EP0318352A1 - Carbon dioxide lance for metallurgy - Google Patents

Abstract

Lance for carbon dioxide snow intended, particularly, for making a molten metal inert. It comprises a pipe for supplying pressurised liquid carbon dioxide intended to be connected to a source for supplying pressurised liquid carbon dioxide, a T-shaped piece (1), one end of which is connected to the said pipe and the other two ends of which are connected, respectively, to at least one liquid carbon dioxide transportation line (31, 33; 32, 34) and to a controlled valve (151, 152), the exit orifices of the controlled valves (151, 152) being connected together by means of a V-shaped connection piece (35) which emerges in a discharge pipe (51, 56, 58) for the snow. <IMAGE>

Description

The present invention relates to a carbon dioxide snow lance intended in particular for inerting a molten metal such as steel during the pouring of a first container into a second container, in order to provide protection against oxidation and / or nitriding the molten metal. It relates more particularly to a lance usable during a steel casting of a converter or an electric oven in a ladle, of a ladle in a distributor, of a distributor in a continuous casting mold, etc ...

At the start of a pouring of liquid steel, for example from a converter or an oven into a ladle or a ladle into a distributor, or during the first casting of a ladle into this distributor in the event of a sequence , the liquid metal is in contact with the atmosphere.

The height of fall of the liquid metal in the receiving container and the turbulence lead to fairly large nitriding and / or oxidation reactions. These occur during casting in a distributor generally until the nozzle is completely immersed in the liquid metal poured into the distributor, said nozzle being placed at the lower end of the bag and surrounding the jet of casting. When the immersion of the lower part of the nozzle has been carried out, the problems of nitriding and / or oxidation are less of a problem since cover powders are generally used which are spread on the surface of the liquid metal in the distributor, or any other similar known means. On the other hand, during a casting of an oven or converter in a ladle, the oxidation and / or the nitriding arise from the beginning to the end of the casting.

Generally, during a ladle - distributor casting, the nitriding and / or oxidation phenomenon mentioned above generally lasts from 45 seconds to approximately 4 minutes depending on the size and shape of the distributor. The metal poured into the distributor before immersion of the nozzle is thus more or less strongly oxidized and / or nitrided and the steel billets or ingots formed from this metal do not have the desired metallurgical qualities.

Among the processes known to avoid these drawbacks is the process known under the trade name "SPAL", developed by the company L'AIR LIQUIDE and using cryogenic liquids such as argon or liquid nitrogen which very effectively protect the impact zone of the metal jet by inerting the bottom of the containers before start of casting and then covering the surface of the liquid metal to be protected during casting.

However, when it is also desired to produce steels having a low percentage of nitrogen, that is to say when it is desired to avoid nitriding of the steel, it is not possible to use liquid nitrogen for protection of molten metal. In this case, the only method currently available consists in using liquid argon spilled on the surface of the liquid metal. However, argon is a relatively expensive gas and a more economical solution is currently being sought which makes it possible to obtain metallurgical results which are substantially identical to those obtained when using liquid argon.

US Patent 4,614,216 to Savard et al. describes the protection of a casting jet using carbon dioxide gas. But the use of gas is not very effective when it is a question of projecting it into a pocket or a distributor to make a screen between the atmosphere and the molten metal, because the turbulences formed do not allow eliminate ambient air.

