Silica removal reagent for reverse osmosis membrane pretreatment

Abstract

A method for reducing silica contamination in aqueous systems is provided. The method includes adding a composition including a ferric salt and a magnesium salt to an aqueous medium to precipitate silica in the aqueous medium to form precipitated silica. The method also includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

Description

Technical Field

[0001] The present invention generally relates to removing silica from an aqueous medium. More specifically, the present invention relates to a composition comprising a ferric salt and a magnesium salt for removing silica in an aqueous medium.

Background Art

[0002] (Description of Related Art) Worldwide, when industrial water contains large amounts of silica, amorphous silica scale causes significant fouling problems. In most cases, large amounts of silica mean that industrial water contains at least 5 ppm to up to about 500 ppm of dissolved silica and may contain more silica in any of its dissolved, colloidal, or particulate forms.

[0003] The solubility of silica unfavorably limits the efficient use of water in industrial applications such as cooling, boilers, geothermal, reverse osmosis, and papermaking. Specifically, when minerals are concentrated during processing, the water treatment operation is limited because the solubility of silica may exceed about 150 ppm. This excess can result in the precipitation and deposition of amorphous silica and silicates, which can in turn result in a loss of equipment efficiency. Furthermore, the accumulation of silica on the internal surfaces of water treatment devices such as boilers, cooling, and purification systems reduces heat transfer and fluid flow through heat exchange tubes and membranes.

[0004] When silica scale forms on a water treatment device, the removal of such scale is very difficult and costly. Thus, in high-silica water, cooling and reverse osmosis systems typically operate at low water utilization efficiency to ensure that the solubility of silica is not exceeded. However, under these conditions, reverse osmosis systems must limit their pure water recovery rate, and cooling systems must limit water recirculation. In either case, the amount of drainage is large.

[0005] Over the years, various additives have been used to suppress silica deposition. Current techniques for silica scale control in industrial cooling systems involve the use of either colloidal silica dispersants or silica polymerization inhibitors. Unlike common scales / deposits such as calcium carbonate and calcium phosphate, silica polymerization can only be slowed down when its concentration significantly exceeds the supersaturation level, and it cannot be stopped completely. Therefore, silica dispersants / scale inhibitors have certain limitations. The maximum permissible level of dissolved silica is typically around 200–400 ppm. [Overview of the project] [Problems that the invention aims to solve]

[0006] Silica contamination is often a major bottleneck in high-recovery reverse osmosis systems. In zero liquid discharge (ZLD) or near-ZLD processes, silica concentrations can reach levels significantly higher than the limits of dispersants / scale inhibitors. Membranes heavily contaminated with silica are extremely difficult to clean and nearly impossible to recover completely. Therefore, reducing the silica concentration in the membrane influent is often necessary. [Means for solving the problem]

[0007] A method for reducing silica contamination in aqueous systems is provided. This method involves adding a composition containing a ferric salt and a magnesium salt to an aqueous medium to precipitate silica in the aqueous medium and form precipitated silica. The method also includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

[0008] In some embodiments, the ferric salt is ferric chloride, ferric chloride hydrate, polyferric chloride, ferric sulfate, polyferric sulfate, or a combination thereof.

[0009] In some embodiments, the magnesium salt is magnesium oxide, magnesium chloride, magnesium chloride hexahydrate, magnesium sulfate, magnesium sulfate monohydrate, magnesium sulfate heptahydrate, magnesium carbonate, or a combination thereof.

[0010] In some forms, magnesium salts are anhydrous magnesium salts.

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

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

[0013] In some embodiments, the composition is prepared by mixing a ferric salt and a magnesium salt before adding them to an aqueous medium.

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

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

[0016] In some embodiments, the polyacrylamide flocculant is added in an amount of about 0.05 ppm to about 5 ppm.

[0017] In some embodiments, the aqueous medium has a pH of about 9.5 to about 12.5.

[0018] In some embodiments, the method includes adding an aluminum compound to an aqueous medium.

[0019] In some embodiments, the method includes supplying the aqueous supernatant to a filtration system and then to a reverse osmosis system.

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

[0021] In some embodiments, the aqueous medium has a temperature of from about 10 °C to about 100 °C.

[0022] In some embodiments, the aqueous medium has a total hardness of from about 0 mg / L to about 2,000 mg / L as CaCO3.

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

[0024] In some embodiments, the method includes adding a base after adding the composition.

[0025] In some embodiments, the base is an alkali metal hydroxide.

