1. M. Mihovsky
Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010, 3-18
THERMAL PLASMA APPLICATION IN METALLURGY
(REVIEW)
M. Mihovsky
University of Chemical Technology and Metallurgy Received 05 January 2010
8 Kl. Ohridski, 1756 Sofia, Bulgaria Accepted 12 February 2010
E-mail: mihovsky@uctm.edu
ABSTRACT
A classification of the most perspective plasma technologies and equipment are presented including the initial
working raw materials, and the type of resulting materials. The main pilot plasma technologies and such with industrial
application are indicated.
The important factors predetermining plasma technology development in metallurgy are evaluated.
The basic works of Plasma Metallurgy Research Laboratory “PLASMALAB” for thermal plasma application in
metallurgy are shown, based on actual results, experience, raw materials state, technological, economical and ecological
conditions.
Keywords: plasma technology, plasma torch, plasma furnace, metallurgy.
INTRODUCTION • Widespread use of alternative electroenergy
sources.
The actual state concerning raw materials, energy, • Tendency for an expanding application of hydro-
technology and ecology worldwide predetermines the gen as reducing agent obtained from water dissociation
development of new metallurgical technologies and and a gradual replacement of traditional fossile reductans.
equipment based on thermal plasma application as a • Gradual decrease of the absolute volume of
concentrated heat source and for chemically active com- metallurgical production in favor of improved quality
ponents of the processes. of produced metals and alloys.
Contemporary unfavourable state of resources for • Gradual replacement of presently used classi-
metallurgy, serious energy and ecology problems and cal alloys by new advanced metal-based materials with
incessant increase of metals and alloys demand lead to a better properties.
new strategic concept concerning the world metallurgy. • Development of new nonwaste, clesn metallur-
According to a present prognosis on the devel- gical technologies and equipment, providing a maxi-
opment of 21st century metallurgy we find that the fol- mum utilization of the valuable components in natural
lowing main principals would be valid: and waste raw materials.
• Processing of low quality, poor polymetal raw All of these principals can be observed in the
materials and waste. case of plasma metallurgy technologies. Its perspectives
3

2. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
are confirmed by numerous theoretical and practical as well as solid (fine coke, coal powder and others) re-
researches carried out in the field of thermal plasma ducing agents are used.
metallurgy application during the last 20 years in tech- The characteristic feature of these technologies
nically advanced countries. is that the materials are processed to a finely powdered
The application of thermal plasma in metallurgy at condition and in most cases are subjected to prior par-
present is developing in two directions - improvement of tial reduction (Fig.1). The aim in all the different plasma
classical metallurgical technologies and development of units is to provide through various technologies a short
principally new plasma metallurgical technologies. melting time of the raw material and a good contact
According to the scope of their application, plasma tech- (maximum area) between the reducing gas and the melted
nologies are classified generally as smelting and reducing oxide material.
(extracting) processes upon which existing classifica- In general plasma reduction technologies are
tions are based. classified according to the heat-exchange type between
The growing limitation of quality raw materials for the plasma arc and the processed material and accord-
the metallurgical industry, and the serious ecological prob- ing to the conditions under which the reduction pro-
lems imposed on them require to keep in mind as a clas- cesses run.
sification characteristics not only the existing plasma pro- Industrial production of metals and alloys
cesses, but also both the type of initial raw materials source through reduction - from natural and waste raw materi-
and the type of the final material (Fig.1). The basic plasma als is done in plasma furnaces, similar to the plasma-
metallurgical processes used in industry as well as the most arc ones, working in “open bath” 3 (Fig.1). The indus-
perspective plasma pilot units and technologies are in- trial furnaces TETRONICS-FOSTER WHEELER (EPP-
cluded in this scheme. Expanded Precessive Plasma, Fig. 2) [5, 6] are well
The main idea of the Swedish PLASMABLAST known, in which the plasma torch rotates with a veloc-
[1] technology for improvement of the blast furnace ity of 50-1500 min-1 around the furnace vertical axis at
process is overheating of the blow air, which aims mainly 5-15° angle and of arc length 500-750 mm while the
cutting down of coke consumption. Besides, plasma powdered material is introduced around the heating
overheating of the blast makes possible a more flexible surface of the resulting plasma cone. The latest data, ob-
control of the furnace heating process. tained from processing of powder slimes and slags in a 2
The developed process PIROGAS (Plasma In- MW plasma furnace are: specific energy consumption
jection of Reducing Overheated Gas System) provides 1300 kWh/t powder; reducing agent consumption 90-
also low quality pulverized coal to be blown into the 300 kg depending on the type of processed material [7].
