Brine valorisation method for a brine originating from seawater desalination

Abstract

The present invention relates to a method for preparing magnesium sulphate by treating a brine typically originating from seawater desalination.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for recovering brine from seawater desalination.

[0003] TECHNICAL FIELD OF THE INVENTION

[0004] The present invention relates to the preparation of magnesium sulfate by treatment of a brine typically derived from the desalination of seawater. The process can furthermore allow the selective recovery of NaCl and / or kainite.

[0005] STATE OF THE ART

[0006] Magnesium sulfate is used in particular in the production of animal feed, pulp and paper. In the pharmaceutical field it is also used as a chemical agent or additive.

[0007] The most widely used process for producing magnesium sulfate is based on the reaction between sulfuric acid and magnesium oxide or magnesium carbonate. However, this process is expensive and produces magnesium sulfate of insufficient quality with a purity of approximately 90%.

[0008] Another method involves mixing epsomite (MgSC .TFW) and halite (NaCl) at 83°C until clusters of less hydrated magnesium sulfate crystals form. High pressure is then applied while stirring while hot to cause the clusters of crystals to collapse into fine, less hydrated crystals. These can then be separated from the relatively coarse halite crystals.

[0009] Another method proposes the production of magnesium sulfates from seawater and brines. This process consists of producing MgSO4 by adding NaOH at 90°C until a pH of 10 is reached and precipitating the magnesium in the form of Mg(OH)2. The precipitate formed is then brought into contact with sulfuric acid (H2SO4) to produce MgSO4 with low purity. However, the presence of Ca ions 2+ also leads to the formation of secondary products such as Ca(OH)2 and CaSO4. Furthermore, this process makes it possible to achieve yields not exceeding 80% at a very high cost.

[0010] US 3,536,444A proposes a process for the recovery of hydrated magnesium sulfate from brines formed after the Carnallite precipitation step (KMgCI3.6H2O). The process is based on the mixing of two brines of different densities 1.36 (after Carnallite crystallization) and 1.32 (before Carnallite crystallization) at 35°C in order to promote the formation of magnesium crystals. The main objective of this document is the recycling of the brine formed after the Carnallite precipitation. However, the yield of this process is generally very low at around 12%.

[0011] Seawater desalination is increasingly used worldwide to provide freshwater for agriculture and industry. The major ionic species present in seawater are Cl; Na + , Mg 2+ , SO4 2 ; K + and that 2+They represent more than 99.9% of the total mass of substances dissolved in seawater. Their total mass can vary from one seawater to another but generally their relative proportions remain constant.

[0012] Reverse osmosis (RO) is currently the desalination technology of choice. In 2020, RO dominated the market and accounted for over 55% of global revenue. This is attributed to its advantageous properties, such as ease of operation, low installation cost, the ability to treat different types of feedwater, and minimal chemical use. The major drawback of this technology is the large quantities of brines produced by desalination plants. These brines typically represent 50% of the feed stream. The salinity of brines is twice that of seawater (approximately 70 g / L). They are generally discharged into the sea, typically after being reprocessed, for example, by dilution with non-potable water. Given the large quantities of brine produced, there is therefore a strong interest in recovering the salts they contain.

[0013] Thus, there is a need for new methods to recover brine from seawater desalination by enabling the production of magnesium sulfate salts at lower cost, without the aforementioned drawbacks and without generating effluents harmful to the environment. Advantageously, the proposed process will allow the recovery of salts from brines from reverse osmosis desalination plants.

[0014] SUMMARY OF THE INVENTION

[0015] The present invention relates to a process for producing magnesium sulfate, in hydrate form, from a brine resulting from the desalination of seawater, the process comprising the following steps: (1) removal of 80 to 94% of the volume of water initially contained in the brine leading to the formation of a suspension comprising NaCl crystals, followed by separation of the suspension into a solution (S1) and a solid residue comprising the NaCl crystals;

[0016] (2) removal of a portion of the water contained in the solution (S1) so as to reach a total volume of water removed in steps (1) and (2) corresponding to 95 to 99% of the volume of water initially contained in the brine leading to the formation of a suspension comprising kainite crystals, followed by the separation of the suspension into a solution (S2) and a solid residue comprising kainite crystals and possibly NaCl crystals;

[0017] (3) cooling the solution (S2) obtained at the end of step (2) to a temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of hydrated magnesium sulfate; and

[0018] (4) separation of the suspension (S3) resulting from step (3), so as to obtain a solution (S4) and a solid M comprising the hydrated magnesium sulfate crystals.

