HK1237378B - Recovery of lithium from silicate minerals - Google Patents

Description

Lithium recovery from silicate minerals

Technical Field

The present invention discloses a method, system and apparatus for recovering lithium from lithium-containing silicate minerals, including hard rock minerals as well as clay and mica minerals. The primary chemical consumed in the method may be carbon dioxide recovered from a flue gas stream, including any substances produced from process equipment used elsewhere in the process of converting lithium silicate minerals into products desired by users of lithium chemicals, particularly lithium carbonate and lithium hydroxide.

Background Art

Lithium is widely present throughout the Earth's crust, with an average concentration of about 20 parts per million. This concentration is comparable to that of other valuable metals such as cobalt, but much lower than iron and aluminum, and it is much more abundant than the precious metals gold and platinum. However, the metal is widely dispersed, and high-grade deposits appear to be rare and generally small, so known economic resources may not be sufficient to meet demand for applications (particularly batteries) where its use is expected to grow significantly in the coming years and decades.

Even now, lithium batteries allow for the storage of significantly larger quantities of electricity per unit of battery weight: at least 150 watt-hours per kilogram (Wh/kg), preferably 250 Wh/kg, and perhaps 1500 Wh/kg for extended periods. Such storage intensities will allow electricity to penetrate the hitherto entirely petroleum-fueled road transport market and accelerate the development and deployment of power generation systems that exploit inherently intermittent forms of renewable energy: wind and solar.

Harvesting lithium from seawater may remain too expensive due to its extremely low concentration (less than 0.2 parts per million by weight), even though the total amount in seawater far exceeds any foreseeable demand (more than 200 billion tons of the metal).

The economic supply of lithium chemicals needed to manufacture lithium batteries currently relies primarily on brine from South American salt lakes (salinas) in the so-called "lithium triangle" spanning Argentina, Bolivia, and Chile. However, the security of supply from these salt lakes is jeopardized by political risks, environmental challenges, and uncertainty about how much economically recoverable lithium is contained in these salt lakes.

Lithium is also recovered from certain hard-rock silicate minerals. However, until recently, there has been little interest in exploring hard-rock lithium deposits, firstly because the dry salt lakes of the Lithium Triangle have long been considered to be rich in lithium, and secondly because the current refining process for hard-rock lithium ores (which has remained largely unchanged since before World War II) is expensive, complex, hazardous, and environmentally challenging.

With the benefit of significantly superior processing, the world's hard rock lithium resources can be developed to the benefit of battery manufacturers, providing them with greater confidence that the supply of lithium for batteries will be secure for a longer period of time and at a lower overall cost than would otherwise be the case.

Reference to background or prior art herein does not constitute an admission that such art forms part of the conventional and/or general knowledge of a person of ordinary skill in the art. Such reference is not intended to limit the methods and systems as described herein in any way.

Summary of the Invention

Disclosed herein are improved methods for recovering lithium from lithium-bearing silicate minerals (e.g., hard rock minerals, which are often found in the type of crystalline rocks known as pegmatites). The hard rock lithium-bearing silicate minerals may include _spodumene ( _LiAlSi2O6 ) and/or any of a range of other lithium-bearing silicate minerals, including but not _limited to petalite _LiAlSi4O10 , eucryptite _LiAlSiO4 , apatite (Li _, Na) _AlPO4 (F,OH), and various minerals in the mica group including lepidolite K(Li,Al,Rb) ₃ (Al,Si) _4O10 (F,OH) ₂ and ferroleum mica KLiFeAl( _AlSi3 ) _O10 (OH,F) ₂ . Lithium may also be present in certain clays and in the newly discovered (2006) sodium-lithium borosilicate mineral _{jadarite LiNaSiB3O7} (OH), which is the result of partial weathering of such minerals including hectorite _Na0.3 (Mg _, Li) _3Si4O10 (OH) _{2 .}

The method may include mixing silicate minerals and nitric acid. As will be described below, the use of nitric acid as a leaching agent allows the extracted lithium value (lithium value) to be converted into a form such as pure lithium hydroxide. Lithium hydroxide is suitable for direct sale to customers who may prefer to purchase their lithium needs in the form of hydroxide (e.g., for the manufacture of lithium batteries). Lithium hydroxide may also allow pH to be controlled during each stage of the method, as will be described below. Lithium hydroxide may further allow the capture of carbon dioxide produced in the method, converting the hydroxide into a desired amount of carbonate (wherein lithium carbonate is another form suitable for sale, such as for the manufacture of lithium batteries). The lithium hydroxide-lithium carbonate gas scrubbing system will also be described below.

The process recovers lithium in marketable forms, particularly as hydroxide (LiOH in anhydrous form or _LiOH.H2O in monohydrate form) and carbonate (lithium calculations are usually expressed in the industry in terms of sodium carbonate equivalents or LCE).

The method can avoid the need to purchase and consume expensive and dangerous chemicals such as sulfuric acid and sodium carbonate (soda ash). The method can avoid the production of undesirable by-products such as sodium sulfate or analcime. The method can also involve minimizing the number of processing steps. The method can also be environmentally friendly, including limiting emissions of the greenhouse gas carbon dioxide.

In one embodiment, prior to mixing the silicate mineral and the nitric acid solution, the silicate mineral may be pretreated to make the lithium value therein more suitable for leaching by nitric acid. For example, the natural (as-mined) lithium aluminum silicate mineral spodumene is typically in the inert α (alpha) form. Pretreatment of the silicate mineral may include thermal treatment (e.g., calcination or roasting). Alternatively, the pretreatment may be non-thermal (e.g., mechanical treatment), for example, by ultrafine or high-intensity grinding of the mineral to produce strong mechanical shear (e.g., in an agitated mill such as Isamill ^™ ).

When the silicate mineral is heat-treated, it can then be ground (e.g., in a roller mill) and subsequently separated from the resulting hot gas stream (e.g., in a cyclone) before being mixed with the nitric acid solution. Mechanical treatment, on the other hand, can avoid the separation stage. Thus, depending on the process setup, such grinding may or may not be necessary.

When the step of pre-treating the silicate mineral comprises a thermal treatment, such as may involve, for example, the combustion of a fossil fuel (e.g. natural gas), the gaseous fluid stream produced, as a resultant flue gas, may comprise carbon dioxide, which in turn may be scrubbed with a slurry comprising a portion of the lithium hydroxide and/or lithium carbonate in an aqueous phase.

The methods disclosed herein may include subjecting a mixture of silicate minerals and nitric acid to a leaching process. The leaching process may be carried out in one or more stages and may be carried out under conditions such that lithium values in the silicate minerals are leached into the aqueous phase as lithium nitrate. The conditions of the leaching process may be controlled to minimize the amount of non-lithium values that may be present in the silicate minerals (e.g., aluminum, iron, and other transition metals including nickel, chromium, manganese, and cobalt; alkaline earth metals calcium and magnesium; and phosphate ions) and leached into the aqueous phase. If leached, the non-lithium values may be separated from the aqueous phase (e.g., precipitated, etc.).

In one embodiment, the conditions of the leaching process may include increasing the temperature and/or pressure of the leaching process to accelerate the leaching of lithium ions into the aqueous phase as lithium nitrate.

For example, silicate minerals can be reacted with nitric acid at elevated temperature (e.g., ∼170° C.) and elevated pressure (e.g., ∼20 bar), for example, in a digestion reactor (e.g., an autoclave). It has been shown that, for example, 95% of the lithium in a sample of calcined (β) spodumene can be extracted in one hour at a temperature of 170° C. in such a reactor under pressure.

Alternatively, the silicate minerals can be reacted with nitric acid at elevated temperature (e.g. 100°-120°C) but at atmospheric pressure. Such a reaction can also be carried out in a digestion reactor, in this case not a pressure vessel but a reactor in the form of a vertical silo reactor, for example.

In one embodiment, the conditions of the leaching process may include reacting the silicate mineral in a stoichiometric excess of nitric acid to ensure maximum extraction of lithium from the silicate mineral. The leaching time may be controlled to help maximize the extraction of lithium values from the silicate mineral while also helping to minimize the extraction of "impurities" (including aluminum, iron, other transition and alkaline earth metals, and phosphate ions) present in the lithium-rich silicate mineral.

The controlled time can be terminated by neutralizing or otherwise removing the remaining free nitric acid. This can occur in one or more stages (e.g., vessels) separate from the leaching stage. Many alternatives for terminating the leaching process can be used, each of which can lead to different embodiments of the method. It should be understood that such embodiments can consist of combinations of the variations described herein.

In one embodiment to terminate leaching, neutralization alone can be used. This embodiment can include adjusting the pH of the leach solution, which is initially highly acidic due to the presence of residual free, excess, unreacted nitric acid, to near neutral pH. For example, the residual free nitric acid can be neutralized by recycling a portion of _a basic lithium compound (e.g., one or more of _Li₂O , LiOH, and _Li₂CO₃ , in solution or as fine crystals in suspension) produced as part of the lithium recovery process.

Advantageous results of neutralization by lithium hydroxide and/or lithium carbonate may include conversion of free nitric acid to more lithium nitrate, and conversion of nitrates of aluminum, iron, and other transition and alkaline earth metals to their insoluble oxide or hydroxide forms.

Suitable equipment for conducting the neutralization reaction may include simple covered tanks (which may be just one, or two or more tanks operating in series). Each tank may be equipped with an agitator or other device (e.g., an air injection system) to maintain any insoluble solids in suspension (in suspension form). Extended mixing times may be required. This is because reactions (e.g., in which aluminum nitrates are particularly hydrolyzed) are preferably carried out slowly so that the resulting aluminum hydroxide particles, for example, are discrete and crystalline (rather than gel-like) and have favorable sedimentation, filtration, and washing properties. This allows such particles to be handled and removed in typical solid-liquid separation equipment and systems.

In an alternative embodiment, instead of relying solely on the use of a basic intermediate (such as lithium hydroxide) to neutralize all of the remaining nitric acid, the leaching process can be substantially terminated and most of the excess nitric acid recovered by physical means.

For example, in one embodiment, the product of the leaching process may first be heated to distill off excess nitric acid and any free water present as steam. In one embodiment, the dried solid material may then be further heated to a temperature of about 200° C., sufficient to decompose aluminum, iron, and other transition metal nitrates into their respective insoluble oxides or hydroxides. The heating may also cause the release of further nitrogen oxides and oxygen, which may be captured and transferred to join (combine) with the nitric acid and water vapor produced during the drying stage.

Thus, by converting the aluminum, iron, and transition metal nitrate impurities into insoluble products, they can be added to the remaining solid material remaining after the leaching stage. In contrast, the lithium values can be retained in a soluble form so that they can be easily extracted into an aqueous solution (e.g., by extraction into a pH-neutral solution) in a subsequent stage. The pH-neutral solution can be prepared by, for example, recycling a relatively small portion of the lithium hydroxide and/or lithium carbonate product as an aqueous solution or slurry.

