M. Mihovsky                 Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010, 3-18            ...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010are confirmed by numerous theoretical and prac...
M. Mihovsky                 Fig. 1. PLASMALAB scheme of pilot and industrial plasma metallurgical technologies.       Usin...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010                                              ...
M. Mihovsky                                                Charge Feeding          Filter       Charge                    ...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010                                              ...
M. Mihovsky                                                                Fig. 11 shows the behaviour of the arc and heat...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010Fig. 10. Scheme of the experimental “PLASMALAB...
M. Mihovsky                                    a)                                                           b)Fig. 12. a) ...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010Fig. 14. The graphite particles moving away fr...
M. Mihovsky          Fig. 16. Linde DC plasma furnace [24].                                                               ...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010                                              ...
M. Mihovskynace (max expansion at 50-60 Hz). While in the case of                     using a plasma torch this effect is ...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010                                              ...
M. MihovskyREFERENCES                                                          Raw Steel, presented at the Office of Techn...
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010    nace into a Plasma-Induction One, 8th Inte...
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PLASMA

  1. 1. M. Mihovsky Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010, 3-18 THERMAL PLASMA APPLICATION IN METALLURGY (REVIEW) M. MihovskyUniversity of Chemical Technology and Metallurgy Received 05 January 20108 Kl. Ohridski, 1756 Sofia, Bulgaria Accepted 12 February 2010E-mail: mihovsky@uctm.eduABSTRACT A classification of the most perspective plasma technologies and equipment are presented including the initialworking raw materials, and the type of resulting materials. The main pilot plasma technologies and such with industrialapplication are indicated. The important factors predetermining plasma technology development in metallurgy are evaluated. The basic works of Plasma Metallurgy Research Laboratory “PLASMALAB” for thermal plasma application inmetallurgy are shown, based on actual results, experience, raw materials state, technological, economical and ecologicalconditions. Keywords: plasma technology, plasma torch, plasma furnace, metallurgy.INTRODUCTION • Widespread use of alternative electroenergy sources. The actual state concerning raw materials, energy, • Tendency for an expanding application of hydro-technology and ecology worldwide predetermines the gen as reducing agent obtained from water dissociationdevelopment of new metallurgical technologies and and a gradual replacement of traditional fossile reductans.equipment based on thermal plasma application as a • Gradual decrease of the absolute volume ofconcentrated heat source and for chemically active com- metallurgical production in favor of improved qualityponents of the processes. of produced metals and alloys. Contemporary unfavourable state of resources for • Gradual replacement of presently used classi-metallurgy, serious energy and ecology problems and cal alloys by new advanced metal-based materials withincessant increase of metals and alloys demand lead to a better properties.new strategic concept concerning the world metallurgy. • Development of new nonwaste, clesn metallur- According to a present prognosis on the devel- gical technologies and equipment, providing a maxi-opment of 21st century metallurgy we find that the fol- mum utilization of the valuable components in naturallowing main principals would be valid: and waste raw materials. • Processing of low quality, poor polymetal raw All of these principals can be observed in thematerials and waste. case of plasma metallurgy technologies. Its perspectives 3