US Pat. No. 4,666,511 to Naud describes a method of protection against oxidation and / or nitriding of a molten metal poured into a container, by spraying carbon dioxide snow into the container before pouring and / or onto the bath. molten metal during casting. Liquid carbon dioxide is expanded through a valve to form carbon dioxide snow. However, the solution described in this patent does not make it possible to adjust and orient the carbon dioxide snow flow towards the foot of the casting jet. This problem is particularly difficult to solve because it involves injecting carbon dioxide snow near a molten metal at a temperature of the order of 1000 ° C to 1500 ° C.
French Application 86/16475 filed on November 26, 1986 describes a method of protection against oxidation and / or nitriding of a liquid metal, carried out in particular by injecting carbon dioxide snow into the distributor, the injection being carried out by two successive stages:
a first step of purging the distributor, taking place before the start of the pouring of the liquid metal, during which carbon dioxide snow is injected at a purge rate such that said snow at least partially reaches the bottom of the distributor in which it sublimates at least partially, so as to gradually expel the air present in it, this step being completed when the concentration of oxygen in the vicinity of the zone corresponding to the foot of liquid metal jet at the start of the casting is less than about 0.5%,
- a second step of maintaining the atmosphere around the jet foot, starting when the liquid metal begins to flow in the distributor, during which carbon dioxide snow is injected at a maintenance rate, lower than the rate purge, such as the presence of this snow or of the gas resulting from the sublimation thereof, in an area located in the vicinity of the jet foot and / or on the surface of the liquid metal in said distributor, maintains an atmosphere containing less 0.5% by volume of oxygen in said zone, the pouring of the liquid steel starting substantially at the end of the first stage, preferably from the end of the latter.

According to a preferred embodiment of this process in which the bag is provided with a nozzle placed around its pouring orifice, the second step ends as soon as the lower end of the nozzle is substantially immersed in the liquid metal, the surface of the liquid metal bath in the distributor then being covered with a means of protection against oxidation and / or nitriding, known per se. Preferably, the maintenance flow is at most equal to about 50% of the purge flow.

According to one embodiment of the method described in this French application, a quantity of snow sufficient to produce snow is injected into the ladle for receiving the pouring of the liquid steel coming from the converter or from the electric furnace. a purge of said pocket, this quantity of carbon dioxide snow preferably being between 0.2 and 5 kg per tonne of metal poured.

Of course, in general, the lance according to the invention finds applications in the casting of a jet of liquid steel from a first container into a second container, the casting jet and / or the surface of the bath. of liquid metal from the second container being protected against oxidation and / or nitriding with carbon dioxide, in the form of snow.

Other applications of ladle metallurgy provide protection of the surface of the liquid metal bath against oxidation and / or nitriding by covering said surface with a layer of carbon dioxide snow.

All these applications require the availability of a carbon dioxide snow injection lance, which is handy, reliable, usable in a high temperature environment, with a variable flow rate.

However, it has been found that, when one relaxes liquid carbon dioxide at atmospheric pressure and according to an adiabatic expansion, in order to create snow, one encountered various problems:
- if the speed of ejection of the gas - solid mixture is too low, the convection linked to the temperature above the pocket is such that little or no snow arrives at the bottom of the pocket or on the surface of the liquid metal.
- if the ejection speed is too fast, the convection near the nose of the lance is such that a mixture of solid and gaseous carbon dioxide is made such that air is drawn into the bag and such that the amount of snow in the pocket is very small due to the heat exchange between the hot air and the snow.

The invention solves the above problems.

The snow lance according to the invention is characterized in that it comprises a supply pipe for pressurized liquid carbon dioxide intended to be connected to a supply source for pressurized carbon dioxide, a T-piece one end of which is connected to said pipe and the other two ends of which are respectively connected successively to at least one liquid carbon dioxide transport line and to a controlled valve, the outlet orifices of the controlled valves being connected to each other by a V-shaped connecting piece which opens into a snow evacuation pipe.

Preferably, the snow evacuation pipe has in its front part a divergent part of frustoconical shape, the small base of which is situated on the side of the V-shaped connecting part and the large base of which is situated on the side of the opening. outlet placed at the end of the snow disposal pipe. This frustoconical part, preferably interchangeable, determines the speed of ejection of the carbon dioxide snow transported pneumatically by the carbon dioxide produced directly by the trigger. It also makes it possible to reduce the formation of two-phase (gas / solid mixture), that is to say to limit the amount of gas generated during expansion.

It has been found, in practice, that this frustoconical part should not, in most cases, have a length less than 300 mm. The angle at the top of the corresponding cone will preferably be of the order of approximately 6 ° (between approximately 5 ° and 7 °).

Preferably, the speed of ejection of the gases and of the solid particles will remain less than or equal to 30 m / s while the snow flow will be between 40 kg / min and 150 kg / min.

