[0001] This invention relates to a method of refining molten metal.
[0002] It is well known to refine molten metal by introducing one or more jets of oxygen
into it. For example, steel is made from iron in this way. In some processes the jets
of oxygen are introduced into a bath of the molten metal from tuyeres whose tips are
situated below the surface of the molten metal. The use of tuyeres is considered to
be less than ideal, being associated as a cause of damage to the refractory lining
of the vessel in which the metal refining operation is carried out. It is therefore
common practice to introduce some or all the jets of oxygen into the molten metal
from above. The commercially important Basic Oxygen Steelmaking (BOS) process is one
that uses oxygen jets "blown" into the molten metal from above. The oxygen reacts
with carbon that is found in dissolved form in the molten metal to form carbon monoxide.
In addition the oxygen reacts with impurities or minor components of the molten metal
(for example, silicon) so as to form a slag on the surface of the molten metal.
[0003] In general, the reaction between oxygen and, say, carbon in the molten metal is not
rate limited. It is therefore desirable to maximise the rate at which the oxygen is
introduced into the molten metal. Practical issues do, however, limit this rate. One
of these issues is the degree of turbulence caused by the introduction of the oxygen
into the molten metal. If there is excessive splashing of molten metal, it is believed
that wear of the lance through which the oxygen is blown may become excessive resulting
in a need to replace the head of the lance so frequently that the economical manufacture
of the metal is prejudiced. Excessive splashing of the molten metal may also cause
it to be ejected from the vessel giving rise to loss of yield and an increased need
for maintenance of downstream equipment. On the other hand, in order to facilitate
reaction between oxygen and carbon it is desirable that there is a certain amount
of turbulence in the molten metal particularly during the latter stages, typically
the last 20% of the blow, when mass transfer limitations can be encountered.
[0004] It is disclosed in patents applications such as
EP-A-866 138 and
EP-A-866 139 that a jet of oxygen as it travels through a stationary atmosphere tends to entrain
the stationary atmosphere into it. This entrainment has the effect of reducing the
velocity and the thrust of the oxygen jet. It is postulated that as a result of the
entrainment the lance head has to be positioned closer to the surface of the molten
metal than is ideal, therefore making it more vulnerable to damage by the splashing
molten metal.
EP-A-866 138 and
EP-A-866 139 propose that the lance head is in effect converted into a burner. There is still
a primary jet of oxygen, which is ejected from the lance at supersonic velocity, but
now the primary jet of oxygen is surrounded by a secondary flame envelope. The secondary
flame envelope is formed by ejecting a fuel gas (or liquid fuel) and secondary oxygen
from the lance head. The fuel gas and secondary oxygen mix to form a flame. The flame
envelope is stated to prevent entrainment of stationary atmosphere into the oxygen
jet. Therefore, so it is explained, the oxygen jet does not diverge or lose velocity
in the way it would were there to be no flame envelope. Accordingly, the oxygen jet
is able to penetrate well beneath the surface of the molten metal, thereby facilitating
its reaction with carbon dissolved in the molten metal. Further, the lance head can
be readily positioned sufficiently above the molten metal that its rate of wear can
be kept to acceptable levels.
[0005] Although this is a beguiling theory, we believe it overestimates the disadvantages
of conventional practice. Further, the formation of the flame envelope has one substantial
disadvantage, namely the fuel gas or liquid fuel has to be supplied to the overhead
lance. The degree of disadvantage may vary from metal melting or refining process
to metal melting or refining process. In, for example, the BOS process the disadvantage
is quite substantial because the teachings of
EP-A-866 138 and
EP-A-866 139 require an overhead fuel supply to be laid on specially for the creation of the flame
envelope. The engineering difficulties of doing this are considerable, particularly
as the lance normally has to be manipulated between upper and lower positions. In
addition, the likelihood of hydrogen pick-up by the molten metal is significantly
increased and for many grades of steel is unacceptable.
[0006] US4 396 182 discloses a lance for blowing an oxidising gas onto a bath of molten metal, in which
the lance has a set of primary oxygen ports 3 and a set of secondary oxygen ports
6. It is not agreed, however, that entrainment of this secondary jet into the primary
jet is disclosed. The main idea on which the invention is based is to create of the
outlet of lance a continuous sheet of oxygen [from the secondary jets] which expands
in the form of an "umbrella" around the principle jets while being directed toward
the bath.