  2. 2. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010are confirmed by numerous theoretical and practical as well as solid (fine coke, coal powder and others) re-researches carried out in the field of thermal plasma ducing agents are used.metallurgy application during the last 20 years in tech- The characteristic feature of these technologiesnically advanced countries. is that the materials are processed to a finely powdered The application of thermal plasma in metallurgy at condition and in most cases are subjected to prior par-present is developing in two directions - improvement of tial reduction (Fig.1). The aim in all the different plasmaclassical metallurgical technologies and development of units is to provide through various technologies a shortprincipally new plasma metallurgical technologies. melting time of the raw material and a good contactAccording to the scope of their application, plasma tech- (maximum area) between the reducing gas and the meltednologies are classified generally as smelting and reducing oxide material.(extracting) processes upon which existing classifica- In general plasma reduction technologies aretions are based. classified according to the heat-exchange type between The growing limitation of quality raw materials for the plasma arc and the processed material and accord-the metallurgical industry, and the serious ecological prob- ing to the conditions under which the reduction pro-lems imposed on them require to keep in mind as a clas- cesses run.sification characteristics not only the existing plasma pro- Industrial production of metals and alloyscesses, but also both the type of initial raw materials source through reduction - from natural and waste raw materi-and the type of the final material (Fig.1). The basic plasma als is done in plasma furnaces, similar to the plasma-metallurgical processes used in industry as well as the most arc ones, working in “open bath” 3 (Fig.1). The indus-perspective plasma pilot units and technologies are in- trial furnaces TETRONICS-FOSTER WHEELER (EPP-cluded in this scheme. Expanded Precessive Plasma, Fig. 2) [5, 6] are well The main idea of the Swedish PLASMABLAST known, in which the plasma torch rotates with a veloc-[1] technology for improvement of the blast furnace ity of 50-1500 min-1 around the furnace vertical axis atprocess is overheating of the blow air, which aims mainly 5-15° angle and of arc length 500-750 mm while thecutting down of coke consumption. Besides, plasma powdered material is introduced around the heatingoverheating of the blast makes possible a more flexible surface of the resulting plasma cone. The latest data, ob-control of the furnace heating process. tained from processing of powder slimes and slags in a 2 The developed process PIROGAS (Plasma In- MW plasma furnace are: specific energy consumptionjection of Reducing Overheated Gas System) provides 1300 kWh/t powder; reducing agent consumption 90-also low quality pulverized coal to be blown into the 300 kg depending on the type of processed material [7].furnace through the plasma torches. The experimental Some plasma units [8,9] use an extended arc 4data show that this technology can reduce coke con- (Fig.1) in combination with a vertically located tubesumption to the necessary minimum for reduction pro- reactor (EAFR-Extended Arc Flash Reactor, Fig.3) incesses in the blast furnace, i.e. from 470 kg/t pig iron to whose upper section the powdered charge is introduced.385 kg/t [ 3]. The plasma arc is generated between three hollow graph- Calculations show that the development of new ite electrodes (three-phase-power supply).advanced technologies corresponding with raw materi- The basic ELRED concept [10] is the reductionals, energy and ecology requirements in many cases are of fine (size below 0,1mm) iron ore concentrate (moresounder from an economical point of view compared to than 65 % Fe) in two stages: an initial stage for pre-reconstruction of the classical technologies. This reduction in solid state in the presence of an excess ofprompted the development of alternative reduction carbon to a partially metallized product containing car-plasma metallurgical technologies. bon and a final reduction stage for smelting of this prod- These technologies are intended for processing uct using electrical energy.of raw low quality polymetal ores, as well as of waste The prereduction takes place under pressure andmaterials (slags, slimes, chemical industry waste mate- temperature 970-990oC in a circulating fluidized bedrials, etc.). Gaseous (hydrogen, methane, carbon oxide) in a reactor vessel with internal refractory lining.4
