GB2162932A - Cooling metallurgical apparatus - Google Patents

Abstract

A lance, tuyere or other metallurgical apparatus is provided with an indirect cooling system using gas as the coolant. In the illustrated system a lance 30 has an outer cooling jacket forming part of a closed circuit in which heated gas emerging from the jacket is cooled by heat exchanger 32 and fed back to the jacket by a compressor 34 via a further heat exchanger 35. Suitable gases are He, N2, CO2, steam and air. <IMAGE>

Description

SPECIFICATION Metallurgical apparatus This invention relates to metallurgical apparatus and is particularly, but not exclusively, concerned with lances for use in metallurgical processes.

It is well known tousle gas, oil or solid fuel-fired lances to achieve or promote intense combustion in certain metallurgical processes. Where the process involves high operational temperatures, and the lance is to be submerged in, or otherwise contacted by, molten materials or exposed to high back radiation, it is necessary to cool the lance since otherwise it will be damaged. Traditionally, lances have been cooled by providing them ith an indirect cooling circuit such as a water jacket or coil through which cooling water is circulated. Alternatively, or in addition, the lance may be provided with a refractory sheath to shield it from the intense heat. In practice, both these arrangements have serious disadvantages. The use of a water coil or jacket is potentially very dangerous since any leakage of water could cause an explosion.The use of liquids, in general, reduces the problem only slightly. Refractory sheaths have the disadvantage that, in use, an accretion of material builds up on them.

A quite different method of cooling a lance has been used with some success. It involves utilising the gas flow through the lance for cooling purposes which provides so-cailed direct cooling. Thus, for example, air is pumped through the lance in very large volume with she dual intention of supplying oxygen for combustion or for the metallurgical process, while the nitrogen effectively cools the lance. There are a number of problems. The very large volume and velocity of the gas cause excessive splashing of the metallurgical materials (e.g. molten metals) and although some splashing may be advantageous, excessive splashing is not. Also, the large volume and velocity of the gas result in a poor heat balance.Furthermore, there is practically no possibility of lance turn-down, i.e. of reducing the amount of gas passing through the lance to the metallurgical process, since any such reduction will. almost inevitably result in inadequate cooling of the lance. Lance turn-down is an important operational control in the use of lances, and a turn-down ratio of up to 5:1 is used in water-cooled lances. The impossibility of achieving this with the enhanced gas-feed cooling technique is a serious disadvantage of the procedure.

We have now devised a new way of effecting cooling of metallurgical apparatus such as lances, by which the disadvantages noted above of the liquid cooling jacket technique and of the enhanced gas-feed technique are substantially reduced or overcome.

In accordance with the present invention, a lance or other metallurgical apparatus such as a tuyere, is cooled by providing it with an indirect cooling circuit, such as a jacket or coil, and pumping the cooling gas through the circuit. It is surprising that adequate cooling can be achieved in this way, since very large amounts of heat are involved and it is well known that liquids are better conductors of heat than gases and that large mass flows of gas are required in the known enhanced gas-feed cooling technique to achieve the desired control of lance temperature.However, despite these contra-indications, we have found that gas indirect cooling can be operated very satisfactorily, without the risk of explosion inherent in the prior art use of coolant liquids, and with the possibility of lance turn-down (which is practically impossible with the prior known enhanced gas feed technique).

The invention also includes metallurgical apparatus such as a lance or tuyere for example, which comprises a jacket or coil and means for circulating a coolant gas through the jacket or coil to cool the apparatus.

The invention will hereafter be described with reference to lances, but it is to be understood that it can be applied to other apparatus, used in metallurgical operations, where cooling is required.

In the use of a lance, the end portion from which the operating fluids (e.g. oxygen, natural gas etc.) are emitted is exposed to the greatest heat and thus requires the most cooling. In accordance with a preferred feature of the invention, the cooling circuit includes one or more channels or pipes to conduct the coolant gas to the said end portion, and one or more further channels or pipes to guide it away from the end portion. One simple way of achieving this is to provide an annular cooling jacket, with a cylindrical dividing wall therein to divide the inside volume of the jacket into an inner and an outer region, the two regions meeting at the forward end portion of the lance. Thus, gas may be pumped to the forward end portion along, for example, the inner region of the jacket, and returned away from the forward end along the outer region of the jacket.This is merely one possible arrangement: many other different arrangements can equally be used.

Preferably, the lances of the invention have an outer gas-cooling jacket. They may also include one or more further jackets or coils internally of the lance, but these are not usually necessary.

