Melting method for an electric ARC furnace with alternative sources of energy and relative electric ARC furnace with special burner positioning

Melting method for an electric arc furnace with alternative sources of energy for the melting of iron-based alloys, the electric furnace (10) including tuyeres (13) positioned on the bottom to deliver oxygen, at least one tuyere (15) to deliver coal dust which works in the area of contact between the bath of molten metal (16) and the layer of slag (22), at least one supersonic lance (12) cooperating with a lance (29) to deliver coal dust, these lances (12, 29) having a working position (12c, 29c) in which the supersonic lance (12) is positioned in close proximity to the surface of the molten metal (16) and the lance (29) to deliver coal dust is positioned in the vicinity of the surface of the layer of slag (22), and a plurality of burners (28) positioned on the cooled sidewalls (31) of the furnace (10) and delivering oxygen-based gases and combustible substances, whereby at least two first burners (28) work with the action of one burner supporting the action of the next one, the furnace (10) being charged with one or more charges which provide for a first step of start-up of the melting and a second step of melting, a refining step and a tapping step being included, in which method in the first step of start-up of the melting at least one second burner (28e) collaborates in freeing quickly the path of introduction of the supersonic lance (12) and of the lance (29) to deliver coal dust.

BACKGROUND OF THE INVENTION 
This invention concerns a melting method for an electric arc furnace with 
alternative sources of energy and the relative electric arc furnace. 
This invention is applied to the field of electric arc furnaces for the 
melting of iron-based alloys and has been conceived and designed to 
optimize the efficiency and output of a plant by using alternative sources 
of energy. 
This invention aims to achieve a saving of electrical energy during the 
melting cycle and to reduce the cycle times by increasing the number of 
castings which can be achieved per day; moreover, the invention tends to 
increase the yield factor of the alternative energy introduced. 
The invention is applied both to furnaces working with direct current and 
to furnaces working with alternating current. 
The furnaces to which the invention is applied can have a tapping channel 
or a tap hole without any constraint regarding the positioning of the 
hole. 
A typical and preferred, but not exclusive, application of the invention is 
its use in furnaces which are caused to work with the "pond" method, that 
is to say, with a liquid heel always present. 
This means that, when the molten metal is withdrawn from the furnace, that 
molten metal is not completely withdrawn. 
The invention is also suitable for the melting of completely cold charges. 
The invention is applied both to furnace charges of scrap in skips or 
continuous charges of scrap or to continuous charges of pre-reduced 
material or to mixed charges. 
The state of the art of methods for melting metals contains the procedure 
of injecting into the furnace gaseous elements based on oxygen, possibly 
in combination with carbonaceous fuels, to obtain a reaction of oxidation 
with those fuels and with the oxidizable chemical elements included in the 
molten metallic mass and in the scrap. 
This reaction of oxidation theoretically makes possible the recovery of the 
intrinsic energy potential of the carbon and of the chemical elements in 
the molten metal and in the scrap by means of an exothermic oxidation 
reaction. 
These gaseous elements with an oxygen base which are injected may consist 
of air, air enriched with oxygen or even of pure oxygen. 
These gaseous elements can be blown onto the molten metal by means of 
lances located above the surface of the bath and/or by means of nozzles or 
tuyeres positioned in the hearth of the furnace. 
U.S. Pat. No. 3,459,867 teaches the use of burners which deliver oxygen in 
a stoichiometric dose and therefore with a desired excess of oxygen. 
These burners act along a circumference positioned between the electrodes 
and the sidewall of the furnace, and their method of working is with the 
action of one burner working as a support for the action of the next one 
so as to create a vortex in the empty part of the furnace. 
GB-A-2,115,011 teaches the blowing of the stirring and conversion gas from 
below the bath of molten metal. 
It also teaches the introduction from above of solid carbonaceous materials 
and the use of tuyeres positioned on the sidewall and delivering mixtures 
of gas towards the bath. 
