WO1996018869A1

WO1996018869A1 - A level measurement system for liquid metal baths

Info

Publication number: WO1996018869A1
Application number: PCT/NO1994/000202
Authority: WO
Inventors: Dag Baekkedal
Original assignee: Dag Baekkedal
Priority date: 1994-12-12
Filing date: 1994-12-12
Publication date: 1996-06-20
Also published as: AU1392895A

Abstract

In a method and device for measuring the surface level in a smelting bath (3) at high temperatures and simultaneously feeding in raw metal in the form of a metal bar (7) which is lowered into the smelting bath (3) by means of a lowering mechanism (4), the number of withdrawals from the smelting bath (3) controls the introduction of a new bar (7). The metal bar which supplies the raw metal to the smelting bath is electrically insulated and constitutes one electrode in the level measuring system and the metal mass is the second electrode. By lowering the bar down to the surface of the smelting bath the surface level is measured and the lowest smelting bath level in the level measurement is used as indication for the supply of another bar before the smelting bath is tapped.

Description

A level measurement system for liquid metal baths

The present invention concerns a level measurement system for measuring the metal level in a liquid metal bath, for use within the foundry industry, e.g. in magnesium casting.

Modern mass production of metal objects is normally performed by means of pressure die-casting, wherein a casting mould is filled with molten metal by means of a displacement pump. This displacement pump is filled by means of gravity from a liquid, heated metal bath in a furnace, which requires a certain even static pressure which is maintained by ensuring that the level in the metal bath is at a certain constant height above the inlet of the pump. New metal is added to the liquid metal bath in the form of bars which are placed in the bath manually, by means of feed tongs or a robot.

In their molten state various metals have to be protected to a greater or less degree, depending on their reactivity with the oxygen in the air. Magnesium, whose area of application is gradually increasing due to its low weight, especially within the car industry and other fields allied to transport, is particularly reactive. In fact molten magnesium is so reactive that it burns in air if it is not protected.

Smelting of magnesium previously took place in open furnaces, where the surface of the magnesium was protected against combustion in the air by means of foundry salt which floated on top of the molten mass. However, this carried the risk of pollution of the magnesium, since some salt could find its way into the casting moulds along with the molten mass, thus causing cavities to be formed in the castings, with the result that the castings had to be discarded. Developments within the field in recent times have therefore involved a transition substantially from the use of foundry salt to the use of protecting gas, which is not encumbered by the said drawbacks. The use of gas as a protective medium necessitates a transition from open to closed furnaces, since an open furnace cannot retain a gas.

The supply of metal bars is often controlled manually by an operator who determines the supply rate on the basis of a measurement of the metal level in the molten mass and information on the withdrawal rate. In the case of open furnaces the measurement of the metal level is often performed manually, either by means of a measuring rod which is held down to the surface of the molten mass, or by the operator's assessment of the level. This naturally leads to some variation in the level. On the basis of the above-mentioned requirement for a constant level, the result of this is that the method of feeding in bars manually has been shown in practice to be unsatisfactory.

In the case of closed furnaces, measuring methods are required in order to ascertain the level of the molten mass.

Various methods are employed for measuring the level of the molten mass. One known method consists in the use of a float which by means of a mechanism or suspension arms supplies information to a scale instrument or a switch. This type of measuring equipment is extremely vulnerable to incrustation, and consequently requires frequent maintenance. During maintenance the float has to be removed from the furnace, which means that the slag comes into contact with the oxygen in the air and is ignited. Consequently this type of measuring equipment is not desirable from the safety point of view or with regard to the working environment.

A second method which is used for measuring the level in a molten mass consists in the use of one or more stationary or movable electrodes, wherein an electrical circuit is closed and emits a signal when the electrode comes into contact with the molten mass. US 4 728 875 describes such a method wherein two electrodes are mounted on an arm which is lowered into a smelting bath. The electrodes are mounted at a certain vertical distance in relation to each other, thus indicating the highest and the lowest level in the smelting bath. The arm is adjusted in such a manner that one electrode is in contact with the bath and one is located above the bath. If the level drops causing both electrodes to be out of the smelting bath, impulses are supplied for replenishing of metal. Should the level rise causing both the electrodes to come into contact with the smelting bath, the filling process is reduced or closed down completely.

However, the combination of measuring by means of electrodes and the use of protecting gas has the disadvantage that metal becomes attached to the electrodes to a greater or less extent due to the surface tension in the molten metal. This results in unreliable measurements where the level will often be registered as higher than it actually is. Modern mass production of castings makes great demands on accuracy and uniform quality in the castings, which makes even static pressure at the inlet to the pump essential, and which in turn requires a relatively constant level in the smelting bath. Due to the above-mentioned phenomenon whereby molten metal becomes attached to the electrodes due to the surface tension, it is difficult to obtain adequate accuracy of measurement, which means that measuring by means of electrodes is unsuitable for mass production of castings where there are stringent requirements with regard to accuracy and uniform quality.

