SYSTEM FOR OPTICALLY ANALYZING A MOLTEN METAL BATH

Technical Field

[0001] This invention relates generally to refining molten metal, e.g. steel, and, more particularly, to analyzing the molten metal bath during the refining.

Background Art

[0002] Metals such as steel are typically produced and refined in a refractory lined vessel by heating charge materials such as metal bearing scrap, pig iron, ore, limestone, dolomite, etc. to a molten state and blowing oxygen into the resulting molten metal bath in order to oxidize impurities. It is not always possible to know the precise chemical composition of the charge materials prior to the start of processing. Therefore, the composition must be determined after the charge materials have become molten and thoroughly mixed. Moreover, the changing composition of the molten metal bath must be at least periodically determined so as to know the timing and quantity of additives made to the refining vessel contents. The standard method for determining the composition of a molten metal bath is to stop the production process, withdrawn a small sample of material, and analyze this sample using a mass spectrometer.

[0003] Continuous on-line measurement is more desirable but the high temperature and the presence of dust, fume, and slag do not permit locating measuring devices inside the molten metal bath. Those skilled in the art have attempted to deal with these problems by

using optical fibers close to the surface of the molten metal bath or using such aids as lenses, mirrors and prisms in order to pass data from the molten metal bath to an analyzer. However such arrangements are unsatisfactory because they are complicated to set up and difficult to maintain during the refining process, thus compromising the accuracy of the data gathered and compromising the integrity of the analysis based on such data.

Summary Qf The Invention

[0004] One aspect of the invention is: [0005] A method for optically analyzing a molten metal bath comprising:

(A) forming a coherent argon gas stream by passing an argon gas stream out from a lance and surrounding the argon gas stream with a flame envelope;

(B) passing the coherent argon gas stream to a molten metal bath;

(C) sighting longitudinally through the coherent argon gas stream to view the molten metal bath and obtain optical data therefrom; and

(D) passing the optical data to an analyzer. [0006] Another aspect of the invention is: [0007] Apparatus for optically analyzing a molten metal bath comprising:

(A) a molten metal furnace containing a molten metal bath;

(B) a lance having an ejection end for passing a coherent argon gas stream to the molten metal bath;

(C) a sight glass mounted on the lance on the end opposite the ejection end to provide a pressure seal to

prevent leakage of argon gas from the lance while providing an optically transparent view port and aligned so as to view the molten metal bath longitudinally through the coherent argon gas stream to obtain optical data; and

(D) an analyzer and means for passing the optical data to the analyzer.

[0008] As used herein, the term "flame envelope" means a combusting stream around at least one other non-combusting gas stream.

[0009] As used herein, the term "coherent gas stream" means a gas stream whose diameter remains substantially constant.

[0010] As used herein, the term "molten metal bath" means the contents of a metal refining furnace comprising molten metal and which also may comprise slag.

[0011] As used herein, the term "optical data" means a value describing a characteristic of a molten metal bath which can be sensed by a receiver spaced from the molten metal bath.

[0012] As used herein, the term "longitudinally" means in line with the major axis.

[0013] As used herein, the term "sight glass" means an optically transparent material, such as sapphire or quartz, capable of providing a seal between a pressurized stream of argon gas in a lance and the fiber optic cable or other optical components. A light source, such as a laser, may be fitted to the sight glass to increase the energy of the molten metal bath observed through the coherent argon gas jet so as to improve the effectiveness of the analysis.

Brief Description Qf The Drawing [0014] The sole Figure is a simplified cross sectional representation of one preferred arrangement which may be used in the practice of the invention.

Detailed Description

[0015] The invention will be described in detail with reference to the Drawing. Referring now to the Figure, there is shown molten metal furnace 10 which contains a molten metal bath comprising molten metal 4 and a slag layer 5, which may be molten and/or solid, above the pool of molten metal. Typically the molten metal will comprise iron or steel. The slag layer generally comprises one or more of calcium oxide, silicon dioxide, magnesium oxide, aluminum oxide and iron oxide.

[0016] Lance 1 is positioned so as to provide argon gas to the molten metal bath. The embodiment illustrated in the Figure is a preferred embodiment wherein the lance is positioned so as to provide the argon gas to the molten metal bath in a direction perpendicular to the surface of the molten metal bath. Alternatively, the lance could be positioned through a sidewall of furnace 10 so as to provide the argon gas angularly to the surface of the bath.

[0017] In the practice of this invention, argon is used as the gas through which an optical sighting is made. Unlike conventional sensing systems which employ oxygen or another reactive gas, argon, due to its inertness relative to the molten metal, provides for a much clearer optical view of the molten metal from the

remote sight position. In addition, the heaviness of the argon gas makes for a better defined impact site at the molten metal than the conventional more lighter gases employed with conventional systems . The combination of reduced splashing and other visual impediments at the gas-metal impact site due to the non-reactivity of the argon gas, coupled with the better defined impact site due to the density of the argon gas, enables a much clearer optical view than is possible with conventional systems. This clearer optical view enables better data acquisition and improved data analysis.

[0018] The argon gas is provided from the lance at a high velocity, preferably at sonic or supersonic velocity. Generally the velocity of the argon gas stream 3 provided from the lance has a velocity of at least 1000 feet per second (fps) and preferably at least 1500 fps. Most preferably the argon gas stream has a supersonic velocity upon ejection from the lance and also has a supersonic velocity when it contacts the bath surface.

