Salt production method using RO membrane

Abstract

In a salt production system, by appropriately combining an NF membrane and an RO membrane, the problem of clogging caused by scale (SO 4 ) in seawater of the RO membrane is improved, and a salt production system with good salt production efficiency using the RO membrane is provided. 【Solution means】In a salt production system, first, seawater is permeated through an NF membrane to remove most of the scale (SO 4 ) present in the seawater. Next, the seawater is concentrated by the RO membrane, and then the seawater is further concentrated by a BC permeation device using the RO membrane.

Description

Technical Field

[0001] The present invention relates to salt production characterized by using a reverse osmosis membrane method and a brine concentration method for seawater. Method It relates to.

Background Art

[0002] Conventionally, as salt production methods, for example, there have been a vacuum salt production apparatus of Patent Document 1 and a method using RO membranes and NF membranes (Patent Document 2), etc. On the other hand, a salt production method combining a reverse osmosis membrane method using an RO membrane and a brine concentration method using a semi-transparent membrane (Patent Document 3) has been studied. However, since the semi-transparent membrane is clogged by scale (SO 4 ) present in seawater, it has been difficult to put it into practical use as a salt production method.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] The present invention has been made in view of the above circumstances, and improves the problem of clogging due to scale (SO 4 ) present in seawater in the semi-transparent membrane, and solves the problem of providing salt production with significantly better salt production efficiency than conventional production methods. Method To provide.

[0005] The inventor noticed that an NF membrane used in the salt production method disclosed in Patent Document 2 can remove about 98% of scale, and by appropriately combining an RO membrane with this salt production method, a salt production method with significantly improved salt production efficiency was developed. Method was developed. Also, according to this salt production Method method, salt with adjusted Ca and Mg contents, high palatability, and no quality variation is provided.

Means for Solving the Problems

[0006] The invention according to claim 1 is a salt production method for obtaining salt from seawater, Method characterized by sequentially performing the following steps 1 to 6. Method It relates to a salt production method. (Step 1) A step of permeating seawater through an NF membrane to remove SO 4 (Step 2) A step of concentrating the seawater that has permeated through the NF membrane by the reverse osmosis membrane method, which is a step of concentrating the seawater that has permeated through the NF membrane by permeating it through an RO membrane to obtain first concentrated water with a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by permeating it through an RO membrane to obtain second concentrated water with a Baume specific gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water, which is a step of using a vacuum concentration device for the second concentrated water to obtain third concentrated water with a Baume specific gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, which is a step of performing salt production on the third concentrated water using a salt production steam kettle. (Step 6) A step of further performing salt production on the salt produced, which is a step of further performing salt production by separating the produced salt into salt and mineral water using a centrifugal dehydrator.

[0007] The invention according to claim 2 is the salt production method according to claim 1, characterized in that the average pore diameter of the NF membrane in step 1 is 10 nm or less. Method It relates to a salt production method.

[0008] ​The invention according to claim 3 is the salt production according to any one of claims 1 and 2, characterized in that the seawater is deep ocean water. Method It relates to.

Effect of the Invention

[0009] According to the invention according to claim 1, by performing the following steps 1 to 6 in order, salt production from seawater Method can be carried out with significantly higher salt production efficiency than before. In addition, this also has the effect of reducing the energy consumption of the conventional salt production process. (Step 1) A step of permeating seawater through an NF membrane to remove SO 4 (Step 2) A step of concentrating the seawater that has permeated through the NF membrane by the reverse osmosis membrane method, wherein the seawater that has permeated through the NF membrane is concentrated by permeating it through an RO membrane to obtain first concentrated water with a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, wherein the first concentrated water is concentrated by permeating it through an RO membrane to obtain second concentrated water with a Baume specific gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water, wherein a vacuum concentration device is used for the second concentrated water to obtain third concentrated water with a Baume specific gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, wherein salt production is carried out using a salt production steam kettle for the third concentrated water. (Step 6) A step of further producing salt from the salt produced, wherein the salt produced is separated into salt and mineral water using a centrifuge to further produce salt.

[0010] According to the invention according to claim 2, since the average pore diameter of the NF membrane is limited to 10 nm or less, salt production with even higher salt production efficiency than before can be carried out. In addition, this also has the effect of further reducing the energy consumption of the conventional salt production process.

