Method and device for measuring levels of cast iron and slag in a blast furnace

Abstract

The present invention relates to a method for measuring the liquid-metal surface level (13) and the slag surface level (14) in the crucible (1) of a metallurgical shaft furnace comprising the following steps: measuring, at one or more points on the external wall (2) of the crucible, the following variables: the circumferential strain in said external wall (2) by means of a number of strain-gauge sensors (6) fixed to the armor (4) of the external wall (2) of the crucible; and the temperature of said external wall (2) by means of one or more temperature sensors (7) fixed to the armor (4) of the external wall (2) of the crucible; introducing said variables measured at a number of points on the external wall of the crucible into the general equation governing circumferential strain, the solution of which is analytical, and which contains two unknowns, the liquid metal level and the overall liquid metal/slag level, considering set parameters; and solving said equation and obtaining an analytical solution giving the liquid metal surface level (13) and the slag surface level (14) in the crucible (1).

Claims

1. A method for measuring the surface level of liquid metal (13) and the surface level of slag (14) in a crucible (1) of a metallurgical shaft furnace having an external wall (2) provided with a shielding made of steel (4), as well as a tap hole, the shielding (4) of the external wall surrounding a refractory thickness (3) in contact with the liquid metal, the shielding (4) being provided on its external surface with a plurality of strain gauge sensors (6) and with one or more thermal sensors (7), said sensors (6,7) being mounted so as to be aligned in a vertical plane, on either sides of the tap hole and such that the two types of sensors are alternated in this vertical plane, said method comprising the following steps of: measuring at a number of points of the external wall (2) of the crucible the following variables: the circumferential strain of said external wall (2) by means of the strain gauge sensors (6) attached onto said shielding (4); the temperature of said external wall (2) by means of thermal sensors (7) attached onto said shielding (4); measuring the circular pressure or static pressure of the hot blast in the furnace if the furnace is operated under overpressure; introducing said variables measured at said number of points on the external wall of the crucible in a general equation governing the continuous circumferential strain the solution of which is analytical, and which contains two unknown values, the level of liquid metal and the overall level of liquid metal-slag, said general equation involving the following established parameters; the geometry of the shaft furnace, the parameters representative of the nature of the constituent materials of the shaft furnace, the thickness of the external wall of the shaft furnace at each point of measurement, the density of liquid metal and of slag; and solving said equation and obtaining the surface level of liquid metal (13) and the surface level of slag (14) in the crucible (1); characterized in that the General equation governing the continuous circumferential strain of the external wall is, in the case of a blast furnace, as follows: $y=y_0{F}_1+\frac{}{2}{F}_2+\mathrm{LT}$ where F.sub.1 and F.sub.2 are constants for a given blast furnace at each measurement height; $\begin{array}{c}{F}_1=\cos h\left(x\right)\cos \left(x\right)\\ F_2=\cos h\left(x\right)\sin \left(x\right)+\sin h\left(x\right)\cos \left(x\right)\mathrm{and}\\ ={\left[\frac{3\left(1-v^2\right)}{R^2{t}^2}\right]}^{1\text{/}4}\end{array}$ where y is the horizontal deflection (or the displacement) measured and corrected as a function of the tem orange measured, x is the vertical position, R is the mean radius at each height, t is the thickness of the wall at each height and is Poisson's ratio; the other terms depending on the pressure applied to the wall of the shaft furnace: y.sub.0 is the deflection or displacement at the level of at least one nozzle; is the angle of the strained wall at the level of at least one nozzle; LT is the load term which is proportional to the force applied over the wall of the furnace; a final transformation being performed over the pressures P in order to take into account the fact that the crucible of the blast furnace is constructed with materials having different mechanical properties, refractory material and steel shell, and with various thicknesses differing as a function of the height (Ex=Young's modulus of the material x): $P_{\mathrm{across}\mathrm{steel}\mathrm{shell}}=P_{\mathrm{int}.\mathrm{across}\mathrm{refractory}}=\frac{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{Steel}\mathrm{shell}}}{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{Steel}\mathrm{shell}}+E_{\mathrm{refract}.}{\mathrm{Thickness}}_{\mathrm{refract}.}};$ with the relationship between the static pressure P and the level of liquid metal or slag h being given by a relation of the type P=*g*h, where is the average density and g is the acceleration due to gravity.

