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Title:
PROCESSING OF LOW HEATING VALUE OFF-GAS
Document Type and Number:
WIPO Patent Application WO/2016/123666
Kind Code:
A1
Abstract:
A direct smelting process for processing low heating-value off-gas is disclosed. The method includes supplying carbonaceous material to a molten bath of metal and slag in a direct smelting vessel and supplying a hot gas blast from stoves into a top space above the molten bath to post-combust reaction gas from the molten bath in the top space and, thereby, produce an off-gas. The method includes supplying fuel gas, off-gas and oxygen-containing combustion gas to the stoves and combusting the fuel gas, off-gas and the combustion gas to heat the stoves and controlling the dome temperature in the stoves by supplying the off-gas to the stoves together with the fuel gas and the combusting gas when the heating value of the off-gas is below 1.8 MJ/Nm3 (HHV basis). a direct smelting plant for smelting metalliferous material to molten metal in a molten bath of metal and slag. The plant operates the method described above.

Inventors:
DRY, Rodney James (326 The Boulevard, City Beach, Western Australia 6015, 6015, AU)
GOODMAN, Neil John (4/16 Kintail Road, Applecross, Western Australia 6153, 6153, AU)
PARKINSON, Anthony John (19/569 Wellington Street, Perth, Western Australia 6000, 6000, AU)
Application Number:
AU2016/050057
Publication Date:
August 11, 2016
Filing Date:
February 03, 2016
Export Citation:
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Assignee:
TECHNOLOGICAL RESOURCES PTY. LIMITED (123 Albert Street, Brisbane, Queensland 4000, 4000, AU)
International Classes:
C21B13/14; C21C5/38; C21C5/40; F27D17/00
Domestic Patent References:
WO2007121530A12007-11-01
Attorney, Agent or Firm:
GRIFFITH HCK (GPO Box 1285, Melbourne, Victoria 3001, 3001, AU)
Download PDF:
Claims:
1. A direct smelting process including supplying carbonaceous material to a molten bath of metal and slag in a direct smelting vessel and supplying a hot gas blast from stoves into a top space above the molten bath to post-combust reaction gas from the molten bath in the top space and, thereby, produce an off-gas, the process including:

(a) supplying fuel gas, off-gas and oxygen-containing combustion gas to the stoves and combusting the fuel gas, off-gas and the combustion gas to heat the stoves; and

(b) controlling the dome temperature in the stoves by supplying the off-gas to the stoves, together with the fuel gas and the combusting gas, when the heating value of the off-gas is below 1.8 MJ/Nm3 (HHV basis).

2. The process defined in claim 1 includes controlling the volume of off-gas supplied to the stoves in response to the oxygen content in flue gas leaving the stoves and/or the dome temperature in the stoves. 3. The process defined in claim 2 includes continuously monitoring the dome temperature and the oxygen content in the flue gas and controlling the volume of off- gas supplied to the stoves in response to the oxygen content in flue gas leaving the stoves and/or the dome temperature in the stoves. 4. The process defined in claim 3, wherein controlling the volume of off-gas supplied to the stoves includes supplying the off-gas at a rate that controls the dome temperature to be in the range of 100 to 250°C above the hot blast temperature.

5. The process defined in claim 3 or claim 4, wherein controlling the volume of off-gas supplied to the stoves includes supplying the off-gas at a rate that maintains the oxygen content in the flue gas above 0.5% v/v.

6. The process defined in claim 3 or claim 4, wherein the process further includes adjusting the supply of combustion gas to the stoves in response to the oxygen content in the flue gas to a rate that, in combination with the supply of off-gas, maintains the oxygen content in the flue gas above 1.0% v/v.

7. The process defined in any one of the preceding claims, including splitting the off-gas into streams and supplying one stream to the stoves and supplying another stream to a waste heat recovery unit and combusting the off-gas with fuel gas to generate heat.

8. The process defined in any one of the preceding claims, wherein the stream of off-gas supplied to the stoves is 5 to 30% v/v of the combustion gas supplied to the stoves and the balance of the off-gas is supplied to the waste heat recover}' unit.

