WO2013181675A1

WO2013181675A1 - Arc furnace electrode operation

Info

Publication number: WO2013181675A1
Application number: PCT/ZA2013/000039
Authority: WO
Inventors: Paul James BRERETON-STILES; Daniel Jacobus OOSTHUIZEN; Christopher James HOCKADAY
Original assignee: Mintek
Priority date: 2012-05-31
Filing date: 2013-05-30
Publication date: 2013-12-05

Abstract

In a submerged arc furnace deviation of a current path is detected by identifying a variation of a parameter of a magnetic field generated by current flow in the furnace.

Description

ARC FURNACE ELECTRODE OPERATION

BACKGROUND OF THE INVENTION

[0001] This invention relates to the operation of a submerged arc electric furnace. The invention is described hereinafter with reference to the operation of a three phase arc furnace but it is to be understood that this is exemplary only and that the principles of the invention can be applied to a furnace which uses a different number of electrodes or to a furnace which makes use of direct current (DC) instead of alternating current (AC).

[0002] Figure 1 illustrates aspects of a typical submerged arc furnace 10. In the furnace a bath 2 of molten metal is overlaid by a slag 14 and by a substantially solid mixture 16 of feed materials which are colder than the molten metal and which are fed into the furnace from above through a plurality of feed apertures 18 formed in a roof 20.

[0003] Electrodes 22, 24 etc. extend into the furnace through a roof 20. Each electrode usually penetrates the mixture 16 to a point at which a tip 22A, 24A etc. of the electrode is above an upper surface 12A of the molten metal 12.

[0004] In order to operate the furnace efficiently it is important that electrical power is delivered by the electrodes to the furnace charge mixture in an optimal manner. In this way a target power density is produced within the charge and an appropriate temperature is reached at a correct location or locations within the charge so as to drive smelting reactions, and _sto minimise energy losses. [0005] The distribution of the power delivered by the electrodes is determined by paths taken by currents which flow from the electrodes and by the resistivity of the material through which the currents flow. The current paths, from one electrode to another, are dependent at least on the positions of the electrode tips (22A, 24A etc.) within the furnace charge and on the resistivity of the furnace charge.

[0006] It is desirable to blend and feed a homogeneous mixture of materials into the furnace in an attempt to maintain a uniform chemical composition and resistivity. This objective is however difficult to achieve. The feed materials can segregate as they fall into the furnace due to differing physical properties of the feed materials such as density and particle size. Additionally, conditions within the furnace can be turbulent due to gas evolution from reaction zones and extreme temperature variations. As a consequence of these factors the feed materials are not normally uniformly mixed and this, coupled with temperature variations within the furnace charge, affect the resistivity of the material and hence the preferred current paths.

[0007] In order to achieve a desired power density and optimal energy efficiency a current path should be from a tip of an electrode, into and then across the metal bath and then up to a tip of an adjacent electrode. If the electrodes are not correctly positioned or if the resistivity of the burden is inappropriate the current can take alternative paths through the charge mixture and this results in inefficiencies. For this reason at least it is important in the operation of a submerged arc furnace to maintain the electrodes at adequate lengths so that the electrodes can penetrate, to a desired extent, into the charge whereby the electrode tips are then correctly positioned. [0008] Erosion and breakage of the electrodes and inconsistent slippage of the electrodes into the furnace make it difficult however to achieve this objective. The electrodes are consumed at their tips, often at different rates, due to physical factors inside the furnace. Also it is not possible to observe the position of an electrode tip and the location of the surface of the melt due to the slag and furnace charge mixture which floats on the molten metal. If an electrode is at a location which is determined by a limiting point of movement of its holder 26 it is difficult for a furnace operator to determine whether this is due to an incorrect electrode length, incorrect furnace charge material resistivity, or an abnormal metal bath position.

[0009] Knowledge of a dominant current path taken from each electrode may allow for more effective furnace operation as a technician might be able to identify problematic situations, such as direct inter-electrode short-circuiting of the current, and metal bath build-up and, if so, then to take corrective action such as adjusting the position of the electrode, stoking the charge mixture, or altering the composition of the feed material recipe. Furthermore, accurate information on the positions of the electrode tips and on the location of the surface of the metal bath can be used for the direct control of electrode penetration to a set point to ensure efficient and stable furnace operation, and the accurate control of electrode slippage to ensure adequate electrode length is maintained.

