JP3406290B2 - Method for treating wastewater containing silica gel - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention mainly relates to wastewater containing hydrous silica gel (hereinafter referred to as "silica gel") generated during ground improvement work by a chemical injection method, particularly ground improvement work using a water glass grout agent. The present invention relates to a method for treating wastewater that is treated with an alkali hydroxide and a coagulant to reduce the volume.

[0002]

2. Description of the Related Art Ground improvement work by a chemical injection method is to strengthen soft ground with silica gel produced by reacting sodium silicate and sulfuric acid. In this reaction process, first, a silica solution was prepared from a sodium silicate solution and sulfuric acid using a special manufacturing apparatus to prepare a solution A, and a diluted sodium silicate solution B solution was prepared with a special monitor at a ratio of 1: 1 (2: 1, 3: 1, 1). : 0) Mix and infiltrate the target ground from the tip to form silica gel between soil particles, but in this process unreacted silica solution is discharged to the ground along with groundwater and reacts, silica gel is formed, and waste water is generated. Occurs. In addition, unreacted substances on the ground are mixed with rainwater and snowmelt water to generate silica gel and generate wastewater.

Silica gel contains about 87% water and is suspended in wastewater. The weight ratio of silica gel to the waste water is about 5% at maximum, and most of it is water except earth and sand.

Therefore, as one of the methods for disposing of waste water containing silica gel, public sewers are installed at the construction site, and when the SS concentration in the waste water is smaller than the discharge regulation value of the sewers, the waste water is discharged to the public sewer. it can. However, there is a problem that the filtration device is clogged when the contained silica gel is treated as sewage.

If the public sewer is not maintained or if the SS concentration is too high to flow into the public sewer, it is discarded as industrial waste at an industrial waste site, and water is removed on the sun.

However, this method requires a large amount of cost for transporting the waste water, and it is also difficult to secure an industrial waste site of a sufficient size for disposal.

[0007]

An object of the present invention is to provide a method of precipitating wastewater containing silica gel, which is generated when a chemical solution injection method using sodium silicate and sulfuric acid is applied, by using an alkali hydroxide and a coagulant, The purpose of the present invention is to provide a means for facilitating the separation of the silica gel and the supernatant, reducing the water content of the precipitate, reducing the volume of waste water generated, and reducing the waste treatment cost.

[0008]

According to the present invention, wastewater containing silica gel generated by a chemical injection method using sodium silicate and sulfuric acid is used to form a precipitate with a chemical mainly containing an alkali hydroxide, an acid and a coagulant. In the treatment method to form, the precipitate part and the supernatant part are easily separated and the supernatant part is drained,
A treatment method for reducing the volume of waste water is provided.

Preferably, the alkali hydroxide concentration is 0.1 mol / L
Above (pH = 12.5 or higher), silica compounds produced by adding each to silica gel-containing wastewater to dissolve the silica gel so that the concentration of the coagulant becomes 0.5 g / L or more, and neutralizing by adding an acid. Is a method for reducing the volume of silica gel-containing wastewater, in which the wastewater is aggregated and precipitated.

More preferably, the aggregating agent is added in the process of dissolving the silica gel in the alkali hydroxide.

[0011]

When the alkali hydroxide is added to the waste water according to the present invention, the silica gel is dissolved. Then, the gel containing silica is generated again by neutralizing with, for example, sulfuric acid, and the waste water is suspended. By adding an appropriate coagulant before the silica gel is regenerated, it is possible to form a silica-containing gel that easily precipitates. Silica, which is not involved in gelation, is dissolved in the solution, but gelation does not occur even if the pH of the solution is changed to a pH range that meets the river discharge standards, so there is no problem in discharging the supernatant into the river. .

Potassium hydroxide, sodium hydroxide and the like can be used as the alkali hydroxide, but sodium hydroxide and the like have a longer dissolution time, and therefore potassium hydroxide is preferably used.

