How to make silica sol?

Chemical compound

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Sodium metasilicate is the chemical substance with formula Na
2SiO
3, which is the main component of commercial sodium silicate solutions. It is an ionic compound consisting of sodium cations Na+
and the polymeric metasilicate anions [SiO2
3]n. It is a colorless crystalline hygroscopic and deliquescent solid, soluble in water (giving an alkaline solution) but not in alcohols.[1]

Preparation and properties

The anhydrous compound can be prepared by fusing silicon dioxide SiO
2 (silica, quartz) with sodium oxide Na
2O in 1:1 molar ratio.[2]

The compound crystallizes from solution as various hydrates, such as

• pentahydrate

  Na

  
  2

  SiO

  
  3

  ·5

  H

  
  2

  O (CAS -79-3, EC 229-912-9, PubChem )
• nonahydrate

  Na

  
  2

  SiO

  
  3

  ·9

  H

  
  2

  Hebei Silicon Research Electronic Materials Co., L Product Page

  O (CAS -24-3, EC 229-912-9, PubChem )[3]

Structure

In the anhydrous solid, the metasilicate anion is actually polymeric, consisting of corner-shared {SiO4} tetrahedra, and not a discrete SiO32 ion.[4]

In addition to the anhydrous form, there are hydrates with the formula Na2SiO3·nH2O (where n = 5, 6, 8, 9), which contain the discrete, approximately tetrahedral anion SiO2(OH)22 with water of hydration. For example, the commercially available sodium silicate pentahydrate Na2SiO3·5H2O is formulated as Na2SiO2(OH)2·4H2O, and the nonahydrate Na2SiO3·9H2O is formulated as Na2SiO2(OH)2·8H2O.[5] The pentahydrate and nonahydrate forms have their own CAS Numbers, -79-3 and -24-3 respectively.

Uses

Sodium Metasilicate reacts with acids to produce silica gel.[6]

• Cements and Binders - dehydrated sodium metasilicate forms cement or binding agent.
• Pulp and Par - sizing agent and buffer/stabilizing agent when mixed with hydrogen peroxide.
• Soaps and Detergents - as an emulsifying and suspension agent.
• Automotive applications - decommissioning of old engines (CARS program), cooling system sealant, exhaust repair.
• Egg Preservative - seals eggs increasing shelf life.
• Crafts - forms "stalagmites" by reacting with and precipitating metal ions. Also used as a glue called "soluble glass".
• Hair coloring kits

See also

References

12,260 ~IELD OF THE INVENTION
This invention relates to a process for the preparation of a stable, colloidal silica sol.

DESCRIPTION OF THE PRIOR ART
Silica sol and silica gel have for many years been employed in a variety of industries, such as those re~uiring the use of absorbents, catalyst bases, binders, antisoiling agents, thickening agents and the like. In particular silica sol is widely used as binders for in-sulating materials and catalyst manufacture.
The present commercial processes for theproduction of colloidal silica sol all use sodium silicate as the starting material. However7 the increasing cost of sodium silicate has escalated the ultimate cost of the silica sol or gel to the consumer. Hence, the large volume of silica sol used commercially dictates that the most economical process be employed in its production.
In U. S. Patent 3,494,874 which issued on February 10, to E. M. Flanigen and R. W. Grose there is disclosed a process for preparing a silica gel agglomerate. The silica gel is prepared by the controlled extraction of a narrow class of metal silicates with aqueous acids having PKa values of less than about 3. It is also indicated in the patent that the properties of silica gels can ~ary over a wide range depending upon their method of preparation and only a relatively few can be considered as practical adsorbent materials. One con~entional method 12,260 referred to in the patent for preparing adsorbent quality silica gels is the acidification and gelation of water solutions of soluble silicates, such as codium silicate.
The patentees indicate that they can obtain siliceous materials which are ideally suited as adsorbents but ex-tracting certain metal silicates with inorganic acids having the above pKa values. Among the many metal silicate compounds disclosed is calcium metasilicate, such as wollastonite. The cited patent, however, is directed to the preparation of a silica gel agglomerate as opposed to a stable, colloidal silica sol. Hence, prior to the present invention, all of the known, commercial methods for preparing stable, colloidal silica sol used sodium silicate as the starting material.
Accordingly, one or more of the following objects can be achieved by the practice of this invention. It is an object of this invention to provide a process for the preparation of stable, colloidal silica sol from relatively inexpensive starting materials. A further obj ect of this invention is to provide a process for preparing a silica sol having a particle size and weight percent silica which renders it ideally suitable as a binder. Another object of the invention is to provide a process for the preparation of a stable silica sol of a desired particle size and weight percent whic~ involves a simple precipitation and ion exchange technique. These and other objects can readily be achieved in accordance with the teachings herein set forth.