2. A method according to claim 1, characterized in that the measurements of circumferential strain and temperature are carried out continuously and/or in real time.

3. An external wall (2) of a crucible of the metallurgical shaft furnace, comprising a steel shielding (4) as well as a tap hole, the shielding (4) being provided with a device for measuring the surface level of liquid metal (13) and the surface level of slag (14) in the crucible (1) of the furnace, for the implementation of a method for measuring the surface level of liquid metal (13) and the surface level of slag (14) in said crucible, said measuring device comprising a plurality of strain gauge sensors (6) and one or more thermal sensors (7) attached onto the external surface of said shielding (4), the sensors (6,7) being mounted so as to be aligned in a vertical plane, on either sides of the tap hole, and the two types of sensors being alternated in the vertical plane, in such a manner that the strain gauge sensors (6) are configured to measure a first variable, the circumferential strain of said external wall (2), at a plurality of points of the latter, and that the thermal sensors can measure a second variable, the temperature of said external wall (2), at one or more points of the latter, so as to be able to introduce said first and second variables measured at a plurality of points of the external wall of the crucible in a general equation governing the continuous circumferential strain the solution of which is analytical, and which contains two unknown values, the level of liquid metal and the overall level of liquid metal-slag, said general equation involving the following established parameters: the geometry of the shaft furnace, the parameters representative of the nature of the constituent materials of the shaft furnace, the thickness of the external wall of the shaft furnace at each point of measurement, the density of liquid metal and of slag; and to finally solve said general equation and obtain a value for the surface level of liquid metal (13) and the surface level of slag (14) in the crucible (1); characterized in that the general equation governing the continuous circumferential strain of the external wall is in the case of a blast furnace, as follows: $y=y_0{F}_1.Math.\begin{array}{c}\\ 2\end{array}{F}_2.Math.LT$ where F.sub.1 and F.sub.2 are constants for a given blast furnace at each measurement height: $F_1=\cosh \left(x\right)\cos \left(x\right)$ $F_2=\cosh \left(x\right)\sin \left(x\right)+\sinh \left(x\right)\cos \left(x\right)$ $\mathrm{and}={\left[\frac{3\left(1-v^2\right)}{R^2{t}^2}\right]}^{1/4}$ where y is the horizontal deflection (or the displacement) measured and corrected as a function of the temperature measured, x is the vertical position, R is the mean radius at each height, t is the thickness of the wall at each height and is Poisson's ratio; the other terms depending on the pressure applied to the wall of the shaft furnace: y.sub.0 is the deflection or displacement at the level of at least one nozzle; is the angle of the strained wall at the level of at least one nozzle; LT is the load term which is proportional to the force applied over the wall of the furnace; a final transformation being performed over the pressures P in order to take into account the fact that the crucible of the blast furnace is constructed with materials having different mechanical properties, refractory material and steel shell, and with various thicknesses differing as a function of the height (Ex=Young's modulus of the material x): $P_{\mathrm{across}\mathrm{steel}\mathrm{shell}}=P_{\mathrm{int}.\mathrm{across}\mathrm{refractory}}=\frac{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{steel}\mathrm{shell}}}{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{steel}\mathrm{shell}}+E_{\mathrm{refract}}{\mathrm{Thickness}}_{\mathrm{refract}.}}$ with the relationship between the static pressure P and the level of liquid metal or slag h being given by a relation of the type P=*g*h, where is the average density and g is the acceleration due to gravity.

4. A wall according to claim 3, characterized in that the strain gauges (10) are welded to said shielding (4) itself and in that the strain gauge sensor (6) comprises a protective cover (11) attached mechanically onto the shielding (4), said protective cover (11) comprising a sealing with respect to the shielding which is ensured by means of a resilient sealing gasket.