  3. 3. M. Mihovsky Fig. 1. PLASMALAB scheme of pilot and industrial plasma metallurgical technologies. Using shaft furnace 5 (Fig. 1) ASEA SKF persed flow forms a melt film on the wall of the reac-founded the basic plasma process PLASMASMELT [12] tor-anode. This film falls down to the bottom part of(Fig.4) for pig iron production, which was further de- the reactor getting reduced on the way. Thanks to theveloped in the PLASMADUST [13] modification - for high temperature and speed of the plasma reducing gasnon-ferrous metals dusts processing and the surface processes in the liquid film take place com-PLASMACHROME [14] - for FeCr production. The paratively quickly and the time of the stay of the mate-shaft furnace is filled with coke, while in its stove zone rial in the reactor is long enough for a nearly full re-are installed three plasma torches of 6 MW each. The duction of the raw materials. The 1 MW FFP pilot unitpowdered initial raw material is subjected to two-grade was constructed after this scheme. Hematite from the Carolpre-reduction in two furnaces of “fluidized bath” type, Lake source was processed in it, resulting in a metal ofusing reducing agent, the gas (carbon oxide), obtained chemical composition: 0.005 % S; 0.001 % P; 0.006 % C;in the shaft of the furnace. This gas acts both as trans- 0.06 % Si; 0.07 % Cu under electrical energy consump-porting medium (to deliver the pre-reduced material tion of 3.9 kWh/kg Fe. A mixture of hydrogen and natu-and the low quality powder coals into the plasma torch ral gas with ratio of 2:1 was used as a reducing andzone) and as a plasma gas. transport agent. In spite of the promising results of this Bethlehem Steel developed a FFP (Falling Film original idea on plasma reactor for reduced productionPlasma) reactor 6 (Fig.1), which actually gives the best of metals and alloys from fine raw materials it did notof technological technical and economical results [17]. find industrial application. The main reasons for this areThe essence of this method is concluded in the reduc- the lack of arc plasma torch of high power with sufficienttion of a thin layer (film) of melted oxides (Fig.5). The working life and the high working voltage (over 1000 V)fine raw material with powdered solid reducing agent is needed for obtaining a stable maximum long arc.tangentially introduced by reducing plasma gas in a ver- Plasma furnace technology was first applied intical water-cooled cylinder camera, acting as a plasma South Africa in the mid-to late 1970 s, when it wastorch anode section. The intensively whirled gas dis- realized that advantages could be obtained in the pro- 5
  4. 4. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010 Fig. 4. PLASMASMELT [12]. Fig. 2. EPP [5, 6]. Fig. 3. EAFR [8, 9]. Fig. 5. FFP-reactor [17].cessing of fines for the production of ferro-alloys. A search Laboratory “PLASMALAB” proposed a new con-number of processes have been investigated, and a 40 ception on design of plasma furnace and of technologyMVA DC transferred-arc ferrochromium furnace has for scrap remelting (Fig. 6) and production of metals andbeen implemented on an industrial scale at Palmiet alloys from deferent materials by reduction (Fig. 7).Ferrochrome, Krugesdorp. South Africa has well-de- According to the general conception [21] our firstveloped plasma-furnace research facilities, which include PLASMALAB FFP-reactor improved design offers athe 3.2 MVA DC transferred-arc plasma furnace at combination between a hollow graphite cathode and aMintek, Randburg [18-20]. copper water cooled nozzle, Fig. 8, [22]. A special fea- All described reducing processes have some tech- ture of this construction is that the plasma gas is intro-nological and design advantages and disadvantages. Af- duced from both sides (interior and exterior) of the tubeter critical estimation of them Plasma Metallurgy Re- graphite cathode, i.e. the gas is bilaterally blown, and6
  5. 5. M. Mihovsky Charge Feeding Filter Charge System Gas V2 G as Supply Arc Plasma Gas V1 System Preliminary Heating Torch Chamber Water Cooling System Power Inductor Supply Mould Computer Condensersà òîð Continues casting machine OFF Gases IngotFig. 6. Conception for plasma installation for remelting of disperse scrap and production a continuous caste metal ingot [21, 39].the relation in the capacity, respectively the velocity of reactor zone is provided mainly by tangentially blow-the two gas streams (V1 and V2) is regulated in such a ing plasma gas V2 and in the lower zone - through DCway that the plasma arc reaches a tube-like configura- magnetic field.tion. Such organization of the plasma arc enables the Several undesirable effects were registered inintroduction of the powder feed into a volume surrounded these experiments:with highly heated plasma. The adopted type of charge • Obtaining a stable burning plasma arc withintroduction ensures its maximal convection and radia- length up to 280 mm requires high consumption of thetion heat exchange with the plasma arc on its way be- tangentially blowing plasma gas (16 m3/h) [22].tween the cathode and the anode. The anode spot, ac- • Tangentially blowing gas V2 is inadequately ef-cording to the general concept offered, is in a continuous fective for cathode spot rotation.motion with a controlled velocity and trajectory. • Gas