Metalliferous ore direct reduction

Abstract

An apparatus for heating a microwave absorbent material comprises a furnace 3 that has a microwave energy zone 30 and an endless conveyor 50 for transporting the material through the microwave energy zone 30 so that material on the conveyor can be exposed to microwave energy. The conveyor forms a part of a microwave energy barrier that prevents loss of microwave energy from the microwave energy zone. The conveyor comprises a base for carrying material. The base comprises a plurality of metal sections 88 extending transverse to a direction of travel through the furnace that are coupled together with a gap 90 between adjacent metal sections so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path. The gap defines or comprises a choking structure 112 that restricts microwave leakage through the gap.

Description

[0001] METALLIFEROUS ORE DIRECT REDUCTION

[0002] TECHNICAL FIELD

[0003] The present invention relates to an apparatus for heating a microwave absorbent material, the apparatus comprising a furnace, such as a linear hearth furnace, through which, in use, the material is transported on an endless conveyor and microwave energy provides at least a part of the energy for heating the material in the furnace.

[0004] The present invention also relates to an endless conveyor that is suitable for use in a furnace, such as a linear hearth furnace, that includes a means for supplying microwave energy to heat a microwave absorbent material, with the conveyor being configured to form a barrier to loss of microwave energy from gaps in the conveyor.

[0005] The present invention relates more particularly, although by no means exclusively, to an apparatus for continuously producing direct reduced metal material from a metalliferous ore and a carbon rich solid reductant in a furnace, such as a linear hearth furnace, through which, in use, the ore and the solid reductant are transported on an endless conveyor and microwave energy is applied for heating the metalliferous ore in the furnace.

[0006] The present invention relates particularly, although by no means exclusively, to an apparatus for continuously producing direct reduced iron from iron ore and biomass that comprises a furnace, such as a linear hearth furnace, having an endless conveyor comprising interlinked metal pans for carrying ore and biomass through the furnace, with the interlinked metal pans being configured to form a barrier to loss of microwave energy from the furnace.

[0007] Such direct reduced iron, for example while hot, may be subsequently melted in a furnace to create hot metal, then cast as pig iron or refined further to steel in a metallurgical furnace.

[0008] The term “microwave absorbent material” is understood herein to mean a solid substance that can absorb microwave energy (typically producing heat), comprising completely or in part a mined material that has been formed by a geological process. For the avoidance of doubt, microwave absorbent material may be a blend of mined materials or combinations of mined and non-mined material(s), may be in a loose or bound form, and / or may have already undergone some solid-state chemical or thermal process. It is not necessary that the microwave absorbent material be particularly absorbent to microwave energy on entry to the furnace, only that it readily absorbs microwave energy at the time of application of microwave energy, and such absorbency is created by bringing such material to predetermined temperatures. Iron ore and biomass are examples of microwave absorbent materials. Naturally occurring spodumene is another example of a microwave absorbent material.

[0009] The term “direct reduced metal material” is understood herein to mean solid metal(s) produced in the direct reduction of metalliferous ore to metal(s) by a reducing agent at temperatures below the bulk melting temperature of the metal(s). For the purposes of the discussion herein “solid direct reduced metal materials” (DRM) is understood herein to mean materials that are at least substantially metal.

[0010] The term “linear hearth furnace” is understood herein to mean a furnace that includes a lengthwise extending refractory-lined heating chamber having at least one reaction zone with an inlet (feed) end at one end and an outlet (discharge) end at the opposite end and an endless conveyor that extends along the length of the chamber and carries material through the chamber from the inlet to the outlet ends for thermal processing in the chamber, with the conveyor returning to the inlet end from the discharge end and repeating the process of loading material on the conveyor to be transported through the chamber.

[0011] The term “direct reduced iron” is understood herein to mean solid iron produced from the direct reduction of iron ore to iron by a reducing agent at temperatures below the bulk melting temperature of the iron. For the purposes of the discussion herein “direct reduced iron” (DRI) is understood to have at least 85% metallisation.

[0012] The term “metallisation” is understood herein to mean 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. The use of the term “iron ore” as used herein is not solely limited to its mineral forms, such as magnetite, hematite and / or goethite. It includes all iron rich ores from which iron may be extracted, as well as other desirable metals recovered or extracted, such as limonitic laterites, titaniferous magnetite and vanadiferous magnetite.

[0013] BACKGROUND

[0014] With global efforts to reduce overall atmospheric CO2 there is pressure on primary metal producers to find means to produce metals from metalliferous ores without causing net emissions of greenhouse gases. In particular, there is pressure not to use geological origin carbonaceous material, like coal and natural gas, collectively often referred to as ‘fossil fuels’, which are non-renewable and result in increased concentrations of atmospheric greenhouse gases. Iron and steel making processes are no exception to this trend and are historically carbon intensive processes in which carbon, usually from carbonaceous geological materials, is used with the carbon (except for residual amounts retained in iron and / or steel products) eventually being discharged to the atmosphere as CO2.

[0015] The majority of iron produced globally for conversion to steel is currently produced in the blast furnace route, which is a technology that has existed since prior to the industrial revolution. Even with technology advances, a blast furnace still requires around 800kg of metallurgical coal for every tonne of iron produced and emits high levels of CO2, roughly 1.8- 2.2 t CO2 per tonne of hot metal. Fossil fuels, in particular the requirement for coal (in the form of coke), are an essential feed material for a blast furnace, and it is not possible simply to use hydrogen as a complete substitute therein.

[0016] An alternative approach to the blast furnace route is the direct reduction of iron ore in the solid state by carbon monoxide and hydrogen derived from natural gas or coal. While such direct reduction plants are (outside of India) small in number compared to blast furnaces, there are many known processes for the direct reduction of iron ore. In India, coal based rotary kiln furnaces are typically fed with lump iron ore and coal to produce DRI, also known as sponge iron (approaching 20% of world production of DRI). Elsewhere gas-based shaft furnace processes are generally used. In these processes, indurated pellets produced from agglomerated finely ground iron ore are reduced to DRI (approaching 80% of world production of DRI).

[0017] One proposal to make steel in a ‘green manner’ i.e., without significant involvement of fossil fuels, is through the production of DRI using hydrogen as the reductant. This could, for example, be through using the indurated pellet approach or through the reduction of iron ore fines in a fluidized bed system, such as the Metso hydrogen-based Circored ™ process, followed typically (but not exclusively) by smelting in a special form of electric arc furnace (a submerged arc furnace (SAF) or an electric smelting furnace (ESF)) to produce iron containing carbon or steel. For such routes to be low carbon users / emitters as possible requires the use of non-fossil fuel produced energy, like 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, as well as the use of non-fossil fuel produced energy to power the furnace. However, the final smelting / melting step by definition requires some carbon to produce a final metallurgical structure of a desired end product. Only high grade DRI with very low gangue and other impurities can be supplied to a conventional EAF for steelmaking by mixing such DRI with steel scrap.

