Method and system for lithium production

Abstract

A method and system for producing a lithium product from a solution comprising lithium nitrate is disclosed. The lithium nitrate-containing solution can be obtained by reacting a lithium-containing metal silicate with nitric acid. The methods and systems include subjecting the lithium nitrate-containing solution to a first heat treatment process (in one or more heated vessels) in which water and nitric acid, when present, are removed, and thereby heating the resulting lithium nitrate-rich crystal slurry to produce a molten liquid. The methods and systems also include passing the molten liquid to a second heat treatment process (in a further heated vessel) in which the molten liquid is heated to substantially decompose lithium nitrate into lithium oxide.

Description

[0001] This application is a divisional application of the Chinese patent application with application number 202280093238.6 (application date: 2022 / 12 / 19, invention name: Method and system for lithium production). Technical Field

[0002] A method and system for producing a lithium product from a solution containing lithium nitrate is disclosed. The solution containing lithium nitrate is generally produced by a method of extracting lithium from a lithium silicate ore using nitric acid. The lithium product can also be converted into lithium chemical products, particularly lithium carbonate and lithium hydroxide, according to the needs of the user. Lithium metal can also be produced. Background Art

[0003] Lithium is widely found throughout the Earth's crust, with an average concentration of about 20 parts per million, making it less abundant than other valuable metals such as iron and aluminum, but far more abundant than precious metals such as gold and platinum. One of the fastest growing end uses for lithium is in the battery industry. The use of lithium-ion batteries for electric vehicles (EVs) is expected to continue to drive the lithium market as demand for EVs continues to increase. In addition, demand for lithium is also gradually increasing in other applications including energy storage systems and as an alloying metal. The end result is that in order for lithium supply to keep up with growing demand, sustainable methods for producing lithium are needed.

[0004] Recovery of lithium from seawater is unlikely to be economically viable due to its very low concentration (less than 0.2 parts per million by weight), even though seawater contains more than 200 billion tons of lithium.

[0005] At present, lithium produced from brine accounts for most of the lithium produced in the world. Most of these brines originate from South American salt lakes (salars) in the so-called "lithium triangle" region extending across Argentina, Bolivia and Chile. However, there are several disadvantages in the lithium market's heavy reliance on South American brines as the main source of lithium. These disadvantages include environmental challenges and uncertainty in the amount of economically recoverable lithium contained in these brines. In addition, when using solar evaporation as a stage for recovering lithium from brines, the processing time is very long (up to several years), which means that these methods need time to respond to fluctuations in lithium demand.

[0006] Increasingly, the world's lithium supply is met by extracting lithium from lithium-bearing hard-rock silicate minerals. Traditional routes to recover lithium from hard-rock silicate minerals are expensive, complex, hazardous and environmentally challenging. Until recently, this meant that there was very little interest in pursuing these sources of lithium.

[0007] The inventors have invented (WO 2017 / 106925) an improved method for recovering lithium from silicate minerals using nitric acid, providing an improvement over the prior art that has been in use for more than half a century. The relevant contents of WO 2017 / 106925 are incorporated into the present disclosure by cross-reference. The method of WO 2017 / 106925 provides a "closed" method in which the main chemical used to extract lithium value from metal silicates, namely nitric acid, can be recovered for reuse in extracting additional lithium value from metal silicates. WO 2017 / 106925 describes how lithium nitrate formed by reacting metal silicates with nitric acid is purified and further converted into lithium products (i.e. lithium carbonate and lithium hydroxide) for lithium battery production. WO 2017 / 106925 also describes how the lithium oxide intermediate product is converted (i.e. reduced) to lithium metal. However, in the process of WO 2017 / 106925, the purification of lithium nitrate involves subjecting a lithium nitrate solution to a crystallization stage, wherein intermediate lithium nitrate crystals are produced. This intermediate is a substantially pure lithium nitrate crystal product, which is then further processed into lithium hydroxide. Crystallization is an energy intensive process and does not necessarily provide any additional benefit in terms of the desired purity of the final product for its intended end use.

[0008] Other considerations in lithium production include the geographic location of the source of the mined ore, the type of processing plant, consumer requirements, and the associated costs of transporting lithium between the source, plant, and consumer. For example, it is less economical to transport lithium in the form of ore than in the form of oxide, in terms of lithium. In addition, it is more economical to transport lithium in the form of oxide than in the form of hydroxide or carbonate, in terms of lithium. Dividing the process into multiple stages so that part of the process is carried out near the source of the ore and the rest is carried out near or by the battery plant can provide a means to minimize the cost of lithium transportation. For example, the ore can be mined and processed into a lithium oxide product in a processing plant near the mine. The lithium oxide can then be transported to the battery plant for further processing into hydroxides, etc.

[0009] The above references to background art do not constitute an admission that such art forms part of the common knowledge and / or general knowledge of a person skilled in the art. The above references are also not intended to limit the application of the methods and systems disclosed herein. Summary of the invention

[0010] The present invention discloses a method for producing a lithium product from a solution containing lithium nitrate in a first aspect. The solution containing lithium nitrate can be obtained by reacting a metal silicate containing lithium with nitric acid. For example, the solution containing lithium nitrate can be obtained by the method disclosed in WO 2017 / 106925. In this article, the term "lithium product" can include lithium nitrate, lithium oxide, lithium carbonate, lithium hydroxide and lithium metal (and other products containing lithium).

[0011] The method may include subjecting the solution comprising lithium nitrate to a first heat treatment process. In the first heat treatment process, water and nitric acid (when present) may be removed. The resulting lithium nitrate-rich crystal slurry may be heated to produce a molten liquid. The heating to produce the molten liquid may be performed in a separate vessel / stage.

[0012] The method may further include conveying the molten liquid to a second thermal treatment process. In the second thermal treatment process, the molten liquid may be heated to substantially decompose the lithium nitrate into lithium oxide. As described above, the lithium oxide may be the "lithium product" produced by the method, or the lithium oxide may be further processed to produce, for example, lithium carbonate, lithium hydroxide and / or lithium metal (among other products).

[0013] The method and system disclosed herein have a significant advantage over the prior art in that battery-grade lithium hydroxide and commercial-grade lithium carbonate can be produced from a solution containing lithium nitrate using a relatively simple process. For example, the method and system can have fewer purification steps than previous methods known in the prior art (including the method disclosed by the applicant in WO 2017 / 106925), thereby reducing capital / operating costs and operational complexity.

[0014] In addition, the relative simplicity allows the method and system as disclosed herein to be installed near existing lithium mines and concentrators. For example, the method and system can be "modularized / containerized", as explained in more detail below. The facilities for the method and system can be owned and operated by the same owner / operator as the mine / concentrator, allowing them to add value to their initial products. The flexibility of the method and system can further allow the owner / operator to terminate the method at a stage where, for example, lithium oxide is the final product. Since lithium oxide contains a high weight percentage of lithium, transporting lithium oxide products ensures even greater reductions in freight costs than transporting more familiar final lithium products (hydroxides / carbonates). The lithium oxide can be sold directly to, for example, a battery plant, which can convert less pure oxides into high-purity lithium hydroxide in its own facilities as needed. Alternatively, the owner / operator may decide to process high-purity lithium hydroxide (and carbonates) on-site by simply adding additional equipment (e.g., a lithium hydroxide crystallization tank).

[0015] In some embodiments, during the second thermal treatment process, the lithium oxide produced during the decomposition of the lithium nitrate can be allowed to float to the top of the molten liquid, thereby generating an overflow of lithium oxide and lithium nitrate slurry and an underflow of lithium nitrate liquid. Such a flotation stage can facilitate the separation of lithium nitrate from the lithium oxide.

[0016] In this regard, in some embodiments, the method may further include conveying the lithium oxide and lithium nitrate slurry to a separation stage. In the separation stage, the lithium nitrate may be substantially removed from the lithium oxide. For example, the separation stage may produce lithium oxide crystals.

[0017] For example, in some embodiments, the separation stage may include a liquid removal stage in which the lithium nitrate is substantially removed from the lithium oxide solid as a liquid, thereby producing lithium oxide crystals. For example, the liquid removal stage may include a screw press.

[0018] In some embodiments, the lithium oxide crystals may be conveyed to a maturation stage. In the maturation stage, a controlled amount of water may be added to the lithium oxide crystals, thereby producing a solution comprising lithium hydroxide monohydrate. The solution comprising lithium hydroxide monohydrate may constitute the product of the method.

[0019] In some embodiments, the solution comprising lithium hydroxide may be subjected to a crystallization stage (e.g., to produce lithium hydroxide monohydrate in a purer form). In the crystallization stage, the solution of lithium hydroxide may be concentrated and crystallized to form crystalline lithium hydroxide monohydrate.

[0020] In some embodiments, the crystallized lithium hydroxide monohydrate can be separated from the solution to produce a crystallized lithium hydroxide product that is substantially free of liquid. For example, the separation can be achieved by centrifugation. The resulting lithium hydroxide monohydrate crystals can constitute the product of the method.

[0021] In some embodiments, the lithium nitrate solution removed from the lithium oxide during the separation stage may be collected for further processing. For example, the further processing may include adding a carbonate compound to the lithium nitrate solution (e.g., during a lithium carbonate precipitation stage) such that the lithium may react with the carbonate to produce lithium carbonate. The further processing may be performed under conditions that promote precipitation of lithium carbonate.

[0022] In some embodiments, the lithium carbonate precipitate can be separated from the solution. For example, the separation can be achieved by using a dewatering centrifuge. The lithium carbonate precipitate can constitute the product of the method.

[0023] In some embodiments, the solution separated from the lithium carbonate precipitate can be recycled as an alkaline solution. The alkaline solution can be used in the process stage of reacting the lithium-containing metal silicate with nitric acid. The alkaline solution can be used as a neutralizing solution.

[0024] In some embodiments, the carbonate compound may include sodium carbonate and / or potassium carbonate, such as may be produced by adding sodium carbonate or potassium carbonate as the carbonate compound.

[0025] In some embodiments, the solution separated from the crystallized lithium hydroxide monohydrate can be sent to the carbonate compound for further processing. For example, the separated solution can be sent to the lithium carbonate precipitation stage.

[0026] In some embodiments, the first heat treatment process may include a two-stage heat treatment. The two-stage heat treatment may include a first heating stage, in which the solution containing lithium nitrate is heated to near boiling to remove water and nitric acid (when present). The two-stage heat treatment may further include a second heating stage, in which the resulting near-boiling lithium nitrate slurry is heated to a temperature of about 400° C. to melt the lithium nitrate in the slurry. Typically, these stages (e.g., containers) are separated because they operate at different temperatures. However, these stages can be performed in a single container operated at different conditions.

[0027] In some embodiments, the first thermal treatment process can produce a vapor stream comprising water and nitric acid (when present). The vapor stream can be collected and condensed. For example, the condensed vapor stream can provide process water and / or a dilute nitric acid solution.

[0028] In some embodiments, the temperature within the molten lithium nitrate in the second thermal treatment process may be about 600°C. Such a temperature may cause the molten lithium nitrate to decompose into lithium oxide and nitrogen oxides. The gaseous stream containing nitrogen oxides may be collected and, for example, conveyed to a nitric acid production process. The nitric acid may be recycled for use in extracting more lithium from the lithium silicate mineral feedstock to the process.

[0029] In some embodiments, the solution containing lithium nitrate that is sent to the first thermal treatment process may be substantially free of contaminants. For example, the lithium nitrate solution may have been treated according to a purification process as described in WO 2017 / 106925.

[0030] For example, the solution containing lithium nitrate that is conveyed to the first thermal treatment process may be produced by an extraction process in which a lithium-containing metal silicate is reacted with nitric acid. The extraction process may include mixing the silicate mineral with the nitric acid. The extraction process may also include subjecting the mixture to a leaching process, the conditions of the leaching process being such that the lithium values ​​in the silicate mineral are leached as lithium nitrate into an aqueous phase.

[0031] The extraction method may further comprise passing the leachate solution to a solid-liquid separation stage to remove remaining solids.

[0032] The extraction method may further include optionally heating the separated leachate to distill off water and nitric acid as a vapor stream. The water and nitric acid vapor stream may be collected and used for regeneration of nitric acid. The nitric acid may be recycled for extracting more lithium from the lithium silicate mineral raw material.

[0033] The extraction method may further comprise subjecting the leachate to a series of reaction conditions such that contaminants may be substantially precipitated from the leachate. The extraction method may ultimately comprise passing the solution through a filtration stage to remove the precipitate, thereby producing a substantially contaminant-free solution comprising lithium nitrate, which may then be passed to the first thermal treatment process.

[0034] In some embodiments, the nitric acid produced from the vapor stream of the second thermal treatment process may form at least a portion of the nitric acid that is mixed with the silicate mineral in the extraction process.

[0035] In some embodiments, the lithium-containing metal silicate may include spodumene (LiAlSi2O6). The lithium-containing metal silicate may also include any of a series of other lithium-containing silicate minerals, including but not limited to petalite LiAlSi4O 10 ; eucryptite LiAlSiO4; lithium phosphate aluminum (Li,Na)AlPO4(F,OH); and various minerals in the mica family, including lepidolite K(Li,Al,Rb)3(Al,Si)4O 10 (F,OH)2 and lithium iron mica KLiFeAl(AlSi3)O 10 (OH, F)2. The lithium-containing metal silicate may also include the lithium-boron mineral jaddariite LiNaSiB3O7(OH), or also written as Na2O.Li2O.(SiO2) 2. (B2O3)3.H2O.