furnace through the plasma torches. The experimental Some plasma units [8,9] use an extended arc 4
data show that this technology can reduce coke con- (Fig.1) in combination with a vertically located tube
sumption to the necessary minimum for reduction pro- reactor (EAFR-Extended Arc Flash Reactor, Fig.3) in
cesses in the blast furnace, i.e. from 470 kg/t pig iron to whose upper section the powdered charge is introduced.
385 kg/t [ 3]. The plasma arc is generated between three hollow graph-
Calculations show that the development of new ite electrodes (three-phase-power supply).
advanced technologies corresponding with raw materi- The basic ELRED concept [10] is the reduction
als, energy and ecology requirements in many cases are of fine (size below 0,1mm) iron ore concentrate (more
sounder from an economical point of view compared to than 65 % Fe) in two stages: an initial stage for pre-
reconstruction of the classical technologies. This reduction in solid state in the presence of an excess of
prompted the development of alternative reduction carbon to a partially metallized product containing car-
plasma metallurgical technologies. bon and a final reduction stage for smelting of this prod-
These technologies are intended for processing uct using electrical energy.
of raw low quality polymetal ores, as well as of waste The prereduction takes place under pressure and
materials (slags, slimes, chemical industry waste mate- temperature 970-990oC in a circulating fluidized bed
rials, etc.). Gaseous (hydrogen, methane, carbon oxide) in a reactor vessel with internal refractory lining.
4

3. M. Mihovsky
Fig. 1. PLASMALAB scheme of pilot and industrial plasma metallurgical technologies.
Using shaft furnace 5 (Fig. 1) ASEA SKF persed flow forms a melt film on the wall of the reac-
founded the basic plasma process PLASMASMELT [12] tor-anode. This film falls down to the bottom part of
(Fig.4) for pig iron production, which was further de- the reactor getting reduced on the way. Thanks to the
veloped in the PLASMADUST [13] modification - for high temperature and speed of the plasma reducing gas
non-ferrous metals dusts processing and the surface processes in the liquid film take place com-
PLASMACHROME [14] - for FeCr production. The paratively quickly and the time of the stay of the mate-
shaft furnace is filled with coke, while in its stove zone rial in the reactor is long enough for a nearly full re-
are installed three plasma torches of 6 MW each. The duction of the raw materials. The 1 MW FFP pilot unit
powdered initial raw material is subjected to two-grade was constructed after this scheme. Hematite from the Carol
pre-reduction in two furnaces of “fluidized bath” type, Lake source was processed in it, resulting in a metal of
using reducing agent, the gas (carbon oxide), obtained chemical composition: 0.005 % S; 0.001 % P; 0.006 % C;
in the shaft of the furnace. This gas acts both as trans- 0.06 % Si; 0.07 % Cu under electrical energy consump-
porting medium (to deliver the pre-reduced material tion of 3.9 kWh/kg Fe. A mixture of hydrogen and natu-
and the low quality powder coals into the plasma torch ral gas with ratio of 2:1 was used as a reducing and
zone) and as a plasma gas. transport agent. In spite of the promising results of this
Bethlehem Steel developed a FFP (Falling Film original idea on plasma reactor for reduced production
Plasma) reactor 6 (Fig.1), which actually gives the best of metals and alloys from fine raw materials it did not
of technological technical and economical results [17]. find industrial application. The main reasons for this are
The essence of this method is concluded in the reduc- the lack of arc plasma torch of high power with sufficient
tion of a thin layer (film) of melted oxides (Fig.5). The working life and the high working voltage (over 1000 V)
fine raw material with powdered solid reducing agent is needed for obtaining a stable maximum long arc.
tangentially introduced by reducing plasma gas in a ver- Plasma furnace technology was first applied in
tical water-cooled cylinder camera, acting as a plasma South Africa in the mid-to late 1970 s, when it was
torch anode section. The intensively whirled gas dis- realized that advantages could be obtained in the pro-
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4. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
Fig. 4. PLASMASMELT [12].
