Pun & Appl. CMm., Vol. 48, pp. 179-194. Pergarnon Press, 1976. Printed in Great Britain.
PLASMA ENGINEERING IN METALLURGY AND INORGANIC MATERIALS TECHNOLOGY
N. N. RYKAUN A. 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, plasma melting, and plasma jet processes. The various methods of plasma generation are reviewed, such as arc plasma generators and high frequency (HF) plasma . generators including HF-induction plasmatrons, HF-capacity plasmatrons and HF-flame plasmatrons. Plasma melting techniques covered include plasma-arc remelting and reduction 'melting. Plasma jet reactors, multi-jet reactors, and processes such as product extraction, dispersed material behaviour in plasma jets, production of disperse materials, reduction of metals, synthesis of metal compounds, and production of composite materials are briefty described.
INTRODUCTION ment for their realisation are especially closely related) Thermal plasma engineering enables new inorganic may be broadly classified, by the aggregate state of the materials, with pre-determined mechanical and chemical material to be processed, into the following four groups: properties, shape and structure to be produced, such as the processes involving the effect of plasma jets on a metallic alloys, chemical metal compounds, ultra-disperse compact solid phase, on a compact liquid phase, on and spherical powders and refractory and composite dispersed condensed material transformed to a certain materials. Thermal plasma processes can play an impor degree into vapour and on gaseous phases (which in pure tant role in extraction metallurgy, both in the effective form is a typical plasmochemical process) (Table 1). ulilisation of polymetal ores and concentrates, and in the If one neglects the overlap of typical characteristics processing of industrial wastes, particularly environmen between these classes, and the complications arising from tal pollutants. chemical reactions, during the process, this simplified The most promising application of thermal plasma classification may help to systemize the data and to assess engineering is in the production of materials with new the main advantages and shortcomings of each type of specific properties, which cannot be synthesized by any process. other method. A number of processes affecting a compact solid body The use of thermal plasmas, in a metallurgical has already been realised on an industrial scale: cutting of installation, can essentially intensify many metallurgical metallic and non-organic materials, welding and building processes, as chemical reactions occur in the gas phase, up, realizing predetermined surface properties by thermal between the vaporised condensed phases, and the or chemical means, processing and drilling of rocks, dissociated and activated vapours, and not on the surface. spraying on protective coatings (heat-resistant, wear The kinetics of such reactions is therefore intensified, resistant and corrosion-resistant), producing composite resulting in milliseconds being sufficient for.completion of materials by building-up matrix material on reinforcement processes. The productivity of the installation per unit fibres, producing refractory metal workpieces by spraying time, area and volume is remarkably improved if both the layers on the model subsequently smelting out. The response time and volume are minimised. hardware and engineering problems of these processes The possibility of realizing thermal plasma processes, have to a certain extent been solved, and they are widely depends on the development of the appropriate plasma used in industrial material processing technology. equipment, i.e. the plasma generators, furnaces and reactors. The general requirements of process engineers 1. THERMAL PLASMA GENERATION are sufficient power, the possibility of utilising different Thermal plasma jets for technological applications are active gases, sqch as hydrogen, oxygen, chlorine, methane generated in direct and altemating current arc plasmot etc. and a durable service life. rons, as weil as in electrodeless high-frequency induction The difficulties involved in realising plasma processes plasmatrons. Research is under way for developing are primarily determined by an insufficient development plasmatrons operating at high (up to 100 bar) and low of both the engineering and technological problems (down to 10-2 torr) pressures, as weil as plasllllltrons of dealing with specific conditions arising in high the ultrahigh frequency, pulsed arc discharge and other temperature rapid-rate processes. Among these are types. problems of jet diagnostics, powder mixing, quenching, Are plasmatrons have a high efficiency (60--90%) and condensation, high temperature filtering etc. A certain provide high power of up to 2-5 MW. Their service life, danger may also arise from non-critical attempts in however, is limited by electrode erosion and, when applying thermal plasma to unsuitable objects and operating with reactive gases (oxygen, chlorine, air) does processes. It is therefore necessary, first of all, for not exceed 100--200 hr. With electrodes that erode, such as metallurgists and chemical engineers to make a critical graphite, the service life of arc plasmatrons used in the assessment of both the advantages and disadvantages in cracking of petroleum products, may reach several applying a particular plasma route. hundred hours. Metallurgical and engineering plasma processes and At the Institute of Thermal Physics in Novosibirsk devices (in plasma engineering the processes and equip- (Prof. M. F. Zhukov) several types of arc plasmatrons.for
179 180 N.N. RYKALIN
