Method for producing high-purity magnesium carbonate using seawater desalination brine

Abstract

The present invention discloses a method for producing magnesium carbonate using seawater desalination brine. The method for producing magnesium carbonate using seawater desalination according to the present invention comprises the steps of: (A) continuously supplying seawater desalination brine and an alkaline solution to a first reactor to form solid-state magnesium hydroxide through a reaction; (B) continuously separating and discharging the solid-state magnesium hydroxide from the brine that is continuously supplied from the first reactor to a first solid-liquid separator; (C) transferring the brine discharged from the first solid-liquid separator into a second reactor and introducing polymer hydrogel to bind and remove residual magnesium and calcium in the brine; (D) mixing the magnesium hydroxide discharged from the first solid-liquid separator with water to prepare a mixture solution and supplying the mixture solution along with carbon dioxide to a third reactor to precipitate magnesium carbonate; (E) supplying the mixture solution from a third reactor into a third solid-liquid separator to separate and discharge the magnesium carbonate; and (F) producing sodium hydroxide, hydrogen, and chlorine through a chloralkali process with respect to the brine discharged from the second reactor to reduce the sodium concentration in the brine.

Description

Method for producing high-purity magnesium carbonate using seawater desalination concentrate

[0001] The present invention relates to a technology for producing magnesium carbonate from brine, which is discharged as a by-product of seawater desalination and is concentrated in magnesium and sodium, and for treating sodium.

[0002]

[0003] Desalination plants produce freshwater from seawater through reverse osmosis and other processes. Approximately 50% of the seawater is converted into freshwater, while the remaining 50% is enriched with metal cations at concentrations much higher than the original seawater. Specifically, the concentrated water contains cations of 18,298 mg / L sodium, 2,715 mg / L magnesium, and 774 mg / L calcium. Directly discharging this highly concentrated concentrated water into the ocean can have negative impacts on the ecosystem. Therefore, seawater is drawn in, mixed with the concentrated water, and diluted before discharge. However, environmental concerns regarding the discharge of concentrated water have recently been raised, particularly in Europe, necessitating a fundamental solution.

[0004] Meanwhile, given the high concentration of metals in concentrated water, methods are being proposed to recycle it as a raw material rather than a waste product. Previously, the primary method proposed was to react calcium and magnesium with carbon dioxide to fix it. However, this method primarily aimed to reduce carbon dioxide emissions and had limitations in terms of recycling it as an industrial raw material.

[0005]

[0006] The present invention is intended to solve the above-mentioned problems, and its purpose is to provide a method for recycling magnesium in concentrated water as a high-quality industrial raw material while simultaneously providing a method for removing carbon dioxide.

[0007] Meanwhile, other unspecified purposes of the present invention will be additionally considered within the scope that can be easily inferred from the detailed description and effects thereof below.

[0008]

[0009] To solve the problem described above, the following solutions are proposed.

[0010] In order to achieve the above object, the present invention provides a method for producing magnesium carbonate using concentrated seawater desalination water, comprising the steps of: (A) continuously supplying concentrated seawater desalination water and an alkaline solution to a first reactor for reaction to form solid magnesium hydroxide; (B) continuously separating and discharging solid magnesium hydroxide from concentrated water continuously supplied from the first reactor to a first solid-liquid separator; (C) transferring the concentrated water discharged from the first solid-liquid separator to a second reactor and introducing a polymer hydrogel to combine and remove residual magnesium and calcium in the concentrated water; (D) mixing water with the magnesium hydroxide discharged from the first solid-liquid separator to form a mixture, and supplying the mixture to a third reactor together with carbon dioxide to precipitate magnesium carbonate; (E) supplying the mixture from the third reactor to a third solid-liquid separator to separate and discharge magnesium carbonate; And (F) a step of producing sodium hydroxide, hydrogen, and chlorine through a chlor-alkali process for the concentrated water discharged from the second reactor to reduce the concentration of sodium in the concentrated water; is provided.

[0011] Meanwhile, another method for producing magnesium carbonate using concentrated seawater desalination water of the present invention is characterized by using a hydrogel. That is, the present invention comprises the steps of: (A) continuously supplying concentrated seawater desalination water and an alkaline solution to a first reactor for reaction to form magnesium hydroxide in a solid state; (B) continuously separating and discharging solid magnesium hydroxide from the concentrated water continuously supplied from the first reactor to a first solid-liquid separator; (C) supplying an alkaline solution and carbon dioxide together with the concentrated water discharged from the first solid-liquid separator to a second reactor to form calcium carbonate in a solid state; (D) separating calcium carbonate from the concentrated water supplied from the second reactor to the second solid-liquid separator; (E) mixing water with the magnesium hydroxide discharged from the first solid-liquid separator to form a mixture, and supplying the mixture together with carbon dioxide to a third reactor to precipitate magnesium carbonate; (F) a step of supplying the mixed solution from the third reactor to a third solid-liquid separator to separate and discharge magnesium carbonate; and (G) a step of producing sodium hydroxide, hydrogen, and chlorine through a chlor-alkali process for the concentrated water discharged in a liquid phase from the second solid-liquid separator to reduce the concentration of sodium in the concentrated water.

[0012] In the embodiments of the above two inventions, the following technical limitations and additions are possible.