[0018] Another proposal to make steel in a ‘green manner’ i.e., without any involvement of fossil fuels, is through the production of DRI using biomass as the reductant. 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). A Group company of the applicant is developing technology to produce a biomass-reduced DRI from iron ore. The biomass technology, including a specific process known as Bioiron™ technology, uses raw biomass instead of metallurgical coal as a reductant to convert iron ore to metallic iron for use in steelmaking processes. The patent portfolio for Bioiron technology includes, for example, International patent applications

[0019] PCT / AU2017 / 051163 (WO2018 / 076048), PCT / AU2021 / 050526 (WO2021 / 237308), PCT / AU2021 / 051094 (WO2022 / 061398), and PCT / AU2021 / 051398 (WO2022 / 109663). To date there is no large-scale commercial ironmaking process that uses biomass directly, including for the DRI production route. Previous attempts to add some biomass into processes originally designed for coal (e.g., 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 metallurgical process around the fundamental nature of biomass.

[0020] 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 - and 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.

[0021] The use of electromagnetic energy, such as microwave (MW) energy and radio frequency (RF) energy, in iron ore reduction processes, whether simply in 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 been previously proposed.

[0022] One proposal to use electromagnetic energy as part of the direct reduction of iron ore in a solid state is the Bioiron ™ process, as exemplified in International application PCT / AU2021 / 051398 (WO2022 / 109663) in the name of the applicant. That application describes a process and an apparatus for direct reduction of iron ore in a solid state under anoxic conditions with biomass as a reductant and with electromagnetic energy as a source of energy where gases arising from reduction of the ore in a reduction zone flow into a preheat zone and iron ore in the preheat zone is initially heated from combustion of oxygen in the zone, while anoxic conditions are maintained in the reduction zone. This is referred to by the applicant as a ‘countercurrent’ approach, with briquettes (of a composite of iron ore fragments and biomass) traveling in one direction and reduction gases from the reduction zone traveling in the opposite direction. Electromagnetic compatibility (EMC) requirements impose stringent limits on any emissions outside the internationally agreed and recognised bands for the application of MW energy and RF energy. These limits are much lower than those imposed by health and safety and are typically equivalent to pWs of power at any frequency outside permitted bands.

[0023] It is known to use choking (attenuation) tunnels for continuous-flow, conveyor-based applications for the passage of “workloads” to be treated in industrial scale microwave ovens. Such choke tunnels are provided at the entrance and exit thereof to help minimise power wastage by effectively reflecting leakage power back into the oven and to ensure the applied high field strength in the oven is reduced to a residual microwave level that is less than the maximum prescribed for health and safety purposes, as well as at least one step towards ensuring that any residual leakage arising from such ovens does not lead to interference with communication equipment and the like.

[0024] However, there is a number of technical challenges when the microwave oven is within a furnace such as a linear heath furnace and there are furnace temperatures varying from room temperature to well in excess of 1000°C, as is the case with the Bioiron™ process. One challenge is to contain the microwaves to a desired zone within the furnace, while not unnecessarily inhibiting material flows, such as reduction gases, within the furnace. Another challenge is not to unnecessarily create impedance points for microwaves in any gaps between components of the conveying equipment in the furnace that may trigger electrical arcing leading to excessive heating of such componentry, while still ensuring that the microwaves are contained to the desired zone. Significant variations in temperatures that will occur in furnaces operating processes such as the Bioiron process will cause different thermal expansions of furnace equipment and this will have an impact on any gaps between different furnace components, such as gaps between moving conveyors and fixed furnace walls. These technical challenges become increasing difficult on scale up.

[0025] Further development work has been carried out on the invention described in International application PCT / AU2021 / 051398 (WO2022 / 109663). It is understood that the above description is not to be taken as an admission of the common general knowledge in Australia or elsewhere.

[0026] SUMMARY OF THE DISCLOSURE

[0027] The invention is the result of work to develop an apparatus for the invention described in International application PCT / AU2021 / 051398 (WO2022 / 109663).

[0028] The invention provides an apparatus for heating a microwave absorbent material that comprises a furnace, such as a linear hearth furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone, and an endless conveyor for transporting the material through the microwave energy zone and being configured to form a part of a barrier to loss of microwave energy from the zone.

[0029] The invention also provides an apparatus for continuously producing direct reduced metal material (DRM) from a metalliferous ore and a carbon rich solid reductant, such as biomass, the apparatus comprising a furnace, such as a linear hearth furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating the ore and the reductant and reducing the ore to DRM in the zone, and an endless conveyor for transporting the ore and the reductant through the microwave energy zone and being configured to form a part of a barrier to loss of microwave energy from the zone.

[0030] The invention also provides an apparatus for continuously producing direct reduced iron (DRI) from iron ore and biomass comprising a furnace, such as a linear hearth furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating the ore and biomass and reducing the ore to DRI, and an endless conveyor for transporting the ore and biomass through the microwave energy zone and being configured to form a part of a barrier to loss of microwave energy from the zone.

[0031] The term “endless conveyor” is understood herein to mean a type of conveyor has a base for carrying material that moves in a continuous loop.

[0032] The term “microwave energy barrier” is understood herein to mean a structure that is capable of reducing the amount of microwave energy escaping the furnace by a factor of at least 100 (20 dB), typically a factor of at least 1000 (30 dB), and more typically a factor of at least 10,000 (40 dB). If the barrier was the only barrier to microwave energy being emitted from the furnace, then for personnel safety reasons alone the furnace would need to be designed to be capable of ensuring that leakage at 50 mm or more from the furnace does not exceed 5 milliwatt (5 mW) of energy per cm2.

[0033] The conveyor may comprise a plurality of metal sections that are not transparent to microwaves extending transverse to a direction of travel of the conveyor and coupled together with a gap between adjacent metal sections.

[0034] The metal sections may be coupled together so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path.

[0035] The invention also provides an apparatus for heating a microwave absorbent material, the apparatus comprising a furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating a microwave absorbent material in the zone, and an endless conveyor for carrying a microwave absorbent material through the furnace, the conveyor comprising a base comprising a plurality of metal sections extending transverse to a direction of travel through the furnace and defining a base for carrying the microwave absorbent material and being coupled together so that there is a gap between adjacent metal sections so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path, and with the conveyor being configured to form a part of a microwave energy barrier to loss of microwave energy from the microwave energy zone.

