JPH08276191A - Recovery of silica in aqueous solution and seeds used therein - Google Patents

Abstract

PURPOSE: To enhance silica recovery efficiency in a silica-containing aq. soln. by adjusting the pH of the silica-containing aq. soln. by using seeds selected from the group consisting of preliminarily prepared cation supported silica colloid, a silica gel and a mixture of them so that the adsorbing amt. of silica in the soln. on the seed be comes a specific value. CONSTITUTION: A part of geothermal hot water 1 ejected from the ground and containing a large amt. of silica is allowed to flow in a concn. tank 2-1 and a polyvalent cation T is added to the concn. tank 2-1 to form supporting seeds S1. The greater part of the geothermal hot water 1 is allowed to flow in a reaction tank 3 and the pH thereof is adjusted by pH control agent appropriately supplied from a pH control device 4 so that at least 60% of supersaturated silica in the aq. soln. is adsorbed on the seeds S1. After the pH of the geothermal hot water 1 is adjusted, the supported seeds S1 are 1 allowed to flow in the reaction tank and the geothermal hot water is reacted with the supporting seeds S1 in the reaction tank 3 to obtain silica recovery effect.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for adsorbing and recovering silica in an aqueous solution containing silica (silicic acid) such as geothermal hot water to prevent the formation of silica scale in the flow path of the aqueous solution.

[0002]

2. Description of the Related Art Geothermal power generation is a method in which a high-temperature geothermal fluid in the ground is ejected and power is generated using separated steam. In this case, geothermal heat containing silica at a concentration of several hundred ppm together with steam. Water gushes. The ejected geothermal hot water is returned to the ground through an underground reduction well, but the temperature of the geothermal fluid is 2
While the temperature of the refluxed geothermal hot water is as low as 97 ° C to 98 ° C while the temperature is 50 ° C to 350 ° C, the solubility of silica in the geothermal hot water is relatively low. Moreover, since silica is concentrated as it is separated from water vapor, some of the silica contained in the geothermal hot water becomes supersaturated.

Since this supersaturated silica easily deposits and adheres as a silica scale to the hot water path in the geothermal power plant, the inner wall of the underground reduction well, etc., the heat efficiency of the heat exchanger is lowered, the hot water path is blocked, or the underground This is a cause of reduction in capacity of the return well. Moreover, since this silica scale adheres firmly to the inner wall and the like and is difficult to remove, if the adhesion of the silica scale progresses, the use of the hot water path or the underground reduction well is stopped, and the silica scale is removed. Must be removed. As described above, the presence of silica in the geothermal hot water is a major obstacle to the utilization of the geothermal hot water.

Therefore, for the purpose of removing silica contained in geothermal hot water and preventing adhesion of silica scale to the hot water passage or the underground reduction well, for example, as disclosed in Japanese Patent Laid-Open No. 63-1496. The disclosed method is known. This is because a portion of the geothermal hot water is retained to generate silica colloid, and then the retained geothermal hot water is contacted with the remaining geothermal hot water to adsorb silica in the geothermal hot water to the silica colloid. Then, the grown silica colloid is recovered by ultrafiltration.

In addition, polyvalent cations such as aluminum and iron are added to geothermal hot water to aggregate silica, and the aggregated silica is used as a nucleus to generate a silica colloid, which is then recovered by flotation or the like. There is also a way to do it.

[0006]

However, among the above-mentioned conventional methods, in the method disclosed in Japanese Patent Laid-Open No. 63-1496, the particle size of the obtained silica colloid is small, so that the pore size of the filtration membrane is small. There is a possibility that the filtration membrane may be clogged with colloid having a size similar to the above. In addition, in the method of adding polyvalent cations, it is necessary to add polyvalent cations to all the geothermal hot water to be used, so there is a problem that a large amount of polyvalent cations are required and the cost increases. . Moreover, in any of the above methods, there is a problem in that a white suspended substance floats in the supernatant liquid after silica recovery, and its clarity is low.

Therefore, the main object of the present invention is to improve the recovery efficiency of silica when recovering silica in a silica-containing aqueous solution such as geothermal hot water, and an additive added to the silica-containing aqueous solution when recovering silica. It is a decrease in the consumption of. Another object of the present invention is to improve the clarity of the supernatant after recovering silica. It is a further object of the invention to provide suitable seeds for use in silica recovery.