[0026] In other embodiments, there is provided the use of a composition comprising a ferric salt and a magnesium salt for removing silica from an aqueous medium.

[0027] The foregoing has outlined, in general terms, the features and technical advantages of the present disclosure so as to enable a better understanding of the detailed description that follows. Additional features and advantages of the present disclosure that form the subject matter of the claims of this application will be described hereinafter. It should be understood by those skilled in the art that the concepts and specific embodiments disclosed herein can be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present disclosure. It should also be recognized by those skilled in the art that such equivalent embodiments do not depart from the spirit and scope of the present disclosure as set forth in the appended claims.

Embodiments for Carrying Out the Invention

[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 contamination 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 also includes removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

[0030] The composition added to the aqueous medium comprises a ferric salt. 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 embodiments, the ferric salt is ferric chloride. In some embodiments, the ferric salt is ferric chloride hydrate. In some embodiments, the ferric salt is ferric sulfate. In some embodiments, the ferric salt is polyferric chloride. In some embodiments, the ferric salt is polyferric sulfate.

[0031] The ferric salt may be an anhydride or a hydrate. Examples of ferric salt hydrates include, but are not limited to, hexahydrate, pentahydrate, and dihydrate. For example, iron chloride may be hexahydrate, pentahydrate, dihydrate, or other hydrate.

[0032] The composition added to the aqueous medium contains a magnesium salt. 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 embodiments, the magnesium salt is magnesium oxide. In some embodiments, the magnesium salt is magnesium chloride. In some embodiments, the magnesium salt is magnesium chloride hexahydrate. In some embodiments, the magnesium salt is magnesium sulfate. In some embodiments, the magnesium salt is magnesium sulfate monohydrate. In some embodiments, the magnesium salt is magnesium sulfate heptahydrate. In some embodiments, the magnesium salt is magnesium carbonate.

[0033] The magnesium salt may be anhydrous or hydrated. In some embodiments, the magnesium salt is an anhydrous magnesium salt, such as anhydrous magnesium chloride.

[0034] In some embodiments, the composition is prepared by mixing a ferric salt and a magnesium salt before adding them to an aqueous medium. The mixing may involve combining a powder of magnesium salt with a powder of ferric salt. Pre-mixing the ferric salt and magnesium salt together synergistically promotes the removal of silica and hardness.

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

[0036] While not bound by any particular theory, two mechanisms are thought to exist for silica removal by magnesium salts. Most magnesium precipitates as magnesium hydroxide, which absorbs silica in solution. Some magnesium can co-precipitate with silica to directly form magnesium silicate. The co-precipitation mechanism allows magnesium salts to remove silica more efficiently than silica absorption by magnesium hydroxide. By pre-mixing magnesium with an acidic ferric salt and administering them together, the ferric salt is thought to prevent the magnesium from rapidly precipitating into magnesium hydroxide, thus increasing the overall silica removal efficiency.

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

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

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

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

[0041] This method may further include adding a base to the aqueous medium after adding the composition to adjust the pH to an optimal level. In some embodiments, the base is an alkali metal hydroxide. Examples of alkali metal hydroxides include sodium hydroxide and lime. This method may further include adding a softening agent to reduce the hardness level. A typical softening agent is, for example, sodium carbonate (soda ash). Those skilled in the art can select an appropriate base and a suitable softening agent according to the needs of the system.

[0042] The composition may be added to an aqueous medium and then stirred or mixed to disperse the composition throughout the medium. The means of stirring the medium are not limited to any particular structure or technique. Those skilled in the art can select an appropriate mixing method according to the needs of the system.

[0043] While the aqueous media that can be treated are not particularly limited, compositions containing ferric and magnesium salts have been found to synergistically remove silica from wastewater. Further purification methods, such as filtration and ion exchange, are applied to regenerate the wastewater. Filtration methods include, in particular, multimedia filtration, microfiltration, and ultrafiltration. Ion exchange methods are applied to further reduce hardness levels. In some embodiments, the method involves adding the composition and then feeding the aqueous supernatant through a filtration system, followed by an ion exchange system, and then a reverse osmosis system.

[0044] In high-recovery reverse osmosis systems, silica can be removed as a precipitate in a softening process, primarily used to reduce the hardness of the raw water. During softening, the 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 reagents, such as aluminum and ferric-based coagulants and polyacrylamide flocculants, are also added to facilitate the sedimentation of the precipitate.

