METAL REDUCTION AND MELTING PROCESS

Abstract: A metal reduction and melting process which involves the heating of a burden comprising a metal containing component and a carbon containing component in a channel type induction furnace in order to reduce such metal containing component, in which at least part of the gaseous products of the process is utilised for preheating the burden; and apparatus for carrying out such preheating.

Description: TITLE: METAL REDUCTION AND MELTING PROCESS BACKGROUND TO THE INVENTION THIS invention relates to a metal reduction and melting process, such as for example a steelmaking process, in which a metal and carbon containing burden is heated in a channel type induction furnace in order to cause the reduction and melting of the metal containing part of the burden, and apparatus for carrying out such preheating. PRIOR ART In a known process in this regard, the metal containing component of the burden comprises a metal ore, or a partially reduced metal ore (such as sponge iron in the case of an iron containing burden), and the carbon containing component coal. In this known process the burden is fed substantially vertically into the furnace through one or more feed holes located along the side of the furnace in a manner so that the burden forms a continuous longitudinally extending heap along each side of the furnace which floats on the bath of molten metal in the furnace. In this known process the resulting gaseous products, mainly CO, and the volatile matter associated with the coal, are combusted in a cavity located in the furnace above the floating burden by burning them with oxygen, which is introduced into such area from outside the furnace. The resulting combustion gases from said burning, which are typically at a temperature in the order of 1600 C, are then withdrawn from the furnace and utilised elsewhere. OBJECT OF THE INVENTION It is an object of the present invention to provide a metal reduction and melting process in which such combustion gases are better utilised. SUMMARY OF THE INVENTION According to the invention a metal reduction and melting process which involves the heating of a burden comprising a metal containing component and a carbon containing component in a channel type induction furnace in order to reduce such metal containing component, includes the step of utilising at least part of the gaseous products of such process for preheating the burden, i.e. prior to its introduction to the furnace. It will be appreciated that if as a result of such preheating the temperature of the burden is increased beyond a predetermined value, the metal containing component in the burden will start to be reduced before the burden enters the furnace. This will give rise to at least some gaseous reduction products being formed in the burden prior to its introduction to the furnace. The particular temperature at which such reduction starts to take place in the burden before it enters the furnace, will of course be determined by the nature of the particular metal containing component in the burden. Furthermore, as a result of such preheating of the burden at least some of the volatile matter associated with the carbon containing compound of the burden will also be released in the burden prior to it entering the furnace. The aforesaid gaseous products which enter the furnace may accordingly comprise the aforesaid volatile matter together with the CO and CO2 formed in the said reduction process. It will be appreciated that such preheating of the burden not only gives rise to a reduced energy requirement in the overall process, but also to an increased throughput for the furnace. Further according to the invention at least part of the aforesaid gaseous products are burnt with air and/or oxygen in the furnace and the combustion gases so produced utilised for said preheating of the burden. Further according to the invention the burden is fed to the furnace in an elongated chamber which is provided with one or more passages extending around or through the chamber, through which passages the said combustion gases can pass in order to preheat the burden in the chamber. Preferably the chamber and passage(s) are sealed off relative to one another so that no gaseous product can pass between them. In a preferred form of the invention, such a passage may comprise an annular jacket which extends longitudinally around the chamber. Further according to the invention the one or bottom end of said chamber may extend for such a distance into the furnace that it engages the heaps of burden floating on the bath of molten metal in the furnace. In this manner the burden in the chamber is prevented from falling free into the furnace through such end. In one form of the invention a positive pressure may be maintained in said chamber in order to ensure that most of the gaseous products formed in the chamber will pass into the furnace. Such a pressure may, for example, be