[0036] Each gap may extend across the width of the metal sections.

[0037] Each gap may extend through a thickness of the metal sections.

[0038] Typically, each gap defines a tortuous pathway.

[0039] The tortuous pathway may comprise one or more than one change of direction.

[0040] The one or more than one change of direction may comprise one or more than one bend in the pathway.

[0041] The microwave energy barrier is an important part of the furnace for health and safety reasons, electromagnetic compatibility (EMC), and for process reliability reasons.

[0042] Typically, the metal sections of the conveyor are not transparent to microwaves and therefore are a part of the microwave energy barrier.

[0043] Typically, each gap defines or comprises a choking structure that restricts microwave leakage through the gap and forms a part of a microwave energy barrier.

[0044] Without the choking structure, each gap could define a pathway for microwaves to travel from the microwave energy zone and escape the microwave energy zone.

[0045] More specifically, typically the gap is designed to be a minimum physical distance, preventing arcing. Due to the need for physical separation, energy leakage through this opening can occur.

[0046] The term “choking structure” is understood herein to mean a structure that restricts microwave leakage. The choking structure may be configured to absorb and therefore attenuate rather than reflect microwaves into the microwave energy zone.

[0047] For example, the gap may be at least partially filled with an absorbent material.

[0048] Carbon foam is an example of an absorbent material.

[0049] It is also noted that the feed material for the furnace may be very absorbent.

[0050] The choking structure may be configured to reflect microwaves into the microwave energy zone.

[0051] In that event, each gap may define or comprise a choking structure as a result of the design of the gap.

[0052] The design selection may take into account dimensions and changes of direction of a pathway defined by the gap, noting that the wavelength and intensity of the microwaves will have an impact on dimensions and changes of direction.

[0053] Typically, the design selection is not solely governed by what is necessary to allow adjacent metal sections to change orientation relative to each other when the conveyor moves in a curved path.

[0054] The choking structure may comprise the one or more than one change of direction of the pathway.

[0055] The choking structure may comprise a deadend in the pathway.

[0056] The term “deadend” is understood herein to mean in general terms a reflective structure formed for example by a closed groove.

[0057] The deadend may extend from a bend in the pathway. For example, the deadend may be a branch of a main part of the pathway.

[0058] The deadend may be configured to reflect microwaves into the microwave energy zone.

[0059] The choking structure may be a quarter-wave choking structure.

[0060] The choking structure may be a quarter wave choking structure that is a result of a number of design features of each gap.

[0061] One design feature is that each gap is formed as a tortuous pathway, with multiple bends and changes in direction.

[0062] In addition, another design feature is that the pathway comprises a deadend.

[0063] The deadend may extend from a bend in the pathway and is in the form of a channel extending across the width of each metal section (which may also be described as a closed groove). In effect, the deadend is a branch from a part of the pathway.

[0064] In one embodiment, the choking structure is a quarter wave choking structure.

[0065] The quarter wave choking structure relies on a property of impedance transformation of a transmission line, in which the impedance transforms to its inverse every quarter wavelength from its termination.

[0066] The quarter wave choking structure may comprise an entry section from the microwave energy zone, a deadend (closed groove) extending from the entry section, and an exit section. The deadend section may terminate in a physical short circuit (high microwave reflection) and have a length equal to or approximate to a quarter of the wavelength of an operational microwave frequency (wavelength slightly different than free space). This presents a point of very high impedance at a deadend entry. The length from the deadend entry to an entry gap at a top surface of the metal sections is equal to or approximates a quarter wavelength of the operational microwave frequency, making its impedance very low, forming a virtual electrical connection (i.e. virtual short circuit, high microwave reflection). This allows currents to bridge the entry gap mostly unperturbed, minimising the energy that leaks through the structure. Similarly, the length of the exit section also equals or approximates a quarter wavelength of the operational microwave frequency and ensures a low impedance on the entry gap and, therefore, a high microwave reflection.

[0067] Typically, the choking structure is selected to reduce the amount of microwave energy escaping the furnace by a factor of at least 30 dB, typically by a factor of at least 40 db.

[0068] The choking structure may be configured so that there is very little loss of energy via absorption in deadend.

[0069] Typically, the choking structure is configured so that there is no more than 0.04% loss of energy from the gap.

[0070] The deadend may comprise a channel extending across the width of the metal sections.

[0071] The pathway may comprise a filler element for preventing dust penetrating the pathway.

[0072] The pathway may comprise a filler element for maintaining a desired geometry of the pathway.

[0073] Typically, the filler element is at least substantially transparent to microwaves.

[0074] It is noted that it is not essential that the filler be transparent to microwaves.

[0075] A part of one metal section may overlap at least a part of a successive metal section when the conveyor is travelling through the microwave energy zone, and there is a vertical space between the overlapping sections, with the space forming an initial part of the gap, and with the initial part extending from the microwave energy chamber.

[0076] In other embodiments, the metal sections do not overlap, and there is a space between one metal section and a successive metal section when the conveyor is travelling through the microwave energy zone, with the space forming an initial part of the gap, and with the initial part extending downwardly, typically vertically, from the microwave energy chamber.

[0077] Each metal section may comprise a leading edge element and a trailing edge element extending transverse to the travel direction of the conveyor, with each gap being between the leading edge element of one metal section and the trailing edge element of the successive metal section.

[0078] Each leading edge element and the trailing edge element may have a width, a thickness, and an upper surface that faces the microwave energy zone when the conveyor is travelling through the micro wave energy zone.

[0079] Each gap may extend across the width of each leading edge element and each trailing edge element.

[0080] Each gap may extend through the thickness of each leading edge element and each successive trailing edge element.

[0081] The metal sections may be in the form of pans.

[0082] The pans may comprise a flat base.

[0083] Each pan may comprise end edges, for example in the form of flanges, that extend upwardly from opposite sides of the base of the pan.

[0084] The apparatus may include pans having side edges comprising flanges that have the same gap structure as the base of the pans (for the purposes of forming part of the microwave energy barrier).

[0085] As noted above, the microwave energy barrier is an important part of the furnace for health and safety reasons, electromagnetic compatibility (EMC), and for process reliability reasons. The choking structure of the conveyor is one part of the microwave energy barrier.

[0086] The microwave energy barrier may also comprise a microwave choking structure at a feed inlet of the microwave energy zone.

