FR2814460A1 - Manufacturing lithium titanate micro sintering grain, involves dispersing lithium, titanium in an aqueous binder solution, dropping liquid in bath containing polyvalent metal, drying grains, calcinating, sintering - Google Patents

Abstract

Raw material powder containing lithium and titanium is dispersed in a liquid containing aqueous binder solution of algenic acid. The dispersion liquid is dropped as moist gel grain through a nozzle into a liquid bath containing polyvalent metal. The binder solution is gelled. The grains are dried and subjected to calcination to remove the polyvalent metal. The grains are subsequently sintered.

Description

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départ pulvérisé est dispersé, sécher les particules de gel et fritter les particules ainsi séchées afin d'obtenir de fins granules frittés de titanate de lithium. The present invention relates to a process for producing sintered granules of lithium titanate. In particular, it relates to a process for producing fine sintered granules of lithium titanate, Li2TiO3, of high purity and sphericity by means of a process which includes the steps of forming gel particles in which the product of

initially sprayed is dispersed, dry the gel particles and sinter the particles thus dried in order to obtain fine sintered granules of lithium titanate.

 In the known process for producing sintered granules of metal oxides, wet granulation using gelation has been advantageously used to form raw particles to be sintered. Wet granulation allows the diameter of the raw particles to be easily adjusted and the dimensions of the resulting sintered granules can be finer than those of the granules obtained by fusion granulation or by drum granulation. Wet granulation also makes it possible to avoid drawbacks such as contamination of the heated bath under the effect of impurities coming from the crucible during the melting process of the pulverized starting product, in the case of fusion granulation, or a limitation of the sphericity of the particles as well as a dispersion of the distribution of the dimensions of the particles obtained by the rotating drum in the case of granulation with the drum.

Le brevet français n 2 761 057 décrit une utilisation de la granulation par voie humide mentionnée ci-dessus pour produire des granules frittés de titanate de lithium, Li2TiO3. Cependant, dans la production de fins granules U.S. Patent No. 5,676,919 describes a process for the production of sintered granules of lithium oxides, Li 2 O, in which the gel particles which contain a dispersion of a sprayed starting product are formed by wet granulation, and sintered granules are then obtained from the gel particles. In this process, a water-soluble, polymeric resinous compound with hydroxyl groups is used as the aqueous gelling binder solution in which a sprayed starting material solution is dispersed. The dispersion thus obtained is poured dropwise into a solidification bath in order to form gel particles, and the gel particles are then dried and sintered to form fine sintered granules.

French Patent No. 2,761,057 describes a use of the wet granulation mentioned above to produce sintered granules of lithium titanate, Li2TiO3. However, in the production of fine granules

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qu'un composant qui lance une polymérisation de la solution aqueuse de composé résineux polymère soit présent, en raison de la réaction avec des groupes hydroxyl. Afin d'éviter la solidification indésirable par polyméri- sation, il faut donc limiter le type de produits de départ pulvérisés dont l'utilisation est admissible. sintered with lithium titanate, powders of various components, such as lithium titanate Li2Ti03 which is the source of lithium and titanium oxide TiOz which is the source of titanium, must be selectively mixed in the starting product sprayed in accordance with lithium and titanium contents necessary to produce the product particles. When the sprayed starting product is dispersed in an aqueous binder solution, it is therefore possible, depending on the way used to prepare the sprayed starting product,

that a component which initiates polymerization of the aqueous solution of polymeric resinous compound is present, due to the reaction with hydroxyl groups. In order to avoid undesirable solidification by polymerization, it is therefore necessary to limit the type of sprayed starting products the use of which is admissible.

 Due to the high viscosity of the drop-by-drop solution prepared by dispersing the sprayed starting material in the aqueous binder solution of the water-soluble polymeric resinous compound, pump means must also be used under relatively high discharge pressures to bring the solution to the nozzle drop by drop. In addition, the high viscosity drip solution tends to stick to the nozzle opening in the atmosphere and to form a solid which blocks the nozzle opening and interferes with a constant drip flow. , so it is sometimes impossible to continue the drip flow.

trictions ou limitations à la préparation du produit de départ pulvérisé. In view of the problems set out above, it is a main object of the present invention to provide an improved process for the production of sintered granules of lithium titanate which makes it possible to avoid solidification before gelation of the dropwise solution. drop and eliminate res-

restrictions or limitations on the preparation of the sprayed starting material.

It is a second object of the present invention to provide a process for the production of sintered granules of lithium titanate which makes it possible to use a drip solution whose viscosity is lower than that of conventional processes so that the charge on the feed pump means is reduced and a constant drip can be maintained continuously.