The preferred gases for use in the lances of the invention are helium, nitrogen, carbon dioxide, steam and air, although bthers such as moisture-saturated air or other gases (and mixtures of gases) may be used. Of these, helium has a very high specific heat and good thermal conductivity and is thus preferred although, of course, it is expensive. Hydrogen may also be used as an excellent gas coolant. In the event of jacket rupture, the danger of any explosion is substantially less than with a liquid coolant, but this use of hydrogen is less safe due to the combustibility of the gas than the use of the previously mentioned gases.

In one preferred embodiment of the invention (which is a practical necessity when expensive gases are used as coolants) the coolant gas is-confined and recirculated within a closed system which includes a cooling jacket The system will also comprise one or more heat-exchangers to remove heat from the gas, and a compressor to circulate the gas around the system. Since the compressor will itself heat the gas, a heat-exchanger may be provided downstream of the compressor especially to cool the gas after compression. Preferably, a closed system is maintained at a pressure slightly above atmospheric to preclude any possibility of ingress of air.

It is not essential to use a closed system: an open system may be employed in which, for example, coolant gas is pumped around the cooling jacket and then vented to atmosphere (or otherwise utilised but not recirculated to the lance). A heat exchanger would preferably be provided downstream of the compressor, to cool the gas after compression, the cooled gas then being passed through the jacket of the lance. The heated gas exiting from the lance jacket could be used as such for some purpose, or passed through a heat exchanger to recover thermal energy therefrom.

The method of the invention is particularly useful in lances used to apply heat in a metallurgical process either by the supply of oxygen or another combustion reactant such as a hydrocarbon or hydrogen, or a mixture of oxygen and a combustible material. The lances may advantageously be used, for example, for submerged combustion or otherwise in metallurgical furnaces, top blown rotary converters, ladles or metallurgical pool reactors, for example.

With the lances of the invention, turn-down is of course possible since the cooling rate achievable by the indirect gas cooling circuit is not affected by the rate of supply of reaction fluids through the lance itself. This is a very important feature of the present invention, because it allows the lance to be used for oxygen/fuel combustion purposes (with a turn down of, for example 3:1 for submerged use), whereas the prior known enhanced gas-feed lances could only be used for air/fuel combustion, or at best with rriinimal oxygen enrichment of the air, and only then with very little turn-down facility. This.use of lances of the present invention enables various very high temperature metallurgical processes to be carried out under control and with safety.

In order that the invention may be more fully understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a section through the end region of one embodiment of lance of the invention; and Figure 2 is a schematic illustration of one embodiment of closed loop system for the coolant gas.

Referring to the drawings, the end region of the lance shown in Fig. 1 comprises a central cylindrical duct 1 and, arranged co-axially with this duct 1, a tube 2. The annular space between duct 1 and tube 2 forms a gas supply channel 3. The end (left-hand side in Fig. 1) of duct 1 and channel 3 are closed by a block 4 including a series of nozzle bores 5 through which gases supplied along duct 1 and channel 3 exit from the lance into region 6 where combustion may occur.

Tube 2 extends slight beyond block 4 and is joined to a coaxial tube 7 by an annual end piece 8. Tubes 2 and 7 and end piece 8 define a cooling jacket of the invention. Within this jacket is a partition in the form of a cylinder 9 which divides the inside volume of the jacket into an inner region 10, and an outer region 11, with an end region 1 2.

The letters A, B, C and D on Fig. 1 indicate the respective diameters of the duct 1, tube 2, cylinder 9 and tube 7, and various dimensions are typically as follows: Pipe Size SCH OD Wall thickness A 1 inch 40 1.315 inch 0.133 inch B 2 40 2.375 0.154 C 3 80 3.5 0.30 D 4 80 4 5 .337 In an example of the use of the lance, a gas such as hydrocarbon is passed in duct 1, and oxygen is passed in channel 3, both in the directions of the arrows shown. Both pass through respective nozzles 5 to mix and combust in region 6. The end of the lance is, of course, in a high temperature environment (e.g in a furnace) and the gas-cooling jacket serves to cool the lance. Cooling is effected by pumping helium along region 10, around region 1 2 and back along region 11, in the arrowed directions.Given a sufficient flow of gas at an appropriate temperature, enough heat is removed from the end region of the lance to achieve a satisfactory cooling thereof. For example, when the lance is used to inject oxygen and natural gas into a furnace operating at 1 200 to 1400"C, a typical heat flux to the cooling jacket is about 30,000 B.T.U's per hour per square foot. The metal of the lance must be kept at below about 450"C, for the use of normal materials such as carbon steel or stainless steel, and this can be achieved by the present invention. Generally, the temperature of the (cool) helium gas as supplied to inner region 10 will be no higher than about 50"C and the flow rate will be about 330 Ibs helium per hour.