[0008]

According to the present invention, a seed having silica adsorption is added to an aqueous solution containing silica,
Provided is a method for recovering silica in an aqueous solution, which comprises: adsorbing silica in the aqueous solution onto the seed; then solid-liquid separating the aqueous solution containing the seed, and recovering the obtained solid content as a silica-containing material. Here, a seed selected from the group consisting of a cation-supporting silica colloid, silica gel and a mixture thereof prepared in advance is used, and the pH of the aqueous solution to which the seed is added is adjusted to at least 60% of the supersaturated content of silica in the aqueous solution. Adjust to adsorb to the seed. In the present specification, the removal ratio (%) with respect to the supersaturated component is the solubility of amorphous silica at 90 ° C. of 320
It means the value calculated based on mg / l.

The cation-supporting silica colloid is prepared by bringing polyvalent cations into contact with a silica colloid obtained by heating and concentrating a part of the aqueous solution containing silica such as geothermal hot water. In this case, the polyvalent cation to be supported is preferably selected from any of aluminum, magnesium and calcium ions. On the other hand, silica gel may be a general commercially available granular material,
Those having a particle size of 45 μm or less are preferable.

In the present invention, the pH of the aqueous solution obtained by adding such a seed is adjusted to a predetermined range, and at least 60% of the supersaturated portion of silica in the aqueous solution such as geothermal hot water is adsorbed on the seed. However, such a silica removal ratio is 7 to 7 when the pH of the aqueous solution is in the range of only the seed consisting of the cation-supporting silica colloid.
When 10 is used including a seed made of silica gel, it can be achieved by adjusting to 7 to 8. Further, the seed addition amount in this case is preferably an amount such that the addition amount of silica is 1 g or more per 1 liter of the aqueous solution.

In order to improve the clarity of the supernatant by the above method, a flocculant is added to the supernatant of the reaction product, if necessary, after adjusting the pH range of the aqueous solution.
As the coagulant of this type, for example, polyaluminum chloride or an organic polymer coagulant is preferably used. By the addition of such an aggregating agent, the suspended substance suspended in the supernatant is rapidly aggregated and settled, and the clarity of the supernatant is improved. In addition, the concentration of dissolved silica in the supernatant decreases as the suspended solids aggregate and settle.

Further, a part of the solid content obtained by solid-liquid separation may be added again to the aqueous solution as a regeneration seed. This reduces the consumption of seeds used. Further, for the purpose of preventing the polymerization of silica in the aqueous solution over time and preventing a decrease in the silica recovery rate accompanied therewith, prior to the addition of seeds, the aqueous solution immediately after water sampling is once made alkaline, and then the seeds are added. May be returned to neutral upon addition.

As described above, in the present invention, a cation-supporting silica colloid prepared in advance, preferably having a particle size of 45.
Using a seed selected from the group consisting of sub-μm silica gel, and mixtures thereof, the pH of the aqueous solution to which the seed is added is such that at least 60% of the supersaturated part of the silica of the aqueous solution is adsorbed onto the seed. Preferably, the cation-supporting silica colloid seed alone is adjusted to a range of 7 to 10 and the silica gel seed is adjusted to a range of 7 to 8 to improve the recovery efficiency of silica in the aqueous solution. Here, when the seed is a polyvalent cation-supporting seed, a polyvalent cation is required for its preparation, but in the present invention, the polyvalent cation is not added to the aqueous solution, Since it is added only to the seed before it is added to the aqueous solution, the consumption of polyvalent cations can be small.

The consumption of seeds can also be reduced by adding a part of the solid content obtained by the solid-liquid separation to the aqueous solution again as a regeneration seed. Further, the aqueous solution is once made alkaline prior to the addition of the seeds, and then returned to neutrality at the time of adding the seeds, whereby polymerization of silica in the aqueous solution and accompanying reduction in silica recovery rate are prevented. .