[0019] The method of the invention optionally comprises the following features, alone or in combination.

[0020] Advantageously, the method of the invention further comprises the following steps:

[0021] (5) rinsing the solid (M) by contacting it with an organic solvent leading to the production of MgSC crystals and the organic solvent containing impurities;

[0022] (6) optional recycling of the organic solvent to be sent to step (5);

[0023] (7) optional drying of the solid (M) containing MgSC crystals.

[0024] Preferably, the water removed in steps (1) and / or (2) is recovered, typically as fresh water.

[0025] The brine typically has a pH ranging from 7.5 to 8.5 and / or a salinity ranging from 4.6 to 9.0% by weight. The brine may be derived from the desalination of seawater by distillation or reverse osmosis. In some embodiments, the solution (S2) is cooled in step (3) to a temperature ranging from -15°C to -3°C, preferably from -9°C to -4°C.

[0026] The organic solvent is typically an alcohol or acetone.

[0027] The drying step (7) is typically carried out at a temperature ranging from 30 to 80°C, preferably ranging from 35 to 75°C.

[0028] The MgSC crystals obtained after drying (7) are in particular in the form of a white solid with a purity greater than 95% by weight, preferably greater than 99% by weight.

[0029] In some embodiments, the crystallization step (3) of the MgSC salts is carried out in a container having a volume / surface ratio ranging from 1:1 to 1:10 or from 1:3 to 1:10.

[0030] Typically, magnesium sulfate hydrate has the general formula MgSC .nFW in which n ranges from 1 to 11. Magnesium sulfate hydrate is preferably selected from the group consisting of epsomite (MgSO4.7H2O), magnesium sulfate hexahydrate (MgSC .ôFW), pentahydrate (MgSC .ôFW), alone or in combination.

[0031] Other aspects of the invention are as described below.

[0032] FIGURES

[0033] Figure 1: is a diagram of the brine treatment process according to the invention for the production of NaCI, kainite and MgSC.

[0034] The figures illustrate in a non-limiting manner devices capable of implementing the treatment method using resin according to the invention.

[0035] DETAILED DESCRIPTION OF THE INVENTION

[0036] The inventors have developed a process for treating brine from the desalination of seawater, enabling the production of magnesium sulfate at low cost, with good yields and without generating effluents harmful to the environment.

[0037] Thus, the subject of the present invention is a process for producing magnesium sulfate in hydrate form, from brine obtained from the desalination of seawater, the process comprising the following steps:

[0038] (1) removal of 80 to 94% of the volume of water initially contained in the brine leading to the formation of a suspension comprising NaCI crystals, followed by separation of the suspension into a solution (S1) and a solid residue comprising NaCI crystals;

[0039] (2) removal of a portion of the water contained in the solution (S1) so as to reach a total volume of water removed in steps (1) and (2) corresponding to 95 to 99% of the volume of water initially contained in the brine leading to the formation of a suspension comprising kainite crystals, followed by the separation of the suspension into a solution (S2) and a solid residue comprising kainite crystals and possibly NaCl crystals;

[0040] (3) cooling the solution (S2) obtained at the end of step (2) to a temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of hydrated magnesium sulfate; and

[0041] (4) separation of the suspension (S3) resulting from step (3) leading to the production of a solution (S4) and a solid (M) comprising the crystals of hydrated magnesium sulfate.

[0042] Advantageously, the method according to the present invention can be implemented to recover the salts contained in the brine and thus maximize the recovery of brines from seawater desalination while limiting their environmental impact.

[0043] Advantageously, the method according to the present invention allows the recovery of brine into a wide range of recovered products, and this with great flexibility.

[0044] An advantage of the process according to the invention is that the manufacture of all these products can be carried out without waste requiring reprocessing.

[0045] Magnesium sulfate hydrate advantageously responds to the general formula MgSC .nFW in which n varies from 1 to 11. Magnesium sulfate hydrate is thus generally epsomite (MgSC . FW), magnesium sulfate hexahydrate (MgSC .ôFW), pentahydrate (MgSC .ôFW), alone or in combination. Brine:

[0046] The brine used in the process according to the invention comes from the desalination of sea water.

[0047] The term "sea water" means water from an ocean or water from an inland sea or similar.

[0048] The brine mainly contains the following ionic species CI; Na + , Mg 2+ , SO4 2 ', CO3 2 'K + and that 2+ .

