Biomass-based iron production using a smelt-reduction vessel

Abstract

A process and a plant for producing molten iron from iron ore and biomass using at least one hearth furnace and a smelt reduction vessel (SRV) are disclosed. The process and the plant produce molten iron using a hearth furnace to commence reduction of iron ore and finish off the reduction of ore in a smelt-reduction vessel, with fuel gas produced in the smelt-reduction vessel being combusted to supply heat in the hearth furnace.

Description

[0001] BIOMASS-BASED IRON PRODUCTION USING A SMELT-REDUCTION VESSEL

[0002] TECHNICAL FIELD

[0003] The present invention relates to a process and a plant for producing molten iron from iron ore and biomass.

[0004] The present invention relates particularly, although by no means exclusively, to a process and a plant for producing molten iron from iron ore and biomass using a hearth furnace to commence the reduction of iron ore and finishing off the reduction of ore in a smelt-reduction vessel, with fuel gas produced in the smelt-reduction vessel being combusted to supply heat in the hearth furnace.

[0005] The term “hearth furnace” describes a furnace that includes a lengthwise (whether linear or circular) a generally horizontally extending heating chamber and a base that extends along the length of the chamber from an inlet end to a discharge end that carries material through the chamber for rapid thermal processing in the chamber.

[0006] The term “molten iron” means liquid iron that has within it at least 2% by weight dissolved carbon.

[0007] The term “smelt reduction vessel” (often abbreviated “SRV” and also described as a “direct reduction vessel”) relates to an air or oxygen-blown direct smelting furnace of the type used, for example, in the HIsmelt™ and HIsarna™ processes (1-12) (collectively hereafter “HI SRV based pathways”). It does not contain electric heating elements of any type, and instead derives the necessary process heat from partial combustion of carbon and bath-derived CO / H2 using injection of oxygen-containing gas in the upper region of the vessel. Commercial examples of this type of smelter operate in China (Molong in Shandong Province and Jianlong CISP, Inner Mongolia).

[0008] BACKGROUND

[0009] Iron and steel making are historically carbon intensive processes in which the majority of the carbon used is eventually oxidised to CO2 and discharged to the atmosphere. With the world seeking to reduce overall atmospheric CO2 there is pressure on iron and steel makers to find means to make iron and steel without causing net emissions of greenhouse gases. In particular there is pressure to not use coal and natural gas, collectively often referred to as ‘fossil fuels’, which are non-renewable and result in increased concentrations of atmospheric greenhouse gases.

[0010] The majority of iron in the world is produced by the blast furnace route, which is a technology that has existed since prior to the industrial revolution. Even with technology advances the blast furnace currently still requires around 700-900 kg of metallurgical coal for every tonne of iron produced and emits high levels of CO2, roughly 1.8-2.0 t CO2 per tonne of hot metal. The use of fossil fuels, in particular the requirement for coal (in the form of coke), is an essential feed material for a blast furnace to operate, and it is not possible to simply use hydrogen therein as a complete substitute.

[0011] An alternative approach to blast furnaces is direct reduction of iron ore in the solid state by carbon monoxide and hydrogen derived from natural gas (or coal) to produce direct reduced iron (DRI). This reduction occurs at temperatures below the bulk melting temperature of the solids. For the purposes of the prior art discussion “direct reduced iron” (DRI) is understood to have at least 85% metallisation, where metallisation means the extent of conversion of iron oxide into metallic iron during reduction of the iron oxide, as a percentage of the mass of metallic iron divided by the mass of total iron.

[0012] While such plants are (outside of India) minor in number compared to blast furnaces, there are many processes for the direct reduction of iron ore. In India coal based rotary kiln furnaces are used to produce DRI, also known as sponge iron (approaching 20% of world production of DRI). Elsewhere they tend to be gas-based shaft furnace processes (approaching 80% of world production of DRI). The gas-based direct reduction plants are usually part of integrated steel mini-mills, located adjacent to an electric arc furnace (EAF) steel plant, but some DRI is shipped from captive direct reduction plants (usually Midrex™ or HYL™ process based) to remote steel mills. Because the DRI is usually, along with scrap steel as the other feed material, melted in electric arc furnaces to directly produce steel (typically well below 0.8% by weight carbon), there are strict requirements on the levels of impurities in the DRI such as gangue and phosphorus which are expensive and difficult to remove in the EAF. Hence, the iron ores used to make DRI are often crushed and ground to micron particle sizes to enable removal of gangue minerals. Such fine material is difficult to handle (both transport and operationally wise) so it is then agglomerated using water and / or binder to produce closely sized ‘green’ balls which are, once dried, then fed into furnaces where they are fired into hard pellets (a process known as induration), before eventually being supplied to direct reduction plants as feed material (or sometimes to blast furnaces as a high quality iron ore feed material to help dilute the gangue of the lump or sinter iron ore that a blast furnace uses). The ‘green’ balls that form the pellets have a typical compressive strength of around IO N when wet, and 50 N when dried. As pellets (after induration), they have a compressive strength of around 2000 N.

[0013] One futurist alternative to all of the above is the production of DRI from iron ores (in the form of an indurated pellet feed) using hydrogen, followed by smelting in an EAF to produce steel. For this route to be carbon neutral it requires conversion of renewable (green) energy into hydrogen (particularly in periods when wind / solar power cost is low), with subsequent production of DRI using the hydrogen. This route has strong support in Europe and has the potential to become a significant part of the global solution (13). However, there are limitations, as follows:

[0014] 1. The amount of electricity needed is high (estimated at 3500-4500 kWh / t to the liquid steel stage) and green power costs need to be low (or alternatively a high carbon tax needs to in place) for it to become cost-effective against coal and natural gas based processes.

