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Pure & Appi. Chem., Vol. 48, pp. 179—194. Pergamon Press, 1976. Printed in Great Britain.
PLASMA ENGINEERING IN METALLURGY ANDINORGANIC MATERIALS TECHNOLOGY
N. N. RYKALINA. A. Baikov Institute of Metallurgy, Prospekt Lenina, 49 Moscow, B-334, USSR
Abstract—An account is presented of the work done in the USSR on the generation of thermal plasma, plasmamelting, and plasma jet processes. The various methods of plasma generation are reviewed, such as arc plasmagenerators and high frequency (HF) plasma generators including HF-induction plasmatrons, HF-capacityplasmatrons and HF-flame plasmatrons. Plasma melting techniques covered include plasma-arc remelting andreduction melting. Plasma jet reactors, multi-jet reactors, and processes such as product extraction, dispersedmaterial behaviour in plasma jets, production of disperse materials, reduction of metals, synthesis of metalcompounds, and production of composite materials are briefly described.
INTRODUCTION
Thermal plasma engineering enables new inorganicmaterials, with pre-determined mechanical and chemicalproperties, shape and structure to be produced, such asmetallic alloys, chemical metal compounds, ultra-disperseand spherical powders and refractory and compositematerials. Thermal plasma processes can play an impor-tant role in extraction metallurgy, both in the effectiveulilisation of polymetal ores and concentrates, and in theprocessing of industrial wastes, particularly environmen-tal pollutants.
The most promising application of thermal plasmaengineering is in the production of materials with newspecific properties, which cannot be synthesized by anyother method.
The use of thermal plasmas, in a metallurgicalinstallation, can essentially intensify many metallurgicalprocesses, as chemical reactions occur in the gas phase,between the vaporised condensed phases, and thedissociated and activated vapours, and not on the surface.The kinetics of such reactions is therefore intensified,resulting in milliseconds being sufficient for completion ofprocesses. The productivity of the installation per unittime, area and volume is remarkably improved if both theresponse time and volume are minimised.
The possibility of realizing thermal plasma processes,depends on the development of the appropriate plasmaequipment, i.e. the plasma generators, furnaces andreactors. The general requirements of process engineersare sufficient power, the possibility of utilising differentactive gases, sqch as hydrogen, oxygen, chlorine, methaneetc. and a durable service life.
The difficulties involved in realising plasma processesare primarily determined by an insufficient developmentof both the engineering and technological problemsdealing with specific conditions arising in high-temperature rapid-rate processes. Among these areproblems of jet diagnostics, powder mixing, quenching,condensation, high temperature filtering etc. A certaindanger may also arise from non-critical attempts inapplying thermal plasma to unsuitable objects andprocesses. It is therefore necessary, first of all, formetallurgists and chemical engineers to make a criticalassessment of both the advantages and disadvantages inapplying a particular plasma route.
Metallurgical and engineering plasma processes anddevices (in plasma engineering the processes and equip-
ment for their realisation are especially closely related)may be broadly classified, by the aggregate state of thematerial to be processed, into the following four groups:the processes involving the effect of plasma jets on acompact solid phase, on a compact liquid phase, ondispersed condensed material transformed to a certaindegree into vapour and on gaseous phases (which in pureform is a typical plasmochemical process) (Table 1).
If one neglects the overlap of typical characteristicsbetween these classes, and the complications arising fromchemical reactions, during the process, this simplifiedclassification may help to systemize the data and to assessthe main advantages and shortcomings of each type ofprocess.
A number of processes affecting a compact solid bodyhas already been realised on an industrial scale: cutting ofmetallic and non-organic materials, welding and building-up, realizing predetermined surface properties by thermalor chemical means, processing and drilling of rocks,spraying on protective coatings (heat-resistant, wear-resistant and corrosion-resistant), producing compositematerials by building-up matrix material on reinforcementfibres, producing refractory metal workpieces by sprayinglayers on the model subsequently smelting out. Thehardware and engineering problems of these processeshave to a certain extent been solved, and they are widelyused in industrial material processing technology.
1. THERMAL PLASMA GENERATION
Thermal plasma jets for technological applications aregenerated in direct and alternating current arc plasmot-rons, as well as in electrodeless high-frequency inductionplasmatrons. Research is under way for developingplasmatrons operating at high (up to 100 bar) and low(down to 10_2 torr) pressures, as well as plasmatrons ofthe ultrahigh frequency, pulsed arc discharge and othertypes.