[0007] JP-A-08-246017 discloses a lance head having an axial oxygen port 1 surrounded by a series of secondary
oxygen ports 4. The general arrangement appears to be similar to those of
US 4 396 182, in particular, and there is no disclosure of entraining oxygen from the port 3 into
oxygen from the ports 4, or vice versa.
[0008] US6 432 163 B1 relates to an improvement or modification of the apparatus disclosed in
EP-A-866 138. D5 discloses a lance with central primary oxygen nozzles 5. There is a first ring
8 of secondary nozzles surrounding the primary nozzles 5 and a third ring 9 of tertiary
oxygen nozzles outside the ring 8 of secondary nozzles. In one mode of operation fuel
may be passed through the secondary nozzles. This fuel mixes with the tertiary oxygen
and forms a flame shroud or envelope which surrounds the primary oxygen jets. The
flame shroud or envelope serves the same purpose as that in
EP-A-866138, namely to prevent entrainment of surrounding atmosphere into the primary jets.
[0009] We believe the ability to improve oxygen blowing techniques in metal refining is
contingent upon increasing the rate at which oxidant is made available for reaction
without causing concomitant problems of unduly enhanced rates of lance wear.
[0010] According to the present invention there is provided a method of refining molten
metal in a vessel by the reaction of oxygen with impurities in the molten metal, according
to claim 1.
[0011] It is believed that the method according to the invention can be operated so as to
decrease the time taken to reduce the carbon content of a given volume of molten metal.
It is further believed that this result can be achieved without increasing the initial
pressure, velocity and flow of the primary oxygen, thereby keeping down the risk of
increasing the rate of erosion or damage to the lance head, in comparison with that
usually experienced in conventional methods. Increased pressure, velocity and flow
of the primary oxygen can also cause ejection of some molten metal and slag from the
vessel with attendant loss of yield and maintenance issues. The method and lance head
according to the invention also offer the advantage that no fuel is supplied to the
lance head, thereby avoiding the need for an overhead fuel supply, such an overhead
fuel supply being required for the formation of the shrouded gas jets described in
EP-A-866 138 and
EP-A-866 139.
[0012] The method and lance head according to the invention are particularly intended for
use in the Basic Oxygen Steelmaking (BOS) process, but are also applicable to some
other steelmaking processes and some processes for refining non-ferrous metals.
[0013] The said primary jet of oxygen is desirably ejected from the lance in both step (a)
and step (b) of the method according to the invention at an axial velocity that is
supersonic. In both these steps, a supersonic velocity in the range of Mach 1.5 to
Mach 3 may be used.
[0014] In order to achieve entrainment of each secondary oxygen jet at a suitable intermediate
location of its associated primary jet, the longitudinal axis of each secondary jet
diverges from the longitudinal axis of its associated primary jet in the direction
of travel at an angle of 5° to 25°.
[0015] The angle of divergence of each secondary oxygen jet from its associated primary
oxygen jet is in the range of 5° to 25° depending on the absolute velocity of the
second oxygen jet and its velocity relative to the first oxygen jet. Preferred angles
of divergence are in the range of 10° to 20°.
[0016] Typically, from two to eight secondary oxygen jets are used, with from two to six
being preferred. The exact number of secondary ports may be selected in accordance
with the desired ratio of primary oxygen to secondary oxygen flow. For example, the
secondary oxygen flow may be up to 50% of the primary oxygen flow and up to twelve
secondary ports may be used. Normally the secondary oxygen flow is from 5 to 50% of
the primary oxygen flow.
[0017] The linear separation of each secondary oxygen port from its associated primary oxygen
port is typically less than twice the diameter of the primary oxygen port.
[0018] Each secondary oxygen jet is preferably thinner than the primary oxygen jet.
[0019] The individual secondary oxygen jets preferably travel separately from one another
to their entrainment in the primary jet of oxygen.
[0020] In, for example, Basic Oxygen Steelmaking, the head of the lance is typically immersed
in the slag layer during the said step (b). In this example of Basic Oxygen Steelmaking
and in other examples, there is a plurality of primary oxygen jets, each being associated
with one or more secondary oxygen jets.
