AU2005217667B2

AU2005217667B2 - Direct smelting plant and process

Info

Publication number: AU2005217667B2
Application number: AU2005217667A
Authority: AU
Inventors: Rodney James Dry
Original assignee: Technological Resources Pty Ltd
Current assignee: Technological Resources Pty Ltd
Priority date: 2004-02-27
Filing date: 2005-02-28
Publication date: 2009-12-03
Anticipated expiration: 2025-02-28
Also published as: AU2005217667A1

Description

WO 2005/083130 PCT/AU2005/000284 DIRECT SMELTING PLANT AND PROCESS TECHNICAL FIELD 5 The present invention relates to a direct smelting plant and a direct smelting process for producing molten metal from a metalliferous feed material such as ores, partly reduced ores and metal-containing waste streams. 10 The term "smelting" is herein understood to mean thermal processing wherein chemical reactions that reduce metalliferous feed material take place to produce molten metal. 15 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) 20 and other patent applications, such as the more recently filed International applications PCT/AU2004/000472 (WO 2004/090173) and PCT/AU2004/000473 (WO 2004/090174) (which focuses on producing molten iron from iron ore fines) in the name of the applicant. 25 The HIsmelt process includes: (a) forming a bath of molten metal and slag in a direct smelting vessel; 30 (b) injecting into the bath via solid material injection lances: (i) a metalliferous feed material, 35 typically metal oxides; and (ii) a solid carbonaceous material, WO 2005/083130 PCT/AU2005/000284 2 typically coal, which acts as a reductant of the metalliferous feed material and a source of energy; and 5 (c) smelting the metalliferous feed material to metal in the metal layer. The HIsmelt process also includes post-combusting reaction gases, such as CO and H 2 released from the bath, 10 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 15 smelt the metalliferous feed materials. The HIsmelt process also includes producing the hot air blast in stoves and supplying the hot air blast to the lance or lances via a refractory brick-lined main. The 20 stoves consist of at least two individual stoves that cycle between two main phases, namely a heating phase and a heat exchange phase. In the heat exchange phase a stove provides hot air at greater than 1000 0 C to a hot air injection lance, and in the heating phase the stove 25 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 hot air at any point 30 in time to the direct smelting vessel. The HIsmelt process also includes processing off gas released from the vessel, typically at a temperature of 1450 0 C, to recover heat from the off-gas and clean the off 35 gas before releasing the cooled and cleaned off-gas into the atmosphere. Possible off-gas processing options include (a) using off-gas as a source of energy for heating WO 2005/083130 PCT/AU2005/000284 3 stoves, (b) generating steam and/or electricity from off gas, and (c) using off-gas as a source of energy for pre treating metalliferous feed material prior to supplying the feed material to the vessel. The pretreatment options 5 include pre-heating and/or pre-reducing metalliferous feed material. The HIsmelt process enables large quantities of molten metal, such as molten iron, to be produced by direct 10 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, 15 carbonaceous material, and fluxes, to the solids injection lances, and (b) hot air via the hot air injection lance or lances. Further to the preceding paragraph, the HIsmelt 20 process generates large quantities of off-gas that are at high temperatures and are potentially valuable sources of energy. Off-gas processing that efficiently recovers and utilises energy can have a substantial impact on the operating costs of the process and is a desirable objective 25 on this basis. However, it is important that off-gas processing that involves the use of off-gas in direct smelting plant operations, such as pretreatment units or stoves, does not overly complicate the operation of the plant and is properly integrated with the plant. 30 The present invention provides a direct smelting process that effectively and efficiently processes off-gas in an integrated way within a direct smelting plant. 35 DISCLOSURE OF THE INVENTION In accordance with the invention, a stream of WO 2005/083130 PCT/AU2005/000284 4 off-gas from a direct smelting vessel is split into at least two streams. One of the streams is supplied to a plurality of stoves that produce hot air for the vessel and is used as a source of energy and combusted in the stoves. 5 Another off-gas stream is supplied to a waste heat recovery unit and is used to generate steam and/or electricity. Preferably the steam and/or electricity are used in the direct smelting plant. Each stove has a repeating sequence of operating phases that comprises a heating phase, a so 10 called bottling phase, and a heat exchange phase that is a longer time period than the heating phase. The term "bottling phase" is understood herein to mean a phase in which a stove is closed and the stove is neither heated by combusted off-gas nor cooled by heat exchange with an air 15 stream. In practice, the duration of the bottling phase of a given stove is at least the amount of time required to open and close the valves necessary to change-over the off gas and air streams so as to switch over (a) the given stove from a heating phase to a heat exchange phase and (b) 20 another stove