[0010] Different attempts have been made to address the aforementioned factors. In one approach the length of each electrode is inferred from hydraulic pressure measurements taken on electrode support structures. A difficulty with this technique, however, is that a customised electrode support installation is required which, in many instances, cannot be retrofitted. Additionally, the hydraulic pressure can be affected by factors such as the addition of paste blocks to the electrodes, required to make up for electrode consumption, and horizontal forces acting on electrodes. Also, the correct electrode length and tip position alone do not guarantee the correct current conduction path. Undesirable feed mixture conditions within the furnace can still result in direct inter-electrode conduction higher up in the furnace resulting in inefficiencies.

[0011] The length of an electrode can be measured physically when a furnace is switched off and the burden has been burnt down. This process, in its nature, is disruptive to efficient furnace operation and, from a practical point of view, cannot be performed regularly.

[0012] Another approach used for estimating electrode penetration is to measure current in the electrode to determine the resistance of a conductive path below the electrode tip. However these measurements do not enable a distinction to be made between different conductive paths e.g. between inter-electrode conductive paths versus a conductive path to the bath, nor to distinguish situations in which, say, a short electrode has substantial material build-up of low resistivity below it, and a situation in which a long electrode has negligible material build-up of high resistivity below it.

[0013] It is possible to some extent to make a determination of the electrode length by measuring the extent of electrode slip and by estimating electrode consumption. This approach however is reasonably accurate only over a short term and depends on other measurements to provide corrective factors for compensating for drift in slippage and electrode consumption. [0014] Attempts to determine conductive paths in furnaces are limited to simulations assuming defined charge distributions, dig-outs of solidified furnaces and photography of arcs in open-arc furnaces.

[0015] An object of the present invention is to address at least, to some extent, the aforementioned situation.

SUMMARY OF THE INVENTION

[0016] The invention provides a method of detecting a deviation in a dominant path of current delivered by each electrode in a submerged arc furnace which includes the steps of obtaining a measurement at each of a plurality of locations of at least one parameter of a magnetic field generated, at least, by current flow in a conductive path in the furnace and using at least one variation of a defined type in the parameter to define the deviation.

[0017] The parameter may be selected from any attribute of the electromagnetic field which is variable in response to a change in the current path and the position at which the measurement of the parameter is obtained. For example, the parameter may be selected from the magnitude, gradient and phase of the electromagnetic field. Two or three of the parameters may be measured or calculated.

[0018] The conductive path preferably comprises the electrode in a vertical sense from an upper point, e.g. an upper end of the electrode, at least within the furnace, to the electrode tip, and then downwardly into the metal bath.

[0019] In one form of the invention the parameter is measured at each of a plurality of vertically displaced locations which are positioned generally parallel to but displaced from the electrode. Use may be made of a plurality of sensors which are positioned at the respective aforementioned locations. In a different approach one or more sensors may be moved, in succession, to the different locations. The measurements may be taken discretely i.e. with one measurement at each location or on a continuous basis.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The invention is further described by way of example with reference to the accompanying drawings in which :

Figure 1 , referred to hereinbefore, illustrates aspects of the operation of a submerged arc furnace,

Figure 2 is a representation from one side and in cross section of part of an electric arc furnace in which the method of the invention is employed illustrating possible current flow paths to electrodes,

Figure 3 is a simplified plan view, from above, of the furnace, and

Figure 4 is a curve of a measurement adjacent a furnace as a function of furnace height i.e. in a direction which extends vertically downwards from an upper end of the furnace towards a lower end.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0021] In a submerged arc furnace each electrode enters the furnace, usually through a roof and, with the furnace in an operational stage, each electrode penetrates a mixture that may include a slag layer on an upper surface of a molten metal bath which is held in a lower region of the furnace. A lower end of the electrode penetrates through the mixture to a point at which a tip of the electrode is normally above the metal bath surface. [0022] The electrode carries a high magnitude electric current. In normal operation, the resistivity of the electrode is much lower than that of the mixture. Hence the current flows, to a substantial extent, vertically inside the electrode. The current establishes a magnetic field with a flux density (field strength) which is proportional to the magnitude of the current and inversely proportional to a radial distance from the electrode which is regarded as an elongate conductor. For a given electrode current the magnetic field which is generated around an electrode is substantially constant over a first region which is above the mixture and within a second region inside the mixture where the current remains within the electrode and is substantially constant.