[0049] The brine useful in the process of the invention is conventionally a brine resulting from the desalination of seawater by distillation or reverse osmosis.

[0050] Atlantic Ocean water has an average salinity of 35 g / L, or 3.5% by weight. Ocean water generally has a salinity ranging from 30 to 40 g / L, or 3 to 4% by weight. Inland seas and similar seas have a higher salinity because evaporation concentrates the salt there.

[0051] Brine from seawater desalination generally has a salinity ranging from 1.1 to 2 times, typically 1.2 to 2 times or 1.3 to 1.7 times the salinity of the seawater from which it originates.

[0052] Typically, brine from desalination, typically by reverse osmosis, has an average salinity ranging from 4.6 to 9.0% by weight or 1 to 4.5% by weight.

[0053] The brine useful in the process according to the invention can therefore have an average salinity ranging from 4.6 to 9.0% by weight or from 1 to 4.5% by weight.

[0054] Table 1 describes an example of the chemical composition of the major elements of a brine resulting from desalination compared to the composition of the seawater from which it is derived.

[0055] [Table 1]

[0056] Table 1: Chemical composition of brine compared to seawater Seawater has a pH of 7.65, brine from seawater desalination typically has a pH of 8.16. This basicity is associated with the salts in seawater.

[0057] The brine useful in the process according to the invention may have a pH ranging from 7.5 to 8.5 and typically 8.2.

[0058] The brine useful in the process according to the invention may have a density greater than 1.05 kg / L.

[0059] Brackish waters having characteristics similar to a brine described above can also be treated in the process according to the invention.

[0060] The brine useful in the process according to the invention can be filtered beforehand when solid impurities are present.

[0061] Step (1): formation of NaCI

[0062] The method according to the invention comprises a step (1) of removing 80 to 94% of the volume of water initially contained in the brine leading to the formation of a suspension comprising NaCI crystals, followed by the separation of the suspension into a solution (S1) and a solid residue comprising NaCI crystals.

[0063] Typically, step (1) leads to the elimination of 85 to 90% of the volume and more generally 90% of the volume of water initially contained in the brine.

[0064] The crystallization of NaCl is independent of the means used to remove water from the brine. Typically, water removal can be carried out in the open air. Water removal can also be carried out by any other methods known to those skilled in the art.

[0065] Water removal can be achieved by evaporation. Evaporation is generally carried out at a temperature ranging from 20°C to 250°C, or even from 20 to 100°C or even from 20 to 60°C. Water removal is typically carried out in the open air.

[0066] Generally, the removal of water from the brine to be treated is carried out in such a way as to remove at least 90% of the volume, and up to 94% of the volume of water initially contained in the brine.

[0067] Advantageously, during step (1), typically during water removal, the brine has a controlled salt concentration to be in a range of 300 to 380 g / L, i.e. a salinity of 30 to 38% by weight, which makes it possible to selectively promote the crystallization of NaCl, typically with respect to the magnesium salts, MgSC and MgCh. The salt concentration of the solution (S1) is commonly determined by the density of the solution (S1).

[0068] Generally, the solid residue obtained at the end of step (1) contains at least 95% by weight of NaCl, typically up to 99% by weight of NaCl, or from 95 to 99% by weight of NaCl.

[0069] Separation can be achieved by any method that allows a solid to be separated from a liquid. Typically, separation is achieved by filtration, decantation, centrifugation and / or scraping.

[0070] The solid residue containing NaCI can also be separated from the brine as the water is removed and the NaCI crystallizes. The solution (S1) therefore corresponds to the brine in which at least 90% by weight of NaCI, or even at least 93% by weight of NaCI or even at least 95% by weight of NaCI has been removed relative to the quantity of NaCI contained in the brine to be treated.

[0071] The solution (S1) typically has a density ranging from 1.1 to 1.3 kg / L, and / or a pH ranging from 8 to 9, or even a pH ranging from 8.5 to 9 and / or a conductivity ranging from 140 to 200 mS / cm.

[0072] The solid residue can be used in washing and / or centrifugation steps in order to recover purified NaCI salts which can then be used, for example, in human or animal food, in the manufacture of soda (NaOH), chlorine (Ch) or even bleach (NaCIO).

[0073] The solution (S1) contains a quantity of NaCl salts less than or equal to 10% by weight, typically less than or equal to 7% by weight or even 5% by weight.

[0074] The solution (S1) is then engaged in a kainite formation step.