[0011] According to the invention according to claim 3, by using deep ocean water as seawater, salt production capable of producing salt containing a large amount of mineral components​Method can be set as Deep ocean water generally refers to seawater at a depth of 200 m or more. It contains few bacteria and viruses, and in addition to Ca and Mg, it contains many types of mineral components. The reason why deep ocean water contains a large amount of these components is that the water temperature is low compared to surface water, and in addition, the water temperature fluctuation is small.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0013] The salt produced Method by the salt production according to the present invention will be described. The salt produced Method by the salt production according to the present invention is salt obtained from seawater, and the Mg content per 100 g is adjusted to 3100 mg or more and 5000 mg or less, and the Ca content per 100 g is adjusted to 800 mg or more and 3300 mg or less. The salt concentration in seawater varies depending on the location and the like, but is about 3.5%. Mg in the salt is in the form of MgCl 2 or MgSO 4 and exists in this form. The ratio of Mg itself is around 4% of the total salt content. Ca exists in the form of CaSO 4 and the ratio of Ca itself is around 2% of the total salt content. With such a content, it becomes a salt with a highly palatable taste. As the seawater used to obtain such a salt, deep ocean water is preferred.

[0014] The salt production Method according to the present invention will be described step by step with reference to the drawings. FIG. 1 shows a flowchart of salt production Method and FIG. 2 shows a schematic diagram of equipment used from step 1 to step 3.

[0015] (Project 1) Project 1 is a process of permeating seawater (1) through an NF membrane (2) to remove SO 4 . Seawater raw water with a Baumé specific gravity value of 3.5 is used to obtain seawater with a Baumé specific gravity value of 3.0.

[0016] The NF (Nano Filtration) membrane is a semi-transparent membrane with an average pore diameter of 10 nm or less, preferably 8 nm or less, and more preferably 5 nm or less. It has the property of allowing water, low molecular weight organic substances, monovalent ions, etc. to permeate, but making it difficult for medium and high molecular weight organic substances, polyvalent ions, etc. to permeate. Substances that are difficult to permeate through the NF membrane (2) include SO 4 . Since SO 4 is difficult to permeate through the NF membrane (2), by permeating seawater (1) through the NF membrane (2), the obtained NF membrane permeate water will have a significantly reduced content of SO 4 . Generally, Ca combines with SO 4 to precipitate as CaSO 4 . However, since the NF membrane permeate water has a low content of SO 4 , Ca cannot combine with SO 4 , and will exist in the concentrated water in ionic form even during subsequent concentration processes, etc. Therefore, salts and mineral water with a high Ca content can be obtained.

[0017] The supply of seawater to the NF membrane (2) is carried out by a motor and an NF membrane treatment pump. The NF membrane treatment pump can be any pump that can supply seawater at the required pressure. For example, a multistage pump, a plunger pump, etc. can be used.

[0018] The NF membrane tank (21) filled with the NF membrane (2) is, for example, a cylindrical tank with a diameter of 8 inches and a length of 1.6 m, and a flow rate of 74 m per 24 hours 3 . Two or more such tanks are arranged in series or in parallel for use. The NF membrane (2) is a synthetic polymer-based composite membrane or a crosslinked polyamide-based composite membrane, and is filled in the NF membrane tank (21) in the shape of hollow fibers.

[0019] By allowing seawater (1) to permeate through the NF membrane (2) twice, SO that could not be completely removed during the first permeation 4 can be removed, and it is possible to more reliably prevent Ca from precipitating in the subsequent concentration process. Even if the NF membrane is permeated twice, the contents of Mg and Ca do not decrease. Therefore, even in the salt obtained after passing through the NF membrane twice, it is possible to obtain a salt with a high content of Mg and Ca and a high palatability. The permeation of the NF membrane (2) may be performed more than twice. By permeating more than twice, SO 4 can be removed more reliably, and the contents of Mg and Ca do not decrease either. Therefore, it is possible to obtain a salt with a high content of Ca and Mg and mineral water with a high content of Ca.

[0020] In Step 1, seawater with a Baume specific gravity value of 3.0 is obtained from seawater raw water with a Baume specific gravity value of 3.5. The seawater is stored in the permeate storage tank (3).

[0021] The Baume specific gravity value (hereinafter sometimes referred to as "Be'") is a unit used to indicate the concentration of concentrated water in seawater treatment processes, etc. It can be said that the higher the Baume specific gravity value, the higher the concentration.

[0022] (Step 2) Step 2 is a step of concentrating the seawater (permeate) that has permeated through the NF membrane (2) by the reverse osmosis membrane method. It is a step of concentrating by allowing the seawater that has permeated through the NF membrane (2) to permeate through the RO membrane (4) to obtain first concentrated water with a Baume specific gravity value of 7.0 or more and 12.0 or less.

