GB2259976A - Electric arc furnace bottom stir tuyere assembly - Google Patents

Abstract

A tuyere assembly (18) for injecting gas for stirring the melt in an an electric arc furnace (10) as initially installed extends above the furnace bottom (14) so as to avoid erosion of the furnace bottom (14) by the currents produced in the melt by the stirring gas. Preferably, the tuyere assembly (18) includes a tuyere block (20) retained in the furnace bottom (14) and a tuyere brick (22) removably received in said tuyere block (20) and containing a conduit (24) for the introduction of the stirring gas. <IMAGE>

Description

ELECTRIC ARC FURNACE BOTTOM STIR TUYERE ASSEMBLY This invention relates to an improved means for stirring the contents of an electric arc furnace.

Various means have been proposed for stirring the molten metal contained in electric arc furnaces. In general these fall into two main groups, namely: devices which utilize magnetic or electromagnetic means to circulate the molten metal and devices in which gas is introduced into the furnace through a tuyere or other gas injection device.

The use of magnetic or electromagnetic stirring devices requires expensive and complex additional equipment and is undesirable from a cost standpoint. Tuyeres or lances installed in the furnace lining produce the desired agitation, however, the stirring campaign is typically less than desired due to molten metal penetration and/or the formation of a metal/slag/refractory crust above the tuyere. When the gas flow passage is sufficiently large such that the pressure of the gas exiting the tuyere does not exceed the local pressure due to the weight of the metal bath, gas bubbles form and release at the tuyere tip.

With the release of each bubble, molten metal penetrates and solidifies inside the tuyere until the passage is either completely blocked (requiring oxygen blowing to reopen) or has formed a restriction which enables highly unstable continuous flow at a threshold sonic velocity.

This flow regime has been referred to as incipient jetting and is believed to cause rapid erosion of the local refractory as discussed in a paper entitled Gas Iniection Tuyere Design and Experience by L. Farmer, D. Lach, M.

Lanyi and D. Winchester published in the 1989 Steelmaking Conference Proceeding, pp.487-495, by ISS-AIME. When a sump or depression develops around a tuyere due to local erosion and/or hearth repair, the tuyere is prone to being covered by crust consisting of one or more of the following: metal, slag, and refractory repair material.

Once covered, there is no visible sign of stirring and, presumably, no benefits of stirring are derived during this period. This condition continues until the crust is melted away allowing stirring to resume. This intermittent stirring is highly unpredictable and therefore undesirable for consistent operation.

The present invention provides a means for avoiding the expense of installing electromagnetic stirring means and for avoiding the intermittent blockage which has been experienced with prior art gas injection devices installed in the furnace lining.

The invention provides an improved means for injecting gas into an electric arc furnace which avoids the formation of a sump or pit when gas is injected into the melt and in which an open passageway for the introduction of gas into the melt is available at all times.

The invention also provides gas injection means which is readily installed in a furnace bottom and which has a long service life.

According to a first aspect, the present invention provides an electric arc furnace which includes a refractory lined bottom and at least one tuyere assembly set in said bottom for introducing a stirring gas, wherein the tuyere assembly extends a sufficient distance above the bottom to avoid erosion of said bottom by introduction of said gas.

According to a second aspect, the invention provides a method of melting metal in an electric arc furnace having a refractory lined hearth in which said furnace is charged with metal and a melt is formed in said furnace hearth, and said melt is stirred by introducing a stirring gas into said melt through a conduit disposed in a tuyere assembly located in the refractory lining in the hearth, wherein the tuyere assembly extends a sufficient distance above the bottom to avoid erosion of said bottom by introduction of said gas.

Usually, the tuyere assembly extends at least 2.5 cm (1 in) above the refractory lined bottom and preferably between 2.5 cm (1 in) and 10 cm (4 in) above said refractory furnace bottom at the time it is originally installed.

Advantageously, the tuyere assembly includes a tuyere block retained in the furnace bottom and a tuyere brick removably received in the tuyere block and containing a conduit for the introduction of the stirring gas.

Preferably, the tuyere block has a generally trapezoidal cross-sectional shape tapering in the upward direction (relative to the furnace bottom). It also is preferred that the tuyere brick is connected to a gas supply pipe which extends downwardly through said furnace bottom. The latter arrangement permits the tuyere brick to be readily separated from the furnace bottom by pushing upwardly on said supply pipe, to move said pipe and tuyere brick out of said furnace bottom as a single unit when it is to be replaced. The tuyere block suitably does not extend above the furnace bottom or extends above the furnace bottom but not above the tuyere brick.