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 It is a third object of the present invention to provide a process for the production of sintered granules of lithium titanate which uses a drip solution which does not solidify on contact with air, which allows easy adjustment. of the particle size by allowing a stable drip without causing any blockage of the pouring nozzle, and which is suitable for mass production in which fine particles with high sphericity can be obtained easily.

(e) calciner lesdites particules de gel après séchage afin d'en enlever le métal polyvalent, en obtenant ainsi des particules sphériques de titanate de lithium ; et fritter lesdites particules sphériques de titanate de lithium. According to a first aspect of the present invention, these objects are achieved by a process for the production of sintered granules of lithium titanate starting from a pulverized starting product containing lithium and titanium, characterized in that it comprises the steps consisting in: (a) preparing a dispersion of said sprayed starting product in an aqueous binder solution containing an alginic acid salt, said solution having a gel-forming property when said alginic acid salt contact of polyvalent metal ions; (b) forming drops of the dispersion by pouring said dispersion dropwise through a nozzle; (c) immersing said drops in a liquid bath which contains polyvalent metal ions to cause gelation of said solution by contact with said alginic acid salt in said solution, thereby forming moist spherical gel particles in which the product of sprayed start is dispersed; (d) drying said spherical wet gel particles

(e) calcining said gel particles after drying in order to remove the polyvalent metal therefrom, thereby obtaining spherical particles of lithium titanate; and sintering said spherical particles of lithium titanate.

 In the process according to the process of the present invention, the dispersion or the drip solution is first prepared by mixing / dispersing the starting material sprayed into the aqueous binder solution which contains an alginic acid salt. This solution has the chemical property of forming a gel of an alginic acid salt by reaction with polyvalent metal ions. The dispersion thus prepared is then poured drop by drop.

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métalliques polyvalents qui provoquent la gélification de la solution aqueuse de liant. Par conséquent, la solution aqueuse de liant de chacune des gouttes sphériques se gélifie au contact avec le bain liquide en formant, dans le bain liquide, des particules humides de gel dans lesquelles le produit de départ pulvérisé est dispersé uniformément. Les particules humides de gel ainsi formées sont donc d'une configuration sphérique à sphéricité élevée qui est préservée et elles peuvent parvenir à maturation dans l'état immergé dans le bain liquide. Puis, les particules sphériques humides de gel sont enlevées du bain liquide, rincées si nécessaire et séchées à l'air, en retenant leur forme sphérique, pour obtenir des particules sphériques sèches de gel. Puis, ces particules sphériques sèches de gel sont d'abord calcinées dans l'atmosphère réductrice pour enlever par décomposition thermique le liant et le métal polyvalent des particules sphériques de gel, en formant donc des particules sphériques de titanate de lithium d'où les impuretés ont été complètement enlevées. Les particules sphériques de titanate de lithium sont finalement frittées pour obtenir de fins granules frittés de titanate de lithium. Puisque le liant et le métal polyvalent ont été complètement enlevés des particules sphériques de gel pendant les étapes de calcination et de frittage, les granules frittés qui sont finalement obtenus sont de fins granules sphériques de titanate de lithium de très haute pureté. drop through a nozzle to form drops of the dispersion. When it leaves the nozzle towards the free space, each of the drops takes on a spherical shape under the effect of its surface tension. The spherical drops fall in the form of drops in the liquid bath which contains the ions

polyvalent metals which cause gelation of the aqueous binder solution. Consequently, the aqueous binder solution of each of the spherical drops gels on contact with the liquid bath, forming, in the liquid bath, wet gel particles in which the sprayed starting material is uniformly dispersed. The wet gel particles thus formed are therefore of a spherical configuration with high sphericity which is preserved and they can mature in the immersed state in the liquid bath. Then, the spherical wet gel particles are removed from the liquid bath, rinsed if necessary and air dried, retaining their spherical shape, to obtain dry spherical gel particles. Then, these dry spherical particles of gel are first calcined in the reducing atmosphere to remove by thermal decomposition the binder and the polyvalent metal of the spherical particles of gel, thus forming spherical particles of lithium titanate from where the impurities have been completely removed. The spherical particles of lithium titanate are finally sintered to obtain fine sintered granules of lithium titanate. Since the binder and the polyvalent metal were completely removed from the spherical gel particles during the calcination and sintering steps, the sintered granules which are finally obtained are fine spherical granules of very high purity lithium titanate.