[0023] The RO (Reverse Osmosis) membrane is a semi-transparent membrane with an average pore diameter of about 0.5 nm. Water can permeate through it, but other molecules, etc. are difficult to permeate through it. Therefore, it is suitable for concentrating seawater.

[0024] The reverse osmosis membrane method is a method of concentrating seawater using a reverse osmosis membrane that allows only water and mineral components smaller than a certain size to pass through. Specifically, pressure is applied to seawater to cause it to pass through the reverse osmosis membrane, separating it into permeated water that has passed through the reverse osmosis membrane and concentrated water with reduced water content and increased concentration.

[0025] The supply of seawater that has passed through the NF membrane (2) to the RO membrane (4) is carried out by an electric motor and an RO membrane treatment pump. The RO membrane treatment pump can be any pump that can supply seawater at the required pressure. Similar to the NF membrane treatment pump, a multi-stage pump, a plunger pump, or the like can be used.

[0026] The RO membrane tank (41) filled with the RO membrane (4) is, for example, a cylindrical tank with a diameter of 8 inches and a length of 1.6 m, and a flow rate of 74 m per 24 hours 3 Three or more such tanks are arranged in series or in parallel for use. The RO membrane (4) is a crosslinked fully aromatic polyamide composite membrane and is filled in the RO membrane tank (41) in the shape of hollow fibers.

[0027] As the seawater (1) supplied to the RO membrane (4), deep ocean water is preferred. This is because deep ocean water contains a rich variety of mineral components, so the salt obtained according to this embodiment will also contain a rich variety of mineral components. After passing through the first NF membrane (2) multiple times, it may be concentrated by the RO membrane (4). Even if the permeation through the NF membrane (2) is carried out multiple times, the contents of Mg and Ca do not decrease, and furthermore, SO can be more reliably removed 4 to prevent the precipitation of Ca in the subsequent concentration process. Therefore, salts with a high content of Ca and Mg can be obtained.

[0028] In step 2, first concentrated water with a Baume specific gravity value of 7.0 or more and 12.0 or less is obtained from seawater with a Baume specific gravity value of 3.0 that has passed through the NF membrane (2).

[0029] (Step 3) Step 3 is a step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by permeating it through an RO membrane (5) to obtain a second concentrated water having a Baume specific gravity value of 13.0 or more and 20.0 or less.

[0030] In Step 3, a device (BC permeation device) used in the Brine Concentration method, which is known as a method for concentrating brine, is used.

[0031] Fig. 2 shows a four-stage BC permeation device (51) in which four BC permeation devices (51) are connected in series. The BC permeation device (51) has an RO membrane (5), a first chamber (52) and a second chamber (53) partitioned by the RO membrane (5). The first concentrated water from Step 2 is supplied to the first chamber (52), and by pressurizing the first chamber (52), the water contained in the first concentrated water is permeated through the RO membrane and transferred to the second chamber (53). The first concentrated water concentrated in the first chamber is recovered as the second concentrated water. One BC permeation device (51) may be used, or a plurality of them may be connected in series.

[0032] The concentrated water discharged from the first chamber of the first-stage BC permeation device (51) is sequentially sent to each first chamber (52) so as to be supplied to the first chamber (52) of the second-stage BC permeation device (51) and concentrated. In addition, the permeated water discharged from the second chamber (53) of the last fourth-stage BC permeation device (51) is sent to the second chamber (53) of the second-stage BC permeation device (51). The permeated water discharged from the second chambers (53) of the first-stage, second-stage, and third-stage BC permeation devices (51) is sent to the NF permeated water storage tank (3). By setting the recovery path of the permeated water in this way, it can be recovered without using a pump.

[0033] The RO membrane (5) in Step 3 is a semi-transparent membrane with an average pore diameter of about 0.5 nm. Using the first concentrated water that has permeated through the RO membrane (4) in Step 2 as raw water, the RO membrane (5) generates a second concentrated water with an even higher concentration and discharges diluted water with a salt concentration of about 6.5%. The diluted water is returned to the NF permeated water storage tank (3) again. The RO membrane (4) in Step 2 and the RO membrane (5) in Step 3 may be semi-transparent membranes with the same average pore size or semi-transparent membranes with different average pore sizes.

[0034] The supply of brine (high-concentration salt water) such as the first concentrated water that has passed through the RO membrane (4) in Step 2 to the RO membrane (5) is carried out by a pump for RO membrane treatment. The pump for RO membrane treatment may be any pump that can supply brine at the required pressure. Similar to the pump for NF membrane treatment, a multi-stage pump, a plunger pump, or the like can be used.