Excess nitric acid and water can be distilled off in a drying stage using a hollow-flight screw conveyor, such as the Bepex Thermascrew ^™ or the Metso Holo-Flite ^™ . These and similar screw conveyors can be completely enclosed and can be operated under a slight negative pressure to prevent the emission of nitrogen oxides and nitric acid vapor. Alternatively, the conveyors can be confined within sealed pressure vessels to allow them to operate at elevated pressures, such as 10 to 20 bar (i.e., to match the pressure at which the upstream digestion stage can be carried out). As described below, the nitric acid and water vapor can be collected for use in regenerating the nitric acid (e.g., a nitric acid solution can be produced in a dedicated nitric acid production plant, which can then reuse the acid in the process).

The drying stage may optionally utilize molten anhydrous lithium nitrate as a heating medium. The anhydrous lithium nitrate may contain varying, but typically low, proportions of molten potassium nitrate and sodium nitrate. The molten lithium nitrate may be circulated through the hollow scrapers of the conveyor heating system. The molten lithium nitrate may be produced from the intermediate lithium nitrate product in subsequent process stages.

When compared to embodiments employing only a neutralization method, the alternative embodiment may require additional energy and equipment. However, the alternative embodiment can minimize the formation of additional lithium nitrate. This is because most of the excess nitric acid is removed from the system without having to be neutralized by recycled alkaline lithium compounds (e.g., hydroxides and/or carbonates). This, in turn, can minimize the extent to which chemical intermediates (lithium nitrate and alkaline lithium compounds such as hydroxides and/or carbonates) are recycled through the entire process.

In one embodiment, a primary solid-liquid separation stage (e.g., a filtration stage) may be added immediately after the leaching stage. This may be employed before any operations are performed to neutralize or otherwise remove (e.g., by distillation) free excess nitric acid present in the product slurry from the leaching stage.

In this regard, it has been observed that the leach product of the calcined (β) spodumene concentrate with excess nitric acid forms a filter cake that is easily filterable and highly suitable for washing in the primary solid-liquid separation stage. This means that the lithium values (and other metals made soluble by the nitric acid and therefore leached into solution) can be effectively washed out from the remaining solids with a minimum amount of water. These remaining solids become an inert solid tailings (tailings) product that can be safely and conveniently disposed of, for example, by using it as a landfill (although this is by no means the only possible use that can be found for this material). The filtrate from the solid-liquid separation stage may comprise a strongly acidic (i.e., due to its still high nitric acid concentration) clarified solution (i.e., mother liquor). This solution may substantially contain all of the lithium values leached from the calcined spodumene concentrate as soluble nitrates, as well as some of the aluminum, iron, and other metal values leached from the spodumene.

When a primary solid-liquid separation stage is employed, the treatment of the resulting mother liquor can follow either of two treatment pathways, which share similarities with those described above.

More specifically, and as described above, the first treatment path may include direct neutralization of the free acid using an alkaline solution or slurry containing recycled lithium hydroxide and/or lithium carbonate. As described above, this results in the formation of precipitates of insoluble and poorly soluble oxides and hydroxides, including oxides and hydroxides of aluminum, iron and other transition metals, and the alkaline earth metals magnesium and calcium.

In the first treatment path, these precipitated solids can be removed in a separate (secondary) solid-liquid separation stage. For example, the separation stage may include clarification before filtering the thickened underflow slurry of the precipitated solids. The thickened slurry can be processed separately, for example by filtering it and washing it to produce a dewatered filter cake that is essentially free of soluble lithium values. The dewatered filter cake can be processed together with the washed solids from the primary filtration stage. What remains is a clarified solution of lithium nitrate, which consists of both a solution including the lithium values leached from the spodumene and a solution from the following: neutralizing (by recycling the alkaline lithium hydroxide/lithium carbonate solution/slurry) the nitric acid residue to the amount consumed to extract the lithium and other metal values actually leached from the spodumene.

In a second processing path, the mother liquor may first be heated (rather than being heated in a screw conveyor (e.g., by molten _LiNO3 )) until most of the liquid values, including water and free nitric acid, are evaporated. This first heating may occur, for example, in a mechanical vapor compression evaporator. This first heating may leave a concentrated lithium nitrate solution (plus some other soluble impurities).

The concentrated solution (which is still acidic due to the presence of some nitric acid that has not been distilled off) can then be neutralized, as described above, i.e., using recycled lithium hydroxide and/or lithium carbonate solution/slurry. This has the effect of insolubilizing the aluminum, iron and other transition metals, and importantly the alkaline earth metals magnesium and calcium, that were initially present as nitrates. This has the effect of converting these impurities into insoluble oxides and hydroxides and producing more lithium nitrate in solution. This concentrated, pH-neutral solution can then be processed in equipment similar to that used for further processing of the neutralized solid material (i.e., the solid material obtained from a process variant in which the product of the leaching reactor is not filtered before its remaining nitric acid values are distilled off and/or neutralized).

In all embodiments of the process, nitric acid and water vapor can be collected together with nitrogen oxides and some oxygen from the decomposition of nitrates of aluminum, iron and other transition metals for use in the regeneration of nitric acid (e.g., a nitric acid solution can be produced in a dedicated nitric acid production unit and the acid can then be reused in the process).

In one embodiment, these volatile materials may first be passed into a distillation column where the majority of the nitric acid and water vapor are first condensed to produce a nitric acid solution that is concentrated to a level suitable for recycling to the spodumene leaching stage.

In another embodiment, these volatile materials may be sent directly to a downstream nitric acid production facility.

In all embodiments of the process, the product of the above-described stages can be a hot, concentrated aqueous lithium nitrate solution. This solution can also contain small amounts of alkali metal sodium and potassium nitrates. This solution can be further processed to convert the contained lithium values into the desired chemicals: lithium hydroxide and lithium carbonate. Two alternative variations can be used.

In a first variant, the entire concentrated lithium nitrate solution may be heated gradually in an operational sequence, ultimately reaching a temperature exceeding ~700°C. Whether a single plant and equipment item or a series of plant and equipment items may be used to achieve the heating, the lithium nitrate solution undergoes three changes. First, it is evaporated to dryness as water is distilled off. Second, with further heating to above 260°C, the solid crystals of lithium nitrate melt and become a flowable liquid. Third, with heating of the liquid to above 600°C, the lithium nitrate decomposes into lithium oxide (lithium oxygen) with the emission of nitrogen oxides and some oxygen.

The first and second stages of heating (i.e., heating to above 260°C and preferably to about 400°C) can be carried out in any of a range of commercially available equipment, such as an insulated, covered tank or a series of similar tanks (tanks), each equipped with an agitator to maintain the solids in suspension. Such tanks can collectively hold a large inventory of molten lithium nitrate at a controlled temperature of about 400°C (e.g., with a residence time of at least 1 hour). The contents of the tanks can be maintained at this temperature by continuously circulating (e.g., via specialized pumps and piping) through a heat recovery unit. The heat recovery unit can function as a heater for the molten lithium nitrate. This heat recovery unit can take the form of a convection heat exchanger capable of extracting heat from hot flue gases exiting a lithium silicate (e.g., spodumene) calciner. During this heating stage, any remaining water contained in the feed can be flashed off as vapor (which can be directed to join the vapor and other volatiles from the previous and subsequent heating stages). Any insoluble solids can be retained in suspension by the agitator provided in the tanks.

In one embodiment, a suitable amount of hot molten lithium nitrate can be withdrawn and circulated through equipment items where the material being processed needs to be heated. These equipment items may include, in particular, the thermal drying of the leaching reactor product in the hollow flight screw conveyor described above. Similarly, molten lithium nitrate can be used to heat the filtrate/overflow from the washing and filtration steps (i.e., in the secondary solid-liquid separation stage, where this stage is installed immediately downstream of the leaching/mixing reactor).

In a second variation, the process may further comprise a first crystallization stage, in which the concentrated lithium nitrate solution from leaching (and any additional downstream purification) may be further concentrated and then crystallized to form relatively pure crystalline LiNO ₃ . The first crystallization stage may employ an evaporator/crystallizer.

However, before the first crystallization stage, and in the case of embodiments of the method that do not involve a primary solid-liquid separation stage immediately downstream of the leaching stage, the dried solid produced by distillation of excess nitric acid and water vapor can be mixed with the _separated solution of crystallized LiNO produced in the first crystallization stage, together with a controlled amount of process water. The mixing can occur in a treatment stage arranged after the drying stage (e.g., in one or more treatment vessels). In this mixing stage, a portion of the recycled LiOH _and /or _Li2CO3 product (e.g., as an aqueous solution/slurry) can also be added to neutralize the solution (i.e., to bring the resulting solution mixture close to pH neutrality) and to dissolve any soluble lithium values present in the dried solid. The mixing can be performed to form a slurry of lithium nitrate solution and the remaining insoluble solids.

The resulting crystalline LiNO ₃ from the first stage of crystallization (often referred to as "First Strike" by those familiar with crystallization) is typically in the form of a slurry and can be separated from the solution, for example, by centrifugation. On the other hand, the solution separated from the crystallized LiNO ₃ can be recycled to the first crystallization stage, wherein a portion of the solution is optionally treated to remove soluble impurities present therein, thereby preventing their build-up in the process.

In all process embodiments, a side-stream processing regime may be employed to prevent accumulation of soluble impurities in the process.

In the side stream treatment regime, in order to reject as much as practicable the alkali metal (sodium and potassium) nitrates from the system while minimizing the loss of lithium value from the overall process, a suitable proportion of the concentrated lithium nitrate solution retained after most of the lithium nitrate has crystallized therefrom may be withdrawn for further processing.

In one embodiment, the side stream can be cooled so that additional lithium nitrate will crystallize because the solubility of lithium nitrate in aqueous solution decreases sharply with decreasing temperature. Such a second stage of crystallization may be referred to as a "second capture" by those familiar with crystallization. The resulting crystal-rich slurry can be transferred to a specialized filter centrifuge similar to, but smaller than, the main lithium nitrate crystal separation centrifuge. Alternatively, a separation centrifuge can be operated intermittently and can be comparable in capacity to the separation centrifuge used in the first capture crystal centrifugation process. In either case, the resulting crystal-rich slurry can be placed in a stirred tank for temporary storage. Depending on their purity, the separated crystal mass (lumps) can be returned to the feed tank supplying the primary lithium nitrate crystallizer, where they can be redissolved and recrystallized to add to the main lithium nitrate crystal product. However, if sufficiently pure, the lithium nitrate from the second capture crystallization can simply be added to the product of the primary (first capture) crystallizer.

In the side stream treatment system, the liquid recovered from the centrifugation of the second captured crystal slurry can be treated with an appropriate amount of a soluble carbonate, which is relatively enriched in sodium and potassium. In one embodiment, sodium carbonate (i.e., soda ash) can be used. As is known, soda ash causes lithium to precipitate as insoluble lithium carbonate, leaving additional sodium nitrate/potassium nitrate in solution.

In one embodiment of the side stream treatment system, the concentrate from the second catch crystallizer may be maintained at a temperature exceeding 60°C, and optionally exceeding 80°C, to maximize precipitation of lithium carbonate, which becomes less soluble in aqueous solution with increasing temperature.

