Process for production of a highly pure silicic acid aqueous solution

A process for producing high purity silicic acid aqueous solution is disclosed, wherein alkali metals in an alkali silicate solution are removed by ion-exchanging, the resulting solution is treated in an electrochemical process in the present of a strong acid and oxidizing agent to solubilize the remaining non-alkali metal salts, and the non-alkali metals in the water soluble salts are removed by ion-exchanging to obtain the silicic acid aqueous solution.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a process for production of a highly 
purity silicic acid aqueous solution from a low pure alkali silicate 
aqueous solution. More particularly, it relates to a process for producing 
a highly pure silicic acid aqueous solution. Considering the fact that low 
purity alkali silicates have been thought to be unsuitable as starting 
materials for silica sources of an artificial quartz or an optical fiber, 
it is surprising that the present invention can use a low purity silicate 
for such purposes. 
2. Description of the Prior Art 
Silica sources for an artificial quartz or an optical fiber have been 
produced by pyrolysis of purified silicon tetrachloride or by hydrolysis 
of purified tetraethyl silicate. Though the silicon tetrachloride and 
tetraethyl silicate have high purities, they are very expensive and 
difficult to handle because of their corrosiveness and flammability. 
A Japanese Patent Laid Open No. Sho. 63-21212 discloses a process for 
producing silica comprising, ion exchanging an alkali silicate aqueous 
solution with cation-exchange resin to get an acidic silica sol, adding an 
acid to the sol, ion exchanging the sol with cation-exchange resin, and 
pouring the thus treated sol into an ammonium-containing alkaline solution 
to precipitate silica. 
A Japanese Patent Laid Open No. Sho. 60-191016 also discloses a process for 
producing precipitated silica by adding a strong acid to an alkali 
silicate to obtain a free acid concentration of 1 N or more, and 
precipitating silica at a high temperature from 70.degree. C. to 
90.degree. C. 
Japanese Patents Laid Open No. Sho. 60-2041613 and Japanese Patents Laid 
Open No. Sho. 60-2041614 also disclose processes which comprise producing 
silica gel from a concentrated alkali silicate aqueous solution by simply 
rinsing it, and heating the gel in a strong acid repeatedly. 
These prior processes, however, hardly provide a highly pure silica having 
metal impurity content of 1 ppm or less because they use low purity alkali 
silicates as starting materials. Consequently, a highly pure (low 
impurity) alkali silicate aqueous solution must be used as a starting 
material to produce a highly pure silica. 
Non-alkali metal impurities exist as silicate complexes in an alkali 
silicate solution The complexes are very stable at ordinary temperature; 
they hardly dissolve in water except when a large amount of hydrochloric 
acid is added. When a hydrochloric acid is added, non-alkali metals form 
water soluble metal chlorides. The previously referred Japanese Patent 
Laid Open No. Sho. 60-191016 uses this reaction. According to this 
invention, a large amount of hydrochloric acid is added to an alkali 
silicate solution (silica content, 10% by weight or less) until the 
solution turns acidic, then the solution is contacted with the 
ion-exchange resin to remove metal impurities. The metal impurities are 
removed very efficiently. 
SUMMARY OF THE INVENTION 
In the referred to processes, silicic acid in a silicate solution under 
acidic condition forms an oligomer or gel in a short time. Once non-alkali 
metals are captured in silicon oxide oligomer or gel, they hardly dissolve 
into the aqueous solution. This is the reason why the prior patents failed 
to obtain non-alkali metal-content of 1 ppm or less. 
The inventors of the present application have studied the removal of 
non-alkali metals which exist as complexes with silicic acid oligomers, 
and have discovered an effective method for removing the metals. The 
method comprises an electrochemical process which serves to change the 
metal complexes to water-soluble metallic salts. Once the metallic salts 
are formed, they are easily removed by ion-exchanging. It is therefore a 
primary object of the present invention to provide a silica source for 
highly pure silica material. It is another object of the invention to 
provide a method for removing metal impurities in an alkali silicate 
solution used as a cheap starting material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The gist of the present invention is to remove alkali metals in an alkali 
silicate aqueous solution by ion-exchanging, to apply an electrochemical 
process to the solution in the presence of strong acids and oxidizing 
agents to from water-soluble salts of non-alkali metals, and to remove the 
non-alkali metals in solution by ion-exchanging. 
The ion-exchange resins used in ion-exchanging are of a strong acid type, 
of weak acid type, or of mixed acid types. 
