GB1571484A - Process for melting metal in a vertical shaft furnace - Google Patents

Description

(54) PROCESS FOR MELTING METAL IN A VERTICAL SHAFT FURNACE (71) We, BOC Limited, an English company of Hammersmith House, London, W6 9DX, England, do hereby declare the invention. for which we pray that a patent may be granted to us and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a process for melting metal in a vertical shaft furnace.

In a vertical shaft furnace metal is melted bv the heat evolved when fuel is burnt in the furnace. The molten metal so formed sinks to the bottom of the furnace, from where it can be tapped off. The furnace is regularly recharged so as to keep it operating continuouslv. A blast of air is supplied so as to support combustion of the fuel which is usuallv coke. However, another fuel such as oil or gas can be used instead of or in addition to coke.

It has long been known that the addition of pure oxygen to the air blast whilst the latter is travelling through the main from which it is distributed to the tuyeres of the furnace offers many advantages. One of the advantages is that a reduction in the fuel (usually coke) required per ton of metal melted may be achieved. Thus, the hourly production of molten metal may be increased. or. if desired, the hourly consumption of coke may be reduced. It is also possible to balance the rates of metal production and fuel consumption so that a little more molten metal is formed with a slight but significant saving in fuel. Another advantage is that by enriching the air blast in oxygen the molten metal can be raised to a higher temperature.

Notwithstanding the benefits to be gained from the enrichment in oxygen of the air blast some. but by no means all, operators of furnaces had formed the view that this technique did not have sufficient commercial utility. However. the recent steep increases in the price of coke have made the process fully viable on a commercial scale.

Moreover, since the enrichment in oxygen of the air blast was first practised even more attractive processes using substantially pure oxygen have been found. These processes are based on the direct injection of oxygen into the interior of the furnace in a discrete stream or jet. In UK patent specification 914 904 there is described a process in which oxygen is fed into the cupola through a separate set of tuyeres located below the tuyeres (i.e. nozzles) through which the air blast is introduced into the furnace. By this means regions of the charge not impinged upon by the air blast can be raised to a temperature higher than that of their surroundings.

The same result may be achieved by injecting the oxygen into the charge through nozzles which extend into the main air tuyers and which are at an angle to the plane in which their axes lie. This process is described in UK patent specification 1 006 274. It has been found that both these so-called "direct injection" processes offer, for example, savings in the consumption of fuel of up to 40% greater than those achieved with the so-called "blastenrichment" process.

Another "direct injection" process which is an improvement over blast enrichment involves a modification of the process described in UK specification 1 006274. In this modified process oxygen is injected into the furnace through nozzles which are locally co-axially within the main air tuyeres.

Streams of pure oxygen are injected directly into each tuyere at a velocity greater than that of air. In this manner a localised concentration of pure oxygen is created in the area of combustion. This process, too, offers considerable advantages over the method of blast-enrichment.

The above-mentioned techniques of direct injection of oxygen into the furnace have been adopted recently on a large scale by operators of cupolas in the United Kingdom. However. for these techniques to be fully successful considerable skill and know-how is required in the installation and operation of the equipment for injecting oxygen into the furnace. For instance. the installation of the separate tuyeres which are used in the process described in patent specification 914 904 requires particularly careful engineering.With injection of ox ygen through nozzles in the main air tuvers there is a risk of the lining of the furnace becoming eroded at regions adjacent the tu,'eres if the nozzles are not correctlv designed. Nloreover, the erosion by heat of the nozzles themselves can give rise to problems.

Nluch recent work has been directed at improving - processes involving the direct injection of oxygen into vertical shaft furnaces. Such processes are almost universally acknowledged to be superior to the more traditional technique of enriching the air blast in oxygen. It is generally believed that this superiority resides in the abilitv of the direct injection process to create local concentrations of pure oxygen in the combustion region of the furnace. Such local concentration of pure oxvgen cannot be achieved bv the blast-enrichment process as the air and the oxygen are thoroughly mixed by the time the blast enters the tuyeres.

The present invention relates to a new process for melting metal in a vertical shaft furnace which we believe to be capable of being operated as efficiently as the abovementioned "direct injection'' processes but which does not involve the creation of localised concentrations of pure oxygen in the combustion region of the furnace. Indeed. the process according to the present invention can be viewed as being more akin to the previously mentioned traditional method of blast-enrichment than it is to direct-injection processes.