[0021] The relationship between each primary oxygen jet and its associated secondary oxygen
jets may be such that any or all of the typical or preferred features described above
may be employed.
[0022] A plurality of primary oxygen jets is employed. They typically issue from primary
oxygen ports that are arranged generally circumferentially (or on the perimeter of
another closed geometric figure). The secondary oxygen ports are typically arranged
outside the primary oxygen ports.
[0023] Each primary oxygen port has a group of secondary oxygen ports associated with it
and each group of secondary oxygen ports is arranged on the arc of a circle that is
concentric with the primary oxygen port with which said group is associated. The angle
subtended by the arc is normally less than 180°.
[0024] In a preferred lance head according to the invention each primary oxygen port is
in the form of a convergent-divergent nozzle and each oxygen jet is emitted from the
tip of the lance head.
[0025] Lance heads according to the invention generally have passages for the flow of a
coolant, for example, water.
[0026] In some preferred embodiments of the lance head according to the invention all the
primary and secondary oxygen ports communicate with a common chamber in the lance
head. Such embodiments offer the advantage of mechanical simplicity. In other preferred
embodiments of the lance head according to the invention, the or each primary oxygen
port communicates with a chamber in the lance head that does not communicate with
the secondary oxygen ports. These embodiments offer the advantage of making possible
control of the velocity and flow rate of the secondary oxygen independently of the
primary oxygen.
[0027] On many occasions it is preferred to perform in a method according to the invention
the addition step of
e) mixing with at least one stirring gas upstream of ejection the oxygen from which
the primary jet and/or secondary jet is formed.
[0028] In conventional practice with a conventional lance commercially pure oxygen is simply
blown at supersonic velocity into the molten metal. The rate and velocity are chosen
so as to complete a refining operation in a minimum time without creating excessive
turbulence and splashing. Including a stirring gas in the primary jet in accordance
with the invention tends to facilitate metallurgical reaction between dissolved carbon
in the molten metal and the gaseous oxygen that penetrates the surface of the molten
metal. Further, the use of the secondary jet provides additional oxygen and the rates
of supply of primary oxygen, secondary oxygen and stirring gas may be chosen so as
to maximise the total rate of oxygen input whilst ensuring that the force imparted
by the primary jet is not increased to the point where unacceptable splashing would
occur.
[0029] If a stirring gas is used, the primary jet is preferably formed by premixing the
stirring gas with the oxygen.
[0030] The stirring gas is preferably a noble gas, particularly argon. For some grades of
steel, however, nitrogen may be tolerated as the stirring gas provided it does not
have a deleterious effect on the steel.
[0031] If a stirring gas is used, the said primary jet of oxygen may have the same composition
throughout a heat. Alternatively its composition can be varied, being increased at
one or more instants during a heat. Indeed, there may be during an initial period
no deliberate addition of stirring gas. (Some argon will always be present as an impurity
in the oxygen.) The need for stirring is usually greatest towards the end of a heat
and therefore the mole fraction of stirring gas in the primary jet is preferably greater
in the last part (typically the last fifth) of the heat than in the first half of
the heat. Indeed, it is possible to continue the supply of stirring gas after the
supply of oxygen has been discontinued.
[0032] The method according to the invention and lance heads according to embodiments of
the invention will now be described by way of example with reference to the accompanying
drawings, in which:
Figure 1 is a schema of a BOS vessel adapted to operate the method according to the
invention;
Figure 2 is an end view of a first lance head according to the invention;
Figure 3 is a section through the line N-N of Figure 2;
Figure 4 is a side elevation of the lance head shown in Figure 2;
Figure 5 is an end view of a second lance head according to the invention; and
Figure 6 is a section through the line M-M of Figure 5.
