Plasma Arc Technology

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!!l ...... ;7 ...... y ...... I ...... ,...... I...... I...... ,...... ,...... ,...... I ...... ,...... I...... iuii - Published by The EPRl Center for MaterialsProduction CMP - 076 An Electric Energy Tool for High-Temperature Materials Processing Introduction Over the past 40 years, engineers I worldwide have developed many ! 1. Plasma torch materials processing applications using 2. Plasma plasma arc heaters. Early applications 3. Tundish injection chamber at low power levelsof 10 - 20 kW were 4. Tundish heating chamber for the cutting and welding of metals 5. Tundish casting chamber and for plasma spraying of metal and 6. Plasma power supply refractory powders to form protective 7. Bottom electrode coatings. These are now well-estab- 8. Controller lished commercial processes. Contin- 9. Molten steel temperature sensor ?ddevelopments are being made for 10. Ladle the surface treatment of metals (phase transformations for hardening, carburiz- ing, nitriding) and for metal overlays. Present research efforts at low powers include the synthesis of diamond films as well as the productionof supercon- ductive and engineering ceramic powders. Figure 1 Plasma Tundish Heater and ControlSystem During the 1960s plasma arc heaters were developed at power levels of500 What are Plasma Arc Heaters? hydrogen), oxidizing (oxygen) or inert kW up to35 MW for aerospace materi- (argon). als testing. Subsequent plasma heater All electric arcs are plasmas. Plasma developments have resultedin many simply refers to the presence of ionized Heaters can be designed for both low high-power applications for materials gases. Plasma arc heaters are electri- and high gas-flow rates. The gas can processing.Today, industrial-worthy cal resistance heaters where the be used to convey fine solids, liquids, plasma arc systems are available for resistive element is the electrically- or additional gases into the arc-heated extended reliable operation at high conductive partially ionized gas be- zone for physical transformations and power and current levels. Commercial tween two electrodes. Whereas chemical reactions. This controlled breakthroughs in technology include plasma arcs occurin nature as sudden flow of various gases enhances the simple physical tasks, such as heating lightning discharges, plasma arc versatility of plasma arc heaters when of molten steel in continuous casting heaters provide a continuouslycon- compared to conventional electric arc tundishes for temperature control using trolled electrical arc discharge. furnaces with "free burning arcs" and 1 - 4 MW plasma heaters. Complex no gas control features. multiphase chemical reaction systems Although the arc discharge can be th high specific energy requirements, controlled by magnetic and mechanical The core temperatureof the plasma such as ferrochromium production by means, the key design featureof arc can range from4,000 to 20,OOO"C carbon reduction of chromite ore fines, plasma arc heaters is the controlled (7,200"-36,000"F). Theworking are being processed at power levels upfeed of gas to thearc environment. temperature of the bulk gas, depending to 30 MW. The gas can be either reducing (e.g., on the gas flow ratesand theheater design, is about 2,000 - 3,O0O0C (3,600”-5,400”F). Thehigh ternpera- amperes (as high as 100,000 A with ture of the arc and the working gas graphite electrodes) and a few hundred THE PLASMA ADVANTAGE plus heat transfer at the arc root volts. The workptece is one electrode; attachment area result in heat fluxes shown as the anode (+) in Figure 2b. The unique advantages for the about 10-50 times higher than oxygen - The gas phase electrode, or cathode application of plasma arc fuel flames. (-), can be a water-cooled copper technology include: cylinder or a refractory metal button Gas environment control Most plasma heaters operate on direct (e.g., thoriated-doped tungsten),or it Gas-flow rate control current, with ”nontransferred” and/or can be a hollow graphite electrode Fine particle feed capability “transferred” arcs. For nontransferred without water cooling. The hollow 1 Hightemperature operation the gas phase is the only passage canbe used for the direct \ Highenergy fluxes working resistive element, Figure 2a. introduction of gas and fine solids into : Rapidquenching The arc can be “transferred” attaching the arc environment as shown in Figure i’ No productsof combustion directly to a workpiece (e.g., an object 5 for the reduction of chrome ore fines I to be cut, welded, heated, melted, for ferrochromium production. reacted, etc.), as shown in Figure 2b, with resistive heating of both the gas phase and the workpiecein ,series. Nontransferred arc heaters areused when it is desirable to heat the gas phase, such as for gas phase reactions (e.g., production of acetylene from natural 3as) and for countercurrent solids - gas contact in shaft heaters, melters (cupolas), and reactors (blast furnaces). These heaters typically consist c: water-cooled cylindrical copper electrodes and operateup to 2,000 amperes and 6,000 volts with high flow rates of a reactive gas. Transferred arc heaters are useful for the bulk heating of solids and melts and where the gasphase is not a 1 principal reactant but can be important ..:.:.:.:.:.: ;.?.$.:~:<*:$::::::::j:.:::::::~ ...... in\...,. ,...... , . . . , . I as an inert shield. The gas flow rates ...... are typically very low which canbe important for controlling vapor phase L L reactions and condensation phenom- Figure 2a. Nontransferred Arc Figure 2b. Transferred Arc ena. Transferred arc heaters typically nperate at up to several thousand