brought about by introducing the burden to the chamber by means of a suitable lock system. In addition, or alternatively, such positive pressure may be maintained by introducing a pressurised inert gas, such as nitrogen, to the burden prior to passing it into the chamber. In one form of the invention at least some of the aforesaid gaseous products formed in the burden prior to its introduction to the furnace may be removed from the burden prior to the burden entering the furnace. Thus, for example, at least some of the oxidising gases, such as CO2 and H2O- vapour, formed in the burden may so be removed from the burden. The reason for removing such oxidising gases relates to the fact that if the temperature in the burden reaches a predetermined value, such oxidising gases may react with the metal and carbon containing components of the burden thus resulting in the consumption of the carbon containing compound. Further according to the invention such oxidising gases may be removed from the burden by means of a elongated open ended conduit which extends longitudinally through said elongated chamber, so that its one or bottom end is located towards the bottom end of the chamber and its other or upper end communicates with a suitable gas withdrawal system, the conduit including over its length at least one gas outlet which is located at a predetermined level in the chamber. In use, the conduit will accordingly be surrounded by the burden and its said gas outlet located at a predetermined level in the burden. Further according to the invention said gas outlet of the conduit is located at a level in the chamber where the burden is at that temperature just before which said oxidising gases will start to react with the metal and carbon containing components of the burden. Such preferred level may, for example, be determined experimentally by, for example, analysing the gas withdrawn from the conduit. In addition, or alternatively, the temperature at the gas outlet of the conduit may be measured such as, for example, by means of a thermocouple or the like. In practice, once the preferred level of said gas outlet of the conduit has been determined, its position will remain fixed and the temperature of the burden in said location controlled by, for example, controlling the rate at which the oxidising gases is removed from the conduit, and/or the rate of the preheating, and/or the rate at which the burden is fed through the chamber. In the case where the metal containing compound comprises iron, and the carbon containing component coal, the aforesaid temperature will be in the order of 600°C. The oxidising gases which are so removed may, for example, be utilised elsewhere or they may be passed to the said passage(s) utilised for preheating the burden. Further according to the invention preheated oxygen and/or air are/is introduced to the conduit towards its said bottom end in order to combust any CO present in that part of the burden to CO2, which is then also withdrawn through said gas outlet. In order to ensure that substantially all the CO is converted to CO2 before the gas reaches the said gas outlet, the wall of the chamber immediately below the gas outlet is maintained at a lower temperature relative to the rest of the chamber. Such lower temperature may be obtained through suitable heat insulation, and/or by delaying the rate at which the burden in the chamber moves past such part of the conduit's wall. This latter operation could, for example, be effected by increasing the cross sectional width of the chamber in such areas. Still further according to the invention at least some of the volatile matter associated with the carbon containing components of the burden is circulated from the said upper end of the conduit towards its said bottom end, where it can be combusted simultaneously with the CO by oxygen and/or preheated air. As will be appreciated the heat supplied through such conversion and combustion will assist in the preheating of the burden. Said circulation of the volatile matter may be brought about by means of an elongated open ended pipe of which the one end is located at a predetermined level towards said upper end of the conduit, and the other end towards said bottom end of the conduit. Still further according to the invention, the carbon containing component of the burden may be charged to the chamber in such a manner that it is distributed towards the outer wall of the chamber, while the metal containing component of the burden is distributed towards the outer wall of said conduit. In this way the carbon containing component (which provides the carbon which is required for the endothermic Boudouard reaction which in turn is required to convert carbon dioxide into carbon monoxide, which is ultimately required for reducing the metal in the metal containing component of the burden), is closer to the preheating source than the metal containing component, which in some cases