Fig. 2. EPP [5, 6].
Fig. 3. EAFR [8, 9]. Fig. 5. FFP-reactor [17].
cessing of fines for the production of ferro-alloys. A search Laboratory “PLASMALAB” proposed a new con-
number of processes have been investigated, and a 40 ception on design of plasma furnace and of technology
MVA DC transferred-arc ferrochromium furnace has for scrap remelting (Fig. 6) and production of metals and
been implemented on an industrial scale at Palmiet alloys from deferent materials by reduction (Fig. 7).
Ferrochrome, Krugesdorp. South Africa has well-de- According to the general conception [21] our first
veloped plasma-furnace research facilities, which include PLASMALAB FFP-reactor improved design offers a
the 3.2 MVA DC transferred-arc plasma furnace at combination between a hollow graphite cathode and a
Mintek, Randburg [18-20]. copper water cooled nozzle, Fig. 8, [22]. A special fea-
All described reducing processes have some tech- ture of this construction is that the plasma gas is intro-
nological and design advantages and disadvantages. Af- duced from both sides (interior and exterior) of the tube
ter critical estimation of them Plasma Metallurgy Re- graphite cathode, i.e. the gas is bilaterally blown, and
6

5. M. Mihovsky
Charge Feeding
Filter Charge System
Gas V2
G as Supply
Arc Plasma Gas V1 System
Preliminary
Heating
Torch
Chamber
Water Cooling
System
Power
Inductor Supply
Mould Computer
Condensersà
òîð Continues casting machine
OFF Gases Ingot
Fig. 6. Conception for plasma installation for remelting of disperse scrap and production a continuous caste metal ingot [21, 39].
the relation in the capacity, respectively the velocity of reactor zone is provided mainly by tangentially blow-
the two gas streams (V1 and V2) is regulated in such a ing plasma gas V2 and in the lower zone - through DC
way that the plasma arc reaches a tube-like configura- magnetic field.
tion. Such organization of the plasma arc enables the Several undesirable effects were registered in
introduction of the powder feed into a volume surrounded these experiments:
with highly heated plasma. The adopted type of charge • Obtaining a stable burning plasma arc with
introduction ensures its maximal convection and radia- length up to 280 mm requires high consumption of the
tion heat exchange with the plasma arc on its way be- tangentially blowing plasma gas (16 m3/h) [22].
tween the cathode and the anode. The anode spot, ac- • Tangentially blowing gas V2 is inadequately ef-
cording to the general concept offered, is in a continuous fective for cathode spot rotation.
motion with a controlled velocity and trajectory. • Gas stream V1 blows some powder raw materi-
The motion of the anode spot follows a compli- als out of the reactor.
cated rotary and at the same time reciprocating trajec- To avoid above-mentioned disadvantages the
tory i.e. the anode spot would move as a spiral upwards - Plasma Torch - FFP-Reactor system construction has
downwards along the interior wall of the reactor-anode. been modified [23].
The organization of the motion of the plasma The plasma gas is unilaterally introduced into
arc is realized with simultaneously working electro-, the space between the graphite cathode (Fig.9) and the
electromagnetic and gas dynamic devices, computer nozzle, situated at the top of the plasma reactor. The
controlled and synchronized. plasma gas is tangentially introduced under a gradient
In our first experiments [22] the motion of the of 40°C to the vertical axis of the reactor through an
anode spot along the reactor height is provided by syn- insulation flange. The charge is introduced into the space
chronized increase of the gas flow rates V1 and V2 and between the cathode and special flange through three
the plasma arc current. The arc’s rotation in the upper feeding screw systems. The powder material is swiped
7

6. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
Charge Feeding
System
Filter Charg
Gas V1
Gas Supply
Gas V2 System
Vibroreaktor
FFP-Plasma
Reactor
Water Cooling
System
Scrubbe
Power
Supply
Metal
Slag
Computer
Tundish Ar
Condensers Mould
àòîð
Continues casting machine
OFF Gases
Ingot
Fig. 7. Conception for plasma installation for reduction processing of disperse natural and waste raw materials and production
a continuous caste metal ingot [21 39].
along from the plasma gas and is blown through the magnetic action. An experimental stand has been made
nozzle on the reactor wall. (Fig.10).