[0087] The microwave energy barrier may also comprise a microwave choking structure at a discharge outlet of the micro wave energy zone.

[0088] The furnace may comprise walls that define opposite sides of the microwave energy zone.

[0089] The side walls may be fixed walls in relation to the conveyor when the conveyor is moving through the furnace.

[0090] The side walls and the conveyor may be positioned so that these is a gap between each side wall and the conveyor to allow movement of the conveyor without contacting the side walls.

[0091] Each gap may define a potential pathway for microwaves to travel from the microwave energy zone and escape the microwave energy zone.

[0092] The chamber includes a choking structure to minimise microwave leakage through the gaps that forms a part of the microwave energy barrier.

[0093] In a situation where the pans have side flanges, the microwave energy barrier may comprise choking structures for the gap between each side wall and the side flanges.

[0094] The base of the conveyor may be formed from any suitable material.

[0095] Aluminium (which terms includes aluminium alloys) is a preferred material from the perspective of being a conductor and substantially not absorbing microwaves. However, one limitation of aluminium is being able to withstand furnace temperatures for production of DRE Stainless steel is another option. It is not the best material from the perspective of being a conductor and not absorbing microwaves, but it is able to withstand furnace temperatures for production of DRI.

[0096] Other materials include, by way of example, Inconel and other high temperature metal alloys and ceramic materials.

[0097] The base of the conveyor may further comprise a layer (such as a coating) of a material on an upper surface of the metal sections.

[0098] The layer may be metallic to minimise surfaces losses.

[0099] The layer may be transparent to microwave energy.

[0100] The layer may be configured to lift material off the metal sections.

[0101] The metalliferous ores may be an iron ore.

[0102] The carbon rich solid reductant may be biomass.

[0103] The invention also provides an endless conveyor for carrying a material through a furnace for heating microwave absorbent material in the material with microwave energy, the conveyor comprising a base for carrying the material, the base comprising a plurality of metal sections extending transverse to a direction of travel through the furnace, and being coupled together so that there is a gap between adjacent metal sections with the gap being configured to form a part of a microwave energy barrier to loss of microwave energy from a microwave energy zone of the furnace.

[0104] In broader terms, the invention also provides an endless conveyor that is suitable for carrying a material through a furnace, such as a linear hearth furnace, of an apparatus for heating a material with microwave energy, with the conveyor being configured to form a part of a barrier to loss of microwave energy from the furnace. The conveyor may comprise a plurality of metal sections that are not transparent to microwaves extending transverse to a direction of travel of the conveyor and coupled together with a gap between adjacent metal sections.

[0105] The metal sections may be coupled together so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path.

[0106] Each gap may extend across the width of the metal sections.

[0107] Each gap may extend through a thickness of the metal sections.

[0108] Typically, each gap is a tortuous pathway.

[0109] The pathway may comprise one or more than one change of direction.

[0110] The at least one change of direction may comprise one or more than one bend in the pathway.

[0111] Typically, each gap defines or comprises a choking structure that restricts microwave leakage through the gap and forms a part of the microwave energy barrier.

[0112] The choking structure may be configured to absorb and therefore attenuate rather than reflect microwaves back to the microwave energy zone.

[0113] The choking structure may be configured to reflect microwaves into the microwave energy zone.

[0114] In that event, each gap may define or comprise a choking structure because of the selection of the design of the gap.

[0115] The design selection may take into account dimensions and changes of direction of the pathway defined by the gap, noting that the wavelength and intensity of the microwaves will have an impact on dimensions and changes of direction. Typically, the design selection is not solely governed by what is necessary to allow adjacent metal sections to change orientation relative to each other when the conveyor moves in the curved path.

[0116] The choking structure may comprise the one or more than one change of direction of the pathway.

[0117] The choking structure may comprise a deadend that is in the pathway.

[0118] The deadend may extend from a bend in the pathway.

[0119] For example, the deadend may be a branch of a main part of the pathway.

[0120] The materials selection for the conveyor may be as described above.

[0121] BRIEF DESCRIPTION OF THE DRAWINGS

[0122] The present invention is described further by way of example with reference to the accompanying drawings of a linear hearth, of which:

[0123] Figure 1 is (a) a schematic diagram of one embodiment of a linear hearth furnace of an apparatus for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in accordance with the invention, (b) a temperature profile along the length of a furnace of the apparatus for an embodiment of a method for producing direct reduced iron (DRI) from briquettes of a composite of iron ore fragments and biomass in accordance the invention, and (c) a plot of off-gas volumetric flow rate of gases produced along the length of the furnace during the course of the method;

[0124] Figure 2 is a computer rendered image of a microwave energy zone of the furnace shown in Figure 1 with sections of the furnace removed to show an embodiment of the conveyor of the apparatus within the microwave energy zone, with the conveyor being one embodiment of a conveyor in accordance with the invention; Figure 3 is a computer rendered image similar to that shown in Figure 2, viewed from one end of the microwave energy zone;

[0125] Figure 4 is an enlargement of a vertical cross-section through a part of the furnace shown in Figures 2 and 3 which shows details of the conveyor, particularly details of how successive metal sections of the embodiment of the conveyor are interlinked together via interlinked leading and trailing elements of the metal sections so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path (e.g., travelling from right to left as shown);

[0126] Figure 5 is a side view of an enlargement of a part of Figure 4 that includes additional details of how successive metal sections of the embodiment of the conveyor are interlinked together;

[0127] Figure 6 is a microwave energy (E-field) pattern for the part of the furnace shown in Figure 4 that shows an E-field generated in a part of the microwave energy zone by microwave energy that is delivered via a microwave energy horn of the apparatus, including the E-field in the interlinked section of successive metal sections of the conveyor;

[0128] Figure 7 is a simplified diagram that shows the interlinked end elements of successive metal sections of another embodiment of a conveyor (typically travelling from left to right as shown) in accordance with the invention;

[0129] Figure 8 is a simplified diagram that shows the interlinked end elements of successive metal sections of another embodiment of a conveyor (typically travelling from left to right as shown) in accordance with the invention; and

[0130] Figure 9 is a simplified diagram that shows the interlinked end elements of successive metal sections of another, but not the only other, embodiment of a conveyor (typically travelling from left to right as shown) in accordance with the invention. DESCRIPTION OF EMBODIMENTS

[0131] Figure l is a schematic diagram of an embodiment of a linear hearth furnace of an apparatus for producing direct reduced iron (DRI) of the present invention taken as a longitudinal section through the furnace.