Furthermore, in the present invention, when recovering silica from an aqueous solution containing silica, the seed is added to the aqueous solution containing silica to adsorb silica in the aqueous solution, and a part of the aqueous solution containing silica is heated. Concentration causes the silica to gel and form a silica colloid, which is loaded with polyvalent cations to provide a seed produced. Here, the polyvalent cation is preferably an ion of a substance selected from aluminum, magnesium, and calcium. With such a seed, the silica-containing aqueous solution itself can be effectively used as raw water for the seed.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION Various embodiments of the present invention will now be described in more detail with reference to the drawings. FIG. 1 shows a first embodiment of a method for recovering silica in an aqueous solution according to the present invention.
It is shown in the flowchart. In the present embodiment, first, a part of the geothermal hot water (aqueous solution containing silica) 1 containing a large amount of silica ejected from the ground is introduced into the concentrating tank 2-1 via the route indicated by reference sign A in the figure. Inflow.

The geothermal hot water flowing into the concentrating tank 2-1 is heated and concentrated until the silica contained therein becomes colloidal, and silica colloid is formed. Also, in the process of concentration,
A polyvalent cation (reference symbol T in the figure) is added in a state of, for example, chloride, and as a result, the polyvalent cation containing the silica colloid and supporting the polyvalent cation is provided in the concentrating tank 2-1. Ion-supporting seed (hereinafter abbreviated as "supporting seed") S
₁ is prepared. Then, the carried seed S ₁ is then transferred to the seed storage tank 2-2.

Here, the polyvalent cation T is stored in the concentration tank 2-1.
Is added to form the supporting seed S ₁ because the supporting of the polyvalent cation T improves the silica adsorbing ability of the seed as compared with the case where only the silica colloid is used as the seed. The polyvalent cation T is preferably selected from aluminum ion, magnesium ion or calcium ion.

On the other hand, most of the geothermal hot water 1 flows into the reaction tank 3 through the route indicated by the symbol B in the figure, and its pH is
By the pH adjusting agent appropriately supplied from the pH adjusting device 4,
The pH is maintained within the range of 7-10, more preferably 7-9. Here, the reason why the pH of the geothermal hot water 1 is set within the above range is that the polymerization rate of silica in the geothermal hot water 1 is high within this range, the adsorption of silica to the seeds is relatively easy, and the geothermal heat Since the pH of water 1 is near neutral,
This is because the adjustment of is also easy. Regarding the relationship between the type of polyvalent cation and the adjusted pH, when aluminum ion is used, the silica adsorption capacity is almost unchanged in the above pH range, but in the case of calcium ion, the pH is higher than 9 in the above range. When it is large, the effect of removing silica is enhanced. However, compared with aluminum ion or magnesium ion, calcium ion has a low silica removing effect. On the other hand, magnesium ions have a greater effect of removing silica than aluminum ions when the pH is higher than 8, but the maximum value is around 9 to 10, and the effect is weakened when the pH is higher than that.

After adjusting the pH of the geothermal hot water 1, the supported seed S ₁
Of the supported seed S in the reaction tank 3.
React with ₁ . Regarding the amount of the supported seed S ₁ added to the reaction tank 3, according to the study by the present inventors, 1 liter of geothermal hot water was converted into at least 1 silica.
The result shows that if the supported seed S ₁ corresponding to g is added, the silica recovery effect can be expected to the extent that there is no practical problem.

The mixed liquid after the reaction is flown into the solid-liquid separation device 5 from the reaction tank 3, and in the solid-liquid separation device 5, sludge 6 (solid phase) containing silica as a main component and low silica having a low silica content. It is separated into hot water 7 (liquid phase). Where sludge 6
For separating the low-silica hot water 7 and the low-silica hot water 7, an easy and reliable method such as natural sedimentation can be applied.

A part of the sludge 6 thus separated is
The regenerated seed S ₂ carrying the polyvalent cations T is returned to the reaction tank 3 through the path indicated by the symbol C in the figure, and is again reacted with the geothermal hot water 1. As a result, the regeneration seed S
₂ silica geothermal water in 1 is adsorbed by the further at solid-liquid separator 5, it is separated into a sludge 6 and low silica hydrothermal 7. A part of this sludge 6 is reused as reflux seed S ₂ in the reaction tank 3 again. Then, by repeating the above steps, silica in the geothermal hot water 1 is continuously recovered with high recovery efficiency.

Further, in the method according to the present invention, a part of the sludge 6 obtained by the solid-liquid separation is added to the reaction tank 2 again as the regenerated seed S ₂ , so that the consumption of the seed is reduced. Addition amount of supported seed S ₁ or regenerated seed S ₂ to geothermal hot water 1 is converted to silica
The small amount of at least 1 g per liter also contributes to the reduction of seed consumption.