According to the present invention there is provided a process for melting metal in a vertical shaft furnace including the steps of distributing blast air from a main to several conduits which connect the main to separate tuyeres through which the air is introduced into the furnace. and increasing the concentration of oxygen in the air leaving some but not all of the tuveres bv selecting some of the tuyeres and supplying oxygen to them, which oxygen mixes fullv with the air in the selected tuyeres before the air leaves such tuyeres in them mixing oxygen fully with the air passing therethrough.

Typically, rather than introducing oxygen into the selected tuyeres it may be introduced into those conduits which connect such tuveres to the main. the introduction being effected through lances or diffusers which are positioned with their outlets terminating upstream of the tuyeres in the selected conduits.

Each diffuser may be able to distribute the oxygen supplied to it in a plurality of streams. The diffuser may typically take the form of a perforated cylinder or a perforated cone. The streams of oxygen which flow out of the cylinder or cone readily mix with the air blast. However, if desired, the oxygen may be passed into each selected conduit from a lance which has a single outlet, provided that the oxygen becomes mixed fully with the air before the latter leaves its tuyere. In order to ensure that this takes place the oxygen should be fed to the lance at a pressure well in excess of that of the air blast. As a result the oxygen undergoes a drop in pressure as it leaves the lance.

This causes it to form a divergent stream or divergent jet. Because the oxygen is diverging it readily becomes mixed with the air blast in its respective conduit.

It is important to position each lance sufficiently upstream of the inlet of its associated tuyere for the oxygen to become fully mixed with the air blast before it leaves the tuyere. If each lance is positioned coaxially with its associated tuyere and if the outlet of the lance terminates too near to the inlet of its associated tuvere there will be a tendency for the oxygen to leave the tuyere in the form of a discrete divergent stream, relatively unmixed with the air blast, which creates in the region of the lining of the furnace immediately surrounding the tuyere a local concentration of oxygen that will give rise to a higher-than-average localised temperature. Such a relativelv high temperature may erode the lining of tie furnace to such an extent that a new lining will be required in the region of the tuyere.It is possible to use a steel lance, which if it is positioned such that it protrudes into its associated tuyere, will be eroded by virtue of the high temperature that obtains in the tuyeres. In time. however. there will be no further damage to the lining caused by the stream of oxygen. Nevertheless. in the period in which the lance is relatively uneroded sufficient damage may be caused to make necessary a shut-down of the furnace in order to prepare it for relining.

If the air blast is distributed from the main to an even number of tuyeres, and hence to an even number of conduits oxygen is preferably introduced into every other tuvere. If the furnace is designed to melt ferrous metals it will typically have six, eight, ten or twelve tuyeres through which the air blast is introduced into the furnace.

Therefore. preferably three. four, five or six tuyeres respectively are selected for the introduction of oxygen. If there is an odd number (typically up to 11) of tuyeres (and hence an odd number of conduits) oxygen is preferably introduced into that number of tuyeres which is the integer nearest to, but greater than, the total number of tuyeres through which blast air alone is introduced into the furnace. Thus, if there are seven tuyeres. oxygen should prefeably be introduced into four of them. Furnaces designed to melt non-ferrous metals may, however have up to 1()() tuyeres.Frequently the number of such tuyeres may be more than 70. In such furnaces the oxygen may be introduced into any chosen number of tuyeres. Typically, however. the oxygen may lie introduced into from 2 to 10 tuyeres.

Preferably oxygen is introduced into each selected tuyere ot a rate up to a tenth or a fifth (bv volume) of that at which air passes theretlirough. Suppose that a furnace has eight tuyeres and every other tuyere is selected for the introduction of oxygen. Say, it is desired that the total rate of introduction of oxygen into the furnace should be one twentieth (by volume) of that at which the air blast is passed into the furnace. It will then be necessary to introduce oxygen into each selected conduit at a rate of one-tenth of that at which the air blast passes therethrough. It will be appreciated that an equivalent quantity of oxygen could be introduced into the main at one-twentieth the rate at which air passes therethrough.

However, bv choosing the process of selective introduction of oxygen according to the present invention there can be achieved an improvement in the efficiency of the furnace approximating to. or matching, that which it has previously been believed could be gained onlv bv the direct injection of oxygen into the furnace. Moreover. this level of efficiencv can be achieved by the process according to the invention without the engineering problems that are caused by the injection of oxygen through separate tuyeres. installed below the main air tuyeres. or without the risk of erosion of the lining of the furnace or of the lance used to inject the oxygen.Such erosion has, in practice, occurred when oxygen is introduced directly into the furnace in discrete streams through the main air tuyeres. It is also to be appreciated that any risk of severe local overheating or local explosion caused bv a local concentration of pure oxygen in the furnace is avoided bv the process according to the present invention.

