KR20190028936A - Method for treating seawater desalination concentrates using pH control - Google Patents

Abstract

The present invention is for environmentally friendly treatment of the concentrated water generated in the seawater desalination process. That is, the seawater desalination concentrated water contains cations such as calcium, magnesium, and sodium at a very high concentration, so that environmental pollution may be a problem when it is discharged as it is. In the present invention, the concentrated water is separated from the concentrated water containing monovalent cations of sodium and the concentrated water containing divalent cations, and then individual processes are separately performed on the separated concentrated water. That is, for sodium, carbon dioxide and ammonia are supplied to precipitate in the form of sodium bicarbonate to be removed. After separation of calcium and magnesium, carbon dioxide is injected separately to precipitate calcium carbonate and magnesium carbonate. Through these processes, the concentration of metal ions in desert water can be reduced to 10,000 ppm or less. Incidentally, it is possible to prevent air pollution by removing the carbon dioxide of the flue gas generated in the desalination plant.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating seawater desalination concentrates using pH-

The present invention relates to a method for eco-friendly treatment of seawater desalinated concentrated water, which can reduce ions in seawater desalinated concentrated water so as to satisfy concentrated water discharge standards.

In the seawater desalination plant to solve the water shortage, the brine is produced as fresh water through the reverse osmosis system, and about 50% of the brine is converted into fresh water, and the remaining 50% is generated as concentrated water. Here, the concentrated water means wastewater in which metal cations are concentrated at a concentration much higher than that of the original seawater or brine.

The concentrated water contains dissolved metals such as sodium, magnesium and calcium in ionic form. The concentration of sodium is about 20,000 mg / L, magnesium is about 2,200 mg / L and calcium is about 800 mg / L.

It is anticipated that environmental regulation standards will be prepared for concentrated water containing such ions at a high concentration. Although emission standards have not yet been established in Korea, emission standards are expected to be introduced soon in overseas countries where desalination plants such as the United States and Spain are actively operating, and they are expected to be in the order of several thousand to 10,000 ppm.

Currently, in Korea, concentrated water is treated by concentrating the concentrated water again with the sea water and reducing the concentration before discharging the concentrated water. Therefore, it is required to develop a technology capable of drastically reducing ions in desalinated concentrated water.

An object of the present invention is to provide a method for precipitating calcium, magnesium and sodium from concentrated water generated in a seawater desalination plant, thereby remarkably reducing the ion content in the concentrated water.

On the other hand, other unspecified purposes of the present invention will be further considered within the scope of the following detailed description and easily deduced from the effects thereof.

According to an aspect of the present invention, there is provided a method for removing metal cations from a concentrated water of a seawater desalination plant including metal cations including sodium, calcium and magnesium, A magnesium separation step of raising the pH of the water to a first range to precipitate magnesium in a solid state and then separating the magnesium precipitate from the concentrated water; A calcium separation step of adjusting the pH of the concentrated water from which magnesium is separated to a second range higher than the first range to precipitate calcium in a solid state and then separating the calcium precipitate from the concentrated water; A sodium removal step of mixing the ammonia solution with the concentrated water in which magnesium and calcium are removed, separating and removing the sodium in the concentrated water by injecting carbon dioxide gas, and precipitating the sodium hydroxide with sodium hydrogen carbonate; A magnesium treatment step of reacting the magnesium precipitate with carbon dioxide to precipitate magnesium carbonate; And a calcium treatment step of precipitating calcium carbonate by reacting the calcium precipitate with carbon dioxide; And the like.

According to the present invention, after the sodium removal step, the magnesium treatment step and the calcium treatment step, the concentration of metal ions in the concentrated water can be reduced to at least 10,000 ppm or less.

In one embodiment of the present invention, it is preferable that ammonia is recovered from the ammonium chloride aqueous solution formed together with sodium hydrogencarbonate in the sodium removal step and reused.