Table 1. Metallurgy and inorganic chemical engineeringthermal plasma processes
Plasmainteraction with: III. Dispersed condensed phase I. Compact solid phase . II. Compact liquid phase in carrying gas jet IV. Gas phase
Processes Welding Melting Metals reduction Gas heating Cutting Refining Synthesis of compounds Building-up Alloying Ore benification (Spraying on substrate) Reduction smelting Powders processing Crystals production Spraying
Machinery · Bumers Plasma furnaces Plasmajet Plasmajet CUtters Reactors Reactors
500-1000 kW have been examined.4 Powerful three-phase by diminishing the channel diameter and increasing the plasmatrons have been developed with a net efficiency length of linear plasmatron, results in current shunting to exceeding 90%. At the Paton Institute of Electric welding the tube body and can Iead to the formation of a (Kiev) a series of direct and alternating current arc fluctuating (cascade) arc. The maximum current of the plasmatrons have been constructed. By joining several furnace plasmatron is limited not only by the service life plasmatrons in the reactor the total power may rise up to of the cathode but also by the so-called current of 2-3 MW and more. stationary stability, i.e. the current value at which the arc High frequency induction plasmatrons have at present can bum for a long time without forming a cascade. A relatively small power (up to 1 MW) with efficiencies from fluctuating arc Ieads to descruction of the linear plasmat 50 to 75% and their durability is limited only by the ron assembly and has bindered further development of service life of power sources (up to 2-3 months). At the high power plasma furnaces. One way to decrease the Baikov Instute of Metallurgy (Moscow) high frequency possibility of forming a cascade arc is by arc current induction generators on a power Ievel up to 300 kW have modulation. The fluctuating arc does not occur if the arc been developed (I. D. Kulagin, L. M. Sorokin). burning time is lower than a certain value. The so-called current of dynamic stability can considerably exceed the value of stationary stability current. 1 1. Are plasma generators Some developments of arc plasma generators are Among the electric. arc plasma generators the most promising: widely used are the linear types (Fig. 1). Cathodes are (a) A generator with interelectrode inserts in the made of tungsten rods alloyed with thorium, yttrium or sectioned channel and distributed gas supply (Fig. 2);2 lanthanum and zirconium, generally in a water-cooled (b) A generator with tubular electrodes and distributed copper housing. The cathode service life ranges from gas inflow for heating up nitrogen, air and natural gas; twenty to several hundred hours depending on operation with this type arc power is increased considerably by conditions. The service life of a copper ring-formed or raising the voltage (Fig. 3);4 tubular anode (intensively water-cooled) for currents up (c) A three-phase generator with 3 or 6 tungstenrod or to 10 kA when operating with high enthalph gases reaches tubular electrodes (Fig. 4)3 for heating hydrogen and inert 100-150hr. The magnetic field for the rotation of the arc gases. This type has a rather good service life at power anode spot is provided by a water-cooled solenoid Ievels up to 100 kW. mounted on the anode housing. The arc and solenoid are Rather extensive experience in discharge investigations power-supplied, as a rule, in series from the same source. enables one to calculate, by using criterion relationships, Argon, nitrogen, air, hydrogen, natural gas and their the electric, gasdynamic and geometric parameters of mixtures are used as the plasma forming gas. Depending linear arc plasma generators with gas and magnetic on the type of gas, the efficiency varies within 60-85%. discharge stabilisation for a wide power range and for the The average mass flow gas temperature for hydrogen on falling and rising volt-ampere source characteristics.4 the plasmatron outlet is up to 3700"K, for other gases-up In high (atmospheric) pressure plasma arcs, plasma is to 4500-12,000"K. the main source of heat. Thus, the energy transferred by The tendency to increase jet temperature and flow rate argon plasma can constitute 40-70% of the total value of energy absorbed by the compact heated body. With (a) decreasing pressure (10 torr and lower), the arc spot becomes the main heating source. The convective and radiative components of the heat transfer from plasma to heated body do not exceed 5-10% of the total energy transfer. The drop of potential in the anode area, observed in low pressure discharges in an argon-shielded atmos phere amounts to several volts. The hollow cathode for low-pressure arc (10-3-1 torr) is constructed in the form of a cylinder formed by tungsten sections through which the plasma-forming gas is brought (Fig. 1. Areplasma generator-linear type.• (1) electrodes, (2) arc, in (Fig. 5). 7 The hollow rod tungsten cathode has shown a (3) breakdown of low temperature gas, (4) electromagnetic coils, (5) high Serviceability with argon, helium, hydrogen, nit vortex chamber, (6) thermo-cathode. rogen. The electrode erosion is due only to the Plasmaengineering in metallurgy and inorganic materials technology 181
Fig. 2. Areplasmagenerator with a sectioned channel. 2
Gas o Electrons • Positive ions • Gas molecules Hollow cathode ~.ina•p~-~ .. I l~• \ _ _!:! I "--1 I I I + + +. I ac \,I I II/' power 1 1 : : supply i • I I· 4---- II I 'I Positive ion 'I I • -_ ~l~ron generated in plasma Fig. 3. Are plasma generator with distributed gas injection.• (1, 2) +I I : + Electron from cathode surface electrodes; (8) current-conductive washer; (9) collector for gas input.
