NZ204455A

NZ204455A - Process of decreasing the silica content of a supersaturated silica solution by forming silica colloid and separation thereof

Info

Publication number: NZ204455A
Application number: NZ20445583A
Authority: NZ
Inventors: G Sato; Y Arima; M Komatsu; H Nishida; Y Tanaka; M Hagio
Original assignee: Catalysts & Chem Ind Co
Priority date: 1982-06-14
Filing date: 1983-06-02
Publication date: 1985-07-12
Also published as: IT8367653A0; PH19651A; IT1162875B

Description

<div class="application article clearfix" id="description"> i 204455 Priority Dafe{s}; £?rX?.\.. Complete Specification Filed: Class: £.^>£33 ]Jft% -4 3\© i.Ttffe?.. H 2 JUL 1985 Publication Date: P.O. Journal, No: 12?!. No.: Date: NEW ZEALAND PATENTS ACT, 1953 COMPLETE SPECIFICATION PROCESS OF CONVERTING SUPERSATURATED SILICA INTO SILICA SOL AND SEPARATING SAME |/We, CATALYSTS $ CHEMICALS INDUSTRIES CO. LTD, a Japanese company, of No. 6-2, Ohtemachi 2-chome, Chiyoda-ku, Tokyo, Japan hereby declare the invention for which $ / we pray that a patent may be granted to hm/us, and the method by which it is to be performed, to be particularly described in and by the following statement: - - 1 - (followed by page la) (%-344 5 5 process of converting supersaturated silica into cilica col and geparatinq dame background of the invention 1. Field of the Invention The present invention relates to a process of reducing a supersaturated silica by converting a supersaturated solution silica-containing aqueous solution into a stable silica colloid that is usually called a silica sol. 2. Description of the Prior Art A supersaturated silica-containing aqueous solution appears to be stable, but in fact is difficult to handle because troubles such as scale-deposition/adhesion and the like take place in equipment using such an aqueous solution. For instance, the geothermal water after vapor separation in geothermal power plants, into which silica under the ground is dissolved, comes to have the silica concentration of 4 00 ppm - 1000 ppm. When the geothermal water is discharged in the atmosphere or subjected to heat recovery treatment, the water temperature lowers so that the dissolved silica content is in excess of the solubility at that temperature, namely in the so-called supersaturated state. In this connection, it is to be noted that the solubility of noncrystalline silica in water is several tens - three hundred and several tens ppm at 2 0 - 100°C. Accordingly, the water contains a considerable amount of supersaturated silica which is unstable and very adhesive,, which leads to that the water containing parts of the equipment are scaled. Studies of dissolving the scale trouble have already been reported in the thesis "Geothermal Generation & Use of Geothermal - 101- I 44 5 5 i water" on pages 100 - 104 of Vol. 15 No. 2 of Magazine "Ceramics", on pages 107 - 120 of the separate volume of Vol. 28 No. 2 of Magazine "Shimadzu Review" (1st Report), on pages 121 - 127 of the same, pages 39 - 46 of Vol. 91 No. 12 of "Journal of the Chemical and Society of Japan" (2nd Report) and the like. According to these reports, it is elucidated that correlation exists between the process of the supersaturated silica in a geothermal water being made colloidal and the adhesiveness of silica, and it is stated that silica deposits and adheres markedly to pipe walls under the condition where silica colloid is formed at a high speed. These reports also elucidate that the formation of colloid can be restrained effectively by taking measures of lowering the reaction temperature to 50°C, stirring, filtering, filtering after mixture of river water and the like, and further state that the adhesion of silica to pipe walls is reduced under the condition where silica colloid particles are about 0.3 y in dimensions when measured by the light scattering method and effective in restraining the formation of colloid. It is sure that the adhesion of silica can be restrained effectively by taking the above mentioned measures and can be reduced to some degree. But, this adhesion trouble is not dissolved yet and still remains as a serious problem, SUMMARY OF THE INVENTION The present invention has been accomplished on the basis of an idea of not restraining the polymerization of a supersaturated silica but forcing the supersaturated silica contained in the solution to polymerize in a short time, and further by discovering the factors and conditions for increasing the polymerization rate and forming a stable silica colloid called silica sol. - 2 - 2 044 5 5 In other words, the present invention comprises mixing a supersaturated silica-containing solution to be treated and a silica seed thereby to prepare a mixed solution wherein the total amount of silica is in the range of 0.05 - 5 wt.% and maintaining the resulting mixed solution at the pH of 6 - 10 and at the temperature of 4 0°C or more thereby to promote the polymerization of a dissolved silica(example, monomeric silica and/or dimeric silica etc.)to thus form a silica colloid and separating it by means of an ultrafilter membrane. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a view illustrating the relationship between the specific surface area of a silica colloid group in a silica seed and the amount (concentration) of a supersaturated silica reduced. Fig. 2 is a view illustrating the case where the presence of silica colloid has been detected by the gel-chromatography method. DETAILED DESCRIPTION OF THE INVENTION A polymerization reaction of a supersaturated silica is slow to take place, because the originally contained dissolved silica alone is insufficient to induce said reaction. Even if said polymerization reaction take place, it is unable to lower the supersaturated portion of the silica in a short time. In particular, the aqueous solution, whose silica concentration is in the range of 400 - 500 ppm, has little possibility of ! inducing a reaction. In case the silica concentration is over this range, there takes place a polymerization reaction which, j however, is defective in that its velocity is slow and - 3 - /! 10 20 30 9 11A A ^ k the polymerized silica to be formed is unstable and difficult to separate. Taking the above mentioned disadvantages into consideration, the present invention is devised to mix another silica source (silica seed) with a solution to.be treated, said silica source functioning as the source to induce the polymerization of the dissolved silica contained in the solution to be treated. A series of experiments showed that a group of small diametric substances may be used as the source to induce the polymerization of the dissolved silica. However, such a group of substances include not only those whose particle diameter can be measured but also those whose particle diameter is too fine to be measured by the present measuring technique. On the other hand, within the silica there is included even the one just after hydrolysis of alkali silicate whose properties are still difficult to grasp clearly. Because of this, its characterization is difficult to jdo in the usual manner of defining with the same properties. ! i The silica seeds employed in the present invention may be concretely enumerated as follows: - i) In the case of mixing a solution to be treated and an alkali silicate solution together with acid or a silica solution obtained by previously neutralizing an alkali silicate solution with acid or dealkalizing an alkali silicate solution with an ion-exchange resin, the effect is attained when the total silica concentration is 500 ppm (0.05%) or more after the completion of said mixing, and excellent effects are attained especially when the total silica concentration is 1000 ppm (0.1%) or more. ii) In the case of mixing a solution to be treated and a silica solution containing a silica polymer which can be fractionated through an ultrafilter membrane whose fractionatable - 4 - 10 20 30 204455 molecular weight is 5000 (the particle diameter of said silica polymer is so fine that another measuring method can not be applied), it is observed that the supersaturated portion of the silica decreases and the more the amount of said silica solution is, the earlier the supersaturated portion of the silica decreases. iii) In the case of mixing a solution to be treated and a silica solution in which it is difficult to grasp the colloidal diameter and molecular weight of the low molecular weight-silica polymer but the peak of the silica polymer concentration besides the peak of the dissolved silica concentration can be clearly observed when measured by the use of the gel-chromatography method, it is observed that effects can be achieved when the total silica concentration is 500 ppm or more. iv) In the case of using a silica solution containing an industrially isynthesized silica colloid whose primary average I particle diameter is 4 my or more, it is observed that said silica solution is serviceable in decreasing the supersaturated silica when the total silica concentration is 5 00 ppm or more and that the more said silica solution amount, the higher the decreasing velocity becomes. v) By using, as a silica source, a non-precipitable silica polymer whose specific area can be measured, there was investigated the relationship between the amount of said silica source added and the amount (concentration) of the supersaturated silica decreased to find the results shown in Fig. 1. The obtained |results proved that a close relationship existed between the !specific surface area of the silica colloid group added and the j !amount of the supersaturated silica decreased and made clear 1 I ^ that when the mixed solution was allowed to have a specific - 5 - 10 20 30 204455 surface area of 1 m or more per liter, it was effective in decreasing the supersaturated silica. In this connection, it is to be noted that Fig. 1 is a view plotting Example 7 - Example 11 referred to afterwords. Comparative explanations will be made on i) - iii) and iv). Example i) - Example iii) are concerned with silica polymers which can be measured in terms of molecular weight rather than particle diameter. These polymers are low in molecular weight (about 500 - 5000). Example iv) is concerned with a normally marketable colloidal silica solution in which particles have been grown up to 4 my or more in order to obtain a high-concentrated solution required from the standpoint of handling. This solution is in a generally close particle state as compared with the silica polymers disclosed in Example i) - Example iii). It is essential that the total amount of silica in the mixed solution obtained by mixing the above mentioned silica seeds and the solution to be treated should be in the range of 0.05 wt.% -5 wt.