In addition, in the case of this embodiment, the seed S
A portion of geothermal water 1 by using a colloidal silica obtained by heating and concentrating as _1, can be effectively utilized geothermal water 1 itself as the seed S _1. In addition, polyvalent cation T
Since it may be added only to the hot water used as the seed S ₁ , the consumption of the polyvalent cation T may be small.

On the other hand, the sludge 6 which is not used as the regeneration seed S ₂ in the above step is dehydrated in the dehydration tank 8,
As a high silica product 9, it is effectively used for construction materials and others. Further, the low-silica hot water 7 is sent to an underground reduction well (not shown), a heat exchanger, or the like together with the water discharged from the dehydration tank 8.

FIG. 2 is a flow chart showing a second embodiment of the method for recovering silica in an aqueous solution according to the present invention.
In this embodiment, using the following silica gel particle size 45μm as a seed S _1, without providing the seed tank or the like, that is the seed S ₁ directly added to and reacted with the geothermal water 1 into the reaction vessel 3 Only the difference from the first embodiment. here,
The reason why silica gel having a particle size of 45 μm or less is used as a seed is that the silica adsorption capacity of the seed S ₁ is greatly improved when such a particle size is used, and the silica after the reaction is larger than the case where a seed having a diameter larger than 45 μm is reacted This is because the concentration of silica in the supernatant of the above is considerably reduced. It is considered that this is because the specific surface area is increased due to the reduction in seed particle size.

The amount of seed S ₁ added to the reaction tank 3 is the same as in the first embodiment, but the pH of the geothermal hot water 1 in the reaction tank 3 is 7 to 8 after seed addition.
It is preferable to adjust to the above range. For example, when the pH becomes as high as 9, the effect of making the particle size in the above range does not appear, and silica is dissolved and the silica concentration after the reaction becomes high.

FIG. 3 is a flow chart showing a third embodiment of the method for recovering silica in an aqueous solution according to the present invention.
In this embodiment, after the solid-liquid separation device 5 separates the sludge 6 containing silica as a main component into a supernatant liquid, a predetermined amount of the coagulant G is added to the supernatant liquid to suspend the suspension in the supernatant liquid. The only difference from the first embodiment is that the suspended substance is aggregated and settled.

The coagulant G used here is preferably an organic polymer-based coagulant such as polyaluminum chloride or a polyacrylamide-based or modified polyacrylamide-based coagulant, and these are used alone or in combination. The amount of the flocculant added is appropriately selected depending on the degree of suspension of the supernatant and the degree of clarification required. When the coagulant G is added as described above, not only the suspended substance is coagulated and settled to improve the clarity of the supernatant, but also the concentration of dissolved silica in the supernatant is lowered. The supernatant liquid having improved clarity is sent to low-silica hot water 7 having a low silica content together with water discharged from the dehydration tank 8 to an underground reduction well or a heat exchanger (not shown).

FIG. 4 is a flow chart showing a fourth embodiment of the method for recovering silica in an aqueous solution according to the present invention.
In this embodiment, as in the case of the second embodiment, a seed storage tank or the like is not provided, and a seed S ₁ made of silica gel having a predetermined particle size is directly added into the reaction tank 3 so that the geothermal water 1
In the same manner as in the third embodiment, a predetermined amount of coagulant G is added to the supernatant separated by the solid-liquid separator 5 to remove suspended substances suspended in the supernatant. Aggregated and settled. The amount of seed S ₁ added to the reaction tank 3 and the pH range of the geothermal hot water 1 in the reaction tank 3 are the same as those in the second embodiment, but the required aggregation depends on the particle size of the silica gel. The amount of agent is different. That is, if the particle size of silica gel is made smaller in order to improve the silica recovery efficiency, the turbidity of the supernatant liquid generally increases, and the required amount of the flocculant to be added increases. On the other hand, when silica gel has a large particle size, conversely, the turbidity of the supernatant decreases and the required amount of coagulant added decreases.

[0030]

EXAMPLES The method of the present invention will be described in more detail below with reference to examples. Example 1 40 liters of geothermal hot water was placed in a container and heated by boiling with a heater to evaporate and concentrate to 840 ml, while the chloride reagent of aluminum and calcium was adjusted to 1000 ppm as a metal ion concentration. What was each melt | dissolved in distilled water was added in the process of the said concentration, and each seed which carried each metal ion was prepared. Here, the addition amount of distilled water in which the chloride reagent was dissolved was calculated so that the metal ion concentration would be 50 ppm when added to hot water as a metal ion supporting seed. Further, for comparison, a hot water seed consisting only of the silica colloid produced by the above-mentioned evaporative concentration was prepared.