[0023] At some point within the second region the dominant current flow path changes, as it leaves the confines of the electrode, in its direction, shape (distribution), or density, or a combination of these features. A measurement of the magnetic flux density at a location on a vertical line which is substantially parallel to the electrode and which is adjacent the point at which the current leaves the electrode, also referred to as an inflection point, will register a change in the strength or direction of the magnetic field. This is due to the relationship between the magnetic field strength measured at a particular location and its radial distance from the current flow path and the current flow density. This inflection point normally corresponds to the electrode tip position at which most of the current leaves the electrode and is distributed over an area which is larger than the cross-sectional area of the electrode. However under abnormal furnace charge composition conditions undesired direct inter-electrode conduction can occur and this is reflected in a substantial variation in the inflection point.

[0024] The inflection point in combination with other variables or variations, measured over time, can be used to identify different operating conditions. For example it is proposed to differentiate between an inflection point which is indicative of the electrode tip and an inflection point associated with inter-electrode conduction for, if an electrode is moved in a scenario where conduction from the tip dominates, the inflection point would move as the electrode moves, whereas, if an electrode is moved in a scenario where inter electrode conduction dominates, it is believed that movement of the inflection point would be limited, for the position of the inflection point would be determined mainly by the furnace charge composition and, to a lesser extent, by the position of the electrode.

[0025] In normal furnace operation, most of the current flow continues from the tip of the electrode to the liquid metal bath which has a lower resistivity than the mixture. Inside the metal bath the flow direction of the current changes from mostly vertical to mostly horizontal. This is due to current flowing between electrodes. The vertical current flow component gradually decreases and this results in a corresponding decrease in a measurement of the magnetic field at a defined location on a vertical line which is parallel to the electrode.

[0026] Figure 2 and Figure 3 are graphical depictions of the aforementioned normal operational process. Figure 2 illustrates in cross section and from one side a lower end of an electric arc furnace 30 in an operational mode. Lower ends 32 and 34 respectively of two electrodes 36 and 38 are shown immersed in a mixture 40 which floats on a surface 42 of a liquid metal bath 44 held inside the furnace.

[0027] The electrode 36 is short compared to the electrode 38. Thus its lower end 32 is relatively far from the surface 42 compared to the distance between the lower end 34 and the surface 42. It is this type of situation which the invention attempts to address. [0028] The directions of current flows within the electrodes, the mixture 40 and the melt 44 are indicated by means of dotted lines 50. Within the electrodes 36 and 38 the current flow paths are substantially vertical, and are confined to the cross section of each respective electrode. Directly below the electrode tips, within the mixture 40, the current flow paths are, generally, vertically aligned but, due to the capability of the mixture 40 to conduct throughout its cross sectional area, the current flow paths start spreading radially outwardly.

[0029] Within the melt 44, which has a low resistivity, current flows into the melt and then between electrodes.

[0030] The aforementioned process has been illustrated for two electrodes but similar effects arise in a furnace which has three or more electrodes wherein each electrode interacts with at least one other electrode. In a single electrode furnace with an anode or cathode hearth connection, similar effects to the multi-electrode furnace arise at a tip of the electrode.

[0031] Figure 3 illustrates, in plan, and on a reduced scale compared to Figure 2, the furnace 30 with three electrodes 36, 38 and 54 displaced symmetrically from one another. Measurement locations are established outside of the furnace along respective vertical lines 60, 62 and 64 which are associated, respectively, with the electrodes 36, 38 and 54 and which are radially displaced outwardly from the electrodes. The lines are notional in that their positions could vary due to a number of factors. Figure 2 illustrates the lines 60 and 62 which are associated respectively with the electrodes 36 and 38. In the ensuing description reference is made only to the measurement line 62. However a similar explanation applies to the electrode 36 and, for that matter, to the electrode 54 and the measurement line 64. [0032] The measurement line 62 is parallel to the electrode 38 and extends vertically. At each of a plurality of locations 70.1 , 70.2 ... 70. N, which are vertically displaced from each other along the length of the line 62, a measurement is taken of one or more of a designated set of electromagnetic parameters e.g. a magnetic flux density, and a phase, of a magnetic field which is established by the current which flows through the electrode 38 and thereafter in a conductive path associated, primarily, with this electrode established in the mixture 40 and the melt 44.

