Title of Invention

A METHOD FOR PRODUCING A LOW PHOSPHOROUS CALCIUM-SILICATE BASED SLAG

Abstract

The present invention relates to a method for producing a low phosphoroous calciumsilicate based slag comprising the steps of treating a molten calcium-silicate based slag in a vessel with a molten ferrosilicon calloy; whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy, and removing from the vessel the calcium-silicate based slag with a phosphorus content of less than 3 ppmw.

Full Text

Field of invention The present invention relates to a calcium-silicate based slag having a very low phosphorous content a method for producing such calcium-silicate based slag having a very low phosphorous content and to the use of the calcium-silicate based slag for the removal of boron and phosphorous from molten silicon. Background art In the method of refining metallurgical grade silicon in order to produce pure silicon for solar cell production (solar grade silicon) It is known to remove boron by treatment of molten silicon with a calcium-silicate based slag. Such a method Is disclosed in U.S. patent No. 5,788,945. In order to remove boron from silicon to an acceptable low level It is necessary to use a slag having a low boron content. For solar grade silicon there Is also strict requirements to the content of phosphopDus. Thus the phosphorous content of solar grade silicon should be less than 3 ppmw. The slag treatment of silicon in order to remove boron also influences the phosphorous content of silicon. Thus the distribution coefficient between phosphorous in slag and phosphorous in silicon is very low and in the range between 0.1 and 0.3. If calcium-silicate based slag used to remove boron from i silicon contains too much phosphorous, the phosphorous content of silicon can therefore be increased during the slag treatment. It is thus important to use a low phosphorous containing calcium-silicate based slag for removal of boron from silicon. phosphorous content. In order not to Increase the phosphorous content of silicon during slag treatment the calcium-silicate based slag, the phosphorous content of the calcium-silicate based slag should be as low as possible and preferably well below 3 ppmw. Disclosure of Invention It is an object of the present Invention to provide a calcium-silicate based slag having a very low phosphorous content and a method for removing phosphorous from calcium-silicate based slag where low cost sources of CaO and SiOacan be used to produce the slag. Thus, according to a first aspect the present invention relates to a calcium-silicate based slag for treatment of molten silicon, said slag having a phosphorous content of less than 3 ppmw. According to a preferred embodiment the calcium-silicate based slag has a phosphoHDus content below 1 ppmw. According to a preferred embodiment the calcium-silicate based slag contains CaFz and/or hJigO in an amount o1 up to 30 % by weight in order to lower the viscosity of the slag and to increase removal of phosphorous and boron from silicon. According to another preferred embodiment the calcium-silicate based slag contains one or more of BaFa, BaO, LiF and LI2O in an amount of up to 10 % by weight in order to adjust the density of the slag to facilitate removal of the slag from silicon after slag treatment. According to yet another preferred embodiment the calcium-silicate based slag contains AI2O3 in order to be able to adjust the aluminum content of silicon to be treated with the slag. According to a second aspect the present invention relates to a method for producing a low phosphorous calcium-silicate based slag, which method is characterized in that molten calcium-silicate based slag is treated with a molten ferrosilicon alloy in a vessel whereby phosphorous in the calcium- ) silicate based slag is transferred to the ferrosilicon alloy, and separating the molten low phosphorous calcium-silicate based slag from the molten feaosilicon alloy. According to a preferred embodiment a molten layer of a ferrosilicon alloy is provided in the vessel, whereafter a Si02 source, a CaO source are supplied to the top of the layer of fernosilicon alloy to provide a layer of molten calcium-silicate based slag whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy, and removing the low phosphorous calcium-silicate based slag from the vessel. Preferably the ferrosilicon alloy comprises up to 30 % by weight silicon, the remaining being iron except for normal amounts of impurities. More preferably the ferrosilicon alloy comprises 10 - 20 % by weight silicon. The amount of silicon in the ferrosilicon alloy should balance the amount of Si02 in the liquid calcium-silicate based slag in order to prevent silicon in the slag from entering the fernDsilicon alloy and thus changing the composition of the slag. According to another embodiment, the molten fenxisiiicon alloy can be produced in situ by adding Fe203 and Si together with the slag fonning compounds Si02 and CaO. Upon heating Fe203 will be reduced to Fe by some of the Si