The application of electrochemical process to the silicic acid solution 
serves to shorten reaction time for metal elements to form water soluble 
salts, thus metal atoms can be easily removed by ion-exchanging before the 
acidic silica sol forms gel. 
According to the present invention, commercially available JIS(Japanese 
Industrial Standard)water glass is preferably used as a cheap starting 
material, however, any alkali silicate aqueous solution may be used as a 
starting material. 
According to the present invention, diluted alkali silicate aqueous 
solutions having silica content of 1 to 10% by weight and a pH7 or more 
are preferably used. 
According to the invention, as acidic silicic acid sol is first produced by 
the removal of alkali metals by ion-exchanging of the alkali silicate 
aqueous solution. Then the electrochemical process is applied to the sol. 
The term "electrochemical process" in the present invention means a 
process comprising inserting a pair of electrodes into the silicic acid 
solution, and applying electrical voltage to the electrodes. On the 
surfaces of electrodes, reduction and oxidation of the metals, existing as 
silicate compounds in the aqueous solution, occur, thus water-soluble 
salts are formed. 
Preferable electrodes are platinum, gold, and gold-platinum alloy 
electrode, however, any other electrode may be used in the present 
invention. Especially preferable electrode is a platinum electrode having 
high surface area, produced by treating the platinum electrode in an 
aqueous solution of PtC1.sub.4 salt such as Na.sub.2 (PtC1.sub.4) or 
K.sub.2 (PtC1.sub.4), until the surface of platinum electrode becomes 
rough. The gold electrode treated similarly may be preferably used. 
Both direct and alternating currents are used, however, the alternating 
current is preferable because hydrogen generates when the direct current 
is used. Voltage of from 0.1 V to 100 V is preferable. The alternating 
current of from 1 Hz to 1 MHz frequency, preferably from 0.1 KHz to 10 KHz 
frequency is preferable. Further, the alternating current having 
rectangular wave form is preferable because it has a higher energy 
density. 
Silica sol, to which the electrochemical process is applied, must be 
acidic, and strong acids having high electronegativity must exist in order 
to form water-soluble metalic salts. Accordingly, the strong acid should 
be added previous to the electrolysis reaction. Examples of strong acid 
are hydrochloric acid, nitric acid and hydrobromic acid. The acid content 
of 0.01 to 1% by weight is preferable. In other words, the acid is 
preferably added so as to get pH of 1 to 2 where longer gel time of silica 
sol is obtained. 
According to the present invention, 0.01 to 1% by weight of oxidizing 
agents must be added to silicic acid sol to accelerate oxidation reaction 
of metals. Preferred oxidizing agent is hydrogen peroxide aqueous 
solution, hypochlorous acid or nitrous acid, or a combination thereof. If 
the oxidizing agent is not added, reduction and oxidation reaction of 
metals is delayed, and gelation of silicic acid proceeds faster than 
oxidation reaction. 
Metal atoms forming water soluble salts after the electorochemical process 
can be easily removed from the silicic acid sol by ion-exchanging. 
However, if gelation of silicic acid occurs, ion exchanging becomes 
impossible. Consequently, it is preferable to ion exchange the sol while 
the electrochemical reaction proceeds More particularly, after the silicic 
acid sol is electrochemically treated, it is passed through an 
ion-exchange column, and again the sol is electrochemically treated and 
the thus treated sol is passed through the column. In order to reduce the 
impurity content, it is preferable to repeat the electrochemical process 
and the ion-exchange process. 
A small ion-exchange column suffices for the above ion-exchange steps 
because the amount of metal atoms to be exchanged is very small. 
As described above, the present invention provides a process for producing 
silicic acid having metal content of 2 ppm or less, preferably of 1 ppm or 
less comprising the steps of: 
removing alkali metals in the alkali silicate aqueous solution by ion 
exchanging to obtain the acidic solution, 
adding the strong acid and the oxidizing agent to the solution. 
applying the electrochemical process to the solution to form water soluble 
salts of non-alkali metals existing in the water as silicate compounds, 
ion-exchanging thus treated solution to remove metal atoms existing as 
water soluble salts. 
EXAMPLES 
The following examples and a comparative example will clearly illustrate 
the preferred embodiments of the present invention. 
In the examples, Fe and Al contents per silica were measured by a 
Inductively Coupled Plasma Atomic Emission Spectroscopy. 
EXAMPLE 1 
A commercially available concentrated sodium silicate solution(Nippon 
Chemical Industries Co. Ltd. JIS No.3 sodium silicate) was diluted to 
silica content of 5% by weight. The solution thus diluted was passed 
through an activated cation-ion-exchange resin(Diaion SK1B) bed to remove 
sodium in sodium silicate. 
