Process for recovering brine from seawater desalination.

Abstract

The present invention relates to a process for preparing magnesium sulfate by treating a brine typically obtained from the desalination of seawater.

Description

Title of the invention: Process for recovering brine from the desalination of seawater. TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to the preparation of magnesium sulfate by treatment of a brine typically obtained from the desalination of seawater. The process can furthermore make it possible to selectively recover NaCl and / or kainite. STATE OF THE ART

[0002] Magnesium sulfate is used in particular in the production of animal feed, pulp or paper. In the pharmaceutical field it is also used as a chemical agent or additive.

[0003] The most widely used process for the manufacture of 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 about 90%.

[0004] Another method involves mixing epsomite (MgSO4.7H2O) and halite (NaCl) at 83°C until clusters of less hydrated magnesium sulfate crystals form. High pressure is then applied while stirring while hot so that the clusters of crystals collapse to give fine, less hydrated crystals. These can then be separated from the relatively coarse halite crystals.

[0005] Another process proposes the manufacture of magnesium sulfates from seawater and brines. This process consists of manufacturing 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 Ca2+ ions 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.

[0006] US application 3,536,444A proposes a process for the recovery of hydrated magnesium sulfate from brines formed after the Camallite precipitation step (KMgCl3.6H2O). The process is based on the mixing of two brines of different densities 1.36 (after Camallite crystallization) and 1.32 (before Camallite 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 Camallite precipitation. However, the yield of this process is generally very low by about 12%.

[0007] Seawater desalination is increasingly used worldwide to provide freshwater for agriculture and industry. The major ionic species present in seawater are Cl, Na+, Mg2+, SO42, K+ and Ca2+. They represent more than 99.9% of the total mass of dissolved substances in seawater. Their total mass can vary from one seawater to another but generally their relative proportions remain constant.

[0008] Reverse osmosis (RO) is currently the desalination technology of choice. In 2020, reverse osmosis dominated the market and accounted for more than 55% of the global revenue share. This is attributed to its advantageous properties, such as ease of operation, low installation cost, the ability to treat different types of feed water, and minimal chemical use. The large quantities of brines produced by desalination plants are the major drawback of this technology. These brines generally represent 50% of the feed stream. The brines have a salinity 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.

[0009] Thus, there is a need for new methods for recovering 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 method will enable the recovery of salts from brines from reverse osmosis desalination plants. Summary of the invention

[0010] The present invention relates to a process for producing magnesium sulfate, in hydrate form, from brine obtained from the desalination of seawater, the process comprising the following steps:

[0011] (1) elimination 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 the separation of the suspension into a solution (SI) and a solid residue comprising the NaCl crystals;

[0012] (2) elimination of a part of the water contained in the solution (SI) so as to reaching 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 in a solution (S2) and a solid residue comprising kainite crystals and possibly NaCl crystals;

[0013] (3) cooling the solution (S2) obtained at the end of step (2) to a temperature temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of hydrated magnesium sulfate; and

[0014] (4) separation of the suspension (S3) resulting from step (3), so as to obtain a solution (S4) and a solid M comprising the crystals of hydrated magnesium sulfate.

[0015] Other aspects of the invention are as described below. FIGURES

[0016] [Fig. 1]: is a diagram of the brine treatment process according to the invention for the production of NaCl, kainite and MgSO4.

[0017] The figures illustrate in a non-limiting manner devices capable of implementing the treatment method using resin according to the invention. DETAILED DESCRIPTION OF THE INVENTION

[0018] 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.

[0019] 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:

[0020] (1) elimination 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 the separation of the suspension into a solution (SI) and a solid residue comprising NaCl crystals;

[0021] (2) elimination of a part of the water contained in the solution (SI) so as to reaching 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;

[0022] (3) cooling the solution (S2) obtained at the end of step (2) to a temperature temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of hydrated magnesium sulfate; and

[0023] (4) separation of the suspension (S3) resulting from step (3) leading to obtaining of a solution (S4) and a solid (M) comprising the crystals of hydrated magnesium sulfate.

[0024] 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 the brines resulting from the desalination of seawater while limiting their environmental impact.

