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Vol. 1, No. 3, 1985 Publishedby the Center tor Production

Electric Arc The Energy Efficient Way to Melt Introduction Electric 'arc produce a the balance producedby basic gen furnaces use a metallic charge large and increasing portion of the furnaces, 57%, and open consisting of 60 to 70% blast fur- raw steel produced in the U.S. In hearth furnaces,9%. nace liquid , and 30 to 40% 1984, approximately 34% of the There are significant differ- . Electric furnaces use essen- total of 92 million of steel was ences between these steelmaking tially a 100% scrap steel charge. made in furnaces with processes. For example, basic oxy- Even though liquid iron(hot ) is used in both the basic oxygen and open hearth furnaces, the overall energy required to pro- duce steel by those methodsis considerably greater than with elec- tric furnaces using a solid scrap charge. This is because of the larger quantity of energy requiredin the process to reduce iron to liquid iron. The electrical energy required to ,make1 of liquid steelin an electric furnace at 500 kwh or 1.7 million Btu is only a small fractionof the 16 to 20 million Btu required using the blast furnace/basic oxygen furnace route. The blast furnaceis provided with energyin the form of which is mixed with pellets/ sinter and limestoneto constitute a furnace charge or burden.liquid iron, containing approximately4% dissolved is tapped from the bottomof the furnace into hot- metal and transferred to steel- making furnaces. Steel is an iron-based containing some , vary- ing amountsof carbon (generally less than 0.5%) and other alloying Continued on page 2 l Introduction the injection of oxygen and the or molds. Continuous cast- Continued from page 1 addition of fluxes. On the other ing produce semi-finished hand, the primary function of the sections, i.e., billets, blooms and elements. Thus, the primary func- is to serve as slabs. require an additional tion of the basic oxygen or open a scrap melter. Frequently, steel is operation to produce the hearth furnace is to refine the further refined in a ladle metal- semi-finished shapes. Final steel liquid iron by reducing carbon to a lurgy unit to improve cleanliness products, strip, sheets, bars, rods, prescribed level, as as remov- and provide additional tempera- plates, tubes, etc., are manufac- ing impurities such as , suitur ture and/or composition control. tured from the semi-finished sec- and . In a basic oxy- Liquid steel is solidified in tions using a variety of rolling mills gen furnace, this is accomplished by either continuous machines and processes.

The Electric ArcFurnace

The first commercial electric arc furnace in the U.S., a 4-ton unit, was placed in operation in 1906 by the Holcomb See1Co. at Syracuse, N.Y Production increased signifi- cantly during World WarII, and again after 1960 with the advent of mini steel mills. It increased stead- ily from 8.4 million tons in 1960 to a record of 34.1 million tons in 1981, an increase of more than 300%. The proportion of steel made in electric furnaces has continued to increase from 28%in 1981 to 34% in 1984. Production within the next ten years is expected to reach 40 million tons, representing 40to 42% of total steel production. The rapid growthin electric furnace steel is dueto a number of factors including relatively low investment costs, improved tech- nology which has reduced produc- r This cutaway drawing shows an electric furnace with cahn electrodes attachedto support arms and elactrical cables. Molten steel and the rodter mounting on which the furnace maybe tilted is also shown

lower-priceof steel scrap in com- specialty and mini mill producers. parison with blast furnace hot metal. Electric furnacesalso are used Investment per annual ton (1 982 in operations with their use dollars) is approximately $235 for doubling since 1957. In general, coke ovenhlast fumacehasicoxy- furnace capacities here are smaller, gen furnace production vs.$77 for one to50 tons, than in basic steel electric furnaces.’’ operations. Steel are the Approximately 300 electric arc principal users of electric arc fur- naces. There are approximately 350 ! saap Steel furnaces, rangingin capacity from less than 10to 400 tons, have been foundries with electric arc furnaces tion costs, and the appreciably installed in theU.S. by integrated, in the United States. Blast FurnaceIBasic Oxygen Steelmaking

