WO2014196132A1

WO2014196132A1 - Method and device for treating boron-containing water

Info

Publication number: WO2014196132A1
Application number: PCT/JP2014/002608
Authority: WO
Inventors: 野田　清治; 古川　誠司
Original assignee: 三菱電機株式会社
Priority date: 2013-06-03
Filing date: 2014-05-19
Publication date: 2014-12-11

Abstract

For the purpose of carrying out a treatment for removing boron, which is contained in trace amounts in sea water, by a coprecipitation method using an alkaline component, a sea water treatment method and a sea water treatment device (1) for removing boron from sea water with high efficiency are provided, in which, in a step of adding an alkaline component to water to be treated to accelerate the precipitation of insoluble matter, the rate of precipitation of the insoluble matter is kept within the range of 0.003 to 0.02 [mol/(L·min)] inclusive so that boron can be incorporated into the insoluble matter during the process of growing of a magnesium salt, thereby precipitating the insoluble matter to thereby separate boron.

Description

Method and apparatus for treating boron-containing water

The present invention relates to a water treatment method and apparatus for removing boron contained in boron-containing water such as seawater.

Boron components (about 5 mg / L in terms of boric acid) present as boric acid in seawater are considered to cause plant growth problems and human infertility. According to the WHO Guidelines (2009), The acid concentration needs to be reduced to 2.5 mg / L.

As a conventional method for treating boron-containing water, when precipitating a certain substance in a solution, “co-precipitation” is used, which is known as a phenomenon in which a substance that should not be precipitated alone is mainly precipitated. There is something. This is because the magnesium compound and the silicate compound are added to the water to be treated containing boron, the alkali is added to adjust the pH to 10.5 to 11.5, and boron, silicon and magnesium are added by a coprecipitation method. An insoluble matter was generated, and the insoluble matter was solid-liquid separated to remove boron from the water to be treated (see, for example, Patent Document 1).

JP 2009-82843 A

In the conventional method for treating boron-containing water, when coprecipitating boron, the amount of magnesium added to boron must be 10 to 30 times by weight, and the silicate compound must be 2 to 15 times by weight. It was.

In this way, in the boron removal method using the coprecipitation method, in order to remove boron at a low concentration of 100 mg / L (ppm) or less, a large amount of a magnesium compound and an alkali must be added. There was a problem that a large amount of the dissolved material was also present.

The present invention has been made to solve the above-described problems. By efficiently incorporating boron into the molecules of the main precipitate or the aggregates thereof, the amount of the main precipitate is reduced. The object is to efficiently remove boron from boron-containing water by increasing the boron content.

The method for treating boron-containing water according to the present invention includes a first step of adding an alkali to the water to be treated to precipitate insoluble matter, and a second step of separating the insoluble matter precipitated from the water to be treated. When the correction coefficient at the temperature T [° C.] determined by the equation (1) is D ₂₀ (T), the precipitation rate of the insoluble matter in the first step is 0.003 · D ₂₀ (T) [mol / ( L · min)] to 0.02 · D ₂₀ (T) [mol / (L · min)] or less.

The boron-containing water treatment apparatus of the present invention includes a precipitation tank for precipitating insoluble matter from a solution obtained by adding alkali to the water to be treated, a chemical tank for supplying alkali to the precipitation tank, A pH measurement unit for measuring the pH of the solution, a control unit for controlling the reaction conditions in the solution based on the results measured by the pH measurement unit, and a particle separation tank for separating insoluble matter deposited from the water to be treated Is provided.

According to this invention, since the deposition rate of magnesium hydroxide is controlled to be equal to or lower than the diffusion rate of boron, a large amount of boron can be taken into the molecules or aggregates of the precipitated insoluble matter. For this reason, since the magnesium compound to be added is unnecessary or small, a treatment method with a small amount of insoluble matter can be obtained, and a treatment apparatus with a small capacity of each tank can be obtained.

It is a block diagram which shows the boron containing water treatment apparatus in Embodiment 1 of this invention. Embodiment 1 of the present invention is a diagram showing the precipitation rate dependence of the insoluble matter of the boron reduction amount. It is a figure which shows the precipitation rate dependence of the boron reduction | decrease amount per deposit amount. It is a figure which shows the time change of an insoluble matter density | concentration. It is a figure which shows typically a mode that boron is fully taken in into the molecule | numerator or aggregate of a precipitate. It is a figure which shows the time change of solution pH during precipitation of an insoluble matter. It is a block diagram which shows the boron containing water treatment apparatus in Embodiment 2 of this invention. It is a figure which shows the hydroxide ion density | concentration dependence of the precipitation rate of an insoluble matter. It is a figure which shows the magnesium ion concentration dependence of the precipitation rate of an insoluble matter. It is a figure which shows the dependence with respect to the product of the hydroxide ion concentration and magnesium ion concentration of the precipitation rate of an insoluble matter. It is a figure which shows the relationship between the amount of boron reduction, and the addition amount of the magnesium ion added by equimolar ratio with the hydroxide ion. It is a block diagram which shows the boron containing water treatment apparatus in Embodiment 3 of this invention.

In the description of the present application, the insoluble matter and the precipitate are the same. The precipitate is a precipitate of insoluble matter (precipitate).

Embodiment 1 FIG.
In the course of advancing research on boron separation by the coprecipitation method, the inventors of the present application determined that the boron into the precipitate depends on the size relationship between the insoluble matter formation rate in the coprecipitation method and the diffusion rate of boron molecules in the solution. We found that the uptake efficiency of potato greatly changed and elucidated the mechanism. Therefore, in the first embodiment, the boron removal performance in the case where the production rate of the insoluble matter is changed will be described in detail including theoretical examination.

FIG. 1 shows a boron-containing water treatment apparatus 1 used in the present invention. This apparatus includes a water storage tank 2 for storing water to be treated such as seawater containing boron, a pretreatment tank 3 for pretreating the seawater, and a transport pump 4 for supplying the pretreated seawater to the precipitation tank 5. In this precipitation tank 5, the insoluble matter is deposited from the water to be treated.

