Electromagnetic valve with slag indicator

A valve device for metallurgical containers includes an electric circuit formed by two electrodes opening out into the molten metal at the tapping point. The electric circuit measures the resistive voltage drop across the molten metal at the tapping point to determine the slag content in the tap stream by changes in the measured resistive voltage drop. The rate of tapping of the melt can be controlled simultaneously along with the indication of the presence of slag in the tap stream.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to valve devices used in connection with 
tapping points from metallurgical containers. 
In casting operations it would be of great value to have a signal 
indicating when the surface of the bath has declined to the point that 
slag starts streaming out from the tap hole. A vortex is normally formed 
above the tap hole and sucks down the slag from the surface of the bath, 
and therefore considerable amounts of slag are suddenly drawn down into 
the tap stream. The requirements for accuracy of the measurement signal 
from a slag indicator are therefore moderate. 
SUMMARY OF THE INVENTION 
The invention provides a solution to the aforementioned problem and other 
associated problems whereby an electric circuit is formed by two 
electrodes opening out into the molten metal at the tapping point (the tap 
stream). The electric circuit is provided with means for measuring the 
resistive voltage drop across the molten metal at the tapping point. Any 
change in this resistive voltage drop is a measure of any slag content in 
the tap stream. Since the resistivity of molten slag and molten iron are 
quite different and, in addition, the tap stream completely fills the 
cavity adjacent the magnetic valve, a measurement of the resistive voltage 
drop caused by the valve current provides a good indication of the 
presence of slag in the tap stream. 
In a preferred embodiment of the invention a magnetic circuit is arranged 
in a known manner perpendicular to the current, a resultant force thus 
being obtained with, or against, the tap stream. In the embodiment of the 
invention, the circuit and the magnet are supplied with current, 
preferably direct current, the voltage drop across the tap stream being 
arranged to be measured, and a measure of the slag contents thus being 
obtained. Simultaneously, the rate of tapping of the melt can be 
controlled in a known manner by means of the valve.

DETAILED DESCRIPTION 
In FIG. 1, numeral 1 shows a tap hole from a metallurgical container (not 
shown). Two electrodes 2, 3 are arranged at two diametrically opposed 
locations of the walls of hole 1, and form a circuit together with the 
molten metal of the tap stream between the electrodes. Magnetic circuit B 
is arranged at 4 perpendicular to the current in circuit i, a resultant 
force F thus being obtained with, or against, the tap stream flow. The 
magnetic circuit is supplied with direct current and alternatively with a 
superimposed alternating field (B.about.). 
The electromagnetic valve may be used for continuous control of the rate of 
flow of a metal stream. In continuous casting, for example, it may be used 
in the ladle for controlling the metal flow to the tundish, or in the 
tundish for controlling the flow to the mold. In certain cases it also 
makes possible casting directly from the ladle to the mold, that is, the 
tundish may be omitted. The use of the valve is, however, not limited to 
the continuous casting process. 
The two electrodes 2, 3 are water-cooled. The outflow direction of the melt 
is designated by numeral 5. The valve may be used both for reducing and 
increasing the rate of outflow. The electromagnetic valve is suitably 
supplied with direct current from a thyristor rectifier (FIG. 2), since in 
this way the inductive voltage drop in the leads are eliminated. Typical 
data for a valve are 0.5-1.0 Wb/m.sup.2 and 5-20 kA for a valve in a 
ladle, that is, a valve that is capable of withstanding the ferrostatic 
pressure at a bath height of 3-4 meters. 
FIG. 2 shows a three-phase network R, S, T, to which are connected 
thyristor convertor circuits 6, 7, each of which includes three 
thyristors, one for each phase of the three-phase network. Thyristor 
convertor 7 is connected to excitation magnet coils 8, 9 having a magnetic 
iron core 10. The gap of iron core 10 forms magnetic circuit B as 
illustrated in FIG. 1. Current i is supplied from three-pulse convertor 
circuit 6 between electrodes 2, 3. The conductors from three-pulse 
convertor circuit 6 include reversing switch 11, to provide a means for 
reversing current i, and thereby force F. The voltage drop V in the gap 
between electrodes 2, 3 is measured by voltmeter V. As previously 
mentioned, the resistive voltage drop V can be calibrated to provide an 
indication of the slag contents in the melt stream flowing between 
electrodes 2, 3. 
