Method for producing lithium sulfide in a circulating bed reactor

Abstract

The present invention relates to a method for preparing lithium sulfide (Li2S) from lithium hydroxide (LiOH) and hydrogen sulfide (H2S), characterised in that the lithium hydroxide is sulfided in a reaction zone (9) comprising at least one moving bed tubular reactor (9a) having the configuration of an ascending vibrating spiral in which the lithium hydroxide (4) flows in counter-current to an anhydrous gas mixture (10) containing hydrogen sulfide and at least one inert gas.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for producing lithium sulfide in a circulating bed reactor

[0003] The subject of the present invention is the preparation of lithium sulfide from lithium hydroxide and hydrogen sulfide by a continuous mode process using a moving bed (or circulating bed).

[0004] STATE OF THE PRIOR ART

[0005] Lithium sulfide (Li2S) is a key raw material for the manufacture of solid electrolytes used in next-generation batteries. Solid electrolytes are less flammable than liquid electrolytes and potentially more efficient and lighter.

[0006] There are several known methods for synthesizing lithium sulfide, all of which are hampered by the extremely hydrophilic nature of this compound. Indeed, at room temperature, lithium sulfide is very reactive to moisture and reacts spontaneously upon contact with water to form lithium hydroxide (LiOH) by releasing hydrogen sulfide (H2S). Thus, the reaction conditions must be perfectly controlled to avoid any degradation of the lithium sulfide as it forms. Once the lithium sulfide is formed, strict control of its residual water content is essential, particularly via storage in a dry and inert atmosphere.

[0007] It is also desirable to be able to produce lithium sulfide with the highest possible degree of purity and in the form of solid particles with an average particle size of less than 1 mm. These properties lead to good stability and good electrical capacity of the electrolyte.

[0008] A known method for the synthesis of lithium sulfide is to react lithium hydroxide with hydrogen sulfide, according to the following reaction:

[0009] 2 LiOH + H2S -> Li2S + 2 H2O This route is widely used because the starting raw material LiOH is available and inexpensive and because the reaction, endothermic, can be carried out at a moderate temperature generally between 130 and 450°C.

[0010] However, industrial processes using this synthesis route face several drawbacks, starting with the significant quantity of water produced by the reaction, which must be completely eliminated to avoid the reconversion of LiiS into LiOH.

[0011] Furthermore, the starting raw material LiOH is generally in the hydrated form LiOH.HiO, and an extensive drying step of this is necessary before the actual sulfurization can start.

[0012] In fact, the sulfurization rate of the hydrated form LiOH.HiO is too low.

[0013] Furthermore, the presence of this hydrated form in the sulfidation reactor leads to the formation of lithium sulfide blocks, i.e. larger lithium sulfide particles consisting of LiiS aggregates surrounding an internal core of unsulfided LiOH.HiO. Thus, the quality of the lithium sulfide obtained is greatly reduced: not only is its purity lower, but its average particle size also increases.

[0014] Various processes have been described in the prior art, including in particular batch processes (or so-called "batch" processes), using reactors in which the solid particles can be stirred or placed in a fluidized bed. Such batch processes are described, for example, in patent applications EP 0 802 159, JP2020033259A, JP2015137183 and

[0015] WO2018 / 141919.

[0016] These processes often require a very large quantity of sulfurizing agent (H2S). They also face the problems of maintaining the reactor temperature (the reaction being endothermic) and of eliminating the water formed as the reaction progresses. Finally, processes operating in batch mode are less productive on an industrial scale because they require regular interruptions in production and significant logistics for loading and unloading the reactors.

[0017] These processes are therefore uneconomical and often lead to lithium sulfide of lower quality and purity, which contains residual LiOH and particles of larger particle size due to the presence of LiiS and LiOH.HiO aggregates.

[0018] Few continuous mode processes exist, due to the significant constraints associated with the LiOH sulfidation reaction.