reacts exothermically with carbon monoxide. Further according to the invention the iron containing compound of the burden comprises an iron ore, and the carbon containing component, coal. The invention will now be described further by way of example with reference to the enclosed drawings, in which: BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic cross sectional view of one embodiment of a furnace provided with preheating apparatus according to the invention; and Figure 2 is a similar view as that of figure 1 with parts shown broken away, of another embodiment of said apparatus. DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION In both embodiments of the invention a burden preheating arrangement 10 according to the invention is shown as being utilised with a channel type induction furnace 11. Furnace 11 comprises an elongated tubular vessel 12 which is of circular configuration in cross section and which includes at least two parallel spaced rows of burden feed holes which each extends through the wall of vessel 12 along opposite sides thereof and of which only one, 13, is shown in detail in the drawings. Vessel 12 is heated from below by means of two parallel spaced rows of electric induction heaters 14. When used in a steelmaking process, vessel 12 is charged with a quantity of molten steel to form a bath 16, and a burden 15, comprising a mixture of coal and iron ore, is introduced to vessel 12 through feed holes 13 to float on the bath of molten steel 16 in the form of two spaced apart heaps 17 which each extends along a longitudinal side of vessel 12, and above which a cavity 18 is formed in vessel 12. Vessel 12 is also provided with a plurality of oxygen burners 19, and at least one port (not shown) for removing the reduced and molten steel from vessel 12. In the embodiment shown in figure 1, arrangement 10 includes an elongated chamber 20 of which the one end extends through feed hole 13 into the interior of furnace 11 to such a depth that it engages the upper end of heap 17. Burden 15 is fed to chamber 20 via a burden feed bunker 21 and a series of lock systems 22. Chamber 20 is surrounded by an elongated annular passage in the form of a jacket 23 which has a bottom inlet 24 provided with a nozzle 25 which passes through the wall of vessel 12 to communicate with area 18 of furnace 11. Jacket 23 also includes a top outlet 26 which is connected via a pipe 27 to the duct 28 of a gas exhaust system (not shown). Chamber 20 and jacket 23 are suitably sealed off with respect to each other to prevent any gas from passing from the one to the other. In operation, vessel 12 is charged with a supply of molten steel 16, and induction heaters 14 switched on. A burden 15, comprising a mixture of coal and iron ore, is fed from bunker 21 via lock systems 22 and feed holes 13 into vessel 12, to form heaps 17 which float on top of liquid metal bath 16. The upper end of a heap 17 is engaged by the bottom end of chamber 20 so that the burden at the top of the heap blocks the open end of chamber 20, thus preventing burden 15 from falling freely from the open end of chamber 20. Lock systems 22 ensure that a positive pressure is maintained in chamber 20 so that substantially no gas can escape from the cold upper surface of burden 15. The gases, mainly CO, CO2 and volatile matter resulting from the heating and consequential reduction of the iron containing component of burden 15 are burnt in cavity 18 by the oxygen supplied by burners 19. The resulting combustion products are fed from area 18 via nozzle 25 and inlet 24 into jacket 23 where they serve to preheat the burden 15 located in the bore of chamber 20. As a result of such preheating of burden 15, the throughput of furnace 11 is increased significantly. The spent combustion gasses passing from jacket 23 via outlet 26 and pipe 27 to duct 28, can be used elsewhere in the process in order to utilise their chemical and/or thermal properties. At the same time the coal associated volatile matter located in chamber 20 can as a result of such preheating of burden 14, be cracked to carbon, CO and H2 which can then directly participate in the reduction of burden 15 in pipe 20. As a result of this, the coal and oxygen consumption per ton of product in the process can also be reduced significantly. In the embodiment shown in figure 2, wherein parts corresponding to those shown in figure 1 are indicated by the same number, a burden 15 is fed from feed bunker 21 to the upper end of an elongated chamber 29 of which the bottom end communicates with an inlet 13 in vessel 12 of furnace 11. Chamber 29 is annularly surrounded by an elongated jacket 30 of which the bottom end has an outlet 24 which communicates via nozzle 25 with area 18 of vessel 12. The upper end of jacket 30 has an outlet 26 which communicates via a pipe 27 with the duct 28 of a gas withdrawal system (not shown). The outlet of feed bunker 21 is provided with a gas inlet pipe 