préparer le produit de départ pulvérisé. Selon la présente invention, la puissance exigée du moyen de pompe peut de plus être réduite puisque la viscosité de la solution de dispersion utilisée comme solution de goutte à goutte est moindre que celle d'une solution aqueuse classique de goutte à goutte formée d'un composé résineux polymère. Puisque la dispersion ne se Consequently, since the aqueous binder solution used to form the drop-by-drop solution of the invention does not contain any hydroxyl group, the drop-by-drop solution does not solidify before gelation, whatever the starting materials used in any preparation process and composition, which widens the free choice of design for

prepare the sprayed starting material. According to the present invention, the power required of the pump means can also be reduced since the viscosity of the dispersion solution used as a drip solution is lower than that of a conventional aqueous drip solution formed by a polymer resinous compound. Since the dispersion is not

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 does not solidify on contact with air, any fear of blockage of the drip nozzle under the effect of solidification of the solution can also be avoided.

la forme des particules humides de gel est sphérique sous l'effet de la tension superficielle des gouttes, dès lors que la condition du goutte à goutte a été déterminée, et que cette condition peut être maintenue sans interférence due à un blocage de la buse. Une production en série peut donc être mise en oeuvre en assurant un réglage constant de la dimension des particules. Il est

évidemment facile d'ajuster la dimension des gouttes en elle-même de façon que les fins granules frittés de titanate de lithium produits soient de la dimension souhaitée. Therefore, the present invention makes it possible to produce fine particles of high sphericity lithium titanate which contain high purity lithium titanate. The present invention also makes it possible to constantly create spherical gel particles of the same dimension since

the shape of the wet gel particles is spherical under the effect of the surface tension of the drops, as soon as the condition of the drip has been determined, and this condition can be maintained without interference due to blockage of the nozzle. Mass production can therefore be implemented by ensuring constant adjustment of the particle size. It is

obviously easy to adjust the size of the drops in itself so that the fine sintered granules of lithium titanate produced are of the desired size.

 It should be noted that the process which includes the steps (a) to (f) mentioned above can be divided as a whole into two composite steps which consist in (A) forming particles of the lithium compound, this first composite step (A) comprising steps (a) to (d) mentioned above, and (B) removing a binder from said lithium compound particles, this the second composite step (B) comprising steps (e) and (f) set out above above.

 Steps (A) and (B) can be performed separately from each other, at different locations.

In the composite step (A), the dispersion is prepared which contains the aqueous binder solution containing an alginic acid salt and the pulverized starting product dispersed in the aqueous binder solution. The aqueous binder solution has the property of forming a gel when said alginic acid salt comes into contact with polyvalent metal ions. The sprayed starting material is mixed with and dispersed in the aqueous binder solution to form a dispersion which is used as a drip solution.

It is preferable to use, for this binder or salt of alginic acid, sodium alginate or ammonium alginate intervening as compound of

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 source of alginic acid, since the aqueous solution of sodium alginate or ammonium alginate has the property of easily forming a gel when the salt comes into contact with polyvalent metal ions and can be removed at relatively low temperatures during subsequent stages of calcination and / or sintering.

 Said liquid bath may contain one or more polyvalent metal ions selected from the group consisting of zinc, calcium, aluminum, titanium and zirconium ions.

 Preferably, said liquid bath comprises an aqueous solution of zinc chloride.

 The method may include the additional steps of adding an organic plasticizer to said dispersion; and rinsing said wet spherical gel particles before drying to remove said organic plasticizer from said wet spherical gel particles.

 In fact, during the preparation of the dispersion, the source compound of the binder, for example sodium alginate or ammonium alginate, is first dissolved in water to form the aqueous binder solution which contains the salt. alginic acid. Then, the sprayed starting material is mixed with this aqueous binder solution, and an organic plasticizer, tetrahydrofurfuryl alcohol (4 HF) for example, can be added and mixed by stirring. This makes it possible to prepare a dispersion in which the sprayed starting material is dispersed uniformly in the aqueous binder solution.

 A preferable composition contains, in addition to water, the following elements: - pulverized starting product: 0.1 to 60% by weight - binder source: 0.1 to 5% by weight, relative to the total weight before addition of the organic plasticizer, - organic plasticizer: 60% by weight or less, relative to the total weight of the final dispersion after preparation.

 After the preparation of the dispersion, the concentration of the aqueous binder solution is preferably adjusted to an optimal concentration in the preceding range of compositions, taking into account the type of compound.

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 of the alginic acid used and the reactivity with the liquid bath in the subsequent gelling step. Although it is preferable to use sodium alginate or ammonium alginate as the alginic acid compound, the type of alginic acid compound can be appropriately chosen in combination with the composition of the liquid bath. in the gelation stage.

