LU82301A1 - SCORING LEVEL MEASUREMENT SYSTEM IN A METALLURGICAL CRUCIBLE DURING REFINING OF RAW CAST IRON - Google Patents

Description

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Slag level measurement system in a metallurgical crucible during the refining of pig iron

The present invention relates to a system for measuring the level of slag in a metallurgical crucible during the refining of pig iron.

5 Knowledge of the level of slag in a steelworks converter is important, both from a metallurgical point of view, as a parameter of the degree of refining progress, and from a safety point of view, in order to prevent a overflow.

10 Thus the applicant has developed a system for measuring the level of the slag which allows at the same time an evaluation of the consistency thereof, which constitutes important information, especially for the refining of phosphorous cast irons where a slag from a frothy consistency was considered desirable.

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This measurement system which the applicant described in Luxembourg patent C? - / £ S no. 71,261, comprises a thermostatic acoustic conduit which is placed above the converter and which picks up the noise emitted by the blowing lance. After filtering the frequencies whose origin is foreign to the parameters which are of interest for the measurement proper, we retain a signal whose interpretation allows us to conclude from the degree of noise absorbed by the slag and by this to the degree of the sparkling character present as well as to its level in the crucible. On the one hand, this measurement system where the location of the sensor is situated outside the crucible, is problematic in the case of installations of refining where the crucible can be more or less tightly connected to the chimney, eg using a <50

P

- 2 - mobile skirt fitted with hydraulic cylinders. In this case, these are refining installations where, during blowing, air is prevented as much as possible from entering the chimney, either to remove the gases formed during the preparation for use as a combustion gas, or to prevent post-combustion of carbon monoxide in the immediate vicinity of the cooling system piping installed in the chimney.

• 10 On the other hand, the development of more sophisticated refining processes, such as that described in Luxembourg patent application No. 81.207 of April 30, 1979, which includes in addition to the blowing of oxygen on the surface of the bath, a bubbling of the bath by means of inert gas introduced through the bottom of the crucible, results in the formation of a slag of essentially non-foaming consistency, so that the measuring system described above is no longer able to provide results easily interpretable to determine the level of slag in the crucible.

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The presence of a denser slag, due to the effect of the bubbling of inert gas introduced by the bottom of the crucible, therefore requires the development of a measurement system based on measurement parameters other than the degree of absorption of the noise 25 emitted by the blowing lance.

The object of the invention therefore consisted in proposing a system for measuring the level of slag, largely independent on the one hand of the configuration of the refining installation and on the other hand of the degree of foaming of the slag and whose performance is at least equivalent to that of known systems.

This object is achieved by the measuring system according to the invention which is characterized in that it essentially comprises an electric oscillator whose resonant frequency varies as a function of the proximity of the level of the slag in the crucible, as well as an electronic system for processing the measured signal.

»» - D - The oscillator is mounted on the external lateral face of the crucible carcass, at the level of the average level of the slag, in a water-cooled enclosure, while the electronic treatment system is usefully mounted remote 5

The LC circuit which determines the resonant frequency of the oscillator can in principle be produced in two ways:

According to a first embodiment, the inductance "L" is an integral part of the oscillator block, while the capacity consists of two sheets of suitable dimensions which are mounted in the crucible between the permanent lining and the wear refractory. The electrical connection with the inductance is made by an electrically insulated passage through the carcass of the crucible. The variation in the level of the slag opposite the electrodes causes a variation in the capacitance of the circuit, hence a variation in the resonant frequency of the oscillator, which constitutes the measurement signal.

A second embodiment provides that the capacitor is physically housed on the oscillator block, while the inductor is mounted in the crucible between the permanent lining and the wear refractory. It consists of one or more turns forming either a vertically arranged frame and having appropriate geometric dimensions, or an inductance arranged horizontally, once around the crucible, always between the permanent lining and the refractory * wear. The measurement effect is due, as in the first form, to the variation of the capacity of the LC circuit. 30

In both cases the shape, size and arrangement of the sensor itself i.e. elements mounted in the crucible give the measurement signal either the character of continuously evolving information (inductance or electrodes having an elongated shape and arranged vertically in the direction of the greatest dimension), or the character of a all-or-nothing information (the same elements arranged horizontally).