[0132] Figure 1 is based on Figure 1 of an earlier filed International application PCT / AU2023 / 051350 (WO2024130328A1) in the name of the applicant and the disclosure in that specification of the International application is incorporated herein by cross-reference. It describes producing direct reduced iron, but the apparatus described therein could be applied to any metalliferous ore and carbon rich solid reductant.

[0133] The furnace shown in Figure 1 is part of an apparatus that is configured for continuously producing direct reduced iron (“DRI”) from microwave absorbent material in the form of iron ore and biomass, typically at least initially in the form of briquettes of a composite of iron ore fragments and biomass.

[0134] The linear hearth furnace includes an endless conveyor that, in use, transports iron ore and biomass through a furnace having an inlet for iron ore and biomass and an outlet for DRI and between the inlet and outlet a feed zone, a preheat zone, a microwave energy zone (for heating material with microwave energy) and a discharge zone, and a transition zone between the preheat zone and the microwave energy zone. The transition zone includes a compacting device for compacting preheated material, typically by reducing the height of preheated material, as it passes thereunder, so that it presents a more homogenised as described herein bed of material as a result of compaction which is better suited to processing in the microwave energy zone.

[0135] As noted above, minimising the extent to which microwave energy can “escape” from the furnace is one consideration, and the design of the conveyor that transports material through the apparatus is an important factor in minimising microwave energy loss. This is complicated with potential operating temperatures varying from room temperature to well in excess of 1000°C, as is the case with the Bioiron™ process. The conveyor of the invention is formed as a barrier to microwave energy escaping the microwave energy zone of the furnace. Figures 2-7 focus on embodiments of the conveyor in accordance with the invention. These Figures are described after the following description of Figure 1.

[0136] With reference to Figure 1, the linear hearth furnace, generally identified by the numeral 3, includes an elongate chamber defined by a refractory-lined base wall 82, opposed side walls 84 (see Figures 2 and 3), and a hood 86 that has the following zones along its length: a feed zone 10 that includes an inlet to the chamber and is configured to receive a feed material in the form of briquettes (not shown) of iron ore and biomass, i.e. briquette material, a preheat zone 20 for heating material, i.e. iron ore and biomass, in the briquettes and reducing iron ore and releasing volatiles in biomass and producing a preheated material, with the volatiles being combusted in the preheat zone, a microwave energy zone 30 for heating the preheated material further and reducing iron ore and forming DRI; a transition zone 25 between the preheat zone 20 and the microwave energy zone 30, with the transition zone 25 including a passageway 275 for gas flow from the microwave energy zone 30 to the preheat zone 20 and a compacting device 251 for compacting the preheated material; a discharge zone 40 for DRI that includes an outlet of the chamber.

[0137] The furnace 3 also comprises an endless conveyor 50 having a metallic material base (described further below) that, in use, moves in a continuous loop through the chamber from the inlet to the outlet and transports material that is at least initially in the form of briquettes on the base through the chamber from the inlet and discharges DRI from the outlet and then returns to the inlet to be re-loaded with additional briquettes.

[0138] The furnace 3 also comprises an assembly (not shown) for supporting and moving the conveyor 50 through the chamber. The assembly may be any suitable assembly.

[0139] The furnace 3 also comprises a flue gas outlet 70 in the preheat zone 20 for discharging gases produced in the furnace by heating and / or combustion within the furnace. The furnace 3 also comprises a means for supplying microwave energy to the microwave energy zone that heats iron ore and biomass and reduces ore to DRI.

[0140] The feed zone 10 includes a feed chute 12 and is configured to continuously feed briquettes into the feed zone 10 via the inlet to form a relatively uniform bed of briquettes on the moving conveyor 50 in the feed zone 10 of the chamber, while restricting outflow of furnace gases via the inlet.

[0141] The term “relatively uniform bed of briquettes” is understood herein to mean a relatively uniform layer of briquettes covering the base of the conveyor 50 and typically having a consistent ‘bed’ thickness, at least lengthwise, i.e., in the direction of briquette travel within the furnace. This does not however mean that individual briquettes have to be stacked in anything more than a random way on the base, noting that in some embodiments this may be desirable.

[0142] The discharge zone 40 is configured to continuously discharge DRI from the discharge zone 40 via the outlet, while restricting the inflow of oxygen-containing gases into the microwave energy zone 30 of the chamber from outside the chamber. The discharge zone 40 includes an enclosed discharge chute 42 that has a downwardly directed opening that has a flow control valve 44 that can be selectively operated to allow DRI to flow through the opening.

[0143] The preheat zone 20 has a plurality of air or oxygen-enriched air fed burners 22 for generating heat by burning combustible gases in a top space of the preheat zone 20. The burners 22 are spaced along the length of the preheat zone 20. The optimal spacing can be readily determined by a skilled person for any given operating conditions, such as the amount and type of biomass and the amount and type of iron ore and the required metallisation.

[0144] The combustible gases generated in the furnace include combustible gases originating within the furnace. The combustible gases include: volatiles in biomass in material moving through the preheat zone 20; and combustible reduction gases, such as CO, generated by reduction of iron ore in material in: (i) the preheat zone 20; and

[0145] (ii) the microwave energy zone 30, with the combustible gases generated in the microwave energy zone 30 flowing from the microwave energy zone 30 to the preheat zone 20.

[0146] There may be additional combustible gases supplied to the burners 22 depending on the operating conditions in the furnace.

[0147] In use, the microwave energy zone 30 is an anoxic environment. Heat is provided by microwaves. Microwaves heat iron ore and biomass and reduce ore to DRI. Figure 1(b) illustrates the typical operating temperatures along the length of the furnace, varying from room temperature at the inlet end to 1100°C at the DRI discharge outlet end.

[0148] The means for supplying microwave energy to the microwave energy zone comprises a source of microwave energy (not shown) and pipework that transfers microwave to the microwave energy zone. The pipework includes a plurality of microwave energy input units 32 (waveguides 64 and horns 66) in a top space of the microwave energy zone.

[0149] The microwave energy may have any suitable microwave frequency and vary by country, but the current industrial frequencies of around 2450MHz, 915MHz, 443MHz and 330 MHz are of most interest.

