Silica removal agent for reverse osmosis membrane pretreatment

Abstract

A method of reducing silica fouling in an aqueous system is provided. The method includes adding a composition including a trivalent iron salt and a magnesium salt to an aqueous medium to precipitate silica in the aqueous medium and form precipitated silica. The method further includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

Description

Technical Field

[0001] This invention generally relates to the removal of silica from aqueous media. More specifically, this invention relates to a composition comprising ferric and magnesium salts for removing silica from aqueous media. Background Technology

[0002] In many parts of the world, amorphous silica scale causes significant fouling problems when industrial water contains high levels of silica. Typically, high silica levels mean that industrial water contains at least 5 ppm and up to about 500 ppm of dissolved silica, and may contain even higher levels of dissolved, colloidal, or particulate silica.

[0003] In industrial applications such as cooling, boilers, geothermal energy, reverse osmosis, and papermaking, the solubility of silica adversely limits the efficient use of water. Specifically, water treatment operations are restricted because the solubility of silica can exceed approximately 150 ppm when minerals are concentrated during treatment. This excess leads to the precipitation and deposition of amorphous silica and silicates, resulting in a loss of equipment efficiency. Furthermore, the accumulation of silica on the internal surfaces of water treatment equipment (such as boilers, cooling, and purification systems) reduces heat transfer and fluid flow through heat exchange tubes and membranes.

[0004] Once silica scale forms on water treatment equipment, removing it is extremely difficult and expensive. Therefore, when using water with high silica content, cooling and reverse osmosis systems typically operate at low water utilization efficiency to ensure that silica solubility is not exceeded. However, under these conditions, the reverse osmosis system must limit its pure water recovery rate, and the cooling system must limit water recirculation. In both cases, the water discharge volume is very large.

[0005] Various additives have been used over the years to inhibit silica deposition. Current techniques for controlling silica scale in industrial cooling systems involve the use of colloidal silica dispersants or silica polymerization inhibitors. Unlike common scale / deposits such as calcium carbonate and calcium phosphate, silica polymerization can only be slowed down, not completely stopped, if the silica concentration is significantly higher than supersaturation levels. Therefore, there are certain limitations to silica dispersants / scale inhibitors. The maximum permissible dissolved silica level is typically around 200-400 ppm.

[0006] Silica fouling is a major bottleneck in high-recovery reverse osmosis systems. For zero liquid discharge (ZLD) or near-ZLD processes, silica concentrations can reach levels significantly exceeding dispersant / antiscalant limits. Membranes heavily fouled with silica are difficult to clean and almost impossible to fully recover. Therefore, it is typically necessary to reduce the silica concentration in the membrane feed. Summary of the Invention

[0007] A method for reducing silica fouling in an aqueous system is provided. The method includes adding a composition comprising a ferric salt and a magnesium salt to an aqueous medium to precipitate silica in the aqueous medium and form precipitated silica. The method further includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

[0008] In some respects, ferric salts are ferric chloride, ferric chloride hydrate, polyferric chloride, ferric sulfate, polyferric sulfate, or combinations thereof.

[0009] In some respects, magnesium salts are magnesium oxide, magnesium chloride, magnesium chloride hexahydrate, magnesium sulfate, magnesium sulfate monohydrate, magnesium sulfate heptahydrate, magnesium carbonate, or combinations thereof.

[0010] In some respects, the magnesium salt is anhydrous.

[0011] In some respects, the magnesium salt is magnesium chloride and the ferric salt is ferric chloride.

[0012] In some respects, the magnesium salt is magnesium sulfate and the ferric salt is polyferric sulfate.

[0013] In some respects, the composition is prepared by mixing a ferric salt and a magnesium salt before being added to an aqueous medium.

[0014] In some respects, the composition is added to an aqueous medium in an amount ranging from about 10 ppm to about 2,000 ppm.

[0015] In some aspects, the method involves adding a polyacrylamide-based flocculant to an aqueous medium.

[0016] In some cases, polyacrylamide-based flocculants are added in amounts from about 0.05 ppm to about 5 ppm.

[0017] In some respects, the pH of aqueous media is approximately 9.5 to approximately 12.5.

[0018] In some aspects, the method involves adding aluminum compounds to the aqueous medium.

[0019] In some aspects, the method involves feeding the aqueous supernatant into a filtration system, followed by feeding it into a reverse osmosis system.

