the lance then dips to a varying depth into the liquid aluminium.

In a crust breaker of the type described above and known from DE 202006002727 U1, for the reasons given above, the higher force is used over virtually the whole length of the cylinder stroke, and only on reaching the end positions, which are scanned by sensors, does a switch-over to a lower pressure level take place, to reduce air consumption. However, the pressure applied during virtually the whole of the stroke, and designed to break through the crust, leads to relatively high consumption of air or fluid power, not least due to leaks in the fluid system.
A problem of the present invention is to reduce the fluid consumption of the crust-breaking cylinder by applying the higher pressure only as required.
According to one embodiment of the present invention, there is provided a crust breaker for breaking through the crust formed on a metal melting bath, with a fluidic double-acting cylinder with a piston rod provided with an impact element, and which has two operating chambers separated by a piston and able to be supplied with either a higher or a lower fluid pressure by means of a valve assembly, wherein the cylinder is provided with a position measuring unit, wherein monitoring means are provided for monitoring a presettable distance-time curve or a presettable speed curve during the forward movement of the impact element towards the crust, wherein a control unit is connected to the valve assembly to switch over from the lower to the higher fluid pressure after a detectable deviation from the prescribed course and wherein the control unit is also designed for corresponding monitoring of the return movement of the impact element.
The benefits of the invention are in particular that, in the normal situation, the cylinder is operated only with the minimum operating pressure required, so that the desired outward and return travel times are maintained precisely. If now the cylinder strikes an obstacle, i.e. in particular the crust, then the speed reduces, or the outward movement stops. This is detected by the exceeding of a presettable deviation from the preset distance-time or speed curve. Only in this case is the higher pressure, designed to break through the crust, applied. A special benefit is provided by the fact that a changeover to the higher pressure level may be made at any desired point on the cylinder stroke, without the need to know the distance of the crust breaker from the metal melting bath or the crust respectively. If, since the last impact, no new crust has formed or else only a relatively thin crust, then the set deviation from the preset course is not exceeded, and no switchover to the higher pressure level is made. This leads to an altogether very high saving in fluid, in particular air.
Since according to the invention any desired obstacle may be detected through the reduction in speed of movement and where applicable the exceeding of the set deviation, this also applies to the return movement of the impact element, when any residues on the impact element may be detected when it strikes against a scraper device, and higher pressure is needed to overcome these residues, depending on their extent.
A scraper device, which may be of tubular or annular shape, is expediently provided to scrape off any metal from the metal melt which remains hanging on the impact element.
In an advantageous manner, a pressure setting device is also provided for the lower fluid pressure which presets the speed of the impact element, to make it possible to set this speed.
In an expedient variant, the valve assembly comprises a first valve for switching between the high and the lower fluid pressure, and a second valve for supplying either the first or the second operating chamber with the preselected fluid pressure.
This means that only a very small number of valves is required, namely in particular two.
Means of forming the actual speed as a variation in time of the distance signal of the position measuring unit are preferably provided, these means may be located in the electronic control unit.