[0036] In a second aspect, a system for producing a lithium product from a solution containing lithium nitrate is also disclosed herein. As described above, the lithium product produced by the system may include lithium nitrate, lithium oxide, lithium carbonate, lithium hydroxide, and lithium metal (and other products containing lithium). The system includes one or more heated containers in which the solution containing lithium nitrate can be heated to the extent that water and nitric acid (when present) can be removed as vapor and the resulting lithium nitrate-rich crystal slurry can form a molten liquid.

[0037] The system may also include a further heated vessel in which the molten liquid may be heated to substantially decompose the lithium nitrate into lithium oxide.

[0038] In some embodiments, the further heated container can be configured as a flotation tank. In the further heated container thus configured, as the lithium nitrate decomposes, the lithium oxide can float, thereby producing an overflow of lithium oxide and lithium nitrate slurry and an underflow of lithium nitrate. The overflow can be conveyed to a solid-liquid separation stage.

[0039] In some embodiments, the further heated container can produce a gaseous stream comprising oxides of nitrogen. The further heated container can also be sealed to prevent such gases from escaping into the atmosphere. In some embodiments, the further heated container can be heated indirectly, such as by electric induction heating.

[0040] In some embodiments, the solid-liquid separation stage may include a screw press in which the lithium oxide may be substantially separated from the lithium nitrate.

[0041] In some embodiments, the one or more heated containers may include an evaporator. In the evaporator, the solution containing lithium nitrate may be heated to cause water and nitric acid (when present) to evaporate as a vapor, thereby producing a supersaturated solution of lithium nitrate.

[0042] In some embodiments, the one or more heated containers may further include a melting vessel to which the saturated to supersaturated lithium nitrate solution may be transferred. In the melting vessel, the supersaturated solution of lithium nitrate may be heated to melt the lithium nitrate. The melting vessel may be heated directly or indirectly, such as by electric induction heating.

[0043] As described above, the one or more heated vessels may include an evaporator separate from the melting vessel. However, in some embodiments, evaporation and melting may be performed in a single vessel operated at different conditions.

[0044] In some embodiments, the system may further include a nitric acid plant. The gaseous stream containing nitrogen oxides produced primarily by the further heated vessel may be conveyed to the nitric acid plant to produce nitric acid. As described above, the nitric acid may be recycled for use in extracting more lithium from the lithium silicate mineral feedstock.

[0045] In some embodiments, a condenser may be arranged to collect and condense vapor from the one or more heated vessels and, when present, from the evaporator.

[0046] In some embodiments, the system may further comprise an apparatus arranged to produce a crystalline lithium hydroxide product from the lithium oxide produced in the further heated vessel.

[0047] In some embodiments, the system may further include an apparatus arranged to produce a lithium carbonate product from the lithium nitrate-containing underflow. The apparatus may also be arranged to produce a lithium carbonate product from the lithium nitrate produced in the solid-liquid separation stage. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Embodiments of the method and system will now be described, by way of example only, with reference to the following drawings, in which:

[0049] Figure 1 is a generalized flow chart of an overall process for recovering lithium products from lithium-bearing silicate minerals.

[0050] Figure 2 is a schematic diagram of a process flow diagram of a process for recovering lithium from a solution comprising lithium nitrate.

[0051] Figure 3 It is a schematic diagram of a method flow chart of the first heat treatment step.

[0052] Figure 4 It is a schematic diagram of a method flow chart of the second heat treatment step.

[0053] Figure 5 is a schematic diagram of a process flow diagram for a nitric acid production facility.

[0054] Figure 6 is a schematic diagram of a process flow diagram for a lithium hydroxide production area for converting lithium oxide into lithium hydroxide monohydrate.

[0055] Figure 7 is a schematic diagram of a process flow diagram for a lithium carbonate production area for converting lithium nitrate into lithium carbonate.

[0056] Figure 8 and Fig. 9is a schematic diagram of a process flow diagram for producing purified lithium nitrate from lithium-containing silicate minerals, wherein the purified lithium nitrate is conveyed to Figure 2 method. DETAILED DESCRIPTION

[0057] In the following detailed description, reference is made to the accompanying drawings which form a part of the detailed description. The illustrative embodiments described in the detailed description, depicted in the accompanying drawings, and defined in the claims are not intended to be limiting. Other embodiments may be utilized and other changes may be made without departing from the spirit or scope of the subject matter presented. It will be readily understood that aspects of the present disclosure as generally described herein and shown in the accompanying drawings may be arranged, substituted, combined, separated, and designed in a variety of different configurations, all of which are contemplated in the present disclosure.

[0058] The following description discloses embodiments of a method and system for recovering a lithium product from a solution containing lithium nitrate. At least three lithium products (lithium oxide, lithium hydroxide, and lithium carbonate) and nitric acid can be produced from a solution containing lithium nitrate. The method and system are designed to be attached to an upstream process in which lithium is leached from a lithium-containing metal silicate to form a solution containing lithium nitrate. Figure 8 and Fig. 9 An example of this upstream process is described. The method and system has an advantage over the prior art in that no crystallization of lithium nitrate is required prior to producing the lithium product.

[0059] Figure 1 is a conceptual flow diagram of a method 10 for lithium extraction, shown in simple block diagram form, in which a solution 22 containing lithium nitrate is produced from a lithium-containing metal silicate 12 and nitric acid 18. Figure 8 and Fig. 9 The method 10 for lithium extraction and production of a purified lithium nitrate solution 22 is illustrated and described in greater detail.

[0060] Additional to the lithium extraction process 10 is a process 20 for recovering lithium product from the purified lithium nitrate solution 22. Figures 2 to 7 The method 20 is described in more detail.

[0061] Typically, the method 10 for extracting lithium from lithium-containing metal silicates using nitric acid will be the first stage of a method as outlined in the method described by the applicant in WO 2017 / 106925 (the relevant contents of which are incorporated herein by reference). In this regard, after the initial purification of the solution containing lithium nitrate, the solution is conveyed to the first heat treatment stage of the method 20, i.e., not to the crystallization tank of WO 2017 / 106925.

[0062] However, it should be understood that a solution containing lithium nitrate can be produced by other means. For example, a solution containing lithium nitrate can be produced by chemically treating a salt lake brine containing lithium using the method disclosed in the applicant's co-pending patent WO 2020 / 107074 (the relevant contents of which are incorporated herein by reference).

[0063] It should be understood that Figures 1 to 9 Methods 10 and 20 presented in the drawings are modularized / containerized, whereby each module / container can define one or more different stages of the overall method. In addition, each module / container can be largely manufactured and assembled in a factory environment, and can then be transported to the new lithium refinery site and assembled there quickly and conveniently. This represents an attractive option because lithium ore refineries are often located in remote areas (e.g., near their host mines and concentrators). In remote areas, refinery construction costs are typically much higher than in more densely populated areas. In addition, in remote areas, securing qualified operating personnel is more challenging and expensive; and construction time may be delayed longer due to inclement weather. On the other hand, the advantages of building a refinery adjacent to or otherwise close to a host lithium ore mine and concentrator are obvious, at least for Figures 1 to 9 This is true for methods 10 and 20 of the invention. For example, this uniquely and substantially eliminates the need to transport large quantities of expensive and hazardous chemicals from mines / mineral processing plants to remote processing locations, and also eliminates the need to transport the largely worthless chemical byproducts of said chemical use to suitable harmless disposal sites. Similarly, Figures 1 to 9 The only residue of both Process 10 and Process 20 is the leached spodumene concentrate which is an inert, free-draining, finely divided solid which can be conveniently returned to the main mine where it can be blended with the typically significantly larger amounts of beneficiation waste that is also produced.

[0064] For example, at pilot scale, a modular / containerized plant may include up to four such modules. The process equipment may be mounted on three skids, each with the floor area of ​​a standard "20-foot" (TEU) freight container. The TEU size may be such that the height of all process equipment is less than 2.8 meters, allowing for road transport. The equipment may be secured to an acid-resistant FRP base / floor. The chassis may form a bank to contain any spills and leaks. The floor may be reinforced to withstand the work mass and normal stresses of equipment operation.

[0065] Modules / containers can be assembled in sequence. This can, for example, allow upstream processes (e.g. Figure 8 and Fig. 9The first module / container could even be ready within three months of the decision to proceed with construction, and the other modules / containers could follow in sequence (e.g. at 4-6 week intervals), so that the whole process could be demonstrated within 6 months of the decision to proceed with construction.

[0066] Production of purified lithium nitrate-rich bull leachate (PLS)

[0067] As outlined in WO 2017 / 106925, and as Figure 1 , Figure 8 and Fig. 9 As further described in detail in Figures 2 to 7 The purified lithium nitrate solution 22 used in the method 20 is first activated by activating the lithium-containing metal silicate 12 (such as α spodumene), such as by calcination, to produce mainly β spodumene. β spodumene ( Figure 8 ) is formed for Figure 8 and Fig. 9 The starting material 800 for the method shown in FIG.

[0068] Digestion of spodumene in nitric acid (module 1 of the pilot scale plant)

[0069] Beta spodumene 800 is fed to a feed mixer 802 where it is mixed with a stream of concentrated nitric acid 804, nominally 50%, but ranging between 28% and 68%, and in excess (e.g. this is in Figure 1 804 may be produced in a nitric acid production plant 50 (described below).

[0070] Once the batch is mixed in feed mixer 802, the contents of the feed mixer are transferred to feed storage tank 806, which takes the form of a covered polypropylene tank equipped with an agitator to prevent separation of the feed material. In tank 806, the contents are mixed and maintained at ambient temperature (e.g., about 25° C.). In process 10, tank 806 can be used to provide a flow "buffer" whereby its contents can be intermittently topped up but then continuously fed to digestion reactor 820 where the beta spodumene is "digested," i.e., leached, by nitric acid.

[0071] In the digestion reactor 820, the mixture of nitric acid and beta spodumene is subjected to elevated temperatures and pressures to release lithium from the beta spodumene matrix and form a solution containing lithium nitrate and other impurities. Other contaminants present in the beta spodumene that may be leached with the lithium to form part of the solution may include iron, aluminum, silica, potassium, and sodium.

[0072] The contents of tank 806 are pumped from the tank to digestion reactor 820 via a suitable reactor feed pump 808. Pump 808 may include a peristaltic pump equipped with a variable speed drive to regulate the flow of β spodumene / nitric acid slurry to digestion reactor 820. Pump 808 typically pressurizes the slurry to 6-8 bar gauge and forces the slurry through a regenerative spiral heat exchanger 810 (a suitable spiral heat exchanger is a titanium spiral heat exchanger supplied by Alfa Laval). In heat exchanger 810, the hot slurry stream 812 discharged from digestion reactor 820 preheats the β spodumene / nitric acid slurry so that a temperature close to 180°C maximum can be reached when the slurry is transferred to digestion reactor 820. An auxiliary electric heater 814 may be provided. Typically, electric heater 814 is used only during startup and process instability. In any case, heat exchanger 810 and optional electric heater 814 are combined to increase the slurry feed temperature entering digestion reactor 820 to 180°C maximum.

[0073] As depicted digestion reactor 820 comprises a three-compartment autoclave (and is substantially made of titanium). During normal operation, approximately one-third of the total volume of reactor 820 is dedicated to the gas phase. Autoclave digestion reactor 820 can also be operated with significantly lower liquid phase: gas phase volume to allow exploration of lower residence time. The nominal residence time of slurry in autoclave digestion reactor 820 is at most 60 minutes. Correct filling of reactor 820 is ensured by continuously monitoring the quality of operation of the reactor, and in the case of its deviation from the set point, the speed of reactor feed pump 808 is adjusted accordingly.

[0074] In operation of digestion reactor 820, the concentration of free nitric acid in the aqueous phase of the reactor contents moving along its length gradually decreases, descending from its feed end. Operation of digestion reactor 820 is entirely continuous. During operation, excess water vapor and nitric acid are distilled off as stream 809, which can be sent, for example, to nitric acid plant 50 for regeneration of concentrated nitric acid for reuse in digestion reactor 820. Additionally or alternatively, stream 809 can be condensed to provide (e.g., weak acid) process water, the use of which is referred to hereinafter.

[0075] The reaction time for leaching of lithium values ​​in the digestion reactor 820 is short. For example, extraction of lithium values ​​from calcined spodumene concentrate is typically greater than 95% within 15 minutes, as ensured by using a significant excess of nitric acid. Alternatively, the digestion reactor 820 can be operated with a longer reaction time to promote secondary reactions, such as autohydrolysis of ferric nitrate. Notably, under conditions marked by a large excess of nitric acid, the equilibrium for autohydrolysis of ferric nitrate is (ideally) maintained on the left:

[0076] Fe(NO3)3+3H2O→Fe(OH)3+3HNO3

[0077] However, depending on how much nitric acid is added beyond the stoichiometric amount required to leach lithium values ​​from the spodumene concentrate, and how much of this excess acid is used to leach other metals (particularly iron and aluminum) into solution as nitrates, the total free nitric acid concentration may fall to a point where the equilibrium in the above reaction may begin to shift to the right, resulting in the formation of, in particular, iron hydroxide precipitates, even when there is still significant free nitric acid in solution.