Thus, when it is desired to reduce the speed of ejection of the gas / solid mixture, by the same upstream pipe diameter, it will be sufficient to lengthen the frustoconical part (beyond 300 mm) so as to increase the diameter of the downstream pipeline.

According to a preferred embodiment of the invention, the transport lines are rigid and / or are bent over a radius of the order of 3 to 5 times their diameter.

The snow lance according to the invention, by its simplicity, can be installed directly under an electric oven or under a converter or any system located near or above a ladle.

• La figure 1, une vue d'une lance selon l'invention.
• La figure 2, une vue d'une vanne de détente utilisée dans la lance de la figure 1.

The invention will be better understood with the aid of the following embodiments, given without limitation, together with the figures which represent:

• Figure 1, a view of a lance according to the invention.
• FIG. 2, a view of an expansion valve used in the lance of FIG. 1.

In Figure 1 A is shown a side view of an embodiment of the lance according to the invention while in Figure 1 B is shown a top view thereof.

The supply of liquid carbon dioxide, stored in a tank not shown in the figure, is carried out using a supply hose (also not shown) which ensures the flexibility required to orient the snow created by spear. A T-shaped element 2 is connected by its branch 21 to the hose and distributes the liquid carbon dioxide, according to two parallel circuits, respectively in its branches 22 and 23. Each of these latter is extended by a rigid line comprising a curved part 31, 32 of radius, preferably between 3 and 5 times the diameter of the line, so as to minimize the pressure losses in this part of the lance then by a rectilinear part 33, 34. The branches 22, 23 as well as the parts 31, 32, 33, 34 of the rigid line have a diameter slightly greater than that of the branch 21 connected to the flexible.

The end of the rectilinear parts 33 and 34 of the rigid lines is connected to the means 151, 152 forming a solenoid valve which will be described below with the aid of FIG. 2. These means 151, 152 have their outlet connected respectively to a piece of link V-shaped which provides the connection between the two dry ice flows. This part 35 therefore comprises two identical branches respectively composed of a divergent frustoconical part 36, 39 whose small base is located on the side of the means 151, 152, extended by a cylindrical tube 37, 40, whose diameter is that of the large base of the frustoconical part, the two branches joining symmetrically to form a connection end 38, of section slightly greater than or equal to the sum of the sections of the cylindrical tubes 37, 40. The angle of convergence A between the axes cylindrical tubes 37, 40 is minimized, depending on the arrangement and the size, in particular, of the means 151 and 152, in order to avoid a loss of energy during the junction of the two sets, because this loss of energy generates gas phase, which we try to avoid.
The assembly 12 comprising the T-shaped part, the rigid lines, the solenoid valves 151, 152 and the V-shaped part 35 form a standard assembly constituting means for generating carbon dioxide snow which can be connected to means of projection of this snow which can be interchangeable. For this purpose, the downstream end (in the direction of flow of the snow) of the part 35 can be connected by a removable flange 50 to an intermediate part 51 which determines, by its geometry, the speed of ejection of the dry ice, transported pneumatically by the gas produced directly by the trigger. This part 51 successively comprises from upstream to downstream, a cylindrical part 52 of section equal to the downstream end of the part 35 and detachably secured to the part 35 by means of the flange 50, an intermediate frustoconical part 53 whose length determines the speed of ejection of the snow, extended by a new downstream cylindrical part 54 (of cross section greater than that of the part 52) connected in a removable manner by a flange 55 to the barrel 56 proper, which is a cylindrical tube whose the section is identical to that of the part 54, the value of this section depending on the desired speed of ejection of the solid / gas mixture for a given diameter of the pipe 52 which is a function of the desired maximum flow rate of the lance.

All the proposed equipment, whatever its length, is dimensioned so that the exit speed of the snow particles is such that the jet has the kinetic energy necessary to throw the carbon dioxide snow at the desired location and so as not to induce parasitic air suction phenomena, which would cause untimely sublimation of part of the solid particles generated, thus affecting the efficiency of the installation.

