WO2023070234A1 - Method for producing high-purity mg(oh)2 from industrial discard bischofite - Google Patents

Abstract

The present invention relates to a method for producing 99.7% high-purity magnesium hydroxide (Mg(OH)2) from discard bischofite, said method comprising: a) preparing reagents, dissolving 88% pure discard bischofite in water, and forming a 36-37% bischofite solution by weight; and at the same time preparing a high-purity NaOH alkaline solution in water, forming a 23-24% NaOH solution by weight: b) conducting a reactive crystallisation to form Mg(OH)2, controlling supersaturation by change of pH, and using a 12-13% NaCl solution by weight, the NaCl/MgCl2 (v/v) proportion being 0.45, introducing the NaCl solution to the reactor before the reagents prepared in a); c) separating from the solution obtained in b), using filtration, preferably in a filter press; d) washing the Mg(OH)2 pulp with fresh water to remove impurities; and e) drying at 80°C.

Description

PRODUCTION METHOD OF HIGH PURITY MG(OH)2 FROM INDUSTRIAL DISCARD BISCHOFITE.

FIELD OF THE INVENTION

The present invention refers to the field of inorganic chemical industry, especially an effective method to obtain magnesium hydroxide (Mg(OH) ₂ ) with a purity of 99.7%, from waste bischofite (MgCI ₂ x6H ₂ O) purity of 88.0 %, and using sodium hydroxide (NaOH) as reagent and sodium chloride (NaCI) as reaction medium.

BACKGROUND

Mg(OH) ₂ , in addition to the application in lithium-ion batteries, has a wide market with various uses, such as a flame retardant (fireproof filler) that does not develop toxic and corrosive substances on combustion. Mg(OH) ₂ also serves as a precursor for the synthesis of MgO refractory ceramics (C. Henrist, JP Mathieu, C. Vogels, A. Rulmont, and R. Cloots, "Morphological study of magnesium hydroxide nanoparticles precipitated in dilute aqueous solution," Journal of Crystal Growth, vol. 249, pp. 321-330, 2003/02/01 ). Likewise, magnesium hydroxide (Mg(OH) ₂ ) can be applied in different areas such as medicine, industry and battery manufacturing. For these applications, the material must be of high purity. However, with the traditional method only 95% purity is achieved.

The conventional process for the production of Mg(OH) ₂ is based on the reaction of MgCI ₂ with Ca(OH) ₂ . This process is economical due to the low prices of the CaO used in the reaction, however, the product contains many impurities that remain as reaction residue. The Mg(OH) ₂ obtained has a colloidal character since it is found in aqueous suspensions, due to its small crystal size, which makes it difficult to filter and very difficult to separate, retaining impurities in greater quantities. The most predominant impurity is Ca(OH) ₂ due to its low solubility (H. Tsuge, K. Okada, T. Yano, and N. Fukushi, "Reactive Crystalization of Magnesium Hydroxide," Department of Applied Chemistry 2014).

Much research has been done to produce high purity Mg(OH) ₂ , studying the effect of the type of alkali on the reaction. In these works, the incidence of solubility and alkali concentration in reactive crystallization has been studied, and it has been obtained that NaOH is a suitable alkali for the process, producing large crystals and insoluble by-products in the reaction, which facilitate operations. filtering. Stirring, temperature, and reagent feed flow are operational parameters that also have a significant effect on crystal size. In addition, the use of additives favors crystal growth, limiting the nucleation rate (H. Tsuge, K. Okada, T. Yano, and N. Fukushi, "Reactive Crystalization of Magnesium Hydroxide," Department of Applied Chemistry 2014; A Cipollina, M. Bevacqua, P. Dolcimascolo, A. Tamburini, A. Brucato, H. Glade, et al., "Reactive crystallization process for magnesium recovery from concentrated brines," Desalination and Water Treatment, vol. 55, pp. 2377-2388, 2015/08/28, and X. Song, S. Sun, D. Zhang, J. Wang, and J. Yu, "Synthesis and characterization of magnesium hydroxide by batch reaction crystallization," Frontiers of Chemical Science and Engineering, vol. 5, p. 416-421, December 01 2011).

On the other hand, the treatment of brines for the concentration of lithium in the non-metallic mining industry is done through evaporation pools. In the different stages of concentration, different salts precipitate, where some are used in the same process, marketed or discarded. Within the main waste salts, is Magnesium Chloride Hexahydrate (MgCI ₂ x6H ₂ O), known as Bischofite. A fraction of said salt is used as a dust suppressant for dirt roads; however, a large amount of this salt is stored as waste material.

There are some attempts to produce Mg(OH) ₂ from bischofite but MgCI ₂ solutions that use Ca(OH) ₂ to precipitate Mg(OH) ₂ directly from brines are used. However, as explained above, this process does not control the crystal size, which makes the filtering operation difficult, and the purity of the product in the form of Mg(OH) ₂ ) only reaches 95%. Different Mg compounds are also used, such as oxides, acetates or dolomites, which require different treatment, such as grinding, leaching and the use of lime or other bases, and may even recommend the use of magnesium salt sources such as bischofite but which They include other processes such as the previous purification treatment to eliminate a priori the impurities present in the waste bischofite.

CN101224901 B (Liaoning Jiayi Hardware and Mineral CO LTD; Univ. Dalian Maritime) refers to a method for continuously preparing high purity magnesium hydroxide from bischofite as a raw material and ammonium as a precipitant, whereby treated bischofite is prepared in a solution with a concentration of 0.5 mol/L-4 mol/L, the bischofite solution and ammonium gas are added continuously after the reaction starts and produces the continuous deposition of magnesium hydroxide. From this method, magnesium hydroxide with a granulometry of 10 microns-100 microns and a purity of 99-99.999% is obtained, that is, it has a large granule size and high purity.