The metal ion-supporting seed prepared as described above was added to the geothermal hot water at a ratio of 960 ml of hot water to 40 ml of the metal ion-supporting seed. The amount of silica at this time is 2 with the adjusted hot water seed.
5.1g silica / liter (silica concentration 5 for geothermal hot water
Since it is 28 ppm, 47.6 times), 1.006 g silica / liter is added to the solution after the addition. In addition, seeds consisting only of silica colloid were similarly added to geothermal hot water, and the silica removal effects of both were compared. The results are shown in Table 1 below.

[Table 1]

From Table 1, the metal ion-supporting seeds are
It can be seen that the silica concentration in the supernatant after the reaction is reduced by about 10 to 100 ppm as compared with the case where only the silica colloid is used as the seed. Further, it can be seen from Table 1 that the silica adsorption capacity of the supported seeds is higher when aluminum ions are used as metal ions than when calcium ions are used in the case of pH 7 where the experiment was conducted.

Example 2 As a seed, silica gel having a particle size of 45 μm or less and 45 μm
Seeds of larger diameter were prepared and directly reacted with geothermal hot water as in the case of FIG. 2 to compare silica removal effects. The results are shown in Table 2. Note that in Table 2, the pH is unadjusted (about 7.2).

[Table 2]

As is clear from this table, when the silica gel seed having a particle size of 45 μm or less was used, the silica concentration in the supernatant after the reaction was higher than that when the silica gel seed having a diameter of more than 45 μm was reacted. It decreased by about 70 ppm.
The cause of this is considered to be an increase in specific surface area as the seed particle size is reduced.

Further, regarding the influence of the particle size of the seeds, silica recovery seeds of geothermal hot water were further recovered by using silica gel seeds (“Wakogel LC” manufactured by Wako Pharmaceutical Co., Ltd.) having four particle sizes of 5, 10, 30, and 50 μm. The test was conducted. The test procedure was the same as that described above, but the seed addition amount and reaction time were changed as shown in Table 3. The results are also shown in Table 3.

[Table 3]

As is apparent from the above, when the particle diameters are 5 and 10 μm, the effect of removing silica is large, and 518 to
Initial silica concentration of 522 ppm is 379-400 ppm
However, when the particle size was 30 and 50 μm, it was only reduced to about 430 ppm. on the other hand,
Regarding the turbidity, the clarity was poor when the particle size was 5 and 10 μm, but it was transparent when the particle size was 30 and 50 μm. Although the pH was not adjusted in the above experiment, an experiment was conducted under the same conditions as above except that the pH was set to 9. As a result, the silica concentration after the reaction was about 450 ppm, and a sufficient result was not obtained. It is considered that this is because the silica dissolves as the pH increases to 9. Further, the influence of the difference in particle size of the silica gel seeds at pH 9 on the silica concentration was hardly recognized.

Example 3 To illustrate the embodiment shown in FIG. 3 or 4, the effect of the addition of a flocculant on the supernatant was investigated. From one example using the supported seed, a supernatant liquid was prepared which was relatively cloudy and in which a suspended matter was observed. As the coagulant, polyaluminum chloride (hereinafter abbreviated as "PAC") and a commercially available polyacrylamide-based coagulant (Mitsui Cytec "A130", hereinafter abbreviated as "A130") are used in combination, and the above supernatant is used. Liquid 1
A total of 0.5-5 mg was added per ml. The results are shown in Table 4 below.

[Table 4]

As is clear from Table 4, when the flocculant is added, not only the suspended substance aggregates and sediments to improve the clarity of the supernatant, but also the concentration of dissolved silica in the supernatant decreases. There is.

Example 4 Geothermal hot water was sampled, made alkaline immediately thereafter, and p
The H was returned to neutral upon seed addition and then reacted. The results are shown in Table 5.

[Table 5]

In this case, the silica recovery efficiency in the geothermal hot water is slightly improved as compared with the geothermal hot water which is not treated immediately after water sampling. This is presumably because by making the geothermal hot water alkaline immediately after water collection, polymerization of silica during the period from water collection to reaction and the accompanying reduction in silica recovery rate were prevented.

