Title of Invention

A PROCESS FOR INTENSIFICATION OF SCRAP MELTING IN AN ELECTRIC ARC FURNACE

Abstract

This invention relates to the process for intensification of scrap melting in an electric Arc furnace. The process for intensification of scrap melting in an electric arc furnace for producing molten metal comprising providing the scrap metal in the electric arc furnace., subjecting the scrap metal to heating by electric arcing in said furnace wherein the electric arc is shielded by slag/ scrap metal cover during melting of the molten metal using the said electric arc.

Full Text

The present invention relates to the process for intensification of scrap melting in an Electric Arc Furnace. In the presently known art of scrap melting in electric arc furnace the available cost effective scrap mix alongwith non-metallic charges like lime and iron ore is charged in a refractory lined EAF or in an EAF with provision of water cooled panels. The roof, after charging, is swung back in arcing position and arc is struck initially at low voltage and gradually increasing the voltage to maximum. Electrodes gradually bore down into the charge. Second and even third charge is made depending on scrap quality and capacity of the furnace whenever partial melting of the scrap makes room for subsequent charge. During melting, metal pool is formed just under electrodes and with passing of time standing scrap- column slowly collapses. Towards the end of melting (when 70-80% of scrap is molten) voltage is reduced so that long arc is avoided. After attaining all melt (about 1550-60°C bath temperature), the bath is completely flat and covered with slag. At this stage, metal sample and temperature is taken. The temperature is then raised to about 1600°C and refining process follows thereafter. During refining, since, only temperature of the bath needs to be maintained by arcing the voltage is reduced to minimum and flaring of arc becomes insignificant. Thus under the known art discussed above the electric arc furnace is charged with scrap, sponge iron, non-metallic fluxes like lime, iron ore etc. For one heat, in general, two bucket charging practice is employed as the density of scrap is low. After about 70% melting is over the electric arcs which were covered with scrap become exposed. The radiation emanating from the arcs lead to loss of energy and badly affects the furnace lining. The heat transfer to metal bath is also reduced. Traditionally, this phenomenon is minimized by reducing the arc voltage and thereby the arc length. Even then there is significant loss of energy. It is thus the basic objective of the present invention to provide a process for scrap melting in electric arc furnace to produce molten metal which would avoid the loss of expensive energy due to exposure of the electric arc as experienced in the known art and also improve heat transfer to the bath during arcing. Another objective of the present invention is directed to provide a process for scrap melting in electric arc furnace which would consume less power as compared to conventional processes and thereby provide for a more economical scrap melting in electric arc furnaces. Yet another objective of the present invention is to provide a process for scrap melting in electric arc furnace which would save melting time and hence will be cost effective. Yet further objective of the present invention is directed to provide a process for scrap melting in electric arc furnace which would not damage the side wall linings of the furnace and would thereby provide for increased life of the furnace wall lining. Yet further objective of the invention is directed to provide a process of scrap melting in electric arc furnace which would provide for engulfing the arc by slag and thereby also prevent pick-up of nitrogen by steel from the ionised atmosphere in the vicinity of the arcs. Thus according to the present invention there is provided a process for producing molten metal from scrap metal in an Electric Arc Furnace comprising : providing the scrap metal in the Electric Arc Furnace ; subjecting the scrap metal to heating by electrical arcing ; and shielding the electrical arc by slag cover during the melting process ; followed by refining of the melt following conventional techniques. In particular, in the process of the invention the step of shielding of the electrical arc comprise : constantly monitoring the slag level in said furnace and ensure the the electric arcs generated are covered by said slag ; and injecting coke breeze with air as carrier gas with or without oxygen blowing when about 70-80% of the scrap has melted with continued run of the furnace at maximum voltage thereby generate CO gas bubbles to swell up the slag for continued shielding of the electric arc during the melting process. The process of the invention is thus distinguished from the known art of scrap melting in electric arc furnaces in that the same provides for a slag cover for the arc even towards the end of melting in an electric arc furnace when about 70-80% of scrap has melted to thereby save substantial amount of energy loss. In particular, the process reduces power consumption in electric arc furnaces through generation of increased volume of slag even when 70-80% of slag is melted by developing a sustainable gas-slag emulsion. The swelled slag so generated is capable of protecting the arc from radiating out. The process developed involves injection of coke at the slag-metal interface at a controlled rate with or without simultaneous oxygen lancing depending on slag fluidity. This generates carbon monoxide gas at controlled rate by C-O reaction or C-FeO (slag) reaction under a slag condition conducive for entrapping the CO gas bubble resulting in foaming slag sufficient to cover the arc underneath. Parameters like point of coke injection, time of injection start, injection rate, number of injection stages, need for oxygen lancing, and conducive slag characteristics for sustained bubble