Patent Number: 058964290
Section: description

DESCRIPTION OF PREFERRED EMBODIMENTS A. Introduction A pair of methods is used to safely and reliably evaluate the thickness and integrity of the liners of the hearth of a blast furnace. The first method involves the transport of gamma rays (specifically, bremsstrahlung radiation) directed into the hearth-wall liner. The second involves the die away of thermal neutrons directed into the hearth-wall liner. Either method can be used independently to evaluate the condition of a hearth-wall liner. However, the two methods are used to greatest advantage when they are used together because of the complementary information that each provides. The term, "gamma rays," may be used interchangeably herein with "photons" and "electromagnetic radiation." However, the term, "gamma rays," most correctly refers to radiation produced by nuclear processes. An embodiment of the apparatus for performing the disclosed methods is illustrated in FIG. 2. The radiation source 32 is a small compact 6 MeV electron linear accelerator (LINAC) directed toward the wall 13 opposite the molten iron 36. The hearth-wall liner 14 typically comprises carbon and is surrounded along its exterior by a one-to-two-inch-thick steel shell 11. The radiation source 32 is one meter long with a weight of approximately 40 kg and is designed for field use. It produces gamma rays of up to 6 MeV in energy. Moreover, a beryllium target 33 may be moved into the path of the gamma rays, as shown by the broken-line image, to produce neutrons for the decay-time measurements. Alternatively, the radiation source 32 may be supplemented or replaced by a small sealed pulsed neutron generator. The source 32 is surrounded by gamma-ray detectors 34 positioned to detect gamma radiation emitted from the wall 13 as a result of the gamma rays and neutrons directed into the wall 13. Any suitable gamma-ray detector may be used. For example, the detectors may be semiconductor detectors, such as germanium or cadmium zinc telluride. Alternatively, the detectors may be scintillators, such as sodium iodide (thallium-doped), barium fluoride, bismuth germanate, one of the rare earth oxyorthosilicates (cerium-doped) or a rare earth aluminate (cerium-doped) porovskite, for example, with an associated photosensitive device such as a photomultiplier tube (PMT) or avalanche photo diode (APD). As another alternative, the detectors may be gas detectors, such as high pressure xenon. The radiation/neutron source 32 and its associated detectors 34 are portable and may be moved vertically up the wall 13 of the furnace as well as horizontally to traverse the entire area to be scanned. The cylindrical side wall of the crucible-shaped hearth 12 of a blast furnace 10, illustrated in FIG. 1, is particularly well suited for evaluation by these methods. B. Gamma-Ray Transport In the first of the two methods for evaluating the hearth-wall liner, high-energy bremsstrahlung radiation 38 is directed at the hearth sidewall 13 from outside the furnace 10, and the radiation emitted back out of the wall 13 is measured and analyzed. As noted previously, the bremsstrahlung radiation is produced by a LINAC and comprises gamma ray photons of at least 1.02 MeV, and, preferably, between about 3 and 8 MeV, emitted by accelerated electrons after colliding with the nuclei of atoms in a target within the LINAC. The gamma rays penetrate the hearth 12, allowing testing to be performed without draining the furnace, without placing probes in the furnace, without breaching the integrity of the outer steel shell 11 and without suspending equipment down into the furnace from above. Using gamma rays as described herein, the test can be performed without any disruption of furnace operation. As the photons collide with the atoms that comprise the hearth-wall liner 14, and, perhaps also, the iron bath 36, as shown in FIG. 3, each photon 40 may interact with an atom to produce at least one of the following three results: pair production, Compton scattering or photoelectric absorption. First, if the energy of the photon is above 1.02 MeV, the photon may be completely absorbed by the nucleus resulting in pair production. Pair production occurs when the energy of the photon is converted into an electron and a positron, each with a rest-mass energy of 511 keV and a kinetic energy that depends on the energy of the incident gamma ray. A positron is a particle with the mass of an electron but with a positive charge. As the positron and electron move away from their point of origin, each loses energy by ionization. When the positron energy drops low enough, the positron combines with an electron, and both are annihilated to produce a pair of 511 keV photons radiated in opposite directions. Pair production is illustrated by the photon-interaction representation, A, in FIG. 3. Annihilation is illustrated by the representation, B, along the same path. The probability of pair production is directly proportional to the square of the atomic number of the medium. Though the exact energy dependence of pair production is complex, a reasonable approximation of the probability of pair production per unit track length, .SIGMA..sub.pair, at higher energies is given by: EQU .SIGMA..sub.pair =K.sub.pair .rho..sub.nuc Z.sup.2 lnE.sub..gamma., where K.sub.pair is a constant, .rho..sub.nuc is the nuclear density, Z is the atomic number and E.sub..gamma. is the energy of the incident photon. Second, the photon may collide with an electron and transfer all of its energy to the electron in the form of kinetic energy. In such a case, the electron is ejected from its atom, and the photon disappears in what is known as the photoelectric