WO2025118030A1

WO2025118030A1 - Lithium ore calcination

Info

Publication number: WO2025118030A1
Application number: PCT/AU2024/051316
Authority: WO
Inventors: Iain FARR
Original assignee: Technological Resources Pty Ltd
Current assignee: Technological Resources Pty Ltd
Priority date: 2023-12-06
Filing date: 2024-12-06
Publication date: 2025-06-12
Anticipated expiration: 2026-06-06

Abstract

A method for producing a calcined material from lithium bearing ore (or concentrates thereof) in a hearth furnace is disclosed. A preheat stage of the method comprises preheating feed material using radiant heating. A subsequent decrepitation stage of the method comprises passing the preheated feed material on a moving base through a contained microwave zone within a hearth furnace and transmitting microwaves into the microwave zone and heating and decrepitating the preheated feed material and forming calcined material. The preheated feed material enters the decrepitation stage at a bulk temperature of at least 600°C and calcined material leaves the decrepitation stage at a bulk temperature of no more than 1100°C.

Description

LITHIUM ORE CALCINATION

TECHNICAL FIELD

The present invention relates to a calcination method for producing, typically continuously, calcined material from feed material containing lithium bearing ore or concentrates thereof for subsequent recovery of lithium.

The present invention relates particularly, although by no means exclusively, to a method for continuously producing calcined material containing B phase spodumene from feed material containing naturally occurring spodumene (i.e., a phase) for subsequent recovery of lithium.

The present invention relates particularly, although by no means exclusively, to a calcination method that comprises two stages for producing calcined material from feed material comprising lithium bearing ore or concentrates thereof for subsequent recovery of lithium. An initial preheat stage comprises preheating feed material using radiant heating. A subsequent decrepitation stage comprises heating and decrepitating preheated material in a hearth furnace by passing preheated feed material (while preheated) on a moving base through a contained microwave zone within the hearth furnace and transmitting microwaves into the microwave zone and heating and decrepitating the preheated feed material and forming calcined material.

The present invention relates particularly, although by no means exclusively, to a method for producing calcined material, typically continuously, from feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium, the method comprising distributing preheated feed material containing lithium bearing ore or concentrates thereof over a moving base and forming a relatively uniform bed of material before the material enters a contained microwave zone and is exposed to microwave energy that is supplied to the microwave zone through a series of horns spaced transversely across the moving base and in close proximity to the uniform bed of material, with the microwave energy heating and decrepitating material and forming calcined material. The present invention relates particularly, although by no means exclusively, to a method for continuously producing calcined material using a linear hearth furnace for the initial preheat stage and a linear hearth furnace for the decrepitation stage.

The present invention relates particularly, although by no means exclusively, to a method for continuously producing calcined material using a rotary kiln furnace for the initial preheat stage and a linear hearth furnace for the decrepitation stage.

The calcined material when produced, for example, may be subsequently acid roasted and leached or leached with alkaline reagents in an autoclave for recovery of the lithium in a leachate.

Alternatively, the calcined material may be further crushed to a smaller size material before the calcined material undergoes any leaching process, including the aforementioned processes.

The term “calcined material” is understood herein to mean lithium bearing ores or concentrates thereof, that have been thermally processed to temperatures above 800°C in a constrained atmosphere while being below the melting temperature of any lithium containing mineral therein and/or the melting temperature of any material/phase that forms during the decrepitation stage of the calcination process.

The term “decrepitation stage of the calcination process” is understood herein to mean when lithium containing mineral crystal structures are broken/shattered.

The term “hearth furnace” is understood herein to mean a furnace in which feed material rests on a moving base while being thermally processed that includes a lengthwise extending refractory-lined heating chamber with an inlet (feed) point and an outlet (discharge) point and an endless moving base that extends along the length of the chamber and carries feed ore through the chamber from the inlet point to the outlet point for thermal processing in the chamber, with the moving base returning to the inlet point and repeating the process of loading feed material onto the moving base to be transported through the chamber. A “linear hearth furnace” is understood herein to mean a hearth furnace in which the chamber has a quadrilateral, for example, rectangular transverse, cross-section and the moving base is a series of segmented sections that form a generally contiguous base while passing through the chamber.

The term “rotary kiln furnace” is understood herein to mean a hearth furnace comprising a cylindrical vessel, typically inclined slightly to the horizontal, which is rotated about its longitudinal axis. A feed material is fed into an upper end of the cylinder. As the kiln rotates, material gradually moves down toward a lower end and may undergo a certain amount of stirring and mixing as it moves through the kiln. Hot gases pass along the kiln, sometimes in the same direction as the process material (co-current), but usually in the opposite direction (counter-current) .

The term “contained micro wave zone” is understood to mean a zone in which micro waves are generally contained to meet international electromagnetic compatibility (EMC) requirements which impose stringent limits on any emissions outside agreed and recognised bands for the application of microwave (MW) energy and radio frequency (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.

BACKGROUND

With global efforts to reduce overall atmospheric CO2 there is pressure on all producers of manufacturing feed materials used in subsequent downstream processes to find means to produce such feed materials without causing net emissions of greenhouse gases. In particular, there is pressure in any heating and/or reduction step involving such feed materials not to use, or significantly reduce the use of, geological origin carbonaceous material, like coal or natural gas, collectively often referred to as ‘fossil fuels’, which are non-renewable and result in increased concentrations of atmospheric greenhouse gases. Processes for the production of battery precursor materials from ores formed from geological processes are no exception to this trend and are historically carbon intensive processes in which carbon, from carbonaceous geological materials, is used with the carbon eventually discharged to the atmosphere as CO2. Much of the lithium compounds produced globally for batteries have their origin from mining spodumene mineral as a hard rock. It has been reported that CO2 emissions from the extraction of the lithium sourced from spodumene is nine tonnes of CO2 per tonne of lithium carbonate equivalent (LCE) produced. This is nearly triple that of LCE for lithium from brine sources, although such sources can have their own environmental issues from extraction.

Spodumene (Li A IS i IOO) is a lithium aluminosilicate that is named from the Greek word “spodumene” (burnt to ashes) due to its ash-like grey colour post-grinding. Spodumene is perhaps the most important lithium mineral to produce lithium compounds from hard rock because it has a significant Li content. Its theoretical chemical composition is approximately 8% LiiO, 27.4% AhO3, and 64.6% SiCL. Naturally occurring spodumene exists in what is known as an a phase. However, that a phase is quite compact with a density of 3.27 g/cm³, due to its monoclinic crystal structure. The lithium in such rock, even when well commuted, is not very amendable to lithium recovery when acid roasted and leached. Two other phases can exist, being the P phase and y phase (a metastable phase). The P phase occurs through recrystallisation when spodumene is homogeneously heated to between 900 to 1100°C and then allowed to return to room temperature. Its structure is that of an open tetragonal, with a lower density of 2.45 g/cm³. y phase occurs through recrystallisation when spodumene has been heated only between 700 to 900°C.

The advantages of converting spodumene from a phase to P phase are disclosed in US patent 2,516,109 in the names of RB Ellestad and LK Milne. The approach disclosed in the US patent has become an accepted industry process, with current industrial practice being conventional heating (calcination) at 1000 to 1100°C for about 2 hours. However, this is a particularly energy intensive process, which is a disadvantage.