[0045] The magnesium concentration in influent water is often far lower than the amount required for satisfactory silica removal. Therefore, it is common practice to add an additional magnesium source to the influent water to promote 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 having a pH of about 9.5 to about 12.5. In some embodiments, the pH of the aqueous media may be about 10.0 to about 11.5.

[0047] In some embodiments, the aqueous medium has an conductivity of about 100 μs / cm to about 100,000 μs / cm. In some embodiments, the aqueous medium has an conductivity of 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 embodiments, the aqueous medium has a total hardness of about 0 mg / L to about 2,000 mg / L as CaCO3. In some embodiments, the hardness of the aqueous medium may be about 50 mg / L to about 1,500 mg / L as CaCO3. In some embodiments, the hardness of the aqueous medium may be about 10 mg / L to about 600 mg / L as CaCO3. In some embodiments, the hardness of the aqueous medium may be about 200 mg / L to about 500 mg / L as CaCO3.

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

[0050] Silica exists as dissolved silicic acid species, silicates, or complex ions thereof, and can also exist as colloidal silica or suspended silica. The higher the total concentration of silica from all these sources in water, the more difficult the problems caused by amorphous silica scale formation become.

[0051] In some embodiments, the aqueous medium has a temperature of about 10°C to about 50°C. In some embodiments, the temperature of the aqueous medium may be about 20°C to about 40°C.

[0052] In water pretreatment for high-recovery reverse osmosis systems, wastewater inflows often contain scale inhibitors. Scale inhibitors can be added to upstream cooling water treatment and / or reverse osmosis processes to prevent scaling and buildup in these processes. Unfortunately, scale inhibitors interfere with downstream softening and silica removal processes, resulting in higher operating pH and higher chemical usage.

[0053] While not bound by any particular theory, pre-mixing magnesium salts and ferric salts and administering them together allows the newly precipitated ferric hydroxide to efficiently bind with the scale inhibitor, eliminating their adverse effects. As a result, hardness and silica removal efficiency are increased.

[0054] Although not necessary for carrying out the present invention, the compositions disclosed herein are intended to 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 this method may, in certain embodiments, be combined with other practical tools known in the industry. Typical practical tools include sensors for measuring the content of various additives in a system, dissolution or particulate contaminant sensors, other sensors based on resistance, capacitance, spectral absorption or transmittance, colorimetric measurements, and fluorescence; and mathematical tools for analyzing sensor / controller results (e.g., multivariate analysis, metric chemistry, on / off dose control, PID dose control, and combinations thereof).

[0056] In another embodiment, the inert fluorescent tracer is included in a synergistic blend to provide a means for determining the dosage level. A known proportion of the fluorescent tracer is added simultaneously with or consecutively to the blend. An effective inert fluorescent tracer comprises a substance that is chemically nonreactive with other components in the system and does not degrade significantly over time. Such a tracer should also be completely (or essentially completely) soluble in the blend at all relevant concentration levels, and preferably, the fluorescence intensity should be substantially proportional to its concentration and not significantly quenched or otherwise reduced by other components in the system. Furthermore, the inert fluorescent tracer should not be noticeably or significantly affected by any other chemical properties in the system. The phrase "not noticeably or significantly affected" means that the inert fluorescent compound generally has a change of about 10% or less in its fluorescence signal under the conditions normally encountered in fuel ethanol.

[0057] Desirable properties of an inert fluorescent tracer include, but are not limited to, the following: a fluorescence excitation / emission wavelength that does not significantly overlap with light-absorbing substances, other additives, contaminants, etc., present in the water of the system; high solubility; excellent chemical stability; appropriate fluorescence properties at controllable wavelengths (for example, other components in the system should not interfere with the fluorescence properties at those wavelengths); and excitation / emission wavelengths that are separated from other fluorescent components that may be present in the system to prevent interference; as well as avoiding negative effects on the properties of the system.

[0058] Typical inactive fluorescent tracers include fluorescein or fluorescein derivatives, rhodamine or rhodamine derivatives, naphthalene sulfonic acid (mono-, di-, tri-, etc.), pyrene sulfonic acid (mono-, di-, tri-, tetra-, etc.), stilbene derivatives containing sulfonic acid (including fluorescent whitening agents), biphenyl sulfonic acid, phenylalanine, tryptophan, tyrosine, vitamin B2 (riboflavin), vitamin B6 (pyridoxine), vitamin E (α-tocopherol), ethoxyquin, and café. Examples include polymers containing ions, vanillin, naphthalene sulfonic acid formaldehyde condensate polymers, phenyl sulfonic acid formaldehyde condensate, lignin sulfonic acid, polycyclic aromatic hydrocarbons, amines, phenols, sulfonic acids, aromatic (poly)cyclic hydrocarbons containing carboxylic acid functional groups in any combination, (poly)heterocyclic aromatic hydrocarbons having N, O, or S, and polymers containing at least one of the following: naphthalene sulfonic acid, pyrene sulfonic acid, biphenyl sulfonic acid, or stilbene sulfonic acid.