[0025] 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.

[0026] 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.

[0027] Magnesium sulfate hydrate advantageously corresponds to the general formula MgSO4.nH2O in which n varies from 1 to 11. Magnesium sulfate hydrate is thus generally epsomite (MgSO4.7H2O), magnesium sulfate hexahydrate (MgSO4.6H2O), pentahydrate (MgSO4.5H2O), alone or in combination. Brine:

[0028] The brine useful in the process according to the invention comes from the desalination of sea water.

[0029] The terms "sea water" designate the water of an ocean or the water of an inland sea or similar.

[0030] The brine mainly contains the following ionic species Cl, Na+, Mg2+, SO42, CO32 K+ and Ca2+.

[0031] The brine useful in the process of the invention is conventionally a brine resulting from the desalination of seawater by distillation or reverse osmosis.

[0032] The water of the Atlantic Ocean 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 or similar have a higher salinity, because evaporation concentrates the salt there.

[0033] A brine resulting from the desalination of seawater generally has a salinity ranging from 1.1 to 2 times, typically from 1.2 to 2 times or from 1.3 to 1.7 times the salinity of the seawater from which it originates.

[0034] Commonly, brine from desalination, typically by reverse osmosis, has an average salinity ranging from 4.6 to 9.0% by weight or from 1 to 4.5% by weight.

[0035] 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.

[0036] Table 1 describes an example of the chemical composition of the major elements of a brine resulting from desalination compared to a composition of the seawater from which it is derived.

[0037] [Tables 1] Seawater (g / L) Brine (g / L) Na+ 10.56 16.87 K+ 0.38 0.88 Mg2+ 1.30 1.93 Ca2+ 0.40 0.67 Cl 19.36 40.02 SO42 2.70 6.56

[0038] Table 1: Chemical composition of brine compared to that of seawater

[0039] Seawater has a pH of 7.65, brine from desalination of seawater has generally a pH of 8.16. This basicity is associated with the salts contained in seawater.

[0040] The brine useful in the process according to the invention can have a pH ranging from 7.5 to 8.5 and typically 8.2.

[0041] The brine useful in the process according to the invention may have a density greater than 1.05 kg / L.

[0042] Brackish waters having characteristics similar to a brine described above can also be treated in the process according to the invention.

[0043] The brine useful in the process according to the invention may be filtered beforehand when solid impurities are present. Step (1): formation of NaCl

[0044] 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 NaCl crystals, followed by the separation of the suspension into a solution (SI) and a solid residue comprising NaCl crystals.

[0045] 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.

[0046] 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. The removal of water can also be carried out by any other methods known to those skilled in the art.

[0047] Water removal can be carried out by evaporation. Generally, evaporation is 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. Typically, water removal is carried out in the open air.

[0048] Generally, the removal of water contained in 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.

[0049] Advantageously, during step (1), typically during the removal of water, the brine has a controlled salt concentration so as to be in a range from 300 to 380 g / L, i.e. a salinity ranging from 30 to 38% by weight, which makes it possible to selectively promote the crystallization of NaCl, typically with respect to the magnesium salts, MgSO4 and MgCl2. The salt concentration of the solution (SI) is commonly determined by the density of the solution (SI).

[0050] 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 even from 95 to 99% by weight of NaCl.

[0051] The separation can be carried out by any method allowing the separation of a solid from a liquid. Typically, the separation is carried out by filtration, decantation, centrifugation and / or scraping.

[0052] The solid residue containing NaCl can also be separated from the brine as the water is removed and the NaCl crystallizes. The solution (SI) therefore corresponds to the brine in which at least 90% by weight of NaCl, or even at least 93% by weight of NaCl or even at least 95% by weight of NaCl has been removed relative to the quantity of NaCl contained in the brine to be treated.

[0053] The solution (SI) 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.

[0054] The solid residue can be used in washing and / or centrifugation steps in order to recover purified NaCl salts which can then be used, for example, in human or animal food, the manufacture of soda (NaOH), chlorine Cl2) or even bleach (NaCIO).

[0055] The solution (SI) 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.