In one embodiment of the side stream treatment system, the lithium carbonate crystals can be filtered out of the liquid and washed with hot water. Depending on the quality and their purity, these crystals can be added to the final lithium carbonate product. In another embodiment, these crystals can be returned to the neutralization device to neutralize excess nitric acid. Although this amount will generally be small (i.e., this will ultimately depend on the composition of the lithium silicate concentrate (concentrates) originally entering the refining process), the remaining blend of sodium nitrate and potassium nitrate can be found to have value as a fertilizer. In this way, the process can be operated so that the loss of lithium value from the side stream purification can be very small (i.e., well below 1% of the total amount of lithium entering the refining (as lithium silicate mineral concentrate)).

In another embodiment of the side stream treatment system, ammonium carbonate can be used to precipitate lithium values as lithium carbonate. In this embodiment, the ammonium carbonate can be prepared using an aqueous ammonia solution (ammonia can also be used elsewhere in the process) to scrub a portion of the flue gas from the lithium silicate mineral concentrate calcination reactor. The barren liquor obtained from the lithium carbonate precipitation (which is primarily a mixture of ammonium nitrate, potassium nitrate, and sodium nitrate) is also valuable as a fertilizer and can at least aid in the revegetation of mining spoil heaps and tailings disposal sites.

The combined crystalline LiNO ₃ separated from the solution (i.e., from primary and secondary capture) can be subjected to a thermal treatment, for example, at a temperature that decomposes the LiNO ₃ into solid lithium oxide (lithium oxygen) Li ₂ O, i.e., a temperature above 600° C. During the thermal decomposition, a gas stream comprising nitrogen oxides can be generated. This stream can be sent to a nitric acid production unit to produce the nitric acid used in the process.

Prior to thermal decomposition of the crystallized LiNO ₃ , the crystallized LiNO ₃ can be melted, for example, in a heated holding vessel (i.e., similar to that described for the first variant of the method, in which lithium nitrate is not crystallized). As previously described, the LiNO ₃ can be maintained in a molten state by heat exchange with, for example, gaseous by-products resulting from the initial thermal activation treatment (e.g., calcination) of the silicate mineral. Also as previously described, a portion of the molten LiNO ₃ can be conveyed to be used as a heating medium in the drying stage (e.g., circulating through the hollow scraper of the drying conveyor) and possibly elsewhere in the apparatus, as described below.

The heat treatment may be performed using a roaster, such as a bubbling fluidized bed roaster, which is directly heated by air heated by the combustion of a fuel. In order to avoid contamination of the lithium nitrate/lithium oxide system with carbon dioxide (i.e., which would result in the formation of an undesirable intermediate, lithium carbonate, at this stage of the method), the heat treatment may include the combustion of a fuel that does not form carbon dioxide during combustion. For example, the fuel may include ammonia (i.e., anhydrous gaseous ammonia), which may be burned in excess air using equipment familiar to those skilled in the art of nitric acid manufacturing in the presence of a suitable catalyst (e.g., a platinum-rhodium matrix or mesh).

The combustion of ammonia in air in the presence of such a suitable catalyst has the following advantages in that additional nitrogen oxides (instead of nitrogen) are produced:

4NH ₃ +5O ₂ →4NO+6H ₂ O

These nitrogen oxides can be conveyed, along with the remainder of the flue gases (including that from the decomposition of lithium nitrate), to a nitric acid production unit, where they contribute to the production of additional nitric acid, to the extent (position) where they partially (if not completely) compensate for the losses of nitric acid that inevitably occur as it is recycled throughout the process. Here, it can be seen that the combustion of ammonia used to fuel the lithium nitrate decomposition process increases the production of nitric acid used in the leaching of silicate minerals.

In one embodiment, the method may further include a slaking stage, in which a controlled amount of typically pure water (e.g., distilled or demineralized water) may be added to the _Li2O produced during the thermal treatment. The amount added may be sufficient to convert the _Li2O to LiOH and dissolve all of the LiOH in solution.

In this embodiment, the slurry obtained from the aging stage (i.e., comprising LiOH in a nearly saturated solution) may be subjected to a second crystallization stage. In the second crystallization stage, the solution of lithium hydroxide may be concentrated and crystallized to form pure crystalline lithium hydroxide monohydrate (LiOH.H ₂ O). This may form a product of the process.

The second crystallization stage can provide additional purification of the lithium hydroxide. Further, the method and system can produce lithium hydroxide early in the overall flowsheet, in contrast to prior art methods where it must be produced from lithium carbonate via the more complex lithium counterpart of the known causticization reaction: _Li2CO3 +Ca(OH) ₂ → _2LiOH + _CaCO3 .

In one embodiment, the crystallized LiOH.H ₂ O can be separated from the solution, for example, by centrifugation. The separated crystallized LiOH.H ₂ O can be further processed as desired. The further processing can include (a) drying the crystals and optionally grinding them to a specified particle size. The further processing can also include (b) further heating the dried crystals to a temperature of at least 180° C. under reduced pressure. This can drive off the water of crystallization, thereby producing the anhydrous lithium hydroxide product of the process. The distilled water vapor can be collected and condensed to produce additional pure process water for use elsewhere in the process.

In one embodiment, the heating medium used for the concentration and crystallization of lithium hydroxide monohydrate, and if desired, the dehydration of the crystallized lithium hydroxide monohydrate and the removal of water of crystallization, can be molten lithium nitrate, such as that produced as described above.

In one embodiment, the lithium hydroxide solution separated from the crystallized LiOH.H ₂ O can be divided so that a first portion of the solution can be recycled to the leaching process for use in terminating the reaction of the silicate minerals with the nitric acid (i.e., for neutralizing the remaining nitric acid that has not been consumed in the leaching (digestion) stage). As previously described, this terminates the reaction of the silicate minerals with the nitric acid, preventing other impurities from being leached into the solution. This recycling is dependent on whether there is already a preheating step, in which excess nitric acid contained in the leach reactor product is neutralized directly, or is first heated to a point where most of the volatile materials (including nitric acid and water) are distilled off. As previously described, if most of the free nitric acid is distilled off rather than having to be neutralized, the amount of lithium hydroxide recycled can be much less.

In one embodiment, a second portion of the separated lithium hydroxide solution may be used to scrub carbon dioxide from process off-gas. For example, the second portion of the lithium hydroxide solution may be used to scrub carbon dioxide from flue gas generated during pre-treatment (e.g., calcination) of the lithium-containing silicate mineral prior to mixing the mineral with the nitric acid solution.

In this embodiment, lithium hydroxide solution is used to wash the carbon dioxide from flue gas etc. to produce a rich lithium carbonate logistics. A part thereof can be the lithium carbonate in solid form. Solid lithium carbonate can be used as the lithium carbonate product of the method from the logistics separation. For example, the lithium carbonate in solid form can be classified (for example using a hydrocyclone, and afterwards for example using a non-porous drum sedimentation type (solid-bowl decanter type) centrifuge or a rotary vacuum drum type (rotaryvacuum-drum) filter or a horizontal belt type (horizontal belt) vacuum filter to separate). The thicker fraction of the solid lithium carbonate classified and separated can form the lithium carbonate product of the method. Finer fraction can be recycled to reuse in the washing of the carbon dioxide from flue gas etc.

Therefore, the method disclosed herein also uses a lithium hydroxide-lithium carbonate system. Lithium hydroxide is moderately soluble in water, wherein the resulting solution has a strong affinity for carbon dioxide. Lithium hydroxide can react with carbon dioxide to form lithium carbonate. On the other hand, lithium carbonate is poorly soluble in water. Therefore, when a relatively concentrated lithium hydroxide solution is brought into contact with a gas stream containing carbon dioxide, the lithium carbonate formed and exceeding its solubility under the prevailing conditions will precipitate from the solution as crystals. The conditions under which the reaction can be carried out include a temperature in the system that is higher than the temperature at which the metastable salt lithium bicarbonate can be formed, i.e. a solution temperature of more than 60°C. Therefore, scrubbers for flue gases and the like are typically operated to a temperature above the solution temperature.

Also disclosed herein is a system for recovering lithium from a lithium-containing silicate mineral. The system includes a leaching reactor in which a mixture of a silicate mineral and nitric acid is subjected to conditions such that the lithium values in the silicate mineral are leached into an aqueous phase as lithium nitrate.

The system may further include various process devices as described above and in further detail below.

Also disclosed herein is a method for scrubbing carbon dioxide from a process off-gas. The method comprises passing the off-gas and a LiOH solution through a scrubbing vessel to allow the LiOH to react with the carbon dioxide to form Li ₂ CO ₃ .

The method may form part of the methods and systems described above and in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the method and system will now be described with reference to the following drawings which are intended to be exemplary only, in which:

FIG1 is a basic block diagram of a process for recovering lithium from lithium-bearing silicate minerals.

2 is a generalized block diagram of a method and system for recovering lithium as lithium hydroxide and lithium carbonate from lithium-bearing silicate minerals.

FIG3 is a schematic flow chart schematically illustrating a specific embodiment of the method and system.

FIG4 is a schematic flow chart schematically illustrating another specific embodiment of the method and system.

DETAILED DESCRIPTION

The following description discloses various embodiments of methods and systems for producing lithium hydroxide and lithium carbonate in various proportions from spodumene and other lithium-rich metal silicate ores using a recyclable nitric acid leach solution. The methods and systems can also capture carbon dioxide from flue gases generated elsewhere in the overall process while simultaneously producing lithium carbonate.

Referring first to FIG. 1 , and generally speaking, the methods disclosed herein may be viewed as comprising the following stages or steps:

1. Digestion (impregnation) of lithium-rich metal silicate minerals in nitric acid to produce pure lithium nitrate.

2. Decomposition of lithium nitrate into lithium oxide (lithium oxygen) and volatile substances including nitrogen oxides.

3. Convert lithium oxygen into pure lithium hydroxide.

4. Collect gases and volatiles from processes 1, 2 and 3 to reform the nitric acid used in 1.

Thus, the process can be efficient in that nitric acid can be recycled therein. Furthermore, when ammonia is used as the fuel in stage 2 of Figure 1, other nitrogen oxides can be produced to reform the nitric acid and thereby account for any losses of this acid elsewhere in the overall process.

Two variations of the method and system will be described in detail herein. The first variation will be described with respect to the embodiment of the method and system of FIG. 3 . The second variation will be described with respect to the embodiment of the method and system of FIG. 4 . However, it should be understood that the two variations of the method and system should not be construed as mutually exclusive, as aspects of one may be applicable to the other, may be combined, etc.

Before describing the specific embodiments of Figures 3 and 4, we first describe the generalized flow chart implementation of the method and system (i.e., covering two method variants) with respect to the block diagram of Figure 2. The method and system of Figure 2 can be considered to include the following stages:

Preprocessing stage 10 (Figure 2)

At this stage (indicated at 10 in FIG. 2 ), the silicate mineral is pre-treated to produce a treated (eg activated) silicate mineral.

For example, the pretreatment stage can be used to convert an alpha form of a mineral (e.g., alpha spodumene) into a beta form of a mineral (e.g., beta spodumene). Typically, the pretreatment stage includes a thermal pretreatment step, but it may include only non-thermal (e.g., mechanical) pretreatment steps.

