Method for preparing silica nanoparticles

1. A method for preparing a silica nanoparticle precursor solution, comprising the steps of:

a) pre-treating a siliceous rock to form a sodium silicate solution;

b) slowly titrating the sodium silicate solution with an acid solution under stirring until a pH value of the sodium silicate solution is reduced by a predetermined value so as to obtain a silicic acid-containing solution;

c) continuously stirring the silicic acid-containing solution until a pH value of the silicic acid-containing solution becomes constant;

d) slowly titrating the silicic acid-containing solution with the acid solution under stirring until the pH value of the silicic acid-containing solution is reduced by the predetermined value so as to obtain a silicic acid-enriched solution having a silicic acid amount which is greater than that of the silicic acid-containing solution by a predetermined increment; and

e) continuously stirring the silicic acid-enriched solution until the pH value of the silicic acid-enriched solution becomes constant if a pH value of the silicic acid-enriched solution does not reach a target pH value.

2. The method according to claim 1 , further comprising a step of collecting the silicic acid-enriched solution as the silica nanoparticle precursor solution when the pH value of the silicic acid-enriched solution reaches the target pH value.

3. The method according to claim 1 , further comprising, after step e), N additional steps of slowly titrating the silicic acid-enriched solution with the acid solution to permit the silicic acid-enriched solution to reach the target pH value, wherein N is an integer larger than 1.

4. The method according to claim 3 , wherein each of the N additional steps of slowly titrating the silicic acid-enriched solution is performed until the pH value of the silicic acid-enriched solution is reduced by the predetermined value.

5. The method according to claim 3 , further comprising, between two successive steps of the N additional steps of slowly titrating the silicic acid-enriched solution, a step of continuously stirring the silicic acid-enriched solution until the pH value of the silicic acid-enriched solution becomes constant.

6. The method according to claim 1 , wherein the predetermined value is about 1.

7. The method according to claim 1 , wherein the target pH value is about 7.

8. The method according to claim 1 , wherein the continuously stirring the silicic acid-enriched solution is performed for a period ranging from about 25 minutes to about 35 minutes.

9. The method according to claim 1 , wherein the acid solution is a hydrochloric acid solution having a concentration ranging from about 3 wt % to about 7 wt %.

10. The method according to claim 9 , wherein the titrating is performed at a temperature ranging from about 55° C. to about 75° C.

11. The method according to claim 1 , wherein step a) includes the sub-steps of:

a-1) crushing and grinding the siliceous rock to form siliceous rock powders; and

a-2) leaching the siliceous rock powders with an acidic leaching solution at a temperature below a boiling point of the acidic leaching solution to remove non-siliceous components so as to form acid-leached siliceous rock powders.

12. The method according to claim 11 , wherein the acidic leaching solution is a sulfuric acid solution having a concentration ranging from about 3 M to about 6 M, and sub-step a-2) is performed at a temperature ranging from about 80° C. to about 95° C. for a period ranging from about 22 hours to about 26 hours.

13. The method according to claim 11 , wherein step a) further includes, after sub-step a-2), a sub-step of a-3) leaching the acid-leached siliceous rock powders with a basic leaching solution at a temperature below a boiling point of the basic leaching solution to obtain the sodium silicate solution.

14. The method according to claim 13 , wherein the basic leaching solution is a sodium hydroxide solution having a concentration ranging from about 25 wt % to about 35 w %, and sub-step a-3) is performed at a temperature ranging from about 80° C. to about 95° C. for a period ranging from about 28 hours to about 32 hours.

15. The method according to claim 1 , wherein the siliceous rock is siliceous mudstone.

16. A method for preparing silica nanoparticles, comprising the steps of:

a) pre-treating a siliceous rock to form a sodium silicate solution;

b) slowly titrating the sodium silicate solution with an acid solution under stirring until a pH value of the sodium silicate solution is reduced by a predetermined value so as to obtain a silicic acid-containing solution;

c) continuously stirring the silicic acid-containing solution until a pH value of the silicic acid-containing solution becomes constant;

d) slowly titrating the silicic acid-containing solution with the acid solution under stirring until the pH value of the silicic acid-containing solution is reduced by the predetermined value so as to obtain a silicic acid-enriched solution having a silicic acid amount which is greater than that of the silicic acid-containing solution by a predetermined increment;

e) continuously stirring the silicic acid-enriched solution until the pH value of the silicic acid-enriched solution becomes constant if a pH value of the silicic acid-enriched solution does not reach a target pH value;

f) collecting the silicic acid-enriched solution as the silica nanoparticle precursor solution when the pH value of the silicic acid-enriched solution reaches the target pH value; and

g) subjecting the silica nanoparticle precursor solution to a flame spray pyrolysis or a drying-grinding-calcining treatment to form the silica nanoparticles.

17. The method according to claim 16 , further comprising, between step f) and g), a step of continuously stirring the silica nanoparticle precursor solution for a period ranging from about 3 hours to about 13 hours.

18. The method according to claim 16 , wherein the drying-grinding-calcining treatment includes the sub-steps of:

i) drying the silica nanoparticle precursor solution to form a dried silica nanoparticle precursor;

ii) grinding the dried silica nanoparticle precursor to form silica nanoparticle precursor powders; and

iii) calcining the silica nanoparticle precursor powders at a temperature ranging from about 900° C. to about 1000° C. to obtain the silica nanoparticles.

19. A paint composition, comprising:

the silica nanoparticles prepared by the method according to claim 16 in an amount ranging from about 0.5 part by weight to about 1 part by weight;

a crosslinking agent selected from the group consisting of polyisocyanate, melamine, aziridine, carbodiimide, and combinations thereof in an amount ranging from about 4 parts by weight to about 5 parts by weight; and

a balance of a water-based paint based on 100 parts by weight of the paint composition.