Lance for blow-refinement in converter

A lance for blow-refinement in a converter comprises a primary nozzle generating a high-velocity, high-pressure primary oxygen jet, and an auxiliary nozzle generating an auxiliary oxygen jet. The auxiliary oxygen jet formed by the auxiliary nozzle has a velocity lower than the speed of sound. The auxiliary nozzle is configured so as to impede but not prevent oxygen flow therethrough. In the preferred construction, deceleration of gas-metal reaction induced by the oxygen jet.

BACKGROUND ON THE INVENTION 
The present invention relates generally to a lance for blow-refinement in a 
converter such as a Bessemer converter. More specifically, the invention 
relates to a lance having an auxiliary nozzle which can improve the 
thermal efficiency of secondary combustion in a converter. 
As is well known, a lance for blow-refinement installed in the converter is 
directed to a molten metal bath for injecting a high-pressure, 
high-velocity jet of oxygen to cause strong churning and rapid reaction 
near the molten metal bath surface. High-purity, high-energy gaseous 
oxygen injected toward the molten metal bath surface causes a gas-metal 
reaction, specifically carbon reduction. At the same time, the oxygen flow 
causes a slag-metal reaction, such as slagging of lime, and scavenging of 
phosphorus. When the proportion of pig iron in the source material is 
relatively high, specifically approximately 95%, the carbon content in the 
pig iron is sufficient as a heat source to heat the molten metal. At lower 
proportions of pig iron and high proportions of scrap and/or iron ore, it 
becomes necessary to heat the molten metal externally to compensate for 
the lack of an internal heat source. There are two ways to do this: one is 
to supply a carboniferous material, such as coke; the other way is to 
induce combustion of the carbon monoxide (CO) generated by the 
carbon-reducing gas-metal reaction, by supplying oxygen (O.sub.2) through 
an auxiliary nozzle. 
Various lances have been proposed and which include an auxiliary nozzle for 
supplying the oxygen needed for secondary combustion of carbon monoxide. A 
typical structure of this kind of lance has been disclosed in Japanese 
Patent First Publication (Tokkai) No. shows 53-102205. The lance disclosed 
has a plurality of primary nozzles and a plurality of auxiliary nozzles 
arranged alternatingly. The injecting outlets of the auxiliary nozzles are 
located higher, i.e. further from the bath surface than the primary 
nozzles. These primary and auxiliary nozzles adjoin and oxygen passage 
through the lance. The lance is also provided with a cooling medium 
circuit for a cooling medium, such as cooling water. 
In this known arrangement, the refining operation in the converter is 
mediated by secondary combustion of carbon monoxide generated in the 
primary gas-metal reaction. The internal pressure in the converter is held 
at about atmospheric pressure. On the other hand, the internal pressure in 
the oxygen passage of the lance is several kg/cm.sup.2 to several tens of 
kg/cm.sup.2. The primary nozzles are in the form of Laval nozzles. The 
velocity of the oxygen discharged through the primary nozzle is 
supersonic. The high discharge velocity of the oxygen ensures that the 
pressure of the oxygen stream at the molten metal surface will be higher 
than the static pressure of the slag on the molten metal surface, even 
though the oxygen is injected from a distance from the molten metal 
surface of about 1 to 3 m. Specifically, this oxygen jet flows at velocity 
of over 100 m/sec. Therefore, the oxygen jet churns up the molten metal 
bath and induces rapid reaction. 
On the other hand, the auxiliary nozzles are located higher than the 
primary nozzles and are essentially straight and untapered. The auxiliary 
nozzles discharge oxygen at near the speed of sound. Because of their 
greater distance from the molten metal bath and their straight shape, the 
auxiliary nozzles produce lower-energy oxygen jets. Thus the oxygen 
discharged through the auxiliary nozzles can more easily react with the 
carbon monoxide gas generated by the gas-metal reaction induced by the 
oxygen jet. 