As the coagulant, calcium hydroxide, calcium carbonate or the like can be used.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in more detail below. The following general method was used in the following experiments. As silica gel-containing waste water, silica gel synthesized from sodium silicate and sulfuric acid (dry solid content 13%) suspended in pure water was used. The concentration of silica gel was adjusted to 5 wet g / L when mixed with an alkaline solution. To dissolve silica gel in waste water, add alkali hydroxide to the silica gel suspension as a solid or liquid,
It was prepared with pure water so as to have a volume of 200 mL, and then stirred with a jar tester at a stirring speed of 50 rpm for 1 hour.
Sulfuric acid was added to the solution thus treated to neutralize it, and after stirring, the solution was transferred to a graduated test tube so that the liquid level was 10 cm and allowed to stand, and the state of precipitate formation was observed. . The absorbance at λ = 600 nm was measured as the silica gel turbidity in the solution.

Example: Silica gel-containing wastewater generated in ground improvement work using a water glass grout material can be treated with an alkali hydroxide and a coagulant. For example, when potassium hydroxide and calcium hydroxide are used, the content of potassium hydroxide is more preferably 0.5 mol / L or more, and the content of calcium hydroxide is 0.5 g / L or more. The temperature at this time is not limited. If the concentration of potassium hydroxide is low, it takes a long time to dissolve, and excessive addition of calcium hydroxide does not improve the cohesiveness, but rather increases the cost.

Although other alkali hydroxides are added and dissolved, potassium hydroxide is preferable as the alkali. Sodium hydroxide can also be used although it has poor solubility. The flocculant is added to the wastewater containing silica gel containing potassium hydroxide, and the addition time is effective before the addition of potassium hydroxide and during the dissolution of the silica gel, preferably when potassium hydroxide is added.

The preferred conditions of the wastewater treatment method according to the present invention are pH = 12.5 or more (potassium hydroxide concentration is 0.1 mol / L or more) and coagulant concentration is 0.5 g / L. In the form of a mixture and stirred. This dissolves the silica gel, but the time depends on the concentration of potassium hydroxide added. Silica gel is also dissolved by dilute potassium hydroxide, but 0.5m is needed to shorten the dissolution time.
It is desirable to contain more than ol / L. On the contrary, if the potassium hydroxide is in excess, a large amount of acid for neutralization is required, which is costly. The coagulant is required to be added in an amount of 0.5 g / L or more in order to exert the effect. However, when the addition amount is large, the aggregation and sedimentation time does not change, but the amount of the sediment containing the aggregating agent increases, and the treatment cost increases.

The treatment of the silica gel-containing wastewater of the present invention can be conducted with at least one stirring layer.

<Experimental Example 1> Examination of Solubility Depending on Alkali Type Potassium hydroxide or sodium hydroxide was added to 0.5 mL / L of model wastewater 100 mL and stirred,
The dissolution started. At this time, the change with time of the turbidity of the model wastewater was measured at λ = 660 nm. The results are shown in Figure 1.

From these results, it is understood that silica gel can be dissolved by alkali hydroxide and potassium hydroxide has a large dissolving power.

<Experimental Example 2> Examination of potassium hydroxide concentration (pH of waste water) Further, the stirring speed was set to 50 rpm and the stirring time was set to 1 hour,
The relationship between the dissolution of silica gel and the pH of the solution was investigated. Figure the result
Shown in 2.

From these results, an increase in the concentration of potassium hydroxide brings about an improvement in the solubility of silica gel, and it is possible to dissolve the silica gel almost completely in 1 hour at pH = 12.5 (potassium hydroxide concentration 0.1 mol / L) or higher. all right.

<Experimental Example 3> Selection of time for adding coagulant The time for adding coagulant in the process of dissolving silica gel and re-gelling by neutralization was examined using calcium hydroxide as a coagulant. The addition timing is (1) when adding potassium hydroxide at the same time when adding potassium hydroxide to the model waste liquid,
(2) When potassium hydroxide was added to the model waste liquid and dissolved for 1 hour, and then calcium hydroxide was added, the solutions were neutralized for each and the regelation and the precipitation formation were compared.