In a first variant, the position measuring unit may be provided as a microwave measuring system in an operating chamber of the cylinder to measure the distance between the piston and one cylinder end wall. In another variant, the position measuring unit may also have a resistance measuring element mounted in or on the piston rod and able to be tapped by at least one electrical pick-off element.
In view of the high temperatures which occur, this position measuring system is preferably in encapsulated form.
In an advantageous manner, a measuring arrangement may also be provided to measure the electrical connection between the molten metal or the electrically conductive container for the latter on the one hand, and the impact element or the piston rod connected to it on the other hand, for the purpose of detecting the dipping of the impact element into the molten metal. By this means, in combination with the position measuring system, the distance between the crust breaker and the crust, or the molten metal lying below the crust, may be measured.
The electronic control unit may also be integrated in or on the cylinder in a compact form.
In order to obtain higher speeds of operation and/or movement of the impact element, the electronic control unit has means of presetting the higher fluid pressure at the start of the piston movement, and of switching over to the lower fluid pressure after completion of the acceleration stage.
An embodiment of the invention is shown in the drawing and explained in detail in the description below. The drawing shows in:
Figure 1 a view of a crust breaker positioned over molten metal, together with the relevant electrical and fluid lines, as an embodiment of the invention Figure 2 a signal diagram to explain the mode of operation when the impact element of the crust breaker strikes and dips into the metal melting bath, and Figure 3 a corresponding signal diagram for the return movement.
The crust breaker shown in Figure 1 is comprised substantially of a fluidic, double-acting cylinder 10, the interior of which is divided by a piston 11 into two operating chambers 12, 13. From the piston 11, a piston rod 14 extends with sealing through an end wall 15 of the cylinder 10, and carries at its free end an impact element 16 in the form of an impact lance for breaking through a crust 17 which commonly forms on a metal melting bath 18, which may be for example an aluminium melting bath. The metal melting bath 18 is contained in a bath container 19 made of a metal which melts at a higher temperature than the metal melt.
Attached or fixed to the end wall 15 of the cylinder 10 with the passage for the piston rod is a tubular scraper device 20, in which the impact element 16 is able to slide with minimal play. The free end of the scraper device 20 furthest away from the end wall has a scraping edge 21 by means of which, during the return movement of the impact element 16 into the scraper device 20, any remaining hanging deposits 22 of 15 the crust 17 or the metal melt are scraped off. Instead of a tubular scraper device 20, an annular scraper device may also be used.
At the second end wall 23 of the cylinder 10 opposite the first end wall 15, a bar-shaped resistance measuring element 24 is attached centrally, and extends from that point into the interior of the piston rod 14, in a longitudinal passage 25.
The resistance measuring element 24 may be made of or coated with a resistance material, or a foil resistance element may be applied to it. Located inside the piston 11 is a pick-off element 26 to tap the resistance measuring element 24. This may also involve a slip ring arrangement or other flexible slip elements. The pick-off element 26 may of course also be located outside the piston 11. Another pick-off element 27 in or on the first end wall 15 detects the electrical connection between the electrically conductive piston rod 14 and the electrically conductive first end wall 15.
The resistance value which may be obtained between the pick-off element 27 in the first end wall 15 and the end section of the resistance measuring element 24 attached to the second end wall 23 is a measure of the position of the piston 11 and therefore of the impact element 16, and is fed over lines 28, 29 to an electronic control unit 30.
The position measuring unit formed substantially by the resistance measuring element 24 may also for example be in the form described in DE 29713825 U1, in which case the second pick-off element 27 is not required. Other known position measuring units may also be used in place of the position measuring unit described.
For example, a measuring element capable of movement in the piston rod 14 like the resistance measuring element 24 may have magnetic or other markings which can be scanned digitally. Such markings may also be made directly on and tapped from the piston rod 14. It is also possible to provide in the operating chamber 12 or in the other operating chamber 13 a distance measuring unit working on the basis of ultrasound or microwaves, for measuring the distance between the piston 11 and the respective end wall 15 or 23. Other known potentiometric position measuring units may also be used in a comparable manner.
To detect the position at which the impact element 16 has broken through the crust 17 on the metal melting bath 18, a lamp 31 is connected in series to a voltage source 32 between the pick-off element 27 and the bath container 19. After breaking through the crust 17, the impact element 16 reaches the metal melt, thereby closing the circuit and causing the lamp 31 to light up. Instead of a lamp, it is of course possible to use any other optical or acoustic signal device. The resultant signal is fed back to the electronic control unit 30 via the line 29, so that the measured position value at this point in time is a measure of the position of the crust breaker relative to the metal melting bath 18.
The lamp 31 and the external voltage source 32 may of course also be provided by a suitable circuit in conjunction with the electronic control unit 30, in which case only the resistance value between the pick-off element 27 and the bath container 19 is detected.

To actuate the cylinder 10, a lower pressure P1 from a first pressure source 34 or a higher pressure P2 from a second pressure source 35 may be preset via an electrically controllable 3/2-way valve 33. The operating pressure preset in each case by the valve setting of the directional control valve 33 is fed by means of a second 5/2-way valve 36 to one of the two operating chambers 12, 13, while the other operating chamber in each case is vented. In this process, the two directional control valves are controlled by the electronic control unit 30. The lower pressure P1 is chosen so that, in the unloaded state, the piston 11 moves at the desired speed. This speed may be varied using an adjustable restrictor 37 between the first pressure source 34 and the directional control valve 33. The higher pressure P2 is chosen so that it exerts a force on the piston 11 which is sufficient to break through a crust 17 of the maximum thickness which occurs. Here too a suitable restrictor 36 or another pressure setting device may be provided to vary the pressure P2.
The mode of operation of the crust breaker assembly described is explained below with the aid of the signal diagrams shown in Figures 2 and 3.
For the forward movement of the piston 11 and impact element 16 respectively towards the metal melting bath 18, the operating chambers 12 is supplied with the lower pressure P1 by means of suitable valve settings of the directional control valves 33, 36. This causes the impact element 16 to move, at first unhindered, towards the crust 17. The relevant distance-time curve (w-t curve) is stored in the electronic control unit 30 as specified value curve Sv. This specified value curve follows a linear course, as if there were no crust 17 or other obstacle in the movement path of the impact element. Formed along this specified value curve Sv is a tolerance range Tv, shown in Figure 2 as a broken line. The movement of the impact element 16, i.e. the movement actual value Iv, which is shown as a broken line, initially follows substantially the specified value curve Sv. If the impact element 16 reaches the crust 17 and the force supplied by the lower pressure P1 is insufficient to break through this crust, then the movement of the impact element 16 is inhibited, and the movement actual value lv leaves the tolerance range Tv. This is detected in the electronic control unit 30 by a setpoint/actual value comparison and, by reversal of the directional control valve 33, the higher pressure P2 is set, which is sufficient to break through the crust 17. If during the movement of the impact element 16 no crust has yet formed, or else the crust is so thin that even the lower pressure P1 is sufficient to break through it, then the movement actual value lv does not leave the tolerance range Tv and there is therefore no reversal to the higher pressure P2.
The specified value curve Sv may be stored in the electronic control unit 30 for example as part of a learning cycle. The setpoint/actual value comparison may be made most easily through a speed comparison. This means that a desired speed pattern is stored, and during movement is compared with the actual speed. The actual speed may be obtained from the variation over time of the distance signal of the position measuring unit.
During the return movement of the impact element 16 from the metal melting bath 18, deposits 22 from the metal of the metal melting bath 18 or the crust 17 may remain on the impact element 16, and are removed by the scraper device 20. The relationships involved in the return movement are shown in Figure 3. Here, instead of the specified value curve Sv, there is the specified value curve Sr, while the tolerance range Tv is replaced by the tolerance range Tr and the movement actual value lv by the movement actual value Ir. The movement takes place again initially with the lower pressure P1, which is supplied to the other operating chamber 13. If any remaining deposits 22 reach the scraper device 20, then it is possible that the force available from the lower pressure P1 is insufficient to carry out the scraping process.
In this case, once again, the movement is inhibited, and the movement actual value Ir moves outside the tolerance range Tr. This in turn effects a switch to the higher pressure P2 which is sufficient to carry out the scraping process. If there are no, or only minimal, deposits on the impact element 16, then the lower pressure P1 may be adequate for the scraping process, and there is no switch to the higher pressure P2.

