COOLING OF A METALLURGICAL SMELTING REDUCTION VESSEL

Technical field

[0001 ] The present invention generally relates to metallurgical smelting reduction vessels and more specifically to cooling their refractory lining.

Background of the invention

[0002] At present, the steelmaking industry is witnessing a slow but steady shift from large integrated steel plants to a variety of smaller plant types, serving either as a replacement of or as a valuable complement to integrated steel plants. In this context, alternative methods of ironmaking generically named "smelting reduction" are attracting increased interest. Among the advantages of smelting reduction processes is their suitability for producing hot metal using low grade raw materials, e.g. ore fines or other waste iron oxides (such as steel plant wastes) and low grade carbonaceous reducing agents such as coal, in other words, without the need for increasingly scarce high grade materials essential to blast furnace ironmaking i.e. high grade lump iron ore and coke.

[0003] Smelting reduction (SR) in hot metal (pig iron) production, as its name suggests, involves both reduction and smelting of iron oxides, i.e. melting accompanied by chemical reaction(s). Currently, most SR processes use two distinct reactors to perform two (or three) subsequent basic steps: partial pre-reduction of the iron oxides (normally in solid state) in the first stage and removal of the remaining oxygen in liquid phase reduction reactions in the second (and third) stage(s).

[0004] Generally speaking, the present invention is concerned with the vessel (reactor) used to perform the second and/or third stage i.e. completion of the reduction and smelting of the iron oxides into hot metal. The latter stage(s) of SR processes typically involve creation of a bath of molten metal and a layer of molten slag on top of the bath in a refractory lined vessel. Reduction reactions occur primarily within the lower region of the slag layer whereas oxidation reactions, in particular post-combustion of CO generated by reduction of the iron oxides in the bath, take place primarily in and above the upper region of the slag layer in order to generate the heat required for driving the process. Due to the process-intrinsic features of SR, the slag layer in a SR vessel usually consists of a slag that has a considerable proportion of gas phase, a relatively high basicity (enabling efficient desulfurization of the metal) and an extremely high temperature (since it serves as a site of (post-)combustion heat generation) that well may amount to >1700°C in the region of heat generation of the slag layer (upper region). Due to the considerable

proportion of gas phase, the slag is commonly referred to as being "foaming" or "foamy" and thus quite voluminous when compared to the volume of the bath, an ample slag volume being beneficial to (post-)combustion and thus energy efficiency. Moreover, during operation, the slag layer above the bath is typically actively agitated, in order to promote post-combustion heat transfer as well as and other reactions occurring within the slag layer by strong turbulence.

[0005] From what is stated above, it will be appreciated that in the special case of a SR vessel, the refractory lining, i.e. the inward brickwork made of heat-resistant material required for containing the high-temperature reactants is subjected to very adverse conditions, especially in the zone of the turbulent slag layer of the SR vessel. Noteworthy sources of considerable wear being mechanical wear from the abrasive washing action of the strongly turbulent molten slag, chemical wear from the slag constituents and thermal wear from exposure to extreme temperatures in the zone of concern.

[0006] It follows that, as already stated in WO2007/134382, the extent of erosion of the refractory in a smelting reduction vessel, in particular in the zone of the turbulent slag layer, is greater than typically experienced in blast furnaces, in which the molten metal bath and the slag layer are relatively quiescent. The same holds true when comparing smelting reduction vessels to conventional electric arc furnaces for steelmaking by scrap melting. Although some degree of stirring of the bath and slag is occasionally provided in an EAF, namely in order to spread the electric arc power throughout the bath, such stirring is typically by far less intense than in smelting reduction vessels because it is aimed at agitating only the bath itself and the interface between the slag layer and the bath of molten metal. In an EAF the slag layer itself in turn serves a thermal blanket (avoiding excessive heat loss) and is thus, compared to the slag layer in a SR process, relatively quiescent and usually much less voluminous since it is not required to support (post-) combustion. Furthermore, the thermal load per unit area is typically much less intense in a classic EAF furnace than in an SR vessel.

