WO1990010720A1

WO1990010720A1 - Detection of the molten steel-slag interface by spectrophotometry during tapping of the basic oxygen converter

Info

Publication number: WO1990010720A1
Application number: PCT/AU1990/000098
Authority: WO
Inventors: Peter John Sampson; Kenneth Garry Kemlo
Original assignee: The Broken Hill Proprietary Company Limited
Priority date: 1989-03-10
Filing date: 1990-03-09
Publication date: 1990-09-20

Abstract

During the tapping of molten steel from the converter prior to continuous casting, the onset of slag from the taphole is difficult to detect. Excessive slag carryover from the converter results in a rich slag cover for the steel, favouring reversion of phosphorus, manganese (and other constituents if present) in the quiescent state. The onset of slag is detected remotely, based on the fact that the spectral emission lines of sodium and potassium may be resolved in the light emission from the steel, which characteristic is absent in the slag. The optical emissions (1) from the furnace taphole are accessed by an optical head (200) about 20 metres distant, focussed by a lens (2) and divided by a beamsplitter (3). Each path after the beamsplitter (3) has an optical interference filter (4, 6) placed before a photodetector (5, 7). Optical interference filter (6) passes wavelengths centred on the emission line (589.3 nm for Na or 768.2 nm for K) and optical interference filter (4) passes wavelengths centred adjacent to the emission line, so that the output of photodetector (5) is proportional to background radiation only. Light not passed by the filters (4, 6) is reflected back towards the furnace. Some light (11) is incident upon a translucent screen (8) and this is used to align the instrument during installation. The outputs of the photodiodes are fed into an automatic control and output system with fibre optics, alarm indicators and computer control.

Description

DETECTION OF THE MOLTEN STEEL-SLAG INTERFACE BY

SPECTROPHOTOMETRY DURING TAPPING OF THE BASIC OXYGEN CONVERTER

Field of the Invention

This invention relates to the detection of the metal- slag transition and has application to detection of the presence or absence of slag in iron or molten steel, especially but not exclusively during the tapping of steel from a converter.

Background Art

The high quality of steel required for continuous casting has highlighted the detrimental effects of slag carryover from the BOS converter to the steel ladle at tapping. Slag carryover into the ladle forms a layer on the surface of the molten metal which directly affects both the steel chemistry and the production requirements of the BOS plant. Excess slag causes rephosphorisation, reduction of the yield of alloys added during tapping, erosion of the ladle refractory lining and an increase in the amount of non-metallic inclusions present in the steel. Too little slag, however, can also cause problems such as excess heat loss during transfer of the ladle to the continuous caster and increased reoxidation and nitrogen levels due to contact of the steel with the air. The greatest contributor to variations in slag levels in the ladle is the inconsistency of operators in detecting the onset of slag at the end of tapping. Under current practice, this onset is determined by the operator using visual (colour, texture, viscosity) and/or acoustic differences between the steel and the slag. Consistent recognition of the metal-slag interface requires considerable experience and is affected by the grade of steel, the temperature, the type of slag and the presence of fumes from the addition of alloys. To reduce the problem of operator consistency, two approaches have been previously attempted. The first is to use a refractory ball with a density between those of the metal and the slag. It floats at the metal-slag interface in the converter and, at the end of tapping, plugs the taphole. The second approach is to use detectors which can detect the transition from metal to slag. These detectors can either contact the converter or be remote from it. The contact methods require the installation of sensors on the converter taphole and the measurement of differences in the density, magnetic permeability, electrical conductivity, optical emission or acoustic emission. These methods are vulnerable to the harsh environment of the BOS converter and the need to pass electrical signals to the control room. Signal interference from skull build-up (solidified metal and slag from previous heats) is a particular problem as is access to such sensors in around-the-clock production. The remote type of sensor uses optical methods such as pyrometry and laser spectroscop . These methods rely on the differences in the optical emission spectra of the metal and the slag streams, and can be used remote from the converter vessel. Problems arise in these sensors from variations in the stream's position, dimensions and temperature, and from interferences to the emitted spectra by fume.

