JPS5918116B2 - Water production method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for desalinating seawater or brine using a permeable membrane or an ion exchange membrane.

Typical methods for recovering fresh water from seawater or underground brine include evaporation, electrodialysis, and freezing.

The evaporation method has the oldest history, and is technically perfect as a reliable method, and is now being put into practical use in regions with water shortages, such as the Middle East, but it has reached its limits in terms of reducing the cost of producing fresh water However, the utilization of raw seawater is low and more than half of it is wasted.

In addition, electrodialysis uses an ion exchange membrane to electrically move coexisting ions in water, collect water with weak ions, and recover drinking water through multiple stages of desalination.
According to Faraday's law, the amount of electricity consumed increases in proportion to the ion concentration, and it is not easy to adapt to highly concentrated salts such as seawater and brine, and the running cost is relatively high, making it uneconomical.

Moreover, since desalination is progressed by passing ions through the membrane, various electrochemical scale problems often occur.For example, if contaminants adhere to the membrane surface and cause polarization, the membrane Electrolysis may occur within the tank and it may become alkaline, which may generate carbonate-based scale or hydroxide-based scale.

In addition, reverse osmosis membrane method has recently been attracting attention.

This method uses a semipermeable membrane to force desalinated water out of the semipermeable membrane by applying a pressure higher than its inherent osmotic pressure to the salt water. This method requires very little energy for desalination and is easy to operate, so it is hoped that it will be put into practical use in the future.

However, since the membrane separates ion-sized particles and the medium, water, is separated using pressure, various problems occur on the membrane surface.

That is, it is a concentration polarization phenomenon on the membrane surface, and furthermore, it is a problem of contamination on the membrane surface.

This image phenomenon is closely related; if concentration polarization is likely to occur, membrane surface contamination is likely to occur, and if contaminants adhere to the membrane surface, concentration polarization is also likely to occur.

Membrane surface contamination can be broadly divided into two types: one is when suspended solids or colloidal substances already present in the liquid to be treated immediately adhere to the membrane surface as the medium permeates through the membrane, preventing membrane permeation. In the other case, the substance existed as a soluble substance in the liquid to be treated, not as a suspended or colloidal substance, but as concentration progresses, the concentration increases and reaches the supersaturated region, where it finally precipitates. They may adhere to the membrane surface or precipitate on the membrane surface, thereby blocking membrane permeation.

In particular, when a concentration polarization phenomenon occurs, a supersaturated concentration tends to occur near the membrane surface, and a precipitation phenomenon tends to occur.

To deal with the former problem, conventional methods of pretreatment for desalination using reverse osmosis membranes include coagulation sedimentation, sand filtration, and if necessary, activated carbon treatment or precision filtration using precoat filters or cartridge filters. ,
Some are in practical use.

In other words, most of the solids are removed by coagulation sedimentation and sand filtration,
Furthermore, it may be necessary to perform precision finishing using a pre-coat filter or cartridge filter to remove organic matter,
When it is necessary to remove chemicals, it is customary to perform filtration using activated carbon or the like.

In addition, for the latter problem, removing SiO2 or
Partial softening of Ca'', Mg'', SO4-rHCO3- etc. is performed using an ion exchange resin, and the flow rate of the liquid passing through the membrane surface is increased to prevent concentration polarization at the membrane surface. method is implemented.

The present invention aims to improve the separation efficiency of subsequent permeable membranes and electrodialysis by simultaneously removing these contaminating factors very easily.

The present invention involves adding at least one precipitant selected from carbonate, hydroxide, and phosphate of barium as a hardness ion removing precipitant to seawater or brine to form a precipitate, and then separating the precipitate. and, if necessary, the pH of the separated liquid
Set the value within the range where alkaline scale does not precipitate (approximately 4 to approximately 6
), and then separated into a concentrated liquid and a membrane-permeated liquid using a permeable membrane or an ion exchange membrane.

That is, the present invention deals with suspended solids, which are membrane-fouling factors,
In addition to chemically and physically separating colloidal substances and scale substances generated when concentrating through the membrane to prevent contamination of the membrane, the ion concentration is further reduced through the chemical and physical treatment. , thereby reducing the ion concentration load in membrane separation, which will be described later.

To explain the embodiment of the present invention with reference to the drawings, in the first illustrated example, brine 1 to be treated is placed in a reaction tank A.
- Here, a hardness ion removing precipitation forming agent 2 for removing scale components is injected and reacted for a predetermined time.

Reaction tank A is equipped with a stirrer 3, and is a reaction/separation device that combines a circulating draft tube type reaction tank and a subsequent coagulation sedimentation tank B, especially one that can circulate slurry in the reaction system. is convenient.

In this case, the hardness ion removing precipitant used is Ba(OH)2 for the purpose of removing temporary hardness, and BaC for the purpose of removing permanent hardness.
Os is used and Ca++, which is a temporary hardness scale component, by the following reactions, respectively.

HCO3 etc. have a permanent hardness of C a +, SO
4// etc. can be removed.