[0007] Replacement of a worn out refractory lining is cost intensive, in terms material and labour cost, and also in terms of productivity loss due to reactor down time. As in any kind of metallurgical vessel, there is thus a need for extending the service life of the refractory, which need is however particularly pronounced in the case of SR vessels. Many approaches are known for cooling the refractory lining in other types of metallurgical vessels or furnaces, in particular in blast furnaces and conventional electric arc furnaces. So far however, few viable suggestions have been made as regards the cooling the refractory lining of a SR vessel.

Background Art

[0008] European patent EP 0735146 discloses a specific example of a SR vessel, namely a cyclone converter furnace (CCF). With reference to FIG.2 of this patent, it discloses a CCF smelting reduction vessel (1 1 ) for producing hot metal. The vessel comprises, besides a top part (13) connected to a pre-reduction melting cyclone (12), a bottom part (14) having an outer shell and an inner refractory lining (14, 15) for containing a bath of molten metal (17) and a layer of slag (18, 19) above the bath. The vessel further comprises a bottom gas blowing device for bubbling (24) the bath of molten metal (17) and the slag layer (18, 19) as well as oxidant gas injection devices in the form of oxygen lances (23) for injecting oxygen directly into the slag layer (18, 19). Bottom gas bubbling and (hard blow) oxygen injection allow creating a voluminous and turbulent slag layer (19), which improves transfer of post-combustion heat within the slag layer (18, 19). Carbonaceous material is injected into the slag using gravity-feed chutes (22). In order to contain the voluminous and strongly turbulent slag layer (19), the refractory lining (14, 15) has a capacity of at least twice the maximum volume of the bath of molten metal (17) in a SR vessel, e.g. a CCF vessel according to this patent. With the aim of prolonging the service life of the refractory at its most vulnerable point in the SR vessel, this patents proposes a water cooling installation (16) be arranged in the zone of the turbulent slag layer (19), a major portion of the installation being located above the maximum level of the bath of molten metal (17) for cooling the inner refractory lining (14, 15) in this zone. The suggested cooling is either of the stave-cooler type (FIG.2) or the plate-cooler type (FIG.3), both being part of forced-circulation ducted water cooling systems that are per se well known for cooling blast furnace refractory brickwork. A similar plate-cooler type solution for a SR vessel is disclosed in European patent application EP 1477573. While EP 0735146 and EP 1477573 teach solutions that appear to allow increasing the service life of the refractory lining, these solution are cost intensive in terms of constructional cost.

[0009] United States patent US 5,708,785 discloses another type of SR vessel. As regards cooling of the refractory lining, this patent proposes restricting cooling to a minimal zone, namely cooling only the portion of the refractory wall above the slag surface between the upper surface of the slag and the furnace charging chute (i.e. above the zone of cooling suggested by EP 0735146) on the ground that cooling on too extensive a scale results in prohibitively high constructional and operational costs. In order to reduce the wear rate of the refractory below the slag surface, US 5,708,785 basically suggests lowering the hot metal and slag temperatures, the suggested cooling installation aiming at reducing wear related to hot off-gas temperatures and slag spattering above the slag surface. Lowering the process temperatures is however not generally viable in SR

process, and thus US 5,708,785 fails to provide a generally applicable solution for extending the refractory service-life, especially in the zone of the turbulent slag.

Technical problem

[0010] Accordingly, it is an object of the present invention to provide a cooling solution that efficiently extends the service life of the refractory while being applicable to various types of SR processes/vessels and less expensive in terms of constructional costs.

[001 1 ] This object is achieved by a smelting reduction vessel as claimed in claim 1 and a method of producing hot metal as claimed in claim 1 1 respectively.