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Intermediate shapes and arrangements make it possible to influence the shape of the response of the sensor, eg bias mounting.

The prerequisites for the method described are a relatively high electrical conductivity of the slag relative to that of the refractory surrounding the electrodes or inductance and a lowest possible electrical conductivity of the refractory.

10 As the electrical conductivity of the slag is given, one has an influence only on that of the refractory. An appropriate choice of the refractory material intended to be mounted at the measurement location provides eg bricks in magnesia.

15 Considering that it is obvious that the mechanical and electrical stability of the oscillator, determined by the construction and the quality of the components, the stability of the temperature around the oscillator block and the stability of the supply voltage must be ensured as far as possible, there remain two parasitic influences which must be compensated for.

These are, on the one hand, the variation of the electrical conductivity of the refractory as a function of the temperature and, on the other hand, the progressive wear of the refractory during the campaign.

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The effects of noise on the final measurement signal are as follows: The increase in the temperature of the refractory, with which an increase in the conductivity is associated, causes a decrease in the basic resonant frequency of the oscillator and mask. partly the influence of the frothy slag on the capacity of the circuit, which results in a decrease in the relative variation of the capacity, in other words a decrease in the sensitivity.

35 On the other hand, following the wear of the refractory, the distance between the fixed electrode (sheets or inductance) and the mobile electrode (frothy slag) is reduced, which results in an increase in the relative variation of capacity under the influence of slag, therefore an increase in the sensitivity of the system - 5 -

The two factors act in opposite directions on the useful signal

As regards the evolution of the parasitic influences over time, these can be considered to vary slowly with respect to the duration of a steel-making phase, from which it follows that continuous compensation can be dispensed with in favor of "punctual" compensation at the start of each blowing phase.

The preceding description of the physical phenomena described determines the dimensioning of the oscillator and the design of the signal processing system.

Having a variation of the natural frequency of an oscillator-15 as an image of the quantity to be measured has two advantages: a) The frequency is a quantity which is measured if not necessarily, then at least in practice by methods digi-20 pillowcases, which lend themselves directly as inputs to a computer. This computer being supposed to be existing, one can envisage a solution in which this one takes care of the information processing.

B) The weakness of the measurement effect to be expected poses the problem of the sensitivity of the system. Again, the frequency being a quantity available in digital form allows a more subtle treatment than an analog quantity and gives the whole a greater sensitivity. We raet-30 tra to advantage, as described below, the principle of frequency change.

With regard to the design of the oscillator, it should be noted that the basic frequency of the oscillator, the choice of which is free in principle, is subject to the following constraints: - 6 -

On the assumption that this frequency is located in the range between 5 and 10 MHz, we will have to admit a basic instability estimated at 1 kHz. In other words, kHz must be considered as a unit of measurement, any variation of less than 51 kHz is not taken into account.

If we set a resolution of $ 1, the stroke of the natural frequency of the oscillator will have to be at least equal to 100 kHz. In addition, it will be advantageous to reduce to a minimum the 10 contrary effects exerted by the rise in the temperature of the refractory and the wear thereof during the campaign on the measurement effect, provided that this stress is compatible with the two preceding ones and with the possibilities of realization dictated by the geometric data of the crucible, by the thickness and the electrical properties of the two types of refractory.

In the case of an independent sensor independent of a computer, the processing system will essentially comprise two counters, one of which is of the up / down counter type which can be prepositioned to a given value, the necessary memories, a crystal time base with counter and comparator allowing adjustment of the duration of a measurement cycle per step-unit, a counter control unit (start, stop and reset to zero) and the time base in function external criteria and a digital / analog converter for displaying the measurement value to the operator. A final signal shaping unit may however be included when using a computer.

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A particularly advantageous embodiment provides that two oscillators located at different levels are used and that one works in differential; the signal of one of the oscillators, sheltered from the influence of the foaming slag, can then serve as a reference for the other. The processing electronics will be more complex, but it will always consist of the same types of components.