In one embodiment of the present invention, in the magnesium treatment step and the calcium treatment step, water is supplied to each of magnesium and calcium precipitates to form an aqueous solution, and carbon dioxide is supplied to the aqueous solution to form magnesium carbonate and calcium carbonate, It is preferable that the water obtained by solid-liquid separation of the precipitate formed by the reaction with the carbon dioxide and the aqueous solution is used for making the aqueous solution or recycled as the raw water for the desalination process.

Likewise, it is preferable that the concentrated water separated through solid-liquid separation after removing sodium is used as raw water for the desalination process or is obtained by obtaining sodium hydroxide through electrolysis and reused as a pH regulator.

On the other hand, even if the effects are not explicitly mentioned here, the effect described in the following specification, which is expected by the technical features of the present invention, and its potential effects are treated as described in the specification of the present invention.

1 is a schematic flowchart of a method for eco-friendly treatment of seawater desalinated concentrated water according to the present invention.
Figure 2 is a schematic flow diagram of the process for treating the divalent cations shown in Figure 1;
* The accompanying drawings illustrate examples of the present invention in order to facilitate understanding of the technical idea of the present invention, and thus the scope of the present invention is not limited thereto.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may obscure the subject matter of the present invention.

FIG. 1 is a schematic flow chart of a method for eco-friendly treatment of seawater desalination concentrated water according to an embodiment of the present invention.

The present invention is a technique for reducing sodium, magnesium, and calcium from concentrated water generated in a seawater desalination plant to reduce the cation content of concentrated water. In the concentrated water to which the present invention is applied, metal cations are abundantly dissolved. For example, the concentrated water discharged from a single domestic desalination plant contains 800 mg / L of calcium, 2,200 mg / L of magnesium and 20,000 mg / L of sodium (2%). In addition, potassium and lithium are dissolved .

In the present invention, TDS of the concentrated water is reduced by precipitating and removing sodium, magnesium and calcium, which occupy the largest portion of the metal concentration of the concentrated water, in a solid state.

Of the major metals in concentrated water, sodium is a monovalent ion, while calcium and magnesium are bivalent ions. The removal methods of magnesium and calcium, which are divalent ions, and sodium, which is a monovalent ion, are different from each other. In the present invention, first, a pretreatment step of separating a divalent cation and a monovalent cation from each other is performed.

The pretreatment step in this embodiment uses electrodialysis. Electrodialysis is performed by installing an ion exchange membrane in the concentrated water and then applying a voltage to move ions to perform separation. That is, monovalent cations can pass through the ion exchange membrane and move to the compartment provided with the negative electrode. However, since the divalent cation does not pass through the ion exchange membrane, the univalent cation remains in the compartment in which the positive electrode is disposed, And the two cations are separated from each other. Electrodialysis is a well-known technique and will not be described in further detail.

After the monovalent cation and the divalent cation are separated from each other by electrodialysis, a process of precipitating the solid into the concentrated water containing monovalent cation and divalent cation through different methods is performed.

First, the monovalent cation, sodium, is precipitated and removed.

As shown in FIG. 1, ammonia is supplied together with carbon dioxide to concentrated water containing sodium, and sodium is synthesized and precipitated in the form of sodium hydrogen carbonate. The reaction is shown in the following reaction formula.

CO ₂ + 2 NH ₃ → NH ₂ COO ^- + NH ₄ ⁺ ... Scheme 1

NH ₂ COO ^- + H ₂ O → NH ₃ + HCO ₃ ^- ... Scheme 2

NH ₄ ⁺ + HCO ₃ ^- + NaCl ^- > NaHCO ₃ (↓) + NH ₄ Cl ... Scheme 3

That is, carbon dioxide meets and melts with ammonia to form an intermediate between NH ₂ COO ^- and NH ₄ ⁺ , and forms carbonic acid (HCO ₃ ^- ) again as shown in Scheme 2. In the presence of ammonium and carbonic acid, sodium-enriched water (NaCl) precipitates while forming sodium bicarbonate (NaHCO ₃ ) and forms ammonium chloride together. After sodium hydrogencarbonate is precipitated, it is subjected to solid-liquid separation such as filtration to remove sodium hydrogencarbonate.