General Description of the Invention

[0012] The present invention proposes a smelting reduction vessel for producing hot metal that comprises an outer shell and an inner refractory lining made of brickwork for containing, during operation, a bath of molten metal and a layer of slag above the bath, In order to create a voluminous and turbulent slag layer for improving post-combustion heat transfer within the slag layer, the vessel includes at least one of the following devices: a bottom gas blowing device for bubbling the bath of molten metal and [he layer of slag; a pneumatic injection device for Injecting carbonaceous materia! into the layer of slag: or an oxidant gas injection device for injecting an oxidant gas into the layer of slag; in order to safely contain (except for unavoidable splashing) the voluminous strongly turbulent slag layer, the refractory lining is dimensioned to define a capacity of at least twice the maximum volume of [he bath of molten metal contained by the lining. Normally, the height of the refractory lining in an SR vessel as measured from the hearth bottom is at least twice the maximum metal bath level. The vessel according to the invention further includes a cooling installation arranged in the zone of the turbulent siag layer with at ieast a major portion of the installation above the maximum level of the bath of molten metal for cooling the inner refractory lining in this zone, where wear of the lining is most pronounced in an SR vessel. In order to achieve the aforementioned object, the present invention proposes that the cooling installation comprises at least one row of copper slabs mounted onto openings provided in the outer shell and so as to be in thermo-conductive contact with the inner refractory lining in the zone of the turbulent slag layer and at least one row

of spray cooling devices associated to the row of copper slabs for spraying liquid coolant onto the copper slabs through the openings.

[0013] As will be appreciated, the proposed kind of cooling installation, especially in view of its large scale, allows notable savings in constructional cost compared to known cooling systems for SR vessels. In fact, both stave coolers and plate coolers have a relatively high cost per item, whereas the present invention replaces their function by relatively inexpensive massive copper slabs devoid of internal cooling channels (and related circuit connections) using associated external spray cooling means.

[0014] To eliminate the need for vapour treatment installations, the cooling installation comprises sensors, e.g. temperature sensors measuring slab temperatures at selected copper slabs, and a process control system to which the sensors are connected and which is adapted to control the spraying of liquid coolant onto the copper slabs and maintaining average or maximum slab temperature below the evaporation temperature of the liquid coolant. In case water is used as coolant, the slab temperature is preferably maintained below 8O ⁰ C ₁ more preferably below 8O ⁰ C during operation. Advantageously, the installation further comprises spraying nozzles with the control system controlling the spraying of liquid coolant onto the copper slabs so as to avoid creation of a continuous coolant film on the copper slabs by creating a plurality of streamlets of coolant trickling on the copper slabs.

[0015] In order to enhance heat dissipation, the refractory lining may comprise MgO or MgO-C based refractory bricks being in thermo-conduclive contact with the copper slabs. Typically, in order to cover the considerable scale of the critical zone of turbulence, the row(s) of copper slabs cover a vertical zone of the refractory lining that extends substantially from the maximum level of the bath, or alternatively from the minimum level of the bath, to at least 1 m (usually as far as 1.5-2m or even further) above this maximum level. To enable use of less expensive slabs of reduced height and/or to adapt to non cylindrical vessel shapes, a plurality of rows of copper slabs can be arranged one above another so as to extend substantially from the hot metal tapping level (i.e. the minimum bath level) of the vessel to the upper edge of the refractory lining, each row preferably entirely surrounding the refractory lining. In order to enable use of spray cooling nozzles with limited spray cone angle (e.g. hydraulic spillback nozzles), at least two rows of spray cooling devices arranged one above another can be associated to one given row of copper slabs. Depending on the specified slag volume, the smelting reduction vessel may have a refractory lining dimensioned to define a capacity of at least three times the maximum volume of the bath of molten metal and/or with a refractory lining that has a

height exceeding the maximum depth of the bath of molten metal by at least 1 m in order to contain the voluminous turbulent siag layer.

[0016] The invention also concerns the method of operating the SR vessel as set out above in order produce hot metal. In case of a oxy-coal or hot-blast coal smelter using no electric power for heat generation, carbonaceous material, preferably non-coking coal, is fed into siag layer at a rate of at least 700kg C / (h ^* m ² ) for using combustion of carbonaceous material as process heat source. The carbonaceous fuel and reducing agent can be pneumatically injected or gravity fed. As a consequence of post-combustion in the upper region of the slag layer and heat transfer due to turbulence, the turbulent siag layer is (on spatial average) overheated by at least 100 ⁰ C, possibly >200 ^< € with respect to the melting temperature of the molten metal bath, and thereby improves iron smelting and reduction.

