The term"smelting"is herein understood to mean thermal processing wherein chemical reactions that reduce metalliferous feed material take place to produce molten metal.

A known direct smelting process, which relies principally on a molten bath as a reaction medium, and is generally referred to as the HIsmelt process, is described in International application PCT/AU96/00197 (WO 96/31627) and other patent applications, such as the more recently filed International applications PCT/AU2004/000473 (W02004/090174) and PCT/AU2004/000472 (W02004/090173) (which focuses on producing molten iron from iron ore fines) in the name of the applicant.

The HIsmelt process includes the steps of: (a) forming a bath of molten metal and slag in a direct smelting vessel; (b) injecting into the bath: (i) a metalliferous feed material, typically metal oxides ; and (ii) a solid carbonaceous material,

typically coal, which acts as a reductant of the metalliferous feed material and a source of energy; and (c) smelting the metalliferous feed material to metal in the metal layer.

In the HIsmelt process the metalliferous feed material and solid carbonaceous material are injected into the molten bath through solids delivery means in the form of lances which are inclined to the vertical so as to extend downwardly and inwardly through the side wall of the direct smelting vessel and into a lower region of the vessel so as to deliver at least part of the solids material into the metal layer in the bottom of the vessel.

The HIsmelt process also includes post-combusting reaction gases, such as CO and H2 released from the bath, with a blast of hot air, which may be oxygen-enriched, that is injected into an upper region of the vessel through at least one downwardly extending hot air injection lance and transferring the heat generated by the post-combustion to the bath to contribute to the thermal energy required to smelt the metalliferous feed materials.

The hot air is produced in stoves and is supplied to the lance or lances via a refractory brick-lined hot blast main. The stoves consist of at least two individual stoves that cycle between two phases, a heating phase and a heat exchange phase. In the heat exchange phase a stove provides hot air at greater than 1000°C (herein after called"pre-heated air") to the hot air injection lance, and in the heating phase the stove regenerates the heat within its internal construction via combustion of a fuel and passing combustion products through the stove. The operation of the stoves is coordinated so that there is always at least one stove in its heat exchange phase and

providing pre-heated air at any point in time.

Off-gases resulting from the post-combustion of reaction gases in the vessel are taken away from the upper part of the vessel through an off-gas duct. The vessel includes refractory-lined water cooled panels in the side wall and the roof of the vessel, and water is circulated continuously through the panels in a continuous circuit.

The HIsmelt process enables large quantities of molten metal, such as molten iron, to be produced by direct smelting in a single compact vessel.

However, in order to achieve this it is necessary to supply to the vessel large quantities of (a) solid feed materials, such as iron-containing feed materials, carbonaceous material, and fluxes, to the solids injection lances, and (b) pre-heated air via the hot air injection lance or lances.

The supply of solid feed materials and pre-heated air to the direct smelting vessel must continue throughout a smelting campaign, which desirably is at least 12 months, and it is important that the supply of these materials be provided reliably during the period of a smelting campaign.

At the end of a smelting campaign the direct smelting vessel is shut-down to allow maintenance work, which typically includes a partial re-line or a complete re-line of the internal refractory lining of the vessel.

The shut-down period may vary considerably depending on the circumstances, ranging from periods as short as 1 month to considerably longer periods. Typically, the shut-down periods will be 8 weeks. Preferably the shut-down period is the shortest possible time.

One of the issues that face operators of the

HIsmelt process is that it is not a desirable option to completely shut-down stoves that are used to produce pre- heated air for the process at the end of a smelting campaign of only 12-18 months. This is an entirely different situation to that with hot air stoves used with blast furnaces. Blast furnaces typically operate for 20 years before requiring a re-line, and it is a viable option to completely shut-down blast furnace stoves after this length of service.

It is also not a practical option to continue to operate stoves during a shut-down of a direct smelting vessel in the same way that the stoves operate during a smelting campaign, i. e. producing very high flow rates of pre-heated air. Specifically, it is an entirely uneconomic proposition to operate stoves in that way while there is no production of metal in the direct smelting vessel.

Further, the gas used as a fuel during normal operation of stoves is usually off-gas from the smelting vessel, and this is typically not available during a shut- down.