[0078] The hot slurry stream 812 exiting the digestion reactor 820 is rapidly counter-currently cooled to below 100° C. by the incoming digestion reactor feed slurry in the regenerative spiral heat exchanger 810. This cooling further prevents excess nitric acid from digesting the impurity-metal values ​​in the leached spodumene.

[0079] The slurry leaving the spiral heat exchanger 810 flows through a pressure reducing control valve 816 to reduce its pressure to ambient pressure and then flows into a covered and stirred filter feed tank 818. The effective volume of the feed tank 818 is sufficient to accommodate the flow of the reaction slurry for up to two hours under the operating parameters of the process. The partially cooled ambient pressure reaction slurry is pumped from the filter feed tank 818 to a tailings filter press in the form of a plate and frame filter press 824 via a tailings filter feed pump 822.

[0080] At the beginning of each filtration cycle, the filter feed pump 822 is turned on to fill the plate and frame filter press 824. Once the back pressure in the filter press 824 rises to be substantially equal to the back pressure in the input line of the filter feed pump 822, the filling is established to be complete, whereupon the filter feed pump is turned off. Using the plate and frame filter press 824, the solids present in the partially cooled product slurry from the digestion reactor 820 are filtered to form a filter cake in the filter press.

[0081] Next, the solenoid valve 826 that receives hot washing water 828 is opened for the desired washing time to wash the filter cake. The amount of washing water used will be set according to the degree of required filter cake washing, but washing water of at least the same quality as the filter cake is usually used. When washing is completed, the washing water supply valve 826 is closed, and the electromagnetically controlled air flow valve 830 is opened to allow compressed air 832 to be sent to the filter press 824, thereby blowing off excess moisture from the filter cake. A variety of control schemes can be followed, but a preferred filtering method is to continue washing and / or air drying cycles until enough slurry is accumulated in the filter feed tank 818 for the next complete filtering cycle. At the end of the final drying cycle, the air flow valve 830 is closed, and the filter press 824 is opened for discharging the filter cake. The low-moisture filter cake (leached spodumene concentrate) is discharged from the filter cloth to be collected in a hopper or other receiving container, thereby forming a barren residue / tailings stream 834. This can be disposed of subsequently (for example, by using it as a component for producing cement). At least a portion of wash water stream 828 may also be recovered for use elsewhere in the process.

[0082] The filtrate from filter press 824 is periodically discharged as a pregnant leachate (PLS) stream 836, which is a substantially clear solution containing lithium nitrate, aluminum nitrate and lesser amounts of iron, alkali metals sodium and potassium and calcium and even lesser amounts of a range of other metal nitrates, as well as nitric acid, resulting in a pH below zero. Stream 836 is collected in a crude PLS storage tank 838 (e.g., another polypropylene tank equipped with a dust cover). The stream is periodically transferred from tank 838 to the purification stage of the process (see below) by a crude PLS transfer pump 840. Fig. 9 described).

[0083] Purification of the rich leachate (Module 2)

[0084] Reference now Fig. 9 , a crude PLS (pregnant leachate) 900 containing lithium nitrate and other impurities is pumped to a series of purification and neutralization stages via a transfer pump 840. In these stages, reagents are added to promote the precipitation of base metals and to neutralize residual nitric acid. The solid precipitate is removed by conveying the solution and precipitate to a solid-liquid separation stage. The final result of these purification and neutralization stages is a substantially contaminant-free solution 22 containing lithium nitrate. The solution 22 is also substantially free of nitric acid. The typical concentration of remaining contaminants in a solution containing lithium nitrate and the suitability of the solution as a feed to the method 20 are also described in more detail below with reference to Example 1.

[0085] As described above, the filtrate 838 (i.e., crude PLS) from the tailings filter press 824 is pumped from the crude PLS storage tank 838 by the transfer pump 840, and the filtrate is a hot (near boiling) aqueous solution of the following substances: lithium nitrate; together with the nitrates of other metals also leached from the mineral by the nitric acid: aluminum, some residual iron and other transition metals, calcium, magnesium, sodium, potassium and phosphate ions; plus the remaining free nitric acid in excess of the amount required to convert the various cations leached from the concentrate to nitrates; together with the wash water added in the tailings filter press 824.

[0086] This hot crude PLS 900 enters an electrically heated PLS evaporator 904 (i.e., heated via an electrically heated jacket 902), where most of the volatiles (i.e., water and nitric acid) are boiled off as stream 906. The PLS evaporator 904 (which in a commercial scale plant may be a multiple effect evaporator consisting of three or more effects, or a multistage flash system having 12 or more stages) is made large enough to accommodate up to one hour's throughput of the concentrate to allow time for the hydrolysis of most of the aluminum and ferric iron values ​​to proceed to substantial completion, as described below.

[0087] The PLS evaporator system 904 is operated to the point where only a saturated solution of primarily lithium nitrate plus crystals thereof remains, and the solution and crystals form a dense but pumpable slurry. Even though the evaporation is carried out essentially at ambient pressure, the temperature in the final saturated solution may exceed 180°C, after which the slurry is no longer pumpable, because the nitrate solution is highly concentrated, even saturated, resulting in a significant increase in its boiling point. Test work has shown that before this point is reached, almost all of the free nitric acid is boiled off due to the high deliquescent nature of lithium nitrate: the strong attraction of lithium nitrate to water allows nitric acid to be preferentially boiled off. In practice, the operation of the PLS evaporator 904 is such that the concentration of solubles in the PLS does not advance beyond the point where it is no longer a pumpable slurry, nor significantly beyond the point where lithium nitrate crystals begin to form (e.g., when the temperature of the boiled liquor reaches about 180°C).

[0088] The combined nitric acid-rich vapor phase 906, which is removed by boiling off at a temperature of about 120° C. in the PLS evaporator, flows to the nitric acid production stage 50 (located, for example, in module 3, as described below) at near ambient pressure. Embodiments involving evaporation systems (such as multiple-effect evaporation or multi-stage flash evaporation) can result in the production of not just one nitric acid-rich vapor phase, but several such phases, which can range from essentially pure water vapor to concentrated nitric acid vapor. These separated phases can be condensed alone or in combination with other phases and recycled to various parts of the overall process 10 and process 20, including direct recycling to the feed mixer 802, recycling to the nitric acid plant 50, or recycling to multiple storage tanks each for storing process water or nitric acid solutions of varying concentrations.

[0089] Stripping of the majority of the unreacted (excess) nitric acid from the liquid phase in the evaporator system 904 reduces the pH concentration of the PLS (i.e., increases the pH), but only to about pH 1-2. This allows the iron and aluminum values ​​extracted from the spodumene concentrate and absorbed into solution as nitrates by the nitric acid in the digestion reactor 820 to largely autoprecipitate. More specifically, the equilibrium in the following reversible reaction is shifted to the right (this is observed to be essentially the same for ferric iron described above):

[0090] Al(NO3)3+3H2O←→Al(OH)3+3HNO3.

[0091] This equation is a simplification; aluminum hydroxide contains a large amount of lithium values, approximately 5% of which are recovered from beta spodumene in digestion reactor 820, to the point where "hydroxide" may best be described as hydrated lithium aluminate, having a molecular formula close to LiAlO(OH)2.

[0092] In the process, recovery of the lithium is also sought. In this regard, the saturated liquid / crystal slurry from the PLS evaporator (including its load of iron hydroxide and (mainly) aluminum hydroxide crystals) is pumped by a suitable volumetric pump 908 to a closed stirred storage container in this embodiment, namely the filter feed tank 910. The filter feed tank 910 is maintained at a pressure of 6-8 bar and a temperature of about 180°C. The filter feed tank 910 is operated with a flow of sufficient hot wash water 912, such as to dilute the PLS liquor to redissolve any lithium nitrate crystals and ensure that crystallization of lithium nitrate or other nitrates does not occur at temperatures as low as 80°C. The flow of wash water 912 is controlled by a solenoid valve and check valve pair 914 to maintain the nitric acid concentration in the aqueous phase below 5% HNO3. Pressurizing the blend in tank 910 allows the boiling temperature to be maintained above 180°C. The pressure in tank 910 may also be controlled / regulated via compressed air line 916 (compressed up to approximately 1,000 kPa), controlled by solenoid and check valve pair 918 .

[0093] As required, a weak nitric acid feed stream 920 may also be diverted to tank 910 via a solenoid valve and check valve pair 922 to maintain the nitric acid concentration in tank 910 at a desired level below 5%. This weak nitric acid itself is prepared and held in a storage tank 924, and may be prepared by feeding a strong nitric acid stream 926 (generated elsewhere in the process, such as in plant 50) via valve 928. The strong nitric acid 926 is mixed with a portion of the hot wash water 912 in the storage tank 924, the feed of the hot water portion being controlled via valve 930. The produced weak nitric acid 920 is pumped out of the storage tank 924 via pump 932.

[0094] The dilute nitric acid (retained from the PLS evaporation step / stage 904) leaches the lithium values ​​in the aluminum hydroxide precipitate to form more lithium nitrate in solution under the high temperature and pressure conditions maintained in the pressurized filter feed tank 910. The residence time provided in the filter feed tank 910 is about 2 hours, which is sufficient to precipitate more aluminum values ​​and completely leach lithium values ​​from all of them.

[0095] The liquid phase from the filter feed tank 910 flows under pressure to a further filter press 936 (other filtration or arrangements including centrifugation are also possible) via a check valve and control valve pair 934. In a similar manner to the filter press 824, the filter press 936 is fed by hot wash water from stream 912 and compressed air from line 916 via a corresponding solenoid valve and check valve pair 938. Weak nitric acid 920 for filtrate washing (i.e., rinsing the filter cake of residual nitrate values) can also be fed to the filter press 936 via its corresponding solenoid valve and check valve pair 938.

[0096] The filter cake 940 from the filter press 936 is periodically discharged from the filter cloth to be collected in a hopper or other receiving container and thereby again forms a barren residue / tailings stream. This stream 940 can be combined with the leaching stage (module 1, Figure 8 )'s tailings residues are combined and can subsequently be disposed of similarly.

[0097] The filtrate stream 942 from the filter press 936 is passed to a subsequent purification stage. As illustrated, the purification stage comprises a series of three stirred settling tanks arranged to sequentially remove residual aluminum values ​​by adding lithium hydroxide solution 944 (ie, from a downstream product process).

[0098] The first such precipitation tank 946 is configured for Al(OH)3 precipitation. The second (next) precipitation tank 947 is configured for Mg(OH)2 precipitation. The third (next) precipitation tank 948 is configured for CaCO3 precipitation.

[0099] The filtrate stream 942 is sent to a first precipitation tank 946 where the solution pH is raised to about 2-3 so that any remaining aluminum values ​​precipitate as hydroxide (due to the high lithium nitrate concentration, the hydroxide precipitates as a filterable crystalline solid rather than a gel). The resulting supernatant is then sent to a second precipitation tank 947 where the solution pH is raised to about pH 9.5 to precipitate magnesium values ​​as insoluble magnesium hydroxide according to the formula:

[0100] Mg(NO3)2+2LiOH→2LiNO3+Mg(OH)2.

[0101] If desired (or required), residual calcium is removed by precipitating it as insoluble calcium carbonate (calcite) in a third precipitation tank 948, but this time the correct amount of lithium carbonate 950 (i.e. from a downstream product process) is also added. The lithium carbonate stream 950 causes the following reaction:

[0102] Li2CO3+Ca(NO3)2→2LiNO3+CaCO3.

[0103] The liquid is passed through the settling tanks 946, 947, 948 and is driven by the fine filter feed pump 952 (i.e., the pump 952 draws the liquid through the tanks 946, 947, 948). The precipitated solids accumulated in the liquid when the liquid passes through all three tanks (and when pumped by the fine filter feed pump 952) are transferred to the fourth agitation tank (i.e., the filter feed tank 954). Through this stage, the precipitated solids are present in low amounts and are removed and washed by transferring the liquid from the filter feed tank 954 to a suitable fine filter 956. Similarly, due to the high concentration of lithium and nitrate ions, all precipitated solids are removed and washed as dehydrated crystalline solids rather than gels. The solids filtered out by the fine filter 956 are collected and fed as stream 958 to the combined tailings stream from the leaching stage and the filter press tailings 940 for disposal.

[0104] It should be noted that there is an option to operate the four tanks (ie, three precipitation tanks 946, 947, 948 and a single filter feed tank 954) in pairs: the first two tanks precipitate aluminum values ​​and the second two tanks precipitate magnesium values.

[0105] Purification stages 946-948 produce a purified lithium nitrate solution 22. Although this solution may comprise the product of the process, the purified lithium nitrate solution 22 is typically further processed to produce lithium oxide and downstream products including lithium hydroxide, lithium carbonate, and lithium metal.

[0106] Method (20) - Recovery of lithium product from purified PLS ( Figure 1-7 ).

[0107] The following section describes a preferred embodiment of the overall process in which crystallization is avoided throughout the production of crude PLS to the production of solid lithium oxide. Thus, this overall description represents an alternative to the techniques described in U.S. Pat. No. 10,131,968 and U.S. Pat. No. 10,883,156, since the production of high purity lithium oxide intermediates requires the crystallization of pure crystalline lithium nitrate from purified PLS to remove the alkali metals remaining after all Group 2 and Group 3 metals have been removed.