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A non-limiting illustration of the measurement system according to the invention is provided by way of the drawings, in which the fig. 1 shows a diagrammatic view of the embodiment using electrodes as capacity housed in the re-5 fractairo of the crucible, while FIG. 2 shows the embodiment where the inductance is housed in the refractory. Fig. 3 shows the block diagram of the electronic system for processing the measured signal.

10 Ln fig. 1 we distinguish the enclosure (l) cooled with water (8.8 ') and mounted on the carcass (3) of the crucible. This oscillator block (2) is connected to the electrodes (5) which can be arranged either vertically, horizontally, or even at an angle, which is not shown. The connection between the block (2) and the electrodes 15 (5) crosses the permanent lining (4) of the crucible. The electrodes (5) are housed between the permanent lining (4) and the wear refractory (6). The level of the slag (7) is shown at the height of the electrodes (5) · 20 The block (2) is connected to the signal processing system, not shown. To this end, the enclosure (l) has a conduit (9) for the connecting cables.

The G. 2 shows the same enclosure (l), cooled with water (8.8 ').

The block (2) is represented so as to indicate that the capacity is integrated therein.

*

In this embodiment, a coil (5) acting as an inductance of the LC circuit is housed between the permanent lining (4) and the wear refractory (6). The coil (5) can be arranged either to form a vertical or horizontal frame; the dotted line symbolizes that the coil turns around the crucible.

35 Fig. 3 shows the block diagram of the system for processing the measured signal.

The references represent the following: - 8 - 1) Arrival of the "frequency" signal from the oscillator 2) Shaping circuit 3) Counter 4) Selection switch for the smallest significant unit 5 5) Counter 6) Memory 7) Up / down counter with prepositioning 8) Memory 9) Digital / analog converter, 10 10) Analog / digital converter with computer output 11) Analog recorder 12) Control circuit for counters, memories including time base 13) Input of external signals (criteria) 15 14) Control frequency meter t

Claims (9)

1. A system for measuring the level of slag in a metallurgical crucible during the refining of pig iron, characterized in that it essentially comprises an electric oscillator whose resonant frequency varies as a function of the proximity of the level of the slag in the crucible, as well as an electronic system for processing the measured signal, '2. System according to Claim 1, characterized in that the oscillator block is mounted on the external lateral face of the crucible and that the signal processing is mounted at a distance from the crucible, 15 3. System according to claims 1-2, characterized in that two electrodes in the form of elongated metal sheets which constitute the capacity of the LC circuit are housed between the wear refractory and the permanent lining of the crucible and that the inductance of said circuit is an integral part of the oscillator block. k. System according to claims 1-3, characterized in that the electrodes are arranged so that their axis is situated in the plane of the axis of the crucible. 5. System according to claims 1—5, characterized in that the electrodes are arranged so that their axis is located at an angle to the plane of the axis of the crucible. 30 6. System according to claims 1-3, characterized in that the electrodes are arranged so that their axis is located vertically on the plane of the axis of the crucible. 7. System according to claims 1-2, characterized in that at least one turn acting as an inductance of the LC circuit is housed between the wear refractory and the permanent lining of the crucible and that the capacity of said circuit is part integral of the oscillator block. - 2 - 8. System according to claims 1-2 and 7, characterized in that the turn is arranged in the horizontal plane and makes the complete turn of the crucible. 5 9 · System according to claims 1-2 and 7, characterized in that the coil is arranged in the vertical plane in the refractory of the crucible. 10. System according to claims 1-9, characterized in that the oscillator block is installed in a water-cooled chamber which is raised to an appropriate level which is usefully the average level of the slag. 11. System according to claims 1-10, characterized in that the refractory material which is in direct contact with the constituent elements of the LC circuit has a low electrical conductivity compared to that of the slag. 12. System according to claims 1-11, characterized in that the refractory material is advantageously magnesia. 13. System according to claim 1, characterized in that the signal processing system essentially comprises two counters, one of which is of the up / down counter type which can be prepositioned to a given value, the necessary memories, a base of crystal time with counter and comparator allowing adjustment of the duration of a measurement cycle per step-unit, a counter control unit (start, stop and reset) and the time base according to criteria and a digital / analog converter for displaying the measured value. 35