[0019] The present invention aims to provide a process which makes it possible to prepare lithium sulfide on an industrial scale in continuous mode.

[0020] The invention also aims to enable the preparation of lithium sulfide of high chemical purity, in the form of small particle size.

[0021] SUMMARY OF THE INVENTION

[0022] The Applicant has now developed an innovative process which makes it possible to produce excellent quality lithium sulfide from lithium hydroxide and hydrogen sulfide by a continuous mode process using a moving bed (or circulating bed) circulating in at least one reactor having a particular configuration.

[0023] The process according to the invention is characterized in that the key step of sulfurization of lithium hydroxide is carried out in a reaction zone comprising at least one tubular reactor whose configuration is that of an ascending vibrating spiral, in which the lithium hydroxide circulates countercurrent to a flow of anhydrous sulfurizing gas containing hydrogen sulfide and an inert gas.

[0024] Thus, the subject of the present invention is a process for preparing lithium sulfide (LiiS) from lithium hydroxide (LiOH) and hydrogen sulfide (H2S), characterized in that the sulfurization of the lithium hydroxide is carried out in a reaction zone comprising at least one tubular moving bed reactor having the configuration of an ascending vibrating spiral, in which the lithium hydroxide circulates countercurrently to an anhydrous gas mixture containing hydrogen sulfide and at least one inert gas. The process according to the invention makes it possible to prepare lithium sulfide with high yields, and in particular with a mass yield greater than 80%.

[0025] In addition, it allows the sulfurization of lithium hydroxide to be carried out under controlled temperature conditions. In particular, maintaining a temperature in the reactor within the desired operating range of 150 to 450°C is facilitated, the endothermic nature of the reaction being compensated by the circulation of the bed of lithium hydroxide particles.

[0026] Furthermore, unlike the methods of the prior art, the lithium sulfide formed as it progresses in the reactor does not remain in contact with the water, which is eliminated with the gas flow circulating in the opposite direction.

[0027] Thus, the process according to the invention makes it possible to obtain lithium sulfide of high purity, greater than 99.0% by mass.

[0028] In addition, the lithium sulfide leaving the reactor only encounters the incoming flow of anhydrous sulfur gas based on hydrogen sulfide and inert gas, which guarantees the production of a product with a particularly low residual lithium hydroxide content, less than 1% by mass.

[0029] According to the present invention, the reaction zone comprises at least one tubular reactor whose configuration is that of an ascending vibrating spiral. In such a reactor, the LiOH particles progress upwards along a vibrating helical turn, countercurrent to the descending flow of anhydrous sulfur gas.

[0030] This configuration has the additional advantage of making it possible to avoid the formation of lithium sulfide blocks in the form of LiiS aggregates surrounding an internal core of LiOH.HiO. Indeed, the LiOH particles move in upward jumps ensured by the vibrations of the helical coil, which ensures high mixing of these within the sulfurizing gas flow and leads to better contact of the LiOH particles with the sulfurizing gas, while avoiding the formation of clusters by impacts of the particles on the walls of the reactor. Compared to the methods described in the prior art, this configuration also makes it possible to reduce the sulfurizing gas flow rate and the quantity of hydrogen sulfide used, thanks to the better contact of the sulfurizing gas and the lithium hydroxide. It makes it possible to accelerate the sulfurization reaction of the latter and to eliminate water efficiently as it is formed.

[0031] Other objects, characteristics, aspects and advantages of the invention will appear even more clearly on reading the description and the attached non-limiting figure: [Fig 1] illustrates an example of a lithium sulfide production unit in accordance with the process of the invention.

[0032] In what follows, and unless otherwise indicated, the limits of a domain of values ​​are included in this domain, notably in the expressions "between" and "ranging from ... to ..."

[0033] Furthermore, the expressions "at least one" and "at least" used in this description are respectively equivalent to the expressions "one or more" and "greater than or equal to".