stream V1 blows some powder raw materi- The motion of the anode spot follows a compli- als out of the reactor.cated rotary and at the same time reciprocating trajec- To avoid above-mentioned disadvantages thetory i.e. the anode spot would move as a spiral upwards - Plasma Torch - FFP-Reactor system construction hasdownwards along the interior wall of the reactor-anode. been modified [23]. The organization of the motion of the plasma The plasma gas is unilaterally introduced intoarc is realized with simultaneously working electro-, the space between the graphite cathode (Fig.9) and theelectromagnetic and gas dynamic devices, computer nozzle, situated at the top of the plasma reactor. Thecontrolled and synchronized. plasma gas is tangentially introduced under a gradient In our first experiments [22] the motion of the of 40°C to the vertical axis of the reactor through ananode spot along the reactor height is provided by syn- insulation flange. The charge is introduced into the spacechronized increase of the gas flow rates V1 and V2 and between the cathode and special flange through threethe plasma arc current. The arc’s rotation in the upper feeding screw systems. The powder material is swiped 7
  6. 6. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010 Charge Feeding System Filter Charg Gas V1 Gas Supply Gas V2 System Vibroreaktor FFP-Plasma Reactor Water Cooling System Scrubbe Power Supply Metal Slag Computer Tundish Ar Condensers Mould àòîð Continues casting machine OFF Gases IngotFig. 7. Conception for plasma installation for reduction processing of disperse natural and waste raw materials and productiona continuous caste metal ingot [21 39].along from the plasma gas and is blown through the magnetic action. An experimental stand has been madenozzle on the reactor wall. (Fig.10). The centrifugal force caused by the plasma gas It includes DC plasma torch with hollow graph-blown out tangentially orients the particles on the reac- ite cathode and graphite tubular reactor-anode, withtor wall. which the processes of electromagnetic and gas dynamic The rotating motion of the cathode and the an- control of the plasma arc are simulated.ode spot is realized with a circular magnetic field, ob- A major requirement to this system (“plasmatained from a monophase AC power supply stator, situ- torch–plasma reactor”) is to provide a possibility for aated coaxially to the tube reactor-anode. Besides, the conveniently observation and photographing of thestator moves under controlled velocity upwards-down- plasma arc in the volume of the tube reactor-anode, aswards the reactor height with the help of hydrocylinders. well as measuring of its main electric, gas dynamic and The next step is synchronization of the gas flow geometrical parameters.rate change, the plasma torch current and the stator One of the original solutions in the present work isposition along the reactor height. For this purpose a the series inductor connection in the anode electrical chainspecial gas valve (Fig.9) is constructed to regulate the of the system ”plasma torch cathode-reactor-anode”plasma gas flow rate, combined with the inductive trans- (Fig.10). Thus, the “PLASMALAB” system for Auto-ducer for the arc current control. Electro-Magnetic Rotation (AEMR) of DC transferred The main target is to study the energetic param- plasma arc generates in the inductor from plasma arceters of the DC transfer plasma arc and the possibility current, i.e. an additional DC source is not necessaryfor its effective rotating under the influence electro- for arc electromagnetic control.8
  7. 7. M. Mihovsky Fig. 11 shows the behaviour of the arc and heat- ing of graphite reactor-anode without axial magnetic field (the anode chain is connected directly to the graph- ite reactor). It is seen that the arc burns calmly between the clearly outlined static/immobile anode and cathode spots. At series connection of the inductor, in the an- ode chain (Figs.10, 12) the arc unscrews under the in- fluence of the created by the inductor, axial magnetic field (under the action of Laurence force). The graphite reactor-anode is heated evenly (Fig.12a), because of the high rotational speed of the anode spot, which is the main target of the electromagnetic control of the arc in FFP-plasma reactor “PLASMALAB”. The cathode spot also rotates, but its speed is lower. An evidence for this is the observed spiral trajectory of the arc column (Fig.12b), i.e. the cathode spot drops behind in its rota- tional speed compared to the anode one. The rotational speed of the arc depends on flow- ing current which is the same that creates the magnetic field in the inductor-as the arc current grows, the mag- netic induction of the electromagnetic field grows too. In accordance