A reduction in the energy intensity in this process through using microwaves as a heating source has been suggested by, among others, A N Nikoloski et al, as published in Minerals Engineering in 2017 under the title “ Mineralogical transformations of spodumene concentrate from Greenbushes, Western Australia. Part 2: Microwave heating" and more recently by M Rezaee et al, as published in Powder Technology in 2022 under the title “Microwave-assisted calcination of spodumene for efficient, low -cost and environmentally friendly extraction of lithium". A challenge, as noted from their laboratory experiments, is that spodumene is difficult to heat using microwaves from room temperature due to their low absorption of incident energy. Spodumene does however absorb microwaves at elevated temperatures as its dielectric loss increases with temperature, but uncontrolled input of microwaves at such temperatures can lead to thermal runaway with melting of the spodumene, which would negate the phase transformation and lead to smooth particles (glass like) that are less susceptible to acid roasting and leaching or the equivalents thereof.

Another group (and earlier to those referred to above) that suggested the use of microwaves is Olli Peltosaari et al as published in Minerals Engineering in 2015 under the title “a -> y fl phase transformation of spodumene with hybrid microwave and conventional furnaces'". Noting that spodumene is a transparent (to microwaves) material at room temperature and only became absorbent at relatively high temperatures, they proposed using another absorbent (to microwaves) material placed next to the spodumene. This material absorbed energy from microwaves and heated up the material and some of the heat transferred to surrounding material through conduction and radiation. While such an approach is workable in the laboratory, there is a risk that a majority of the microwave energy continues to be taken up by this more absorbent material rather than the material to be microwaved. Thus, the potential benefits of internal heating of the desired material to be microwaved may in fact be muted. The authors in considering how this approach could be applied industrially speculated that a rotary kiln (a cylindrical furnace) with half the cylinder lined with SiC might be considered (the remainder of the cylinder would have a transparent material). However, there are numerous challenges to such an approach, noting as an example that dust is the enemy of microwave reliability, and as well that any layer of microwave absorbent material building up on a transparent material can quickly change the microwave dynamics within a furnace, with heat being transferred to that later material.

Notwithstanding the above, a challenge is how to industrially undertake calcination of feed material containing lithium oxide for subsequent recovery of lithium using microwaves as an energy source, to reduce the overall energy requirements and calcination time, both technically and in a manner that is not detrimental to the environment i.e., reduced or no fossil fuels are necessarily required. For example, the feed material containing lithium bearing ore needs to be presented to any microwave source in a form where there is capacity /capability for the microwaves to be absorbed by all the feed material containing lithium bearing ore in a manner that ensures relatively uniformed heating of all such material, thus avoiding overheating of material (leading to melting) and/or underheating of material (leading to no beneficial effect re downstream acid roasting and leaching).

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

SUMMARY OF THE DISCLOSURE

The present invention comprises a method for producing a calcined material from lithium bearing ore (or concentrates thereof) in a hearth furnace. A preheat stage of the method comprises preheating feed material using radiant heating. A subsequent decrepitation stage of the method comprises passing the preheated feed material on a moving base through a contained microwave zone within a hearth furnace and transmitting microwaves into the microwave zone and heating and decrepitating the preheated feed material and forming calcined material. The preheated feed material enters the decrepitation stage at a bulk temperature of at least 600°C and calcined material leaves the decrepitation stage at a bulk temperature of no more than 1100°C.

The present invention provides a method for producing a calcined material, typically continuously, from a feed material, with the feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium, the method comprising two stages, with a preheat stage comprising preheating feed material using radiant heating and a decrepitation stage comprising heating and decrepitating preheated feed material in a hearth furnace by passing preheated feed material (while preheated) on a moving base through a contained microwave zone within the hearth furnace and transmitting microwaves into the microwave zone and heating and decrepitating preheated feed material and forming calcined material. The present invention also provides a method of forming a relatively uniform bed of a preheated material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium on a moving base before moving the material into and exposing the material to microwave energy in a microwave zone and decrepitating the material and forming a calcined material, the method comprising distributing preheated feed material on the moving base, for example by smoothing the material on the moving base, for example by using a roller or other suitable smoothing device positioned above and contacting the preheated material.

The present invention relates more particularly, although by no means exclusively, to a method for producing a calcined material, typically continuously, from a feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium, the method comprising distributing preheated feed material containing lithium bearing ore or concentrates thereof over a moving base and forming a relatively uniform bed of material before the material enters a contained microwave zone and is exposed to microwave energy that is supplied to the microwave zone through a series of horns, for example spaced transversely across the moving base and in close proximity to the uniform bed of material, with the microwave energy heating and decrepitating material and forming calcined material.

The term “relatively uniform” is understood herein to mean a relatively uniform layer of feed material covering the base and typically having a consistent ‘bed’ depth, at least length ways, i.e., in the direction of travel of feed material within the furnace. When the feed material is in the form of briquettes of ore fragments and biomass, 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.

The microwave energy may be transmitted to the microwave energy zone via a series of horns suspended from a roof of the furnace.

The horns may be placed transversely across the moving base. The horns may have outlets that are in close proximity to the uniform bed of preheated material.

The term “in close proximity” is understood herein to mean any suitable spacing between outlets of the microwave horns and a uniform bed of preheated material in any given situation. The spacing will depend on a number of factors, including any one or more than one of the composition of the material on the bed, the thickness of the bed, the frequency of the microwave energy, the size of the microwave zone (i.e. the numbers of horns, the length of the zone, etc.) and the target temperature.

It is desirable that the bed be relatively uniform for a number of reasons. First, a relatively uniform bed aids in ensuring that the material is heated uniformly across the moving base. Secondly, a relatively uniform bed aids in ensuring that the heating occurring is predictable and repeatable. Thirdly, a relatively uniform bed allows for the horns to be placed in close proximity to the uniform bed of material, without the risk of contact.

The present invention also provides a method for continuously producing a calcined material containing B phase spodumene from feed material comprising naturally occurring spodumene (i.e., a phase) for subsequent recovery of lithium, the method comprising passing preheated feed material on a moving base through a hearth furnace that comprises a contained microwave zone and transmitting microwaves into the microwave zone and heating and decrepitating the material and forming calcined material.

The method may comprise carrying out the preheat stage and the decrepitation stage in the same hearth furnace, with a distinct preheat zone and a distinct and separate decrepitation zone within the furnace.

The method may comprise carrying out the preheat stage and the decrepitation stage in two separate furnaces. It is envisaged that, at least until non-fossil fuel based options become readily available commercially, the two stages will be carried out in two separate linked furnaces. Regardless of which approach, the decrepitation stage may comprise feeding the preheated feed material into the contained microwave zone at a bulk temperature of at least 600°C, typically at least 650°C, more typically at least 700°C, and more typically again at least 800°C.

The “bulk temperature” may be determined, for example, by making a plurality of temperature measurements across and along and at different heights of a section of a bed of material and determining an average of the temperatures, with the average being the “bulk temperature” of the volume covered by the measurements, with the volume being the “bulk” and the number and locations of the measurements being determined based on experience and taking into account factors such as the bed density and composition variation through the bed.