[0034] DETAILED DESCRIPTION

[0035] Sulfur dioxide

[0036] The present invention uses an anhydrous gas mixture containing hydrogen sulfide and at least one inert gas.

[0037] This gas mixture is anhydrous, that is to say its water content is less than or equal to 1% by volume.

[0038] The gas mixture contains hydrogen sulfide (H2S), which is the sulfurizing agent reacting with lithium hydroxide to form lithium sulfide, releasing water.

[0039] The hydrogen sulfide content of the gas mixture is advantageously in the range from 30 to 90% by volume, preferably from 40 to 80%, more preferably from 50 to 70%, and better still from 55 to 65% by volume, relative to the total volume of said mixture. The gas mixture also comprises at least one inert gas, i.e. a non-reactive gas. Inert gases are well known to those skilled in the art.

[0040] The inert gas(es) may in particular be chosen from nitrogen (N2) and noble gases such as argon, helium, krypton, neon and xenon, and mixtures thereof.

[0041] Preferably, the inert gas(es) are chosen from argon, nitrogen and their mixtures, and more preferably the inert gas is nitrogen.

[0042] The inert gas content is advantageously in the range from 10 to 70% by volume, relative to the total volume of the gas mixture.

[0043] Preferably, the gas mixture further comprises hydrogen (H2). In this case, the hydrogen content of the gas mixture is advantageously in the range from 5 to 30% by volume, preferably from 10 to 20%, relative to the total volume of the mixture.

[0044] The reaction zone

[0045] In the present invention, the sulfurization of lithium hydroxide is carried out in a reaction zone comprising at least one tubular moving bed reactor having the shape of an ascending vibrating spiral, in which the lithium hydroxide circulates countercurrently to the sulfurizing gas.

[0046] A moving bed reactor is a term known per se to mean any reactor in which solid particles circulate from the inlet to the outlet of the reactor. In the reactor in the form of an ascending vibrating spiral implemented in the invention, the particles progress upwards along a spiral. Their progression is ensured by the vibrations of the tubular reactor.

[0047] A feature of the process of the invention is that the conversion of lithium hydroxide into lithium sulfide is carried out in a tubular reactor at the inlet of which the lithium hydroxide particles are introduced and circulated towards the outlet. Conversely, the sulfurous gas is introduced at the outlet of the reactor and circulates towards the inlet thereof.

[0048] In this description, the terms reactor inlet and outlet are defined with respect to the upward flow direction of the solid lithium hydroxide particles in the tubular reactor.

[0049] So inside the reactor, two flows circulate counter-current to each other, one solid and the other gaseous:

[0050] - The solid stream consists of lithium hydroxide particles. As it progresses upward in the reactor, the solid stream becomes depleted in lithium hydroxide and enriched in lithium sulfide;

[0051] - The gas stream consists of sulfurous gas, i.e. the anhydrous gas mixture containing hydrogen sulfide and at least one inert gas. As it progresses downward in the reactor, the gas stream becomes depleted in hydrogen sulfide and enriched in water vapor.

[0052] According to a preferred embodiment, said anhydrous gas mixture is introduced into the reaction zone at at least two points thereof: at the outlet of the reaction zone and at at least one point located between the outlet and the inlet of the reaction zone.

[0053] More preferably, said anhydrous gas mixture is introduced at the outlet of the reaction zone and at at least two different successive points positioned between the outlet and the inlet of the reaction zone.

[0054] The introduction of the anhydrous gas mixture at several successive points along the reaction zone provides in particular the following advantages:

[0055] - It allows working locally in stoichiometric excess of H2S to guarantee a maximum conversion rate of LiOH into LiiS;

[0056] - It allows to locally lower the partial pressure in H2O and therefore to limit its inhibitory effect on the conversion of LiOH into Li2S.

[0057] In this embodiment, the composition of said anhydrous gas mixture may be different between the different points of introduction. In particular, the H2S content may be different, for example the higher the closer the point of injection of the mixture is to the outlet of the reaction zone.