%. The more the amount of silica seed is, the more the reaction is promoted, whereby the supersaturated silica is reduced in amount and comes near the saturated state at that time. In order to accelerate the reaction rate after mixing, furthermore, it is essential that the mixed solution should be maintained at the pH in the range of 6 - 10 and at the temperature of 4 0°C or more. When the pH is preferably in the range of 7-9 and the temperature is 6 0°C or more, the reaction rate is more accelerated. The silica colloid formed according to the present invention are so stable that it does not re-dissolve easily and it does not deposite and adhere to pipe walls. Further, said silica polymers - 6 - 204455 can' be obtained in the form of silica colloid having dimensions sufficient for separation and removal. It is desirable that said silica colloid should be grown so as to have the particle diameter of 5 my or more. In order to separate the silica colloid from the solution, it is desirable that ultrafiltration should be employed for that purpose. As the ultrafilter membrane there may be used any one which is made of cellulose, polyimide, polyolefin, polysulfone or the like and whose fractionatable molecular weight is in the wide range of about 5000 - 300,000. Said membrane may be used in the normal, not especially specified conditions. The filtrate, which is obtained by filtering through an ultrafilter membrane and contains a reduced amount of super-jsaturated silica, comprises a saturated silica and a small amount i |of supersaturated silica but has no longer possibility to form I silica scale and a fresh silica colloid. t | ij The non-precipitable silica colloid, which is in a state i I I concentrated by means of an ultrafilter membrane and is usually j jcalled a silica sol, has been built up to be larger than the seed. I The silica colloid may become a stable silica sol free from aggregation and sedimentation because it is said to have a negative electric charge on the surface and so the repelling power acts to prevent mutual approaching and coupling. The silica colloid, which has been separated and concentrated by the process of the present invention, permits to obtain 0.1% -5 0% silica sol depending on how to combine the ultrafilter i i |membranes and arranging them in how much stages. Depending on I i how to determine the particle dimensions of silica colloid and the concentration of silica at the starting time, it is also possible to change the pH of the silica colloid being concentrated - 7 - 204455 in the range of 6 - 10. The silica sol is of low viscosity and liquid even when the concentration of silica is raised up to 20% • 50%,,,-and is free from gelation and sedimentation when stored for a long period of time. The present invention is further advantageous in that as mentioned in Example 14, even when the solution to be treated contains arsenic there is no possibility of the arsenic being concentrated in the silica colloid. The effects of the present invention will be clarified in the manner of showing how the silica colloid is formed with reference to Example 1 - Example 14 and Comparative Example 1 -Comparative Example 4 using an artificial geothermal water obtained according to a manufacturing process referred to hereinafter and Example 15 using a real geothermal water. Process for manufacturing an artificial geothermal water 263 g of sodium silicate (silica concentration: 24 wt.%) was dissolved in 100 kg of an ionexchanged water to prepare a dilute sodium silicate solution (silica concentration: 0.063 wt.%) This dilute solution was allowed to pass through a 10 cm-across and 2 00 cm-long column charged with 5 £ of a previously regenerated cation exchange resin (SK-1B) at the rate of S.V.5 to thereby prepare 100 I of a 0.06 wt.% silica solution. 100 I of this 0.06 wt.% silica solution was mixed with DESCRIPTION OF THE PREFERRED EMBODIMENTS 99.5% potassium chloride 38 g 99.5% sodium chloride 244 g 95.0% calcium chloride 2.3 g 99.5% sodium sulfate 14 • 8 9' 10.0 g, 99.5% boric acid 8 204455 99.5% sodium arsenite 0.9 g, and 98.0% sodium hydroxide 8.0 g, and then heated at 80°C for 10 minutes. Thereafter, the same was treated with 6N-HC£ to thereby prepare an artificial geothermal water with the pH of 7.5. The dissolved silica concentration of this water was 600 ppm. This water will be called Water A hereinafter. Example 1 100 I of water A was added with 212.5 g of sodium silicate (silica concentration: 24 wt.%) so that the total silica concentration of this mixed solution was made 1100 ppm. The pH of this mixed solution was made 7.8 using 5N-HC£. Thereafter, said solution was maintained at 80°C for 60 minutes successively. Then, 90 £ of this solution was filtered by means of an ultra-filter membrane (fractionatable molecular weight: 6000) to obtain a filtrate (yield: 89 I). The thus obtained silica colloid was I found to be a stable colloidal solution (silica concentration: 7.4 wt.% and pH: 7.6). Example 2 138 m£ of 1.4N-HCJI was added to a solution obtained by diluting 67.5 g of sodium silicate (silica concentration: 24 wt.%) . in 1415 g of an ionexchanged water to thereby prepare a solution (pH: 7.0 and silica concentration: 1%) in the yield of 1620.5 g. This solution was added to 100 I of Water A so that the total silica concentration of this mixed solution was made 75 0 ppm. The) pH of this mixed solution at this time was 7.3. Thereafter, this solution was maintained at 80°C for 60 minutes successively. Then, 90 £ of this solution was filtered by means of an ultra-filter membrane (fractionatable molecular weight: 6000) to obtain a filtrate (yield: 89 £) . The thus obtained silica colloid was 2 044 5 5 found to be a stable colloidal solution (silica concentration: 3.9 wt.