9. The process defined in any one of the preceding claims, wherein the off-gas supply to the stoves is 15 to 30% v/v of the off-gas from the direct smelting vessel and the balance of the off-gas is supplied to the waste heat recovery unit.

10. The process defined in any one of the preceding claims, including injecting metalliferous material into the molten bath and smelting the metalliferous material to metal and generating additional reaction gas to an extent that causes buoyant uplift of the molten bath so that molten material is projected into the top space and is exposed to heat generated by post-combustion of the reaction gas.

1 1 . The process defined in claim 0, wherein the metalliferous material and the carbonaceous material are in the form of fines of less than 6 mm in a major dimension.

12. The process defined in claim 10 or claim 1 1, including pre-treating the metalliferous material by heating and pre-reducing the metalliferous material.

13. The process defined in claim 12, wherein the metalliferous material is preheated such that it is 300°C or hotter at a feed point into solids injection lances that delivers the metalliferous material into the molten bath.

14. The process defined in claim 12 or claim 13, including pre-reducing the metalliferous material.

15. The process defined in any one of claims 10 to 14, wherein the direct smelting vessel includes a refractory-lined main chamber and a refractory-lined forehearth connected to the main smelting chamber via a forehearth connection and wherein the process includes smelting the metalliferous material to molten metal and removing molten metal semi-continuously or continuously via the forehearth and removing slag periodically.

16. The process defined in any one of claims 10 to 14, wherein the metalliferous material is iron ore.

17. The process defined in any one of the preceding claims, wherein controlling the volume of off-gas supplied to the stoves includes supplying the off-gas at a rate that maintains the oxygen content in the flue gas in the range of 0.5 to 5.0% v./v.

18. The process defined in any one of the preceding claims, wherein the process includes supplying the off-gas, having a heating value below 1.8 MJ Nm3 (HHV basis), to the stoves without additional fuel gas.

19. A direct smelting plant for smelting metalliferous material to molten metal in a molten bath of metal and slag, the plant including:

(a) a direct smelting vessel containing a molten bath of slag and metal and a top space above the molten bath, the vessel including one or more gas injection lances injecting an oxygen-containing gas and two or more solids injection lances injecting carbonaceous material and post-combusting reaction gas from the molten bath, thereby producing an off-gas with a heating value below 1.8 Mj Χη (HHV basis);

(b) an off-gas hood and scrubber in communication with the top space receiving the off-gas from the vessel and cooling and cleaning the off-gas;

(c) a waste heat recovery unit receiving and combusting a portion of the off-gas from the direct smelting vessel to recover heat from the off-gas; and

(d) stoves receiving a mixture of a fuel gas, an oxygen-containing combustion and a portion of the cooled and cleaned off-gas sufficient to control the dome temperature of the stoves, and combusting the mixture of gases to heat the stoves.

20. The direct smelting plant defined in claim 19, wherein one or more of the solids injection lances injects metalliferous material.

A direct smelting process for processing low heating-value off-gas is disclosed. The method includes supplying carbonaceous material to a molten bath of metal and slag in a direct smelting vessel and supplying a hot gas blast from stoves into a top space abov the molten bath to post-combust reaction gas from the molten bath in the top space and thereby, produce an off-gas. The method includes supplying fuel gas, off-gas and oxygen-containing combustion gas to the stoves and combusting the fuel gas, off-gas and the combustion gas to heat the stoves and controlling the dome temperature in the stoves by supplying the off-gas to the stoves together with the fuel gas and the combusting gas when the heating value of the off-gas is below 1.8 MJ/Nm ' (HHV basis), a direct smelting plant for smelting metalliferous material to molten metal in a molten bath of metal and slag. The plant operates the method described above.

Description:
The present invention relates to a process for smelting a metalliferous material.

The term "metalliferous material" is understood herein to include solid feed material and also includes within its scope partially reduced metalliferous material.