Fig. 5. Scheme of hollow cathode arc plasma generator.7
very important to investigate the electrode phenomena in d.c. and a.c. arcs, in order to increase the heating efficiency and the electrodes' service life. lncreasing the power of plasmatrons is an urgent problem, especially for big metallurgical and chemical installations. However, several plasmatrons of smaller unit power may be arranged in the same reactor. In this case, it is necessary to have several independent power supply sources and control units. The power supply scheme is much simpler with a.c. plasma generators. Thyristors with automatic arc current stabilisation are now mostly used as power supply sources for d.c. plasma generators with parameters 1000V/1000A; more power ful sources are available up to 7 MV A. For small plasma generators silicon-diode power supply sources are rated at Fig. 4. Three-phase arc plasma generators.' (1) plasmatron body, 350V/600A. (2) isolation insertion, (3) electrode holder, (4) electrodes, (5) current input, (6) nozzle chamber. 2. High frequency plasma generators The main practical advantage of electrodeless HF evaporation of tungsten and is in agreement with plasma generators lies in that the service life of plasma Langmuir's law. installation is limited only by life time of electro-vacuum The low-pressure plasma is essentially on a non parts of a transformer and of an electromagnetic energy equi1ibrium state. The electron temperature measured by source-approx. 2-3 x Hr hr. a probe method amounts to 40 X Hr-100 X Hr°K. The Energy generators and transformers providing the temperature of the neutral species does not exceed necessary constant anode voltage (usually 10-12 kv), 1500-3000"K. This rather high electron temperature plays assembled on thyristors or semi-conductor diodes, have an important role in transferring energy from the high efficiency (99%) and are practically unlimited in discharge to the heated body. power. The HF generators of electromagnetic energy For further development of arc plasma generators, it is circuits ·also use electro-vacuum parts: high power 182 N. N. RYKALIN generator triodes, tetrodes, magnetrons etc., their power increasing the power of HFC-plasmatrons and in develop reaching at present to approx 500 kW. Conventional ing a HFC-systems of 100 and 1000 kW power Ievels. For industrial generators have high anode losses, up to HFC-plasmatrons, any plasma forming gases are suitable. 20-40%, thus sharply reducing the HF system efficiency HFF -flame plasmatrons are essentially of a combined which does not exceed 40-60%. Two ways of diminishing type, as the electrode discharge current is grounded the anode Iosses to between 5-8% are being developed. through the distributed capacity. The efficiency of the These are (a) operating the generator under overload system is near to 50%. Even lower minimum power is conditions and (b) using special generator lamps with necessary to maintain the HFF-discharge. The presence magnetic focussing. HF industrial generator ·efficiency of an erodable electrode Iimits the choice of a plasma may increase by these means up to 70-85%. forming gas, though at a power Ievel of up to 10kW, Energy generated by a high frequency electromagnetic erosion of this electrode is unessential. field is used for gas heating in different types of HF Plasmatrons with combined energy supply: HF+ direct plasmatrons: induction, capacity, flame and combined current; HF+ altemating current; HF+ LF (low fre (Fig. 6). quency) are not yet fully developed, but many present a HFI -induction plasmatrons have been ·developed the certain interest. most. Their power inpilot plants has reached 200-300 kW; In the field of HF plasma industrial engineering the . in laboratories 500-1000kW units are being tested. The following problems have to be solved: increasing the minimum power necessary for seH-sustained induction efficiency of the anode circuit up to 90-95%; increasing discharge is determined by the gas, pressure and the power of HF-plasmatrons up to 3-5 MW; developing frequency of electromagnetic field. As the frequency is combined energy supply plasmatrons. On theoretical side reduced from the MHz range to the hundreds of KHz of HF-discharges it is important to develop engineering range, the power increases from less than 10 kW to methods of calculation HF-plasmatrons taking into hundreds of kW, and then rises hyperbolically on further account dynamics of a plasma gas flow, especially in frequency reduction. Difficulties in supplying the power turbulent conditions. Attention should also be paid to for discharges on standard industrial frequencies (50- development of tubeless generation of HF 60 Hz) are explained by this very phenomenon. To reduce electromagnetic oscillations. the minimum power for sustaining an induction discharge, it is necessary to increase the plasma conductivity by 2. PLASMA MELTING lowering the pressure or by adding ionizing mixtures. The processes concerned with the effect of thermal Electrodynamics of HFI-discharges is governed by the plasma on compact molten material, the melting of metals ~aws of induction heating of conductive materials. and ceramics, the alloying and refining remelting of metals However gas dynamic phenomena in HFI~discharge are and alloys, the reduction smelting of metals and the rather complicated and can only be qualitatively growing of metallic and ceramic crystals, are carried out evaluated. That is why engineering methods to calculate üi ·plasma furnaces. Processes in industrial use at the gas flow HFI-discharges, have yet to be developed. present are: the continuous remelting of bars or rods HFI-plasmatrons can operate with quartz or metallic (electrodes) in the water-cooled crystalliser, the intermit discharge chambers for different plasma forming gases. tent melting ot materials in a ceramic crucible, and The most promising is the Operation on chemically active combined methods, e.g. induction plasma melting of gases: oxygen, chlorine hydrogen and vapours of reactive metals and alloys. substances. HFC-capacity-plasmatrons have no wearing parts, as 1. Plasma fumaces . the electrodes are placed outside the discharge chamber. A number of types of plasma furnaces for Iabaratory Capacity coupling of an HFC-discharge with. the elec and industrial applicati'ons have been developed. trodes voltage Ieads to the formation of a phase shift Industrial plasma furnaces for semi-continuous Opera between the electrode and discharge current. The tion have been developed at the Paton Electric Welding electrodynamic conditions of HCF discharge are worse~ Institute (Kiev)-Table 2.5 From 3 to 6 d.c. or a.c. ned by a phase shift and so the efficiency of the discharge plasmatrons are radially arranged (Fig. 7). The fumaces is reduced. To maintain a seH-supporting HFC-discharge are designed for remelting of axially located ingots. These comparatively small power is necessary: in the range of furnaces are used for· refining of precision and heat 10-20 MHz it equals 0.2 kW for air and 1.0 kW for resistant. alloys, high-temperature metals, ball bearing hydrogen operation. This presents an essential advantage. steels, high tensile special steels as weH as for nitrogen An efficiency of about 40% has been achieved on a 10 kW alloying of metals CFig. 8). power level.We do not envisage any major difficulties in Pumaces with three-phase power supply have been developed by Electrotherme (Belgium).6 The furnaces are .designed for the refining of niobium, tantalum, bolyb denum, titanium and other metals as weH as of heat-resistant alloys based o~ nickel and cobalt.
