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Slag Detection By Infrared Thermal Imaging

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A Temperature Sensor Application in Steel Making

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Page Index 

Introduction to the Measurement

Wavelength Effects: Near-IR, Mid-IR and Long IR

Attenuation by Absorption & Scattering Vs Wavelength

Observations and Open Questions

References and Links to Vendor Pages


Introduction to the Measurement

In basic terms, the presence of slag on the surface of a stream of liquid steel or liquid iron (pretty hot stuff~ 1500°-1650°C), can be seen due to a difference in brightness between the two materials even though they are both at about the same temperature. This fact has been used for many, many years by operators of steel melters to detect when slag is coming out of a melting furnace or ladle. Until 20 or so years ago, it was about the only way one could see when it was time to change the furnace pouring angle to limit the amount of slag poured into the next vessel. The original detector was, and in many plants still is, the human eye. Liquid steel is very, bright because of the high temperature. Thus, the person observing the stream looked through a special filter which reduced the level of visible radiation reaching one's eyes.

The reason for the difference in brightness between the two materials, liquid slag and liquid steel or liquid slag and liquid iron or liquid slag and just about any liquid metal, is because liquid slag and liquid metals have different spectral emissivities (Note: if you are unsure what the term spectral emissivity means, visit the E-Trail to learn something about it).

The spectral emissivity of slag is reasonably constant over the thermal infrared portion of the Electromagnetic Spectrum, and has a larger value than that of the steel. Furthermore, the spectral emissivity of liquid steel drops in value at longer and longer wavelengths. Over the same wavelength range, say from a wavelength of about 0.5 micrometers (microns) to a wavelength of about 15 micrometers, the spectral emissivity for steel will reduce from about 0.5 to about 0.05. The value for slag, on the other hand, remains nearly constant at about 0.8 to 0.9, depending upon its composition.

Wavelength effects-Peak Wavelenth of Emitted Thermal Radiation

Due to the nonlinearity of the emission of radiant thermal power over the above wavelength passband (0.5 to 15 micons), as described mathematically by The Planck's Law equation, the peak in the Planck curve of Radiance vs Wavelength, occurs at about 1.5 microns.

Wavelength effects-Near IR

Instruments measuring at wavelengths shorter than the wavelength of peak emission will have a very non-linear response to temperature versus a linear response to radiance (brightness). This fact allows a thermal imager operating at short wavelengths to discriminate between slag and steel many times easier than a human eye, even under conditions of serious attenuation in the sighting path. So, basically, if a human can see the slag, a short wavelength thermal imager can see it even more easily. Further, since most Silicon CID and CCD detector arrays (the detector used in several instruments) operate at about a 0.9 micron peak sensitivity in the Near IR, they may see better than the human eye if the dust in the sight path is composed of particles in the sub 1 micron size range because of reduced scattering effects.

Instruments operating at or near the wavelength of peak emission wavelength (~1.5 microns), also Near IR, will still have a very non-linear response, but will have the possible added advantage of even lower sensitivity to a major component of dust, if any, in the sub 1 micron size range.

Any Near IR Thermal Imager has several other,very significant advantages over the alternative devices described below. The final choice of an instrument is not usually based on just one or two advantages, however, but an overall assement of the pros and cons of a given situation. So, rather than making any recommendations, we have tried to lay out all the parameters and related theory and tradeoffs below to inform a user of all the factors so they can compare all the parameters and arrive at a conclusion that suits their need.

Wavelength effects-Mid IR

Instruments operating in the MW -IR at about 3-5 microns, are on the long wavelength side of the Planck curve. They have a reduced sensitivity to the apparent temperature difference between the steel and slag, but there is an increased difference in the perceived temperatures because of the reduced spectral emissivity value of the steel. The steel will look colder in this wavelength range than in the Near-IR. This improves "contrast" for an imager but this advantage is somewhat offset by an increases sensitivity to attenuation due to any sight path attenuation. Instruments working in this waveband will have even less sensitivity to scattering by dust in the sub 1 micron waveband region, but can be severely affected by scattering and direct blockage of radiance due to larger dust, smoke and macroscopic-sized particles, just as the other two wavelength range devices would be.

Wavelength effects-Long IR

Instruments operating in the LW-IR region at about 8-12 and 8-14 microns, are far out on the wing of the long wavelength part of the Planck curve. Their response is nearly linear in terms of radiance and apparent temperature. The "contrast" is further improved between steel and slag by the much reduced spectral emissivity value of the steel. The steel will look very much colder than the slag. This 'advantage' is even more significantly offset by a very much increased sensitivity in the temperature difference to the effects of any attenuation in the sight path. In these wavelength passbands, the effects of sight path attenuation due to scattering from sub 1 micron sized particles is very small, but, as mentioned above, larger particels can affect the attenaution significantly.

Attenuation by Absorption & Scattering Vs Wavelength

Clearly the effects of absorption of radiation from the stream by absorption and scattering of IR radiation in the sight path between the steel stream and the thermal imaging camera is one of the parameters involved in choosing an instrument to perform the measurement. It is not the only parameter, however, but about the only one that is not well described in the literature and one that is still subject to many untested hypotheses in the year 2001.

The problem exits because there is an atmosphere containing Water Vapor and much smoke and dust. The latter is generated in the pouring of liquid steel into a vessel like a refractory-lined ladle. Early in most pours, various solid, cold additives and fluxes are also added to the ladle. Some portion of the additives and refractory linings of the ladle are vaporized when the steel first arrives and some parts are physically ejected upwards and out the ladle. The net result is a rapidly-generated, billowing cloud of dust, smoke and possibly steam that rises up from the ladle, surrounding and obscurring a view of the stream of steel-slag.