It is known in a temporary shut-down of a smelting vessel, during which shut-down pre-heated air from stoves is not required, to continue to combust gas in the combustion chamber of a stove temporarily and to vent the combusted gas through the dome of the stove. However this does not maintain the refractory brick-lined hot blast main in a hot state, which can lead to problems with brick-work and expansion joints in the hot blast main.

In the circumstances, there is a need for a cost effective process that maintains the stoves and the hot blast main during the course of a shut-down in a way that minimises damage to the equipment.

The present invention provides a cost effective and reliable process and plant for maintaining the stoves and the hot blast main during a shut-down of a direct smelting vessel.

DISCLOSURE OF THE INVENTION The present invention provides a process and an apparatus that maintains stoves and a hot blast main that connects the stoves to a hot air (or hot oxygen-enriched air) injection lance or lances of a direct smelting vessel during the course of a shut-down of the vessel.

In particular, the process maintains the temperatures of the stoves and the hot blast main within temperature ranges that minimise damage to the stoves and the hot blast main.

According to the present invention there is provided a process for maintaining stoves and a hot blast main that connects the stoves to a hot air (or hot oxygen- enriched air) injection lance or lances of a direct smelting vessel in a hot state during the course of a shut- down of the vessel, which process comprises: (a) isolating the hot blast main from the hot air (or hot oxygen-enriched air) injection lance or lances; (b) operating a burner of each stove using a fuel gas and a stream of air and generating a stream of combustion products that flow along a gas pathway of the stove from one end towards an opposite end thereof and thereby heat refractory checkerwork of the stove in a heating phase of the stove; and

(c) transferring heat from each stove to the hot blast main in a heat exchange phase of the stove by supplying a stream of air to the said opposite end of the gas pathway and thereafter successively passing the stream of air through the gas pathway of the stove and the hot blast main, whereby the air stream is heated by heat exchange with refractory checkerwork of the stove and the stove is cooled by such heat exchange and the resultant hot air stream heats the hot blast main.

During a shut-down of the vessel the main objective is to ensure that the stoves and the hot blast main are maintained within operating temperature ranges that avoid damage to the stoves and the hot blast main.

The applicant has realised that this objective can be achieved by operating the stoves during a shut-down to create significantly different heat transfer conditions than the heat transfer conditions that are required during normal operation of the stoves to supply a hot air blast to the direct smelting vessel. The applicant has realised further that the volumetric flow rates of combustion products and hot air through the stoves are important factors in creating the required heat transfer conditions during a shut-down. In addition, the applicant has realised that the volumetric flow rates of combustion products and hot air through the stoves can be delivered by the combustion air fan that is used conventionally to supply combustion air to the burner of a stove for the heating phase of the stove. Thus, the combustion air fan can be used advantageously as a dual function fan to supply air for the heating phase and the heat exchange phase during a shut-down. In addition, the applicant has realised that a modified construction of the hot blast main is advantageous to optimise maintaining the stoves and the

hot blast main during a shut-down.

Preferably the process includes coordinating operation of the heating phase and the heat exchange phase of each stove during the shut-down so that the stoves supply a continuous hot air stream to the hot blast main during the shut-down.

Preferably the volumetric flow rate of the combustion products produced in step (b) during the shut- down is relatively small compared to the volumetric flow rate of the combustion products produced during heating phases of the stoves when the stoves are operating under normal operating conditions with the hot blast main connected to the hot air injection lance or lances.

Preferably the volumetric flow rate of the combustion products produced in step (b) during the shut- down is 50% or less of the volumetric flow rate of the combustion products produced during the normal heating phases of the stoves.

More preferably the volumetric flow rate of the combustion products produced in step (b) during the shut- down is 40% or less of the volumetric flow rate of the combustion products produced during the normal heating phases of the stoves.

Preferably the volumetric flow rate of the hot air produced in step (c) during the shut-down is relatively small compared to the volumetric flow rate of the hot air produced during heat exchange phases of the stoves when the stoves are operating under normal operating conditions with the hot blast main connected to the hot air injection lance or lances.

Preferably the volumetric flow rate of the hot

air produced in step (c) during the shut-down is 50% or less of the volumetric flow rate of the hot air produced during the normal heat exchange phases of the stoves.

More preferably the volumetric flow rate of the hot air produced in step (c) during the shut-down is 40% or less of the volumetric flow rate of the hot air produced during the normal heat exchange phases of the stoves.