Summary of the Invention

It is an object of the invention to provide an improved method and apparatus for the remote monitoring of a metal- slag transition in metallurgical systems or processes. The invention is based on the most surprising finding that there are intense narrow band characteristic spectral lines in the emission spectra of a molten metal, and that these characteristic lines disappear at slag onset: the lines are not present in the emission spectra of the slag. In the case of molten steel in or tapped from a converter, these lines are found to arise from the presence of the alkali metals sodium and potassium in the molten steel. The invention accordingly provides a method of detecting a metal-slag transition in a molten metal comprising monitoring light emitted by the molten metal and detecting a change in one or more predetermined narrow band lines of the spectrum of the emitted light, which change is indicative of a metal-slag transition. The invention further affords apparatus for detecting a metal-slag transition in a molten metal comprising means for receiving light emitted by the molten metal and producing a signal representative of a preselected segment of the spectrum of said light, and means to detect a change in one or more predetermined narrow band lines of said spectrum, which change is indicative of a metal-slag transition. In one application, the molten metal is iron or steel and the narrow band spectral lines comprise alkali metal lines. The change may then be the disappearance of the predetermined spectral line(s), indicating slag onset in the molten steel, or the appearance of the line, indicating the absence of slag or movement of the interface out of the field of view. The invention may thereby be employed to detect a metal-slag interface in iron or steel. The preferred spectral lines, one or both of whose disappearance is detected, are then the sodium and potassium doublet lines respectively in the visible and infrared bands of the emission spectra, more particularly at 589 .0/589 . 6 nm and 766.5/769.9 nm. The method may further include adding to the slag a small amount of a substance selected to give rise to said predetermined spectral lines(s). The apparatus preferably further includes means responsive to said detection of the disappearance of the one or more predetermined spectral lines to display a record of the detection, and/or to sound an alarm, and/or to modify the handling or treatment of the molten metal. The method of the invention preferably entails comparison of the emission intensity at the or each spectral line and at a nearby reference wavelength in the black body portion of the spectrum. The ratio of these intensities might be measured. Comparative monitoring in this manner overcomes difficulties which would otherwise arise, for example, from the variation in absolute critical energy emitted by a molten metal stream, due for example to the different temperature of each heat in the BOS converter and to different tapping stream dimensions and position. The effect of fumes, which will typically interfere substantially equally with closely spaced wavelengths, will also be compensated for.