Through the above reaction, sodium in concentrated water can be precipitated with sodium hydrogencarbonate. In the above process, the supply of ammonia (NH ₃ ) and carbon dioxide is requested, and the carbon dioxide can use the flue gas of the seawater desalination plant. Although the flue gas contains about 15% of carbon dioxide, it can be used by collecting only carbon dioxide separately from the flue gas. However, considering the economical efficiency, it is preferable to use the flue gas as a carbon dioxide source as it is.

And ammonia can be recovered and recycled using the following equation (4), not continuously.

2 NH ₄ Cl + Ca (OH) ₂ ? CaCl ₂ + 2 NH ₃ + 2H ₂ O ... Scheme 4

Referring to Scheme 4, calcium hydroxide is added to the ammonium chloride solution to produce calcium chloride and ammonia. Only ammonia can be recovered and recycled. Since the ammonia can be continuously recycled, the process can be economically operated.

The concentrated water separated by solid-liquid separation after sedimentation of sodium can be used as raw water for the desalination process or can be obtained by electrolysis to obtain sodium hydroxide, which can be reused as a pH regulator in a divalent cation removal process described later.

On the other hand, a process of precipitating and removing calcium and magnesium from concentrated water containing divalent cations will be described. This process is illustrated in FIG.

The present invention provides a method for maximizing carbonation of divalent cations in concentrated water. In metal carbonation techniques, there are often only certain cations present in the starting material. For example, calcium is dominant in limestone, and magnesium is overwhelmingly predominant in serpentinite. However, concentrated water contains both calcium and magnesium at significant levels of metal carbonation. Because of these various ions, special processes are required to carbonate them all.

In the present invention, a step of raising the pH step by step in consideration of the characteristics of the concentrated water and carrying out carbonation for each metal has been developed.

Referring to FIG. 2, magnesium in the concentrated water is first precipitated in the hydroxide / hydrate form by raising the pH of the concentrated water containing divalent cations to the first range. Here, the first range means a range in which magnesium can be precipitated, and in this embodiment, the range is about pH 10 to 11.5. Alkali solutions for increasing the pH are various, but sodium hydroxide is used in this embodiment. When the pH of the concentrated water is adjusted to the first range, magnesium precipitates with Mg (OH) ₂ , and the concentrated water and the magnesium precipitate are separated from each other by filtration. The magnesium precipitate is stored in suspension in water. When the magnesium precipitate is introduced into the water, the magnesium again melts and is present in the suspension.

After the magnesium is separated, the alkaline solution is added to the concentrated water to raise the pH to the second range. Here, the second range is a range in which calcium can be precipitated, which is higher than the first range, and in this embodiment, the pH is adjusted to a range of more than 11.5 and 13.5 or less. Again, sodium hydroxide is used as the alkali solution. When the pH is raised to the second range, calcium in the concentrated water precipitates as Ca (OH) ₂ . The precipitated calcium is separated from the concentrated water by filtration as before, and it is stored in a suspension state with water. When water is added, calcium is again dissolved and remains in the ionic state.

Carbon dioxide is then injected into each of the suspensions in which the magnesium is dissolved and the suspension in which the calcium is dissolved. The process in which carbon dioxide in the gaseous phase is converted to carbonic acid and carbonized is subjected to the following reaction (A-E).

That is, as shown in reaction (A), the gaseous carbon dioxide dissolves, and the dissolved carbon dioxide reacts with water to form carbonic acid as shown in the following reaction (B).

CO _{2 (g)} ? CO _{2 (aq)} (A)

CO _{2 (aq)} + H ₂ O → H ₂ CO ₃ (B)

Then, carbonic acid is decomposed into hydrogen and carbonic acid ions (CO ₃ ^2- ) through the following reaction (C) and reaction (D).