[0108] refer to Figure 1 The solution 22 containing lithium nitrate purified to a degree free of metals other than alkali metals is transferred to a first heating process 30, which will be referred to below. Figure 3This is described in more detail. In the heating process 30, water 32 is evaporated from the solution. The evaporated water 32 is collected and condensed and stored 36 for reuse in the process. The remaining nitric acid vapor can be entrained with the evaporated water 32. In the heating process 30, the resulting lithium nitrate-rich crystal slurry is heated to form a lithium nitrate melt 34 (molten liquid).

[0109] The molten lithium nitrate melt 34 is transferred to a second heating process 40, as described below. Figure 4 This is described in more detail. In the second heating step 40, the lithium nitrate melt 34 is heated more intensely to decompose the lithium nitrate substantially into lithium oxide 44 and a gaseous stream 42 containing nitrogen oxides (NOx) plus some oxygen.

[0110] The gaseous NOx stream 42 formed from the decomposition of lithium nitrate in the second heating stage 40 is collected and conveyed to a nitric acid production facility 50, referred to below. Figure 5 Describe it in more detail.

[0111] As the lithium nitrate decomposes in the second heating process 40, the lithium oxide formed tends to float to the top, forming an overflow of lithium oxide slurry in the lithium nitrate, which is filled with bubbles of other decomposition products: a blend of gaseous nitric oxide, nitrogen dioxide and oxygen. The overflowing slurry first enters a circumferential launder, where substantially all of the bubbles burst, thereby releasing gaseous products to leave a two-phase slurry of solid lithium oxide crystals in molten lithium nitrate. This slurry flows to a solid-liquid separation system, where the undecomposed lithium nitrate liquid is separated from the solid lithium oxide by (in one embodiment) using a screw press. The screw press "squeezes" the lithium nitrate-rich liquid from the lithium oxide to produce a lithium oxide product 44 in the form of a cake. The lithium nitrate-rich liquid is returned to the decomposition vessel 404 for recycling.

[0112] Since the lithium nitrate-rich liquor also contains nitrates of sodium and potassium (and perhaps small amounts of nitrates of rubidium and cesium, to the extent that these normally scarce alkali metals are contained in the spodumene concentrate), these alkali nitrates will accumulate in the contents of the decomposition vessel 404, their concentration relative to the lithium nitrate steadily increasing as more and more lithium nitrate is decomposed. In practice, this approach may reach a limit, as the lithium oxide cake from the screw press (in this embodiment) will inevitably include some of these alkali nitrates in the interstices between the compacted lithium oxide crystals that form the cake. Eventually a situation will arise where the amount of such alkali nitrates removed from the circulating lithium nitrate liquor (decomposition vessel to screw press liquor and back to the decomposition vessel) will match the rate at which these alkali nitrates enter this circulating stream as fresh feed to the decomposition vessel.

[0113] Therefore, the lithium oxide cake formed as described is not a pure product, but rather "crude" lithium nitrate. Further purification is required before it can be considered acceptable as a feed for lithium battery manufacturing.

[0114] Reference has been made to previous patents issued to the inventors, such as US 10,131,968 and US 10,883,156, which describe techniques for producing high purity lithium oxide as a key process intermediate or final product. They include an additional process step: crystallization of a partially purified pregnant leach solution (PLS), i.e., a lithium nitrate solution so purified that the only impurity elements present in this PLS are sodium and potassium (and possibly rubidium and cesium, if present in the spodumene concentrate feed to the refinery), also present as nitrates. However, although significantly simpler than the crystallization process that other lithium mineral refining processes must undergo (e.g., by using sulfuric acid rather than conveniently recyclable nitric acid to crystallize the sodium sulfate by-product, as in the inventors' related patents and applications), crystallization is a complex and expensive process that is preferably not carried out at remote mine sites.

[0115] A feature of the present application is the production of battery-grade lithium chemicals with minimal crystallization stages, in particular the production of pure crystals of lithium hydroxide monohydrate with only one crystallization stage: the crystallization stage for the final lithium hydroxide monohydrate product.

[0116] Another feature of the present application is that it enables the overall process of refining lithium-rich minerals into battery-grade lithium chemicals to be conveniently divided into two locations: production of a crude lithium oxide product at or near the mine / concentrator site, which is the source of refinery feed (e.g., spodumene concentrate with a 6% lithium oxide content, commonly abbreviated as SC6); and a "finishing" stage located at or near the lithium battery manufacturing facility, where the crude lithium oxide is converted into pure, battery-grade lithium hydroxide (or, if desired, into carbonate, by resorting to the techniques described, inter alia, in the inventor's U.S. Patent No. 10,131,968).

[0117] This is referenced below Figure 4 Describe in more detail.

[0118] Even in the "crude" solid lithium oxide produced by this version of method 20, the high concentration of elemental lithium minimizes the transportation cost per ton of elemental lithium, especially compared to transporting spodumene concentrate (of which 94% is waste rock). Such waste rock inevitably presents disposal problems for the customer / battery manufacturer. In the context of transportation, lithium oxide represents the most convenient and lowest cost option: one ton of pure lithium oxide contains as much elemental lithium as 18 or more tons of SC6, allowing for losses in processing.

[0119] Whether the upstream process for producing the solid lithium oxide product 44 is substantially adjacent to the lithium hydroxide production region 60 or is separated (perhaps by thousands of kilometers), reference is made below to the upstream process for producing the solid lithium oxide product 44. Figure 6 The methods involved are described in more detail.

[0120] In the lithium hydroxide production area 60, the lithium oxide is mixed (cured and dissolved) with sufficient water to form a lithium hydroxide solution. Figure 6 The lithium hydroxide is then transferred to a crystallization tank where water is evaporated to produce a slurry of lithium hydroxide monohydrate crystals. A dewatering centrifuge is used to produce a lithium hydroxide monohydrate product that is substantially free of liquid.

[0121] In one embodiment, in the lithium nitrate decomposition stage 40, most of the lithium values ​​are converted to solid lithium oxide, so this lithium nitrate-rich liquid (which may contain sodium and potassium nitrates) only accounts for a small portion of the total lithium nitrate fed to the decomposition reactor vessel 404.

[0122] In one embodiment, the lithium nitrate contained in the residual liquor 46 may be subjected to further processing stages 70 to form a lithium carbonate product. The reasons for such further processing are firstly to provide additional marketable products and secondly to minimize the amount of lithium values ​​remaining in the material which becomes a blend of primarily lithium nitrate and sodium and potassium nitrates.

[0123] Reference below Figure 7 In the lithium carbonate production stage 70 described in more detail, soda ash and / or potash are added to the squeezed residual lithium nitrate liquor 46. This results in the precipitation of slightly soluble lithium carbonate. Liquid is removed from the lithium carbonate using a dewatering centrifuge to produce a lithium carbonate product that is substantially free of liquid.

[0124] Specific details of the method 20 for implementing each of the operational stages 30 to 70 described above will now be described. Figure 2 A general schematic diagram of method 20 is provided. Figure 2 The overview illustrates: a first lithium nitrate heating process 30 according to the present disclosure; a second lithium nitrate heating process 40 according to the present disclosure; nitric acid production 50; lithium hydroxide monohydrate production 60; and lithium carbonate production 70.

[0125] Figures 3 to 7 Each of the figures in provides a more detailed breakdown of these individual method operations 30 - 70 within method 20 , respectively.

[0126] Example 2 further provides preliminary calculations and design parameters for a pilot scale plant.

[0127] Lithium nitrate first heating step 30 ( Figure 3 )

[0128] Reference now Figure 3 , a solution 22 containing lithium nitrate (e.g., purified PLS, such as can be obtained by reacting lithium-containing metal silicate with nitric acid in the lithium extraction process 10) is pumped into a storage tank 300 of the lithium nitrate first heating process 30. The lithium nitrate first heating process 30 is a two-stage process including an evaporator 302 and a melting vessel 320.

[0129] The storage tank 300 serves as a feed tank for the evaporator 302. When the control valve 304 is open, the solution 22 comprising lithium nitrate flows from the storage tank 300 into the main compartment 312 of the evaporator 302. When the evaporator 302 needs to be isolated from the storage tank 300, such as if maintenance is required or in the event of equipment failure or if the flow to the evaporator 302 needs to be reduced, the valve 304 can be partially or completely closed.

[0130] In the evaporator 302, the solution 22 containing lithium nitrate is heated. The evaporator 302 is operated at a temperature close to the boiling point of the solution 22, at a given operating pressure of the evaporator 302. The evaporator 302 can be configured to operate at an elevated pressure or under a vacuum. At a temperature close to the boiling point of the solution 22, water is evaporated. The evaporated water is removed as a vapor stream 306. The vapor stream 306 may contain very low concentrations of nitric acid, for example, when nitric acid is still present in the solution 22 containing lithium nitrate.

[0131] The evaporator 302 also includes an outer jacket 310 surrounding a main compartment 312 of the evaporator 302. Steam is fed into the outer jacket 310. The steam condenses in the outer jacket 310, thereby releasing energy. The energy released from the condensing steam 310 is transferred to the solution in the main compartment 312, thereby heating the solution and evaporating water. The condensed steam from the evaporator jacket 310 can be collected for use as process water, or returned to the boiler for reuse in the evaporator 302.

[0132] As water and nitric acid (when present) continue to evaporate in the evaporator 302, a supersaturated solution of lithium nitrate is produced. The supersaturated solution of lithium nitrate contains a portion of lithium nitrate crystals in a slurry collected at the bottom of the evaporator. This slurry 308 of lithium nitrate crystals is pumped out of the evaporator 302 by a pump (not shown).

[0133] Depending on the volume of solution comprising lithium nitrate to be processed, evaporator 302 may include a single effect evaporator (for smaller volumes, such as in a pilot plant) or a multiple effect evaporator (for larger volumes, such as in a commercial scale system).

[0134] In a single-effect evaporator configuration, there is only one such evaporator 302, wherein the water vapor 306 produced by the evaporator 302 is collected immediately. In a multiple-effect evaporator configuration, there are several such evaporators 302 configured in series, wherein the solution containing lithium nitrate is gradually concentrated as it passes through a series of evaporators. The water vapor produced by the first evaporator in the series is used as the steam stream in the next evaporator in the series, and so on. Each evaporator may require a small volume of make-up steam. The last evaporator in the series produces a vapor stream 306.

[0135] The evaporator 302 (single effect or multiple effect) typically includes a mechanical vapor recompression system in which the vapor 306 is recompressed in a compressor. The recompressed vapor is then used as steam for the outer jacket 310. A small volume of make-up steam is also typically required. The use of mechanical vapor recompression can significantly reduce the total live steam demand, thereby reducing the power required by the boiler. This can significantly save capital and ongoing costs of the method and system, especially if electricity is cheap and produced from renewable resources such as hydroelectric power.

[0136] A control valve 324 between the evaporator 302 and the melt vessel 320 allows the evaporator 302 and the melt vessel 320 to be isolated from each other. When the valve 324 is closed, the slurry 308 cannot be transferred from the evaporator 302 to the melt vessel 320. When the valve 324 is open, the slurry 308 of lithium nitrate crystals can be moved (e.g., by pumping) from the evaporator 302 to the melt vessel 320.

[0137] The melt vessel 320 operates at a temperature of about 400°C, which is above the melting point of lithium nitrate. The melt vessel 320 is enclosed in a jacket 324, which electrically indirectly heats the melt vessel 320. The vessel 320 is typically heated by induction to a temperature of about 400°C.

[0138] The high temperature of the melting vessel 320 causes the lithium nitrate crystals present in the slurry to melt. Other salts present in the slurry (by this stage of the process, only nitrates of other alkali metals, especially sodium and potassium, and trace amounts of rubidium and cesium (if present in the spodumene concentrate), and possibly some calcium) may also melt if the melting point of the salts is below 400° C. A molten liquid comprising molten lithium nitrate and molten other salts is thereby produced. The molten liquid 34 comprising molten lithium nitrate and molten other salts is collected at the bottom 322 of the melting vessel 320. The molten liquid 34 is conveyed to the second thermal treatment process 40.

[0139] The remaining water in the slurry 308, along with any remaining nitric acid, is vaporized in the melt vessel 320, producing a vapor stream 314. The vapor 314 produced by the melt vessel 320 is collected and combined with the vapor 306 from the evaporator 302 (when mechanical vapor recompression is not part of the system) to form a combined vapor stream 32. The combined vapor stream 32 is collected and sent to the condenser 80 ( Figure 2 The combined vapor stream 32 is condensed in condenser 80 and stored in storage tank 36 ( Figure 2 ). The water recovered in the storage tank 36 is used as process water elsewhere in the process, for example as wash water, as feed to a boiler, etc. The condenser 80 may employ chilled or cold water (e.g., from a cooling tower) as a second cooling stream in the condenser 80. Energy is transferred from the condensed combined vapor stream 32 to the chilled or cold water stream, resulting in a significant heat change within the chilled or cold water.

[0140] Lithium nitrate second heating step 40 ( Figure 4 )

[0141] like Figure 4 As shown, the molten liquid 34 containing lithium nitrate and other molten salts from the first heat treatment process 30 is transferred to the second heat treatment process 40.

[0142] The control valve 402 is placed between the melt vessel 320 and the second heat treatment process 40 so that when the valve 402 is partially or completely closed, the molten liquid 34 cannot be transferred from the melt vessel 320. This allows the melt vessel 320 to be isolated from the second heat treatment process 40, such as when maintenance or stopping the second heat treatment process 40 or controlling the flow to the second heat treatment process 40 is required.