An alkaline chemical tank 11 is connected to the precipitation tank 5 via a chemical pump 12 in order to supply the alkaline chemical to the pretreated seawater, and an insoluble matter is precipitated from the seawater by adding alkali to the seawater. . A stainless steel stirring blade W equipped with a stirring motor 9 is inserted into the precipitation tank 5 and rotates at a predetermined speed to mix the solution of water to be treated with alkali added thereto. Further, a pH electrode 8 is inserted into the solution in the precipitation tank 5, and the change in pH of the solution accompanying the addition of alkali or precipitation of insoluble matter is continuously measured. The measurement result of the pH electrode 8 is sent to the controller 10 through the signal line SL1, and the state of the precipitation reaction is judged from the time change of pH, and the operations of the drug pump 12, the agitation motor 9 and the transport pump 4 are controlled as necessary. By doing so, the precipitation reaction in the precipitation tank 5 can be controlled. The agitation motor 9, the transport pump 4, and the drug pump 12 are connected to the controller 10 by a signal line SL2, a signal line SL3, and a signal line SL4, respectively. These pumps have a flow rate determined by operating conditions, but may be configured separately with a flow meter. That is, the controller 10 controls the reaction rate in the tank by controlling the speed of supplying the water to be treated to the precipitation tank 5, the speed of supplying alkali from the chemical tank 11 to the precipitation tank 5, the speed of stirring the precipitation tank 5, and the like. It has the function of keeping speed properly. In the precipitation tank 5, boron is removed from the seawater by efficiently incorporating boron into the molecules of the precipitate or into the aggregates thereof.

The outflow pipe for discharging the solution from the precipitation tank 5 includes a transport pump 6 for supplying the processed seawater to the particle separation tank 7, and the seawater after the insoluble matter containing boron is precipitated and separated is separated. It is transported to the intermediate tank 14 by the transport pump 13. The particle separation tank 7 in FIG. 1 schematically shows a method of taking out a supernatant portion. The operation of the transport pump 6 may be controlled by the controller 10.
The intermediate tank 14 is connected to a reverse osmosis separation module 16 that performs desalting treatment via a transport pump 15. The reverse osmosis separation module 16 includes a permeate outflow pipe 19 for sending treated permeate, that is, demineralized water, as drinking water to a use point, for example, and a discharge pipe 17 for discharging non-permeate water out of the system. Is connected. The reverse osmosis separation module 16 is connected to a return pipe 18 for returning a part of the non-permeated water to the upstream side of the transport pump 4 in front of the precipitation tank 5 as necessary.

The pretreatment tank 3 performs sand filtration or microfiltration on the seawater to be treated, and further adds chlorine or a flocculant to the seawater to remove suspended substances and organic substances in the seawater. This not only prevents the materials of the precipitation tank 5, the particle separation tank 7, and the reverse osmosis separation module 16 from adhering to and contaminating the membrane surface of the reverse osmosis membrane, but also the precipitation tank. The factor that adversely affects the boron removal reaction in 5 can be removed.

In the particle separation tank 7, the insoluble matter (particle diameter of 0.1 to 100 μm) containing boron generated in the precipitation tank 5 is separated. Specifically, in addition to a sedimentation tank using gravity sedimentation (residence time 1 to 60 minutes), a liquid cyclone using centrifugal force, a porous membrane having a retained particle diameter of 0.1 μm or more, and the like can be used. .

In the intermediate tank 14, the water quality is adjusted as necessary in order to reduce foreign matter that enters the reverse osmosis separation module 16 in the subsequent stage. For example, an acidic chemical solution is injected (not shown) in order to dissolve fine particles that have not been completely separated in the particle separation tank 7. This is caused by clogging when a large amount of solid foreign matter enters the reverse osmosis separation module 16 and it is necessary to replace the internal reverse osmosis membrane. Of the particles, fine particles that cannot be removed by the separation tank 7 are redissolved. However, redissolving the particles containing boron once precipitated is useless from the viewpoint of boron removal, which is the object of the present invention, and the amount of redissolving must be limited to a minimum. It is preferable that the particles to be re-dissolved are separated in the particle separation tank 7 so as to be 1% or less of the total.
The treatment liquid in the particle separation tank 7 usually has a pH of 9.5 or higher, but an acid chemical solution is added thereto to adjust the pH of the treatment water to 6.0 to 8.5, preferably 6.5 to The neutral region is adjusted to 8.5 and supplied to the reverse osmosis separation module 16 at a predetermined pressure by the transport pump 13.

In the present invention, the reverse osmosis membrane in the reverse osmosis separation module 16 is not particularly limited in its shape, structure, etc. For example, any of spiral type, hollow fiber type, tubular type, frame and plate type, etc. Things are used. Further, the material constituting the reverse osmosis membrane in the reverse osmosis separation module 16 is preferably made of, for example, cellulose acetate, polyvinyl alcohol, polyamide or the like. What consists of polyamide with a high function is suitable. When the above spiral type, hollow fiber type, tubular type, and frame and plate type modules are used, the reverse osmosis membrane is a composite in which a separation active layer (reverse osmosis membrane) is supported on a porous support membrane. A reverse osmosis membrane is used.

According to the present invention, the procedure for desalinating seawater containing boron with the above-described apparatus configuration is as follows. After the seawater in the water storage tank 2 is pretreated in the pretreatment tank 3, it is continuously transported to the particle separation tank 7 by the transport pump 6 while being supplied to the precipitation tank 5 at a predetermined flow rate by the transport pump 4. Remove boron. Further, in the reverse osmosis separation module 16 in the subsequent stage, the salt in the seawater is blocked, and the permeated water from which the salt has been removed at a predetermined blocking rate is obtained from the permeated water flow pipe 19. In the present invention, the reverse osmosis membrane in the reverse osmosis separation module 16 is, for example, a saline solution having a pH of 6.5 and a concentration of 3.5% as raw water at a temperature of 25 ° C. and an operating pressure of 10 to 56 kgf / cm 2 for 1 hour. It is preferable to use a salt blocking rate of 70 to 95% or more when operated.