Ca (HCO3)2 +Ba (OH)2 → CaCO
3↓+Ba COa↓+2H20 Ca804+BaCO3→Ba5o4↓+Ca COa
↓Additionally, other precipitation agents other than those listed above include PO4 salt, H
Barium phosphates such as 2PO4 salt, HPO, salt, etc. are also available.

Note that when bicarbonate-based calcium and sulfate-based calcium are simultaneously removed using B a (OH) 2 and B a COa, it is advisable to use this solution as a mixed solution.

In this way, in the method of the present invention, all solutes that can become scale components contained in the liquid to be treated can be completely separated and removed as precipitates by adding chemicals, thereby eliminating the generation of by-product salts that occur in normal chemical reactions. Therefore, the concentration of the solute itself in the liquid to be treated can be reduced.

Therefore, since the osmotic pressure of the liquid to be treated to be supplied to the subsequent reverse osmosis membrane device can be reduced, it is possible to increase the permeation effect of the device.

In addition to simply precipitating and separating solutes that can become scale components, in the present invention, colloidal substances already suspended in the liquid to be treated, such as fine precipitated colloids such as metal hydroxides and sulfides, silicic acid, Alternatively, it is also possible to co-precipitate and separate clay minerals, suspended solids, etc., and it can be said to be an extremely effective pretreatment for the treatment of both layers.

Next, if necessary, a precipitation separation aid 5 such as a flocculant is added to the reaction slurry 4 in the reaction tank A, and the slurry is allowed to flow into the coagulation-sedimentation tank B or flotation separation device to separate the precipitate.

As the precipitation separation aid, inorganic metal salts can be used in addition to organic polymer compounds.

The concentrated sludge 6 precipitated in this coagulation-sedimentation tank B is discharged outside the system, while the separated liquid 7 is sent to the sand filtration tank C to remove suspended components in the liquid, and the filtrate 8 is collected in a receiving tank. .

Next, the filtrate 8 in the receiving tank is pumped to the reverse osmosis membrane device E by the pump P-1, where it is separated into a concentrated liquid 9 and a membrane permeated liquid 10.

Since this concentrated liquid 9 has pressure, it is possible to rotate a turbine and recover about 30 to 40% of the energy.

Although it may not be necessary depending on the membrane material, water recovery rate of the system, and liquid properties of the liquid to be treated, in many cases reverse osmosis membrane equipment E
When water is passed through, a pH adjuster 11 is added.
For example, HCl, H2SO4゜N a OH, Ca (O
H) 2 etc. are used, but acid agents are often used.
Adjust the pH value to a range where alkaline scale does not precipitate, for example 4 to 6, and add HCO3-+, co3-+ to the solution.
, etc. remain, CO2 can be removed by aeration after acid injection.
is effectively removed.

In addition, the membrane materials of the reverse osmosis membrane device E include organic membranes such as acetyl cellulose, ethyl cellulose, polyester, polyacrylonitrile, and polyamide, and inorganic membranes such as carbon material membranes, graphite oxide membranes, and dynamic membranes. A membrane can be used, and the membrane module can be of a hollow type, a tube type, a spiral type, a filament type, a flat plate type, or a rond type.
0 kgf/ffl, preferably 20/70 kgf/ffl.

In the second illustrated example, an ultrafiltration membrane device F is installed immediately before the reverse osmosis membrane device E, and the filtrate in the receiving tank of the first illustrated example is pumped to the pump p-1.
Once introduced into the ultrafiltration membrane device F, the concentrated liquid 12
is discharged out of the system, and the membrane permeate liquid 13 is sent via a receiving tank G to a reverse osmosis membrane device E by a pump P-2.
The rest is the same as the first illustrated example.

The membranes used in the ultrafiltration membrane device F are as follows:
In addition to ultrafiltration membranes, microporous membranes of 0.1 to 2.5μ can be used, and the membrane material may be the same as the reverse osmosis membrane described above, and a tube-type membrane module is most suitable. The operating pressure is 0.2 to 50 kgf/cIIt, preferably several to 15 kgf/cfIl.

In addition, in the examples shown in FIGS. 1 and 2 above, a reverse osmosis membrane device was finally used, but when an electrodialysis device equipped with an ion exchange membrane was used instead, positive and anion exchange membranes were used. It is used to perform direct current electrolysis.

In addition, in the embodiment of the present invention, a combination of a reaction tank and a separation device is shown in removing scale components, but the reaction and separation are not limited to this combination. The reaction liquid is brought into contact with a contact granule such as a granule mainly composed of CaCO3, a granule mainly composed of calcium phosphate such as phosphate rock, or an ore powder mainly composed of B a S 04. Alternatively, the separated liquid from the coagulation-sedimentation tank B' can be flowed through the packed bed filled with the contact particles to promote the reaction of the scale components and extremely reduce dissolved calcium ions. When used in a device, the agitation reaction is carried out in the coexistence of contact particles in a reaction tank equipped with a paddle-type stirrer, and the reaction slurry that does not contain the contact particles in the upper layer is sent to a separation device such as a sedimentation tank for precipitation treatment. Alternatively, a fluidized bed filled with contact particles may be formed, and the upper layer of the reaction slurry, which does not contain the particles, may be sent to a separation tank such as a settling tank for treatment.