Brief Description of the Drawings

[0017] Further details and advantages of the present invention will be apparent from the following detailed description of several not limiting embodiments with reference to the attached drawings, wherein:

FIG.1 is a schematic vertical cross sectional view of a first embodiment of a metallurgical smelting reduction vessel according to the invention;

FIG.2 is a schematic vertical cross sectional view of a second embodiment of a metallurgical smelting reduction vessel according to the invention;

FIG.3 is a schematic horizontal cross sectional view of the embodiment of FIG.2;

FIG.4 is a vertical cross sectional view of a cooling installation suitable for use in a smelting reduction vessel according to the invention.

Description of Preferred Embodiments

[0018] A smelting reduction (SR) vessel 10 of the oxy-coal smelter type is schematically shown in FIG.1. The SR vessel 10 comprises an outer shell 12 made of steel. The shell 12 is inwardly lined by means of brickwork made of refractory bricks 14. The bricks 14 form a lateral inner refractory lining 16. For reasons that will become apparent below, it is preferable to use thermally conductive bricks such as MgO or MgO-C based refractory bricks 14. Although not shown in FIG.1 , similar refractory bricks typically also line the bottom of the SR vessel 10. Other parts of the vessel 10, such as the vessel roof 17 and the lateral walls above the refractory lining 16 are protected by other measures e.g. by gunited refractory or cooling panels know as such (and not shown).

[0019] As shown in FIG.1 , the inner refractory lining 16 defines a crucible, i.e. a reaction space containing a bath of molten metal 18, typically 3-4% carbon content pig iron, and a layer of predominantly liquid slag 20 above the bath during operation. The SR vessel 10 has a charging chute 22 arranged in the vessel roof 17 for gravity feeding raw materials such as a feed 24 of iron ore fines/droplets or other iron oxides into the reaction space defined by the refractory lining 16. Typically, the iron oxides in the feed 24 have previously been pre-reduced and pre-heated e.g. in a hearth furnace for solid-state direct reduction or in a melting cyclone as known from the CCF process. A number of oxygen lances 26 (only one being shown in FIG.1 ) are distributed in the SR vessel 10 through the vessel roof 17 or the lateral shell 12 to provide secondary oxygen for post-combustion in the slag layer 20. The oxygen lances 26 are configured to inject a (soft) blow of oxygen 28 directly into the slag layer 20. The blow of oxygen 28 is generally slanting and more precisely directed downwards and toward the centre of the reaction space within the lining 16. Alternatively, the lances 26 can be configured as combined hard blow and soft blow lances additionally providing a high velocity primary oxygen blow (as shown in FIG.2) for enhancing carbon combustion. FIG.1 further shows one of a set of pneumatic carbon injection lances 30 for feeding carbonaceous material such as non-coking coal serving as reductive for iron oxides and as a fuel for heat generation, normally accompanied by flux additives for adjusting the chemical composition of the bath 18 and the slag 20. The injection lances 30 provide a hard blow of pulverized carbonaceous material 32 at high velocity and high pressure i.e. a blow that goes directly into and through the slag layer 20 to penetrate the surface of the metal bath 18. In order to further promote chemical reactions at the interface between the bath and the slag layer 20, the SR vessel 10 is equipped with bottom gas blowing devices 34, e.g. gas permeable refractory bricks equipped with an inert gas supply, of known configuration for bubbling the bath 18 so as to agitate the interface between bath 18 and slag layer 20 and thereby also the slag layer 20 itself. As an alternative to the central gravity feed 24 of FIG.1 , raw material could be fed by pneumatic injection lances or gravity feed chutes passing laterally through the shell (both not shown). Furthermore, provided sufficient turbulence of the slag layer 20 is warranted, the oxygen injection lances 26 may be replaced by a hot blast or oxygen enriched hot blast injection system.

[0020] Chemical and metallurgical aspects of SR reactions taking place in the vessel 10 are well known to the skilled person and will therefore not be detailed here. Noteworthy in the context of the present invention however are certain characteristics of the slag layer 20.