[0015] 2. Hydrogen consumption requirements for the production of DRI are likely to be steady, whilst the generation of hydrogen itself is likely to be periodic in line with the availability of intermittent renewable energy, like wind and solar. This calls for a buffering approach to balance supply and demand. Storage and delivery of large amounts of hydrogen is a serious technical challenge. Underground salt caverns and exhausted natural gas reservoirs appear to show good potential. However, not all geographical locations may be amenable to this type of hydrogen storage. Moreover, suitable storage locations may not be close to DRI facilities for existing EAF steel mills and / or integrated steelmaking facilities, resulting in supply challenges.

[0016] 3. Only low-gangue ore types (or those able to readily be upgraded to remove gangue) can be used with the DRI / EAF combination. The EAF will penalise high gangue ore types strongly, rendering them essentially non-competitive. This implies much of the ore currently used in blast furnaces could become sub-economic for such a process route.

[0017] It is known that sustainable biomass could be a complementary part of the solution, acting as a substitute for fossil fuels. Burning of either fossil fuels or biomass will release CO2 when used. However, when fast growing plants are the source of the biomass, they are largely a carbon-neutral energy source (since through photosynthesis around the same amount of CO2 is taken up when the plants are regrown).

[0018] To date the only commercial ironmaking process that uses biomass directly is the charcoal mini-blast furnace (mostly found in in Brazil). This option has certain limitations related to charcoal production. Previous attempts to insert some biomass into processes originally designed for coal (e.g., conventional blast furnaces and coke ovens) are marginal at best, typically relying on a pre-charring step for the biomass and usually quite disappointing in terms of overall CO2 impact. This is largely because the nature of biomass is vastly different to that of coal. To use biomass successfully it is necessary to re-design the process around the fundamental nature of biomass.

[0019] Biomass can take many forms and avoiding competition with food production is key for biomass selection. Examples of biomass that might meet the selection criteria include elephant grass, sugar cane bagasse, forestry by-products, excess straw, azolla and seaweed / macroalgae. Such biomass availability varies considerably from one geographic location to another. This will most likely be a significant factor in determining the size and location of future biomass-based iron plants given the volume of material required and the economic challenges in transporting such material long distances.

[0020] Biomass, such as wood chips, has been shown in lab-scale studies (14) to be able to reduce iron ore to solid iron by the intermingling thereof with iron ore and placing in a furnace that heats the ore up to over 800 °C within a controlled atmosphere that prevents re-oxidation of the reduced material. While intermingling assists with the efficacy of the reduction process, on an industrial scale as a continuous process it potentially creates challenges, where gas flow created by convection heating as part of the reduction process picks up fine particles of char, leading to massive gas processing / char recycling challenges, or a lot of carbon being wasted through the need to clean up the offgas from the process, before discharge to the atmosphere.

[0021] Another example (described from laboratory phase experiments) is disclosed in

[0022] AU 2007227635 B2 (5). The patent discloses the use of briquettes (for example, in the shape of coherent spherical balls) produced by mixing iron ore concentrate comprising magnetite (FesC ), wood chips that have passed through a 4.75 mm sieve, a small amount of flour, and slight moistening (to achieve agglomeration). Such composites were dried at 105 °C (to provide strength and rigidity) in handling. The composites were then placed in a furnace (that was electrically heated) at temperatures in excess of 1375 °C to undertake the reduction of the iron ore. The patent discloses that preferably fine iron ore particles should be used and that while ‘particles as large as 0.25 inch in diameter’ (i.e., the typical top size of iron ore fines, being 6.35 mm) ‘or larger could be used, processing times would be unnecessarily long, and particles would not lend themselves to being formed into a coherent mass’ .

[0023] The application of electromagnetic energy, such as microwave (MW) energy and radio frequency (RF) energy in iron ore reduction processes, whether as simply a form of heating energy or as a means to enhance reaction rates or provide additional heating at crucial times in the reaction process, to produce DRI has also been considered. One of the first laboratory attempts known to the applicant is described in US patent 4,906,290 where briquettes containing a mixture of iron ore fines, coal and burnt lime were subjected to microwaves until they glowed red and then rapidly placed in a crucible where they were smelted to produce a molten iron containing 3.8% carbon. While no examination of the microwave product from the microwaving appears to have been carried out, it is likely that DRI was produced.

[0024] Thus, various lab-scale studies have shown that iron ores tested by mixing the ores with biomass and heating the mixtures in a small furnace can produce DRI in a manner that appears (superficially) somewhat better than that expected from first principles. Although the reasons may not be clear, the result stands as a technical “sweet spot”. The technical challenge remains how to perform this efficiently at large scale.

[0025] There are many possible approaches. One of these approaches (described in International Application PCT / AU2021 / 051398) involves briquetting ore and biomass, then using a linear or rotary heath furnace to preheat the material to around 600-900 °C to devolatilise it. Ore pre-reduction is expected to reach around 10-70% under these conditions. This is followed by a microwave treatment stage where the briquettes are heated to around 1000-1100 °C and reduced (using residual bio-carbon) further, with reductions typically around 90-95% and in some instances up to almost full metallisation. This DRI may then be fed to an electric smelting furnace or an induction furnace where it is melted to produce pig iron.