from a heat exchange phase to a heating phase. The duration of the heating, bottling, and heat exchange phases of the stoves are selected to ensure that there is a continuous, uninterrupted flow of hot, oxygen enriched air to the vessel. In a situation in which there 25 are two stoves only, the stoves do not otherwise require off-gas during the bottling phases of the stoves and thus all of the stove off-gas is in effect diverted to the waste gas recovery unit during the bottling phases and is used in that unit. In a situation in which there are three or more 30 stoves, and there is overlap between the respective time periods of heating, bottling and heat exchange phases of the stoves, the off-gas requirements for the stoves are more complicated. Specifically, the overlap of the phases of the stoves makes it probable that one of the stoves will 35 require off-gas for a heating phase during at least part of the time that one of the other stoves is in a bottling phase. Thus, the off-gas requirements for three or more WO 2005/083130 PCT/AU2005/000284 5 stove operations are more variable than for a two stove only operation. The end result for both two and three or more stove operations is that varying amounts of off-gas are supplied to the waste gas recovery unit as the off-gas 5 requirements for the stoves change. Accordingly, the waste gas recovery unit is arranged to adjust its operating parameters to accommodate the resultant substantially different flow rates of off-gas to the unit. 10 The direct smelting process of the invention comprises supplying solid metalliferous feed material (such as ferrous feed material) and solid carbonaceous material to a direct smelting vessel containing a bath of molten metal and slag and smelting the metalliferous feed material 15 to molten metal in the vessel, supplying hot air to the vessel and post combusting reaction gases generated in the vessel and thereby producing heat required to continue process reactions in the vessel, and releasing off-gas from the vessel, producing hot air for the vessel by'operating a 20 plurality of stoves so that each stove has a sequence of operating phases that comprise a heating phase, a bottling phase, and a heat exchange phase that is a longer time period than the heating phase, and processing at least part of the off-gas released from the vessel by; 25 (a) cooling the off-gas and particulate material carried in the off-gas; (b) removing particulate material from the 30 cooled off-gas; (c) using at least part of the cooled and cleaned off-gas as required as a source of energy for heating the stoves in the heating WO 2005/083130 PCT/AU2005/000284 6 phases of the stoves for producing hot air for the vessel; (d) using at least part of the unused balance of 5 the cooled and cleaned off-gas as a source of energy in a waste heat recovery unit and thereby generating steam and/or electricity; and 10 (e) adjusting operating conditions in the waste heat recovery unit to accommodate variations in off-gas supplied to the waste heat recovery unit. 15 One consequence of step (c) above and the operation of the stoves with heating, bottling, and heat exchange phases is that there are substantial variations in off-gas flow rates to the waste heat recovery unit. 20 In a typical operation, there is an increase in off-gas flow rate to the waste heat recovery unit exceeding 20% in periods of low off-gas demand in the stoves. Typically, the increase in off-gas flow rate to 25 the waste heat recovery unit in periods of low off-gas demand is at least 40%. Preferably the waste heat recovery unit comprises a boiler for combusting off-gas with air and generating 30 superheated steam and the process comprises varying the flow rate of air to the waste heat recovery unit to accommodate varying flow rates of unused off-gas to the boiler in step (d) as a consequence of varying off-gas requirements of the stoves. 35 WO 2005/083130 PCT/AU2005/000284 7 Preferably the process also comprises increasing the flow rate of air to the boiler a predetermined time period before a bottling phase commences and there is a consequential reduction in the demand for off-gas in the 5 stoves and an increase in off-gas flow rate to the waste heat recovery unit. The predetermined time period may be any suitable time period given the operational constraints of the plant 10 and process. A typical time period is 30 seconds. Preferably the off-gas processed in steps (b) to (d) above comprises 55-65% by volume of the total amount of off-gas released from the vessel. 15 Preferably step (c) comprises using the off-gas as the source of energy for heating the stoves in the heating phases of the stoves by combusting the off-gas with air in burners of the stoves. 20 Preferably the process also comprises preheating the cooled and cleaned off-gas prior to using the off-gas in the stove burners. 25 Preferably the process also comprises using the combustion products produced in the stove burners after the products have been used to heat the stoves to preheat cooled and cleaned off-gas prior to using the off-gas in the stove burners. 30 Preferably the process also comprises processing at least some of the off-gas released from the vessel by using the off-gas for pretreating solid metalliferous feed material in a pretreatment unit by heating and partially 35 reducing the solid metalliferous feed material prior to supplying the solid metalliferous feed material to the vessel.