1 12,260 SUMMARY OF THE INVENTION
In its broad aspect, the present invention is directed to a process for the preparation of stable, colloidal silica sol which comprises the steps of:
(a) forming an aqueous slurry of calcium silicate, (b) vigorously mixing the slurry and simultaneously adding a dilute and having a PKa value of less than about 8, the anion of which forms a precipitate with the calcium ion, (c) heating the slurry at a tempera-ture of up to about 100C., and for a period of time until precipitation is complete, (d) filtering the slurry and contacting the filtrate with at least one ion exchange resin, (e) recovering the silica sol, and (f) thereafter stabilizing and con-centrating the sol to provide the colloidal silica sol.

DESCR IPT ION OF THE PREFERRED EMBOD IMENT
It has now been found that a stable, colloidal silica sol, for example, one comprised of 15 weight per-cent silica and having average particle sizes of from 3-13 m~, can readily be formed by reacting calcium silicate and a dilute acid having a pKa ~alue of less than about 8, such as 0.5 - lN sulfuric acid. The term pKa is used herein in its conventional sense as representing 1 12,260 the negative logarithm of the acidic concentration ionization constant.
The procedure involves extraction of calcium from calcium s~licate, separation of silica sol from calcium sulfate, removal of impurity ions by ion exchange method and finally stabilization and concentration. This method provides an alternative way to the present commercial process of making colloidal silica sol by using sodium silicate as a starting material, which as hereinbefore indicated has economic disadvantages. Calcium silicate is an especially good choice, since the cost of the starting materials for making calcium silicates, i.e., lime and silica, has remained low in the pennies per pound range Also the readily available mineral material (wollastonite) is in the 1 - 3 cent per pound range.
As in the case of making silica gel from calcium silicate, calcium can be extracted from the "framework"
to form soluble or insoluble salts with different acids.
For making silica sol, those acids which form insoluble calcium salts are favored, because the formation of soluble salt could cause coagulation of colloidal silica particles and result in large size particles or silica gel. The critical concentration of calcium ions in solution re-quired to coagulate colloidal silica with respect to pH
and particle size is known in the art.- For example, the concentration of calcium ions needed to completely coagulate the colloidal silica is only in the milli lar range.
Thus, if the acid used to extract calcium from calcium silicate forms a soluble calcium salt, the coagulation of colloidal silica would be una~oidable, unless cation 12,260 exchange resin is em~loyed during the extraction process to remove calcium ions as soon as they are formed. Such a process is very similar to the present commercial process of making silica sol from sodium silicate. The commercial process of making silica sol is by neutralizing the basic sodium silicate with acid and meanwhile remove sodium ions by ion exchange process.
In the present process, sulfuric acid was ex-tensively studied because it is the cheapest acid on the market. However, as previously indicated, any dilute acid having a pKa value of less than about 8 can be employed as long as it forms an insoluble salt with calcium and otherwise does not interfere with the preparation of the sol. Illustration acids in addition to sulfuric, include phosphoric and similar inorganic acids, and strong organic acids.
~ rom experience in using sulfuric acid to extract calcium out of calcium silicate, it was found that the optimal ratio of calcium silicate to water is around 0.56 on weight basis (i.e., 25 grams of calcium silicate hydrate in 450 ml of distilled water). If the ratio is higher than 0.056, the final mix after extraction is very thick (due to the formation of insoluble calcium sulfate) and efficient mixing becomes very difficult. On the other hand, if the ratio is lower than 0.056, the~amount of silica in the aqueous sol is reduced. Extraction of calcium by dilute (lt2 - lN) sulfuric acid can be achieved in less than one minute in a Waring blender at ambient temperature. After com~lete extraction, separation of silica sol from insoluble calcium sulfate can be easily accomplished by heating the _ ~ _ 12,260 mix from ambient to about 100C., and preferably to about 60~C and then filtering. The purpose of heating is to coagulate the calcium sulfate but not the colloidal silica particles. Several coagulation temperatures were tried (1~0C, 80C, 60C and 40C) and the re-sults set forth in the Examples indicate that at 40-60C
