Materials Used in Metallurgy

 

Metallurgy is a science and engineering which studies the methods used to obtain metals in free condition from compounds that occur in nature.

Only some metals are mined in the native state. Among these are gold, silver, platinum, mercury, tin and, partly, copper. Most of the metals, however, are found in the earth in the form of oxides, silicates, sulphides, carbonates, etc. Native metals and metal compounds are associated with considerable amount of foreign materials, such as rock, gravel, sand, clay and other impurities that require removal.

Metal ore is its chemical compounds plus foreign impurities. Ore may be rich or poor. In the last case it must be dressed (concentrated) to remove undesirable impurities. When ore may be mined and converted in metal with commercial profit it is called pay or able ore. Ores of different metals are put through various dressing processes to obtain them with small quantity of impurities, i.e. to obtain almost pure metal compound and then to obtain metal in the free condition.

To convert ore in metal the high temperature is usually required. To obtain high temperature we use fuel, which may be:

- gaseous (native gas, blast-furnace gas);

- liquid (black mineral oil, or mazut);

- solid (anthracite, coke, charcoal).

Electric energy is also used in metallurgy to receive heat.

Some impurities, in the main oxides, find their way into a furnace together with ores. As a rule, oxides have high melting point: A1₂O₃-2040°C, CaO-2570°C, MgO-2800°C, etc. Besides that, an ash is formed on account of fuel burning.

To remove these impurities and ashes from the furnace fluxes are used. Fluxes render the impurities fusible at operating temperatures, combine with them and carry them off into slag.

All metallurgical processes are accompanied by slag formation, representing oxides, suphides, nitrides, and other chemical compounds. Slags are formed on the account of added fluxes as well as damage of furnace lining. The slag importance in metallurgy is very high: such important reactions as oxidation and deoxidation are possible because of slags, sulphur and phosphorous are assimilated and carried away with slag. Slags protect metal against gas saturation, facilitate heat accumulation in metal. The principal components of ferrous metallurgy slags are: CaO, SiO₂, P₂O₅, Al₂O₃, FeO, MnO, CaS, MnS, etc. The main slag characteric is its basicity mostly determined as CaO/SiO₂.

Metallurgical furnaces are operated at high temperatures. To prevent damage their walls are covered with refractory or fireproof materials. Refractory are materials that can stand at high temperatures from 1580 to 2400°C and more without considerable mechanical damage and withstand chemical attack of molten metal and slag.

According to chemical composition refractories are divided into:

 

acid materials:

basic materials:

inert refractory:

Correspondingly, slags may be acid, neutral or basic. Only acid slag may be built up in a furnace with acid lining and, on the contrary, in the basic furnace basic slag must be formed, because of acid slag reaction with basic lining, or basic slag reaction with acid lining with formation of easy fusible compound: SiO₂+CaO=CaSiO₃ and with destruction of lining.

 

Blast-Furnace Process

 

Blast-furnace process is used in ferrous metallurgy for cast iron production. The main product of ferrous metallurgy is steel, but two-stage process of steel production is now predominantly used in the metallurgy: Fe-ore®cast iron®steel.

Cast iron (iron) is a general term applied to iron-carbon alloys, containing more than
2.14 %C.

So iron is obtained in blast furnace by reducing from ores by carbon. The following raw materials, named charge, are commonly used in the blast furnace process: iron ore, fuel, flux.

Four chief types of iron ore are used:

- hematite Fe₂O₃;

- limonite 2Fe₂O₃·3H₂O;

- magnetite Fe₃O₄;

- siderite FeCO₃;

After mining the iron ore is crushed to powder, dressed from impurities and sintered in pieces. Such sintered ore is called agglomerate. During agglomeration main part of sulphur is removed from the ore and limestone CaCO₃ is added to the ore. Hence, we receive and use in blast furnace so-called fluxed ore.

The main fuel in blast furnace is coke, which is produced of coking coal by preheating it at temperature ~ 1000°C without air during 14…16 hours. Coke has the chemical composition: 80…88 %C, 8…12 % ash, 2…5 % moisture, 0.5…1.8%S, 0.02…0.2 %P. Part of coke may be replaced by natural gas (CH₄), or black mineral oil, or powder coal, or blast furnace gas.

Limestone CaCO₃ is used as a flux in the blast furnace.

The modern blast furnace constitutes the largest and most complicated type of metallurgical plant. Such a plant is capable to produce more than ten thousand tons of iron a day and night
(24 hours). It works continuously from 7 to 10 years.