[0041]

As described above, according to the method for recovering silica in an aqueous solution of the present invention, a seed selected from the group consisting of a cation-supporting silica colloid and silica gel prepared in advance is used, and the seed is added. The pH of the aqueous solution should be at least 60% of the supersaturated portion of the silica in the aqueous solution.
%, So that the seeds are adsorbed on the seeds, the recovery efficiency of silica in the aqueous solution can be sufficiently improved. Further, the consumption of additives such as polyvalent cations used for recovering silica can be reduced.

When the seed is a polyvalent cation-supporting seed, polyvalent cation is required for its preparation.
In the present invention, the polyvalent cation is not added to the aqueous solution, but is added only to the seed before being added to the geothermal hot water solution, so that the consumption of the polyvalent cation can be small.

[Brief description of drawings]

FIG. 1 is a flowchart showing a first embodiment of the present invention.

FIG. 2 is a flowchart showing a second embodiment of the present invention.

FIG. 3 is a flowchart showing a third embodiment of the present invention.

FIG. 4 is a flowchart showing a fourth embodiment of the present invention.

[Explanation of symbols]

 1 Geothermal hot water 2-1 Concentration tank 2-2 Seed storage tank 3 Reaction tank 4 pH adjuster 5 Solid-liquid separator

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number within the agency FI Technical display location F03G 4/00 551 F03G 4/00 551 E21B 43/00 (72) Inventor Takafumi Furukawa Omiya City, Saitama Prefecture 1-297 Kitabukuro-cho, Mitsubishi Materials Corporation

Claims (12)

[Claims] 1. A silica-adsorptive seed is added to an aqueous solution containing silica and reacted to adsorb the silica in the aqueous solution onto the seed, and then the obtained aqueous solution containing the seed is subjected to solid-liquid separation. In the method for recovering silica in an aqueous solution for recovering solids as a silica-containing material, a seed selected from the group consisting of cation-supporting silica colloid, silica gel and a mixture thereof is used as the seed, and the seed is added. A method for recovering silica, wherein the pH of the aqueous solution is adjusted such that at least 60% of the supersaturated content of silica in the aqueous solution is adsorbed on the seed. 2. The cation-supporting silica colloid is produced by heating and concentrating a part of the silica-containing aqueous solution to gelate the silica to produce a silica colloid, which is supported by a polyvalent cation. The method for recovering silica according to claim 1, wherein the silica is recovered. 3. The method for recovering silica according to claim 2, wherein the polyvalent cation is an ion of a substance selected from aluminum, magnesium and calcium. 4. When only a cation-supporting silica colloid is used as the seed, the pH range of the aqueous solution is 7
10. It adjusts to 10 and makes it react, It is characterized by the above-mentioned.
4. A method for recovering silica in an aqueous solution according to any one of 1 to 3. 5. The silica recovery method according to claim 1, wherein when only silica gel is used as a seed, the silica gel has a particle size of 45 μm or less, and the pH range of the aqueous solution is adjusted to 7-8. Method. 6. The method for recovering silica according to claim 1, wherein an aggregating agent is added to the supernatant of the reaction product. 7. The silica recovery method according to claim 1, wherein a part of the solid content obtained by solid-liquid separation is added again to the aqueous solution as a regeneration seed. 8. The aqueous solution to which the seed is added,
8. The method according to claim 1, wherein after the water is taken, the silica is made alkaline to stop the polymerization of the silica in the aqueous solution and then returned to neutral when the seed is added.
The method for recovering silica according to item. 9. The method for recovering silica according to claim 1, wherein the seed is added to the aqueous solution so that the amount of silica added is 1 g or more per 1 liter of the aqueous solution. . 10. The method according to claim 6, wherein a substance selected from polyaluminum chloride and an organic polymer-based aggregating agent is used as the aggregating agent. 11. When recovering silica from an aqueous solution containing silica, the silica is added to the aqueous solution containing silica,
In a seed for adsorbing silica in an aqueous solution, a seed produced by heating and condensing a part of the silica-containing aqueous solution to gel silica to form a silica colloid, and supporting the polyvalent cation on the silica colloid. . 12. The seed according to claim 11, wherein the polyvalent cation is an ion of a substance selected from aluminum, magnesium, and calcium.