Arc plasmatrons have a high efficiency (60—90%) andprovide high power of up to 2—5MW. Their service life,however, is limited by electrode erosion and, whenoperating with reactive gases (oxygen, chlorine, air) doesnot exceed 100—200 hr. With electrodes that erode, such asgraphite, the service life of arc plasmatrons used in thecracking of petroleum products, may reach severalhundred hours.
At the Institute of Thermal Physics in Novosibirsk(Prof. M. F. Zhukov) several types of arc plasmatrons for
179
180 N. N. RYKALIN
Table 1. Metallurgy and inorganic chemical engineering thermal plasma processes
500-1000 kW have been examined.4 Powerful three-phaseplasmatrons have been developed with a net efficiencyexceeding 90%. At the Paton Institute of Electric welding(Kiev) a series of direct and alternating current arcplasmatrons have been constructed. By joining severalplasmatrons in the reactor the total power may rise up to2—3 MW and more.
High frequency induction plasmatrons have at presentrelatively small power (up to 1 MW) with efficiencies from50 to 75% and their durability is limited only by theservice life of power sources (up to 2—3 months). At theBaikov Instute of Metallurgy (Moscow) high frequencyinduction generators on a power level up to 300 kW havebeen developed (I. D. Kulagin, L. M. Sorokin).
1. Arc plasma generatorsAmong the electric arc plasma generators the most
widely used are the linear types (Fig. 1). Cathodes aremade of tungsten rods alloyed with thorium, yttrium orlanthanum and zirconium, generally in a water-cooledcopper housing. The cathode service life ranges fromtwenty to several hundred hours depending on operationconditions. The service life of a copper ring-formed ortubular anode (intensively water-cooled) for currents upto 10 kA when operating with high enthalph gases reaches100—150 hr. The magnetic field for the rotation of the arcanode spot is provided by a water-cooled solenoidmounted on the anode housing. The arc and solenoid arepower-supplied, as a rule, in series from the same source.Argon, nitrogen, air, hydrogen, natural gas and theirmixtures are used as the plasma forming gas. Dependingon the type of gas, the efficiency varies within 60—85%.The average mass flow gas temperature for hydrogen onthe plasmatron outlet is up to 3700°K, for other gases—upto 4500—12,000°K.
The tendency to increase jet temperature and flow rate
(a)
by diminishing the channel diameter and increasing thelength of linear plasmatron, results in current shunting tothe tube body and can lead to the formation of afluctuating (cascade) arc. The maximum current of thefurnace plasmatron is limited not only by the service lifeof the cathode but also by the so-called current ofstationary stability, i.e. the current value at which the arccan burn for a long time without forming a cascade. Afluctuating arc leads to descruction of the linear plasmat-ron assembly and has hindered further development ofhigh power plasma furnaces. One way to decrease thepossibility of forming a cascade arc is by arc currentmodulation. The fluctuating arc does not occur if the arcburning time is lower than a certain value. The so-calledcurrent of dynamic stability can considerably exceed thevalue of stationary stability current.'
Some developments of arc plasma generators arepromising:
(a) A generator with interelectrode inserts in thesectioned channel and distributed gas supply (Fig. 2);2
(b) A generator with tubular electrodes and distributedgas inflow for heating up nitrogen, air and natural gas;with this type arc power is increased considerably byraising the voltage (Fig. 3);4
(c) A three-phase generator with 3 or 6 tungsten rod ortubular electrodes (Fig. 4)3 for heating hydrogen and inertgases. This type has a rather good service life at powerlevels up to 100 kW.
Rather extensive experience in discharge investigationsenables one to calculate, by using criterion relationships,the electric, gasdynamic and geometric parameters oflinear arc plasma generators with gas and magneticdischarge stabilisation for a wide power range and for thefalling and rising volt—ampere source characteristics.4
In high (atmospheric) pressure plasma arcs, plasma isthe main source of heat. Thus, the energy transferred byargon plasma can constitute 40-70% of the total value ofenergy absorbed by the compact heated body. Withdecreasing pressure (10 torr and lower), the arc spotbecomes the main heating source. The convective andradiative components of the heat transfer from plasma toheated body do not exceed 5—10% of the total energytransfer. The drop of potential in the anode area, observedin low pressure discharges in an argon-shielded atmos-phere amounts to several volts.
The hollow cathode for low-pressure arc (10-—1 torr) isconstructed in the form of a cylinder formed by tungstensections through which the plasma-forming gas is broughtin (Fig. 5)7 The hollow rod tungsten cathode has shown ahigh serviceability with argon, helium, hydrogen, nit-rogen. The electrode erosion is due only to the
I. Compact solid phase
Plasma interaction with:III. Dispersed condensed phase
II. Compact liquid phase in carrying gas jet IV. Gas phase
WeldingCuttingBuilding-up(Spraying on substrate)
ProcessesMelting Metals reductionRefining Synthesis of compoundsAlloying Ore benificationReduction smelting Powders processingCrystals production Spraying
Gas heating
BurnersCutters
MachineryPlasma furnaces Plasma jet
ReactorsPlasma jetReactors
2 3/
(Fig. 1. Arc plasma generator—linear type.4 (1) electrodes, (2) arc,(3) breakdown of low temperature gas, (4) electromagnetic coils, (5)
vortex chamber, (6) thermo-cathode.