Since the boron-containing water treatment apparatus 1 of the present embodiment controls the insoluble matter generation rate to be equal to or less than the boron diffusion rate, there is an effect that the boron can be sufficiently taken into the precipitate molecules or aggregates. . Next, the process of incorporating boron into the precipitate will be described in detail.

Example 1.
In Example 1, an example in which an experiment was conducted on the boron removal performance in the precipitation tank 5 of the boron-containing water treatment apparatus 1 shown in Embodiment 1 will be described.
Artificial seawater 20L was supplied to the precipitation tank 5 which is a stainless steel water tank having a capacity of 25 L (liter) by the transport pump 4, and the water temperature was kept at about 20 ° C. while stirring the rotating speed of the stirring motor 9 at about 600 rpm. When the water temperature was stabilized, the chemical pump 12 was operated, and a predetermined amount of an aqueous sodium hydroxide solution or an aqueous sodium carbonate solution was added from the alkaline chemical tank 11 to start a precipitation reaction. Changes in the pH and temperature of seawater during precipitation were measured with a pH electrode (GST-2729C, Toa DKK). Table 1 shows the water quality of the aqueous solution used in the experiment. Solution No. 1-No. 3 is an aqueous solution simulating the ionic composition of seawater, and the boron concentrations were adjusted to 5 mg / L, 10 mg / L, and 24 mg / L, respectively. The reason why H ₃ BO ₃ is shown in Table 1 is that boron does not dissociate into anions in seawater, and the weight concentration values in the table are calculated for boron alone. In addition, Solution No. No. 4 does not contain seawater components and contains a high concentration of boron, and was prepared for the purpose of evaluating the influence of direct reaction between boron and an alkaline substance.

As the reagents, primary reagents were used, sodium hydroxide was an 8 mol / L aqueous solution, and sodium carbonate was an 8 mol / L aqueous solution prepared from ultrapure water and a powder reagent. In order to examine the water quality change accompanying the precipitation reaction accompanying the addition of alkali, suction filtration was performed using a membrane filter (φ50 mm, Millipore) having a retention particle size of 0.45 μm, and the filtrate was analyzed. An ion chromatography apparatus was used for the cation composition analysis, and boron was quantitatively analyzed using a plasma emission (ICP) analyzer. The artificial seawater containing the precipitated particles was sufficiently stirred and then suction filtered using a membrane filter having a retained particle diameter of 0.45 μm to collect particles from 30 mL of the solution. The filter was dried at 50 ° C. for 12 hours or more, and the particle concentration was calculated from the change in dry weight. In order to confirm the progress of the precipitation reaction, a certain amount of solution was collected from the precipitation tank 5 at regular intervals after the alkali was added. By the chemical analysis, the constituent ion components of the filtrate were quantified, and the degree of progress of the precipitation reaction was quantified.
For example, if the dry weight (g) of the filtrate collected every 30 seconds is set to a ₁ , a ₂ , a ₃ , a ₄ ,..., A _N , the precipitation rate R _i ( mol / (L · min)) is

M: Molecular weight of insoluble matter [g / mol]
Given in.
However, in accordance with the precipitation reaction proceeds, gradually deposition rate R _i is the reactant is consumed is reduced. In the present Example 1 conditions, the precipitation reaction was completed in 10-20 minutes, as the representative value of R _i, employing the arithmetic mean of the measured values of i = 20 the following R _i.
SEM / EDX analysis (acceleration voltage 15 kV) was performed in order to investigate the constituent elements and crystal structure of the precipitated particles collected on the filter. The precipitated particles were redissolved with hydrochloric acid, and boron contained in the particles was quantified using a plasma emission (ICP) analyzer.
Since fine particles of inorganic salt are adhered inside the precipitation tank 5 after the precipitation reaction, the precipitation tank 5, the pH electrode 8, and the stirring blade W are immersed in 0.01 mol / L hydrochloric acid for 60 minutes or more at room temperature. The fine particles were completely dissolved.

FIG. 2 is a graph showing the insoluble dissolution rate dependency of the boron reduction amount in Example 1. This is the solution no. 3 shows the amount of boron decrease before and after the reaction when sodium hydroxide was added in an amount of 0 to 0.23 mol / L to 3 and the deposition rate was controlled. The amount of boron decrease increased in proportion to the precipitation rate until the precipitation rate reached 0.01 mol / (L · min), and reached a maximum of 4 mg / L. The boron removal rate at this time was 80%, and it was proved that a high removal rate could be realized without using a magnesium compound, which had to be added by a conventional method.

Further, when the precipitation rate is 0.01 mol / (L · min) or more, the boron decrease amount gradually decreases, and when the precipitation rate exceeds 0.02 mol / (L · min) and 0.03 mol / (L · min), 1 Reduced to 5 mg / L. In the figure, δ represents the difference in boron decrease between the precipitation rate of 0.03 mol / (L · min) and 0.02 mol / (L · min), and 0.02 mol / (L · min). The value of about 10% in terms of the amount of boron reduction in min). Therefore, even if measurement accuracy is taken into consideration, it can be recognized that there is a sufficient difference between the two.

From this, if it is considered that there is a boundary at 0.02 mol / (L · min), it is considered that the deposition rate is desirably controlled below this value. Moreover, since a boron reduction amount of 1.1 mg / L or more is necessary from a practical viewpoint, it is desirable to control the deposition rate to, for example, 0.003 mol / (L · min) or more. In FIG. 2, the recommended precipitation rate range OP1 is shown.

When the precipitation rate is 0.01 mol / (L · min) or more, the amount of boron decrease decreases and exceeds 0.02 mol / (L · min) and is saturated. This is considered to be because the amount of insoluble matter produced was saturated due to the lack of ions that produced the product. Considering from a theoretical point of view, the point at a deposition rate of 0.02 mol / (L · min) can be considered as a convenient coincidence between the diffusion rate of boron and the deposition rate. Since the boron diffusion rate is a constant determined by the water temperature, if the precipitation rate exceeds the boron diffusion rate, the boron uptake amount does not change even if the precipitation rate increases, so the boron removal amount is saturated. Can be interpreted.