The barrel 56 is then connected via the movable flange 57, to a consumable endpiece 58, comprising a bent upstream cylindrical part 59 connected to the flange 57 and a downstream cylindrical part 60, substantially rectilinear, the axis of which forms a angle B of approximately 60 °, in the present example, with the axis of the barrel 56. This consumable end piece 58 can take any desired shape: straight cylinder, bent, of diameter identical or greater than that of the barrel, divergent truncated cone, possibly bent, etc., so as to correctly target a specific place (jet foot: cylindrical part) or a larger surface (divergent truncated cone for the total surface of a pocket). The mobility of the flange 57 is determined beforehand as a function of the operating conditions or can be controlled mechanically, electrically or by any other device.

FIG. 2 represents a partially exploded view of the solenoid valves 151 or 152, a figure in which the details of the valve 70 itself have not been shown. The electro-pneumatic control of this valve is carried out for example using a WORCHESTER type control device provided with a pneumatic operator which controls the movement of the valve (not shown) of the valve 70 connected to the pipeline. rigid 33 by a flange 75. The ball valve system, known per se, of the cryogenic valve 70 is housed in the space 76 (details not shown). Only the bore 77 is shown, which is extended by a bore 78 with a diameter greater than this, in which an injector 72 is housed which limits the flow of liquid CO₂ to be transported. This injector abuts at the bottom of the bore 78 and its upstream face is close to the valve flap (bore 77 of short length) so as to avoid any untimely formation of snow between the injector 72 and the valve (not shown). The internal structure of the injector 72 is that of a venturi device with a converging upstream part, a central part of constant diameter and a diverging downstream part. The downstream face of the injector 72 abuts in the connection device 73 which covers the latter and is screwed into the bore 78 which is tapped. A connection flange 79 of the divergent frustoconical part 36 is placed at one of the upstream ends of the V-shaped part 35.

All of the equipment described above is dimensioned so as to present an optimum yield for the transformation of liquid carbon dioxide into a solid and gas mixture, taking into account, on the one hand, the thermodynamic conditions for storage of liquid CO2, because the pressure regulation of the liquid CO2 reservoir is carried out in a minimum-maximum pressure range as low as possible, which makes it possible to overcome flow fluctuations due to possible pressure fluctuations, and on the other hand, the characteristics of insulation specific to the installation so as to avoid any entry of heat and consequently limit the rate of two-phase, that is to say snow / carbon dioxide mixture (better control of the flow).

The above criteria therefore make it possible to determine the flow rates of liquid CO2 passing through the injector, itself calculated for a given pressure and for 100% of liquid.

By way of nonlimiting example, the characteristics of a lance are indicated below which make it possible to successfully implement the process which is the subject of the aforementioned French patent application.

This lance is sized to generate a solid CO2 flow corresponding to the transformation of 120 kg / min of liquid CO2, using an injector 72 with a diameter of 10 mm in the assembly 151 and an injector 72 with a diameter of 12 mm in the assembly 152.

The lance therefore allows 3 possibilities of flow rates:
When the valve 151 is opened alone, only the 10 mm diameter injector is used. The flow rate is then 80 kg / min of liquid CO₂.
When the valve 152 is opened alone, only the 12 mm diameter injector is used. The flow of liquid CO est is then 105 kg / min.
When the two valves are opened simultaneously, the two injectors are used and the liquid CO₂ flow rate is 120 kg / min.
Liquid CO2 storage takes place at a pressure of around 20 bar and a temperature of around - 20 ° C.

Between the liquid storage means and the T-piece, in the present example, a supply hose with a diameter of 2.54 cm (1 inch), a length of 30 m, is used, inducing a pressure drop of 5 bar. .
. For a flow rate of 120 kg / min (2 injectors simultaneously) of liquid CO₂, the volume of gas to be evacuated is around 40%, ie 30 m³ / min = 0.5 m³ / s.

The exit diameter of the lance at 60 is 150 mm and the gas exit speed is 30 m / s.
. For a flow rate of 80 kg / min (1 injector diameter 10 mm), the volume of the gas to be evacuated is 0.32 m³ / s, i.e. for a diameter leaving the lance of 150 mm a gas exit speed of 19 m / s.

