Radio Wave Interferometer Measurements of Slag Depth
Lars B. Bååth
RWI Radio Wave System AB Knäredsgatan 21A SE-302 50 Halmstad
Sweden and
University of Halmstad Kristian IV väg 3 SE-302 50 Halmstad
Sweden +46 35 16 12 60
lars@rwi.se
Key words: Slag depth, Interferometers, Hot metal level measurements, Quasars
INTRODUCTION
Changes in the pattern and polarization of electromagnetic wave fronts represent the most sensitive probes in physics. Electromagnetic waves may penetrate media of varying physical properties, changing its amplitude, phase, and polarization in a way that is specific to the content and structure of the media. Thus molecular gas, and solids, will emit or absorb electromagnetic radiation mainly depending on its composition, density, the physical temperature, the molecular structure, or the electric and/or magnetic field in the area where the gas resides. Continuum radiation will also be affected when penetrating a media in the sense that the amplitude will be attenuated and the propagation velocity will change, also depending on the orientation of the molecular di- poles, resulting in a sudden change of phase in the interface area. Radio astronomical techniques, and especially interferometer techniques where one has direct access to the wave front data, are very well suited to be used for any such probes.
Interferometer techniques have been used for imaging in radio astronomy for nearly 50 years. The original discovery of its use can be traced back to 1948 [1] when it was discovered that the strong radio source Cygnus A showed interferometer fringes in a single antenna on the cliffs overlooking the ocean outside Sydney, Australia. The fringes were the results of interference between the radio wave going directly into the antenna and the wave reflected in the water below and into the antenna.
Since then radio astronomy has developed elaborate techniques for interferometers [2]. Data are sampled at sites distanced from 100 m to 5000 km from each other and then brought together to a special processor unit [3]. The data are then delayed so that the optical axis of the interferometer is pointing towards the radio source, and each combination of element, or antenna, pairs is cross correlated. Thereafter a focused image is calculated using a calibration technique where the phase fluctuation of the atmosphere is reduced [4, 5, and 6]. With radio interferometers it is therefore possible to make images which are completely diffraction limited in resolution.
The resolution can be chosen to fit the physics one wants to study and range from 10-100 arc seconds for connected element interferometers to 50 micro arc seconds at high frequencies and very long, intercontinental baselines [7]. Figure 1 below shows a composite of images of the quasar 3C273 obtained with global sets of interferometers at wavelengths 6 cm [8] and 3 mm [9] wavelength, respectively.
Figure 1: Global radio interferometer images of the quasar 3C273 at 6cm and 3mm (insert).
Interferometers at earth can be significantly simplified in that the phase reference can be transmitted via a way which does not pass the object, or through a cable or optical fiber. In the radio band this can achieved by splitting a signal into a reference and an object signal and then either transmit the object signal through the media to be investigated, or towards the media and be reflected back at a surface behind the media. If the signal is transmitted towards and reflected at a surface, then the phase of the signal will change linearly with frequency since a delay between the reference and object signals offset from zero in the time plane will correspond to a phase slope in the frequency plane. If the signal is instead transmitted towards a semitransparent medium, then part of the signal will be reflected, and part of the signal will propagate through the medium to be reflected at the next surface where the index of reflection is again changing. These double, or multiple, reflected waves will, when cross-correlated with the reference signal, show a more complicated curve of phase as a function of frequency. If data therefore are sampled as complex cross-correlated amplitudes in frequency channels over a frequency band, then the distances to both or all the surfaces can be recovered. If then, as in the astrophysical
case, the signal is transmitted and received by an interferometer in the aperture plane, then the full three dimensional structure of the volume can be reconstructed. If the data are sampled from a single point only, then only the depth information can be reconstructed.
TECHNOLOGY
In a rare case of technology transfer, the Photonics group, with research in radio astronomy, at the University of Halmstad and MEFOS of Luleå have together developed a technology for dynamically and in real-time measure multiple levels in metallurgical processes. The technology is protected under US and world patents. The development started with a clear description of the problems to be solved and the industrial robustness required of any system to be industrially viable. Based on these parameters, possible measurement techniques in the electromagnetic field area were specified and investigated. A number of trials and experiments decided the final technology which then was optimized for wavelength, power, and polarization properties as well as for cost- efficiency. The development was performed in close collaboration with Swedish and European steel and metal industries, in some cases within the frame works of the European Cole and Steel Collaboration (ECSC).
The theoretical work and experiments showed very early that the interferometer technology to be superior in that all information of the electromagnetic signal is available, as the phase is measured in the frequency plane (dimension). Data can therefore freely be treated in a computer depending on application or the conditions at the measured object. Furthermore, interferometers are intrinsically very robust and virtually insensible for external disturbances, as only the signal coherent with the one sent will be monitored. The robustness has been proven at numerous tests in a variety of industrial processes.
