Calcium-silicate based slag for treatment of molten steel

1. A calcium-silicate based slag comprising phosphorous, and the phosphorous is present in an amount of less than 3 ppmw for treatment of molten silicon for solar cell production.

2. The calcium-silicate based slag according to claim 1 , wherein the slag has a phosphorous content of less than 1 ppmw.

3. The calcium-silicate based slag according to claim 1 , the slag further contains up to 30% by weight of CaF 2 and/or MgO in order to lower the viscosity of the slag.

4. The calcium-silicate based slag according to claim 1 , the slag further contains up to 10% by weight of one or more of BaF 2 , BaO, LiF and Li 2 O in order to adjust the density of the slag.

5. The calcium-silicate based slag according to claim 1 , wherein the slag contains Al 2 O 3 in order to control the aluminum content of silicon treated by the slag.

6. A low phosphorous calcium-silicate based slag containing phosphorous in an amount less than 3 ppm and made by 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 containing phosphorous in an amount of less than 3 ppmw is produced;

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

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

(f) repeating steps (b)-(e) to make a low phosphorous calcium-silicate based slag containing phosphorous in an amount of less than 3 ppmw.

7. The slag according to claim 6 , 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.

8. The slag according to claim 6 , 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.

9. The slag according to claim 6 , wherein the ferrosilicon alloy contains up to 30% by weight of silicon, the remainder, except for normal impurities, being iron.

10. The slag according to claim 9 , wherein the ferrosilicon alloy contains 10 to 20% by weight of silicon.

11. The slag according to claim 6 , 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.

12. The slag according to claim 6 , wherein carbon monoxide and hydrogen are supplied as reducing gases.

13. The slag according to claim 12 , wherein argon and nitrogen are supplied as inert gases.

14. A low phosphorus calcium-silicate based slag containing phosphorus in an amount less than 3 ppmw and made by 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 tap 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 tap 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 phosphorus from the layer of molten calcium-silicate based slag to the layer of molten ferrosilicon alloy and produce a layer of molten low phosphorus calcium-silicate based slag containing phosphorus in an amount of less than 3 ppmw;

(d) tapping the layer of molten low phosphorus 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 phosphorus calcium-silicate based slag containing phosphorus in an amount of less than 3 ppmw.