CN101224902B (Liaoning Jiayi Metals & Mimerals Co Ltd; Univ. Dalian Maritime) refers to a method for depositing high-purity magnesium hydroxide, with an ammonia liquid double phase to continuously produce magnesium hydroxide from bischofite and ammonia - water ammoniacal as a precipitant. The treated bischofite is prepared in a solution with a concentration of 0.5 mol/L-4 mol/L. Magnesium hydroxide with large granulometry of 10 pm-100 pm and high purity of 99-99.999% is produced.

uso directo en el proceso de bischofita de descarte con una composición determinada y sin tratamiento previo, uso exclusivo de NaOH como álcali y una solución de NaCI como medio de reacción y aditivo de cristalización, y el control del flujo de alimentación de los reactivos para la obtención de Mg(OH)₂ con una pureza del 99,7%. CN1663913A (Univ. Beijing Science & Tech) refers to a method for preparing metallic magnesium from bischofite, and from which magnesium hydroxide is prepared by a process with ammonia at a temperature between 45-55°C for 20-30 minutes. , with a range of Mg:NH ₃ of 1:1,2-1,5:2.2 and a magnesium chloride concentration of 5-55 g/L, calcining the magnesium hydroxide at a temperature between 900-1000°C for 3-4 hours, achieving magnesium hydroxide with a purity greater than 99.5%. Although many methods for obtaining Mg(OH) ₂ have been developed, there is still a need for an effective method to obtain high purity Mg(OH) ₂ using waste bischofite as a Magnesium source. The present invention is distinguished by the

direct use in the process of discarded bischofite with a determined composition and without prior treatment, exclusive use of NaOH as alkali and a NaCl solution as reaction medium and crystallization additive, and control of the feed flow of the reagents for the obtaining Mg(OH) ₂ with a purity of 99.7%.

BRIEF DESCRIPTION OF THE FIGURES

Figures 1A and 1B. Show the process and flowchart for obtaining Mg(OH) ₂ from industrial waste bischofite.

Figure 2. Thermodynamic data of the reaction for the formation of Mg(OH) ₂ from MgCI ₂ *6H ₂ O. Figure 3. Granulometric analysis of the Mg(OH) ₂ products.

Figure 4. XRD analysis of the Mg(OH) ₂ samples.

Figure 5. Granulometric distribution of Mg(OH) ₂ crystals (% Retained) for different sources of

Figure 6. Washing stages for the purification of the Mg(OH) ₂ obtained at different feed flows.

Figure 7. Granulometric distribution of the Mg(OH) ₂ products obtained at different reagent feed flows (retained percentage).

Figure 8. E-pH diagrams: (A) Mg-CI-H ₂ O system and (B) B-H2O system

Figure 9A-9C. BSE analysis of Mg(OH) ₂ samples: Fig. 9A, Mg(control); Fig. 9B, Mg(comparative); and Fig. 9C, Mg(inventive).

Figures 10A and 10B. SEM image of Mg(control) by SE (secondary electrons).

Figures 11 A and 11 B. SEM image of Mg (comparative) by SE (secondary electrons).

Figures 12A and 12B. SEM image of Mg(inventive) by SE (secondary electrons).

Figure 13. Evolution of washing stages of the Mg(OH) ₂ crystals produced as a function of conductivity.

DETAILED DESCRIPTION OF THE INVENTION

pureza que alcanza el 99,7%. El método para obtener Mg(OH)₂ emplea además los reactivos NaOH como álcali y NaCI como un aditivo para mejorar la cristalización. The present invention relates to a method for obtaining magnesium hydroxide, Mg(OH) ₂ from discarded bischofite from the industry, which ensures a product with a

purity reaching 99.7%. The method to obtain Mg(OH) ₂ also uses the reagents NaOH as alkali and NaCl as an additive to improve crystallization.

It is known that the reaction of MgCI ₂ and NaOH requires that the reagents be in the ionic state and corresponds to a homogeneous chemical reaction, see the chemical equation (Eq. 1).

The main product of the reaction is Mg(OH) ₂ in the solid (s) state and NaCl in the aqueous (aq.) state, which can be removed by filtering and washing the crystals. It is also known that the preparation of a NaOH solution is an exothermic solution, and that its handling must be careful. The thermodynamics of the reaction for the formation of Mg(OH) ₂ is shown in Figure 2, which shows that the chemical reaction for the formation of Mg(OH) ₂ is endothermic, therefore energy must be introduced into the system in the form of heat. for the reaction to proceed optimally. The AG values show that the reaction is spontaneous and directly proportional to the temperature, thus a higher temperature favors the formation of Mg(OH) ₂ . In the same way, the equilibrium of the reaction is favored at high temperatures, as shown in Figure 2, and it increases with the increase in temperature, displacing the equilibrium towards the formation of Mg(OH) ₂ , thus being able to reach high conversions. Consequently, in the studied process, an operating temperature of 80°C is used, to avoid a high evaporation rate and supersaturation of the solution with NaCI.

Figures 1 A and 1 B show the process and flow diagram of the Mg(OH) ₂ production process.

The method for obtaining magnesium hydroxide Mg(OH) ₂ according to the embodiment of the invention proposed in this application is described in detail below. In this method of the invention, bischofite from the Salar de Atacama is used as a source of Magnesium, which responds to industrial discard bischofite with a purity of 88.04%. In the comparative method for obtaining Mg(OH) ₂ , purified bischofite of 94.54% purity is used; and for its part in the control method MgCI ₂ *6H ₂ O analytical grade of 99% purity, purchased from the company SIGMA ALDRICH, is used.

In the comparative method and the control method, the same procedure is used as that used in the method of the invention, only the starting MgCI ₂ *6H ₂ O is modified as indicated in the previous paragraph.

Table 1 presents the chemical composition of discarded bischofite of the invention and purified bischofite used in the comparative method.

Table 1. Chemical composition of discarded bischofite of the invention and purified bischofite used in the comparative method.

Table 2 shows the symbols used to identify the Mg(OH) ₂ products using: (1) industrial bischofite from the discard area, with a purity of 88.08%, (2) purified industrial bischofite, having a purity of 94.52%; and (3) MgCI ₂ *6H ₂ O, 99% pure analytical grade (Sigma Aldrich).

Table 2. Symbology used to identify the Mg(OH) ₂ product.

Step a), preparation of aqueous solutions of bischofite and sodium hydroxide.

agua (corriente 1 10), formando una disolución con una concentración de 36 a 37% en peso de bischofita. The reagents are prepared, for which bischofite (current 100) is dissolved in

water (current 1 10), forming a solution with a concentration of 36 to 37% by weight of bischofite.

In parallel, the alkali is prepared, which is selected exclusively from NaOH (current 200), and for which analytical grade NaOH is used, with a purity of 99% purchased from the company SIGMA ALDRICH. The NaOH dissolves in water (stream 210) forming a 23 to 24% by weight NaOH solution.

Step b), conduction of reactive crystallization for the formation of Mg(OH) ₂ .