5. A wall according to claim 3, characterized in that each thermal sensor (7) is a platinum-resistance thermometer.

6. A wall according to claim 3, characterized in that the measuring device is arranged between channels (9) of a channel-based cooling circuit mounted onto or in the wall.

7. A blast furnace comprising a crucible provided with an external wall (2), said external wall comprising a shielding (4) provided with a device for measuring the surface level of cast iron (13) and the surface level of slag (14) in the crucible (1), according to claim 3.

8. A wall according to claim 3, characterized in that the strain gauge sensor (6) comprises four strain gauges (10).

9. A wall according to claim 8, characterized in that two of the gauges (10) are completely welded onto the shielding and two gauges are not completely welded onto the shielding, these latter two gauges allowing for a correction of temperature.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic representation of the vertical cross-section of the crucible of a blast furnace.

(2) FIGS. 2 and 3 are elevation views of two variants of the external steel shielding of the crucible of a blast furnace equipped with the measuring device according to the invention and which differ from each other in their cooling systems.

(3) FIG. 4 is a graphical representation of the measurement of the surface level of cast iron as well as the surface level of slag over time, obtained according to the method of the invention.

(4) FIG. 5 is a detailed view of a strain gauge sensor welded onto the steel shielding of the crucible of a blast furnace, according to the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(5) FIG. 1 is a schematic representation of the vertical cross-section of the wall 2 of the crucible 1 of a blast furnace. The wall is made up of refractory materials 3 and of a steel shielding 4. According to a preferred embodiment of the device of the invention, the steel shielding of the external wall is equipped with strain gauge sensors 6 represented by circles alternating with thermal sensors 7 represented by triangles. FIG. 1 shows a non-limiting example with six strain gages sensors 6 and five thermal sensors 7, attached onto the external surface of the steel shielding 4. The positioning of the tap hole 5 is also indicated.

(6) At the high temperatures prevailing within the crucible 1, the refractories 3 may soften and/or be subject to erosion and corrosion. Without a cooling system, their operational life would be limited. The cooling is achieved by circulating water through the plates and pipes encased in the walls of the blast furnace. The crucible 1 of a blast furnace may be cooled either by water flowing through the cooling plates inside the wall of the crucible or by water flowing in the open air along the external wall.

(7) In FIG. 2, a cooling system based on the use of water channels is implemented, the channels 9 being schematically represented by vertical lines.

(8) In FIG. 3, the cooling is achieved by means of water flowing in the open air along the external wall.

(9) In FIGS. 2 and 3, the steel shielding 4 of the external wall 2 of the crucible 1 is provided, according to the device of the invention, by way of illustration, with two strain gauge sensors 6 represented by arrangements in the form of crosses and one thermal sensor 7 represented by a rectangle. The arrows represent the wiring 8 of the sensors to a device for recording and storing data.

(10) FIG. 4 is a graphical representation of the measurement, according to the invention, of the surface level of slag 14 (upper curve indicating the cumulative level of cast iron and slag) as well as the level of cast iron 13 (lower curve) as a function of time, the distance of these levels being measured relative to the tap hole 5.

(11) FIG. 5 is detailed view of a sensor based on stress or strain gauges 6 attached onto the steel shielding 4 of the external wall 2 of the crucible 1. The sensor comprises four individual measurement gauges 10 represented by rectangles. Preferably, two gauges are completely welded onto the shielding 4, while the other two gauges are not completely welded onto the wall so as to allow for a correction of temperature (expansion of the gauges). The gauge comprises a cover 11 fastened by means of a system of screws and bolts 12 onto the shielding 4. The channels 9 of the cooling system are represented by vertical lines. The sealing of the cover 11 relative to the shielding is ensured by means of a resilient sealing gasket, for example made of silicone (not shown). The cross-shaped form of the sensor in the figures is the result of a form of cover 11 that is substantially vertical and of a substantially horizontal cross piece to fasten the cover to the shielding. Thus, it is the gauges that are rigidly secured onto the shielding and not the cover.