[0033] Referring to Figure 1 of the drawings, there is shown a Basic Oxygen Steelmaking
(BOS) vessel 102. The vessel 102 has a refractory lining (not shown). In operation,
the vessel is charged with a batch of molten iron. This volume of molten iron is shown
by the reference numeral 106 in Figure 1. The molten iron is refined by reaction with
oxygen. The oxygen is supplied through a vertical lance 110 having a head 112. The
lance 110 is typically made of stainless steel and has a plurality of primary ports
114 in its head 112 for the discharge of oxygen. The ports 114 communicate with an
oxygen passage 115 through the lance 110. The lance 110 and head 112 are also provided
with passages 116 for the flow of a coolant (typically, water) to protect it against
catastrophic damage in the hot environment of the BOS vessel 102. The lance 110 is
also associated with a lance manipulator (not shown) which is able to raise and lower
the lance 110. In typical practice, the lance 110 is operated in two positions. One
is a so-called "soft blowing" position, in which the lance 110 is operated with its
tip relatively distant from the surface of the molten metal, and the other is a so-called
"hard blowing" position, in which the lance 110 is operated with its tip relatively
close to the surface of the molten metal and typically with the lance head 112 immersed
in a volume 118 of molten slag that is formed on the surface of the volume 106 of
molten metal during the refining of the molten metal. It is one of the advantages
of the method according to the present invention that it does not necessarily involve
any change to the soft and hard blowing lance head positions that are sometimes conventionally
used.
[0034] The method according to the invention may equally be used with a lance whose position
is controlled dynamically in response to decarburisation rate and other factors. Such
dynamic control is well known in the art.
[0035] The refining of the molten iron commences with the supply of oxygen from the lance
head 112 in a soft blowing position. The oxygen is ejected from the head 112 at a
supersonic velocity, typically in the range of Mach 1.5 to 3. The oxygen is typically
supplied to the lance head at a temperature in the range of 0°C to 50°C. There is
no need to preheat the oxygen, but a small amount of incidental preheating may take
place as the ambient environmental of a BOS vessel is usually at a substantially higher
temperature than normal room temperature. The oxygen is also typically supplied at
a pressure in the range of 5 bar to 20 bar so as to enable it to be ejected from the
lance head at a supersonic velocity. The primary oxygen penetrates the surface of
the molten metal 106 and reacts with carbon and other impurities such as silicon and
phosphorus therein. The chemistry of steelmaking is well known and need not be described
in detail herein. Suffice it to say that the dissolved carbon in the bath of molten
metal has a high affinity for oxygen and reacts rapidly with it to form carbon monoxide,
while other impurities react with the oxygen to form a molten slag which, being lighter
than the molten ferrous metal, rises to the surface to form a molten slag layer. The
velocity of the primary oxygen is such as to cause agitation of the molten metal and
there is typically a degree of turbulence at its surface. The slag layer 118 will
also be turbulent and will contain a considerable volume of carbon monoxide bubbles
as a result of the reaction between the carbon dissolved in the molten ferrous metal
and the oxygen.
[0036] After the formation of the slag layer it is desirable to increase the rate of decarburisation
of the molten metal. The lance 110 is thus lowered into its hard blowing position
with the head 112 immersed in the molten slag 118. (It is this position which is illustrated
in Figure 1.) The primary oxygen is supplied at a supersonic ejection velocity during
the hard blowing stage. In addition, in accordance with the invention, secondary oxygen
is also supplied. If desired, or as an inevitable consequence of the configuration
of the lance head, the secondary oxygen may be supplied during the soft blowing phase
and may then help to form the slag. There are a number of different options. For example,
the primary oxygen flow rate and velocity may be increased from the soft blowing phase
to the hard blowing phase.
[0037] A plurality of primary oxygen jets is employed and each is associated with a plurality
of secondary oxygen jets. One such primary oxygen jet 120 is illustrated schematically
in Figure 1. The primary oxygen jets 120 diverge in the direction of flow of oxygen.
Two secondary oxygen jets 122 are shown in Figure 1. The secondary oxygen jets 122
travel separately from one another. They diverge from the primary oxygen jets 120.
The angle of divergence depends on the absolute and relative velocities of the primary
and secondary oxygen jets. In general, the lower the absolute and relative velocity
of the secondary oxygen jets, the wider can be the angle of divergence. The purpose
of the angle of divergence is to ensure that most of the secondary jets are entrained
back into the primary jets upstream of the surface of the molten metal. However, it
is preferred to avoid this entrainment from taking place too near the tip of the lance
head itself. This may happen if the angle of divergence is too small. On the other
hand, if the angle of divergence is too large the secondary oxygen jets may simply
continue to diverge and peter out without being entrained into the primary jets and
without penetrating the molten metal. An angle of divergence in the range of 5° to
25°, preferably 10° to 20° is employed.