formation were established for long and stable foaming slag capable of covering the arc leading to reduction in power consumption. Importantly, therefore the process of scrap melting of this invention provides for cover of the arc radiation during melting period by generating, a swelled up slag cover to engulf the arcs completely. Such step of generation of swelled up slag cover is directed to achieve the following advantages i) shielding of the arcs to improve heat transfer to bath for saving in power consumption ii) saving in melting time iii) protection of side wall lining, iv) prevent pick up of nitrogen by steel from the ionised atmosphere in the vicinity of the arcs. In accordance with a preferred aspect of the process of the invention, the step of shielding the electric arc comprise a first stage injection of coke breeze with air as carrier gas when 70-80% scrap has melted with or without simultaneous blowing of oxygen in the bath ; continuing the arcing at 1550 - 1560°C metal temperature upto melt down under constant monitoring of the melt sample and temperature ; a second stage injection of coke which is continued until the bath temperature of 1600°C is reached to thereby ensure engulfing of the arc by slag during the melting process. The most preferred operating conditions for achieving proper shielding of the electric arc following the process of the invention are identified as follows : Coke Size : is less than 3 mm Carrier gas = Compressed air No. of phases of injection = 2 In the process of this invention raw materials and equipment include : a) Dry coke breeze of less than 3 mm size. b) Utilization of oxygen blowing facility (regularly used for decarburisation during refining period), if required. c) Dispenser unit for injecting coke with air as carrier gas. Preferably, the steps of shielding the arc in the process of the invention comprise : start of injection of coke breeze (about 3 kg/t, 15-25 kg/min, less than less than 3 mm size) with air as carrier gas when 70-80% scrap has melted and a slag-metal pool is available to introduce the injection lance at slag/metal interface with the furnace continued to run at maximum voltage throughout; simultaneous blowing of oxygen in the bath only if slag is found to be viscous wherebv the carbon of coke reacts with oxygen of slag FeO or directly blown oxygen to form CO gas bubbles which are entrapped in viscous slag and provide for swelling up of the slag. continuing arcing in that temperature range of 1550-1560°C upto melt down and taking all melt sample and temperature in the process ; second stage injection of coke of about 3 kg/t, similar to first stage injection, which is continued till bath temperature of upto about 1600°C is reached and finally, subjecting the melt to refining as per conventional techniques. In the above process of the invention oxygen is required for generation of CO gas by carbon-oxygen reaction. If the initial slag has a high FeO content (in excess of 18%), the slag, inspite of being highly fluid, provides enough oxygen, as FeO for. CO generation. CO gas generated by reaction of injected oxygen and carbon escapes through high FeO fluid slag. On the other hand, if FeO is utilized as a source of oxygen for generation of CO gas, the slag fluidity decreases due to the lowering of FeO content and also the slag temperature (reaction of C with FeO is endothermic). This provides for the swelling up of the slag. Alternately, simultaneous oxygen injection may be used when initial slag FeO is less than 18% and the slag is not sufficiently fluid. This depends on visual judgement of slag fluidity. Thus following the process of the invention an initial slag with high FeO content (>18%) arising from usual charge mix may be made foamy simply by coke injection and without supplementary gaseous oxygen. With visual judgement of slag viscosity, supplementary oxygen may be used occasionally. The details of the invention, its objectives and advantages are explained hereunder in greater detail with reference to non-limiting exemplary embodiments of a conventional process of metal scrap melting and the process of this invention in relation to accompanying figure 1. EXAMPLES For the purpose identical batches of scrap were melted in EAF following conventional technique and that following the process of the present invention. The conventional process of scrap melting followed is detailed under Example 1 while the process of scrap melting in accordance with the present invention is detailed under Example 2 hereunder. EXAMPLE 1 (Conventional Process): i) Charging the EAF with a) Scrap : 106kg/100 kg liquid steel output in two batches. (Scrap may be partially replaced by Sponge Iron) b) Iron Ore : 20 kg/t, Lime : 30 kg/t ii) Melting the charge in (i) above with highest voltage. iii) Reducing the voltage after about 80% completion of melting. vi) Melting down further at about 1550°C and v) Continuing heating further to reach 1600°C. EXAMPLE 2 (Process of the invention): i) Charging the EAF with a) Scrap : 106 kg/100 kg. Liquid steel output in two batches. (Scrap may be partially replaced by Sponge Iron) b) Iron ore : 20 kg/t, Lime : 30 kg/t ii) Melting the charge in (i) above with highest voltage. iii) After about 70% melting introducing coke injection lance at slag/metal interface carrying out first stage coke injection by injecting 3 kg/t coke breeze of less than 3 mm size. vi) Continuing melting with highest voltage operation. v) Melting down further at about 1550°C vi) Carrying out second stage coke injection by Injecting coke 3 kg/t of less than 3 mm size until attainment of 1600°C when the injection is ended. Initial slag characteristics were found to be identical in both conventional process and the process of invention as the charge materials including slag forming non-metallics remained unchanged. Nominal slag characteristics in both the cases were identified as follows : Basicity = 1.4-2.5 FeO = 12 - 30% The energy consumed in the respective processes