From the above, in order to sufficiently secure the boron incorporation efficiency into the precipitate and realize the practically required boron removal capability, the deposition rate is 0.003 mol / (L · min) or more, 0.02 mol / It has been found desirable to control to (L · min) or less. When the boron removal amount is 2.0 mg / L or more, the deposition rate is 0.005 mol / (L · min) or more and 0.016 mol / (L · min) or less, and the boron removal amount is 3.0 mg / L or more. It was found that the deposition rate is desirably controlled to 0.007 mol / (L · min) or more and 0.013 mol / (L · min) or less when necessary.

FIG. 3 is a diagram showing the deposition rate dependence of the insoluble matter of the boron removal performance in Example 1. As with FIG. 3 shows the boron removal amount per precipitate amount before and after the reaction when sodium hydroxide is added up to 0.23 mol / L and the precipitation rate is controlled. The value on the vertical axis in FIG. 3 is obtained by normalizing the boron reduction amount in FIG. 2 with the amount of precipitates, and corresponds to the boron removal efficiency per precipitate. The unit “mg-boron / (L · g-precipitate)” is obtained by dividing the weight of the boron (mg) by the volume of the solution (L) and the weight of the precipitate (g).
Until the precipitation rate is 0.008 mol / (L · min), the decrease in boron per amount of precipitate is approximately 2.8 mg-boron / (L · g-precipitate), but is 0.008 mol / (L · min). ) Above, the amount of boron decrease gradually decreased. When the deposition rate was 0.02 mol / (L · min) or more, the precipitation rate decreased to 1.0 mg-boron / (L · g-precipitate). Therefore, in the region where the precipitation rate exceeds 0.02 mol / (L · min), even if the precipitate is obtained quickly, it is not advantageous with respect to the boron reduction amount per precipitate. This supports the above-mentioned consideration regarding the diffusion rate of boron, and in order to increase the efficiency of boron incorporation into the precipitate, the precipitation rate should be controlled to 0.02 mol / (L · min) or less. I understand that.

Solution No. 1, no. When a similar experiment was performed on No. 2, the boron removal amount changed in proportion to the initial boron concentration, but the influence of the deposition rate on the relative change tendency of the boron removal amount was the same. Therefore, regardless of the boron concentration contained in seawater, the precipitation rate is 0.003 mol / (L · min.) In order to secure sufficient boron incorporation efficiency into the precipitate and to achieve the practically required boron removal capability. It is desirable to control it to 0.02 mol / (L · min) or less.

According to the water quality standard value published by the Ministry of Health and Welfare, the boron concentration should be 1.0 mg / L or less. Since the boron concentration contained in seawater is typically 5 mg / L, the method of the present invention is also effective in water supply applications. In some sea areas, the boron concentration may be higher than 5 mg / L. 1, no. From the experimental results of 2, even when such seawater is treated, the above tendency does not change. This can be explained qualitatively because the diffusion rate of boron does not depend on the boron concentration.
The same results as in FIGS. 2 and 3 were obtained even when potassium hydroxide was used in addition to sodium hydroxide as the alkaline agent.

Next, ancillary experiments conducted to support the above interpretation of the diffusion rate will be described.
Solution No. The precipitate from 1 was collected, and the dried precipitated particles were dissolved in hydrochloric acid and analyzed by ICP. The amount of boron contained in the particles was 11 mg / L. The boron concentration in the artificial seawater was a decrease of 10 mg / L because what was 24 mg / L before the treatment was 14 mg / L after the treatment. The above 11 mg / L and 10 mg / L are considered to coincide with each other when experimental errors are taken into consideration. Therefore, the amount of boron reduced in artificial seawater and the amount of boron contained in the precipitate are almost equal, It was confirmed that the decrease in boron concentration was caused by the incorporation of boron into the precipitate.

Solution No. From the results of elemental analysis (SEM / EDX) of the precipitate from No. 1, Cl, Na, Mg, and O soot were detected as constituent elements of the precipitate. Among the detected combinations of constituent elements, magnesium hydroxide has the lowest solubility, and the solubility of magnesium hydroxide in water at 18 ° C is only 9.8 mg / L (5th edition of RIKEN Dictionary). The main component is considered to be magnesium hydroxide or other magnesium salt.

As Comparative Example 1, solution no. Sodium hydroxide 0.029 mol / L was added to 4, and the change of the boron concentration was investigated. As a result, the boron concentration did not change within the measurement error.
Boron exists as boric acid in seawater, and this boric acid does not dissociate when the pH is near neutral, but when the pH value becomes high, that is, in the alkaline region, boric acid has the following formula:

It is known to dissociate into borate ions as shown in FIG. Even if such ionization of boric acid progressed, it was confirmed that removal of boron would not proceed without the generation of insoluble matter.

As Comparative Example 2, when an amount of sodium carbonate having the same alkalinity as sodium hydroxide is added based on Solution 1, the precipitation rate is 0.003 mol / (L · min) or more and 0.02 mol / (L · min). ) The following could be realized, but the boron removal amount was 0.05 mg / L or less. As a result of chemical analysis of the precipitate, the main component of the insoluble material is calcium carbonate, which is considered to be caused by a structure that hardly incorporates boron into the molecule unlike the magnesium salt.

The purpose of the present invention is to incorporate boron in the solution into the particles along with the precipitation. However, since boron (boric acid) is present in the form of molecules in the aqueous solution, it itself cannot be changed into a hardly soluble compound. Therefore, it is necessary to incorporate boron into the precipitate in the molecular state. The fact that calcium carbonate has a clear crystal structure while the magnesium salt that is a precipitate does not have a clear crystal structure means that the crystal structure (arrangement of constituent atoms) has a degree of freedom, It is presumed that it was easy to take up boric acid as a foreign substance.

