[001 ] The invention is related to a lance for blowing oxygen onto a bath of molten steel, and more particularly to a lance for post-combustion in steelmaking.

[002] In steel-refining, the main starting materials are usually a mixture of liquid pig-iron and scrap. The quantity of scrap which can be added, i.e. the scrap addition or scrap rate, depends notably on the temperature of the liquid pig iron and on the quantity of heat generated in the converter by oxidation of chemical elements. Most of it concerns the transformation of carbon into carbon monoxide CO and then into dioxide CO2. The more CO2 is formed, the more heat is created and may be transferred to bath so as to provide energy for additional scrap melting. The transformation of CO to CO2 is known as postcombustion.

[003] Typically, with usual single oxygen flow, very little CO is post-combusted into CO2 inside the vessel. By injecting a secondary flow of oxygen during the process, the unburned CO moving upward meets additional O2 provided by this secondary flow and is then combusted into CO2. The reaction is defined by the commonly known equation: CO + I/2O2 = CO ₂ .

[004] There are two different technologies which have been developed to provide the secondary flow of oxygen. The first one consists in having a single oxygen flow supply and then split it in a primary flux for standard decarburization and a secondary flow for enhancing post-combustion.

[005] This first technology has the advantage of requiring few modifications of existing lances and for example to keep same lance diameter and weight, thus not impairing the overall support structure of the lance and reducing investment costs. Disadvantage is that the secondary flow rate of oxygen being defined by the surface ratio between primary and secondary oxygen ejection means, it cannot be managed independently form the primary flow according to the process phases. Also, if oxygen supply is limited, primary oxygen flow is reduced, which impairs the decarburization process and productivity.

[006] The second technology consists in having a dual flow lance, wherein primary and secondary flows of oxygen have their own supply and are independently controlled. An example of a lance according to this technology is illustrated in patent US 5,681 ,526. Main advantage of this technology is that primary and secondary flows of oxygen are independently controlled which allows to more accurately control the post-combustion process and thus to increase the post-combustion rate. Disadvantage of this technology is that it requires an overall change on the installation and thus high investment cost.

[007] However, whatever the chosen technology, the possibility of loading more scrap without impairing the productivity is still limited.

[008] There is so a need for a post-combustion lance which allows to to promote the postcombustion reaction while not impairing the decarburization process nor damaging the decarburization equipment. In particular, the post combustion lance must be able to inject the secondary flux of oxygen so as to provide enough oxygen at the right location for the post-combustion to happen without damaging the internal refractory layer of the steelmaking vessel. Preferentially the lance according to the invention must allow to reach a Post combustion rate comprised between 12 and 25%. Upper limit is needed as part of energy will be transferred to the bath, but another part will be transferred to the exhaust gas, whose temperature will thus increase which could damage exhaust gas treatment system.

[009] This problem is solved by a post-combustion lance according to the invention comprising a plurality of tubes which surround one another and are concentric to a central longitudinal axis of the lance Z, among this plurality of tubes being a first central tube forming a supply duct for a primary flux of oxygen, and at least one second tube surrounding said first central tube so as to form a gap for the circulation of a secondary flux of oxygen. The lance further comprises a tip located at one end of the lance and provided with at least a primary oxygen ejection mean for ejection of the primary flow of oxygen, a distributor comprising at least four channels in fluid connection with the gap for the circulation of the secondary flux of oxygen of the lance, each channel being provided with at least one secondary oxygen ejection mean, said secondary oxygen ejection mean being designed so that it forms an ejection angle with the central longitudinal axis Z of the lance from 65 to 75° and said secondary oxygen ejection mean being located at a distance d above the primary oxygen ejection mean of the tip such as the ratio between the distance d and the internal diameter D of the converter is from 0,04 to 0,15.

[0010] The lance of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: - the secondary oxygen ejection mean is located at a distance d above the primary oxygen ejection mean of the tip such as the ratio between the distance d and the internal diameter D of the converter 2 is from 0,08 to 0,15.

- the distance d is from 250mm to 750mm.

- the distance d is from 500mm to 750mm.

- the ejection angle p is from 65 to 70°.

- the secondary oxygen ejection means have an exit with an oblong shape.

- the biggest dimension of the secondary oxygen ejection means exits is from 10 to 25mm.

- the biggest dimension of the secondary oxygen ejection means exits is from 12,5 to

20 mm.

- the distributor comprises five channels,

- the number of primary oxygen ejection means is the same as the number of secondary oxygen ejection means,

- the exits of the secondary oxygen ejection means of the distributor are shifted from the exits of the primary oxygen ejection means of the tip.