GB-A-1,421,203 teaches the delivery of oxygen or other gases from the 
bottom into a zone between the electrodes and the sidewall of the furnace. 
FR-B-2.208.988 teaches the delivery of gas into the furnace from above 
downwards and also teaches the delivery of gas from below the scrap and/or 
bath of molten metal both in the zone of the electrodes and between the 
electrodes and the sidewall of the furnace. 
This document includes, next, in the sidewall one or more lances which 
deliver oxygen and/or other gases, in a configuration such that the action 
of one lance acts as a support for the action of the next one, between the 
electrodes and the sidewall of the furnace, the purpose being to create a 
great mixing of the gases in the empty part of the furnace. 
EP-B1-257.450 discloses a method whereby a plurality of lances are used 
which deliver oxygen or mixtures of oxygen between the electrodes and the 
inner sidewalls of the furnace, and whereby a plurality of nozzles are 
used which are arranged below the molten bath and in the zones where the 
lances work. 
These nozzles positioned on the bottom are employed also to stir the bath 
and to make uniform the blowing action of the lances inasmuch as, even 
when lances of a supersonic type are used, the jet of the combustion gases 
cannot reach a depth greater than 20 to 30 cms. in the molten bath unless 
expendable lances are used. 
The nozzles and tuyeres of the state of the art function typically with 
pressures up to 60 bar, but normally between 5 and 20 bar. 
This high pressure has the effect that the exothermic reaction takes place 
generally in the empty part of the furnace, thus entailing a substantial 
loss of heat energy through the outgoing fumes. 
As can be seen from U.S. Pat. No. 3,902,889 and from EP-B1-257.450 these 
nozzles or tuyeres on the bottom have an oxygen emission diameter between 
3 and 6 mm. and consist advantageously of a double tube, with a central 
tube to emit O.sub.2 and an annular surrounding slit to emit hydrocarbons 
and/or inert gases for cooling purposes. 
The use of tuyeres to blow oxygen from the bottom, combined, for instance, 
with different fluids such as argon, nitrogen and methane, enables the 
oxidation reaction to be improved and made uniform. 
The technology of tuyeres makes possible a better homogenization of the 
molten steel, thus permitting intensive working under a condition of a 
"long arc" without generating lack of uniformity of the temperature, and 
also permitting a reduction of the melting time and still other 
advantages. 
In the state of the art the blowing from the bottom has always been carried 
out in practice with relatively high pressures and with relatively low 
rates of flow of gas through a plurality of tuyeres distributed on the 
bottom and generally having a small diameter. 
This method entails a swift passage of the oxygen through the bath of 
molten metal, this oxygen being mostly burnt in the empty part of the 
furnace up to the roof of the furnace. 
Moreover, the adjustment of the blowing during the various working cycles 
is carried out on the flow rate of the gases, and the pressure is variable 
as a fact resulting from the variation of the flow rate. 
It should be noted that in the state of the art the tuyeres are placed 
substantially symmetrically on the hearth of the furnace, often in a 
position corresponding to that of the oxygen emission lances, which are 
located, moreover, in the high part of the furnace. 
Such an arrangement of the tuyeres and of the lances delivering oxygen and 
other gases does not lead to great improvements in the efficiency and 
output of the melting plant since these gases become burnt in the empty 
part of the furnace and often finish burning in the fumes discharge 
conduit. 
The state of the art detailed above is therefore not suitable to give an 
accurate idea of the true position of the techniques and technology of the 
state of the art, so that, in fact, at the present time there is still no 
electric arc furnace which uses on a great scale an alternative energy and 
which functions with appreciable results in an efficient and continuous 
manner. 
The experiments of the state of the art have all ended with unsatisfactory 
results, so that in fact electric arc furnaces working industrially with 
assistance from an alternative energy do not now exist. 