range the growth of silica particles is minimal (~ 3 m~
The particle growth rate is correlated to the amount of silica sol passing through Whatman No. 41 filter paper as summarized in Table 2. The more the wt. % of silica in the filtrate, the smaller the particle size and thus the slower the growth rate.
In preparing the silica sol of the present inven-tion it was first thought that the silica sol filtrate thus obtained could be first stabilized and then concentrated down to the desired concentration. Commercial silica sols are usually stabilized by the addition of a small quantity of either sodium hydroxide or ammonium hydroxide to adjust the pH of the sol to the ran~e of 8-10. Addition of sodium hydroxide to the dilute sol supposedly can be achieved by adding acidic sol slowly into a dilute sodium hydroxide solution with vigorous stirring. Howe~er, it was unexpected-ly and surprisingly found that silica gel instead of silica sol was o~tained. The reason for the gelation of silica sol is believed to be due to t~e excess amount of impurity ions in the sol. Since the starting materials, lime, sand and wollastonite, all contain fairly large amounts of impurity (87-95% CaO in lime, ~ 98% SiO2 in sand and -v98% CaSiO3 in wollastonite), one would expect 0.3% or 12,260 more impurities ~Mg, Al, etc.) in the dilute sol. If ~he dilute sol was first treated with cation and anion exchange resin and then stabilized with NaOH solution, a stable dilute silica sol was obtained. To make concentràted silica sol (20~/o or more SiO2 by weight), the dilute sol is concentrated by evaporation.
By conducting the process as hereinbefore indicated it was observed that the separation of silica and calcium sulfate by a one step filtration was very success-ful. The amount of silica in the filtrate is about 77 wt. /cof the total silica content of the starting material. And the amount of calcium sulfate in the filtrate is very small C0.2 wt. %~. However, in the filter cake, 86 wt. % is calcium sulfate and only S wt. % is silica. Chemical analy-sis of sol samples prepared by the process of this invention also indicates that a fairly large amount of silica (~-0.6 wt. %) was trapped in the ion exchange columns during the ion exchange processes. Thus resulted a final yield of silica of only 56%. On the other hand, the ion exchange columns trapped almost all of the calcium sulfate particles and resulted in a very pure final silica sol product (CaS04 wt. % <0.034%). The particle size remained relatively constant in all the dilute sols (all around 3 - 5 m~ ~ and increased some (3 - 13 m~) only when the dilute silica sol was condensed down ~o ~ 15 wt. % of silica. Such a particle growth is mainly due to the long heating effect during the conversion process.
One of the starting materials, hydrated calcium sulfate, used in the following examples WBS msde 12,260 in the laboratory under hydrothermal condition (CaO/SiO2 =
O.9, autoclaved at 385F for 24 hours), and it was determined by analysis to be a xonotlite-rich material. Powder was obtained by breaking up a monolith in a Waring blender.
The other one, wollastonite, a (Grade P-4) calcium silicate, was obtained from Interpace Corporation.
Reagent grade sulfuric acid was used for extraction.
The desired concentration was achieved by diluting concen-trated acid with distilled water. pH measurements were obtained by using a Sargent-Welch pH/activity meter ~Model PAX). Orion calcium ion specific electrode was used to detect the calcium ion activity in the silica sol.
All the filtratior, processes were done with Buchner funnel and Whatman No. 41 filter paper under vacuum (aspirator). The ion exchange resin was purchased from Rohm and Haas Company under the tradename of Amberlite~P
(IR-123 and IR-45). Ion exchange was performed in a Pyrex glass column.
Transmission electron microscopic studies in-~ol~ed dilution of silica sol by distilled water followedby ultrasonic dispersion. The turbidity measurements were obtained by passing 410A wavelength light through a one centimeter quartz sample cell with a slit width of O.032 mm in a Cary 14 instrument.
The following examples illu~trate the best mode presently contemplated for the practice of the present invention:

~r.~

12,260 The following Examples are illustrative.

PREPARATION OF HYDRATED CALCIUM
SILICATE
Hydrated calcium silicate was prepared from calcium silicate (CaO/SiO2 = 0.9) by autoclaving the material at 385F. for 24 hours and then drying overnight at 110~C. The starting material had a porosity of approximately 90 percent. X-ray crystalline composition and chemical analysis of the hydrated material are set forth in Table I below:

Z ~ o ~ ~
~ a~

E~
Z ~ o P~ U'~

~1 Z ,~
P: oo z E~
o U~
:4 C~ ~ ~

o~
., ~ ~ ~ C~
~ rl ~ .~ ¢ O O
u~ o a) ~ ~ ~
~ x ~ 3 ~ ~

O9Z ' Zl 12,260 PREPARATION OF STABLE SILICA SOL
The process was conducted by mixing 75 grams of the hydrated calcium silicate prepared in accordance with Example 1, with ml of distilled water in a Waring blender. (Due to the size of the Waring blender used, the mixing was completed in three consecutive runs.) While stirring vigorously, 34.5 ml of 95-98% sulfuric acid was added at once. An additional minute of stirring was carried out after addition of sulfuric acid. Then the slurry was poured into a glass beaker and heated to 60~C
with stirring (magnetic bar). The whole hot mix was filtered through a Buchner funnel with a Whatman No. 41 filter paper under vacuum drawn by an aspirator. The pH
value of the filtrate collected at this point was approximately 1.5. After the filtrate cooled down to ambient temperature, the mix was poured first into a cation exchange column (filled with Amberlite IR-120) and then into an anion exchange column (filled with Amberlite IR-45).
The pH values of the silica sol after passing the cation and anion exchange column were in the range of 1.3 - 1.5 and 5.7 to 7.6 respectively. The final pH of the dilute sol was adjusted by addition of 0.1 N ~aOH to about 8.6.
Concentration of the stabilized dilute sol was carried out in a rotavapcr pumped with an aspirator.- The temperature was maintained at 60C durin~ the entire evaporation process.
The product of each preparation step was studied by means of pH measurements. Particle size measurement was done by transmission electron microscope, turbidity by light scattering and percent silica and calcium sulfate by wet 12,260 chemical analysis. The results are summarized in Table II
below:

115~728 e e ~ '`

H ~ : 5 3 _ ¦ e e ¢ e e e e o o e ~) Z Z o o o o Z
~ I O' U~ 'O C

~1 ~ e ," O ~ O o ~ ' o~ ~ = o sl z z ~ O

. ~ ~5'b6-tfy~

O9Z' Zl 12,260 PREPARATION OF STABLE SILICA
SOL FROM WOLLASTONITE
The process was conducted by mixing 30 grams of wollastonite powder (Interpace Corporation Grade P-4) with 500 ml of distilled water in a Waring blender (due to the size of the Waring blender used, the mixing was repeated four times). While stirring vigorously, 13.6 ml of ~5-98% sulfuric acid was added at once. The slurry was stirred for an additional five minutes after addition of sulfuric acid. Then the slurry was filtered without further heating (the temperature of the slurry after mixing was 40C) through a Buchner funnel with a Whatman No. 41 filter paper under vacuum drawn by an aspirator. The pH value of the filtrate collected at this point was approximately 2.85 and the solid residue in the filtrate was determined to be 2.3 wt. %. After the filtrate cooled down to ambient temperature, the mix was poured first into a cation exchange column (filled with Amberlite IR-120) and then into an anion exchange column (filled with Amberlite IR-45). The pH value of the silica sol after passing the cation and anion exchange column were 2.3 and 7.9 respectively. The final pH of the dilute sol was adjusted by addition of lN NaOH to about 8.5. Concentration of the stabilized.dilute sol was carried out in a Rotavapor pumped with an aspirator.
The temperature was maintained at 65C during the entire evaporation process.
The product of each preparation step was studied by means of pH measurements. Particle size measurement was done by transmission electron microscope and percent 11~ 12,260 silica and calcium sulfaee by wet chemical analysis. The results are summarized in Table III below:

-- Ll --U C~
,,, _ p,_ ~ ~ ~ ~r a~ N Z
n :

~P
æl ~ ~ ~
Z Z z æ o _ c~
~ I` o o ~
o I~S _I o o o U~
~ Z o o o o U V V
U~
C
o o~
_ , ~n -- d .~1 ~. ~ o 1`
o . . . . .
r~ O ~ _l ~ CO e U~ ~ _~
H ~J

3:1 ~ a~ ~ ~ ~ a o z ~ u C
o ~ 3 ~U ~o c eu o .,.1 ~ .~ L~
c -~ U ~c C ~ X
o a~ ~ ~ ~ x ~ ~ ~ E~ ~ ~~ ,. _, r~ ~ e ~ e ~
h (~ 6 0 ~ C ~ e Q~ ~ ~ O
-rlC U C
~ o ~ o ~ o C o 5: 3 ~ c ~ ~ l a~ ." ~:
_1 '~
3 ~: z a~ o ~ ~ ~

OgZ' Zl 1 12,260 Although the invention has been illustrated by the preceding examples, it is not to be construed as being li~ited to the materials employed therein, but rather the invention encompasses the generic area as hereinbefore dîsclosed. Various modifications and embodiments thereof, can be made without departing from the spirit and scope thereof.

Sodium metasilicate - Wikipedia

CAA - Process for the preparation of silica sol