The stirring usually will be argon or nitrogen. It is preferred that the tuyere opening is sized to operate such that the stirring gas exists as an underexpanded jet.

The following is a description by way of example only and with reference to the accompanying drawings of a presently preferred embodiment of the invention. In the drawings: Figure 1 is a fragmentary cross-section of a furnace with a tuyere constructed in accordance with the present invention; Figure 2 is an enlarged fragmentary view of the tuyere of Figure 1, and Figures 3(a) to 3(e) illustrate one possible mechanism which may cause intermittent loss of stirring in prior art devices.

As shown schematically in Figure 3(a), a prior art stirring device or tuyere comprises a gas injecting pipe 1 embedded in a refractory sleeve 2 set in refractory bricks 3 and a refractory lining 4. As installed, the tuyere 1,2 is flush with the top of the lining 4 and when the tuyere is first installed good stirring occurs when gas is fed through the pipe 1. As successive heats are melted in the furnace, a combination of loose refractory material falling into the area of the tuyere during furnace repair between heats, and erosion in the area adjacent to the exit tip 5 of pipe 1, possibly due to the currents set in motion as the metal is stirred by the gas injected through pipe 1, results in the condition shown in Fig. 3(b).

Between heats the lining is repaired e.g., by shovelling or gunning loose refractory onto the furnace hearth or bottom, and this refractory plus any residual molten metal or slag in the furnace can form a crust 6 and produce the condition shown in Figure 3(c). This blocks the exit from pipe 1 and no stirring occurs until an opening 7 is created e.g., either by permitting the heat supplied to the furnace to cause remelting of the crust 6, as shown in Figure 3(d), or oxygen can be introduced into pipe 1 to open a passage through the crust 6.

Eventually the blockage becomes more severe, e.g., because of a skull 8 of metal being formed on top of the crust 6. This condition is shown in Figure 3(e). As the blockage progresses there is no indication of a problem because gas continues to flow through the pipe 1 and the back pressure does not elevate until the initially porous skull 8 becomes dense. It was observed that once the blockage occurred, the gas would flow but stirring of the metal could not be observed and therefore it is believed that the gas was not stirring the melt possibly because it was entering the furnace over an extensive area through widely dispersed cracks, fissures, and pores.

To avoid this sequence of events the present invention provides a tuyere shown in Figures 1 and 2. As shown in Figure 1, a furnace 10 having a lining 12 of refractory brick as is well known in the art has a hearth 14 of refractory material which may be rammed in place. Furnace 10 may also include rammed refractory base 16 for the brick work 12. According to the invention, a tuyere assembly 18 is positioned in the bottom of furnace 10 with a portion projecting above the hearth 14. Referring to Figure 2 the tuyere assembly 18 includes a chrome magnesite tuyere block 20 having a cross-sectional shape in the form of a trapezoid set in the lining or hearth 14 as the remainder of the lining is laid down on the furnace hearth.Tuyere block 20 is so constructed and arranged to receive a tuyere brick 22 made from a refractory material (e.g., MgO-Graphite mixture) suitable for being formed around the small diameter stainless steel tubular nozzle pipe 24.

Nozzle pipe 24 is in turn connected to a larger diameter gas supply pipe 26 as by a circumferential weld 28 at the upper end 30 of supply pipe 26. This construction is designed so that when the tuyere brick 22 erodes or the nozzle pipe 24 becomes blocked and can not be cleared by oxygen injection or there is a routine change of the tuyere, the tuyere brick 22, nozzle and pipe assembly 24-32 may be removed easily from the furnace by pushing on the lower end 32 of the supply pipe 26 to move tuyere brick 22 and nozzle and pipe assembly 24-32 upwardly into the furnace hearth area. A new nozzle and pipe assembly is then installed by inserting a new tuyere brick 22 and pipe assembly 24-32 into the opening created by removal of the used assembly from inside the furnace.Tuyere brick 22 and alternately tuyere block 20 and tuyere brick 22, initially extend approximately at least one inch (2.5 cm) and preferably four inches (10 cm) above the hearth lining 14 to optimize the mass of block 20 projecting above hearth 14. The lower end 32 of supply 26 is adapted for connection to a source of stirring gas such as argon or other inert gas by conventional flexible hose connection as known in the art and therefore not shown.