[0020] In some respects, the conductivity of the aqueous medium is from about 100 μs / cm to about 100,000 μs / cm and the silica concentration is from about 30 mg / L to about 250 mg / L. The aqueous medium contains a scale inhibitor.

[0021] In some respects, the temperature of the aqueous medium is from about 10°C to about 100°C.

[0022] In some respects, the total hardness of aqueous media, calculated as CaCO3, ranges from about 0 mg / L to about 2,000 mg / L.

[0023] In some respects, the method includes stirring the aqueous medium after adding the composition.

[0024] In some respects, the method involves adding a base after adding the composition.

[0025] In some respects, a base is an alkali metal hydroxide.

[0026] In other respects, compositions comprising ferric and magnesium salts are provided for use in removing silica from aqueous media.

[0027] The foregoing has provided a fairly broad overview of the features and technical advantages of this disclosure in order to better understand the following detailed description. Additional features and advantages of this disclosure, which form the subject matter of the claims of this application, will be described below. Those skilled in the art will understand that the disclosed concepts and specific embodiments can be readily used as the basis for modifications or the design of other embodiments for achieving the same purpose of this disclosure. Those skilled in the art will also recognize that such equivalent embodiments do not depart from the spirit and scope of this disclosure as set forth in the appended claims. Detailed Implementation

[0028] The relationships and functions of the various elements of the embodiments can be better understood by referring to the following detailed description.

[0029] A method for reducing silica fouling in an aqueous system is provided. The method includes adding a composition comprising a ferric salt and a magnesium salt to an aqueous medium to precipitate silica in the aqueous medium and form precipitated silica. The method further includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

[0030] Compositions added to aqueous media include ferric salts. Examples of ferric salts include, but are not limited to, ferric chloride (FeCl3), ferric chloride hydrate, polyferric chloride, ferric sulfate, or polyferric sulfate. In some aspects, the ferric salt is ferric chloride. In some aspects, the ferric salt is ferric chloride hydrate. In some aspects, the ferric salt is ferric sulfate. In some aspects, the ferric salt is polyferric chloride. In some aspects, the ferric salt is polyferric sulfate.

[0031] Ferric salts can be anhydrous or hydrated. Examples of ferric salt hydrates include, but are not limited to, hexahydrate, pentahydrate, and dihydrate. For example, ferric chloride can be hexahydrate, pentahydrate, dihydrate, or other hydrates.

[0032] Compositions added to aqueous media include magnesium salts. Examples of magnesium salts include, but are not limited to, magnesium oxide, magnesium chloride, magnesium chloride hexahydrate, magnesium sulfate, magnesium sulfate monohydrate, magnesium sulfate heptahydrate, or magnesium carbonate. In some aspects, the magnesium salt is magnesium oxide. In some aspects, the magnesium salt is magnesium chloride. In some aspects, the magnesium salt is magnesium chloride hexahydrate. In some aspects, the magnesium salt is magnesium sulfate. In some aspects, the magnesium salt is magnesium sulfate monohydrate. In some aspects, the magnesium salt is magnesium sulfate heptahydrate. In some aspects, the magnesium salt is magnesium carbonate.

[0033] Magnesium salts can be anhydrous or hydrated. In some respects, for example, magnesium salts are anhydrous magnesium salts, such as anhydrous magnesium chloride.

[0034] In some respects, the composition is prepared by mixing a ferric salt and a magnesium salt before being added to an aqueous medium. Mixing may include combining powdered magnesium salt with powdered ferric salt. Premixing the ferric salt and magnesium salt together synergistically improves the removal of silica and hardness.

[0035] In some respects, the weight ratio of magnesium salt to ferric salt is from about 0.1:1 to about 10:1. In other respects, the weight ratio of magnesium salt to ferric salt is about 4:1, about 2:1, about 3:1, about 5:1 or about 6:1.

[0036] Unbound by any particular theory, two mechanisms for removing silica via magnesium salts are believed to exist. Most magnesium precipitates as magnesium hydroxide, thereby absorbing silica in solution. Some magnesium can co-precipitate with silica, directly forming magnesium silicate. Compared to magnesium hydroxide absorbing silica, the co-precipitation mechanism allows magnesium salts to remove silica more effectively. By premixing magnesium salts with acidic ferric salts and feeding them together, it is believed that the ferric salts protect magnesium from rapid precipitation into magnesium hydroxide, thus improving the overall silica removal efficiency.

[0037] The composition can be added to an aqueous medium in an effective amount sufficient to precipitate silica from solution. In some aspects, the composition is added to the aqueous medium in an amount ranging from about 10 ppm to about 2,000 ppm.