[0058] The temperature inside the reactor(s) is advantageously maintained in the range from 150 to 450°C, preferably from 300 to 450°C and better still from 350 to 400°C.

[0059] The temperature inside the reactor(s) can be determined in a manner known per se, using thermocouples.

[0060] Due to the endothermic nature of the reaction, it is important to be able to control the temperature to ensure the most constant temperature possible in the reactor, and in any case, to avoid having reaction zones at a temperature lower than or equal to 100°C. This is facilitated by the implementation of the countercurrent process and by the potential presence of multiple reactors in the reaction zone, as described below.

[0061] According to the present invention, the temperature at the inlet of the reaction zone is greater than or equal to 350°C and less than or equal to 450°C.

[0062] At the exit of the reaction zone, the temperature is generally less than or equal to 450°C, or even less than or equal to 350°C.

[0063] The pressure inside the reactor(s) is maintained at a value below 3 bars (3.10 5 Pa), preferably less than 2 bars (2.10 5Pa), and more preferably still less than 1.3 bars (1.3. 10 5 Pa).

[0064] The Applicant has surprisingly found that the process of the invention makes it possible to obtain conversion efficiencies of LiOH into LiiS using smaller quantities of H2S and with lower circulation speeds than in the prior art. In the process of the invention, the hourly space velocity of hydrogen sulfide is advantageously in the range from 30 to 450 h' 1 .

[0065] A particular reactor implemented in the invention consists of a vibrating spiral of substantially tubular shape winding helically around a vertical axis, and comprising at least two pitches. The cross-section of the spiral is preferably circular and in this case, the spiral is a tube. In general, the tube has a diameter of between 100 and 300 mm. It typically has a developed length of up to 400 m.

[0066] The tube is hollow, that is to say it does not include any elements in its interior part.

[0067] The total height of the spiral can range from 5 to 40 m, preferably 10 to 20 m.

[0068] The spiral rise angle can range from 1 degree to 10 degrees, preferably from 1 to 5 degrees, and even more preferably from 1 to 4 degrees.

[0069] The reactor has a number of turns preferably ranging from 15 to 60, more preferably from 25 to 40.

[0070] Typically, the number of turns is such that it allows a particle circulation speed ranging from 50 to 6,000 kg / h, preferably from 50 to 500 kg / h, and a gas hourly space velocity (GHSV) typically from 50 to 1,500 h 1 , preferably 50 to 500 h 1 . Solid particles typically occupy 5 to 80% of the volume of the coils, preferably 10 to 50%.

[0071] Said vibrating spiral is advantageously made of a metallic material. Preferably, it is made of a metal tube made of a metal alloy, more preferably steel.

[0072] It can be obtained, for example, by shaping a metal tube into a helix around a substantially vertical axis. According to an advantageous embodiment, a central shaft makes it possible to stiffen and support the helix formed by the spiral. The spiral can be electrically insulated from the central shaft by the fixing system.

[0073] According to a preferred embodiment, a transformer supplies the vibrating spiral in at least one step (i.e. at least one turn) with low voltage current, less than 50V, which makes it possible to directly heat the metallic mass of the tube by Joule effect to the required temperature in the reactor.

[0074] In particular, one or more steps (one or more coils) are heated by Joule effect to a temperature between 150 and 450°C, particularly in the lower part of the reactor, in the LiOH particle entry zone. The Joule effect has the direct consequence of generating heat in the mass of the coil. It allows for greater flexibility in controlling the temperature at the heart of the coil compared to indirect heating, for example using a heat transfer fluid.

[0075] The spiral vibrations of the reactor may be produced by at least one system placed at any suitable level, for example at the base or top of the drum or positioned around the drum. Suitable vibration systems include unbalanced motors, electromagnetic vibrators (excited by a variable cycle, with pulse generation) and unbalanced excitations. Preferably, the vibrations are produced by a table supporting the central drum and driven by two unbalanced motors.