[0143] When valve 402 is opened, molten liquid 34 is conveyed into decomposition vessel 404, typically toward its upper end. Decomposition vessel 404 is enclosed by dome 406 to help prevent toxic gas leakage. The inner surface 408 of decomposition vessel 404 is completely lined with periclase (dead-burned magnesium oxide) tiles. Periclase is a suitable material for lining decomposition vessel 404 because it is resistant to high temperatures and has no amphoteric properties, i.e., it does not react with lithium oxide. Decomposition vessel 404 operates at a temperature of about 600° C. (i.e., near the decomposition temperature of lithium nitrate). Jacket 410 surrounds decomposition vessel 404. Jacket 410 indirectly heats decomposition vessel 404 by electrical induction, thereby maintaining a temperature of about 600° C.

[0144] The contents of the decomposition vessel 404 (i.e., the molten liquid 34) are maintained at a temperature of about 600°C, promoting the decomposition of lithium nitrate into lithium oxide crystals according to the following reaction:

[0145]

[0146] Usually, more NO is produced than NO2. NO and NO2 are collectively referred to as NO x .

[0147] Although a large amount of lithium nitrate present in the molten liquid 34 is decomposed in the decomposition vessel 404, some molten lithium nitrate will remain. The residual lithium nitrate can be removed from the decomposition vessel 404 as an underflow 422 by carefully opening the control valve 424 as described below.

[0148] The decomposition vessel 404 is configured to also function as a flotation cell, albeit one designed to operate at elevated temperatures and capable of handling the chemicals involved in the process. The molten liquid feed 34 enters the decomposition vessel 404 via a feed port 412 towards its upper end.

[0149] The lithium oxide crystals formed during the decomposition of the lithium nitrate have a lower density than the residual lithium nitrate. Therefore, the lithium oxide crystals will tend to move upward 416 (ie, "float") through the container 404 to the top 418 of the container.

[0150] When lithium nitrate decomposes, NO x The oxygen and gas form bubbles 414 in the vessel 404, thereby simulating bubbles introduced by aeration in a conventional flotation cell. The bubbles 414 move upward 416 through the vessel to the top 418 of the vessel. The upward movement of the bubbles 414 also helps promote the separation of the less dense lithium oxide crystals.

[0151] Due to the lithium oxide crystals and NO x With the upward movement of the oxygen bubbles 414, a foam of lithium oxide crystals in the lithium nitrate-rich bubbles forms on top of the residual lithium nitrate slurry. The foam forms an overflow of the decomposition vessel 404 and overflows into the peripheral collection trough 420. The residual lithium nitrate slurry forms an underflow 422, which can be removed from the decomposition vessel 404 by opening the valve 424.

[0152] Collection launder 420 directs the overflow of lithium oxide crystals in lithium nitrate-rich bubbles to outlet port 425 and tube 426. As the overflow is directed to outlet port 425 and tube 426, the bubbles collapse, forming a concentrated slurry of lithium oxide crystals in molten lithium nitrate.

[0153] The cover 406 of the decomposition vessel 404 extends beyond the collection trough 420 and the outlet port 426 to ensure that NO x and oxygen cannot escape into the atmosphere. In contrast, gaseous NO x and oxygen stream 42 is directed to outlet pipe 438. Gaseous NO x and oxygen stream 42 is sent to nitric acid production facility 50 ( Figure 1 and Figure 5 ).

[0154] The concentrated slurry of lithium oxide crystals in molten lithium nitrate is directed to a screw pressure conveyor 428. Depending on the capacity required, one or more screw pressure conveyors operating in parallel may be used.

[0155] In the screw pressure conveyor 428, molten lithium nitrate is squeezed out of the lithium oxide crystals. The residual molten lithium nitrate is collected at the bottom outlet 430 of the screw pressure conveyor 428. The molten lithium nitrate collected from the screw pressure conveyor is combined with the lithium nitrate underflow 422 from the decomposition vessel 404 to form a combined lithium nitrate stream 46. The combined lithium nitrate stream 46 is pumped to the lithium carbonate precipitation stage ( Figure 1 and Figure 6 ).

[0156] When the molten lithium nitrate is squeezed out of the lithium oxide crystals, the lithium oxide crystals are compressed into a lithium oxide crystal mass substantially free of liquid phase. The lithium oxide crystals 44 are collected from the upper end of the screw pressure conveyor 428 .

[0157] The screw pressure conveyor 428 is a variation of the type widely used for pressing edible oil from oilseeds, with the necessary modifications to enable it to withstand process conditions. The rotor is made of high chrome steel covered with periclase applied using plasma technology, and the stator is made of porous sintered periclase. Other surfaces are lined internally with periclase tiles as required, including (but not limited to) where the surface may come into contact with the molten liquid.

[0158] In one version of the process, lithium oxide crystals 44 are the final product. The lithium oxide has 46.5% lithium by weight. This represents the highest content of elemental lithium compared to other lithium products (e.g., hydroxides, carbonates), and is significantly higher than the lithium content of the original ore. The cost of shipping the lithium oxide product is relatively cheap in terms of lithium. For example, it can be economically shipped by air, such as from a remote operating site.

[0159] The lithium oxide crystal product 44 is also an ideal precursor for the production of lithium metal. Depending on the purity of the initial solution containing lithium nitrate, the lithium oxide crystal product may not be able to produce battery grade lithium hydroxide monohydrate by adding water alone, i.e., a lithium hydroxide monohydrate crystallization step may be employed. By ending the process with a lithium oxide crystal product, the capital and operating costs in the process may be reduced at this point (i.e., no crystallization tank is required).

[0160] Furthermore, the process lends itself naturally to being split into two parts. In the first part, nearly pure lithium oxide is produced, which, due to its high lithium content, requires that the bulk of the refinery be built adjacent to the primary lithium mine / concentrator. Due to the high lithium content by weight, the freight costs of transporting the lithium value as lithium oxide to the battery plant are particularly low. In the second part, the remainder of the process (which is the crystallization of lithium hydroxide monohydrate) can then be carried out at the battery plant or a secondary refinery.

[0161] In the process shown, the lithium oxide crystals are typically further processed to produce crystalline lithium hydroxide monohydrate. In this version of the process, the liquid-free lithium oxide crystals 44 are conveyed to a lithium hydroxide production section 60 ( Figure 1 and Figure 6 ).

[0162] Nitric acid production 50( Figure 5 )

[0163] like Figure 5 As shown, gaseous NO is generated by decomposing lithium nitrate in the decomposition vessel 404. x The gaseous stream 42 is passed to a nitric acid production facility 50. The gaseous stream may also contain water vapor and trace amounts of nitric acid.

[0164] The gaseous stream 42 first passes through a cooler / condenser 502. In the cooler / condenser 502, cold water 504 is passed countercurrently to the gaseous stream 42. Energy (heat) is transferred between the gaseous stream 42 and the cold water 504, thereby producing a cooled gaseous stream 506 and warm water 508. Some or all of the water and nitric acid present as vapor in the gaseous stream 42 may be condensed in the cooler / condenser 502.

[0165] The cooled gaseous / liquid stream 506 (which may also contain some condensed water and acid) then passes through the nitric acid synthesis heat exchanger 508. In the heat exchanger 508, chilled water 510 is passed countercurrently to the cooled gaseous stream 506. Energy (heat) is transferred between the cooled gaseous stream 506 and the chilled water 510, thereby further condensing and cooling the gas in the gaseous stream to ambient temperature. At the same time, the chilled water 510 is heated. The heated chilled water 512 is returned to be used as makeup water for the chiller.

[0166] As the cold gaseous stream 506 is further cooled and condensed, the following reactions occur:

[0167]

[0168] The NO formed in reaction (3) reacts with additional oxygen to form more NO2 via the first reaction. Since the cooled gaseous stream 506 contains primarily reactants with very little diluent gas, reactions (2) and (3) proceed rapidly enough that nitric acid formation is substantially complete within the heat exchanger. Small amounts of oxygen may be added to maintain oxidative conditions within the heat exchanger.

[0169] The nitric acid formed by reaction (3) is transferred to a nitric acid storage tank 514. The nitric acid storage tank 514 is covered and airtight to prevent vapors from leaking into the atmosphere. Make-up nitric acid 52 is added to the storage tank 514 as needed to maintain an acid concentration of approximately 50% nitric acid (by weight) in the storage tank 514. Nitric acid 18 from the storage tank 514 is pumped by pump 516 to the process 10 for producing a solution comprising lithium nitrate from the reaction of a lithium-containing metal silicate with nitric acid.

[0170] Lithium hydroxide monohydrate production 60( Figure 6 )

[0171] Reference now Figure 6 , liquid-free lithium oxide crystals 44 (from Figure 4 method, i.e. from the screw press 428) is gravity fed into the lithium oxide aging tank 602 of the lithium hydroxide production section 60 ( Figure 1 and Figure 6 ). Water 604 is also added to the slaking tank 602. The water 604 is typically desalinated water with very low levels of impurities, thereby ensuring that a high purity lithium hydroxide product can be obtained.

[0172] One option is to add enough water 604 to the aging tank 602 so that anhydrous lithium hydroxide product is produced. The process ends here and anhydrous lithium hydroxide is sold as the final product. The lithium oxide crystals 44 react with water 604 to produce anhydrous lithium hydroxide according to the following reaction:

[0173]

[0174] The advantage of terminating the process at such a lithium hydroxide product 608 is that the process is simpler and less costly because no further equipment or processing is required. The slaking is controlled to produce anhydrous lithium hydroxide having an extremely fine crystal size. The anhydrous lithium hydroxide product 608 contains approximately 29% elemental lithium, making it cheaper to transport on a lithium basis than the original ore. Therefore, the anhydrous lithium hydroxide product 608 can be economically transported elsewhere for further processing.

[0175] However, a stoichiometric excess of water 604 is typically added to the maturation tank 602. In the maturation tank 602, the lithium oxide crystals 44 react with the excess water 604 to produce a lithium hydroxide monohydrate solution according to the following reaction:

[0176]

[0177] Excess water 604 is also required to ensure that a pumpable solids-free solution 608 of lithium hydroxide monohydrate is produced. The maturation tank 602 further includes an agitator 606 that promotes homogeneity within the maturation tank 602.

[0178] The solution 608 of lithium hydroxide monohydrate is pumped by pump 610 to a lithium hydroxide crystallization tank 612. In the crystallization tank 612, the solution is heated 618 to evaporate water, thereby forming a vapor stream 616 and a slurry 614 of concentrated and crystallized lithium hydroxide monohydrate crystals.

[0179] The vapor stream 616 can be combined with the vapor from the first thermal treatment step 316 ( Figure 3 ) are combined and directed to condenser 80. In condenser 80 ( Figure 1 ), the vapor stream is condensed to produce process water which is stored in storage tank 36. The process water is reused elsewhere in the process as required.

[0180] Depending on the volume of lithium hydroxide solution, crystallizer 612 can be an electrically heated evaporator or can be heated using steam. Typically, electrically heated single-effect crystallizer 612 is used in smaller scale operations, such as in a pilot plant. Crystallizer 612 includes jacket 618. Jacket 618 is powered, causing crystallizer 612 to heat.

[0181] For larger operation, a steam-heated crystallizer including mechanical vapor recompression will be used. When the crystallizer 612 is steam-heated, the crystallizer 612 also includes an outer jacket 618 around the main compartment 620 of the crystallizer 612. The outer jacket 618 allows steam to be fed to and flow through the outer jacket 618. The steam in the outer jacket 618 condenses, thereby releasing energy. The energy released from the condensed steam is transferred to the solution in the main compartment 620, thereby making the solution hot and evaporating water. The condensed steam from the crystallizer jacket 618 can be collected and used as process water or returned to the boiler.

[0182] The steam 616 is recovered and at least a portion thereof may be sent to a compressor 622. The compressor 622 increases the pressure of the steam 616, thereby producing high pressure steam 624. The high pressure steam 624 may be reused as a steam source in the outer jacket 618 of the crystallizer 612. Due to losses, a small volume of make-up steam is usually required in the loop. However, in a crystallizer equipped with mechanical vapor recompression, the demand for fresh steam is significantly reduced. This also reduces the size of the boiler required in the plant, thereby greatly saving the capital and ongoing costs of the method / system.

[0183] The lithium hydroxide monohydrate crystal slurry 614 is discharged from the bottom 617 of the crystallization tank 612 in a controlled manner. When the control valve 618 is open, the crystal slurry 614 is transferred to the dehydration centrifuge 626. When the valve 618 is closed, the crystallization tank 612 is isolated from the dehydration centrifuge 626, and the crystal slurry 614 cannot flow to the dehydration centrifuge 626.

[0184] In the dewatering centrifuge 626, a lithium hydroxide monohydrate crystal product 628 is separated from the slurry. The lithium hydroxide monohydrate crystal product 628 is substantially free of liquid. The dewatering centrifuge 626 is generally operated so that the lithium hydroxide monohydrate crystal product 628 can be filtered and washed with warm wash water 630.

[0185] In the initial filtration stage 626 , the liquid separated from the lithium hydroxide monohydrate crystals product 628 (centrate) is collected and forms part of the combined recovery liquid stream 632 .

[0186] During washing, the interstitial liquid contained in the lithium hydroxide monohydrate crystal product 628 is substantially replaced by wash water. The interstitial liquid (wash liquid) recovered as part of the washing cycle is collected and forms another part of the combined recovery liquid stream 632.