[0075] Step (2): formation of kainite

[0076] The method according to the invention comprises a step (2) of removing a portion of the water contained in the solution (S1) so as to reach a total volume of water removed in steps (1) and (2) ranging from 95 to 99% of the volume, or even 99% of the volume of water initially contained in the brine leading to the formation of a suspension comprising kainite crystals, followed by the separation of the suspension into a solution (S2) and a solid residue comprising kainite crystals and optionally NaCl crystals.

[0077] The removal of water in step (2) can be carried out by evaporation, so as to reach 95 to 99% of the volume, or even 99% of the volume of water compared to the quantity of water initially contained in the brine. Typically, the removal of water is carried out gradually, that is to say, it is carried out slowly to promote the crystallization of the kainite salts. For example, the slow removal of water can be carried out by evaporation in the open air.

[0078] Typically, water removal by evaporation is carried out at a temperature of 50°C or less, typically ranging from 20 to 50°C or from 25 to 45°C. Evaporation can be carried out under solar radiation, typically for 8 hours.

[0079] A cooling step can also be carried out to promote the crystallization of kainite salts, typically at a temperature ranging from 5 to 25°C, or even 10 and 20°C, or even 10 to 15°C.

[0080] The crystallized kainite salts can be recovered by any methods known to those skilled in the art, for example by scraping.

[0081] Separation can be achieved by any method that allows a solid to be separated from a liquid. Typically, separation is achieved by filtration, decantation, centrifugation, or scraping.

[0082] Thus, step (2) according to the invention allows the formation of kainite salts. The kainite salts are obtained by crystallization and separation of the kainite salts contained in the suspension.

[0083] Kainite is a mixed salt of potassium and magnesium with the formula KCIMgSO4.3H2O.

[0084] Step (2) therefore makes it possible to obtain a solid residue containing at least 50% by weight of kainite, typically the solid residue contains at least 60% by weight of kainite or at least 73% by weight of kainite and typically from 50 to 90% or from 70 to 90% by weight of kainite. Optionally the solid residue may also contain NaCl, for example from 10 to 50% by weight of NaCl and typically less than 20% by weight of NaCl. Conventionally, the solution (S2) contains a quantity of kainite salts less than or equal to 5% by weight, typically less than or equal to 3% by weight or even less than or equal to 1% by weight.

[0085] The solution (S2) typically has a density ranging from 1.1 to 1.4 kg / L, and / or a pH ranging from 8 to 9, or even a pH ranging from 8.5 to 9 and / or a conductivity ranging from 160 to 240 mS / cm.

[0086] The water removed in steps (1) and / or (2) is typically recovered as fresh water.

[0087] The method according to the invention comprises a step (3) of cooling the solution (S2) obtained at the end of step (2) to a temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of magnesium sulfate hydrate, typically of formula MgSCU, n H2O with n varying from 1 to 11.

[0088] Conventionally, the solution (S2) which is sent to the cooling step (3) has a water evaporation rate greater than or equal to 99% by volume compared to the volume of water initially contained in the brine.

[0089] The solution (S2) may further have a density ranging from 1.1 kg / l to 1.4 kg / l, and typically ranging from 1.28 kg / l to 1.35 kg / l.

[0090] The water evaporation rate refers to the volume of water that has been eliminated relative to the volume of water initially contained in the brine to be treated in the process according to the invention.

[0091] The solution (S2) can be cooled by any means, for example by a freezer, or any other suitable cooling system to promote the crystallization of the MgSC salts.

[0092] The solution (S2) is cooled to a temperature ranging from -30°C to 0°C, typically to a temperature ranging from -15°C to -3°C, or even from -9°C to -4°C. Generally, the crystallization (3) is carried out for a period ranging from 1 h to 10 h (hour), typically ranging from 2 h to 6 h.

[0093] Typically, the formation of MgSC crystals is carried out without stirring the solution (S2) so as to promote the appearance of MgSC crystals.

[0094] The step of forming MgSCU crystals can be carried out in a container having a volume / surface ratio ranging from 1:1 to 1:10 or from 1:3 to 1:10. The method according to the invention comprises a step (4) of separating the suspension (S3) resulting from step (3) leading to the production of a solution (S4) and a solid (M) comprising the hydrated magnesium sulfate crystals.

[0095] Typically, the solid (M) contains at least 95% by weight of MgSCU.

[0096] Generally, the separation (4) of the suspension (S3) from step (3) into a solution (S4) and a solid (M) is carried out by cold filtration.

[0097] Cold filtration can be carried out at a temperature less than or equal to 0°C, typically less than or equal to -5°C, so as to promote the formation of MgSC crystals.