[0150] The horns 66 form an interface 80 (see the horizontal dotted line in Figure 1) that separates the microwave energy zone 30 into an upper sub zone 56 and a lower sub zone 58. The horns 66 are pyramidal horns, more particularly sectoral horns 66 in the embodiment shown in Figure 1. The horns 66 are arranged in rows (see Figure 1(b)) having microwave outlets 70 for microwave energy. The microwave outlets 70 are rectangular in transverse section. The rows extend across a width of a section of the microwave energy zone 30 and along a length of the section above a top surface of the conveyor 50 and, in use above a top surface of material carried on the conveyor 50. The section may be any suitable length and any suitable width. The horns 66 are not in contact with each other at the microwave outlets 70 of the horns 66 and there are gaps 76 between the horns 66 at this location (see Figure 3). The interface 80, as shown as a horizontal dotted line in Figure 1, is a sharp transition between the upper sub zone 56 and the lower sub zone 58. It is noted that the invention is not confined to a sharp transition.

[0151] The horns 66 are defined by side walls 72, 74 that are typically formed from metal sheet material.

[0152] The horns 66 are rectangular in transverse section and are formed with only one pair of opposing side walls 72 being flared and diverging with distance from the waveguides 64 and the other pair of opposing side walls 74 being parallel to each other which, in use, produces a fan-shaped beam, which is narrow in the plane of the flared side walls, but wide in the plane of the narrow side walls. The flaring may be in the E-plane (electric field) or H-plane (magnetic field) direction to form a rectangular opening at its output end.

[0153] The horns 66 in each row are placed above the conveyor 50 so that the shorter sides of the rectangular microwave outlets 70 (with the shorter sides being the side walls 74) are parallel to the direction of moment of the conveyor 50 within the microwave energy zone 30.

[0154] The horns 66 are arranged and configured so that the cumulative effect of the field patterns of the horns is to maximise the homogeneity of treatment of the material on the conveyor 50.

[0155] The transition zone 25 includes a passageway 275 for gas flow from the microwave energy zone 30 to the preheat zone 20 and a compacting device 251 for compacting the preheated material.

[0156] The compacting device 251 is in the form of a driven roller.

[0157] The compacting device 251 is provided to break-up and compact material on the conveyor 50 as it moves through the transition zone 25 from the preheat zone 20 to the microwave energy zone 30 so that it presents a more homogenised and uniform height bed of material which is better suited to processing with microwave energy in the microwave energy zone 30. The compacting device 251 does this by applying a downward force onto material and thereby breaking and compacting by reducing the height of material passing through the gap between the compacting device 251 and the conveyor 50. It is noted that there may be a combination of surface and profile and range of densities in the bed. Typically, the bed is a high density packed bed.

[0158] The transition zone 25 also includes a microwave choke, generally identified by the numeral 263, that at least substantially prevents microwave energy from passing from the microwave energy zone 30 to the preheat zone 20. While not show in Figure 1, the microwave energy barrier may also comprise a like microwave choking structure at the outlet of the microwave energy zone.

[0159] A suspended refractory wall 253 sits in front of the compacting device 251. A gap between a lower end of the wall and the conveyor 50 defines a passage for material to pass therethrough prior to being compacted by the compacting device 251. It also forms a barrier over which upper end reduction gases generated in the microwave energy zone 30 that enter the upper sub zone 56 can pass from the microwave energy zone 30 to the preheat zone 20.

[0160] In use of the apparatus, reduction gases generated in the microwave energy zone 30 flow into the preheat zone 20 counter-current to the direction of movement of briquettes on the conveyor 50 through the furnace from the inlet to the outlet.

[0161] The counter-current flow of reduction gases from the microwave energy zone 30 into the preheat zone 20 is caused by a higher gas pressure in the microwave energy zone 30 compared to gas pressure in the preheat zone 20.

[0162] While such pressure effect will be largely caused by the suction effect of a required exhaust fan linked to a dust extraction (baghouse) system at the atmosphere discharge end of the process the higher gas pressure is also the result of several structural and operational factors in the described embodiments of the method and the apparatus of the invention. The volume of reduction gases generated in the microwave energy zone 30 is illustrated by the plot of off-gas volumetric flow rate shown in Figure 1(c). The Figure shows a peak in gas produced in the microwave energy zone 30.

[0163] The counter-current flow of reduction gases from the microwave energy zone 30 to the preheat zone 20 transfers combustible gases, such as CO, that are generated in reactions that reduce iron ore in the microwave energy zone 30 to the preheat zone 20. The combustible gases in the gas flow from the microwave energy zone 30 are combusted by the plurality of air or oxygen-enriched air fed burners 22 spaced along the length of the preheat zone 20.

[0164] The temperature profile shown in Figure 1 is an example of a suitable temperature profile along the length of the furnace.

[0165] In use, the conveyor 50 transports material that is initially in the form of briquettes (not shown) of iron ore and biomass that are on the base successively and continuously through the zones 10, 20, 25, 30, 40 in a sequential manner and eventually circles back in its endless pathway so that each portion of the refractory or metallic base material of the conveyor 50 eventually presents itself at the feed zone 10 to be loaded with more briquettes. Preferably, the refractory or metallic base material has residual heat from the chamber when the conveyor 50 returns to the feed zone 10.

[0166] In use, reduction and other gases generated in the chamber are discharged as a flue gas via the flue gas outlet 70 in the preheat zone 20.

[0167] The briquettes may be manufactured by any suitable method. By way of example, measured amounts of iron ore fines and biomass and water (which may be at least partially present as moisture in the biomass) and optionally flux is charged into a suitable size mixing drum (not shown) and the drum rotated to form a homogeneous mixture. Thereafter, the mixture may be transferred to a suitable briquette-making apparatus (not shown) and cold-formed into briquettes. In one embodiment of the invention, the briquettes are roughly 20 cm3in volume and contain 30-40% biomass (e.g., elephant grass at 20% moisture). A small amount of flux material (such as limestone) may be included, with the balance comprising iron ore fines. The physical structure of the DRI at the end of the process is not critical. The physical structure may be friable and break easily or it could resemble a robust 3D “chocolate bar”.

[0168] As noted above, minimising the extent to which microwave energy can “escape” from the furnace is an important consideration for furnace design, and

[0169] - the design of the conveyor 50 is an important factor in minimising microwave energy loss from the furnace.

[0170] It is also relevant to note that operating a process in a furnace that has a conveyor 50 that moves through the microwave energy zone 30 (microwave energy zone) and fixed side walls (in relation to the moving conveyor) presents a particular challenge for minimising leakage of microwaves from the microwave energy zone 30.

[0171] Figures 2 and 3 are images of a part of the microwave energy zone 30 of the linear hearth furnace shown in Figure 1 with sections of the furnace removed to show an embodiment of the conveyor of the furnace within the microwave energy zone, with the conveyor being one embodiment of a conveyor in accordance with the invention. Figure 4 is an enlargement of a vertical cross-section through a part of the furnace shown in Figures 2 and 3. Figure 5 is a side view of an enlargement of a part of Figure 4 that includes additional details not shown in Figure 4. Figure 6 is a microwave energy (E-field) pattern for the part of the furnace shown in Figure 4.