[0026] The applicant has since carried out further development work into the approach described above to better establish how to perform at scale in an efficient manner. In doing so the applicant has come across an alternative approach, where the process does not rely on the use of any type of electrical or electromagnetic energy for heating. Instead, heating is provided via partial combustion reactions in a smelt-reduction vessel.

[0027] The above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

[0028] SUMMARY OF THE DISCLOSURE

[0029] The applicant has been carrying out research and development work for an effective and efficient method for producing molten pig iron from iron ore using biomass (as a source of reductant and as a heating source of the iron ore).

[0030] The use of electromagnetic energy for heating in the form of microwaves has been a central requirement of a main research and development pathway of the applicant (the “EM energy pathway”).

[0031] The applicant has considered an alternative research and development pathway that includes a molten bath -based smelt-reduction vessel ("SRV”) of the type used in HI SRV based pathways.

[0032] The initial view of the applicant was that HI SRV based pathways would (i) require a substantial portion of its fuel to be in the form of coal, and (ii) produce an offgas that would not be suitable for use in an ore-biomass preheating step. The initial view was an informed view given that the applicant developed the HIsmelt technology and has considerable knowledge in relation to this and related technologies.

[0033] The present invention is based on a realisation of the applicant that the HI SRV based pathways approach is an effective and efficient alternative method for producing molten iron from iron ore to the EM energy pathway if the HI SRV based pathways include:

[0034] (i) deliberately using more, typically substantially more, biomass in briquettes (or other suitable composite forms of iron ore and biomass) than that proposed for the EM energy pathway, to avoid having to use coal in a smelt-reduction vessel, and

[0035] (ii) using technical -grade (pure) oxygen as an oxidant in a smelt-reduction vessel in order to provide an offgas suitable (in terms of heating value and flame temperature) for countercurrent back-integration into the iron ore-biomass preheating stage.

[0036] The term “composites” in the context of iron ore and biomass is understood herein to mean any physically bound combinations of iron ore and biomass, such as briquettes. For example, the iron ore and biomass may be physically bound as a result of a compaction process, with or without a binder, with or without fluxes.

[0037] The term “industrially produced oxygen”, which is also known as “technical grade oxygen”, is understood herein to mean oxygen at purity levels of 95% or greater, for example when produced by an Air Separation Unit (ASU).

[0038] The term “heating value”, often referred to as “calorific value”, is understood herein to mean the amount of heat released during its combustion.

[0039] While composites are desirable for increasing the contact between reductants (released from biomass) and the ore, the applicant has further realised that using types of furnaces other than hearth furnaces for pre-heating ores, at least with composites, has a number of limitations. For example, a rotary kiln will generally be unsuitable because of the breakdown of composites as they are rotated through the furnace, likely leading to excessive dust make. Likewise, shaft furnaces will generally be unsuitable as the composite will typically be unable to bear the weight of the burden in the shaft.

[0040] In broad terms, the invention provides a process and a plant for producing molten iron from iron ore and biomass using a hearth furnace to commence reduction of iron ore and finishing off the reduction of ore in a smelt-reduction vessel, with fuel gas produced in the smeltreduction vessel being combusted to supply heat in the hearth furnace.

[0041] In broad terms, the invention provides a process for producing molten iron from iron ore and biomass using at least one hearth furnace and a smelt reduction vessel (SRV) comprising:

[0042] (a) supplying a plurality of composites of iron ore and biomass containing 40-55% biomass (on a dry weight basis) to a hearth furnace, and heating the composites and combusting combustible gases released from biomass in the composites and partially reducing iron ore in the composites and producing a heated and partly reduced iron ore product and discharging the product from the hearth furnace, with the product having a temperature in the range 550-950 °C and being pre-reduced to between 20 and 70% by predominantly radiant heat from burners in the upper zones of the heath furnace;

[0043] (b) supplying the partly reduced iron ore product, typically hot, through solids injection lances into a molten bath in the smelt reduction vessel and melting and reducing iron ore in the product to molten iron;

[0044] (c) feeding industrially produced oxygen into a top of the smelt reduction vessel and partially combusting an offgas arising from reactions of material in the molten bath and discharging a partially combusted and hot offgas from the smelt reduction vessel with a postcombustion degree (as defined herein) of between 50 and 75%; and

[0045] (d) cooling and cleaning at least a part of the partially combusted and hot offgas discharged from the smelt reduction vessel and using this as a fuel gas in the hearth furnace.

[0046] The terms: (a) “dry weight basis” in the context of “biomass” is understood to generally mean ‘as dry’ at constant selected ambient conditions. While the procedure to determine can vary depending on the biomass selected, for consistency of determination reference is made to ASTM E1757-19 “Standard Practice for Preparation of Biomass for Compositional Analysis" and

[0047] (b) “predominantly” in the context of “radiant heat” is understood herein to mean “for the most part”, in accordance with the standard dictionary meaning of the term.

[0048] The hot offgas discharged from the hearth furnace may be further combusted (incinerated) in a boiler to remove residual CO, then used while hot to generate steam and then electric power.

[0049] The hot offgas, optionally after steam raising for power generation, may be cooled, de-dusted, and then processed in a CO2 removal system, such as an amine scrubber, and the resulting captured CO2 sent to a geo-sequestration facility.