[0076] The reaction zone may consist of one or more moving bed reactors.

[0077] Preferably, the reaction zone contains at least two moving bed reactors. The reaction zone may thus consist of several moving bed reactors, which may be arranged in series and / or in parallel. Said moving bed reactors may be of identical configurations (in particular when they are arranged in parallel) or different. Thus, a plurality of moving bed reactors may be used, all or some of which may consist of tubular reactors having the configuration of vibrating spirals. When several vibrating spirals are used, they may be of different sizes.

[0078] Lithium sulfide LiS

[0079] At the outlet of the reaction zone, the lithium sulfide is recovered in the form of small solid particles such as, for example, beads, particles of more or less cylindrical shape or irregular shape.

[0080] The number average size of the lithium sulfide particles, corresponding to the diameter of the equivalent spherical volume, is preferably less than or equal to 4 mm, and more preferably less than or equal to 1 mm.

[0081] The average size here refers to the number-average value of the particle diameter by assimilating them to spheres, and is defined by the median diameter d50, measured by laser diffraction granulometry, for example using a known device such as a laser diffraction particle size analyzer (in English "Laser Diffraction Particle Size Analyzer"), which makes it possible to determine the size distribution of a population of particles.

[0082] In the case where the particles are indeed spherical, the average size is equal to the average diameter.

[0083] According to an advantageous embodiment, part of the flow of particles leaving the reaction zone is recycled into said zone, either at its inlet or at an intermediate point thereof. Recycling at an intermediate point is particularly facilitated when the reaction zone comprises several reactors in series, the recycled particles being introduced for example between two successive individual reactors.

[0084] Such recycling of the particle flow at the outlet of the reaction zone is advantageous when the conversion of LiOH into LiiS is not complete after a single pass through the reaction zone.

[0085] Lithium hydroxide LiOH

[0086] The lithium hydroxide introduced at the inlet of the reaction zone is in the form of small solid particles such as, for example, beads, particles of more or less cylindrical shape or of irregular shape.

[0087] The number average size of the lithium hydroxide particles, corresponding to the diameter of the equivalent spherical volume, is preferably in the range from 10 pm to 4 mm.

[0088] The average size here refers to the number-average value of the particle diameter by assimilating them to spheres, and is defined by the median diameter d50, measured by laser diffraction granulometry, for example using a known device such as a laser diffraction particle size analyzer (in English "Laser Diffraction Particle Size Analyzer"), which makes it possible to determine the size distribution of a population of particles.

[0089] In the case where the particles are indeed spherical, the average size is equal to the average diameter.

[0090] According to a preferred embodiment, the lithium hydroxide is introduced into the reaction zone with a water content of less than 10 mol%.

[0091] The preliminary drying step

[0092] According to a preferred embodiment, prior to its introduction into the reaction zone, the lithium hydroxide undergoes a drying step. This step is typically carried out in a drying zone and aims to produce lithium hydroxide in anhydrous form according to the following process:

[0093] LiOH.H2O - LiOH + H2O

[0094] The drying step is preferably carried out by subjecting the lithium hydroxide to a heat treatment at a temperature in the range of 150 to 350°C, preferably 175 to 250°C, and circulating at least one inert gas in the drying zone so as to remove water.

[0095] The inert gas(es) may in particular be chosen from nitrogen (N2) and noble gases such as argon, helium, krypton, neon and xenon, and mixtures thereof.

[0096] Preferably, the inert gas(es) are chosen from argon, nitrogen and their mixtures, and more preferably the inert gas is nitrogen.

[0097] The pressure inside the drying zone is advantageously maintained at a value lower than 3.10 5 Pa (3 bars), preferably less than 2.10 5 Pa (2 bars) and better still less than or equal to 1.3. 10 5 Pa (1.3 bars).