to the conception developed in “PLASMALAB”, for plasma installation and technol-Fig. 8. PLASMALAB FFP- reactor with hollow graphite ogy for reduction production of metals and alloys, theelectrode [21]. processed disperse charge is blown through a hollow graphite cathode on a gravitational principle and gets in FFP-reactor, in the zone of DC plasma arc. It is proved that the arc has transport phenomena [24]. The main idea is that the charge particles are sucked by the rotating arc at the exit of the hollow cath- ode and are transferred to the wall of the concentrically situated tube reactor-anode. It is supposed that in the volume of the arc column, the particles will be heated and melted while getting on the reactor-anode wall. The melted metal-slag suspension runs down the reactor anode wall and reduction processes run. The obtained metal-slag melt drops in the rector bath. Fig. 13 shows the behaviour of the arc at differ- ent current values. At minimal current-20 A (Fig.13h), due to a substantial difference in the rotational speed of the cathode and anode spots, the arc column is seen as a part of the helical line. With the increase of the current up to 80 A, the arc column gradually becomes fade (it is not noticeable on the pictures due to its high rotational speed). AtFig. 9. New PLASMALAB FFP – reactor [23]. 9
  8. 8. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010Fig. 10. Scheme of the experimental “PLASMALAB” stand for investigation of system “plasma torch-FFP-plasma reactor”[40]. 1-argon cylinder; 2-throttle valve; 3-gas collector; 4-flow meter; 5-plasma torch; 6-refractory insulator; A7-graphitetube-anode; K-cathode; 8-water-cooling copper inductor; 9-contact clamp; 10-protective screen;. 11-video-camera; 12-transferred plasma arc; E-DC power supply; Sh-shunt. a) b)Fig. 11. a) Heating of the tube graphite anode; b) DC transferred plasma arc in the system “hollow graphite cathode-tube graphiteanode”, without axial magnetic field [40].current 80A, three cathode spots appear on the cathode adjacent edge is heated evenly. In the whole investigated(three sources of intensive thermionic emission), and their current range, the motion speed of the anode spot is muchnumber grows with the current increase. Simultaneously, higher than the one of the cathode.the cathode spots start rotating faster on the adjacent edge The arc column crosses the cathode axis withof the cathode. Under these circumstances of increased frequency equal to the revolutions of the anode spot,current, the anode spot rotates at speed extremely higher i.e. the particles falling through the hollow cathode arethan the one of the cathode. At current 250 A the rota- taken by the arc column and transferred to the tubetional speed of the cathode spot is so high that the whole graphite anode wall. A demonstration of this is the pic-10
  9. 9. M. Mihovsky a) b)Fig. 12. a) Heating of the tube graphite anode; b) DC transferred plasma arc in the system “hollow graphite cathode-tubegraphite anode”, under the action of axial magnetic field [40]. a b ? c d #) ) #) ) A e B f C g D h ) $) ) )Fig. 13. Burning of DC transferred plasma arc in the system “hollow graphite cathode-tube graphite anode” – arc (inductor)current from 20 A to 250 A [40].tured trajectory of graphite particles that are moving Plasma-arc melting (Fig. 1, position 8) is de-away from the cathode (Fig. 14). The trajectory of the veloped as an alternative of the classical electric arcmoving away particles follows the trajectory of the arc melting. Three types of plasma furnaces are working incolumn, which is clearly shown on Fig. 14. the world today differing in the way of power supply The results from the experiments give us con- (DC and AC) and in the way of plasma torch location.fidence that the idea about charge feed through a hol- The first plasma melting furnaces Linde type [25]low graphite cathode in “PLASMALAB” FFP-reactor (Fig.16), working still in Russia, are actually recon-is perspective. structed classical electric arc furnaces, where the graphite The dimensions of the model graphite tube re- electrodes are substituted by plasma torches (cathodes)actor are used for design of a working laboratory FFP- with an independent DC electrical power supply and areactor “PLASMALAB”, shown on Fig.15. Its techno- water cooled bottom electrode (anode). The next steplogical potentialities lie ahead. in the plasma-arc melting development is the Freital - The smelting plasma technologies (positions 8 to Voest Alpine system, where four DC plasma torches12 in Fig.1) find wider application at present. are located at the furnace jacket walls under a certain 11