Brief Description of the Drawings

The invention will now be further described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagram depicting the emission spectrum of a stream of molten steel as it is tapped from a BOS converter, recorded with an IRIS speσtro-radiometer; Figure 2 is a histogram of measured delays between the disappearance of the sodium emission line in the spectrum of Figure 1 and the raising of the furnace; Figure 3 is cross-sectional ray diagram of an optical head forming part of a first embodiment of apparatus according to the invention; Figure 4 is a block diagram of the electrical circuit of the apparatus; Figure 5 is a representation of the response of apparatus according to Figures 3 and 4 as it monitors steel being tapped from a BOS converter; Figure 6 is a diagram o f a second embodiment of apparatus according to the invention, utilising fibre optics and computer processing; and Figure 7 i s a diagram of a third embodiment of apparatus according to the invention. Referring to Figure 1 , it will be observed that the maj ority of the visible infrared radiation in the emission spectra of the steel stream at tapping i s blackbody radiation, a smooth and broadband radiation common to all materials at raised temperatures . Superimposed on this are numerous very narrow emission ( and sometimes absorption ) lines due to the presence of elemental constituents. These emissions would have a spectral width of approx. 0.2 to 0.3 nm. The surprising elements of the spectrum are the two intense narrow band emissions at about 590 and 770 nm. The spectral width of these lines, as indicated in Figure 1, is limited by the spectral resolution of the IRIS spectro- radiometer (approx. 6 to 8 nm) by which the spectrum is recorded. The line at 590 nm corresponds to the visible yellow line while the line at 770 nm is invisible to the naked eye. The two emission lines of Figure 1 have been identified as due to sodium ( doublet at 589 . 0 and 589 . 6 nm ) and potassium ( 766.5 and 769.9 nm) . The presence of the alkali metals sodium and potassium in the melt of a steel converter , evidenced by the aforementioned intense lines in the visible and infrared bands of the measured emi ssion spectrum , was most unexpected . No alkali metals could be measured in steel samples to the limits of detectability, 5 ppm, by a variety of analytical techniques . Alkali metals are abundant , however, as oxides in the slag used in a BOS converter. It is believed that the emissions originate from alkali metals forced into the steel in the converter by the vigorous agitation with the slag . These alkali metals then rapidly diffuse from the steel surface at tapping. The intense alkali metal lines in the steel emission spectrum are not present in the emission spectrum of slag. Thus their disappearance from a spectrum forms the basis for detecting the onset of slag at the end of tapping. Figure 2 shows the measured delay times from the disappearance of the sodium line to the commencement of raising of the converter for 13 heats of a specific BOS converter . It will be observed that, in most cases, the yellow line did disappear before the furnace was raised and various amounts of slag were observed in the ladles on these occasions . However, on three occasions , shown in the column " <0" , the furnace was raised before the yellow line disappeared . In these cases little or no slag was observed in the ladle and hot metal was found to be remaining in the furnace. A preferred detector based on the principle of the invention consists of an optical head, a master control unit and a remote alarm unit for the tapping station . In use, the optical head is positioned about 20 m from the tapping steel stream . A suitable optical head 100 is depicted schematically in Figure 3. The optical emissions 1 from the furnace are focussed by a lens 2 , and divided by a beamsplitter 3. Each path after the beamsplitter has an optical interference f ilter 6 ( 4 ) placed bef ore a photodetector 7 ( 5 ) . Only light passed by the optical interference f ilter i s incident on its respective photodetector . The optical interference filter 6 passes wavelengths centered on the emission line that is to be recorded ( e . g . 589 .3 nm for the sodium line in the molten steel case ) so that the output of the photodiode 7 is proportional to the intensity of the emission line plus any background radiation at that wavelength . Optical interference filter 4 passes wavelengths centered near to but not including the emission line so that the output of photodetector 5 is proportional to the background radiation only. Light not passed by the optical interference filters is reflected back towards the furnace and some light 11 is incident upon a translucent screen 8. The image formed on this screen is used to align the instrument during installation. The electrical part of the unit is shown in Figure 4. The outputs of the two photodiodes 7(5) are amplified to produce signals 12(13). Both signals 12 and 13 are fed to level detectors 14, 15 which produce signals respectively indicating when too little light is available. A signal 17 is output when either of the level detectors 14, 15 is activated. Signal 17 thus indicates a "LOW LIGHT" alarm condition. The two signals 12, 13 are also fed into a ratiometer 16 to produce a signal 16a proportional to the ratio of the intensity of light at the emission wavelength to the background radiation. Signal 16a is fed to two low pass filters 18, 19, one of which has a longer time constant so that the two filters have different bandwidths, e.g. one may have ten times the bandwidth of the other. The outputs of the two low pass filters 18, 19 are fed to a difference amplifier 20 which produces a signal 20a indicating any sudden change in the relative level of the emission line. Signal 20a is then fed to a level detector which generates a "SLAG" signal, 21a. A "SLAG ALARM", 22, is then generated provided the "LOW LIGHT" signal, 17, is not activated to close a switch 23. The "SLAG ALARM" and "LOW LIGHT" conditions can be displayed in the operator's room at a remote indicator panel 24. Figure 5 shows the ratio recorded for one heat of a BOS converter. Alloy additions at the beginning of the tap show a significant effect on the ratio. This is due to the several orders of magnitude reduction in light intensity reaching the instrument. When alloy addition is finished and only minor fume is present, the system recovers quickly. A closer spacing of reference and sodium emission wavelengths may improve this response. When a clear stream is available the ratio is approximately constant. Fluctuations in the ratio are due to variations in the amount of alkali metal in the steel stream. The reference wavelength does not show the same amount of fluctuation as it is due to the blackbody radiation only. The onset of slag has produced a notable change in the ratio (at X in Figure 5) which can be detected by the instrument. The instrument is adjusted to distinguish between the complete disappearance of the sodium line (and hence the onset of slag) and the normal variation in this line's intensity. As mentioned, a "LOW LIGHT" alarm is generated by the instrument when insufficient light to determine slag onset is received. A "SLAG ALARM" cannot be generated during a "LOW LIGHT" condition. The instrument just described is not affected by variations in the stream's temperature or position and can handle moderate amounts of fume. The unit is completely passive and remote from the BOS converter, requiring only line-of-sight access to the tapping stream. Instead of the circuit depicted in Figure 4, an alternative detection arrangement may be employed. The two signals 12, 13 may be fed to respective programmable gain amplifiers which amplify the signal intensities for input to a computer. The computer can separately set the gains of the programmable gain amplifiers. The computer digitises the amplified emission line and reference intensities and uses these digitised forms to determine their ratios and hence the metal slag transition. If the computer detects a sudden change in the ratio of the emission line and reference intensities, it can generate a slag alarm to operate a buzzer or alarm at the indicator panel as before. A further embodiment is depicted in Figure 6. In this case, fibre optics is used to remote the optical head detection electronics from the furnace area. The incoming light 1' from the hot metal/slag system is focussed by the lens 43 onto the input end 44a of an optical fibre 44. At a remote optical head 100' , the light emerging from the output end 44b of the fibre 44 is collimated by lens 45 to be incident upon a dispersive element 36, e.g. a prism or grating, which deflects the incoming light according to wavelength. Lens 2' focuses the deflected light onto a charge-coupled device (CCD) array detector 7' , or any other practicable array of detectors, such that each element of the array receives different wavelengths of light 37,38. The array detector 7 ' is controlled by control electronics 41 which feeds the intensity signals from each element of the array detector 7 ' to a computer 28. The computer 28 can control the array detector via control signals 23. Intensities corresponding to the emission line and reference intensities can be recorded from different elements of the array detector. If the computer 28 detects a sudden change in the ratio of the emission line intensity and the reference intensity it generates a slag alarm 34 to operate a buzzer or other alarm 35 at an indicator panel 33. An optical fibre head system similar to that shown in Figure 6 could also be used with the interference filter system of Figure 3. However an alternate system to that combination is shown in Figure 7. The incoming light 1" from the hot metal/slag stream is focussed by the lens 43" onto the input end 44a" of an optical fibre 44". Optical fibre 44" is input to an optical fibre coupler 46 which splits the light into two optical fibre paths 47 and 48. The output light 55 of optical fibre 47 is collimated by lens 49 and incident upon an emission line interference filter 4". The light 9" passed by the signal interference filter 4" is focussed by lens 51 onto the signal detector 5". Similarly, the output light 56 of optical fibre 48 is collimated by lens 57 and incident upon the reference interference filter 6". The light 10 passed by the reference interference filter 6" is focussed by lens 59 onto the reference detector 7". The electronics of Figure 4, or the computer based alternative arrangement described above, could be used with the system of Figure 7. It will of course be appreciated that the invention is not limited in application to the use of alkali metal spectral lines. In the steel application, other suitable emission lines may include, e.g., lines for metals such as manganese, phosphorous, carbon, sulphur or iron, or for iron oxides. The invention is also not limited to the detection of slag onset in BOS tapping but may also be applied generally to metal-slag transition detection, for example in blast furnace tapping, steel casting and slag granulation rigs, in aluminium, silver, lead, zinc and copper smelting and a wide variety of metallurgical systems and processes. In a modification of the invention, a substance such as an alkali or metal oxide may be added to the slag to give rise to a suitable spectral line, in a small amount sufficient to permit detection.