Center for Materials OTHER PLASMA AND ELECTRODELESS ARC DISCHARGES ProductionRole In 1988, CMP published “A Survey of In additionto arc discharges between electrodesit is possibleto sustain plasma Thermal Processing Research Facili- discharges by high-frequency powered induction coils. These electrodeless induction- ties“ as an aid to planning research coupled plasmas (ICP)have the unique capabilityof producing an ultraclean arc and activities. In March 1990, CMP plasma with no contamination. These systems, which can operate at atmospheric or organized the first EPRI-sponsored reduced pressures, are ideal as a heat source for chemical analysis instruments, and plasma symposium on “Industrial and also for powder treatment. Conventional Systems operate upto about 30 kW, but Environmental Applicationsof Plasma” developmental lCPs have operated on air at900 kW. attended by specialists from plasma Plasma arc discharges are typically produced at normal pressures. Plasma discharges vendors and developers, as well as also can be produced at low pressures such as the familiar fluorescent lamp. Low- from electrical utilities and industry. pressure discharge devices are in wide commercial use in microelectronic fabrication for etching patterns in integrated circuits. CMP has supported laboratory, pilot plant, and industrial scale plasma CMP’s activities have been focusedon the development of plasma arc processes activities for treating electric arc rather than on induction-coupled or low-pressure plasmas. furnace dust and specialty steelmaking dusts, smelting of silicon metal and increased steelmaking vessel(oxygen FeMn , FeCr ferrosilicon production from taconite converter, electric arc furnace)refrac- In South Africa the plasma remelting of fines, melting of aluminum scrap, tory life due to lower steeltap tempera- ferroalloy crush finesof melting / separation of aluminum from tures, ferromanganese and ferrochromiumin scrap melter dross, and the heating/ (2) improved steel quality, e.g., preven- 4 MW and 14 MW plasma furnaces, refining of steel ladle melts. In addition, tion of center segregationsfor high- respectively, was initiated in 1983. EPRl has funded the development of carbon steels such ashard-drawn wire, the plasma-assistedcupola for iron (3) higher yields due toelimination of Titanium melting and recycling of mill scale (iron strand freeze-off and lessmold powder United States-fosteredplasma technol- oxide). related product defects. ogy is used at five locations for “inert gas”- melting and remeltingof Plasma heatingof steel ladles has titanium scrap and sponge, see Figure Applications of Plasma Arcs been developed industrially on melts 3. Plasma heaters rangefrom 300 kW The plasma processing of gases, from 30 - 220 tons and heaters from 1 - to 2.25 MW. In one case Oregon liquids, and solids coversa wide range 15 MW. CMP has funded the develop- Mejallurgical Corp. melts titanium scrap of applications including both physical ment of heating and refiningof foundry and sponge using an 800 kW Retech and chemical changes with increasing ladle melts. heater to produce 6 million pounds per specific energy requirements. The year of titanium electrodes for least energy-demanding systemis that remelting. A unique cost saving of heating solids, liquids, or gases, with Melting and Remelting of High- feature of this installation is the inert no change of state. For a change of Alloy and Refractory Metals gas recyle system. state, greater energy inputs are needed to melt solids and then to High-Alloy Steels Reclaiming of Metals from vaporize liquids. The most demanding The first large-scale industrial applica- Scrap Materials and Residues are complex chemical reaction systems tion of scrap melting wasin 1973 in with changes of state suchas vaporiza- Freital, Germany at 20 MW with 30- E. Reclaiming of metals from consumer tion of liquids produced from solid ton heats of alloy steel. The plasma discards and industrial residues by feeds. All c; these systems havebeen- advantage was the ability to melt with plasma melting is attractive due to the successfully demonstrated withthe use inert argon to prevent reoxidation and low gas flow rates which minimizeoff- 2f plasma arc technology. yield loss to the slag of expensive gas cleaning and handling systems in alloys such as chromium,and molyb- A complete plasma system willinclude comparison to combustion heating denum. a plasma heater, a processing unit (e.g., furnace, reactor, spray chamber) .a. and requisite auxiliary equipment, such n as the power supply, working gas, reactant feeds, productcollector, and cleaning and cooling of the product gas.