The relationship between the diffusion rate of boron and the precipitation rate of insoluble matter was estimated by the following method.
Solution No. 1, no. 2, no. When the particle size of the insoluble matter precipitated from No. 3 was measured with a light scattering type particle size distribution meter, the particle size was in the range of 0.5 to 100 μm, and the average particle size was about 60 μm.

FIG. 4 is a diagram showing the time change of the insoluble matter concentration (concentration of precipitated particles) in the experiment of Example 1. Under the conditions where the precipitation rate was 0.02 mol / (L · min), the change over time in the concentration of the insoluble matter after addition of sodium hydroxide in the sealed system was measured. Specifically, at 0 minutes on the horizontal axis in FIG. When the chemical solution was added so that the sodium hydroxide concentration was 0.112 mol / L with respect to 3, the deposition rate Ri measured immediately was 0.02 mol / (L · min). As shown in FIG. 4, the concentration of insoluble matter increased rapidly to about 1.0 g / L in the first about 5 minutes, and gradually increased after 10 minutes. When the time constant τ _R for the precipitation reaction was determined from the change in the concentration of the insoluble matter, it was found to be about 5 minutes.

This precipitate was collected, and the dried precipitated particles were dissolved in hydrochloric acid and analyzed by ICP. The amount of boron contained in the precipitated particles was 4 mg / L with respect to seawater. On the other hand, since the amount of precipitate per 1 L of seawater is about 1.4 g from FIG. 4, the boron content per unit volume of the precipitated particles was 6.75 mg / cm ³ considering the density of the precipitate. Solution No. Since the boron concentration in 3 is 5 mg / L, the boron-containing density of the precipitated particles is about 1,350 times greater than the proportion of boron present in the solution.

FIG. 5 is a diagram schematically showing a state in which boron is sufficiently taken into precipitate molecules or aggregates, and shows an image of boron 101 taken into an insoluble matter 102 mainly composed of a magnesium salt. The insoluble matter 102 grows toward the region G by the reaction between magnesium ions (Mg ²⁺ ) 103 in seawater and the added alkali component (OH ⁻ ) 104. At this time, since the insoluble matter 102 grows while taking in boron 101 present in the vicinity, the boron 101 is removed from the seawater.

As described above, the solution No. 1, no. 2, No. Since the average particle diameter of the insoluble matter precipitated from 3 is about 60 μm, the boron present in the range of 1,350 times the volume ratio, that is, about 11 times the diameter ratio (≈1350) is taken in by the diffusion phenomenon. There is a need. Therefore, in the process in which the precipitated particles of the insoluble matter 102 grow in the region G in FIG. 5, it is possible to take in all boron contained in the region S having a diameter ratio of 11 times.

When the diffusion constant D in water of boron at room temperature D = 1.5 × 10 ⁻⁹ (m ² / s) and the diffusion distance 660 μm (= 60 μm × 11), the time constant τ _D required for boron diffusion is calculated.

This is about 5 minutes.

Therefore, in consideration of measurement errors and the like, it was confirmed that the deposition rate of 0.02 mol / (L · min) was equal to the time constant τ _R of the precipitation reaction and the diffusion time constant τ _{D of} boron. Therefore, as shown in FIG. 3, when the precipitation rate was 0.02 mol / (L · min) or more, the boron removal amount per precipitate amount became constant because the generation rate of insoluble matter was the boron molecule in the solution. Therefore, it can be concluded that the generation of insoluble matter has progressed in a state where it is not possible to take in boron sufficient for the deposition rate.

From the above discussion, in the boron-containing water treatment apparatus 1 according to the first embodiment, it became clear that the deposition rate is a parameter that determines the boron removal performance. Therefore, as shown in FIG. 2, by controlling the deposition rate within the range of 0.003 mol / (L · min) or more and 0.02 mol / (L · min) or less, the insoluble matter production rate is diffused by boron. It is possible to obtain an effect that the boron can be sufficiently taken into the precipitate molecules or aggregates by controlling the speed below the speed. Therefore, since an insoluble matter hardly containing boron is not generated, an efficient boron removing apparatus can be obtained.

Next, as a means for controlling the precipitation rate in the boron-containing water treatment apparatus 1 of Embodiment 1, a specific example of a method by pH control will be shown.

FIG. 6 is a diagram showing the change over time of the pH of the alkaline solution during the precipitation of insoluble matter. Specifically, Solution No. 1 shows the pH change of the solution when sodium hydroxide is experimentally added, which corresponds to the state inside the precipitation tank 5 in a state where the transport pumps 4 and 6 of the boron-containing water treatment apparatus 1 are stopped.
At this time, the solution No. The initial water quality of No. 1 was pH 8.2 and electric conductivity 4.3 S / m (salt concentration 2.67%). Addition of sodium hydroxide was started at about 3 minutes along the horizontal axis (time axis) in FIG. By adding sodium hydroxide, the pH increased to about 10.9 and the precipitation reaction started. Thereafter, the pH gradually decreases because Mg (OH) ₂ particles are generated by the reaction of Mg ²⁺ and OH ⁻ , and OH ⁻ decreases as precipitation proceeds.

As a result of the experiment, a pH reduction rate of 0.02 per minute corresponds to a deposition rate of 0.003 mol / (L · min), and a pH reduction rate of 0.07 per minute is 0.00. It was found to correspond to 02 mol / (L · min). Since the actual seawater composition is complex, the relationship between the two is not a simple proportional relationship, but the corresponding relationship tends to increase monotonously.
At the initial stage of the reaction, the alkaline chemical pump 12 is controlled to adjust the pH to the range of 10.0 to 11.5, whereby the deposition rate is 0.003 mol / (L · min) or more and 0.02 mol / ( L · min) or less. Although the pH of the solution gradually decreases, by maintaining the pH decrease rate in the range of 0.02 to 0.07 per minute, the precipitation rate is 0.003 mol / (L · min) or more and 0.02 mol. / (L · min) or less can be held.