 The sprayed starting product to be dispersed in the aqueous binder solution is preferably a mixture of titanium oxide, TiO2 and lithium titanate Li2Ti03. If it is particularly desirable to increase the lithium content of the final product, the additional amount of titanium oxide is increased. It is also possible to use lithium titanate powder as a sprayed starting material.

 The quantity of sprayed starting material dispersed in the aqueous binder solution can be determined as a function of the following two elements: 1) the balance between the dimensions of the drops to be formed in the subsequent step of forming the drops and the dimensions required particles of the final product consisting of sintered granules of lithium titanate, and 2) the flow properties of the dispersion.

l'intérieur d'une plage dans laquelle des gouttes peuvent être facilement formées par la buse, par exemple une plage de 0, 1 mm à 3 mm de diamètre, la quantité de produit de départ pulvérisé dans la dispersion est ajustée pour être comprise entre 0, 1 % et 60% en poids afin que la dimension de particules des granules frittés finalement produits soit correcte. Il faut noter que la raison du choix de cette plage réside dans le fait que la quantité d'humidité qui s'évapore des particules humides de gel dans l'étape de séchage devient excessivement grande lorsque la quantité de produit de départ pulvérisé est inférieure à 0, 1%, même pour une dimension minimale de particules, de sorte que les particules de gel se rident et qu'il devient donc difficile la production

de particules frittées à excellente sphéricité. En revanche, si la quantité de produit de départ pulvérisé dépasse 60% en poids, la viscosité de la dispersion devient trop élevée et il est difficile de faire couler la dispersion goutte à goutte à travers la buse. For example, when the drop size is selected at

within a range in which drops can be easily formed by the nozzle, for example a range of 0.1 mm to 3 mm in diameter, the amount of starting material sprayed into the dispersion is adjusted to be between 0.1% and 60% by weight so that the particle size of the sintered granules finally produced is correct. It should be noted that the reason for the choice of this range lies in the fact that the amount of moisture which evaporates from the wet gel particles in the drying step becomes excessively large when the amount of sprayed starting material is less than 0, 1%, even for a minimum particle size, so that the gel particles become wrinkled and therefore production becomes difficult

sintered particles with excellent sphericity. On the other hand, if the quantity of sprayed starting product exceeds 60% by weight, the viscosity of the dispersion becomes too high and it is difficult to pour the dispersion dropwise through the nozzle.

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The formation of drops is carried out by the nozzle so that the dispersion drops drop by drop into the air and that each of the drops takes on the spherical shape under the effect of its surface tension. It may be advantageous to use a vibrating nozzle for this purpose. Electrostatic spraying can also be applied as required.

régler la dimension ou le diamètre des gouttes en excitant la buse pour former la vibration à une certaine fréquence et en ajustant la fréquence de vibration de la buse et/ou le débit volumétrique de la dispersion qui traverse la buse. During the formation of the drops by a vibrating nozzle, it is possible to

adjust the size or diameter of the drops by exciting the nozzle to form vibration at a certain frequency and adjusting the vibration frequency of the nozzle and / or the volumetric flow rate of the dispersion passing through the nozzle.

où Q représente le débit de la dispersion déchargée à travers la buse, d le diamètre des gouttes, et f la fréquence de vibration de la buse vibrante. In this case, the following equation (1) is satisfied:
Q = (7t / 6) d3. f ... (1)

where Q represents the flow rate of the dispersion discharged through the nozzle, d the diameter of the drops, and f the vibration frequency of the vibrating nozzle.

 The size of the drops can therefore be adjusted by adjusting the flow rate Q of the dispersion and the vibration frequency of the vibrating nozzle. To form droplets with a diameter d in the range 0.1 mm to 3 mm, it is preferable to choose the flow rate Q as an arbitrary variable parameter and to adjust the vibration frequency f in a range from 10 Hz to 1000 Hz.

 The drops thus formed fall into the liquid bath containing the polyvalent metal ions and are immersed there, which causes the aqueous binder solution to gel, in contact with the alginic acid salt of the solution, and the formation of wet spherical particles. gel by solidification of the aqueous binder solution. For the liquid bath where gelation takes place, an optimal concentration for gelation is chosen according to the type of alginic acid compound used as binder.

gélification est optimale pour provoquer la gélification des gouttes de la solution aqueuse de liant, il est important que les propriétés du bain évitent toute précipitation, dans le bain, de la dissolution de produit de départ pulvérisé dispersé dans les gouttes de la solution aqueuse de liant. En effet, si le bain possède la propriété de dissoudre la substance de produit de départ utilisé comme source de lithium ou de titane, la teneur en lithium ou en titane dans les gouttes varie pendant la réaction de gélification, ce qui entraîne un risque Even if the composition of the liquid bath used for the reaction of

gelation is optimal to cause gelation of the drops of the aqueous binder solution, it is important that the properties of the bath avoid any precipitation, in the bath, of the dissolution of sprayed starting product dispersed in the drops of the aqueous binder solution . Indeed, if the bath has the property of dissolving the starting material substance used as a source of lithium or titanium, the content of lithium or titanium in the drops varies during the gelling reaction, which involves a risk

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 of non-uniform quality or size for the sintered granules of lithium titanate of the final product.