WO 2005/083130 PCT/AU2005/000284 8 Preferably the off-gas processed in the pretreatment unit comprises 35-45% by volume of the total amount of off-gas released from the vessel. 5 Preferably the process also comprises processing off-gas discharged from the pretreatment unit by using the off-gas as an additional source of energy in the waste heat recovery unit. 10 Preferably the process comprises processing the off-gas discharged from the pre-treatment unit by removing particulate material from the off-gas and cooling the off gas and removing moisture from the off-gas and forming a 15 fuel gas for the waste heat recovery unit. Preferably the process comprises splitting the off-gas released from the vessel into at least two streams, with one stream being processed by using the off-gas in the 20 stoves and the waste heat recovery unit and the other stream being processed by using the off-gas in the pretreatment unit. Preferably step (a) comprises cooling the off-gas 25 to a temperature of 1000*C or below. In addition to removing particulate material, preferably step (a) comprises cooling the off-gas from a vessel discharge temperature of at least 1400 0 C. 30 Preferably step (b) also comprises removing soluble gases and metal vapors from the cooled off-gas. Preferably step (b) also comprises further 35 cooling the off-gas to a temperature less than 100 0 C, more preferably in the range of 65-90 0 C, and removing moisture from the off-gas.

WO 2005/083130 PCT/AU2005/000284 9 Preferably step (b) also comprises further cooling the off-gas to a temperature less than 50*C, more preferably in the range of 30-45 0 C, and removing moisture 5 from the off-gas and forming a cooled and cleaned gas that is suitable to be used as a fuel gas for the waste heat recovery unit and the stoves. According to the present invention there is also 10 provided a direct smelting plant for producing molten metal from a metalliferous feed material which comprises: (a) a direct smelting vessel for containing a bath of molten metal and slag and smelting 15 the metalliferous feed material to metal in the vessel, the vessel comprising a solids feed means for supplying solid feed material into the vessel and a gas injection means for injecting hot air into the vessel; 20 (b) a plurality of stoves for producing hot air for the vessel; (c) a means for processing off-gas from the 25 vessel, the processing means comprising gas cooling means for cooling off-gas, particulate material removal means for removing particulate material from cooled off-gas, the stoves for combusting cooled 30 and cleaned off-gas in heating phases of the stoves, a waste gas recovery unit for combusting cooled and cleaned off-gas and generating steam and/or electricity, and the waste gas recovery unit comprising a means 35 for adjusting operating conditions in the waste heat recovery unit to accommodate variations in-off-gas supplied to the waste WO 2005/083130 PCT/AU2005/000284 10 heat recovery unit. Preferably the waste heat recovery unit comprises a boiler for combusting off-gas with air and generating 5 superheated steam. Preferably the means for adjusting operating conditions in the waste heat recovery unit comprises a means for varying the flow rate of air to the waste heat 10 recovery unit to accommodate varying flow rates of off-gas to the boiler as a consequence of varying off-gas requirements of the stoves. Preferably the means for varying the flow rate of 15 air to the waste heat recovery unit is adapted to increase the flow rate of air to the boiler a predetermined time period before a bottling phase commences and there is a consequential reduction in the demand for off-gas in the stoves and an increase in off-gas flow rate to the waste 20 heat recovery unit. Preferably the means for processing off-gas from the vessel further comprises a means for removing moisture from the cooled and cleaned off-gas. 25 Preferably the plant further comprises a pretreatment unit for pretreating solid metalliferous feed material by heating and partially reducing the solid metalliferous feed material prior to supplying the solid metalliferous feed material to the vessel. 30 Preferably the means for processing off-gas from the vessel further comprises the pretreatment unit, wherein the pre-treatment unit is adaped for processing some of the off-gas from the vessel by using the off-gas to heat and 35 partially reduce the solid metalliferous feed material. Preferably the the waste heat recovery unit is WO 2005/083130 PCT/AU2005/000284 11 adapted for combusting off-gas from the pretreatment unit as well as off-gas that has been processed in the off-gas cooling and the particulate material removal means and for generating steam and/or electricity. 