% and pH: 7.2). Example 3 7 0 m£ of 5N-HC£ was added to a solution obtained by diluting 135 g of sodium silicate (silica concentration: 24 wt.%) in 1415 g of an ionexchanged water to thereby prepare a solution (pH: 7.1 and silica concentration: 2%) in the yield of 1620.5 g. This solution was added to 10 0 £ of Water A so that the total silica concentration of this mixed solution was made 15 00 ppm. The pH thereof at that time was 7.2. Thereafter, this mixed solution was maintained at 80°C for 60 minutes successively. Then, 90 £ of this solution was filtered by means of an ultrafilter membrane (fractionatable molecular weight: 5000) to obtain 1 £ of a concentrated solution (silica concentration: 7.5 wt.%). Next, 0.5 £ of this solution was added to 100 £ of Water A and maintained at 80°C for 60 minutes successilvely. Then, 90 I iof this mixed solution was filtered by means of an ultrafilter 'j jmembrane (fractionatable molecular weight: 6000) to obtain a i jfiltrate (yield: 89 £) . The thus obtained silica colloid was (found to be a stable colloidal solution (silica concentration: |6.2 wt.% and pH: 7.0). |Example 4 588 g of 99.5% sodium chloride was dissolved in 100 £ of an ionexchanged water. 416.7 g of sodium silicate (silica concentration: 24 wt.%) was added to this solution while stirring, heated up to 8 0°C and maintained at that temperature for 10 minutes. Thereafter, 6N-HC£ was added thereto so that the pH was made 7.5. This solution was left cooling at room temperatures for 12 hours. This solution was observed by the gel-chromato-graphy method to contain colloid in addition to 14 3 ppm of - 10 - : ^ v 7 J- Zl (.J a dissolved silica (see Fig. 2). 42.9.kg of this solution was added to 100 £ of Water A so that the total silica concentration of this mixed solution was made 720 ppm. The pH at this time was 7.4. Subsequently, this mixed solution was maintained at 80°C for 60 minutes successively. Then, 121 I of this solution was j filtered by means of an ultrafilter membrane (fractionatable molecular weight: 6000) to obtain a filtrate (yield: 120 £). The thus obtained silica colloid was found to be a stable collidal solution (silica concentration: 5.4 wt.% and pH: 7.3). Example 5 25 g of solution containing silica colloid (silica'concentration: 4 0 wt.% and average particle diameter: 16 my) was added to 100 & of Water A so that the total silica concentration of this mixed solution was made 700 ppm. Its pH at this time was 7.7. Subsequently, this mixed solution was maintained at 8 0°C for 60 minutes successively and then filtered by means of"an ! ultrafilter membrane (fractionatable molecular weight: 6000) to !! jobtain a filtrate (yield: 89 I) . The thus obtained silica colloid : jwas found to be a stable colloidal solution (silica concentration: |3.4 wt.% and pH: 7.5). x Example 6 20 g of a solution containing silica colloid (silica concentration: 30 wt.% and average particle diameter: 7 my) was added to 100 I of Water A so that the total silica concentration of this mixed solution was made 660 ppm. Its pH at this time was 7.8. Subsequently, this mixed solution was maintained at 80°C jfor 60 minutes successively and then filtered by means of an j |ultrafilter membrane (fractionatable molecular weight: 6000) to 1 obtain a filtrate (yield: 89 Z) . The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: i - 11 - 2 044 5 5 3.0' wt.% and pH: 7.7). w Example 7 A colloidal solution containing a silica colloid group (silica concentration: 30 wt.% and specific surface area per gram of SiC^: 227 m^) was added to 100 £ of Water A until the specific 2 2 surface area of said silica colloid group became 680 m (6.8 m per £ of the mixed solution) so that the total silica concentration of this mixed solution was made 630 ppm). The pH of this mixed solution at this time was 7.7. Subsequently, the mixed solution was maintained at 80°C for 60 minutes successively, and then 90 £ of this solution was filtered by means of an ultrafilter membrane (fractionatable molecular weight: 6 000) to obtain a filtrate in the yield of 8 9 £. The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 3.7 wt.% and pH: 7.6). Example 8 A filtrate was obtained in the yield of 89 £ according to the exactly same procedure as Example 7 except that the colloidal || solution used in Example 7 was replaced by a colloidal solution !| |j containing a silica colloid group (silica concentration: 20 wt.% •f ! 2 and specific surface area per g of SiC^: 634 m ) and the total silica concentration was made 720 ppm by adding this solution until the specific surface area of said silica colloid group 2 2 became 7600 m (76 m per £ of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 3.6 wt.% and pH: 7.8). Example 9 A filtrate was obtained in the yield of 8 9 £ according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution - 12 - 2 044 5 5 con'taining a silica colloid group (silica concentration: 4 0 wt.% 2 and specific surface area per g of Si02: 61 m ) and the total silica concentration was made 718 ppm by adding this solution until the specific surface area of said silica colloid group 2 2 became 720 m (7.2 m per I of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 3.6 wt.