The blast furnace is like a vertical pipe, lining by refractory inside, in which fluxed ore and coke, named a charge, are charged from the top and preheated air (1100°C) is blown into the furnace below. Iron and slag are tapped from the furnace periodically through a tap hole and a slag hole.

The blast furnace derived its name from the fact that air to support combustion must be blown into it under pressure, because of the resistance offered by the column of material within the shaft to passage of the combustion gases. A typical blast furnace is shown in Fig. 2.1.

Chemical reactions between carbon, oxygen, iron and its oxides occur within the blast-furnace by combustion of coke and temperature equal from 1500 to 2000°C.

Nearby tuyeres carbon of coke combines with oxygen of air with evolution of heat:

 

                                                      (2.1)

 

Fig. 2.1. Blast furnace: 1 – iron taphole; 2 – tuyeres; 3 – exhaust pipes; 4 – top;

5 – air blast pipe; 6 – slag hole

 

Reduction of iron is performed in the first turn by CO in succession from higher to lower oxydes and to pure iron (Fe₂O₃®Fe₃O₄®FeO®Fe):

                                  (2.2)

                                        (2.3)

                                                 (2.4)

 

Reduction by CO is called indirect one, reduction by C and H₂ is called direct one:

                              (2.5)

                              (2.6)

 

At temperature above 1000°C the carburizing of iron takes place:

                                           (2.7)

                                                           (2.8)

 

Hence, because of the carburizing we have cast iron with approximately 4 % of carbon instead of pure iron.

The reduction of Mn, Si, P also takes place, and S from coke dissolves in molten cast iron. As a result, cast iron has the following chemical composition: 4.0…4.4 %C, 0.6…3.0 %Si,
0.3…1.0 %Mn, 0.15…0.30 %P, 0.03…0.07 %S.

The blast-furnace produces:

- conversion iron, or steelmaking pig iron, or pig iron used for steel-making practice (contains ~l %Si);

- foundry iron, poured in pigs and used for remelting in foundry shops (contains ~3 %Si);

- ferromanganese - alloys used for deoxidation and for alloying of steel. FeMn has average chemical composition: 7 % C, 70 % Mn, the rest-Fe;

- ferrosilicon - alloy used for deoxidation and alloying of steel: 2%C, 13%Si, the rest-Fe;

- slag (CaO, MgO, A1₂O₃, SiO₂, FeO, MnO, etc.) used in building industry;

- blast-furnace gas (14…18 % CO₂, 22…28 % CO, 2…6 %H₂, 50…55% N₂) has low calorific value (3350…4000 kJ/m³).

 

Steel production

 

Steel contains lower amount of carbon and impurities than steelmaking pig iron (table 2.1).

 

Table 2.1 - Chemical Compositions of Steel and Cast Iron

Alloy	C,%	Mn,%	Si, %	S,%	P,%
Steel	0.05…1.3	0.4…0.8	0.2…0.4	to 0.05	to 0.05
Pig iron	4…4.2	0.75…1.25	0.75…1.75	to 0.3	to 0.07

 

To produce steel, utilizing pig iron, it is necessary to decrease content of carbon, other elements and impurities.

There are three main steel production methods in metallurgy: (l)oxygen-converter process, (2) open-hearth process and (3) electric-furnace melting.

 

2.3.1. Oxygen-Converter Process

 

Oxygen converter is pear-like tank made of steel sheets and having a refractory lining inside (Fig. 2.2).

Fig. 2.2. Oxygen converter: 1-steel construction; 2-refractory lining; 3-hole for steel tapping;

4-tilting device; 5-water-cooled copper lance; 6 – oxygen jet; 7 – liquid metal

 

The liquid pig iron (conversion iron) is poured into converter and by water-cooled lance oxygen less than 10…12 atmospheres pressure is given on metal surface. When the blowing is started, the slagforming components (lime CaO, iron ore Fe₂O₃, etc.) are introduced into the converter.

Oxygen penetrates into liquid metal and oxidizes iron according to the law of mass:

                                                   (2.9)

where component is: in metal [], in slag (), in gas atmosphere {}.