30 by means of which an inert gas such as nitrogen can be passed into burden 15 and so into chamber 29 in order to put the latter under a positive pressure. Chamber 29 is provided with an elongated open ended conduit 31 of which the upper end 32 communicates via a pipe 33 and pipe 27 with duct 28. The lower end 34 of conduit 31 extends a predetermined depth into chamber 29. More details of this are given later. When burden 15 is fed through chamber 29 into vessel 12, the burden in chamber 29 will annularly surround conduit 31 and engage its bottom open end 34. Conduit 31 is provided towards the middle of its length with a series of apertures 35 which are of such size that gas, but not solid burden 15, can pass through them. Conduit 31 is also provided with an elongated open ended pipe 36 of which the upper end 37 is located towards the upper end 32 of conduit 31 and its lower end 38 towards the lower end 34 of conduit 31. Conduit 31 is also provided at its open end 34 with the inlet 39 of an oxygen preheated/air burner (not shown). Conduit 31 is so positioned in chamber 29 that its apertures 35 are located in that part of burden 15 where its temperature is in the order of 600 °C. Conduit 31 is furthermore of such a length that its upper end 32 is located in that part of burden 15 where its temperature is in the order of 200 °C, and its lower end 34 in that part of burden 15 where its temperature is in excess of 800 °C. Furnace 12 is operated in the same manner as that described above in respect of the embodiment of figure 1. During such operation, preheating of the lower end of chamber 29 by jacket 30 to a temperature in the order of 850 C causes the Boudoard reaction to take place, in this part of burden 15, i.e. C + CO → 2CO. Part of such CO will react with the iron component of burden 15 to form CO2, thus causing the partial reduction of the iron component, i.e. 3Fe 2 0 3 + CO → 2 Fe 3 0 + C0 2 . The formed CO2 will flow upwardly in burden 15 until it reaches apertures 35 of conduit 31 through which it is then withdrawn via the bore of conduit 31 and pipes 33 and 27 to duct 28 of the gas exhaust system (not shown). Another part of the CO will flow upwardly in the bore of conduit 31 where it will be burnt to CO2 by oxygen/preheated air burner 39, i.e. 2CO + O 2 → C0 2 , which CO2 will then, with the CO2 passing from apertures 35, pass via pipes 33 and 27 to duct 28. At the same time any volatile matter and water vapour present in the upper parts of burden 15 in chamber 29 will pass from the upper end 37 of pipe 36 through its bottom end 38 into the bore of conduit 31 where the volatile matter will be combusted by oxygen/preheated air burner 39. It will be appreciated that the heat developed in the conversion of the CO to CO2, and the combustion of the volatile matter, will assist in heating up burden 15 in chamber 29. It will be appreciated further that by locating conduit 31 in the said position in chamber 29 where its apertures 35 are located at that position where the temperature of the burden 15 is in the order of 600 °C, the said CO2 will be removed from burden 15 before it can react with the metal and/or carbon containing components of burden 15. In order to further prevent such reaction from taking place, those parts of the wall of conduit 31 located immediately below apertures 35 may be maintained at a lower temperature relative to the rest of the wall. This, may, for example be effected through proper heat insulation of the wall and/or by reducing the flow rate of burden 15 through chamber 29 in such area. The latter operation may, for example, be effected by increasing the cross sectional width of chamber 29 in such area. It will be appreciated further that the temperature of the burden at the lower end 34 of conduit 31 can be controlled by controlling any one or more of the following integers: (1) the rate at which burden 15 is fed through chamber 29; (2) the nature of the combustion in conduit 31, and (3) the rate at which the combustion gases are passed through jacket 30. It will be appreciated still further that, if required, at least part of the gases formed in vessel 12 during the reduction of burden 15 can be sucked back via inlet 13 into chamber 29 in order for them to be utilised in the reduction and/or preheating of burden 15. The molten metal product of furnace 12 can further be treated and collected in the conventional manner. It will be appreciated that the invention also includes within its scope apparatus for carrying out the process of the invention substantially as herein described. It will be appreciated further that there are no doubt many variations in detail possible with a process and apparatus according to the invention without departing from the spirit and/or scope of the claims. Thus, for example, burden 15 may be charged to chamber 29 in such a manner that it is distributed towards the outer wall of the chamber for the reasons set out above.