As a result, in the case of (1), the sedimentation was almost completed about 10 minutes after the neutralization, and about 25 vol% of the total amount of the waste liquid was formed. The supernatant was colorless and transparent, and the state did not change even after standing for 1 day. In the case of (2), only a slight amount of aggregation occurred, the supernatant liquid became cloudy immediately after neutralization, and the generated fine gel-like substance did not settle, and the entire waste liquid gelled after 1 day. From this result, it was desirable to add the coagulant in the process of dissolving silica gel with potassium hydroxide, and it can be said that there is almost no effect even if it is added after completely dissolving.

<Experimental Example 4> Examination of the addition amount of calcium hydroxide as a coagulant. The amount of calcium hydroxide added to agglomerate the silica gel that regels by neutralization was examined. Calcium hydroxide was added at the same time as potassium hydroxide. The results are shown in Figure 3.

Waste liquid was placed in a test tube so that the liquid surface was 10 cm high, and a flocculant was added. Precipitation occurred rapidly at any concentration and became constant in about 10 minutes. A flocculant concentration of 0.1 g / L has a sufficient effect on sedimentation, but the sedimentation height is
It was found to be constant above 0.5 g / L, and it was found that the added amount of calcium hydroxide should be 0.5 g / L after the addition.

<Experimental Example 5> Treatment of ground improvement work wastewater Flake potassium hydroxide was added to actual work wastewater (pH = 9.6, silica gel concentration 43 wet g / L) to a concentration of 0.5 mol / L. . At the same time, the concentration of calcium hydroxide is 0.5g
/ L was added to the mixture so that the mixture was stirred for 1 hour. After dissolution, it was neutralized with 60% sulfuric acid and left to stand. At this time, the amount of precipitate formed and the appearance of the supernatant were observed. Moreover, the amount of silica gel contained in the precipitate was measured.

As a result, the colorless and transparent supernatant was separated into a precipitate part that was easy to separate, and the volume ratio of the supernatant was 7: 3. At this time, the silica gel contained in the waste liquid is about 40% in the precipitation part
It was found that about 60% was dissolved in the supernatant. However, this supernatant contained almost no SS, and no re-gelation was observed even after standing for several days in the pH range where river flow was possible. It can be drained.

[0029]

According to the present invention, by dissolving silica gel with an alkali hydroxide and precipitating the silica gel regenerated at the time of neutralization with an aggregating agent, it is possible to remove silica gel which is difficult to separate by filter press or ordinary precipitation treatment. It is possible to efficiently separate the waste water containing it into a supernatant and a precipitation part to reduce the volume.

Thus, in the treatment of wastewater containing silica gel, which occurs when the chemical solution injection method using sodium silicate and sulfuric acid is applied, the separation of silica gel and the supernatant liquid is facilitated, the water content of the precipitate is reduced, and It is possible to reduce the volume of the waste water and reduce the waste treatment cost.

[Brief description of drawings]

FIG. 1 shows the relationship between solution turbidity and stirring time when adding potassium hydroxide and sodium hydroxide to the silica gel-containing wastewater such that the concentration of each is 0.5 mol / L and stirring.

FIG. 2 shows the effect of potassium hydroxide concentration on silica gel dissolution in the relationship between the pH of the solution and the turbidity after stirring for 1 hour.

FIG. 3 shows a process in which silica gel dissolved in alkali hydroxide is neutralized and then regenerated silica gel is precipitated, in terms of the relationship between the calcium hydroxide concentration and the precipitation height after standing for 10 minutes. Is.

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Claims (2)

(57) [Claims] 1. The alkali hydroxide concentration is 0.1 mol / L or more (pH
= 12.5 or more), add each to the silica gel-containing wastewater so that the concentration of the coagulant is 0.5 g / L or more, dissolve the silica gel, and add an acid to neutralize the resulting silica compound. A method for reducing the volume of silica gel-containing wastewater to be precipitated. 2. The method for reducing volume of silica gel-containing wastewater according to claim 1, wherein the flocculant is added in the process of dissolving the silica gel in the alkali hydroxide.