[0013] In one example of the above inventions, the carbon dioxide supplied to the third reactor is a microbubble having a particle size of 50 micrometers or less, and the microbubble can be supplied by pressurizing it to 5 bar or less.

[0014] And in one example of the present invention, the alkaline solution supplied to the first reactor is supplied at a ratio of 1.8 to 2.2 times the number of moles (mol) of magnesium in the concentrated water.

[0015] In addition, in one example of the present invention, the first reactor has an inlet formed on one side, an outlet formed on the other side, and a tubular stirring portion formed with a plurality of protrusions protruding from the inner surface to generate turbulence in the concentrated water. In particular, the first reactor is formed with a reaction portion formed at the rear end of the tubular stirring portion to form magnesium hydroxide. In addition, it is preferable to further include a spray nozzle inserted into the inside of the tubular stirring portion to supply an alkaline solution along the flow direction of the concentrated water.

[0016] In one example of the present invention, the first solid-liquid separator is configured with a shape in which the diameter increases from the top to the bottom and then decreases again, and a space is provided inside the solid and liquid of the concentrated water to mutually separate them, and an inlet for the concentrated water to be introduced is formed at the bottom, a discharge hole is formed at the upper central portion, and a chamber in which a plurality of slits in the shape of long holes are formed along the circumferential direction in the area with the largest diameter, and a shutter is installed in the space portion inside the chamber so as to be able to move up and down between a first position for closing the slit and a second position for opening the slit, and a cover portion in which a flow path for guiding the solid discharged through the slit of the chamber is formed, so that when the shutter rotates with the slit closed, the liquid in the concentrated water is continuously discharged through the discharge hole, and the solid in the concentrated water moves to the area with the largest diameter in the chamber and is discharged through the slit when the shutter moves downward.

[0017] In one example of the present invention, the mixed liquid is continuously supplied and discharged to the third reactor,

[0018] ... Equation (1)

[0019] (Here, V gas is the gas injection volume per unit time, V w is the amount of mixture injected per unit time, C mg is the molar concentration of magnesium in the mixture, A is the carbon dioxide volume correction factor, Tco2 is the conversion rate (0.01~1) in which carbon dioxide participates in the carbonation reaction, P co2 is the concentration of carbon dioxide in the exhaust gas (0.01 to 1 as a volume ratio), z is the target mineral coefficient (0.5, 0.8, 1) that determines the specific form of magnesium carbonate to be produced, and 22.4 is the volume of 1 mole of ideal gas (L).

[0020] The amount of gas containing carbon dioxide supplied to the third reactor per unit time can be determined by the above equation (1).

[0021] In one example of the present invention, the water mixed with the magnesium hydroxide is clear water, and it is preferable that the mixed liquid discharged from the third solid-liquid separator be supplied again to the third reactor and circulated.

[0022] In one example of the present invention, sodium hydroxide and chlorine generated in the chlor-alkali process can be mixed to form sodium hypochlorite.

[0023] In one example of the present invention, when the concentration of sodium hydroxide produced in the chlor-alkali process is 10 mass% or more, it is possible to additionally produce sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3) by injecting carbon dioxide into sodium hydroxide.

[0024]

[0025] The present invention involves supplying an alkaline solution to concentrated water generated in a seawater desalination plant to form magnesium hydroxide. The magnesium hydroxide is then reacted with carbon dioxide to produce high-quality magnesium carbonate, usable as an industrial raw material. Carbon dioxide, a greenhouse gas, is removed during this process. Notably, the above process is performed continuously, enabling the mass production of magnesium carbonate.

[0026] Additionally, calcium in the concentrated water can be adsorbed to the hydrogel to form a hydrogel composition capable of removing carbon dioxide, ultimately increasing the carbon dioxide treatment capacity.

[0027] The chlor-alkali process produces sodium hydroxide and chlorine gas from sodium-rich concentrated water (from which magnesium and calcium have been removed), creating a cyclical process. Furthermore, since sodium in the concentrated water is consumed as sodium hydroxide is formed, the TDS standard can be met when the concentrated water is disposed of.

[0028] Meanwhile, even if the effect is not explicitly mentioned herein, it is added that the effect and its provisional effect described in the following specification expected by the technical features of the present invention are treated as described in the specification of the present invention.

[0029]

[0030] Figure 1 is a schematic flow chart of a method for producing magnesium carbonate using seawater desalination concentrate according to a first embodiment of the present invention.

[0031] Figure 2 is a schematic diagram of a continuous reactor.

[0032] Figure 3 is a schematic cross-sectional view taken along line aa of Figure 2.

[0033] Figure 4 is a schematic drawing of the first solid-liquid separator.

[0034] Figure 5 is for explaining cross-linking by substitution of calcium in sodium alginate.

[0035] Figure 6 is a schematic flow chart of a method for producing magnesium carbonate using seawater desalination concentrate according to a second embodiment of the present invention.

[0036] ※ It is to be noted that the attached drawings are provided for reference only to help understand the technical concept of the present invention, and the scope of the rights of the present invention is not limited thereby.

[0037]

[0038] In describing the present invention, if it is judged that the detailed description of related known functions that are obvious to those skilled in the art and may unnecessarily obscure the gist of the present invention, will be omitted.