To this solution, hydrochloric acid and hydrogen peroxide aqueous solution 
were added to obtain a solution having silica content of 4% by weight, 
hydrochloric acid content of 0.1% by weight, and hydrogen peroxide aqueous 
solution content of 0.1% by weight. The pH value of solution was 0.5, and 
the gelation time was about 72 hours at room temperature. 
Then the solution of 2000 cc was taken into a polypropylene container, and 
was electrochemically treated in the apparatus shown in FIG. 1. 
In FIG. 1, 1 is an electrolytic cell, 2 is an ion-exchange tower filled 
with 2000 ml of Diaion SK1B, and 3 is a storage container, and each of 1, 
2, and 3 is connected with pipes and a pump. The ion-exchange tower 2 is 
provided with an inlet 5 for distilled water and hydrochloric acid. An 
outlet 6 for sample picking-up is provided between the ion-exchange tower 
2 and the storage container 3. 
A silicic acid aqueous solution was pumped to circulate the solution 
through the electrolytic cell 1, the ion-exchange tower 2, and the storage 
container 3. A pair of electrodes (golden plates, 20 cm.sup.2) were placed 
at distance of 2 cm to let the solution flow between the electrodes. An 
alternating current having a rectangular wave from and frequency of 1 KHz, 
voltage of 2 V was applied between the electrodes. Sending speed was 
adjusted to obtain a linear speed in the tower of 2 cm/min. The 
circulation was continued for totally 24 hours. Table 1 shows circulation 
time and Fe and Al contents per silica in the silicic acid solution. 
EXAMPLE 2 
A purification process as described in Example 1 was repeated except that 
platinum electrodes were used instead of gold electrodes. The circulation 
was continued for 24 hours. Fe and Al contents per silica in the silicic 
acid aqueous solution, after the circulation, are shown in Table 1. 
EXAMPLE 3 
The purification process as described in Example 1 was repeated except that 
the alternative current frequency of 100 Hz was used. The circulation was 
continued for 24 hours. Fe and Al contents per silica in the silicic acid 
aqueous solution, after the circulation, are shown in Table 1. 
COMATIVE EXAMPLE 
The commercially available concentrated sodium silicate soluton(Nippon 
Chemical Industries Co., Ltd. JIS No.3 sodium silicate) was diluted to 
silica content of 5% by weight. The aqueous solution thus obtained was 
passed through the activated caton ion-exchange resin(Diaion SK1B) to 
remove sodium in sodium silicate. The silicic acid solution thus obtained 
was treated with the ion-exchange resin two times. Fe and Al contents per 
silica in the silicic acid sol, after ion exchanging, are shown in Table 
1. 
TABLE 1 
______________________________________ 
Example Fe (ppm) Al (ppm) 
______________________________________ 
Circulation time (hour) 
1 0 124 406 
1 20 40 
4 2.5 4 
12 1 or less 
1 
24 1 or less 
1 or less 
2 0 124 406 
1 30 60 
4 10 15 
12 1 or less 
2 
24 1 or less 
1 or less 
3 0 124 406 
1 24 48 
4 8 13 
12 1 or less 
2 
24 1 or less 
1 or less 
number of ion 
exchange times 
Comparative 
0 124 406 
Example 1 54 198 
2 48 198 
3 48 198 
______________________________________ 
The above results clearly show that the process comprising, ion exchanging 
the alkali silicate aqueous solution to obtain the acidic solution, adding 
the strong acid and the oxidizing agent to the acidic solution, applying 
the electrochemical process to the solution to form water soluble salts of 
non-alkali metals which dissolve in water as silicate compounds, and ion 
exchanging the solution to remove non-alkali metals, is a very excellent 
process for eliminating metals from sodium silicate. 
Removing metal elements from an alkali silicate aqueous solution to a metal 
content of 1 ppm or less has been thought to be impossible. However, 
according to the present invention, metal elements in an alkali silicate 
aqueous solution can be removed very easily and effectively to a metal 
content of 1 ppm or less. 
Highly pure silica can be produced from the low metal-content-silica sol, 
produced thus by adding a large amounts of ammonium salt and organic 
alkali to the sol to precipitate silica, and rinsing precipitates thus 
formed with pure water. Further, highly pure colloidal silica can be 
produced from silica sol of the present invention. 
Highly pure silica produced according to the present invention is used as 
all kinds of fillers, fillers for simiconductors, and for quartz glass.