Patent Number: 058964290
Section: summary

BACKGROUND OF THE INVENTION In a typical iron production process, a blast furnace is used to reduce iron ore to liquid iron for subsequent processing. A typical blast furnace 10, shown in FIG. 1, can be as high as 100 feet and have a diameter of 50 feet. Inside a steel shell 11 of the furnace 10, three-foot-thick refractory carbon blocks 14 form a hearth-wall liner providing thermal insulation between the molten iron (not shown) and the shell 11. When the furnace 10 is operating, a burden is fed into the top of the furnace 10. The burden typically includes iron ore, coke and limestone. The iron ore provides iron which serves as the predominant component of steel. The coke combusts to provide the heat required for smelting. Moreover, the coke also supplies needed carbon and carbon dioxide. The limestone serves as a flux to form a fluid slag that can be readily drained from the hearth 12. As the burden is fed to the furnace 10, it fills the stack 16 as a solid aggregate. The burden is then forced through the stack 16 down into an inverted conical section, known as the bosh 18, where melting starts. A blast of heated air and fuel are introduced through openings 20 at the bottom of the bosh 18, just above the hearth 12 to melt the burden. The resulting iron melt and slag accumulate in the hearth 12 to form a molten bath until drained through a tapping hole 22. As iron is processed in the furnace 10, the carbon blocks 14 are gradually worn away or weakened due to erosion caused by the mechanical motion of molten iron and also due to infiltration of the molten iron into cracks which develop in the blocks 14. Infiltration and thinning of the carbon blocks 14 has conventionally required that the entire furnace 10 be shut down and relined approximately every six years. For a large furnace, relining requires three to four months downtime of the furnace and consequent loss of production. The cost of relining and shutdown can be $120 million or more. If relining is delayed, the mechanical integrity of the blocks 14 can fail catastrophically allowing the molten iron to escape through the hearth-wall liner. Furnace failure can easily cost $5 million to $50 million, depending on the extent of damage. In recognition of this danger, blast furnace operators typically err on the conservative side and often replace the hearth-wall lining prematurely. In an effort to better assess the appropriate time to replace the hearth-wall liner, a variety of techniques have been used to evaluate its condition. A common technique uses thermocouples embedded into the hearth-wall liner at various locations. To augment the reliability of these measurements, the use of thermocouples is usually supplemented by periodically drilling through the hearth wall. Although the drilling damages the hearth, it nevertheless enhances the determination of hearth-wall liner thickness. Additionally, invasive optical methods have been developed to evaluate the inner surface of the hearth for wear from inside the furnace when the furnace is shut down and emptied. SUMMARY The use of invasive thermocouples and other methods for determining hearth-wall liner thickness often proves inaccurate and difficult to interpret. When thermocouples, for example, are relied upon, thin fissures of iron through the wall may not be detected. In fact, catastrophic furnace failures, in which the molten iron breaks out of the containment, are still reported. The cost of these failures is enormous. The methods of this invention are based on two techniques for probing with radiation. When used to evaluate the hearth-wall liner of a furnace, the methods of this invention improve the furnace operator's ability to forecast the onset of a failure and, consequently, allow the operator to better assess the appropriate time for hearth-wall liner replacement. The resulting benefits include cost savings, improved planning, decreased downtime, and improved safety. Moreover, the methods are non-invasive and can therefore be performed during normal furnace operation and with greater ease and with less interference than those of the prior art. Further, the methods can be repeated periodically to monitor for changes in the hearth-wall liner. A method for inspecting a wall in accordance with the invention includes the steps of directing neutrons and photons of radiation into the wall. Radiation is then emitted from the wall as a result of Compton scattering, photoelectric absorption, pair production and neutron absorption. This radiation is measured and analyzed to evaluate the remaining thickness of the wall and the extent to which the wall has been infiltrated by another material. Alternatively, either the step of directing radiation into the wall or the step of directing neutrons into the wall can be used in the absence of the other. In a preferred embodiment, the radiation directed into the wall is in the form of gamma rays and the inspected wall includes a hearth-wall liner comprising carbon. The hearth-wall liner is adapted to contain a molten metal, such as iron, in a furnace, and the inspection is performed from a position outside the furnace. In this embodiment, the material for which infiltration into the wall is evaluated is the molten metal contained by the hearth-wall liner. When neutrons are directed into the wall, the neutrons scatter until absorbed, at which point, signature gamma rays are produced. The rate at which neutrons are absorbed is a function of the neutron-absorption cross section of the medium in which the neutrons travel. In short, the neutrons will be absorbed more quickly in a medium of larger cross section. In accordance with one aspect of the invention, the analysis includes evaluating, as a function of time, measurements of emitted gamma rays produced by neutron absorption to determine the amount of iron infiltration into the carbon hearth-wall liner. The analysis of radiation emitted from the wall as a result of directing radiation into the wall is more complex than that for neutrons. However, the methods of this invention also encompass techniques for evaluating the measurements of radiation emitted from the wall as a result of the incident radiation. For example, when a gamma ray/nucleus interaction leads to pair production, gamma rays having an energy of 511 keV are produced. In one embodiment, the rate of pair production, which reflects the composition of the matter through which the gamma rays travel and is therefore sensitive to the overall thickness of a hearth-wall liner, is determined by monitoring for the effects of 511 keV gamma rays produced within the hearth wall and molten iron. Further, the emitted radiation can be plotted as a function of the energy of the emitted photons. The peak of this plot typically falls in the range of about 200 keV to about 300 keV. The magnitude of this peak, in particular, as well as the magnitude of other parts of the spectrum, is dependent upon the thickness of different compositions penetrated by the gamma rays. Another embodiment utilizes this dependance to determine the thickness of a hearth-wall liner by analyzing the magnitude of the different portions of the spectrum, particularly the range between 200 and 511 keV, for a given set of measurements. A third embodiment uses the ratio of the number of photons emitted from the wall to the number of photons directed into the wall to determine the thickness of the hearth-wall liner because this ratio, as well, is dependent upon the composition of penetrated matter. Further, these measurements are preferably performed by a plurality of detectors and the measurements recorded by each detector can then be compared to determine the extent to which the gamma rays have scattered and, as a result, determine the thickness of the hearth-wall liner. In accordance with another aspect of this invention, a computer Monte Carlo simulation program, such as MCNP or COG, is used to simulate the behavior of photons in media of specified composition and of varying thicknesses. Use of a suitable Monte Carlo program is described in MCNP.TM.--A GENERAL MONTE CARLO N- PARTICLE TRANSPORT CODE (Judith F. Briesmeister, ed., Version 4A, 1993), incorporated herein in its entirety by reference. The program is then used to produce a simulated readout of radiation detected outside the furnace. When the photons are actually directed into a wall in accordance with the methods of this invention, the radiation measurements are then compared to the simulated results to determine the thickness of the hearth-wall liner. In a preferred embodiment of an apparatus used to perform the methods of this invention, an electron accelerator is used to direct bremsstrahlung radiation having an energy at least as great as the pair production threshold of 1.022 MeV, and preferably 3 to 8 MeV, into the hearth-wall liner. Further, the electron accelerator can also be used to trigger the emission of neutrons directed into the wall by, for example, placing a beryllium target in the path of the stream of photons emitted by the electron accelerator. Alternatively, a pulsed sealed neutron source can be used to provide the neutrons that are directed into the wall.