The present invention relates more particularly, although by no means exclusively, to a molten bath-based smelting process for producing molten metal from a metalliferous feed material which is injected into a smelting vessel that has a strong bath/slag fountain generated by gas evolution in the molten bath, with the gas evolution being at least partly the result of carbonaceous material injected into the molten bath.

In particular, although by no means exclusively, the present invention relates to a process for smelting an iron-containing material, such as an iron ore, and producing molten iron.

The present invention relates particularly, although by no means exclusively, to a smelting process in a smelting vessel that includes a main chamber for smelting metalliferous material.

More particularly, the present invention relates to processing of off-gas from the smelting process in a direct smelting plant during times when the off-gas has a low calorific value (CV), such as during start-up of the smelting process.

A known molten bath-based smelting process is generally referred to as the HIsmelt process and is described in a considerable number of patents and patent applications in the name of the applicant.

The HIsmelt process is associated particularly with producing molten iron from iron ore or another iron-containing material .

In the context of producing molten iron, the HIsmelt process includes the steps of:

(a) forming a bath of molten iron and slag in a main chamber of a smeltin (b) injecting into the bath: (i) iron ore, typically in the form of fines; and (ii) a solid carbonaceous material, typically coal, which acts as a reductant of the iron ore feed material and a source of energy; and

(c) smelting iron ore to iron in the bath.

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

In the Hlsme!t process solid feed materials in the form of metalliferous material and solid carbonaceous material are injected with a carrier gas into the molten bath through a number of lances which are inclined to the vertical so as to extend downwardly and inwardly through the side wall of the main chamber of the smelting vessel and into a lower region of the vessel so as to deliver at least part of the solid feed materials into the metal layer in the bottom of the main chamber. The solid feed materials and the carrier gas penetrate the molten bath and cause molten metal and/or slag to be projected into a space above the surface of the bath and form a transition zone. A blast of oxygen-containing gas, typically oxygen-enriched air or pure oxygen, is injected into an upper region of the main chamber of the vessel through a

downwardly extending lance to cause post-combustion of reaction gases released from the molten bath in the upper region of the vessel. In the transition zone there is a favourable mass of ascending and thereafter descending droplets or splashes or streams of molten metal and/or slag which provide an effective medium to transfer to the bath the thermal energy generated by post-combusting reaction gases above the bath.

Typically, in the case of producing molten iron, when oxygen-enriched air is used, it is generated in hot blast stoves and fed at a temperature of about 1200°C into the upper region of the main chamber of the vessel.

Off-gases resulting from the post-combustion of reaction gases in the smelting vessel are taken away from the upper region of the smelting vessel through an off-gas duct.

The smelting vessel includes refractory-lined sections in the lower hearth and water-cooled panels in the side walls and the roof of the main chamber of the vessel, and water is circulated continuously through the panels in a continuous circuit. The HIsmelt process enables l arge quantities of molten iron, typically at least 0.5 Mt/a, to be produced by smelting in a single compact vessel.

The HIsmelt process includes solids injection into a molten bath in a smelting vessel via water-cooled solids injection lances.

In addition, a key feature this process is that it operates in smelting vessels that include a main chamber for smelting metalliferous material and a forehearth connected to the main chamber via a forehearth connection that allows continuous metal product outflow from the vessels, A forehearth operates as a molten metal-filled siphon seal, naturally "spilling" excess molten metal from the smelting vessel as it is produced. This al lows the molten metal level in the main chamber of the smelting vessel to be known and controlled to within a small tolerance - this is essential for plant safety. Molten metal level must (at all times) be kept at a safe distance below water-cooled elements such as solids injection lances extending into the main chamber, otherwise steam explosions become possible. It is for this reason that the forehearth is considered an inherent part of a smelting vessel for the HIsmelt process.

The term "forehearth" i s understood herein to mean a chamber of a smelting vessel that is open to the atmosphere and is connected to a main smelting chamber of the smelting vessel via a passageway (referred to herein as a "forehearth connection") and, under standard operating conditions, contains molten metal in the chamber, with the forehearth connection being completely filled with molten metal.