Preferably the process includes using the same fan or fans to supply the streams of air to each stove during the heating and the heat exchange phases of the stove during the shutdown.

Preferably the hot air produced in step (c) vents through a vent means connected to the hot blast main.

Preferably the vent means is located proximate a forward end of the hot blast main, ie the end that is connected to the hot air injection lance or lances.

Preferably the fuel gas is natural gas.

The process may comprise additional steps during the shut-down to the above-described steps.

For example, the process may comprise a further step of transferring heat from one or more of the stoves by supplying a stream of air to the opposite end of the gas pathway of the stove or stoves and thereafter successively passing the air stream through the gas pathway and thereafter venting the air stream without passing the air stream through the hot blast main, whereby the air stream is heated by heat exchange with refractory checkerwork of the stove or stoves and the stove or stoves is cooled by such heat exchange. This process step is appropriate in situations where the temperature of the hot blast main is

within a suitable temperature range and further heat transfer to the main is not required and the stove or stoves in question are above a minimum shut-down temperature and can accommodate further heat transfer to the air stream. More preferably, this process step includes venting the hot air stream from the stove or stoves by passing the stream through off gas supply mains for the stove or stoves.

By way of further example, the process may comprise a further step of"bottling"one or more of the stoves altogether for a time period during the shut-down.

As with the preceding paragraph, this process step is appropriate in situations where the temperature of the hot blast main is within a suitable temperature range and further heat transfer to the main is not required at that time and the stove or stoves in question are above a minimum shut-down temperature.

In any situation, the duration of the above- described process steps during the shut-down will be determined by reference to a range of factors, including the factors discussed in the following paragraphs.

Typically, each stove has a main heat exchange chamber that is packed with refractory checkerwork and the said opposite end section of the gas pathway is in a lower section of the chamber and extends in a tortuous path upwardly through the checkerwork. Typically further, the checkerwork is supported on a metal grid in the lower section of the chamber.

It is important that the heating phase during a shut-down does not heat the checkerwork support grid to temperatures at which the grid loses appreciable mechanical strength, ie loss of mechanical strength to an extent that the internal structural integrity of the refractory

checkerwork is compromised.

In such a situation, preferably the process includes operating the heating phase of each stove during the shut-down until the temperature in the lower section of the main chamber, and more preferably the checkerwork support grid, approaches but does not reach a temperature at which the checkerwork support grid loses appreciable mechanical strength.

Typically, the checkerwork support grid is formed from cast iron. Cast iron starts to lose mechanical strength to an extent that is cause for concern at temperatures above 350°C.

In such situations, preferably the process includes operating the heating phase of each stove during the shut-down until the temperature in the lower section of the main chamber of the stove approaches but does not reach 350°C.

By way of further example, typically, each stove includes a dome section that is lined with silica bricks.

Silica bricks undergo a phase change at 875°C that results in a volume change and is undesirable on this basis to cool the dome section to temperatures at or below the phase change temperature during the shut-down.

In a situation in which one or more than one stove includes silica bricks in the dome section, preferably the process includes controlling the process during the shut-down so that the temperature of the dome section of the stove or stoves remains above the phase change temperature.

By way of further example, typically, the hot blast main includes a plurality of refractory brick lined

sections and a plurality of expansion joints that interconnect the bricked sections. In such a situation, thermal cycling can cause damage to the brickwork and the joints and is undesirable on this basis.

Accordingly, preferably the process includes controlling the process during the shut-down so that there is minimal temperature cycling within the hot blast main.

According to the present invention there is also provided an apparatus for pre-heating air for a direct smelting plant for producing molten metal from a metalliferous feed material, which apparatus comprises: (a) a plurality of stoves for producing streams of pre-heated air for a direct smelting plant; (b) a hot blast main for supplying pre-heated air from the stoves to a gas injection means extending into a direct smelting vessel when the plant is operating and producing molten metal from a metalliferous feed material in the vessel when the plant is operating under normal operating conditions; (c) a fuel gas supply means for supplying fuel gas to a burner of each stove during normal operating conditions of the plant and during a shut-down of the vessel; (d) a first air supply means for supplying air (i) to the burner of each stove during a heating phase of the stove during normal operating conditions of the plant, and (ii) to the burner of each stove during a heating phase of the stove during a shut-down of the

vessel; (e) a second air supply means for supplying air to each stove during a heat exchange phase of the stove during normal operating conditions of the plant; (f) a vent in the hot blast main for allowing streams of hot air generated in the heat exchange phase of each stove to flow from the hot blast main after flowing through and heating the main.