Plasma engineering in metallurgy and inorganic materials technology 181
Fig. 3. Arc plasma generator with distributed gas injection.4 (1,2)electrodes; (8) current-conductive washer; (9) collector for gas
input.
Fig. 4. Three-phase arc plasma generators.3 (1) plasmatron body,(2) isolation insertion, (3) electrode holder, (4) electrodes, (5)
current input, (6) nozzle chamber.
evaporation of tungsten and is in agreement withLangmuir's law.
The low-pressure plasma is essentially on a non-equilibrium state. The electron temperature measured bya probe method amounts to 40 x i0—i00 x 103°K. Thetemperature of the neutral species does not exceed1500—3000°K. This rather high electron temperature playsan important role in transferring energy from thedischarge to the heated body.
For further development of arc plasma generators, it is
Positive ion
Electron generated in plasma
Electron from cathode surface
Fig. 5. Scheme of hollow cathode arc plasma generator.7
very important to investigate the electrode phenomena ind.c. and a.c. arcs, in order to increase the heatingefficiency and the electrodes' service life. Increasing thepower of plasmatrons is an urgent problem, especially forbig metallurgical and chemical installations. However,several plasmatrons of smaller unit power may bearranged in the same reactor. In this case, it is necessaryto have several independent power supply sources andcontrol units. The power supply scheme is much simplerwith a.c. plasma generators.
Thyristors with automatic arc current stabilisation arenow mostly used as power supply sources for d.c. plasmagenerators with parameters 1000 V/1000 A; more power-ful sources are available up to 7 MVA. For small plasmagenerators silicon-diode power supply sources are rated at350 V/600 A.
2. High frequency plasma generatorsThe main practical advantage of electrodeless HF
plasma generators lies in that the service life of plasmainstallation is limited only by life time of electro-vacuumparts of a transformer and of an electromagnetic energysource—approx. 2—3 x iO hr.
Energy generators and transformers providing thenecessary constant anode voltage (usually 10—12 kv),assembled on thyristors or semi-conductor diodes, havehigh efficiency (99%) and are practically unlimited inpower. The HF generators of electromagnetic energycircuits also use electro-vacuum parts: high power
Fig. 2. Arc plasma generator with a sectioned channel.2
11109 163
0 Electrons
• Positive ions• Gas molecules
182 N. N. RYKALIN
generator triodes, tetrodes, magnetrons etc., their powerreaching at present to approx 500 kW. Conventionalindustrial generators have high anode losses, up to20—40%, thus sharply reducing the HF system efficiencywhich does not exceed 40-60%. Two ways of diminishingthe anode losses to between 5—8% are being developed.These are (a) operating the generator under overloadconditions and (b) using special generator lamps withmagnetic focussing. HF industrial generator efficiencymay increase by these means up to 70—85%.
Energy generated by a high frequency electromagneticfield is used for gas heating in different types of HFplasmatrons: induction, capacity, flame and combined(Fig. 6).
HFI-induction plasmatrons have been developed themost. Their power in pilot plants has reached 200—300 kW;in laboratories 500—1000 kW units are being tested. Theminimum power necessary for self-sustained inductiondischarge is determined by the gas, pressure andfrequency of electromagnetic field. As the frequency isreduced from the MHz range to the hundreds of KHzrange, the power increases from less than 10 kW tohundreds of kW, and then rises hyperbolically on furtherfrequency reduction. Difficulties in supplying the powerfor discharges on standard industrial frequencies (50—60 Hz) are explained by this very phenomenon. To reducethe minimum power for sustaining an induction discharge,it is necessary to increase the plasma conductivity bylowering the pressure or by adding ionizing mixtures.
Electrodynamics of HFI-discharges is governed by thelaws of induction heating of conductive materials.However gas dynamic phenomena in HFI-discharge arerather complicated and can only be qualitativelyevaluated. That is why engineering methods to calculategas flow HFI-discharges, have yet to be developed.HFI-plasmatrons can operate with quartz or metallicdischarge chambers for different plasma forming gases.The most promising is the operation on chemically activegases: oxygen, chlorine hydrogen and vapours of reactivesubstances.