[0038] In some aspects, the amount of the composition added to the aqueous medium is from about 10 ppm to about 1,000 ppm, from about 50 ppm to about 500 ppm, or from about 100 ppm to about 400 ppm. In some aspects, the amount of the composition added is from about 100 ppm, from about 200 ppm, from about 300 ppm, or from about 400 ppm.

[0039] The method may further include adding a polyacrylamide-based flocculant to an aqueous medium. In some aspects, the polyacrylamide-based flocculant is anionic polyacrylamide. The polyacrylamide-based flocculant may be added alone. The flocculant may be added after the composition has been added to the aqueous medium. The polyacrylamide-based flocculant may be added in an amount from about 0.05 ppm to about 5 ppm.

[0040] In some aspects, the method optionally includes adding an aluminum compound to an aqueous medium. The aluminum compound may be polyaluminum chloride or other aluminum salts.

[0041] The method may further include adding an alkali to an aqueous medium after adding the composition to adjust the pH to an optimal level. In some aspects, the alkali is an alkali metal hydroxide. Examples of alkali metal hydroxides include sodium hydroxide and lime. The method may further include adding a softener to reduce the hardness level. Typical softeners are, for example, sodium carbonate (soda ash). Those skilled in the art can select a suitable alkali and a suitable softener according to the needs of the system.

[0042] After the composition is added to the aqueous medium, the aqueous medium can be stirred or mixed to disperse the composition throughout the medium. The means of stirring the medium are not limited to any particular structure or technology. Those skilled in the art can select a suitable mixing method according to the needs of the system.

[0043] There are no particular limitations on the treatable aqueous media, but compositions containing ferric and magnesium salts have been found to synergistically remove silica from wastewater. To recover the wastewater, additional purification measures, such as filtration and ion exchange, are applied. Filtration methods include multi-media filtration, microfiltration, and ultrafiltration. Applying ion exchange further reduces hardness levels. In some aspects, the method involves feeding the aqueous supernatant into a filtration system, followed by an ion exchange system, and then into a reverse osmosis system after the addition of the composition.

[0044] For high-recovery reverse osmosis systems, silica can be removed as a precipitate in the softening process, which is primarily used to reduce the hardness of raw water. During softening, water is typically treated with a combination of lime, caustic soda, and soda ash. Calcium precipitates as calcium carbonate, and magnesium precipitates as magnesium hydroxide. Silica can precipitate with magnesium to form magnesium silicate, and to a lesser extent with calcium to form calcium silicate. Other agents, such as aluminum- and ferric coagulants and polyacrylamide-based flocculants, are also added to improve the settling of the precipitate.

[0045] The magnesium concentration in influent water is typically far below the amount required for satisfactory silica removal. Therefore, it is common practice to add an additional magnesium source to the influent water to improve silica removal. Magnesium oxide, magnesium chloride, magnesium sulfate, magnesium carbonate, and dolomite lime are commercially available as magnesium sources, with magnesium oxide being the most commonly used.

[0046] The compositions disclosed herein are particularly effective for removing silica from aqueous media with a pH of about 9.5 to about 12.5. In some aspects, the pH of the aqueous media may be about 10.0 to about 11.5.

[0047] In some aspects, the conductivity of the aqueous medium is about 100 μs / cm to about 100,000 μs / cm. In other aspects, the conductivity of the aqueous medium is about 100 μs / cm to about 300,000 μs / cm, about 100 μs / cm to about 10,000 μs / cm, or about 5,000 μs / cm to about 10,000 μs / cm.

[0048] The hardness of an aqueous medium can be measured in mg / L as CaCO3. In some cases, the total hardness of the aqueous medium as CaCO3 is from about 0 mg / L to about 2,000 mg / L. In some cases, the hardness of the aqueous medium can be from about 50 mg / L to about 1,500 mg / L as CaCO3. In some cases, the hardness of the aqueous medium can be from about 10 mg / L to about 600 mg / L as CaCO3. In some cases, the hardness of the aqueous medium can be from about 200 mg / L to about 500 mg / L as CaCO3.

[0049] Prior to treatment with the compositions disclosed herein, the silica concentration in the aqueous medium may be from about 30 mg / L to about 250 mg / L. In some aspects, the silica concentration in the aqueous medium may be from about 50 mg / L to about 250 mg / L. In some aspects, the silica concentration in the aqueous medium may be from about 60 mg / L to about 250 mg / L.