[0050] A small amount of coal or biochar may be co-fed to the smelt reduction vessel for metal carbon control purposes, such that it comprises no more than 10%, typically no more than 5%, of total carbon input to the SRV on an annually averaged basis.

[0051] An amount of natural gas or other combustible fuel may be fed into a product discharge end of the hearth furnace to supplement the combustible gases released from biomass in providing radiant heat, with the overall amount of this fuel not exceeding 3 GJ / tonne of molten iron, typically not exceeding 2 GJ / tonne of molten iron.

[0052] Step (b) may comprise supplying the partly reduced iron ore product through solids injection lances into a molten bath at a temperature above 300 °C, typically above 400 °C, and more typically 500 °C.

[0053] Step (c) may comprise carrying out smelt reduction of iron ore in accordance with the operating principles for the HIsmelt™ and HIsama™ processes as described in references (1- 12) and the disclosures in these references are incorporated herein by cross-reference. The hearth furnace may be a linear hearth furnace or a rotary hearth furnace or any other suitable hearth furnace.

[0054] The composites, as originally fed into the hearth furnace, may be any suitable size.

[0055] By way of example, the composites may have a volume of less than 25 cm3and greater than 2 cm3. Typically, they may have a volume of 3-20 cm3.

[0056] By way of example, as briquettes, the composites may have a major dimension of l-10cm, typically 2-6cm and more typically 2-4cm.

[0057] By way of further example, such briquettes may be generally cuboid, i.e., box-shaped, with six sides and all angles between sides being right angles. By way of example, the briquettes may be “pillow-shaped” briquettes.

[0058] While not limited thereto, the briquettes may be formed as a result of the iron ore fragments and biomass being brought into close contact through compaction.

[0059] An embodiment of the process in accordance with HI SRV based pathways typically comprises the following steps:

[0060] 1. Ore and biomass (optionally with flux materials) are formed into briquettes or other suitable composite forms, with the amounts of the ore and biomass being selected based on factors including having enough biomass to produce sufficient energy for the process. One consideration is selecting an amount of biomass capable of producing sufficient energy for generating heat for a hearth furnace and a smelt-reduction vessel used in the process and for reducing iron ore in the hearth furnace and the smelt reduction vessel. Typically, when biomass is the only source of energy in the process, the amount of biomass is 40-55% (dry ore + dry biomass basis) of the briquettes or other suitable composite forms. In all such calculates, relevant properties of the ore and the biomass are taken into account.

[0061] 2. Briquettes or other suitable composite forms are fed to a hearth furnace, where they are preheated to about 550-950 °C average temperature and pre-reduced to about 20- 70% (this percentage relating to the degree of oxygen removal from ore from an assumed starting point of Fe20s). The term “average” temperature is understood herein to take into account that there may be temperature variations through composites and therefor a temperature of, say 600 °C takes into account that the temperature may be higher on a surface of a composite than inside the composite. Typically, the hearth furnace operating conditions are selected so that the biomass in briquettes or other composite forms is completely devolatilised and is in the form of char or other graphitic type material when the briquettes or other composite forms are discharged from the hearth furnace. Cleaned offgas from a smelt-reduction vessel with a heating value of around 2.5-4.0 MJ / Nm3(LHV basis) is ducted to burners in a top space of the hearth furnace where it is burned with air (which may be preheated and / or partially enriched with oxygen), thereby delivering radiant heat to the briquettes for preheating and pre-reduction in the hearth furnace. Briquette-derived (or other composite forms-derived) bed gas is an additional source of fuel for these combustion reactions and the hearth furnace top space burner system is operated in an oxygen-deficient manner (meaning O2 is only present in an immediate plume region of each burner). Briquettes or other composite forms are removed from the hearth furnace at 550-950 °C average temperature and fed into a smelt-reduction vessel via solids injection lances, typically while still hot, typically at a temperature above 300 °C,. Appropriate hot solids handling equipment is used to manage the transfer and regulation of this hot partially reduced material into the smelt-reduction vessel. It is noted that the briquettes or other composite forms may have broken down to an extent while moving through the hearth furnace. It is also noted that in some situations it may be necessary to breakdown the briquettes or other composite forms to be suitable for injection into the smelt-reduction vessel via solids injection lances. Technical-grade oxygen, as described herein, is used as oxidant in the smelt-reduction vessel top space to generate heat in accordance with known principles of the HIsmelt™-based pathways. Hot metal and slag are tapped from the smelt-reduction vessel in the normal way.

[0062] 7. Smelt-reduction vessel offgas is cooled from about 1500 °C to about 600 °C in a membrane-wall offgas hood or any other suitable option, with heat from this cooling step being used to generate saturated steam.

[0063] 8. Smelt-reduction vessel offgas is subsequently wet-scrubbed and cleaned to deliver a fuel gas with a heating value of about 2.5-4.0 MJ / Nm3(LHV basis). This fuel gas is used as described in Step 3 above.

[0064] 9. After smelt-reduction vessel offgas (together with briquette bed-derived or other composition form bed-derived gas) is burned in the hearth furnace to provide the necessary process heat, offgas from the hearth furnace is sent to an afterburner to combust residual CO via addition of extra air. Offgas leaving the hearth furnace typically has a post-combustion degree of 85-98% (defined as [CO2+H2O] / [CO+CO2+H2+H2O] as mole fractions, expressed as a percentage).