During normal operation clean, wet-scrubbed SRV off-gas typically contains around 20-22% CO and has a heating value in the range 3-3.5 MJ/Nm 3 (HHV basis). This type of gas is able to burn with a stable flame temperature around 1000-1 100°C in an appropriately designed burner system without using any significant support fuel.

During start-up, however, there are significant periods when SRV off-gas is lean - typically in the range 0.5-2 MJ/Nm 3 . This off-gas, once it has been wet-scrubbed and cleaned, is very difficult to burn. The CO content (6-12% v/v) and H 2 S content (300 ppm or more) are generally too high to allow safe venting. Flaring is generally not attractive because excessive amounts of support fuel (e.g. natural gas) would be needed to achieve a stable flame. On the demonstration plant, the solution was to send this off- gas to a burner drum of a main boiler (to recover as much of the remaining chemical energy in the off-gas as possible) and to increase the natural gas supply to the boiler as necessary to achieve a flame temperature of at least 900-1000°C. This consumed additional natural gas but was tolerated due to the lack of any practical alternatives.

The present invention is partly the result of experience gained on a

demonstration plant that operated the HIsmelt process. This plant was constructed in Perth, Western Australia in 2002-2003 when natural gas prices were below $A3/GJ. By the time this first-of-a-kind plant was fully operational some years later, natural gas had risen above SA8/GJ. As a consequence, the "as built" configuration (which was designed for high natural gas consumption due to the original low cost) came under severe economic pressure.

The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

Summary of the Disclosure

The present invention is based on the realisation that, during such periods of low CV gas production (such as start-up or re-start of the direct smelting process), the hot blast stoves offer an opportunity to absorb a significant portion of this gas with no associated increase in support fuel consumption.

Hot blast stoves generally heat air or oxygen-enriched air from ambient conditions to about 1200 °C. They achieve this by having (usually) either two or three stoves containing a large mass of specially engineered gas-permeable refractory elements which alternately heat and cool in duty cycles of about 30-60 minutes. This refractory is first heated by burning a gaseous fuel (e.g. natural gas, normal-strength SRV off-gas or blast furnace gas in the case of a blast furnace). Once the refractory is hot enough the stove is pressurised and cold air (or oxygen-enriched air) is admitted. This cold stream then heats up to around 1200-1250 °C (usually with a cold bypass and mixing system to keep outlet temperature constant) by direct contact with hot refractory. This "heat reservoir" is drained over about 30-60 minutes and at the end of this period one of the other stoves is ready to take over, allowing the depleted one to be re-fired for its next cycle.

An important consideration with stoves of this type is dome (or flame) temperature. Typically, stoves are operated with a maximum dome temperature around 100-250 °C above that of the hot blast temperature. If hot blast is being produced at 1200 °C, then the flame or dome temperature typically needs to be in the range 1300- 1450 °C. In particular, operation with higher dome temperatures is not advisable due to engineering/thermal containment limitations. Well-tuned stoves are typically operated with 10-15% excess air, resulting in around 0.8-1.3% oxygen (v/v) in the flue gas.

During start-up the stoves are traditionally 100% fired on support fuel - usually natural gas. Combustion at or near stoichiometric conditions with this fuel type would result in excessively high dome temperatures. To avoid this, it is generally necessary to add extra cold air into the combustion chamber such that the resulting dome temperature is in the normal range (or only modestly elevated). Excess air is typically 50-60% under these conditions, with flue oxygen around 6-9% v/v.

When stoves are operated in this manner, the amount of air over and above that needed for combustion (i.e. the "excess air" part, typically around 30-40% of the total air flow) is present essentially as a cooling medium to dilute the flame and reduce the dome temperature. The core of the current invention is realisation that lean SRV off-gas can substitute for most of this "excess" combustion air without incurring any fuel gas penalty (i.e. without requiring additional support fuel). In fact, the combustible material (CO gas) that is present in the lean SRV off-gas can still be combusted and is able to contribute in a positive manner, resulting in a proportional reduction in fuel gas usage.