Claims

1. A method of detecting a metal-slag transition in a molten metal comprising monitoring light emitted by the molten metal and detecting a change in one or more predetermined narrow band lines of the spectrum of the emitted light, which change is indicative of a metal-slag transition.

2. A method according to claim 1 wherein said change comprises the substantial disappearance of said predetermined spectral line(s), indicating the presence of slag in the field of view.

3. A method according to claim 1 or 2 wherein the molten metal is iron or steel.

4. A method according to claim 3 wherein said predetermined narrow band spectral lines comprise alkali metal lines.

5. A method according to claim 4 comprising detecting a change in one or more of the sodium and potassium doublet lines at 589.0/589.6 nm and 766.5/769.9 nm.

6. A method according to any preceding claim wherein said detecting includes comparing the intensities at the or each spectral line and at a nearby reference wavelength in the black body portion of the spectrum.

7. A method according to any preceding claim further including adding to the slag a small amount of a substance selected to give rise to said predetermined spectral lines(s).

8. A method according to claim 7 wherein the substance is a selected oxide.

9. Apparatus for detecting a metal-slag transition in a molten metal comprising means for receiving light emitted by the molten metal and producing a signal representative of a preselected segment of the spectrum of said light, and means to detect a change in one or more predetermined narrow band lines of said spectrum, which change is indicative of a metal-slag transition.

10. Apparatus according to claim 9 wherein said detection means is arranged to detect substantial disappearance of said predetermined spectral line(s), indicating the presence of slag in the field of view.

11. Apparatus according to claim 9 or 10 for detecting a metal-slag transition in molten iron or steel, wherein said detection means is arranged to detect a change in one or more alkali metal lines of the spectrum of the light emitted by the molten steel.

12. Apparatus according to claim 11 wherein said alkali metal lines includes the sodium and potassium doublet lines at 589.0/589.6 nm and 766.5/769.9 nm.

13. Apparatus according to any one of claims 9 to 12 wherein said detection means includes means to compare the intensities at the or each spectral line and at a nearby reference wavelength in the black body portion of the spectrum.

14. Apparatus according to any one of claims 9 to 13 further including means responsive to said detection of the disappearance of the one or more predetermined spectral lines to display a record of the detection, and/or to sound an alarm, and/or to modify the handling or treatment of the molten metal.

15. Apparatus according to any one of claims 9 to 14 wherein said light receiving means includes optical fibre means and said signal producing means coupled to receive said light via said optical fibre means at a location remote from the molten metal.

16. Metal treatment, processing or handling plant including apparatus according to any one of claims 9 to 15.