Calcium-silicate based slag for treatment of molten silicon

1. A method for producing a low phosphorous calcium-silicate based slag comprising the steps of:

(a) providing a layer of molten ferrosilicon alloy in a vessel;

(b) forming a layer of molten calcium-silicate based slag on top of the layer of molten ferrosilicon alloy in the vessel;

(c) stirring said layer of molten ferrosilicon alloy and layer of molten calcium-silicate based slag by supplying a reducing and/or an inert gas to the vessel, whereby phosphorous in the layer of molten calcium-silicate based slag is transferred to the layer of molten ferrosilicon alloy and a molten low phosphorous calcium-silicate based slag having a phosphorous content of less than 3 ppmw is produced;

(d) removing molten low phosphorous calcium-silicate based slag containing less than 3 ppmw phosphorous from the vessel;

(e) maintaining the layer of molten ferrosilicon alloy in the vessel; and

(f) repeating steps (b)-(e) to produce low phosphorous calcium-silicate based slag having a phosphorous content of less than 3 ppmw.

2. The method according to claim 1 , wherein the layer of molten calcium-silicate based slag in step (b) is produced in situ in the vessel by adding a SiO 2 source and a CaO source to the top of the layer of ferrosilicon alloy.

3. The method according to claim 1 , wherein the layer of molten ferrosilicon alloys in step (a) is produced in situ in the 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 vessel in step (b) is used calcium-silicate based slag which has been used for slag treatment of molten silicon.

7. The method according to claim 1 , wherein carbon monoxide and hydrogen are supplied as reducing gases.

8. The method according to claim 7 , wherein argon and nitrogen are supplied as inert gases.

9. A method for producing a low phosphorous calcium-silicate based slag comprising the steps of:

(a) forming a layer of molten ferrosilicon alloy in an arc furnace having vertical electrodes, the furnace having a lower tap hole, and an upper top hole, the layer of molten ferrosilicon alloy being below the upper tap hole;

(b) forming a layer of molten calcium-silicate based slag on top of the layer of molten ferrosilicon alloy in the furnace, the layer of molten calcium-silicate based slag extending above the upper top hole;

(c) maintaining the layer of molten calcium-silicate based slag and the layer of molten ferrosilicon alloy in the furnace for a predetermined period of time to transfer phosphorous from the layer of molten calcium-silicate based slag to the layer of molten ferrosilicon alloy and produce a layer of molten low phosphorous calcium-silicate based slag having a phosphorous content of less than 3 ppmw;

(d) tapping the layer of molten low phosphorous calcium-silicate based slag from the furnace through the upper tap hole;

(e) maintaining the layer of molten ferrosilicon alloy in the furnace; and

(f) repeating steps (b)-(e), to produce low phosphorous calcium-silicate based slag having a phosphorous content of less than 3 ppmw.