It is a preferred feature of the invention to so design and position the dividing cylinder 9 as to achieve the desired gas flow rates at local regions in the jacket. Thus, where the greatest cooling effect is needed, and taking account of the gas temperature, the cylinder 9 will be positioned to provide the appropriate cross-sectional area for the desired flow rates in the regions 10, 11 and 1 2 as is desired. As will be appreciated, reductions in cross-section will cause increased coolant gas flow velocities, and vice versa.In considering the desired flow rates and velocities and the overall cooling effect required in different parts of the lance, it should be borne in mind that, for example, the oxygen and other gases and/or oils or particulate carbonaceous materials passed throngh the lance into the metallurgical process should not be excessively preheated by the coolant jacket since otherwise the oxygen may react with the metal of the lance itself or the oil suffer coking, for example.

Fig. 2 shows schematically a closed gas loop system which is suitable when helium is used as the coolant gas. The loop includes the lance 30 in which the helium is passed through the cooling jacket or coil, and from which the heated gas exits via line 31 to the first heat exchanger 32. Between lance 30 and heat exchanger 32, the gas may typically be at about 240-250"C and at a pressure of 7 psig. The temperature of the gas exiting heat exchanger 32 may typically be about 40"C, the pressure being about 1 psig. The gas from heat exchanger 32 then passes via line 33 to compressor 34, and from there to a second heat exchanger 35, where heat generated during the compression step is removed.The gas may typically enter heat exchanger 35 at about 130-140"C and 16 psig, and leave at about 50"C and 11 psig. From exchanger 35, the cooled gas is returned to the cooling jacket of the lance. At some point in the loop, preferably on line 33, a helium reservoir 50 is provided to keep the system topped-up.

Typically, the secondary fluid in heat exchanger 32 would rise from about 20"C to 40"C, and in heat exchanger 35 from 20 to 40"C.

In the static state, the system would normally be at a pressure of about one psi above atmospheric or a small pressure in excess of atmospheric, to preclude dilution of the helium gas from in-leakage of air. The compressor (34) provides the motive power for the circuit and, in compression, generates some heat manifest in the outlet temperature of the helium gas. The heat exchanger (35) cools the helium gas prior to delivery to the lance, to less than about 50"C.

In passage through the lance, the helium may pick up approximately 140,000 B.T.U's per hour, which raises the helium temperature to about 240"C. With this design, it is possible to keep the metal temperature of the lance down to about 350"C which enables the selection of normal materials of construction such as carbon steel, 304 or 31 6 stainless steel. The heat picked up in the lance is then dissipated through the heat exchanger (32) which is preferably a water cooled exchanger which drops the helium temperature on discharge to approximately 40"C. In operating the complete circuit, pressure is lost through heat exchanger 35, the lance and the heat exchanger (32) making a total pressure loss of approximately 1 5 psi. With the configuration proposed the lance may be operated with no more than a 4 psi pressure differential. The above temperature pressures and heat flow rate are given by way of illustration only.

Claims (14)

1. Metallurgical apparatus having an indirect cooling circuit, and means for pumping a coolant gas through said circuit.

2. A metallurgical lance or tuyere apparatus which comprising an indirect cooling circuit comprising a jacket or coil for the circulation of coolant gas therethrough.

3. A lance according to claim 2, wherein the cooling circuit includes one or more channels or pipes to conduct coolant gas to an end of the lance, and one or more further channels or pipes to conduct the gas away from said end.

4. A lance according to claim 2 or 3, which has an outer said jacket or coil.

5. Apparatus according to any of claims 1 to 4, which comprises a closed recirculatory system for said coolant gas, said system including said circuit, at least one heat exchanger and at least one compressor.

6. Metallurgical apparatus substantially as herein described with reference to Fig. 1 or 2 of the accompanying drawings.

7. A method of cooling metallurgical apparatus comprising an indirect cooling circuit which comprises passing a coolant gas through said circuit.

8. A A method according to claim 7, wherein said apparatus comprises a lance or tuyere provided with a cooling jacket or coil.

9. A method according to claim 8, wherein the gas comprises helium, nitrogen, carbon dioxide, steam, or air.

10. A method according to claim 7; 8 or 9, wherein a mixture of coolant gases is used.

11. A method according to claim 7, 8, 9 or 10, wherein the coolant gas is circulated around a closed system including said indirect cooling circuit, a compressor and a heat exchanger.

12. A method of cooling metallurgical apparatus substantially as herein described with reference to Fig. 1 or Fig. 2 of the accompanying drawings.

13. A metallurgical process wherein there is used a cooled metallurgical apparatus as claimed in any of claims 1 to 6.

14. A process according to claim 13, wherein the apparatus is a gas-feed lance and a turndown ratio of up to 5:1 is used.