[0172] Figures 2 and 3 show the conveyor 50 passing through the microwave energy zone 30. The conveyor 50 passes under a section of the hood 86 and between the side walls 84 of the chamber. The Figures also show a series of vertical waveguides 64 extending through openings in the hood 86. The waveguides are connected to horns 66, as shown in Figure 1. Two such horns 66 are shown in Figure 3, with a further three horns 66 in the background of the Figure. One such horn 66 is shown in Figure 6.

[0173] The conveyor 50 is configured to form a part of a microwave energy barrier to loss of micro wave energy from the micro wave energy zone 30. The base of the conveyor 50 comprises a plurality of metal sections 88 extending transverse to a direction of travel of the conveyor 50 through the furnace 3. Typically, the metal sections 88 are perpendicular to the travel direction.

[0174] The metal sections 88 are not transparent to microwaves and therefore are a part of the microwave energy barrier. Thus, such conveyors can be used for the transport of any material where microwave energy provides at least part of the of the energy for heating the material in a furnace 3, and the furnace is configured to have a microwave energy zone 30.

[0175] The metal sections 88 are coupled together so that there is a gap, generally identified by the numeral 90, between adjacent metal sections 88.

[0176] The metal sections 88 are in the form of pans that have a flat base 100 and flanges 102 that extend upwardly and outwardly from opposite ends of the base 100.

[0177] As can best be seen in Figure 3, the side walls 84 of the chamber are inboard of the flanges 102. There is a gap 110 between each side wall 84 and the flanges 102 on that side of the conveyor 50 to allow the conveyor 50 to move relative to the side walls 84 without contacting the side walls.

[0178] The gaps 110 define potential pathways for microwaves to travel from the microwave energy zone 30 and escape the zone 30.

[0179] The chamber includes a choking structure 112 to minimise microwave leakage through the gaps 110. The choking structure 112 comprises a tortuous pathway that is configured to reflect microwaves back through the gaps 110 into the microwave energy zone 30.

[0180] As can best be seen in Figure 4, the flat section 100 of each metal section 88 comprises a leading edge element, generally identified by the numeral 92, and a trailing edge element, generally identified by the numeral 94, extending transverse to the travel direction of the conveyor 50 through the microwave energy zone 30. Each gap 90 is between the leading and trailing edges elements 92, 94 and extends across the width and through a thickness of the metal sections 88.

[0181] The gaps 90 make it possible for each metal section 88 to rotate relative to successive and preceding metal sections 88 as the metal sections move around the curved path, shown for example on the right side of Figure 2, without the metal sections 88 contacting each other and interfering with movement in the curved path.

[0182] It can be appreciated that providing each gap 90 for this purpose means that each gap is a potential pathway for microwaves to travel from the microwave energy zone 30 and escape the zone 30.

[0183] In order to minimise microwave leakage through the gaps 90, each gap 90 defines or comprises a choking structure.

[0184] The choking structure in this embodiment returns microwaves by reflecting microwaves into the micro wave energy zone.

[0185] In other embodiments not shown in the Figures, the choking structure is configured to absorb microwaves and therefore attenuates rather than reflects microwaves.

[0186] In the embodiment shown in Figures 2-6, as can best be seen in Figures 4 and 5, the choking structure is a quarter wave choking structure.

[0187] One design feature is that each gap 90 is formed as a tortuous pathway, with multiple bends and changes in direction, with the lengths of some sections of the pathway being formed to be one quarter of the wavelength(s) of the micro waves.

[0188] Another design feature is that the pathway includes a deadend 68. The deadend 68 extends from a bend in the pathway and is in the form of a channel extending across the width of each metal section 88 (which may also be described as a closed groove). In effect, the deadend 68 is a branch from a part of the pathway. The deadend 68 is formed to be one quarter of the wavelength(s) of the microwaves.

[0189] Typically, the choking structure is configured so that there is no more than 0.04% loss of energy from the deadend.

[0190] The choking structure relies on a property of impedance transformation of a transmission line, in which the impedance transforms to its inverse every quarter wavelength from its termination.

[0191] As is shown in Figure 5, the choking structure comprises an entry section (C) - (B), a deadend (closed groove) section (B) - (A), and an exit section (B) - (D). The deadend section (B) - (A) terminates in a physical short circuit (A) (high microwave reflection, of a length equal or close to a quarter of the wavelength of an operational microwave frequency - wavelength slightly different than free space). This presents a point of very high impedance at the deadend entry (B). The distance from this point to an entry gap (C) at a top surface of the metal sections is equal to or also approximates a quarter wavelength of the operational microwave frequency, making its impedance very low, forming a virtual electrical connection (i.e. virtual short circuit, high microwave reflection). This allows currents to bridge the gap mostly unperturbed, minimising the energy that leaks through the structure. Similarly, the distance from (B) to (D) also approximates a quarter wavelength operational microwave frequency and ensures a low impedance on the opening gap and, therefore, a high microwave reflection.

[0192] The effectiveness of the embodiment shown in Figures 2-6 is illustrated by the microwave energy (E-field) pattern shown in Figure 6.

[0193] The microwave energy (E-field) pattern shown in Figure 6 was generated with a sample 104 (that reflects material that has passed through the transition zone) supported by the conveyor 50 in the microwave energy zone 30 (microwave energy zone) and microwaves being supplied to the microwave energy zone 30 via microwave energy input units 32 (waveguides 64 and horns 66), with one horn 66 shown in the Figure. The legend on the right side of the Figure indicates the microwave energy (E-field) strength in different sections of the furnace shown in the Figure. It is evident from the Figure that there is a high E-field in an early part of the deadend 68 and in a part of the pathway of the gap 90 that leads into the deadend 68. It is also evident from the Figure that there are substantially no microwaves in the last part 98 of the pathway. It follows that the Figure indicates that substantially no microwaves escaping the zone 30.

[0194] In the embodiment shown in Figure 7, the gap 90 is a less tortuous pathway than the gap 90 in the embodiment shown in Figures 2-6 and does not include a deadend. The pathway is formed as a quarter wave choking structure that comprises two changes of direction that reduce loss of microwaves from the microwave energy zone 30 by reflecting microwaves into the micro wave energy zone 30.