Preferably the vent includes an end plug that closes an outlet end of the vent when a direct smelting process is operating and is removed from the vent when there is a shut-down of the vessel.

Preferably the vent defines a serpentine pathway between the hot blast main and the outlet end of the vent.

The purpose of the serpentine pathway is to avoid straight line exposure of the end plug to radiant heat from the hot blast main during operation of a direct smelting process when the plug is in place and closes the outlet end.

Preferably the vent extends horizontally outwardly from the hot blast main and then upwardly and inwardly to a position above the hot blast main and thereafter upwardly to the outlet end.

The term"horizontally"is understood herein to include arrangements that are within 15° above or below a horizontal arrangement.

Preferably the vent is located proximate a forward end of the hot blast main, ie the end that is connected to the hot air injection lance or lances.

Preferably the first air supply means is adapted to supply air to a separate inlet of each stove during a heat exchange phase of the stove during a shut-down of the vessel when the second air supply means is not operational.

Preferably the apparatus comprises a valve means enables the first air supply means to switch from supplying air to the burner of each stove to the separate inlet of the stove as required during a shut-down of the vessel.

According to the present invention there is also provided a direct smelting plant for producing molten metal from a metalliferous feed material which comprises: (a) a direct smelting vessel to hold a molten bath of metal and slag and a gas space above the bath; (b) a solids feed means to supply solid feed material into the vessel; (c) a gas injection means extending downwardly into the vessel to inject pre-heated air into the gas space above the bath; (d) an off-gas duct means for facilitating flow of off-gas from the vessel away from the vessel; (e) a metal and slag tapping means for tapping molten metal and slag from the bath and transporting that molten metal away from the vessel; and (f) the above-described apparatus for pre- heating air for the vessel.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the present invention are described in more detail hereinafter with reference to the accompanying drawings, of which: Figure 1 is a diagram that illustrates the main components of one embodiment of a direct smelting plant in accordance with the present invention that are relevant to the description of the embodiment; Figure 2 is a side elevation of the direct smelting vessel of the above plant; Figure 3 is a vertical section through the hot blast main and the vent of the main of the above plant, with the vent arranged for operation of a direct smelting process; Figure 4 is a vertical section through the hot blast main and the vent of the main of the above plant, with the vent arranged for shut-down of the plant; and Figure 5 is a vertical section through the stove of the above plant; and Figure 6 is a diagram that illustrates the main components of another embodiment of a direct smelting plant in accordance with the present invention that are relevant to the description of the embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS The following description is in the context of smelting iron ore fines to produce molten iron in accordance with the HIsmelt process as described in the above-mentioned International patent application

PCT/AU96/00197. The disclosure in the patent specification lodged with the International application is incorporated herein by cross-reference.

The direct smelting plant shown in the Figures includes a direct smelting vessel 11, two stoves 27 for producing streams of hot air, a hot blast main 29 for supplying the hot air streams from the stoves 27 to the vessel 11, a cold blast blower 31, cold blast supply main 38 and cold blast transfer lines 37 for supplying air at pressure to the stoves 27 during normal operation of the stoves 27, two combustion air fans 35 and combustion air transfer lines 39a and 39b for supplying air at ambient temperature and pressure to the stoves 27 during both normal operation of the vessel and also during a shut-down of the vessel 11. The transfer lines 37,39a and 39b include control valves to control flow of air through the lines.

The vessel 11 is of the type described in detail in the above-mentioned International applications PCT/AU2004/000473 (W02004/090174) and PCT/AU2004/000472 (W02004/090173), and the disclosure in the patent specifications lodged with these applications is incorporated herein by cross-reference.

With reference initially to Figure 2, the vessel 11 has a hearth 13, a generally cylindrical barrel 15 extending upwardly from the hearth, an annular roof 17, an off-gas chamber 19, an off-gas duct 21 for discharging off- gases, a forehearth 23 for discharging molten metal continuously, and a tap-hole (not shown) for discharging molten slag during smelting.