HFC -capacity -plasmatrons have no wearing parts, asthe electrodes are placed outside the discharge chamber.Capacity coupling of an HFC-discharge with the elec-trodes voltage leads to the formation of a phase shiftbetween the electrode and discharge current. Theelectrodynamic conditions of HCF discharge are worse-ned by a phase shift and so the efficiency of the dischargeis reduced. To maintain a self-supporting HFC-dischargecomparatively small power is necessary: in the range of10—20 MHz it equals 0.2 kW for air and 1.0 kW forhydrogen operation. This presents an essential advantage.An efficiency of about 40% has been achieved on a 10 kWpower level. -We do not envisage any major difficulties in
increasing the power of HFC-plasmatrons and in develop-ing a HFC-systems of 100 and 1000 kW power levels. ForHFC-plasmatrons, any plasma forming gases are suitable.
HFF -flame plasmatrons are essentially of a combinedtype, as the electrode discharge current is groundedthrough the distributed capacity. The efficiency of thesystem is near to 50%. Even lower minimum power isnecessary to maintain the HFF-discharge. The presenceof an erodable electrode limits the choice of a plasmaforming gas, though at a power level of up to 10 kW,erosion of this electrode is unessential.
Plasmatrons with combined energy supply: HF + directcurrent; HF + alternating current; HF + LF (low fre-quency) are not yet fully developed, but many present acertain interest.
In the field of HF plasma industrial engineering the -following problems have to be solved: increasing theefficiency of the anode circuit up to 90—95%; increasingthe power of HF-plasmatrons up to 3—5 MW; developingcombined energy supply plasmatrons. On theoretical sideof HF-discharges it is important to develop engineeringmethods of calculation HF-plasmatrons taking intoaccount dynamics of a plasma gas flow, especially inturbulent conditions. Attention should also be paid todevelopment of tubeless generation of HF-electromagnetic oscillations.
2. PLASMA MELTING
The processes concerned with the effect of thermalplasma on compact molten material, the melting of metalsand ceramics, the alloying and refining remelting of metalsand alloys, the reduction smelting of metals and thegrowing of metallic and ceramic crystals, are carried outin plasma furnaces. Processes in industrial use at thepresent are: the continuous remelting of bars or rods(electrodes) in the water-cooled crystalliser, the intermit-tent melting of materials in a ceramic crucible, andcombined methods, e.g. induction plasma melting ofmetals and alloys.
1. Plasma furnacesA number of types of plasma furnaces for laboratory
and industrial applications have been developed.Industrial plasma furnaces for semi-continuous opera-
tion have been developed at the Paton Electric WeldingInstitute (Kiev)—Table 2. From 3 to 6 d.c. or a.c.plasmatrons are radially arranged (Fig. 7). The furnacesare designed for remelting of axially located ingots. Thesefurnaces are used for refining of precision and heat-resistant - alloys, high-temperature metals, ball bearingsteels high tensile special steels as well as for nitrogenalloying of metals (Fig. 8).
Furnaces with three-phase power supply have beendeveloped by Electrotherme (Belgium).6 The furnaces aredesigned for the refining of niobium, tantalum, bolyb-denum, titanium - and other metals as well as ofheat-resistant alloys based on nickel and cobalt.
Table 2. Paton electric welding institute plasma furnaces5
Furnace type Y-461 Y-467 Y-600
Total power, kW 160 360 1800Number of plasmatrons 6 6 6Maximum weight of ingot, kg 30 460 5000Maximum diameter of ingot, mm 100 250 630Extrusion rate of ingot, mm/mm 1.5—15 1.5—15 2—20
2 3
Fig. 6. Schemes of high frequency plasma generators. (1)Induction—HF!; (2) Capacity—HFC; (3) Flame—HFF.
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184 N. N. RYKALIN
Table 3. IMET laboratory plasma furnaces
No.Layout
Numberplasmatrons
Diameterof ingot
(mm)
Plasma-forming gas Gas consump-tion
(cbm/hr)
Operatingpressure in
melting space
Power(kW)
Efficiency(%)
1. Axial 1—3 60-100 argon, mixture of argon andnitrogen and hydrogen
2-4 (1—3) bar 30 30-40
2. Axial 1 60—100 argon, helium, nitrogen,hydrogen
10_3_10_1 10—10 ton 100 30—70
3. Radial 4 100 argon, argon—nitrogen, ammonia 2—4 1—2 bar 50 30—50
Note: The efficiency was measured on water-cooled copper anode.
(anode). This type is especially promising for metalrefining, melting steel and special alloys and for producingbig-size high-quality castings. Furnaces for batch workhaving a capacity of up to 10t9"° have been also built inUSSR, GDR.