[0039] With reference to the attached drawings below, a method for producing magnesium carbonate using seawater desalination concentrate according to a first embodiment of the present invention (hereinafter referred to as “magnesium carbonate producing method”) will be described in more detail.

[0040] FIG. 1 is a schematic flow diagram of a method for producing magnesium carbonate using seawater desalination concentrate according to a first embodiment of the present invention, FIG. 2 is a schematic drawing of a continuous reactor, FIG. 3 is a schematic cross-sectional view taken along line aa of FIG. 2, and FIG. 4 is a schematic drawing of a first solid-liquid separator.

[0041] The method for producing magnesium carbonate according to the first embodiment of the present invention involves reacting carbon dioxide with concentrated seawater, a byproduct of a desalination plant, to produce magnesium carbonate. While the purpose of the present invention is to remove carbon dioxide, a more important goal is to produce high-quality magnesium carbonate. Furthermore, the goal is to implement a process capable of commercial mass production through a continuous process.

[0042] In this specification, "carbon dioxide" refers to the flue gas generated during the power generation process of a seawater desalination plant, and also includes flue gas generated at other industrial sites. While flue gas contains various components, with carbon dioxide accounting for approximately 15%, for convenience of explanation, it is referred to as carbon dioxide gas (gas). While the present invention does not exclude the use of 100% carbon dioxide gas through means such as separate capture of carbon dioxide from the flue gas, direct use of the flue gas is preferable, considering both the economic feasibility of the process and the efficiency of carbonation.

[0043] The concentrated water used as the starting material in the present invention is rich in dissolved metal cations. For example, concentrated water discharged from a single domestic desalination plant contains 18,298 mg / L of sodium, 2,715 mg / L of magnesium, 774 mg / L of calcium, and 711 mg / L of potassium. Additionally, lithium and strontium are dissolved in low concentrations.

[0044] In this example, an alkaline solution (NaOH in this example) is first supplied to the concentrated water to raise the pH to approximately 10.5, thereby generating magnesium in the concentrated water as solid magnesium hydroxide. In particular, the purpose of this example is to implement a continuous process so that commercial mass production is possible. That is, in this example, the process of mixing and stirring the concentrated water and the alkaline solution to react, and the subsequent solid-liquid separation process of separating the magnesium hydroxide generated in a solid state are all performed continuously. In the past, the concentrated water and the alkaline solution were fed together into a reactor, stirred, and allowed to settle for a certain period of time to generate magnesium hydroxide, and then the concentrated water in the reactor was passed all at once to a solid-liquid separator. However, in this example, the concentrated water passes through the first reactor and flows directly into the first solid-liquid separator. In other words, the concentrated water does not settle within the first reactor, but continuously moves, generating magnesium hydroxide and discharging it to the first solid-liquid separator.

[0045] For this purpose, the first reactor (10) is formed in a long tube shape as illustrated in Fig. 2. Concentrated water flows into one side of the first reactor (10) and is discharged from the other side. In addition, sodium hydroxide solution is supplied into the first reactor. The important point is that the concentrated water and sodium hydroxide must be mixed evenly within a short period of time. Since the concentrated water is not allowed to remain in one place as in the past but is continuously moved, it must be evenly mixed with the sodium hydroxide solution within a short period of time. Accordingly, in this example, a spray nozzle (17) is inserted into the front part of the tubular first reactor (10) and arranged coaxially. The spray nozzle (17) supplies an alkaline solution along the flow direction of the concentrated water. In this example, in particular, the spray nozzle is formed in the form of a showerhead to spray the alkaline solution radially so that the alkaline solution can be mixed throughout the concentrated water. It is formed in the form of a showerhead to spray the alkaline solution radially so that the alkaline solution can be mixed throughout the concentrated water. Of course, in other examples, the injection nozzle may not be inserted into the first reactor, but as in this example, inserting it into the reactor and coaxially arranging it can improve the stirring efficiency.

[0046] In addition, a stirring part (12) having a protrusion (11) formed on the inner surface is provided in the first half of the tubular first reactor (10), and a reaction part (13) without a protrusion is provided in the second half. The protrusion (11) has the advantage of promoting mixing of the concentrated water and the alkaline solution by interfering with the flow of the concentrated water and forming turbulence. Of course, depending on the embodiment, the protrusions may be formed on the inner surface along the entire length of the first reactor, or it may be formed only in a tubular shape without protrusions. However, as in this example, if the stirring part is formed in the first half, the mixing efficiency increases, and if there are no protrusions in the second half, it is effective because magnesium hydroxide can be stably produced.

[0047] Meanwhile, the amount of alkaline solution supplied is determined in proportion to the amount of concentrated water supplied. When using a stationary reactor, the pH inside the reactor can be continuously measured using a pH sensor while injecting the alkaline solution. However, in a continuous reactor like this example, since the concentrated water continuously flows, it is not appropriate to control the pH using a sensor. Therefore, the amount of alkaline solution injected (injection speed) is determined in advance before the concentrated water is injected into the first reactor and supplied continuously. In this example, the sodium hydroxide solution is supplied at a ratio of 1.8 to 2.2 times the mole number (mol) of magnesium in the concentrated water. In other words, once the amount of concentrated water supplied per unit time (e.g., 1 second, 1 minute) is determined, the mole number of magnesium in the concentrated water can be known, and the sodium hydroxide solution is supplied so that the mole number of sodium hydroxide is 1.8 to 2.2 times the mole number of magnesium. During the set unit time, the concentrated water and the alkaline solution are each supplied at a constant rate. When the mole fraction is within the above range, the pH of the concentrated water can be formed to be approximately 10.5.

