Process for the manufacture of potassium stabilized silica sols

A method for producing a silica sols containing less than 150 ppm sodium haivng an average particle size of 2-110 nm, and a particle size distribution with a coefficient dispersion of 0.16-0.4. The silica sols are produced using a KOH heal to which aliquots of modified silicic acid containing acidic anions are added. The aliquots are added to maintain a substantially constant concentration of colloidal silica in the product. Subsequent to production, the sols may be concentrated by ultrafiltration. Preparation of low sodium silica sols utilizing a constant silica technique by heating the initial heel to a temperature greater than 80.degree. C. and adding additional silica maintaining an approximately constant total silica. The stabilization is effected with a KOH.

THE INVENTION 
The field of the invention is the preparation of low sodium silica sols. 
More particularly the invention is directed to low sodium silica sols 
having less than 100ppm sodium. 
BACKGROUND 
Recent advances in the application of colloidal silica have brought an 
increase in the demand for low sodium colloidal silica products. Producers 
of electronic chips currently view the presence of sodium as an 
undesirable contaminant. Their belief is that sodium ions present in the 
final product result in defective microchips. Thus, manufacturers of 
electronic components have begun to request that the materials used in 
manufacturing have lower sodium levels. 
The areas of coatings and catalysts also have seen an increase in demand 
for low sodium colloidal silica products. Excessive sodium can lead to 
unwanted haze or cracking in many coating applications. Sodium has been 
shown to be detrimental to catalyst behavior by lessening the selectivity 
and activity of catalysts. In many instances, the presence of sodium leads 
to total catalyst deactivation. 
The following patents teach the production of low sodium silica sols: 
Weldes, U.S. Pat. No. 3,239,521 discloses a method of forming soluble 
sodium free hydroxylated organic quaternary nitrogen silicates. While 
producing a silica containing compounds substantially free sodium, Weldes 
does not form a sols and his ratio of quaternary to silica content is 
high. Another Weldes Patent U.S. Pat. No. 3,326,910 also discloses a 
method for preparation of amine silicates which are substantially free of 
alkali metal ions. Again, the silicate used is reacted with an amine and 
the resulting amine silicate solution is apparently soluble in water. 
Yates, U.S. Pat. No. 3,597,248 discloses a method of producing guanidine 
silicates. Again, the form of the silicate is water soluble and contains a 
large percentage of the organic amine. A second Yates Patent, U.S. Pat. 
No. 3,630,954 discloses colloidal silica sols having a high surface are 
which are stabilized by an organic amine and strong base. 
Vossos et al. U.S. Pat. No. 3,582,494 discloses a method for producing 
alkaline aqueous colloidal silica sols from salt free acidic colloidal 
sols by treating such acidic sols with at least 0.003% by weight, based on 
the weight of the silica in such sols, of a salt whose anion is derived 
from a weak acid and whose ionization constant should not exceed that of 
carbonic acid in order to stabilize the sols. 
Rule, U.S. Pat. No. 2,577,484 discloses a process for producing stable 
silica sols having an SiO.sub.2 to "base" molar ration, expressed as 
"M.sub.2 O" being from 130:1 to 500:1. Rule also discloses a method for 
utilizing alkanol amines as a stabilizer. 
Schaefer, U.S. Pat. No. 4,054,536 discloses a process producing silica sols 
free of metal oxides thorough the use of organic amines. Such sols having 
broad particle size ranges and short stabilities. 
SUMMARY OF THE INVENTION 
The invention includes a potassium based method of preparing silica sols 
which contain less than 150 ppm sodium (w/w) on a product basis, and 
preferably less than 100. 
A method of producing the low sodium silica sols comprising the following 
steps: 
A. Providing a 0.1%-5% aqueous solution solution of a potassium, 
B. Heating the aqueous solution of the potassium hydroxide to at least 
80.degree. C., 
C. Preparing a modified acid sol by mixing aqueous silicic acid and another 
acid in a weight ratio of silicic acid to the other acid with the range of 
from 10:1 to 150:1; 
D. Adding to the heated aqueous solution of the KOH a 2 to 10% by weight 
modified acid sols in a volume ratio of from 2:1 to 15:1, the mole ratio 
of SiO.sub.2 : KOH being in the range of 1:1 to 150:1 thereby producing 
the silica sols; and then, 
D. Concentrating the silica sols by either distillation or ultrafiltration 
so as to recover an aqueous colloidal silica sols which contains 5-55% by 
weight SiO.sub.2 having pH of 8-10.5 and containing less than 150ppm 
sodium metal. 
The process wherein the heel is composed of a KOH and anion modified 
silicic acid such that the ratio of anion modified SiO.sub.2 to KOH ratio 
is 1:1 to 50:1. 
The silica sols generally have an average particle size of 2-110 nm and 
preferably of from 10-30nm and a particle size distribution with 
coefficient of dispersion of 0.16-0.4 and preferably 0.16 to 0.25. These 
sols exhibit long-term stability. 
The potassium based low sodium silica sols are prepared employing KOH and 
acidic anions (such as Cl.sup.--, SO.sub.4.sup.--2, NO.sub.3.sup.--). The 
heel includes KOH and/or water and modified silicic acid (i.e. acid sol), 
where the acid sol has been modified by the addition of an acid or acid 
salt. Generally, the heel may contain from 0.1 to 5 wt.% aqueous KOH, and 
preferably from 0.5 to 2.2 wt. % KOH. The heel may include from 0.-10% 
modified acid sol. 
The invention includes the steps of heating a heel containing from 0.1 to 5 
wt. % of a KOH to a temperature within the range of from 80.degree. C. to 
130.degree. C. and preferably within the range of 80.degree. C. to 
110.degree. C. and most preferably within the range of 90.degree. C. to 
105.degree. C. temperature and then adding aliquots of modified acid sols 
to the heel. The aliquots are added at a measured rate designed to 
maintain substantially constant concentration of SiO.sub.2 (2-6 wt.%) in 
the mixture. 
The method of preparation of the new potassium based low sodium sols of 
this invention differs from the previous work in two important ways. The 
first difference is that the particle growth process and the concentration 
process are separate steps. Previous work combined the particle growth and 
concentration steps into a simultaneous procedure. The second difference 
occurs in that the silica sol is directly prepared from a potassium source 
rather than removing the sodium then replacing it with potassium or using 
drastic synthetic preparatory conditions.