When the pretreatment stage comprises a thermal pretreatment, the step of thermally treating the silicate mineral may result in a thermal phase change in the silicate mineral and/or result in the removal of volatile components of the silicate mineral.The thermal treatment step may be carried out in a first reactor such as a calciner or roaster.

When the thermal pretreatment step includes calcination, the calcination is typically carried out in the presence of air or oxygen but at a temperature below the melting point of the silicate mineral. The calcination can be carried out in various calciners, such as rotary kilns, fluidized bed calciners, flash calciners, transport calciners, or other suitable devices commonly known to those skilled in the art of high temperature processing of mineral materials.

The thermal pretreatment stage is operated to raise the temperature of the silicate mineral to a temperature well above ambient temperature. For example, the thermal pretreatment stage may raise the temperature of the silicate mineral to at least about 1000°C or 1100°C. As will be appreciated by those skilled in the art, the maximum temperature of the thermal treatment step is limited to a temperature that does not risk vitrification of the solids, which renders them resistant to leaching. For example, a calcination temperature of about 1050°C is required for the "decrepitation" of alpha spodumene to the more reactive beta form.

Once heat treated, the silicate mineral is in a more reactive (eg, beta) form. Consequently, the more reactive form is more susceptible to chemical attack, including by acids in the presence or absence of water.

As an option, the heat treatment step may include an additional non-thermal treatment step after the heat treatment. For example, this may involve fine or even ultrafine grinding. The additional fine grinding may be performed in a roller mill to form the silicate mineral into particles. The particle nature of the treated silicate mineral may provide a larger surface area for subsequent reactions. The size of the particles after grinding may be less than approximately 300, 200, 100 or 70 microns or even finer. The optimal size distribution may be determined based on the specific circumstances.

When the pretreatment stage comprises entirely non-thermal pretreatment steps, it may include mechanical treatment steps such as ultrafine grinding. For example, the entire conversion of, for example, α-spodumene to β-spodumene can be achieved entirely by mechanical means (i.e., without thermal treatment such as by calcination or roasting). The application of intense mechanical shear caused by high-intensity/ultrafine grinding in a stirred mill (e.g., Isamill ^™ ) can lead to the desired activation of, for example, α-spodumene.

Mixing and digestion stage 12 (Figure 2)

In this stage (indicated by reference numeral 12 in FIG. 2 ), the pretreated silicate mineral (β-spodumene in FIG. 1 ) and the mineral acid, in this case nitric acid, are mixed (“nitric acid” recycle in FIG. 1 ). The nitric acid can be produced by an on-site nitric acid production plant (stage 22 in FIG. 2 ). The mixing stage 12 can comprise a tank (e.g., a continuously stirred tank) or an in-line mixer. In the mixing stage 12, the silicate mineral is blended into or slurried with an aqueous phase containing nitric acid (slurrying). The resulting mixture/blend can take the form of a solution, slurry, or paste (“slurry/paste” in FIG. 2 ).

For example, the calcined beta spodumene solid can be slurried with concentrated nitric acid (at least 40%, preferably at least 60% acid) from a nitric acid plant (along with process water as needed) to obtain a suitable slurry or paste, for example containing 50-60% by weight of insoluble solids (i.e., insoluble solids of the calcined beta spodumene). The amount of nitric acid added is sufficient to convert all the lithium in the spodumene into lithium nitrate (stoichiometric amount); the excess can be as much as 40% or even 100% of the stoichiometric amount.

Depending on whether the mineral leach is a pressure leaching or not, the pressure of the slurry leaving the mixing stage 12 may be raised by a pump to the operating pressure of the digester/leaching reactor.

The mixture (solution, slurry or paste) of the pretreated silicate mineral and nitric acid ("slurry/paste") is now subjected to a digestion/leaching reaction. Reaction conditions are adjusted so that the mixture reacts rapidly to produce a slurry comprising, for example, lithium nitrate in an aqueous phase and remaining mineral solids.

As will be described in greater detail below, the reactor in which the treated silicate ore is reacted with nitric acid may comprise a digester, which may take the form of a single or continuous pressure vessel (e.g., a single or continuous autoclave), or a non-pressure vessel such as a tank or tower (e.g., a vertical hopper or silo reactor). The reactor may also take the form of one or more pipelines, or one or a series of stirred and pressurized vessels, or a plurality of interconnected and agitated chambers contained within a single pressure vessel, etc. For a particular project, the specific reactor configuration selected from the range may depend on the characteristics of the lithium-rich metal silicate ore.

In the digester/leaching reactor, concentrated nitric acid (i.e., in excess/stoichiometric excess) continuously, but more rapidly, reacts with the silicate minerals to ensure complete leaching of the lithium values in the silicate minerals into the aqueous phase as a soluble salt, lithium nitrate. The pH of the reactor contents can be controlled at this stage to be strongly acidic, preferably between pH 1 and pH 3, to maximize lithium leaching in the shortest possible time.

As mentioned above, the reaction can be carried out at ambient/atmospheric pressure. Alternatively, the reaction can be carried out at elevated pressures of at least 5 bar, possibly 10 bar, or even 20 bar. The reaction typically employs elevated temperatures >100°C, such as 120°C, possibly 160°C, or even up to 200°C, depending on the specific lithium-rich metal silicate ore being leached.

Termination and solids separation stage 14 (Figure 2)

In the termination and separation stage 14 of Figure 2, the reaction of nitric acid with the remaining mineral solids in the "Li-rich slurry" (Figure 2) is terminated, and the solid residue is separated. This is used to minimize the leaching of non-lithium values into the aqueous phase. The non-lithium values present in the silicate minerals may include aluminum, iron, nickel, chromium, manganese, cobalt, calcium, magnesium, sodium, potassium and phosphate ions. However, by adjusting the conditions in the termination sub-stage, any non-lithium values can be separated from the aqueous phase (e.g., precipitated, etc.) and returned to the solid residue (i.e., removed from the process as tailings).

termination

In one variation, the slurry can be neutralized (e.g., using a portion of the final product of the process/system, such as lithium hydroxide, lithium carbonate, etc.). The amount of neutralizing solution added to the product stream is controlled to bring the pH conditions to near neutral (i.e., between pH 6 and pH 7). This immediately results in the termination of all acid leaching activity.

In the terminator stage, in order to minimize the amount of lithium hydroxide and/or lithium carbonate that needs to be recycled, the majority of the nitric acid in excess relative to the amount consumed in converting the lithium value in the lithium-rich ore into lithium nitrate can be removed. In another variation, the removal is performed by heating and substantially drying the slurry. The dried cake resulting from the heating can be re-slurried and subsequently neutralized.

In either variation, heating and/or neutralization is used to terminate the reaction of the nitric acid and the mineral solids, thereby minimizing the extraction of non-lithium values.

It is known that nitric acid forms an azeotropic mixture with water, consisting of 68% nitric acid and 32% water. Therefore, when heated in the terminator stage, the mixture boils (i.e., distills) at about 120°C under atmospheric pressure to form a gas phase. The distilled gas phase can be collected and transferred to the nitric acid production unit (stage 22 in Figure 2). Distillation will also be added to the gas phase to remove excess water present in the stream compared to the water required to form the azeotropic mixture. It will be understood that if a higher pressure (e.g., 5 to 20 bar) is used in the digester/leaching reactor, the temperature at which these gas phases are formed will be higher than the temperature required if only atmospheric pressure is applied to the digester/leaching reactor (i.e., in accordance with the law that the boiling point of a solution increases under elevated pressure conditions).

The heating in the terminator stage can be carried out in a dryer (e.g., a hollow scraper conveyor). The dryer can be heated, for example, using molten lithium nitrate, an intermediate product of the process/system, as its heating medium (e.g., the molten lithium nitrate can be circulated through the hollow scrapers of the conveyor). The heating can be carried out stepwise to a temperature that is firstly sufficient to remove most of the excess nitric acid and water by evaporation to form a gas phase, and secondly sufficient to decompose any nitrates of aluminum, iron, and other base metals that are present as impurities in the metal silicate ore. The heating can produce a relatively dry solid cake, which can then be transferred to a neutralization vessel where it can be slurried with a solution/slurry of, for example, process water and a lithium hydroxide/lithium carbonate product solution to be neutralized to a pH of ~neutral.

The neutralization in the terminator stage can be carried out in a neutralization vessel, such as a continuous stirred tank reactor or a series of such reactors, in which the slurry is allowed sufficient time to completely neutralize the excess nitric acid.

However, when only neutralization is employed in the reaction termination substage, the outlet stream from the digestion/leaching substage can first be mixed with a neutralizing solution (e.g., recycled lithium hydroxide solution) in an in-line mixer before the mixture is passed to and enters the reactor/buffer vessel. When a pressure vessel is employed in the digestion/leaching substage, this passage can occur with minimal pressure drop. The pressure in the reactor/buffer vessel can be maintained and controlled (e.g., by compressed air from an air compressor).

Solid separation

In the solid separation sub-stage, lithium nitrate in an aqueous phase comprising primarily concentrated lithium nitrate solution is separated from a slurry comprising insoluble residues of the treated mineral (primarily silicon and aluminum values). The lithium nitrate can then be recovered from the solution, for example as crystallized _LiNO3 in the evaporation/crystallization sub-stage of stage 16 (see below).

In a variant (described in detail below with reference to FIG4 ), the insoluble residue of the treated mineral can be subjected to a primary solid-liquid separation sub-stage, such as filtration, before a termination sub-stage comprising neutralization of the mixture. The separated solid residue can be removed from the process as tailings.

After the termination sub-stage (i.e., after neutralization or drying), the solids separation sub-stage can separate the lithium-rich aqueous phase (as a clear solution) from the now depleted insoluble mineral residue concentrate. This solids separation sub-stage employs a separation device to separate and wash the insoluble solids from the slurry and then produces a depleted tailings in solid form that can be safely and permanently disposed of.

Removal of solids may be by filtration, resulting in a washed filter cake. More than one filter stage may be used (eg plate and frame filters, rotary vacuum drum filters, etc.).

In one variant (described in detail below with reference to FIG. 4 ), when a primary solid-liquid separation substage has been employed, and before the final filtration stage, any remaining solid residues in the process solution/slurry may first be subjected to a clarification stage (e.g., in one or more settling vessels). This ensures that the resulting clarified liquor is substantially free of solid residues, which is conveyed to the subsequent stages of the process. The solid underflow from the clarification stage may be processed separately, for example, by filtering and washing it, to produce a dewatered cake substantially free of soluble lithium values. The dewatered cake may be processed together with the washed solids from the primary solid-liquid separation substage.

In the solids separation sub-stage, when the digestion stage 12 is operated at elevated pressure, a plate and frame filter press equipped with cake washing facilities can be used; the pressure of the feed stream entering the filter can be close to the digester operating pressure, thereby avoiding the need to reduce the pressure of the stream through elaborate pressure-reducing equipment. At atmospheric pressure, filters such as rotary vacuum drum filters with cake washing facilities and horizontal belt filters with cake washing facilities can be used. The equipment ultimately selected for removing insoluble solid materials will depend on the characteristics of the solids present, such as whether they are free-draining. The washed filter cake can form a stable residue composed primarily of silica and alumina, plus some other insolubles (depending on the composition of the original mineral concentrate), but may also contain other silicate minerals, iron ore (primarily goethite), magnesite, limestone (calcite and aragonite), and ilmenite.