[0035] The BC permeation device (51) is, for example, a cylindrical device with a diameter of 10 inches and a length of 1.6 m, and a flow rate of 14 m per 24 hours 3 Four such devices are arranged in series and used. When a flow rate of more than 14 m 3 is to be output, the BC permeation device (51) is added. The RO membrane (5) is a hollow fiber type made of cellulose triacetate and is filled in the BC permeation device (51).

[0036] By concentrating two or more times with the RO membrane (5), a higher-concentration brine can be obtained.

[0037] From the first concentrated water concentrated in Step 2, a second concentrated water with a Baume specific gravity value of 13.0 or more and 20.0 or less is obtained in Step 3. The second concentrated water is stored in the concentrated water storage tank (6).

[0038] In the salt production device of the present application, since about 98% of the scale (SO 4 ) is removed by the NF membrane (2) in Step 1, although the frequency of clogging of the RO membrane (5) in Step 3 is low, it is necessary to wash the RO membrane (5) with water by a cleaning system and perform maintenance when the salt production device stops.

[0039] (Step 4) Step 4 is a step of further concentrating the second concentrated water obtained in Step 3, and is a step of obtaining a third concentrated water with a Baume specific gravity value of 24.0 or more by using a vacuum concentration device (7) for the second concentrated water. As the vacuum concentration device (7), for example, a heat pump system can be used.

[0040] Each of steps 1 to 4 is controlled by a level sensor and is automatically operated.

[0041] The vacuum concentration device (7) will be described. The role of the vacuum concentration device (7) in this salt production device is to obtain third concentrated water with a Baume specific gravity value of 24.0 or more by heating the second concentrated water at a temperature of 80°C or lower while reducing the pressure. The reason for setting the heating temperature of the second concentrated water to 80°C or lower is that if the second concentrated water is heated above 80°C, although the concentration time is shortened, trace components contained in the second concentrated water, such as umami components like amino acids and sugars, may be altered, which may have an adverse effect on the taste of the produced salt, and this is not preferable. The degree of pressure reduction of the second concentrated water is not particularly limited, but it may be reduced to -94 KPa or less, preferably about -98 to -100 KPa. If the pressure reduction exceeds -94 KPa, a long time is required to concentrate the second concentrated water, which is not practical. By cooling the water vapor generated in the process of concentrating the second concentrated water, distilled water can be recovered, and the recovered distilled water can be used as water for pharmaceutical production or water for alcohol production.

[0042] In step 4, third concentrated water with a Baume specific gravity value of 24.0 or more is obtained from the second concentrated water that has passed through the RO membrane (5). The third concentrated water is stored in the water storage tank (8).

[0043] When Be’ is 24.0 or more, the content of Na in the third concentrated water decreases, and the contents of components such as Mg and Ca increase. That is, the contents of Mg and Ca in all mineral components increase.

[0044] (Step 5) Step 5 is a step of obtaining salt from the third concentrated water obtained in step 4, and is a step of producing salt using a salt production steam kettle (9) for the third concentrated water. For example, four salt-making steam kettles (9) with a capacity of 900 L per unit are used. Also, for example, one salt-making steam kettle (9) with a capacity of 3600 L is used.

[0045] In Step 5, brine with Be’ of 29.5% is obtained. For example, 360 kg of brine can be obtained from each of the 900 L-capacity salt-making steam kettles (9). Also, for example, 1440 kg of brine can be obtained from one 3600 L-capacity salt-making steam kettle (9).

[0046] (Step 6) Step 6 is a step of further performing salt production by separating the salt product obtained in Step 5 into salt and moisture (bittern) using a centrifugal dehydrator (10). For example, salt and moisture (bittern) are separated by the high dehydration efficiency due to the centrifugal force caused by rotation at 1000 - 1200 rpm. For example, from the 360 kg of brine, 200 kg of salt and 160 kg of bittern are obtained. Also, for example, from the 1440 kg of brine, 800 kg of salt and 640 kg of bittern are obtained.

[0047] In Step 6, further produced salt and bittern are obtained.

[0048] <Example 1> Table 1 shows data when the permeation order of the NF membrane and the RO membrane is swapped. JPEG0007683869000002.jpg41151 Permeation through the RO membrane after permeation through the NF membrane is denoted as (NF membrane → RO membrane), and permeation through the NF membrane after permeation through the RO membrane is denoted as (RO membrane → NF membrane). Table 1 shows the following results. (1) For both (NF membrane → RO membrane) and (RO membrane → NF membrane), Na, K, Mg, and Ca are higher than in the raw water. (2) However, for (NF membrane → RO membrane), Na, K, Mg, and Ca are higher than for (RO membrane → NF membrane). (3) The Baumé degree is 4.5 for (RO membrane → NF membrane), but it increases to 6.4 for (NF membrane → RO membrane). From the above, (NF membrane → RO membrane) is more preferable than (RO membrane → NF membrane).