In addition, regardless of the approach, the decrepitation stage may comprise heating the feed material in the decrepitation stage to a bulk temperature of no more than 1100°C.

It is noted that it is important that the temperature of material does not generally exceed a temperature at which the material being calcined melts, which for lithium bearing ore containing spodumene, or concentrates thereof, is nominally set at 1100°C, as this will impact on material handling and lithium recovery.

Given that “bulk temperature” is an average of a plurality of temperatures in a given volume of material, the reference to a bulk temperature of 1100 °C should be regarded as an indication of a maximum temperature and an assessment should be made to check whether a lower maximum bulk temperature is appropriate in a given situation.

While it will depend on numerous variable factors, it is expected that the range of temperatures leading to the establishment of the bulk temperature for calcined material leaving the decrepitation stage will be relatively narrow given the use of microwave heating for the decrepitation stage.

As an example of a variable factor, the uniformity of temperature within preheated feed material as it enters the decrepitation stage will influence the uniformity of temperatures i.e., the range of temperatures, of the calcined material leaving the decrepitation stage. It is expected that feed material that has been preheated in a rotary kiln (introduced in the description later) will have a more uniform temperature than feed material that is preheated in a linear heath furnace (also introduced in the description later), due to the inherent mixing of material that occurs in a rotary kiln.

The decrepitation stage may comprise heating the preheated material to bulk temperatures of between 1075°C and 1100°C in the decrepitation stage.

The lithium bearing ore may be in the form of ore fragments.

The term “ore fragments” is understood herein to mean suitable size pieces of lithium bearing ore (as passed through an appropriately screen mesh of 3.35mm 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 lithium mineral in the ore may be of any suitable type, but in particular is directed to exploitable minerals occurring in pegmatites. Spodumene (Li A IS i aOe) , petalite (LiAlSi40io) and lepidolite (K(Li,AL,Rb)3(AlSi)40io(F,OH)2) are lithium containing hard rock which have been known to have been mined for their lithium content, although spodumene is consider the main mineral for extraction purposes.

Typically, the lithium bearing ore is ore produced in any suitable mine and mining operation.

Mined ore from a mine may be processed in any suitable downstream processing operation to produce a suitable lithium bearing ore (or concentrates thereof) for the method.

Typically, the mined ore comprises various minerals and it is not unusual for the ore to be commuted to a particular size or size range as part of the mineral separation for the specific concentration of desirable minerals like spodumene. As an example, ores (as a concentrate) may present with a nominal LiOi content of greater than 6.0% and an iron oxide content of less than 1% through being crushed/ground down to a size of - 250 pm, gravity separated, followed by scrubbing with a wet high intensity magnetic separator. It is to be noted that the residual iron oxide like FC2O3, is naturally non-magnetic, but exhibits paramagnetic properties under high intensity magnetic fields.

Typically, the method includes heating ore fragments in the preheat stage so that such fragments reach an entry temperature for the decrepitation stage of at least 600°C, more typically at least 650°C, more typically again at least 700°C, and more typically again at least 800°C.

The preheat stage may use radiant heating produced by various means.

While, typically, those means ideally are without the use of fossil fuel, recognising that nonfossil energy, either directly or indirectly, should where possible be used for the benefit of society in undertaking the method, the method is not limited thereby.

For example, the method may comprise carrying out the preheat stage in a rotary kiln furnace, such as a conventional rotary kiln, with a central gas fired burner, and carrying out the decrepitation stage in a linear hearth furnace.

The method may comprise transferring preheated ore fragments from the rotary kiln while hot into the linear hearth furnace.

Where a greener energy approach is desired, an indirectly heated rotary kiln may be used for the preheat stage, with an electrical heating system that heats an outer shell of the kiln through radiative heat transfer to the outer shell. Such a system has recently become available commercially and is offered by Metso under the trademark RotarEkiln. Metso states that this furnace uses “resistive emitters to provide radiative heat transfer to the kiln shell”. Thus, the radiative heat may also be indirect, as in this example the heat has to pass through the shell of the kiln to heat the feed material.

Where a singular linear hearth furnace is used for the whole calcination method, the radiant heating may be undertaken using a series of electric heaters or plasma torches running along a roof of the linear hearth furnace in the preheat zone. Alternatively, the radiant heating may be undertaken using a series of fuel gas burners running along sides of the linear hearth furnace in the preheat zone.

The method may comprise forming a relatively uniform bed of preheated material from the preheat stage before moving the material into and exposing the material to microwave energy in the decrepitation stage by distributing preheated feed material on the moving base, for example by smoothing the material on the moving base, for example by using a roller or other suitable smoothing device positioned above and contacting the preheated material.

The microwaves for the decrepitation stage may be delivered in a multimodal fashion or a resonant fashion or in the manner subsequently described in this specification. The contained microwave area within a linear hearth typically excludes the inlet (feed) point and an outlet (discharge) point. Containment is done by the use of what are known as microwave chokes. For example, where the feed material enters the decrepitation zone, a microwave baffle (a form of microwave choke) is placed just above the feed material on the moving base to ensure that there is minimal microwave escape outside the zone through the opening needed for entry of feed material. While it would be possible not to have the same or a similar baffle before the outlet (discharge) point and to simply rely only on microwaves being contained by a discharge mechanism in the outlet (discharge) point, for example a lock hopper system, it is preferred that the contained microwave area be more confined by having a baffle before the outlet (discharge) point, thereby providing a multilayer layered containment approach.

The moving base for conveying the ore fragments through the linear hearth furnace typically comprises a plurality of sections extending transverse to a direction of travel of the moving base, with the sections being in the form of flat metal pans.

These metal pans typically are constructed of heat resistant stainless steel that generally form a contiguous base while passing through the chamber of the furnace. An example of heat resistant stainless is the grade commonly referred to as SS31O.

The metal pans may have flanges, ridges or other forms of physical restraints to aid in carrying ore fragments through the furnace, and in particular to prevent spillage from the sides of the moving base.

Metal pans are desirable because any microwave energy that passes through the ore fragment bed without being absorbed may then be reflected back by the metal into the ore fragment bed for further opportunity to be absorbed.

As the aim is to have all the ore fragments at a bulk temperature of between 1000 to 1100°C by the end of it decrepitation stage, it may be desirable to protect the metal with a layer of refractory material that could be transparent or absorbing to micro waves. It is speculated that if it is an absorbing material that this material, because the microwaves have to first pass through the ore fragments which are now themselves absorbent, will not preferentially take up the microwave energy to those ore fragments within the decrepitation zone.

While the calcination of ore fragments can be undertaken in a linear hearth furnace in which all heating is done by the conversion of electricity, directly to heat or indirectly though conversion to electromagnetic energy, there are potentially good reasons to undertake the heating of the preheat zone though the combustion of non-fossil fuel gases. Such non fossil fuel gases can be directly fed into the preheat zone or can be created in the furnace by the heating of biomass and/or the production of off-gases from reduction reactions that occur between elemental components of the biomass and any reducible oxides, particularly in the microwave containment zone. The use of biomass to produce non fossil fuel gases is a cheaper option than for example using hydrogen gas. However, biomass placed in the linear hearth furnace should essentially emit all its volatiles (for combustion and thereby provide radiant heating) before leaving the preheat zone.