SUMMARY OF THE INVENTION 
The present applicants have therefore tackled for some time now the task of 
perfecting a method and an electric furnace which, with the normal 
practice of a steelworks, provide a continuous production with an 
effective and efficient use of an alternative energy and with very short 
tap-to-tap times so as to make possible even up to 26/30 castings per day 
with an electric arc furnace. 
The present applicants have therefore designed and tested for a long time 
and have thereafter obtained this invention so as to achieve high plant 
output, short melting times, modest consumption of energy and 
homogenization of the bath. 
This invention employs in an innovatory manner the technology of the 
tuyeres and the blowing devices for the injection of gases and fuels into 
the furnace. 
This injection of oxygen and fuels based on carbon has the purpose of 
causing a quick start-up of the exothermic chemical reactions of 
re-combination, by oxidation, of the chemical elements in the molten bath 
immediately above the bath and in the scrap during the progress of the 
melting. 
According to the invention this injection of oxygen and fuels is carried 
out in such a way as to increase the surface area affected by the reaction 
so as to obtain a wide direct distribution of the heat, a considerable 
reduction of the consumption of electrical energy and a reduction of the 
times of melting of the charge. 
The present applicants have found to their surprise that the best results 
are achieved when it is possible to obtain the reactions of re-combination 
of CO+O=CO.sub.2 in the area between the surface of the bath of molten 
metal and the area immediately above the slag, so as to make use, on the 
one hand, of the effect of retaining and distributing the foamy 
conformation of the slag and, on the other hand, of the effect of direct 
transmission of the heat which the slag has in relation to the bath of 
molten metal. 
The production cycle according to this invention can be applied to any type 
of charge even if, as an example, we shall dwell in this description 
substantially on three types of starting material to arrive at the molten 
steel. 
The first type of example of a starting material provides for the use of 
scrap alone by charging the furnace with a plurality of charges, that is 
to say, by introducing the desired quantity of scrap into the furnace in a 
plurality of steps; in the cases considered hereinafter, the number of 
charges is typically two. 
The second type of example of the starting material provides for a mixture 
of scrap and molten cast iron in desired percentages and advantageously 
with alternate charges, namely typically one charge of scrap, one charge 
of molten cast iron and a successive charge of scrap. 
The third type of example of the starting material provides for a mixture 
of scrap and sponge-iron (pre-reduced iron) in desired percentages, 
advantageously with a first single charge of scrap together with a 
quantity of sponge-iron and a successive continuous charging of the 
remaining quantity of sponge-iron. 
The invention provides for the injection into the furnace of: 
an oxidizing gas from the bottom through oxygen tuyeres located in the 
hearth of the furnace, the gas being delivered at low pressures and with 
high flow rates; 
oxygen and oxygen plus fuel (according to the working phases of the 
furnace) from above downwards by means of burners located on the sidewalls 
of the furnace in cooperation with the cooled panels included immediately 
above the layer of refractory material; this delivery takes place at an 
angle between 20.degree. and 45.degree. to the horizontal; 
oxygen into the molten bath by means of supersonic lances working 
immediately above the surface of the molten metal and delivering the 
oxygen at an angle between 40.degree. and 50.degree., but advantageously 
45.degree., to the central axis of the furnace; 
carbonaceous fuels on a gaseous carrier onto the layer of slag by means of 
lances delivering coal dust and positioned just above the surface of the 
slag and delivering this carbonaceous fuel from above downwards at an 
angle between 20.degree. and 35.degree., but advantageously 30.degree., to 
the horizontal; 
carbonaceous fuels on a gaseous carrier by means of tuyeres delivering coal 
dust and sending the fuels between the molten metal and the layer of slag. 
The supersonic lances or the tuyeres delivering coal dust, or both, may be 
of a type with a fixed position or may be of a type which can be moved, 
actuated and positioned as desired either directly or by means of remote 
controls. 