With the removal of insolubles, the soluble lithium cations, along with any other soluble cations and soluble anions (primarily nitrates), can be collected as a clear solution for use in other reactions. This solution can be referred to as a clear liquor or mother liquor. Other alkali metal (sodium and potassium) and even trace amounts of alkaline earth metal (calcium and magnesium) ions may also be present in the solution (if such metals were present in the original silicate mineral fed to the second reactor).

Lithium nitrate manufacturing stage 16 (Figure 2)

In this stage, shown in Figure 2 at 16, an intermediate lithium nitrate product is produced. The lithium nitrate may be produced as a clear concentrated solution (Figure 4), or may be produced in an evaporation and crystallization sub-stage (Figure 3).

In the evaporation/crystallization substage, the clarified liquor containing lithium nitrate in solution is subjected to evaporation and crystallization, wherein the lithium nitrate solution is concentrated by evaporation to produce lithium nitrate crystals. This substage may include a mechanical vapor recompression mechanism, wherein a vacuum pump reduces the pressure above the contents of the vessel until the aqueous phase begins to boil. The water vapor is compressed by the vacuum pump and returned to the shell side of a shell and tube heat exchanger (calandria) in the vessel as adiabatically heated steam. The condensed water is collected for reuse elsewhere in the process. Lithium nitrate is highly soluble in water, and its solubility increases rapidly with increasing temperature. Therefore, the evaporator/crystallizer includes a section in which the contents are further slowly cooled (e.g., in a spiral-type heat exchanger, followed by cooling by a cooling fluid (e.g., cooling water), followed by cooling by an air-cooled condenser, or by cooling by an evaporative cooling tower), so that more lithium nitrate crystallizes from the solution to form a thick crystal slurry.

The thick crystal slurry is then conveyed to a device for separating and dehydrating the lithium nitrate crystals from the crystallizer slurry and for returning the largely solids-free lithium nitrate solution to the crystallizer system. Such a device may include a centrifuge, such as a flat drum decanter, screen-bowl decanter, conical-screen, or pusher-screen type. The dehydrated crystal material is conveyed to the next stage (lithium hydroxide production), while the filtrate, concentrated lithium nitrate solution, is returned to the evaporator/crystallizer.

Lateral flow purification

The evaporation/crystallization substage may also include a device for treating a side stream (extracting steam) of the filtrate produced by a separation device for dehydrating the lithium nitrate crystals. The treatment device may include further crystallization of the lithium nitrate (returned to the process), leaving a solution enriched in sodium and potassium ions (whose concentrations will continue to increase unless these metals are removed from time to time using the treatment device) and only a minimum of lithium ions. The concentrated solution may then be treated with a soluble carbonate, such as sodium carbonate or ammonium carbonate. Ammonium carbonate can be prepared by reacting an aqueous ammonia solution with carbon dioxide present in flue gases (e.g., from the calcination stage). In either case, the addition of the soluble carbonate precipitates most of the remaining lithium values as insoluble lithium carbonate, which can then be recycled. The remaining solution is suitable for disposal or as a fertilizer due to its relatively high available nitrogen content and potassium value.

Lithium hydroxide manufacturing, stage 18 (Figure 2)

In this stage, shown at 18 in Figure 2, lithium nitrate from the lithium nitrate production stage 16 is converted into lithium oxide. During this conversion, the waste gases produced (comprising nitrogen dioxide and other nitrogen oxides, as well as oxygen) are collected and used to produce more nitric acid; i.e., they are transferred to the nitric acid production unit (stage 22 in Figure 2).

More specifically, the dehydrated lithium nitrate crystals from the separation device of stage 16 are transferred to a stirred and heated container (e.g., a covered, insulated/jacketed tank) where the lithium nitrate crystals are heated as they are added to the contents of the tank. The contents of the tank are maintained at a temperature sufficient to melt the lithium nitrate crystals (>250°C; at least up to 260°C and typically up to 400°C). The tank is partially filled with molten lithium nitrate so that when the molten lithium nitrate enters, the lithium nitrate crystals quickly melt and join the contents of the tank. The temperature of the contents of the tank can be maintained by continuously circulating the contents of the tank to and then through the tubes of a convection heater. These tubes can in turn be heated by hot flue gases exiting, for example, the calciner of the pretreatment stage 10 (i.e., the flue gases exiting the calciner can be at a temperature of ~800-900°C). The resulting heated molten salt can be returned to the tank.

Melting the lithium nitrate facilitates feeding it to the subsequent decomposition reactor, where the lithium nitrate can then be better decomposed to form lithium oxide. Furthermore, the molten lithium nitrate can be used as an efficient heat transfer medium (e.g., in the dryer (hollow scraper conveyor) of the termination sub-stage; and in the drying device (hollow scraper conveyor) of the lithium hydroxide evaporation/crystallization sub-stage).

The contents of the molten lithium nitrate salt tank are pumped to a lithium nitrate decomposition reactor where the molten lithium nitrate is further heated, either directly (e.g., by a stream of hot gas generated from the combustion of a fuel in air) or indirectly (where the hot gas stream can be generated, for example, by the combustion of natural gas or any other suitable clean fuel, including carbon-containing fuels, in air and does not come into contact with the lithium nitrate). The decomposition produces solid lithium oxide ( _Li2O ).

The decomposition reactor can take the form of a bubbling fluidized bed roaster, which burns by the combustion of fuel in air/oxygen. In this case, the fuel should preferably be carbon-free, such as hydrogen or ammonia, typically anhydrous ammonia. For both, ammonia has many advantages. These advantages include that it is relatively cheap and easy to transport and store. In addition, when it burns in air (relative to oxygen), in the presence of a suitable catalyst (such as platinum-rhodium alloy wire gauze), it forms additional nitrogen oxides, which will increase the manufacture of total nitric acid there when being transferred to a nitric acid plant. Therefore, ammonia can be used for partially or fully supplementing any nitric acid loss in the method.

The decomposition reactor operates at a temperature of at least 600°C, preferably 650°C and up to 750°C. At these temperatures, lithium nitrate decomposes to form a mixture of lithium oxides, which naturally form small pellets or granules within the fluidized bed in the reactor environment. The reactor emits a gas stream comprising nitrogen oxides of nitrogen dioxide and nitric oxide, along with some oxygen from the decomposition of nitrate ions. Some water vapor and additional nitric oxide and other nitrogen oxides from the combustion of ammonia, plus nitrogen from the combustion air and a depleted level of oxygen, may also be included in this gas stream.

Lithium hydroxide manufacturing stage 20 (Figure 2)

In this stage, shown at 20 in Figure 2, the lithium oxide from the decomposition reactor is first converted to lithium hydroxide by adding and mixing it with an appropriate amount of water. This can be done in a maturation vessel (e.g., a continuously stirred tank) to produce a concentrated solution of lithium hydroxide.

The concentrated solution of lithium hydroxide from the slaking vessel is then transferred to a second evaporator/crystallizer unit (e.g., also of the mechanical vapor recompression type). At this point, the solution is further concentrated, resulting in lithium hydroxide crystallizing in the solution and forming crystalline lithium hydroxide monohydrate. If desired, for example, to meet customer demand for lithium chemicals in the form of lithium hydroxide, the amount of lithium hydroxide monohydrate crystals produced (as a portion of all lithium hydroxide entering the crystallizer unit) can be controlled. The slurry of crystallized lithium hydroxide monohydrate obtained from the evaporator/crystallizer is then separated and dehydrated to produce high-purity lithium hydroxide monohydrate crystals from the remaining lithium hydroxide retained as an aqueous solution. The separation and dehydration device may include a centrifuge, such as a non-porous drum decanter, a sieve bowl decanter, or a continuous cone screen centrifuge, or it may include a propeller or vibrating screen centrifuge.

The lithium hydroxide production stage 20 may additionally include a device for drying and driving off the water of crystallization from the produced lithium hydroxide monohydrate crystals to produce a pure anhydrous lithium hydroxide product that can meet specific market specifications. The drying device may include a fully enclosed hollow scraper conveyor, wherein hot molten lithium nitrate can be circulated through the hollow scrapers. A stream of nitrogen is circulated through the gaps of the hollow scraper screw conveyor in a closed-loop arrangement, whereby the lithium hydroxide monohydrate crystals are ultimately heated to a temperature exceeding 160°C, which is sufficient to drive off the water of crystallization. The pure anhydrous lithium hydroxide obtained can then be ground and packaged as the product of the method/system.

The lithium hydroxide crystallization stage 20 may further include a device (e.g., a covered tank "LiOH solution reservoir" in FIG2 ) for collecting and containing the saturated lithium hydroxide solution (i.e., filtrate/centrifuge filtration) remaining after the lithium hydroxide monohydrate crystals have been removed. The filtrate/centrifuge filtration comprises a saturated lithium hydroxide aqueous solution, which is collected in the covered tank. A small amount of water (as well as other liquid streams) is added to dilute the contents of the tank so that there is no risk of continued crystallization of lithium hydroxide from the solution.

This solution is then conveyed in suitable amounts (e.g. pumped from the tank using a separate pump) for recirculation to the nitric acid distillation dryer and to neutralize the pH of any residual/excess/excess nitric acid in the digestion/leaching product in the terminator stage. The remainder of the solution may be conveyed to a flue gas scrubber (of the scrubbing stage 24) where it is used to adsorb/capture carbon dioxide contained in flue gases etc. (carbon dioxide etc. gas in the flue) by converting it into sparingly soluble lithium carbonate.

Nitric acid production stage 22 (Figure 2)

At this stage, indicated by reference numeral 22 in FIG. 2 , the offgases from the decomposition of the lithium nitrate can be conveyed to a "nitric acid plant." Excess nitric acid and water vapor distilled off in the distillation dryer (hollow scraper conveyor) of the terminator stage can also be conveyed to the nitric acid plant. The nitric acid plant can take the form of one or a series of absorption towers, such as those used in conventional nitric acid plants.

In the nitric acid plant (the operation of which will be familiar to those familiar with the commercial production of nitric acid by the Ostwald process), the off-gases and distillation vapors are absorbed in a recycle stream of a continuously cooled solution of nitric acid in water to produce more nitric acid suitable for recirculation to the digestion/leaching reactor. This produces a concentrated nitric acid solution (at least 40% acid and preferably at least 60% acid) suitable for use in the digestion/leaching reactor. The nitrogen oxides formed by the catalytic combustion of ammonia in air (in the lithium nitrate decomposition sub-stage) are added to the total amount of nitric acid produced, and in this way, nitric acid losses from the overall process (caused by, for example, incomplete washing of tailings or incomplete conversion of nitrogen oxides to nitric acid in the nitric acid plant) can be compensated.