The vessel 11 also includes a hot air injection lance 41 for delivering a hot air blast into an upper region of the vessel 11. The lance is positioned centrally

to extend downwardly through the off-gas chamber 19 into an upper region of the barrel 15. Only an upper section of the hot air injection lance 41 is visible in Figure 2. The lance is connected to the hot blast main 29.

With reference to the Figures generally, the vessel 11 also includes a plurality of solids injection lances (not shown) extending downwardly and inwardly through openings (not shown) in the side walls of the lower barrel 15 for injecting iron ore fines, solid carbonaceous material, and fluxes entrained in an oxygen-deficient carrier gas into the vessel.

The off-gas duct 21 of the vessel 11 transports the off-gas away from the vessel 11. The off-gas is split into two streams, with one stream going to the stoves 27 and the other stream going to a treatment station (not shown) for preheating the iron ore fed to the vessel 11.

The off-gas duct 21 includes a gently inclined first section 21a extending from the upper barrel 19 of the vessel 11 and a vertically extending second section 21b that extends from the first section 21a.

The hot blast main 29 is a refractory brick lined main that, typically at least is 75m long, of circular cross-section-as shown in Figures 2 to 4.

The hot blast main 29 includes a vent 61 near the downstream end thereof, proximate the hot air injection lance 41.

Figure 3 illustrates the vent 61 as it is arranged for operation of the HIsmelt process and Figure 4 illustrates the vent 61 as it is arranged during a shut- down of the HIsmelt process.

The main difference between the 2 arrangements is

that the Figure 3 arrangement includes an end plug 91 that seals the vent 61 during operation of the HIsmelt process and the Figure 4 arrangement includes an elbow section 93 that replaces the end plug 91 during a shut-down period.

The purpose of the elbow section 93 is to direct hot air flow from the vent 61 away from equipment in the vicinity of the vent and to prevent water flow into the vent 61.

A further difference is that in the arrangement of Figure 4, a blanking plate (not shown) is typically installed into the hot blast main 29 adjacent the hot air blast lance 41 during a shut down of the HIsmelt process.

The blanking plate serves to isolate the hot blast main 29 from the hot air blast lance 41 and thereby ensures that the entire hot air flow supplied to the hot blast main during a shut-down of the vessel, as described hereinafter, flows through vent 61.

With reference to Figures 3 and 4, the vent 61 defines a serpentine pathway for hot air to flow from the hot blast main 29 to atmosphere during non-operation of the vessel. The purpose of the serpentine pathway is to avoid straight line exposure of the end plug 91 to radiant heat from the hot blast main 29 during operation of the HIsmelt process when the plug 91 is in place.

The vent 61 includes a U-shaped section that has one arm 68 that extends horizontally outwardly from the hot blast main 27, a base 65 that extends vertically upwardly, and another arm 67 that extends horizontally inwardly to a position above the hot blast main 27. The vent 61 also includes a vertical section 69 that extends upwardly from the arm 67 of the U-shaped section.

In the arrangements shown in Figures 3 and 4, the arm 68 and half of the base 65 of the U-shaped section of the vent 61 are lined with refractory bricks. The

remainder of the vent 61 includes a lining of a castable material.

In a smelting operation in accordance with the HIsmelt process, the vessel 11 contains a molten bath of iron and slag which includes a layer of molten metal and a layer of molten slag on the metal layer. A suitable carrier gas transports iron ore fines, coal and flux into the molten bath through the solids injection lances. The momentum of the solid materials and the carrier gas causes the solid materials to penetrate the metal layer in the vessel 11. The coal is devolatilised and thereby produces gas in the metal layer. Carbon partially dissolves in the metal and partially remains as solid carbon. The ore fines are smelted to metal and the smelting reaction generates carbon monoxide. The gases transported into the metal layer and generated by devolatilisation and smelting reactions produce significant buoyancy uplift of molten metal, solid carbon and slag (drawn into the metal layer as a consequence of solid/gas injection) that generates upward movement of splashes, droplets and streams of molten metal, solid carbon, and slag. These splashes, droplets and streams entrain slag as they move through the slag layer.

The buoyancy uplift of molten metal, solid carbon and slag causes substantial agitation of the slag layer in the vessel, with the result that the slag layer expands in volume. In addition, the upward movement of splashes, droplets and streams of molten metal, solid carbon and slag extend into the space above the molten bath and form a transition zone.

