Flux composition useful in directional solidification for purifying silicon

1. A method comprising:

(a) forming a molten liquid from silicon and a flux composition, the flux composition comprising silicon dioxide (SiO 2 ), sodium carbonate (Na 2 CO 3 ), calcium oxide (CaO) present in an amount that is between 1.50 wt. % to 1.90 wt. % of the flux composition, and at least one of calcium fluoride (CaF 2 ) and calcium chloride (CaCl 2 );

(b) forming a slag, from the flux composition and impurities in the molten liquid; and

(c) optionally removing at least a portion of the slag from the molten liquid, to provide a purified molten liquid.

2. The method of claim 1 , which provides for silicon with at least about a 99.5 wt. % reduction in aluminum compared to the amount of aluminum in the silicon used to form the molten liquid.

3. The method of claim 1 , which provides for silicon comprising less than about 10 ppmw aluminum.

4. The method of claim 1 , wherein the method purifies silicon from aluminum and boron.

5. The method of claim 1 , wherein forming the molten liquid from the silicon and the flux composition comprises one of:

initially forming a molten liquid of silicon and subsequently adding the flux composition to the molten silicon, or

initially contacting solid silicon with the flux composition and heating both the solid silicon and the flux composition to form the molten liquid.

6. The method claim 1 , further comprising (d) directionally solidifying the purified molten liquid, to form solid silicon.

7. The method of claim 1 , wherein the flux composition is present on an internal surface of a furnace refractory prior to forming a molten liquid.

8. The method claim 6 , further comprising (e) removing a portion of the solid silicon, to provide purified solid silicon.

9. The method of claim 1 , wherein any one or more steps is independently carried out multiple times.

10. The method of claim 1 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 25 wt. % to 75 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 37.6 wt. % to 56.4 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) and the calcium chloride (CaC 1 2 ) are, when present, each independently present in an amount that is up to 6.00 wt. % of the flux composition.

11. The method of claim 10 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 35 wt. % and 50 wt. %.

12. The method of claim 1 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 38.43 wt. % to 46.97 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 45.54 wt. % to 55.66 wt. % of the flux composition, the calcium oxide (CaO) is present in an amount that is between 1.53 wt. % to 1.87 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) and the calcium chloride (CaCl 2 ) are, when present, each independently present in an amount that is between 4.00 wt. % to 6.00 wt. % of the flux composition.

13. The method of claim 1 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 35 wt. % and 80 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 40 wt. % and 55 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) and the calcium chloride (CaCl 2 ) are, when present, each independently present in an amount that is between 0.50 wt. % and 6.00 wt. % of the flux composition.

14. The method of claim 13 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 35 wt. % and 50 wt. %.

15. The method of claim 1 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 35 wt. % and 50 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 45 wt. % and 55 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) and the calcium chloride (CaCl 2 ) are, when present, each independently present in an amount that is between 4.00 wt. % and 6.00 wt. % of the flux composition.

16. The method of claim 1 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 40.565 wt. % to 44.835 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 48.07 wt. % to 53.13 wt. % of the flux composition, the calcium oxide (CaO) is present in an amount that is between 1.615 wt. % to 1.785 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) and the calcium chloride (CaCl 2 ) are, when present, each independently present in an amount that is between 4.50 wt. % to 5.50 wt. % of the flux composition.

17. The method of claim 16 , wherein the calcium fluoride (CaF 2 ) and the calcium chloride (CaCl 2 ) are both present in the flux composition.

18. A method comprising:

(a) forming a molten liquid from silicon and a flux composition, the flux composition comprising silicon dioxide (SiO 2 ) present in an amount that is between 35 wt. % to 50 wt. % of the flux composition, sodium carbonate (Na 2 CO 3 ) present in an amount that is between 45 wt. % to 55 wt. % of the flux composition, calcium chloride (CaCl 2 ) present in an amount that is between 4.00 wt. % to 6.00 wt. % of the flux composition, calcium oxide (CaO) present in an amount that is between 1.50 wt. % to 1.90 wt. % of the flux composition, and optionally calcium fluoride (CaF 2 ), wherein the calcium fluoride is, when present, present in an amount that is between 4.00 wt. % to 6.00 wt. % of the flux composition;

(b) forming a slag, from the flux composition and impurities in the molten liquid; and

(c) optionally removing at least a portion of the slag from the molten liquid, to provide a purified molten liquid.

19. The method of claim 18 , wherein the silicon dioxide (SiO 2 ) is present in an amount that is between 40.565 wt. % to 44.835 wt. % of the flux composition, the sodium carbonate (Na 2 CO 3 ) is present in an amount that is between 48.07 wt. % to 53.13 wt. % of the flux composition, the calcium chloride (CaCl 2 ) is present in an amount that is between 4.50 wt. % to 5.50 wt. % of the flux composition, the calcium oxide (CaO) is present in an amount that is between 1.615 wt. % to 1.785 wt. % of the flux composition, and the calcium fluoride (CaF 2 ) is, when present, present in an amount that is between 4.50 wt. % to 5.50 wt. % of the flux composition.

20. The method of claim 19 , wherein the calcium fluoride (CaF 2 ) is present in the flux composition.