The invention tends to produce induced and controlled chemical reactions in 
a very specific zone of the furnace so as to make maximum use of the 
energy potential of the individual elements and, in particular, of the 
post-combustion phenomenon with a great effectiveness from the energy 
point of view towards the bath of molten metal. 
In particular, the invention tends to induce and enhance a plurality of 
chemical reactions within the molten bath in the layer of slag and just 
above that layer of slag. 
The oxygen injected from the bottom by the oxygen tuyeres rises from the 
bottom according to a wide cone of dispersion and reacts mostly with the 
Fe in the molten metal to create FeO, which re-combines with the C coming 
from the charge and present in the bath. 
This re-combination frees the Fe and generates CO, which tends to rise 
above the molten metal into the slag. 
The oxygen delivered by the supersonic lance and entering in depth into the 
bath reacts with the Fe to cause substantially the same reaction as that 
generated by oxygen from the oxygen tuyeres. 
The supersonic lance sends the oxygen in the same direction as the 
direction of rotation of the molten bath, so that there take place a 
distribution of the heat produced and a honogenization over the whole 
surface of the bath. 
The carbonaceous substances delivered by the tuyeres delivering the coal 
dust reduce the excess of FeO and generate CO, which contributes to the 
desired formation of foamy slag. 
The lance delivering coal dust together with the supersonic oxygen lance 
causes a strongly reducing zone above the layer of slag, and owing to the 
rotation of the slag this reducing zone is distributed into an area 
heavily sprinkled with oxygen, thus contributing to the formation of 
further CO, which increases the foamy conformation of the slag. 
The oxygen delivered by the burners involves the CO in the foamy slag and 
the CO emerging from that slag, thus generating a post-combustion reaction 
with a strong delivery of heat in close proximity to the foamy slag. 
This foamy slag causes the transfer of the greater part of that heat to the 
molten metal. 
This situation enables better advantages to be achieved in terms of 
transmission of heat to the molten bath and of uniformity of the resulting 
heat potential substantially over the whole surface of the bath. 
According to the invention the blowing devices work substantially at the 
central zone of the furnace. 
A first group of these blowing devices acts at a tangent to a ring defined 
by a first outer circumference, which has a maximum value of about 0.70 
times the upper inner diameter of the furnace defined at the cooled 
panels, and by a second inner circumference, which has a minimum value of 
about 0.25 times the upper inner diameter of the furnace. 
A second group of these blowing devices acts directly towards the center of 
the furnace. 
This orientation of the blowing devices has the result of making uniform 
and homogeneous the transfer of thermal energy to the whole mass of molten 
metal without causing the problems of wear, which arise in the state of 
the art when the jets of the blowing devices work close to or against the 
refractory sidewalls of the furnace, against the electrodes and against 
the roof of the furnace, or when the post-combustion takes place in the 
free space of the furnace. 
The melting method according to the invention can be divided substantially 
into at least two separate steps which characterize each single cycle of 
charging the material, plus a third step which takes place when the charge 
has been melted. 
The first step corresponds to the charging of the furnace and to the 
start-up of the melting of the charge. This first step requires a great 
contribution of energy to start the melting of the charge, particularly 
when the charge is cold. 
The first step is characterized by an accentuated use of the burners in 
their specific function, that is to say, in this first step the burners 
inject oxygen and a fuel, generally methane, to prime a reaction of 
combustion which generates a huge quantity of heat. 
In this first step, according to the invention, at least one burner is 
employed in contributing to the speedy freeing from scrap of the zone in 
front of the supersonic lance and the lance delivering coal dust, the 
purpose of this being to free the space required for introduction of those 
lances into the furnace so that they can be quickly put to work. 
Moreover, this burner affects a zone which would otherwise be a cold zone. 
In this first step the oxygen tuyeres deliver oxidizing gas from the bottom 
at high rates of flow and at a low pressure. This pressure of the oxygen 
may be constant or continuously variable or variable in one or more steps. 