Injection of the hot air via the hot air injection lance 41 post-combusts reaction gases, such as carbon monoxide and hydrogen (which are liberated during coal devolatilisation and smelting reactions), in the upper part of the vessel. Off-gases resulting from the post- combustion of reaction gases in the vessel are taken away

from the upper part of the vessel through the off-gas duct 21. Hot metal produced during a smelting operation is discharged from the vessel 11 through a metal tapping system that includes the forehearth 23.

Post-combustion of reaction gases generates substantial heat and a proportion of the heat transfers to the splashes, droplets and streams of molten metal, solid carbon and slag and the heat transfers to the molten bath when the splashes droplets and streams return to the bath.

The transferred heat to the bath facilitates the endothermic smelting reactions in the bath.

With reference to Figure 5, each stove 27 is of a conventional form and includes a burner (not shown) and an upright cylindrical structure (with a domed top 81) formed from an outer metal shell 83 and a refractory brick internal lining 85 and an internal vertical partition 87 that divides the structure into a combustion chamber 51 on one side of the partition and a main heat exchange chamber 57 on the other side of the partition. The heat exchange chamber 57 and the combustion chamber 51 are interconnected by a domed section 55. Together, the heat exchange chamber 57, the domed section 55, and the combustion chamber 51 define a gas pathway through the stove.

During a heating phase of each stove 27 when the vessel 11 is smelting, the burner produces a stream of combustion products which pass to the combustion chamber 51 and flow upwardly through the combustion chamber 51 into the domed section 55 of the stove 27. The combustion products then flow downwardly through a network of refractory checkerwork in the main heat exchange chamber 57 of the stove 27 and heat the checkers. Thereafter, the now-cooler combustion products flow from the stove 27 via an opening 59 in a lower section of the heat exchange chamber 57. The lower section of the heat exchange chamber

57 is formed as a plenum chamber 64 to facilitate gas flow.

In this context, the stove 27 includes a horizontally- disposed grid 63 supported by columns 65 that supports the checkers. The grid 63 and the columns 65 are formed from cast iron.

During the heating phase of each stove 27 when the vessel 11 is smelting, fuel gas in the form of discharged off-gas from the vessel 11 is supplied to the burner (not shown) and ambient temperature combustion air is supplied to the burner via the combustion air fan 35 and the transfer line 39b for the stove 27 (Figure 1) and the combustion products produced by the burner heat the stove 27.

In a heat exchange phase of each stove 27 when the vessel 11 is smelting, the burner is not operated and a stream of air is directed through the stove 27 in the opposite direction to the stream of combustion products.

Specifically, air is supplied to the opening 59 in the stove 27 and flows upwardly from the plenum chamber 64 through the heat exchange chamber 57. The air stream is heated by heat exchange with the checkers as the air stream flows through the heat exchange chamber 57. The hot air flows around the dome section 55 and downwardly through the combustion chamber 51 and leaves the chamber via hot blast opening 71 in a lower section of the combustion chamber 51.

The hot blast opening 71 is connected to the hot blast main 29.

During the heat exchange phase of each stove 27 when the vessel 11 is smelting, air under pressure (referred to as"cold blast") is supplied to the transfer line 37 for the stove 27 (Figure 1) from the cold blast blower 31, which is a high pressure fan that can deliver high flow rates of pressurised air. The resulting stream of heated air that exits the stove 27 through hot blast

opening 71 is referred to as"hot blast"or"hot air blast". The hot blast flows along the hot blast main 29 to the hot air injection lance 41 in the direct smelting vessel 11.

Typically, the HIsmelt process requires a constant flow of hot blast at a temperature of 1200°C when the vessel 11 is smelting. To achieve this, the refractory in the domed section 55 of each stove 27 is heated to temperatures above 1200°C during heating phases of each stove 27 so that the initial hot blast from the stove 7 has a temperature above the required 1200°C. The cold blast is supplied to the stove 27 until its temperature drops to 1200°C, whereupon the stove re-enters the heating phase and hot blast is obtained from the other stove 27. To achieve a constant hot blast temperature of 1200°C, some of the cold blast is mixed with the hot blast via a mixing valve 43 (see the Figure 6 embodiment) so that the average temperature of the hot blast is the required 1200°C.