% and pH: 7.5). Example 10 A filtrate was obtained in the yield of 8 9 I according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution containing a silica colloid group (silica concentration: 4 0 wt.% 2 and specific surface area per g of SiC^: 34 m ) and the total silica concentration was made 717 ppm by adding this solution until the specific surface area of said silica colloid group 2 2 became 400 m ' (4.0 m per I of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 3.6 wt.% and pH: 7.6). Example 11 A filtrate was obtained in the yield of 89 £ according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution containing a silica colloid group (silica concentration: 30 wt.% 2 and specific surface area per g of SiC>2: 227 m ) and the total silica was made 1197 ppm by adding this solution until the 2 specific surface area of said silica colloid group became 136 00 m 2 (136 m per I of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 8.3 wt.% and pH: 7.3). - 13 - Example 12 204455 A filtrate was obtained in the yield of 89 Jl according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution containing a silica colloid group (silica concentration: 30 wt.% 2 and specific surface area per g of SiC^: 3 90 m ) and the total silica concentration was made 7 07 ppm by adding this solution until the specific surface area of said silica colloid group 2 2 became 4200 m (42 m per £ of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 3.5 wt.% and pH: 7.5). Example 13 A filtrate was obtained in the yield of 8 9 £ according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution containing a silica colloid group (silica concentration: 5 wt.% 2 and specific surface area per g of SiC>2: 1174 m ) and the total silica concentration was made 1182 ppm by adding this solution until the specific surface area of said silica colloid group 2 2 became 70000 m (700 m per £ of the mixed solution). The thus obtained silica colloid was found to be a stable colloidal solution (silica concentration: 8.3 wt.% and pH: 7.6). Example 14 A filtrate was obtained in the yield of 89 £ according to the exactly same procedure as Example 7 except that the colloidal solution used in Example 7 was replaced by a colloidal solution containing a silica colloid group (silica concentration: 30 wt.% 2 and specific surface area per g of SiC^: 3 90 m ) and the total silica concentration was made 7 00 ppm and the arseric concentration was made 5 ppm respectively by adding this solution until - 14 - 204455 the- specific surface area of said silica colloid group became 2 2 3900 m (39 m per £ of the mixed solution). The thus obtained silica colloid was found to be a stable silica sol in which SiC>2 concentration was 3.4 wt.%, pH 7.6, As concentration 5 ppm and As was not deposited. Example 15 A colloidal solution containing a silica colloid group (silica concentration: 30 wt.% and specific surface area per g 2 of SiC^: 250 m ) was added to 100 £ of a geothermal water taken out of Otake in Oita Prefecture until the specific surface area 2 2 of said silica colloid group became 12500 m (125 m per £ of the mixed solution), whereby the total silica concentration of this mixed solution was made 1000 ppm. Its pH at this time was 8.8. Subsequently, this mixed solution was maintained at 8 0°C for 6 0 minutes successively, and then 90 £ of this solution was filtered by means of an ultrafilter membrane (fractionatable molecular i jweight: 6000) to obtain a filtrate in the yield of 89 £. The thus i obtained silica colloid was found to be a stable colloid (Si02 concentration: 6.3 wt.% and pH: 8.7). Comparative Example 1 100 £ of Water A was maintained at 8 0°C successively while stirring. Thereafter, 90 filtered by means of an ultrafilter membrane molecular weight: 6000) to obtain a filtrate This silica solution was observed to be 0.06 and unstable. !Comparative Example 2 I j By repeating the exactly same procedure I ! Example 1 except that Water A was maintained minutes there was obtained a filtrate in the for 20 minutes £ of this water was (fractionatable in the yield of 8 9 £. wt.% in concentration as Comparative at 80°C for 60 yield of 89 £. - 15 - 204455 Thi's silica solution was observed to be 0.06 wt.% in concentration and unstable. Comparative Example 3 50 g of rock crystal powder (silica concentration: 99 wt.% 2 and specific surface area: 1 m /g) was added to 100 £ of Water A, and thereafter this mixed solution was heated for 60 minutes successively while maintaining it at 8 0°C. This mixed solution had to be mingled and stirred ceaselessly due to deposition of said rock crystal powder. 90 % of this solution was filtered by means of an ultrafilter membrane (fractionatable molecular weight: 6000) to obtain a filtrate in the yield of 89 £. This silica solution was found to be 0.06 wt.% in concentration and unstable. Comparative Example 4 50 g of silica powder (silica concentration: 98 wt.% and 2 specific surface area: 620 m /g) was added to 100 SL of Water A, and thereafter this mixed solution was heated for 6 0 minutes successively while maintaining it at 8 0°C. This mixed solution had to be mingled and stirred ceaselessly due to deposition of said silica powder. 90 I of this solution was filtered by means of an ultrafilter membrane (fractionatable molecular weight: 6000) to obtain a filtrate in the yield of 90 Z. This silica solution was found to be 0.06 wt.