The maximum secondary combustion rate of this conventional blow-refinement 
lance is about 30% and its heating efficiency is limited to about 20%. 
However, the effective heating efficiency is significantly lower than 20%. 
Although this heating efficiency can be improved by adjusting the ratio of 
pig iron to scrap, the maximum possible increase in heating efficiency is 
only about 5%. 
On the other hand, on the market, the price of scrap is dropping due to 
continuing increases in supply. Therefore, from the viewpoint of cost, the 
need for increasing the proportion of scrap is urgent. This requires an 
improvement in lance design to achieve a higher secondary combustion rate 
and higher heating efficiency for the molten metal. 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a 
blow-refinement lance for a converter which can achieve a higher secondary 
combustion rate and a higher heating efficiency. 
Another and more specific object of the present invention is to provide an 
improved lance which can slow down the oxygen jet discharged through the 
auxiliary nozzle in order to achieve a higher secondary combustion rate 
and a higher heating efficiency. 
In order to accomplish the aforementioned and other objects, a lance for 
blow-refinement in a converter comprises a primary nozzle generating a 
high-velocity, high-pressure primary oxygen jet, and an auxiliary nozzle 
generating an auxiliary oxygen jet. The auxiliary oxygen jet formed by the 
auxiliary nozzle has a velocity lower than the speed of sound. The 
auxiliary nozzle is configured so as to impede but not prevent oxygen flow 
therethrough. 
In the preferred construction, deceleration of the oxygen jet from the 
auxiliary nozzle is achieved by exerting resistance to oxygen flow. 
According to one aspect of the invention a lance for blow-refinement in a 
converter comprises a pressurized oxygen source, a primary nozzle having 
an outlet directed toward the surface of a molten metal bath in the 
converter and forming a high-pressure high-velocity primary oxygen jet 
capable of agitating the molten metal and inducing a chemical reaction 
therewith, an auxiliary nozzle for forming an auxiliary oxygen jet for 
inducing secondary combustion of carbon monoxide generated in the reaction 
induced by the primary oxygen jet, and means, incorporated in the 
auxiliary nozzle, for limiting the velocity of oxygen flow through the 
auxiliary nozzle to a point where the resulting jet forms a combustion 
zone in which the carbon monoxide oxidizes above the molten metal surface 
and for adjusting the velocity of the auxiliary oxygen jet within the 
combusting zone to approximately the flame propagation speed therein. 
The flow velocity limiting means controls the velocity of the auxiliary 
oxygen jet at the outlet of the auxiliary nozzle to below the speed of 
sound, preferably, no greater than 100 m/sec. 
The diameter at the outlet of the auxiliary nozzle is greater than that at 
an inlet opening into the pressurized oxygen source. 
The flow velocity limiting means comprises means for defining a taper in 
the auxiliary nozzle by which the diameter of the auxiliary nozzle 
gradually increases toward the outlet. In the alternative embodiment, the 
flow velocity limiting means comprises a member exerting resistance to 
oxygen flow through the auxiliary nozzle. The auxiliary nozzle has a first 
section adjoining the pressurized oxygen source in which the inner 
diameter increases toward the outlet, a second section adjoining the 
larger-diameter end of the first section and having a constant diameter, 
and a third section adjoining the end of the second section remote from 
the first section, including the outlet and having inner diameter 
gradually increasing toward the outlet. Preferably, the flow resistance 
member is disposed within the second section. 
In the preferred construction, the flow resistance member is a 
multi-conduit assembly defining a plurality of small-diameter conduits 
exerting resistance to oxygen flow through the second section. 
Alternatively the flow resistance member defines a zig-zag path for oxygen 
flow through the second section. 
The first section has an inlet at the point of juncture with the 
pressurized oxygen source and the ratio of the diameters of its distal end 
and the inlet is in the range of 1.1 to 10.0 and the diameter of the 
outlet is 1.1 to 20.0 times the diameter of the inlet 10, wherein the 
axial length of the auxiliary nozzle is between 1 and 200 times the 
diameter of the inlet. 