Table 2. Paton electric welding institute plasllia furnaces'
Furnace type Y-461 Y-467 y.{j()()
Total power, kW 160 360 1800 Number of plasmatrons 6 6 6 2 3 Maximum weight of ingot; kg 30 460 5000 Maximum diameter of ingot, mm 100 250 630 Fig. 6. S' w vqi Abc 9LC bJ92W 1(IUJ9CG 101 CoJJflJflOfl2 UJGIUIJ ObGL900w (j)wcj o pwcq () bJ92wLou () rno () GUGL2OfIICG' 2udG OQ — — v bJ92w9 1nw9cG (j)WC9JjO pG WCC( () bj22wLOu2 (3) () G1JGLA 20111CC pao __ _1!00) — 1 I ido oni I ri ri cpawpG —------? - - - E! J0 fOM bLG2ntc 91C btw9 111 W2CG M!W pOJJOMCJOqG 101 UJGJ1JIJ IWflW -2bo1JG jjJcpac[2 [U M9GL-coocq 110W MQDCG pC jidfIEJ WGJ jJOM2 H0 CtA29fl!CL O CItCflpiL 01 LGCWUfl1L CL0-GC4iOU j€ wA uq pc cLA9fl!2c1 91G pGq pA iuqJ?iqnJ bj92urn juo nb o C9U pG btoqnccq iD jpi cn.uc _1 JJJG 9x19J bJ92w IflW9CG 101 pcp I0LJ( ThIflJ CGI9UJJC cu1cipjin qicjobqpA (n2v) (E! J) LP!2 1fIUI9C 101WCJWCJJL W1!2 92 CGLUJ1C I!U!U 0U 01ict qcbj wtou uq poow jccoq E 8 yc bJ92wJ tW96 -Q—j9pJ J H!1uqj0i bL22rn 1I1UJCC IAIIJJ XIJ bJ92wLou 101 UJpiU p4'UI (L!y /ACJJ 92 1fILDC iiup Lqj bpi2ui9Lou2 oi pipoLio1A WGJU o pqj cjrnLG !° uJo pi pccu qcAGJobGq pc iKoA IU!UI 0 WJjflL1 (INE!) (yocoM)—j9pc q!uGq oi. uwiqpi wj uq jjoA LGUJJpU8 pIJJ CWbGLfl1c uq CAG ll0 IJ OL bLociu pn uq IUqfl4I.piJ J'o/ibL2nL bjurn ip pojoiA cpoqc 101 wjiu pi p-cuibini qJAG uq 2bCCij UJJoA uuo llCL2 JJAC pccu qAcjobcq p? jic (b9u) (E! ioY VU wqnLJI ILJJCG M!flJ 2I) LCi w v b192w1i 1I1W9C JW CC19W!C CLf1CIfC 0L b1wLou uq oøj bo1L o P Pq!1 WCpiIJ8 ObcLiOu 184 N. N. RYKALIN Table 3. IMET Iabaratory plasma furnaces Diameter Plasma-forming gas Gas consump- Operating Power Efficiency No. Number of ingot tion pressure in (kW) (%) Layout plasmatrons (mm) (cbm/hr) melting space 1. Axial 1-3 60-100 argon, mixture of argon and 2-4 (1-3) bar 30 30-40 nitrogen and hydrogen 2. Axial 60-100 argon, helium, nitrogen, m-•-1o-1 10-'-10torr 100 30-70 hydrogen 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 metal shown the possibility of alloying metal by nitrogen from refining, melting steel and special alloys and for producing the gaseous phase. The plasma alloying enables one to big-size high-quality castings. Pumaces for batch work obtain higher concentrations of nitrogen in the ingot and a having a capacity of up to 10t 9'10 have been also built in rather uniform distribution of the nitride phase, both of USSR, GDR. which are unattainable by other methods. 12·s A plasma induction furnace, with the induction heating The Paton Electric Welding Institute (Kiev) has being combined with plasma arc heating (Fig. 12) has been developed the industrial technology of producing more developed by Daido Steel (Japan). 11 A plasma arc using nitrided grades of stainless steel. The way is thus open for some 35% out of total power of 100-500 kW increases producing new alloys with an increased nitrogen content, considerably the output of the furnace, and intensifi.es the e.g. alloys of b;c.c. metals with internal alloying im refi.ning action of slags. These fUrnaces are used for purities. Plasma-are remelting enables one to control the melting stairuess steels, non-ferrous metals and special alloying phase content within the prescribed Iimits and is alloys. now an established industrial method for obtaining such compositions. 2. Plasma -arc remelting A plasma-arc method of growing large monocrystais of This substantially improves the quality of metal. Unlike refractory metals, up to 50 mm in diameter and weighing vacuum-arc and electron-beam remelting, the Iosses of more than 10 kg, has been developed in the Baikov highly vaporizable components (manganese, molyb Institute of metallurgy (Prof. E. M. Savitsky) and realized denum, magnesium etc.) by the plasma process are very in industry. 13 Tungsten monocrystals produced with low. Plasma arc remelting makes it possible to refine plasma melting are characterized by a high purity (Fig. alloys with readily oxidizable and chemically active 13): high technological plasticity, resistance to recrystalli components-titlmium, aluminium. The most widely used zation and creep, and anisotropy of emissive properties gases are argon, argon-hydrogen (for iron-nickel and reaching 30-70%. nickel alloys) and argon-nitrogen (for alloying from the gas phase). 3. Reduction melting In a plasma furnace the liquid metal bath is affected by A plasma melting process can be combined with metal activated gas particles of the plasma jet. Therefore, the reduction by gaseous or solid reducers: hydrogen, equilibrium concentrations of reagents in this case will ammonia, natural gas, petroleum cracking products and differ from the equilibrium concentrations with the carbon. The furnaces for reduction melting have to be non-activated gas. lnvestigation of the interaction be tween liquid metal and nitrogen containing plasma has ~ g~ 12r--T--,_--r--+~~~~~--,_~ 0 E ..c t; 8 r--T...... ~"--r--Q- ~F-....J.,!;~,_--1---1 c ...::J ....E ..c +c 0 u c ~ 8 0 2 4 6 8 Rate of monocrystal growth, mm/min Fig. 13. Decarbonization efficiency of plasma remelting by tungsten monocrystals production. 