[0187] The volume of wash water 630 added to the dewatering centrifuge 626 is based on the volume of liquid contained in the lithium hydroxide monohydrate crystals product 628 and the desired impurity content of the interstitial liquid contained in the lithium hydroxide monohydrate crystals product 628. Depending on the desired moisture content of the lithium hydroxide monohydrate crystals product 628, the lithium hydroxide monohydrate crystals product 628 may be further dried prior to packaging and shipment.

[0188] The combined recycle stream 632 is a concentrated aqueous solution containing lithium hydroxide and small amounts of carbonates and nitrates of sodium, potassium and lithium. The combined recycle stream 632 can be reused in several areas of the process as required. A first portion can be sent back to the purification stage of the initial lithium nitrate solution obtained by leaching beta spodumene with nitric acid. A second portion can be recycled back to the lithium oxide maturation tank 602. A third portion can be sent to the lithium carbonate production area 70 ( Figure 1 and Figure 7 ).

[0189] Lithium carbonate production 70( Figure 7 )

[0190] like Figure 7 As shown, the residual stream 46 squeezed out from the screw press 430 still contains lithium nitrate ( Figure 1 and Figure 4 ). The lithium values ​​contained in this stream are recovered as lithium carbonate. The stream is pumped to the lithium carbonate production section 70 where it enters the lithium carbonate precipitation tank 702.

[0191] A third portion ( Figure 6 ) is also pumped to the lithium carbonate precipitation tank 702.

[0192] The lithium nitrate stream 46 from the screw press is a saturated solution of lithium nitrate plus sodium nitrate and potassium nitrate. Nitrates of rubidium and cesium may also be present, to the extent that these alkali metals are present in the spodumene concentrate. This blend of mainly lithium, potassium and sodium nitrates as anhydrous salts can be used as a molten salt heat storage and transfer medium in a solar thermal power plant. The presence of lithium values ​​can reduce the melting point of the eutectic blend of sodium nitrate, potassium nitrate and lithium nitrate to less than 100°C, while remaining stable at temperatures in excess of 500°C. Therefore, one option is to further concentrate the blended lithium nitrate stream 46 and the recovered liquid stream 632 to form a mixed nitrate product.

[0193] Typically, however, the lithium nitrate stream 46 and the reclaim stream 632 are further processed to recover lithium values. This is accomplished by precipitating the lithium as sparingly soluble lithium carbonate. To accomplish this, a combination of soda ash (Na2CO3) and / or potash (K2CO3) 704 is added to the lithium carbonate precipitation tank 702. The tank 702 is continuously stirred by means of an agitator 706 to promote mixing between the lithium nitrate stream 432, the centrate / wash liquor 632, and the added soda ash and / or potash 704.

[0194] When the streams are mixed in tank 702, the lithium nitrate present reacts with the soda ash according to reaction (6) to form lithium carbonate (Li2CO3) and sodium nitrate (NaNO3). The lithium nitrate present also reacts with the potash according to reaction (7) to form lithium carbonate and potassium nitrate (KNO3).

[0195] 2LiNO3+Na2CO3→Li2CO3+2NaNO3(6)

[0196] 2LiNO3+K2CO3→Li2CO3+2KNO3(7)

[0197] The tank conditions (temperature, pressure, reagent addition) are selected to optimize the precipitation of lithium carbonate from solution formed by reactions (6) and (7). For example, since the solubility of lithium carbonate decreases with increasing temperature, the lithium carbonate precipitation tank 702 can be heated to increase the temperature of the contents, thereby increasing the mass of the lithium carbonate precipitate formed in the lithium carbonate precipitation tank 702.

[0198] Lithium carbonate is a strong base. A portion 950 of the lithium carbonate slurry from the lithium carbonate precipitation tank 702 may be recycled back to the lithium extraction process 10 ( Figure 1 and Fig. 9 ). This compensates for the need for fresh alkali in the process.

[0199] exist Fig. 9 In the purification stage (i.e., tank 948), lithium carbonate 950 is used to adjust the pH of the solution containing lithium nitrate, thereby precipitating contaminants in the solution and forming a solution containing lithium nitrate that is essentially free of contaminants, which can then be transmitted to method 20.

[0200] Typically, however, the majority of the lithium carbonate slurry is pumped from the lithium carbonate precipitation tank 702 to a dewatering centrifuge 708 via a slurry pump 710. The dewatering centrifuge 708 operates to remove liquid from the contents of the slurry, thereby producing a lithium carbonate product 712 that is substantially free of liquid and a centrate 714. The lithium carbonate product 712 of marketable purity or better is dried, packaged, and sold.

[0201] The centrifuge liquid 714 consisting mainly of lithium, sodium and potassium nitrates is discharged into a nitrate storage tank 716. The liquid in the nitrate storage tank 716 can be used in the lithium recovery process 20 or the first lithium extraction process 10 ( Figure 1 ) for reuse elsewhere in the process. However, some solution will need to be discharged from the process to prevent accumulation of pollutants (e.g. sodium, potassium, rubidium, cesium) in the process. The discharged solution represents a small by-product nitrate stream.

[0202] The byproduct nitrate stream is pumped from the nitrate storage tank 716 by pump 718. The byproduct nitrate stream can be discharged to a waste area or can be further processed. The further processing can include recovering heavier alkali metal salts (potassium, rubidium and cesium). These alkali metal salts can be evaporated to dryness and converted into pellets for sale. For example, potassium nitrate is particularly valuable as a fertilizer.

[0203] Example

[0204] Non-limiting embodiments of the method and system will now be described.

[0205] Example 1

[0206] In this example, the production of a solution comprising lithium nitrate (such as may be used as a feed to the above-described process) is provided.

[0207] It is noted that the solution comprising lithium nitrate is produced in substantially the same manner as the first stage of the method and system outlined in the applicant's co-pending patent WO 2017 / 106925. However, the method is terminated before the lithium nitrate crystallization stage of WO 2017 / 106925 (i.e. after thermal hydrolysis, pH adjustment and lithium carbonate addition).

[0208] Crude PLS production

[0209] Figure 8 The main items of plant and equipment for extracting lithium values ​​from calcined (β) spodumene are shown. Figure 8 In a setup of , beta spodumene is fed at a rate of one tonne per day (up to 50 kg / h, dry basis) and the leached solids are separated from the lithium nitrate-rich pregnant leachate (PLS).

[0210] Beta spodumene was added to the feed mixer (a conventional concrete mixer equipped with a 304L stainless steel bowl) at a rate of 50 kg / h together with 63 kg of nominal 50% nitric acid by weight to prepare 123 kg / h of slurry, 75 l / h by volume. In practice, the batch with the correct proportions of calcined spodumene and nitric acid was replenished regularly (e.g. every hour).

[0211] Nitric acid is added in significant stoichiometric excess: up to 150% to maximize leaching of lithium values ​​from the calcined spodumene. Nitric acid concentrations range from 30% to 50% HNO3 by weight, the balance being water, but tend to be towards the upper end of this range to minimize downstream evaporation loads. In practice, the strength of the acid used is determined by the rheological properties of the blend of acid and calcined spodumene, taking into account the changes that the aqueous phase undergoes during the leaching reaction:

[0212] LiAlSi2O6+HNO3→LiNO3+AlSi2O5(OH)-75kCal / mol.

[0213] In practice, this reaction shows that a liquid (nitric acid) can be easily converted to a solid (lithium nitrate), despite being highly soluble in hot water. The slurry leaving the digestion reactor (i.e., a three-compartment autoclave, made essentially of titanium) is controlled to have enough liquid phase to ensure that the slurry is pumpable, but not so much liquid that the solid and liquid can be significantly separated.

[0214] Note that the use of 50% acid is close to the upper limit of safe concentration. The overall reaction is exothermic, which is based on (molecular weight 63), converting to a total feed slurry of 135kCal / kg. The heat energy released heats the feed blend from 25°C to 80°C net, which consumes about 35kCal / kg of the total feed slurry, and the remainder (100kCal) is consumed in boiling off water plus nitric acid vapor; the amount of the latter removed by boiling is about 0.2kg water equivalent, reducing 1kg of feed (solid plus liquid phase) to 0.8kg product, of which 0.44kg is spodumene concentrate, and 0.36kg is 0.12kg lithium nitrate, plus 0.8kg of aluminum nitrate and other soluble nitrates, plus 0.16kg of nitric acid plus a blend of water.

[0215] The pressure of the resulting lithium nitrate slurry is reduced and it is transferred to a 100-liter storage tank for storage. The lithium nitrate slurry is transferred from the storage tank to a plate and frame filter press. The filtration method employed includes repeated washing and air drying cycles. At the end of the final drying cycle, the air flow to the filter press is stopped and the filter press is opened for discharging the filter cake. The resulting low-moisture filter cake (leached spodumene concentrate) is discharged from the filter cloth for disposal. The amount of washing water used in the filter press is set according to the desired degree of filter cake washing, but initial tests were conducted using water of the same mass as the filter cake, i.e. 16 liters per cycle or 50 liters per hour, resulting in a net 75 liters / hour of crude rich leachate (crude PLS). This crude PLS is transferred to a 200-liter storage tank.

[0216] Based on a pilot scale plant design capacity of 50 kg / h beta spodumene (dry basis), the following preliminary equipment dimensions and parameters for crude PLS production were recorded:

[0217] • Nitric acid (as 65% HNO3) flow rate to digestion reactor feed mixing tank: 63 kg / h.

[0218] • Feed to digestion reactor, total by weight: 113 kg / h.

[0219] Digestion reactor feed pump discharge pressure (maximum operating pressure): 8 bar gauge.

[0220] Digestion reactor feed pump combined feed temperature: close to ambient temperature.

[0221] Feed volume to the digestion reactor and to the dewatering filter: 80 l / h.

[0222] Weight of tailings (volume 30 l / h, 10% moisture): 50 kg / h.

[0223] Report hot wash water to filtrate: 27 l / h.

[0224] Total filtrate (crude PLS, nitric acid plus washing water, no filter cake): 77 kg / h.

[0225] Feed mixer, standard concrete mixer, stainless steel bowl: 200 litre bowl capacity.

[0226] Feed storage tank, with agitator, HDPE or PP, with cover: 0.5m dia x 1.0m SWD.

[0227] • Digestion reactor (200 liters total volume): 0.35 m diameter x 2.0 m long, titanium.

[0228] Filter feed pump 100 l / h, up to 8 bar operating pressure: peristaltic.

[0229] Filter feed tank (200l capacity, polypropylene, stirred, with lid): 0.5m diameter x 1.0m SWD.

[0230] Plate and frame filter press (20 minutes cycle time): 5 plates, each 0.5m x 0.5m.

[0231] · Coarse PLS (filter press filtrate) storage tank (with lid, 200l capacity): 0.5m diameter x 1.0m SWD.

[0232] Instrument air compressor capacity: 100 l / h at 8 bar, 25 °C.

[0233] Purified PLS production

[0234] The crude PLS from the 200-liter storage tank is then sent to multiple purification and neutralization stages to produce purified PLS. In these stages, reagents are added to promote the precipitation of base metals and neutralize residual nitric acid. Solid precipitates are removed by sending the solution and precipitate to a plate and frame filtration stage.

[0235] The crude PLS from the 200 liter storage tank is pumped as a hot (near boiling) aqueous solution to an electrically heated single effect 100 liter evaporator formed of heat resistant glass, which is heated via an electrically heated jacket and can accommodate several hours of production of the concentrate. In the evaporator, water and nitric acid are boiled off and the vapor stream is collected for use elsewhere in the process (e.g. for nitric acid production). Evaporation is performed up to 140°C and for a sufficient time to allow hydrolysis of aluminum and ferric iron values.

[0236] The dense but pumpable slurry was pumped from the evaporator by a positive displacement pump to a 120 liter filter feed tank where it was maintained at a pressure of 6-8 bar and a temperature of about 180°C. Hot wash water was added to the tank to maintain the nitric acid concentration in the aqueous phase below 5% HNO3. Compressed air (at about 1,000 kPa) was added to the tank as needed. The liquid residence time provided in the tank was about 2 hours, sufficient to allow precipitation of other aluminum values ​​and complete leaching of lithium values ​​therefrom.

[0237] The resulting liquid phase from the filter feed tank is flowed under pressure to another plate and frame filter press, which operates with a washing and air drying cycle similar to the aforementioned plate and frame filter press. The filter cake from the filter press is regularly discharged for disposal. The filtrate stream from the filter press is continued to the purification stage comprising a series of three stirred settling tanks. These stirred settling tanks are arranged to sequentially remove residual aluminum, magnesium and calcium impurities from the PLS by adding lithium hydroxide solution (downstream product).

[0238] The filtrate stream is sent to a first 300 liter precipitation tank, which is configured for Al(OH)3 precipitation. In this tank, the solution pH is raised to about 2-3, whereby the residual aluminum values ​​are precipitated as aluminum hydroxide crystals. The resulting supernatant is then sent to a second 300 liter precipitation tank, which is configured for Mg(OH)2 precipitation. In this tank, the solution pH is raised to about pH 10.5, so that the magnesium values ​​are precipitated as insoluble magnesium hydroxide crystals. Again, the resulting supernatant solution is sent to a third 300 liter precipitation tank, where the residual calcium is removed by precipitating it as insoluble calcium carbonate (CaCO3-calcite). In addition, the correct amount of lithium carbonate (i.e., downstream product) is added.