[0098] The solution (S4) can be recycled to step (1) for example by mixing it with the brine (1) treated in step (1). Thus the recycling of the solution (S4) makes it possible to optimize the production of salts.

[0099] The method may include any or all of the following optional steps.

[0100] The solid (M) from step (4) can be purified by rinsing (5) by contacting with an organic solvent to obtain MgSC crystals and organic solvent containing impurities.

[0101] The organic solvent is typically miscible with water and may be an alcohol such as methanol or ethanol, or acetone. Any other water-miscible organic solvent that can purify MgSC crystals by solubilizing impurities is suitable for carrying out step (5).

[0102] The organic solvent can be recycled (6). Optionally, the organic solvent containing impurities before being recycled is purified, for example by distillation. Conventionally, the organic solvent is recycled (6) to step (5).

[0103] The organic solvent is optionally recycled to be sent to step (5). The MgSCU crystals from step (5) can be dried (7), typically in an oven, for example at a temperature ranging from 30 to 80°C. Typically, drying the MgSCU crystals helps preserve the quality of the crystallized MgSCU.

[0104] The drying step (7) can be carried out at a temperature ranging from 35 to 75°C, typically ranging from 35 to 70°C or even ranging from 40 to 60°C.

[0105] Typically, the MgSCU crystals obtained after drying (7) are in the form of a white solid, typically with a purity greater than 95% by weight, or even greater than 99% by weight relative to the weight of the crystals.

[0106] Mg ions 2+ are analyzed by flame emission spectrophotometry.

[0107] For example, magnesium can be analyzed by flame spectrophotometry and / or SO4 2 ' can be analyzed gravimetrically. Purity can be determined by XRD (X-ray Diffraction) analysis.

[0108] The MgSCU crystals obtained at the end of the drying step (7) are commonly of general formula MgSCU. nFW in which n varies from 1 to 11. In particular, the crystals obtained have an "n" equal to 5, 6 or 7 or a mixture of these values. For example, MgSC can be in the form of epsomite (MgSO4.7H2O), hexahydrate (MgSC .ôFW), pentahydrate (MgSC .5H2O) alone or in combination.

[0109] The value of n corresponding to the hydration rate of MgSC crystals is classically determined by DRX.

[0110] The various embodiments presented throughout the description may be used alone or in combination with each other, without limitation of combination.

[0111] Methods

[0112] An XRD analysis can for example be performed using a Bruker D8 diffractometer with Cu Ko radiation in the range of 15 to 55 20, with a step size of 0.02 and a collection time of 2.5 s / step. Processing can be performed using the Highscore Plus software based on COD19 data.

[0113] The methods for carrying out chemical analyses can, for example, be carried out for:

[0114] - Sodium, potassium and calcium, flame spectrophotometry.

[0115] - Magnesium, complexometric titration. - SO4 sulfate 2 ' a gravimetric method.

[0116] - Chlorine Cl' a potentiometric method.

[0117] EXAMPLES

[0118] The following non-restrictive examples illustrate exemplary embodiments of the invention.

[0119] Example 1:

[0120] In this example, the useful brine is derived from seawater desalination by reverse osmosis. The brine has a density of 1.045 kg / L, a concentration of 1.38 g / L of potassium chloride (KCl), 6.08 g / L of magnesium chloride (MgCh), and 48 g / L of sodium chloride (NaCl).

[0121] In a first step (1), 90% of the water volume of the brine is removed by evaporation so as to crystallize the NaCl. Evaporation is facilitated by external heating using a Bunsen burner.

[0122] The solid NaCl is separated and a solution (S1) is obtained.

[0123] In a second step (2), 99% of the volume of water in relation to the quantity of water initially contained in the brine is eliminated by evaporation of the water contained in the solution (S1). The evaporation is carried out in the open air.

[0124] Evaporation is carried out discontinuously, after contact with solar radiation for a maximum of 12 hours, the solution is cooled overnight for a minimum of 8 hours. The liquid is then filtered. The cycle is repeated several times until 99% by volume of water compared to the amount of water initially contained in the brine is eliminated.

[0125] Table 2 below lists the composition of the solids obtained in each cycle.

[0126] [Table 2] The obtained filtrate has a density of 1.3 kg / l, a final pH of 8.68 and a conductivity of 169.3 mS / cm.

[0127] The filtrate is cooled to (-5°C) for 3 hours to promote the formation of crystals. The mixture is separated, step (4), by filtration into a solution (S4) and a solid in the form of crystals.