A NaCI solution with a concentration of 12 to 13%, prepared from analytical grade NaCI with a purity of 99% purchased from the company SIGMA ALDRICH, is incorporated into the reactor, and the reactor is fed in parallel and gradually. reagents prepared in Step a) (current 120 and 220) maintaining a volume ratio of the NaCl/MgCl ₂ solutions of 0.45, for the formation of Mg(OH) ₂ by reactive crystallization, whose supersaturation level is mainly controlled by the change in pH.

This NaCI solution must be placed inside the Mg(OH) ₂ formation reactor before starting the process, playing the role of reaction medium. The presence of the NaCI solution inside the reactor allows the pH change not to be sudden when the reagents are fed, which favors the growth of the crystals.

The alkali and bischofite feed flows are established from 1.5 and 3.75 ml/min (0.1 1 and 0.27 reactor volume/h), up to 4.5 and 1 1.25 ml/min ( 0.33 and 0.82 reactor volume/h), respectively; and preferably these alkali and bischofite feed flows are 4.5 and 11.25 ml/min (0.33 and 0.82 reactor volume/h), respectively. Alkali feeding must be carried out until the system reaches a pH of 12, which is the isoelectric point of Mg(OH) ₂ . A low reagent feed flow favors a pH change that is slow, obtaining larger crystals. With the feed flow of the reagents it is possible to control the size of the particles, therefore, if smaller crystals are desired, the feed flow of the reagents must be higher.

While the reaction occurs, NaCl plays the role of reaction medium and additive for crystallization, because the small hydration sphere of Na ⁺ ion in solution is significantly absorbed in all facets of the crystal nuclei, which which allows a more orderly ionic diffusion of Mg, improving the growth and physical properties of the crystal.

The operational conditions inside the reactor are: temperature 80°C, mechanical stirring from 400 to 600 rpm, preferably 450 rpm; and a reaction time of 2.5 to 3 hours.

The reaction products are mainly Mg(OH) ₂ and NaCl (stream 300). The advantage of using NaOH as alkali is that the reaction by-products, such as Na, K, Li and Ca hydroxides or chlorides, are solubilized and these can be easily removed from the system by filtration since Mg(OH) ₂ has a low solubility.

To reduce the generation of aggregates, an aging time of 3 hours should be applied to the Mg(OH) ₂ crystals after the reaction has concluded. The aging of the crystals helps the smaller crystals to dissolve in the solution and precipitate again, being incorporated into the matrix of other larger crystals.

Step c), solid-liquid separation by filtering process.

The crystals obtained in the process have large dimensions, which makes the filtering operation easy.

Table 3 shows the results of the granulometric analysis performed on the products obtained from the bischophytes shown in Table 1 .

Table 3. Dimensions of the Mg(OH) ₂ crystals produced.

The granulometric distribution of the products is shown in Figure 5.

As can be seen from Table 3 and Figures 3 and 5, it is noted that with the bischofite used in the method of the invention it is possible to obtain larger crystals. This is attributed to the greater presence of impurities in the raw material that favors crystalline growth. In this way, the product is adequately separated by filtration, preferably by filter press. Subsequently, the filtrate (stream 410), which consists mainly of a NaCl solution, can be recycled to the reactor after a workover, to work as an additive and reaction medium instead of feeding a fresh NaCl solution after each Mg(OH) ₂ formation process. Stream 400, consisting of the Mg(OH) ₂ pulp, goes to the wash step.

Step d), washing the Mg(OH) ₂ pulp (current 400) with fresh water to eliminate the soluble impurities impregnated in the cake.

This stage is controlled by measuring the conductivity of the filtrate, since the higher the conductivity, the greater the presence of dissolved ions in it, and washing must be carried out until the conductivity of the filtrate is reduced to the lowest and constant value possible. These minimum conductivity values range from 450 to 600 pS/cm.

Step e), drying of the Mg(OH) ₂ obtained from Step d).

The washed product from Step d) (stream 600) is dried in an oven at 80°C for two days. The purity of this product is determined by XRD and chemical analysis, which is given in Figure 4.

As observed in Figure 4, the diffractograms show only the presence of Mg(OH) ₂ , not being able to detect any other phase formed, which indicates a high purity of the product.

Table 4. Chemical analysis of the Mg(OH) ₂ products.

Table 4 shows the result of the chemical analyzes performed on both samples of Mg(OH) ₂ . As can be seen, when using the bischofite of the inventive embodiment, a Mg(OH) ₂ product with a higher purity reaching 99.7% is obtained. For its part, when using the bischofite used in the comparative method, a lower purity Mg(OH) ₂ product is obtained, reaching 99.2%.

This difference in the purity of the Mg(OH) ₂ products obtained is attributed to the fact that precisely the impurities present in the discarded bischofite used in the method of the invention allow obtaining larger crystals, which leads to the cake formed have lesser amounts of retained water. Table 5. % of water retained in the Mg(OH) ₂ cake.

As observed in Table 5, the bischofite cake of the invention has a lower percentage of retained water, which implies a lower amount of impurities impregnated in it, achieving a higher purity in the product.

- Determination of the feed flows in the method proposed for obtaining magnesium hydroxide Mg(OH) ₂ .

A MgCl2 solution with a concentration of 36 to 37% by weight is used, prepared from an 88% pure industrial waste bischofite and distilled water. A 23 to 24% by weight NaOH solution prepared with 99% pure NaOH and distilled water is used as the basic solution. Inside the reactor, a 12 to 13% by weight NaCl solution is prepared, in a volume ratio of NaCl/MgCl ₂ equal to 0.45.

Both the NaOH solution and MgCl ₂ must be fed at the same time into the reactor containing the NaCl solution. The feeding speed of the MgCI ₂ solution is based on the feeding speed of the NaOH solution, since the volume of both reagents is different due to their established concentration; therefore, for the process to work properly it is necessary that both reagents finish feeding at the same time, thus, the speed of both reagents will be different. Initially the flow was expressed in [ml/min], however, for scaling purposes it is convenient to express it in [V/h] of solution (where V represents the volume of the reactor). In the development of the experimental tests, three reagent feed flows are used (V1, V2 and V3) as detailed in the following Table 6.

Table 6. Feed flows of reagents studied

The experimental conditions are detailed in Table 7. Table 7. Operating conditions for the production of Mg(OH) ₂ from industrial waste bischofite

The Mg(OH) ₂ products obtained from the different feed streams were analyzed based on their separation properties and granulometric distribution. The first aspect evaluated was the filtration time of each sample. Table 8 shows the data obtained in each case, where it can be seen that the shortest filtration time was for the Mg(OH) ₂ produced at the feeding speed V ₂ , taking a time of 1.18 hours for its filtration. . Following this is the product produced at Vi and finally V ₃ . With these tests it was possible to show that a speed that is too low or high causes the filtration time to be prolonged, but an average speed between these values (optimum speed) allows the filtration time to be shorter. This allowed the product produced with feed flows of the reagents represented through V ₂ to require fewer filtration stages, which implies less energy costs in product washing operations, as observed in Figure 6.