[0038] As the secondary oxygen jets 122 travel through the slag layer 118 so they encounter
bubbles of carbon monoxide therein. Such is the temperature in the slag layer that
we predict that there will probably be reaction between carbon monoxide and oxygen
to form carbon dioxide. Indeed, this reaction may quite possibly be sufficiently intense
to form each secondary oxygen jet into a flame. The formation of carbon dioxide in
this way is not intrinsically disadvantageous as carbon dioxide can act as a decarburising
agent. The entrainment of secondary oxidant into the primary oxygen jets brings more
oxidant into contact with the molten metal. As a result the rate of oxidation of carbon
and other impurities is enhanced and it therefore is possible to reduce the time taken
to reduce the concentration of dissolved carbon to a chosen level. It matters little
whether the secondary oxidant enters as the primary oxygen jets as secondary oxygen,
secondary carbon dioxide, or a secondary mixture of the two gases. Thus, any formation
of carbon dioxide is incidental to the invention.
[0039] It is believed that the time taken to reduce the carbon level in the molten metal
to a given value is dependent upon the rate at which oxidant molecules are brought
into contact with dissolved carbon molecules. Enhancing the rate at which oxidant
molecules come into intimate contact with the dissolved carbon reduces the refining
time. Accordingly, there are advantages to be had in employing a sizeable quantity
of secondary oxidant. In general terms, the rate of flow of secondary oxidant can
be up to 50% of the rate of flow of primary oxidant.
[0040] If the primary oxygen and the secondary oxygen flow via the same chamber in the lance
head, there is no freedom in setting the secondary oxygen jet velocity independently
of the primary jet velocity; it tends to be a little less than the primary oxygen
jet velocity, there being greater "frictional losses" associated with the secondary
jets because they are normally generated from thinner passages than the primary jets.
In such an example, the secondary jets would typically exit the secondary passages
at sonic velocities but in an underexpanded condition, leading to an immediate strong
shock to supersonic flow conditions and a series of shock waves dissipating the kinetic
energy of the jets. On the other hand, if the secondary oxygen comes from a separate
source than the primary oxygen there is a much greater freedom to vary the secondary
oxygen velocity. In general, a secondary oxygen velocity substantially less than the
primary oxygen velocity facilitates entrainment of the secondary jets into the primary
oxygen.
[0041] During or throughout the hard blowing stage argon or other stirring gas may be added
to the primary oxygen upstream of the lance 110. Typically the stirring gas is added
at a rate up to or equal to that at which secondary oxygen is used. The total flow
of gas to the primary jets preferably remains unaltered throughout the hard blowing
phase of a heat. The stirring gas may be supplied at constant rate throughout a heat,
or may be supplied towards the end of a heat when the level of dissolved carbon is
approaching what is desired. Stirring gas may be substituted for some of the primary
oxygen and supplied to the primary jets.
[0042] When the carbon level in the ferrous metal has been reduced to a desired value, the
supply of oxygen (both primary and secondary) and stirring gas may be stopped and
the lance 110 withdrawn from the steelmaking vessel 102. The molten metal may then
be tapped off from the vessel 102 in a conventional manner.
[0043] The ability to select when and how much stirring gas to supply helps the steelmaker
to optimise the steelmaking process. During an initial period of the hard blow phase
of a heat, the carbon levels are relatively high and the substitution of stirring
gas for oxygen may simply retard the refining. Towards the end of the heat, when carbon
levels are lower, the addition of a stirring gas is believed to be beneficial.
[0044] The configuration of a first lance head 200 for use in the method according to the
invention is illustrated in Figures 2 to 4. With reference to Figures 2 to 4, the
lance head 200 has a nose 202 at its forward end or tip 204. The nose 202 is surrounded
by a sloping annular face 206 which has its inner circumferential edge more forward
than its outer circumferential edge. As shown in Figure 2, four primary oxygen ports
208 are formed in the annular face 206. Each of the primary oxygen ports 208 has its
axis normal to the face. Thus, each of the primary oxygen ports 208 has an axis that
diverges in the direction of oxygen flow from the longitudinal axis of the lance head
200 itself. The angle of divergence is typically in the order of 5 to 15°. The oxygen
ports 208 are arranged circumferentially being equally spaced form one another. As
a result of this arrangement the primary oxygen jets penetrate the surface of the
molten metal at four different regions, thereby facilitating a good dispersal of the
oxygen. If desired, fewer or more primary oxygen ports 208 than the four illustrated
may be employed.