In most conventional vertical shaft furnaces there is a blower which creates the air blast and supplies the air to a ring main. The furnace is surrounded by this ring main which is usually positioned above the level of the tuyeres. The conduits which connect the tuyeres to the ring main are referred to as "downdrops" and have a generally vertical limb. the top of which terminates in the ring main, and a generally horizontal limb which supplies the air blast to its respective tuyere. It has been previously found that direct injection of the oxygen into the furnace through a tuyere requires a lance to be inserted through the horizontal limli of the downdrop. Sometimes there may be no convenient wall through which the lance can be inserted and it will be necessary to form an aperture through which the lance may be inserted.Each additional seam or joint may not be entirely air-tight. In the process according to the present invention the lance or diffuser may be inserted through either limb of the downdrop. Moreover, the lance or diffuser may be situated anywhere in each downdrop so long as the oxygen is mixed with the air before it leaves its associated tuyere. In consequence it may often prove possible to insert the lance or diffuser through an existant seam or other form of joint between walls of the downdrop without the need to create an entirely separate hole for it.

Thus, by avoiding creating an extra seam or joint the process according to the present invention may in practical terms give rise to a leakage of air less than that which sometimes occurs with direct injection of oxygen through the tuyeres. In this respect, therefore, the process according to the present invention may be more efficient than a comparable process in which oxygen is injected directly through the tuyeres. It will be appreciated that if blast enrichment is carried out in the main any leakage that does take place through the downdrops will be particularly serious as part of the added oxygen will be lost by virtue of the leak.

It is preferred to introduce the oxygen continuously (rather than intermittently) into the selected tuyeres during operation of the furnace. The oxygen may be distributed to the conduits from any convenient source of supply. Many vertical shaft furnaces have a capacity such that oxygen is best supplied thereto from a "tonnage" plant in which oxygen is separated from air by rectification at cryogenic temperatures. Although it is not always essential that the oxygen supplied to the selected conduits be pure, any nitrogen that it not extracted therefrom in a plant which separates oxygen from air will fail to have any beneficial effect. Thus, to supply such nitrogen is merely a waste of energy. The oxygen may if desired be supplied to its own ring main to which the diffusers or lances are connected.

The process according to the present invention is particularly suited for the melting of ferruous metal in a hot-blast or cold blast cupola. It may also be performed in a blast furnace or in any vertical shaft furnace employed to produce non-ferrous metals such as lead, copper and antimony.

Manv if not all of the advantages of the process according to the present invention may be achieved if mixing of the oxygen with the air takes place in tuyeres itself.

Indeed. if the oxygen is introduced into selected tuyeres through perforated diffusers actually situated in the tuyeres themselves it may be possible to achieve sufficient mixing of the air and oxygen before the stream enters the combustion region of the furnace.

Accordingly, in association with each selected tuyere there may be an oxygen diffuser which has many small apertures or small perforations in its walls, at least part of each diffuser extending into its associated tuyere. Oxygen may be introduced into the selected tuyeres from the diffusers in the form of fine streams, whereby the oxyen and the blast air become mixed before they enter the combustion region of the furnace.

The diffusers positioned in the tuyeres should desirablv be made of a metal which is not eroded by the temperatures that obtain therein. Copper is such a metal.

Each diffuser may be located coaxially within its associated tuyere or may be located parallel or at an angle to the axis of the tuvere.

The process according to the present invention will now be described by way of example with reference to the accompanying drawings in which: F4tiiie 1 is a side elevation, partly in section of the lower part of a cupola adapted to perform the process according to the present invention: Figure 9 is a section through the line II - II in Figure 1: Figlçre 3 is a schema illustrating apparatus for supplying air to the cupola; and Figllre 4 is a schema illustrating apparatus for supplying oxygen to the cupola.

Referring to the drawings a cupola 2 is provided in the usual manner with a hearth 4 having tap hole 6 for the removal of molten metal from the cupola. Cupola 2 also has a hole 8 which is situated above the hole 6 so as during operation of the furnace to communicate with the layer of slag that forms above the molten metal and below the main charge. Slag may therefore be tapped off from time to time or continuously through the hole 8.