During a smelting operation the HIsmelt process requires substantial amounts of hot air. Therefore, the cold blast blower 31 must be capable of producing a substantial flow rate of air to and then through the stoves 27 and along the hot blast main 29 to the hot air injection lance 41. In addition, the stoves 27 and the hot blast main 29 must be substantial in size in order to accommodate the large flow rate of air. Typically, the cold blast blower 31 delivers approximately 110,000 Nm3/h of air pressurised at approximately 170kPa (gauge). The cold blast may be enriched with approximately 30,000 Nm3/h of Oxygen so that the stoves produce approximately 140,000 Nm3/h of hot air that is supplied to the hot blast main and smelt reduction vessel during normal operation. The combustion air fans deliver approximately 74,000 Nm3/h of air at a pressure of approximately 13kPa (gauge).

The stoves 27 apso operate during a shut-down of the vessel 11 in order to maintain the temperature in the stoves 27 and the hot blast main 29.

Specifically, each stove 27 operates with heating and heat exchange phases during a shut-down of the vessel.

These phases maintain the temperature of the stoves 27 within a required temperature range and transfer heat to the hot blast main 29 to maintain the temperature of the hot blast main 29 within a required temperature range.

During the heating phase of each stove 27 when the vessel 11 is shut-down (and there is no off-gas available as a source of energy), natural gas is supplied to the burner from a source (not shown) via natural gas main 91 and a transfer line 93 and ambient temperature combustion air is supplied to the burner via the combustion air fan 35 and the transfer line 39b (Figure 1) and the combustion products produced by the burner heat the stove 27. Typically, the combustion products heat the domed section 55 of the stove 27 to temperatures of the order of 1250°C. The heating phase continues until the temperature of the cast iron horizontally-disposed checker support grid 63 and columns 65 approaches but does not reach 350°C. The basis for the selection of the temperature of 350°C is that cast iron starts to lose appreciable mechanical strength above this temperature.

During the heat exchange phase of each stove 27 during a shut-down of the vessel 11 the cold blast supplied to the opening 59 of the stove 27 via the cold blast blower 31 when the vessel 11 is smelting is replaced by air at ambient temperature and pressure. This air is supplied to the transfer line 37 from the combustion air fan 35 for the stove 27 via the transfer line 39a (Figure 1). The resulting hot air stream exits the stoves 27, via hot blast opening 71, and flows along the hot blast main 29 to the

vent 61 from which it discharges. The hot air stream heats the main 29 so that the temperature in the main 29 is above a predetermined minimum temperature. The combustion air fan 35 delivers a sufficient flow rate of air to meet the heat transfer requirements during a shut-down. The heat exhange phase continues until the dome section 55 of the stove cools to 900°C. At temperatures below this temperature silica bricks in the domed section 55 undergo a phase changes that results in an undesirable volume change of the bricks.

Preferably the timing of the heating and heat exchange phases for both stoves 27 during a shut-down are controlled so that there is no overlap of these phases and one stove 27 operates in the heating phase while the other stove operates in the heat exchange phase and vice versa.

The process also includes an optional step of diverting the heated air streams produced in the heat exchange phases of the stoves 27 away from the hot blast main 29 in situations where the main is within a required temperature range and further heating is not required.

The process also includes an optional step of bottling the stoves 27 altogether, again in situations where the stoves and the hot blast main 29 are within a required temperature range and further heating is not required.

Figure 6 illustrates an alternative, although not the only possible alternative embodiment, to the embodiment shown in Figure 1. Both embodiments include combustion air fans. However, in Figure 1, the fans operate independently and supply separate combustion air transfer lines 39a and 39b. In Figure 6 the fans operate to supply a single combustion air main 42 which then feeds combustion air transfer lines 39a and 39b. This provides some redundancy

in the combustion air system and allows for maintenance on the fans during a smelting campaign. It also allows the fans to operate in tandem so that a combined air flow can be provided.

Many modifications may be made to the embodiments of the present invention described above without departing from the spirit and scope of the invention.

By way of example, whilst the present invention has been described in the context of a direct smelting process, it can readily be appreciated that the described process for maintaining the stoves and hot blast main is not so limited and extends to stoves and hot blast mains that are used in other applications.