13 Carbon content in starting tungsten--0.014-0.016%. (x-axis-rate of monocrystal growth mm/min; y-axis-carbon content in tungsten monocrystal, Fig. 12. lnduction furnace with arc plasma generator. 11 x1o-•wt%.) Plasmaengineering in metallurgy and inorganic materials technology 185 provided with appliances for the formation of the ingot metals from simple compounds, the direct and oxidising and the removal of condensed and gaseous reaction reducing synthesis of metal compounds, the processing products from the furnace. and decomposition of i:aw materials, are realised in Hydrogenplasma reduction melting with deep deoxida plasma jet reactors. tion, which has made it possible to discard the subsequent deoxidation process by ·producing soft magnetic alloys (50% Fe and 50% Ni) altogether, 13 was carried out at the 1. Plasma jet reactors Paton Institute (Kiev). Reduction melting of a material Chemical and metallurgical plasma jet processes are containing 80% metallic iron was performed at Baikov carried out as a rule on dispersed particles of condensed Institute of metallurgy (Moscow) in a radial plasma materials. The introduction of dispersed material into the fumace. Ammonia admixture to plasma forming argon high · temperature jet zone and its extraction from the acted as a reducer. Mter melting a 100% high purity iron wake of hot gases stream represent complex engineering ingot was obtained. problems, in view of the high temperatures and rates of Refractory materials (oxides, nitrides) are melted in gas flow. Complete processing of the starting material and Belgium, France and Great Britain in plasma furnaces a maximum fixation of the product must be achieved. with rotating ceramic crucible. 14' 13 These furnaces have a In research and development of plasma processes, horizontally or vertically located crucible of heat-resistant simple direct ftow cylindrical reactors with water or refractory material. The inner cavity of the crucible has a gas-cooled metal walls and with a single plasmatron are barrel shape and is heated by arc plasma colurnn (Fig. 14). mostly used (Fig. 15). The reactor diameter at the jet inlet The charged material is melted by convective and lies generally within 2-10 jet diameters. The starting radiative heat from the plasma arc column. Oxidising, material is introduced into the jet by the transporting gas reducing and vapourizing processes can be carried out in (dispersed raw material) or by overpressure Oiquid and these furnaces under batch or continuous Operating vapour materials). Cooling gas is sometimes blown in conditions. through the reäctor walls for terminating the high temperature reaction and fixing the condensed phase product. 3. PLASMA JET PROCESSES The disadvantages of simple cylindrical reactors are as Chemical and metallurgical processes progressing follows. The dispersed raw material, deposits on the under the effect of thermal plasma jets on the condensed outlet nozzle of the plasma generator and on the reactor phase of the dispersed material, such as the reduction of walls. The optimum conditions that will eliminate or minimize these disadvantages for introducing the mater ial into the jet that will eliminate or minimize these disadvantages are to be determined for each reactor type by special investigations. The deposit formation on the reactor walls can be dealt with in different ways; by I increasing the reactor diameter by raising the temperature of its inner wall, by blowing on the walls with bailast gas, and by imposing ultrasonic vibrations. The quenching, i.e. rapid chilling of the reaction products for small scale processes, is usually realized by cold gas jets, on the cooling surface of a rotating metal drum. (a) Reactors in which the dispersed raw material is brought in directly to the zone of electric discharge have not yet gained wide application, though a number of interesting suggestions have been put forward. One of them is a Water cooled reactor involving a fountain layer with high frequency Iid discharge torch (Fig. 16). 16 Another one is a magneto hydrodynamic spatial discharge reactor (Fig. 17). The plasma jet formed by a conventional arc generator acts as a cathode for a more powerful spatial discharge in the zone of the solenoid magnetic field. The powder to be processed is brought into the same space. The particles residence time in the high temperature zone is essentially increased due to the comparatively low gas flow rate and drift due to the tangential component of the velocity. Since the concentration of raw material in the space is relatively low, the discharge remains stable, and varia tions of its parameters do not exceed 10%. With the bottoin arrangement of the plasma generator the material residence time in the high temperature zone is stilllonger. 