H ₂ CO _{3 -} > H ⁺ + HCO ₃ ^- (C)

HCO ₃ ^{- &} gt ^; H ⁺ + CO ₃ ^2- (D)

When magnesium and calcium are supplied in the state in which carbonate ions are formed, magnesium carbonate is formed as shown in the following reactions (E) and (F), and the mixture is precipitated in a solid state.

Mg ^{2 +} + CO ₃ ^2- → MgCO ₃ ↓ (E)

CA ²⁺ + CO ₃ ^2- → CaCO ₃ ↓ (F)

Although carbon dioxide is injected here, in this embodiment, the flue gas generated in the desalination plant is directly injected. In this embodiment, it is not excluded to inject the separately collected high concentration of carbon dioxide, but the exhaust gas of approximately 15% concentration is directly or indirectly used without additional processing to improve the economical efficiency. When the concentration of carbon dioxide is low, the yield may be lowered. However, it is preferable to directly use the flue gas in terms of economy / efficiency as compared with the process of collecting carbon dioxide separately.

As described above, when magnesium and calcium precipitate in carbonate form, they do not melt again and remain in a stable state to fix carbon dioxide. If the two suspensions are each filtered to separate calcium carbonate and magnesium carbonate, the filtrate can be used as the raw water for the desalination process, or can be recycled back into the water to make the suspension. And calcium carbonate and magnesium carbonate can be used as industrial materials such as fillers.

Then there is a question about why calcium and magnesium should be separated first. For example, when the pH is increased stepwise, magnesium is first precipitated in the concentrated water and remains in that state. If the pH is further increased, calcium precipitates and precipitates of magnesium and calcium may be present in the concentrated water. In this way, calcium and magnesium are precipitated and filtered together in a single reaction tank, and a suspension is prepared to inject carbon dioxide. The above process can be considered because filtration of the precipitate by each step of the pH is not economical because the process is added. In addition, since the starting material used in the existing carbonation technology contains a large amount of one metal such as limestone (calcium source) and serpentine (magnesium source), there is no reason to consider the metal separation process. Research has been conducted. There is no reason to consider the process in the concentrated water in which the metal is contained in combination as in the present invention.

There are two reasons why metal separation processes should be performed.

First, to use carbonated minerals as individual industrial raw materials. Since calcium carbonate and magnesium carbonate have different industrial uses, the calcium carbonate and magnesium carbonate must be separated from each other to increase their purity in order to sell them as a raw material. However, if the two substances are mixed as described above, calcium carbonate and magnesium carbonate should be separately separated from each other. Such a process is not easy. Therefore, it can be concluded that there is no need to remove calcium carbonate or magnesium carbonate when there is no industrial reuse purpose, that is, when it is used only for fixing carbon dioxide. However, considering the yield of carbonation of metal, this also does not hold.

In other words, the second reason for the metal separation process is that the carbonation of calcium and magnesium together is disadvantageous in terms of yield. When the pH of the concentrated water is raised to the first range, the magnesium precipitates in the form of a hydrate and the magnesium concentration in the concentrate drops. However, when the pH is in the first range, the concentration change is not large. This phenomenon is sufficiently predictable. When injecting carbon dioxide in this state, a completely different pattern appears. That is, magnesium is not carbonated, but calcium and carbon dioxide in the suspension first react with calcium carbonate to precipitate calcium carbonate, and the magnesium precipitated as a hydrate is rather re-dissolved to increase the concentration of magnesium in the concentrated water. The same procedure is then repeated if the pH is again raised to the second range. In other words, as calcium is carbonated, the concentration in the concentrated water is gradually lowered. The concentration of magnesium is lowered when the pH is increased, but is repeated when the carbon dioxide is injected. The concentration of magnesium repeats up and down. Magnesium stably precipitates only after calcium is completely precipitated. In condensed water in which magnesium and calcium are dissolved together, magnesium easily precipitates as hydrate as compared to calcium, whereas calcium carbonate is more prevalent in carbonation when carbon dioxide is injected. After all, all the calcium is converted into calcium carbonate, and the carbonation of magnesium proceeds steadily. Thus, the process of carbonating two materials together is very disadvantageous in terms of yield.

The inventors of the present invention discovered the above phenomenon while studying a single process for carbonizing magnesium and calcium together, and introduced the carbonation process of each mineral in consideration of the yield of carbonation per metal and the industrial reuse of the product.

The method of carbonating metal has been widely used. It mainly reacts solid state limestone, slag (calcium source) or serpentine (magnesium source) with carbon dioxide. There is a method of directly reacting solid and gas, and a method of eluting calcium or magnesium from solid and then reacting with carbon dioxide. In the latter case, there is a problem in that energy is introduced into the process of eluting calcium or magnesium from minerals once, although the carbonation reaction efficiency is excellent. However, the concentrated water discharged from a seawater desalination plant has an advantage of satisfying both economical efficiency and efficiency of carbonation since the metal cation is already dissolved.

Particularly, in order to elute metal from a starting material in a solid state, an acid solution is used to dissolve the metal after dissolution, and the solution is acidic. To carbonate the solution, the pH of the solution must be changed to a basic solution. do. However, seawater desalted concentrated water is discharged at a pH of 8, which is very economical since no additional process is required.

As described above, in the present invention, TDS of the concentrated water can be drastically reduced by precipitating sodium, calcium and magnesium in the concentrated water generated in the seawater desalination process, respectively, in a solid state. It is expected that the concentration of the cation in the metal will be reduced to 10,000 ppm or less. In addition, it is expected that carbon dioxide generated from seawater desalination plants can be fixed, which will also be effective in preventing air pollution.

The scope of protection of the present invention is not limited to the description and the expression of the embodiments explicitly described in the foregoing. It is again to be understood that the present invention is not limited by the modifications or substitutions that are obvious to those skilled in the art.

Claims (5)

A method for removing metal cations from a seawater desalination plant concentrated water containing metal cations, including sodium, calcium and magnesium,
A magnesium separation step of raising the pH of the concentrated water to a first range to precipitate magnesium in a solid state and then separating the magnesium precipitate from the concentrated water;
A calcium separation step of adjusting the pH of the concentrated water from which magnesium is separated to a second range higher than the first range to precipitate calcium in a solid state and then separating the calcium precipitate from the concentrated water;
A sodium removal step of mixing the ammonia solution with the concentrated water in which magnesium and calcium are removed, separating and removing the sodium in the concentrated water by injecting carbon dioxide gas, and precipitating the sodium hydroxide with sodium hydrogen carbonate;
A magnesium treatment step of reacting the magnesium precipitate with carbon dioxide to precipitate magnesium carbonate; And
A calcium treatment step of reacting the calcium precipitate with carbon dioxide to precipitate calcium carbonate; Wherein the method comprises the steps of: The method according to claim 1,
Wherein the concentration of the metal ion in the concentrated water after the sodium removal step, the magnesium treatment step, and the calcium treatment step is 10,000 ppm or less. The method according to claim 1,
Wherein the ammonia is recovered from the ammonium chloride aqueous solution formed together with the sodium hydrogencarbonate in the sodium removal step. The method according to claim 1,
In the magnesium treatment step and the calcium treatment step, water is supplied to each of the magnesium and calcium precipitates to form an aqueous solution, and carbon dioxide is supplied to the aqueous solution to form magnesium carbonate and calcium carbonate,
Wherein the water obtained by solid-liquid separation of the precipitate formed by reacting with the carbon dioxide and the aqueous solution is used for making the aqueous solution or recycled as the raw water for the desalination process. The method according to claim 1,
Wherein the concentrated water separated through solid-liquid separation after sodium removal is used as raw water for desalination process or sodium hydroxide is obtained through electrolysis to reuse as a pH regulator.

Images