Patent Publication Number: US-2009232181-A1

Title: Systems and methods for controlling the electrode position in an arc furnace

Description:
BACKGROUND 
     Embodiments of the present invention are generally directed to systems utilizing arc furnaces in steel production, and are specifically directed to the control systems used to control the electrode position inside the arc furnace. 
     SUMMARY 
     Electric arc furnaces (e.g. DC arc furnaces and AC arc furnaces) are widely used devices in the production of steel. As would be familiar to one of ordinary skill in the art, direct current (DC) arc furnaces utilize a single electrode, which generate an electric arc to heat the material therein. The present inventors have recognized the importance of positioning the lower tip of the electrode below the upper surface of the slag, because that ensures that the electric arc is submerged inside the slag. Properly submerging the electric arc maximizes the power that the electric arc can deliver without damaging the furnace walls. As a result, the present invention is directed to a control system configured to control the position of the lower tip of the electrode to maximize the power of the electric arc without damaging the furnace. 
     According to one embodiment, a system for controlling the position of an electrode in an arc furnace is provided. The system comprises an arc furnace comprising a molten bath and slag disposed over the molten bath, wherein the slag contacts the molten bath at an interface. The system also comprises an electrode having a lower tip configured to be disposed below the upper surface of the slag. The current forms an electric arc between the upper surface of the slag and the interface. The system also comprises a control system configured to determine the position of the lower tip of the electrode relative to the upper surface of the slag based on harmonic frequencies associated with the current, wherein the lower tip position in relation to the slag surface correlates to the harmonic frequencies. 
     According to further embodiments, the control system may comprise a sensor configured to detect the current and output harmonic frequencies corresponding to the current, and a filtering device in communication with the sensor and configured to output only the harmonic frequencies which fall within a predefined filter range. The control system may also comprise a digital processor in communication with the filtering device configured to determine the position of the lower tip of the electrode relative to the upper surface of the slag based on harmonic frequencies associated with the current. 
     According to yet another embodiment, a method for controlling position of an electrode lower tip in an arc furnace is provided. The method comprises the steps of positioning the electrode tip below the upper surface of the slag, forming an electric arc directed by delivering current via the electrode to the upper surface of the slag, determining the position of the electrode lower tip relative to the upper surface of the slag by measuring harmonic frequencies associated with the current, wherein the position of the electrode lower tip relative to the upper surface of the slag correlates to the harmonic frequencies, and repositioning the lower tip of the electrode when the lower tip of the electrode is disposed above the upper surface of the slag or when a portion of the electrode in addition to the lower tip is submerged below the upper surface of the slag. 
     These and additional objects and advantages provided by the embodiments of the present invention will be more fully understood in view of the following detailed description, in conjunction with the drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description of specific embodiments of the present invention can be best understood when read in conjunction with the drawings enclosed herewith. The drawing sheets include: 
         FIG. 1  is a schematic view illustrating an arc furnace wherein the electrode lower tip is disposed below the upper surface of the slag according to one or more embodiments of the present invention; and 
         FIG. 2  is a schematic view illustrating an arc furnace wherein the electrode lower tip is disposed above the upper surface of the slag; and 
         FIG. 3  is a schematic view illustrating an arc furnace wherein the electrode lower tip is submerged below the upper surface of the slag. 
     
    
    
     The embodiments set forth in the drawings are illustrative in nature and not intended to be limiting of the invention defined by the claims. Moreover, individual features of the drawings and the invention will be more fully apparent and understood in view of the detailed description. 