In fact, it is desired to achieve the position that the oxygen injected 
from the bottom will expand over a wide zone and will be displaced slowly 
into the molten metal so as to achieve the best and most complete 
combination with the iron molecules. 
In this first step the emission of the oxygen by the supersonic lances 
takes place not at once but when a minimum head of molten metal is present 
and the whole relative zone is free of scrap. 
The second step concerns the completion of the melting of the charge. 
In this second step the feed of fuel to the burners is interrupted and the 
burners are caused to function substantially as subsonic oxygen lances 
operating as far as possible in the vicinity of the surface of the foamy 
slag. 
In this second step there is an intense use of the supersonic lances and of 
the means blowing the coal dust so as to intensify the chemical reactions 
detailed above. 
According to the invention the pressure of blowing the oxygen from the 
oxygen tuyeres may be constant or may be increased progressively (as said 
above, continuously or according to one or more steps) so as to supply 
also a greater quantity of oxygen according to the requirements of the 
bath. 
However, this blowing pressure always remains within about limited values 
and never exceeds a value of 8 to 10 bar at the inlet of the oxygen 
tuyere. 
The rate of flow of the oxygen in the oxygen tuyeres, at least in the first 
and second melting steps, may vary between a minimum value of about 2.8 
Nm.sup.3 /min. per each single oxygen tuyere and a maximum value of about 
6.0 Nm.sup.3 /min. 
The low blowing pressure at which the oxygen is delivered does not create 
problems of spurting of the molten steel and, above all, does not create 
problems of the thermal lance type with a resulting perforation of the 
roof. 
Moreover, the great quantity of oxygen and the low blowing pressure lead to 
the creation of a wide zone of diffusion and a strong re-mixing in the 
molten metal. 
The exothermic reactions arising therefrom transfer a substantial quantity 
of heat directly into the mass of molten metal and in a huge area thereof. 
The positioning and orientation of the subsonic oxygen lances are such as 
to ensure a wide effect of covering the surface of the slag, bearing in 
mind also the convective motions of the molten bath and of the slag on a 
substantially horizontal plane. 
It should be noted that the low blowing pressure from the bottom causes 
also less turbulence in the bath and therefore less problems for the 
electric arc, thereby improving the yield of the same. 
The third step in the method is the refining step. 
In this third step the burners, either acting as such or as subsonic oxygen 
lances, are normally disactivated since the yield of the burners becomes 
negligible. 
Lastly, there is the tapping of the molten metal; in this step, if working 
is carried out with a liquid heel, a desired and determined quantity of 
molten metal is kept within the furnace. 
Where there is a liquid heel, the tuyeres on the bottom are kept working 
with a minimum blowing pressure of about 3 to 4 bar and with a minimum 
flow rate of 1.5 to 1.8 Nm.sup.3 /min., with possibly only inert gases 
being blown in. 
This minimum pressure is the pressure enough to overcome the ferrostatic 
pressure of the liquid heel and to withstand the dynamic action which the 
charge exerts on the molten metal at the moment of its introduction. 
This minimum value is therefore calculated so that there will be no 
flow-back of molten metal into the tuyeres, for a flow-back would lead to 
blockage thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An electric arc furnace 10 shown in FIGS. 1 to 5 comprises in this case 
three upper electrodes 11a, 11b and 11c respectively for generation of the 
electric arc. 
The description which follows therefore concerns an electric arc furnace 
working with alternating current and with a liquid heel, but remains valid 
also in the event of an electric arc furnace working with direct current 
or a furnace without a liquid heel. 
The electric arc furnace 10 is equipped with oxygen tuyeres 13, four in 
number in this case and positioned in the hearth 14. 
According to the invention the oxygen tuyeres 13 are positioned within a 
ring which surrounds a circle 34 enclosing the electrodes 11a, 11b, 11c 
and is outside that circle 34. 
The ring, which, depending on the structure of the hearth 14 of the furnace 
10, may be circular or may have another similar configuration, is defined 
by an outer perimeter and an inner perimeter. 