EXAMPLES 
Materials 
Reagent grade KOH potassium hydroxide was used for all preparations. 
Acid sols (decationized sodium silicate) was prepared for synthesis by 
cation exchanging diluted sodium silicate by passing it through Dow HGR-W2 
resin in the Hydrogen form. The starting sodium silicate concentration was 
adjusted such that the acid sols produced had a silica concentration of 
5.0.+-.0.1 (w/w). The acid sols for the process area synthesis varied in 
concentration from 4.5 to 6.2 percent SiO.sub.2. 
Laboratory Preparations 
Modified acid sols was prepared by adding to the freshly prepared aqueous 
silica acid, an acid or acid salt. Although HNO.sub.3, HCl, H.sub.3 
PO.sub.4, H.sub.2 SO.sub.3 acetic acid and the potassium salts of these 
acids are examplory other acids or acid salts are usable. 
EXAMPLE 1 
In the laboratory, a mixture of 10.58 gm 45% KOH (w/w) and 338 gm H.sub.2 O 
was heated to 95.degree. C. Modified acid sol containing 157.9 g of 
SiO.sub.2 and 2.68 g of concentrated HNO.sub.3 was added over a 21/4 hour 
period with heating and stirring. The reaction produced a 5% silica sols 
with an average particle size of 18 nm. 
EXAMPLE 2 
In the laboratory, a mixture of 10.58 g KOH (45% w/w) and 305 ml H.sub.2 O 
was heated to 85.degree. C. Oven a 135 minute period a mixture of 2788.4g 
silicic acid (5.66% SiO.sub.2 w/w) and 1.34 g nitric acid was added with 
heating and stirring. After completion of the modified acid sols mixture 
the reaction was heated and stirred for 30 minutes. This produced a 2 nm 
product at a SiO.sub.2 concentration of 5% (w/w). 
EXAMPLE 3 
In the laboratory a mixture of 289.7g H.sub.2 O and 10.58 g of 45% KOH 
(w/w) was heated to 95.degree. C. Modified acid sols containing acid sols 
(2710.3 g 5.83% SiO.sub.2) and acetic acid (1.50g) was added over a 135 
minute period with heating and stirring. After completion of the modified 
acid sols addition the mixture was stirred and heated for an additional 30 
minutes. This produced a 16 nm product at a SiO.sub.2 concentration of 5% 
(w/w). 
EXAMPLE 4 
In the laboratory, a mixture of 10.58gm 45% KOH (w/w) and 289.7g H.sub.2 
was heated to 95.degree. C. Modified acid sols containing acid sols 
(2810.3g, 5.83% SiO.sub.2) and acetic acid (2.86g) was added over a 135 
minute period with heating and stirring. After completion of the modified 
acid sols addition the reaction was heated and stirred for 30 minutes. 
This produced a 17 nm product at a SiO.sub.2 concentration of 5% (w/w). 
EXAMPLE 5 
IN a 50 gallon reactor five gallons of H.sub.2 O and 559.0 g of 45% KOH 
(w/w) were added and mixed. After heating the mixture to 97.degree. C. a 
mixture of silicic acid (19.54 Kg, 5.34% SiO.sub.2) and nitric acid 
(141.6g) was added to the reactor over a 131 minute period maintaining the 
temperature at 97.degree. C. After completing the addition of the acid 
sols mixture the reaction was stirred and heated for 30 minutes. 
After cooling the mixture to 30.degree. C. the product was concentrated to 
30% SiO.sub.2 (w/w) using normally concentrating methods. The average 
particle size was 20 nm. 
EXAMPLE 6 
In a 50 gallon reactor five gallons of H.sub.2 O and 515g of 45% KOH (w/w) 
were added and mixed. After heating to 98.degree. C., a mixture of silicic 
acid (19.54 Kg, 4.92% SiO.sub.2) and nitric acid (130.5g) was added to the 
reactor over a 139 minute period maintaining the temperature in the range 
of 95.degree.-96.degree. C. After completing the addition of the acid sols 
mixture, the reaction was stirred and heated for 30 minutes. 
After cooling the mixture to 30.degree. C. the product was concentrated to 
30% SiO.sub.2 (w/w) using normal concentrating methods. The average 
particle size was 18 nm.