The invention is also related to a steelmaking process using a lance according to nay of the previous embodiments wherein the flow of secondary oxygen is comprised between 50 and 135 Nm3/min.

[001 1] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication and which is in no way restrictive, with reference to the appended figures in which:

Figure 1 illustrates a post-combustion process in a converter,

Figure 2 illustrates a first embodiment of a lance according to the invention

Figure 3 illustrates the distributor of the lance of figure 2

Figure 4 illustrates a second embodiment of a lance according to the invention

[0012] Elements in the figures are illustration and may not have been drawn to scale.

[0013] Figure 1 illustrates a converter 2 containing a bath of molten metal 20. The converter is internally covered with a wall of refractories 3 and have an internal diameter D. The molten metal is pig iron which needs to be decarburized to produce steel. To perform such a decarburization, a lance 1 is inserted into the converter and blows a primary flux of oxygen

21 towards the molten metal 20 through an outlet provided in the tip 15 of the lance. This decarburization allows to remove carbon from the bath as CO. To combust the unburned CO into CO2, a second flux of oxygen 22 is injected towards uprising exhaust gas. This reaction is exothermic and releases a lot of energy which can be further used to melt scrap into the molten bath.

[0014] This double oxygen injection is performed with a post combustion lance, as the one illustrated in figure 2, according to one embodiment of the invention. As a purpose of information, such a lance is usually more than 20 meters long. The post-combustion lance 1 according to the invention comprises a plurality of tubes which surround one another and are concentric to a central longitudinal axis of the lance Z. The lance according to the invention is made of an upper part 1 A and of a lower part 1 B joined together by a distributor 17. The lower part 1 B of the lance is the one closest to the bath 20 when inserted into the steelmaking vessel 2. The lance is composed of a first tube 1 1 which supplies the primary flux of oxygen 21 , a second tube 12, which surrounds the main tube 11 thus forming a first annular gap 31 for the supply of cooling water within the lance 1 . The lance being subjected to high temperature during the steelmaking process it needs to be constantly cooled down so as to avoid being quickly damaged. Those two first tubes go along the whole length of the lance, each in a single part, which allows reducing risks of tightness issues. The first tube 1 1 is preferentially made of a material allowing the passage of a flow at a speed of at least 60m/s, such as stainless steel.

[0015] The lance 1 then comprises a third tube 13, surrounding the second tube 12 to form a second annular gap 32 for the supply of the secondary flux of oxygen 22 necessary for the post-combustion. This third tube does not extend all along the length of the lance 1 but only along the upper part 1 A. This third tube is preferentially designed so that there is a ratio of 1/5 between the section of the gap for the circulation of the primary of oxygen and the section of the gap for the circulation of the secondary flux of oxygen. A fourth tube 14, comprising two parts, a first part 14A, which surrounds the third tube 13 along the upper part 1 A of the lance, and a second part 14B surrounding the second tube 12 along the lower part 1 B of the lance. This fourth tube 14 thus form a third annular gap 33 allowing to draw off the cooling water. In another embodiment, this first annular gap 31 may be designed for the drawn off of the cooling water from the lance 1 while the third annular gap 33 allows the entry of the water within the lance 1 .

[0016] The lance 1 further comprises a tip 15, closing the lower part of the lance 1 B. This tip is in fluid connexion with both first 31 and third annular gaps 33 so as to close the water circuit and provide circulation of water within the lance. This furthermore allows the cooling down of the tip 15 itself which is the closest part to the molten steel and thus subjected to the highest temperatures.

[0017] The tip is provided with at least one primary oxygen ejection mean 16 for blowing primary flow of oxygen 21 onto the bath of molten steel and allowing decarburization. In a preferred embodiment the tip is provided with at least four primary oxygen ejection means 16, the optimal number depending notably on the size of the ladle and thus of the circumference of the molten bath. The diameter of the primary oxygen ejection means exit depends on the same parameters. In a preferred embodiment those primary oxygen ejection means 16 have an exit diameter comprised between 40 and 50mm, preferentially between 40 and 45mm. In a preferred embodiment these ejection means are designed so as to eject the primary flux of oxygen with an ejection angle a with the central axis Z of the lance 1 comprised between 10 and 20°, preferentially between 14 and 18°. This allows to find the good compromise between maximisation of the surface of the molten bath receiving oxygen ang keeping sufficient distance from the refractory walls to avoid damaging them.