% in concentration and unstable. The above obtained filtrates according to Example 1 to Example 15 and Comparative Example 1 to Comparative Example 4 were measured with reference to the silica amount respectively. The obtained results were shown in Table-1 and Table-2. - 16 - Table-1 ?. "445 5 Mixed Solution (maintained at 80°C) Filtrate (80°C) Total silica concentration Dissolved silica concentration To tal silica concentration Dissolved silica concentration (ppm) (ppm) (ppm) (ppm) Example 1 1100 400 401 400 Example 2 750 396 397 395 Example 3 951 385 392 386 Example 4 720 397 399 397 Example 5 700 4 04 410 405 Example 6 660 4 01 403 401 Table-2 \ Specific surface area of colloid group per i of the mixed solution Mixed Solution (maintained at 80°C) Filtrate (80°C) To tal silica concentration Dissolved silica concentration Total silica concentration Dissolved silica concentration (m2) (ppm) (ppm) (ppm) (ppm) Example 7 6.8 630 431 431 430 Example 8 7 6 720 390 390 390 Example 9 7.2 718 421 422 420 Example 10 4.0 717 431 433 432 Example 11 136 1197 360 362 360 Example 12 42 707 401 404 400 Example 13 700 1182 340 343 340 - 17 - •; r»44 5 5 Table-2 (continued) \ Specific surface area of Mixed solution (maintained at 80°C) Filtrate (80°C) \ colloid group per Z of the mixed solution Total silica concentration Dissolved silica concentration Total silica concentration Dissolved silica concentration (m2) (ppm) (ppm) (ppm) (ppm) Example 14 39 700 355 356 354 Example 15 125 1000 353 353 350 Comparative Example 1 - 6 00 (Water A) 598 598 597 Comparative Example 2 - 600 (Water A) 596 599 596 Comparative Example 3 0.5 1094 ( * ) 599 598 598 Comparative Example 4 310 1090 ( * ) 597 597 596 Note) * : (including the deposit amount) It was observed therefrom that in the cases of Comparative Example 1 to Comparative Example 4 some supersaturated silica could be reduced but the reduced silica could hardly be removed because it was unstable and so dissolved again upon ultrafiltration, whilst in the cases of Example 1 to Example 15 a large amount of silica was reduced and could be removed effectively because it was stable. - 18 - 204455 i In this connection, it is to be noted that the above mentioned measurement using the gel-chromatography method was conducted according to T. Tarutuni, (J. Chromatogr., 523 (1970)). The specific surface area of the silica colloid was measured according to the colloidal titration method, namely calculated from the amount of caustic soda (pH: 4.0 - 9.0) titrated. The concentration of dissolved silica was calculated according to the testing method of industrial water (JIS K0101). The total Si02 amount in a sample is estimated previously, and said sample is taken out in an amount of 0.05 g as SiC^- This sample is diluted with a distilled water to 5 0 m£ and its pH is adjusted with 2N-HC& to 1.0. This sample is then placed in a 250 mil-measuring flask and diluted to about 230 mil. 10 mil of a 10% solution of ammonium molybdate was added thereto, diluted with a distilled water to 250 mil and mixed. Its absorbancy is measured with a wavelength of 420 my after 20 minutes' left standing. The amount of dissolved silica is calculated using a calibration curve obtained previously from the absorbancy of the sample (the i dissolved silica referred to in this method means monomeric silica acid and dimeric silica acid). Next, the effects of the present invention will be clarified in the manner of showing the silica scale amounts with reference to Example 16 - Example 19 and Comparative Example 5 and Comparative Example 6 using an artificial geothermal water obtained according to a manufacturing process referred to hereinafter. The particle diameters used herein are all mean values. I Process for manufacturing an artificial geothermal water 425 g of sodium silicate (silica concentration: 24 wt.%) was dissolved in 100 Kg of an ionexchanged water to prepare 100.4 Kg of a dilute sodium silicate (silica concentration: 0.1 wt.%). - 19 - 204455 This dilute solution was allowed to pass through a 10 cm-across and 200 cm-long column charged with 5 £ of a previously regenerated cation exchange resin (SK-lB) at the space velocity of 5 to thereby prepare 100 £ of a 0.1 wt.% silica solution. 100 Z of this 0.10 wt.% silica solution was mixed with 38 g, 244 g, 2.3 g, 14.8 g, and 10.0 g, thereby 99.5% potassium chloride 99.5% sodium chloride 95.0% calcium chloride 99.5% sodium sulfate 99.5% boric acid preparing an artificial geothermal water. This will be called Water B hereinafter. Example 16 1.14 g of 99.5% potassium chloride, 7.33 g of 99.5% sodium chloride, 0.07 g of 95.0% calcium chloride, 0.44 g of 99.5% sodium sulfate, 0.30 g of 99.5% boric acid and 2.73 g of 98% sodium hydroxide were weighed out respectively, then added into j 2 . 9£ of an ionexchanged water measured previously in a 5 £-vessel, i ! and stirred for 1 hour. This solution was mixed with 10.0 g of a colloidal silica (particle diameter: 11.0 my and silica concentration: 30%), and an ionexchanged water was added thereto while stirring so that the total amount might become 3.0 £. Thereafter, the same was mingled for 10 minutes to the full. This solution is employed as a seed solution. 