(12) The principle of the method is as follows: the cast iron and the slag exert a differential pressure on the wall of the refractories 3 inside the crucible 1, which pressure will be transferred to the shielding 4 of the external steel wall 2.

(13) The variation in static pressure in the blast furnace caused by the variation in the surface levels of cast iron 13 and slag 14 will induce a variation in the strain of the shielding 4.

(14) The direct measurements of the strain and temperature of the shielding, as well as the measurement of circular pressure (static pressure of the hot blast, i.e. the gases circulating in the blast furnace) are the input data for a system of equations of which the analytical solution will give the surface levels of cast iron 13 and slag 14 in the crucible 1.

(15) The system of equations takes into account the following established parameters: the geometry of the blast furnace; the nature of the constituent materials of the blast furnace (via Young's modulus, Poisson's ratio, etc, of the steel and the refractory); the thickness of the external wall (including the refractory) of the blast furnace at each point of measurement; the density of cast iron and slag.

(16) It will be noted that the temperature values measured at the level of the thermal sensors installed on the external surface of the shielding allow the correction of temperature of the strains measured according to the following equation:
Corrected Strain=Measured Strain.Math.T,
where is the thermal expansion coefficient and T is the difference between the measured temperature and the reference temperature.

(17) The results of calculation with several points of measurement are provided in FIG. 4 (solid line: overall slag and cast iron level; dotted line: cast iron level). FIG. 4 also shows the different steps of a casting process: opening of the tap hole (casting starts), flow of molten cast iron, the slag possibly still accumulating, flow of molten cast iron and slag (slag inflow), low levels reached and outlet of gas, re-plugging of the tap hole (casting ends).

(18) The measurements of circumferential strain and temperature are preferably carried out on a continuous basis and/or in real time.

(19) In the variant embodiment where there is only one point of strain measurement, it should be near the level of the tap hole (for example at a distance of one meter) and only allows to calculate the overall level of cast iron and slag. In this case, the overall level is obtained from the static pressure value at the tap hole. This latter is obtained as the unknown value from an equation wherein the measurement given by the strain gauge is a function of the static pressure, the circular pressure, the geometry of the crucible (radius), the thickness of the walls (steel, refractory), and the respective Young's moduli (steel, refractory). To obtain the level as a function of the static pressure, a value of 3300 kg/m.sup.3 for density is adopted, which is an intermediate value between the density of cast iron and that of pure slag. The signal from the strain gauges is filtered to eliminate the medium-term fluctuations linked to the changes in temperature. This gives a good correlation with the results of the digital models.

(20) In the second variant embodiment, there are several strain gauge sensors disposed over a generatrix of the furnace, on either sides of the tap hole, at a suitable height. The problem is simplified by using the superposition method. Three assumptions are formulated: Hooke's law is valid for all materials involved, the strains are small as compared to the size of the structure, and the deflection does not alter the action of the forces applied. The forces acting on the furnace are the following: pressure of gas acting on the internal surface; static pressure acting on the internal wall up to the liquid metal-slag interface; static pressure of the liquid metal-slag interface on the hearth; stress on the bottom; stress at the level of the nozzles.

(21) The two unknown values of the problem are at all times the level of liquid metal and the overall level of liquid metal-slag. The problem is solved by the presence of several sensors over one single generatrix.

(22) The general equation governing the continuous circumferential strain of the external wall (shielding) takes into account the continuity of the material and depends on the deflection at the nozzles, on the angle formed by the strained wall to the nozzles and on the load term which depend on the force applied to the structure. The parameters of the equation are constants that depend on Poisson's ratio and Young's modulus of the constituent materials of the wall (steel shielding and refractories), on the thickness of the wall at each height and on the mean radius at each height. Also taken into account is the variable thickness of the refractory with the height (General references: Roark's Formulas for Strain and Stress; Strength of materials, S. Timoshenko).