The interferometer technology developed in this project is based on that a very narrow frequency channel in the microwave range is beamed onto a metallurgical bath. The signal will be reflected and partially transmitted through each surface in its path. Such a surface is found at every interface between two materials of different index of refraction, or in some cases also at the interface between solid and liquid phases of the same material.
The signal is received by the same antenna as the sum of the signals reflected from each such interface. The phase of the received signal is measured against the phase of the transmitted signal as the phase of the summed standing waves between the transmitter and the surfaces. The phases of standing waves are measured over a wider frequency range and a Fourier transform is then performed to give the required reflection spectrum.
Each material has a different velocity for the microwaves. The velocity is dependent on the composition of the material rather than the density. The velocity is usually determined as c/v, where c is the speed of light in vacuum and v is the speed of the microwaves in the material. This ratio is usually denoted n, or the index of refraction for the material. The real distance between e.g. slag and steel surfaces is automatically determined from the measured, optical distance by compensating for the lower velocity through the slag. This is also done for each material the signal has penetrated before the surface to be measured. The index varies between compositions, but our experiments have shown that the variation is minor within each type of e.g. slag. The index is also frequency dependent and we are presently working on a method to determine a slag quality through this dependency.
RESULTS
Some of our earlier work has been previously discussed in other proceedings, e.g. [10] and [11].
Figure 2 shows data from comparison between RWI measured and manual measured slag depths for a variety of slag. The RWI slag depths have been determined by using the index of refraction appropriate to each slag type.
The manual measurements were made with a burn-off method. All measurements were made on a ladle. The standard deviation of the difference is about 10 mm, which is well within the error bars estimated for the manual measurements (25 – 30 mm).
RWI - Manual slag depth
-50 -40 -30 -20 -10 0 10 20 30 40 50
deviation (mm)
deviation (mm)
Figure 2: The difference between RWI and manually measured slag depths is shown for each observation.
Figure 3 below shows the reflection spectrum from a smelter. A number of levels are detected and four of these are tracked as the top of the roast (or black top), the slag interface, the matte interface, and finally the bottom of the furnace. Note also that there is at least one additional layer between the slag and matte. It is quite common that such additional layers are detected, but it is up to the plant metallurgists to interpret such data.
Figure 3: Screen dump of the reflection spectrum from a smelter furnace.
Figure 4 below shows slag and steel levels observed on an electric arc furnace during melting. The very first levels show the lid being put on. Thereafter follows scrap melting and motion of the scrap until about 13:49 when the refining process starts and foaming of slag can be seen. The furnace is tilted and tapped at about 14:09. The complete process is therefore monitored, including the foaming of the slag and the tapping. With this approach it is therefore also possible to measure the hot heal after tapping.
EAF 1
1,5
2 2,5 3 3,5 4 4,5
5
13:40:00 13:50:00 14:00:00 14:10:00
distance (m)
Time (h)
slag steel
Figure 4: Reflection spectrum from a smelter furnace.
Experiments and industrial scale tests also show that the foaming slag and emulsion layers can be monitored in a converter during blowing [12]. Typical data from such industrial tests are shown in figure 5 below. The data
Blacktop Top of slag
Top of matte
Bottom of furnace
starts with the converter being turned into up-right position at about 15:41. The converter is then turned at the end for tapping. Note that the turning for tapping in this case is started before the foaming slag has settled down.
Figure 5: Foaming slag in a converter during blowing.
Note also that the foaming is affected by a number of events during the blow. The motion of the lance and lime/dolomite additions clearly has an effect. Turning the oxygen on and off has, naturally, very rapid impact on the foam and changes the foam height by a meter in a few seconds at e.g. 16:01. The start of the foam is also very rapid and the slag height increases by about 3 meters in 10 seconds. Dynamic, fast, and direct measurement technology such as the one presented here, are necessary to fully monitor and control such violent processes as in a converter.
Finally, we have further developed the technology to also monitor the inside of Soederberg electrodes. The instrument is mounted on top and inside the live electrode to continuously, during full production, measure and follow primarily four levels: the top of the paste, the top of the liquid paste, the top of the baked zone, and finally the tip of the electrode. Figure 6 below shows example of a screen dump of the measurements of 3 such electrodes in a furnace. The positions of the levels are in this case all relative to the position of the bottom of the clamping shoe. Sporadically, also the material below the tip of the electrode can be measured and we are presently developing methods to track also this. Continuous monitoring of electrodes is important for a number of reasons, e.g. to keep the baked zone within a certain region, to detect cracks in the electrode before the break happens, and to position the tips of the electrodes at a desired position to optimize the process.