[0056] The solution (SI) is then engaged in a kainite formation step. Step (2): kainite formation

[0057] The method according to the invention comprises a step (2) of removing a portion of the water contained in the solution (SI) 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.

[0058] 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 relative to the quantity of water initially contained in the brine. Typically, the removal of water is carried out progressively, 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.

[0059] Commonly, the removal of water by evaporation is carried out at a temperature less than or equal to 50°C, typically ranging from 20 to 50°C or from 25 to 45°C. The evaporation can be carried out under solar radiation, typically for 8 hours.

[0060] A cooling step can also be carried out to promote the crystallization of the kainite salts, typically at a temperature ranging from 5 to 25°C, or even from 10 to 20°C, or even from 10 to 15°C.

[0061] The crystallized kainite salts can be recovered by any methods known to those skilled in the art, for example by scraping.

[0062] The separation can be carried out by any method allowing the separation of a solid from a liquid. Typically, the separation is carried out by filtration, decantation, centrifugation or scraping.

[0063] 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.

[0064] Kainite is a mixed salt of potassium and magnesium having the formula KClMgSO4.3H2O.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] The water removed in steps (1) and / or (2) is typically recovered as fresh water. Step 3)

[0069] The method according to the invention comprises a step (3) of cooling the solution (S2) obtained at the end of step (2) at a temperature ranging from -30°C to 0°C leading to a suspension (S3) comprising crystals of magnesium sulfate hydrate, typically of formula MgSO4, n H2O with n varying from 1 to 11.

[0070] 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 relative to the volume of water initially contained in the brine.

[0071] 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.

[0072] By water evaporation rate, we mean the volume of water which has been eliminated in relation to the volume of water initially contained in the brine to be treated in the process according to the invention.

[0073] 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 MgSO4 salts.

[0074] 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 1h to 10h (hour), typically ranging from 2h to 6h.

[0075] Typically, the formation of MgSO4 crystals is carried out without stirring the solution (S2) so as to promote the appearance of MgSO4 crystals.

[0076] The step of forming MgSO4 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. Step 4)

[0077] 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.

[0078] Typically, the solid (M) contains at least 95% by weight of MgSO4.

[0079] Generally, the separation (4) of the suspension (S3) resulting from step (3) into a solution (S4) and a solid (M) is carried out by cold filtration.

[0080] 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 MgSO4 crystals.

[0081] 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.

[0082] The method may comprise one or the other, or even all of the following optional steps. Step (5)

[0083] The solid (M) from step (4) can be purified by rinsing (5) by contacting with an organic solvent to obtain MgSO4 crystals and organic solvent containing impurities.

[0084] 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 MgSO4 crystals by solubilizing the impurities is suitable for carrying out step (5). Step (6)

[0085] 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).

[0086] The organic solvent is optionally recycled to be sent to step (5). Step (7)

[0087] The MgSO4 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 MgSO4 crystals makes it possible to preserve the quality of the crystallized MgSO4.

[0088] 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.

[0089] Typically, the MgSO4 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.

[0090] Mg2+ ions are analyzed by flame emission spectrophotometry.

[0091] For example, magnesium can be analyzed by flame spectrophotometry and / or SO42 can be analyzed gravimetrically. Purity can be determined by XRD (X-ray Diffractometric) analysis.

[0092] The MgSO4 crystals obtained at the end of the drying step (7) are commonly of general formula MgSO4.nH2O 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, the MgSO4 may be in the form of epsomite (MgSO4.7H2O), hexahydrate (MgSO4.6H2O), pentahydrate (MgSO4.5H2O) alone or in combination.

[0093] The value of n corresponding to the hydration rate of MgSO4 crystals is conventionally determined by DRX.

[0094] The various embodiments presented throughout the description may be used alone or in combination with each other, without limitation of combination. Methods

[0095] An XRD analysis can for example be carried out using a Bruker D8 diffractometer with Cu Ka 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. The processing can be carried out using the Highscore Plus software on the basis of the COD19 data.

[0096] The methods for carrying out chemical analyses can, for example, be carried out for:

[0097] - Sodium, potassium and calcium, flame spectrophotometry.

[0098] - Magnesium, complexometric titration.