[0050] Silica exists in the form of dissolved silicon, silicates, or their complexed ions, and can also exist as colloidal or suspended silica. The higher the total concentration of silica from all sources in these waters, the more difficult it is to resolve the problems caused by the formation of amorphous silica scale.

[0051] In some cases, the temperature of the aqueous medium is from about 10°C to about 50°C. In other cases, the temperature of the aqueous medium can be from about 20°C to about 40°C.

[0052] In the water pretreatment of high-recovery reverse osmosis systems, the wastewater influent typically contains scale inhibitors. Scale inhibitors can be added to upstream cooling water treatment and / or reverse osmosis processes to prevent scaling and deposits in these processes. Disadvantageously, scale inhibitors can interfere with downstream softening and silica removal processes, leading to higher operating pH and higher chemical dosages.

[0053] Unbound by any particular theory, by premixing and co-concentrating magnesium and ferric salts, the newly precipitated ferric hydroxide effectively binds to scale inhibitors and removes their negative effects. As a result, the removal efficiency of hardness and silica increases.

[0054] Although it is not necessary to implement this invention, it is envisioned that the compositions disclosed herein may be combined with one or more corrosion inhibitors, one or more other scale inhibitors, one or more fluorescent tracers, or one or more water treatment polymers.

[0055] It should be understood that, in some embodiments, the method may be combined with other practical tools known in industry. Representative practical tools include sensors for measuring the content of various additives in a system; sensors for dissolved or particulate contaminants; other sensors based on resistance, capacitance, spectral absorbance or transmittance, colorimetry, and fluorescence; and mathematical tools for analyzing sensor / controller results (e.g., multivariate analysis, chemometrics, on / off dosing control, PID dosing control, and combinations thereof).

[0056] In another embodiment, the co-blended compound includes an inert fluorescent tracer to provide a means of determining dosage levels. A known proportion of the fluorescent tracer is added to the blend simultaneously or sequentially. Effective inert fluorescent tracers include substances that do not chemically react with other components in the system and do not significantly degrade over time. Such tracers should also be completely (or substantially completely) soluble in the blend at all relevant concentration levels, and preferably, their fluorescence intensity should be substantially proportional to their concentration and not significantly quenched or otherwise weakened by other components in the system. Furthermore, the inert fluorescent tracer should not be noticeably or significantly affected by any other chemical substance in the system. The expression "not noticeably or significantly affected" means that the fluorescence signal of the inert fluorescent compound typically changes by no more than about 10% under conditions commonly encountered with fuel ethanol.

[0057] The desired characteristics of an inert fluorescent tracer preferably include: fluorescence excitation / emission wavelengths that do not significantly overlap with light-absorbing substances present in the system, such as water, other additives, and contaminants; high solubility; excellent chemical stability; suitable fluorescence properties at manageable wavelengths (e.g., other components in the system should not interfere with fluorescence properties at those wavelengths); and excitation / emission wavelengths that are separate from other fluorescent components that may be present in the system to prevent interference; and avoid negative impacts on the properties of the system.

[0058] Representative inert fluorescent tracers include fluorescein or fluorescein derivatives; rhodamine or rhodamine derivatives; naphthalenesulfonic acid (mono-, bis-, tri-, etc.); pyrenesulfonic acid (mono-, bis-, tri-, tetra-, etc.); stilbene derivatives containing sulfonic acids (including optical brighteners); biphenylsulfonic acid; phenylalanine; tryptophan; tyrosine; vitamin B2 (riboflavin); vitamin B6 (pyridoxine); vitamin E (α-tocopherol); ethoxyquin; caffeine; vanilla extract; naphthalenesulfonic acid formaldehyde condensate polymer; benzenesulfonic acid formaldehyde condensate; lignin sulfonic acid; polycyclic aromatic hydrocarbons; aromatic (poly)cyclic hydrocarbons containing amine, phenol, sulfonic acid, or carboxylic acid functional groups in any combination; (poly)heterocyclic aromatic hydrocarbons having N, O, or S; polymers containing at least one of the following moieties: naphthalenesulfonic acid, pyrenesulfonic acid, biphenylsulfonic acid, or stilbenesulfonic acid.

[0059] Representative scale inhibitors include, but are not limited to, inorganic and organic polyphosphates, phosphonates, and polycarboxylates. These inhibitors help to inhibit or disperse other types of scale, such as calcium carbonate, calcium sulfate, calcium phosphate, calcium fluoride, barium sulfate, and calcium oxalate.