[0098] The drying step is preferably carried out in continuous mode, and more preferably in a drying zone comprising one or more moving bed reactors in which the lithium hydroxide particles circulate. The inert gas flow can then circulate, in the drying zone, co-currently or counter-currently with the flow of lithium hydroxide particles. Preferably, the inert gas flow circulates in the drying zone co-currently with the flow of lithium hydroxide particles.

[0099] According to a first particularly preferred embodiment, at least one moving bed reactor of the drying zone is a tubular reactor in the form of an ascending vibrating spiral. In such a reactor, the particles progress upwards along a spiral, in which they are gradually converted into anhydrous LiOH with removal of water.

[0100] Tubular reactors in the form of a vibrating spiral have been described above. In this embodiment, the inert gas flow preferably circulates upwards in said vibrating spiral (i.e. co-current with the flow of solid particles). However, a circulation of the inert gas flow downwards in the vibrating spiral (i.e. counter-current with the flow of solid particles) can also be implemented.

[0101] According to a second embodiment, at least one moving bed reactor of the drying zone is a horizontal tubular reactor comprising a thermal screw, i.e. an endless screw, or Archimedes screw, in which the particles are conveyed and dried in a continuous flow, with removal of water. In such a reactor, the LiOH.HiO particles progress along the blades of a screw, in which they are gradually converted into anhydrous LiOH with removal of water.

[0102] The thermal screw can be heated electrically or by a heat transfer fluid and the heat exchange can occur through the trough, the central core or the coils. The inert gas flow can be injected co- or counter-current to the flow of solid particles, preferably co-current.

[0103] The presulfurization stage

[0104] Preferably, a pre-sulfurization of the lithium hydroxide is additionally carried out during the drying step as described above. Pre-sulfurization means a partial sulfurization of the lithium hydroxide, such that the anhydrous lithium hydroxide at the end of the drying step contains from 5 to 40% by mass of lithium sulfide Li2S.

[0105] This pre-sulfurization is advantageously carried out by placing the lithium hydroxide particles during their circulation in the drying zone in contact with an anhydrous gas mixture containing at least one inert gas and from 5 to 30% by volume of hydrogen sulfide relative to the total volume of said mixture.

[0106] This gas mixture is anhydrous, that is to say its water content is less than or equal to 1% by volume.

[0107] The hydrogen sulfide content of the gas mixture used for pre-sulfurization is more preferably in the range of 10 to 15% by volume, relative to the total volume of said mixture.

[0108] The inert gas(es) are chosen from those described above for the drying step, including nitrogen (N2), noble gases and their mixtures. Preferably, the inert gas(es) are chosen from argon, nitrogen and their mixtures, and more preferably the inert gas is nitrogen.

[0109] According to a preferred embodiment, the inert gas(es) present in the gas mixture used for the pre-sulfurization of lithium hydroxide is (are) that (those) used for drying. Thus, for example, the pre-sulfurization can be carried out by directly adding hydrogen sulfide, or a mixture comprising hydrogen sulfide, inert gas and optionally hydrogen, to the gas stream circulating in the drying zone as described below.

[0110] Preferably, the gas mixture used for pre-sulfurization further comprises hydrogen (H2). In this case, the hydrogen content of the gas mixture is advantageously in the range from 1 to 10% by volume, preferably from 2 to 5% by volume, relative to the total volume of said mixture.

[0111] According to a preferred embodiment, said gas mixture used for the pre-sulfurization of lithium hydroxide consists entirely or partly of the gas mixture recovered at the outlet of the reaction zone, previously dried to remove the water. This gas mixture can be diluted if necessary with inert gas to adjust the hydrogen sulfide content to the concentration required for the pre-sulfurization.

[0112] The gas mixture used for pre-sulfurization can be introduced at one or more points in the drying zone, preferably located downstream of the lithium hydroxide particle inlet zone. Indeed, the pre-sulfurization step is advantageously initiated at a point in the drying zone where the water content of the lithium hydroxide is sufficiently low.

