Process for the production of medium and high purity silicon from metallurgical grade silicon
A process for purifying low-purity metallurgical grade silicon, contains at least one contaminant and obtains a higher-purity solid polycrystalline silicon. The process includes containing a melt of low-purity metallurgical grade silicon in a mold having insulated bottom and side walls, and an open top; solidifying the melt by unidirectional solidification from the open top towards the bottom wall while electromagnetically stirring the melt; controlling a rate of the unidirectional solidification; stopping the unidirectional solidification when the melt has partially solidified to produce an ingot having an exterior shell including the higher-purity solid polycrystalline silicon and a center including an impurity-enriched liquid silicon; and creating an opening in the exterior shell of the ingot to outflow the impurity-enriched liquid silicon and leave the exterior shell which has the higher-purity solid polycrystalline silicon.
1. A batch casting process for purifying low-purity metallurgical grade silicon, containing at least one of the following contaminants Al, As, Ba, Bi, Ca, Cd, Fe, Co, Cr, Cu, Fe, La, Mg, Mn, Mo, Na, Ni, P, Pb, Sb, Sc, Sn, Sr, Ti, V, Zn, Zr, and B, and obtaining a higher-purity solid polycrystalline silicon, said process comprising the steps of:
a. containing a melt of low-purity metallurgical grade silicon in a mould, said mould having an insulated bottom wall, insulated side walls, and an open top;
b. solidifying the melt by unidirectional solidification from said open top towards said insulated bottom wall of said mould while electromagnetically stirring the melt so as to wash a solidification interface and prevent impurity rich liquid to be trapped by dendrite formation;
c. controlling a rate of said unidirectional solidification;
d. stopping said unidirectional solidification when the melt has partially solidified to produce an ingot having an exterior shell comprising said higher-purity solid polycrystalline silicon and a center comprising an impurity-enriched liquid silicon; and
e. creating an opening in said exterior shell of said ingot to outflow said impurity-enriched liquid silicon and leave the exterior shell comprising said higher-purity solid polycrystalline silicon thereby obtaining said higher-purity solid polycrystalline silicon.
2. A process according to claim 1 , wherein said insulated bottom wall and said insulated side walls are heated to minimize a formation of a skin of silicon on said insulated bottom wall and said insulated side walls of said mould.
3. A process according to claim 1 , wherein said rate of unidirectional solidification is controlled by an insulation of said insulated bottom wall and said insulated side walls.
4. A process according to claim 1 , wherein said rate of unidirectional solidification is controlled by controlling a rate of heat extraction from the melt at said open top of said mould.
5. A process according to claim 4 , wherein said controlling a rate of heat extraction comprises placing a cooling medium in contact with a free surface of the melt at said open top of said mould.
6. A process according to claim 5 , wherein said cooling medium is water or air.
7. A process according to claim 1 , wherein said rate of unidirectional solidification is determined so as to minimize dendrite formation.
8. A process according to claim 7 , wherein said rate of unidirectional solidification is less than 1×10 4 m/s.
9. A process according to claim 7 , wherein said rate of unidirectional solidification is comprised between 1×10 4 m/s and 5×10 6 m/s.
10. A process according to claim 1 , wherein said stopping the unidirectional solidification is done when 40 to 80% of the melt has solidified.
11. A process according to claim 1 , wherein said creating an opening in said exterior shell of said ingot of step (e) comprises piercing said exterior shell of said ingot by mechanical means or thermal lance.
12. A process according to claim 11 , wherein said impurity-enriched liquid silicon flowing out of the ingot is extracted from the mould through a tap opening in said mould.
13. A process according to claim 1 , wherein said process further comprises an additional step before step (e) of removing said ingot from said mould.
14. A process according to claim 1 , wherein said process further comprises an additional step (f) of melting said higher-purity solid polycrystalline silicon and repeating steps (b) to (e).
15. A process according to claim 1 , wherein a top portion of said higher-purity solid polycrystalline silicon contains at least 90% less metal contaminants Al, As, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, K, La, Mg, Mn, Mo, Na, Ni, Pb, Sb, Sc, Sn, Sr, Ti, V, Zn, and Zr than the low-purity metallurgical grade silicon.
16. A process according to claim 1 , wherein said higher-purity solid polycrystalline silicon contains approximately at least 45% less phosphorus and approximately at least 10% less boron than the low-purity metallurgical grade silicon.
17. A process according to claim 1 , wherein said impurity-enriched liquid silicon contains less than 60 ppm of carbon and less than 20 ppm of oxygen and is a low carbon and low oxygen grade of silicon.
18. A process according to claim 13 , wherein the step of removing said ingot from said mould includes the step of lifting the into out of the mould using a suspension mechanism.