(23) More precisely, the general equation governing the continuous circumferential strain of the external wall is:

(24) $\begin{array}{cc}y=y_0{F}_1+\frac{}{2}{F}_2+\mathrm{LT}& \left(1\right)\end{array}$
where F.sub.1 and F.sub.2 are constants for a given blast furnace at each measurement height:

(25) $\begin{array}{cc}{F}_1=\cos h\left(x\right)\cos \left(x\right)& \left(2\right)\\ F_2=\cos h\left(x\right)\sin \left(x\right)+\sin h\left(x\right)\cos \left(x\right)\mathrm{and}& \left(3\right)\\ ={\left[\frac{3\left(1-v^2\right)}{R^2{t}^2}\right]}^{1\text{/}4}& \left(4\right)\end{array}$
where y is the horizontal deflection or displacement measured, x is the vertical position, R is the mean radius at each height, t is the thickness of the wall at each height and is Poisson's ratio.

(26) The other terms depend on the pressure applied to the wall of the blast furnace: y.sub.0 is the deflection or displacement at the level of the nozzles; is the angle formed by the strained wall at the level of the nozzles; LT is the load term which is proportional to the force applied over the structure.

(27) A final transformation is performed to take into account the fact that the crucible of the blast furnace is constructed with materials having different mechanical properties (refractory and steel shell) and with various thicknesses differing as a function of the height (E.sub.x=Young's modulus of material x):

(28) $\begin{array}{cc}{P}_{\mathrm{across}\mathrm{steel}\mathrm{shell}}=P_{\mathrm{int}.\mathrm{across}\mathrm{refractory}}=\frac{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{Steel}\mathrm{shell}}}{E_{\mathrm{steel}}{\mathrm{Thickness}}_{\mathrm{Steel}\mathrm{shell}}+E_{\mathrm{refract}.}{\mathrm{Thickness}}_{\mathrm{refract}.}};& \left(5\right)\end{array}$

(29) Finally, the relationship between static pressure P and level of liquid metal or slag h is given by a relation of the type P=.Math.g.Math.h, where is the average density and g is the acceleration due to gravity.

(30) The role of the stress or strain gauges is to translate the strain undergone by a test body (here, a substantially cylindrical body) into variation in electrical resistance. More precisely, the variation in electrical resistance of the gauge is proportional to its strain (piezo resistor). It is the coefficient or gauge factor k which translates this proportionality, according to the following relationship:
R/R=k.Math.L/L,
where k is a constant that depends on the materials considered and on the temperature. It characterises the sensitivity of the gauge.

(31) Each gauge consists of a set of closely spaced turns of resistance wire, made from a thin metal sheet bonded onto a flexible and insulating support, obtained by means of photogravure according to the technique used for printed circuits.

(32) The strain gauge may be made of different materials: alloy steels, stainless steels, aluminum, semiconductors, etc.

(33) The different types of strain gauge sensors and their mounting are indeed well known by the person skilled in the art and are not limiting the scope of the present invention.

(34) The thermal sensors according to the invention are preferably platinum-resistance thermometers, and more preferably Pt100 thermometers (resistance of 100 ohms at 0 C. and 138.5 ohms at 100 C.)

(35) The device of the invention presents the following advantages. Its installation is simple and entails modest costs. The device allows for easy and rapid installation and possible replacement given that it is placed on the external wall of the blast furnace, which does not get heated to high temperatures.

(36) The measurement system yields good results on crucibles that are cooled both by means of open-air sprayers (spray cooling) as well as by water-channel circuits (channel cooling). However, during implementation, for spray cooling, the cooling of the sector where the gauges are installed must be temporarily interrupted, which involves the temporary shutdown of the blast furnace. This problem does not exist for blast furnaces having systems of the channel cooling type, if the measuring device is installed, for example, in the space between two channels, and the installation can be carried out during the normal operation of the blast furnace.

(37) The sensors do not necessarily have to be spaced at equidistance over a generatrix. If the density of sensors is greater near the tap hole, a far greater precision in calculation is obtained.

Key

(38) 1. blast-furnace crucible 2. wall of the crucible 3. refractories 4. steel shielding 5. tap hole 6. strain gauge sensor 7. thermal sensor 8. wiring 9. cooling channels 10. strain gauge 11. cover 12. bolts 13. surface level of cast iron 14. surface level of slag