A plasma induction furnace, with the induction heatingbeing combined with plasma arc heating (Fig. 12) has beendeveloped by Daido Steel (Japan)." A plasma arc usingsome 35% out of total power of 100—500 kW increasesconsiderably the output of the furnace, and intensifies therefining action of slags. These furnaces are used formelting stainless steels, non-ferrous metals and specialalloys.
2. Plasma -arc remeltingThis substantially improves the quality of metal. Unlike
vacuum-arc and electron-beam remelting, the losses ofhighly vaporizable components (manganese, molyb-denum, magnesium etc.) by the plasma process are verylow. Plasma arc remelting makes it possible to refinealloys with readily oxidizable and chemically activecomponents—tittinium, aluminium. The most widely usedgases are argon, argon—hydrogen (for iron—nickel andnickel alloys) and argon—nitrogen (for alloying from thegas phase).
In a plasma furnace the liquid metal bath is affected byactivated gas particles of the plasma jet. Therefore, theequilibrium concentrations of reagents in this case willdiffer from the equilibrium concentrations with thenon-activated gas. Investigation of the interaction be-tween liquid metal and nitrogen containing plasma has
Fig. 12. Induction furnace with arc plasma generator.1'
shown the possibility of alloying metal by nitrogen fromthe gaseous phase. The plasma alloying enables one toobtain higher concentrations of nitrogen in the ingot and arather uniform distribution of the nitride phase, both ofwhich are unattainable by other methods.'2'5
The Paton Electric Welding Institute (Kiev) hasdeveloped the industrial technology of producing morenitrided grades of stainless steel. The way is thus open forproducing new alloys with an increased nitrogen content,e.g. alloys of bc.c. metals with internal alloying im-purities. Plasma-arc remelting enables one to control thealloying phase content within the prescribed limits and isnow an established industrial method for obtaining suchcompositions.
A plasma-arc method of growing large monocrystals ofrefractory metals, up to 50 mm in diameter and weighingmore than 10kg, has been developed in the BaikovInstitute of metallurgy (Prof. E. M. Savitsky) and realizedin industry.'3 Tungsten monocrystals produced withplasma melting are characterized by a high purity (Fig.13): high technological plasticity, resistance to recrystalli-zation and creep, and anisotropy of emissive propertiesreaching 30—70%.
3. Reduction meltingA plasma melting process can be combined with metal
reduction by gaseous or solid reducers: hydrogen,ammonia, natural gas, petroleum cracking products andcarbon. The furnaces for reduction melting have to be
b0U)>.U00EC0)1-C
C
C0)4-C0UC0aU
Rate of monocrystal growth, mm/mm
Fig. 13. Decarbonization efficiency of plasma remelting bytungsten monocrystals production.'3 Carbon content in startingtungsten—0.014—0.016%. (x-axis—rate of monocrystal growthmm/mm; y-axis—carbon content in tungsten monocrystal,
xlllY3 wt%.)
Plasma arc torch
Peeping window for
Hopper for alloyaddition
Coil for
heating
Electrode
Plasma engineering in metallurgy and inorganic materials technology 185
provided with appliances for the formation of the ingotand the removal of condensed and gaseous reactionproducts from the furnace.
Hydrogen plasma reduction melting with deep deoxida-tion, which has made it possible to discard the subsequentdeoxidation process by producing soft magnetic alloys(50% Fe and 50% Ni) altogether,'3 was carried out at thePaton Institute (Kiev). Reduction melting of a materialcontaining 80% metallic iron was performed at BaikovInstitute of metallurgy (Moscow) in a radial plasmafurnace. Ammonia admixture to plasma forming argonacted as a reducer. After melting a 100% high purity ironingot was obtained.
Refractory materials (oxides, nitrides) are melted inBelgium, France and Great Britain in plasma furnaceswith rotating ceramic crucible.'4'15 These furnaces have ahorizontally or vertically located crucible of heat-resistantrefractory material. The inner cavity of the crucible has abarrel shape and is heated by arc plasma column (Fig. 14).The charged material is melted by convective andradiative heat from the plasma arc column. Oxidising,reducing and vapourizing processes can be carried out inthese furnaces under batch or continuous operatingconditions.
3. PLASMA JET PROCESSES
Chemical and metallurgical processes progressingunder the effect of thermal plasma jets on the condensedphase of the dispersed material, such as the reduction of
Fig. 14. Arc plasma furnaces with rotating ceramic crucible. (a)horizontal or inclined axis.'4 (b) vertical axis.'5.
metals from simple compounds, the direct and oxidising—reducing synthesis of metal compounds, the processingand decomposition of raw materials, are realised inplasma jet reactors.