[0049] <Example 2> Table 2 shows the test examples related to the salt production of the present invention. Method In Table 2, the data of seawater after Process 1, Process 2, and Process 3 are shown. TIFF0007683869000003.tif54165 The permeation pressures in these Processes 1, 2, and 3 are as follows. Process 1 (NF membrane tank): 2 Mpa or less Process 2 (RO membrane tank): 8 Mpa or less Process 3 (BC permeation device): 7 Mpa or less

[0050] The results in Table 2 are as follows. (1) After Process 1, the Baumé degree has decreased by 0.6. (2) After Process 2, Na, K, Mg, and Ca have increased to about twice that of the raw water, and the Baumé degree has also increased to 8.2. (3) After Process 3, Na, K, Mg, and Ca have further increased to about twice that of after Process 2, and the Baumé degree has increased to 14.9. In this test, the BC permeation device (51) has 4 stages, but if it is changed to 6 stages, the Baumé degree will increase to 17 - 18. (If it is 3 stages, the Baumé degree will increase to 10 - 12.) <Comparison between the present invention and other processes> For comparison of the present invention, the order of the NF membrane tank and the RO membrane tank of the present invention was reversed, and the cases of concentration in the order of RO membrane tank → NF membrane tank → BC permeation device and the case of concentration in the order of RO membrane tank → BC permeation device without using the NF membrane tank of the present invention were carried out. As a result, the following was obtained. 1. When concentrating in the order of RO membrane tank → NF membrane tank → BC permeation device, the Baumé degree after the BC permeation device only increased to the 7th - 8th place. When concentrated in the order of the RO membrane tank → BC permeation device, clogging occurred in the first and second stages of the BC permeation device, and the concentration operation could not be performed.

[0051] By combining steps 1, 2, and 3 in this way, the Baumé degree can be increased, so salt can be produced without much energy or labor in the subsequent steps 4 to 6. Conventionally, since steps 4 to 6 were performed without performing step 3 (BC permeation device) to produce salt, a large amount of energy and labor were required for salt production. In the present invention, by combining the NF membrane capable of removing about 98% of the scale as described above, the RO membrane of the reverse osmosis pressure method, and the RO membrane of the BC permeation device, salt production with extremely high salt production efficiency Method can be obtained.

Industrial Applicability

[0052] The salt obtained by the present invention is used as table salt. In particular, since a salt with a highly palatable taste can be obtained, it can be suitably used in various dishes. In addition, the bitter salt obtained as a by-product by the present invention can be suitably used as an additive for drinking water, a health food, or a plant activator for agriculture.

Explanation of Signs

[0053] 1 Seawater, raw seawater 2 NF membrane 21 NF membrane tank 3 Permeate water storage tank 4 RO membrane 41 RO membrane tank 5 RO membrane 51 BC permeation device 52 First chamber 53 Second chamber 6 Concentrated water storage tank 7 Vacuum concentration device 8 Storage tank 9 Salt production steam kettle 10 Centrifugal dehydrator

Claims

1. A method for producing salt from seawater, comprising the steps of: (Step 1) Seawater is passed through an NF membrane to remove SO ４ The process of removing (Step 2) A step of concentrating the seawater permeated through the NF membrane by a reverse osmosis membrane method, in which the seawater permeated through the NF membrane is concentrated by passing it through an RO membrane to obtain a first concentrated water having a Baume specific gravity value of 7.0 or more and 12.0 or less. (Step 3) A step of further concentrating the first concentrated water, which is a step of concentrating the first concentrated water by permeating the first concentrated water through an RO membrane to obtain a second concentrated water having a Baume specific gravity value of 13.0 or more and 20.0 or less. (Step 4) A step of further concentrating the second concentrated water, using a vacuum concentrator for the second concentrated water to obtain a third concentrated water having a Baume specific gravity value of 24.0 or more. (Step 5) A step of obtaining salt from the third concentrated water, which comprises subjecting the third concentrated water to salt production using a salt production steam boiler. (Step 6) A step of further refining the salt produced by the above process, in which the salt produced by the above process is separated into salt and mineral water using a centrifugal dehydrator, thereby further producing salt.

2. The salt production method according to claim 1, characterized in that the average pore size of the NF membrane in step 1 is 10 nm or less.

3. 3. The method for producing salt according to claim 1, wherein the seawater is deep sea water.

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