Scrubber stage 24 (Figure 2)

In this stage, shown as reference numeral 24 in FIG2 , the filtered flue gas containing carbon dioxide is scrubbed with the remainder of the concentrated lithium hydroxide solution produced in the lithium hydroxide production stage 20. Although flue gases are primarily generated during the thermal treatment of silicate minerals, they can also be of external origin. The circulating and distributed solution absorbs/captures the carbon dioxide contained in the flue gas, etc., by converting it into insoluble lithium carbonate.

The flue gas scrubber can be in the form of a primarily hollow chamber (e.g., a tower) through which a concentrated lithium hydroxide solution is circulated and distributed via banks of sprays at a relatively high volumetric rate. The lithium hydroxide reacts with the carbon dioxide present in the flue gas and is converted into lithium carbonate in the process. Since the lithium carbonate is poorly soluble, most of it precipitates out of the solution, converting the circulating scrubbing medium into a lithium carbonate slurry in an aqueous phase rich in lithium hydroxide.

Lithium carbonate filtration stage 26 (Figure 2)

At this stage, indicated at 26 in FIG. 2 , the precipitated (“crude”) lithium carbonate crystals are classified in order to remove a portion of the precipitated lithium carbonate crystals continuously formed in the lithium hydroxide-rich slurry circulating through the flue gas scrubber.

More specifically, a slurry of lithium carbonate in a lithium hydroxide-rich aqueous phase is pumped through the reservoir of a hydrocyclone. The spigot product (underflow stream) of the hydrocyclone comprises a thick slurry of a coarser size fraction of lithium carbonate crystals, which can be further separated (i.e., dewatered and washed) from their associated solution in, for example, a centrifuge (e.g., a flat drum decanter centrifuge) or in a rotary drum vacuum filter apparatus.

The bulk (balance) of the liquid phase (lithium hydroxide solution) from the reservoir of the hydrocyclone (overflow stream) is recycled through the flue gas scrubber of the scrubbing stage 24 together with the finer lithium carbonate crystals and the solution separated from the thick slurry of coarser-sized lithium carbonate (i.e. as a result of dewatering and washing).

Controllers are fitted to hydrocyclones to adjust the diameter of their casings, thereby allowing the volumetric split between the casing and the overflow stream to be adjusted as required.

Lithium carbonate drying stage 28 (Figure 2)

At this stage, indicated at 28 in FIG. 2 , the coarser size fraction of the separated lithium carbonate crystals is then dried and packaged as a suitable (eg pure) lithium carbonate product of the process/system.

Other unit operations may be included in the overall process and system shown in FIG2 , consistent with good engineering practice, in particular, for the provision of services and utilities, efficient use of waste heat, conversion of water, and minimization of all waste streams. These may include bicarbonation and carbonation units, which are well known to those familiar with conventional methods for refining lithium-rich materials including brine, as shown in the dashed lines of FIG2 . These units may be justified if there is a market for particularly pure lithium nitrate.

Implementation of the First Method & System (Variant 1)

Referring now to FIG. 3 , a schematic depiction of an embodiment of a first specific method and system for recovering lithium from lithium-bearing silicate minerals is provided.

In the first method and system embodiment of Figure 3, alpha spodumene is fed as a filter cake containing an average of 10% by weight water to a first reactor in the form of a natural gas-fueled rotary kiln 1, operated at a temperature of about 1050°C, as required to "explode" the alpha spodumene into the more reactive beta form. Excessively high temperatures risk subjecting the solid to vitrification, which makes it resistant to leaching.

Most of the calcined beta spodumene product from the calciner 1 (partially cooled by the countercurrent flow of hot gases and solids through the rotary kiln) is conveyed to an air-swept dry grinding mill 2, such as a roller or table mill commonly used for grinding (pulverizing) coal and other relatively soft rocks such as limestone.

The hot combustion gases comprise the remainder of the calcined beta spodumene product from the calciner 1. These gases are passed through one or more convection-type molten salt heaters 3 to be heated to a temperature of about 400°C. A stream of molten lithium nitrate is used as a heat transfer medium in the heater 3. As described below, the molten lithium nitrate can be used at various locations throughout the plant. The heated gases are then passed to a waste heat boiler 4 to produce high pressure steam for use elsewhere in the process and (in one embodiment) for power generation. As a result, the hot combustion gases from the calciner 1 are partially cooled in the waste heat boiler 4. They are further cooled in the primary air heater 5, where some of their more sensible heat is transferred to the ambient air intended for use as combustion air in the calciner 1.

The cooled combustion gases from the primary air heater 5 are cleaned of the soot they carry (the finer fraction of the calcined beta spodumene) by passing them through a fabric filter station 6. The calcined beta spodumene solids removed at station 6 are pneumatically transferred (using air as a carrier) to join the main stream of calcined beta spodumene from the calciner 1 and then conveyed to the grinder 2. Using heated air from the air preheater 5, the ground calcined beta spodumene solids are conveyed to the storage of a dust cyclone 8, where they are separated from the air used to transport them. This now heated air is conveyed to the calciner 1 to serve as combustion air.

The densified underflow of the dust cyclone reservoir, the calcined beta spodumene solids, is passed to a mixing vessel in the form of an inline mixer 9 to be slurried with concentrated nitric acid from the nitric acid plant 7 and process water as required to achieve appropriate conditions, thereby forming a slurry or paste containing approximately 60% by weight of insoluble solids (calcined beta spodumene). The mixing vessel may also take the form of a continuously stirred tank reactor.

The amount of nitric acid added to the mixing vessel exceeds the amount of nitric acid required to convert all of the lithium in the spodumene into lithium nitrate (the stoichiometric amount). In this embodiment, the reaction between the nitric acid and the calcined spodumene is carried out at ambient pressure; other embodiments may utilize elevated pressure.

The slurry is then conveyed to the digestion reactor 10. The still hot calcined solids from the dust cyclone 8 transfer their heat to the slurry, heating it to the operating temperature of the reactor 10. In the reactor 10, under the prevailing conditions of ambient pressure and temperature of 100-120°C, the lithium values in the silicate mineral ore are leached according to the following reaction (1).

2LiAlSi ₂ O ₆ +2HNO ₃ →2LiNO ₃ +H ₂ O+Al ₂ O ₃ +4SiO ₂ (1)

β-spodumene, nitric acid, lithium nitrate

In practice, the temperature of the outlet stream from reactor 10 is at least as high as the temperature of its inlet, since reaction (1) is exothermic.

The outlet stream from the reactor 10 (which in this embodiment takes the form of a vertical closed silo) is conveyed to a solid heating system 11, which in this embodiment takes the form of a completely enclosed hollow scraper screw conveyor. The molten lithium carbonate is circulated through the hollow scrapers of the heating system 11, which also serves to convey the outlet stream from the reactor 10 along its length. During its passage, the outlet stream (paste) is gradually heated, initially to at least 100°C and preferably 120°C, whereupon water and nitric acid are evaporated; in most cases an azeotropic mixture consisting of 68% nitric acid and 32% water. Once the paste has thus been dried to a cake, the cake is further heated to a temperature exceeding 155°C by means of molten salts circulating through the hollow scrapers of the heating system/conveyor. At this temperature, any aluminum nitrate formed (derived from the reaction between nitric acid and the aluminum values in the spodumene ore) is decomposed:

Al(NO ₃ ) ₃ +3H ₂ O→Al(OH) ₃ +3HNO ₃ (2)

The nitric acid is evaporated, leaving behind solid aluminum hydroxide.

In this way, most of the free acid is driven off from the cake, so that the tendency of aluminum and other impurity base metals (such as ferric iron, nickel, cobalt and others) to continue leaching is greatly slowed down (if not completely stopped). The nitric acid-rich vapor from the reactor 10 and (partially) the heating system 11 is conveyed to the nitric acid plant 7.

The dried product from reactor 11 and the distilled water (condensate) are now slurried in a mixing/leaching tank 12 (which may be two or more tanks arranged in series). The amount of water added is sufficient to convert the contents into a pumpable slurry. An appropriate amount of lithium hydroxide solution, produced elsewhere in the process as described below, is also added to the contents. The strongly alkaline lithium hydroxide solution is used to neutralize any excess nitric acid that remains after the lithium values in the leached spodumene have been converted to lithium nitrate under the prevailing conditions according to reaction (1) and the excess nitric acid has been distilled off according to reaction (2). If the excess acid is not neutralized, it tends to continue to attack the now depleted spodumene, potentially resulting in an increase in the amount of aluminum, silicon and any base metals (any transition metals, including but not limited to chromium, manganese, iron, cobalt and nickel), and alkaline earth metals (particularly magnesium and calcium) that are leached and converted into soluble salts and thus present in a dissolved state in the aqueous phase. The amount of lithium hydroxide added is sufficient to raise the pH to near neutrality, i.e., to a pH between 6 and 7. The neutralization reaction can therefore be written as follows:

HNO ₃ +LiOH→LiNO ₃ +H ₂ O (3)

It can be seen from this that the product of the reaction is more lithium nitrate, as the lithium nitrate obtained by reaction (1) is added.

In the mixing/leaching tank 12, the highly soluble lithium nitrate present in the solid material from the reactor 11 is dissolved in water, which is mixed with it to form a slurry consisting of a concentrated lithium nitrate solution and lean solids (primarily those on the right side of reaction (1)).

The contents of the mixing/leaching tank 12 are pumped to a solid-liquid separation stage. As shown in Figure 3, this is in the form of a rotary drum vacuum filter 15, but it is also possible to use a horizontal belt vacuum filter. The filtration stage increases the solids concentration of the filter cake to a maximum of ~85% by weight, and with the aid of hot wash water, substantially all of the solubles (including all soluble lithium values) are washed out of the filter cake. The filtrate will therefore contain substantially all of the lithium values leached from the spodumene concentrate, but now as soluble lithium nitrate. The filter cake, which is substantially free of lithium and consists of stable silica, alumina and possibly other metal oxides present in much lower amounts, can be safely placed for long-term storage (i.e. tailings).

The filtrate (which may also be referred to as the mother liquor) flows (e.g., via pumps, piping, and storage tanks, not shown in FIG. 3 for clarity) into a unit in the form of an evaporator/crystallizer 13. In the embodiment shown in FIG. 3 , this vessel 13 is based on the principle of mechanical vapor recompression, wherein evaporation occurs at near-atmospheric pressure and the vapor is recompressed for reuse in heated tubes (not shown in FIG. 3 , inside the evaporator). The water vapor condensed in the tubes is condensed and collected for reuse as pure process water elsewhere in the process, particularly for recovering the lithium values in the filter cake formed in the dewatering filter 15. The other product is a slurry of lithium nitrate crystals (perhaps together with minor impurities also in solution) in a saturated lithium nitrate solution. During operation, this slurry is circulated through the evaporator/crystallizer 13. A portion of this slurry is withdrawn from the main circulation stream and sent to a centrifuge 14, such as a solid drum decanter, a bowl decanter, a continuous cone screen, or a propeller or vibrating screen type centrifuge. The mass flow rate at which the crystal slurry is withdrawn and fed to the dewatering centrifuge is set to match the mass rate of crystal production (as filter cake from centrifuge 14 ) with the rate at which new lithium nitrate solution is fed to the evaporator/crystallizer 13 .