Heating, Melting, and Remelting

Heating of steel melts in tundishes and ladles In 1987, a 1 MW tundish heater was installed at Nippon Steel’s Hirohata Works. By mid-1991 there were 12 plasma installations worldwide for temperature control of steel in continuous casting tundishes. These units range in power level from 350 kW to 2.4 MW for tundish melts of 5 tons to 80 tons, respectively. The unit, as shown in Figure 1, equipped with a sensor in the melt, can control the steel

temperature to k 1O C of the target temperature. Plasma tundish heating .. has several advantages including(1 ) Figure 3 Plasma Titanium Melting .. .- \\ systems. In addition, the higher temperatures and heat fluxes increase To equipment throughput anda plasma Wetscrubber inert gas minimizes metal reoxidation. - n Iron Scrap & The ability to lower the volumeof gas Coke In flow is the plasma advantage for the General Motors foundryplasma-fired cupola in Defiance, Ohio. The cupola uses six 1.5 MW Westinghouse plasma torches as shown in Figure 4. In conventional coke-fired cupolas the plant-generated scrap chips, and borings wouldbe swept out due tothe high flow rates of the combustion off- Plasma Torch gases. This development, commercial- ized in 1989, was funded in part by Plasma Air Metal Blast Air EPRI. In a related development in out France, a 2 MW Aerospatiale heater has been used since 1989 to increase + the blast air from 400 to1,OOO"C (750"- .. 1,800"F)on a Peugeot cupola resulting in decreased cokeconsumption, Figure 4 PlasmaCupola reduced dust emissions, and increased cupola capacity. Fine chromite feed Platinum Since 1984 a 3 MW Tetronics R&D plasma furnace has reclaimed platinum from automobile emission catalysts by a simple melting process atthe 70,000 Cathode oz./year capacity Multi-Metco facility in Anniston, Alabama. Hollow electrode Aluminum In the remelting of aluminum scrap large quantities of dross are generated which contain substantial quantities of aluminum droplets. An experimental program by Hydro-Quebec using a plasma-fired rotary kiln for aluminum recovery from dross has been com- mercialized by Alcan usinga Plasma Energy Corporation (PEC) heater. The Center for Materials Production has funded further development.of plasma processes to treat aluminum drosses containing lead and cadmium contami- nants. Figure 5 Plasma Ferrochromium Production Smelting of Ores Africa for Middleburg Steeland Alloy. In 1986, two shaft furnaces, each In 1989, this installation, Figure 5, was equipped with four 6 MW Chromite ores increased to a 30 MW furnace with nontransferred SKF plasma gas The ability to smelt low-cost friable smelting of chromite ore and heaters, wereinstalled for smelting of chromite ores as fines hasbeen one of ferrochrome crush finesfed through the chromite ores in Sweden. This facility the most successful applicationsof hollow electrode. Significant develop- which also required development work plasma systems. In December 1983,a ment work was required forthe auxil- on the ore feed system operated 14 MW dc hollow graphite electrode satisfactorily by 1988, but was shut .. iary systems for fines feedingas well .. transferred-arc ASEA Brown Boveri/ as for off-gas removal. down in 1990 due to unfavorable Mintek system wasinstalled in South chromium prices. Manganese ores In France in 1984, three 1.5 MW nontransferred Aerospatiale plasma gas heaters were retrofitted to a ferromanganese blast furnace to replace coke as fuel during electrical off-peak periods. This was increased to eight 1.5 MW heaters in 1986.

Iron ores In 1987, six2.0 MW Aerospatiale heaters were retrofitted to increase the blast temperatureof an iron blast furnace at Lorfonte, Lorraine, France. In 1981, a SKF system withthree 2 MW plasma gas heaters wasused to produce a reduction gas toproduce metallized pellets. The SKF shaft reducer in Hofors, Sweden was Figure 6.