2 "-.Fused I product High frequency and ftame (ultra high-frequency) Liquid layer plasma generators are usually combined with the direct flow reactors, the raw material being introduced into the (b) discharge area or below the discharge. An exception is the Fig. 14. Are plasma furnaces with rotating ceramic crucible. (a) high frequency torch discharge with the rountain layer horizontal or inclined axis. •• (b) vertical axis. ". reactors. 186 N. N. RYKALIN 4 -water (a) Fig. 16. Fountain layerreactor with a discharge torch16 (1) housing, (2) feeder, (3) re~ucer, (4) plasmatron, (5) nozzle, (6) separation device, (7) hopper. (b) Fig. 15. (a) Direct ftow reactor with cooled walls for processing Fig. 17. Reactor with magnetic and hydrodynamic spatial dispersed materials. (1) plasma generator; (2) powder feeder; (3) discharge! (1) plasmatron, (2) plasma jet-cathode, (3) main anode, reactor body; (4) power supply source. (b) Direct ftow reactor with (4) housing, (5) solenoid, (6) zone of the spatial discharge, (7) high frequency generator and axi3I material injection. (1) body; (2) quenching device, (8) cooling gas supply, (9, 10, 11) current lines, inductor; (3) discharge chamber; (4) vortex chamber; (5) feeder; (6) (12, 13, 14) power supply sources, (15) oscillator, (16) plasma- quenching device; (7) reactor. forminggas supply (17) powder supply. 2. Multi-jet reactors out of the reactor, while the large unprocessed particles of The reactor with two confticting plasma jets is used for the raw material oscillate in the high temperature turbulent processing polydispersed raw materials (Fig. 18).17 The wake of the opposite by directed gas jets; the larger the finely dispersed material formed in the process is carried particle the Ionger it stays in the high temperature zone;. Plasma engineering in metallurgy and inorganic materials technology 187 Fig. 18 Scheme of a reactor with confticting plasma jets.-17 P,.,-plasma generators; G,.,-gas input; S-powder feeder; R-reactor body. The reactor with the three-arc mixing chamber shown in Fig. 19" provides a rather uniform temperature Fig. 20. Cyclone reactor.'" (1) reaction chamber, (2) mixing distribution across the section of 0.85 dia, at a distance of chamber, (3) plasmatron, (4) gas supply, (5) raw material input, (6) about two diameters from chamber outlet. Usually the reactor cone, (7) hopper, (8) gas phase outlet, (9) branch pipe. plasma generators are placed normally to the chamber axis, but installation at an angle of 60° to the axis 3. Dispersed material behaviour in plasma jets facilitates the axial injection of raw material. Interaction between the disperse material and the Installatiohs are available in which several plasmatrons heated gas jets, as weil as gas jets mixing phenomena, are are symmetrically arranged on a conical head attached to investigated in order to develop optimal reactor designs the cylindrical portion of the reactor. The inaterial is for various plasma processes. Efficiency of chemico supplied to the top of the cone, closer to the plasma jets, metallurgical plasma processes as weil as product quality so that fraction of processed raw material is being are primarily determined by heating up and transforming increased. the raw material (fusion, evaporation, chemical re~tc Reactor with several plasmatrons fixed tangentially tions), by condensation of the vapours formed, and by (Fig. 20i6 ensures a uniform temperature distribution coagulation of the condensed product particles. across the sections of the reactor, although the heat Iosses Material is held in the reactor zone for rather brief through the walls are rather high. residence times. The complexity of experiments makes it Product extraction. H the product is formed in molten impossible so far to obtain fiill information on the materiai state and accumulated on a liquid bath its removal from behaviour, for the range of temperiltures and rates for the the apparatus can be carried out either periodically or state of substance in which we are interested. Certain continuously. It is also rather easy to withdraw large-size results have been obtained on the heating ·up. and .the friable powders. Rather complex problems arise with motion of rather large particles (over 100-150 mkm) in a ultra-dispersed powders tending tostick together, or with plasma jet. Pictures of the jet were taken through a pyrophoric powders. Such powders require highly effec rotating perforated disk using high-speed filming and tive filters, thereby increasing considerably their size. photometry at different wave lengths. Laser diagnostics Special maintenance is required. Stringent requirements of the bi-phase jet is very promising too. ·. are imposed on reactor sealing; when producing highly' Mathematical modelling of the disperse material active powders that are easily oxidized in the air; the behaviour in the plasma jet brings rather encouraging extraction of such powders can be accomplished by results. Undoubtly, a complete model taking into account intermediate lock chambers. Self -ignition of the powder in all known phenomena in the particle loaded jet would the air can be eliminated