JPS61263607A - Electrodialytic method - Google Patents

Abstract

PURPOSE:To prevent the contamination of an electrodialyser by the contaminants in a raw soln. for a long period by primarily filtering an electrolyte soln. contg. a flocculant and an oxidizing substance, adding a reducing substance to the filtrate, secondarily filtering the filtrate and then carrying out electrodialysis. CONSTITUTION:A flocculant such as ferric chloride and an oxidizing substance such as sodium hypochlorite are added to a raw soln. such as seawater and the soln. is primarily filtered by a primary sand filter. A reducing agent such as sodium bisulfite is added to the filtrate to neutralize the residual chlorine, etc., and the filtrate is filtered by the secondary sand filter. The filtrate is sent to an ion-exchange membrane electrodialyser and electrodialysis is carried out.

Description

Detailed Description of the Invention (Technical Field of the Invention) The present invention relates to a method for electrodialyzing an electrolyte solution C (hereinafter also referred to as a raw material solution) using an ion exchange membrane electrodialysis device (hereinafter also simply referred to as a dialysis device). In particular, the present invention relates to a pretreatment method for a raw material solution that is suitable for preventing contamination of a dialysis apparatus by contaminants contained in the raw material solution and for stably performing electrodialysis.

(Prior art and its problems) Electrodialysis is used, for example, in the field of concentrating seawater to obtain salt;
It is used in a variety of fields, including the field of desalinating seawater to obtain drinking water, and the field of separating and purifying solutions containing impurities. In recent years, in dialysis machines used in these fields, attempts have been made to reduce the electrical energy by narrowing the layer spacing to q#.

On the other hand, the raw material solution supplied to such a dialysis machine contains contaminants such as various microorganisms, suspended matter (hereinafter referred to as S8), and colloidal substances, so it is not directly supplied to the dialysis machine. When the membrane is supplied to the membrane, the dialysis performance deteriorates due to the contaminants staining the membrane surface and clogging the flow path. Therefore, the raw solution is generally filtered before being supplied to the dialysis machine.

The filtration method is generally to simply pass the raw material solution through a filter, but recently, flocculants and oxidizing substances have been added for the purpose of promoting the aggregation of SS contained in the raw material solution and for the purpose of sterilizing microorganisms. There is also a method in which the mixture is added and then filtered using a filter such as a sand filter.

Furthermore, the filtration method is such that the filtrate after the primary filtration as described above is free from dirt on the filter layer, flocculants and S, which are generated after backwashing the filter layer.
It is generally adopted to subject the filtrate to secondary filtration due to contamination with S-based glue.

However, even when electrodialysis is performed using a dialysis machine after neutralizing the multi-stage filtered raw material solution as described above, there is a problem in that the dialysis performance deteriorates after a certain period of time.

Means for Solving Problem C) The present inventors have conducted various studies regarding the above problem. As a result, although the amount of flocculant added together with the oxidizing substance was generally controlled by the properties of the raw material solution, it was difficult to accurately control the amount, so excessive flocculant was mixed into the raw material solution. It has been found that its presence in the filtrate after secondary filtration is unavoidable. Next, it has been found that when the filtrate is neutralized with a reducing substance and then supplied to a dialysis apparatus, the flocculant present in the filtrate forms a precipitate, which causes a decrease in dialysis performance.

The present invention is based on the above-mentioned findings, and consists of firstly filtering an electrolyte solution containing a flocculant and an oxidizing substance, then adding a reducing substance to the filtrate, performing secondary filtration, and then performing ion exchange membrane electrodialysis. This is an electrodialysis method characterized by supplying it to a device and performing electrodialysis.

In the present invention, first, a raw material solution containing a flocculant and an oxidizing substance is primarily filtered. That is, generally, a raw material solution containing contaminants is added with at least a flocculant, an oxidizing substance, and a pH control agent if necessary, and then supplied to a fire source filter and filtered.

The raw material solution mentioned above is generally seawater, but underground brine, lake water, industrial wastewater, etc. can also be used. Further, as the flocculant, ferric chloride is suitable, but other known agents such as iron compounds such as ferric sulfate and aluminum compounds such as aluminum sulfate can be used without particular limitation.