9
10
11
12
13
14
15
15:30 15:31 15:32 15:33 15:34 15:35 15:36 15:37 15:38 15:39 15:40 15:41 15:42 15:43 15:44 15:45 15:46 15:47 15:48 15:49 15:50 15:51 15:52 15:53 15:54 15:55 15:56 15:57 15:58 15:59 16:00 16:01 16:02 16:03 16:04 16:05 16:06 16:07 16:08 16:09 16:10 16:11 16:12 16:13 16:14 16:15 16:16 16:17 16:18 16:19 16:20 16:21 16:22 16:23 16:24 16:25 16:26 16:27 16:28 16:29 16:30
Slag Height
Charged Metal
Emergency Stop OG
Ignition
OG Ignition Lime/Dolomet Additions
Lance Lowered
Sublance Sampling
Oxygen Off
End Tap Start
Tap
Figure 6: Screen dump of the measurements of three electrodes.
ACKNOWLEDGEMENTS
We acknowledge the help of a number of dedicated and illustrious scientists and engineers at a large variety of process plants in the world. Without all of you, this technology would not have been developed into a proper industrial tool. We look forward to working with you in the future and together present metallurgical papers.
The author also acknowledges the great work done by Dr Donald Malmberg of MEFOS with collaborators and Mr. Les Bedser of RWI South Africa who originated the idea to measure in electrodes.
REFERENCES
1. J.G.Bolton and G.J.Stanley, “Observations on the Variable Source Radio-frequency Radiation in the Constellation Cygnus”, Australian Journal of Scientific Research Ser. A, vol. 1, p. 58, 1948
2. A.R.Thompson, J.M.Moran, and G.W.Swenson,”Interferometry and Synthesis in Radio Astronomy”, ISBN 0-471-80614-5, publ. John Wiley and Sons
3. L.B.Bååth, “Mapping in practice”, Very Long Baseline Interferometry: Techniques and Applications, eds. Felli and Spencer, NATO ASI Series, p199, 1989
4. L.B.Bååth and F.Mantovani, “EVN as a Phase Stable Interferometer”, IAU Colloqium 131: Radio Interferometry, eds. T.Cornwell and R.Perley, p.298, 1991
5. Akujor,C.E. and Bååth,L.B., “Combined array imaging of extragalactic radio sources”, Astronomy with millimeter and submillimeter wave interferometry", IAU Coll. 140, eds. Ishiguro,N. and Welch,Wm.J, 1993
6. Bååth,L.B., “Global Fringe Fitting applied to 100GHz data”, Frontiers of VLBI eds. Hirayabashi and Inoue, p353, 1991
7. L.B.Bååth, S.Padin, D.Woody, A.E.E.Rogers, M.C.H.Wright, A.Zensus, A.J.Kus, D.C.Backer, R.S.Booth, J.E.Carlstrom, R.L.Dickman, D.T.Emerson, H.Hirabyashi, M.W.Hodges, M.Inoue, J.M.Moran, M.Morimoto, J.Payne, R.L.Plambeck, C.R.Predmore, and B.Rönnäng, “VLBI Observations of Active Galactic Nuclei at 3mm”, Astronomy & Astrophysics, p. 257, 1992
8. A.J.Zensus, L.B.Bååth, M.H.Cohen, and G.Nicholson, “The Inner Radio Jet of 3C273”, Nature. p. 410, 1988
9. L.B.Bååth, S.Padin, D.Woody, A.E.E.Rogers, M.C.H.Wright, A.Zensus, A.J.Kus, D.C.Backer, R.S.Booth, J.E.Carlstrom, R.L.Dickman, D.T.Emerson, H.Hirabyashi, M.W.Hodges, M.Inoue, J.M.Moran, M.Morimoto, J.Payne, R.L.Plambeck, C.R.Predmore, and B.Rönnäng, “The Microarcsecond Structure of 3C273 at 3mm”, Astronomy & Astrophysics, p. 47, 1991
10. D.Malmberg and L.B.Bååth, “Radio-wave technology in metallurgic and mining industry”, Scaninject VI: 7th conference on Refining Processes, 1995
11. D.Malmberg and L.B.Bååth, “Slag level detection in EAFs using microwave technology”. Scandinavian Journal of Metallurgy ISSN 0371-0459, 2000
12. Millman, S., Malmberg,D. and Bååth,L.B., “Radio Wave Interferometry for BOS slag control”, ECSC report Contract no. 7210-CB/905 ref. No. 96/C1.04, 2000