[0019] Fuel and oxidant are provided out from the lance around the argon gas stream and combust to form a flame envelope 2 around the argon gas stream 3. Preferably, as shown in the Figure, the flame envelope extends for the entire length of the argon gas stream within the furnace from the lance ejection end to the bath. The fuel used to form flame envelope 2 is preferably gaseous and may be any fuel such as methane or natural gas. The oxidant used to form flame envelope 2 may be air, oxygen-enriched air having an oxygen concentration exceeding that of air, or

commercial oxygen having an oxygen concentration of at least 90 mole percent.

[0020] Flame envelope 2 serves to keep ambient gas, e.g. furnace gases, from being drawn into or entrained into argon gas stream 3, thereby keeping the velocity of argon gas stream 3 from significantly decreasing and keeping the diameter of argon gas stream 3 from significantly increasing, generally for a distance of at least 2Od where d is the diameter of the nozzle at the lance ejection end from which gas stream 3 is ejected. That is, flame envelope 2 serves to establish and maintain argon gas stream 3 as a coherent gas stream generally for a distance of at least 2Od. Preferably, as shown in the Figure, argon gas stream 3 is a coherent gas stream from the lance to the bath. [0021] The use of a coherent jet of argon gas to penetrate through the slag layer and fume above the bath is not envisioned by conventional practice. The gas stream issuing from a standard lance does not penetrate the slag layer from a long distance and does not provide a clear view of a molten metal bath to accurately measure its properties. The use of a shroud fuel gas is required to produce the concentrated or coherent stream of argon gas. The shroud gas also generates light signals at specific wavelengths due to the combustion of elements and molecules such as sodium, potassium, CaO, and MnO, which can be used to determine whether the slag is being completely penetrated.

[0022] The use of a spectrometer or other instrument capable of measuring light intensity at several wavelengths is employed. Two separate wavelengths are

used for measuring temperature. Other wavelengths are used for measuring the quantity of various elements, such as carbon, silicon, copper, chromium, etc. Yet other wavelengths indicate the presence of oxides such as CaO, MnO, and MgO in the field of view, and can be used to determine whether the slag containing these oxides is being completely penetrated. A further indicator of penetration of the slag layer is the conversion of light signals from the combustion of sodium and potassium by the shroud fuel, from emission spectra to absorption spectra. This has been shown to occur when the inert argon gas penetrates completely through the slag layer.

[0023] The argon gas passed to the bath in gas stream 3 serves to help refine the molten metal by mixing the bath. Preferably, as shown in the Figure, the high velocity and coherent nature of argon gas stream 3 serves to drive gas stream 3 through slag layer 5 and deep into molten metal 4 so as to enhance the mixing action of the gas delivered to the bath in argon gas stream 3.

[0024] As has been discussed above, it is desirable at least periodically, and preferably continuously, to monitor the condition of the molten metal to determine, for example, its composition, temperature and/or the proportion of scrap that has been melted. In the practice of this invention these parameters are monitored by sighting through sight glass 9. As shown in the Figure, sight glass 9 is mounted on lance 1 on the end opposite the ejection end to provide a pressure seal to prevent leakage of argon gas from the lance while providing an optically transparent view port.

This leakage prevention serves not only to reduce gas losses but also serves to reduce the chance of pressure imbalances which could negatively impact the formation and maintenance of the coherency of the argon gas stream. The formation and the maintenance of a coherent gas stream is not attainable with conventional sensing systems.

[0025] The coherent nature of argon gas stream 3, which keeps furnace gases, fumes, particles, etc. from being entrained into argon gas stream 3, enables a clear line of sight to form from sight glass 9 to the molten metal bath. This enables viewing the molten metal bath by sighting longitudinally through the unobstructed pathway provided by coherent argon gas stream 3. This viewing enables the gathering of optical data from the bath. Data that can be gathered by viewing the molten metal through the coherent argon jet include temperature by way of optical pyrometry, measurement of the quantities of various elements contained in the molten metal bath and slag by way of spectroscopic analysis, and conditions of the process such as the proportion of melted scrap by analysis of the temperature trends.

[0026] The optical data is passed to an analyzer 1, such as by light guide assembly 8 which may comprise fiber optic cable or a system of lenses and mirrors. Analyzer 7 may be, for example, an optical spectrometer optical pyrometer, or a combination of these instruments. Analyzer 7 employs the data to provide measurements of temperature and composition of the molten metal bath, thereby enabling the operator to make adjustments to the amounts and timing of

additional charge materials, fluxing agents, alloys, electrical energy, and reactive agents such as oxygen, to facilitate arriving at the desired endpoint of the refining process.

[0027] By observing the current temperature of the molten bath and the quantity of carbon, chromium, manganese or other elements remaining in the molten metal bath, the operator can determine when the processing of the metal has reached the conditions specified for the type of metal being produced. Further, if the quantity of certain trace elements such as copper are observed to be outside the quality limitations for the metal being produced, the operator will be able to make adjustments to bring the product into specification before the completion of processing. By knowing the proportion of scrap melted, the operator will know the appropriate time to add additional scrap to the furnace.

[0028] By the use of the invention one can obtain continuous and on-line measurement of molten metal bath properties without need for using optical fibers close to the surface of the molten metal bath or using such aids as lenses, mirrors and prisms. Although the invention has been described in detail with reference to a preferred embodiment, those skilled in the art will recognize that there are other embodiments of the invention within the spirit and the scope of the claims .