The term “volatiles” is usually understood in respect to carbonaceous material to mean gases, other than those arising from water (whether bound or free) being initially driven off, that are formed or released by heating of the carbonaceous material to cause breakdown of organic components therein to gases or liquids. The inventor believes that it is desirable generally for low-boiling-point organic compounds that will condense into oils on cooling to not be present in any residual biomass that passes into the decrepitation zone, where such compounds have the potential to interfere and/or interact with the microwave delivery equipment. Accordingly, the term “volatiles” is understood herein to mean only low-boiling-point organic compounds that are driven off at temperatures below 600°C upon heating in an oxygen deficient environment.

Typically, such biomass is supplied to the linear hearth furnace in the form of briquettes of ore fragments and biomass. This is generally considered more desirable than simply placing ore fragments and biomass loose on the moving base. Briquettes provide a natural matrix which allows the thermal radiation to more readily penetrate the feed material layer, than say a finer layer of mixed biomass and ore fragments.

Nevertheless, the invention includes embodiments in which the feed material is a mixture of ore fragments and biomass and separate layers of ore fragments and biomass.

The term “briquette”, as originally fed into a linear hearth furnace, is understood herein as a broad term that means a composite of ore fragments and biomass that has been formed as a result of the ore fragments and biomass being brought into close contact through compaction, or alternatively through mixing and binding in a wet state of the ore fragments and biomass together. Those skilled in the art would probably describe the latter (particularly when in a spherical form) as pellets. While the inventor believes “raw” 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 early structural failure of the pellet; pellets are not excluded.

The term “biomass” is understood herein to mean living or recently living organic matter. Specific biomass products for a composite of 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. To achieve the desired removal of the majority of volatiles from the biomass within heated briquettes prior to briquettes leaving the preheat zone, the finish preheat temperature for the briquettes (as a collective as they leave the preheat zone, i.e., bulk temperature) should be at least in a range of 600 to 800°C. Because of the nature of a bed of briquettes, the temperature throughout the bed (at least in the preheat zone) will not be uniform and will definitely vary through the bed and may also vary across the bed.

Where only a linear hearth furnace is used for both stages, the method may comprise supplying briquettes at ambient temperature to the preheat zone of the linear hearth furnace and progressively heating briquettes to a finish preheat temperature as briquettes are transported through the preheat zone.

The method may include controlling the operation so that at least 90%, typically at least 95%, of volatiles in biomass in the briquettes is released as a gas in the preheat zone.

The control options for achieving volatilisation mentioned in the preceding paragraph include controlling, by way of example, any one or more than one of the temperature profile in the furnace, the residence time of briquettes in the preheat zone, the length of the preheat zone, the travelling speed of the moving base, the briquette loading on to the moving base, and the amount of biomass in the briquettes, noting that a number of the factors are inter-related.

By way of example, a travelling speed i.e., velocity, of the moving base may be controlled to give briquettes sufficient time in the preheat zone for at least 90%, typically at least 95%, of the volatiles to be released from biomass in briquettes.

The briquettes, as originally fed into the linear hearth furnace (or, more generally, any suitable hearth furnace), may be any suitable size and shape.

By way of example, the briquettes may have a volume of less than 25 cm³ and greater than 2 cm³. Typically, the briquettes may have a volume of 3-20 cm³. By way of example, the briquettes may have a major dimension of 1-10 cm, typically 2-6 cm and more typically 2-4 cm.

By way of example, the 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. By way of further example, the briquettes may be “ice hockey puck-shaped” briquettes.

The briquettes, as originally fed into the linear hearth furnace (or, more generally, any suitable hearth furnace), may include any suitable relative amounts of ore fragments and biomass. The briquettes may include 20-45% by weight on a wet (as-charged) basis, typically 25-35% by weight on a wet (as-charged) basis, of biomass. When choosing to use compacted briquettes ideally the biomass chosen has a significant lignocellulosic component within.

This aids in holding the briquette together, through what is speculated to be a plastic deformation of the biomass material.

In any given situation, the preferred proportions of the ore fragments and biomass will depend on a range of factors, including but not limited to the ore fragments particular characteristics (such as fragment size and mineralogy), the type and characteristics of the biomass, the operating process constraints, and materials handling considerations.

The method may include, when using a linear hearth furnace for both stages, generating a higher pressure of gases in the decrepitation zone compared to gas pressure in the preheat zone and thereby causing gases generated in the decrepitation zone to flow counter-current to the direction of movement of briquettes on the moving base through the linear hearth furnace.

The method may include creating higher pressure in the decrepitation zone by means of a gas flow “choke” between the decrepitation zone and the preheat zone of the furnace.

The gas flow “choke” in the decrepitation zone may be configured so as to increase the flow rate of gases generated from the decrepitation zone to the preheat zone by a factor of 2-3 compared to what the flow rate would have been without the gas flow “choke” in order to ensure that there is no substantial gas flow from the preheat zone to the decrepitation zone of the furnace.

The travelling speed may also be controlled so that heating briquettes in the decrepitation zone using microwave energy alone increases the temperature of briquettes by at least a further 200°C, and preferably at least 250°C, and more preferably at least 275°C.

It is noted that travelling speed is not the only factor relevant to achieving the at least 200°C temperature increase of briquettes in the decrepitation zone. Other factors include controlling, by way of example, any one or more than one of the residence time of briquettes in the decrepitation zone, the length of the decrepitation zone, the briquette loading on to the moving base, the type and power of the microwave energy, noting that a number of the factors are inter-related.

The microwave energy may have any suitable microwave frequency, but the current industrial frequencies of around 2450 MHz, 915 MHz, 922 MHz, 896 MHz and 433 MHz are of most interest. For example, in Australia and South Africa 922 MHz is the allocated frequency. In USA and Europe, 915 MHz is the allocated frequency. In UK, 896 MHz is the allocated frequency. A key requirement however is that the linear hearth furnace be designed so that the energy is contained within.

As noted above, the briquette heating in the preheat zone may include generating heat by burning combustible gases generated in the furnace via the plurality of air or oxygen enriched air fed top space burners, typically preheated air or oxygen enriched air fed top space burners, within the preheat zone. Typically, that step includes combusting at least 90% by volume, more typically at least 95%, of combustible gases generated in the furnace.

The burners may be either (i) spaced along the top of the linear heath furnace in the preheat zone or (ii) aligned more or less horizontally along the long axis to assist in ensuring a generally uniform heating pattern along the length of the preheat zone and to achieve direct radiant heat transfer from the top of the furnace. The amount of preheated air or oxygen enriched air fed to each burner may be adjusted to compensate for established variations in fuel gas flow across and along the chamber.

In use, combustible gases in the hot gas flowing into the preheat zone from the decrepitation zone combust as the gases passes each of the plurality of air or oxygen enriched air fed top space burners.

The combustion profile, i.e., the profile of post-combustion of combustible gas along the length of the preheat zone, may be 35-45% at a hot end of the preheat zone, i.e., at the end adjacent the decrepitation zone, increasing to 90-95% at a cold end of the preheat zone, i.e., at the end adjacent the feed entry. The combustion profile may be any suitable profile.