[0060] Example

[0061] Example 1: Anhydrous magnesium chloride and ferric chloride were mixed together in a weight ratio of approximately 4:1 to form a fine powder. This powder was added to a wastewater sample from a coal chemical plant. The wastewater sample had the following characteristics: pH approximately 9.21, conductivity approximately 212,000 μs / cm, SiO2 concentration approximately 90.5 mg / L, total hardness as CaCO3 approximately 450 mg / L, calcium hardness as CaCO3 approximately 290 mg / L, and total alkalinity as CaCO3 approximately 710 mg / L. The wastewater contained an unknown amount of scale inhibitor from upstream reverse osmosis and cooling water treatment. The temperature was set at approximately 21°C. The pH of the wastewater solution was pre-adjusted using a fixed amount of NaOH, and then the premixed salt was added either via a 10 wt% solution of a total powder dosage of approximately 200 ppm, or individually via a 10 wt% solution of each salt at approximately 1-minute intervals. After all chemicals were added, the solution was allowed to stir for another 10 minutes. Then, anionic polyacrylamide solution was added at a dose of approximately 0.5 ppm, the solution was stirred for another minute, and then allowed to settle for approximately 60 minutes. The final pH of the solution was approximately 11.0, and the supernatant sample was taken for analysis. The analytical results are shown in Table 1.

[0062] Table 1

[0063] unit premix First Mg, then Fe First Fe, then Mg <![CDATA[SiO2]]> mg / L 59.1 64.1 66.3 Total Hardness <![CDATA[mg / L CaCO3]]> 119 131 142 Ca hardness <![CDATA[mg / L CaCO3]]> 73 65 63 Turbidity NTU 4.3 5.7 9.5 Total Railway mg / L 0.05 0.07 0.12

[0064] Example 2: Anhydrous magnesium sulfate and polysulfate were mixed together in a weight ratio of approximately 1:1 to form a fine powder. This powder was added to a wastewater sample from a coal chemical plant. The wastewater sample had the following characteristics: pH approximately 9.64, conductivity approximately 8440 μs / cm, SiO2 concentration approximately 64.3 mg / L, total hardness (calculated as CaCO3) approximately 247 mg / L, calcium hardness (calculated as CaCO3) approximately 217 mg / L, and total alkalinity (calculated as CaCO3) approximately 730 mg / L. The wastewater primarily originated from cooling water sludge and contained an unknown amount of scale inhibitor. The temperature was set at approximately 27°C. The premixed salt was added via a 10 wt% solution of approximately 200 ppm of total powder, or individually via a 10 wt% solution of each salt at approximately 1-minute intervals. After salt addition, a 30 wt% NaOH solution was added dropwise to allow the pH of the solution to increase to approximately 11.3, with pH adjustment taking approximately 0.5 minutes. The solution was then allowed to stir for another 10 minutes, and anionic polyacrylamide solution was added at a dose of approximately 0.5 ppm. The solution was stirred for another minute and then allowed to settle for approximately 60 minutes. The final pH of the solution was approximately 10.90, and the supernatant sample was collected for analysis. The analytical results are shown in Table 2.

[0065] Table 2

[0066] unit premix First Mg, then Fe First Fe, then Mg <![CDATA[SiO2]]> mg / L 39.1 41.1 45.5 Total Hardness <![CDATA[mg / L CaCO3]]> 72 71 114 Ca hardness <![CDATA[mg / L CaCO3]]> 57 55 52 Turbidity NTU 1.6 1.8 2.4 Total Railway mg / L 0.03 0.03 0.04

[0067] All compositions and methods disclosed or claimed herein can be made and performed without excessive experimentation based on this disclosure. While the invention may be embodied in many different forms, specific preferred embodiments are described in detail herein. This disclosure is exemplary of the principles of the invention and is not intended to limit the invention to the specific embodiments illustrated. Furthermore, unless expressly stated to the contrary, the term "a" is intended to include "at least one" or "one or more". For example, "ferric salt" is intended to include "at least one ferric salt" or "one or more ferric salts".

[0068] Any range given in absolute or approximate terms is intended to encompass both, and any definitions used herein are intended to be clarifying and not restrictive. While the numerical ranges and parameters illustrating the broad scope of the invention are approximate, the numerical values ​​described in specific examples are reported as accurately as possible. However, any numerical value inherently contains some error necessarily caused by the standard deviation found in its corresponding test measurement. Furthermore, all ranges disclosed herein should be understood to encompass any and all subranges contained therein (including all fractional and overall values).