1. Plasma jet reactorsChemical and metallurgical plasma jet processes are
carried out as a rule on dispersed particles of condensedmaterials. The introduction of dispersed material into thehigh temperature jet zone and its extraction from thewake of hot gases stream represent complex engineeringproblems, in view of the high temperatures and rates ofgas flow. Complete processing of the starting material anda maximum fixation of the product must be achieved.
In research and development of plasma processes,simple direct flow cylindrical reactors with water orgas-cooled metal walls and with a single plasmatron aremostly used (Fig. 15). The reactor diameter at the jet inletlies generally within 2—10 jet diameters. The startingmaterial is introduced into the jet by the transporting gas(dispersed raw material) or by overpressure (liquid andvapour materials). Cooling gas is sometimes blown inthrough the reactor walls for terminating the hightemperature reaction and fixing the condensed phaseproduct.
The disadvantages of simple cylindrical reactors are asfollows. The dispersed raw material, deposits on theoutlet nozzle of the plasma generator and on the reactorwalls. The optimum conditions that will eliminate orminimize these disadvantages for introducing the mater-ial into the jet that will eliminate or minimize thesedisadvantages are to be determined for each reactor typeby special investigations. The deposit formation on thereactor walls can be dealt with in different ways; byincreasing the reactor diameter by raising the temperatureof its inner wall, by blowing on the walls with ballast gas,and by imposing ultrasonic vibrations.
The quenching, i.e. rapid chilling of the reactionproducts for small scale processes, is usually realized bycold gas jets, on the cooling surface of a rotating metaldrum.
Reactors in which the dispersed raw material is broughtin directly to the zone of electric discharge have not yetgained wide application, though a number of interestingsuggestions have been put forward. One of them is areactor involving a fountain layer with high frequencydischarge torch (Fig. 16).16 Another one is a magneto-hydrodynamic spatial discharge reactor (Fig. 17). Theplasma jet formed by a conventional arc generator acts asa cathode for a more powerful spatial discharge in thezone of the solenoid magnetic field. The powder to beprocessed is brought into the same space. The particlesresidence time in the high temperature zone is essentiallyincreased due to the comparatively low gas flow rate anddrift due to the tangential component of the velocity.Since the concentration of raw material in the space isrelatively low, the discharge remains stable, and varia-tions of its parameters do not exceed 10%. With thebottom arrangement of the plasma generator the materialresidence time in the high temperature zone is still longer.2
High frequency and flame (ultra high-frequency)plasma generators are usually combined with the directflow reactors, the raw material being introduced into thedischarge area or below the discharge. An exception is thehigh frequency torch discharge with the rountain layerreactors.
(b)
186 N. N. RYKALIN
(b)
Fig. 15. (a) Direct flow reactor with cooled walls for processingdispersed materials. (1) plasma generator; (2) powder feeder; (3)reactor body; (4) power supply source. (b)Direct flow reactor withhigh frequency generator and axial material injection. (1) body; (2)inductor; (3) discharge chamber; (4) vortex chamber; (5)feeder;(6)
quenching device; (7) reactor.
2. Multi-jet reactorsThe reactor with two conflicting plasma jets is used for
processing polydispersed raw materials (Fig. 18).17 Thefinely dispersed material formed in the process is carried
out of the reactor, while the large unprocessed particles ofthe raw material oscillate in the high temperature turbulentwake of the opposite by directed gas jets; the larger theparticle the longer it stays inthe high temperature zone..
— 4
+6
5
(a)
3
4
7
Fig. 16. Fountainlayerreactorwith a dischargetorch'6 (1) housing,(2) feeder, (3) reducer, (4) plasmatron, (5) nozzle, (6) separation
device, (7) hopper.
Fig. 17. Reactor with magnetic and hydrodynamic spatialdischarge.2 (1) plasmatron, (2) plasma jet-cathode, (3) main anode,(4) housing, (5) solenoid, (6) zone of the spatial discharge, (7)quenching device, (8) cooling gas supply, (9, 10, 11) current lines,(12, 13, 14) power supply sources, (15) oscillator, (6) plasma-
forming gas supply (17) powder supply.
Plasma engineering in metallurgy and inorganic materials technology 187
The reactor with the three-arc mixing chamber shownin Fig. l9 provides a rather uniform temperaturedistribution across the section of 0.85 dia, at a distance ofabout two diameters from chamber outlet. Usually theplasma generators are placed normally to the chamberaxis, but installation at an angle of 60° to the axisfacilitates the axial injection of raw material.