5 Preferably the waste heat recovery unit comprises a means for mixing off-gas from the pretreatment unit and off-gas that has been processed in the off-gas cooling and the particulate material removal means and thereafter 10 injecting the mixed off-gas into a burner of the waste heat recovery unit. BRIEF DESCRIPTION OF THE DRAWINGS 15 The present invention is described in more detail hereinafter with reference to the accompanying drawing which is a diagram that shows an embodiment of a direct smelting plant in accordance with the present invention. 20 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following description of the plant shown in the figure is in the context of using the plant to smelt iron ore fines to produce molten iron in accordance with 25 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. 30 The process is based on the use of a smelt reduction vessel 3. The vessel 3 is of the type described in detail 35 in the above-mentioned International applications PCT/AU2004/000472 and PCT/AU2004/000473 and the disclosure in the patent specifications lodged with these applications WO 2005/083130 PCT/AU2005/000284 12 is incorporated herein by cross-reference. In use, the vessel 3 contains a molten iron bath. Ferrous feed material (such as iron ore fines, iron-bearing 5 steel plant wastes or DRI fines), coal and fluxes (lime and dolomite) are directly injected into the bath via a plurality of water-cooled solids injection lances 5. Specifically, one set of lances 5 is used for 10 injecting hot, prereduced and preheated ferrous feed material and another set of lances 5 is used for injecting coal and fluxes. The lances 5 are water cooled to protect them 15 from the high temperatures inside the vessel 3. The lances are typically lined with a high wear resistant material in order to protect them from abrasion by the gas/solids mixture being injected at high velocity. 20 Ferrous feed material is pretreated by being preheated and prereduced in a fluidised bed preheater 17 before being injected into the bath. Coal and fluxes are stored in a series of lock 25 hoppers 25 before being injected into the bath. The coal is supplied to the lock hoppers 25 via a coal drying and milling plant 71. The injected coal de-volatilises in the bath, 30 thereby liberating H 2 and CO. These gases act as reductants and sources of energy. The carbon in the coal is rapidly dissolved in the bath. The dissolved carbon and the solid carbon also act as reductants, producing CO as a product of reduction. The injected ferrous feed material 35 is smelted to molten iron in the bath. The typical reduction reactions involved in WO 2005/083130 PCT/AU2005/000284 13 smelting injected ferrous feed material to molten iron that occur in the bath are endothermic. The energy required to sustain the process and, more particularly these endothermic reactions, is provided by reacting CO and H 2 5 released from the bath with oxygen-enriched air injected at high temperatures, typically 1200 0 C, into the vessel 3 via a hot air blast ("HAB") lance 7 extending into a top space of the vessel 3. 10 Energy released from the above-described post combustion reactions in the vessel top space is transferred to the molten iron bath via a "transition zone" in the form of highly turbulent regions above the bath that contain droplets of slag and iron. The droplets are heated in the 15 transition zone by the heat generated from post combustion reactions and return to the slag/iron bath thereby transferring energy to the bath. The hot, oxygen-enriched air injected into the 20 vessel 3 via the HAB lance 7 is generated in a pair of hot blast stoves 11 by passing a stream of oxygen-enriched air (nominally containing 30 to 35% by volume 02) through the stoves 11 and heating the air and thereafter transferring the hot oxygen-enriched air to the HAB lance 7 via a hot 25 blast main 41. The operation of the pair of stoves 11 is coordinated to ensure that there is a continuous, uninterrupted flow of hot, oxygen-enriched air at a 30 constant straight line temperature in the main 41 to the HAB lance 7. Each stove 11 operates in accordance with a repeating sequence of phases that comprises a heating 35 phase, a bottling phase, and a heat exchange phase that is a longer time period than the heating phase.