The use of the lance for inerting a distributor is carried out according to the method described in the aforementioned French application. This lance thus allows in particular the implementation of the double flow recommended in said patent application.

There may be cases where the user needs a low jet energy to inert a metallurgical container where the thermal, capacity, surface / volume ratio effects are very different from the furnace - ladle or converter - ladle casting. A more homogeneous distribution of solid CO2 is therefore essential. This can be the case of the inerting of the surface of a pocket during treatment, of an induction furnace, etc.

In this case, it is preferable to use an injector with two calibrated orifices opposite at 180 ° C. and arranged tangentially to the wall of the tube of the part 73 in which it is housed to cause the gas and solid assembly to rotate ( the axial downstream end of the injector 72 is then closed and replaced by two divergent frustoconical openings, the axis of which is oriented tangentially with respect to the wall of the tube). The speed obtained is sufficient to avoid sticking of the particles on the wall of the barrel 56 of the barrel.

In addition, said rotation of the particles allows the faster reduction of the kinetic energy of the jet by increasing the pressure drops.

At the exit of the barrel, the solid particles in rotation are distributed in the form of a cone thus ensuring a more homogeneous distribution of the snow in the bottom of a distributor for example.

Claims (15)

1.- Dry ice lance intended, in particular, for inerting a molten metal, characterized in that it comprises a part (2) in the shape of a T, a first end (21) of which is intended to be connected to a source of liquid carbon dioxide supply via a flexible pipe and the second and third ends (22,23) of which are connected respectively to at least one liquid carbon dioxide transport line (31,33 ; 32,34) and to a controlled valve (151,152), the outlet orifices of the controlled valves (151,152) being interconnected by a V-shaped connecting piece (35) which opens into a drain pipe for the snow (51,56,58). 2.- carbon dioxide snow lance intended, in particular, for inerting a molten metal according to claim 1, characterized in that the snow evacuation pipe (51,56,58) comprises in its front part a diverging part of frustoconical shape (53) whose small base is located on the side of the V-shaped connecting piece (35) and whose large base is located on the side of the opening (61) for evacuation placed at the end of said snow evacuation pipe (51,56,58). 3.- Dry ice lance according to one of the preceding claims intended, in particular, for the inerting of a molten metal characterized in that the transport lines (31,33; 32,34) are rigid. 4.- dry ice lance according to one of the preceding claims, intended, in particular, for inerting a molten metal characterized in that the transport lines (31,33; 32,34) have a curved part according to a radius of the order of 3 to 5 times their diameter. 5.- carbon dioxide snow lance according to one of the preceding claims intended, in particular, for the inerting of a molten metal characterized in that the valves (151,152) are direct passage. 6.- Dry ice lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the valves (151,152) are controlled by an electro-pneumatic operator (71). 7.- carbon dioxide snow lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the valves (151,152) include an injector (72) which limits the flow of liquid CO₂. 8.- Dry ice lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the valve (151,152) is of the pivoting ball type and in that the injector (72) is placed near said ball. 9.- Dry ice lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the injector (72) has the characteristics of a venturi device. 10.- Dry ice lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the connecting part of the injector to the part (35) in the form of V is divergent. 11.- Dry ice lance according to one of the preceding claims intended, in particular, for the inerting of a molten metal characterized in that each branch of the V-shaped connecting part has a divergent end part, then constant section. 12.- dry ice lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the inlet section of the intermediate piece (51) is greater than or equal to the sum of the sections of the branches (37.40) of the V-shaped part (35) 13.- carbon dioxide snow lance according to one of the preceding claims intended, in particular, for the inerting of a molten metal characterized in that the intermediate part (13) is removable. 14.- carbon dioxide snow lance according to one of the preceding claims intended, in particular, for the inerting of a molten metal characterized in that the length of the part 13 is variable according to the speed of ejection of the gases. 15.- carbon dioxide snow lance according to one of the preceding claims intended, in particular, for inerting a molten metal characterized in that the end of the lance is a consumable, interchangeable and possibly maneuverable part.

Images