The amount of flocculant added to the raw material solution is appropriately selected depending on the concentration of SS in the raw material solution, but if it is too small, the flocculation effect will be insufficient, and if it is present in excess, the addition of the reducing substance described below will occur. As the amount increases, the filtration process after addition becomes complicated. Furthermore, the oxidizing substance to be present in the raw water together with the flocculant is a substance that increases the flocculating effect of the flocculant and has a sterilizing effect, and generally has a standard redox potential of 900 tnV or more. For example, chlorine: hypochlorous acid; hypochlorites such as sodium hypochlorite, potassium hypochlorite, and calcium hypochlorite;
Chlorine dioxide, etc., especially chlorine-free hypochlorous acid, or its salts, exhibits particularly excellent pollution prevention effects when combined with the reducing substances mentioned below, and chlorine, etc. generated by electrodialysis can also be recycled. Preferably.

The amount of the oxidizing substance added to the raw material solution varies depending on the properties and content of contaminants contained in the raw material solution, and cannot be generally limited, but if the amount added is too small, the oxidation effect may not be sufficient. Therefore, even if a reducing substance described below is added, a sufficient pollution prevention effect cannot be obtained.

Furthermore, even if the amount of oxidizing substances added is increased too much, the effect will reach a plateau and the amount of reducing substances required to neutralize the oxidizing substances will increase, making it uneconomical and possibly requiring secondary filtration. It becomes complicated. Therefore, in general, if a busy oxidizing substance is added so that the concentration of the oxidizing substance in the raw water becomes 1 to 10 ppm, preferably 2 to 7 ppm, the treatment time of the raw material solution with the flocculant and oxidizing substance will be longer. Although it is moderately effective, it causes the long length and intermediate tank to become huge.
10 to 60 minutes is appropriate.

As the primary filter used for primary filtration of the raw material water and the secondary filter for secondary filtration described below, any known filter can be used without particular limitation, but sand filters are particularly preferred. Any known sand filter can be used without particular limitation as long as it has a sand filter layer that can be backwashed. for example,
Commonly used filters include those in which the entire filtration layer is made of sand, and those in which the filtration layer is made of sand and anthracite. -The structure and processing capacity of the fire source filter are selected depending on the type of raw material solution and the desired throughput, etc., but when raw water is supplied at a rate of 5 to 50 m/hour, 8S in the filtrate is The concentration of 0.05-0.
Generally, the concentration of the oxidizing substance is 50 ppm or less, the oxidizing substance concentration is 7 to 10 ppm or less, and the F I (Fouling Index) is 5 to 6.

The most important feature of the present invention is that a reducing substance is added to the filtered water obtained by primary filtration as described above, and then secondary filtration is performed. By doing this, according to the present invention, even if the flocculant present in the filtrate from the fire source produces a precipitate due to the addition of a reducing substance, the precipitate can be removed from the secondary filter. , it is possible to prevent a decline in dialysis performance due to contamination of the dialysis equipment over a long period of time.

In addition, in the present invention, the primary filtration and the secondary filtration are not limited to a single stage filtration, but may be a multistage filtration of two or more stages.

The reducing substance used in the present invention is not particularly limited as long as it has the purpose of neutralizing the above-mentioned oxidizing substance, but in general, a substance having a standard redox potential of 400 mV or less is used.

Such substances include, for example, sodium thiosulfate,
Sodium sulfite, sodium bisulfite, sulfuric compounds such as hydrosulfide, hydrazine hydrate, and hydrazine sulfate.

Hydrazines such as hydrazine phosphate, hydroxylamines such as hydroxylamine sulfate and hydroxylamine hydrochloride, and hydrogen compounds such as sodium borohydride and lithium aluminum hydride are used. In particular, sulfite-containing compounds are preferred because they can be easily decomposed and rendered harmless after electrodialysis, and exhibit particularly good effects compared to the above-mentioned reducing substances. The amount of the reducing substance added may be appropriately selected depending on the oxidizing substance contained in the filtered water obtained by the second filtration so as to achieve the above-mentioned purpose.

The dialysis device used in the present invention is not particularly limited.

Known dialysis devices such as filter press type and unit cell type are generally used. The membrane spacing of the dialysis machine is 0.2 to 2
The range (2) is generally suitable, but is particularly suitable for dialysis machines with a diameter of 0.61 or less.

Moreover, the conditions for electrodialysis that have been conventionally applied are applied as they are. Particularly suitable conditions include a current density of 0.5 to 6 A/eLn? .

The liquid temperature is 5 to 40°C.

In addition, in the present invention, any known method can be employed without particular limitation regarding the method of adding the flocculant, oxidizing substance, and reducing substance, or the use of an associated intermediate tank, etc.