[0239] The resulting liquid from the precipitation tank is transferred to a 300 liter fine filter feed tank where the liquid is held ready for filtering solids. The liquid along with the precipitated solids is pumped from this tank by a feed pump to a candle fine filter to be filtered from the liquid as a dehydratable crystalline solid. The solids filtered out by the fine filter are collected and disposed of. The filtrate from the fine filter contains a purified lithium nitrate solution.

[0240] Record the following preliminary equipment size and parameters for purified PLS production:

[0241] Feed to evaporator (crude PLS, 77 kg / h): 13 kg / h HNO3, 12.5 kg / h LiNO3, 6.5 kg / h other nitrates, 45 kg / h H2O.

[0242] ·Evaporator exhaust steam: 17kg / h HNO3, 40kg H2O.

[0243] Evaporator capacity: 52 l / h water equivalent.

[0244] Evaporator power (energy input / hour): 140MJ (40kW).

[0245] Cooling water (CW) flow to tower condenser (ΔT = 20°C): 2,000 l / h (0.6 l / sec, 25 mm diameter tube required).

[0246] Crystal slurry from evaporator to filter feed tank: 25 kg / h (12.5 kg / h LiNO3, 2.5 kg / h Al(OH)3, 5 kg / h HNO3 solution).

[0247] Positive displacement (PD) pump: 20 l / h maximum.

[0248] Dilution water flow rate to filter feed tank: 5 kg / h.

[0249] Total feed to filter press: 30 kg / h.

[0250] Filter feed tank (0.12m 3 , titanium, 10 bar max): 0.25m diameter x 2.2m high.

[0251] Washing water to filter press: 5kg / h.

[0252] · Filter cake from filter press: 3 kg / h.

[0253] Plate and frame filter press (60 minute cycle time): 2 plates, each 0.5m x 0.5m.

[0254] · Filtrate from filter press (combined): 32 kg / h.

[0255] · Sedimentation tank (3, each 0.3m 3 Capacity, HDPE / PP: 0.6m diameter x 1.6m SWD).

[0256] Candle filters, (7 filter sticks, each 60cm x 10cm OD: 0.5m diameter x 1.0m total height).

[0257] Candle filter feed tank, 304L ss: 0.6m diameter X 1.2m SWD.

[0258] Candle filter feed pump: 40kg / h, 10 bar.

[0259] Purified PLS: 30 kg / h (15 kg / h LiNO3, 1 kg / h K / NaNO3, 14 kg / h water).

[0260] The resulting PLS was observed to be substantially free of contaminants and to have a negligible concentration of impurity elements. The purified PLS is ready to be transferred to the first thermal treatment step of the method as described herein.

[0261] The concentrations of several elements in the purified PLS were analyzed using standard techniques. The results of this analysis are presented in

[0262] Table 1.

[0263] Next, a portion of the PLS was boiled down to produce a crude lithium nitrate product. The crude product was not washed. The crude lithium nitrate product was again analyzed for mass fractions of the same elements using standard techniques. The results of this analysis are also presented in

[0264] Table 1.

[0265] Table 1 (Concentration (mg / L) and mass fraction (%) of the most common elements in PLS and crude lithium nitrate, respectively; concentration / fraction of lithium nitrate equivalent is also provided):

[0266]

[0267]

[0268] It is noted that the PLS has a high concentration of lithium, which accounts for >9 wt% of the crude lithium nitrate (>98 wt% based on lithium nitrate). Sodium is the most common contaminant, accounting for about 0.6 wt% of the crude lithium nitrate. The next most common contaminant is potassium, which accounts for about 0.1 wt% of the crude lithium nitrate. The remaining eighteen elements cumulatively account for about 0.15 wt% of the crude lithium nitrate.

[0269] Therefore, sodium and potassium are the main contaminants of concern in terms of their impact on the purity of the final lithium hydroxide monohydrate and lithium carbonate. However, it is also noted that the presence of sodium and potassium in these concentrations within the PLS does not have a detrimental effect on the purity of the lithium hydroxide monohydrate product. It is noted that the ionic radius of sodium and potassium is significantly larger than that of lithium, so they are not easily incorporated into the lithium hydroxide monohydrate crystal structure. Washing the crystallized lithium hydroxide monohydrate also helps to redissolve any sodium and / or potassium salts that may have co-precipitated during the crystallization process.

[0270] Similarly, it is observed that the presence of sodium and potassium does not have a detrimental effect on the purity of the lithium carbonate product. It is noted that lithium carbonate is slightly soluble. Its solubility is about 1.5g / 100mL water at 0°C, and decreases to about 0.7g / 100mL water at 100°C, i.e. it has a negative slope temperature-solubility curve. On the other hand, sodium carbonate and potassium carbonate are very soluble in water. For example, sodium carbonate has a solubility of about 7g / 100mL water at 0°C, and increases to about 45g / 100mL water at 100°C. This means that, due to the presence of low concentrations of sodium carbonate and potassium carbonate, even if lithium carbonate precipitates, these carbonates remain in the solution. By carrying out lithium carbonate precipitation at an elevated temperature, the possibility of sodium carbonate and potassium carbonate co-precipitation is further reduced. This has the additional benefit of increasing the recovery rate of lithium carbonate. It is observed that the sodium carbonate and potassium carbonate that do co-precipitate can be washed away from the lithium carbonate product during centrifugation.

[0271] Thus, battery grade lithium hydroxide and commercial grade lithium carbonate can be produced from purified PLS using the methods described herein.It is also noted that high purity lithium hydroxide and lithium carbonate can be produced from solutions containing lithium nitrate having similar characteristics to the PLS described above.

[0272] Example 2

[0273] In this example, several calculations and design parameters for a pilot plant are generated in which a lithium product is recovered from a purified solution comprising lithium nitrate using a method such as that described in Example 1.

[0274] The purified solution containing lithium nitrate is fed to a lithium recovery process (e.g. Figure 1 and Figure 2 The solution comprises 15 kg / h LiNO3, 1 kg / h KNO3 and NaNO3 in total and 14 kg / h water. The solution comprising lithium nitrate is transferred to a purified lithium nitrate storage tank having a capacity of 100 L, a diameter of 0.6 m and a side water depth (SWD) of 1.2 m.

[0275] The solution containing lithium nitrate is discharged from the storage tank to an electrically heated single-effect evaporator (see e.g. Figure 2 and Figure 3 In the evaporator, 8 kg / h of water is evaporated. As a result, the remaining 6 kg / h of water, 15 kg / h of LiNO3 and a total of 1 kg / h of KNO3 and NaNO3 are conveyed from the evaporator to a melting vessel (see, for example Figure 2 and Figure 3 The melt vessel includes electric induction heating and operates at a temperature up to about 400° C. (i.e., above the melting point of lithium nitrate). The melt vessel drives off most of the remaining water and produces molten lithium nitrate liquid, which is collected at the bottom of the vessel. The molten liquid is transferred to a decomposition vessel (see, e.g., Figure 2 and Figure 3 404 in FIG.

[0276] The decomposition vessel is completely lined with periclase on the inside and is jacketed to indirectly heat the vessel contents by electric induction so that the contents reach a temperature of about 600°C (the decomposition temperature of lithium nitrate). As the lithium nitrate decomposes, NOx and oxygen gases are produced. The maximum flow rate of NOx and oxygen from the decomposition vessel is 12 kg / h HNO3 equivalent at about 600°C. The NOx and oxygen from the decomposition vessel are combined with water and nitric acid vapor from the lithium extraction process (when used), resulting in a total gas flow rate of about 50 kg / h to the nitric acid production facility at 300°C. Of this 50 kg / h gas stream, 29 kg / h is HNO3.

[0277] As mentioned above, both the melting vessel and the decomposition vessel are heated indirectly. Note that the latent heat of fusion of anhydrous LiNO is 0.36 kJ / kg. Note that the heat capacity of molten LiNO is 0.93 J / kg / °C. The total energy required to melt 15 kg / h of LiNO and heat it from 60°C to 600°C (i.e., the operating temperature of the decomposition vessel) is 12.9 MJ / h. The heat of decomposition of 15 kg / h of LiNO is 47 MJ / h. Therefore, the total energy required for heating, melting, and decomposition of LiNO is 60 MJ / h.

[0278] In a variant, the melting vessel and the decomposition vessel are heated by gas. In the case where the vessel is electrically heated, a power of about 20 kW is required. In the case where the vessel is gas heated, a gas of about 100 MJ / h is required.

[0279] While it performs its decomposition function, the decomposition vessel can also function as a froth flotation cell operating at high temperature. Because the lithium oxide crystals formed during the decomposition of the lithium nitrate have a lower density than the residual lithium nitrate, the lithium oxide crystals tend to move upward (i.e., "float") to the top of the decomposition vessel. In this regard, the NOx and oxygen gases generated during the decomposition form bubbles in the vessel, and the bubbles move upward to the top of the vessel, entraining and thereby separating the less dense lithium oxide crystals.

[0280] The resulting foam of lithium oxide crystals in the lithium nitrate-rich bubbles forms on top of the residual lithium nitrate slurry and overflows into the peripheral collection trough, while the residual lithium nitrate slurry forms the underflow of the decomposition vessel. The collection trough directs the overflow of lithium oxide crystals in the lithium nitrate-rich bubbles to a screw pressure conveyor (i.e., one or more screw pressure conveyors operating in parallel) via an outlet port and a pipe. The screw pressure conveyor includes a high chromium steel rotor covered with periclase (applied using plasma technology) and a stator made of porous sintered periclase. Other surfaces are lined internally with periclase tiles as required.

[0281] In the screw pressure conveyor, molten lithium nitrate is squeezed out of the lithium oxide crystals and collected at its bottom outlet. The combined lithium nitrate stream from the screw pressure conveyor is combined with the lithium nitrate underflow from the decomposition vessel and pumped to the lithium carbonate precipitation stage (see e.g. Figure 1 , Figure 2 and Figure 6 stage 60).

[0282] By extruding the molten lithium nitrate, the screw press also produces a compacted mass of lithium oxide crystals that is essentially free of liquid, wherein the crystals leave the upper end of the screw press conveyor at a rate of about 3 kg / h of lithium oxide. The lithium oxide is collected and discharged to a lithium oxide ripening tank (see, e.g. Figure 2 and Figure 6The lithium oxide aging tank has a capacity of 100 L. When dry lithium hydroxide is the final product, 1.8 kg / h of process water is added to the aging tank to convert the lithium oxide into dry lithium hydroxide. However, usually lithium hydroxide monohydrate is the final product, in which case 3 kg / h of process water is added to the aging tank to convert the lithium oxide into lithium hydroxide monohydrate. An additional 25 kg / h of hot process water is added to the aging tank to dissolve the lithium hydroxide monohydrate, thereby forming a pumpable solution.

[0283] This lithium hydroxide monohydrate solution is pumped to a lithium hydroxide crystallization tank (see e.g. Figure 2 and Figure 6 The crystallization tank is an electrically heated single-effect unit and requires 20 kW of electricity to evaporate the water.

[0284] The crystallized lithium hydroxide slurry is then transferred from the crystallization tank to a dewatering centrifuge at a rate of 5 kg / h (see, e.g. Figure 2 and Figure 6 The centrifuge produces a lithium hydroxide monohydrate crystal product, wherein the separated liquid is collected to form a part of the recovered liquid stream. In the dewatering centrifuge, the interstitial liquid contained in the lithium hydroxide monohydrate crystal product is washed with wash water and substantially replaced by the wash water, wherein this interstitial liquid is also collected and recovered to form another part of the combined recovered liquid stream. The combined recovered liquid stream is reused in other areas of the process. The first part is sent back to the purification stage of the initial lithium nitrate solution obtained by leaching beta spodumene with nitric acid. The second part is recycled back to the lithium oxide maturation tank. The third part is sent to lithium carbonate production (see, for example Figure 1 , Figure 2 and Figure 7 area 70).

[0285] The combined lithium nitrate stream from the decomposition vessel is collected and sent to the lithium carbonate production stage (see e.g. Figure 1 , Figure 2 and Figure 7 Area 70 in the figure). Approximately 1 kg / h of commercial grade lithium carbonate can be produced.

[0286] First, the lithium nitrate stream containing the centrate and wash liquor from the lithium hydroxide dehydration centrifuge is pumped to a lithium carbonate precipitation tank (see e.g. Figure 2 and Figure 7 The lithium nitrate is present in the tank together with a small amount of sodium nitrate and potassium nitrate. The nitrates of rubidium and cesium are sometimes also present in small amounts.

[0287] A combination of soda ash (Na2CO3) and / or potash (K2CO3) is added to a lithium carbonate precipitation tank to precipitate lithium as slightly soluble lithium carbonate, thereby forming lithium carbonate (Li2CO3), sodium nitrate (NaNO3), and potassium nitrate (KNO3). Tank conditions are optimized for precipitation of lithium carbonate (i.e., noting that the solubility of lithium carbonate decreases with increasing temperature). Therefore, the lithium carbonate precipitation tank is heated to increase the temperature of the contents, thereby increasing the mass of the lithium carbonate precipitate formed.