[0059] These additional scale inhibitors may include, but are not limited to, inorganic and organic polyphosphates, phosphonic acids, and polycarboxylic acids. These inhibitors may help inhibit or disperse other scales such as calcium carbonate, calcium sulfate, calcium phosphate, calcium fluoride, barium sulfate, and calcium oxalate. [Examples]

[0060] Example 1: Anhydrous magnesium chloride and ferric chloride were mixed together as fine powders in a weight ratio of approximately 4:1. These were added to a wastewater sample from a coal-fired chemical plant. The wastewater sample had the following characteristics: pH of approximately 9.21, conductivity of approximately 212,000 μs / cm, SiO2 concentration of approximately 90.5 mg / L, total hardness of approximately 450 mg / L as CaCO3, calcium hardness of approximately 290 mg / L as CaCO3, and total alkalinity of approximately 710 mg / L as CaCO3. The wastewater contained an unknown amount of scale inhibitor from upstream reverse osmosis and cooling water treatment. The temperature was set to approximately 21°C. The pH of the wastewater solution was pre-adjusted using a certain amount of NaOH, and then the pre-mixed salts were added via a 10 wt% solution at a total powder dose of approximately 200 ppm, or the components were added separately via a 10 wt% solution of each salt at intervals of approximately 1 minute. After adding all the chemicals, the solution was stirred for a further 10 minutes. Next, an anionic polyacrylamide solution was added at a dose of approximately 0.5 ppm, and the solution was stirred for a further 1 minute, followed by precipitation for approximately 60 minutes. The final solution had a pH of approximately 11.0, and the supernatant sample was taken for analysis. The results of this analysis are shown in Table 1. [Table 1]

[0061] Example 2: Anhydrous magnesium sulfate and ferric polysulfate were mixed together as fine powders in a weight ratio of approximately 1:1. These were added to a wastewater sample from a coal-fired chemical plant. The wastewater sample had the following characteristics: pH of approximately 9.64, conductivity of approximately 8440 μs / cm, SiO2 concentration of approximately 64.3 mg / L, total hardness of approximately 247 mg / L as CaCO3, calcium hardness of approximately 217 mg / L as CaCO3, and total alkalinity of approximately 730 mg / L as CaCO3. The wastewater was mainly from cooling water blowdown and contained an unknown amount of scale inhibitor. The temperature was set to approximately 27°C. The pre-mixed salts were added via a 10 wt% solution at a total powder dose of approximately 200 ppm, or the components were added separately via a 10 wt% solution of each salt at intervals of approximately 1 minute. After adding the salt, a 30% by weight NaOH solution was added dropwise to raise the pH of the solution to approximately 11.3, which took about 0.5 minutes. The solution was then stirred for a further 10 minutes, and an anionic polyacrylamide solution was added at a dose of approximately 0.5 ppm. The solution was continued to stir for a further minute, and then allowed to settle for approximately 60 minutes. The final solution had a pH of approximately 10.90, and the supernatant sample was taken for analysis. The results of this analysis are shown in Table 2. [Table 2]

[0062] All compositions and methods disclosed and claimed herein can be prepared and performed without undue experimentation, taking into consideration this disclosure. The present invention can be embodied in many different forms, and certain preferred embodiments of the present invention are described in detail herein. This disclosure is illustrative of the principles of the present invention and is not intended to limit the present invention to the specific embodiments illustrated. In addition, unless expressly stated otherwise, the term “a” is intended to include “at least one” or “one or more.” For example, “a ferric salt” is intended to include “at least one ferric salt” or “one or more ferric salts.”

[0063] Any range given by either an absolute or approximate term is intended to encompass both, and any definitions used herein are intended to clarify, not limit. Numerical ranges and parameters that specify the broad scope of the invention are approximate, but the numerical values ​​specified in specific examples are reported as accurately as possible. However, any numerical value inherently contains a certain error that inevitably results from the standard deviation observed in their respective test measurements. Furthermore, all ranges disclosed herein should be understood to encompass all subranges contained therein (including all decimal values ​​and whole values).