Then, there is a continuous accumulation (albeit at a slow rate) of other soluble salts, particularly sodium and potassium ions, in the aqueous phase of the crystal slurry circulating through the evaporator/crystallizer 13. Not shown in FIG3 , but still part of the process, is further cooling of a portion of the filtrate from the centrifuge to a relatively low temperature, for example, below 40° C. Upon such cooling, most of the lithium nitrate remaining in solution precipitates in the form of crystals to form a slurry of lithium crystals together with sodium and potassium ions and possible other trace ions in solution in the aqueous phase, which now contains significantly less lithium nitrate in solution. A small centrifuge, such as a non-porous drum decanter centrifuge, is used to separate the crystals from the aqueous phase. The separated crystals are returned to the evaporator/crystallizer 13, while the filtrate, along with its loaded sodium and potassium ions, is collected in a tank and periodically removed for benign disposal.

Further processing to recover additional lithium value may be advisable. For example, by mimicking the method widely used to recover lithium value from lithium-rich brines found in the "dry salt lakes" of the "lithium triangle" of South America, sodium carbonate (soda ash) may be added as a solution to precipitate most of the residual lithium as insoluble lithium carbonate:

2LiNO ₃ +Na ₂ CO ₃ →Li ₂ CO ₃ +2NaNO ₃ (4)

The lithium carbonate precipitate can be removed by conventional solid-liquid separation methods, such as by vacuum filtration and washing with hot water. Depending on its quality and purity, this lithium carbonate can be added to the final lithium carbonate product, or it can be recycled to the digestion reactor 10. Sodium nitrate is simply added to the sodium nitrate already present in the barren solution.

The dehydrated mass of lithium nitrate crystals from the centrifuge 14 is conveyed to a heated storage vessel in the form of a molten lithium nitrate storage tank 16. The contents of the tank are maintained at approximately 400°C, a temperature at which the salt is a transparent, colorless, highly mobile liquid. The lithium nitrate crystals that enter the storage tank 16 and then fall into the molten lithium nitrate melt immediately to join the molten lithium nitrate mass. The contents of the tank are maintained at this temperature by continuously circulating the contents of the tank through a molten lithium nitrate heater 3 (for clarity, FIG3 does not show the interconnection of pumps and piping). Also from the storage tank 16, the molten lithium nitrate is pumped or otherwise distributed as needed to supply the thermal energy necessary for the proper operation of the dryer 11 and potentially to other parts of the overall process, including, if necessary, preheating the feed to the digestion reactor 10.

The molten lithium nitrate is transferred as needed to a lithium oxide roaster 18 where the crystals are rapidly heated to a temperature exceeding 600° C., ideally 700° C. The lithium oxide roaster can be a bubbling fluidized bed reactor comprising two vertically stacked fluidized beds. The upper bed is formed of pellets or granules of lithium oxide and is operated at a temperature of about 700° C. As soon as the molten lithium nitrate sprayed or otherwise distributed on the bubbling bed solids contacts the solids, they are rapidly heated and, in the process, decompose to form lithium oxide according to reaction (5) with the release of nitrogen dioxide and oxygen:

4LiNO ₃ →2Li ₂ O+4NO ₂ +O ₂ (5)

In FIG3 , the roaster 18 is directly fired with a fuel that does not form carbon dioxide when burned; in this embodiment, the fuel is anhydrous ammonia. It is burned in air over a heated platinum-rhodium mesh catalyst, whereupon the combustion products, namely water vapor, nitric oxide, and oxygen-depleted air, mix with nitrogen dioxide and oxygen from the decomposition of lithium nitrate. The reaction for the catalytic combustion of ammonia in air can be written as follows:

4NH ₃ +5O ₂ →4NO+6H ₂ O (6)

The nitric oxide, after cooling (e.g., by a convection gas cooler, which is used to heat the boiler feed water (BFW in Figure 2)), combines with the free oxygen present in the combustion gases to form nitrogen dioxide:

2NO+O ₂ →2NO ₂ (7)

The nitrogen dioxide, together with the nitrogen dioxide formed by the decomposition of the lithium nitrate according to reaction (4), plus water and free oxygen, plus the off-gases from the dryer 11, are conveyed to the nitric acid plant, where they all combine to form nitric acid:

H ₂ O+2NO ₂ +O ₂ →2HNO ₃ (8)

The nitric acid plant 7 can be sourced from a company experienced in the design and construction of nitric acid plants starting from ammonia. However, most of the infrastructure required for the catalytic combustion of ammonia to form nitrogen oxides in the same manner as that presented by reaction (6) will no longer be required (except in a much more abridged form - i.e., the lithium oxide roaster 18 will be fueled only for the combustion of the required ammonia).

Nitric acid plant 7 comprises one or more columns arranged in series, each equipped with sieve plates and bubble caps, through which a cooled mixture of nitric acid and water is continuously circulated. It rapidly absorbs nitrogen dioxide (from the combustion of the waste gas) and oxygen to form more nitric acid, the concentration of which, under steady-state conditions, can be, for example, 40% acid or higher (a preferred product is at least 60% nitric acid). Nitric acid plant 7 may also include a separate fractionation column, in which the relatively dilute nitric acid produced in the plant is separated into two streams: a concentrated acid stream (nominally 68% nitric acid) and an aqueous stream containing virtually no nitric acid, which can be used as process water elsewhere in the plant. The acid is withdrawn at an appropriate rate and transferred to a storage tank (not shown in FIG. 3 ), from which it can be pumped to digester 10 as needed.

The process disclosed herein is characterized in that the unavoidable loss of nitric acid from the overall system is partially or completely compensated by the additional nitric acid formed by using ammonia as the heat source required for reaction (6).

The lithium oxide (lithium hydroxide) (pellets formed in the upper fluidized bed of the lithium oxide roaster 18) are partially cooled by arranging them to fall into the bottom fluidized bed through which the combustion air necessary for reaction (6) is passed. The partially cooled pellets of pure lithium oxide (lithium hydroxide) are further cooled and quenched in a mixing vessel/agitated storage tank 19. In this regard, a controlled volume of distilled water is added to the storage tank 19 (including, for example, condensate from the evaporator/crystallizer 13), which is then converted to lithium hydroxide:

Li ₂ O+H ₂ O→2LiOH (9)

The process is highly exothermic, so circulating cooling water is used to continuously cool the vessel. The amount of water added to the storage tank 19 is sufficient to dissolve all of the lithium nitrate and convert it to hydroxide according to reaction (9). The lithium hydroxide completely goes into solution to form a nearly saturated lithium hydroxide solution.

The nearly saturated lithium hydroxide solution is then transferred to another crystallizer 20. In the lithium hydroxide crystallizer 20 (which is also of the mechanical vapor recompression type in the embodiment shown in FIG3 ), some water vapor is evaporated, which results in the formation of some crystals of lithium hydroxide monohydrate, LiOH.H ₂ O, in the suspension of the now saturated lithium hydroxide solution. The amount of water evaporated is carefully controlled so that the amount of lithium hydroxide monohydrate crystals produced matches the amount of lithium hydroxide required to meet the specific contracted demand for lithium hydroxide. An appropriate proportion of the slurry is removed from the crystallizer 20 and sent to a centrifuge 21. In the embodiment of FIG3 , the centrifuge is of the continuous conical screen type, or it can be of the non-porous drum settling type, or the sieve bowl settling type, or the propeller or vibrating screen type.

The solid crystal cake produced by centrifugation can be further processed (equipment not shown in Figure 3). First, it can be dried and then packaged for dispatch. Alternatively, it can be further heated to drive off the water of crystallization. In both cases, the heating can be carried out in a completely enclosed hollow scraper screw conveyor (similar to the hollow scraper screw conveyor used for dryer 11), in which the molten lithium nitrate circulates through the hollow scrapers. A nitrogen stream circulates through the interstices of the hollow scraper screw conveyor in a closed-circuit arrangement. The lithium hydroxide monohydrate is finally heated to a temperature exceeding 160°C, which is sufficient to drive off the water of crystallization:

LiOH.H ₂ O→LiOH+H ₂ O (10)

The pure, anhydrous lithium hydroxide can then be ground and packaged, as required by the terms of the sales contract to the customer. A circulating stream of nitrogen is used to transport the water vapor through a condenser (which can be of the indirect contact type); its purpose is to cool the gas/vapor mixture to a temperature sufficient to condense most of the water vapor, producing pure water that can be used elsewhere in the process. The now less humid nitrogen is returned to the hollow scraped screw dryer/conveyor, and the process is repeated.

The centrifuge filtrate, a saturated lithium hydroxide aqueous solution, is collected in another covered tank 22, to which some process water (and other liquid streams also entering the tank 22) is added to dilute the solution so that there is no risk of continued crystallization of lithium hydroxide from the solution. The lithium hydroxide solution is pumped from this tank 22 using a separate pump as follows:

to the leach tank 22 in an amount sufficient to neutralize any remaining excess nitric acid in the product stream from the dryer 11 according to reaction (3), i.e., to raise the pH in the leach tank 12 to a value between 6 and 7;

- The remainder, to the flue gas scrubber 30 , using the pump 23 , where it is used to absorb the carbon dioxide contained in the flue gases.

The reaction between the relatively concentrated lithium hydroxide solution circulating through scrubber 10 can be written as:

2LiOH+CO ₂ →Li ₂ CO ₃ +H ₂ O (11)

The temperature of the circulating slurry (i.e., maintained in circulation by pump 23) is maintained at a temperature above 60°C, preferably 80°C, to ensure that lithium bicarbonate is not formed. Lithium carbonate is much less soluble than lithium hydroxide, so most of the lithium carbonate formed according to reaction (10) precipitates out of the solution as pure lithium carbonate crystals. These lithium carbonate crystals are circulated through the scrubber 30 as a component of the lithium carbonate slurry in the lithium hydroxide solution (plus some lithium carbonate also in solution). During such circulation, the lithium carbonate crystals tend to grow in size. As the slurry circulates, it passes through a classifying device, in FIG3 the reservoir of a hydrocyclone 24, which sorts out the larger crystals and concentrates them to thicken the slurry into a casing product. The remaining slurry, which includes most of the solution and the finer lithium carbonate crystals, is returned to the scrubber 10 through a receiving tank 22.

The casing product is conveyed to a dewatering device 25, which in one embodiment is a non-porous rotary drum decanter centrifuge (or in the embodiment shown in FIG3 , a vacuum drum filter). The resulting solid filter cake of pure lithium carbonate is dried, ground, and packaged according to the terms of sale to the customer.

Implementation Method of the Second Method & System (Variant 2)

Referring now to FIG. 4 , a schematic depiction of an embodiment of a second specific method and system for recovering lithium from lithium-bearing silicate minerals is provided.

In Figure 4 , the same reference numerals are used to denote devices that are the same or similar as in Figure 3 and are therefore not described in detail. With respect to Figure 4 , only the following method and system variants are described here.

In the variation shown in Figure 4, from the digestion reactor 10, the leached product material is now directly conveyed to the tailings filtration unit 15, wherein the insoluble solids are washed to remove their solubles (primarily lithium nitrate plus excess nitric acid) and other soluble nitrates of other metals (i.e., as previously described). The insoluble solids are then dewatered and transported away as an inert tailings.