The outer perimeter is defined by a diameter which is advantageously about 
0.5 to 0.7 times the upper inner diameter defined in the high part of the 
furnace 10 which includes the cooled panels 31, whereas the inner 
perimeter is defined by a diameter which is advantageously about 0.25 to 
0.35 times that upper inner diameter. 
According to the invention the vertical axis of the oxygen tuyere 13 in 
relation to the closest vertical refractory sidewall 30 has to be distant 
from that sidewall 30 by a value between at least 0.7 and 1.4 times the 
height of the liquid head at the specific oxygen tuyere 13. 
In the example shown in FIG. 1, the oxygen tuyeres 13 are arranged 
substantially along a circumference equal to about 0.55 times the upper 
inner diameter and are located in the front semi-circle of the hearth 14 
toward the tap hole 32. 
This semi-circle is defined by the vertical plane positioned perpendicular 
to the longitudinal vertical plane which passes through the tap hole 32 
and through the central axis of the furnace 10. 
At least one oxygen tuyere 13 is included in a position of substantial 
cooperation with a hole 36 for the aspiration of fumes located in the roof 
of the furnace 10 and shown with lines of dashes in FIG. 1; 
According to the invention (FIG. 9) the oxygen tuyeres 13 include a central 
pipe 18 to deliver oxygen, this pipe being provided by means of a bore in 
a polygonal copper section 19. 
This polygonal copper section 19 is surrounded by a cylindrical pipe 20 so 
as to create a plurality of sectors 21 between each side 17 of the 
polygonal copper section 19 and the inner circumference of the cylindrical 
pipe 20. 
The sectors 21 form passages for the gaseous cooling mixture. 
The central pipe 18 to deliver oxygen advantageously has a diameter between 
10 and 20 mm., but preferably between 10 and 14 mm. 
The cooling mixture passing through the sectors 21 consists of at least one 
gas of a high cooling power, methane in this case, and of a diluting or 
filling gas, CO.sub.2 in this case, or else N.sub.2 or a mixture of these 
gases. 
Hereinafter only CO.sub.2 is indicated as the diluting gas but the 
conditions of CO.sub.2, N.sub.2 gases or a mixture shall be understood as 
being included. 
FIG. 10 shows with a block diagram the system for immediate adjustment of 
the percentages of these components of the cooling mixture. 
The adjustment of the percentages is carried out so as to keep unchanged, 
or substantially unchanged, the ratio between the quantity of cooling gas 
in the mixture and the flow rate of the oxygen, with a value determined 
beforehand as being indispensable for ensuring the necessary and desired 
cooling of the oxygen tuyeres 13. 
In this case the primary adjustment takes place, by means of an adjustment 
circuit 27, on the pressure of the oxygen injected through the central 
delivery pipe 18 of the oxygen tuyeres 13. 
This adjustment of pressure affects the flow rate of the oxygen, this flow 
rate being read continuously by a flow rate reader 23 and being sent to an 
adjuster 24. 
This adjuster 24 acts on a valve 25 located on the feed line of CO.sub.2 so 
as to vary the pressure of the same. 
Variation of the pressure of the CO.sub.2 causes a corresponding variation 
of the rate of flow of the CO.sub.2, this variation of the rate of flow 
being compensated in a mixer 26 by a variation of the CH.sub.4. 
According to the invention the minimum rate of flow of oxidizing gas in the 
melting steps in each of the oxygen tuyeres 13 is about 2.8 Nm.sup.3 
/min., whereas the maximum rate of flow may reach about 6.0 Nm.sup.3 
/min.. 
In the tapping step, or in the periods of awaiting a charge, particularly 
when there is a liquid heel in the furnace 10, the blowing pressure is 
kept at about 3 to 4 bar, with minimum rates of flow of about 1.5 to 1.8 
Nm.sup.3 /min.. 