Blast Furnace Electric Furnace Steelmaking

L Scrap Rec

Electric arc furnaces consist of a -lined hearth, vertical cylindrical sidewalls and a remov- able . Three electrodes, which pass through holesin the roof, are clamped to arms which move vertically on masts mounted to the furnace assembly. The elec- trodes and roof can be raised and swung to one sideto permit furnace charging. In conventionalfumaccs, a horizontal tapping spout is built into the hearth structure with a working located diametrically opposite in the sidewall. The entire furnace unit can betilted on rockers for tapping liquid steel into a ladle. is supplied from3- a multi-voltage tap . When the roof of the furnace is in place the threecarton The electrodes are connected by electrodes are lowered until they approach thecold scrap. Electric arcs produce heatto melt the scrap. ’ ,f THE Casting STEEL CYCLE

~~ r‘ ntinuous Casting 1 Sheet

Rolling and other t, Mill ProcessingMill I Bar .,

Shapes Products

:ling

heavy flexible cablesto the trans- former which is located as close to the furnace as possible to avoid excessive transmissionloss with the heavy currents employed. Many modern electric arc furnaces also are equipped with oxy- burners. Energy input consists of ap- proximately 70% electrical and30% chemical whichis derived from oxy-fuel combustion, oxidation of carbon and other chemical reactions. Approximately 53% of the total en- ergy is retained in the liquid steel; heat loss in the gases2070, is cooling losses from walls and roof 17%, with 10% lost in the . Operations and Costs At the start of a heat cycle, with the electrodes and roof raised and swung to one side, a chargeof steel scrap is dropped into the furnace from a clamshell bucket. The roof is replaced, electrodes lowered and an arc struck. Arc lengthis opti- mized to meet the changing condi- tions during the process by selecting the appropriate voltage tap. Two, and sometimes three buckets of scrap, are used in mak- ing a single heat of steel. After melting and are completed the heat is tapped into a ladle for casting. A typical modern melt shop, The roof of a fumace is pivoted asideso that a charging bucketof scrap may be lowered into containing one 194. dia., 120-ton position for bottom-dumping. furnace with a 70-Mva transformer, would have an annual melting tion of 475 kWh/ton, the monthly The cost of producing steel in capacity of approximately600,000 electrical consumption would be ap- electric furnaces varies considerably. tons. Average heat times would proximately 24 millionkwh. The major component is raw mater- be closeto 2 hours. Power con- Total usage of electric power ials, typically63% followed by elec- sumption for an efficient operation the 31 million tons produced in the tric power 16%, labor 1 1YO, elec- should bein the rangeof 450 to U.S. in 1984 was approximately trodes 8%, and 2%”. 500 kWh/ton. Thus,for a consump- 14,725 billion kwh. Special Features Modern furnaces are equipped with furnace condition and power de- ture losses and avoid slag con- a variety of featuresto increase mand. More complex systems tamination in the ladle. production rates, reduce heat times provide control of metallurgical pa-0 Oxygen and carbon injection to and lower operating costs. They rameters (tap temperature, timing provide additional source of heat include: of process events), data logging from oxidationof carbon. and least-cost charge calcu- 0 Coated/water-cooled electrodesto 0 Ultra High Power (UHP) trans- lations, etc. reduce electrode consumption. formers. Power levels of 600to 0 Scrap preheatingto recover en- Arc stability is an important 900 kvdton are being installed. ergy from furnace waste gases. 0 Water-cooled sidewalls and roofs factor in the operation of an electric 0 Single electrode d-c furnacesto to reduce refractory costs. furnace. At the beginning of the reduce electrode consumption 0 Oxy-fuel burners to supplement melting period, power input is limited and noise. heat input and improve melting by unstable arcs which can also 0 Continuous charging using pre- cause flickerin the primary voltage efficiency. heated scrap. line. Flicker is of mounting con- 0 Oxygen injection for decarburi- In summary, modern electric cern with increasing transformer zation to reduce refining time. furnace steelmaking is typified by: 0 injection to reduce process- power. Most new UHP meltshops ing time and heatloss. are equipped with staticVAR 0 Productivity in excess of 60 tondhr. 0 Foamy to shield sidewalls generators for this reason. 0 Steel scrap as a raw material. 0 Flexibility. Shop capacity can and roof from heat radiation from Additional state-of-the-art the arcs. This practice permits the be increased in relatively small developments currently being intro- increments at one-third the cost use of maximum available second- duced to improve furnace per- ary voltage through the use of of coke /blast furnace/ formance as well as steel qual- basic oxygen furnace installa- long arcs with high power factors. ity include: 0 Computer controlto optimize elec- tions. Shut down and start-up trical power programming, auto- 0 Eccentric bottom tapping to re- costs to market demands matic tap changing based on duce tap times, reduce tempera- are relatively small. Bibliography