 

Iron protoxide reacts with C, Si, Mn, oxidizes them and their oxides pass from metal into slag and atmosphere:

 

Direct oxidation by O₂ also proceeds:

                                                           (2.13)

                                                  (2.14)

                                                         (2.15)

 

Due to high-basic slag formation, reactions of desulphurization and dephosphorization take place:

                                               (2.16)

                        (2.17)

 

By oxidation of alloying elements and impurities (Fig. 2.3), accompanied with great amount of heat evolved, metal is overheated to high temperature t. To cool metal coolers in form of steel scrap are added during melting.

Fig. 2.3. Variation of temperature t and chemical content of steel vs time in oxygen converter process

 

Any fuel is not needed for oxygen-converter process. It is carried out very rapidly, during from 25 to 40 min. High production rate creates some problems with control and getting required chemical composition of steel.

When the carbon content is dropped to required level the blowing is stopped and alloying is accomplished. So far as steel is saturated with oxygen the operation of deoxidation must be performed.

The FeMn and FeSi are added into the converter and Al is placed into the ladle, where the steel is poured from the converter. When steel mixes with deoxidizers the deoxidizing reaction occurs by the formula:

                            (2.18)

 

The capacity of oxygen converters ranges from 50 to 400 tons, the tap-to-tap time is 25...45 minutes, providing thereby the highest productivity.

 

 

2.3.2. Open - Hearth Process

 

The main parts of modern open-hearth furnace (Fig. 2.4) is a reaction chamber 7 formed by bottom 6 below, roof at the top and side walls, all made of refractory materials. The front wall has doors 5 through which scrap is charged into the reaction chamber, samples of steel are taken, and the process of melting is inspected. The back wall has an opening (taphole) closed with refractory mass during melting and opened when the metal in the furnace is ready for tapping. Ports 3, 4 and 8, 9 connect the reaction chamber with regenerators 1, 2 and 10, 11 which are brick-lined chambers filled with a checkerwork of refractory brick. The regenerators serve to utilize waste heat of the combustion product leaving the furnace, so that the temperature during melting can be increased from 1400°C (in a furnace without regenerators) to 1800°C.

  Ports 8 and 9 are conduits for supplying the gaseous fuel and air that form the flame in the furnace and for removing the combustion products.

Reverse valves 12 and 14 are used to reverse periodically, every 10 or 20 minutes, the direction of the air and gas flow from one end of the furnace to the other. The valves are connected to a flue for directing combustion products to the stack 13 after they have given up most of their heat to the checkers. During the furnace operation, the waste gases from the furnace are conveyed through downtakes 3 and 4 (left in Fig. 2.4) into regenerators 1, 2. After heating the checkers the gases are taken by flues 17 and 18 to the stack 13.

Fig. 2.4. Open-hearth furnace: 1, 2, 10, 11 – regenerators; 3, 4, 8, 9 ports; 5 – door; 6 – bottom;

7 – reaction chamber; 12, 14 – reverse values; 13 – stack; 15 – air; 16 – combustion products;

17, 18 – flues; 19 – gas

 

At the same time, cold gas 19 and air 15 pass through right flues, regenerators 10 and 11 and uptakes 8 and 9 and enter the reaction chamber 7 at a temperature of 1.000...1.200°C to heat and melt the metal and slag. When one part of regenerators is substantially cooled and the other heated, the direction of gas and air flow is reversed, so that the temperature in the reaction chamber is always near stable.

According to the charging materials used for melting in an open-hearth furnace, a distinction is made between ore, pig-and-ore and pig-and-scrapprocesses.

The ore process uses molten pig iron with addition of iron ore to oxidize the impurities in the iron.

The pig-and-iron process employs a charge consisting mainly of molten pig iron with the addition of scrap and iron ore.

In the pig-and-scrap process the charge is solid. It consists mainly of steel scrap and solid pig iron.

The reactions of metal oxidizing by FeO (iron ore) in open-hearth furnace are the same as in an oxygen converter.

After oxidation steel is deoxidized, alloyed and then tapped.

Open-hearth furnaces have capacity from 20 to 900 tons, the tap-to-tap time is 5…10 hours. The steel quality is the same for oxygen converter and for open hearth furnace. But the open-hearth process considerably lost in productivity.

The open-hearth process is a dying process; it is not used in the United States, Germany, and Japan. But the main amount of common and quality steel in Ukraine is still produced in the open-hearth furnaces.

 

2.3.3. Electric Steel Making

 

Electrometallurgy is a branch of engineering concerned with the reduction of metals from their oxides and manufacture of various steels and alloys with electric energy being used as the source of heat.