[0099] - Sulfate SO42 a gravimetric method.

[0100] - Chlorine Cl a potentiometric method. EXAMPLES

[0101] The following non-restrictive examples illustrate examples of embodiments of the invention.

[0102] Example 1:

[0103] In this example, the useful brine comes from the desalination of seawater by reverse osmosis. The brine has a density of 1.045 kg / L, a concentration of 1.38 g / L of potassium chloride (KC1), 6.08 g / L of magnesium chloride (MgCl2) and 48 g / L of sodium chloride (NaCl).

[0104] In a first step (1), 90% of the volume of water in the brine is removed by evaporation so as to crystallize the NaCl. Evaporation is facilitated by external heating using a Bunsen burner.

[0105] The solid NaCl is separated and a solution (SI) is obtained.

[0106] In a second step (2), 99% volume of water relative to the initial quantity of water initially contained in the brine is eliminated by evaporation of the water contained in the solution (SI). Evaporation is carried out in the open air.

[0107] 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 relative to the quantity of water initially contained in the brine is eliminated.

[0108] Table 2 below lists the composition of the solids obtained in each cycle.

[0109] [Tables2] Cycle water removed in % volume Density Crystallized solids 1 90 1.26 NaCl 2 93 1.29 NaCl 3 96 1.35 MgSO4.6(H2O); KCLMg(SO4).2.75H2O 4 99 1.266 NaCl

[0110] The filtrate obtained has a density of 1.3 kg / L, a final pH of 8.68 and a conductivity of 169.3 mS / cm.

[0111] 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.

[0112] The solid crystals formed are then dried at a temperature of 60°C so as to obtain a solid in the form of fine, white crystals mainly made up of MgSO4 having a purity by DRX analysis of approximately 85%. The DRX analysis shows that the solid contains a mixture of several phases MgSO4.6H2O, NaCl, KC1 and kainite (KCl.MgSO4.2.75H2O). Example 2:

[0113] This example illustrates an embodiment shown schematically in [Fig. 1].

[0114] In this example, a method identical to that of example 1 is implemented, except the exception that before the drying step, the solid crystals are rinsed with an organic solvent, acetone.

[0115] The drying step is carried out on the rinsed solid, at 60°C.

[0116] DRX analyses confirm that the final solid is magnesium sulfate from the chemical formula: pure MgSO4.6H2O. 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 (MgSO4) obtained has a purity of 99% by weight relative to the weight of the solid. Example 3:

[0117] 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.

[0118] DRX analyses show that the product obtained by drying at 40°C is composed of a mixture of MgSO4 crystals at different hydration rates; 57% epsomite (MgSO4.7H2O) and 43% hexahydrate (MgSO4.6H2O). Thus, the drying temperature can help control the degree of hydration of MgSO4 crystals.

Claims

Claims

1. A process for producing magnesium sulfate hydrate from a brine obtained from the desalination of seawater, the process comprising the following steps: (1) removing 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 the separation of the suspension into a solution (SI) and a solid residue comprising NaCl crystals; (2) removing a portion of the water contained in the solution (SI) so as to achieve 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 optionally 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) separating the suspension (S3) resulting from step (3) leading to obtaining a solution (S4) and a solid (M) comprising the crystals of hydrated magnesium sulfate.;

2. A method according to claim 1, further comprising the following steps: (5) rinsing the solid (M) by contacting with an organic solvent resulting in the production of MgSO4 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 MgSO4 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. A method according to any one of claims 1 to 4, wherein the brine is derived from the desalination of seawater by distillation or reverse osmosis.

6. A 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. A method according to any one of claims 2 to 7, wherein the drying step (7) is carried out at a temperature ranging from 30 to 80°C, preferably ranging from 35 to 75°C.

9. A method according to any one of claims 2 to 8, wherein the MgSO4 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. A method according to any one of claims 1 to 9, wherein the crystallization step (3) of the MgSO4 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 MgSO4.nH2 O 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 (MgSO4.7H2O), magnesium sulfate hexahydrate (MgSO4.6H2O), pentahydrate (MgSO4.5H2O), alone or in combination.

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