[0048] The concentrated water discharged from the first reactor is supplied to the first solid-liquid separator to separate the solid magnesium hydroxide. In this example, the first solid-liquid separator also uses a continuous separator, so that the concentrated water is continuously supplied and the solid magnesium hydroxide is continuously separated. The configuration of the centrifuge used in this example is briefly described.

[0049] Referring to Fig. 4, the continuous centrifuge (50) used in this example has a chamber (20), a shutter (30), and a cover (40). The chamber (20) is installed to rotate by being coupled to a rotation shaft (21). The chamber (20) has an upper half (22) that is formed into a cone shape with a diameter that gradually increases from the top to the center, and a lower half (23) that extends downward from the upper half (22) and has a diameter that gradually decreases. A space (24) is formed inside the chamber (20) for separating solids and liquids among the concentrate. In addition, an inlet pipe (25) is inserted from the upper side of the chamber (20) to the space (24). Concentrated water flows into the lower side of the space (24) through an inlet port opened at the lower end of the inlet pipe (25). An outlet port (26) is formed at the upper side of the chamber (20) through which liquids separated from solids are discharged. And a slit (27) is formed at the boundary point between the upper half (22) and the lower half (23), i.e., the point with the largest diameter. The slit (27) is a passage through which the solid magnesium hydroxide separated from the concentrated water is discharged. A plurality of slits (27) are arranged spaced apart from each other along the circumference of the chamber (20). In addition, in this example, the slits are formed in the shape of long holes along the circumference of the chamber so that the magnesium hydroxide is efficiently discharged while the chamber rotates.

[0050] The shutter (30) is installed in the space (24) so ​​as to be able to move between a first position and a second position. When the shutter (30) is in the first position, which is raised, the shutter blocks the slit (27) and seals it. On the other hand, when the shutter is in the second position, which is lowered, the slit (27) is opened. The shutter (30) is in the first position, which is the basic position, and periodically opens the slit (27) for a short time and then seals it again.

[0051] The cover part (40) is arranged to surround the area with the largest diameter in the chamber (20), i.e., the point where a plurality of slits (27) are formed, and forms a cylindrical flow path (41) into which magnesium hydroxide discharged through the slits (27) of the chamber (20) flows. The flow path (41) is connected to a pump (not shown).

[0052] In addition, a plurality of disks (28) are stacked within the space (24), and a passage (29) is formed between the disks (28). The disks (28) are also formed in a cone shape corresponding to the upper half (22) of the chamber, and their diameters increase from the top to the bottom. As indicated by the arrows in Fig. 4, the disks (27) guide the liquid in the concentrate to the center of the chamber.

[0053] In a continuous centrifuge having the above-described configuration, the chamber (20) continuously rotates to separate liquid and solid from the concentrated water. That is, concentrated water containing magnesium hydroxide is continuously introduced through the inlet pipe (25), and the chamber (20) also continuously rotates. Magnesium hydroxide in a solid state among the concentrated water is pushed outward within the space (24) by centrifugal force and accumulates near the point with the largest diameter in the chamber, that is, the slit (27). However, since the slit (27) is closed by the shutter (30), the magnesium hydroxide cannot be discharged. The liquid component moves to the center of the chamber through the passage (29) between the plurality of disks (28) and is then continuously discharged through the discharge hole (26). The magnesium hydroxide accumulated near the slit is discharged through the slit (27) to the flow path (41) when the shutter (30) descends and temporarily opens the slit (27). In this way, the supply of concentrated water, the separation of solids from the concentrated water, and the discharge of liquid and solid components are all performed continuously. However, the first solid-liquid separator may be of any type that enables continuous solid-liquid separation, even if it is not of the above type.

[0054] The concentrated water, from which solid magnesium hydroxide is separated through the first solid-liquid separation, is fed to the second reactor. In the second reactor, a polymer hydrogel or polymer resin is added to the concentrated water to bind and remove residual magnesium and calcium within the concentrated water.

[0055] In this example, sodium alginate (SA) is used as the polymer hydrogel. However, in addition to this, at least one of polyacrylic acid, polyacrylamide, kappa-carrageenan, karaya gum, amidopectin, carboxymethylcellulose, polytetrafluoroethylene, polyethylene, polyurethane, polyethylene glycol, and polyester may be used as the synthetic polymer material. At least one of hyaluronic acid, heparin, dextran, collagen, albumin, gelatin, and chitosan may be used as the natural polymer material.

[0056] Polymer hydrogels become viscous when dissolved in an alkaline solution. Since the concentrated water contains sodium hydroxide solution and is alkaline, the polymer hydrogel dissolves in the concentrated water. As shown in Figure 5, the sodium sites in the dissolved sodium alginate are replaced by calcium or magnesium in the concentrated water, which acts as a cross-linker to interconnect the sodium alginate chains. Accordingly, sodium alginate is converted into calcium alginate or magnesium alginate. A plurality of alginate chains are linked and solidified by calcium or magnesium, forming a gel. The hydrogel material formed in this way can remove carbon dioxide from the air. That is, the calcium or magnesium contained in the hydrogel material combines with carbon dioxide in the air and is converted into calcium carbonate or magnesium carbonate, thereby removing it.

[0057] Meanwhile, in the case of polymer hydrogels other than sodium alginate, calcium and magnesium can cause polymer chains to clump together through polymerization, bonding, and cross-linking, thereby gelling. The expression that the hydrogel binds and removes calcium and magnesium can be understood as a general term for the chemical reactions described above. It should be noted that, rather than adding the hydrogel directly to the concentrated solution, it is also possible to dissolve it in a separate alkaline solution and then add the dissolved solution to the concentrated solution.

[0058] Meanwhile, the magnesium hydroxide separated from solid in the first solid-liquid separator is mixed with water (clear water) in a mixing tank to form a mixed solution, which is then supplied to the third reactor. In another variation, the mixed solution may be formed by mixing magnesium hydroxide and the concentrated water discharged from the second solid-liquid separator, but in the first embodiment, the mixed solution is formed by mixing clear water. Although the use of concentrated water may be economical because clear water is not used, it is not desirable in terms of improving the purity of magnesium carbonate because it contains various ions as impurities. Since the technical goal of this embodiment is to produce high-purity, high-quality magnesium carbonate without impurities, it is advantageous to use clear water without impurities to form the mixed solution.

[0059] In addition, carbon dioxide is supplied to the third reactor, and the carbon dioxide and magnesium react to produce magnesium carbonate, which is then precipitated. In this embodiment, the third reactor is also a continuous reactor in which the mixture is continuously supplied and discharged. For example, a tubular reactor that is formed long in one direction and has an inlet on one side and an outlet on the other side can be used. The length of the continuous reactor can be adjusted to ensure a residence time sufficient for the carbon dioxide and magnesium in the mixture to react to form magnesium carbonate. In addition, to promote the formation of magnesium carbonate, the carbon dioxide is pressurized at a pressure of 5 bar or less (4 to 5 bar) and injected into a particle size of 50 μm or less (1 to 50 μm), preferably 10 μm or less. A micro size increases the surface area of ​​the carbon dioxide, which promotes contact with magnesium, thereby improving the efficiency of producing magnesium carbonate.

[0060] Meanwhile, in a continuous process, it is important to determine the carbon dioxide injection rate per unit time in advance. In a continuous process, since the mixture containing magnesium continuously passes through, for example, a tubular reactor, carbon dioxide must also be continuously injected. In the past, when a stationary reaction was performed using a batch reactor, a pH sensor was used. That is, when carbon dioxide was injected, the pH of the mixture gradually decreased, and a method was used to stop the carbon dioxide injection when the pH reached around 8-9. However, this method is difficult to apply in a continuous process. Therefore, the carbon dioxide injection rate or the injection rate per unit time must be determined in advance and supplied to the third reactor at a constant rate. Therefore, the present invention uses the following equation (1) to determine the carbon dioxide injection rate per unit time in advance to implement a continuous process.

[0061] ... Equation (1)

[0062] (Here, V gas is the gas injection volume per unit time, V w is the amount of mixture injected per unit time, C mg is the molar concentration of magnesium in the mixture, A is the carbon dioxide volume correction factor, T co2 is the conversion rate (0.01~1) in which carbon dioxide participates in the carbonation reaction, P co2 is the concentration of carbon dioxide in the exhaust gas (0.01 to 1 as a volume ratio), z is the target mineral coefficient (0.5, 0.8, 1) that determines the specific form of magnesium carbonate to be produced, and 22.4 is the volume of 1 mole of ideal gas (L).

[0063] The above equation (1) is derived from equation (2) below, and basically corresponds the number of moles of carbon dioxide supplied per unit time to the number of moles of magnesium.

[0064] ... Equation (2)

[0065] Excluding A and z, the left side is the number of moles of carbon dioxide that enters per unit time and participates in the carbonation reaction, and the right side is the number of moles of magnesium that enters per unit time. Multiplying the concentration of carbon dioxide in the flue gas (vol%, ex: 0.15) to obtain the volume of carbon dioxide and dividing by the volume of 1 mole of carbon dioxide, 22.4 (L), gives the number of moles of carbon dioxide gas entering per unit time. In the case of the right side, multiplying the flow rate per unit time of the mixture by the molar concentration of magnesium in the mixture (mol / L) gives the number of moles of magnesium per unit time. However, since the volume of lmol of carbon dioxide gas may differ from 22.4 L like an ideal gas, a correction factor of 0.85 to 1.15 is possible.

[0066] In the above equation, the z value is related to the specific form of magnesium carbonate. The z value is determined depending on the type of magnesium carbonate to be produced. For example, if the target mineral is artinite, z = 0.5 is substituted to reduce the amount of carbon dioxide injected, and if the target mineral is hydromagnesite or dypingite, z = 0.8 is set. Additionally, z = 1 is used for magnesite, barringtonite, nesquehonite, and lansfordite. The above target mineral coefficient is derived from research results based on the ratio of magnesium to carbonate ions in each mineral. As above, if the flow rate of the mixture (including magnesium) per unit time and the concentration of magnesium in the mixture are known, the injection rate of the gas containing carbon dioxide per unit time can be determined in advance using the above equation (1). The flow rate of the mixture per unit time can be controlled by a pump. In addition, since the concentration of magnesium in the mixture can be monitored in real time by installing a sensor, even if the amount of mixture per unit time and the concentration of magnesium change in real time, the amount of carbon dioxide injected per unit time can be changed in real time.