Alternatively, the supernatant liquid that has passed through a normal reaction tank-separation layer may be allowed to flow directly through a packed bed filled with contact particles to cause a contact reaction.

As described above, the present invention enables barium carbonate, barium hydroxide, barium phosphate, etc. Scale-generating components such as Ca'', SO, -THC03-, etc. caused by injection are chemically removed by precipitation, and membrane-contaminating substances in the liquid to be treated are separated and removed physically and chemically. Afterwards, the pH is kept within a range where alkaline scale does not precipitate, and the treatment is carried out by a desorption method, in which scale components are chemically removed along with turbid components, etc., using an extremely easy operation without the need for special equipment.
It can be physically separated, and the solute that can become a scale component can be separated and removed as a precipitate without causing scale problems or by-product salt generation in the removal method, so it is possible to reduce the solute concentration in the liquid to be treated, i.e. It is possible to reduce the osmotic pressure, reduce the load on the reverse osmosis membrane device based on the solute concentration, and furthermore, it is possible to extremely increase the water recovery rate.

Next, examples will be shown.

17-20 pIl as suspended solids, 25 pIm as total SiO2 Ca + 10 concentration 145-150 pIl [l
.

SO, - concentration 470pIllll, other C1l,
“Na”.

1100-1300 as TDS containing Mg + 10 etc.
A mixed slurry of 50 g/l of Ba(OH)2 and BaCOs (1:4.2) was added to brine with a pH of 7-8/25°C and a liquid temperature of 45-50°C in an amount equivalent to 10 Ca concentrations. After stirring for about 25 minutes in a slurry circulation type reaction tank with a slurry concentration of 25 g/l, 1 pp ID of acrylic polyamide was added and precipitated and separated. At 30°C, the Ca+ concentration is 1.
0~2.01)I)+1180. ”Concentration is 190ppH
Met.

After this separated liquid was sand filtered at a superficial velocity of 7 m/h,
Hydrochloric acid was injected into the filtrate to adjust the pH to 5.7/30°C.

Almost no suspended solids were detected in this filtrate, and 5
iQ2 was about 7-ioppm.

The TDS of this liquid was 600-800 degrees.

Next, this liquid was transferred to an acetyl cellulose pre-type reverse osmosis membrane module with a NaC# removal rate of 90.3% at a pressure of 30 kg f.
/cwt, water was passed through the liquid at a temperature of 45 to 47°C to separate the concentrate and the membrane permeate, but the Cl+ in the raw brine and the membrane concentrate was
Even at a concentration ratio of 11 to 12, gypsum did not precipitate on the membrane surface and a high water recovery rate could be obtained.

On the other hand, a positive and anion exchange membrane electrodialyzer was used in place of the reverse osmosis membrane module, but the operation was extremely good and no increase in electrical resistance due to scale trouble was observed.

In addition, regardless of the method of the present invention, by simply adding a flocculant to coagulate sedimentation, sand filtration, and pH adjustment, gypsum becomes saturated at a Ca + 10 concentration ratio of 5.3 to 5.6. Although the supersaturated concentration was maintained, gypsum was gradually deposited on the membrane surface, and the amount of water permeating through the membrane began to decrease.

[Brief explanation of drawings]

FIG. 1 is a system explanatory diagram showing one embodiment of the present invention,
Adding hardness ion removing precipitation agent to seawater or brine,
A method of forming a precipitate, separating it, adjusting the pH, and separating it into a concentrated liquid and a membrane-permeated liquid using a reverse osmosis membrane is shown. FIG. 2 is also a system explanatory diagram showing one embodiment of the present invention,
In the first illustrated example, a method is shown in which ultrafiltration is performed immediately before separation using a reverse osmosis membrane. A: Reaction tank, B: Coagulation sedimentation tank, C
...Sand filter tank, D...Receiver tank, E...
... Reverse osmosis membrane device, F ... Ultrafiltration device, G
...Receiving tank, 1...Brine, 2...
・・Hardness ion removing precipitation agent, 3・・Stirrer, 4・・Reaction slurry, 5・・Precipitation separation aid, 6・・・Concentrated sludge ,7...
Separated liquid, 8...filtrate, 9...concentrated liquid,
10... Membrane permeate liquid, 11... pH adjuster, 12... Concentrate liquid, 13... Membrane permeate liquid.

Claims (1)

[Claims] 1. After adding at least one precipitation-forming agent selected from barium carbonate, hydroxide, and phosphate as a hardness ion-removing precipitation-forming agent to seawater or brine to form a precipitate. This is separated, and if necessary, the pH value of the separated liquid is adjusted to a range in which alkaline scale does not precipitate, and then separated into a concentrated liquid and a membrane-permeated liquid using a permeable membrane or an ion exchange membrane. Method. 2. The watering method according to claim 1, wherein the pH value of the separated liquid is adjusted to 4 to 6.