[0065] 10. Hot flue gas from the afterburner, typically with 1-8% free oxygen present, is used for steam raising and power generation in a power plant. The target for power generation is energy self-sufficiency, such that little or no net electric power is imported from elsewhere.

[0066] 11. Optionally (depending on local conditions), flue gas from the power plant may be sent to a CO2 removal system where it is scrubbed for removal of CO2 (for example, using an amine scrubber). Captured CO2 can then be sent via a pipeline to a CO2 sequestration (CCS) resource such as a deep saline aquifer. Additional (imported) electric power for CO2 capture and sequestration is needed under these conditions.

[0067] If electricity-based heating is used in the process (e,g., microwaves combined with electric smelting) then typically around 30-40% biomass is used in briquettes (calculated on the basis of dry ore and biomass, excluding any flux materials). This provides sufficient carbon for production of hot metal containing 2-4% dissolved carbon. If the electric heating component is removed and a smelt-reduction vessel is used instead, then biomass in briquettes or other suitable composite forms needs to increase to around 40-55% (dry ore + dry biomass basis). At this level, there is sufficient carbon for the SRV. For smelter process control purposes, a small amount of coal (or biochar) feed to the SRV may still be used, but this will amount to no more than about 10% of total SRV fuel input (LHV basis) on an annual average basis.

[0068] If the CCS option as described above (in Step 10) is utilised, a vast majority of sequestered CO2 will be biomass-derived. The net effect is removal of CO2 from the atmosphere via biomass as an intermediate, with final CO2 sequestration in suitable underground geological structures. This performs essentially the same function as direct air capture (DAC) with CCS whilst avoiding high costs associated with it and producing molten pig iron from iron ore at the same time.

[0069] The invention also provides a plant for producing molten iron from iron ore and biomass comprising:

[0070] (a) at least one preheater, such as a hearth furnace, comprising a composite feed inlet and a product outlet, a device for transporting a plurality of composites along a path from the inlet to the outlet, a top space above the path, and a plurality of burners in upper zones of the top space, the hearth furnace being configured to produce a heated and partly reduced iron ore product from the composites, with the product having a temperature in a range 550-950 °C and being pre-reduced to between 20 and 70% by predominantly radiant heat from the burners; and

[0071] (b) a smelt reduction vessel for containing a molten bath and for melting and reducing the heated and partly reduced iron ore product from the hearth furnace and producing a molten iron product and an offgas, and

[0072] (c) a connection between the hearth furnace and the smelt reduction vessel for transferring at least a part of the offgas from the smelt reduction vessel to the burners of the hearth furnace for use as a fuel gas for the burners.

[0073] The hearth furnace may be a linear hearth furnace or a rotary hearth furnace or any other suitable hearth furnace.

[0074] The device for transporting the composites along the path may be a conveyor or any other suitable option for supporting and transporting the composites. The smelt reduction vessel may be based on smelt reduction vessels for the HIsmelt™ and HIsarna™ processes as described in references (1-12) and the disclosures in these references are incorporated herein by cross-reference.

[0075] For example, the smelt reduction vessel may comprise a hearth, upwardly extending side walls, a roof that includes an offgas hood, at least one solids injection lance, and at least one gas injection lance, and separate outlets for discharging molten iron and molten slag, respectively.

[0076] The connection between the hearth furnace and the smelt reduction vessel may be any suitable connection.

[0077] Typically, the connection comprises pipework connecting burners of the hearth furnace and the smelt reduction vessel for transferring offgas from the smelt reduction vessel to the burners of the hearth furnace.

[0078] Typically, the connection comprises a unit, such as a steam-membrane hood, for cooling offgas from the smelt reduction vessel from about 1500 °C to about 600 °C.

[0079] Typically, the connection comprises a unit, such as a wet scrubber, for scrubbing and cleaning the offgas.

[0080] The composites may be any suitable size.

[0081] The composites may be any suitable shape.

[0082] The composites of iron ore and biomass may be in the form of briquettes.

[0083] The briquettes may be formed as a result of the iron ore fragments and biomass being brought into close contact through compaction. The term “briquette”, as originally fed into the preheater such as a hearth furnace, is understood herein as a broad term that means a composite of iron ore fragments and biomass that has formed as a result of the iron ore fragments and biomass being brought into close contact through compaction, or alternatively through mixing and binding for example in an agglomeration process, of the iron ore and biomass together. Those skilled in the art would typically describe the latter (particularly when in a spherical form) as pellets. While it is believed “green” pellets have some inherent challenges, not least being they usually need to be carefully dried first (thereby avoiding any sudden steam evolution) and any chosen binder used cannot be one where massive instantaneous devolatilization occurs during heating - both events potentially leading to structural failure of the pellet; pellets are not excluded, but the term briquette does not include indurated pellets, as a feed material according to the method, as such pellets basically get their increased compressive strength by oxidation of the iron ore fragments at temperature back to a higher state of oxidation and through sintering with at least some cross bonding between such fragments. As such they cannot contain biomass (at least not in a uncarbonized form, i.e., any residual carbon remaining could only be there simply as a function of oxidation reactions not being provided with sufficient time to reach equilibrium).

[0084] It will be understood that the composition, size, shape and / or integrity of briquettes will vary as they are processed through the hearth furnace to eventually exit. The briquettes may lose structural integrity and not be in a form that resembles the feed briquettes.