[0021 ] To warrant efficient use of heat released by (post-)combustion, especially within the upper region of the slag layer 20, this heat must be transported to a lower region, i.e. the main region of reduction reactions while ensuring minimal heat losses e.g. through the off-gases. Additionally, local overheating detrimental to the vessel 10 and in particular the lining 16 must be avoided. The latter aims require (among others) that there is considerable circulation i.e. strong turbulence as indicated by arrows 36 within the slag layer 20 itself and also at the boundary between the slag layer 20 and the molten metal bath 18. Such turbulence at the boundary creates a zone of intense mixing of slag, molten iron and carbon particles. This mixing zone is where most of the iron oxide is reduced. Thus, due to the nature of the SR process, the slag layer 20 is typically strongly turbulent during operation. In the SR vessel 10 as shown in FIG.1 the required turbulence is ensured by a combination of measures, namely by hard blow injections of carbon, with the blow directions and origins suitably chosen of the lances 30, as well as by bottom bubbling using the bottom gas blowing devices 34.

[0022] For the purposes of the present description, the maximum level M of the bath 18 as illustrated in FIGS.1 &2 is measured from the hearth bottom 38 and considered at quiescent i.e. non-agitated bath and corresponds either to the height of the bath 18 reached immediately before tapping in a tapped vessel or to the substantially constant bath level in a vessel equipped with skimmer (siphon type metal discharge). As one of the consequences of strong turbulence, when compared to the maximum molten metal volume (at level M) of the bath 18, the layer of slag 20 in an SR process occupies considerable volume and may typically attain a considerable thickness due to gas inclusions (causing "foaming"), e.g. in the order of 1 -2m or more. Therefore, the refractory lining 16 in the vessel 10 is dimensioned to define a capacity of at least twice the maximum volume of the bath 18 or more, e.g. three times the molten metal capacity. In a typical SR vessel, the height H of the lateral refractory lining 16 as measured from the hearth bottom 38 is therefore 2-4m depending on the useful area and vessel size. In order to increase the total capacity, the shell 12 and refractory lining 16 may be configured to define a reaction space whose cross-section increases with height as seen in FIGS.1 &2.

[0023] Another noteworthy characteristic of the slag layer 20 is due to the fact that at least part of, if not all (e.g. in case of an oxy-coal or hot blast-coal smelter) the heat required for SR reactions is provided by combustion (C + Vz O ₂ -> CO) and (post-)combustion (CO + Vz O ₂ -> CO ₂ ). To this end carbon and oxygen are injected into the slag layer 20 using the lances 26 and 30 respectively. In an oxy-coal or hot blast-coal smelter for example, carbon is injected at a rate of typically 700-1300kg C/(h ^* m ² ). As a consequence, the slag layer 20 has a very high temperature and may be overheated (with respect to the melting

point of the pig iron in the bath 18) by at least 100°C attaining temperatures of >1700 ^< € in the upper region of the layer 20. Also noteworthy, as regards chemical composition, is that the slag forming the layer 20 normally has comparatively high basicity/alkalinity.

[0024] As will be understood, the aforementioned slag characteristics intrinsic to an SR process would, without appropriate counter-measures, result in considerable wear of the refractory lining 16, especially in the zone of turbulence T of the slag layer 20. In order to reduce the wear and thus increase the service life of the refractory lining 16 in this zone T of most pronounced wear, the present invention proposes an installation and method for cooling the refractory lining 16 in the zone T, which in the presently proposed configuration were heretofore unknown.

[0025] As schematically illustrated in FIG.1 , the SR vessel 10 is equipped with copper slabs 40. The major part of the cooling installation, i.e. of the copper slabs 40 is arranged above the level M so as to be in thermo-conductive contact with the refractory lining 16 in its most wear prone zone T. In other words, the copper slabs 40 are arranged so that the range of action of the cooling system covers substantially the entire zone T. If necessary an appropriate filling compound that is thermally stable and conductive such as graphite may be used for improving thermal conduction between the slabs 40 and the bricks 40 in the zone T. In the embodiment of FIG.1 , two separate rows 41 , 42 of copper slabs 40 are arranged one above the other in stacked manner so as to vertically cover the entire zone T. Although not apparent from FIG.1 , it will be understood that irrespective of the shape of the SR vessel 10 in horizontal projection (e.g. square, rectangular or circular) each row 41 , 42 surrounds a respective partial zone of the refractory lining 16 in substantially continuous manner (except e.g. at the slag notch location), with the horizontal section of each slab 40 being appropriately chosen. The copper slabs 40 in each row 41 , 42 are disposed to be generally parallel to the outer contour of the refractory lining 16, and thus may be vertical (row 42) or slightly slanted (row 41 ) as seen in FIG.1. As will be appreciated, it has been found that cooling limited to the zone of turbulence T will considerable limit wear of the refractory lining 16 even though little if any cooling (i.e. besides thermal conduction in vertical direction) of the refractory bricks 14 below the level M is provided.