[0064] Any composition disclosed herein may contain, consist of, or essentially consist of, any element, component, and / or raw material disclosed herein, or any combination of two or more of the elements, components, or raw materials disclosed herein.

[0065] Any method disclosed herein may include, consist of, or essentially consist of, any method step disclosed herein, or any combination of two or more method steps disclosed herein.

[0066] The transitional phrase "comprising," which is synonymous with "including," "containing," or "characterized by," is comprehensive or open-ended and does not exclude additional unlisted elements, components, raw materials, and / or method steps.

[0067] The transitional phrase "consisting of" excludes any elements, components, raw materials, and / or method steps not specified in the claims.

[0068] The transitional phrase "consisting essentially of" limits the scope of the claims to any particular element, component, raw material and / or step, and to any fundamental and novel features of the claimed invention.

[0069] Unless otherwise specified, all molecular weights mentioned herein are weight-average molecular weights, and all viscosities were measured at 25°C using neat (undiluted) polymers.

[0070] As used herein, the term “approximately” means a cited value that is within the error resulting from the standard deviation observed in each of those test measurements, and if those errors cannot be determined, “approximately” may mean, for example, within 5% of the cited value.

[0071] Furthermore, the present invention encompasses 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 present invention described herein will be obvious to those skilled in the art. Such changes and modifications may be made without departing from the spirit and scope of the invention and without diminishing its intended advantages. Accordingly, such changes and modifications are intended to be covered by the appended claims.

Claims

1. A method for reducing silica contamination in an aqueous system, wherein the method is The composition is formed by mixing a ferric salt and a magnesium salt before adding the composition to an aqueous medium, wherein the weight ratio of the magnesium salt to the ferric salt is 2:1 to 6:

1. The composition is added to the aqueous medium to precipitate silica in the aqueous medium and form precipitated silica. A method comprising removing at least a portion of the precipitated silica from the aqueous medium to form an aqueous supernatant.

2. The method according to claim 1, wherein the ferric salt is ferric chloride, ferric chloride hydrate, polyferric chloride, ferric sulfate, polyferric sulfate, or a combination thereof.

3. The method according to claim 1 or 2, wherein the magnesium salt is magnesium oxide, magnesium chloride, magnesium chloride hexahydrate, magnesium sulfate, magnesium sulfate monohydrate, magnesium sulfate heptahydrate, magnesium carbonate, or a combination thereof.

4. The method according to any one of claims 1 to 3, wherein the magnesium salt is an anhydrous magnesium salt.

5. The method according to any one of claims 1 to 4, wherein the magnesium salt is magnesium chloride and the ferric salt is ferric chloride.

6. The method according to any one of claims 1 to 4, wherein the magnesium salt is magnesium sulfate and the ferric salt is polyferric sulfate.

7. The method according to any one of claims 1 to 6, wherein the composition is added to the aqueous medium in an amount ranging from 10 ppm to 2,000 ppm.

8. The method according to any one of claims 1 to 7, further comprising adding a polyacrylamide-based flocculant to the aqueous medium.

9. The method according to claim 8, wherein the polyacrylamide-based flocculant is added in an amount of 0.05 ppm to 5 ppm.

10. The method according to any one of claims 1 to 9, wherein the aqueous medium has a pH of 9.5 to 12.

5.

11. The method according to any one of claims 1 to 10, further comprising adding an aluminum compound to the aqueous medium.

12. The method according to any one of claims 1 to 11, further comprising supplying the aqueous supernatant to a filtration system and subsequently to a reverse osmosis system.

13. The method according to any one of claims 1 to 12, wherein the aqueous medium has an conductivity of 100 μs / cm to 100,000 μs / cm and a silica concentration of 30 mg / L to 250 mg / L, and the aqueous medium further comprises a scale inhibitor.

14. The method according to any one of claims 1 to 13, wherein the aqueous medium has a temperature of 10°C to 50°C.

15. The aqueous medium is CaCO ３ The method according to any one of claims 1 to 14, having a total hardness of 0 mg / L to 2,000 mg / L.

16. The method according to any one of claims 1 to 15, further comprising stirring the aqueous medium after adding the composition.

17. The method according to any one of claims 1 to 15, further comprising adding a base after adding the composition.

18. The method according to claim 17, wherein the base is an alkali metal hydroxide.

19. Use of a composition comprising a ferric salt and a magnesium salt for removing silica from an aqueous medium, wherein the weight ratio of the magnesium salt to the ferric salt is 2:1 to 6:1.

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