Electric furnaces have a number of substantial advantages over other types of melting plants: some types of high-quality steel, such as high-alloy tool steel, stainless, refractory and heat-resistance steels and many structural steels can be smelted only in electric furnaces. It is easy to form an oxidizing, reducing or neutral atmosphere in the electric furnace. Steels with lower content of sulphur and phosphorus, deoxidized and poorly contaminated by nonmetallic inclusions may be easily produced in such furnaces.

All metal-melting electric furnaces can be divided into three groups according to the methods by which electric energy is transformed into heat:

- electric-arc furnaces;

- induction furnaces;

- resistance furnaces.

The method of heating may be used to classify all electric-arc furnaces into direct-arc, indirect-arc and plasma furnaces. In direct-arc furnace electric arc is drawn between electrodes and metal being heated. In indirect-arc furnace the arc strikes between electrodes (as a rule, 2 electrodes) and metal is heated by radiation from the arc. A plasma furnace is similar direct-arc furnace, if plasmatrons are used instead electrodes.

Figure 2.5 shows a direct-arc furnace. The furnace has a steel shell 4 in the form of a tapered cylinder with a spherical bottom 12. The shell has a refractory lining 5 inside. The reaction chamber of the furnace is covered from above by removable roof 6 made of refractory bricks. The furnace has a charging window 10 with a door, and taphole 2 with a tapping spout. The furnace is fed with three-phase alternating current and has three electrodes 9 fastened in electrode clamps 8. Current is supplied via water-cooled flexible cables 7. Arcs are formed between electrodes and metal. The metal is covered by slag. The furnace has rollers 11 to turn it for tapping or for charging.

Fig. 2.5. Direct-arc furnace: 1 – fire-proof bricks, 2 – taphole; 3 – charge; 4 – steel shell; 5 – lining;

6 – roof; 7 – flexible cable; 8 – clamp; 9 – electrode; 10 – window; 11 – tilting mechanism; 12 – bottom

The furnace charge consists of steel and cast iron scrap, foundry iron, ferroalloys, oxidizers, deoxidizers and slag forming materials (limestone CaCO₃, lime CaO, fluorspar CaF₂, sand SiO₂, broken chamotte Al₂O₃+SiO₂); from 10 to 20 % of liquid conversion (pig) iron is added sometimes.

The furnace is charged from the top by means of a drop-bottombucket. To open the reaction chamber for charging, the furnace roof (together with electrodes) is raised and moved to the side. The door 10 is used for small additions during heat.

The lining of walls 5 and bottom 12 may be acid or basic. By this melting process in the electric furnace can be carried out by one of the following main methods:

- in the basic furnace, oxidizing the admixtures with iron ore;

- in the basic furnace without oxidizing the admixtures (remelting process or fusion of steel scrap);

- in the acid furnace with oxidizing;

- in the acid furnace without oxidizing.

Only the electric arc with basic lining allows removing S and P, because lime CaO is needed for dephosphorization and for desulphurization. In acid furnace lime reacts with acid lining and destroys it:

                                         (2.19)

slag    lining

 

The melting process in the basic arc furnace may be divided into stages as follows:

- fettling of the furnace, i.e. small repair by powder fireproof materials (refractories);

- charging of the main charge (steel scrap iron, iron ore, lime);

- melting of the main charge and dephosphorization:

                         (2.20)

 

- bath boil and heating of the metal to required temperature; ferrous ore or oxygen is given info furnace and the process of oxidizing of carbon, silicon, manganese is started:

                      (2.21)

                                (2.22)

 

The bubbles of CO₂ are formed in metal, which are named "metal boils".

- skimming of the oxidized slag to remove phosphorus;

- formation of the reducing slag by adding CaO, CaF₂, FeSi, coal (C) and others;

- deoxidation of metal by reducing slag and desulphurization:

                                          (2.23);

 

- final deoxidation of metal by Mn, Si, Al, Ca, Ce, e.g.:

                                               (2.24)

 

- tapping of metal.

As we can see (reaction 2.20), the amount of phosphorus passing to slag is proportional to the concentration of ferrous oxide FeO and lime CaO in the slag. The reaction is preceded with heat uptake. By this reason, the degree of dephosphorization is higher at low temperature. In electric steelmaking, the temperature of the metal increases gradually and it is therefore essential that the greatest part of the phosphorus had time to pass to slag at a low temperature, i.e. during melting of charge and in the initial 10...15 minutes of the oxidizing stage. To remove phosphorus the part of slag is skimmed off from the furnace.