[0085] Any use of the term ‘briquette’ when describing the passage of material through the hearth furnace is not intended to be self-limiting to what was originally fed into the furnace but is used merely for descriptive convenience.

[0086] The term “fragment” is understood herein to mean any suitable size piece of iron ore (as passed through an appropriately screen mesh of 6.35mm spacing or below) and as used herein may be understood by some persons skilled in the art to be better described as “particles” and / or “fines”. The intention herein is that such terms be used as synonyms. The iron ore may be any suitable type such as magnetite, hematite and / or goethite. However, it does not preclude other iron rich ores from which iron may be extracted such as limonitic laterites, titaniferous magnetite and vanadiferous magnetite due to the local unavailability of the more usual forms of iron ore from which iron is traditionally extracted.

[0087] The term “biomass” is understood herein to mean living or recently living organic matter. Specific biomass products for a composite of iron ore fragments and biomass include, by way of example, forestry products and their by-products (in the form of woodchips, sawdust and residues therefrom), agricultural products and their by-products (like sorghum, hay, straw and sugar cane bagasse), agricultural residues (like almond hull and nut shells), purpose grown energy crops such as Miscanthus Giganteus and switchgrass, macro and micro algae produced in an aquatic environment, as well as recovered municipal wood and paper wastes.

[0088] The invention also provides a hot metal product made by the above-described method.

[0089] The invention also provides a hot metal product made in the above-described plant.

[0090] BRIEF DESCRIPTION OF THE DRAWINGS

[0091] The present invention is described further by way of example with reference to the accompanying drawings, of which:

[0092] Figure l is a schematic diagram of one embodiment of a process and an apparatus for producing molten iron from briquettes of a composite of iron ore fragments and biomass in accordance with the invention, without CO2 capture and sequestration (CCS); and

[0093] Figure 2 is a schematic diagram of a further embodiment, although not the only other embodiment, of a process and an apparatus for producing molten iron from briquettes of a composite of iron ore fragments and biomass in accordance with the invention, with CO2 capture and sequestration (CCS).

[0094] DESCRIPTION OF EMBODIMENTS

[0095] Figures 1 and 2 include considerable process information about the embodiments of the invention shown in these Figures. The process information in the Figures is derived from a process model accessible to the applicant. The embodiments of the invention shown in these Figures are technically-viable embodiments based on the model. With reference to Figure 1, iron ore-biomass composite briquettes 1 (or any other suitable forms of composites of ore and biomass) are formed in a cold briquetting plant 31 by blending together 50% (wet) biomass 33, 45.6% (wet) iron ore 35, 2.4% (wet) of dolomite 37, and 2.0% (wet) limestone 39 and forming a mixture and compacting the mixture into briquettes. The resulting briquettes 1 have 48.5% biomass on the basis of the mass of dry ore plus dry biomass in the mixture.

[0096] Briquettes 1 are fed into a linear hearth furnace 2 that comprises a main chamber 71 and move through the furnace 2 on an endless conveyor 43 from a feed end 41 to a discharge end 45 and are heated over 24 minutes to an average temperature of 806 °C and pre-reduced 34.3% to produce a product with 34.3% pre-reduction (corresponding to 10.2 % metallisation, as defined earlier), 22.41% C, and small amounts of P and S that is discharged as a hot solid DRI product 3 from the discharge end. The carbon content in the hot solid DRI product 3 is 22.4%.

[0097] It is noted that there may be some breakdown of the briquettes 1 as they move through the linear hearth furnace 2 and therefore the briquettes 1 may not have the same shape as at the feed end 41.

[0098] In this embodiment, three identical linear hearth furnaces (LHFs) are used (not shown) in parallel and the hot solid DRI product 3 from each LHF is combined into a single stream for further downstream processing.

[0099] It is noted that any suitable numbers of LHFs may be used. It is also noted that a skilled person would understand what is required to combine multiple streams.

[0100] Hot solid DRI product 3 is fed into a hot solids handing system, identified by the line 47 in the Figure and from there into a HIsmelt™ smelt-reduction vessel (SRV) 4 via solids injection lances 49 of the vessel.

[0101] The hot solids handing system 47 can be any suitable system for transporting hot material.

[0102] The hot solid DRI product 3 is transported by a carrier gas 55 supplied to the line 47. Only two solids injection lances 49 is shown in the Figure. It can readily be appreciated that the SRV 4 may have multiple additional solids injection lances 49 positioned around the side wall of the SRV 4.

[0103] The SRV 4 is as described in References 1-12 and the disclosures in these references, are incorporated by cross-reference.

[0104] Additional feeds into the SRV 4 via the solids injection lances 49 are small amounts of (i) anthracite coal 5 and (ii) calcined lime 6. These feeds 5, 6 are combined and transported by a N2 carrier gas via a line 51 into the supply line 47. The resultant mixed SRV feed in line 47 comprises 23.6% C. Injected solids via line 47 generate a fountain in the SRV according to standard descriptions of the HI SRV based pathways as described in References 1-12.

[0105] It is noted that in this embodiment, biomass is the only heat source in the linear hearth furnace 2 and the SRV 4. In particular, in this embodiment (although not necessarily in all embodiments) there is no electrically-based heating system involving electrodes or any type of electromagnetic radiation (such as microwaves). Other embodiments of the invention may include electrically-based heating systems.

[0106] Industrially produced oxygen 7 at purity levels of 95% or greater produced, for example, by an Air Separation Unit (ASU) (not shown), is injected into the SRV top space for the purpose of heat generation.