Table 8. Filtration times of the Mg(OH) ₂ samples produced at different reagent feed rates.

Figure 6 shows that the Mg(OH) ₂ produced at V ₂ requires 6 washing steps compared to Vi which required 10 and V ₃ which required 7 steps to reach the lowest conductivity in the filtrate. This is due to the different sizes of crystals obtained in the different reagent feed flows, as shown in Figure 7. Large crystals allow reducing the colloidal effect formed by Mg(OH) ₂ suspensions, thus way the sedimentation and filtration operations are facilitated.

With a feed rate of reagents V ₂ , the granulometric distribution of the product has a higher percentage of larger crystals compared to the other products. As observed in Table 9, the D ₅₀ and D ₉₀ of the product obtained with flows of feeding of the reagents represented through V ₂ presents dimensions with values of 70.05 pm and 197.8 pm respectively, higher than the other cases.

Table 9. Percentiles of granulometric distribution of the Mg(OH) ₂ products obtained at different reagent feed flows.

For a complete characterization of the products obtained, a chemical analysis was carried out to determine the level of purity reached under the conditions studied.

Table 10. Chemical analysis of the Mg(OH) ₂ products obtained at different feed flows.

b) alimentar los reactivos preparados en a) al reactor para conducir una cristalización reactiva para la formación de Mg(OH)₂, controlando la sobresaturación por cambio de pH, y empleando una solución de NaCI con una concentración de 12 a 13% en peso, y una relación en volumen de NaCI/MgCI₂ de 0,45; y donde la solución de NaCI se encuentra dentro del reactor de formación de Mg(OH)₂ antes del ingreso de los reactivos preparados en a) para evitar cambios significativos de pH cuando se ingresen los reactivos al reactor, favoreciendo el crecimiento de los cristales de Mg(OH)₂, y donde los reactivos son alimentados al reactor de manera paralela, lentamente y a bajo flujo preferiblemente de 4,5 y 1 1 ,25 ml/min (0,33 y 0,82 volumen de reactor/h) para el álcali y bischofita respectivamente, manteniendo la alimentación de NaOH (ac) hasta alcanzar un pH de 12 o punto isoeléctrico del Mg(OH)₂ dentro del reactor; y la temperatura a 80°C, con agitación constante y un tiempo de reacción de 2,5 a 3 horas; c) separar de la solución obtenida en b), sólido y líquido, mediante filtración, preferentemente mediante filtro prensa; d) lavado de la pulpa de Mg(OH)₂ obtenida en c) con agua fresca para eliminar las impurezas solubles impregnadas en la torta, y controlando la presencia de iones disueltos mediante la medición de la conductividad del filtrado, y manteniendo el lavado hasta que la conductividad del filtrado sea reducida hasta el valor más bajo y constante posible, incluyendo valores mínimos de conductividad en el rango desde 450 a 600 pS/cm; e) secado del Mg(OH)₂ obtenido en d) a una temperatura de 80°C, preferentemente en un horno de secado. f) antes de la etapa c), reducir la generación de agregados, mediante tiempos prolongados de envejecimiento de los cristales de Mg(OH)₂ una vez concluida la reacción dentro del reactor, lo que favorece la disolución de cristales en la solución y su precipitación posterior, incorporándose en la matriz de otros cristales de mayores dimensiones, y opcionalmente g) después de la etapa c), recircular al reactor la solución de NaCI reacondicionada, usándola como aditivo y medio de reacción en lugar de alimentar una solución fresca de NaCI luego de cada proceso de formación de Mg(OH)₂. As can be seen in Table 10, in all cases the purity of the product reached a value close to 99.7%. Therefore, it can be inferred that by synthesizing Mg(OH) ₂ with industrial waste bischofite according to the proposed method, it is possible to achieve a high purity in the product. However, as observed in the previous sections, the difference lies in the separation operations, because although all the products are of high purity, some require more washing stages and longer filtering time. Therefore, it is possible to conclude that the optimal feed flows of the reagents are those described in Table 6 by V ₂ , for which a filtering time of 1.18 hours on average can be achieved, 6 stages of washing, a larger particle size distribution, for a purity of the Mg(OH) ₂ product of 99.7%. Thus, the present invention relates to a method for obtaining magnesium hydroxide Mg(OH) ₂ with a high purity of 99.7% from discarded bischofite without prior treatment, which comprises the following steps: a) preparation of the reagents consisting of: dissolving discarded bischofite in water, forming a solution with a concentration of 36 to 37% by weight of bischofite; and in parallel prepare an alkaline solution of NaOH of 99% purity, in water forming a solution of 23 to 24% by weight of NaOH; where the discarded bischofite contains the following percentage composition:

b) feeding the reagents prepared in a) to the reactor to conduct a reactive crystallization for the formation of Mg(OH) ₂ , controlling the supersaturation by changing the pH, and using a NaCl solution with a concentration of 12 to 13% by weight , and a NaCI/MgCI ₂ volume ratio of 0.45; and where the NaCI solution is inside the Mg(OH) ₂ formation reactor before the reagents prepared in a) enter to avoid significant pH changes when the reagents enter the reactor, favoring the growth of NaCl crystals. Mg(OH) ₂ , and where the reactants are fed to the reactor in parallel, slowly and at a flow preferably of 4.5 and 11.25 ml/min (0.33 and 0.82 reactor volume/h) to the alkali and bischofite respectively, maintaining the NaOH (aq) feed until reaching a pH of 12 or the isoelectric point of Mg(OH) ₂ inside the reactor; and the temperature at 80°C, with constant stirring and a reaction time of 2.5 to 3 hours; c) separating solid and liquid from the solution obtained in b), by filtration, preferably by filter press; d) washing the Mg(OH) ₂ pulp obtained in c) with fresh water to eliminate the soluble impurities impregnated in the cake, and controlling the presence of dissolved ions by measuring the conductivity of the filtrate, and maintaining the washing until that the conductivity of the filtrate be reduced to the lowest and constant value possible, including minimum values of conductivity in the range from 450 to 600 pS/cm; e) drying the Mg(OH) ₂ obtained in d) at a temperature of 80°C, preferably in a drying oven. f) before stage c), reduce the generation of aggregates, through prolonged aging times of the Mg(OH) ₂ crystals once the reaction has concluded within the reactor, which favors the dissolution of crystals in the solution and their subsequent precipitation, incorporating other larger crystals into the matrix, and optionally g) after stage c), recirculating the reconditioned NaCl solution to the reactor, using it as additive and reaction medium instead of feeding a fresh NaCl solution after each Mg(OH) ₂ formation process.