Claims: CLAIMS Claim 1 A metal reduction and melting process which involves the heating of a burden comprising a metal containing component and a carbon containing component in a channel type induction furnace in order to reduce such metal containing component, the process including the step of utilising at least part of the gaseous products of such process for preheating the burden. Claim 2 The process of claim 1 wherein at least part of the gaseous products are burnt with air and/or oxygen in the furnace and the combustion gases so produced utilised for the preheating of the burden. Claim 3 The process of any one of claims 1 or 2 wherein the burden is fed to the furnace in an elongated chamber which is provided with one or more passages extending around or through the chamber, through which passages the said combustion gases can pass in order to preheat the burden in the chamber. Claim 4 The process of claim 3 wherein the chamber and the or each passage is sealed off relative to one another so that no gaseous product can pass between them. Claim 5 The process of claims 3 or 4 wherein the or each passage comprises an annular jacket which extends longitudinally around the chamber. Claim 6 The process of any of claims 3 to 5 wherein the one or bottom end of the chamber extends for such a distance into the furnace that it can engage the heaps of burden floating on the bath of molten metal in the furnace. Claim 7 The process of any one of claims 3 to 6 wherein a positive pressure is maintained in the chamber in order to ensure that most of the gaseous products formed in the chamber will pass into the furnace. Claim 8 The process of claim 7 wherein the said pressure is brought about by introducing the burden to the chamber by means of a suitable lock system. Claim 9 The process of claims 7 or 8 wherein the said pressure is maintained by introducing a pressurised inert gas, such as nitrogen, to the burden prior to passing it into the chamber. Claim 10 The process of any one of the preceding claims wherein at least some of the gaseous products formed in the burden prior to its introduction to the furnace is removed from the burden prior to the burden entering the furnace. Claim 11 The process of claim 10 wherein at least some of the oxidising gases, such as CO2 and H2O vapour, formed in the burden are so removed from the burden. Claim 12 The process of claims 10 or 11 in so far as they are dependent on any one of claims 3-9 wherein the said gaseous products are removed from the burden by means of an elongated open ended conduit which extends longitudinally through said elongated chamber, so that its one or bottom end is located towards the bottom end of the chamber and its other or upper end communicates with a suitable gas exhaust system, the conduit including over its length at least one gas outlet which is located at a predetermined level in the chamber. Claim 13 The process of claim 12 wherein said gas outlet of the conduit is located at a level in the chamber where the burden is at that temperature just before which oxidising gases in the burden will start to react with the metal and carbon containing components of the burden. Claim 14 The process of claim 13 wherein the preferred level of the gas outlet of the conduit is determined by analysing the gas withdrawn for the conduit Claim 15 The process of claims 13 or 14 wherein the temperature of the gas outlet of the conduit is measured by means of a thermocouple or the like. Claim 16 The process of any one of claims 13 to 15 wherein once the preferred level of outlet of the conduit has been determined, its position remains fixed, and the temperature of the burden in said location is controlled by controlling the rate at which the oxidising gases is removed from the conduit, and/or the rate of the preheating, and/or the rate at which the burden is fed through the chamber. Claim 17 The process of any one of claims 11 to 16 wherein the gaseous products so removed are utilised elsewhere or passed to the said passage(s) utilised for preheating the burden. Claim 18 The process of any one of claims 1 1 to 17 wherein preheated oxygen and/or air are/is introduced to the conduit towards its said bottom end in order to combust any CO present in that part of the burden to CO2. Claim 19 The process of claim 18 wherein the wall of the chamber immediately below the gas outlet is maintained at a lower temperature relative to the rest of the chamber in order to ensure that substantially all the CO is converted to CO2 before the gas reaches said gas outlet. Claim 20 The process of claim 19 wherein the cooling is effected through suitable heat insulation of the chamber, and/or by delaying the rate at which the burden in the chamber moves past such part of the chamber's wall by, for example, increasing the cross sectional width of the chamber in such area. Claim 21 The process of any one of claims 12 to 20 wherein at least some of the volatile matter associated with the carbon containing components of the burden is circulated from the said other end of the conduit towards the said one end of the conduit, where they can be combusted simultaneously with any CO present by