[0018] The lance is designed to receive a distributor 17 making the junction between the upper 1 A and the lower part 1 B of the lance and ensuring the circulation of water between the upper 14A and the lower 14B parts of the fourth tube. This distributor 17 comprises at least four channels 6 in fluid connexion with the gap 32 for the circulation of the secondary flux of oxygen 22 of the lance. Each channel 6 is provided with at least one secondary oxygen ejection mean for blowing the secondary flux of oxygen 22 towards the uprising gas flux. This secondary flux of oxygen will provide necessary fuel for the further combustion of CO and the release of additional energy for scrap melting. Secondary oxygen ejection means 18 are located at a distance d above the primary oxygen ejection means 16 of the tip 15 such as the ratio between the distance d and the internal diameter D of the converter 2 is from 0,04 to 0,15, preferentially from 0,08 to 0,15.

[0019] They may be located between 250 and 750mm above the first oxygen ejection means 16 of the tip 15, preferentially between 500 and 750mm.

[0020] The secondary 18 ejection mean may be simple ports located at the exit of the channels 6 or nozzles inserted at the exit of the channel 6.

[0021] In a preferred embodiment the distributor 17 is provided with the same number of secondary ejection means 18 as the number of primary ejection means 16 provided on the tip 15. Their respective exist are preferentially shifted. The exit diameter of the secondary ejection mean may be comprised between 10 and 25mm. The exits may have an oblong or circular shape. [0022] The secondary ejection means 18 are designed so as to have an ejection angle p with a plan perpendicular to the central longitudinal axis Z of the lance comprised between 15 and 25°. This range of angles allows to avoid damaging both the lance itself, with angles inferior to 155 and the refractory wall whose temperature would increase too much with angles higher than 25°.

[0023] In a most preferred embodiment the distributor is mounted on the lance 1 so as to be able to slide of few centimetres, less than 5 cm, along the pipe 12 in order to follow the thermal expansion of the external tube 14 due to thermal constraints it is subjected to. This is done by appropriate means, such as O-rings. The distributor is furthermore provided with sealing means preventing water leakage in the annular gaps supplying the oxygen flows. These sealing means are for example O-rings.

[0024] Figure 3 illustrates a distributor adapted for a lance according to figure 2.

[0025] Figure 4 illustrates a lance 1 according to a second embodiment of the invention. In this embodiment the layout of the different tubes forming gaps for fluid supply is different from the previous one. As visible on the figure, instead of having, starting from the central duct the supply of primary oxygen, water, secondary flux of oxygen and water, in this configuration the lance provides primary oxygen, secondary oxygen and inlet and outlet water. Configuration of the tip 15 and of the distributor 17 are adapted to be compatible with this embodiment but all parameters of the previous embodiment, namely size, shape, angle and location of the ejection means apply to this embodiment.

[0026] In both embodiments of figures 2 and 3, the primary 16 and secondary 18 oxygen ejection means may be conical or parabolic nozzles.

[0027] When using a lance according to the invention in a steelmaking process, flow of secondary oxygen is preferably comprised between 50 and 135 Nm ³ /min.

Trials

[0028] ANSYS Fluent software was used, which is a commercial finite volume solver used for CFD. The model is independent of time, i.e. steady state. The governing equations are conservation of mass, momentum and energy. The turbulence was described by the standard k-e turbulence model.

[0029] Calculations were done for a converter having an internal diameter of 6m to estimate the Post Combustion Ratio, the average refractories wall temperature T _wa ii, the absorbed energy by the bath and the temperature of the exhaust gas T _Gas outlet for different flow rates of secondary flow of oxygen when using a post combustion lance wherein the distributor comprises five secondary oxygen ejection means which are ports having an oblong shape and a length of 20mm, an ejection angle p with the longitudinal axis Z of 65° and being located 500mm above the primary oxygen ejection means. Results are illustrated in table 1 . Reference case is a lance without secondary oxygen flow.

[0030] Those parameters have been chosen as the Post Combustion rate is representative of the amount of CO gas which is transformed in to CO2, thus releasing energy. The temperature of the refractory wall must be controlled as too high temperatures may damage them. Absorbed energy by the bath will impact the amount of scrap which can be additionally loaded into the bath. Temperature of exhaust gas must be monitored to avoid releasing gas at a too high temperature which could damage the overall exhaust gas recovery and treatment system.

Table 1

Same calculations were performed with ten circular secondary oxygen ejection means with a diameter of 20mm located at 500mm above the above the primary oxygen ejection means and a flow rate fixed at 115 Nm ³ /min, only the angle p was varied. Results are illustrated in Table 2.

Table 2 As we can see from those calculations, with a lance according to the invention it is possible to reach a PCR that is much higher than the one obtained with a lance according to prior art.