by introducing passivating have been too complicated for carculation and analysis. additions or by thermal annealing in the reduction Therefore several simplified models have been suggested. atmosphere. · A rather complete model worked out by Yu. V. Tsvetkov and S. A. Panfilov considers heating up, phase transformations (melting, evaporation) and acceleration of spherical particles less than 50mkm diameter, which are uniformly distributed across a jet-section having no radial gradients of velocity and temperature, 18 This model enables one to analyse the kinetics of gas jet and particle velocity, to evaluate the degree, of material .evaporation and to choose the process parameters and the length of the direct-flow reactor Fig. 2L Length of the complete evaporation path of the tungsten trioxide particles of different diameter in a hydrogert-argon jet rises with initial jet temperature and with .· particle (al (b) diameter, Fig. 22. Calculated and experimental data for the degree of reduction of .tungsten oxide wo3 in a Fig. 19. Three-arc mixing chamber (a) and scheme of jet hydrogen-argon jet, and in an argon ·jet with carbon interaction (b).4 particles rises with initial jet temperaturt}...,.Fig. 23. The 188 N.N.RYKALIN temperature Tg of a gas jet in the course of its interaction with particles remain constant, which is reasonable for the 3 hiSh-enthalpy gases and taking the Nusselt criterion in order of 2, then for the small diameter particulates (<50 mkm) it is possible to express the dependence of particle velocity w" ern/sec and temperature TP •c on time u .. t in explicit form (Nikolaev)19 E" 'u ~ .. Tp = Tg -(Tg- Tpo) exp (-t/t'); Bi~ 1 (1) Q 2 ..: VP=Vg[1-exp(-t/t')]; Re<1 (2) 3 (3) Tpo-initial particle temperature, •c; t'-time constant of particle temperature rise; t" and t"'-time constants of particles acceleration, sec t' = Cpypd, t" = ~. t'" = 4dyp (4) 6a ' 18Vyg' 31/Jvg'Yg · Here: d-particle diameter, cm; a-heat exchange coefficient, cal/cmsec•grad; Cp-specific heat of particle material, cal/g grad; 'YP• y8 --densities of p~icle material Fig. 21. Kinetics of gas (Ar) and particle (W) parameters.'" · of jet gas, g/cm3 ; V-gas kinematic viscosity, Re = 0.5 cm; c. = 0.1 g/sec; GM= 1.18 g/sec. T• ..,-temperature and of gas and particles, "K; w"., -rate of gas and particles, ern/sec; cm2/sec; tf1--drag coefficient. 1-length of particle's path, cm; r-particle's radius, cm; a-heat These expressions can be used, e.g. for evaluating the exchange coefficient of gas with particle, cal/cm' ·sec· deg; time of heating particles up to the melting temperature t -time, sec. and length of heating path. discrepancy between the calculated and experimental 4. Production of disperse materials results is observed at low average mass temperatures of Disperse metallic and non-metallic materials are used in gas jet because of jet temperature non-uniformity. This manufacturing powder metallurgy products and porous model is at present being extended to take into account parts (filters), to strengthen metals and alloys, to produce the gradients of temperature and gas velocity across the special ceramies and plastics, components for electronic jet section. appliances and also directly as abrasives, catalysts, Simplified models have been developed: a model of propellant components and pigments. When manufactur heating up of moving particles in the isothermal jet of ing powder materials in plasma jets, the dispersity and gas, 19 a model of heating up of moving particles in a jet shape of powder particles, their purity and surface witb parameters changing along its length, 20 a model of physico-chemical properties are controlled by the · jet condensation and coagulation of particulates in an parameters (power, temperature, :ftow' rate, gas partial isothermal jet.21 Assuming that · the velocity w" and press'ure) and the tempering intensity. As the reactions proceed within the plasma jet and its wake, the processed materials have no contact with the reactor walls, so the reaction products are not contaminated by the lining material. Therefore, a jet of low-temperature plasma, arid, 8 especially, that of high-frequency induction plasma, makes it possibleto obtain high-purity powders (ultradis 7 persed, spheroidi.zed, composite etc.) based on metals, alloys, oxides, nitrides, carbides, borides, hydrides and 6 E compounds~ E complex 5 Spherical particulates of pre-determined size are _; produced by plasma heating: from wire or bars butt 4 melted by plasma arc (Fig. 24) or by supplying standard or granular powders to the plasma jet (Fig. 15). The process 3 has been realised in both versions on installations rated up 2 to100kW.22 ' Partieles · of tungsten, molybdenum, nickel and other metals and high-temperature oxides from 0.1 up to 2 mm, with an output up to 15 kg/hr are produced by wire or rod melting. By powder surface melting particulates are T, "K obtained of high-temperature metals and their alloys, Fig. 22. Length of complete evaporation path of WO, particles titanium and chromium carbides, tungsten, aluminium and de~ding on their radii and the initial temperature To of the zirconium oxides measuring from 1 mkm up to 1 mm witli hydrogen-argon jet:'" Re = 0.5 cm; C,.,. = 1.2.g/sec; Ca,= an output of the spherical fraction of over 90% (Fig. 25). 