[0113] Thus, the gas mixture used for pre-sulfurization is preferentially brought into contact with the lithium hydroxide circulating in the drying zone at one or more injection points located in the downstream part of the latter, that is to say in its second half, relative to the injection point of the solid.

[0114] Preferably, the gas mixture used for pre-sulfurization circulates in the drying zone co-currently with the flow of lithium hydroxide particles.

[0115] In the case where the drying zone is in the form of an ascending vibrating spiral, typically the gas mixture used for pre-sulfurization is introduced at one, two or three points, preferably located in the upper half of the spiral.

[0116] It circulates advantageously in an ascending manner, co-current with the ascending flow of solid particles.

[0117] The lithium hydroxide pre-sulfurization step is advantageously carried out at a temperature in the range from 150 to 350°C, preferably from 175 to 250°C.

[0118] This pre-sulfurization step improves the drying rate of lithium hydroxide and increases the sulfurization rate in the reaction zone located downstream.

[0119] Figure 1 in the appendix illustrates a non-limiting example of a lithium sulfide production unit. In accordance with the present invention, lithium hydroxide (LiOH) is converted into lithium sulfide (LiiS) in a reaction zone 9, which comprises a tubular moving bed reactor consisting of an ascending vibrating spiral 9a. The lithium hydroxide particles are introduced into the lower part of the reaction zone 9 via line 4. The particles progress upward in the spiral 9a, this progression being caused by the vibrations of the spiral.

[0120] An anhydrous gaseous mixture containing hydrogen sulfide in stoichiometric excess and nitrogen is introduced into the upper part of the reaction zone 9. This mixture is conveyed via line 10 and introduced into the spiral reactor 9a at two successive injection points 10a and 10b located in the upper part of said reactor. It circulates downwards in the spiral reactor 9a, counter-current to the ascending flow of particles.

[0121] At the outlet of reaction zone 9, the lithium sulfide particles are recovered and evacuated via line 11.

[0122] In the lower part of the reaction zone 9, a gaseous mixture containing nitrogen, water and a hydrogen sulfide residue is evacuated at two successive points 12a and 12b and conveyed via line 12 to a treatment unit 13.

[0123] In unit 13, the mixture from line 12 is treated to remove the water, which is separated by line 14. It is also possible to carry out dust removal (not shown) of the gas mixture, in order to remove any dust particles that may be entrained. The residual nitrogen and hydrogen sulfide mixture is then discharged via line 15.

[0124] Prior to its introduction into the reaction zone, the lithium hydroxide undergoes a drying step carried out in drying zone 2 which comprises a tubular moving bed reactor which in this example consists of an ascending vibrating spiral 2a.

[0125] The hydrated lithium hydroxide particles (LiOH.HiO) are introduced into the lower part of drying zone 2 through line 1. The particles progress upwards in spiral 2a, this progression being caused by the vibrations of the spiral.

[0126] An inert gas consisting of nitrogen supplied by line 3 is also introduced into the lower part of the drying zone 2, and injected into the spiral 2a at the injection points 3a and 3b. The nitrogen progresses upwards in the spiral reactor 2a, co-current with the particle flow.

[0127] In the upper part of drying zone 2, the dried lithium hydroxide particles are discharged via line 4 and transferred to reaction zone 9. Also in the upper part of drying zone 2, a mixture of water and nitrogen is discharged at successive points 5a and 5b, and conveyed via line 5 to a treatment unit 6.

[0128] In unit 6, the mixture from line 5 is treated to remove the water which is separated by line 7. It is also possible to carry out dust removal (not shown) of the gas mixture, in order to remove any dust particles which may be entrained. The nitrogen is then evacuated via line 8. According to an advantageous embodiment not shown, the nitrogen thus recovered at the outlet of the drying zone is recycled to the unit, either at the level of drying zone 2 or at the level of reaction zone 9.