WO 2005/083130 PCT/AU2005/000284 14 The stoves 11 are heated during heating phases of the stoves 11 by combusting (a) cooled and cleaned off-gas from the vessel 3 and (b) combustion air in burners of the stoves 11 and thereafter passing the combustion products 5 through the stoves 11. During heat exchange phases of the stoves 11, oxygen from an oxygen plant 29 is mixed into streams of pressurised air generated by a blower 31. These oxygen 10 enriched air streams are passed through the stoves 11 and are heated in the stoves 11 and thereby produce the hot, oxygen-enriched air streams for the vessel 3. These hot, oxygen-enriched air streams are often referred to as "hot blast" or "hot air blast". 15 The bottling phases of the stoves 11 are phases in which one of the stoves is essentially closed and is neither heated by combusted off-gas nor cooled by heat exchange with air streams. 20 The duration of the bottling phases of a given stove 11 is at least the amount of time required to open and close the valves necessary to change-over off-gas and hot air streams so as to switch over (a) the given stove 25 from a heating phase to a heat exchange phase and (b) the other stove from a heat exchange phase to a heating phase. Combustion products released from the stoves 11 during heating phases of the stoves 11 are cleaned in a 30 flue gas desulphurisation (FGD) system 13. The FGD removes sulphur, which typically occurs in the form of hydrogen sulphide (H 2 S) and sulphur dioxide (SO 2 ), from the combustion products. The off-gas produced in the vessel 3 contains sulphur and the sulphur is not totally removed in 35 the off-gas cleaning that occurs downstream of the vessel 3 before the off-gas reaches the stoves 11, as described hereinafter.

WO 2005/083130 PCT/AU2005/000284 15 Prior to being passed to the FGD system, combustion products released from the stoves 11 during heating phases of the stoves 11 are passed through heat 5 exchangers (not shown) and preheat cooled and cleaned off gas from the vessel 3 and combustion air before the heated off-gas and combustion air is supplied as feed materials to the burners of the stoves 11 during heating phases. The vessel off-gas and combustion air may be preheated to a 10 temperature of around 180 0 C. Off-gas is released from the vessel 3 via an off gas duct 9 in an upper section of the vessel 3 and passes initially through a radiation cooler, hereinafter referred 15 to as an "off-gas hood", 15. The off-gas is cooled as it passes through the off-gas hood 15 and thereby results in the generation of steam which accumulates in steam drum 35. The off-gas hood may be of a type described in US patent 6,585,929 that cools and partially cleans off-gas. 20 The off-gas stream leaving the off-gas hood 15 is at a temperature of approximately 1000 0 C and is split into two streams. 25 One split stream, which contains approximately 35-45% by volume of the off-gas stream, is passed through the fluidised bed preheater 17 for ferrous feed material. The preheater 17 removes moisture from and preheats and prereduces ferrous feed material. The off-gas is a source 30 of energy and a fluidising gas in the preheater 17. The off-gas released from the preheater 17 is passed through a cyclone 61 and entrained dust is separated from the off-gas. The off-gas then passes through a wet 35 cone scrubber 63 that removes particulate material and soluble gaseous species and metal vapours from the off-gas and cools the off-gas from between 500 0 C and 200 0 C to below WO 2005/083130 PCT/AU2005/000284 16 100*C and typically between 65*C and 90 0 C. The off-gas from the scrubber 63 then passes through an off-gas cooler 65 that further cools the off-gas to below 50 0 C, typically between 30 0 C and 45 0 C, to remove sufficient moisture from 5 the off-gas for it to be used as a fuel gas. Typically the off-gas leaving the cooler has 5% or less H 2 O and a mist content of less than 10mg/Nm3 and typically 5.0 mg/Nm 3 . As is described further hereinafter, the cooled and cleaned off-gas is then used as a fuel gas in a waste heat recovery 10 (WHR) system 25. The other split off-gas stream leaving the off gas hood 15, which comprises between 55-65% of the off-gas from the vessel 3, passes through a wet cone scrubber 21. 15 The scrubber 21 removes particulate material and soluble gaseous species and metal vapours from the off-gas and further cools the off-gas from approximately 1000*C to below 100 0 C and typically between 65 0 C and 90 0 C. The off gas from the scrubber 21 then passes through an off-gas 20 cooler 23 that further cools the off-gas to below 50 0 C, typically between 30 0 C and 45 0 C, to remove sufficient moisture from the off-gas for it to be used as a fuel gas. Typically the off-gas leaving the cooler has 5% or less H 2 0 and a mist content of less than 10mg/Nm 3 and typically 5.0 25 mg/Nm 3 . The resulting off-gas is suitable for use as a fuel gas (a) in the stoves 11 (as described above) and (b) the WHR system 25. In addition, the scrubbed and cooled 30 off-gas is suitable for drying coal in the drying and milling plant 71. For the above purposes, the off-gas from the off gas cooler 23 is split into three streams, with one stream 35 being passed to the stoves 11, another stream being passed to the WHR system 25, and the third stream being passed to the drying and milling plant 71.