Example.

Production of Mo(OH) ₂ from discarded bischofite from the Atacama salt flat.

Mg(OH) ₂ is obtained by crystallization by chemical reaction from discarded bischofite with the composition given in Table 1. The process and flowchart of figures 1 A and 1 B are used.

As already mentioned before, the production of Mg(OH) ₂ presents drawbacks since, being a product with colloidal characteristics, its filtration is a complex operation to carry out. This makes it difficult to wash properly, so many impurities remain impregnated. The colloidal effect is also manifested in the amount of energy and time spent on filtration. To overcome these limitations, NaCl is used as reaction medium and additive in crystallization, and low flows of reagent feed to the reactor to control crystal size. With these control parameters, larger crystals are obtained, filtering time is reduced and effective washing is performed, being a key step to obtain a highly pure product. The operating conditions used in this example are noted below.

1-A Preparation of reagents for the production of Mg(OH) ₂ and establishment of operating parameters.

The following Magnesium sources were used:

1 - industrial bischofite from the discard area, having a purity of 88.08%, (used in the inventive method).

2- purified industrial bischofite, having a purity of 94.52% (used in the comparative method)

control). 3- , analytical grade of 99% purity (Sigma Aldrich) (used in the method

control).

The NaOH used as reagent for the conversion of MgCI ₂ to Mg(OH) ₂ is of analytical grade, with a purity of 99% (Sigma Aldrich).

The NaCI used as additive has a purity of 99% (Sigma Aldrich).

The concentrations of the reagents used for the development of the Mg(OH) ₂ production process are detailed in Table 1 1 .

The operating conditions used in the process are detailed in Table 12.

Table 12. Operating conditions for the production of Mg(OH) ₂

A thermally insulated 2-liter glass reactor is used to avoid heat loss, the temperature was kept constant with a thermostatic bath (Lauda RE 107 thermostatic bath), stirring was permanent (IKA RW20 stirrer), the flows of feeding of each reagent were driven with peristaltic pumps (2, Watson-Marlow 520 peristaltic pumps). The feeding of MgCl2 and NaOH in solution, to the reactor that already contains the NaCl solution, was simultaneous. The volume ratio of NaCl/MgCh is equal to 0.45.

As observed in Table 12, the feed flow of the reagents is low, since care must be taken in the supersaturation stage for the Mg(OH) ₂ production process. It is important to control the rate at which the precipitant is added, because if a medium with a degree of supersaturation is generated, the nucleation rate will be greater, causing the formation of a greater number of nuclei per unit of time, and therefore, obtaining smaller crystals with undefined geometric shapes. Small crystals also affect the quality of the precipitate, this is because they have a greater surface distribution than large crystals. A large surface distribution in the crystals makes them susceptible to a higher degree of adsorption of impurities, causing the precipitate to become contaminated and alter its quality, structure or solid appearance (GD Christian, Analytical Chemistry, Sixth ed. Mexico, 2009). 1-B Production of Mg(OH) ₂ from discarded bischofite.

As can be seen from the Pourbaix diagram (Figure 8 (A)), the pH at which Mg(OH) ₂ begins to form is 6.8, from this value the reaction begins to develop until reaching equilibrium. In this case, the feeding of NaOH to increase the pH of the system was carried out until reaching the value of the isoelectric point, that is, pH of 12 in the case of Mg(OH) ₂ . In this way, a high conversion of the reaction was ensured without excess reagents. To determine the yield of the reaction, a chemical analysis of the Mg ²⁺ ion was carried out, using an ionic balance and it was determined that the yield was 98.37% on average.

The different Mg(OH) ₂ products were characterized to determine their physical and chemical properties. The results are shown below:

Chemical characterization of Mq(OH) ₂ .

To achieve a high purity Mg(OH) ₂ product, the most important step is washing, since this single operation allows the crystals to be freed from impregnated solutions that are laden with impurities. As it is a product with a very low solubility, it is very convenient that the impurities are in the aqueous state, therefore, the use of NaOH as a reactant allowed obtaining the impurities in the aqueous state.

As observed in Table 1 , the main impurities in the system are Na ⁺ and K, which, when reacting with NaOH and MgCI ₂ *6H ₂ O, are obtained as possible by-products: NaOH, KOH, KCI and NaCI, this The latter as the main by-product of the reaction for the formation of Mg(OH) ₂ and the remainder of the additive solution, which are largely eliminated in the filtration of the product, which separates the cake formed from Mg(OH) ₂ from the solution that contains impurities in aqueous state. The remaining impurities impregnated in the crystals were removed by washing with distilled water at 80°C, with a 2:1 weight ratio of distilled water and Mg(OH) ₂ cake, respectively, until a high purity product was obtained.

In the washing process there are some factors that affect its performance, the most important are the size and shape of the crystal. The purification stage of the Mg(OH) ₂ crystals and the effect of their granulometry and morphology are described in detail later.

Another important impurity is H3BO3, which is in the solid state in the system. Boric acid is slightly soluble in water, but its solubility increases with increasing temperature. In addition, its solubility is strongly decreased by the presence of certain ions such as NaCI and LiCI (/W. O and DAN, "Analysis of the solubility of boric acid," Central Nuclear Atucha U22007). Since there is a high concentration of NaCI in the system, the solubility of H3BO3 is very low. On the other hand, B(OH) ₄ , which is an ionic state of boric acid, is highly soluble in water. To determine the conditions under which this phase is stable in an aqueous system, a Pourbaix diagram was used (Figure 8 (B)). This diagram shows us that at a pH greater than 7.4, the stable phase of boric acid is B(OH) ₄ ^_ , this is very convenient since, as mentioned above, the working pH of system is 12, so boron will be removed in the filtration in the B(OH) ₄ form, as well as in the washing stage, since the amount of Mg(OH) ₂ that dissolves in this stage makes the solution reaches a pH of 10.35.

Chemical analysis.