[0129] According to a preferred optional embodiment, the residual nitrogen and hydrogen sulfide gas mixture from the reaction zone 9 and dried, recovered via line 15, is recycled via line 16 to the drying zone 2 where it is introduced into the second upper half of the spiral reactor 2a. This recycled gas mixture contains a lower hydrogen sulfide content than the gas mixture introduced into the reaction zone 9. This recycling makes it possible to carry out in the second upper half of the drying zone 2 a presulfurization of the lithium hydroxide particles, upstream of the reaction zone 9.

Claims

CLAIMS 1. Process for the preparation of lithium sulfide (LiiS) from lithium hydroxide (LiOH) and hydrogen sulfide (H2S), characterized in that the sulfurization of the lithium hydroxide is carried out in a reaction zone (9) comprising at least one tubular moving bed reactor (9a) having the configuration of an ascending vibrating spiral in which the lithium hydroxide (4) circulates countercurrent to an anhydrous gas mixture (10) containing hydrogen sulfide and at least one inert gas.

2. Method according to the preceding claim, characterized in that the hydrogen sulfide content of the gas mixture (10) is in the range from 30 to 90% by volume, preferably from 40 to 80%, more preferably from 50 to 70%, and better still from 55 to 65% by volume, relative to the total volume of said mixture.

3. Method according to any one of the preceding claims, characterized in that the inert gas(es) are chosen from nitrogen, noble gases, and their mixtures, preferably from argon, nitrogen and their mixtures, and more preferably the inert gas is nitrogen.

4. Method according to any one of the preceding claims, characterized in that said gaseous mixture (10) further comprises hydrogen at a content in the range from 5 to 30% by volume, preferably from 10 to 20%, relative to the total volume of the mixture.

5. Method according to any one of the preceding claims, characterized in that said anhydrous gaseous mixture (10) is introduced into the reaction zone (9) at at least two points (10a, 10b) thereof: at the outlet of the reaction zone (10a) and at at least one point located between the outlet and the inlet of the reaction zone (10b).

6. Method according to any one of the preceding claims, characterized in that the reaction zone (9) comprises at least two moving bed reactors, arranged in series and / or in parallel.

7. Method according to any one of the preceding claims, characterized in that the reaction zone (9) comprises a plurality of moving bed reactors which may all, or some of them, consist of vibrating spirals.

8. Method according to any one of the preceding claims, characterized in that prior to its introduction into the reaction zone (9), the lithium hydroxide (1) undergoes a drying step in a drying zone (2), this step preferably being carried out in continuous mode.

9. Method according to the preceding claim, characterized in that the drying zone (2) comprises one or more moving bed reactors (2a) in which the lithium hydroxide particles circulate.

10. Method according to the preceding claim, characterized in that at least one moving bed reactor of the drying zone (2) is a tubular reactor in the form of an ascending vibrating spiral (2a).

11. Method according to one of claims 9 and 10, characterized in that a pre-sulfuration of the lithium hydroxide is carried out during the drying step, by putting the lithium hydroxide particles (1) during their circulation in the drying zone (2) in contact with an anhydrous gas mixture containing at least one inert gas and from 5 to 30% by volume of hydrogen sulfide relative to the total volume of said mixture.

12. Method according to the preceding claim, characterized in that the hydrogen sulfide content of the gas mixture used for pre-sulfurization is in the range from 10 to 15% by volume, relative to the total volume of said mixture.

13. Method according to one of claims 11 and 12, characterized in that said gaseous mixture used for the pre-sulfurization of lithium hydroxide (1) consists in whole or in part of the gaseous mixture (16) recovered at the outlet of the reaction zone (9), previously dried to remove the water.

14. Method according to one of claims 11 to 13, characterized in that the gas mixture used for pre-sulfurization circulates in the drying zone (2) in co-current with the flow (1) of lithium hydroxide particles.

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