If necessary, the pressurized oxygen source comprises a primary oxygen 
source connected to the primary nozzle and an auxiliary oxygen source 
connected to the auxiliary nozzle, the primary and auxiliary sources 
supplying pressurized oxygen to the primary and auxiliary nozzles 
independently.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, particularly to FIGS. 1 to 3, the first 
embodiment of a lance 2 for blow-refinement in a converter, according to 
the present invention, has a plurality of primary nozzles 4 and a 
plurality of auxiliary nozzles 6. In practice, there will be 3 to 5 of the 
primary and auxiliary nozzles 4 and 6. The primary and auxiliary nozzles 4 
and 6 are arranged alternating at given intervals radially around the 
lance 2. Each of the primary and auxiliary nozzles 4 and 6 has an outer or 
upper end adjoining an oxygen passage 8 through the axis of the lance 2. 
Essentially annular cooling medium passages 10 surround the oxygen passage 
8 and the primary and the auxiliary nozzles 4 and 6. 
The oxygen passage 8 is connected to an oxygen source (not shown) in a per 
se well-known manner. Therefore, high-purity and high-pressure of oxygen 
(O.sub.2) is supplied through the oxygen passage 8. In practice, the 
pressure of the oxygen within the oxygen passage 8 is several kg/cm.sup.2 
to several tens of kg/cm.sup.2. On the other hand, the cooling medium 
passages 10 are connected to a cooling medium source (not shown) to 
conduct a cooling medium, such as coolant, cooling water or the like. 
Each primary nozzle 4 is in the form of a Laval nozzle and has an inner or 
lower end located near the central axis of the lance and directed toward 
the upper surface of a molten metal bath in the converter. The primary 
nozzles 4 thus direct oxygen jets toward the upper surface of the molten 
metal bath, which oxygen jets discharged through the primary nozzles will 
be hereafter referred to as "primary oxygen jets" or "primary jets". The 
configuration of the primary nozzles 4 is determined so that the velocity 
of the primary oxygen jets discharged or injected therethrough will be 
supersonic. The high velocity and resulting high kinetic energy of the 
primary oxygen jets causes strong churning in the molten metal bath and an 
accordingly rapid reaction. This reaction generates carbon monoxide, which 
becomes available for secondary combustion. 
On the other hand, the inner or lower ends of the auxiliary nozzles 6 open 
onto the sides of the lance 2 rather than on its lower face. The inner 
ends of the auxiliary nozzles 6 are thus located further from the molten 
bath than the inner ends of the primary nozzles 4. The auxiliary nozzles 6 
are so arranged and configured to discharge oxygen at a velocity lower 
than the speed of sound, preferable lower than 100 m/sec. The oxygen jets 
formed by the auxiliary nozzles 6 will be hereafter referred to as 
"auxiliary oxygen jets" or "auxiliary jets". When the inner ends of the 
auxiliary nozzles 6 lie 1.5 to 4.0 m distance from the upper surface of 
the molten metal bath, the velocity of the auxiliary oxygen jets 
discharged through the auxiliary nozzles 6 must be adjusted so as to 
induce flame propagation at distances of 1.0 to 4.0 m from the inner ends 
of the auxiliary nozzles 6. 
According to the first embodiment of the lance 2 according to the present 
invention, the auxiliary nozzles 6 gradually increase in internal diameter 
toward their inner ends, as shown in FIG. 3. In this configuration, the 
velocity of the oxygen jet at the outer end of the auxiliary nozzle 6 is 
about the speed of sound due to the high pressure, i.e., several 
kg/cm.sup.2 to several tens of kg/cm.sup.2 and the high velocity, i.e. 
about 200 m/sec. to 300 m/sec, in the oxygen passage 8. The gradual 
expansion of the internal diameter of the auxiliary passage 8 lowers both 
the pressure of the oxygen in the auxiliary nozzle 6 and the velocity of 
the discharged oxygen jet. By adjusting the rate of the expansion of the 
internal diameter between the outer and inner ends, the velocity of the 
auxiliary oxygen jet can be adjusted to below the speed of sound. 
