Metallurgical-grade silicon with impurities such as boron and phosphorous is dissolved in an aluminum smelt at a temperature of nearly 800⁰ C. This material has a purity of 97.0% or higher.
As the liquid aluminum-silicon alloy crystallizes, the silicon hardens into flakes. The impurities then remain in the aluminum, which represents the first stage in the purification process. Notice how the silicon has been “drawn” together and has a coating of aluminum.
The mixture cools, and the silicon will solidify first. The aluminum is then poured out. Since the aluminum now has traces of silicon in it, it can be marketed as an aluminum-based master alloy Master Alloys.
The silicon flakes are still coated with a thin layer of aluminum. To remove this layer, acid is poured onto the silicon.
In the acid bath, the aluminum layer separates from the surface of the silicon flakes. This is another layer of purification for the silicon.
The bath is drained, with the liquid containing trace amounts of aluminum. This liquid is polyaluminum chloride, which is used commercially by wastewater treatment facilities. The remaining silicon still has traces of aluminum.
The flakes are then melted. The heat causes the last remaining aluminum atoms to separate and migrate to the surface.
Finally, the combination of aluminum and silicon cools. The remaining aluminum atoms form a thin layer on the surface. This layer is removed from the silicon, and it is the final purification stage necessary to create solar silicon.