(Effects) As understood from the above explanation, according to the method of the present invention, electrodialysis can be performed while preventing contamination of the dialysis apparatus by contaminants contained in the raw material solution for a long period of time.

(Example) Hereinafter, the present invention will be explained in detail based on Examples, but the present invention is not particularly limited to the following Example K.

Comparative Example 1 Raw seawater (8 to 30°C, H = 7.7 to 8.1° SS min =
5 to 7"?/l) is pumped up with a raw seawater pump, and sodium hypochlorite (7.5% as residual chlorine) is used as an oxidizing substance.
was injected with a metering pump to a concentration of 3.0 kg/l, and ferric chloride (55% as FeCl2) was added as a flocculant at a concentration of 3.0 kg/l. The raw material water was poured into the tank at a concentration of 1.5 m/l. This seawater is passed through the primary sand filter pump.
The mixture was fed to a primary sand filter having the specifications shown in Table 1, and then the filtrate was fed to a secondary sand filter having specifications shown in Table 1.

This filtrate was neutralized with acidic sulfite 1 (45% industrial grade) (oxidation-reduction potential of 200 to 300 mV or less), and the effective area was 200 mV or less.
d. Supply to an ion exchange membrane electrodialysis device with 200 pairs of membranes, 6A/drr? It was operated continuously for 75 days.

During this period, the quality of the seawater at the inlet of the dialysis machine was FI = 5.2.
~4.0,58=0.01~0.0511P/L,
Fe+++ = 0.05 to 0.2 [189/L], and the inlet pressure of the dilution chamber of the dialysis machine was 0.75 Kt/ej at the start of operation, and 1.551/- after 75 days.

Example 1 The same raw seawater as in Comparative Example 1 was pumped up, and oxidizing substances and flocculants were injected under the same conditions as in Comparative Example 1 (residual CL2 = 3'F/
L, Fe"" 5 q/L) and passed through a primary sand filter having the specifications shown in Table G1, and this filtrate was received into a 5-fire source filtered seawater tank. This is neutralized by adding a reducing agent (acidic sodium sulfite of Comparative Example 1) for neutralizing residual chlorine in the piping to the secondary filter (oxidation-reduction potential of 200 to 300 mV).
)t, The liquid was passed through and filtered through a secondary sand filter having the specifications shown in Table 1.

The filtrate was continuously operated for 90 days using the same dialysis apparatus as in Comparative Example 1, and evaluated.

During this period, the quality of the seawater at the inlet of the dialysis machine is F I =
2.4~3.2, 88=0.001~0.00811
1f/L, Fe””=0.05'IP/
The inlet pressure of the dialyzer was 0.75 to 4/d at the start of operation and 1.25 to 4/d after 90 days.

Table 1 Example of filter specifications 2 Pump up raw seawater in the same way as Example 1, inject sodium hypochlorite at 6 wq/L and ferric chloride at 51111/L, and add the following to Table 1. Pass the liquid through the primary sand filter with the specifications shown.
Filter, and reduce and neutralize with acidic sodium sulfite in the inlet pipe of the secondary sand filter as in Example 1 (oxidation-reduction potential: 200-30
0 mV)L, aE1 The liquid was passed through and filtered through a secondary sand filter having the specifications shown in the table.

This filtrate was continuously operated for 110 days using the same dialysis apparatus as in Example 1, and evaluated.

During this period, the quality of the seawater at the inlet of the dialysis machine was the same as in Example 1, and the pressure at the dialysis machine inlet was 0.70K at the start of operation.
t/c11.110 days later it was 1.20 Kt/cIi.

Claims (1)

[Scope of Claims] 1) An electrolyte solution containing a flocculant and an oxidizing substance is first filtered, then a reducing substance is added to the filtrate, secondly filtered, and then supplied to an ion exchange membrane electrodialysis device. An electrodialysis method characterized by electrodialysis. 2) The electrodialysis method according to claim 1, wherein the electrolyte solution is seawater. 3) The electrodialysis method according to claim 1, wherein the flocculant is ferric chloride. 4) The electrodialysis method according to claim 1, wherein the oxidizing substance is chlorine, hypochlorous acid, or a salt thereof. 5) The electrodialysis method according to claim 1, in which a sand filter is used for primary filtration and secondary filtration. 6) The electrodialysis method according to claim 1, wherein the reducing substance is a sulfite-containing compound.