For desulphurization low concentracion of FeO, high concentration of CaO (high basity of slag) and high temperature are required (reaction 2.23).

The basic lining is more expensive and has smaller life as compared with acid one. That is why it is mainly used in electric arc furnaces for production of quality and high-quality steels.

2.3.4. Tapping and Teeming

 

The spout of the furnace is lined with fireclay bricks. During melting time the spout should be cleaned from scrap and slag, well dried and blown with compressed air.

Steel is tapped from the furnace into a teeming ladle, whose construction may be seen from the Figure 2.6.

Fig. 2.6. Teeming ladle: 1-steel construction; 2-fireclay brick lining; 3-stopper; 4-stopper end;

5-nozzle; 6-stopper moving mechanism

 

The teeming ladle is used for pouring or teeming of steel. There are three methods of pouring steel in metallurgy:

- top (direct) pouring;

- uphill teeming;

- continuous and semicontinuous casting.

According to the first and the second methods (Fig. 2.7) the cast iron moulds, named ingot moulds, are used. In the first case one ingot mould is filled with metal. In the second case from 2 to 32 ingot moulds are simultaneously filled with steel by using gating system.

In the first case we use all metal for ingot, but because of metal splashing the surface of ingot may have some defects. In the second case we have smooth filling of the mould and good quality of ingot surface, but metal is partly wasted on gating system.

Fig. 2.7. Top (a) and uphill (b) teeming: 1 - bottom plate; 2 – mould; 3 – metallic shell of central

downgate; 4, 6 – chamotte tube; 5 – central brick; 7 – funnel; 8 - teeming ladle

Nevertheless, labor productivity is higher in the second case.

But in both cases the ingot has non-uniform structure (Fig. 2.8): shrinkage cavity (pipe)-4, heterogeneous crystal structure (fine crystals 1, fringe crystals 2, coarse crystals 3).

Shrinkage cavity forms in the riser, because it freezes last. The riser and bottom ends are cut off and undergo remelting.

The ingot has droplet (dendritic) and zone segregation. For example, a content of C, S, P in top part of the ingot is in several times much as their content in lower part (zone segregation).

Fig. 2.8. Steel ingot structure: 1 – fine crystals; 2 – fringe crystals; 3 – coarse crystals;

4 – shrinkage cavity

 

The continuous casting (pouring) was advanced in order to get rid of disadvantages. The schemes of two types of machines for producing of continuous billets (blanks) are shown in
Fig. 2.9 a) vertical continuous caster and b) curved type continuous caster. The molten metal from a ladle 1 is poured into a cooper water-cooled mould 3 through intermediate ladle 2. By cooling action water 4 liquid metal starts to solidity in mould 3 and solidifies finally in cooling zone 5. A steel billet 7 is drawn from the mould by rollers 6 and then is cut by a cutting mechanism 8 into measured sections.

Fig. 2.9. Continuous casting: 1 – stopper ladle; 2 – intermediate ladle (tundish);

3 – water-cooled mould; 4 – water; 5 – zone of secondary cooling; 6 – drawing rollers;

7 – billet; 8 – cutting mechanism

The positive aspects of continuous casting are follows. The losses of metal owing to shrinkage pipe (cavity) in common ingots amounts to 10…16 per cent and 4 per cent are lost as cropping of the bottom end of ingots. With continuous casting the total loss is only 4...5 per cent. Owing to accelerated solidification billets have no segregation and are more homogeneous in structure, which improves metal quality. The method requires less labor and can readily be controlled automatically.

 

2.3.5. Production of High-Quality and Super-High Quality Steels

 

Many branches of modern engineering require metals of the highest quality, which cannot be produced in electric furnaces. As a rule, high-quality steels contain small amounts of non-metallic inclusion and gases. They also have fine and dense (lack of pipes) structure without segregation.

There are two main directions of improving of steel quality in metallurgy:

- by treatment of liquid steel melted in ordinary furnaces;

- by remelting of steel in special furnaces (this branch is named special electrical metallurgy).

The first direction is connected with: vacuum degassing of molten steel (a) and treatment of molten steel by synthetic slag (b).

There are numerous methods of vacuum degassing of steel in metallurgy. The main is degassing in the ladle (Fig. 2.10).