 When the drops enter the liquid bath, the temperature of the bath can be in the range of 0 C to 100 C. In practice, the temperature of the liquid bath can be room temperature. It is then preferable that the liquid bath which receives the drops from the nozzle is at room temperature and that the liquid bath is heated, when a certain quantity of drops has entered the bath, in order to promote maturation of the gel.

déterminer au préalable le temps optimal d'immersion en fonction du cas individuel, et de s'assurer que le gel a atteint sa maturation totale en réglant un temps d'immersion supérieur au précédent. From the point of view of production yield, it is desirable that the time of immersion of the drops in the liquid bath is as short as possible, but it is preferable that the time of immersion is not less than the time necessary for the completion of the gelling reaction of the drops, because the gel particles could otherwise deform during the subsequent drying step if the immersion time is too short. The speed or pace of the gelling reaction is determined by the temperature of the liquid bath, the composition of the dispersion, the size of the drops and the concentration of polyvalent metal ions in the liquid bath. It is therefore preferable to

determine beforehand the optimal immersion time according to the individual case, and ensure that the gel has reached its full maturation by setting an immersion time greater than the previous one.

indésirables et de n'effectuer qu'ensuite le processus de séchage. Le processus de séchage peut être exécuté par séchage naturel à l'air ou par chauffage dans un courant d'air. After completion of the gelling reaction, the wet gel particles caught in the liquid bath are subjected to a drying process to remove the moisture contained both on the surface and inside. If the gel particles tend to stick together during drying, it is preferable to rinse the wet gel particles with water and ethanol before the drying process in order to remove impurities and residual organic plasticizer

undesirable and only carry out the drying process afterwards. The drying process can be carried out by natural air drying or by heating in an air stream.

 The dried gel particles are then sent to the composite step (B) where calcination is first performed by heating the dried gel particles in a reducing atmosphere. In fact, the dried gel particles which come from the drying stage contain a residual polyvalent metal

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 which was occluded during the gelling reaction in the liquid bath. The residual polyvalent metal is dissipated and removed from the dried gel particles by calcination in the reducing atmosphere and the calcined particles form spherical particles of lithium titanate which contain no undesirable impurities.

 During calcination, it is important to avoid any significant crystal growth of lithium titanate in the particles. If there is a significant crystal growth of lithium titanate in the calcination step, the contraction of lithium titanate into fine particles in the subsequent sintering step is hampered, which makes it difficult to obtain sintered granules of titanate high density lithium.

 It is therefore preferable to carry out the calcination for a sufficient time, depending on the quantity and size of the particles, so that the polyvalent metal is effectively completely dissipated under heating conditions which prevent any growth of titanate particles from lithium above the dissipation temperature of the polyvalent metal used. For example, in the case of an aqueous solution of zinc chloride used as a liquid bath, where the polyvalent metal is zinc, the calcination can be carried out at a heating temperature of 600 C to 1000 C for 1 hour to 12 hours depending on the quantity and size of the particles. Indeed, if the heating temperature was below 6000 (, the zinc would not be completely removed whereas, if the temperature exceeded 10000 (, the crystal growth would be significant and sufficient to prevent a high density of the final product of sintered granules of lithium titanate.

 The spherical lithium titanate particles which result from the calcination are then further heated to a higher temperature and sintered.

Under the effect of sintering, the density of the lithium titanate particles increases and their size decreases, which makes it possible to obtain fine sintered granules of lithium titanate. While the sintering atmosphere preferably consists in this sintering step in a refined and adjusted inert gas such as argon, for example, this step is not necessarily carried out in such an inert gas atmosphere: it can be in the air, that is to say in an atmosphere that contains oxygen.