[0067] For reference, if the magnesium carbonate production process is conducted in a semi-batch mode rather than a continuous mode, i.e., the process can be performed in units of a fixed amount of mixed solution. In this case, the carbon dioxide injection time in the third batch reactor can be determined using Equation (3) below. Equation (3) below is also a modification of Equation (1) adapted to the batch mode.

[0068] ... Equation (3)

[0069] In the above equation (3), the left side is the injection time of the exhaust gas, Qw on the right side is the volume of the mixture that flows into the third reactor for one reaction process, Cmg is the molar concentration of magnesium in the mixture, A is the carbon dioxide volume correction factor, V gas is the flue gas flow rate per unit time, P co2 is the concentration of carbon dioxide in the exhaust gas (0.01 to 1 as a volume ratio), z is the target mineral coefficient that determines the specific form of magnesium carbonate to be produced, T co2 is the conversion rate of carbon dioxide participating in the carbonation reaction, and 22.4 is the volume (L) of 1 mole of ideal gas. For reference, the above equation (3) can also be used when producing calcium carbonate in a semi-batch manner by modifying it in the same way as equation (1).

[0070] As described above, after carbon dioxide and magnesium react with each other, the mixed liquid discharged from the third reactor is supplied to the third solid-liquid separator, where solid magnesium carbonate is separated from the mixed liquid. In this embodiment, the third solid-liquid separator can implement a continuous process by using a filter press. The mixed liquid after magnesium carbonate is separated in the third solid-liquid separator is in a clear water state or has some residual magnesium remaining, so it is transferred back to the mixing tank and reused, thereby creating a circulating process.

[0071] Meanwhile, the concentrated water discharged from the second reactor contains a high concentration of sodium, making it undesirable to discharge it directly into the ocean. Therefore, in the present invention, the concentrated water is introduced into a chlor-alkali process to produce sodium hydroxide, hydrogen, and chlorine, thereby reducing the sodium concentration within the concentrated water. Furthermore, the sodium hydroxide produced here can be reused as an alkaline solution in the first and second reactors, thereby completing a cyclic process.

[0072] The chlor-alkali process is primarily used to produce chlorine, with sodium hydroxide as a byproduct. The chlor-alkali process typically involves mixing salt with water to create brine, which is then electrolyzed. However, in the present invention, the concentrated water itself contains sodium at a concentration of approximately 20,000 mg / L, allowing the chlor-alkali process to proceed by electrolyzing the concentrated water as is. Through the chlor-alkali process, sodium hydroxide, hydrogen, and chlorine can be recovered, respectively.

[0073] Sodium hydroxide produced in the chlor-alkali process is reused as an alkaline solution to adjust the pH of the concentrated water, but it can also be mixed with chlorine gas to produce sodium hypochlorite. Furthermore, if the concentration of sodium hydroxide produced in the chlor-alkali process is 10% (mass %) or higher, carbon dioxide can be injected into the sodium hydroxide to additionally produce sodium bicarbonate (NaHCO3) or sodium carbonate (Na2CO3).

[0074] As explained above, the chlor-alkali process is a process that dissolves NaCl in water to create brine and then electrolyzes it, so only monovalent ions are dissolved in the brine. However, in the case of using concentrated desalination water as in the present invention, various ions are dissolved in it. In the previous process, magnesium was removed from the concentrated water using magnesium hydroxide and calcium was removed from the concentrated water using hydrogel, but residual ions may remain. If concentrated water containing divalent ions is used in the chlor-alkali process, there is a problem that the electrolysis efficiency is reduced. Therefore, in the present invention, after separating divalent cations and monovalent cations from the concentrated water separated in the second reactor through electrodialysis, only the concentrated water containing monovalent cations can be used in the chlor-alkali process.

[0075] Meanwhile, a second embodiment of the present invention is illustrated in FIG. 6.

[0076] Referring to Fig. 6, the second embodiment has a process for producing magnesium carbonate by sequentially moving seawater desalination concentrate through the first reactor, the first solid-liquid separator, the mixing tank, the third reaction tank, and the third solid-liquid separation tank. The first reactor, the first solid-liquid separator, the mixing tank, the third reaction tank, the third solid-liquid separator, and the specific processes described in the first embodiment are also used in the second embodiment. Therefore, the magnesium treatment process will be replaced with the description for the first embodiment.

[0077] The difference between the second embodiment and the first embodiment lies in the treatment process for the liquid component (concentrated water) after solid-liquid separation in the first solid-liquid separator. In the first embodiment, hydrogel was added to the concentrated water to remove divalent ions, including calcium ions. However, in the second embodiment, calcium ions are reacted with carbon dioxide to form calcium carbonate.