[0107] Bath-derived CO and char are partially combusted in the SRV 4, resulting in a SRV offgas that is roughly 65% post-combusted (meaning about 65% of the CO has been converted to CO2) that is discharged via line 57 extending from an offgas hood 59 of the SRV 4.

[0108] The primary molten product stream from the SRV 4 is hot metal 8 that is discharged via an overflow weir 61 of the SRV 4. In accordance with normal HI SRV based pathways expectations, the hot metal 8 has 4.0% C and low concentrations of impurities (silicon, manganese and phosphorous in particular). It is either cast into pigs for shipment to EAF based steel plants or the like as cold iron feed or used as hot metal and processed directly to steel in a co-located downstream steelmaking furnace.

[0109] The other molten product stream from the SRV 4 is molten slag 9 that is periodically tapped via line 63 from the SRV 4. This material is similar to blast furnace slag in terms of usability in cement making or other applications usually associated with such slag.

[0110] Hot SRV offgas is first cooled from about 1500 °C to about 600 °C in a steam-membrane hood 65, then wet-scrubbed and cleaned in unit 67 to produce fuel gas 11 with a heating value of around 3.15 MJ / Nm3(LHV basis). From there, it is divided into three equal streams and sent to the three LHFs 2 (only one of which is shown in the Figure) where it is burned in a set of top space burners 69 in the chambers 71 of the LHFs 2.

[0111] The LHF 2 shown in the Figure has 16 burners 69 spaced along the length of the LHF 2. It can readily be appreciated that the LHF 2 may have any suitable number of burners 69. In each LHF 2, air is fed via heat exchanger 12 to produce preheated air 13 at a temperature of 400 °C. This is also added to LHF burners 69 to generate heat for a preheating stage in the LHFs.

[0112] LHF 2 top space temperatures of around 1200-1400 °C are maintained and heat transfer to the briquette bed occurs primarily via radiation in order to minimise briquette carbon oxidation and dust generation.

[0113] The right-most burner 69 (nearest the hot discharge end 45 of the unit and identified by the number 16 in the Figure) receives an additional 2000 Nm3 / h of natural gas 14 to provide the necessary heat at the starting point of the combustion gas train.

[0114] Hot offgas is generated along the length of the chamber of the LHF 2 and moves to the left of the LHF 2 as drawn in the Figure.

[0115] At an offgas discharge end of the LHF 2 (on the left as drawn in in the Figure), the offgas has a post-combustion degree of about 94%, meaning most of the CO has been burned to CO2 but there is still a small residual amount of CO present. This offgas is fed via a line 75 to an afterburner 15, where additional preheated air is added, and full combustion is accomplished. From here, the fully-combusted offgas passes in a line 77via a heat exchanger 12 to a boiler 16.

[0116] If hot metal from the SRV 4 is processed directly to steel in a BOF shop, then there is an opportunity to capture and use BOF gas in this boiler 16 as supplementary fuel (together with a very small amount of natural gas for pilot flame maintenance). Heat is extracted from hot flue gas in boiler 16 and, in this example, 132 MW of electric power is generated. This amount of power is more than enough to provide electricity for the entire plant (including the air separation unit providing oxygen to the SRV 4).

[0117] Flue gas from the boiler 16 is de-dusted in a baghouse 79. From here clean flue gas 17 is vented to atmosphere.

[0118] In the second embodiment shown in Figure 2, the only significant difference relative to the description above in relation to Figure 1 is that CO2 is captured from the boiler flue gas stream for sequestration (CCS).

[0119] With reference to Figure 2, warm gas 20 from the LHF 2 shown in Figure 1 and hot combusted BOF gas / air 21 and combusted and provide heat to a steam circuit generally identified by the numeral 81. Boiler flue gas from the circuit 81 passes via a line 83 to a baghouse 85 at 220 °C. Details of the flue gas are shown in the box 23 in the Figure. The flue gas passes through the baghouse 85 and is then pressurised to 0.2 bar gauge in a fan 24. It then passes via a gas cooler 25 to an amine scrubber 26 where most of the CO2 is transferred from the gas to the amine-containing scrubber liquor. Absorbed CO2 is released back to the gas phase in a stripper 27, from where is passes to cleaning and compression plant 28 for delivery to a suitable CO2 sequestration resource.

[0120] Stripper 27 requires a large amount of low-grade heat for boiling off absorbed CO2. This heat is delivered to reboiler 29 from the main steam circuit. The main circuit effectively uses amine reboiler 29 as a condenser at 5 bar absolute, but in doing so it sacrifices steam cycle efficiency and only generates 82.7 MW of electric power (compared to 132 MW in Example 1). As a consequence, the overall plant needs to import around 96 MW of electrical power (offset against 302 t / h of CO2 sent to CCS). Many modifications may be made to the embodiments described above in relation to Figures 1 and 2 without departing from the spirit and scope of the invention.

[0121] By way of example, whilst the embodiments described above in relation to Figures 1 and 2 are quite specific about the values of temperatures and other parameters in the respective flow sheets of the Figures, as evidenced by the detailed information in the Figures, the invention is not confined to this specific operational data.

[0122] Equally, the invention is not confined to the specific information about the linear hearth furnace 2 and the smelt reduction vessel 4 and other equipment in the Figures.

[0123] The embodiments are two or many possible worked examples of the process and plant of the invention. A skilled person would appreciate variations to these embodiments that are within the scope of the invention.