A similar deceleration of the auxiliary oxygen jet can be obtained by 
various configurations of the auxiliary nozzles 6. 
For instance, in the second embodiment of the auxiliary nozzle 6 of FIG. 4, 
the auxiliary nozzle has sections 6a and 6b of differing diameter. The 
smaller-diameter section 6a adjoins the outer end and has a diameter 
d.sub.1. On the other hand, the larger-diameter section 6b is located 
downstream of the smaller-diameter section 6a and adjoins the inner end. 
The diameter d.sub.2 of the larger-diameter section 6b is significantly 
greater than that of the smaller-diameter section. In the preferred 
embodiment, the ratio of the diameters d.sub.1 and d.sub.2 is in the range 
of d.sub.2 /d.sub.1 =1.1 to 7.0. Furthermore, the length C of the larger 
diameter section 6b should fall in the range d.sub.2 &lt;C&lt;200d.sub.2 based 
on empirical observations. 
On the other hand, in the third embodiment of FIG. 5, the auxiliary nozzle 
6 increases in internal diameter gradually toward the inner end. The 
auxiliary nozzle 6 of FIG. 5 also has a fixed- diameter section 6c 
separating tapering upper and lower sections 6d and 6e. A flow-restriction 
conduit assembly 12 is disposed within the fixed-diameter section 6c. The 
conduit assembly 12 comprises a plurality of a small-diameter or capillary 
conduits 12a, as shown in FIG. 6. These small-diameter conduits 12a exert 
resistance against the oxygen flow through the auxiliary nozzle 6 and so 
lowers the velocity of the oxygen to below the speed of sound. This 
conduit assembly 12 thus augments the effect of the taper of the auxiliary 
nozzle 6 which gradually increases in diameter toward the inner end in the 
sections 6d and 6e. This achieves a more pronounced deceleration than in 
the first and second embodiments of FIGS. 3 and 4. 
A similar effect can be achieved by the fourth embodiment of the auxiliary 
nozzle 6 of FIGS. 7(A) to 7(E). In this fourth embodiment, a plurality of 
flow-restricting vanes 14 extend inward from the inner periphery of the 
fixed-diameter section 6c of the auxiliary nozzle 6. The flow-restricting 
vanes 14 lie perpendicular to the longitudinal axis of the auxiliary 
nozzle. Each vane 14 occludes the center of the auxiliary nozzle 6, 
leaving a peripheral section open for oxygen flow. The vanes 14 are 
arranged so that they overlap as viewed along the axis of the auxiliary 
nozzle 6. Therefore, a zig-zag path is defined through the fixed- diameter 
section 6c of the auxiliary passage 6. This further slows down the oxygen 
flow. 
FIGS. 8 and 9 show a practical application of the auxiliary nozzle 6 of the 
fourth embodiment of FIGS. 7(A) to 7(E). As shown in FIG. 9, three 
auxiliary nozzles 6 are arranged in the lance 2 at regular angular 
intervals, i.e. 120.degree.. Similarly, three primary nozzles 4 are 
arranged radially symmetrically between pairs of auxiliary nozzles 6. 