[0078] Referring to the second embodiment illustrated in FIG. 6, the concentrated water after solid magnesium hydroxide is separated through the first solid-liquid separator is supplied to the second reactor. In addition, an alkaline solution (e.g., sodium hydroxide solution) is also supplied to the second reactor, and carbon dioxide is injected. In the second reactor, calcium in the concentrated water reacts with carbon dioxide to produce solid calcium carbonate. In the second embodiment, the second reactor also uses a continuous reactor like the first reactor. However, since the main process of the present invention is to produce and separate magnesium hydroxide, and subsequently produce magnesium carbonate from magnesium hydroxide, the calcium carbonate process, which is not the main process, may not be configured as a continuous process, but may be configured as a batch type in which the concentrated water is collected and processed at regular time intervals. In the second embodiment, a continuous process is implemented, in which the concentrated water is continuously supplied to and discharged from the second reactor, and during this process, the alkaline solution is first supplied and stirred, and then carbon dioxide is injected. According to an embodiment, the second reactor can be divided into two parts, with an alkaline solution supplied to the first part for mixing, and carbon dioxide injected into the second part. When the alkaline solution is supplied and the pH rises to 12.5, the calcium dissolved in the concentrated water is converted to calcium hydroxide, which then reacts with carbon dioxide to form calcium carbonate.

[0079] As mentioned above, in a continuous process, it is difficult to maintain an appropriate pH using a sensor, so it is necessary to determine the alkaline solution supply amount in advance. Therefore, in this embodiment, the amount of sodium hydroxide solution is determined so that the number of moles of sodium hydroxide is 1.8 to 2.2 times the number of moles of calcium contained in the concentrated water supplied per unit time, and it is continuously supplied at a constant rate for a unit time. In addition, Equation (1) described above can be applied to the calcium carbonate conversion process to determine the amount of carbon dioxide injected (injection amount / sec). However, in Equation (1), the matters related to magnesium, i.e., concentration and molecular weight, can be replaced with those related to calcium, and the target mineral coefficient can be deleted.

[0080] Additionally, carbon dioxide is injected in the form of microbubbles to enhance the reaction efficiency with calcium. Specifically, carbon dioxide is pressurized to a pressure of 5 bar or less (4-5 bar) to create particle sizes of 50 μm or less (1-50 μm), and more preferably, 10 μm or less. This is because the micro size increases the surface area of ​​carbon dioxide, promoting active contact with calcium, thereby enhancing the efficiency of calcium carbonate production.

[0081] Once calcium carbonate is formed in the second reactor, the concentrate is transferred to a second solid-liquid separator to separate the solid calcium carbonate. In this embodiment, the second solid-liquid separator is operated continuously using a filter press. Of course, the calcium carbonate separation process can also be operated in a batch mode.

[0082] Meanwhile, the concentrated water discharged from the second solid-liquid separator can be treated with a chlor-alkali process, as in the first embodiment. That is, in the second embodiment, as in the first embodiment, the liquid concentrated water discharged from the second solid-liquid separator is fed into a chlor-alkali process to produce sodium hydroxide, hydrogen, and chlorine, thereby reducing the sodium concentration in the concentrated water. In addition, the sodium hydroxide produced here can be reused as an alkaline solution in the first reactor and the second reactor to form a circulation process. Since the chlor-alkali process is the same as that described in the first embodiment, a separate description is omitted.

[0083] And before feeding the concentrated water into the chlor-alkali process, the previously described hydrogel can be added to the concentrated water discharged through the second solid-liquid separator to remove any divalent ions remaining in the concentrated water. Of course, in the second embodiment, magnesium and calcium were already removed through the carbonation process, but since some residual divalent ions remain, the hydrogel can be additionally added.

[0084] As described above, in the first and second embodiments of the present invention, an alkaline solution is supplied to concentrated water generated in a seawater desalination plant to form magnesium hydroxide, and then the magnesium hydroxide and carbon dioxide are reacted to produce high-purity magnesium carbonate that can be used as an industrial raw material, and carbon dioxide, a greenhouse gas, is removed in the process.

[0085] In the first embodiment, calcium in the concentrated water is reacted with a hydrogel to produce a hydrogel composition for carbon dioxide removal. This calcium-containing hydrogel composition is then reacted with carbon dioxide to form calcium carbonate, thereby increasing carbon dioxide treatment capacity. Furthermore, in the second embodiment, calcium in the concentrated water is further reacted with carbon dioxide to form calcium carbonate, thereby increasing carbon dioxide treatment capacity.

[0086] The chlor-alkali process produces sodium hydroxide and chlorine gas from sodium-rich concentrated water (from which magnesium and calcium have been removed), creating a cyclical process. Furthermore, since sodium in the concentrated water is consumed as sodium hydroxide is formed, the TDS standard can be met when the concentrated water is disposed of.

[0087] The scope of protection of the present invention is not limited to the description and expression of the embodiments explicitly described above. Furthermore, it should be noted that the scope of protection of the present invention may not be limited by obvious modifications or substitutions within the technical field to which the present invention pertains.

Claims

1. (A) A step of forming solid magnesium hydroxide by continuously supplying seawater desalination concentrate and an alkaline solution to a first reactor and causing a reaction; (B) A step of continuously separating and discharging solid magnesium hydroxide from the concentrated water continuously supplied from the first reactor to the first solid-liquid separator; (C) A step of transferring the concentrated water discharged from the first solid-liquid separator to a second reactor and adding a polymer hydrogel to combine and remove residual magnesium and calcium in the concentrated water; (D) A step of mixing water with the magnesium hydroxide discharged from the first solid-liquid separator to create a mixture, and supplying it to a third reactor together with carbon dioxide to precipitate magnesium carbonate; (E) a step of supplying the mixture from the third reactor to the third solid-liquid separator to separate and discharge magnesium carbonate; and (F) A method for producing magnesium carbonate using concentrated seawater desalination water, characterized by comprising a step of producing sodium hydroxide, hydrogen, and chlorine through a chlor-alkali process for the concentrated water discharged from the second reactor to reduce the concentration of sodium in the concentrated water.