As shown in Figures 1 and 2, tuyere brick 22 typically is installed so its top extends above the top surface of the hearth 14 a distance of two (2) to four (4) inches (510 cm). Based upon a wear rate of about .02 inches (0.5 mm) per heat the tuyere of the invention will survive for more than 100 heats. When the tuyere brick 22 erodes, or the hearth lining surface 14 builds up so that it is now flush with the top surface of the tuyere brick 22, the tuyere brick 22 and nozzle and pipe assembly 24-32 should be replaced. As a result of this construction, sump formation is avoided and a continuous flow of stirring gas has been found to be ensured.

Additionally, the tuyere opening is sized to operate such that the gas exits the tuyere as an underexpanded jet.

In this mode, the gas pressure upon exiting the tuyere exceeds the local pressure due to the weight of the liquid metal. Upon exiting the tuyere, the gas will accelerate as it expands to the local pressure. The tuyere is designed so that the gas velocity at the tuyere exit is sonic (Mach No. = 1.0) and upon expanding outside of the tuyere, supersonic velocities are attained (Mach No. > 1.0). Our studies have shown that at fully expanded velocities greater than Mach No. = 1.25, a very stable jet is produced and the degree of local erosion is minimized.

Employing an elevated tuyere according to the invention will enable steady and continuous operation of sufficient duration.

It will be appreciated that the invention is not limited to the specific description above with reference to the drawing and that modifications and variations can be made without departing from the invention as defined in the following claims.

Claims (18)

1. An electric arc furnace which includes a refractory lined bottom and at least one tuyere assembly set in said bottom for introducing a stirring gas, wherein the tuyere assembly extends a sufficient distance above the bottom to avoid erosion of said bottom by introduction of said gas.

2. An apparatus as claimed in Claim 1, wherein said tuyere assembly extends at least 2.5 cm (1 in) above said refractory lined bottom.

3. An apparatus as claimed in Claim 2, wherein said tuyere assembly extends a distance between 2.5 cm (1 in) and 10 cm (4 in) above said refractory furnace bottom at the time it is originally installed.

4. An apparatus as claimed in any one of the preceding claims, wherein said tuyere assembly includes a tuyere block retained in said furnace bottom and a tuyere brick removably received in said tuyere block and containing a conduit for the introduction of the stirring gas.

5. An apparatus as claimed in Claim 4, wherein said tuyere block has a generally trapezoidal cross-sectional shape tapering in the upward direction (relative to the furnace bottom).

6. An apparatus as claimed in Claim 4 or Claim 5, wherein said tuyere brick is connected to a gas supply pipe which extends downwardly through said furnace bottom.

7. An apparatus as claimed in Claim 6, wherein said tuyere brick is readily separated from the furnace bottom by pushing upwardly on said supply pipe, to move said pipe and tuyere brick out of said furnace bottom as a single unit when it is to be replaced.

8. An apparatus as claimed in any one of Claims 4 to 7, wherein the tuyere block does not extend beyond the furnace bottom.

9. An apparatus as claimed in any one of Claims 4 to 7, wherein the tuyere block extends above the furnace bottom but not above the tuyere brick.

10. An apparatus as claimed in any one of the preceding claims, wherein the tuyere opening is sized to operate such that the stirring gas exists as an underexpanded jet.

11. An apparatus as claimed in Claim 1 and substantially as hereinbefore described with reference and as shown in the drawings.

12. A method of melting metal in an electric arc furnace having a refractory lined hearth in which said furnace is charged with metal and a melt is formed in said furnace hearth, and said melt is stirred by introducing a stirring gas into said melt through a conduit disposed in a tuyere assembly located in the refractory lining in the hearth, wherein the tuyere assembly extends a sufficient distance above the bottom to avoid erosion of said bottom by introduction of said gas.

13. A method as claimed in Claim 12, wherein said tuyere assembly extends at least 2.5 cm (1 in) above said hearth.

14. A method as claimed in Claim 13, wherein said tuyere assembly extends a distance between 2.5 cm (1 in) and 10 cm (4 in) above said hearth at the time prior to said furnace being used for a first melt after installation of said tuyere assembly.

15. A method as claimed in any one of Claims 12 to 14, wherein the tuyere assembly is as defined in any one of Claims 4 to 11.

16. A method as claimed in any one of Claims 12 to 15, wherein the gas is argon.

17. A method as claimed in any one of Claims 12 to 15, wherein the gas is nitrogen.

18. A method as claimed in Claim 12 and substantially as hereinbefore described with reference to the drawings.