A third measurement method is measuring by means of infrared radiation from the molten mass, wherein the measuring equipment is in the form of a disc-like container located under the lid of the smelting furnace. From the technical point of view this measurement method works satisfactorily, and is used to some extent. However, since there is usually very little space between the molten mass and the lid of the molten mass, typical 10 cm, it is necessary to take this measuring equipment apart during cleaning. The frequency of this cleaning will vary, but with round-the-clock operation it may generally involve between 3 and 9 cleaning operations every 24 hours, depending on the amount of pollution and incrustation. This measurement method is therefore extremely cumbersome to use.

It is also known that attempts have been made to use ultrasonics in order to record the level in a molten metal bath. However, these have led nowhere, since the temperature of the smelting bath affects the wavelength in such a manner that it is impossible to obtain adequate accuracy of measurement.

Another measurement method which is in use is measuring by means of a laser. This method functions per se without any special problems, but due to its complexity it is expensive to instal, and like any method which is based on electronics and/or precision mechanics in the vicinity of a smelting bath, it is sensitive to the influence of high temperatures and possibly spatter from the molten mass. Measuring by means of laser normally requires a relatively large equipment, and is therefore better suited within the smelting plant industry, where the furnaces are larger than in the foundry industry.

A further measurement method which should be mentioned is measuring by means of radioactive isotopes which emit either gamma or particle radiation. The radioactive source is located at one side of the liquid level which has to be measured, above the level of the liquid, and a measuring probe is located at the opposite side. During operation the liquid will absorb some of the radiation, depending on the level of the liquid. The intensity of the radioactive radiation is recorded by the measuring probe, which then passes a signal on to a calculation unit which calculates the level of the liquid. This is a "universal method" which can be used within a number of technical fields, including the foundry industry, since it will work independently of high temperature, surface tension, incrustation and other features which are characteristic of a molten metal mass. The employment of isotopes which emit radioactive radiation, however, is extremely undesirable from a work environmental and pollution point of view, and thus the method is not relevant for use except under very special circumstances. In addition there will be more equipment in the bath, which is undesirable due to problems connected with cleaning.

The object of the present invention is to provide a simple and reliable measurement method for measuring the level in a molten metal mass, which will have a reasonable cost both in installation and operation. A measurement method will thereby be provided wherein the level measurement with associated feeding in of metal bars has sufficient accuracy to provide a level in the molten mass which is sufficiently constant to give an even static pressure which is suitable for mass production of die-castings with uniform quality.

The above-mentioned objects of the invention are achieved with a level measuring system for a liquid metal bath of the type mentioned in the introduction, characterized by the features which are indicated in the patent claims.

The measuring principle is based on lowering an electrode into a smelting bath. As an electrode a metal bar is used which supplies raw metal to the smelting bath. There must be a voltage potential between the bar and the smelting bath, which means that the bar's lowering mechanism must be connected to a voltage source via a wire, but in other respects be electrically insulated from the smelting bath. The electrode makes contact when it touches the smelting bath, thus causing a relay to be connected.

In connection with the lowering mechanism for the raw metal bar there is a measuring system for measuring the lowering mechanism's and consequently the metal bar's position in relation to a fixed reference point. By means of this secondary measurement the level of the smelting bath will be known when the bar closes a circuit as it touches the smelting bath.

A metal bar can be lowered into the metal bath in various known ways, e.g. by the use of a hydraulically operated arm with a claw or tongs. A robot may also be employed. The invention is not dependent on the design of the lowering mechanism as long as the bar is electrically insulated from the smelting bath. The measurement of the position of the lowering mechanism in relation to a fixed reference point is not subject to the above-mentioned problems which arise during the measurement of the level of the molten metal mass, and can therefore be performed in a number of known ways. If, e.g., the bar is lowered into the smelting bath by means of a hydraulically operated arm, level switches can be mounted on the hydraulic cylinder, these switches being located in such a manner that they indicate the highest and the lowest smelting bath level.

Thus the invention is not dependent on how the measurement of the level of the lowering mechanism is performed in relation to a reference point.

From the said US patent publication it is known in the prior art to measure the liquid level in a container by means of an electrode which is lowered into the container and makes contact when it touches the surface of the liquid. A new feature, however, is the use of the metal bar which is employed for feeding raw metal into the smelting bath as an electrode, and the combination of the supply of the raw metal with the level measurement.

By using the metal bar as an electrode the measuring device is only in contact with the metal bath during measuring, and the problem of liquid metal becoming attached to the electrode and causing errors in level measurement is avoided. At the same time the need for maintenance is greatly reduced, since there are no parts of the measuring system which require to be taken up for inspection and cleaning.

The invention will now be explained in more detail by means of the drawing which illustrates a preferred embodiment of the invention.