effect. An example of the photoelectric effect is designated by the photon path representation, C, in FIG. 3. The ejection of the electron leaves a vacancy in one of the atom's orbitals which will typically be filled by an electron in an outer orbital of higher energy level. As the outer electron changes orbitals to fill the vacancy, energy is released in a photon which can be easily recognized because its energy will be that of the difference in energy between the respective orbitals of the atom. The ejected electron has an energy which is the difference between the energy of the incident gamma ray and the binding energy of the electron in its atomic shell. Like pair production, photoelectric absorption increases with increasing atomic number. The absorption probability, .SIGMA..sub.a, per track length of photons is approximately given by: EQU .SIGMA..sub.a =K.sub.a Z.sup.3.6 .SIGMA..sub.e /E.sub..gamma..sup.3.15, where .rho..sub.e is the electron density. Third, the photon may collide with an electron and transfer some of its energy to the electron, ejecting the electron from its orbit. Meanwhile, the photon deflects and continues through the medium, albeit at a lower energy. This process occurs mainly with electrons in the outer orbital of an atom and is known as Compton scattering. Compton scattering dominates at energies above those where photoelectric absorption is dominant yet below those where pair production becomes important. Compton scattering is illustrated by the photon-interaction representation, D, in FIG. 3. The difference between Compton scattering and photoelectric absorption is that the photon loses only part of its energy when it undergoes Compton scattering. The likelihood of scatter per unit track length, .SIGMA..sub.s, is a function only of electron density and photon energy and is given by: EQU .SIGMA..sub.s =K.sub.s .rho..sub.e /E.sub..gamma.. The energy of the photon after it is scattered is inversely related to the angle by which it is scattered. After the photon scatters, the reduced-energy photon will likely interact with another atom in one of the three ways discussed. Because carbon, of which the hearth-wall liner 13 of the hearth 12 is comprised, has a relatively low atomic number, 6, pair production and photoelectric absorption rarely occur in carbon. Instead, a photon traveling through carbon is much more likely to undergo Compton scattering. In contrast, the iron of the bath has a much higher atomic number, 26. As a result, the probability of pair production in iron is at least a factor of ten larger than in carbon, especially where the photon energy is greater than 3-4 MeV. Thus, we expect pair production at the surface of the iron and subsequent production of photons with the energy characteristic of this process, 511 keV. The equations, provided above, are utilized to establish a strategy for distinguishing carbon from iron. In a thick piece of carbon, the dominant form of photon interaction will be Compton scattering, whereas in iron, pair production will be much larger. These differences are even more pronounced when one accounts for the difference in density between iron and carbon in establishing absolute rates. Accordingly, the radiation emitted from the wall will vary according to the thickness of carbon that the photon must pass through before reaching the iron. The signature of the photons detected at or near the surface of the wall can then be matched to radiation patterns characteristic of walls having hearth-wall liners of varying thicknesses to closely estimate the actual thickness of the hearth-wall liner measured. The gamma-ray source and detectors are then moved to different locations on the sidewall for repeated measurements to better evaluate the entirety of the sidewall. The exact details of the multiple scatter and its dependence on carbon thickness can best be predicted using simulation, as for example by the MONTE CARLO N-PARTICLE CODE.TM. (MCNP). MCNP is designed to simulate the transport of neutrons, photons and/or electrons through a medium or plurality of media. When a user enters the appropriate description (geometry and materials) of the media and the source, the code sequentially simulates each of the individual probabilistic events that comprise the transport of the particle through the medium. When simulating the interaction or sequence of interactions between a photon and the medium, the code is used to statistically sample a probability distribution of each of the possible forms of photon/atomic interaction, described above, using a random number generator. As the simulated particle proceeds from a source through each simulated interaction, the code is used to track the decrease in energy and the change in direction that each interaction produces until the eventual death of the particle in some terminal category, such as absorption, pair production, or escape. As the histories of more simulated particles are collected, the release of the various types of detectable radiation is tallied to produce a model estimating actual behavior in a system having the specified characteristics. By modifying the characteristics of a carbon hearth-wall liner that are input to the MCNP--particularly, the thickness of the carbon--a range of results can be obtained. Since the area tested will normally be surrounded by a steel shell, the simulation should be configured to include the characteristics of this shell so that its influence can be accounted for. After confirming the accuracy of these results by comparing the MCNP values with actual test data, the MCNP values can be matched to field measurements to provide a reliable estimate of carbon thickness. Alternatively, algorithms can be