Installatiohs are available in which several plasmatronsare symmetrically arranged on a conical head attached tothe cylindrical portion of the reactor. The hrnterial issupplied to the top of the cone, closer to the plasma jets,so that fraction of processed raw material is beingincreased.
Reactor with several plasmatrons fixed tangentially(Fig. 20)16 ensures a uniform temperature distributionacross the sections of the reactor, although the heat lossesthrough the walls are rather high.
Product extraction. If the product is formed in moltenstate and accumulated on a liquid bath its removal fromthe apparatus can be carried out either periodically orcontinuously. It is also rather easy to withdraw large-sizefriable powders. Rather complex problems arise withultra-dispersed powders tending to stick together, or withpyrophoric powders. Such powders require highly effec-tive filters, thereby increasing considerably their size.Special maintenance is required. Stringent requirementsare imposed on reactor sealing; when producing highly'active powders that are easily oxidized in the air; theextraction of such powders can be accomplished byintermediate lock chambers. Self-ignition of the powder inthe air can be eliminated by introducing passivatingadditions or by thermal annealing in the reductionatmosphere.
(a) (b)
Fig. 19. Three-arc mixing chamber (a) and scheme of jetinteraction (b).4
3. Dispersed material behaviour in plasma jetsInteraction between the disperse material and the
heated gas jets, as well as gas jets mixing phenomena, areinvestigated in order to develop optimal reactor designsfor various plasma processes. Efficiency of chemico-metallurgical plasma processes as well as product qualityare primarily determined by heating up and transformingthe raw material (fusion, evaporation, chemical reac-tions), by condensation of the vapours formed, and bycoagulation of the condensed product particles.
Material is held in the reactor zone for rather briefresidence times. The complexity of experiments makes itimpossible so far to obtain full information on the materialbehaviour, for the range of temperatures and rates for thestate of substance in which we are interested. Certainresults have been obtained on the heating up and themotion of rather large particles (over 100—150 mkm) maplasma jet. Pictures of the jet were taken through arotating perforated disk using high-speed filming andphotometry at different wave lengths. Laser diagnosticsof the bi-phase jet is very promising too.
Mathematical modelling of the disperse materialbehaviour in the plasma jet brings rather encouragingresults. Undoubtly, a complete model taking into accountall known phenomena in the particle loaded jet wouldhave been too complicated for carculation and analysis.Therefore several simplified models have been suggested.
A rather complete model worked out by Yu. V.Tsvetkov and S. A. Panfilov considers heating up, phasetransformations (melting, evaporation) and accelerationof spherical particles less than 50 mkm diameter, whichare uniformly distributed across a jet-section having noradial gradients of velocity and temperature.18
This model enables one to analyse the kinetics of gas jetand particle velocity, to evaluate the degree of materialevaporation and to choose the process parameters and thelength of the direct-flow reactor Fig. 21. Length of thecomplete evaporation path of the tungsten trioxideparticles of different diameter in a hydrogen—argon jetrises with initial jet temperature and with particlediameter, Fig. 22. Calculated and experimental data forthe degree of reduction of tungsten oxide W03 in ahydrogen—argon jet, and in an argon jet with carbonparticles rises with initial jet temperature—Fig. 23. The
-s
Fig. 18 Scheme of a reactor with conflicting plasma jets.17P1,2—plasma generators; G,,2—gas input; S—powder feeder;
R—reactor body.
Fig. 20. Cyclone reactor.'6 (1) reaction chamber, (2) mixingchamber, (3) plasmatron, (4) gas supply, (5) raw material input, (6)
reactor cone, (7) hopper, (8) gas phase outlet, (9)branchpipe.
6P
5
04
P
3
2
5 20 25
r, 105sec
Fig. 21. Kinetics of gas (Ar) and particle (W) parameters.'8=0.5 cm; C,, = 0.1 g/sec; GA = 1.18 g/sec. T8,—temperature
of gas and particles, °K; w8,,, —rate of gas and particles, cm/sec;i—length of particle's path, cm; r—particle's radius, cm; a—heatexchange coefficient of gas with particle, cal/cm2 Sec deg;
t—time, sec.
discrepancy between the calculated and experimentalresults is observed at low average mass temperatures ofgas jet because of jet temperature non-uniformity. Thismodel is at present being extended to take into accountthe gradients of temperature and gas velocity across thejet section.