The now clarified, still highly acidic filtrate from unit 15 is sent to leach tank 12 (which can be a series of two or more tanks). Here, its excess acidity is directly neutralized by the recycled lithium hydroxide solution from the lithium hydroxide dewatering centrifuge 21. This tends to precipitate aluminum, iron and other transition metals, magnesium and calcium as insoluble oxides and hydroxides, all of which will settle. The resulting slurry can be sent to a clarifier 26, where the relatively small volume of thickened underflow is filtered separately in a filtration unit 29 (wherein the filter cake is also washed and dehydrated to collect its lithium value in the filtrate). The filter cake is then directly added to the main tailings product from the tailings filtration unit 15, and the filtrate is recycled to the feed stream of the clarifier 26.

The concentrated lithium nitrate solution is then transferred (e.g. by a pump, not shown) to the molten LiNO3 _tank 16, the contents of which are maintained at a temperature of 400°C. A special cyclone mixer/blender 27 is provided to allow safe mixing of the relatively cool stream containing water and the hot 400°C molten lithium carbonate.

The method and system of FIG4 removes solids from the leach product material early after digestion. This avoids the drying stage 11 and crystallization stage 13 of FIG3 . As a result, lithium and nitric acid losses may be greater, but investment and operating costs may be lower. In addition, the lithium nitrate may not be as high-purity.

In contrast, the method and system of FIG3 can minimize the formation of additional lithium nitrate. This is because most of the excess nitric acid is removed within the dryer 11 without having to be neutralized by the recycled basic lithium compound. This, in turn, can minimize the amount of chemical intermediate (lithium nitrate) recycled through the entire process, as occurs using the method and system of FIG4.

In other respects, the method and system of FIG. 4 are the same as the method and system of FIG. 3 .

Other variants

It will be appreciated that the characteristics of spodumene, whether in its raw (α) or activated (β) form, may vary to the extent that variations of the above-described methods and systems may be suitable.

Other unit operations may be included in the whole method, which conform to excellent engineering practices, particularly, for providing services and utilities, effectively utilizing waste heat, converting water, and minimizing all waste streams. When digestion stage 10 is pressurized, the reactor may take the form of one or more autoclaves. Alternatively, it may take the form of one or more vertical silos, fed at the top, and discharged at the bottom. In order to avoid the demand for a complicated depressurization system, the contents of the digestion stage may be directly transferred to the drying and termination stages also designed to operate under high pressure, and may finally be transferred to the plate and frame filter press (for separating and cleaning tailings solids) that also uses pressurized feed operation.

_CO2 scrubbing

The methods and systems described herein may also be considered to include a discrete process for scrubbing carbon dioxide from process off-gases. Such off-gases may originate from a variety of sources, including, in particular, from flue gases generated during pre-treatment (e.g., thermal treatment, such as by calcining or roasting) of lithium-containing silicate minerals.

The separate process may include passing the waste gas and LiOH solution through a scrubbing vessel. The scrubbing vessel may be configured and operated to allow the LiOH to react with carbon dioxide to form _{Li2CO3 . Li2CO3} is another valuable form of lithium that is suitable for use as a _raw material, for example, in battery manufacturing.

The LiOH solution may be a concentrated solution, such as obtained from controlled maturation of a lithium oxide intermediate material. The lithium oxide intermediate material may in turn be produced by heat treating a lithium nitrate crystalline material produced by leaching a lithium-containing silicate mineral with a solution of nitric acid.

The LiOH solution can be passed countercurrently through a scrubbing vessel, such as tower 30 of Figures 3 & 4. For example, the LiOH solution can be sprayed downwardly into the incoming waste gas stream. Tower 30 can include intermediate beds, plates, screens, etc., which help promote the reaction between the _CO2- containing waste gas and the LiOH solution. The temperature of the lithium hydroxide solution can be maintained above 60°C, optionally above 80°C, to reduce or prevent the formation of lithium bicarbonate.

The LiOH solution may take the form of a slurry including a relatively fine fraction of _{Li2CO3 crystals} therein. _The relatively fine fraction of _Li2CO3 may help seed the formation of _{Li2CO3 crystals} . The relatively fine fraction of _Li2CO3 crystals may be prepared by fractionating the slurry (e.g., using a hydrocyclone such as stage ₂₄ in Figures 3 & 4) to produce an overflow stream for feeding/returning to the scrubber 30. From this, the slurry may be circulated through the scrubbing vessel.

The fractionation (ie, the casing product underflow from the hydrocyclone 24) also produces a stream comprising a relatively coarse fraction of _{Li2CO3 crystals} , which may be separated (eg, at stage 25 in Figures 3 and 4 ₎ to form the lithium carbonate ( _Li2CO3 ) product of the discrete process.

In the appended claims and in the preceding description, unless the context indicates otherwise due to language expression or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. specifying the presence of stated features but not excluding the presence or addition of other features.

Claims (23)

1. A method for recovering lithium from lithium-containing silicate minerals, the method comprising: Mix the silicate minerals and nitric acid; The mixture is subjected to a leaching process under conditions that allow lithium in the silicate mineral to leach into the aqueous phase as lithium nitrate. Lithium nitrate is separated from the aqueous phase by crystallizing lithium nitrate. The separated lithium nitrate is subjected to heat treatment at a temperature that causes lithium nitrate to decompose into solid lithium oxide and produce a gas stream including nitrogen oxides. The gas stream containing nitrogen oxides is conveyed to the nitric acid manufacturing stage, where nitric acid is produced for reuse in the leaching process. 2. The method of claim 1, wherein the conditions of the leaching process include increasing the temperature and/or pressure of the leaching process to accelerate the leaching of lithium as lithium nitrate into the aqueous phase. 3. The method of claim 1, wherein the conditions of the leaching process are controlled in such a manner that non-lithium materials present in the silicate mineral tend not to be leached into the aqueous phase. 4. The method of claim 1, wherein the leaching process further comprises a controlled time for reacting the silicate mineral in a stoichiometric excess of nitric acid, wherein the controlled time is terminated by: (i) Neutralize the remaining free nitric acid; or (ii) Heat the leaching product to distill the excess nitric acid along with water as steam. 5. The method according to claim 4, wherein in (i) the remaining free nitric acid is neutralized by recycling a portion of an alkaline lithium compound produced as part of a method for recovering lithium, wherein the recycled alkaline lithium compound comprises one or _more of _Li₂O , LiOH and _Li₂CO₃ . 6. The method of claim 4, wherein in (i), the leaching product material obtained from the leaching process is first conveyed to a solid-liquid separation stage, wherein the solids in the leaching product material are separated from a solution comprising lithium nitrate, wherein the solution is then conveyed to a neutralization stage, and wherein the solids are separated from the process as tailings. 7. The method of claim 4, wherein stage (ii) heating includes a drying stage in which excess nitric acid and water vapor are distilled off as steam. 8. The method of claim 7, wherein the heating medium for the drying stage comprises molten lithium nitrate produced in a subsequent stage of the method. 9. The method of claim 7, wherein the distilled nitric acid and water vapor generated in the drying stage are collected and conveyed to the nitric acid manufacturing stage to produce nitric acid. 10. The method of claim 7, further comprising a crystallization stage in which a solution comprising lithium nitrate is concentrated and crystallized to form crystalline _LiNO3 . 11. The method of claim 10, wherein the leached lithium nitrate solution is separated from the residue of insoluble solids of the silicate mineral before crystallizing _LiNO3 . 12. The method of claim 11, wherein in (ii), a solution separate from the crystalline _LiNO3 produced in the crystallization stage and additional process water are used to reform the dried solids produced by distilling excess nitric acid and water into a slurry. 13. The method of claim 12, wherein the crystalline _LiNO3 is separated from the solution, and wherein the solution separated from the crystalline _LiNO3 is transferred to a container to be mixed with the dried solid, the dried solid being produced by distilling off the excess nitric acid. 14. The method of claim 13, wherein the crystalline _LiNO3 is melted, wherein optionally a portion of the molten _LiNO3 is conveyed as a heating medium in the drying stage of (ii). 15. The method of claim 14, wherein the molten _LiNO3 is conveyed to a heat treatment stage, wherein the temperature of the molten _LiNO3 is increased to cause the _LiNO3 to decompose into solid _Li2O and to generate a gas stream comprising nitrogen oxides. 16. The method of claim 15, wherein the heat treatment stage comprises burning ammonia in excess air and in the presence of a suitable catalyst, wherein the resulting gas stream comprising nitrogen oxides is conveyed to a nitric acid manufacturing stage to produce nitric acid for reuse in the leaching process. 17. The method of claim 15, further comprising a aging stage in which a controlled amount of pure water is added to the _Li₂O generated in the heat treatment stage, the amount of pure water added being sufficient to convert the _Li₂O into LiOH and dissolve all of the LiOH in the solution. 18. The method of claim 17, wherein the pure water comprises distilled water. 19. The method according to claim 17, wherein, in a further crystallization stage, the lithium hydroxide solution is concentrated and crystallized to form pure crystalline lithium hydroxide monohydrate LiOH· _H₂O , and wherein the crystalline LiOH· _H₂O is separated from the solution. 20. The method of claim 19, wherein the crystalline LiOH· _H₂O is further treated by: (a) Dry crystals; and (b) The dried crystals are further heated under reduced pressure to a temperature of at least 180°C to remove the water of crystallization, thereby forming an anhydrous lithium hydroxide product, and the distilled water vapor is collected and condensed to produce additional pure process water. 21. The method of claim 19, wherein the lithium hydroxide solution separated from the crystallized lithium hydroxide monohydrate is dispensed such that: A first portion of the solution is recycled to the leaching process to terminate the reaction between the silicate minerals and nitric acid, and the lithium hydroxide solution neutralizes any remaining or residual free nitric acid; and The second portion of the solution is used in the washing of carbon dioxide from process waste gas. 22. The method of claim 21, wherein the second portion of the solution is used for washing with carbon dioxide derived from: Flue gas generated during the pretreatment of the minerals prior to mixing lithium-containing silicate minerals with nitric acid solution; and/or The flue gas that can be generated during the heat treatment of the lithium nitrate intermediate in this method; and The process involves washing carbon dioxide from flue gas with a lithium hydroxide solution to generate a lithium carbonate stream, a portion of which is in solid form. This solid lithium carbonate can be separated from the stream as a lithium carbonate product of the method. 23. A system for recovering lithium from lithium-containing silicate minerals, the system comprising: A leaching reactor in which the mixture of silicate minerals and nitric acid is subjected to conditions that allow lithium in the silicate minerals to leach as lithium nitrate into the aqueous phase. In the separation stage, lithium nitrate is separated from the aqueous phase by crystallizing lithium nitrate. In the heat treatment stage, the separated lithium nitrate is subjected to heat treatment at a temperature that causes lithium nitrate to decompose into solid lithium oxide and generate a gas stream including nitrogen oxides. In the nitric acid manufacturing stage, a gas stream comprising nitrogen oxides is conveyed to the nitric acid manufacturing stage, and nitric acid is manufactured in the nitric acid manufacturing stage for reuse in the leaching process.