[0033] A respective sensor 72 can be positioned at each location 70.1 to 70. N. Alternatively, the single sensor can be moved automatically or manually between the locations 70.1 , 70.2 etc. and at each location a separate measurement can be taken. It is also possible to move the sensor continuously along the measurement line and to take a continuous measurement of the chosen electromagnetic field parameter or parameters. Measurements taken from several sensors at different locations may be analysed and combined to reduce the impact of noise and interference.

[0034] Figure 4 is a curve of measured magnetic flux density M as a function of vertical distance D on the vertical measurement line 62, going from an upper region of the furnace to a lower region. The magnetic flux density is substantially constant from an origin O up to a location TP which is directly radially displaced from the tip 34 of the electrode 38. When moving downwardly below the tip 34, it is believed that the current in the mixture 40 typically gradually flares outwardly resulting in a change in current density and effective radial distance from the respective sensor at the measurement location. The measured parameter could gradually increase from the position TP to a position B which is directly radially displaced from the surface 42 of the melt. With increasing distance downwards from the surface 42 the measured parameter would decrease substantially as the current flows through the melt towards the other electrodes i.e. as the current flow path changes in direction from vertical to generally horizontal.

[0035] Measurements similar to those shown in Figure 4 can be taken continuously or at regular time intervals using appropriate techniques for each of the electrodes. Significant inflection points in the resulting curves are indicative of physical changes in the conductive path along which the electrode current flows.

[0036] To some extent it is also possible to correlate the measurement of the inflection points with other physical events such as movement of an electrode or tapping of the furnace, to validate whether the inflections are caused by inter- electrode conduction, electrode tip position, or the metal bath position.

[0037] The method of the invention makes it possible to determine the dominant current path from each electrode submerged within a submerged arc furnace. This enables factors such as interior metal build-up, direct inter-electrode conduction between electrodes in the burden resulting from poor feed mixture composition, the location of the electrode tip position and other factors to be accounted for. This is not the case when the extent of electrode penetration into the mixture is to be inferred using existing techniques. Furthermore, methods used to infer electrode length are unable to detect direct inter-electrode conduction, which can cause catastrophic furnace failure if left undetected. The more accurate information which is made available by the method of the invention enables operation of the furnace to be controlled more accurately and helps to avoid inefficiencies and catastrophic events.

[0038] The invention has been described primarily with reference to the measurement of the magnetic flux density. However, as indicated variations in the magnetic field phase may also be used, optionally with the magnitude variations, to provide more accurate information of the dominant conduction path from each electrode. The sensors can be varied in position and need not be aligned vertically, for other configurations are possible.

Claims

1. A method of detecting a deviation in a dominant path of current delivered by each electrode in a submerged arc furnace which includes the steps of obtaining a measurement at each of a plurality of locations of at least one parameter of a magnetic field generated, at least, by current flow in a conductive path in the furnace and using at least one variation of a defined type in the parameter to define the deviation.

2. A method according to claim 1 wherein the parameter is an attribute of the electromagnetic field which is variable in response to a change in the current path and the position at which the measurement of the parameter is obtained.

3. A method according to claim 2 wherein the parameter is at least one of the following: the magnitude, gradient, and phase, of the electromagnetic field.

4. A method according to any one of claims 1 to 3 wherein the conductive path comprises the electrode in a vertical sense from an upper point of the electrode, at least within the furnace, to the electrode tip, and then downwardly into a metal bath.

5. A method according to any one of claims 1 to 4 wherein the at least one parameter is measured at each of a plurality of vertically displaced locations which are positioned generally parallel to but displaced from the electrode.

6. A method according to claim 5 wherein use is made of a plurality of sensors which are positioned at the respective locations.

7. A method according to any one of claims 1 to 4 wherein the parameter is measured at each of a plurality of vertically displaced locations, which are positioned generally parallel to but displaced from the electrode, by moving at least one sensor in succession, to the different locations.

8. A method according to claim 7 wherein a respective measurement is taken at each location.

9. A method according to claim 7 wherein the measurement is taken continually while the at least one sensor is being moved to the different locations.