Post combustion (PC) is defined herein as:

PC % = 100 x (CO2+H₂O)/(CO+CO₂+H₂+H₂O), where the symbol for each species (CO, CO₂ etc) represents the molar concentration (or partial pressure) of that particular species in the gas phase. Given there may be still unoxidised gaseous volatiles in the preheat zone at any point in time throughout the pre-heat zone, it is to be appreciated that the above discussion re PC and the ranges thereof is not absolute, but more a relative estimation/comparison.

In simple terms, PC is a measure of the combustion of combustible gas, with zero indicating no combustion and 100% indicating likely fully combusted.

It follows from the preceding paragraphs that the above combustion profile maintains the preheat zone top space in a bulk reducing condition along the length of the preheat zone, with feed oxygen being consumed rapidly in the vicinity of each burner (in a small, localised region).

The method may include discharging gas produced in the furnace by heating and/or combustion within the furnace as a flue gas through a flue gas outlet at the beginning of the preheat zone, i.e., feed material inlet end. This is advantageous to creating a higher pressure in the preheat zone over the decrepitation zone when using a linear hearth furnace for both stages.

The method may include processing the flue gas in a flue gas system before discharging the processed flue gas to the atmosphere.

The method may include recovering heat from the flue gas and using the heat for preheating air to the burners in the preheat zone.

By way of example, gas discharged from the preheat zone via the flue gas outlet is typically ducted (hot, around 1100-1300°C) to an afterburning chamber where there is final combustion of combustible gas in the flue gas and consequential heat generation.

The method may include discharging calcined material via an outlet into a vessel that is configured to restrict substantial ingress of oxygen-containing gases into the vessel. Positive nitrogen gas streams can be used to assist this process.

Where the vessel is in part a container, that is exchanged on filling with a replacement container, it is preferred that such container remain sealed for a time after filling. Without steps being taken to control the amount of oxygen available to the calcined material, the oxygen can rapidly combust any remanent char arising from the use of briquettes containing biomass and/or re oxidise other reduced material, such as residual iron.

One example of a vessel is a vessel that has (a) an opening to receive calcined material, (b) forms an integral seal with the outlet of the furnace at least during filling the vessel, and (c) a closure that can close that opening after receiving the hot calcined material.

It is not necessary that the closure forms an absolutely gas-tight seal with the vessel, only that the closure be sufficient that it is sealed enough to restrict ingress of air that causes unacceptable levels of oxidation of any residual char. A skilled person will understand the requirements required for a suitable gas-tight seal.

Positive nitrogen gas streams can be used to limit access of air into the vessel, although it is speculated that with a reasonable gas tight seal the atmosphere in the vessel will result in the atmosphere being almost totally nitrogen.

After cooling (natural or otherwise) of the calcined material, may be further processed before being subsequently being acid roasted and leached or leached with alkaline reagents in an autoclave for recovery of the lithium in a leachate. For example, it may be ground. Post grinding has potential benefits, in that it is likely that the ore fragments will be easier to grind at this stage, and thus the overall comminution energy is reduced.

Where biomass has been present as part of the process, the residual char may be separated from the calcined material. Such char has potential commercial application, like being returned to the soil of the biomass provider as a soil improver.

The threshold bulk gas velocity for gases passing from the decrepitation zone to preheat zone may be any suitable velocity having regard to factors such as the material being processed and the size and shape and other structural characteristics of the hearth furnace. Desirably, the velocity is not higher enough to significantly entrain feed material that is being conveyed into the decrepitation zone.

Typically, the microwave energy is delivered in close proximity to the ore fragments through the use of a series of horns placed transversely across the moving base.

The rows of horns may be spaced apart along the length of the section of the length of the decrepitation zone so that there are gaps between the successive rows.

Typically, there will be more than one row of horns.

The horns in at least some of the rows may be spaced apart so that there are gaps between the horns.

The horns of at least some rows may be offset laterally relative to the horns of at least some of the other rows - i.e. laterally relative to the direction of movement of briquettes through the decrepitation zone.

The horns of each row may be offset with respect to the horns of successive rows.

The horns may be pyramidal horns.

The pyramidal horns may include sectorial horns, each with one pair of opposing sides being flared and the other pair of opposing sides being parallel.

The sectoral horns in each row may be placed across the moving base so that the shorter sides of rectangular openings of the sectoral horns are parallel with the direction of moment of the moving base within the reduction furnace.

In other embodiments, at least some of the sectoral horns in each row may be placed across the moving base so that the longer sides of rectangular openings of the sectoral horns are parallel with the direction of moment of the moving base within the reduction furnace.

The moving base may have residual heat as a result of passing through the furnace when it returns to the preheat zone of the furnace.

The method may comprise generating a higher pressure of gas in the decrepitation zone compared to gas pressure in the preheat zone and causing gases generated in the decrepitation zone to flow counter-current to the direction of movement of briquettes on the moving base through the furnace.

The method may comprise using a gas flow “choke” between the preheat zone and the decrepitation zone that contributes to generating the higher gas pressure for causing gases in the decrepitation zone to flow counter-current to the direction of movement of briquettes on the moving base through the furnace.

The gas flow “choke” may be configured to increase the flow rate of the gas from the decrepitation zone to the preheat zone by a factor of 2-3 compared to what the flow rate would be without the gas flow “choke” in order to ensure that there is no substantial gas flow from the decrepitation zone to the preheat zone of the furnace.

The gas flow “choke” may be the result of forming the transverse cross-sectional area of the decrepitation zone to be less than the transverse cross-sectional area of the preheat zone.

The method may comprise discharging gas produced in the furnace that flows in the countercurrent direction to the outlet via a flue gas outlet in the preheat zone.

The method may comprise combusting combustible gas in the gas discharged via the flue gas outlet, for example in an afterburning chamber.

The invention also provides an apparatus for producing a calcined material, typically continuously, from feed material, with the feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium.

The apparatus comprises:

(a) a preheat furnace for preheating feed material using radiant heating, and

(b) a decrepitation furnace for decrepitating preheated feed material in a hearth furnace, the decrepitation furnace comprising a contained microwave and a microwave assembly for transmitting microwaves into the microwave zone and heating and decrepitating preheated feed material and forming calcined material; and

(c) a moving base for passing preheated feed material (while preheated) through the contained microwave zone.

The preheat furnace may be a linear heath furnace.

The linear hearth furnace may comprise the preheat furnace in one zone of the linear heath furnace and the decrepitation furnace in a downstream zone of the linear heath furnace.

The preheat furnace may be a rotary kiln furnace and the decrepitation furnace may be in a separate linear heath furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

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

Figure 1 is (a) a schematic diagram of one embodiment of an apparatus for producing calcined material from briquettes of a composite of 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 calcined material from briquettes of a composite of ore fragments and biomass in accordance the invention, and (c) an example plot of the volumetric flow rate of gas produced along the length of the furnace during the course of the method;

Figure 2 is (a) a schematic diagram of a segment of a decrepitation zone of another, although not the only other, embodiment of the apparatus in accordance with the invention, showing two rows of horns, (b) is a theoretical static heating pattern showing a likely temperature profile of the bed of ore fragments passing under the horns shown in (a) as a result of heating via microwaves from the horns, and (c) is a theoretical dynamic heat pattern showing the temperature profile of the bed of material to be calcined after having moved passed the two rows of horns;

Figures 3(a) and (b) are schematic diagrams of part of the transition zone of the apparatus shown in Figure 1, showing a roller for smoothing the preheated material on the moving base and a microwave baffle that resides between the roller and the decrepitation zone; and

Figure 4 is a schematic diagram of another, but not the only other, embodiment of an apparatus for producing calcined material from ore fragments in accordance with the invention.

DESCRIPTION OF EMBODIMENTS

As noted above, in broad terms, the present invention is a method and an apparatus for continuously producing calcined material from ore fragments and biomass, typically at least initially in the form of briquettes of a composite of ore fragments and biomass, that includes transporting ore fragments and biomass through a furnace having an inlet for ore fragments and biomass and an outlet for calcined material, a preheat zone for a preheat stage of the method, a decrepitation zone for a decrepitation stage of the method, as well a transition zone between the preheat zone and the decrepitation zone, wherein there is a smoothing device for reducing the height of preheated material as it passes thereunder.

Figure 1 is a schematic diagram of one of a number of embodiments of an apparatus of the present invention taken as a longitudinal section through a linear heath furnace.

As is described in more detail below, a key feature of the embodiments of the linear hearth furnace shown in the Figures is a plurality of perforated horns arranged in rows across a width of and along a section of a length of a decrepitation zone of the linear hearth furnace for supplying microwave energy into the decrepitation zone for heating briquettes in the described embodiments (or what remains of the material in the form of briquettes at this stage) passing through the decrepitation zone, the horns having outlet openings for microwave energy, the horns within each row being arranged in a co-polarisation (same orientation) manner to form a regular field pattern, and with perforated skirts between the horns being configured to at least substantially prevent microwave energy passing upwardly through the perforations and to allow reaction gases generated by the material in the decrepitation zone to flow upwardly through the perforations. Allowing reaction gases to flow upwardly through the perforations means that there is a smaller volume of reaction gases in a space between the material and the horns that can flow from the decrepitation zone through the transition zone to the preheat zone and therefore a lower flow rate and less risk of entrainment of solid material (i.e. dust make) with the reaction gases. Previous modelling work of the applicant for such a designed furnace has suggested keeping the flowrate below a threshold of 5 m/s. The actual threshold in any given situation may be a different flowrate.

With reference to Figure 1, the linear hearth furnace, generally identified by the numeral 3, includes an elongate refractory-lined chamber that has the following zones along its length:

(a) an inlet 10 that includes an inlet for feeding feed material into the chamber and is configured to receive briquettes 120 of ore fragments and biomass, noting that the briquettes 120 are shown within the chamber,

(b) a preheat zone 20 for heating material, i.e. lithium oxide-containing ore fragments and biomass, in the briquettes 120 and partly reducing any iron oxides and releasing volatiles in the biomass, with the volatiles being combusted in the preheat zone,

(c) a decrepitation zone 30 for heating material further and commencing the decrepitation stage of the calcination process, and potentially reducing any incidental iron oxides to direct reduced iron (DRI);

(d) a transition zone 25 that sits between the preheat zone 20 and the decrepitation zone 30 and has within it a smoothing device 251;

(e) a discharge 40 for calcined material that includes an outlet for discharging calcined material from the chamber;

(f) an endless moving base 50 having a metallic material base that moves through the chamber from the inlet to the outlet and transports material that is at least initially in the form of briquettes through the chamber from the inlet and discharges calcined material from the outlet and then returns to the inlet to be re-loaded with additional briquettes; and

(g) a flue gas outlet 70 in the preheat zone 20 for discharging gas produced in the furnace by heating and/or combustion within the furnace.

The inlet 10 is configured to continuously feed briquettes 120 into the linear hearth furnace via the inlet to form a relatively uniform bed of briquettes on the moving base 50, while restricting outflow of furnace gases via the inlet. The inlet 10 includes a feed chute 12 that can receive and direct briquettes 120 onto the moving base 50. As noted above, the term “relatively uniform bed of briquettes” is understood herein to mean a relatively uniform layer of briquettes covering the base and typically having a consistent ‘bed’ depth, at least length ways, 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.

The discharge outlet 40 is configured to continuously discharge calcined material via the outlet, while restricting the inflow of oxygen-containing gases into the decrepitation zone 30 of the chamber. The discharge outlet 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 calcined material to flow through the opening.

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.

The combustible gases originating within the furnace include:

(a) volatiles from biomass in material moving through the preheat zone 20; and

(b) combustible gases, such as CO, generated by reduction of any incidental iron oxides in ore fragments in:

(i) the preheat zone 20 and

(ii) the decrepitation zone 30, with the combustible gases generated in the decrepitation zone 30 flowing from the decrepitation zone 30 to the preheat zone 20, as described further below.

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

In use, the decrepitation zone 30 is an anoxic environment.

The decrepitation zone 30 includes a plurality of microwave energy input units 32 (waveguides 64 and horns 66) in a top space thereof for heating briquettes. The micro wave energy input units 32 are operatively connected to a microwave energy generator 34 (see Figure 2 - in which the generator is a microwave energy generator).

The horns 66 are pyramidal horns, more particularly sectoral horns 66 in the embodiment shown in the Figures.

The horns 66 are arranged in rows 68 (see Figure 3(a) which shows two rows only) having microwave outlets 70 for microwave energy. The rows 68 extend across a width of a section of the decrepitation zone 30 and along a length of the section above a top surface of the moving base 50 and, in use above a top surface of briquettes carried on the moving base 50. The section may be any suitable length and any suitable width.

The horns 66 are in contact with or in close proximity with each other at the microwave outlets 70 of the horns 66.

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

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 sides 74 being parallel to each other which, in use, produces a fanshaped 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.

The horns 66 in each row are placed across the moving base 50 so that the shorter sides of the rectangular microwave outlets 70 of the sectoral horns 66 are parallel with the direction of moment of the moving base 50 within the decrepitation zone 30.

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 moving base 50 - see Figure 3(c) where this is illustrated by the uniform temperature profile of material that have passed through the decrepitation zone 30.

The horns 66 within each row 68 are arranged in a co-polarisation (same orientation) manner to form a regular, typically highly uniform, field pattern, and with the horns in at least some of the rows 68 being offset laterally in relation to the horns of at least one of the other rows 68. Figure 2(a) shows an embodiment of an off-set arrangement. This is not the only possible embodiment.

The side walls 72, 74 of the horns 66 may have a plurality of perforations (not shown), for example in the form of holes or slots, punched/cut through the side walls.

The arrangement and the size of the perforations will in part be determined by the wavelength of the microwave energy and the thickness and material of the side walls 72, 74 of the horns. The perforations may be of any size and/or shape but must be selected to prevent microwaves from passing through the perforations while not unduly restricting gases passing therethrough.

The transition zone 25 is configured to have a smoothing device 251 in the form of a roller for reducing the height of material passing through it on the moving base 50. A microwave choke 263 sits behind the smoothing device 251 and is positioned so as to have a set maximum distance above the smoothed bed of material on the moving base 50. The microwave choke 263 is a ‘corrugated’ type with flat peaks 261 and flat troughs 273. The transition zone 25 is separated from the preheat zone 20 by a suspended refractory wall 253, which sits in front of the smoothing device and forms a passage between itself and the moving base 50 so that material may pass thereunder prior to smoothing. It also forms a barrier over which gases can pass from the decrepitation zone 30 to the preheat zone 20.

In use of the apparatus, gases in the decrepitation zone 30 flow into the preheat zone 20 counter-current to the direction of movement of briquettes on the moving base 50 through the furnace from the inlet to the outlet.

The counter-current flow of gas from the decrepitation zone 30 into the preheat zone 20 is caused by a higher gas pressure in the decrepitation zone 30 compared to gas pressure in the preheat zone 20. While such pressure effect is 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. One factor is injecting nitrogen gas (or any other suitable gas) into the horns in the decrepitation zone 30 to ensure that dust does interfere with operation of the horns.

Another factor is gas generated via potential reduction of any iron oxides in the decrepitation zone 30. This reduction gas may contribute to generating and maintaining the higher pressure in the zone (and the anoxic environment).

The volume of reduction gas generated in the decrepitation zone 30 is illustrated by the plot of off-gas volumetric flow rate shown in Figure 1.

Another factor is the induced draft fan at the end of the off-gas train, which depending on its size may have a significant influence.

The counter-current flow of gases from the decrepitation zone 30 to the preheat zone 20 transfers combustible gases, such as CO, generated in reactions that reduce iron oxides in the decrepitation zone 30 to the preheat zone 20. The combustible gases in the gas flow from the decrepitation 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. The combustion profile may be 35-45% at a hot end of the preheat zone 20, i.e., at the end adjacent the decrepitation zone 30, increasing to 90-95% at a cold end of the preheat zone 20, i.e. at the end adjacent the inlet 10.

The combustion of (a) any combustible gases generated in the decrepitation zone 30, and (b) combustion of volatiles released from biomass in the preheat zone provides an important component of the heat requirements for the method, when biomass is part of the feed material.

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

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

In use, gases generated in the chamber are discharged as a flue gas via the flue gas outlet 70 in the preheat zone 20.

As described above in relation to the term “briquettes”, it is desirable for the invention that ore fragments and biomass be in quite close contact. Any approach to achieving this close contact may be used. Ore-biomass mixing followed by compaction of the materials to form briquettes between two rolls in which there are naturally aligning pockets, is one example. Alternative such compaction option is ore-biomass mixing followed by roll pressing using rolls without pockets into compressed slabs containing the ore fragments and biomass that break up naturally (or are deliberately broken up) prior to feeding into the feed station zone.

The briquettes may be manufactured by any suitable method. By way of example, measured amounts of ore fragments and biomass and water (which may be at least partially present as moisture in the biomass) 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 cm³ in volume and contain 30-40% biomass (e.g., elephant grass at 20% moisture), with the balance comprising ore fragments fines.

The physical structure of the calcined material at the end of the process is not critical.

With further reference to Figure 1, the calcined material is fed into an insulated vessel (not shown) which is configured ideally to transport the calcined material (hot) to a feed bins for either further comminution or supply to acid roasting and leaching facility. It is noted that those structural components that are not specifically shown in Figure 1 (excluding the componentry of the transition zone) are generally standard components within the industry and the skilled person would be able to make an appropriate selection of the components.

As shown in Figure 1, gases generated in the chamber flow into and along the preheat zone 20, counter-current to the movement of briquettes, and combustible gas in the gases is subjected to incremental combustion as it passes through the plurality of air or oxygen- enriched air fed burners 22 which, in this embodiment, receive preheated (and/or oxyenriched) air.

The post-combustion profile in the preheat zone 20 is typically 35-45% at the hot end (i.e., the decrepitation zone 30 end), increasing gradually to around 90-95% at the flue gas outlet 70 end. The preheat zone top space is therefore maintained in a bulk reducing condition all the way along its length in the embodiment, with feed oxygen being consumed rapidly in the vicinity of each burner 22 (in a small-localised region).

Off-gas at the flue gas outlet 70 end is then ducted (hot) to an afterburning chamber (not shown), where final combustion of combustible gas in the gas is performed.

The gas from the afterburning chamber is then used (in the example provided) to preheat air for the burners 22 in the preheat zone 20 via a heat exchanger (not shown), before passing to a boiler (not shown) for final heat recovery and then discharge as flue gases to the atmosphere.

This example necessarily contains multiple assumptions regarding kinetic parameters - precise details may shift as a result of different kinetics. However, the principles are not expected to change. Although the current example is based on preheated air, additional oxygen could be added to the air mixture prior to heating so that the ratio of air to oxygen could be varied as an additional control parameter to further optimise the process.

Figure 2(a) is a schematic of the inside of a segment of the decrepitation zone 30 of a linear hearth furnace of one embodiment of an apparatus in accordance with the invention. Figure 2(a) shows two rows 68 of off-set sectoral horns 66, each row placed above and extending across a top surface of a moving base 50 so that the shorter sides 72 of the micro wave outlets 70 of the horns are parallel with and the longer sides 74 of the micro wave outlets 70 are perpendicular to the direction of moment of the moving base within the linear hearth furnace.

Figure 2(a) also shows an interface 80 that separates the decrepitation zone 30 into an upper sub zone 56 and a lower sub zone 58. There are small gaps 76 between the horns 66 in each row 68 and there is a small gap 78 between the rows 68 at the level of the microwave outlets 70.

Whilst not shown in the Figure, these small gaps 76, 78 are closed by a microwave energy barrier that is configured to allow reduction gases to pass therethrough and to at least substantially prevent microwave energy passing therethrough.

The microwave energy barrier may be in the form of perforated metal elements that are connected to the horns 66 close to the microwave outlets 70. The end result is that the interface 80 comprises the horns 66 and the microwave energy barrier in the gaps, and the interface 80 is a single continuous interface at this height of the decrepitation zone 30.

Figure 2(b) is the theoretical temperature profile of a bed of iron containing material that receives micro waves under such a horn structure (i.e., as a static heating pattern without the moving base moving).

Figure 2(c) is the theoretical temperature profile of the same bed of iron containing material having moved on the moving base past the two rows of horns.

Figure 3(a) and (b) are schematic diagrams of the lower part of the transition zone 25 (there is a passageway for gases (not shown) that sits above this apparatus), showing the above- mentioned smoothing device 251 in the form of a water cooled (hollow) roller for smoothing the preheated material on the moving base 50 and a microwave choke 263 that resides between the roller and the decrepitation zone.

The roller 251 is ideally driven in an anticlockwise direction so that the rotational speed of the water-cooled driven roller will be such that the velocity of moment of its outer surface will generally align with the velocity of the material travelling on the moving base 50. However, its velocity may be varied to suit operational circumstances.

Typically, the roller 251 operates with a constant downward pressure approach (apparatus for such not shown) within a defined range of heights above the moving base. Such constant downward pressure can be achieved for example by the use of springs. While a hollow roller 251 is shown, it may be necessary (for spanning across the furnace purposes) to have a shaft supported roller set up, where for example the outer sleeve is made of SS310 and there are series of channels between that sleeve and the main body of the shaft of the roller, which may be made of medium carbon steel.

It can be seen that the driven roller 251 is located sufficiently within the transition zone 25 that it is not likely to be substantially exposed to direct radiation arising from the combusting of combustible gases via air or oxygen-enriched air fed burners in the preheat zone 10.