[0045] Each primary oxygen port 208 forms the termination of a convergent-divergent nozzle
210 formed through the lance head 200. The nozzle 210 has an upstream convergent portion
212, an intermediate portion 214 of constant diameter and a divergent potion 216.
The convergent portion 212 communicates with an oxygen chamber 218 which is formed
as an extension of the head 200. The convergent-divergent nozzles 210 (sometimes referred
to as Laval nozzles) are able to eject at supersonic velocity oxygen supplied at elevated
pressure to the chamber 218. The design of the convergent-divergent nozzles 210 is
preferably such that the oxygen is perfectly expanded on exit from the primary oxygen
ports 208.
[0046] Each primary oxygen port is associated with a plurality of secondary oxygen ports
220. As shown in Figure 2, each primary oxygen port 208 is associated with two secondary
oxygen ports 220. Each port 220 is formed in the annular face 206. The secondary ports
220 are all positioned intermediate the primary oxygen ports 208 and the outer circumferential
edge of the annular face 206. Each secondary port 220 is of a considerably smaller
diameter than the primary oxygen ports 208. Each secondary port 220 has an axis which
in the direction of oxygen flow diverges from the corresponding axis of the primary
oxygen port 208 with which it is associated. The angle of divergence may be up to
45° provided that the criteria discussed above with reference to Figure 1 are fulfilled.
Typically, however, the angle of divergence is in the range of 5 to 25°, more typically
in the range of 10 to 20°.
[0047] Each secondary oxygen port 220 is at the termination of a secondary oxygen passage
222. The secondary oxygen passages 222 are each formed with an upstream leg 224 and
a downstream leg 226. The downstream leg 226 is preferably at an angle to the upstream
leg 224. Each upstream leg 224 communicates with the chamber 218. As shown in Figure
3, the downstream legs 226 are of smaller diameter than the upstream legs 224. If
desired, however, the opposite arrangement can be employed with the upstream legs
224 being of smaller diameter than the downstream legs 226. Such an arrangement may
be used if lower secondary oxygen jet velocities are desired. The passages 222 are
typically all formed as bore and counterbore.
[0048] For ease of illustration, each primary oxygen port 208 is shown as associated with
only two secondary oxygen ports 220. Typically, however, each primary oxygen port
208 is associated with a greater number than two of secondary oxygen ports 220. Thus,
typically, each primary oxygen port 208 is associated with from two to eight secondary
oxygen ports 220. Each group of secondary oxygen ports 220 is preferably arranged
on the circumference of a circle that is concentric with the axis of the associated
primary oxygen port 208. The spacing of the secondary oxygen ports 220 is such that,
in operation, the jets of oxygen that issue therefrom do not merge with one another.
Each group of secondary oxygen ports 220 is typically arranged so that the ports 220
do not extend around the entire circumference but instead subtend an arc that is less
than 360° and normally less than 180°.
[0049] The head 200 is formed with an inner integral sleeve 228 and an outer integral sleeve
230 surrounding the oxygen chamber 218. The sleeves 228 and 230 define passages for
the flow of a coolant, normally water, through the lance head in its normal operation.
These passages extend into the nose 202 of the lance 200.
[0050] The lance head 200 may simply be welded or otherwise fixed fluid tight to a lance
(not shown in Figures 2 to 4) which has three concentric passages, an inner one being
for oxygen and the two others for coolant. After use to refine a large number of batches
of molten metal, it becomes necessary to replace the lance head 200. This may simply
be done by cutting off the used head from the lance and welding on a new lance head.
[0051] The operation of the lance head shown in Figures 2 to 4 is essentially as described
herein with reference to Figure 1. The secondary oxygen jets are entrained in each
primary oxygen jet and, as discussed above, enhance the flow of decarburising agent
that comes into intimate contact with the molten metal being refined. As a result,
it is believed that the time taken to refine a given volume of molten metal of given
composition may be reduced in comparison with conventional practice in which only
primary jets of oxygen are used. Any combustion of the secondary oxygen that might
take place as a result of entrainment of carbon monoxide into the secondary oxygen
jets is incidental to the invention.