Fig. 2.10. Scheme and draught of chamber for vacuum degassing in the ladle:

1 – ladle; 2 – steel; 3 – chamber; 4 – cover of chamber

 

The ladle 1 with molten steel 2 is positioned in vacuum chamber 3, closed by the cover 4. Then the air is exhausted from the chamber by a vacuum pump and gases (O₂, H₂, N₂) in consequence are extracted from steel. The disadvantage is that only gases are removed.

Synthetic slag (40 %Al₂O₃, 55 %CaO, rest-SiO₂, MgO, and others) 1 is melted in electric furnace and then is poured into a ladle 2 (Fig. 2.11). Then steel 3 is poured into the ladle from the height 5...8 m. Large contact surface is formed during mixing of steel with slag. Thus, slag absorbes impurities from steel (sulphur, oxygen, phosphorus). The disadvantage is that special furnace is necessary for slag melting.

Main special electrometallurgical processes are (1) vacuum arc remitting (Fig. 2.12); (2) electroslag remelting (Fig. 2.13); (3) plasma arc remelting (Fig. 2.14, 2.15).

 

Fig. 2.11. Treatment of molten steel by synthetic slag: 1 – slag; 2 – ladle; 3 – steel

Fig 2.12. Vacuum arc remelting: 1 – d.c. generator, 2 – vacuum chamber; 3 – electrode holder;

4 – electrode-moving gear; 5 – exhaust of air; 6 – comsumable electrode; 7 – liquid metal;

8 – ingot; 9 – mould; 10 – carriage for moving the ingot down; 11 – bottom of the mould

Fig 2.13 Elecrtoslag remelting: 1 – mould lifting carriage; 2 – transformer;

3 – electrode replacing mechanism; 4 – holder; 5 – comcumable electrode; 6 – liquid slag;

7 – liquid metal; 8 – mould; 9 – ingot; 10 – bottom

Fig. 2.14. Plasmatron construction: 1 – electric arc; 2 – tungsten electrode; 3 – insulator;

4 – body; 5 – water-cooled nozzle; 6 – plasma jet

Fig. 2.15. Plasma-arc furnace: 1 - lining; 2 - plasma arc; 3 - roof; 4 - plasmatron;

5 - power supply; 6 - bottom electrode

 

A consumable electrode 6 is remelted in vacuum chamber 2 by electric arc (Fig. 2.12). Drops of steel are degassed in vacuum and steel solidifies in water-cooled metallic mold 9, forming the ingot 8. Ingot has a good structure (fine grain, high density) and low content of gases and non-metallic inclusions. A main disadvantage of the process is the complexity of vacuum installation.

A simple method, electroslag remelting for improving the quality of metal, has been developed at the E.G. Paton Institute of Electric Welding. In this process (Fig. 2.13), metal droplets, formed during the melting of a consumable electrode 5, pass through a layer of specially prepared slag 6 and solidify into an ingot 9 in a mold 8. Slag refines metal from sulphur, phosphorus, oxygen, nitrogen and hydrogen.

A new branch of metallurgy-metal melting by means of electric plasma -has been developed in recent years. It employs both plasma arc furnaces of original design and plasma heaters used in conventional type furnaces. Plasma is an ionized gas with total charge equal to zero. A distinction is made between hot plasma with temperature up to a few hundred thousand degrees K and cold plasma, in which temperature reaches 30,000 K and its degree of ionization is around 1 percent. The latter is used in metallurgy. Let us consider the construction of the simplest plasmatron (Fig.2.14). It has an internal rod electrode 2 and a concentric water-cooled annular external electrode 5 in the form of a nozzle. If direct current is applied from d. c. generator, the internal electrode is the cathode and the external one is the anode. The dielectric part 3 joins parts 2, 4 and 5. A flow of gas supplied in chamber of the plasmatron blows out the electric arc, that burns between cathode and anode through the nozzle to the outside. The electric arc 1 is transformed into a plasma jet 6, as a result of squeezing, which is directed on the object to be heated.

The melting in plasma-arc furnaces offers the following advantages as compared with the melding in electric arc furnaces:

- it avoids contamination of the metal with carbon of the electrodes and with hydrogen from the furnace atmosphere;

- the plasma jet may be composed of any mixture of gases which also may be used for metal alloying (such as nitrogen);

- the rate of melting is rather high due to the high concentration of energy.

Figure 2.15 shows schematically a plasma arc furnace with refractory crucible 1. The furnace is hermetically sealed. The plasmatron 4 is fed with direct current. The shape of the furnace resembles that of a steelmaking arc furnace.