  10. 10. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010Fig. 14. The graphite particles moving away from thecathode to the anode follow the plasma column trajectory[39, 40].angle towards the metal bath (Fig.17) [25]. A 45-t VostAlpine plasma furnace went to operation in November1983. The main disadvantage of this furnace is situa-tion of the plasma torches. At the beginning of the melt-ing, plasma arcs melt a horizontal craters in the scrap.Due to plasma arc length limit the torches body enterin the furnace space under the unmelted scrap. Oftenthe scrap falls on the plasma torches and breaks them. Fig.15 “PLASMALAB” FFP-plasma reactor with chargeAs a result this plasma furnace construction did not feeding system [39, 40]find further industrial application but it remains a sig-nificant step in plasma metallurgy development. tively silent work of the plasma torches and the high KRUPP [26] developed AC melting plasma fur- airtightness of the furnace working space). The mainnaces, where the bottom electrode was avoided (Fig.18). problem of the high capacity power steelmaking (ACA starter DC plasma torch works as an electrode in and DC) plasma furnaces is the short working life ofmain powerfull AC plasma torch (Fig.19). The possi- the plasma torch electrodes (cathodes) under high cur-bility for regulation of the plasma torches inclination rent density.towards the vertical furnace axis improves the furnace Plasma-induction melting (Fig. 1, position 9)heating work and decreases the refractory lining con- takes place in the classical induction furnace which hassumption. a plasma torch installed over its crucible and works The plasma-arc melting compared with classical after the transferred arc scheme with bottom waterelectrical arc melting leads to an improved quality of the cooled electrode-anode (Fig. 20) [27].produced metal, decrease of the specific electric energy The use of two principally different methods forconsumption under increased output, enables the pro- heating - plasma and induction - leads to a decrease ofduction of low carbon alloys, increases the alloy elements the smelting time (respectively an increase of the fur-assimilation grade and the output in general, improves nace output) by 20-50 % in comparison with the classi-the production ecology (decreases the noise degree and cal induction furnace. In addition this heating combi-the quantity of the harmful emissions due to the rela- nation decreases the specific electrical energy consump-12
  11. 11. M. Mihovsky Fig. 16. Linde DC plasma furnace [24]. Fig. 19. KRUPP AC plasma torch [25]. Fig. 17. Freital-Voest AlpineDC plasma furnace [25]. The good pilot results obtained in the 60 kg plasma-induction furnace led to the design and recon- struction of an industrial 2,5 tons induction furnace operated in the metallurgical complex Kremikovtsy Ltd. to a plasma-induction one [29]. This furnace consists of two independent devices - an 1 MVA classical main frequency Russian induction furnace and transferred arc metallurgical plasma torch (PLASMALAB-3000) fit- ted into its cover over the crucible (Fig. 21). The plasma torch works with the plasma gas argon or nitrogen or their mixture at maximum currents of 3000 A. The power Fig. 18. KRUPP AC plasma furnace [26]. supply is realized by two DC units of 300 kVA each working parallel.tion by 10-18 %. The highly active slag (heated from The possibility for effectively melting industrialthe plasma torch) resulting from the process and the bronze turnings in this furnace and for producing higheffective stirring makes possible the production of a metal quality bronze castings has been proven. The plasma torchof low sulphur, gases and non metallic inclusion con- current during the melting was kept constant at 1500 Atents. The use of argon as plasma gas leads to a substan- and the voltage varied from 120 to 230 V depending ontial decrease of the burn-off alloy elements. the arc length. Depending on bronze turnings bulk den- Experiments on melting of copper, brass, bronze sity and preliminary processing the yield was 90-93 %;and high-speed steel turnings were made in a plasma the specific power consumption for melting was 420-induction furnace [28] at the plasma research labora- 464 kWh/t; the specific melting time was 1,2-1,55 h/t;tory PLASMALAB during the last few years. the aluminium oxidation was 0,7-0,9 % [30]. 13