empirically developed that relate changes in the spectral response to changes in the carbon thickness. Using MCNP, simulations of gamma ray transport through a hearth wall with a 5-cm-thick steel shell and a carbon liner having a thickness of either 90 or 100 cm were conducted. The simulated energy spectra produced at a point 20 cm from the point on the wall where radiation is incident are shown in FIGS. 4 and 5. The peak of the simulated energy spectrum, between 200 and 300 keV is approximately 5% greater in the simulation for the 100-cm-thick carbon hearth-wall liner, shown in FIG. 5, than the peak for the 90-cm-thick carbon hearth-wall liner. These results validate the premise that the methods of this invention can be used to differentiate between hearth-wall liners of different thicknesses. As an alternative to MCNP, a similar simulation program, known as COG.TM., developed at Lawrence Livermore National Laboratory may be used, as may other Monte Carlo programs. C. Neutron Decay Time A second procedure for inspecting the carbon hearth-wall liner of a blast furnace involves directing neutrons into the hearth-wall liner and measuring the time between emission and absorption of the neutron. The neutron scatters through the medium of the wall, and sometimes through the molten iron, losing energy until it reaches thermal energies. At thermal energies, the neutron diffuses through the medium until captured by a nucleus, thereby producing a gamma ray that can the furnace outside the furnace. The basis for this approach is the fixed rate at which thermal neutrons will be absorbed in a given medium. The rate at which the neutrons are absorbed depends on the macroscopic thermal-neutron-absorption cross section, .SIGMA., of the medium. The macroscopic thermal-neutron-absorption cross section is the average microscopic thermal-neutron-absorption cross section per unit volume of the medium. The rate at which thermal neutrons are absorbed in a medium is given by: EQU dN/dt=N.sub.0 e.sup.-.SIGMA.vt, where N.sub.0 is the number of neutrons at time t=0 and v is the velocity of the neutrons. The major advantage of using this technique in this particular context is that the neutron absorption cross sections for carbon and iron are dramatically different. The microscopic thermal neutron absorption cross section for iron is 2.56 barns (10.sup.-24 cm.sup.2), while that for carbon is 3.4 millibarns. Thus, the sensitivity of a measurement of the thermal neutron decay time for iron in a predominantly carbon environment is about a factor of a thousand. As a result, the detection of a 5 percent change in the thermal decay time should allow the operator to detect an infiltration of iron into carbon at about 50 parts per million. The measurement is performed by producing a short burst of high-energy neutrons. The burst in this situation should be about one millisecond in duration, though narrower burst widths are quite acceptable. The high-energy neutrons can be provided using the same compact electron accelerator used to produce the bremsstrahlung radiation for the gamma-ray transport measurement. The neutrons are supplied by placing a beryllium secondary target in the path of the gamma rays. Alternatively, the neutrons can be supplied by a small sealed neutron generator, such as those used in the oil well logging industry. The high-energy neutrons scatter in the furnace material, gradually losing energy until they approach thermal energies. While at thermal energies, the neutrons diffuse through the furnace material until they are captured by nuclei. Each capture event is accompanied by the essentially instantaneous production of one or more gamma rays that are detected by a gamma-ray detector outside the furnace, where the gamma-ray detector is of a type mentioned previously. The detection of gamma rays is recorded as a function of time relative to the end of the burst of neutrons. Following a sufficient interim, additional neutron bursts are provided to produce more gamma rays, which are detected and recorded to build the necessary level of statistical accuracy for the lifetime determination. The number of gamma rays detected as a function of time is fit by an appropriate mathematical function to determine the decay time of neutrons directed into the wall. Moreover, the neutron source and detectors are moved to different locations on the sidewall, where the measurements are repeated and a better evaluation of the entirety of the sidewall is obtained. The recorded measurements are then fed to a personal computer 35 to determine the amount of infiltrated iron by correlating the neutron decay times (or the time dependence of the gamma-ray spectrum) with known iron infiltration parameters. The proper correlation can be established through mathematical analysis, actual testing, and/or Monte Carlo simulation, using codes such as MCNP. Once the computer 35 has performed this correlation, a value for the estimated thickness of the hearth-wall liner is transmitted to a display 37. Neutron decay measurements have been used to probe earth formation properties behind steel casings. However, they have not been applied in an industrial application, as considered here. The expected depth sensitivity in this environment is expected to be about 70-90 cm which is sufficiently deep to monitor the region from the outer surface of the furnace to a depth near the iron-carbon interface. D. Equivalents While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention as defined by the appended claims. For example, and without limitation, infiltration of other molten metals, besides iron may be analyzed in a similar fashion utilizing the apparatus and methods described herein.