1. The Making, Shaping and Treating 5. Wunsche, E.R., and Simcoe,R., “Elec- 9. Miske, J.C., ”Cost-Effective Foundry of Steel, Tenth Edition, Association tric Arc Furnace Steelmaking with Melting-Part I, Part II”, Foundry M & T, of Iron and Steel Engineers, , Quasi-submerged Arcsand Foamy August 1984, pp. 22-25 and September Pa., 1985. Slags”, AlSE Year Book, 1984, 1984, pp. 32-40. pp. 166-1 73. 2. Electric Furnace Steelmaking, AIME, 10. The Competitive Status of the U.S. Iron and Steel Society, Warrendale, 6. The Electric Arc Furnace, International Steel Industry, National Academy Pa., 1985. Iron and Steel Institute, Brussels, Press, Washington, D.C., 1985. , 1983. 3. Tomizawa, E, and Howard, E.C., “Arc 11. Stubbles, J.R., “Tonnage Maximization Furnace Productivityin the 1980s”, 7. Hogan, W.T., “The Expanding Electric of Electric Arc Fumace Steel Produc- Iron andSteel Engineer,May, 1985, Furnace: A Threat to the BOF?”,AlSE tion”, Iron and Steelmaker, March, pp. 34-37. Year Book, 1983, pp. 395-398. 1984, p. 46. 4. Annual Statistical Report, American 8. Caine, K.E., Jr., “A Review of New Elec- 12. Steelmaking Flowlines, American Iron and Steel Institute, Washington, tric Furnace ”,AlSE Year Iron and Steel Institute, Washington, D.C., 1984. 423-425.Book, pp. 1983, D.C.,1982 1 Other CMP Reports This Tech Commentary is one of acontinuing series of reports writtento describe of I interest to electric and their customers. Reports presently availableare: .. 84-1 Arc‘Fumace Power Delivery (1984, 145 pages) A detailed analysis of the technical problems relating to large electric furnaces and power grids.. 85-1 Ladle Refining Furnaces for the Steel Industry(1985, 32 pages) A review of supplemental steel heating and refining units often usedin conjunction with electric arc furnaces. 85-2 Electric Arc Furnace Dust Disposal(1 985, 80 pages) An analysis of dusts generated by electric furnaces anda review of methods for treatment and disposal. I Future topics for reports will include Applications,Arc Stability, Vacuum Melting and DC Furnaces. I

The Electric Power Research Institute (EPRI) conductsa technical research and The Center for Metals Production LEGAL NOTICE development program for theUS. electric (CMP) is anR8D application center This report was prepared by the Association of sponsored by the Electric Power iron and Steel Engineers,(AISE). Three Gate- utility industry. EPRl promotes thedevelop way Center, Pittsburgh, PA and sponsored by ment of new and improved technologiesto Research Institute (EPRI)and admin- the Center for Metals Production (CMP). Neither help the utility industry meet present and istered through Mellon Instituteof members of CMFl AISE, nor any person acting future electric energy needsin environ- Camegie-Mellon University.CMPs on their behalf: (a) makes any warranty ex- goal is to developand transfer tech- pressed or implied, with respect tothe use of mentally and economically acceptable ways. any informallon, apparatus. method. or process EPRl conducts researchon all aspects of nical information that improves the dlsclosed in this repon or that such use may not electric power productionand use, including productivity and energy efficiency of infringe privately owned rlghts; or (b) assumes , generation, delivery, energy manage- U.S. primary metals producing com- any liabilities with res* to the use of, or for panies (SIC33). Target areas are damages resulting from the use of, any informa- ment and conservation, environmentalef- tion, apparatus, memod, or process disclosed fects, and energy analysis. reductionismelting; refiningremelting; In this report. and surfaceconditionincJprotection. EPRl I. Leslie Harry, Project Manager CMP R.M. Hurd, Director Joseph E. Goodwill, Associate Director Constance M. Reid, Coordinator

Center for Metals Production Mellon Institute 4400 Fifth Avenue Pittsburgh, PA 1521 3 0 1985 Center for Metals Production (412) 268-3243