Use of acid washing to provide purified silicon crystals
View Patent ↗A method for purifying silicon wherein silicon is crystallized from a solvent metal. The method comprises the steps of providing a molten liquid containing silicon, a solvent metal and impurities, cooling the molten liquid to form first silicon crystals and a first mother liquor, separating the first silicon crystals from the first mother liquor, contacting the first silicon crystals with compound which will dissolve the first mother liquor and separating the washed crystals from the wash solution.
1. A method for purifying metallurgical grade silicon with a phosphorous level below 60 ppmw and boron level below 15 ppmw, the method comprising:
(a) forming a first molten liquid from a source silicon and a solvent metal selected from the group of copper, tin, zinc, antimony, silver, bismuth, aluminum, cadmium, gallium, indium, magnesium, lead, an alloy thereof, and combinations thereof;
(b) contacting the first molten liquid with a first gas, such that a vortex is formed in the melt, to provide dross and a second molten liquid, wherein the first gas is selected from the group consisting of oxygen (O 2 ), nitrogen (N 2 ), helium (He), argon (Ar), hydrogen (H 2 ), sulfur hexafluoride (SF 6 ), water vapor (H 2 O), oxygen (O 2 ), carbon dioxide (CO 2 ), carbon monoxide (CO), tetrafluorosilane (SiF4), and combinations thereof;
wherein the contacting the first molten liquid with the first gas employs a gas rotary degasser, thereby creating a vortex of the second molten liquid, which effectively mixes the dross in the second molten liquid;
(c) separating the dross and the second molten liquid;
(d) cooling the second molten liquid to form first silicon crystals and a first mother liquor;
(e) separating the first silicon crystals and the first mother liquor;
(f) contacting the first silicon crystals with an acid to provide washed silicon crystals and used acid; and
(g) separating the washed silicon crystals and the used acid;
wherein the washed silicon crystals have a boron level below 3 ppmw and a phosphorous level below 9 ppmw.
2. The method of claim 1 , further comprising before forming a first molten liquid, pre-treating source silicon by slagging, gas injection, plasma torch, vacuum treatment, or a combination thereof.
3. The method of claim 1 , wherein in step (a), the first molten liquid is formed by heating above the liquidus temperature.
4. The method of claim 1 , wherein in step (a), metallurgical grade silicon is employed that includes about 5 ppm wt % to about 15 ppm wt % boron and about 30 ppm wt % to about 60 ppm wt % phosphorous.
5. The method of claim 1 , wherein in step (a), silicon is employed in about 20 wt. % to about 60 wt. %.
6. The method of claim 1 , wherein in step (a), aluminum, or an alloy thereof, is employed as the solvent metal, in about 40 wt. % to about 80 wt. %.
7. The method of claim 1 , wherein after step (a) and before step (b), the first molten liquid is cooled to above the liquidus temperature.
8. The method of claim 1 , wherein in step (d), the second molten liquid is cooled to above the solidus temperature and below the liquidus temperature.
9. The method of claim 1 , wherein the washed silicon crystals includes less than about 4 ppmw phosphorous (P) and less than about 2 ppmw boron (B).
10. A method for purifying metallurgical grade silicon with a phosphorous level below 60 ppmw and boron level below 15 ppmw, the method comprising:
(a) forming a first molten liquid from source silicon and aluminum;
(b) contacting the first molten liquid with a first gas, such that a vortex is formed in the melt, to provide dross and a second molten liquid, wherein the first gas is selected from the group consisting of oxygen (O 2 ), nitrogen (N 2 ), helium (He), argon (Ar), hydrogen (H 2 ), sulfur hexafluoride (SF 6 ), water vapor (H 2 O), oxygen (O 2 ), carbon dioxide (CO 2 ), carbon monoxide (CO), tetrafluorosilane (SiF4), and combinations thereof;
wherein the contacting the first molten liquid with the first gas employs a gas rotary degasser, thereby creating a vortex of the second molten liquid, which effectively mixes the dross in the second molten liquid;
(c) separating the dross and the second molten liquid;
(d) cooling the second molten liquid to form first silicon crystals and a first mother liquor;
(e) separating the first silicon crystals and the first mother liquor;
(f) optionally melting the first silicon crystals with a solvent metal and repeating steps (a)-(e);
(g) contacting the first silicon crystals with an acid, to provide washed silicon crystals and used acid; and
(h) separating the washed silicon crystals and the used acid, to provide purified silicon crystals;
(i) melting the purified silicon crystals, to provide a silicon melt;
(j) contacting the silicon melt with a second gas; and
(k) directionally solidifying the silicon melt; wherein the washed silicon crystals have a boron level below 3 ppmw and a phosphorous level below 9 ppmw.
11. The method of claim 10 , wherein an ingot or boule is produced.
12. The method of claim 11 , further comprising removing the top of the ingot.
13. The method of claim 11 , further comprising directionally solidifying the ingot or boule to form a multicrystalline ingot or monocrystalline boule.
14. The method of claim 1 or 10 , wherein the washed silicon crystals have a boron level below 2 ppmw, and a phosphorous level below 4 ppmw.
15. The method of claim 1 or 10 , wherein the first gas comprises at least one of helium (He), Neon (Ne), Argon (Ar) and nitrogen (N 2 ).
16. The method of claim 1 or 10 , wherein the first gas comprises bubbles of about 1 mm to about 5 mm.
17. The method of claim 1 or 10 , wherein the acid is hydrochloric (HCl).
18. The method of claim 1 or 10 , wherein at least about 240 kg of purified silicon crystals is obtained.
19. The method of claim 1 or 10 , wherein the first gas is selected from the group consisting of oxygen (O 2 ), nitrogen (N 2 ), helium (He), argon (Ar), hydrogen (H 2 ), water vapor (H 2 O), carbon dioxide (CO 2 ), carbon monoxide (CO), and combinations thereof.