[0026] In order to provide cooling of the copper slabs 40 and consequently - by virtue of thermal conduction - cooling of the refractory bricks 14 in the zone T, each row 41 , 42 of copper slabs 40 has a number of associated rows of spray cooling devices 44. While their actual arrangement depends on the height of the copper slabs 40, two rows 45, 46 of spray cooling devices 44 are associated to each row 41 , 42 of copper slabs 40 respectively in the embodiment of FIG.1. The number of spray cooling devices 44 of a

given row that are associated to one particular slab 40 depends on the width of the slab 40 in question, with typically at least one spray cooling device 44 in each row 45, 46 being associated to each slab 40. To enable spray cooling from outside the shell 12 and secure mounting of the slabs 40, a separate and respective opening 48, e.g. in form of a cut-out, is provided in the shell 12 at least for each vertical set of slabs 40 or for each individual slab 40 respectively. As illustrated in FIG.1 , during operation of the SR vessel 10, the refractory lining is actively cooled by virtue of thermal conduction using spray cooling devices 44 spraying liquid coolant 50 onto the outer side of the copper slabs 40 through the openings 48.

[0027] In order to ensure thorough cooling of the refractory lining 16 over substantially the entire zone T, the copper slab rows 41 , 42 cover a vertical zone of the refractory lining that generally corresponds to the zone T, i.e. a zone that extends substantially from the minimum level of the bath of molten metal 18 (which in a tapped vessel may correspond to the level of hot metal immediately after tapping or, in a skimmed vessel to the constant bath level), to normally at least 1 m above the level M, and generally up to the upper edge of the refractory lining 16 as seen in FIG.1. To give a specific example, with a maximum level M of the bath 18 at 1 100mm and a refractory height H of 2600mm (measured from hearth bottom 38), the rows 41 , 42 of copper slabs span a vertical range of about 1400- 1500mm. In general, the arrangement of copper slabs will span a vertical range of about 1 -2m, depending on the vessel size.

[0028] FIG.2 shows another type of SR vessel 1 10 for illustrating a second embodiment of the invention. Features corresponding to those of the embodiment of FIG.1 are identified by identical references signs in FIG.2 and will not be detailed again hereinafter. As regards the vessel construction as such, i.e. except the cooling installation, the SR vessel 1 10 of FIG.2 basically differs from the vessel 10 of FIG.1 in that the heat supply is a dual mode heat supply, namely an electric arc supported carbon combustion heat supply. To this end, the SR vessel is equipped with three electrodes 1 1 1 , delivering a heating power that is approximately equal to the heating power developed by carbon combustion and thereby allows savings as regards the required amount of carbonaceous fuel. In the embodiment of FIG.2, carbonaceous material is fed into the vessel 1 10 together with the raw material feed 24 via the gravity feed charging chute 22, e.g. by adding excess coal to DRI originating from a hearth furnace. As a further difference, the vessel 1 10 is equipped with combined primary and secondary oxygen injection lances 126 adapted to inject a hard blow 129 through the slag layer 20 into the bath 18 (usually at a pressure in the order of 5-10bar) and a soft blow 128 into the slag layer 20 only for heat supply by carbon combustion and post-combustion respectively. Irrespective of these