decommissioned in 1985 due to a ~~ switch to a 100% scrap feed for the electric arc steelmaking furnace. REFERENCES Treatment of Dusts “Plasma Technology in Metallurgical Processing”, Iron &. Steel SOC.,AIME, Warrendale, Pa., ;1,987. Plasma systems for treating dusts have “Arc Plasma Processes”, International Union for Electroheat, Paris, 1988. substantial economic potential for the disposal of hazardous materials. In “Plasma Arc Process Systems, Reactors and Applications” Plasma Chemis- addition, there isa possible credit for try.and Plasma Processing 9 (1) 85s - 118s. D. R. Mac Rae. Presenta- reclamation of useful metals from the tion, EPRl Plasma Technology Workshop, Nov. 1988. Palo Alto. dusts. “First International EPRl Plasma Symposium” Report 90-9, Center for Zinc, Lead, Cadmium-bearing Materials Production, Pittsburgh, 1990. electric arc furnace dustsIn 1989, two Tetronics R & D systems of 2 and -“Plasma for Industry and Environment”, British National Committee for 3 MW were installed in the United Electroheat, London, 1990. States for zinc recpvery from EAF “Plasma Processes for Metals Production”,Report 85-3, Center for Materials dusts with the production of a nontoxic Production, Pittsburgh, 1985. slag. This development was cofunded by CMP and 30 steel companies. Further plant modifications have 1960s using a one MW Linde arc processing of soil contaminated by included the use of hollow graphite heater. Tioxide (UK) presently pro- hazardous organics. electrodes. See Figure6. duces not onlyTiO, but other high-tech Future Developments Chromium, Nickel, Molybdenum- ceramic powders includingnitrides of titanium, silicon, and aluminum,as well Widespread useof plasma heaters will bearing steel plant dustsIn 1989, a 3 occur for the simpler applications, i.e., MW Tetronics system was installed for as yttria-coated zirconia. These pow- heating and melting, such as the British Steel in Sheffield to smelt and ders are synthesized by gaseous reactants in nontransferred plasma tundish and ladle heating, reactive recover the alloy values in dusts metal melting, and reclaiming ofmetals collected from steel refining vessels. A heaters. Commercial powders are also produced centrifugally under ultra- from molten residues. As the complex- SKF system in Sweden is smelting clean conditions by arcing to spinning ity of thesystem increases,the applica- stainless steel dusts in a shaft furnace electrodes of the desired powder tions will be more site-specific depend- fired by three 6 MW heaters. .. composition. ing on local conditions for rawmaterial Powder Synthesis availability, electrical costs, and Additional Applications environmental considerations. A major commercial area for plasma Other current developmentsinclude arc systems has been the production’of plasma heating as an alternative to Progress is being made in research powders, such astitanium oxides and natural gas-fired glass melters, incin- and developmentof unique plasma zirconium oxides. Titanium oxides synthesized products which have . .. eration of hospital wastes, treatment of .. were produced by PP&G in the late municipal incinerator fly ash, and the outstanding commercial promise.

\ The Electrtc Power Research lnstttute (EPRI) Tire Center for blnterlds I’roducnon (CMl’) ISan LEGAL NOllCE conducts a technical rescarch and developnrent R&D appllcatlons center fundcd by the Electric Thls TechComnwntary was prepared and program tor the US. electric utillty Industry. Power Research lnstltute And operated bv sponsored bv the Center for Matertals Produc- EPRl promote, the developmentoi new and Carnrpe ivlrllon Research Inst~tutrof Carnegle tion tCMP). Nelther membersof CMP nor any miproved technologies to help theuthty Mellon Unlverslty CMP‘s mission is to discover. person actlng on thelrbehalf: (a) makes any miustry meet present and hturr electric energy develop, and deliver advancesin the scrence and warranv expressed or implied with respect to needs In envuonmmtally and economlcally technology of materials productlon ior the the use of any informatlon, apparatus, method, acceptable wavs. EPN conduck research on all benehts of EPRI-member utilities, their or processdisclased in this Techcommentary or aspects of electrlc power production anduse, customers, and society. that such use may not infringe privately owned including fuels, generation, delivery energy EPRI rights, or (b) assumes any liabilities with respect management and conservation, envlronmental Patrlck McDonough to the use of, or for damages resulting iromthe effects, and energy analysis. Manager, Materials Production and Fabricatlon use of, any information, apparatus, method, or CMP process disclosed in this TechCommentary. Joseph E. Coodrv~ll Director

This Techcommentary was prepared by Dr. Don R. Mac Rae, CMP Consultant. Technlcal review was provided by Joseph E. Goodwill, Director, CMP. Edited by John Kollar. CMP.

Applicable SIC Codes: 3312.3321, 3322, 3324,3325,3341,3361,3362,3369,

For ordering information, call EPRl’s Affiliate Member Program 1-800-4320-AMP For technical information contact THE EPRI CENTER FOR MATERIALS PRODUCTION arc applications is available on Carnegie Mellon Research Institute request from the Center for 4400 Fifth Avenue Materials Production publications Pittsburgh, PA 15213-2683 office. 41 2-268-3243