The key safety-related (practical) requirement is that the stoves are not allowed to drift into an oxygen-lean mode with insufficient combustion air. SRV off-gas composition can vary suddenly and the system needs to be able to respond accordingly. This is essentially a process control issue which can be addressed in the following ways:

1. Stoves dome temperature and flue oxygen content can be monitored in realtime. If, for example, oxygen drops below a given set-point (e.g. 1.0% v/v), air flow can automatically be increased whilst SRV off-gas flow to the stoves is automatically decreased.

2, The stoves can be operated with injection of technical -grade oxygen into the combustion air stream during this part of the operation. The oxygen plant will generally have spare capacity at this time, since SRV demand for oxygen is well below that associated with normal operation. By adjusting the amount of oxygen injection into the stoves combustion air stream, it is possible to ensure that, even if SRV off-gas suddenly increases in calorific value, there will still be enough oxygen in the stoves to ensure safe operation.

A significant benefit associated with this mode of operation relates to the amount of support fuel that is available at a given location. If the total amount of support fuel is limited, then an ability to "incinerate" a certain amount of off-gas in the stoves (without incurring any related fuel gas consumption penalty) means that the balance of off-gas which needs to be incinerated elsewhere (typically in the burner drum of the main boiler) will have greater relative access to the remaining quantity support fuel, thereby rendering it easier to bum. In this manner, the direct smelting plant can be operated more easily and within a smaller total supply maximum for support fuel.

In the context of a metalliferous feed material in the form of iron ore, the present invention provides a method of starting-up (including re-starting) a smelting process utilising a smelting vessel and a set of hot blast stoves.

The present invention, more specifically, is based on the realisation that, for example under start-up or restart conditions, when SRV fuel gas quality is below 1.8 MJ/Nm 3 (i.e. too low to allow normal SRV gas operation in the hot blast stoves without large amounts of support fuel), off-gas is still admitted to the stoves. In other words, this includes admitting off-gas to the stoves when the calorific value of the off-gas is 0 to less than 1.8 MJ Nm 3 . The amount of off-gas admitted under these conditions may be 5-30% of the main combustion air flow rate.

One aspect of the present invention provides a direct smelting process including supplying carbonaceous material to a molten bath of metal and slag in a direct smelting vessel and supplying a hot gas blast from stoves into a top space above the molten bath to post-combust reaction gas from the molten bath in the top space and, thereby, produce an off-gas, the process including:

(a) supplying fuel gas, off-gas and oxygen-containing combustion gas to the stoves and combusting the fuel gas, off-gas and the combustion gas to heat the stoves; and (b) controlling the dome temperature in the stoves by supplying the off-gas to the stoves together with the fuel gas and the combusting gas when the heating value of the off-gas is below 1.8 MJ/Nm 3 (HHV basis).

The process is particularly relevant to process start-up or re-start after a break when the vessel off-gas has a low calorific value, i.e. less than 1.8 MJ Nm 3 (HHV basis) and, at times, zero calorific value. However, the process is applicable to other process states that produce low calorific value off-gas, such as "hold" and "idle" states. The "hold" state is characterised by carbonaceous material and a hot blast being supplied to the vessel in order to keep the molten bath hot. The "idle" state is characterised by supplying a hot blast without injecting coal into the bath. The calorific value of off-gas from an idle state is relatively low and the calorific value of off-gas from a hold state is variable. When the off-gas has a low calorific value, considerable fuel gas is supplied to the stoves to heat them. This provides an opportunity to fully combust the off-gas (i.e. combust any CO or C remaining in the off-gas to produce C0 2 ) and, at the same time, control the dome temperature. Additionally, sending less off-gas to the waste heat recovery unit means that less fuel gas is used in the waste heat recovery unit. The direct smelting process, therefore, uses less fuel gas overall.

The process may include controlling the volume of off-gas supplied to the stoves in response to the oxygen content in flue gas leaving the stoves and/or the dome temperature in the stoves.

The process may include continuously monitoring the dome temperature and the oxygen content in the flue gas and controlling the volume of off-gas supplied to the stoves in response to the to the oxygen content in flue gas leaving the stoves and/or the dome temperature in the stoves.