differences, the slag of the layer 20 also presents the aforementioned SR-typical characteristics during operation of the vessel 1 10, namely strong turbulence (due to hard blow oxygen injection from the lances 126 and bottom blowing by the devices 34), high temperature and chemical aggressiveness. Consequently, a cooling installation of the proposed type is provided for cooling the refractory lining 16 in the turbulence zone T of the slag layer 20. In the embodiment of FIG.2, the cooling installation comprises a single row 141 of copper slabs 140 of comparatively larger height for covering the zone T. The row 141 of copper slabs 140 is arranged mainly above the level M as seen in FIG.2 and in circumferentially surrounding manner as seen in FIG.3, which schematically shows the circular horizontal projection of the vessel 1 10. As seen in FIG.2, three rows of cooling devices 144 are associated to the single row 141 of copper slabs 140 for spraying liquid coolant 50 onto the outward face of the copper slabs 140 through corresponding openings 148 provided in the shell 12. As a further difference, the spray cooling devices 144 are provided with a water collection casing 152 for draining liquid coolant 50.

[0029] FIG.3 schematically shows the curved copper slabs 140 having a circular arc shaped section so as to adapt to the horizontal projection of the vessel 1 10 and the lining 16 in particular. Alternatively, the rear face only of the slabs can be machined to conform to the shape of the shell 12. The copper slabs 140 are arranged to completely surround the lining 16. One spray cooling device 144 in each row is associated to one copper slab 140, higher numbers being possible. Arrows in FIG.3 further illustrate the impulses conferred to the slag layer 20 and thus contributing to strong turbulence of the latter. FIG.3 also schematically illustrates the finding that a spray profile covering only a fraction (e.g. 60-90%, preferably <80%) of the outer horizontal extent (in case of a circular vessel: a fraction of the radian measure of the outer face of the slab) of the copper slabs 140 is sufficient for efficient cooling, the high thermal conductivity of copper warranting a substantially uniform temperature distribution across the slabs 140. Thus the total number of spray cooling devices 144 can be reduced.

[0030] FIG.4 shows a specific embodiment of a copper slab 240 and spray cooling device 244 combination. Material composition and shape of the copper slab 244 itself may substantially correspond to what has been described in international patent application no. PCT/EP2006/060337 with one possible difference being an increased height of the slab 244 in order to reduce the number of required rows for covering the entire zone T, the slabs having a height in the order of 500-800mm, preferably 600-750mm, for example. Thickness of the slab 244 may be in the order of 20-80mm with 20mm as a minimum in case the slab has a non uniform thickness (e.g. for adapting to a circular vessel section as shown in FIG.3 without curving the entire slab). FIG.4 further shows two spraying nozzles

260 arranged one above the other. The nozzles 260 are preferably energy efficient hydraulic spray nozzles utilizing coolant pressure as energy source to break the coolant into droplets and thus eliminating the need for a pressurized gas supply as required for atomized spray nozzles. More preferably, the nozzles 260 are hydraulic spillback nozzles allowing adjustment of the coolant discharge rate without affecting droplet size and coolant pressure. Due to the cone angle of the spraying nozzles 260 (e.g. 80-120°) and the limited distance between the nozzle tip and the copper slab 240, a number of nozzles 260 is required to cover a substantial vertical range of the slab 240 with coolant. The nozzles 260 are to be connected to a pressurized water supply and are conveniently mounted to a removable rear panel of a water collection casing 252 equipped with a drain 254 for draining used coolant. Except for the dimensions, other features of the casing 252 (e.g. the mount for a temperature sensor for measuring the temperature of the slab 240) correspond to what has been described in application PCT/EP2006/060337 and are therefore omitted here.

[0031] In relation to the preceding description, several aspects relating to the preferred mode of operation of the proposed cooling installation shall be noted. For the purpose of temperature measurement, a selection of copper slabs 40, 140, 240 is made e.g. in accordance with the hottest regions of the temperature profile. The selected slabs 40, 140, 240 are equipped with dedicated temperature sensors (not shown) in order to monitor slab temperatures. Alternatively, each individual slab 40, 140, 240 can be equipped with a temperature sensor. The temperature sensors are connected to a process control system (not shown), which is also connected to actuators such as valves and pumps for setting at least the flow rate and preferably also the temperature and fluid pressure of the coolant fed to the spray cooling devices 44, 144, 244. Depending on the desired degree of control selectivity, operation of the spray cooling devices 44, 144, 244 may be controlled for each set associated to a particular slab individually, for a number of sets associated a number of slabs collectively or, most economically, for the sets of all slabs 40, 140, 240 in common.