The electronic control unit 30 is optionally equipped with a display 37, to show position values and other processes during the operation of the crust breaker.
The crust breaker shown in Figure 1 has only a single cylinder 10 with one impact element 16. Naturally, in particular for larger metal melting baths 18, crust breakers with several cylinders 11 and therefore several impact elements 16 may be used.
In the vicinity of the metal melting bath 18, there are basically higher temperatures because of the molten metal. Temperature-sensitive components and sub-assemblies, such as for example the position measuring unit, must therefore be suitably designed, for example shielded or encapsulated. The directional control valves 33, 36 and/or the electronic control unit 30 may be located at a distance from the cylinder 10, or else may be integrated in or on it, for example on or in the end walls 15, 23.
Also, in order to speed up the operating processes and varying from the description above, at the start of the forward movement and at the start of the return movement of the impact element 16, the respective operating chamber 12 or 13 may be supplied with the higher pressure P2 during an acceleration phase, with a switch to the lower operating pressure P1 being effected after the desired speed of movement has been reached.
The lower pressure P1 may be for example between 1 and 1.5 bar, and the higher pressure may be approximately 8 bar. Here the pressure is a pneumatic pressure, for example from compressed air. In principle, of course, an hydraulic fluid and an hydraulic arrangement may also be used.

Claims

CLAIMS:

1. Crust breaker for breaking through the crust formed on a metal melting bath, with a fluidic double-acting cylinder with a piston rod provided with an impact element, and which has two operating chambers separated by a piston and able to be supplied with either a higher or a lower fluid pressure by means of a valve assembly, wherein the cylinder is provided with a position measuring unit, wherein monitoring means are provided for monitoring a presettable distance-time curve or a presettable speed curve during the forward movement of the impact element towards the crust, wherein a control unit is connected to the valve assembly to switch over from the lower to the higher fluid pressure after a detectable deviation from the prescribed course and wherein the control unit is also designed for corresponding monitoring of the return movement of the impact element.

2. Crust breaker according to claim 1, wherein a scraper device is provided for metal or pieces of crust from the metal melting bath which remain hanging on the impact element.

3. Crust breaker according to any of claims 1 and 2, wherein a pressure setting device for the lower fluid pressure presetting the speed of the impact element is provided.

4. Crust breaker according to any of claims 1 to 3, wherein the valve assembly comprises a first valve for switching between the higher and the lower fluid pressure, and a second valve for supplying either the first or the second operating chamber with the preselected fluid pressure.

5. Crust breaker according to any of claims 1 to 4, wherein means are provided for forming the actual speed as a variation over time of the distance signal of the position measuring unit.

6. Crust breaker according to any of claims 1 to 5, wherein the position measuring unit is in the form of a microwave measuring system in an operating chamber of the cylinder, for measuring the distance between the piston and a cylinder end wall.

7. Crust breaker according to any of claims 1 to 5, wherein the position measuring unit has a resistance measuring element located in or on the piston rod, which may be tapped by at least one electrical pick-off element.

8. Crust breaker according to any of claims 1 to 7, wherein a measuring arrangement measuring the electrical connection between the metal melting bath or the electrically conductive bath container for the former on the one hand, and the impact element or the piston rod connected to it on the other hand, is provided to detect the dipping of the impact element into the metal melt.

9. Crust breaker according to any of claims 1 to 8, wherein the electronic control unit is integrated in or on the cylinder.

10. Crust breaker according to any of claims 1 to 9, wherein the electronic control unit has means of presetting the higher fluid pressure at the start of the piston movement, and of switching over to the lower fluid pressure after completion of the acceleration phase.

11. Crust breaker according to claim 2, wherein the scraper device is of tubular or annular shape.

12. Crust breaker according to claim 5, wherein the means for forming the actual speed as a variation over time of the distance signal of the position measuring unit are provided in the electronic control unit.