means of oxygen and/or preheated air. Claim 22 The process of claim 21 wherein said circulation of the volatile matter is brought about by means of an elongated open ended pipe of which the one end is located at a predetermined level towards said upper end of the conduit, and the other end towards said bottom end of the conduit. Claim 23 The process of any one of claims 3 to 22 wherein the carbon containing component of the burden is charged to the chamber in such a manner that it is distributed towards the outer wall of the chamber. Claim 24 The process of any one of the preceding claims wherein the iron containing compound of the burden comprises an iron ore, and the carbon containing component, coal. Claim 25 A metal reduction and melting process substantially as herein described with reference to the drawings. Claim 26 Apparatus for preheating the burden of a metal reduction and melting process which involves the heating of a burden comprising a metal containing component and a carbon containing component in a channel type induction furnace in order to reduce such metal containing component; the apparatus comprising an elongated chamber through which the burden can be fed to the furnace, and one or more passages extending around or through the chamber, through which passages gaseous products formed in the process can pass in order to preheat the burden in the chamber. Claim 27 The apparatus of claim 26 wherein the chamber and the or each passage is sealed off relative to one another so that no gaseous product can pass between them. Claim 28 The apparatus of any one of claims 26 or 27 wherein the or each passage comprises an annular jacket which extends longitudinally around the chamber. Claim 29 The apparatus of any one of claims 26 to 28 wherein the one end of the chamber can extend for such a distance into the furnace that it can engage a heap of burden floating on the bath of molten metal in the furnace. Claim 30 The apparatus of any one of claims 26 to 29 including a lock system for feeding the burden to the chamber in a manner so that a positive pressure is maintained in the chamber in order to ensure that most of the gaseous products formed in the chamber will pass into the furnace. Claim 31 The apparatus of any one of claims 26 to 30 including means for introducing a pressurised inert gas, such as nitrogen, to the burden prior to passing it into the chamber in order to maintain a positive pressure in the chamber. Claim 32 The apparatus of any one of claims 26 to 31 including an elongated open ended conduit which extends longitudinally through said elongated chamber, so that its one or bottom end is located towards the bottom end of the chamber and its other or upper end communicates with a suitable gas withdrawal system, the conduit including over its length at least one gas outlet which is located at a predetermined level in the chamber and through which at least some of the gaseous products formed in the burden prior to its introduction to the furnace can be removed from the burden prior to it entering the furnace. Claim 33 The apparatus of claim 32 wherein said gas outlet of the conduit is located at a level in the chamber where the burden is at that temperature just before which oxidising gases in the burden will start to react with the metal and carbon containing components of the burden. Claim 34 The apparatus of any one of claims 32 or 33 including means for introducing oxygen and/or preheated air to the conduit towards its said bottom, end in order to combust any CO present in that part of the burden to CO2. Claim 35 The apparatus of claim 34 wherein the wall of the chamber immediately below the gas outlet is maintained at a lower temperature relative to the rest of the chamber in order to ensure that substantially all the CO is converted to CO2 before the gas reaches the gas outlet. Claim 36 The apparatus of claim 35 wherein the lower temperature is effected through suitable heat insulation applied to the chamber's wall. Claim 37 The apparatus of anyone of claims 26 to 36 including an elongated open ended pipe of which the one end is located at a predetermined level towards said upper end of the conduit, and its other end towards said bottom end of the conduit, the pipe being intended for circulating at least some of the volatile matter associated with the carbon containing component of the burden from said upper end of the conduit towards said bottom end of the conduit, where it can be combusted simultaneously with any CO present by means of oxygen and/or preheated air. Claim 38 Apparatus for preheating the burden of a metal reduction and melting process which involves the heating of a burden comprising a metal containing component and a carbon containing component in a channel type induction furnace in order to reduce such metal containing component substantially as herein described, and as illustrated in either figure 1 or figure 2 of the drawings.

Inventors: FOURIE LOUIS JOHANNES, FOURIE, LOUIS, JOHANNES
Applicants: FOURIE LOUIS JOHANNES, IPCOR NV, FOURIE, LOUIS, JOHANNES, IPCOR N.V.
Application year: 1997
Main classification: C21B13
IPC codes: C21B11, C21B13, F27B14, F27D13