  12. 12. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010 The replacing of DC (W-cathode) plasma torch in pilot PLASMALAB 60 kg plasma induction furnace with hollow graphite electrode reduces the running cost for copper and copper alloys production (Fig. 23) [31]. This system has turned out to be simple, very reliable and easy to maintain. On the other hand the productivity and the yield are practically the same. As a result of the pilot experiments the DC transferred arc plasma torch in 2.5 tons plasma induction furnace was replaced with 100 mm graphite electrode with 16 mm hole for plasma gas blowing. In the case of graphite electrode operation and argon blowing through the electrode hole, the arc length and the voltage are decreased compared to the length and voltage of the plasma torch arc under otherwise Fig. 20. PIF [27]. equal conditions due to the lack of the cooling and iso- lating effect of the plasma gas. This is the reason for substantial arc deviation to the crucible wall. The cath- ode spot moves on the edge of the hole, the anode spot has no clear contour. The V-A characteristics of graph- ite electrode arc have been measured in the current range 500-2300 A, constant arc length 300 mm and gas con- sumption 3800 l/h (Fig. 24). The graphite electrode curve is of a rising type similar to the V-A characteristics of the plasma torch arc, but voltage is 30 to 50 V lower. In the case of si- multaneous operation of plasma and induction heating the plasma arc expands due to interaction between the magnetic field of the induction furnace and the plasma Fig. 21. PLASMALAB 2,5-t PIF [30]. arc. The expansion of the arc in this case strongly de- pends on the current frequency of the induction fur-Fig. 22. 2,5-t plasma-induction furnace with metallurgical Fig. 23. 2,5-t plasma-induction furnace with 100 mm hollowplasma torch [31]. graphite electrode-cathode [31].14
  13. 13. M. Mihovskynace (max expansion at 50-60 Hz). While in the case of using a plasma torch this effect is undesirable – as it % graphite electrode+induction furnaceleads to the appearance of non-controllable secondary #arcs and higher errosion of nozzle and plasma torch plasma torch [5]body, in the case of graphite electrode the expanded arc ! Voltage, Vleads an increase of the working arc voltage (Fig. 24), respectively to an increase of arc power at constant cur- graphite electrode rent. Compared with the classical plasma torch, thehollow graphite electrode (both working at nominal %current 500 A) decreases the average furnace produc- #tivity with 5-8 %. On the other hand, when the two # # #systems - arc and induction heating work simultaneously Current, Athe furnace productivity increases with 9-11%, the melted Fig. 24. Comparison of V-A-characteristics of plasma torchcrater in the scrap is wider and the local overheating of and hollow graphite electrode at constant arc length 300 mm.the metal decreases. Plasma arc remelting (Fig.1, position 10 ) is a plasma arc furnaces and in addition leads to a fasterplasma technology, which has found widest industrial liquid metal heating, due to the high specific heat plasmaapplication at present. Patented in the institute Paton capacity. The plasma ladle reheater is used at U.S. STEEL,(Ukraine) (Fig. 25) [33] and developed by some leading Lorain, OH, to save aborts or to homogenize the tem-world companies as Daido, Retech and others, this tech- perature of the molten steel in the ladle [36].nology is recognized as a basic refining process, suc- The quantity of steel produced by continuos cast-cessfully competing with electro-slag remelting and ing has reached over 90 % of total world-wide produc-vacuum-arc remelting. The technology essence consists tion. The conventional characteristics of the continuousin melting of a metal ingot with plasma heating in a casting processes are: high heat losses from the melt in thecopper water cooled crystallizer, and obtaining of a re- tundish at the start of the casting of the comparatively highfined ingot of desired structure and with a decreased degree of average superheating DT during the further cast-gas and non-metallic inclusions content. The main ad- ing processes and the continuously changing casting pa-vantage of plasma-arc remelting to vacuum-arc remelt- rameters. As a result it is impossible to guarantee constanting is that the process takes place under atmosphere quality of the cast product. The plasma heating systemspressure or under increased pressure and enables an offer an optimal solution of the mentioned problems ineffective gas alloying of the metal. the conventional continuous casting process [37]. The effective nitrogen alloying during melting of Heating of the tundishes in continuous metal cast-high speed steel turnings is carried out by PLASMALAB ing machines enables the decrease of the metal tappingat present in two variations: plasma-induction melting furnace temperature by approximately 20°C, which leadsand plasma-arc melting in a copper water cooled mould to the refractory consumption decrease and increases theunder increased nitrogen pressure. Plasma gas (argon- melting furnace productivity. The intensive metal heat-nitrogen - 3:1) flow rate is 1200 l/h. The melt in the ing in the tundish, makes possible its argon blowing, andplasma-induction furnace is blown with nitrogen dur- in result - an optimal constant temperature and chemicaling the total melting process through the bottom water- composition homogeneity of the cast metal.cooled electrode-anode with a constant gas consump- In addition, this technology faciliates the organi-tion of 180 l/h. The final nitrogen content in solid metal zation of the processes in the melting plants (provides avaries in the range 0,092-0,098 % [34]. simple synchronization between the melting furnace and The rapid development of ladle metallurgy the continuous casting machine).