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an embodiment of a system  1  for controlling the position of an electrode  120  in an arc furnace  100  is illustrated. As shown, the arc furnace  100  is a reactor unit comprising materials (e.g., refractory brick) which can withstand high operation temperatures (e.g., temperatures well above 3000° F.). Although the illustrated embodiments of the figures depict DC furnaces, the arc furnace  100  may be a DC furnace or an AC arc furnace. Referring to  FIG. 1 , the DC arc furnace  100  comprises an electrode  120  disposed within the roof of the furnace  100 . The electrode  120 , which may comprise a consumable or a non-consumable electrode material, is coupled to any suitable power source familiar to one of ordinary skill in the art. Referring to the embodiment of  FIG. 1 , the power source may be a rectifier  124 . As shown in  FIG. 1 , the current used to produce the electric arc  180  is delivered through the electrode  120 , and the current is returned through a bottom electrode  122 . In the  FIG. 1  embodiment, the bottom electrode  122 , which may comprise conductive rods, is positioned in the lower base of the furnace  100 ; however, other suitable locations for the bottom electrode  122  are contemplated herein. 
     During steelmaking, the arc furnace  100  comprises a molten bath  130  and slag  140  disposed over the molten bath  130 . As shown, the slag  140  contacts the molten bath at an interface  135 . In one embodiment, the molten bath  130  may comprise a molten metal such as liquid steel. As would be familiar to one of ordinary skill in the art, slag is the impurity produced when a metal component (ore or scrap) is smelted at high temperatures to improve the purity of the metal component. Despite being an impurity which is eventually removed, the slag may be used to contain the arc power required during steel production. The amount of slag may be altered via chemical additives, etc. 
     As shown in  FIG. 1 , the electrode  120  comprises a lower tip  121  which is disposed below the upper surface  145  of the slag  140 . By disposing the lower tip  121  just below the upper surface  145  of the slag  140 , an electric arc  180  is created which extends from the upper surface  145  of the slag  140  to the interface  135  of the slag  140  and the molten bath  130 . Consequently, in the embodiment of  FIG. 1 , the distance from the lower tip  121  of the electrode  120  to the interface  135  is the arc length. By ensuring the electric arc  180  is disposed inside the slag  140 , the heat transfer of the electric arc is substantially maximized. 
     Referring again to  FIG. 1 , when the lower tip  121  is positioned just below the upper surface  145  of the slag forming material  140 , the height of the slag and the arc length are substantially the same. Because the voltage required to produce the arc (V arc ) is known at any given point, maintaining the lower tip  121  just below the upper surface  145  of the slag  140  enables simple computation of the slag height. The arc length (L arc ) is defined by the equation L arc =k*V arc , wherein k is a constant and V arc  is known. Since the slag height and the arc length are substantially equal when the lower tip  121  is positioned just below the upper surface  145  of the slag  140 , calculating the arc length (L arc ) via the above equation will also yield the slag height. 
     If the electrode lower tip  121  is not positioned just below the upper surface  145  of the slag  140  as in  FIG. 1 , the arc furnace  100  is operating in a non-optimal inefficient manner.  FIGS. 2 and 3  illustrate two scenarios where an arc furnace is running non-optimally. Referring to FIG.  2 , the lower tip  121  of the electrode  120  is positioned above the upper surface  145  of the slag  140 . As a result, the electric arc  181 , which begins at the lower tip  121  of the electrode  120 , is not just delivered to the molten bath  130 . Plasma  105  from the electric arc  181  is also delivered to the walls  110  of the arc furnace  100 , thereby wasting a portion of the heat capacity of the electric arc  181 . Additionally, the plasma  105  also may prematurely degrade the walls  110  of the arc furnace  100 . 
     Referring to  FIG. 3 , more than just the lower tip  121  of the electrode  120  is submerged below the upper surface  145  of the slag  140 . This is non-optimal because the electric arc  182  is not operating at full power. The power of the electric arc (P arc ) is defined by the following equation: P arc =V arc *I=k*L arc *I, wherein I is the current. Utilizing the power equation, increasing the arc length (L arc ) while maintaining the same current increases the power (P arc ) produced by the electric arc. If more than the lower tip  121  of the electrode  120  is submerged below the upper surface  145  of the slag  140 , the arc length (L arc ) is decreased, and thus the power produced by the electric arc  182  is also decreased. By disposing the lower tip  121  of the electrode  120  just below the upper surface  145  of the slag  140  as shown in  FIG. 1 , the arc furnace maximizes arc length and power, without damaging or prematurely degrading the furnace walls  110 . 