The blowing pressure, according to the conformation of the furnace 10, may 
be the same in all the oxygen tuyeres 13 or may be a characteristic 
pressure for each oxygen tuyere 13 according to the relative position 
thereof. 
This pressure is never greater than 10 bar, as measured at the inlet of the 
oxygen tuyeres 13, for a height of the bath of molten metal from 0.7 to 
1.1 meters on the vertical plane of the oxygen tuyere 13. 
A supersonic lance 12 combined with a coal dust lance 29 is included to 
blow the oxygen within the molten bath 16 and thereabove. 
The supersonic lance 12 and the coal dust lance 29 work in the area in 
front of the door 35 of the furnace 10; these lances 12 and 29 work, 
moreover, at a tangent to a circumference within the circle 34 containing 
the electrodes 11 and work in the same direction as the direction of 
rotation of the bath 16 of molten metal and of the slag 22. 
In this case (FIG. 3), the supersonic lance 12 and the coal dust lance 29 
have respective first inactive positions 12a, 29a outside the furnace 10, 
second respective waiting positions 12b, 29b outside the furnace 10 and 
respective third working positions 12c, 29c within the furnace 10. 
In their third working positions 12c, 29c the supersonic lance 12 and coal 
dust lance 29 are inclined towards the bath 16 of molten metal by an angle 
of about 30.degree. to the horizontal; the supersonic lance 12 has its 
outlet in close proximity to the surface of the bath 16 of molten metal, 
whereas the coal dust lance 29 has its outlet in close proximity to the 
surface of the layer of slag 22. 
The supersonic lance 12 delivers the oxygen onto the bath 16 of molten 
metal at an angle (measured on the horizontal plane) between 40.degree. 
and 50.degree., but advantageously 45.degree., to the central axis of the 
furnace 10. 
The supersonic lance 12 injects the oxygen into the bath 16 of molten metal 
through one single outlet hole with a very carefully oriented jet, thus 
defining an area 33a on the surface of the bath 16 of molten metal; the 
part of the oxygen injected by the supersonic lance 12 which does not 
combine with the Fe emerges in an area 33b located generally within the 
circle 34 surrounding the electrodes 11. 
The supersonic lance 12 is oriented so as to work in the bath 16 of molten 
metal in an area not equipped with oxygen tuyeres 13. 
Tuyeres 15 to deliver coal dust, which in this case are two in number (FIG. 
1), are included in cooperation with the refractory sidewalls 30 
substantially at the level of the layer of slag 22. 
These coal dust tuyeres 15 consist advantageously of replaceable 
ceramic-coated pipes, are generally installed immovably and have the 
function of delivering the carbonaceous substances onto the molten bath 16 
below the layer of slag 22. 
These coal dust tuyeres 15 face towards the center of the furnace 10 and 
are fitted in a substantially horizontal position (15a, FIG. 4). 
According to a variant, the coal dust tuyeres 15 (15b, FIG. 3) are inclined 
downwards by an angle between about 30.degree. and 45.degree. to the 
horizontal. 
One of these coal dust tuyeres 15 cooperates with the supersonic lance 12 
in delivering carbonaceous substances below the slag 22. 
The other coal dust tuyere 15 cooperates with a zone located at the side of 
the tap hole 32 in an area heavily sprinkled with the oxygen arriving from 
the oxygen tuyeres 13. 
Burners 28 are installed, according to the invention, on the cooled 
sidewalls 31 of the furnace 10 and act downwards from above and are 
inclined to the horizontal by an angle between 20.degree. and 45.degree.. 
According to the invention the burners 28 emit oxygen at a subsonic speed. 
In this case, there are six burners 28, of which two burners 28a, 28b are 
oriented towards the center of the furnace 10 so as to cooperate, in the 
first melting step, with the electrodes 11 and, in the second melting 
step, with the carbonaceous substances delivered by the coal dust tuyeres 
15 and with the oxygen delivered by the oxygen tuyeres 13. 