2. (A) A step of forming solid magnesium hydroxide by continuously supplying seawater desalination concentrate and an alkaline solution to the first reactor and causing a reaction; (B) A step of continuously separating and discharging solid magnesium hydroxide from the concentrated water continuously supplied from the first reactor to the first solid-liquid separator; (C) A step of supplying an alkaline solution and carbon dioxide together with the concentrated water discharged from the first solid-liquid separator to a second reactor to form calcium carbonate in a solid state; (D) A step of separating calcium carbonate from the concentrated water supplied from the second reactor to the second solid-liquid separator; (E) A step of mixing water with the magnesium hydroxide discharged from the first solid-liquid separator to create a mixture, and supplying it to a third reactor together with carbon dioxide to precipitate magnesium carbonate; (F) a step of supplying the mixture from the third reactor to the third solid-liquid separator to separate and discharge magnesium carbonate; and (G) A method for producing magnesium carbonate using concentrated seawater desalination water, characterized by comprising a step of producing sodium hydroxide, hydrogen, and chlorine through a chlor-alkali process for concentrated water discharged in a liquid phase from the second solid-liquid separator, thereby reducing the concentration of sodium in the concentrated water.

3. In paragraph 1 or 2, The carbon dioxide supplied to the third reactor is microbubbles having a particle size of 50 micrometers or less. A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that the above-mentioned mybro bubbles are supplied under pressure of 5 bar or less.

4. In paragraph 1 or 2, A method for producing magnesium carbonate using concentrated seawater desalination water, characterized in that the alkaline solution supplied to the first reactor is supplied at a ratio of 1.8 to 2.2 times the number of moles (mol) of magnesium in the concentrated water.

5. In paragraph 1 or 2 A method for producing magnesium carbonate using concentrated seawater desalination water, characterized in that the first reactor has an inlet formed on one side, an outlet formed on the other side, and a tube-shaped stirring part formed with a plurality of protrusions protruding from the inner surface to generate turbulence in the concentrated water.

6. In paragraph 5, A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that the first reactor is positioned at the rear end of the tubular stirring section to form a reaction section in which magnesium hydroxide is formed.

7. In paragraph 5, A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that it further comprises a spray nozzle inserted into the inside of the tubular stirring portion and supplies an alkaline solution along the flow direction of the concentrate.

8. In paragraph 1 or 2, The above first solid-liquid separator is configured with a shape in which the diameter increases from the top to the bottom and then decreases again, and a space is provided inside to separate the solid and liquid of the concentrated water from each other, and an inlet is formed at the bottom for the concentrated water to flow in, an outlet is formed at the upper central portion, and a chamber in which a slit is formed in the area with the largest diameter, A shutter installed in a space within the chamber so as to be able to move up and down between a first position for sealing the slit and a second position for opening the slit; A cover part is provided with a path formed to guide the solid discharged through the slit of the chamber. A method for producing magnesium carbonate using concentrated seawater desalination water, characterized in that when the shutter rotates with the slit closed, the liquid in the concentrated water is continuously discharged through the discharge hole, and the solid in the concentrated water moves to the part with the largest diameter in the chamber and is then discharged through the slit when the shutter moves downward.

9. In paragraph 1 or 2, In the third reactor, the mixed liquid is continuously supplied and discharged. ... Equation (1) (Here, V gas is the gas injection volume per unit time, V w is the amount of mixture injected per unit time, C mg is the molar concentration of magnesium in the mixture, A is the carbon dioxide volume correction factor, T co2 is the conversion rate (0.01~1) in which carbon dioxide participates in the carbonation reaction, P co2 is the concentration of carbon dioxide in the flue gas (0.01 to 1 as a volume ratio), z is the target mineral coefficient (0.5, 0.8, 1) that determines the specific form of magnesium carbonate to be produced, and 22.4 is the volume of 1 mole of ideal gas (L). A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that the amount of gas containing carbon dioxide supplied to the third reactor per unit time is determined by the above equation (1).

10. In paragraph 1 or 2 A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that the water mixed with the magnesium hydroxide is clean water.

11. In Article 10 A method for producing magnesium carbonate using seawater desalination concentrate, characterized in that the mixed solution discharged from the third solid-liquid separator is supplied again to the third reactor and circulated.

12. In paragraph 1 or 2, A method for producing magnesium carbonate using concentrated seawater desalination water, characterized in that sodium hydroxide and chlorine generated in the above chlor-alkali process are mixed to form sodium hypochlorite.

13. In paragraph 1 or 2, When the concentration of sodium hydroxide produced in the above chlor-alkali process is 10 mass% or more, carbon dioxide is injected into sodium hydroxide to produce sodium bicarbonate (NaHCO 3 ) or sodium carbonate (Na 2 CO 3 ) is additionally produced. A method for producing magnesium carbonate using seawater desalination concentrate.

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