[0124] References

[0125] 1. PCT / AU99 / 00884 (W02000 / 22176) “A process and apparatus for producing metals and metal alloys”, Priority 14 Oct 1988

[0126] 2. US Patent 6989042 “Direct Smelting Process and Apparatus”, Priority 17 April 2000

[0127] 3. US Patent 8221675 “Smelt-reduction vessel and Cooler Therefor”, Priority 18 May 2006

[0128] 4. US Patent 9175907 “Direct Smelting Process and Apparatus”, Priority 9 Feb 2010

[0129] 5. Australian Patent 2011301784 (WO2012 / 034184) “Direct Smelting Process”, Priority 15 Sept 2011

[0130] 6. US Patent 9359656 “Direct Smelting Process”, Priority 9 Feb 2012

[0131] 7. PCT / AU2012 / 000293 (WO2012 / 126055) “Direct Smelting Process for High Sulphur Feed”, Priority 21 March 2012

[0132] 8. PCT / AU2012 / 001486 (WO2013 / 082658) “Starting a Smelting Process”, Priority 6 Dec 2011

[0133] 9. PCT / AU2012 / 001481 (WO2013 / 082653) “Starting a Smelting Process”, Priority 6 Dec 2011

[0134] 10. PCT / AU2012 / 001487 (WO2013 / 082659) “Starting a Smelting Process”, Priority 6 Dec 2011

[0135] 11. PCT / AU2014 / 001098 (WO2015 / 081376) “Smelting Process and Apparatus”, Priority 4 Dec 2014

[0136] 12. PCT / AU2014 / 001146 (WO2015 / 089563) “Smelting Process and Apparatus”, Priority 19 Dec 2014

[0137] 13. Vogl, V et al, Assessment of hydrogen direct reduction for fossil-free steelmaking, Journal of Cleaner production 203 (218) 736-745

[0138] 14. Strezov, V, Iron ore reduction using sawdust: experimental analysis and kinetic modelling, renewable Energy 31(12) 1892-1905, Oct 2006

Claims

CLAIMS1. A process for producing molten iron from iron ore and biomass using at least one hearth furnace and a smelt reduction vessel (SRV) comprising:(a) supplying a plurality of composites of iron ore and biomass containing 40-55% biomass (on a dry weight basis) to a hearth furnace and heating the composites and combusting combustible gases released from biomass in the composites and partially reducing iron ore in the composites and producing a heated and partly reduced iron ore product and discharging the product from the hearth furnace, with the product having a temperature in the range 550-950 °C and being pre-reduced to between 20 and 70% by predominantly radiant heat from burners in the upper zones of the heath furnace;(b) supplying the partly reduced iron ore product through solids injection lances into a molten bath in the smelt reduction vessel and melting and reducing iron ore in the product to molten iron;(c) feeding industrially produced oxygen into a top of the smelt reduction vessel and partially combusting an offgas arising from reactions of material in the molten bath and discharging a partially combusted and hot offgas from the smelt reduction vessel with a postcombustion degree of between 50 and 75%; and(d) cooling and cleaning the partially combusted and hot offgas discharged from the smelt reduction vessel and using this as a fuel gas in the hearth furnace.

2. The process according to claim 1 wherein hot offgas discharged from the hearth furnace are further combusted (incinerated) in a boiler to remove residual CO, then used while hot to generate steam and thereby electric power.

3. The process according to claims 1 or 2 wherein the hot gases, optionally after steam raising for power generation, are cooled and de-dusted, then processed in a CO2 removal system, such as an amine scrubber, and the resulting captured CO2 sent to a geo-sequestration facility.

4. The process according to claim 1 wherein a small amount of coal or biochar is co-fed to the SRV for metal carbon control purposes, such that it comprises no more that 10% of total carbon input to the SRV on an annually averaged basis.

5. The process according to claim 1 wherein an amount of natural gas or other combustible fuel is fed into the product discharge end of the hearth fumace(s) to supplement the combustible gases released from biomass in providing radiant heat, with the overallamount of this fuel not exceeding 3 GJ / tonne of molten pig iron.

6. The process according to claim 1 where the composites of iron ore and biomass are in the form of briquettes.

7. The process according to claim 6 wherein the briquettes are formed as a result of the iron ore fragments and biomass being brought into close contact through compaction.

8. The process according to any of the preceding claims wherein the partly reduced iron ore product is injected through the solids injection lances into a molten bath while at a temperature above 300°C.

9. The process according to claim 8 wherein the temperature of injection is above 500°C.

10. A plant for producing molten iron from iron ore and biomass comprising:(a) at least one hearth furnace comprising a composite feed inlet and a product outlet, a device for transporting a plurality of composites along a path from the inlet to the outlet, a top space above the path, and a plurality of burners in upper zones of the top space, the hearth furnace being configured to produce a heated and partly reduced iron ore product from the composites, with the product having a temperature in a range 550-950 °C and being prereduced to between 20 and 70% by predominantly radiant heat from the burners;(b) a smelt reduction vessel for containing a molten bath and for melting and reducing the heated and partly reduced iron ore product from the hearth furnace and producing a molten iron product and an offgas; and(c) a connection between the hearth furnace and the smelt reduction vessel for transferring at least a part of the offgas from the smelt reduction vessel to the burners of the hearth furnace for use as a fuel gas for the burners.

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