The most simple connection is obtained if the coils for the generation of 
the magnetic field are placed in series with the current conductors. In 
this way only one rectifier is needed. Also, a device for reversing the 
direction of the current in, for example, the coils may be provided. 
However, the best regulation is obtained if separate thyristor rectifiers 
are used for the generation of the magnetic field as well as the current 
through the tap stream, as is illustrated in FIG. 2. The electromagnetic 
valve should be supplemented with, for example, a disc valve since the 
metal flow cannot be electrically reduced down to zero. In tests 
performed, the metal flow could be controlled .+-.95% from the nominal 
value. 
By superposing a magnetic alternating flux in the magnetic circuit of the 
valve, among other things an induced voltage is obtained across electrodes 
2, 3 and has the same frequency as the superposed flux. This voltage is 
directly proportional to the outflow rate of the metal and can thus be 
employed partly as a casting rate indicator, after integration, for 
indicating the total amount of cast metal E. (E=B.about..times.V.times.l, 
where B.about. is the superposed magnetic flux, V is the rate of flow in 
the casting stream and l is the distance between the conductors.) 
If the magnetic valve is provided with a casting rate indicator, however, 
the following voltages are obtained across the live electrodes: A 
resistive DC voltage drop (of an order of magnitude of 200 mV) caused by 
the electric current through the tap stream. An induced DC voltage (of the 
order of magnitude of 100 mV) caused by the rate of flow of the tap stream 
and the magnetic main flux. An induced AC voltage (of the order of 
magnitude of 10 mV) caused by the rate of flow in the tap stream and the 
superposed magnetic alternating field. 
The resistive DC voltage may be obtained from the following procedures. The 
induced AC voltage is filtered. The induced DC voltage is calculated. This 
voltage is directly proportional to the induced AC voltage at a constant 
magnetic field. The induced DC voltage is subtracted from the total DC 
voltage across the electrodes and the result is thus the resistive voltage 
drop in the tap stream. 
The foregoing may be expressed as follows: 
i.times.R+B.times.V.times.l+B.about..times.V.times.l= the total voltage 
drop between conductors 2, 3, where i is the current strength, R the 
resistance across the tap stream, B the DC voltage field, B.about. the 
alternating field, V the rate of the stream, and l the distance between 
the conductors. 
The induced AC voltage (B.about..times.V.times.l) can be filtered, and may 
be fed back, amplified by a factor K (for example, K=10) so that 
B.times.V.times.l=-B.about..times.V.times.l.times.K, thus producing 
I.times.R which is a measure of the slag contents. 
This device enables an accurate, absolute measurement of the slag contents 
of the tap stream. The magnitudes of the different voltages indicate that 
there will probably be no measurement-technical problems. 
In the case of teeming from, for example, a ladle, the forces from the 
electromagnetic valve will vary very slowly in such a way that the tap 
stream is retarded at the beginning of the tapping and is accelerated at 
the end of the tapping. This means that the induced DC voltage will also 
be varied slowly. When slag suddenly emerges in the tap stream, the 
resistive DC voltage drop will also change very rapidly. It is therefore 
often sufficient for the slag indication to observe when the total DC 
voltage over the electrodes is suddenly changed. In this case therefore 
the valve does not have to be provided with a casting rate indicator. 
The combination of an electromagnetic valve with a slag indicator provides 
a better utilization of the steel in the ladle, because when slag first 
starts flowing down the rate of outflow is reduced and the vortex above 
the tap hole then collapses. The tapping may therefore continue for some 
time until a new vortex is formed and the process is repeated. 
The slag indicator cannot, of course, operate if the valve does not retard 
or accelerate the tap stream. At the end of the tapping, however, the 
valve is normally active so this does not present any problem. 
A slag indicator may, of course, also be provided separately, that is, not 
in combination with an electromagnetic valve, if current is conducted 
through the tap stream and the corresponding voltage drop is measured.