The centrifugal force caused by the plasma gas It includes DC plasma torch with hollow graph-
blown out tangentially orients the particles on the reac- ite cathode and graphite tubular reactor-anode, with
tor wall. which the processes of electromagnetic and gas dynamic
The rotating motion of the cathode and the an- control of the plasma arc are simulated.
ode spot is realized with a circular magnetic field, ob- A major requirement to this system (“plasma
tained from a monophase AC power supply stator, situ- torch–plasma reactor”) is to provide a possibility for a
ated coaxially to the tube reactor-anode. Besides, the conveniently observation and photographing of the
stator moves under controlled velocity upwards-down- plasma arc in the volume of the tube reactor-anode, as
wards the reactor height with the help of hydrocylinders. well as measuring of its main electric, gas dynamic and
The next step is synchronization of the gas flow geometrical parameters.
rate change, the plasma torch current and the stator One of the original solutions in the present work is
position along the reactor height. For this purpose a the series inductor connection in the anode electrical chain
special gas valve (Fig.9) is constructed to regulate the of the system ”plasma torch cathode-reactor-anode”
plasma gas flow rate, combined with the inductive trans- (Fig.10). Thus, the “PLASMALAB” system for Auto-
ducer for the arc current control. Electro-Magnetic Rotation (AEMR) of DC transferred
The main target is to study the energetic param- plasma arc generates in the inductor from plasma arc
eters of the DC transfer plasma arc and the possibility current, i.e. an additional DC source is not necessary
for its effective rotating under the influence electro- for arc electromagnetic control.
8

7. M. Mihovsky
Fig. 11 shows the behaviour of the arc and heat-
ing of graphite reactor-anode without axial magnetic
field (the anode chain is connected directly to the graph-
ite reactor). It is seen that the arc burns calmly between
the clearly outlined static/immobile anode and cathode
spots.
At series connection of the inductor, in the an-
ode chain (Figs.10, 12) the arc unscrews under the in-
fluence of the created by the inductor, axial magnetic
field (under the action of Laurence force). The graphite
reactor-anode is heated evenly (Fig.12a), because of the
high rotational speed of the anode spot, which is the
main target of the electromagnetic control of the arc in
FFP-plasma reactor “PLASMALAB”. The cathode spot
also rotates, but its speed is lower. An evidence for this
is the observed spiral trajectory of the arc column
(Fig.12b), i.e. the cathode spot drops behind in its rota-
tional speed compared to the anode one.
The rotational speed of the arc depends on flow-
ing current which is the same that creates the magnetic
field in the inductor-as the arc current grows, the mag-
netic induction of the electromagnetic field grows too.
In accordance to the conception developed in
“PLASMALAB”, for plasma installation and technol-
Fig. 8. PLASMALAB FFP- reactor with hollow graphite
ogy for reduction production of metals and alloys, the
electrode [21].
processed disperse charge is blown through a hollow
graphite cathode on a gravitational principle and gets
in FFP-reactor, in the zone of DC plasma arc. It is proved
that the arc has transport phenomena [24].
The main idea is that the charge particles are
sucked by the rotating arc at the exit of the hollow cath-
ode and are transferred to the wall of the concentrically
situated tube reactor-anode. It is supposed that in the
volume of the arc column, the particles will be heated
and melted while getting on the reactor-anode wall. The
melted metal-slag suspension runs down the reactor
anode wall and reduction processes run. The obtained
metal-slag melt drops in the rector bath.