Controlling the volume of off-gas supplied to the stoves may include supplying the off-gas at a rate that controls the dome temperature to be in the range of 100 to 250°C above the hot blast temperature.

Controlling the volume of off-gas supplied to the stoves may include supplying the off-gas at a rate that maintains the oxygen content in the flue gas to be above 0.5% v/v.

The process may further includes adjusting the supply of combustion gas to the stoves in response to the oxygen content in the flue gas to a rate that, in combination with the supply of off-gas, maintains the oxygen content in the flue gas above 1.0 % v/v.

The process may include splitting the off-gas into streams and supplying one stream to the stoves and supplying another stream to a waste heat recovery unit and combusting the off-gas with fuel gas to generate heat.

The stream of off-gas supplied to the stoves may be 5 to 30% v/v of the combustion gas suppli ed to the stoves and the balance of the off-gas may be supplied to the waste heat recovery unit.

The off-gas supply to the stoves may by 15 to 30% v/v of the combustion gas supplied to the stoves and the balance of the off-gas may be supplied to the waste heat recovery unit.

The process may further include injecting metalliferous material into the molten bath and smelting the metalliferous material to metal and generating additional reaction gas to an extent that causes buoyant uplift of the molten bath so that molten material is projected into the top space and is exposed to heat generated by post- combustion of the reaction gas.

The metalliferous material and the carbonaceous material may be in the form of fines of less than 6 mm in a major dimension.

The process may include pre-treating the metalliferous material by heating and pre-reducing the metalliferous material ,

The metalliferous material may be preheated such that it is 300°C or hotter at a feed point into solids injection lances that delivers the metalliferous material into the molten bath.

The process may include pre-reducing the metalliferous material.

The direct smelting vessel may include a refractory-lined main chamber and a refractory-lined forehearth connected to the main smelting chamber via a forehearth connection and wherein the process includes smelting the metalliferous material to molten metal and removing molten metal semi-continuously or continuously via the forehearth and removing slag periodically.

The metalliferous material may be iron ore.

The carbonaceous material may be coal .

Controlling the volume of off-gas supplied to the stoves may include supplying the off-gas at a rate that maintains the oxygen content in the flue gas in the range of 0.5 to 5.0% v/v.

The process may include supplying the off-gas, having a heating value below 1.8 MJ/Νητ' (HHV basis), to the stoves without additional fuel gas.

Another aspect of the present invention provides a direct smelting plant for smelting metalliferous material to molten metal in a molten bath of metal and slag, the plant including:

(a) irect smelting vessel containing a molten bath of slag and metal and a top

lances injecting carbonaceous material, and post-combusting reaction gas from the molten bath, thereby producing an off-gas with a heating value below 1.8 MJ/Nm 3 (HHV basis):

an off-gas hood and scrubber in com muni cation with the top space receiving the off-gas from the vessel and cooling and cleaning the off-gas; a waste heat recovery unit receiving and combusting a portion of the off-gas from the direct smelting vessel to recover heat from the off-gas: and stoves receiving a mixture of a fuel gas, an oxygen-containing combustion gas and a portion of the cooled and cleaned off-gas sufficient to control the dome temperature of the stoves, and combusting the mixture of gases to heat the stoves.

One or more of the solids injection lances may inject metalliferous material.

Brief description of the drawing

A direct smelting process in accordance with an embodiment of present invention is described further with reference to the accompanying drawing, of which:

Figure 1 is a schematic flowsheet showing one embodiment of a HIsmelt direct smelting process configured to operate in accordance with the present invention. Description of embodiment

Figure 1 shows a HIsmelt direct smelting process flowsheet. The following description of an embodiment of the present invention is in the context of smelting iron ore fines in accordance with the HIsmelt process and the flowsheet shown in Figure I to produce molten iron.

It will be appreciated, however, that the present invention is applicable to smelting any metalliferous material, including ores, partly reduced ores, and metal- containing waste streams via any suitable molten bath-based smelting process and is not confined to the HIsmelt process.