[0052] Because the secondary oxygen ports 220 communicate with the oxygen chamber 218, the
secondary oxygen jets typically issue at sonic velocity and expand rapidly to supersonic
velocity owing to the pressure of differential between their underexpanded exit condition
and the ambient vessel pressure. This supersonic velocity may be less than that at
which the primary oxygen jets are ejected as a result of "frictional" interaction
between the flowing oxygen and the walls defining the secondary oxygen passages 222.
[0053] An alternative embodiment of the lance head is shown in Figures 5 and 6. Parts in
Figures 5 and 6 that correspond to ones in Figures 2 to 4 are indicated by the same
reference numerals as in Figures 2 to 4. In general, the configuration and operation
of the lance head shown in Figures 5 and 6 are very similar to the configuration and
operation of that shown in Figures 2 to 4. The main difference between the two embodiments
is that in the lance head shown in Figures 5 and 6 the secondary oxygen passages 222
communicate with an annular secondary oxygen chamber 300 that surrounds the chamber
218 and is coaxial therewith. As a result, the secondary oxygen may be ejected at
a velocity independent of that at which the primary oxygen issues from the lance head.
Accordingly, if desired, the secondary oxygen may be ejected at a supersonic velocity
greater than the primary oxygen velocity, a supersonic velocity less than the primary
oxygen velocity, sonic velocity, or a subsonic velocity. One advantage of a subsonic
secondary oxygen velocity is that it facilitates entrainment of the secondary oxygen
jets into the primary oxygen.
[0054] Various changes and modifications to the lance heads may be made. For example, if
desired, the lance head can have at its proximal end apertures formed in the wall
of an oxygen chamber so as to allow some of the oxygen to be ejected for the purpose
of post-combustion of carbon monoxide at a region of the BOS vessel remote from the
surface of the molten metal. Other embodiments of the invention will be readily apparent
to a person skilled in the art. The scope of the invention is defined in the following
claims.
1. Verfahren zum Frischen von geschmolzenem Metall in einem Behälter durch die Reaktion
von Sauerstoff mit Verunreinigungen in dem geschmolzenen Metall, wobei der Behälter
ein Volumen des geschmolzenen Metalls enthält, umfassend die folgenden Schritte:
a) Ausstoßen von mindestens einem primären Sauerstoffstrahl aus einer oberhalb des
geschmolzenen Metalls positionierten Blaslanze in das geschmolzene Metall zur Umsetzung
mit Verunreinigungen darin und zur Bildung einer Schicht aus geschmolzener Schlacke;
b) weiter Ausstoßen des primären Sauerstoffstrahls aus der Blaslanze und dadurch Bewirken, dass der primäre Sauerstoffstrahl durch die Schlackeschicht hindurch in
das geschmolzene Metall gelangt;
c) Ausstoßen einer Vielzahl an sekundären Sauerstoffstrahlen aus der Blaslanze, wobei
der sekundäre Sauerstoffstrahl eine Strecke getrennt von dem primären Sauerstoffstrahl
zurücklegt; und
d) Mitführen der sekundären Sauerstoffstrahlen in den primären Sauerstoffstrahl stromaufwärts
vom Einlass des primären Sauerstoffstrahls in das Volumen des geschmolzenen Metalls,
wobei die Blaslanze ein Kopfteil mit mindestens einer primären Sauerstofföffnung bzw.
Öffnung für primären Sauerstoff und einer Vielzahl von sekundären Sauerstofföffnungen
aufweist, wobei jede sekundäre Sauerstofföffnung mit der primären Sauerstofföffnung
oder einer der primären Sauerstofföffnungen assoziiert bzw. verbunden ist und eine
Achse aufweist, die in Strömungsrichtung von der damit verbundenen primären Sauerstofföffnung
in einem Winkel von 5 bis 25° divergiert, wobei der Divergenzwinkel sicherstellt,
dass die meisten sekundären Strahlen in die primären Strahlen stromaufwärts von der
Oberfläche des geschmolzenen Metalls zurückgeführt werden, wobei es eine Vielzahl
von primären Sauerstofföffnungen und eine Gruppe von zwei bis acht sekundären Sauerstofföffnungen,
die mit jeder primären Sauerstofföffnung verbunden sind, gibt, und wobei jede Gruppe
von sekundären Sauerstofföffnungen auf einem Kreisbogen eines Kreises angeordnet ist,
welcher mit der primären Sauerstofföffnung, mit welcher die Gruppe verbunden ist,
konzentrisch ist.