To measure the washing efficiency, a chemical analysis was performed to determine the purity of Mg(OH) ₂ and the amount of impurities present in the samples. After the washing process, the Mg(OH) ₂ samples were dried in an oven at 100°C. The results are presented in Table 13.

Table 13. Chemical analysis of the Mg(OH) ₂ samples

As observed in Table 13, the product obtained in carrying out the invention reaches a higher purity with respect to the comparative example. Using the most impure bischofite of 88.04% purity, it is possible to obtain a Mg(OH) ₂ product with a higher purity of 0.5%, compared to the product obtained with the bischofite of 94.54% purity. .

Without adhering to any theory, this can be attributed to the effect of the impurities and their concentrations present in the industrial bischofite used, which modified the size and shape of the Mg(OH) ₂ crystals, which is analyzed and described later. when referring to the granulometry and morphology of the product. In summary, the wash turned out to have a good performance.

grado analítico de 99% de pureza, responde al producto de mayor pureza. En este producto, la presencia de Na y Cl es atribuida al subproducto de reacción que se generó y el aditivo de NaCI presente durante la reacción, sin embargo, las concentraciones presentes en el producto son muy bajos, en el orden de los “ppm” como se ve en la Tabla 13. Además, la presencia de B, que se atribuye como impureza de utilizado, es la más baja en comparación a los demás

productos. Como se observa, al trabajar con reactivos de alta pureza la presencia de impurezas es insignificante y no presentan mayor problema en las etapas de filtrado, pudiendo obtener un producto de 99,9% de pureza. Por su parte el producto Mg _comparativo que se obtiene a través del método comparativo que emplea bischofita purificada de pureza 94.54 %, se observa en este producto que mediante el lavado se logró eliminar el 99,54% del Na⁺ presente como impureza en la bischofita. Con respecto al K⁺, se logró eliminar el 99,99% de este ion. Evaluando el Cl, se logra eliminar el 95,2%, y finalmente respecto al boro se elimina un 73,68%, lográndose finalmente una pureza de 99,23% como Mg(OH)₂. The Mg _C ontroi product that is obtained through the control method in which it is used

analytical degree of 99% purity, responds to the highest purity product. In this product, the presence of Na and Cl is attributed to the reaction by-product that was generated and the NaCI additive present during the reaction, however, the concentrations present in the product are very low, in the order of "ppm". as seen in Table 13. In addition, the presence of B, which is attributed as an impurity used, is the lowest compared to the other

products. As can be seen, when working with high purity reagents, the presence of impurities is negligible and they do not present a major problem in the filtration stages, being able to obtain a product of 99.9% purity. For its part, the _comparative Mg product that is obtained through the comparative method that uses purified bischofite of 94.54% purity, it is observed in this product that by washing it was possible to eliminate 99.54% of the Na ⁺ present as impurity in the bischofite. . Regarding K ⁺ , it was possible to eliminate 99.99% of this ion. Evaluating Cl, 95.2% is eliminated, and finally, with respect to boron, 73.68% is eliminated, finally achieving a purity of 99.23% as Mg(OH) ₂ .

Analyzing the product Mg _invention obtained according to the method proposed in this application, and that uses discarded bischofite with a purity of 88%, through washing it was possible to eliminate 99.33% of the Na ⁺ present as impurity in the bischofite . Regarding K ⁺ , 99.9% was eliminated. Evaluating chlorine, 97.1% was eliminated, and finally, regarding boron, 92.14% was eliminated. Finally achieving a product of Mg(OH) ₂ with a purity of 99.72%. The other impurities present in the industrial bischophytes used were not detected by the chemical analysis, so it can be assumed that they are found in less than 1 ppm.

XRD analysis

Analysis was performed by X-ray powder diffraction using a diffractometer. The radiation source (CuKa) was a copper lamp with a wavelength of X = 0.154 nm. Copper Ka radiation was generated at 20 mA and 40 KV. The study was carried out on the Mg(OH) ₂ products obtained, the results are shown in Figure 4.

To identify the spectrograms of the analyzed products, the Match program was used, which could show that the peaks correspond to the Mg(OH) ₂ phase. As shown in Figure 4, in addition to Mg(OH) ₂ , no other phase was identified, and impurities may exist, but in a very low proportion that this method cannot identify.

Analyzing the spectrograms in more detail, it was found that there is no lag in the angles 20, with respect to the highest intensity peaks, this indicates that in the products obtained there is no variation in the interplanar spaces. However, in the spectra of the inventive and comparative Mg(OH) ₂ samples, a broadening in the width of the peaks can be observed. This phenomenon is due to some loss of crystallinity in the Mg(OH) ₂ samples synthesized with bischofite. Crystallinity is related to the degree of order and the size of the crystal of a given crystalline substance (Y. Sa, Y. Guo, X. Feng, M. Wang, P. Li, Y. Gao, et al., "Are different crystallinity-index-calculating methods of hydroxyapatite efficient and consistent?," New Journal of Chemistry, vol. 41, pp. 5723-5731, 2017). As there is a greater amount of impurities in the bischophytes used to produce inventive Mg and _comparative Mg, these were absorbed on the crystalline surface, causing certain dislocations in the row of points of the Bravais lattice (AG Jones, Crystallization Process Systems. Oxfor, 2002 and JPM Syvitski, Principle, methods, and application of particle size analysis, Cambridge, 2007). Physical characterization of Mg(OH) ₂ .

The granulometric analysis by focused beam reflectance measurement of the Mg(OH) ₂ samples was performed by focused beam reflectance measurement with the FBRM probe. The results of the analysis are shown in Figure 5, and Table 14.

Table 14. Dimensions of Mg(OH) ₂ crystals for different sources of d

pura), y el D₅₀ de Mg invento (producido a partir de bischofita de 84,04%) es 48,1% mayor en tamaño en comparación a Mg_control. Analizando el D₉₀ la variación de tamaños se hace más grande. El D₉₀ de Mg_comparativo presenta un tamaño mayor en 53,44% en comparación al D₉₀ para Mg_control. El D90 para Mg_invento presenta un tamaño mayor en 55,39% en comparación al D₉₀ para Mg_control. Con esto podemos inferir que la existencia de mayor impurezas o iones disueltos en el sistema permite obtener cristales de mayor tamaño. Las impurezas de las bischofitas utilizadas actuaron de la misma manera como aditivos en el sistema, estos iones ajenos a la fase cristalizada se absorbieron en la superficie cristalina, permitiendo una difusión adecuada de los iones Mg, como se explicó anteriormente. Las existencias de mayores concentraciones de impurezas que actúen como aditivos permitió obtener cristales más grandes. En promedio, la presencia de impurezas en el