In the example of FIG. 6, alkali was added to maintain the pH decrease rate for 10 minutes immediately after the start of the reaction at 0.02 to 0.05 per minute. As a result, the pH after 20 minutes was 10.6.
On the other hand, the pH after 40 minutes was 10.5, the pH after 60 minutes was 10.4, and the pH decrease was very slow. This is because the reaction was almost completed in about 30 minutes, as is apparent from FIG. Since the actual boron-containing water treatment apparatus 1 operates the transport pumps 4 and 6 and continuously treats seawater, the reaction as shown in FIGS. 4 and 6 proceeds depending on the residence time in the precipitation tank 5. To do.

In this experiment, since the precipitation rate of insoluble matter could be maintained within an appropriate range, a value of 4 mg / L could be obtained as the boron reduction amount. On the other hand, in an experiment conducted separately and maintaining the pH at a constant value of 10, or maintaining the pH at 11, the boron decrease amount was only 1 mg / L.

When performing a continuous process with the boron containing water processing apparatus 1 of FIG. 1, pH of the specific location of the precipitation tank 5 is a steady state. For example, when pH meters are installed at equal intervals along the flow of water at the center of the precipitation tank 5 elongated in the flow direction, the pH value seen from the upstream side decreases at a constant rate per length in the flow direction. It is desirable to go. For example, assuming that the total length L [m] along the flow of the deposition tank 5, the flow rate Q [L / min] of the liquid to be treated, and the cross-sectional area S [m ² ] of the deposition tank 5, the flow rate of the liquid to be treated is v = Q / S [m / min] is given, and the length along the precipitation tank 5 corresponding to the reaction time of 1 minute is v [m].

In addition, when a normal cylindrical tank is used for the precipitation tank 5 and seawater is poured from the bottom of the tank and discharged from the top of the tank, the volume from the bottom of the tank to the height at which the pH electrode 8 is installed is divided by the inflow rate. By defining this value as the average residence time, the rate of pH reduction may be determined. In addition, a reasonable residence time may be calculated as appropriate according to the configuration of the apparatus. In order to advance the initial reaction, the residence time in the precipitation tank 5 is required to be 10 minutes or more, but 30 minutes at the maximum is an appropriate range due to the restriction of the scale of the equipment.

The appropriate range of pH control does not depend on the concentration of boron contained in seawater. In order to sufficiently ensure the efficiency of boron incorporation into the precipitate and to achieve the practically required boron removal capability, the initial pH of the reaction with an average residence time of 10 minutes or less is 10.0 to 11.5, If the reaction conditions in the precipitation tank 5 are adjusted by controlling the operation of the drug pump 12, the transport pump 4 and the transport pump 6 so that the rate of pH decrease is in the range of 0.02 to 0.07 per minute. Good. This makes it possible to control the generation rate of insoluble matter to be equal to or lower than the diffusion rate of boron, and to sufficiently incorporate boron into precipitate molecules or aggregates. In order to process seawater continuously, each conveyance pump is continuously operated and operated so as to meet the above-described conditions.

By using this method, boron can be sufficiently taken into precipitate molecules or aggregates, so that boron can be efficiently removed without using a magnesium compound added separately. As a result, it becomes unnecessary to provide the Mg chemical tank, the Mg chemical pump, and its control mechanism. In addition, since the amount of precipitates necessary for removing boron can be adjusted to a minimum, the size of the particle separation tank 7 can be reduced, the mechanism can be simplified, and the maintenance frequency can be reduced. Moreover, in the intermediate tank 14, the amount of particles to be re-dissolved can be suppressed as a measure against clogging of the reverse osmosis separation module.
In addition, the temperature of said to-be-processed water is based on 20 degreeC. As the temperature increases, the diffusion rate of boron increases, and as the temperature decreases, the diffusion rate decreases. Therefore, the optimum precipitation rate range shifts depending on the temperature. Water temperature 20 ° C. water relative diffusion coefficients D ₂₀ relative to the as approximate function of the temperature T [° C.], it can be described as follows.

By substituting the actual water temperature as the temperature T in this equation to obtain D ₂₀ (T) and multiplying the above precipitation rate as a correction coefficient determined by the temperature T, an appropriate precipitation rate region is set to 0.003 · D. ₂₀ (T) [mol / (L · min)] or more and 0.02 · D ₂₀ (T) [mol / (L · min)] or less. If the precipitation tank 5 of the boron-containing water treatment apparatus 1 includes a thermometer for measuring the water temperature and the controller 10 reads the temperature information measured by the thermometer and performs the calculation, the above correction is performed. It can be done automatically. Equation (4) is the same as equation (1).

Embodiment 2. FIG.
The inventors also studied the process of incorporating boron into the precipitate by a coprecipitation method, and the main precipitate was determined by the ratio of magnesium ion concentration (Mg ²⁺ ) and hydroxide ion concentration (OH ⁻ ) in the liquid to be treated. It was elucidated that the production amount of cereals changed greatly. In Embodiment 2, boron removal performance when the insoluble matter generation rate is changed in the boron-containing water treatment apparatus 1a having a mechanism for adding a magnesium salt to Embodiment 1 shown in FIG. This will be explained in detail, including theoretical considerations.

FIG. 7 shows an example of a boron-containing water treatment apparatus 1a used in the present invention. A magnesium chemical tank 11a is connected to the precipitation tank 5 via a chemical pump 12a in order to supply magnesium salt to the pretreated seawater, and the precipitation of insoluble matter from the seawater is promoted by adding the magnesium salt. The controller 10 determines the state of the precipitation reaction from the change in pH over time, and controls the operation of the drug pump 12, the drug pump 12a, the stirring motor 9 and the transport pump 4 as necessary, so that the precipitation in the precipitation tank 5 is performed. Control the reaction. In the precipitation tank 5, boron is removed from the seawater by efficiently incorporating boron into the molecules of the precipitate or into the aggregates thereof. The configuration other than the drug pump 12a and the magnesium drug tank 11a is the same as that of the first embodiment.
In the boron-containing water treatment apparatus 1a of the second embodiment, the molar ratio of the magnesium ion (Mg ²⁺ ) added to the aqueous solution and the hydroxide ion (OH ⁻ ) is in the range of 2: 1 to 1: 2 (hydroxylation). Therefore, the boron removal rate can be maximized while suppressing the amount of insoluble matter generated within an appropriate range.