[0032] Classical spray cooling methods operate in evaporation mode in order to take advantage of the heat of evaporation of the coolant to achieve high cooling efficiency (considering cooling power per unit mass flow rate of coolant). The present invention in contrast, suggests avoiding evaporation (except for unavoidable remaining natural evaporation) by operating the slabs 40, 140, 240 well below the evaporation temperature of the coolant. For example, in case water is used as coolant, the slabs 40, 140, 240 are operated at temperatures below 8O ⁰ C, preferably below 6O ⁰ C. The process control systems monitors the slab temperatures and maintains slab temperature (on average)

below the set upper limit by appropriate control of the actuators. Such low temperatures at the slabs 40, 140, 240 enable a further decrease of the temperature at the hot face of the refractory lining 16, which is in contact with the layer 20 of molten high temperature slag. Wear of the lining 16 is thereby further reduced. Such low temperatures can be achieved among others by means of a sufficiently high and uninterrupted coolant flow rate (e.g. 1 -4m ³ /h) preferably complemented by sufficiently low coolant inlet temperature (well below the aimed slab temperature, e.g. «60 ⁰ C) at each spray cooling device 44, 144, 244. As will be understood, instead of temperature sensors at the slabs, the control system may also be equipped with any other sensor arrangement suitable for detecting unwanted evaporation of the coolant. Furthermore, the spray defining parameters of coolant flow rate and coolant pressure are preferably controlled to achieve a droplet size spectrum of the spray which is sufficiently large to avoid (rapid) evaporation on impacting onto the slabs the slabs 40, 140, 240 (e.g. a droplet spectrum > 200 μm), noting that too small a droplet size tends to result in evaporation even at slab temperatures below the evaporation point of the liquid coolant. By avoiding evaporation, the liquid coolant stays in liquid aggregation state during the entire cooling cycle. Thus, besides further reducing the hot face temperature of the refractory lining 16, avoiding evaporation (to the technically possible extent) allows eliminating the need for an expensive vapour collection and condensation system typically required with known evaporation mode spray cooling systems.

[0033] As another noteworthy aspect, the process control is configured to avoid formation of a continuous film of liquid coolant on the copper slabs 40, 140, 240, at least within the spray impact area. Substantially eliminating a continuous coolant film allows avoiding loss of cooling capacity due to transition film boiling (a small but heat isolating layer of vapour between the cooled surface and the continuous film). To achieve this, the nozzles of the spray cooling devices 44, 144, 244 are placed at an appropriate distance from the copper slabs 40, 140, 240 and their operating parameters, i.e. flow rate and droplet size spectrum in particular, are set to values that allow formation of a plurality of small streamlets trickling from the spray impact zone downwards on the outer cooled face of the copper slabs 40, 140, 240 towards the drain.

List of reference signs:

10 (oxy-coal type) SR vessel 41 , 42 row of copper slabs

12 outer shell 44 spray cooling device

14 refractory brick 45, 46 row of spray cooling devices

16 inner refractory lining 48 opening in the shell

17 vessel roof 50 (spray of) liquid coolant

18 bath of molten metal 1 10 SR vessel (electric arc assisted)

20 slag layer 1 1 1 electrode

22 charging chute 126 combined primary and secondary oxygen lance

24 feed of iron ore fines/droplets

128 soft blow of (secondary) oxygen

26 oxygen lance

129 hard blow of (primary) oxygen

28 soft blow of oxygen

140 copper slab

30 pneumatic carbon injection lance 141 single row of copper slabs

32 hard blow of pulverized 144 spray cooling devices carbonaceous material

148 opening in the shell

34 bottom gas bubbling devices

152 water collection casing

36 arrows indicating turbulence

240 copper slab

38 hearth bottom

244 spray cooling device

M maximum level of metal bath

252 water collection casing

H height of the refractory lining

254 drain

T zone of turbulence of the slag

260 spraying nozzles layer

40 copper slab