[0195] The embodiment shown in Figure 8 comprises:

[0196] (a) the gap 90 of the embodiment shown in Figure 7 which defines a pathway that forms a quarter wave choking structure, and

[0197] (b) a filler element 106 that fills a section of the gap 90 for preventing dust penetrating the pathway and maintaining a desired geometry of the pathway.

[0198] Typically, the filler element 106 is at least substantially transparent to microwaves.

[0199] The embodiment shown in Figure 9 comprises:

[0200] (a) the gap 90 of the embodiment shown in Figure 7, and

[0201] (b) a deadend in the form of choke chamber 108.

[0202] The gap 90 and the deadend 108 form a quarter wave choking structure.

[0203] The above-described furnace, including the embodiments of the conveyor 50 with the choking structures shown in the Figures, is an effective apparatus for heating a material using micro wave energy as a source of energy. Many modifications may be made to the embodiments described in relation to the Figures without departing from the spirit and scope of the invention.

[0204] By way of example, while the embodiments shown in the Figures are quarter wave choking structures, the invention is not confined to such structures and extends to any suitable choking structures in the gap that can reflect microwaves that would otherwise pass through the gap into the microwave energy zone 30.

[0205] In addition, the invention extends to embodiments not shown in the drawings in which the choking structures 90 of the gaps are configured to absorb rather than reflect microwaves.

[0206] In addition, while the embodiments shown in the Figures relate to microwave absorbent materials in the form of iron ore and biomass, the invention also extends to other microwave absorbent materials such as naturally occurring spodumene.

Claims

CLAIMS1. An apparatus for heating a microwave absorbent material, the apparatus comprising a furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating a microwave absorbent material in the zone, and an endless conveyor for carrying a microwave absorbent material through the furnace, the conveyor comprising a base comprising a plurality of metal sections extending transverse to a direction of travel through the furnace and defining a base for carrying the microwave absorbent material and being coupled together so that there is a gap between adjacent metal sections so that adjacent metal sections can change orientation relative to each other when the conveyor moves in a curved path, and with the conveyor being configured to form a part of a microwave energy barrier to loss of microwave energy from the microwave energy zone.

2. The apparatus defined in claim 1 wherein each gap defines or comprises a choking structure that restricts microwave leakage through the gap and forms a part of a microwave energy barrier.

3. The apparatus defined in claim 2 wherein the choking structure is configured to attenuate rather than reflect microwaves into the microwave energy zone.

4. The apparatus defined in claim 2 wherein the choking structure is configured to reflect microwaves into the microwave energy zone.

5. The apparatus defined in claim 4 wherein each gap defines a pathway from the microwave energy zone, and the choking structure comprises one or more than one change of direction of the pathway.

6. The apparatus defined in claim 5 wherein the choking structure comprises a deadend in the pathway that is configured to reflect microwaves into the microwave energy zone.

7. The apparatus defined in claim 6 wherein the deadend extends from a bend in the pathway.

8. The apparatus defined in claim 6 or claim 7 wherein the deadend is a branch of a main part of the pathway.

9. The apparatus defined in any one of claims 6 to 8 wherein the deadend comprises a channel extending across the width of the metal sections.

10. The apparatus defined in any one of claims 4 to 9 wherein the choking structure is a quarter wave choking structure.

11. The apparatus defined in claim 10 wherein the quarter wave choking structure comprises an entry section from the microwave energy zone, a deadend section, and an exit section, the deadend section terminates in a physical short circuit (high microwave reflection) and has a length equal to or approximates a quarter of the wavelength of an operational microwave frequency thereby presenting a point of high impedance at a deadend entry, the entry section has a length equal to or approximates a quarter of the wavelength of the operational microwave frequency from an entry gap at a top surface of the metal sections to the deadend entry thereby making its impedance low and forming a virtual electrical connection that allows currents to bridge the entry gap mostly unperturbed and thereby minimises energy leakage through the entry gap, and a distance of the exit section is equal to or approximates a quarter wavelength and ensures a low impedance on the opening gap and therefore a high microwave reflection.

12. The apparatus defined in any one of the preceding claims wherein a part of one metal section overlaps at least a part of a successive metal section when the conveyor is travelling through the microwave energy zone, and there is a space between the overlapping sections, with the space forming an initial part of the gap, and with the initial part extending from the microwave energy chamber.

13. The apparatus defined in any one of claims 1 to 11 wherein the metal sections do not overlap, and there is a space between one metal section and a successive metal section when the conveyor is travelling through the microwave energy zone, with the space forming an initial part of the gap, and with the initial part extending downwardly, typically vertically, from the micro wave energy chamber.

14. The apparatus defined in any one of the preceding claims wherein each metal section comprises a leading edge element and a trailing edge element extending transverse to the travel direction of the conveyor, with each gap being between the leading edge element of one metal section and the trailing edge element of the successive metal section.

15. The apparatus defined in any one of the preceding claims wherein the metal sections are in the form of pans.

16. The apparatus defined in claim 15 wherein each pan comprises end edges, for example in the form of flanges, that extend upwardly from opposite sides of the base of the pan.

17. An apparatus for continuously producing direct reduced metal material (DRM) from a metalliferous ore and a carbon rich solid reductant, the apparatus comprising a furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating the ore and the reductant and reducing the ore to DRM in the zone, and an endless conveyor for transporting the ore and the reductant through the microwave energy zone and being configured to form a part of a barrier to loss of microwave energy from the zone.

18. An apparatus for continuously producing direct reduced iron (DRI) from iron ore and biomass, the apparatus comprising a furnace, the furnace comprising: a micro wave energy zone, a means for supplying microwave energy to the microwave energy zone and heating the ore and the biomass and reducing the ore to DRI in the zone, andan endless conveyor for transporting the ore and biomass through the microwave energy zone and being configured to form a part of a barrier to loss of microwave energy from the zone.

19. An endless conveyor for carrying a material through a furnace for heating a microwave absorbent material in the material with microwave energy, the conveyor comprising a base for carrying the material, the base comprising a plurality of metal sections extending transverse to a direction of travel through the furnace and being coupled together so that there is a gap between adjacent metal sections with the gap being configured to form a part of a microwave energy barrier to loss of microwave energy from a microwave energy zone of the furnace.

20. The conveyor defined in claim 19 wherein the base comprises a plurality of metal pans comprising flanges that extend upwardly from opposite sides of the pan that have the same gap structure as the base of the pans (for the purposes of forming part of the microwave energy barrier).

21. The conveyor defined in claim 19 or claim 20 wherein each gap defines or comprises a choking structure that restricts microwave leakage through the gap and forms a part of a micro wave energy barrier

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