Example 2
In this example, an example is shown in which an experiment was conducted on the boron removal performance in the precipitation tank 5 of the boron-containing water treatment apparatus 1a shown in the second embodiment. In this experiment, it was examined in detail how the deposition rate and the boron removal amount change by changing both the addition amount of both the magnesium salt and the alkaline agent.

8, OH of insolubles of boron removal performance in Example 2 ^- is a graph showing the concentration-dependent. Solution No. 1 in Table 1 3 shows the precipitation rate R at the initial stage of the reaction when the concentration of sodium hydroxide to be added is changed with respect to 3 (initial Mg concentration 0.054 mol / L). FIG. 8 shows that the initial precipitation rate R increases in direct proportion to the sodium hydroxide concentration. The same results as in FIG. 8 were obtained when potassium hydroxide was used in addition to sodium hydroxide.

FIG. 9 is a diagram showing the Mg ²⁺ concentration dependence of the insoluble matter in boron removal performance. Solution No. 3 shows the precipitation rate R when 0.058 mol / L of sodium hydroxide is added and the concentration of magnesium added is changed with respect to 3 (initial Mg concentration 0.054 mol / L). As the magnesium salt, the primary reagent magnesium chloride was used. FIG. 9 shows that the deposition rate R increases in direct proportion to the sodium hydroxide concentration.

From the above results, it was found that the precipitation reaction of the insoluble matter produced in Example 2 was proportional to the hydroxide ion (OH ⁻ ) concentration and the magnesium ion (Mg ²⁺ ) concentration. FIG. 10 is a diagram showing the concentration product dependence of the Mg ²⁺ concentration and the OH ⁻ concentration of the insoluble matter in the boron removal performance. The relationship between the product of Mg ²⁺ concentration and OH ⁻ concentration and the precipitation rate is also proportional. From this, the desired deposition rate can be obtained by controlling the product of Mg ²⁺ concentration and OH ⁻ concentration based on the result of FIG.

As described in Example 1, regardless of the boron concentration contained in seawater, the precipitation rate is 0 in order to sufficiently secure the boron incorporation efficiency into the precipitate and to achieve the practically required boron removal capability. It is desirable to control to 0.003 mol / (L · min) or more and 0.02 mol / (L · min) or less. In the boron-containing water treatment apparatus 1a according to the second embodiment, the product of Mg ²⁺ concentration and OH ⁻ concentration contained in the solution in the precipitation tank 5 is 1.0 × 10 6 by controlling the drug pump 12 or the drug pump 12a. It is desirable to control within a range of ⁻³ to 6.78 × 10 ⁻³ (mol ² / L ² ). FIG. 10 shows a recommended ion concentration product range OP2.

In addition, the temperature of said to-be-processed water is based on 20 degreeC. As the temperature increases, the diffusion rate of boron increases, and as the temperature decreases, the diffusion rate decreases. Therefore, the optimum precipitation rate range shifts depending on the temperature.
By substituting the actual water temperature as the temperature T in Equation 3 to obtain D20 (T) and multiplying the above precipitation rate, an appropriate precipitation rate region is calculated as 1.0 × 10 ⁻³ · D ₂₀ (T). It can be determined as [mol / (L · min)] or more and 6.78 × 10 ⁻³ · D ₂₀ (T) [mol / (L · min)] or less.
If the precipitation tank 5 of the boron-containing water treatment apparatus 1a includes a thermometer for measuring the water temperature and the controller 10 reads the temperature information measured by the thermometer and performs the calculation, the above correction is performed. It can be done automatically.

FIG. 11 is a diagram showing the relationship between the boron reduction amount and the addition amount of magnesium ions added at an equimolar ratio with the hydroxide ions, and also shows the amount of precipitates. The amount of boron decrease is determined according to solution No. 3 (initial Mg concentration 0.054 mol / L), the drug pump 12 or drug pump 12a is controlled to increase the respective addition amount while maintaining the molar ratio of Mg ²⁺ and OH ⁻ at 1: 1. The horizontal axis is converted to magnesium ions.

The amount of precipitate increased in direct proportion to the amount of magnesium ion added, whereas the amount of boron decrease tended to be saturated. When the magnesium ion addition amount is 1.0 g / L, the decrease amount of boron is 3.9 mg / L, and when it is more than that, the boron removal amount gradually increases with the addition of Mg ion to 4.6 mg / L. It was. This corresponds to a removal rate of 92% for an initial concentration of 5 mg / L. At this time, the amount of boron remaining in the water to be treated is very small at 0.4 mg / L, realizing an excellent boron removing action. In addition, when the magnesium ion addition amount is 1.0 g / L or more, the decrease amount of boron does not increase for the increase of the precipitate amount, but this is because the precipitation rate is 0.01 mol / (L · min) or more. Is the cause.

Since sea water sometimes contains a buffer substance that consumes OH ⁻ and a component that generates a scale by reacting with Mg, the molar ratio of Mg ²⁺ and OH ⁻ is practically 2: 1 to 1. : It is desirable to add to the precipitation tank 5 while maintaining at 2.
According to the second embodiment of the present invention, the molar ratio of magnesium ions to hydroxide ions added to the aqueous solution is controlled in the vicinity of the reaction stoichiometric ratio, so that the amount of main precipitate containing boron is controlled to control boron. There is an unprecedented remarkable effect that the removal amount can be maximized. That is, in the precipitation tank 5 of the boron-containing water treatment apparatus 1a, by adding magnesium ions and alkalis under the above conditions, it is possible to realize an excellent treatment in which the boron removal rate exceeds 90% in normal seawater. it can. In addition, the amount of magnesium compound input can be minimized, and the amount of insoluble matter to be generated can be minimized, thereby enabling processing with a low running cost.