The auxiliary nozzles 6 turn at the point where the outer (upper) section 
6d and the fixed-diameter section 6c meet. The axis of the section 6d is 
essentially parallel to the axis of the lance 2 and the axis of the 
constant diameter section 6c lies oblique to the axis of the lance. The 
angle of the axis of the fixed-diameter section 6c is determined so as to 
have the inner end of the auxiliary nozzle 6 open at the edge of the lower 
face of the lance. The inner diameter d.sub.1 at the upper end and the 
diameter d.sub.2 of the fixed-diameter section are so proportioned that 
d.sub.2 /d.sub.1 =1.8. Similarly, the inner diameter d.sub.3 at the lower 
end of the auxiliary nozzle 6 and the diameter d.sub.2 of the 
fixed-diameter section satisfy the expression d.sub.3 /d.sub.2 =2.4. The 
overall length .lambda. of the auxiliary nozzle 6 is selected to be 
20d.sub.1. 
Experiments were performed with this auxiliary nozzle 6. The pressure in 
the oxygen passage 8 was held at 9.5 kg/cm.sup.2, which resulted in an 
auxiliary oxygen jet velocity at the lower end of the auxiliary nozzle 6 
of about 70 m/sec. 
The velocity of the primary flow at the lower end of the primary nozzle 4 
should still be higher than the speed of sound in order to maintain the 
effect of churning and rapid reaction. At the same time, effective 
secondary combustion can be achieved by the relatively low-speed auxiliary 
oxygen jet through the auxiliary nozzles 6. 
Experiments have shown that the rate of combustion of the carbon monoxide 
gas is determined by its the flame propagation speed. The flame 
propagation speed of carbon monoxide is lower than or equal to 10 m/sec, 
most commonly several m/sec. Therefore, in order to achieve effective 
combustion, the velocity of the auxiliary oxygen jet must be lower than or 
equal to 10 m/sec at the point where the oxygen mixes with the carbon 
monoxide. Other experiments have shown that it is preferable to define a 
combustion zone in the region above the molten metal bath in the 
converter, where a large amount of foaming slag exists. Toward this end, 
when the lower or inner end of the lance 2 is at a point 1.5 m to 4.0 m 
above the surface of the molten metal bath, the velocity of the auxiliary 
oxygen jet in the region 1.0 m to 4.0 m from the inner end of the lance 
will be approximately equal to the flame propagation speed. To obtain this 
flow velocity, the output velocity of the auxiliary nozzle 6 must be lower 
than the speed of sound, preferable lower than 100 m/sec. 
Therefore, by adjusting the discharge velocity of the auxiliary oxygen jet 
at the inner end of the auxiliary nozzle 6 to a velocity of 70 m/sec, 
effective combustion of the carbon monoxide can be obtained. 
On the other hand, experiments have also shown found that heat transmission 
by the molten metal takes place both by conduction and by radiation. 
Conductive heating is mediated by the foaming slag which is directly 
exposed to combustion of carbon monoxide and so accumulates the heat of 
combustion. When the heated foaming slag returns to the subsurface molten 
metal bath, it heats the molten metal in the bath. On the other hand, 
radiative heating is performed directly by the molten metal in the bath. 
Furthermore, carbon monoxide combustion heats the peripheral walls of the 
converter. This radiated heat is thus transmitted to the molten metal 
through the peripheral walls of the converter by conduction. 
In an example, blow-refinement was performed in a 200 t/ch converter. 
Oxygen is introduced not only from the top of the converter but also from 
below. Oxygen flows at 500N m.sup.3 /min through the primary nozzles 4 and 
at 170N m.sup.3 /min through the auxiliary nozzles. The lower face of the 
lance 2 is set 3.5 m above the surface of the molten metal bath. By 
adjusting the velocity of the auxiliary oxygen jet through the auxiliary 
nozzle 6, the combustion rate of carbon monoxide can be brought to 35% to 
40%. The combustion zone is formed in the region 1 m to 2 m from the inner 
end of the lance 2. This combustion zone lies about 1 m to 2 m above the 
molten metal bath. At this distance, the combustion zone could efficiently 
heat the molten metal. A heating efficiency of 60% to 70% was obtained in 
this experiment. 