In normal operation (i.e. producing molten iron) in a plant operating according to the flowsheet in Figure 1 , the process includes the following steps:

(i) injecting granular coal, flux, and preheated iron ore, typically in the form of fines of less than 6 mm in a major dimension, into the molten bath via injection lances 104, with the preheated iron ore typically being at 300°C or hotter at a feed point into the injection lances 104,

(ii) injecting hot oxygen-enriched air from the stoves into a gas top space above the molten bath in the smelting vessel to generate heat by way of combustion of combustible gases in the top space in order to sustain smelting reactions in the bath

(iii) generating the bath/slag fountain via ascending and thereafter descending droplets and splashes from the molten bath such that heat is transferred from the top space to the bath in order to sustain the smelting reactions;

(iv) removing molten iron semi-continuously or continuously via the forehearth and removing slag periodically via a water-cooled slag tapping device mounted in a side-wall of the vessel;

(v) cooling smelter off-gas discharged from the smelting vessel to below about 300°C and removing dust particles and generating a cool, clean fuel gas with a heating value in the rage 2.5-4 MJ/Nm 3 (HHV basis);

(vi) feeding at least a portion, typically between 15% and 35%, of the clean fuel gas to the hot blast stoves as fuel gas for heating of the refractory in the stoves.

A metalliferous feed material in the form of iron ore 101 (optionally blended with some flux materials such as dolomite) is fed into an ore pre-treatment unit 102 which may be any suitable device such as a CFB preheater, a rotan,' kiln or an ore dryer. Pre-treated iron ore 103 (with temperature and pre-reduction degree dependent on the nature of the ore pre-treatment step) is fed at a temperature of 300°C or hotter to an inlet end of solids injection lances 104 (mounted in the smelting vessel 105) using a suitable conveying gas, such as nitrogen.

Coal 106 is fed into coal drying grinding mill 107, then mixed with flux 108 (typically calcined lime) and fed into injection lances 104. Solids injection lances 104 inject all the solids into the bath and smelting occurs according to the normal HIsmelt process as described earlier. Metal 110, produced by smelting reactions, is discharged via a forehearth 20 and slag 111 is discharged via a slag notch 22 formed in the side wall of the vessel 105 at a level above an interface between the metal layer 24 and the slag layer 26 in the molten bath 28 under quiescent conditions.

Figure 1 shows that the vessel 105 includes two solids injection lances 104. It will be appreciated, however, that the vessel 105 may include more than two solids injection lances 104.

Air 112A and technical-grade oxygen 1 12B (the latter from a cryogenic air separation plant 28) are mixed and heated in hot blast stoves 121 to (typically) 1200°C. The mixture contains 35-40% oxygen by volume and is supplied from the stoves 121 to the vessel 105 as a hot blast stream via line 112 and a hot blast lance 30. The lance 30 extends downwardly from an upper region of the vessel 105 to deliver the hot blast into the top space of smelting vessel 105 and combusts process gas (i.e. reaction gas produced by smelting reactions and by devolatilisation of coal in the molten bath 28) in order to generate heat for the smelting process.

While Figure 1 shows the vessel 105 with one gas injection lance 30, the vessel may include more than one gas injection lance 30 supplied with hot gas blast from the stoves 121.

Off-gas flows (as indicated by the arrow 113) from the vessel 105 and is cooled in an off-gas hood 1 14 and is thereafter scrubbed in wet scrubber 115. Clean gas 116 at a temperature of about 45°C and (in normal operation) with a heating value of 3- 3.5 MJ/Nm 3 is split into two portions, with one portion 117 (typically 25-35% of the total) being used as fuel in the hot blast stoves 121 and the balance 1 18 being burned in a boiler 34 with an air feed 36 in order to recover chemical energy from the off-gas 118.

Optionally, the off-gas flow from the scrubber 1 15 is split into three portions with two portions directed to the hot blast stoves 121 and the boiler 34 as described above and the third portion is supplied to other units in the direct smelting plant, such as the ore pre-heater 102. For example, the third portion of off-gas may be supplied to the ore pre-heater to pre-heat the ore. Although the primary effect of the off-gas stream suppli ed to the ore pre-heater 102 will be to pre-heat the ore, there may be some prereduction.