Figure 3(a) and (b) also show in more detail than Figure 1 the microwave choke 263 in the transition zone 25. As noted above, the microwave choke 263 sits behind the smoothing device 251, is positioned so as to have a set maximum distance above the smoothed bed of material on the moving base 50 and is a ‘corrugated’ type with flat peaks 261 and flat troughs 273 that is positioned above and across the path of the moving base 50.

Figure 4 is a schematic diagram of another, but not the only other, embodiment of an apparatus for producing calcined material from a feed material comprising ore fragments and biomass.

The feed material may be a mixture of ore fragments and biomass, separate layers of ore fragments and biomass stacked one layer on top of a preceding layer, or briquettes of ore fragments and biomass. There are similarities between the embodiments shown in Figures 1 and 4 and the same reference numerals are used to describe the same features.

A major difference between the two embodiments is that the preheat zone 20 and the decrepitation zone 30 are in the same furnace (although as distinct zones within the furnace) in the Figure 1 embodiment and the preheat zone 20 and the decrepitation zone 30 are in separate furnaces in the Figure 4 embodiment.

The Figure 4 embodiment comprises a rotary kiln furnace, generally identified by the numeral 55, as the preheat zone 20 and a linear hearth furnace 3 as the decrepitation zone 30.

With reference to Figure 4, the rotary kiln furnace 53 is a standard construction and includes a cylindrical vessel 57 that defines the preheat zone 20 and has an outer steel shell and an inner refractory lining, an inlet at one end configured to receive briquettes 120 of ore fragments and biomass and an outlet end for preheated material at the other end of the vessel 57.

The rotary kiln furnace 53 also includes a structure, generally identified by the numeral 59, that supports the cylindrical vessel 57 for rotation about an elongate axis of the vessel 57.

The support structure supports the cylindrical vessel 57 at a slight downward angle from the inlet end to the outlet end.

The rotary kiln furnace 53 also includes a feed assembly 10 for feed material at the inlet end and a discharge assembly 40 for preheated material at the outlet end of the vessel 57. In use, the combination of the slight incline and the rotation of the vessel 57 moves feed material from the inlet end to the outlet end of the vessel 57.

The rotary kiln furnace 53 also includes a burner assembly 84 at the outlet end of the vessel 57. The burner assembly 84 is configured to supply heated gas to the vessel 57 that flows in the opposite direction to the direction of moment of feed material through the vessel 57.

The feed assembly 10 also includes a flue gas outlet 70 for discharging gas produced in the preheat zone 20 by heating and/or combustion within the zone and a dust outlet 82 for dust generated in the preheat zone 20 that separates from the flue gas at the inlet end of the vessel 57.

In use, the preheat zone 20 preheats heats material, i.e. lithium oxide-containing ore fragments and biomass, in the briquettes and partly reduces any iron oxides and releases volatiles in biomass, with the volatiles being combusted in the preheat zone.

The discharge assembly 40 discharges preheated material from the vessel 57. The feed material is transferred by any suitable options to the decrepitation zone 30 of the linear hearth furnace 3. It can be appreciated form a comparison of Figures 1 and 4 that the decrepitation zones 30 of the two embodiments are fundamentally the same and operate in the same way, with a result of producing calcined material.

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

By way of example, whilst the above embodiment includes continuous operation, the invention is not so limited.

By way of further example, whilst the embodiments shown in the Figures include feed material in the form of briquettes of ore fragments and biomass, the invention is not so limited and extends to other forms of the material. For example, the material may be a bed of ore fragments and biomass, or even simply a bed of ore fragments alone, where heat for the preheat zone is provided by other means as previously described.

Claims

1. A method for producing a calcined material, typically continuously, from a feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium, the method comprising two stages, with a preheat stage comprising preheating feed material using radiant heating and a decrepitation stage comprising heating and decrepitating feed material in a hearth furnace by passing preheated feed material (while preheated) on a moving base through a contained microwave zone within the hearth furnace and transmitting microwaves into the microwave zone and heating and decrepitating preheated feed material and forming calcined material, and wherein preheated feed material enters the decrepitation stage at a bulk temperature of at least 600°C and calcined material leaves the decrepitation stage at a bulk temperature of no more than 1100°C.

2. The method defined in claim 1 comprises carrying out the initial preheat stage and the decrepitation stage in the same furnace, with a distinct preheat zone and a distinct and separate decrepitation zone within the furnace.

3. The method defined in claim 1 comprises carrying out the preheat stage and the decrepitation stage in two separate furnaces.

4. The method defined in claim 3 comprises carrying out the preheat stage using a rotary kiln furnace and carrying out the decrepitation stage using a linear hearth furnace.

5. The method defined in claim 4 comprises transferring preheated ore fragments from the rotary kiln while hot into the linear hearth furnace.

6. The method defined in any one of the preceding claims comprises heating the preheated material to a bulk temperature of between 1075°C and 1100°C in the decrepitation stage.

7. The method defined in any one of the preceding claims heating the preheated material in the preheat stage so that the bulk entry temperature of the preheated material for the decrepitation stage is at least 650 °C, more typically at least 700 °C, and more typically again at least 800 °C.

8. The method according to any one of the preceding claims comprises forming a relatively uniform bed of preheated material from the preheat stage before moving the material into and exposing the material to microwave energy in the decrepitation stage by distributing preheated feed material on the moving base, for example by smoothing the material on the moving base, for example by using a roller or other suitable smoothing device positioned above and contacting the preheated material.

9. The method according to any one of the preceding claims comprises transmitting the microwave energy to the microwave zone through the use of a series of horns placed transversely across the moving base and in close proximity to the ore fragments.

10. The method defined in claim 9 wherein the average difference in height of each horn above the height of material on the moving base is selected to be sufficiently small so that there is a highly resonant and well-defined field pattern which maximises absorption and homogeneity across a defined area and minimises cross coupling of microwave energy.

11. The method defined in claim 9 or claim 10 wherein the horns are sectorial pyramidal horns each with one pair of opposing sides being flared and the other pair of opposing sides being parallel.

12. The method defined in any one of the preceding claims comprising supplying feed material in the form of briquettes of ore fragments and biomass to the preheat stage.

13. The method defined in claim 12 comprises controlling the preheat stage so that at least 90% of volatiles in biomass in the material are released as a gas in the preheat zone.

14. A method of forming a relatively uniform bed of a preheated material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium on a moving base before moving the material into and exposing the material to microwave energy in a microwave zone and decrepitating the material and forming a calcined material, the method comprising distributing preheated feed material on the moving base, for example by smoothing the material on the moving base, for example by using a roller or other suitable smoothing device positioned above and contacting the preheated material.

15. A method for producing a calcined material, typically continuously, from a feed material comprising lithium bearing ore (or concentrates thereof) for subsequent recovery of lithium, the method comprising distributing preheated feed material containing lithium bearing ore or concentrates thereof over a moving base and forming a relatively uniform bed of material before the material enters a contained microwave zone and is exposed to microwave energy that is supplied to the microwave zone through a series of horns, for example spaced transversely across the moving base and in close proximity to the uniform bed of material, with the microwave energy heating and decrepitating material and forming calcined material.