Simplified models have been developed: a model ofheating up of moving particles in the isothermal jet ofgas,'9 a model of heating up of moving particles in a jetwith parameters changing along its length, 20 a model ofcondensation and coagulation of particulates in anisothermal jet.2' Assuming that the velocity Wg and
25mcm
- I0mcm
.5mcm
4000 4500 5000 5500 6000
T, °K
Fig. 22. Length of complete evaporation path of W03 particlesdepending on their radii and the initial temperature T0 of thehydrogen-argon jet:'8 R, = 0.5 cm; CA = 1.2g/sec; C,2 =
0.015 g/sec; C,, = 0.33 g/sec; r = 2.5;5; 10.0 and 25 mçm.
temperature Tg of a gas jet in the course of its interactionwith particles remain constant, which is reasonable for thehigh-enthalpy gases and taking the Nusselt criterion inorder of 2, then for the small diameter particulates(<50 mkm) it is possible to express the dependence ofparticle velocity w,, cm/sec and temperature T °C on time
in explicit form (Nikolaev)'9
T Tg(TgTpo)exp(tIt'); Bi'1 (1)
Vp = Vg [1 — exp (—t/t")]; Re < 1 (2)
V Y5. Re>2 (3)P
T0—initial particle temperature, °C; t'—time constant ofparticle temperature rise; t" and t"—time constants ofparticles acceleration, sec
,_Cpypd. 1_ d2yp . 4d'yp—
6a ' —
l8Vyg'—
3ifJVgyg
Here: d—particle diameter, cm; a—heat exchangecoefficient, cal/cmsec°grad; C —specific heat of particlematerial, cal/g grad; y, yg—densities of particle materialand ' of jet gas, g/cm3; V—gas kinematic viscosity,cm2/sec; i/i—drag coefficient.
These expressions can be used, e.g. for evaluating thetime of heating particles up to the melting temperatureand length of heating path.
4. Production of disperse materialsDisperse metallic and non-metallic materials are used in
manufacturing powder metallurgy products and porousparts (filters), to strengthen metals and alloys, to producespecial ceramics and plastics, components for electronicappliances and also directly as abrasives, catalysts,propellant components and pigments. When manufactur-ing powder materials in plasma jets, the dispersity andshape of powder particles, their purity and surfacephysico-chemical properties are controlled by the jetparameters (power, temperature, flow rate, gas partialpressure) and the tempering intensity. As the reactionsproceed within the plasma jet and its wake, the processedmaterials have no contact with the reactor walls, so thereaction products are not contaminated by the liningmaterial. Therefore, a jet of low-temperature plasma, and,especially, that of high-frequency induction plasma,makes it possible to obtain high-purity powders (ultradis-persed, spheroidized, composite etc.) based on metals,alloys, oxides, nitrides, carbides, borides, hydrides andcomplex compounds.
Spherical particulates of pre-determined size areproduced by plasma heating: from wire or bars butt-melted by plasma arc (Fig. 24) or by supplying standard orgranular powders to the plasma jet (Fig. 15). The processhas been realised in both versions on installations rated upto 100 kW.22
Particles of tungsten, molybdenum, nickel and othermetals and high-temperature oxides from 0.1 up to 2 mm,with an output up to 15kg/hr are produced by wire or rodmelting. By powder surface melting particulates areobtained of high-temperature metals and their alloys,titanium and chromium carbides, tungsten, aluminium andzirconium oxides measuring from 1 mkm up to 1 mm withan output of the spherical fraction of over 90% (Fig. 25).This process is realized in electric arc plasmatrons with a
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Plasma engineering in metallurgy and inorganic materials technology 193
thermodynamic and transport properties of substances ina dissociated and partially ionized state (Institute of HighTemperatures, Moscow).
Research of non-stationary, modulated and interactingplasma jets with superimposed electric and magneticfields as well as of pulse, cascade, colliding and multi-jetplasmatrons operating in a.c., d.c. HF, UHF at elevatedpressures, and in vacuo, shall enlarge appreciably the fieldof thermal plasma technological applications.
Industrial applicationsThermal plasma processing is in industrial use for
producing special quality alloys, obtaining monocrystals,producing pure powders of specific structure, sphericaland ultradispersed, carrying out direct synthesis ofcompounds and a synthesis combined with oxidising andreducing reactions. Plasma processes are rather suitablefor processing of complex ores such as phosphate,silicon-aluminium and titanium ores and valuable indus-trial wastes ("man-made ores"), e.g. wastes of refractoryand rare metals, cinder wastes, sulphur containinggaseous wastes of non-ferrous metallurgy. The possibilityof varying both the temperature and the mediumcomposition in the reaction zone of a thermal plasmapresents powerful means for developing fundamentallynew processes, as well as modernising the traditionalroutes in metallurgy and inorganic materials technology.
Acknowledgements--Author is grateful for the assistance givenby his co-workers: Dr. A. V. Nikolaev and Dr. S. A. Panfflov inpreparing this lecture.
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