WO 2005/083130 PCT/AU2005/000284 17 The off-gas stream from off-gas cooler 23 is a relatively rich off-gas. The stream that is passed to the WHR system 25 is mixed with the cooled and cleaned off-gas 5 that has passed through the preheater 17, which is a relatively lean off-gas, due to some pre-reduction of the metalliferous feed material in the pre-heater by CO and H 2 in the off-gas. 10 The combined off-gas stream has a calorific value that makes it suitable for combustion as a fuel gas. The combined off-gas stream and air are supplied to and combusted in the WHR system 25. 15 The combined off-gas stream is combusted within the WHR system 25 in a manner that maximises CO destruction, while minimising NOx formation. The off-gas released from the WHR system 25 is 20 combined with off-gas gas from the stoves 11 and then passes to the FGD system 13. SO 2 is removed in the FGD system 23 and the exhaust gas is released to the atmosphere via a stack 45. 25 The WHR system 25 includes: * a thermal oxidiser, ie burner assembly, 37; e a WHR unit, ie boiler, 39; * a steam drum; and 30 0 heat exchange equipment, such as superheat coils and a demineralised water economiser. The WHR system 25 produces saturated steam. The saturated steam is combined with the saturated steam from 35 the steam drum 35 of the off-gas hood 15 and the superheat coils of the WHR system 25 generates superheated steam from the saturated steam.

WO 2005/083130 PCT/AU2005/000284 18 Typically, the WHR thermal oxidiser 37 is a cylindrical carbon steel shell, with internal refractory and insulation. 5 In use, the WHR thermal oxidiser 37 operates with varying combined off-gas flow rates due to the variations in off-gas required by the stoves 11. 10 Specifically, as is described above, the split off-gas stream to the WHR system 25 has a substantially higher flow rate when the stoves 11 are operating in the bottling phases of the stoves. As is described above, substantially lower amounts of off-gas are required by the 15 stoves during bottling phases of the stoves 11 than is required during heating phases of the stoves 11. Consequently, the WHR thermal oxidiser 37 operates with varying flow rates of air to combust the varying flow rates of off-gas to ensure optimum combustion of the off-gas. 20 The process control for the plant commences ramping up the air flow rate to the WHR thermal oxidiser 37 a predetermined time period, typically 30 seconds, before there is an increase in off-gas to the WHR thermal oxidiser 25 37 due to a decrease in demand for off-gas in the stoves 11. Similarly, the process control for the plant commences ramping down the air flow rate to the WHR thermal 30 oxidiser 37 a predetermined time period, typically 30 seconds, before there is an decrease in off-gas to the WHR thermal oxidiser 37 due to an increase in demand for off gas in the stoves 11 35 The thermal oxidiser 37 is in the form of a burner assembly that comprises: WO 2005/083130 PCT/AU2005/000284 19 * a cyclonic combustor; e an auxiliary burner with fuel train and burner management system; e combustion air fans 41; 5 * instruments; and * a control system. Off-gas is introduced tangentially to the cyclonic combustor and imparts a cyclonic swirl pattern .10 within the cylindrical shell of the incinerator chamber. This flow pattern ensures quick and efficient oxidation of the combustibles in the off-gas. A refractory choke ring within the shell helps to recirculate the products of combustion and facilitate mixing with the fresh combustible 15 gases. Air is injected via high-velocity radial jets to penetrate and mix with the combined off-gas stream. This results in staged combustion that assists in minimising NOx 20 generation. The air flow to the thermal oxidiser is set by maintaining a constant oxygen level (1 to 2%) in the exhaust gas. To ensure adequate destruction of CO in the off 25 gas, a minimum combustion temperature of 850 0 C is maintained. The WHR system 25 generates superheated steam from the imported off-gas hood steam and internally 30 generated steam. In producing this steam, the WHR system 25 absorbs heat from the thermal oxidiser combustion products. The steam raising equipment comprises: 35 0 a radiant screen to protect the downstream coils; * a two-stage superheater section with desuperheater controls (where the quantity of superheat is WO 2005/083130 PCT/AU2005/000284 20 controlled by injecting demineralised water as required to maintain the superheated steam at a temperature of 420 0 C); * a main evaporator section, consisting of three 5 modules of convective coils; e an economiser section; and * a steam drum with three element demineralised water control. 10 The steam raised in the WHR system 25 and the off-gas hood 15 is used to drive the HAB blower 31 and the main air compressor (not shown) of oxygen plant 29, with the remainder being passed through a turbo-alternator that generates electrical power required to operate the plant. 15 The turbo-generator system includes a condensing turbine designed to receive superheated steam. The discharge from the turbine passes through a surface condenser operating at vacuum with the resultant condensate 20 being pumped to the de-aerator via condensate pumps. The use of the off-gas as a fuel gas within a plant offsets an amount of electrical power that would otherwise need to be obtained from the grid, which makes 25 the plant generally self sufficient in terms of electrical power. Many modifications may be made to the embodiment of the present invention described above without departing 30 from the spirit and scope of the invention. By way of example, whilst the embodiment comprises a pair of stoves 11, the invention is not so limited and extends to plants having three or more stoves.