     Referring to  FIG. 1 , the system  1  utilizes a control system  150  configured to determine the position of the lower tip  121  of the electrode  120  relative to the upper surface  145  of the slag  140 , and ensure the lower tip  121  is properly positioned. The control system  150  detects harmonic frequencies associated with the current and compares the harmonic frequencies to a predefined filter range. Multiple predefined filter ranges are contemplated based on the processing conditions. The present inventor has recognized via experimentation and mathematical analysis (e.g. Fourier analysis) that a range of about 100 to about 140 Hz is a suitable predefined filter range. If the harmonic frequency value falls within the predefined filter range, the control system  150  knows that the lower tip  121  of the electrode  120  needs to be repositioned. After detecting the position of the lower tip  121  of the electrode  120  relative to the slag interface  145 , the control system  150 , in further embodiments, is operable to instruct that the electrode should be repositioned. 
     The control system  150  comprises multiple components familiar to one of ordinary skill in the art. Referring to  FIG. 1 , the control system  150  may comprise a sensor  152  (e.g. a magnetic field sensor) configured to detect the current. In one embodiment, the magnetic field sensor  152  is a Hall magnetic field sensor. As shown in the embodiment of  FIG. 1 , the magnetic field sensor  152  detects the electric current and outputs harmonic frequencies corresponding to the current. The magnetic field sensor  152  outputs these harmonic frequencies to a filtering device  154 . Although the control system of  FIG. 1  utilizes an analog band pass filter or a digital band pass filter for its filtering device  154 , many other suitable filtering devices  154  are contemplated herein. Optionally, the control system  150  may comprise a rectifier  155  downstream of the filtering device  154 . The rectifier  155  is operable to convert an AC signal from the filtering device to a DC signal. The filtering device  154  only outputs harmonic frequencies which fall within a predefined filter range (e.g., about 100 to about 140 Hz), and purges any values not within that range. As stated above the level of harmonic frequencies within the predefined filter range may indicate the relative position of the lower electrode tip  120  in relation to the slag surface  145 . For example, the harmonic frequencies may indicate that the lower tip  121  of the electrode  120  is above the upper surface  145  of the slag  140  as shown in  FIG. 2 , or that a portion of the electrode  120  in addition to the lower tip  121  is submerged below the upper surface  145  of the slag  140  as shown in  FIG. 3 . 
     Referring again to  FIG. 1 , the outputted harmonic frequencies from the filtering device  154 , are then delivered to a digital processor  156 . The digital processor  156  may comprise a programmable logic controller, a peripheral interface controller, a microprocessor, or another suitable device familiar to one of ordinary skill in the art. The digital processor  156  is configured to receive a signal proportional to the amplitude of the filtered harmonic frequencies, and transforms this signal into a new signal or function that indicates whether the electrode needs to be raised or lowered. In further embodiments, it is contemplated that the digital processor  156  direct communicates with the magnetic field sensor  152 , and is operable to perform the functions of the filtering device  154  without utilizing a rectifier  155 . 
     Referring to  FIG. 1 , the digital processor  156  is also configured to reposition the lower tip  121  of the electrode  120 . In one embodiment, the digital processor  156  is configured to provide information to an operator interface  160 . As used herein, the “operator interface”  160  refers to a suitable user interface or screen operable to display information to a user or operator regarding the position of the lower tip  121  of the electrode  120  in relation to the upper surface of the slag. The operator or the digital processor may raise or lower the electrode  120  using a suitable electrode repositioning device  126 . Referring to the embodiment of  FIG. 1 , the electrode positioning device  126  comprises an arm configured to raise or lower the electrode  120 , for example, an arm fixed to a hydraulic mast configured to be lifted or lowered through a hydraulic column controlled with hydraulic valves. The electrode positioning device  126  may be actuated by the operator, or may be actuated automatically by instruction provided by the digital processor  156 . In an alternative embodiment, the operator may increase or decrease the carbon injection rates to raise or lower the height of the slag in order to ensure the lower tip  121  is disposed below the upper surface  145  of the slag  140 . 
     Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.