3 £ of this seed solution was poured into a 50 £-reactor equipped with a reflux condenser and a stirrer, and heated until it reached 80°C. After having reached 8 0°C, it was maintained at that temperature for 30 minutes. Thereafter, 40 £ of Water B was added to said seed solution at the rate of addition of 333.3 m£/min. After 4 0 £ of Water B had been added, 43 £ of this solution - 20 - 204455 was poured into a 15 0 ^-reactor equipped with a reflux condenser and a stirrer, and further the remainder of Water B (6 0 £) was added thereto at the rate of addition of 333.3 m£/min. After the total amount of Water B had been added, 103 £ of this mixed solution was filtered by means of an ultrafilter membrane to obtain a filtrate in the yield of 102 £. A fixed amount of this filtrate was taken out and an iron plate was dipped therein thereby to measure the amount of silica scale deposited and adhered thereto. Example 17 The amount of silica scale was measured by repeating the exactly same procedure as Example 16 except that the temperature of seed solution and the temperature at which Water B was added I were set 50°C. Example 18 The amount of silica scale was measured by repeating the exactly same procedure as Example 16 except that the rate at which Water B was added was made 6 66.6 m£/min. i Example 19 1.14 g of 99.5% potassium chloride, 7.33 g of 99.5% sodium chloride, 0.07 g of 95.0% calcium chloride, 0.44 g of 99.5% sodium sulfate, 0.30 g of 99.5% boric acid, 0.20 g of 54.2% aluminum chloride and 2.83 g of 98% sodium hydroxide were weighed out respectively, then added into 2.9 £ of an ionexchanged water measured previously in a 5 £-vessel, and stirred for 1 hour. This solution was mixed with 10.0 g of a colloidal silica (particle diameter: 11.0 my and silica concentration: 30%), and ;an ionexchanged water was added thereto while stirring so that the total amount might become 3.0 £. Thereafter, the same was mingled for 10 minutes to the full. This solution is employed - 21 - 2 044 5 5 as a seed solution. 3 £ of this seed solution was poured into a 50 £-reactor equipped with a reflux condenser and a stirrer, and heated until it reached 80°C. After having reached 8 0°C, it was maintained at that temperature for 30 minutes. Then, 40 £ of Water B was added to this seed solution at the rate of 333.3 m£/min. Thereafter, 43 £ of this solution was poured into a 150 £-reactor equipped with a reflux condenser and a stirrer. Further, the remainder of Water B (60 £) was added thereto at the rate of 333.3 m£/min. After the total amount of Water B had been added, 103 £ of this mixed solution was filtered by means of an ultrafilter membrane to separate a filtrate in the yield of 102 £. A fixed amount of this filtrate was taken out and thereafter an iron plate was dipped therein thereby to measure the amount of silica scale deposited and adhered thereto. Comparative Example 5 i f | 100 £ of Water B was filtered by means of an ultrafilter i | jj membrane according to the same procedure as Example 16 to obtain ! la filtrate in the yield of 99 £. A fixed amount of this filtrate j (was taken out, and thereafter an iron plate was dipped therein to measure the amount of silica scale deposited and adhered thereto. Comparative Example 6 100 £ of Water B was heated to 80°C according to the same procedure as Example 16 and maintained for 2 hours at this temperature. Then, the same was filtered by means of an ultra-|filter membrane to obtain a filtrate in the yield of 99 £. A fixed amount of this filtrate was taken out, and thereafter an iron plate was dipped therein to measure the amount of silica I scale deposited and adhered thereto. - 22 - 2044 5 5 The amounts of silica contained in the filtrates obtained by means of ultrafilter membranes according to Example 16 to Example 19 and Comparative Example 5 and Comparative Example 6 were measured respectively. The obtained results were shown in Table 3 together with their treating conditions. Table-3 \ Treating conditions \ Particle diameter of colloid and composition of silica scale Amount of silica scale 2 \ Concen \ Amount of filtrate used (g) Area of iron plate used / 2. (cm ) tration of dissolved silica in filtrate (ppm) \ in the mixed solution (my) per cm of iron plate (mg) Example 16 12 160 18 344 0.19 Example 17 12 160 18 252 0.14 Example 18 13 160 18 367 0.22 Example 19 12 160 18 210 0.09 Comparative Example 5 3 (Water B) 160 18 469 0.68 Comparative Example 6 4 ( * ) 160 18 497 0.33 Note) * : (Water B as-heated) j | Comparison was made as to the amount of silica deposited and ' i adhered to the iron plate on the filtrate side at 4 0°C, though ; 'i it varies considerably by water temperatures. The obtained | results were as shown in Table 3. It was observed therefrom that | the cases where particles were grown and then filtered through ! I i j an ultrafilter membrane (Example 16 to Example 19) showed lower j values as compared with Comparative Example 5 and Comparative j Example 6. i ^04455 The silica concentration of the filtrate separated from the colloidal solution was calculated according to the same procedure as mentioned above. Silica in the scale was determined by the following method. 2 That is, an iron plate (18 cm ) was dipped in the filtrate and left standing at 40°C for 7 days. Thereafter, the iron plate was taken out and washed with a distilled water to the full. Then, this iron plate was placed in a platinum-made vessel, about 50 m£ of a distilled water was added to said vessel and thereafter treated according to the same procedure as employed in the case of measuring the silica amount of the filtrate. - 24 - </div>

Claims

<div class="application article clearfix printTableText" id="claims"> ! , 204455 WHAT WE CLAIM IS: . . I

1. A process of decreasing the silica content of a supersaturated silica- I - - - containing solution which comprises mixing a supersaturated silica-containing solution to be treated and a silical seed to thereby prepare a mixed solution l - ' - whose silica amount is in the range of 0.05 wt.% - 5 wt.%; maintaining this mixed solution at the pH of 6 - 10 and at the temperature of 4 0°C or more thereby promoting the dissolved silica to polymerize; forming a silica colloid, and separating same.

2. A process according to Claim 1 wherein said supersaturated I t silica-containing solution to be treated is a geothermal water.

3. "A process according ..to Claim 1 wherein said silica seed is an alkali silicate solution or a silica solution obtained by I ineutralizing said alkali silicate solution or dealkalizing it ! • iwith an ion-exchange resin. I I i j

4. A process according to Claim 1 wherein the silica seed is a silica solution containing silica polymers that can be fractionated through an ultrafilter membrane whose fractionatable molecular weight is 5000.

5. A process according to Claim 1 wherein the silica seed is a silica solution in which the presence of polymerized silica jean be detected by the gel-chromatography method. I r |

6. A process according to Claim 1 wherein the silica seed is a silica solution containing a silica colloid whose primary |average particle diameter is 4 my or more. - 2S - i 044 55 >w '--J? '"i, I !7.

A process according to Claim 1 wherein the silica seed is a ! |silica solution containing a non-precipitable silica polymer tha i; ji 2 r * j| gives a specific surface area of 1 m or more per £ of the mixed I j solution to a silica colloid group in the mixed solution at the i |time of mixing the supersaturated silica-containing solution to i |be treated and the silica seed. | j8.

A process according to Claim 1 wherein said formed silica I jcolloid is a silica colloid whose particle diameter is 5 my or ! I more. i I j

19. A process according to Claim 1 wherein the formed silica i I colloid can be separated through an ultrafilter membrane. i j j j

10. A process as claimed in any one of the preceding claims j! and substantially as herein described with reference to any i; ji embodiment disclosed in the examples and/or in the accompanying : drawings. casadSfe-taarcaAS0 By His/Their authorised Agents, A, J. PARK & SON Est like r ^ [* "2 JUNI983 I * - 26 - </div>