[0128] The solid crystals formed are then dried at a temperature of 60°C to obtain a solid in the form of fine, white crystals mainly made up of MgSC having a purity by DRX analysis of approximately 85%. The DRX analysis shows that the solid contains a mixture of several phases MgSC .6H2O, NaCI, KCI and kainite (KCI.MgSO4.2,75H2O).

[0129] Example 2:

[0130] This example illustrates an embodiment shown schematically in Figure 1.

[0131] In this example, a process identical to that of Example 1 is implemented, except that before the drying step, the solid crystals are rinsed with an organic solvent, acetone.

[0132] The drying step is carried out on the rinsed solid, at 60°C.

[0133] XRD analyses confirm that the final solid is pure magnesium sulfate with the chemical formula: MgSC .6H2O. Chemical analyses show that the product obtained by the process is composed of 34.52% SO3, 17.12% MgO and 46.42% H2O. The solid (MgSCU) obtained has a purity of 99% by weight relative to the weight of the solid.

[0134] Example 3:

[0135] In this example, a process identical to that of example 2 is implemented, except that the drying is carried out at a temperature of 40°C and not 60°C.

[0136] XRD analyses show that the product obtained by drying at 40°C is composed of a mixture of MgSCU crystals at different hydration levels; 57% epsomite (MgSCU.7H2O) and 43% hexahydrate (MgSCU.6H2O). Thus, the drying temperature can control the hydration level of MgSCU crystals.

Claims

CLAIMS 1. Process for the production of hydrated magnesium sulfate from brine obtained from the desalination of seawater, the process comprising the following steps: (1) removal of 80 to 94% of the volume of water initially contained in the brine leading to the formation of a suspension comprising NaCI crystals, followed by separation of the suspension into a solution (S1) and a solid residue comprising NaCI crystals; (2) removal of a portion of the water contained in the solution (S1) so as to reach a total volume of water removed in steps (1) and (2) ranging from 95 to 99% of the volume of water initially contained in the brine leading to the formation of a suspension comprising kainite crystals, followed by the separation of the suspension into a solution (S2) and a solid residue comprising kainite crystals and possibly NaCl crystals; (3) cooling the solution (S2) obtained at the end of step (2) to a temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of hydrated magnesium sulfate; and (4) separation of the suspension (S3) resulting from step (3) leading to the production of a solution (S4) and a solid (M) comprising the crystals of hydrated magnesium sulfate.

2. The method of claim 1, further comprising the following steps: (5) rinsing the solid (M) by contacting it with an organic solvent leading to the production of MgSC crystals and the organic solvent containing impurities; (6) optional recycling of the organic solvent to be sent to step (5); (7) optional drying of the solid (M) containing MgSC crystals.

3. A method according to claim 1 or 2, wherein the water removed in steps (1) and / or (2) is recovered, typically as fresh water.

4. A method according to any one of claims 1 to 3, wherein the brine has a pH ranging from 7.5 to 8.5 and / or a salinity ranging from 4.6 to 9.0% by weight.

5. Method according to any one of claims 1 to 4, in which the brine comes from the desalination of seawater by distillation or reverse osmosis.

6. Method according to any one of claims 1 to 5, wherein the solution (S2) is cooled in step (3) to a temperature ranging from -15°C to -3°C, preferably ranging from -9°C to -4°C.

7. A method according to any one of claims 2 to 6, wherein the organic solvent is an alcohol or acetone.

8. Method according to any one of claims 2 to 7, in which the drying step (7) is carried out at a temperature ranging from 30 to 80°C, preferably ranging from 35 to 75°C.

9. Method according to any one of claims 2 to 8, in which the MgSC crystals obtained at the end of the drying (7) are in the form of a white solid with a purity greater than 95% by weight, preferably greater than 99% by weight.

10. Method according to any one of claims 1 to 9, in which the crystallization step (3) of the MgSC salts is carried out in a container having a volume / surface ratio ranging from 1:1 to 1:10 or from 1:3 to 1:

10.

11. A method according to any one of claims 1 to 10, wherein the hydrated magnesium sulfate has the general formula MgSCU.nFW in which n varies from 1 to 11.

12. A method according to any one of claims 1 to 11, wherein the magnesium sulfate hydrate is selected from the group consisting of epsomite (MgSCU.7H2O), magnesium sulfate hexahydrate (MgSCU.6H2O), pentahydrate (MgSCU.5H2O), alone or in combination.

Images