permitió incrementar en un 50% el tamaño de la partícula. Evaluating Table 14, the quantitative particle size data shows that the D ₁₀ for all three samples are approximate, with the largest being for Mg _invention . Evaluating the D ₅₀ we can see a large variation in the particle sizes of the Mg invention and _comparative Mg crystals with respect to _control Mg, since the D ₅₀ of _comparative Mg (produced from 94.54% bischofite) is 46.4% larger in size than Mg _control (produced from

pure), and the Mg invention D ₅₀ (produced from 84.04% bischofite) is 48.1% larger in size compared to the Mg _control . Analyzing the D ₉₀ the size variation becomes larger. The D ₉₀ for _comparative Mg presents a larger size by 53.44% compared to the D ₉₀ for Mg _control . The D90 for Mg _invention presents a larger size by 55.39% compared to the D ₉₀ for Mg _control . With this we can infer that the existence of greater impurities or dissolved ions in the system allows obtaining larger crystals. The impurities of the bischophytes used acted in the same way as additives in the system, these ions foreign to the crystallized phase were absorbed on the crystalline surface, allowing an adequate diffusion of the Mg ions, as explained above. Stocks of higher concentrations of impurities that act as additives allowed obtaining larger crystals. On average, the presence of impurities in the

allowed to increase the particle size by 50%.

However, when analyzing the uniformity of the particle distribution, the Mg _control sample is the only one that presents crystals with homogeneous sizes. However, the values of CU (uniformity coefficient) of M _invention and _comparative Mg are greater than 5, which indicates that these crystals do not have homogeneous sizes. This shows that the use of lower purity raw material generates greater dispersion in the granulometric distribution, which can bring complications to the filtering process since, as previously indicated, the adverse effect of the presence of impurities in the system causes the smaller particles clump together in a globular fashion to form aggregates. The presence of aggregates can be reduced with prolonged crystal aging times. SEM analysis for the morphological study of the product

The morphological study of the Mg(OH) ₂ samples is carried out by means of a SEM characterization. The particles were evaluated by backscattered electron (BSE) and secondary electron (SE) analysis. The results are presented in Figures 9A-9C.

BSE analysis has the advantage of being sensitive to variations in the atomic number of the elements present on the surface of the sample. With this analysis we can qualitatively see the existence of some significant impurity, through some color variation on the surface of the sample. As can be seen in Figures 9A-9C, in all three cases a uniform shade of gray is observed on the surface of the samples. This indicates that the impurities are in very small proportions that cannot be detected by the equipment.

Figures 10A and 10B show the SEM analysis of the _control Mg sample from SE secondary electrons, to better analyze the crystalline morphology. Figure 9A shows the homogeneous distribution of these crystals of the _control Mg product, and as can be seen, these crystals have a not very wide range of sizes, with good uniformity, since, as can be seen, the difference between the sizes is not very big. Image 10B shows a very important aspect, the smaller crystals have a quasi-spherical shape, which could be beneficial for the washing process. Another important observation is that Mg _C ontroi does not show a considerable presence of agglomerates. Small particles with almost spherical shapes and the absence of agglomerates are factors that definitely help the suspension to behave better in filtration processes, since the permeability of a cake depends on the shape of the grain, which can be different even for samples that have identical granulometrics (JPM Syvitski, Principle, methods, and application of particle size analysis. Cambridge, 2007).

In Figures 1 1 A and 11 B, the SEM analysis by SE for the _comparative Mg sample is observed. Figures 12A and 12B show the SEM analysis by SE for the Mg _invention sample - Figure 12A shows the difference in crystal size, which, as in Figure 1 1 A corresponding to the _comparative Mg sample It presents a non-uniformity in the distribution of sizes. On the other hand, observing Figure 12B, it is observed that the smallest crystals have irregular polygonal shapes, this is observed both for Mg invention and Mgcomparativa- We can also appreciate the existence of agglomerates for Mg _invention , in less proportion than Mgcomparative- The shapes small irregularities and the presence of agglomerates in the sample Mg _invention , can affect the performance of the filtration and washing of the crystals. With these observations, the effect of the impurities is validated, that the greater the amount of impurities, the larger the crystal size, but they involve the existence of agglomerates.

Unlike the _control Mg sample that does not present agglomerates, the presence of these in the _inventive Mg and _comparative Mg samples is attributed to the impurities of the bischophytes used, since both the hydration sphere of the K ⁺ and B(OH) ions ) ₄ ^_ are different from those of the Na ⁺ ion, which adsorbing on the crystalline surface further favored agglomerate formation as well as increased crystalline growth. In addition, the presence in greater quantities of Na ⁺ and Cl' ions from the impurities of the bischophytes helps the generation of agglomerates. Effect of crystal size and shape on filtration and purity.

The progress of the washing was monitored by measuring the conductivity of the filtrate. The washing stages were carried out until the conductivity of the filtrate was reduced to the minimum possible value. Since Mg(OH) ₂ is a product with very low solubility, it was considered to control the conductivity of the filtrate as a purification parameter, since conductivity is closely related to the presence of ions in the solution; that is, the higher the conductivity of the filtrate, the presence of impurities impregnated in the crystalline mass. As the conductivity of the filtrate decreases, the presence of impurities impregnated in the Mg(OH) ₂ crystals decreases. The filtration time for each Mg(OH) ₂ sample was also controlled to evaluate the effect of the granulometric distribution, the shape and the presence of agglomerates on the washing performance.

For each of the Mg(OH) ₂ produced, an aging time of three hours was used. The crystal aging stage was carried out after finishing the residence time of 3 hours, established in Table 7. Under these conditions, when Mg(OH) ₂ is found in a NaCl solution with a high concentration, its solubility is is increased due to the effect of increasing ionic strength in the solution. The solubility of Mg(OH) ₂ in distilled water is 1.12 x 10 ^-4 M, however, in the presence of a high concentration of NaCl in solution, the solubility increased to 3.36 x 10' ⁴ M. This This fact was very beneficial for the crystalline aging process, since, as solubility increases, the smaller Mg(OH) ₂ crystals formed dissolve and are reincorporated on the surface of the larger crystals (FG Freixedas, AC Bauzá , and O. Sohnel, Crystallization in Solutions- Basic Concepts, Barcelona-Spain: Editorial Reverteré, 2000). By aging it is possible to obtain larger, more stable crystals with fewer agglomerates.