Fig. 13 shows the behaviour of the arc at differ-
ent current values. At minimal current-20 A (Fig.13h),
due to a substantial difference in the rotational speed of
the cathode and anode spots, the arc column is seen as
a part of the helical line.
With the increase of the current up to 80 A, the
arc column gradually becomes fade (it is not noticeable
on the pictures due to its high rotational speed). At
Fig. 9. New PLASMALAB FFP – reactor [23].
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8. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
Fig. 10. Scheme of the experimental “PLASMALAB” stand for investigation of system “plasma torch-FFP-plasma reactor”
[40]. 1-argon cylinder; 2-throttle valve; 3-gas collector; 4-flow meter; 5-plasma torch; 6-refractory insulator; A7-graphite
tube-anode; K-cathode; 8-water-cooling copper inductor; 9-contact clamp; 10-protective screen;. 11-video-camera; 12-
transferred plasma arc; E-DC power supply; Sh-shunt.
a) b)
Fig. 11. a) Heating of the tube graphite anode; b) DC transferred plasma arc in the system “hollow graphite cathode-tube graphite
anode”, without axial magnetic field [40].
current 80A, three cathode spots appear on the cathode adjacent edge is heated evenly. In the whole investigated
(three sources of intensive thermionic emission), and their current range, the motion speed of the anode spot is much
number grows with the current increase. Simultaneously, higher than the one of the cathode.
the cathode spots start rotating faster on the adjacent edge The arc column crosses the cathode axis with
of the cathode. Under these circumstances of increased frequency equal to the revolutions of the anode spot,
current, the anode spot rotates at speed extremely higher i.e. the particles falling through the hollow cathode are
than the one of the cathode. At current 250 A the rota- taken by the arc column and transferred to the tube
tional speed of the cathode spot is so high that the whole graphite anode wall. A demonstration of this is the pic-
10

9. M. Mihovsky
a) b)
Fig. 12. a) Heating of the tube graphite anode; b) DC transferred plasma arc in the system “hollow graphite cathode-tube
graphite anode”, under the action of axial magnetic field [40].
a b
?
c d
#) ) #) )
A
e B
f C
g D
h
) $) ) )
Fig. 13. Burning of DC transferred plasma arc in the system “hollow graphite cathode-tube graphite anode” – arc (inductor)
current from 20 A to 250 A [40].
tured trajectory of graphite particles that are moving Plasma-arc melting (Fig. 1, position 8) is de-
away from the cathode (Fig. 14). The trajectory of the veloped as an alternative of the classical electric arc
moving away particles follows the trajectory of the arc melting. Three types of plasma furnaces are working in
column, which is clearly shown on Fig. 14. the world today differing in the way of power supply
The results from the experiments give us con- (DC and AC) and in the way of plasma torch location.
fidence that the idea about charge feed through a hol- The first plasma melting furnaces Linde type [25]
low graphite cathode in “PLASMALAB” FFP-reactor (Fig.16), working still in Russia, are actually recon-
is perspective. structed classical electric arc furnaces, where the graphite
The dimensions of the model graphite tube re- electrodes are substituted by plasma torches (cathodes)
actor are used for design of a working laboratory FFP- with an independent DC electrical power supply and a
reactor “PLASMALAB”, shown on Fig.15. Its techno- water cooled bottom electrode (anode). The next step
logical potentialities lie ahead. in the plasma-arc melting development is the Freital -
The smelting plasma technologies (positions 8 to Voest Alpine system, where four DC plasma torches
12 in Fig.1) find wider application at present. are located at the furnace jacket walls under a certain
11

10. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
Fig. 14. The graphite particles moving away from the
cathode to the anode follow the plasma column trajectory
[39, 40].
angle towards the metal bath (Fig.17) [25]. A 45-t Vost
Alpine plasma furnace went to operation in November
1983. The main disadvantage of this furnace is situa-
tion of the plasma torches. At the beginning of the melt-
ing, plasma arcs melt a horizontal craters in the scrap.
Due to plasma arc length limit the torches body enter
in the furnace space under the unmelted scrap. Often
the scrap falls on the plasma torches and breaks them. Fig.15 “PLASMALAB” FFP-plasma reactor with charge
As a result this plasma furnace construction did not feeding system [39, 40]
find further industrial application but it remains a sig-
nificant step in plasma metallurgy development. tively silent work of the plasma torches and the high
KRUPP [26] developed AC melting plasma fur- airtightness of the furnace working space). The main
naces, where the bottom electrode was avoided (Fig.18). problem of the high capacity power steelmaking (AC
A starter DC plasma torch works as an electrode in and DC) plasma furnaces is the short working life of
main powerfull AC plasma torch (Fig.19). The possi- the plasma torch electrodes (cathodes) under high cur-
bility for regulation of the plasma torches inclination rent density.
towards the vertical furnace axis improves the furnace Plasma-induction melting (Fig. 1, position 9)
heating work and decreases the refractory lining con- takes place in the classical induction furnace which has
sumption. a plasma torch installed over its crucible and works
The plasma-arc melting compared with classical after the transferred arc scheme with bottom water
electrical arc melting leads to an improved quality of the cooled electrode-anode (Fig. 20) [27].
produced metal, decrease of the specific electric energy The use of two principally different methods for
consumption under increased output, enables the pro- heating - plasma and induction - leads to a decrease of
duction of low carbon alloys, increases the alloy elements the smelting time (respectively an increase of the fur-
assimilation grade and the output in general, improves nace output) by 20-50 % in comparison with the classi-
the production ecology (decreases the noise degree and cal induction furnace. In addition this heating combi-
the quantity of the harmful emissions due to the rela- nation decreases the specific electrical energy consump-
12

11. M. Mihovsky
Fig. 16. Linde DC plasma furnace [24].
Fig. 19. KRUPP AC plasma torch [25].
Fig. 17. Freital-Voest AlpineDC plasma furnace [25].
The good pilot results obtained in the 60 kg
plasma-induction furnace led to the design and recon-
struction of an industrial 2,5 tons induction furnace
operated in the metallurgical complex Kremikovtsy Ltd.
to a plasma-induction one [29]. This furnace consists of
two independent devices - an 1 MVA classical main
frequency Russian induction furnace and transferred arc
metallurgical plasma torch (PLASMALAB-3000) fit-
ted into its cover over the crucible (Fig. 21). The plasma
torch works with the plasma gas argon or nitrogen or
their mixture at maximum currents of 3000 A. The power
Fig. 18. KRUPP AC plasma furnace [26]. supply is realized by two DC units of 300 kVA each
working parallel.
tion by 10-18 %. The highly active slag (heated from The possibility for effectively melting industrial
the plasma torch) resulting from the process and the bronze turnings in this furnace and for producing high
effective stirring makes possible the production of a metal quality bronze castings has been proven. The plasma torch
of low sulphur, gases and non metallic inclusion con- current during the melting was kept constant at 1500 A
tents. The use of argon as plasma gas leads to a substan- and the voltage varied from 120 to 230 V depending on
tial decrease of the burn-off alloy elements. the arc length. Depending on bronze turnings bulk den-
Experiments on melting of copper, brass, bronze sity and preliminary processing the yield was 90-93 %;
and high-speed steel turnings were made in a plasma the specific power consumption for melting was 420-
induction furnace [28] at the plasma research labora- 464 kWh/t; the specific melting time was 1,2-1,55 h/t;
tory PLASMALAB during the last few years. the aluminium oxidation was 0,7-0,9 % [30].
13

12. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
The replacing of DC (W-cathode) plasma torch
in pilot PLASMALAB 60 kg plasma induction furnace
with hollow graphite electrode reduces the running cost
for copper and copper alloys production (Fig. 23) [31].
This system has turned out to be simple, very reliable
and easy to maintain. On the other hand the productivity
and the yield are practically the same. As a result of the
pilot experiments the DC transferred arc plasma torch
in 2.5 tons plasma induction furnace was replaced with
100 mm graphite electrode with 16 mm hole for plasma
gas blowing.
In the case of graphite electrode operation and
argon blowing through the electrode hole, the arc length
and the voltage are decreased compared to the length
and voltage of the plasma torch arc under otherwise
Fig. 20. PIF [27].
equal conditions due to the lack of the cooling and iso-
lating effect of the plasma gas. This is the reason for
substantial arc deviation to the crucible wall. The cath-
ode spot moves on the edge of the hole, the anode spot
has no clear contour. The V-A characteristics of graph-
ite electrode arc have been measured in the current range
500-2300 A, constant arc length 300 mm and gas con-
sumption 3800 l/h (Fig. 24).
The graphite electrode curve is of a rising type
similar to the V-A characteristics of the plasma torch
arc, but voltage is 30 to 50 V lower. In the case of si-
multaneous operation of plasma and induction heating
the plasma arc expands due to interaction between the
magnetic field of the induction furnace and the plasma
Fig. 21. PLASMALAB 2,5-t PIF [30]. arc. The expansion of the arc in this case strongly de-
pends on the current frequency of the induction fur-
Fig. 22. 2,5-t plasma-induction furnace with metallurgical Fig. 23. 2,5-t plasma-induction furnace with 100 mm hollow
plasma torch [31]. graphite electrode-cathode [31].
14

13. M. Mihovsky
nace (max expansion at 50-60 Hz). While in the case of '
using a plasma torch this effect is undesirable – as it %
graphite electrode+induction furnace
leads to the appearance of non-controllable secondary
#
arcs and higher errosion of nozzle and plasma torch plasma torch [5]
body, in the case of graphite electrode the expanded arc !
Voltage, V
leads an increase of the working arc voltage (Fig. 24),
respectively to an increase of arc power at constant cur- graphite electrode
'
rent. Compared with the classical plasma torch, the
hollow graphite electrode (both working at nominal %
current 500 A) decreases the average furnace produc-
#
tivity with 5-8 %. On the other hand, when the two # # #
systems - arc and induction heating work simultaneously Current, A
the furnace productivity increases with 9-11%, the melted Fig. 24. Comparison of V-A-characteristics of plasma torch
crater in the scrap is wider and the local overheating of and hollow graphite electrode at constant arc length 300 mm.
the metal decreases.
Plasma arc remelting (Fig.1, position 10 ) is a plasma arc furnaces and in addition leads to a faster
plasma technology, which has found widest industrial liquid metal heating, due to the high specific heat plasma
application at present. Patented in the institute Paton capacity. The plasma ladle reheater is used at U.S. STEEL,
(Ukraine) (Fig. 25) [33] and developed by some leading Lorain, OH, to save aborts or to homogenize the tem-
world companies as Daido, Retech and others, this tech- perature of the molten steel in the ladle [36].
nology is recognized as a basic refining process, suc- The quantity of steel produced by continuos cast-
cessfully competing with electro-slag remelting and ing has reached over 90 % of total world-wide produc-
vacuum-arc remelting. The technology essence consists tion. The conventional characteristics of the continuous
in melting of a metal ingot with plasma heating in a casting processes are: high heat losses from the melt in the
copper water cooled crystallizer, and obtaining of a re- tundish at the start of the casting of the comparatively high
fined ingot of desired structure and with a decreased degree of average superheating DT during the further cast-
gas and non-metallic inclusions content. The main ad- ing processes and the continuously changing casting pa-
vantage of plasma-arc remelting to vacuum-arc remelt- rameters. As a result it is impossible to guarantee constant
ing is that the process takes place under atmosphere quality of the cast product. The plasma heating systems
pressure or under increased pressure and enables an offer an optimal solution of the mentioned problems in
effective gas alloying of the metal. the conventional continuous casting process [37].
The effective nitrogen alloying during melting of Heating of the tundishes in continuous metal cast-
high speed steel turnings is carried out by PLASMALAB ing machines enables the decrease of the metal tapping
at present in two variations: plasma-induction melting furnace temperature by approximately 20°C, which leads
and plasma-arc melting in a copper water cooled mould to the refractory consumption decrease and increases the
under increased nitrogen pressure. Plasma gas (argon- melting furnace productivity. The intensive metal heat-
nitrogen - 3:1) flow rate is 1200 l/h. The melt in the ing in the tundish, makes possible its argon blowing, and
plasma-induction furnace is blown with nitrogen dur- in result - an optimal constant temperature and chemical
ing the total melting process through the bottom water- composition homogeneity of the cast metal.
cooled electrode-anode with a constant gas consump- In addition, this technology faciliates the organi-
tion of 180 l/h. The final nitrogen content in solid metal zation of the processes in the melting plants (provides a
varies in the range 0,092-0,098 % [34]. simple synchronization between the melting furnace and
The rapid development of ladle metallurgy the continuous casting machine).
(Fig.1, position 12) and of continuous metal casting, The Tetronics Limited has installed 20 Tundish
opened a wide field for plasma heat application. The Heating systems worldwide, including 13 for major Japa-
replacement of the ladle furnace graphite electrodes by nese steel companies, most of which are today heating
plasma torches, assures all the advantages, described for millions of tonnes of steel per year. Until 1993 Tetronics’
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14. Journal of the University of Chemical Technology and Metallurgy, 45, 1, 2010
plasma processes is extremely important for the further
development of plasma manufacturing technologies.
CONCLUSIONS
Based on the review made on the existing pilot
and industrial metallurgical plasma technologies, sources
and energy state and perspectives for world wide metal-
lurgical development the following conclusions can be
made:
• The plasma reduction technologies are still
under development and their industrial application is
limited due to the following main reasons:
− insufficient single power and operating reli-
ability of existing non-transferred arc plasma torches;
− complicated exploitation scheme for maximum
heat and reduction potential utilization of off-gases from
Fig. 25. PAR [33]. plasma units;
tundish plasma heating system used a single polarity − dynamic change of relation between electric
torch with an arc running from the torch to the steel, energy and coke prices and last but not least the con-
with a counter electrode immersed in the steel provid- servative thinking and insufficient financing of big met-
ing the return path for the DC current. Installation of allurgical companies.
the counter electrode requires tundish and/or tundish • The lack of qualitative ores on one side and
car modifications and the refractory lining becomes the significant quantities of stored waste from metal-
complicated with expensive additions. Tetronics has now lurgical and chemical industries on the other are a fa-
successfully introduced an improved avoiding any modi- vorable perspective for future application of plasma tech-
fications to the tundish to incorporate the counter elec- nologies in metallurgy.
trode [38]. • Plasma melting technologies development is
Presently over 35 plasma tundish heating sys- limited mainly by designing and manufacturing of pow-
tems are operating in steelwork world-wide, mainly in erful metallurgical plasma torches with enough single
Japan, China, Europe and USA. Main parameters of power (required currents: 100-130 kA) and reliability.
these systems are tundish size 5-80-t, heating power 0,3- • The cathode and the nozzle are the main ele-
4,3 MW, torch current 1,6-12 kA, current mode 90 % ment.
DC, 10 % AC, plasma gas argon (nitrogen). • The most perspective and easily applicable tech-
During the last decade the thermal plasma pro- nologies in the near future are plasma induction melting,
cesses based on electrical arc or induction plasma gen- plasma remelting, plasma heating in the tundish tech-
eration has been successfully applied in metallurgical nologies, which are already working in the developed
industry (tundish heating for continuous casting, ladle countries and have proved economical effectiveness.
heating for temperature control, plasma heated cupola • In the case of a stable financial support for inten-
for metal recovery), also in materials elaboration (pow- sive design, scientific research and introduction work in the
der synthesis, coatings, spheroidization) and in materi- field of application of plasma in metallurgy, we can expect
als processing (cutting, welding, surface treatment, etc.). actually working plasma technologies for processing of raw
The impressive increase of production of plasma manu- polymetal materials and waste from the metallurgical and
facturing equipment worldwide clearly indicates that the chemical industries in the beginning of the 21st century.
thermal plasma technologies become extensively em- This would facilitate the solving of the complicated raw
ployed by industrial users. Therefore, scaling-up of materials, energy and enviromental problems in industry.
16

15. M. Mihovsky
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