In normal operation, off-gas gas 1 17, together with stoves combustion air 1 19A and a very small amount (or zero) support fuel 120 are burned in the stoves 121 to achieve a dome temperature in the range 1300-1450°C. Oxygen stream 1 19B is zero at this time. The off-gas 1 17 may be delivered to the stoves 121 at roughly the same temperature as it left the scrubber 115. Alternatively, the off-gas 117 may be pre-heated to a temperature in the range of 100 to 300°C in a heat exchanger 32 that is supplied with hot flue gas (i.e. combustion products) from the stoves 121. Although the off-gas 117 and the combustion air 1 19A pass through the heat exchanger 32 separately, they are mixed together and then supplied to the stoves 121 for combustion.

During start-up or re-start from a break in production, when the off-gas gas heating value is below about 2 MJ/Nm 3 , oxygen stream 119B is turned on such that the oxygen concentration of mixed stream 119 i s in the range 21-28%. Off-gas stream 117 is maintained at around 15-30% of the total off-gas flow from the vessel 105, with the balance 1 18 being combusted in the boiler 34 in conjunction with support fuel (not shown).

The variati on in the volume of the off-gas stream 1 17 from 15% to 30% of the total off-gas is a product of the need to keep the oxygen content of the flue gas 122 above 0.5%> v/v to ensure complete combustion of the off-gas 117 and the fuel gas 120 in the stoves. Specifically, off-gas 117 supply to the stoves is reduced in the event that the oxygen content in the flue gas 122 nears or goes below 0.5%> v/v. It is preferable, however, to operate on the basis that the lower limit of oxygen content in the flue gas 122 is 1.0% v/v to provide a greater safety margin in the event of sudden changes in the composition of off-gas 1 17 from the vessel 105.

The oxygen content in the flue gas 122 is also affected by the amount of "excess air" supplied to the stoves 121 in the com bustion air 1 19. Accordingly, control of the oxygen content in the flue gas 122 includes controlling the flow of combustion air 1 19 to the stoves 121 and controlling the oxygen content of the combustion air 119. The combustion air 119 flow is adjusted simultaneously with changes to the flow of off- gas 1 17 in order to ensure that more than the minimum oxygen content is present in the flue gas 122. Additional oxygen 119B may be supplied to the combustion air 1 19A to provide an oxygen-enriched combustion air 1 19.

The variation in the volume of the off-gas stream 1 17 from 15% to 30% of the total off-gas is a product of the need to control the dome temperature in the stoves 121 as described above, i.e. as a substitute for some "excess air" in the combustion air 119. Specifically, the off-gas supply 117 can replace some combustion air 119 to control the dome temperature and can replace some fuel gas 120 that would ordinarily be required to operate the stoves during start-up or re-start as a result of some combustible products, such as CO, being present in the off-gas 17.

Continuous monitoring of the dome temperature and the flue gas 122 oxygen content is carried out and the off-gas 117, combustion air 1 19A and additional oxygen 119B are adjusted in response so that the dome temperature and the oxygen content in the flue gas are maintained in their respective desired ranges. The oxygen content of the gas in the dome (an upper curved roof structure) of each stove 121 may be monitored by a probe placed in the dome. The flue gas composition is effectively the same in the dome and in stream 122 and, therefore, the probe in the dome may replace oxygen content monitoring in the stream 122.

The overall effect is a reduction in fuel gas 120 supplied to the stoves 121 and to the boiler 34, As described above, this is advantageous in terms of reducing fuel costs during start-up and re-start and in terms of process viability in locations where available fuel gas is limited.

Many modifications may be made to the embodiments of the process of the present invention described in relation to the Figures without departing from the spirit and scope of the invention.

By way of example, whilst the embodiment is described in the context of the HIsmelt direct smelting process, it can readily be appreciated that the present invention is not so limited and extends to any molten bath-based smelting process that includes a hot blast from a stove set.