Calcium-silicate based slag for treatment of molten silicon
A method for making solar grade silicon from metallurgical grade silicon is accomplished by treating a calcium silicate based slag with ferrosilicon in the first vessel so as to obtain a purification of the calcium silicate based slag. In a second vessel, the purified calcium based silicate slag is used to treat metallurgical grade silicon in order to obtain a solar grade silicon. The purification of calcium silicate based slag in the first vessel is accomplished to reduce phosphorous to a level of less than 3 ppmw.
1. A method for making solar grade silicon from metallurgical grade silicon comprising,
treating a calcium-silicate based slag with a molten ferrosilicon alloy in a first vessel, whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy producing a calcium-silicate based slag containing less than 3 ppmw phosphorous;
obtaining the calcium-silicate based slag containing less than 3 ppmw phosphorous from the first vessel;
treating a molten metallurgical grade silicon in a second vessel with the calcium-silicate based slag containing less than 3 ppmw phosphorous to reduce the content of phosphorous, boron and iron in the metallurgical grade silicon and produce a solar grade silicon; and
removing the solar grade silicon from the second vessel.
2. The method according to claim 1 , wherein the step of treating a calcium-silicate based slag with the molten ferrosilicon alloy in a first vessel further comprising
providing a molten layer of the ferrosilicon alloy in the first vessel;
supplying a SiO 2 source and a CaO source to the top of the layer of ferrosilicon alloy to generate a layer of molten calcium-silicate based slag, whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy producing a calcium-silicate based slag containing less than 3 ppmw phosphorous.
3. The method according to claim 1 , wherein the molten ferrosilicon alloy is produced in situ in the first vessel by adding Fe 2 O 3 and Si together with the calcium-silicate based slag.
4. The method according to claim 1 , wherein the ferrosilicon alloy contains up to 30% by weight of silicon, the remainder, except for normal impurities, being iron.
5. The method according to claim 4 , wherein the ferrosilicon alloy contains 10 to 20% by weight of silicon.
6. The method according to claim 1 , wherein the calcium-silicate based slag supplied to the first vessel is a calcium-silicate based slag which has been used for treating molten metallurgical grade silicon in the second vessel.
7. The method according to claim 1 , further comprising a step of supplying a reducing and/or an inert gas or a mixture of said gases to the first vessel in order to stir the molten ferrosilicon alloy and the calcium-silicate based slag.
8. The method according to claim 7 , wherein the reducing gases comprise carbon monoxide and hydrogen.
9. The method according to claim 8 , wherein the inert gases comprise argon and nitrogen.