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The sintering temperature is preferably adjusted within a range of 900 C to 13500 (and the sintering is preferably carried out within this temperature range for a period of time between 1 5 minutes and 10 hours. subject, if the sintering temperature is below 9000 (, the sintering does not reach its completion and, if the temperature exceeds 1350 C, undesirable problems can arise, for example a partial melting of fine particles, a junction between the particles and bonding of the particles to the sintering pan. Furthermore, if the sintering time is less than 15 minutes, the sintering may be incomplete, even for the minimum particle sizes and, if this time exceeds 10 hours, the quantity of lithium titanate lost by evaporation increases leading to a poor yield.

davantage de façon à gélifier la solution aqueuse de liant contenue dans les gouttes afin de former les particules sphériques humides de gel dans lesquelles le produit de départ pulvérisé est dispersé ; According to a second aspect, the present invention provides a process for producing sintered granules of lithium titanate starting from a pulverized starting product containing lithium and titanium, characterized in that it comprises the steps consisting in: (a ) prepare an aqueous binder solution containing 0.1 to 5% by weight of sodium alginate or ammonium alginate; (b) preparing a dispersion of 0.1 to 60% by weight of the pulverized starting product dispersed in said aqueous binder solution; (c) adding to said dispersion and mixing therein 60% by weight or less of tetrahydrofurfuryl alcohol; (d) forming drops of said dispersion with a controlled diameter in a range of 0.1 to 3 mm by pouring said dispersion dropwise through a vibrating nozzle at a predetermined frequency in the range of 10 to
1000 Hz; (e) immersing said drops in a liquid bath containing an aqueous solution of zinc chloride with a concentration of 0.5% by weight or

further so as to gel the aqueous binder solution contained in the drops to form the moist spherical gel particles in which the sprayed starting material is dispersed;

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(f) maintenir immergées dans ladite solution aqueuse de chlorure de zinc lesdites particules humides sphériques de gel pour provoquer la maturation des particules de gel ; (g) rincer à l'eau et/ou à l'éthanol lesdites particules sphériques humides de gel après maturation ; (h) sécher par chauffage dans l'air, après rinçage, lesdites particules sphé- riques humides de gel ; (i) chauffer de 1 à 12 heures lesdites particules de gel séchées à une

température de calcination prédéterminée dans la plage de 600 C à 1000 C dans une atmosphère gazeuse réductrice afin d'obtenir des particules de titanate de lithium, d'où le zinc a été enlevé ; et (j) fritter dans l'air lesdites particules de titanate de lithium pendant une période comprise dans une plage de 15 minutes à 10 heures à une température de frittage prédéterminée dans la plage de 900 Cà 1300 C
Les buts, particularités et avantages de la présente invention exposés cidessus ainsi que d'autres ressortiront davantage de la description qui suit de modes de réalisation préférés en conjonction avec les dessins dans lesquels :

- la Figure 1 représente schématiquement le déroulement des étapes selon un premier mode de réalisation de la présente invention ; - la Figure 2 représente conceptuellement une particule sphérique humide de gel obtenue par le procédé de la présente invention ; et - la Figure 3 est un graphe qui illustre un exemple de conditions de calci- nation et de frittage, c'est-à-dire les variations de la température du four.

(f) keeping said spherical wet gel particles immersed in said aqueous zinc chloride solution to cause the gel particles to mature; (g) rinsing said wet spherical gel particles with water and / or ethanol after maturation; (h) drying by heating in air, after rinsing, said spherical wet gel particles; (i) heating said dried gel particles from 1 to 12 hours to a

predetermined calcination temperature in the range of 600 C to 1000 C in a reducing gas atmosphere to obtain particles of lithium titanate, from which the zinc has been removed; and (j) sintering said lithium titanate particles in air for a period in the range of 15 minutes to 10 hours at a predetermined sintering temperature in the range of 900 C to 1300 C
The objects, features and advantages of the present invention set out above as well as others will emerge more from the following description of preferred embodiments in conjunction with the drawings in which:

- Figure 1 schematically shows the progress of the steps according to a first embodiment of the present invention; - Figure 2 conceptually represents a spherical wet gel particle obtained by the process of the present invention; and FIG. 3 is a graph which illustrates an example of calcination and sintering conditions, that is to say the variations in the temperature of the oven.

 In a preferred embodiment shown in Figure 1, sodium alginate is used as a binder and an aqueous solution of zinc chloride is used as a liquid bath. Those skilled in the art will understand, however, that the present invention is not limited to this embodiment.