[0069] Any composition disclosed herein may comprise, consist of, or consist substantially of any element, component, and / or ingredient disclosed herein, or any combination of two or more of the elements, components, or ingredients disclosed herein.

[0070] Any method disclosed herein may comprise, consist of, or consist substantially of: any method steps disclosed herein or any combination of two or more of the method steps disclosed herein.

[0071] The transitional phrase “includes”, “contains”, or “characterized in” is inclusive or open-ended and does not exclude additional unreferenced elements, components, ingredients, and / or method steps.

[0072] The transitional phrase "composed of" excludes any element, component, ingredient, and / or method step not specified in the claims.

[0073] The transitional phrase “consistent essentially of…” limits the scope of the claim to the specified elements, components, ingredients and / or steps, as well as those elements, components, ingredients and / or steps that do not substantially affect the essential and novel features of the claimed invention.

[0074] Unless otherwise specified, all molecular weights referred to herein are weight-average molecular weights, and all viscosities are measured at 25°C with pure (undiluted) polymers.

[0075] As used herein, the term “about” refers to a value being within the error caused by the standard deviation found in their respective test measurements, and if those errors are not definitively determined, then “about” may refer to, for example, less than 5% of the value being referenced.

[0076] Furthermore, this invention covers any and all possible combinations of some or all of the various embodiments described herein. It should also be understood that various changes and modifications to the preferred embodiments of the invention described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. Therefore, such changes and modifications are contemplated to be covered by the appended claims.

Claims

1. A method for reducing silica fouling in an aqueous system, comprising: A ferric salt and a magnesium salt are mixed to form a composition, and then the composition is added to an aqueous medium, wherein the weight ratio of the magnesium salt to the ferric salt is about 2:1 to about 6:

1. The composition is added to the aqueous medium to precipitate silica in the aqueous medium and form precipitated silica; and At least a portion of the precipitated silica is removed from the aqueous medium to form an aqueous supernatant; The ferric salt mentioned above is ferric chloride, ferric chloride hydrate, polyferric chloride, ferric sulfate, polyferric sulfate, or a combination thereof; and The magnesium salt thereon is magnesium chloride, magnesium chloride hexahydrate, magnesium sulfate, magnesium sulfate monohydrate, magnesium sulfate heptahydrate, magnesium carbonate, or a combination thereof.

2. The method according to claim 1, wherein the magnesium salt is an anhydrous magnesium salt.

3. The method according to claim 1, wherein the magnesium salt is magnesium chloride and the ferric salt is ferric chloride.

4. The method according to claim 1, wherein the magnesium salt is magnesium sulfate and the ferric salt is polyferric sulfate.

5. The method according to any one of claims 1 to 4, wherein the composition is added to the aqueous medium in an amount ranging from about 10 ppm to about 2,000 ppm.

6. The method according to any one of claims 1 to 4, further comprising adding a polyacrylamide-based flocculant to the aqueous medium.

7. The method of claim 6, wherein the polyacrylamide-based flocculant is added in an amount of about 0.05 ppm to about 5 ppm.

8. The method according to any one of claims 1 to 4, wherein the pH of the aqueous medium is about 9.5 to about 12.

5.

9. The method according to any one of claims 1 to 4, further comprising adding an aluminum compound to the aqueous medium.

10. The method according to any one of claims 1 to 4, further comprising feeding the aqueous supernatant into a filtration system and then into a reverse osmosis system.

11. The method according to any one of claims 1 to 4, wherein the conductivity of the aqueous medium is from about 100 μs / cm to about 100,000 μs / cm and the silica concentration is from about 30 mg / L to about 250 mg / L, and the aqueous medium further comprises an antiscalant.

12. The method according to any one of claims 1 to 4, wherein the temperature of the aqueous medium is from about 10°C to about 50°C.

13. The method according to any one of claims 1 to 4, wherein the total hardness of the aqueous medium, calculated as CaCO3, is from about 0 mg / L to about 2,000 mg / L.

14. The method according to any one of claims 1 to 4, further comprising stirring the aqueous medium after adding the composition.

15. The method according to any one of claims 1 to 4, further comprising adding an alkali after adding the composition.

16. The method according to claim 15, wherein the base is an alkali metal hydroxide.

17. Use of a composition comprising a ferric salt and a magnesium salt for removing silica from an aqueous medium, said composition being a composition as defined in any one of claims 1 to 16.