Given a high efficiency of combustion of carbon monoxide and a high heating 
efficiency, the amount of the scrap could be increased to a proportion of 
20% relative to other materials. This ratio is about four times as great 
as in the conventional art. 
Although the foregoing embodiments are directed to auxiliary nozzles 
connected to a common oxygen passage together with the primary nozzles, it 
would be possible to connect the auxiliary nozzles to an oxygen passage 
separate from the oxygen passage for the primary nozzles. Separating the 
oxygen passages for the primary nozzles and the auxiliary nozzles would 
facilitate adjustment of the pressure and flow velocity of the oxygen 
through the auxiliary nozzles. 
FIGS. 10, 11 and 12 show the fifth embodiment of the lance according to the 
invention, in which separate oxygen passages 8A and 8B are defined in the 
lance. In this embodiment, the primary nozzles 4 are connected to the 
primary oxygen passage 8A and the auxiliary nozzles 6 are connected to the 
auxiliary oxygen passage 8B. The auxiliary oxygen passage 8B is annular in 
cross-section and surrounds the primary oxygen passage 8A. The auxiliary 
oxygen passage 8B itself is surrounded by the cooling medium passage 10. 
The primary oxygen passage 8A is connected to a primary oxygen source (not 
shown) through an oxygen supply passage which is joined to the outer end 
16 thereof. Similarly, the auxiliary oxygen passage 8B is connected to an 
auxiliary oxygen source (not shown) through an auxiliary oxygen supply 
passage which is connected to the outer end 18 thereof. Also the cooling 
medium passage 10 is connected to a cooling medium source (not shown) at 
the outer end 22 thereof. 
The auxiliary nozzles 6 are all connected to the auxiliary oxygen passage 
8B through small-diameter orifices 6f. The orifice 6f has a diameter 
d.sub.4 substantially smaller than the inner diameter d.sub.2 of the 
essentially fixed-diameter auxiliary nozzles 6. 
In practice, the inner diameter D.sub.1 of the primary oxygen passage 8A 
and the inner diameter D.sub.2 of the auxiliary oxygen passage 8b should 
exhibit the proportions D.sub.2 /D.sub.1 =1.23. On the other hand, the 
diameter d.sub.4 of the orifice 6f and the inner diameter d.sub.2 of the 
auxiliary nozzle 6 should exhibit the proportions d.sub.2 /d.sub.1 =1.65. 
The overall length .lambda. of the auxiliary nozzle should be 20d.sub.2. 
With this construction, the flow velocity of the auxiliary oxygen jet at 
the inner end of the auxiliary nozzle 6 will be about 95 m/sec if oxygen 
at a pressure of about 10 kg/cm.sup.2 is supplied to the auxiliary oxygen 
passage. Therefore, the auxiliary oxygen jet in the converter will be 
below the speed of sound and so will generate a flame front near the 
proper combustion zone. 
Therefore, the effects of the former embodiments can be achieved by this 
embodiment. 
In addition to the effects of the former embodiment, further advantages are 
obtained by this embodiment. For instance, at the beginning and end of 
refining operation, when the oxygen pressure in the primary and auxiliary 
oxygen jets is relatively low, the combustion zone tends to rise toward 
the lance in the former embodiments. This can be prevented by separating 
the primary oxygen passage and the auxiliary oxygen passage and by 
adjusting the timing of the auxiliary oxygen flow. 
Furthermore, separating the primary and auxiliary oxygen passages allows 
precise oxygen flow control through the auxiliary nozzles according to 
combustion conditions in the converter. This further improves the 
efficiency of carbon monoxide combustion and heating of the molten metal. 
While the present invention has been disclosed in terms of the preferred 
embodiment in order to facilitate better understanding of the invention, 
it should be appreciated that the invention can be embodied in various 
ways without departing from the principle of the invention. Therefore, the 
invention should be understood to include all possible embodiments and 
modifications to the shown embodiments which can be embodied without 
departing from the principle of the invention set out in the appended 
claims.