[0288] A portion of the lithium carbonate can be recycled back to the lithium extraction process, for example to adjust the pH of the crude lithium nitrate solution to precipitate contaminants (e.g. calcium) in the solution. However, the majority of the lithium carbonate slurry is pumped via a slurry pump to a dewatering centrifuge. An intermittently operated dewatering centrifuge (see, e.g. Figure 2 and Figure 7 The slurry is then centrifuged in a centrifuge 708 to remove liquid from the contents of the slurry, thereby producing a lithium carbonate product that is substantially free of liquid. The centrate from the centrifuge contains primarily lithium, sodium, and potassium nitrates, which are discharged to a nitrate storage tank (see, e.g., Figure 2 and Figure 7 This byproduct nitrate stream is discharged to waste or further processed to recover heavier alkali metal salts (e.g., potassium, rubidium, and cesium).

[0289] Note that a pilot plant built according to the above principles can be operated as a fully continuous process using modern process control, automation and monitoring techniques. The pilot plant can be built in stages, thereby optimizing the upstream process while the downstream equipment is installed.

[0290] For example, a pilot plant could be built to produce only lithium oxide product (i.e., having a maximum elemental lithium content of 46.5% by weight, making the associated costs of transporting the lithium oxide to customers relatively low). Next, depending on market conditions, anhydrous lithium hydroxide (containing just over 29% elemental lithium by weight) could be produced. Finally, crystallized lithium hydroxide monohydrate could be produced by adding a single-effect electrically heated crystallizer.

[0291] An advantage of the methods and systems as disclosed herein is that the methods can be terminated at any stage, depending on market and customer preferences. For example, a battery producer may prefer to receive lithium as an oxide because this gives them better control over the purity of the lithium hydroxide produced, in which case the rest of the plant does not need to be built. As another example, an operator of a mine / concentrator may decide to install the methods and systems of the present invention near their facility. They may prefer the simplicity of a process that ends with a lithium oxide product because they can avoid the cost / complexity of a lithium hydroxide crystallization tank. In this way, the methods and systems of the present invention can provide a great degree of flexibility, depending on the needs of the lithium customers and plant operators.

[0292] Further changes

[0293] It will be appreciated that the characteristics of the solution comprising lithium nitrate (whether produced by reacting spodumene with nitric acid or from another source) may vary to the extent that variations to the above-described methods and systems may be appropriate.

[0294] In accordance with good engineering practice, other unit operations may be included in the overall process, particularly for the provision of services and utilities, efficient use of waste heat, water conservation, and minimization of all waste streams.

[0295] In the following claims and foregoing description, unless the context requires otherwise due to express language or necessary implication, the word "comprise" and variations such as "comprises" or "comprising" are used in an inclusive sense, i.e. to indicate the presence of recited features but not excluding the presence or addition of additional features.

[0296] In summary, the present invention includes but is not limited to the following items:

[0297] 1. A method for producing a lithium product from a solution containing lithium nitrate, such as can be obtained by reacting a lithium-containing metal silicate with nitric acid, the method comprising:

[0298] a. subjecting the solution comprising lithium nitrate to a first heat treatment process in which water and nitric acid (when present) are removed and the resulting lithium nitrate-rich crystalline slurry is heated to produce a molten liquid;

[0299] b. conveying the molten liquid to a second heat treatment process, in which the molten liquid is heated to substantially decompose the lithium nitrate into lithium oxide.

[0300] 2. A method according to claim 1, wherein, in the second heat treatment step, the lithium oxide produced during the decomposition of the lithium nitrate is allowed to float to the top of the molten liquid, thereby producing an overflow of the lithium oxide and lithium nitrate slurry and an underflow of the lithium nitrate liquid.

[0301] 3. The method according to claim 2, further comprising conveying the lithium oxide and lithium nitrate slurry to a separation stage in which the lithium nitrate is substantially removed from the lithium oxide to produce lithium oxide crystals.

[0302] 4. A method according to claim 3, wherein the separation stage includes a liquid removal stage, such as a screw press, in which the lithium nitrate is substantially removed from the lithium oxide together with the liquid, thereby producing the lithium oxide crystals.

[0303] 5. A method according to item 3 or 4, wherein the lithium oxide crystals are conveyed to a maturation stage in which a controlled amount of water is added to the lithium oxide crystals to produce a solution comprising lithium hydroxide monohydrate.

[0304] 6. A process according to claim 5, wherein the solution comprising lithium hydroxide is subjected to a crystallization stage in which the solution of lithium hydroxide is concentrated and crystallized to form crystalline lithium hydroxide monohydrate.

[0305] 7. A method according to claim 6, wherein the crystallized lithium hydroxide monohydrate is separated from the solution, such as by centrifugation, thereby producing a crystallized lithium hydroxide product that is substantially free of liquid.

[0306] 8. A method according to any one of items 3 to 7, wherein the lithium nitrate liquid removed from the lithium oxide during the separation stage is collected for further processing.

[0307] 9. A method according to claim 8, wherein the further processing comprises adding a carbonate compound to the lithium nitrate solution so that the lithium reacts with the carbonate to produce lithium carbonate, wherein the further processing is carried out under conditions that promote the precipitation of the lithium carbonate.

[0308] 10. A method according to claim 9, wherein the lithium carbonate precipitate is separated from the solution, such as by using a dewatering centrifuge.

[0309] 11. A method according to claim 10, wherein the solution separated from the lithium carbonate precipitate is recycled as an alkaline solution for use in the method stage in which the lithium-containing metal silicate is reacted with nitric acid.

[0310] 12. A method according to any one of items 9 to 11, wherein the carbonate compound comprises sodium carbonate and / or potassium carbonate, such as can be produced by adding sodium carbonate or potassium carbonate as the carbonate compound.

[0311] 13. A method according to any one of items 9 to 12, when dependent on item 7 or 8, wherein the solution separated from the crystallized lithium hydroxide monohydrate is sent to the carbonate compound for further processing.

[0312] 14. The method according to any one of the preceding items, wherein the first heat treatment process comprises a two-stage heat treatment, the two-stage heat treatment comprising:

[0313] i. a first heating stage in which the solution comprising lithium nitrate is heated to near boiling to remove water and nitric acid (when present);

[0314] ii. A second heating stage in which the resulting slurry of near-boiling lithium nitrate is heated to a temperature of about 400° C. to melt the lithium nitrate in the slurry.

[0315] 15. A method according to any one of the preceding clauses, wherein the first thermal treatment step produces a vapor stream comprising water and nitric acid (when present), wherein the vapor stream is collected and condensed.

[0316] 16. The method according to any one of the preceding items, wherein the temperature of the second heat treatment process is about 600°C to cause the molten lithium nitrate to decompose into lithium oxide and nitrogen oxides.

[0317] 17. The method according to any of the preceding clauses, wherein the second thermal treatment step produces a gaseous stream comprising nitrogen oxides, which is collected and conveyed to a nitric acid production process.

[0318] 18. The method according to any of the preceding clauses, wherein the solution comprising lithium nitrate that is conveyed to the first thermal treatment process is substantially free of contaminants.

[0319] 19. The method according to any one of the preceding items, wherein the solution containing lithium nitrate that is sent to the first heat treatment process is produced in a process in which a lithium-containing metal silicate reacts with nitric acid, the method comprising:

[0320] i) mixing the silicate mineral with the nitric acid;

[0321] ii) subjecting the mixture to a leaching process, the conditions of the leaching process being such that the lithium values ​​in the silicate mineral are leached as lithium nitrate into an aqueous phase;

[0322] iii) conveying the leachate solution to a solid-liquid separation stage to remove remaining solids;

[0323] iv) optionally heating the separated leachate to distill off water and nitric acid as a gas stream, wherein the water and nitric acid gas stream is collected and used for regeneration of nitric acid;

[0324] v) subjecting the leachate to a series of reaction conditions such that contaminants are substantially precipitated from the leachate;

[0325] vi) passing the solution through a filtration stage to remove precipitates, thereby producing a substantially contaminant-free solution comprising lithium nitrate for passing to the first thermal treatment process.

[0326] 20. A method according to claim 19, when dependent on claim 17, wherein the nitric acid produced from the gaseous stream of the second thermal treatment step forms at least part of the nitric acid mixed with the silicate mineral in step i).

[0327] 21. A method according to any one of the preceding items, wherein the lithium-containing metal silicate is spodumene LiAlSi2O6, and / or any one of a series of other lithium-containing silicate minerals, including but not limited to petalite LiAlSi4O 10 ; eucryptite LiAlSiO4; lithium phosphate aluminum (Li,Na)AlPO4(F,OH); and various minerals in the mica family, including lepidolite K(Li,Al,Rb)3(Al,Si)4O 10 (F,OH)2 and lithium iron mica KLiFeAl(AlSi3)O 10 (OH,F)2.

[0328] 22. A system for producing a lithium product from a solution comprising lithium nitrate, such as can be obtained by reacting a lithium-containing metal silicate with nitric acid, the system comprising:

[0329] a. one or more heated vessels in which the solution comprising lithium nitrate can be heated to such an extent that water and nitric acid (when present) are removed as vapors and the resulting crystalline slurry rich in lithium nitrate forms a molten liquid;

[0330] b. A further heated vessel in which the molten liquid is heated to substantially decompose the lithium nitrate into lithium oxide.

[0331] 23. A system according to claim 22, wherein the further heated container is configured as a flotation cell in which the lithium oxide floats as the lithium nitrate decomposes, thereby producing an overflow of lithium oxide and lithium nitrate slurry and an underflow of lithium nitrate, and wherein the overflow is conveyed to a solid-liquid separation stage.

[0332] 24. A system according to claim 23, wherein the solid-liquid separation stage includes a screw press in which the lithium oxide is substantially separated from the lithium nitrate.

[0333] 25. A system according to any one of items 22 to 24, wherein the one or more heated containers include an evaporator in which the solution comprising lithium nitrate is heated to cause water and nitric acid (when present) to evaporate as vapor, thereby producing a supersaturated solution of lithium nitrate.

[0334] 26. A system according to claim 25, wherein the one or more heated containers further include a melting container, the supersaturated solution of lithium nitrate is conveyed to the melting container, and the supersaturated solution of lithium nitrate is heated in the melting container to melt the lithium nitrate.

[0335] 27. A system according to item 26, wherein the melting vessel is heated indirectly, such as by electric induction heating.

[0336] 28. A system according to any one of items 22 to 27, wherein the further heated container also produces a gaseous stream comprising nitrogen oxides, wherein the further heated container is sealed to prevent such gases from escaping into the atmosphere.

[0337] 29. A system according to claim 28, further comprising a nitric acid plant, wherein the gaseous stream comprising nitrogen oxides produced by the further heated container is conveyed to the nitric acid plant to produce nitric acid.

[0338] 30. A system according to any one of items 22 to 29, wherein a condenser is arranged to collect and condense vapor from the one or more heated containers and, when present, from the evaporator.

[0339] 31. A system according to any one of items 22 to 30, wherein the further heated container is heated indirectly, such as by electric induction heating.

[0340] 32. A system according to any one of items 22 to 31, further comprising an apparatus arranged to produce a crystalline lithium hydroxide product from the lithium oxide produced in the further heated container.

[0341] 33. A system according to any one of items 23 or 24, or items 25 to 32 when dependent on item 23 or 24, further comprising an apparatus arranged to produce a lithium carbonate product from the lithium nitrate underflow and from the lithium nitrate produced in the solid-liquid separation stage.

Claims

1. A method for producing a lithium product from a solution containing lithium nitrate, the method comprising: a. subjecting the solution comprising lithium nitrate to a first heat treatment process in which water and nitric acid (when present) are removed and the resulting lithium nitrate-rich crystalline slurry is heated to produce a molten liquid; b. conveying the molten liquid to a second heat treatment process, in which the molten liquid is heated to substantially decompose the lithium nitrate into lithium oxide.

2. The method according to claim 1, wherein: In the second heat treatment process, the lithium oxide produced during the decomposition of the lithium nitrate is allowed to float to the top of the molten liquid, thereby generating an overflow of the lithium oxide and lithium nitrate slurry and an underflow of the lithium nitrate liquid.

3. The method of claim 2, further comprising conveying the overflow of the lithium oxide and lithium nitrate slurry to a separation stage in which the lithium nitrate is substantially removed from the lithium oxide.

4. A method according to claim 3, wherein the lithium oxide in the overflow of the lithium oxide and lithium nitrate slurry comprises lithium oxide crystals, and wherein the separation stage comprises a liquid removal stage in which the lithium nitrate is removed from the lithium oxide crystals together with the liquid.

5. The method of claim 4, wherein a screw press is used in the liquid removal stage to substantially remove the lithium nitrate from the lithium oxide crystals together with the liquid.

6. The method of claim 4 or 5, further comprising conveying the lithium oxide crystals to a maturation stage in which water is added to the lithium oxide crystals to produce a solution comprising lithium hydroxide monohydrate.

7. The method of claim 6, further comprising a crystallization stage in which the solution of lithium hydroxide monohydrate is crystallized to form crystalline lithium hydroxide monohydrate.

8. The method of claim 7, further comprising separating the crystallized lithium hydroxide monohydrate from solution to produce a crystallized lithium hydroxide product substantially free of liquid.

9. The process of claim 8, wherein the crystallized lithium hydroxide monohydrate is separated from the solution by centrifugation.

10. A method according to any one of claims 3 to 9, further comprising collecting the lithium nitrate removed from the lithium oxide during the separation stage for further processing.

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