0.015 g/sec; C" =0.33 g/sec; '• =2.S;S; 10.0and25m~m. 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(3) I .giI • Ln3h3rlq 13bwoq lo mUrn031i 3bixoib () O1—OE mIm x mIm (nin bun 3bid1s3 10 n3tugnur (d) bun muin)i) (3) a V— mAm ) x mrn .(s3in 3)E1u3i)mnq noiinsibio13dq 0) b3dinff bubomqd n 31uo1 1611m12 01 )nd) lo dr 1ntmoonom .gniwolg imU 13bwoq lo In)3m bun 311!n)3m-non 2Inn3lnm riuiw 2noin3mib lo 001—01 mn bun 3fli33q 336hU (IU o) 00 g\um 316 b3nrntdosd not)nn3bno3 13116 gniinloqsv8 Initini l3bwoq gnibul3noo rigid mo d n •jq .c n3)2uiuT l3bWOqb33ub31 f1 B n3o1btl nm2nlq )3( moil 31u)nl3qm3) 1n31m3d3 noil3n3l lo noi)3ub31 noi)nbixo 10 muinornmn 3)s12mnn1Eq 1.0—0.0 mAin x I ?. mAcn.(631n Plasma engiDeering in metallurgy and inorganic materials technology 191 ally obtained in the form of powders, or coa~ings may be formed in the process of the synthesis, so that the reaction mixture contains both the synthesized elements and their complex compounds. Plasma processes for obtaining meta! oxides, in particular, those of titanium, ·aluminium, silicon and zirconium are the best investigated. The pigment titanium dioxide modified by additives of aluminium oxide is obtained by buming titanium tetrach loride vapour with an admixture of aluminium trichloride in an atomic oxygen jet generated by a 160 kW high frequency discharge. The installation output may go up to 5.000t peryear with apower consumption about 1.93 kWh per 1 kg of pigment-Fig. 29. With an increase in the temperature of the starting materials, the reaction rate grows considerably and the (a) number of nuclei per unit time increases. Therefore, the degree of dispersity of titanium dioxide particulates (the content of particulates smaller than 1 mkm) rapidly increases and reaches 95-98% (Fig. 30). In the high frequency induction discharge zone, oxygen is heated up to 7000-8000°K and atomized. The average mass tempera ture of the atomic oxygen jet where the tetrachloride vapour is introduced, is controlled by the oxygen flow rate and the discharge power. The best results, from the point of view of titanium dioxide quality and the duration of continuous plant operation, have been obtained with average mass temperatures of 1600-18000C. The physico-chemical properties of titanium pigment dioxide conform with the production requirement: the dispersity is 97.3, the tinting strength-1800 units, the luminosity-over 95. The concentration of chlorine in the (b) exhaust gases amounts to 80-90%. The exhaust gas is Fig. 28. (a) Tungsten powder reduced in a hydrogen jet from used for blowing up the inner walls of the reactor, and for tungsten trioxide 0.2-0.5 mkm (5 x 5 mkm area). (b) Tungsten regeneration of hose filters and is recirculated for ore carbide powders synthesized from plasma tungsten and carbon chlorination. The route makes recycling of chlorine blackpowders0.2-0.5 mkm(5 X 5 mkmarea). metals can be obtained from their oxide compounds in fluidized bed reactors. 25 The plasma route for the production of refractory and rare meta! powders has good industrial prospective due to the comparatively small scale of production and to the complexity of the traditional technology. Reduction of other metals. Research on plasma reduction of non-ferrous and ferrous metals has been carried out. Reducing nicke! and cobalt oxides by hard carbon in 100 kW argon plasma jet in a liquid meta! bath, has been investigated as weil as reduction of nicke! oxides in a hydrogen jet. Silicon powder was obtained from silicon tetrachloride in an argon hydrogen 10 kW high frequency jet.31 Reduction of iron by iron carbonyl decomposition in an argon jet has made it possible to obtain an average size of meta! particles 0.3-0. 7 mcm (Conventional process-about 2 mcm) .. The concentration of carbon in the plasma produced iron powder amounts to 0.1-1:0%. 32 Reduction of iron from its oxides in a natural gas arc plasma jet has been investigated.33 The degree of reducibility baried from 25 to 100% depending on the dispersity of the starting raw material (0.84-0.015 mm; respectively). The product was trapped on a liquid meta! Fig. 29. Installation scheme (a) and HF-generator-reactor (b) for bath without slag accumulation. titanium dioxide plasma production. (I) high frequency generator, (2) titanium chloride meter, (3) titanium chloride evaporator, (4) 6. Synthesis of meta! compounds alumini um chloride meter, (5) alumini um chloride evaporator, (6) The plasma synthesis of meta! compounds has an mixer, (7) discharge chamber, (8) quartz tube reactor, (9} meta! extensive product nomenclature. The products are gener- reactor (10) collector, (II) hopper; (12) lock. .W . 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