Reverse osmosis for purifying mixtures of hydrofluoric acid and nitric acid

Abstract

Disclosed is a method of purifying a solution containing hydrofluoric acid, nitric acid and at least one silicon impurity by treating the solution with at least one reverse osmosis membrane. According to the method of the present invention, silicon impurities contained in the solution containing hydrofluoric acid and nitric acid can be selectively removed or reduced. This method can be advantageously used in the photovoltaic industry or in the battery component industry.

Claims

1. A method of purifying a solution containing hydrofluoric acid, nitric acid and at least one silicon impurity, the method comprising treating the solution with at least one reverse osmosis membrane, wherein at least 90% of the at least one silicon impurity is rejected, and wherein hydrofluoric acid and nitric acid in the solution are not significantly rejected.

2. The method according to claim 1, wherein the solution mainly contains hydrofluoric acid and nitric acid and comprises silicon compound as an impurity.

3. The method according to claim 1, wherein the content of nitric acid in the solution before the purification is from 0.1% w/w to 10% w/w.

4. The method according to claim 1, wherein the content of hydrofluoric acid in the solution before the purification is from 0.001% w/w to 50% w/w.

5. The method according to claim 1, wherein the content of silicon impurity in the solution before the purification is from 0.1% w/w to 30% w/w.

6. The method according to claim 1, wherein one or more reverse osmosis membranes are used, in series or in parallel, or in combination of series and parallel.

7. The method according to claim 1, wherein the silicon impurity is generated by contacting a solution containing hydrofluoric acid and nitric acid with a silicon containing surface.

8. The method according to claim 7, wherein the contacting a solution containing hydrofluoric acid and nitric acid with a silicon containing surface is a surface texturing of solar cells or an etching of silicon based electrode for batteries.

9. The method according to claim 7, further comprising recycling the purified solution to a flow of treatment of the silicon containing surface.

10. The method according to claim 1, further comprising a step of adding hydrofluoric acid and/or nitric acid to set the concentration of hydrofluoric acid and nitric acid contained in the solution to a target level.

11. A process of manufacturing a silicon based solar cell, the process comprising purifying a solution containing hydrofluoric acid, nitric acid and at least one silicon impurity in accordance with the method according to claim 1.

12. A process of manufacturing a silicon based electrode for a battery, the process comprising purifying a solution containing hydrofluoric acid, nitric acid and at least one silicon impurity in accordance with the method according to claim 1.

13. The process according to claim 11, further comprising recycling the purified solution to a flow of treatment of a silicon containing surface during the manufacturing process.

14. The process according to claim 11, wherein the reverse osmosis membrane is made of polyamides, polypiperazine amides, polyacrylonitriles, polysulfones, cellulose acetates, polybenzimidazolines, polyoxadiazoles, polyfuranes, polyether-polyfuranes, polyvinyl amines, polypyrrolidines, carboxylated polysulfones or sulfonated polysulfones.

15. The process according to claim 12, further comprising recycling the purified solution to a flow of treatment of a silicon containing surface during the manufacturing process.

16. The process according to claim 12, wherein the reverse osmosis membrane is made of polyamides, polypiperazine amides, polyacrylonitriles, polysulfones, cellulose acetates, polybenzimidazolines, polyoxadiazoles, polyfuranes, polyether-polyfuranes, polyvinylamines, polypyrrolidines, carboxylated polysulfones or sulfonated polysulfones.

17. The method according to claim 1, wherein the content of nitric acid in the solution before the purification is from 2% w/w to 10% w/w.

18. The method according to claim 1, wherein the content of hydrofluoric acid in the solution before the purification is from 0.5% w/w to 30% w/w.

19. The method according to claim 1, wherein the content of silicon impurity in the solution before the purification is from 0.5% w/w to 20% w/w.

20. The method according to claim 1, wherein ⅔ of the hydrofluoric acid and nitric acid are not rejected.

Description

EXAMPLE

Example 1

(1) In order to confirm the performance of the method according to the present invention, a lab scale device model for membrane system is used. A test is made with a small flat sheet membrane unit working in full recirculation mode. Pressure is applied using the feed/recirculation pump, and static pressure is applied using an inert gas, if necessary. The maximum operating pressure is ˜30 bars.

(2) A thin film composite polyamide membrane is used for testing.

(3) A feed solution matching the typical composition of a spent acid wafer texturing mixture (HNO.sub.3 5% w/w, HF 2% w/w, and H.sub.2SiF.sub.6 2% w/w) is processed through the membrane. It is demonstrated that over 95% of silicon impurities are rejected by the RO membrane, while approximately ⅔ of the acids are maintained in the permeate.

(4) This clearly demonstrates that silicon is almost completely rejected by the membrane while HNO.sub.3 and HF are not significantly rejected by the membrane. Thus, advantage of using reverse osmosis membrane in recycling the acid wafer texturing mixture used in the photovoltaic industry is proven.