Claims

1. A direct smelting process of the invention comprises supplying solid metalliferous feed material and 5 solid carbonaceous material to a direct smelting vessel containing a bath of molten metal and slag and smelting the metalliferous feed material to molten metal in the vessel, supplying hot air to the vessel and post combusting reaction gases generated in the vessel and thereby 10 producing heat required to continue process reactions in the vessel, and releasing off-gas from the vessel, producing hot air for the vessel by operating a plurality of stoves so that each stove has a sequence of operating phases that comprise a heating phase, a bottling phase, and is a heat exchange phase that is a longer time period than the heating phase, and processing at least part of the off-gas released from the vessel by; (a) cooling the off-gas and particulate material 20 carried in the off-gas; (b) removing particulate material from the cooled off-gas; 25 (c) using at least part of the cooled and cleaned off-gas as required as a source of energy for heating the stoves by combusting off-gas with air in burners of the stoves in the heating phases of the stoves for 30 producing hot air for the vessel; (d) using at least part of the unused balance of the cooled and cleaned off-gas as a source of energy in a waste heat recovery unit and 35 21109421 (GHMatters) 4/11/09 - 22 thereby generating steam and/or electricity; and s (e) adjusting operating conditions in the waste heat recovery unit to accommodate variations in off-gas supplied to the waste heat recovery unit. 10

2. The process defined in claim 1, wherein the waste heat recovery unit comprises a boiler for combusting unused off-gas in step (d) with air and generating superheated steam, and the process comprises varying the flow rate of air to the waste heat recovery unit to accommodate varying 15 flow rates of off-gas to the boiler as a consequence of varying off-gas requirements of the stoves.

3. The process defined in claim 2 comprises increasing the flow rate of air to the boiler a 20 predetermined time period before a bottling phase commences and there is a consequential reduction in the demand for off-gas in the stoves and an increase in off-gas flow rate to the waste heat recovery unit. 25

4. The process defined in claim 3 wherein the predetermined time period is 30 seconds.

5. The process defined in any one of the preceding claims wherein the off-gas processed in steps (b) to (d) 30 comprises 55-65% by volume of the total amount of off-gas released from the vessel.