Above the hole 8 is situated a ring of eight tuyeres 10 which are adapted to introduce air through the wall 12 of a cupola into the region in which combustion takes place during operation of the furnace. The tuyeres 10 are equally spaced about the periphery of the wall 12 and their axes all lie in the same plane. The tuyeres as shown in Figure 1 have walls which converge in the direction of the interior of the furnace. The tuyeres may be provided with water cooled copper nozzles.

The tuyeres 10 are connected to a ring main 18 by downdrops 16. Each downdrop has a vertical, limb 20 and a horizontal limb 22.

The ring main 18 is in turn connected to an air blower 24 by a passage 26.

In alternate downdrops 16 are situated diffusers 28 which have enlarged perforated end portions. Each diffuser 28 is situated in the vertical limb of its associated downdrop, although if desired it may equally well be situated in the horizontal limb. The diffusers 28 are connected by conduits 30 to an oxygen ring main 32 which in turn is connected by a pipe 34 to an air separation plant 38 adapted to supply substantially pure oxygen gas.

In operation the cupola is charged with a mixture of coke, steel scrap and pig iron of known composition. Blast air taken from the atomosphere by the blower 26 is introduced into the interior of the cupola through the tuyeres 10. This air supports combustion of the coke which causes a temperature of about 1800"C to be generated. This temperature is sufficient to melt the metal in the charge and this molten metal falls through the furnace and is collected at the bottom thereof. In the cupola various chemical reactions take place which cause a layer of slag to be formed above the molten metal collected at the bottom of the furnace.

Metal and slag may be tapped of from time to time through the holes 6 and 8 respectively.

The temperature of the charge inside the cupola may be raised by passing oxygen from the air separation plant 38 to the ring main 34 and then introducing it into selected downdrops from the diffusers 28. The oxygen leaves each diffuser 28 in a plurality of small streams which readily become mixed with the air passing through the downdrops.

The total rate of oxygen passing through the diffusers 28 may typically be up to one twentieth of the rate of supply of air to the downdrop 16.

For performing the process according to the third aspect of this invention the diffusers 28 should be located within their associated tuyeres 10 rather than in the downdrops.

WHAT WE CLAIM IS: 1. A process for melting metal in a vertical shaft furnace including the steps of distributing blast air from a main to several conduits which connect the main to separate tuyeres through which the air is introduced into the furnace and increasing the concentration of oxygen in the air leaving some, but not all, of the tuyeres by selecting some of the tuyeres and supplying oxygen to them, which oxygen mixes fully with the air in the selected tuyeres before the air leaves

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (17)

**WARNING** start of CLMS field may overlap end of DESC **. Manv if not all of the advantages of the process according to the present invention may be achieved if mixing of the oxygen with the air takes place in tuyeres itself. Indeed. if the oxygen is introduced into selected tuyeres through perforated diffusers actually situated in the tuyeres themselves it may be possible to achieve sufficient mixing of the air and oxygen before the stream enters the combustion region of the furnace. Accordingly, in association with each selected tuyere there may be an oxygen diffuser which has many small apertures or small perforations in its walls, at least part of each diffuser extending into its associated tuyere. Oxygen may be introduced into the selected tuyeres from the diffusers in the form of fine streams, whereby the oxyen and the blast air become mixed before they enter the combustion region of the furnace. The diffusers positioned in the tuyeres should desirablv be made of a metal which is not eroded by the temperatures that obtain therein. Copper is such a metal. Each diffuser may be located coaxially within its associated tuyere or may be located parallel or at an angle to the axis of the tuvere. The process according to the present invention will now be described by way of example with reference to the accompanying drawings in which: F4tiiie 1 is a side elevation, partly in section of the lower part of a cupola adapted to perform the process according to the present invention: Figure 9 is a section through the line II - II in Figure 1: Figlçre 3 is a schema illustrating apparatus for supplying air to the cupola; and Figllre 4 is a schema illustrating apparatus for supplying oxygen to the cupola. Referring to the drawings a cupola 2 is provided in the usual manner with a hearth 4 having tap hole 6 for the removal of molten metal from the cupola. Cupola 2 also has a hole 8 which is situated above the hole 6 so as during operation of the furnace to communicate with the layer of slag that forms above the molten metal and below the main charge. Slag may therefore be tapped off from time to time or continuously through the hole 8. Above the hole 8 is situated a ring of eight tuyeres 10 which are adapted to introduce air through the wall 12 of a cupola into the region in which combustion takes place during operation of the furnace. The tuyeres 10 are equally spaced about the periphery of the wall 12 and their axes all lie in the same plane. The tuyeres as shown in Figure 1 have walls which converge in the direction of the interior of the furnace. The tuyeres may be provided with water cooled copper nozzles. The tuyeres 10 are connected to a ring main 18 by downdrops 16. Each downdrop has a vertical, limb 20 and a horizontal limb 22. The ring main 18 is in turn connected to an air blower 24 by a passage 26. In alternate downdrops 16 are situated diffusers 28 which have enlarged perforated end portions. Each diffuser 28 is situated in the vertical limb of its associated downdrop, although if desired it may equally well be situated in the horizontal limb. The diffusers 28 are connected by conduits 30 to an oxygen ring main 32 which in turn is connected by a pipe 34 to an air separation plant 38 adapted to supply substantially pure oxygen gas. In operation the cupola is charged with a mixture of coke, steel scrap and pig iron of known composition. Blast air taken from the atomosphere by the blower 26 is introduced into the interior of the cupola through the tuyeres 10. This air supports combustion of the coke which causes a temperature of about 1800"C to be generated. This temperature is sufficient to melt the metal in the charge and this molten metal falls through the furnace and is collected at the bottom thereof. In the cupola various chemical reactions take place which cause a layer of slag to be formed above the molten metal collected at the bottom of the furnace. Metal and slag may be tapped of from time to time through the holes 6 and 8 respectively. The temperature of the charge inside the cupola may be raised by passing oxygen from the air separation plant 38 to the ring main 34 and then introducing it into selected downdrops from the diffusers 28. The oxygen leaves each diffuser 28 in a plurality of small streams which readily become mixed with the air passing through the downdrops. The total rate of oxygen passing through the diffusers 28 may typically be up to one twentieth of the rate of supply of air to the downdrop 16. For performing the process according to the third aspect of this invention the diffusers 28 should be located within their associated tuyeres 10 rather than in the downdrops. WHAT WE CLAIM IS:

1. A process for melting metal in a vertical shaft furnace including the steps of distributing blast air from a main to several conduits which connect the main to separate tuyeres through which the air is introduced into the furnace and increasing the concentration of oxygen in the air leaving some, but not all, of the tuyeres by selecting some of the tuyeres and supplying oxygen to them, which oxygen mixes fully with the air in the selected tuyeres before the air leaves

such tuyeres.

2. A process as claimed in claim 1, in which the oxygen is introduced into the selected tuyeres through lances or diffusers positioned with their outlets terminating upstream of such tuyeres in selected conduits.

3. A process as claimed in claim 2, in which each diffuser is able to distribute oxygen supplied to it in a plurality of streams.

4. A process as claimed in claim 3, in which the diffuser is a perforated cylinder or a perforated cone.

5. A process as claimed in claim 2, in which the oxygen is fed to each lance at a pressure in excess of that of the air blast.

6. A process as claimed in any one of the preceding claims, in which several downdrop conduits each having a generally horizontal and a generally vertical limb connect the tuyeres to a ring main. the ring main being positioned above the tuyeres.

7. A process as claimed in any one of the preceding claims, in which the furnace is employed for melting ferrous metal or for smelting ferrous ores and in which oxygen is mixed with the air blast in every other conduit or tuyere. there being an even number of tuyeres.

8. A process as claimed in any one of the preceding claims in which the furnace is employed for melting ferrous metal or for smelting ferrous ores and in which there is an odd number N of tuyeres, oxygen being mixed with the air blast in N+1/2 tuyeres.

9. A process as claimed in claim 8. in which there are 11 or a smaller number of tuyeres.

io. A process as claimed in any one of the preceding claims, in which oxygen is introduced into each selected tuyere at a rate up to a fifth of that at which air passes therethrough.

11. A process as claimed in claim 10, in which the rate of oxygen introduction into each selected tuyere is up to a tenth of that at which air passes therethrough.

12. A process as claimed in any one of claims 1 to 6, in which the furnace is employed to melt non-ferrous metal or to smelt an ore of a non-ferrous metal.

13. A process as claimed in claim 12. in which there are from 20 to 100 tuyeres, the oxygen being mixed with the air blast in from 2 to 10 tuyeres.

14. A process as claimed in any one of the preceding claims, in which the oxygen is introduced into the selected tuyeres continuously during operation of the furnace.

15. A process as claimed in any one of claims 2 to 14 in which the diffusers or lances are connected to their own ring main to which the oxygen is supplied.

16. A process as claimed in any one of the preceding claims, in which the furnace is a hot - or cold - blast cupola.

17. A process for melting metal in a vertical shaft furnace, substantially as herein described with reference to the accompanying drawings.