Next, there is at least one burner, the burner 28e in this case, which 
collaborates also in freeing the path of the supersonic lance 12 so that 
the latter can be quickly put in its working position 12c. 
This burner 28e has also the task of delivering and making uniform the heat 
on the scrap in the zone in front of the door 35 of the furnace 10, for 
this zone would otherwise be cold. 
The other burners 28c, 28d, 28f are arranged according to a configuration 
whereby the action of one burner works as a support for the action of the 
next one and the burners direct their jets in a direction substantially at 
a tangent to a plurality of circumferences. 
Two of these burners 28f, 28c work in surface zones which are positioned on 
the vertical plane of the oxygen tuyeres 13, whereas the other burners 28d 
and, partly, 28e, have the purpose of conveying the heat and combining it 
so as to complete the action of one burner working as a support for the 
action of the next one. 
According to the invention the circumferences along which the burners 28 
work are outside the circle 34 of the electrodes 11 and have a diameter 
substantially between 0.25 and 0.70 times the upper inner diameter of the 
electric arc furnace 10 so as to affect a ring which coincides 
substantially with the ring of positioning of the oxygen tuyeres 13. 
In a first embodiment of the invention, the burners 28c-28f act in a 
direction the same as the normal direction of rotation of the bath 16 of 
molten metal and of the layer of slag 22 during the melting process. 
According to a variant, as can be seen in FIG. 1 where the burners 28 work 
in the direction contrary to the working of the supersonic oxygen lance 12 
and of the coal dust lance 29, the burners 28c-28f act in the opposite 
direction to the direction of rotation of the molten bath 16 and of the 
layer of slag 22. 
The burners 28 have an outlet equipped with a number of holes from six to 
ten for delivery of oxygen. 
In this way the oxygen jet emitted expands to create a huge affected 
lateral zone, referenced with "H" in FIG. 1, which broadens the field 
affected by the oxidization reactions and improves the energy 
contribution. 
The methods of working of the burners 28, the supersonic lance 12, the coal 
dust lance 29, the coal dust tuyeres 15, the oxygen tuyeres 13 and the 
electrodes 11 are shown in FIGS. 6, 7 and 8 for three possible examples of 
application. 
In particular, FIG. 6 shows the case of application of charging of the 
furnace with only scrap in two separate charges; FIG. 7 shows the case of 
charging with scrap and molten cast iron with a first charge of scrap, a 
charge of molten cast iron and a second charge of scrap, whereas FIG. 8 
shows the case of charging with scrap and sponge iron with a first mixed 
charge of scrap/sponge iron and a second charge with sponge iron only. 
The first two cases show also a precise step of refining, which follows the 
step of melting of the metal, whereas in the third case the refining step 
is continuous. 
These illustrative diagrams show that the invention arranges, for each 
cycle of melting of the single charge, to make the burners 28 work, upon 
start-up of the melting, in their specific function by means of a combined 
emission of oxygen and fuel. 
In the second step of melting, or advanced melting, the percentage of 
molten metal as compared to the total of metal within the electric arc 
furnace 10 has reached a significant value, and this situation entails a 
reduction of the transfer of heat from the burners 28 to the scrap and 
molten metal and therefore an appreciable reduction of the efficiency of 
the burners 28. 
In this second step the feed of the fuel is interrupted and the burners 28 
are caused to function substantially as subsonic oxygen lances. 
From the diagrams, which refer to cases of working of the electric arc 
furnace 10 with a molten heel, it is also possible to see how the oxygen 
tuyeres 13 always maintain a minimum blowing pressure even during the 
periods of pauses between one charge and the next charge. 
Moreover, it is possible to see how the supersonic lance 12 and the coal 
dust lance 29 are activated only in a stage of advanced melting, in this 
case too when the percentage of molten metal as compared to the total of 
the metal has reached a value corresponding to the presence of a minimum 
liquid heel.