6. The process defined in claim 5 comprises 35 preheating the cooled and cleaned off-gas prior to using the off-gas in the stove burners in step (c). 21109421 (GHMatters) 4/11/09 - 23

7. The process defined in claim 6 comprises using the combustion products produced in the stove burners after the products have been used to heat the stoves to preheat cooled and cleaned off-gas prior to using the off-gas in 5 the stove burners.

8. The process defined in any one of the preceding claims comprises processing off-gas released from the vessel by using some of the off-gas for pretreating solid 10 metalliferous feed material by heating and partially reducing the solid metalliferous feed material in a pretreatment unit prior to supplying the solid metalliferous feed material to the vessel. 15

9. The process defined in claim 8 comprises processing off-gas discharged from the pretreatment unit by using the off-gas as an additional source of energy in the waste heat recovery unit. 20

10. The process defined in claim 8 or claim 9 comprises splitting the off-gas released from the vessel into at least two streams, with one stream being processed by using the off-gas in the stoves and the waste heat recovery unit and the other stream being processed by using 25 the off-gas in the pretreatment unit and thereafter in the waste heat recovery unit.

11. The process defined in any one of the preceding claims wherein step (a) comprises cooling the off-gas to a 30 temperature of 1000 0 C or below.

12. The process defined in any one of the preceding claims wherein step (a) comprises cooling the off-gas from 35 a vessel discharge temperature of at least 1400*C. 21109421 (GHMatters) 4/11109 - 24

13. A direct smelting plant for producing molten metal from a metalliferous feed material which comprises: 5 (a) a direct smelting vessel for containing a bath of molten metal and slag and smelting the metalliferous feed material to metal in the vessel, the vessel comprising a solids feed means for supplying solid feed material 10 into the vessel and a gas injection means for injecting hot air into the vessel; (b) a plurality of stoves for producing hot air for the vessel; 15 (c) a means for processing off-gas from the vessel, the processing means comprising gas cooling means for cooling off-gas, particulate material removal means for 20 removing particulate material from cooled off-gas, the stoves for combusting cooled and cleaned off-gas in heating phases of the stoves, a waste heat recovery unit for combusting cooled and cleaned off-gas and 25 generating steam and/or electricity, and the waste heat recovery unit comprising a means for adjusting operating conditions in the waste heat recovery unit to accommodate variations in off-gas supplied to the waste 30 heat recovery unit.

14. The plant defined in claim 13 wherein the waste heat recovery unit comprises a boiler for combusting off gas with combustion air and generating superheated steam. 35

15. The plant defined in claim 13 or claim 14 wherein the means for adjusting operating conditions in the waste 21109421 (GHMatters) 4/11/09 - 25 heat recovery unit comprises a means for varying the flow rate of combustion air to the waste heat recovery unit to accommodate varying flow rates of off-gas to the boiler as a consequence of varying off-gas requirements of the 5 stoves.

16. The plant defined in claim 15 wherein the means for varying the flow rate of combustion air to the waste heat recovery unit is adapted to increase the flow rate of 10 combustion air to the boiler a predetermined time period before a bottling phase commences and there is a consequential reduction in the demand for off-gas in the stoves and an increase in off-gas flow rate to the waste heat recovery unit. 15

17. The plant defined in any one of claims 13 to 16 wherein the plant further comprises a pretreatment unit for pre-treating solid metalliferous feed material by heating 20 and partially reducing the solid metalliferous feed material prior to supplying the solid metalliferous feed material to the vessel.

18. The plant defined in claim 17 wherein the means 25 for processing off-gas from the vessel further comprises the pretreatment unit, and the pre-treatment unit is adpated for processing some of the off-gas from the vessel by using the off-gas to heat and partially reduce the solid metalliferous feed material. 30

19. The plant defined in claim 18 wherein the waste heat recovery unit is adapted for combusting off-gas from the pretreatment unit as well as off-gas that has been processed in the off-gas cooling and the particulate 35 material removal means and for generating steam and/or electricity. 21109421 (GHMatters) 4/11/09 - 26

20. The plant defined in claim 19 wherein the waste heat recovery unit comprises a means for mixing off-gas from the pretreatment unit and off-gas that has been processed in the off-gas cooling and the particulate 5 material removal means and thereafter injecting the mixed off-gas into a burner of the waste heat recovery unit. 2110942_1 (GHMatters) 4/11/09