2. Verfahren gemäß Anspruch 1, wobei im Schritt (b) der besagte primäre Sauerstoffstrahl
mit einer axialen Überschallgeschwindigkeit im Bereich von Mach 1,5 bis Mach 3 ausgestoßen
wird.
3. Verfahren gemäß Anspruch 1 oder Anspruch 2, wobei die Längsachse jedes sekundären
Strahls von der Längsachse des damit verbundenen primären Strahls in der Bewegungsrichtung
in einem Winkel von 10 bis 20° divergiert.
4. Verfahren gemäß einem Beliebigen der vorhergehenden Ansprüche, in welchem der sekundäre
Sauerstoffstrom 5 - 50 % des primären Sauerstoffstroms ausmacht.
5. Verfahren gemäß einem Beliebigen der vorhergehenden Ansprüche, welches weiterhin den
Schritt des Mischens mit mindestens einem Rührgas stromaufwärts des Sauerstoffausstoßes,
aus welchem der primäre Strahl und/oder sekundäre Strahl gebildet wird, einschließt.
1. Procédé de raffinage de métal en fusion dans une cuve par la réaction d'oxygène avec
des impuretés dans le métal en fusion, dans lequel la cuve contient un volume du métal
en fusion, comprenant les étapes suivantes:
a) éjecter au moins un jet primaire d'oxygène à partir d'une lance qui est positionnée
au-dessus du métal en fusion dans le métal en fusion pour la réaction avec des impuretés
dans celui-ci et la formation d'une couche de laitier en fusion;
b) continuer à éjecter le jet primaire d'oxygène à partir de la lance et entraîner
ainsi le jet primaire d'oxygène à passer à travers la couche de laitier dans le métal
en fusion;
c) éjecter une pluralité de jets secondaires d'oxygène à partir de la lance, le jet
secondaire d'oxygène parcourant une distance séparément du jet primaire d'oxygène;
et
d) entraîner les jets secondaires d'oxygène dans le jet primaire d'oxygène en amont
de l'entrée du jet primaire d'oxygène dans le volume de métal en fusion,
dans lequel la lance comprend une tête qui présente au moins une sortie d'oxygène
primaire et une pluralité de sorties d'oxygène secondaires, chaque sortie d'oxygène
secondaire étant associée à la sortie d'oxygène primaire ou à l'une des sorties d'oxygène
primaires et présentant un axe qui diverge dans la direction d'écoulement de sa sortie
d'oxygène primaire associée d'un angle de 5° à 25°, l'angle de divergence assurant
que la plupart des jets secondaires sont entraînés à revenir dans les jets primaires
en amont de la surface du métal en fusion, dans lequel il y a une pluralité de sorties
d'oxygène primaires et un groupe de deux à huit sorties d'oxygène secondaires associées
à chaque sortie d'oxygène primaire, et
dans lequel chaque groupe de sorties d'oxygène secondaires est agencé sur un arc d'un
cercle qui est concentrique à la sortie d'oxygène primaire à laquelle ledit groupe
est associé.
2. Procédé selon la revendication 1, dans lequel, à l'étape (b), ledit jet primaire d'oxygène
est éjecté à une vitesse axiale supersonique qui est comprise dans la gamme de Mach
1,5 à Mach 3.
3. Procédé selon la revendication 1 ou la revendication 2, dans lequel l'axe longitudinal
de chaque jet secondaire diverge de l'axe longitudinal de son jet primaire associé
dans la direction de déplacement d'un angle de 10° à 20°.
4. Procédé selon l'une quelconque des revendications précédentes, dans lequel le flux
secondaire d'oxygène équivaut à 5 % à 50 % du flux primaire d'oxygène.
5. Procédé selon l'une quelconque des revendications précédentes, comprenant en outre
l'étape consistant à mélanger avec au moins un gaz d'agitation en amont de l'éjection
l'oxygène à partir duquel le jet primaire et/ou le jet secondaire sont formés.