under Examples 1 and 2 were also monitored and detailed hereunder: These values are based on a 60t heat size. The above TABLE 1 clearly demonstrate the achievement of lower power consumption following the process of scrap melting in EAF in accordance with the present invention vis-a-vis the conventional process of scrap melting in EAF. Importantly, the reduction in power consumption is by about 7% in Electric Arc Furnace following the process of this invention. The process of this invention therefore provides for the following advantages : i) Use of slag FeO as a source of oxygen for in situ generation of controlled carbon monoxide gas bubble through controlled injection of coke in order to swell the ii) Establishes proper time of coke injection and two stage coke injection under plant condition to derive maximum benefit of sustainable arc engulfing foaming slag. iii) Establishes slag characteristics under plant operating condition in order to generate controlled amount of sustainable foaming slag vis-a-vis controlled carbon monoxide gas generation. iv) Two stage coke injection to generate foaming slag twice has the added advantage of using the intervening time for necessary metal sample collection avoiding delay in operation. The process of the invention will therefore provide for power saving of about 7-8% and will be useful for EAF industries for saving electrical energy and therefore render the process of scrap melting in EAF more economical and cost effective. We claim: 1 A process for intensification of scrap melting in an electric arc furnace for producing molten metal comprising: providing the ferrous metal scrap in the electric arc furnace; subjecting the ferrous metal scrap to heating by electrical arcing in said furnace wherein the electrical arc is shielded by slag/ferrous metal scrap cover during melting of the molten metal using the said electric arc. 2. A process as claimed in claim 1, wherein said step of shielding the arc by slag cover comprise: constantly monitoring the slag/metal level in said furnace ; and injecting coke breeze with air as carrier gas with or without oxygen blowing when about 70-80% of the scrap has melted with continued run of the furnace at maximum voltage thereby generating CO gas bubbles to swell up the slag which provide for shielding of the electric arc. 3. A process as claimed in claim 2 wherein the said coke injection is carried out by means of injection lance positioned at the slag/metal interface. 4. A process as claimed in anyone of claims 2 or 3 wherein the coke injected comprise injection of coke breeze of less than 3 mm size. 5. A process as claimed in anyone of claims 2 to 4 wherein said steps of coke injection comprise at least two stage coke injection comprising a first injection of coke breeze with air as carrier gas when 70-80% scrap has melted with or without simultaneous blowing of oxygen in the bath. continuing the arcing at 1550 - 1560°C metal temperature upto melt down under constant monitoring of the melt sample and temperature ; followed by a second stage injection of coke, which is continued until the bath temperature of 1600°C is reached. 6. A process as claimed in anyone of claims 1 to 5 wherein the slag characteristics comprise basicity in the range of 1.4 to 2.5 and FeO in the range of 12-30%. 7. A process as claimed in anyone of claims 1 to 6 wherein said oxygen blowing is carried out when the slag has an FeO content of less than 18% and slag is not sufficiently fluid. 8. A process as claimed in anyone of claims 1 to 7 wherein said oxygen blowing is carried out based upon the FeO content of the slag and visual observation of fluidity of the slag. 9. A process as claimed in anyone of claims 1. to 8 wherein the time for said coke injection is based upon constant visual observation of the level of the slag and the electricity consumption during the process. 10. A process as claimed in anyone of claims 1 to 9 wherein high voltage operation of the furnace is continued throughout melting the process. 11. A process as claimed in anyone of claims 1 to 10 wherein the source of oxygen is selected depending upon the slag fluidity. 12. A process as claimed in anyone of claims 1 to 11 wherein the initial slag is selected by controlled mix of metallic and non-metallic charge to maintain desired FeO levels in the slag. 13. A process as claimed in anyone of claims 1 to 12 wherein the slag viscosity is controlled. 14. A process as claimed in claim 13 wherein the slag viscosity is controlled by the use of burnt dolomite in charge mix or over the fluid slag. 15. A process as claimed in anyone of claims 5 to 14 wherein during said coke injection in the second stage injection coke is injected in amount of 3 kg/t of steel at the rate of 15-25kg/min. 16. A process for intensification of scrap melting in any electric arc furnace substantially as hereindescribed and illustrated with reference to the accompanying examples and figures. This invention relates to the process for intensification of scrap melting in an electric Arc furnace. The process for intensification of scrap melting in an electric arc furnace for producing molten metal comprising providing the scrap metal in the electric arc furnace., subjecting the scrap metal to heating by electric arcing in said furnace wherein the electric arc is shielded by slag/ scrap metal cover during melting of the molten metal using the said electric arc.

Documents:

412-cal-1999-granted-abstract.pdf

412-cal-1999-granted-assignment.pdf

412-cal-1999-granted-claims.pdf

412-cal-1999-granted-correspondence.pdf

412-cal-1999-granted-description (complete).pdf

412-cal-1999-granted-drawings.pdf

412-cal-1999-granted-examination report.pdf

412-cal-1999-granted-form 1.pdf

412-cal-1999-granted-form 18.pdf

412-cal-1999-granted-form 2.pdf

412-cal-1999-granted-form 3.pdf

412-cal-1999-granted-form 6.pdf

412-cal-1999-granted-pa.pdf

412-cal-1999-granted-reply to examination report.pdf

412-cal-1999-granted-specification.pdf