 In the Ski 01 step, sodium alginate is first added to water 8 contained in a tank 2 and the whole is stirred by a stirring paddle 4 to prepare an aqueous binder solution 12 which contains 0.7% by weight of sodium alginate. Then, 20% by weight of tetrahydrofurfuryl alcohol (4HF) acting as a plasticizer as well as powder 6 of

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titanate de ith ! um Li2Ti03 sont ajoutés, et l'ensemble est mélangé intimement par la palette d'agitation 4 pour préparer une dispersion uniforme 14, c'est-à- dire une solution de goutte à goutte contenant 10% en poids de poudre de titanate de lithium Li2Ti03 dispersée dans une solution aqueuse d'alginate de sodium d'une concentration de 0,7% en poids, le pourcentage en poids se référant à la teneur dans la dispersion. Cette dispersion 14 est transférée à un récipient pour être versée goutte à goutte.

ith titanate! um Li2Ti03 are added, and the whole is mixed thoroughly by the stirring paddle 4 to prepare a uniform dispersion 14, that is to say a drip solution containing 10% by weight of lithium titanate powder Li2Ti03 dispersed in an aqueous solution of sodium alginate with a concentration of 0.7% by weight, the percentage by weight referring to the content in the dispersion. This dispersion 14 is transferred to a container to be poured drop by drop.

 In step S 102, the dispersion 14 prepared in step S 10 1 is poured dropwise through a vibrating nozzle 16 placed under the container 10.

16 et le récipient 10 pour régler le débit goutte à goutte de la dispersion. La buse vibrante 16 est équipée d'un mécanisme de vibration qui inclut un oscillateur 20 apte à osciller à une fréquence d'oscillation prédéterminée, un amplificateur 22 pour amplifier la sortie de l'oscillateur 22, et un actionneur 24 de vibration pour communiquer une vibration mécanique à la buse en réponse à l'excitation de sortie amplifiée par l'amplificateur 22. A liquid supply pump 18 is interposed between the vibrating nozzle

16 and the container 10 for adjusting the drop by drop rate of the dispersion. The vibrating nozzle 16 is equipped with a vibration mechanism which includes an oscillator 20 capable of oscillating at a predetermined oscillation frequency, an amplifier 22 for amplifying the output of the oscillator 22, and a vibration actuator 24 for communicating a mechanical vibration at the nozzle in response to the output excitation amplified by the amplifier 22.

 The present embodiment uses a vibrating nozzle 16 with nozzle opening 0.49 mm in diameter. The flow rate of the dispersion is adjusted by the pump 18 to 10 cm3 / minute. The vibrating nozzle 16 is vibrated by an excitation signal with a vibration frequency of 80 Hz.

 In step S103, drops 26 form in the air, fall into the liquid bath and are gelled.

 In fact, a tank 30 filled with an aqueous solution 32 of zinc chloride at 10% by weight was prepared, prior to step S 103, to serve as a liquid bath for gelation. The temperature of the liquid bath 32 of the tank 30 is the ambient temperature, that is to say 20 C. The drops 26 which come from the vibrating nozzle 16 fall on the surface of the liquid bath 32 and sink slightly into the bath 32 and are gelled to form the gel particles 36 by reaction with the bath. The gel particles float in the bath. Subsequently, the liquid bath 32 is heated to 60 ° C. to effect the maturation of the gel particles 36 for 1 hour at this temperature. Figure 2 schematically represents one of the gel particles 36 thus obtained. The gel particle 36 is substantially spherical and the

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 6 lithium titanate powder is uniformly dispersed in a gel 34 of the aqueous binder solution.

 In the next step S104, the gel particles 36 having reached full maturation are removed from the liquid bath 32, transferred to a rinsing tank 34 and rinsed twice with water at 60 ° C. for 10 minutes each time, then with l ethanol at 60 C for 15 minutes in another rinsing tank not shown in the Figure.

 In step S105, the wet particles of gel 46 are then transferred, after rinsing, to a tray or flat tray 50 and dried for 2 hours at 60 ° C. in air to obtain dry particles of gel 56 whose moisture has actually been evaporated. In the present embodiment, it is the natural drying under the conditions mentioned above which is used as the drying method. But any desired drying method can be employed, for example natural drying or drying in a stream of air at room temperature or heated, since the purpose of drying is to remove moisture from the wet gel particles.

 In step S 06, the dry gel particles 56 contained in the tray 50 are introduced into a heating oven, not shown, and a calcination is carried out by gradually raising the temperature of the oven and heating for 4 hours at 1000 ° C. an atmosphere of hydrogen gas.

In this calcination process, the zinc contained in the dry gel particles is dissipated by combustion, and the dry gel particles are converted into particles of lithium titanate, Li2TiO3.

 In the final step S107, the temperature of the oven is further increased and sintering is carried out for 4 hours at 1350 ° C. This sintering process reduces the dimensions of the particles of lithium titanate and produces fine sintered granules with a density equal to 83%. theoretical density and an average diameter of 0.3. FIG. 3 shows, as a function of time, the temperature variation in the oven in the calcination step S 106 and the sintering step S 107.

 As previously described, the aqueous binder solution of the present invention contains no hydroxyl group, so that the drip solution does not solidify before gelation, regardless of the sprayed starting material which is used and whatever its mode of preparation and

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sa composition. Il est donc possible d'éliminer les restrictions ou limitations à la préparation du produit de départ pulvérisé.

its composition. It is therefore possible to eliminate the restrictions or limitations on the preparation of the sprayed starting material.

In addition, since the viscosity of the dispersion solution of the present invention used as a drip solution is lower than that of the aqueous solution of polymeric resinous compound of the prior art, the load on the liquid supply pump can be reduced and the drip nozzle does not block since the dispersion does not solidify on contact with air.

 The present invention therefore makes it possible to obtain fine sintered granules of lithium titanate of high sphericity and high purity. In addition, the method of the present invention forms particles with a spherical configuration due to the surface tension of the drops as they enter the bath. Consequently, once the conditions for the formation of drops have been determined, the particles obtained are constantly of the same size.

This condition is maintained without risk of interference due to blockage of the nozzle, and mass production can therefore be easily implemented according to a constant adjustment of the particle size. The size of the drops is obviously easy to adjust, which makes it possible to obtain fine sintered granules of lithium titanate of the desired size.

Claims (9)

R E V E NDtC A T 1 ONS  1. Process for the production of sintered granules of lithium titanate starting from a pulverized starting product containing lithium and titanium, characterized in that it comprises the steps consisting in: (a) preparing a dispersion of said starting product sprayed into an aqueous binder solution containing an alginic acid salt, said solution having a gel-forming property when said alginic acid salt comes into contact with polyvalent metal ions; (b) forming drops of the dispersion by pouring said dispersion dropwise through a nozzle; (c) immersing said drops in a liquid bath which contains polyvalent metal ions to cause gelation of said solution by contact with said alginic acid salt in said solution, thereby forming moist spherical gel particles in which the product of sprayed start is dispersed; (d) drying said spherical wet gel particles

 (e) calcining said gel particles after drying in order to remove the polyvalent metal therefrom, thereby obtaining spherical particles of lithium titanate; and (f) sintering said spherical particles of lithium titanate.  2. Method according to claim 1, characterized in that said aqueous binder solution comprises an aqueous solution of sodium alginate.  3. Method according to claim 1, characterized in that said aqueous binder solution comprises an aqueous solution of ammonium alginate.  4. Method according to claim 1, characterized in that said liquid bath contains one or more polyvalent metal ions selected from the group consisting of ions of zinc, calcium, aluminum, titanium and zirconium.  5. Method according to claim 1, characterized in that said liquid bath comprises an aqueous solution of zinc chloride. <Desc / Clms Page number 17>  6. Method according to claim 1, characterized in that it comprises the additional steps of adding an organic plasticizer to said dispersion; and rinsing said wet spherical gel particles before drying to remove said organic plasticizer from said wet spherical gel particles.  7. Method according to claim 6, characterized in that said organic plasticizer comprises tetrahydrofurfuryl alcohol (4HF).  8. Method according to claim 1, characterized in that said calcination of the gel particles after drying is carried out in a reducing atmosphere.  9. Method for producing sintered granules of lithium titanate starting from a pulverized starting product containing lithium and titanium, characterized in that it comprises the steps consisting in: (a) preparing an aqueous solution of binder containing from 0.1 to 5% by weight of sodium alginate or ammonium alginate; (b) preparing a dispersion of 0.1 to 60% by weight of the sprayed starting material dispersed in said aqueous binder solution; (c) adding to said dispersion and mixing therein 60% by weight or less of tetrahydrofurfuryl alcohol; (d) forming drops of said dispersion of a controlled diameter in a range of 0.1 to 3 mm by pouring said dispersion dropwise through a vibrating nozzle at a predetermined frequency in the range of 10 to 1000 Hz; (e) immersing said drops in a liquid bath containing an aqueous solution of zinc chloride with a concentration of 0.5% by weight or

 further so as to gel the aqueous binder solution contained in the drops to form the moist spherical gel particles in which the sprayed starting material is dispersed; keeping said spherical moist gel particles immersed in said aqueous zinc chloride solution to cause the gel particles to mature; <Desc / Clms Page number 18>  (g) rinsing said wet spherical gel particles with water and / or ethanol after maturation; (h) drying by heating in air, after rinsing, said wet spherical gel particles; (i) heating said dried gel particles from 1 to 12 hours at a predetermined calcination temperature in the range of 600 C to 1000 C in a reducing gas atmosphere in order to obtain particles of lithium titanate, hence zinc has been kidnapped ; and sintering said lithium titanate particles in air for a period in the range of 15 minutes to 10 hours at a predetermined sintering temperature in the range of 900 C to 1300 C