The question of absorption of emitted infrared by atmospheric components is a relatively small issue since most of the instruments' measuring wavebands are chosen where there is good, not necessarily perfect, transmission in the presence of nominal amounts of Water Vapor and Carbon Dioxide. Given the very large magnitude of apparent temperature difference between the steel and slag in all these passbands, an atmospheric transmission correction or variation will make liitle difference to the overall measurement.

Attenuation by scattering is a different matter since it is wavelength and particle size dependent. Small particles scatter short wavelength radiation widely more than they scatter long wavelength radiation. Thus, it is often observed thatLlong wave IR sees better than Medium wave IR cameras which in turn can can see better in smoke and fog than can shorter, Near IR cameras. That's smoke and fog, which are composed of very small particles, one's that approach the size of the NearIR wavelengths, about 1 micron or less.

We have searched the literature and have not found a characterization or estimate of a typical dust and particle cloud in terms of the particles that make it up and their size distributions in the case of the smoke and dust generated during steel pouring into a ladle. Suffice it to say, if it were composed of a large amount of submicron particles, it is very likely that, at least in the USA, the US Government OHSA organization would be mandating personal protectives measures against sub-micron sized particle inhalation hazards in steel-making plants. Since OSHA does test these plants and since they don't have such a mandate, it is likely that no suchvery small particle distribution exists, at least not in significant quantities.

However, in most cases, this may be a moot point and not really significant in terms of these measurements.

Observations and Open Questions

In the vast majority of pours, the smoke and dust cloud disappears within a few tens of seconds after the beginning of a pour. The view of the stream is clear and unobstructed after that until the presence of slag occurs, usually near the end of a pour. That means that in many plants, the sight path attenuation question never arises. Does it occur in your plant or is the point moot?

In those plants where additives are placed into the ladle near the end of a pour, the sight path is fully or partially obscurred during much of the pour. In these cases, there may indeed be an advantage to the use of a MW or LW thermal imager, but only if a significant micron or sub-micron particle size distribution exists. If this turns out to be the case, then it should also be a clear indication that a careful OSHA particle hazard test may be also warrented. Has OSHA tested your plant atmosphere during additive additions? Might be worth a check.

Is it possible to modify an additive addition practice in order to improve the detection of slag through use of a lower cost, superior imaging system? That, too is an open question which has not been discussed in the literature.


References and Links to Vendor Pages

  • Basic theory and mathematical analysis of the IR slag detection process- 2000 SPIE Thermosense Kantsios Award Paper By G.R. Peacock

    This paper shows why, from the fundamental theory, nearly any thermal imaging system will work in this application and why the Near IR units have a combination of theoretical and practical advantages that add up to make this wavelenth region superior to almost all the longer wavelength passband units. It also shows that the difference in apparent temperature between steel and slag is minimally affected by sight path attenuation due to smoke and dust or nearly any agent, in the Near IR. Given the enormous difference in the amount of thermal radiation present from a 1600°C stream of liquid steel at 1 micron versus 10 microns, that is not hard to understand.

    (Note Added in April 2002-The advent of ruggedized Thermal Imagers based on InGaAs detector arrays operating in the 1.4 to 1.8 micrometer waveband offers another camera source for high performance in the Near-IR with better discrimination between slag and steel than the 0.9 micrometer cameras, albeit at a significantly higher cost. Yet these cameras offer glass optics and the other advantages of operating on the short wavelength side of the Planck curve peak.)

  • Description of The Bethlehem Steel Corporation System & related steel process details & benefits- 2000 I&SS Award winning paper by D.A. Goldstein, A. Sharan and J.A. Stofanak.

    This paper describes the many process advantages of reliable slag detection and describes the unique 8-12 micron wavelength passband system that Bethlehem Steel has developed and patented. They also make some unsupported claims that the long wavelength system is superior to shorter wavelength device choices because the detection process is claimed to be less affected, by dust and smoke in the sight path. They show no evidence or side-by-side comparisons to back up this claim.

    Indeed, there are some steel pouring practices where there is a great deal of smoke and steam throughout the pour. It would be interesting to see such a comparison. However, it is a moot question in most operations since there is usually little to no smoke and dust present near the end of a pour, or tap. That's just the time when slag detection is most needed!

  • Slag Detection and Quantification -IR Ruggedized Systems By Automation Software & Engineering (AS&E) of Twinsburg, Ohio, USA at any waveband desired by the user. System options include: environmental enclosure for camera, rf link across the pouring bay for one or two cameras to minimize installation wiring costs, vision system with self-adapting area-of-interest to avoid stream edge effects, mutiple displays for supervisors and operators, display customization, video archiving of pours for training and process improvement and much more.

  • Slag Detection -IR Commercial System By Mikron Instruments at 1.0 micron waveband (Near IR)

  • New Thermascope-Near IR Thermal Imaging system from Thermoteknix offers slag detection and unique software from this thermal imaging software specialty company.

  • Commercial Slag Detection System by Land Instruments at 3-5 micron wavelength passband (MW IR)

  • Slag Detection System by Itema-Commercial products at 8-12 micron wavelength passband (LW IR)

  • List and description of US patents and prior art papers related to this application of Infrared imaging technology.

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