As observed in Figure 13, the conductivity of the filtrate of the _inventive Mg sample is the highest, with a value of 228 [mS/cm], this is due to the greater presence of ions as impurities in the industrial waste bischofite. used. The _comparative Mg sample filtrate has an initial conductivity of 217.7 [mS/cm], 4.5% less than the Mg filtrate of _{the invention} ; this is due to the purity of the purified bischofite that was used to synthesize this product. Finally, the initial conductivity of the filtrate of the Mg _control sample is the lowest value among the three, with 209.4 [mS/cm], 8.5% lower than the filtrate of the Mg _invention sample; in this case the conductivity for the _control Mg filtrate is only due to the presence of Na ⁺ and Cl- ions in solution. Regarding the variation of conductivity of the filtrate according to each washing stage, it can be observed that up to stage 2 of washing, the conductivity is reduced linearly for the samples Mg _invention , Mg _comparative and Mg _control , with reduction percentages of 80 , 84.2 and 84.6% respectively. From this point the rate of conductivity reduction varies taking another trend. How I know can be seen in Figure 13, the washing for the Mg _control sample had the highest yield, since to reduce the conductivity to the lowest constant value, only 7 washing stages were required. In addition, the conductivity for the _control Mg filtrate was the lowest achieved among the three samples. On the other hand, for the Mg _invention and _comparative Mg samples, it was necessary to carry out 10 washing stages until reaching a minimum and constant conductivity, close to that of the _control Mg sample. The final conductivity of the _control Mg filtrate was on average 61 % lower than the final conductivities of _inventive Mg and _comparative Mg - This variation can be explained due to the effect of the presence of agglomerates in the Mg invention and _comparative Mg samples that, despite Having larger crystals, the presence of agglomerates made the crystalline mass retain a greater amount of solution, this explains the variation in purity obtained in each sample. The effect of the agglomerates is also notable in the values of the Mg invention and _comparative Mg curves, since each one has values higher than those of the _control Mg curve. The time it took to filter for each sample and the percentage of water retained are presented in Table 15.

Table 15. Filtration time and percentage of water retained for the Mg(OH) ₂ samples.

As seen in Table 15, the filtration time was faster for M _invention than for Mgcomparative. This variation in filtration times is largely due to the presence of agglomerates. As can be seen in Figure 1 1 B, the _comparative Mg sample presents a greater amount of agglomerates, which obviously affects the filtering time.

Despite the fact that the Mg _control sample contains the smallest crystals, its uniform granulometric distribution, the absence of agglomerates and the almost spherical shape of the smallest particles allowed the filtration to be carried out in 1.6 hours and also retaining the least amount. of water in the cake, obtaining a 99.88% pure Mg(OH) ₂ product.

Table 15 also shows the values for the percentage of water retained for each sample, this value was obtained by weighing the wet Mg(OH) ₂ pulp after filtering and the weight of the completely dry Mg(OH) ₂ . Although the sample M _invention presents less water retention than the _comparative Mg sample, the values of the retention percentage in both samples are very close.

Despite the fact that both samples contained agglomerates, the Mg _invention sample performed better and achieved a higher product purity with respect to _comparative Mg. This Advantageous variation in the _invention Mg sample is mainly due to the higher amount of agglomerates present in the _comparative Mg sample. However, a factor that also contributes to achieving a higher purity in _inventive Mg is the larger crystal size that is achieved in this sample. Since, as shown in Table 9, the Mg _invention crystals are 4.2% larger than those of _comparative Mg, this small difference significantly influenced the behavior of the Mg _invention pulp, due to the presence of larger particles. , there being less additive force of the solutions on the crystal matrix, causing the percentage of retained H ₂ O to be lower. Due to all these factors, the Mg(OH) ₂ product of the _invention Mg sample is of higher purity than _comparative Mg.

Claims

1 - Effective method to obtain magnesium hydroxide Mg(OH) ₂ with a high purity of 99.7% from discarded bischofite, characterized in that it comprises the following steps: a) preparation of the reagents that consists of dissolving discarded bischofite in water forming a solution with a concentration of 36 to 37% by weight of bischofite; and in parallel prepare an alkaline solution of NaOH in water, forming a solution of 23 to 24% by weight of NaOH, where the discarded bischofite contains the following percentage composition: 88.08% d 1.53% of

3.20% NaCI 0.89% KCI 0.12% CaSO ₄ 0.46% of K ₂ SO ₄ 2.13% H3BO3 b) feeding the reagents prepared in a) to the reactor to conduct a reactive crystallization for the formation of Mg(OH) ₂ , controlling the supersaturation by changing the pH, and using a NaCl solution with a concentration from 12 to 13% by weight, where the volume ratio of NaCI/MgCI ₂ is equal to 0.45, the NaCI solution is inside the Mg(OH) ₂ formation reactor before entering the prepared reagents in a); and the feeding of the reagents is carried out at a low flow of 4.5 and 11.25 ml/min (0.33 and 0.82 reactor volume/h), of alkali and bischofite, respectively; c) separating solid and liquid from the solution obtained in b), by means of filtration; d) washing the Mg(OH) ₂ pulp obtained in c) with fresh water; e) drying of the Mg(OH) ₂ obtained in d). 2. The method of claim 1 characterized in that in step a) the purity of NaOH is 99%. 3. The method of claim 1 characterized in that in step b) the reactants are fed to the reactor simultaneously. 4. The method of claim 1 characterized in that in step b) the feeding of the NaOH solution is maintained until reaching a pH of 12 or the isoelectric point of Mg(OH) ₂ inside the reactor. 5. The method of claim 1 characterized in that in step b) the reactor temperature is maintained at 80 °C and under constant stirring, for a reaction time of 2.5 to 3 hours. 6. The method of claim 1, characterized in that it also comprises, before step c), prolonging the aging time of the Mg(OH) ₂ crystals after the reaction within the reactor has concluded, to reduce the generation of aggregates. 7. The method of claim 1 characterized in that step c) filtration is carried out in a filter press. 8. The method of claim 1 characterized in that it further comprises after step c), recirculating the reconditioned NaCl solution to the reactor. 9. The method of claim 1 characterized in that in step d) the presence of dissolved ions is controlled by measuring the conductivity of the filtrate up to constant minimum values of conductivity in the range of 450 to 600 pS/cm. 10. The method of claim 1 characterized in that in step e) the drying of Mg(OH) ₂ is carried out at 80 °C. eleven . The method of claim 1 characterized in that step e) of drying is carried out in a drying oven.