Embodiment 3 FIG.
FIG. 12 is a configuration diagram showing an example of the boron-containing water treatment device 1b used in the third embodiment, which is different from the second embodiment in that an acid chemical tank 11b and a chemical pump 12b are used. . The acid medicine tank 11b and the medicine pump 12b are used for supplying acid to the pretreated seawater, and the acid medicine tank 11b is connected to the precipitation tank 5 through the medicine pump 12b. Here, the controller 10 controls the operation of the drug pump 12b as needed from the time change of the pH detected by the pH electrode 8. For example, when the rate of decrease in pH is slower than 0.02 to 0.07 per minute, it can be maintained within the above preferred rate range by adding acid. As a result, the generation rate of insoluble matter can be controlled to be lower than the diffusion rate of boron, and boron can be sufficiently taken into the precipitate molecules or aggregates. Can be bigger.

As the type of acid to be added, in addition to inorganic acids such as hydrochloric acid, sulfuric acid and nitric acid, low molecular organic acids such as acetic acid and citric acid can be used.

Further, in addition to the configuration of the third embodiment, as described in the second embodiment, the drug pump 12a for supplying the magnesium salt and the magnesium drug tank 11a may be connected to the precipitation tank 5 in parallel. . Thereby, the degree of freedom of pH adjustment is increased, and it becomes easy to optimize the process of incorporating boron into the aggregate.

The present invention is applicable to seawater desalination treatment.

1, 1a, 1b Boron-containing water treatment device 2 Water storage tank 3 Pretreatment tank 4 Transport pump 5 Precipitation tank 6 Transport pump 7 Particle separation tank 8 pH electrode 9 Stirring motor 10 Controller 11 Alkaline chemical tank 11a Magnesium chemical tank 11b

Acid chemical Tank

12, 12 a, 12 b Drug pump 13 Transport pump 14 Intermediate tank 15 Transport pump 16 Reverse osmosis separation module 17 Discharge pipe 18 Return pipe

Claims

A first step of adding an alkali to the water to be treated to precipitate insoluble matter;
A second step of separating insoluble matter deposited from the water to be treated;
Including
When the correction coefficient at the temperature T [° C.] determined by the equation (1) is D 20 (T),
The precipitation rate of the insoluble matter in the first step is 0.003 · D 20 (T) [mol / (L · min)] or more, 0.02 · D 20 (T) [mol / (L · min)]. A method for treating boron-containing water, which is adjusted within the following range.
The pH of the water to be treated to which alkali has been added in the first step is adjusted to a range of 10.0 to 11.5 at the beginning of the reaction, and the rate of pH decrease after the start of the reaction is adjusted to 0. The method for treating boron-containing water according to claim 1, wherein the treatment is performed within a range of 02 to 0.07.
The method for treating boron-containing water according to claim 1 or 2, wherein a magnesium compound is added in the first step.
In the first step, the product of the magnesium ion concentration and the hydroxide ion concentration contained in the water to be treated with an alkali added is 1.0 × 10 −3 to 6.78 × 10 −3 (mol 2 / L 2 The method for treating boron-containing water according to claim 3, wherein the treatment is performed within the range of
A third step of re-dissolving insoluble matter remaining in the treated water after the second step;
A fourth step of performing a desalting treatment;
The method for treating boron-containing water according to any one of claims 1 to 4, further comprising:
A precipitation tank for precipitating insoluble matter from a solution in which alkali is added to water to be treated containing boron;
Chemical supply means for supplying the alkali to the precipitation tank;
A pH measurement unit for measuring the pH of the solution in the precipitation tank;
A control unit for controlling the drug supply means based on the result measured by the pH measurement unit to adjust the supply amount of alkali;
Particle separation means for separating insoluble matter precipitated from the solution;
An apparatus for treating boron-containing water.
Means for supplying the water to be treated to the precipitation tank;
Means for stirring the solution in the precipitation tank;
A chemical tank for supplying the alkali;
With
The controller is
A speed at which the water to be treated is supplied to the precipitation tank;
A speed at which the alkali is supplied from the chemical tank to the precipitation tank;
Of controlling at least one of them,
The processing apparatus for boron-containing water according to claim 6.
The controller is
The pH of the solution in the precipitation tank is in the range of 10.0 to 11.5 at the beginning of the reaction, and the pH decrease rate immediately after the start of the reaction is in the range of 0.02 to 0.07 per minute of residence time. The treatment apparatus for boron-containing water according to claim 6 or 7, wherein the solution is adjusted so that
A precipitation tank for precipitating insoluble matter from a solution obtained by adding an alkali and a magnesium compound to water to be treated containing boron;
First chemical supply means for supplying the alkali to the precipitation tank;
A second chemical supply means for supplying the magnesium compound to the precipitation tank;
A pH measurement unit for measuring the pH of the solution in the precipitation tank;
A control unit that controls the first drug supply unit and the second drug supply unit based on the result measured by the pH measurement unit to adjust the supply amounts of the alkali and the magnesium compound;
Particle separation means for separating insoluble matter precipitated from the solution;
An apparatus for treating boron-containing water.
The apparatus for treating boron-containing water according to claim 9, wherein the controller adjusts a molar ratio of magnesium ions to hydroxide ions in the solution within a range of 2: 1 to 1: 2. .
The controller adjusts the product of the magnesium ion concentration and the hydroxide ion concentration in the solution within a range of 1.0 × 10 −3 to 6.78 × 10 −3 (mol 2 / L 2 ). The apparatus for treating boron-containing water according to claim 9 or 10.
The boron-containing water treatment apparatus according to any one of claims 6 to 11, further comprising a third chemical supply means for supplying an acid to the precipitation tank.
An intermediate tank for dissolving the insoluble matter remaining in the water to be treated;
A separation module for performing desalting,
The apparatus for treating boron-containing water according to any one of claims 6 to 12, further comprising: