Application of Plasma Technology in Iron and Steelmaking

K. Up"adhya, J.J. Moore, and K.J. Reid

~lUhory

a onoc!Gi" 'f.-.C::=~ b

Figure 1. Constricted plasma arc configura· tlon,. (a)" non-transformed arc and (b) tnlnsferred arc.

46

ABSTRACT

The potential for the application of plasma technology in metal oxide reduction and in iron. and steelmaking is outlined and discussed. Recent evolution and developments in the plasma-based reactors employed in the production of iron, steel, and ferroalloys have been reviewed; the current status is outlined in terms of process control, flexibility in the raw materials consumed, product quality, and energy conservation. The advantag.es and limitations of thermal plasma-based reactors have been critically outlined and their potential to seriously challenge the blast furnace/basic oxygen furnace steelmaking route is considered.

INTRODUCTION

Energy consumption by the iron and steel industry amounts to 11% of the world total energy consumption.! Furthermore, th~ fact that iron and steelmaking processes rely on decreasing coking coal resources both for energy and reductant, the development of alternative technological processes on alternative energy and fuel sources appears critically urgent. One such technology which has emerged recently and which could in the future prove to be an alternative to the conventional blast furnace basic oxygen furnace (BF/BOF) steelmaking processes is the plasma reduction and smelting. By using plasma in metallurgical processes, the energy required to initiate the metallurgical reaction, e.g., heat energy required to attain the minimum reduction temperature, is separated from the energy re­quired to sustain the carbothermic reduction of the metal oxides. Thus, the application of plasma technology provides an extra degree of freedom which is not available within the conventional inetallurgical processes.

The application of thermal plasma systems to the production of iron, steel, ferroalloys, and acetylene has been the subject of intensive research and engineering developments over the past decade. These developments have been reported in the literature in the form of excellent review arti­cles and technical publications by several authors.2-5 The attractiveness of plasma as a processing alternative lies in (1) its independence of oxygen potential, (2) its flexibility in using the source of energy and reductant materials, (3) high-energy density with resultant smaller process vessels, and (4) excellent pollution control. Because of points 3 and 4 above, there is strong potential for greatly decreasing capital costs compared with existing BF/BOF technology systems.

What is a plasma? A gas at room temperature consists of molecules, each usually having two or more atoms combined. When the gas is heated the molecules are dissociated into individual atoms at about 2300°K. At still higher temperatures, at about 3300oK, some of the electrons are displaced from the atoms forming ions and the plasma state reached. Due to the significant degree of ionization, plasma conducts electricity with a conductivity close to that of a molten salt or slag phase, i.e., 10-102 ohm-1

em-1. This is four orders of magnitude less than the conductivity of solid metal, i.e., 10s/ohm-cm.

Diatomic gases such as hydrogen and nitrogen on subjection to the plasma state will contain a significant population of dissociated atoms, e.g., Hand N, which are chemically very much more reactive than the diatomic molecules from which they are formed. Therefore, subjecting reactants to a plasma environment in which the reactants are present in the form of highly reactive ions, reactive atoms and molecules can be used to advan­tage in metallurgical reduction/oxidation processes.

Although the temperatures at which gases become significantly ionized are in the range of 5000-25,000oK, such high temperatures cannot be fully utilized at present in reactors in which solids or liquids must be reacted or

JOURNAL OF METALS • February 1984

transformed. This is because the achievable residence times are only E few microseconds under most circumstances, whereas a few milliseconds are needed under ideal conditions. This ideal condition would be one in which plasma energy could be supplied directly to the condensed phase. However, all condensed phase materials vaporize significantly, and thus reduce the energy available to drive the reaction. This vaporization is the most impor­tant limiting factor to the achievement of particle temperatures above 33000 K in most cases. An upper temperature limit for the metallurgical operations will probably be set by a combination of several factors such as the residence time necessary to achieve extremely high temperatures, the increasing degree of volatilization of condensed phases, and the need for special materials for solid containers.

Furthermore, in a plasma system, a true thermodynamic equilibrium condition never prevails; the plasma gas, as well as carbon atoms, exist in activated states. In addition, by altering the thermodynamic conditions, i.e., using 200% stoichiometric carbon, oxides which were predicted thermodynam­ically unreducible can be reduced to metal in the plasma reactor. For example, chromite ore has been successfully reduced into ferro chrome alloy in the SSP plasma reactor.9

PLASMA IN IRON AND STEELMAKING

Plasmas can be classified into two groups: 1. Thermal or equilibrium plasmas. In this case, the electron temperature

in the plasma is approximately equal to the gas, i.e., ion temperature, and the ratio of the field strength to the pressure (E/P) is smalI.7 High­intensity arcs, shockwaves, and high-pressure radio frequency discharges fall into this group.

2. Nonthermal or nonequilibrium plasmas are characterized by electron temperatures much higher than the gas temperatures and a high value of the E/P ratio. Included in this category of plasmas are low-intensity arc plasmas, low- and medium-pressure microwave discharges and low-pres­sure RF discharges.

Plasma processes of industrial importance in the iron and steel industry are all based on thermal plasmas, in which the bulk of the plasmas approaches a state of local thermodynamic equilibrium (LTE). LTE is defined as a state of plasma which is optically thin, i.e., it does not reabsorb its own radiation, and collision rather than radiation processes govern the transi­tion and reactions in the plasma. Such plasmas are mainly generated from high-intensity electric arc discharge devices.

Although RF discharges have not found wide use in process metallurgy, there have been some developments8 involving material synthesis using RF plasma devices, e.g., sintering green refractory rods (AbOs) and production of nitrides and carbides (AIN,SiC,SbN4). There have been mainly two types of plasma arc arrangements employed in the field of extractive metallurgy. The first is the free burning arcs which are maintained stationary or in circular motion betwen electrodes. The second is derived from one or more plasma torches or plasmatrons. The plasma arcs can be further subdivided into (a) nontransferred arc and (b) transferred arc as shown in Figure 1. In the latter category the plasma arc is transferred to an auxiliary anode which may be a solid electrode or a molten metal bath.

There are various kinds of devices which are employed for the expansion of the plasma arc volume. The beneficial effect of expanding the plasma arc volume is in the enlargement of the area of influence over which the plasma medium can act and, therfore, enhance the reduction efficiency. Several techniques have been brought forward for the expansion of the plasma arc, among them the noteworthy devices are of Gross,9 Whyman,lO King,ll Tylko,12 and Brzozowski,ls This expansion is accomplished essen­tially by two different methods: by rotating the shell containing the plas­ma arc or plasma torch or by imparting orbital motion directly to the plasma arc.

There are numerous devices of this sort in various stages of development: • Russian High Power Plasmatrons for Metallurgy14 • Gross Centrifugal Furnace9 • Westinghouse AC Arc Heater15 • Ionarc Plasma Furnace16 • Tylko Expanded Precessive Plasma Furnace12 • Brzozowski Orbiting Plasma Torch1S • MacRae Plasma Reduction Furnace17 • Extended Arc Flash Reactor18 All the above methods have one thing in common: each is primarily a thermal technique, by which electrical energy is converted into thermal energy which is required to accomplish the chemical reactions.

JOURNAL OF METALS· February 1984

~~;;:"' .. L Figure 2. Comparison of conventional blast furnace/basic oxygen furnace route, direct reduction/electric furnace route, and plas­ma steelmaking route.

47

48

The application of plasma technology in the production of iron and steel is becoming increasingly important. Figure 2 shows the three possible routes to produce steel at the present time. In the conventional BF/BOF route, the agglomerated ore is mixed with coke and smelted in a blast furnace before being refined using oxygen in the basic oxygen furnace. The direct reduction (DRl) processes consist of reducing usually agglomerated ore in the solid state, using either solid or gaseous reducing agents, followed by melting and refining in an electric furnace. Thus, in BF/BOF as well as in DRI processes several stages are required before the steel product is obtained.

The major limitation in the application of plasma technology in the production of iron and steel is the high cost of electrical energy. Therefore, any such application must make full use of the unique activated species existent within the plasma medium to effect a high throughput operation. At the same time, if this can be coupled with the production of a high value product, such as metastable Fe-C powder as recently reported,19 potential commercial applications are distinctly possible. In all such appli­cations optimum use of cogenerative systems, e.g., use of thermal and chemi­cal heat energy of process gases, must be employed. However, even greater potential exists in the application of plasma technology to the production of high value metal such as the refractory metals and ferroalloys. A further advantage of plasma-based extractive metallurgy processes is that low-grade coals may be used effectively as reductants.

APPLICATION OF PLASMA IN THE REDUCTION OF OXIDES

The absorption of energy from a plasma source by an oxide depends to a large extent on the valence state of cations in the oxides. Materials such as silica (Si4+) and alumina (AP+) become more conductive to heat at higher temperatures, their thermal conductivity showing a minimum in the temperature range at which heat transfer by radiation becomes more significant than that by protons (H+). In oxides such as those of iron (Fe2+, Fe3+), chromium (Cr3 +, Cr6+) and manganese (Mn2+, Mn4+), the pres­ence of more than one valence state of the cation makes energy absorption more probable than in the transparent single valency cation oxides. Furthermore, the concentrations of valency states of a variable valency cation are a function of both the temperature and oxygen partial pressure in the surrounding gas phase.

The reduction of metal oxides, in particular iron oxide, by carbon has been studied extensively by numerous investigators,6.10 and generally this can be represented as a solid metal oxide (MO)-solid carbon (C) reaction.

MO(s) + C(s) = M(s) + CO(g) (1)

However, for Reaction 1 to occur to any significant extent, it requires two solid phases to come into physical contact, which is likely to be infrequent and slow due to the lack of any transport medium. Moreover, as soon as some metallization is produced via Reaction 1, the solid metal product acts as a barrier for the diffusion of solid carbon into the inner layer of iron oxides which must occur if the reaction is to continue. At this stage, therefore, the gaseous carbon monoxide produced by this solid-solid reaction becomes a kinetically preferred reductant over carbon since it involves a gas/solid reac­tion. In this way, the solid carbon reductant is being used indirectly as gaseous CO in the reduction of iron oxide, i.e., as:

MO(s) + CO(g) = M(s) + C02(g) (2)

The C02 gas generated can then react with more carbon to renew the CO for further reduction of MO. This is commonly referred to as the Boudouard reaction, i.e.,

CO(2)(g) + C(s) = 2CO(g) (3)

Thermodynamically, the overall reduction reaction can still be represented by Reaction 1 since it is the addition of Reactions 2 and 3 but, kinetically, the reaction follows the more favorable gas-solid two-stage sequence.

The excess gas generated diffuses out of the system through the interparticle pores where the C02/CO ratio can vary over a wide range, depending partly on the thermodynamic stability of metal oxides, whether full combustion to C02 is achieved and the reactivity of the carbon form used (as can be seen from Table I).

However, there is general agreement in the case of iron oxide reduction by carbon, that the rate of reduction is controlled by the oxidation of carbon by C02, i.e.,

C02(g) + C(s) = 2CO(g)

JOURNAL OF METALS· February 1984

Table I: Thermodynamics of the Reactions

.1G~ Reaction Constant (aM = 1 = aMo) Temp., calsl

Reactions oK mole peo Peo2 Peo/Peo PbO+C=Pb+CO 1670 --45,180 8.19x105

2PbO+C=2Pb+C02 1670 -61,320 1.06x 108

PbO+CO=Pb+C02 1670 -76,140 1.29x102

NiO+C=Ni+CO 1670 -39,380 1.42 x 105

2NiO+C=2Ni +C02 1670 --49,720 3.21x106

NiO+CO=Ni+C02 1670 - 9,340 22.6

Fe203+3C=2Fe+3CO 1670 -90,609 7.23x1011

2Fe203 + 3C = 4Fe+ 3C02 1670 -'-94,181 2.12x 1012

Fe203+ 3CO= 2Fe+3C02 1670 - 3,572 2.93

FeO+C=Fe+CO 1670 -24,480 1.59 x 103

2FeO+C=2Fe+C02 1670 -19,920 4.04x102

FeO+CO=Fe+C02 1670 + 4,560 0.254

MnO+C=Mn=Co 1700 - 1,060 1.36

2MnO+C=Mn+C02 1700 +28,080 2.45 x 10.4

MnO+Co=Mn+C02 1700 +27,020 1.80xlO·4

Cr203+3C=2Cr+3CO 1700 -20,980 4.98x102

2CR203+3C =4Cr+ 3C02 1700 +48,640 5.57xlO·7

Cr203+3CO=2Cr+3C02 1700 +27,660 1.11xlO·g

At this point a consideration of the thermodynamics of reduction of oxides by carbon will be helpful and relevant. The conditions for the reduction of oxides by carbon can be obtained by evaluating the thermodynamic data for the individual carbothermic reduction reactions. Table I contains such data for PbO, NiO, Fe203, FeO, MnO, and Cr203. It can be observed from Table 1 that these oxides can be divided into two categories. First, those which are highly reducible oxides and produce a high equilibrium value of the PC02/PCO ratio. In this category fall PbO, NiO and Fe203. Second, those oxides which are relatively difficult to be reduced by carbon and produce a small equilibrium value for the PC02iPCO ratio and in this category fall FeO, MnO, and Cr203.

As mentioned previously, there is a consensus that metal oxide reduction by carbon takes place in two steps, i.e.,

MO + C = M + CO(2)

C02 + C = 2CO

(4)

(5)

Furthermore, the overall rate of reaction is controlled by the gasification of carbon by C02. Therefore, the degree to which reduction of an oxide by carbon in a given system can be carried out is a function of the ferrying capacity of the gas mixture to transport oxygen from the oxide to carbon in the form of C02 + CO mixture. This capacity can be readily judged by comparing the C02/CO ratio in equillibrium with the metal oxide phase in the original and reduced stage with that in equilibrium with carbon at the reaction tempera­ture. The Ellingham diagram for the free energy of formation of oxides6 shows this factor clearly. The free energy of the carbon-oxygen reaction to form CO becomes more negative as the temperature increases, and from the tempera­ture at which this line intersects the various metal/metal oxide lines it is possible to estimate the temperature required for the reduction of oxide to metal when in contact with carbon. The CO/C02 ratio in equilibrium with carbon becomes very large as the temperature increases as does that in equi­librium with the metal/oxide system which is crossed by the free energy line of the carbon-oxygen reaction to form CO at higher temperatures. Thus in these regions of high chemical stability, the capacity of the gas to transport oxygen from oxide to carbon as C02 becomes very small, and although reduc­tion is, in principle, feasible, the rate of reduction will be extremely small. As an example, at 1400°C, the C02/CO ratio in equilibrium with FeO/Fe is about 114 (see Table I), whereas that in equilibrium with CO/C is less than 1110. Hence, carbon particles readily reduce iron oxide particles via the gas phase at this temperature.

JOURNAL OF METALS· February 1984 49

h( ' lelC

co,

0-4ectf'U

L~" " 'htl"e' 'J' ",,,

(00 r"Q

Figure 3. A schematic illustration of the Westinghouse arc heater.

Plt -I\tcI'

Figure 4. The SKF Plasmared process for the production of sponge iron.

50

On the other hand, the C02/CO ratio in equilibrium with MnO/Mn is about 11104 at this temperature and thus the change in C02 partial pressure in the gas phase between an oxide particle and a carbon particle is only about 10-4 atms. Under these circumstances, very little oxygen can be transported as C02 in the gas phase. A similar argument will also hold true for Cr203 reduction by carbon at 1400°C. These considerations suggest that expectations of successful plasma technology for reduction of stable metal oxides at high temperatures which are not otherwise capable of reduction will not be signifi­cant, when this gas phase mechanism of reduction is predominant. However, since many elements which form stable oxides such as chromium, titanium, and silicon also form stable carbides, these carbides play a significant role in the discussion of the carbothermic reduction of metal oxides.

PROCESSES BASED ON NONTRANSFERRED ARC PLASMA SYSTEMS

Nontransferred arc plasma furnaces were originally developed from arc gas heaters. The enthalpy of the arc heated gas provides the energy required by the metallurgical processes. A large number of plasma fur­naces are based on the arc gas heating technique and are typified by the Westinghouse arc gas heaters. The basic design of the arc heater is illus­trated in Figure 3 and is described in detail in the review papers by Sayce2 and Hamblyn.3 The Westinghouse Corporation, in collaboration with vari­ous metallurgical organizations, has been exploring the feasibility of using arc heaters in the conventional iron and steelmaking processes to largely obviate the reliance on coke or natural gas reductant.

SKF Steel in Sweden has introduced two processes based on plasma technology and these are known as Plasmared and Plasmasmelt. The SKF Plasmared process is shown schematically in Figure 4. A plant based on this process is now operating in Sweden at SKF Hofors plant with capacity of 70,000 metric tons of sponge iron per year. The Plasmared process can be divided into four main units: (1) plasma generator, (2) the Plasmared gasifier, (3) sulfur removal, and (4) iron ore reducer.

The Plasma Generator When a gas is heated at around 2500oK, the molecules will break up

into atoms, and at still higher temperatures (--3500 0 K), the atoms will eventually split up into electrons and protons and thus plasma is formed. The break-up of atoms requires a considerable amount of electrical energy, so that a small volume of plasma gas will accumulate a large amount of energy and, when these electrons and ions recombine to form atoms out­side the plasma generator, all this energy is released and available as heat energy. Thus the plasma generator is a device for the efficient conversion of electrical energy into large quantities of heat energy which can be used in reduction, smelting or melting processes. Figure 5 shows the general configuration of the SKF plasma generator. Two tubular water-cooled elec­trodes are separated by an annular process gas inlet. An arc is struck between the electrodes and heats the gas. The gas becomes conductive, forms part of the electrode, and is superheated. On leaving the generator the plasma temperature can vary between 3500 to 10,000°K. The conver­sion efficiency, i.e., electricity to heat ratio, drops at higher temperature, therefore, these plasma generators operate around 5700°K. The electrodes last for 1-2 weeks in continuous service and are easily replaced.

The Plasmared Gasifier The fossil fuel, i.e., coal dust, is injected into the reaction zone of plasma

generators in which the prevailing temperature is around 40000K and a bed of coke, first, provides a compact reaction zone for the gasification and, second, protects the reactor vessel lining from excessive high temperatures. Also, because the raw gas passes through hot coke, the water vapor and C02 content of the product gas is held at considerably lower levels than in conventional coal gasification plants. The product gas is mixed with cleaned and cooled offgas from the reducer and then is used for iron ore reduction. Excess offgas which can contain up to 60% reducing gas is recycled through to a gasifier. As there are no catalysts in this system and the initial gas temperature is high, it is possible to use almost any kind of fossil fuel in this type of reformer, i.e., LPG, heavy fuel oil, or coal + oil slurries. The possibilities to reach equilibrium in this system are good, because all the required heat is in the gas itself. The results of the pilot-plant test show that it is relatively easy to obtain C02 contents down to 2-3%, depending on the type of fuel used; however, C02 and H20 could be completely removed to save electrical energy and fossil fuel consumption. The reduc­ing gas is passed at 11500K through a column of dolomite which reduces the sulfur content of the gas to an acceptable level.

JOURNAL OF METALS • February 1984

Table II: Energy Consumption for the Plasmared Process

Natural Gas LPG Oil

Fossil fuel consumption, 110 120 135 kg/metric ton Fe

Electrical energy, 780 800 860 k Whlmetric ton Fe

Total energy requirement, 8.4 8.8 9.2 GJ/metric ton Fe

Coal

180

850

8.2

The reducer is a simple shaft furnace in which iron ore is charged through a bell or similar top feeding port. The reducing gas enters the lower portion of the furnace at 1073-1123°K. The reduced iron flows down to a discharging system which can either simply cool down the iron to prevent reoxidation or produce iron briquettes. The offgas is cleaned, C02 is scrubbed out, and the gas is again recycled.

The energy consumption figures quoted by SKF per metric ton of sponge iron produced is shown in Table II. An actual result from the Hofors plant, however, provides different figures for the energy consumption, i.e., 100 kg/metric ton of iron of LPG fuel and electricity consumption is 1000 kWh/metric ton of iron at 91-93% metallization with carbon content of 1.5%.20

Plasmasmelt Process

The Plasmasmelt process was developed by SKF Steel in Sweden, with the aim of providing an economic alternative to the blast furnace for the production of molten iron. The main advantages of the Plasmasmelt proc­ess are: (1) the absence of coke ovens, (2) the considerably lower volume of gas" generated, and (3) lower initial investment capital cost. The Plasmasmelt process is shown schematically in Figure 6.

The starting feed materials consist of: coal dust; the reductant; process off gas and electricity for the plasma generator; powdered iron ore, slag formers; and coke for the reaction zone in the shaft furnace.

Reduction of iron ore fines is accomplished .in two stages: prereduction in two fluidized beds to approximately 50-60% metallization, followed by final reduction in a coke-filled shaft furnace from which the product is a molten high-carbon iron. The two fluidized bed prereducers are fired by the proc­ess off gas from the final reduction stages at a temperature of 973-1073°K. The offgas from the fluidized beds are used for the drying and preheating of the incoming concentrate.

In the shaft furnace heat energy is provided by the plasma arc heater which is fed with a small portion of the process offgas bled off from the top of the reduction shaft. The hot (973°K) prereduced ore concentrate is injected into the plasma together with powdered slag formers and pulver­ized coal powders or atomized heavy oil as reducing agent. The tempera­ture sinks rapidly from 3000-5000oK in the plasma arc to 2000-2300oK in the smelt-reduction zone due to the strongly endothermic reduction reactions.

The gas constituents formed during the final reduction are CO, H2, and a small amount of C02 and water vapor. Carbon dioxide and water vapor are reduced by the coke higher up in the shaft and the exit gas thus consists of a mixture of CO + H2 at a temperature of 1273-1473°K. This gas mixture is used in the fluidized bed prereducers and a small portion is preheated in the plasma arc heater and recycled through the shaft.

The coke bed in the shaft is not primarily intended as a reducing agent. Instead, the purpose of the coke is to form a permeable reaction zone in the shaft and also, since it can withstand the high temperature of the order of 2300-3300oK, it could to some extent protect the refractory wall. Furthermore, it provides a uniform carbon content to the hot metal. The entire energy content of the process offgases from both stages is fully utilized. However, a large amount of electrical energy is required for the plaerila arc heater. This may be reduced to some extent by replacing some of the powdered coal with powdered coke and/or oil, as shown in Table III.

The whole system is gas tight and no large compressors are needed. However, a small proportion of offgas is cleaned and compressed for use as a carrier gas for feeding the prereduced ore and coal dust, and also as process gas for the plasma generators. The chemical composition of the product iron is similar to that produced by the blast furnace route. However, the sulfur content is normally 0.01-.015% and thus the iron produced by the Plasmasmelt process can be used directly for steelmaking without any

JOURNAL OF METALS· February 1984

Figure 5. The principle of the plasma generator.

Figure 6. The SKF Plasmas melt process for the production of pig iron.

51

o U1J(IC'eC ll(on

CO" "',.O,

At on q >

Figure 7. lonarc plasma furnace electrode arrangement.

Figure 8. Schematic of the fluid convective cathode arc system.

52

external desulfurization. Sulfur content of BF iron is 0.05-0.08% making the desulfurization step essential. The reason for the low sulfur content of the Plasmasmelt-produced iron could be the low consumption of fossil fuels. Carbon and silicon in the iron are controlled by adjusting temperature and slag composition.

The SKF, Plasmasmelt process21 claims the following advantages over blast furnace iron making: 1. In this process almost any kind of fossil fuels can be used, ie., coke,

coal, charcoal, fuel oil, peat, tar, or gaseous hydrocarbon. 2. The coke for the shaft furnace is not as demanding in mechanical and

chemical properties as that required for a BF and therefore less expensive, and also a much lower amount is needed.

3. In those countries or places where electricity is expensive, it can be partly replaced by injected powdered coal, which will generate gas of high calorific value. If coal is expensive corppared with electricity, the furnace can be operated in equilibrium, so that no excess gas is generated. Results of the tests at Hofors plant show that energy consumption is at its lowest at around 55% prereduction of the ore. At high pre reduction rates excess gas is generated and thus coal consumption is increased but electrical energy consumption is lower.

4. SKF claims that the total costs of electricity, coke, and coal used per ton of hot metal produced by Plasmasmelt process is 16% lower than the cost per ton of iron produced by the blast furnace route.

Plasma has also found application in the. blast furnace. Plasma-heated gas is introduced through an arc heater into the tuyere level. This reduces the coke consumption per ton of hot metal and also improves the furnace 0'peration.22 Thus there is a reduced dependence on expensive metallurgical coke and metal production rates increase.

Processes Based on Transferred Arc Plasma System

In these processes, the plasma arc is transferred to an auxiliary anode which may be a solid electrode or a molten metal bath. Figure 1b illus­trates the principle of transferring an arc to an auxiliary anode. The feedstock is introduced into the interelectrode gap, i.e., between primary and auxiliary anodes. In this way the plasma volume is more efficiently utilized, since energy transfer from the electrically treated gas to the suspended particles takes place both in the discharge and "tail flame" zone.

The Ionarc plasma furnace system is a classic example of the transferred arc arrangement, and this is shown in Figure 7. In the Ionarc plastna process the feed material is injected into the interelectrode gap and the furnace can be operated in a variety of gaseous environments. Associated Minerals Consolidated Ltd. is operating the Ionarc plasma process at their New Hampshire plant to produce various grades of AMA plasma zirconia. 23

The fluid convective cathode arc (FCC) system is another example of the transferred arc plasma arrangement system. The essential feature of. th~ FCC system is the electromagnetically induced gas pumping action caused by the constricted arc attachment, and this is illustrated in Figure 8. An earlier FCC furnace system was applied in the refractory oxide vaporization, metal chloride production, and also carbothermic reduction of magnesium oxide.24-26 Recently this system has been employed for the coal gasification and desulfurization.27,28 At the 120 kW power level, 50% gasification was achieved using a feed rate of 610 g of coal and 450 g of steam per minute. The analysis of this product was: 62% H2, 26% CO, and 12% C02.

A novel, transferred arc plasma system, known as the sustained shockwave plasma reactor (SSP), is currently under investigation at the Mineral Resources Research Center at the University of Minnesota.29,3o Essentially there are three features which are exploited in the SSP system in plasma-particle interaction and in-flight reduction: 1. Very high rates of orbiting of the primary plasma jet issuing from the

plasmatron or similar device produc"ng a plasma cone suitable for use as a reaction medium;

2. Electrically produced pulsations imposed on the plasma core; and 3. Efficient entrainment of a large particle population within the plasma

cone. All of these effects appear to be closely interacting and in some respect

synergistic. In the SSP reactor system the mineral concentrate and the solid reductant, i.e., coal dust, both in the form of powdered fine particles, are injected into an expanded, orbiting plasma, the orbiting of the plasma being achieved by electromagnetic means. The imposition of electrically induced shockwaves creates microfissures in the ore particles and thus provides easy access for the reducing gas and thereby improved reaction kinetics. The main components of the SSP are illustrated schematically in

JOURNAL OF METALS • February 1984

Figure 9. This system differs from the previous two systems in that the stable arc discharge is maintained between the two electrodes even though there is a large population of particles in the space. In fact, in the SSP system the arc stability has been found to increase on injection of mineral particles. The SSP reactor system is thus able to treat large quantities of solids in the in-flight reduction mode without the need of a molten metal/slag bath.

The successful in-flight reduction of Minnesota taconite in an open 40-kW SSP reactor led to more detailed studies in a higher rated enclosed SSP reactor. 31 ,32 Subsequently up to 75% metallization has been achieved in the initial tests at the power level of 60 kW and a feed rate of 76 g/min. Low grade domestic chromites also have been reduced in-flight in the open 40-kW SSP reactor using solid carbon based reductants. 33 It is claimed19 that it may also be possible to produce amorphous metal powder by this technique if rapid quenching of the liquid metal droplets is effected, e.g., by liquid nitrogen spraying, immediately below the plasma tailflame.

Plasma Arc Transferred to a Metal Bath

A large proportion of the electrical energy input to an arc is dissipated in the anode region. This energy dissipation is the main stimulus for the development of this kind of plasma arc furnace, in which the plasma arc is transferred to a molten bath. This is based on the concept· of transferring the arc to a metal bath which acts as the anode and to which a large proportion of energy is transferred near the surface where new feed materi­als continuously land. This facilitates rapid melting and also enhances the slag metal reactions.

The Noranda-McGill University transferred arc plasma furnace system combines the advantages of both the "falling film" mode of prereduction and bath reactions and the main feature of this plasma-furnace system is illustrated in Figure 10. This furnace system has been employed for the decomposition of molybdenite to molybdenum and sulfur. It is claimed that this process eliminates S02 pollution problems. Also, ferrovanadium and ferro niobium have been produced by this technique. 34,35

The expanded precessive plasma furnace12 was originally developed by Tylko as the forerunner to the later SSP reactor to treat particles entrained in the pla~ma volume. The EPP furnace now promoted through Foster Wheeler is illustrated in Figure 11. This is essentially a transferred arc plasma furnace in which the plasma volume is expanded by rotating the cathode gun assembly. However, due to mechanical restriction in rotating the cathode gun assembly beyond about 1500 rpm, only partial reduction of the mineral in the in-flight reduction stage is achieved and thus the final reduction in a molten metal/slag bath is required. The rotation of the cathode gun assembly, however, causes the anode arc attachment to move around on the melt surface and, thus, results in the uniform and efficient heat distribution to the melt surface. In this furnace system, cast iron chips and borings have beE;!n melted in the experimental furnace and the electrical energy consumption was 500 kWh/metric ton of liquid iron. Ferro­chromium has been produced from chromite ore and coal powders at ener­gy levels lower than those of a conventional furnace.36 Using an EEP plasma furnace the chromium to iron ratio has been increased in the chromite ore to a value required to produce high-carbon ferrochrome. Also, this increase in the value of chromium/iron ratio with EPP plasma systems was more than that obtained by the conventional submerged arc furnaces.

Transferred arc plasma furnace systems have also been employed in the melting of high-alloy steels. Techniques such as the melting of metals, superheating and refining, and melting of consumable electrodes in conjunc­tion with progressive ingot solidification and shape casting have been developed. These transferred arc furnace systems either compete with or complement various melting/refining processes, such as electric arc furnace melting, vacuum induction melting, and secondary melting processes, i.e., electroslag refining, vacuum arc refining, and electron beam melting. Ta­ble III show a comparison of the quality of steel obtained on the remelting of a ball bearing steel by various secondary remelting techniques.37

There are several transferred arc plasma remelting furnace either in operating or developing stages. Amongst them are the Linde Plasma arc furnace (Figure 12), PAR patented by the Electric Welding Institute (USSR) and plasma beam remelting furnaces developed in Japan.38 In the plasma beam remelting furnaces, the vacuum conditions required are not as acute as in the conventional electron beam furnace, and it is claimed that the metal purities obtained from both types of melting systems are comparable.39

The technology of falling film plasma reactors has been developed by Chase and Skriven40 to the laboratory scale and a more sophisticated one to the pilot-plant stage by MacRae et aLl7 at Bethlehem Steel Corporation.

JOURNAL OF METALS· February 1984

a

b

____ Feedsfock duCls

Plqsma head sectIOn

Plasma cone

Plasma chamber

Anode rotor

Anode chamber

_ Anular anode

Tall flame section

~=t~~~=~~==:- Secondorygosl ~ liqUid InjecllOn manifold

Short cylinder

~

Free fall chamber

-- Exhaust duct

-- Collector f--

A Power supply for mOIn discharge

B Po~ supply for aUXiliarieS

C Central control unit D Pulsator E Rotor contml 01 cathode F Rotor control at anode G Reactor

Figure 9. The sustained shockwave plas­ma (SSP) reactor.

Anode supporting shell

Plasma torch

cw

FigurE! 1Q. The experlmen~al transferred arc plasma furnace system at McGill University.

53

Figure 11. The 1.5 MW experimental trans­ferred arc plasma furnace.

SonG 'Seol

Pilot 01('

'SwIfC"

Pdolo,e U!SISlor

Figure 12. The Linde plasma arc furnace system.

54

Table III: Energy Requirements for the Various Modes of Operation of the SKF Plasmasmelt Process

Coke, Coal, Oil, Electricity, kg kg kg kWh

Plasma coke 185 30 1020

Plasma, coal 50 170 1120

Plasma, oil 50 135 1080

The main features of the "falling film" plasma reactor is illustrated in Figure 13. In the falling film plasma furnace systems, the mineral parti­cles being treated are entrained in a liquid film which slowly slides down on the furnace wall and acts as a cylindrical anode. The presence of the reactants as a film, rather than as discrete particles entrained in the plasma volume, is said to, first, increase the residence time of the particles and thus enhance the heat-transfer and, second, provide the intimate con­tact of the oxide and carbon which is necessary for the oxide reduction. Iron ore concentrates have been reduced in both their 100-kW and 1-MW plasma furnaces. 41 Hydrogen and methane gas mixtures were used as both the primary plasma gas and the reducing agent. Electrical energy consump~ tion as low as 2.65 kWh/kg of iron produced was achieved in the larger furnace. Ferrovanadium has also been produced in the 500-kW plasma furnace.42 In this case coke fines were used as the reducing agent and the energy consumptions were reported to be as low as 3240 kWh/metric ton of alloy. At the University of Limoges, France, research is currently being conducted to develop a plasma 'furnace system which combines the advan­tages offered by both the falling film plasma reactor and molten bath transferred arc system.43,44 Iron ore has been reduced in a hydrogen plas­ma at just under 5 kWh/kg of iron produced. Initial test results indicate ferrochromium could also be produced by this technique as the iron appeared to evaporate selectively from the bath.

The Extended Arc Flash Reactor (EAFR) was developed at the Universi­ty of Toronto and this is illustrated schematically in Figure 14. The EAFR consists of four zones: the rotary preheater, the flash reactor, the plasma zone, and the hearth. In this process the ore and the reductant move counter-cur­rent to the flow of the rising plasma offgases. The EAFR plasma furnace claims the following advantages over the conventional submerged arc fur­nace.45

1. The quality of the carbonaceous reductant is not important. ~. A wide variety of raw materials can be treated. 3. Agglomeration of the ore fines is not required, thus making this process

more economical and, also, large surface area to volume ratio of the ore fines is used to advantage.

4. Electrical energy from nuclear power plants can be utilized directly. Oolitic ores, titaniferous magnetites, limonites, laterites, chromites, and

iron and steel plant waste oxides have been successfully treated with iron and chromium recovering up to 90%. The quality of the product, it is claimed, is comparable to that obtained by conventional processes such as the BF/BOF route or the submerged arc furnace. In most cases the sulfur, phosphorus, nitrogen, and tramp element levels were lower than those obtained in the conventional processes.

CONCLUSION

Although plasma technology has found commercial applications in blast furnace ironmaking and also in recovery of valuable metals from steel mill baghouse dust, it is considered that considerable research effort and financial resources are needed to explore the vast untapped potential of plasma technology which could successfully be utilized in the iron and steel­making industry. Currently, besides the iron and steel industry, plasma technology has been used for oxide coatings, production of high-purity carbides and nitrides, AMA zirconia, and various other refractory metals from their sulfide ores. Recently, several proposals· have come forward, such as for the production of stainless steel in the Extended Arc Flash Reactor and also for the production of metastable metallic powders directly from the metal oxide ores in the SSP reactor. Also, a Noranda-McGill University plasma reactor has been licensed to Davy McKee for the smelting ofchromite ores. However, it appears appropriate to mention here that only the AMA plasma zirconia process and plasma applied oxide coatings are commercial and all other processes are still either on the laboratory or

JOURNAL OF METALS· February 1984

Table IV: A Comparison of the Steel Quality Obtained on Remelting of Grade SH KH 15 Ball Bearing Steel

Remelting Product Method Den ity Total

Open electric arc 7.8162 0.0226 0.0096 0.0096

Electro lag 7. 239 0.010 0.0076 0.0013

Vacuum arc 7. 190 0.0094 0.0033 0.0042

Electron beam 7. 295 0.005 0.0035 0.0012

Pia ma arc 7. 234 0.0071 0.0029 0.0041

pilot-plant scale. Nevertheless, the application of plasma technology in the improvement of extractive metallurgical operations in the introduction of innovative and radically new processes is inevitable, and this will be reflected in the extraction and refining of metals in the next decade.

References 1. R. S. Barnes. "The Role of Nuclear Steelmaking." Ironmaking and Steelmaking, 2 141 (1975). pp.

271·278. 2. I. G. Sayce, "Plasma Processes in Extractive Metallurgy," Advances in Extractive Metallurgy and Refining,

London, Inst. Min. Met., October 1971. 3. S. M. L. Hamblyn, "Plasma Technology and its Application to Extractive Metallurgy," Mat. Sci. Eng., 9

(3), July 1977. 4. R. G. Gold, W. R. Sandall, P. G. Cheplick and D. R. MacRae, "Plasma Reduction of Iron Oxide with

Hydrogen and Natural Gas at 100 kW and 1MW," Iron and Steelmaking, January 1977. 5. M. G. Fey and W. H. Reed, "Arc Heater Pyrolysis of Hydrocarbons," Presented at 1980 Al Chern. Eng.

Meeting, Philadelphia, Pennsylvania (June 1980). 6. F. D. Richardson and J. H. E. J effes, J.I.S.I., 160 (261 ) (1948). 7. E. Pfender, "Electric Arcs and Arc Gas Heaters," Gaseous Electromcs, vol. 1, edited by M. N. Hirsh and H. J.

Oskam, Academic Press, New York, 1978. 8. G. J. Vogt, "Novel RF-Plasma System for the Synthesis of Ultrafine, Ultrapure, Sic and Si3N.' paper

presented in the Mat. Res. Soc, 1983 Annual Meeting, Boston, Massachusetts. 9. B. Gross, B. Gryca and K. Miklossy, "The Centrifugal Plasma Jet Furnace,," Materials Res. Std., 5

(1965), p. 1973. 10. D. Whyman, "A Rotating Wall DC-Arc Plasma Furnace," Mat. Res. Std., 5 (1965), p. 173. 11. 1. H. A. King, British Patent No. 1,043,384 (1966). 12. J. K. Tylko, British Patent No. 139035113 (1971). 13. W. S. Brzozowski and Z. Celinski, "Plasma Generators," Bull. Acad. Poln. Sci. Ser. Sci. Techn., 5 (1962), p. 7. 14. A. T. Erokhin, "Plasma Processes in Metallurgy and Technology of Inorganic Materials" 70th Anniversary Academician Rykalin, Nauka, Moscow, 1973. 15. D. A. Maniero, Westinghouse Engineer, 26, 1966, p. 66. 16. L. W. Scammon, U.S. Patent No. 3,661,764 (1972). 17. D. R. MacRay, et aI., U.S. Patent to Bethlehem Steel Corporation, No. 4,002 ,466 119771. lB. C. A. Pickles, A. McLean, C. B. Alcock and R. S. Segworth, "Investigation of a New Technique for the Treatment of Steel Plant Waste Oxides in an Extended Arc Flash Reactor," in Advances In Ext. Met. , Inst. Min. Met., London, (1977), p. 69. 19. J . J. Moore, K. J. Reid and J. M. Silvertson, "Production of Metastable Metallic Particles Directly From the Mineral Concentrate by In-Flight Plasma Reduction" paper presented in the Mat. Res. Soc. 1983 Annual Meeting, Boston, Massachusetts. 20. P. A. Lovett, "Plasma Technology at SKF Steel Engineering AB," The Metallurgist and Materials Technologist, October 1983, p. 513-515. 21. P. A. Lovett, "Plasma Technology at SKF Steel Engineering AB," The Metallurgist and Materials Technologist, October 1983, p. 513-515. 22. N. DPonghis, A. Vidal and A. Poos, "Operation of a Blast Furnace with Superhot Reducing Gas," Ironmaking Pro. 38, Detroit, 1979. 23. "AMA Plasma Zirconia Grades," Associated Minerals Consolidated Ltd., Company Brochure. 24. C. Sheer, and S. Korman, U.S. Patent No. 2616843 (19521. 25. C. Sheer, and S. Korman, U.S. Patent No. 2617761 (1952). 26. C. Sheer, and S. Korman, U.S. Patent No. 3099614 (1963). 27. C. Sheer, and S. Korman and T. J. Dougherty, "Arc Gasification of Coal," 4th IntI. Symp. on Plasma Chemistry, Zurich, August 1979. 28. S. Korman, T. J. Doughtery and C. Sheer, "Arc Desulfurization of Coal," 4th IntI. Symp. on Plasma Chemistry, Zurich, August 1979. 29. K. J. Reid, "Direct Steelmaking Based on Solid-Plasma Interactions," Proc. 53rd Annual Meeting, Minn. Section, AIME and 41st Annual Mining Symp., 1980. 30. J. K. Tylko, U.K. Patent No. 7913337 (1979). 31. K. J. Reid, J. J. Moore and J. K. Tylko, "Reduction of Taconite in the Sustained Shockwave Plasma," 5th Intl. Symp. on Plasma Chemistry, Edinburg, August 1981. 32. K. J. Reid, J. J. Moore and J. K. Tylko, "The Application of the Sustained Shockwave Plasma (SSP) Reactor in Process Metallurgy," 2nd World Congress of Chern. Eng., Montreal, October 1981. 33. J. J. Moore, K. J. Reid, and J. K. Tylko, "Reduction of Lean Chromite Ore Using a New Type of Plasma Reactor," paper presented at AIME Conference, Chicago, 1981. 34. W. H. Gauvin, G. R. Kubanek and G. A. Irons, "The Plasma Production of Ferromolybdenum: Process Development and Economics," J. Metals, 33 (1) 119811, pp. 42-46 35. T. Mehmetoglu, W. H. Gauvin and F. Kitzinger, "Characteristics of Plasma Transferred Arcs," 4th IntI. Symp. on Plasma Chemistry, Zurich, August 1979. 36. N. A. Barcza, T. R. Curr, W. D. Winship and C. P. Heanley, "The Production of Ferrochromium in a Transferred-Arc Plasma Furnace," 39th Elec. Fur. Conf. , Houston, December 1981. 37. B. Y. Paton, V. I. Lakomsky, G. F. Torkhov and V. A. Slyshankova, "Metallurgical Features of Plasma-Arc Remelting of High-Alloyed Steels in Water-Cooled Copper Crystallizer," in Plasma Processes in Metallurgy, Joint Pub. Res. Center, Arlington, Virginia, 1974. 38. C. Asada, T. Adachi, "Plasma Induction Heating" Proc. 3rd IntI. Symp. on Electroslag Melting Processes, Pittsburgh, Pennsylvania (1971). 39. T. Konoshito, "Plasma Electron Beam and its Application to Vacuum Metallurgy," Shinku, 17 (12) (1975). 40. J. D. Chase, and J. F. Shriven, "Process for the Beneficiation of Titaniferous Ores Utilizing a Hot Wall Continuous Plasma Reactor," U.S. Patent No. 3856918 (1974).

JOURNAL OF METALS· February 1984

Percent Ga e Nitride Oxygen Nitrogen ----0.0020 0.0033 0.0104

0.0019 0.0024 0.00 2

0.0019 0.0023 0.0070

0.0011 0.0016 0.0033

0.0001 0.0022 0.0075

p, mOr plosma

Elecllit Ci t e

Figure 13. The falling-film plasma reactor.

Ra. 0''1 re neeler

Figure 14. Schematic of the extended arc flash reactor,

55

Kamleshwar Upa­dhya, Mineral Re­sources Research Center, University of Minnesota, 56 East River Road, Minnea­polis, Minnesota 55455.

Dr. Upadhya re­ceived his BSc in metallurgical engineer­ing from B.H.U., India and a PhD in metal­lurgy from the University of Strathclyde, U.K. He is the author of several publica­tions in the areas of slag-metal reactions, corrosion of metals, and plasma processing of minerals. He was the recipient of the John Chipman Award in 1981. His current research interests include thermodynamics and kinetics of metallurgical extraction processes, hot corrosion of metals and alloys, and plasma processing of minerals. Dr. Upadhya is a member of The Metallur­gical Society of AIME,

41. R. G. Gold. W. R. Sandall, P. G. Cheplick and D. R. MacRae, "Plasma Reduction of Iron Oxide with Hydrogen and Natural Gas at 100 kW and 1 MW," Iron and Steelmaking, January 1977. 42. D. R. MacRae, et aI., "Ferrovanadium Production by Plasma Carbothermic Reduction of Vanadium Oxide," 34th Electric Furnace Conf., December 1976. 43. F. Kassabji, "Technical and Economic Studies for Metal Production by Plasma Steelmaking Application," 4th IntI. Symp. on Plasma Chemistry, Zurich, August 1979. 44. B. Pateyron, J. Aubreton, F. Kassabji and P. Fauchais, "New Design of Reduction Plasma Furnaces Including the Electrical Transfer to the Bath and the Falling Film," 5th IntI. Symp. on Plasma Chemistry, Edinburgh, August 1981. 45. C. A. Pickles, "A Review of Plasma Smelting, "Symposium on The Industrial Opportunities for Plasma Technology, Toronto, October 1982.

ABOUT THE AUTHORS

Kenneth J. Reid, Director, Mineral Re­sources Research Center, 56 East Riv­er Road, Minneapolis, Minnesota 55455.

Dr. Reid received his BSc in chemical engineering from the

University of Birmingham, England (1957) and PhD from Cambridge University (1960). Before his appointment to his current posi­tion as director of the Mineral Resources Research Center and professor of mineral engineering at University of Minnesota, he worked for CSIRO in Australia where he conducted basic research on simulation and control of grinding classification and flotation systems; served as associate pro­fessor at McGill University, Montreal; and created and headed IPAC Services, a de­partment which provides technical services in process analysis and control for two principal mining companies in Zambia.

John J. Moore, Asso­ciate Professor, Met­allurgical Engineer­ing, Mineral Re­sources Research Center, University of Minnesota, 56 East River Road, Minnea­polis, Minnesota 55455.

Dr. Moore received his BSc in metallurgy from the University of Surrey, England and PhD in industrial metallurgy from the Uni­versity of Birmingham, England. He has worked as a steelworks metallurgist and an industrial engineering-production control manager. At the University of Minnesota, he is responsible for the Process Technol­ogy Division of the Mineral Resources Re­search Center. He is a member of The Metallurgical Society of AIME.

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JOURNAL OF METALS • February 1984