The drawing illustrates a smelting furnace 1 with a crucible 2 full of liquid magnesium 3 with a temperature of 700 degrees C. Above the smelting furnace there is a lowering mechanism 4, on the outermost part of which there is provided an insulator 5. On to the insulator 5 there is attached a set of tongs or a claw 6 for introducing the bars 7 into the molten mass 3. From the tongs 6, which are electrically insulated from the lowering mechanism 4 by means of the insulator 5, a wire 8 passes to a relay 9. The relay 9 is further connected with the crucible 2, thus causing the circuit to be closed when the bar 7 touches the surface of the smelting bath 3. There is further illustrated a hydraulic cylinder 10 which is used for raising and lowering the lowering mechanism 4. There are two inductive level switches, a high level switch 11 and a low level switch 12, which are permanently mounted in relation to the smelting furnace, and provided with a certain vertical distance between them, which records when the piston in the hydraulic cylinder passes.

With the combination of the bar which acts as an electrode and the two level switches, three levels can be measured in the smelting bath. In addition there is a fixed safety electrode (not shown) whose purpose is to send a signal to stop the process if there is too much metal in the molten mass, typically located 30 mm above normal level.

The approximate mass of each bar and each casting is known in advance, thus enabling a control system (not shown) to regulate the supply of metal bars on the basis of the number of withdrawals from the smelting bath.

During operation the control system will record the number of withdrawals from the molten mass. After a certain number of withdrawals, which correspond as closely as possible to the weight of a bar, the lowering mechanism will fetch a bar from a store (not shown) and lower it into the molten mass. Should the bar make contact during lowering before the high level switch is connected the level of the smelting bath is too high. There is therefore too much metal in the furnace and the control system will wait until a sufficient amount of metal has been removed. If the bar makes contact after the high level switch has been connected, but before the low level switch has been connected, the level of the smelting bath is correct. In this case the feeding in of metal bars will continue to be governed by the number of withdrawals. If the bar makes contact after both the high and the low level switches have been connected the level is too low. In this case the lowering mechanism fetches another bar and records the level once again. If the level is still too low, yet another bar is collected until the correct level is obtained.

As mentioned earlier a constant level in the molten mass is crucial for the quality and uniformity of the castings. With the equipment described above a smelting furnace from Nordiske Industriovner A/S, where a "New Magic" casting pump from Norsk Hydro was used, achieved an accuracy in the level of the molten mass of + 5 mm, with an associated accuracy in the castings of ± 3 weight percent. With this furnace the introduction of a new metal bar into the molten mass represented a level increase of 2.5 mm. In comparison it should be mentioned that the accuracy which is attainable in practice in the molten mass with manual feeding is estimated to be + 5 cm. The method is not limited to magnesium as described in the example, but can be employed for all types of metals.

It will be seen that many modifications are possible within the scope of the invention. One obvious modification is to use a robot to hold the lowering mechanism for the metal bars. It would then be natural to replace the level switches and the separate control system with the robot's positioning system and control system. A positioning system for a robot will usually be of such a nature that the robot's control system is familiar at all times with the robot arm's, and consequently the bar's, exact position, which means that the level switches become superfluous. The result of this will be that the control of the supply process for the bars will be slightly changed in comparison with the procedure described above.

It will, of course, also be possible to have a different type of lowering mechanism than a claw or tongs. The bars can, e.g., be fed into the molten mass by gravity from a pipe or a groove with an internal cross section which corresponds to the cross section of the bars, and with a shut-off arrangement for governing the feed rate.

These examples of modifications, and similar modifications which will be natural for a person skilled in the art, will all lie within the scope of the invention, since the core of the invention consists in the employment of the metal bar which is used for feeding raw metal into the smelting bath as an electrode, and the combination of the supply of raw metal with level measurement.

Claims

PATENT CLAIMS

1. A method for measuring the surface level in a smelting bath (3) at high temperatures and simultaneously feeding in raw metal in the form of a metal bar (7) which is lowered into the smelting bath (3) by means of a lowering mechanism (4), characterized in that the number of withdrawals from the smelting bath (3) governs the introduction of a new bar (7) and that the surface level is measured by lowering the bar down to the surface of the smelting bath.

2. A method according to claim 1, characterized in that the level measurement is employed as a control for the number of withdrawals before the introduction of a new bar.

3. A method according to claim 1, characterized in that the lowest smelting bath level in the level measurement is employed as indication for the supply of another bar before the smelting bath is tapped.

4. A method according to claim 1, characterized in that a continuous positioning system which is familiar at all times with the bar's exact position in relation to a fixed reference point is used to record the level of the molten mass as the bar touches the surface of the smelting bath.

5. A device for measuring the surface level in a smelting bath at high temperatures and simultaneously feeding in raw metal in the form of a metal bar which is lowered into the smelting bath, characterized in that the metal bar which supplies raw metal to the smelting bath is electrically insulated and constitutes one electrode in the level measurement system and the molten metal mass is the second electrode and that in connection with the lowering mechanism (4) for the metal bar there are mounted level switches (11 ,12) which indicate a highest smelting bath level and a lowest smelting bath level.

6. A device according to claim 5, characterized in that the measuring device which indicates high level or low level is moved in parallel with the bar movement.

7. A device according to claim 5, characterized in that a level contact is mounted above the highest level to stop the supply and to secure the system against incorrect adjustment.

8. A device according to claim 5, characterized in that the measuring device which indicates the level comprises a continuous positioning system which is familiar at all times with the bar's exact position in relation to a fixed reference point.