(Fig.1, position 12) and of continuous metal casting, The Tetronics Limited has installed 20 Tundishopened a wide field for plasma heat application. The Heating systems worldwide, including 13 for major Japa-replacement of the ladle furnace graphite electrodes by nese steel companies, most of which are today heatingplasma torches, assures all the advantages, described for millions of tonnes of steel per year. Until 1993 Tetronics’ 15
  14. 14. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010 plasma processes is extremely important for the further development of plasma manufacturing technologies. CONCLUSIONS Based on the review made on the existing pilot and industrial metallurgical plasma technologies, sources and energy state and perspectives for world wide metal- lurgical development the following conclusions can be made: • The plasma reduction technologies are still under development and their industrial application is limited due to the following main reasons: − insufficient single power and operating reli- ability of existing non-transferred arc plasma torches; − complicated exploitation scheme for maximum heat and reduction potential utilization of off-gases from Fig. 25. PAR [33]. plasma units;tundish plasma heating system used a single polarity − dynamic change of relation between electrictorch with an arc running from the torch to the steel, energy and coke prices and last but not least the con-with a counter electrode immersed in the steel provid- servative thinking and insufficient financing of big met-ing the return path for the DC current. Installation of allurgical companies.the counter electrode requires tundish and/or tundish • The lack of qualitative ores on one side andcar modifications and the refractory lining becomes the significant quantities of stored waste from metal-complicated with expensive additions. Tetronics has now lurgical and chemical industries on the other are a fa-successfully introduced an improved avoiding any modi- vorable perspective for future application of plasma tech-fications to the tundish to incorporate the counter elec- nologies in metallurgy.trode [38]. • Plasma melting technologies development is Presently over 35 plasma tundish heating sys- limited mainly by designing and manufacturing of pow-tems are operating in steelwork world-wide, mainly in erful metallurgical plasma torches with enough singleJapan, China, Europe and USA. Main parameters of power (required currents: 100-130 kA) and reliability.these systems are tundish size 5-80-t, heating power 0,3- • The cathode and the nozzle are the main ele-4,3 MW, torch current 1,6-12 kA, current mode 90 % ment.DC, 10 % AC, plasma gas argon (nitrogen). • The most perspective and easily applicable tech- During the last decade the thermal plasma pro- nologies in the near future are plasma induction melting,cesses based on electrical arc or induction plasma gen- plasma remelting, plasma heating in the tundish tech-eration has been successfully applied in metallurgical nologies, which are already working in the developedindustry (tundish heating for continuous casting, ladle countries and have proved economical effectiveness.heating for temperature control, plasma heated cupola • In the case of a stable financial support for inten-for metal recovery), also in materials elaboration (pow- sive design, scientific research and introduction work in theder synthesis, coatings, spheroidization) and in materi- field of application of plasma in metallurgy, we can expectals processing (cutting, welding, surface treatment, etc.). actually working plasma technologies for processing of rawThe impressive increase of production of plasma manu- polymetal materials and waste from the metallurgical andfacturing equipment worldwide clearly indicates that the chemical industries in the beginning of the 21st century.thermal plasma technologies become extensively em- This would facilitate the solving of the complicated rawployed by industrial users. Therefore, scaling-up of materials, energy and enviromental problems in industry.16
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