added and form the ferrosilicon alhy. In this embodiment the amount of Si02 supplied may have to be adjusted in order to compensate for the amount of SiOa produced when FeaOa is reduced by Si, in order to obtain a suitable final composition of the calcium-silicate based slag. In order to increase the speed of reaction between the molten calcium-silicate based slag and the molten ferrosilicon alloy a reducing and/or an inert gas or a mixture of such gases are supplied to the vessel in onder to stir the layers of molten ferrosilicon and liquid calcium-silicate based slag. Typically, carbon monoxide and hydnagen are supplied as reducing gases and argon and nitrogen are supplied as inert gases. By the method of the present invention it has surprisingly been found that practically 100 % of the phosphorous in the calcium-silicate based slag is transferred to the ferrosillcon alloy. Further, only a small amount of iron is being transferred from the fen"osillcon alloy to the calcium-silicate based slag. By the method of the present invention it is thus possible to produce a calcium-silicate based slag having a phosphorous content of below 1 ppmw from a calcium-silicate slag which initially at least 30 ppmw phosphorous. Cheap sources of lime and quarts can thus be used to provide a high quality calcium-silicate based siag which is excellent for the removal of boron and phosphorous from molten silicon. In one embodiment the calcium-silicate based slag treated for removal of ) phosphorous is slag which has been used for slag treatment of molten siiicon and thus has an increased content of phosphorous. In this way calcium-silicate based slag from treatment of molten silicon can be regenerated and recycled thus strongly reducing the costs for slag treatment of molten silicon and avoiding disposal of large volumes of used calcium-silicate based slag. 5 The method of the present invention can be carried out in conventional high temperature furnaces such as induction furnaces and arc furnaces. In a preferred embodiment it is used an arc furnace having vertical electrodes and equipped with a lower tapping hole near its bottom and an upper tapping hole at a higher level. The process Is started by establishing a layer of molten .0 ferrosillcon alloy at the bottom of the furnace, where the top of the layer of ferrosillcon is well below the upper tapping hole. Thereafter slag forming compounds or used slag for regeneration Is added until a layer of molten slag extending a distance above the upper tapping hole Is formed. When the supply of slag forming compounds Is finished, the molten layer of slag is !5 allowed to stay in the furnace for a predetermined period to ensure removal of phosphorous from the slag and into the ferrosillcon alloy, The upper tapping bole is then opened and the slag above the tapping hole is being tapped from the furnace. After tapping, the upper tapping hole is closed and further slag forming compounds are supplied to the furnace. When the phosphorous 30 content in the fen-osilicon alloy has increased to a preset value, the ferrosillcon alloy is tapped from the lower tapping hole and the above described process is repeated. In this way it is obtained a semi-continuous, low cost production of low phosphorous calcium-silicate based slag. The present invention also relates to the use of the slag produced according to the method of the present invention for removal of boron and phosphorous from molten silicon. Detailed description of Invention EXAMPLE 1 A layer of molten ferrosilicon alloy consisting of 85 % by weight Fe and 15 % by weight Si was provided at the bottom of an induction furnace. 120 grams of quarts having a phosphorous content of 4 ppmw and 130 grams of lime having a phosphorous content of 35 ppm were added to the induction furnace and melted on the top of the molten ferrosilicon layer. The initial phosphorous content of the liquid slag was calculated to 20 ppmw based on the phosphorous content of quarts and lime. Three tests were run using different gas compositions for stinging the melt. The stirring gas was supplied through a graphite tube. After treatment the slag was tapped from the induction furnace and analysed with respect to phosphorous, boron, CaO, Si02 and Fe203. The results are shown in Table 1. Table 1 Chemical Analysis Test No. Stirring gas P* [ppmwl B [ppmw] CaO Ewt%] SiOa [wt%] Fe203 [wt%] A Ar-5%H2 B Ar-25%CO C Ar * detection limit for P in slag: 2.5 ppmw (ICP) The results in Table 1 show that the phosphorous content in the produced slags was below the detection limit of 2,5 ppm for all three siags. Based on calculations of mass balances from use of the produced slags in slag treatment of molten silicon, it was found that the phosphorous content of the three slags produced in fact was about 0.1 ppmw. Further, Table 1 shows that the content of FeaOs in the three slags was very low, indicating that only a minor amount of Iron was transfen^ed from the ferrosllicon alloy to the slag phase, EXAMPLE 2 The three slags A, B and C produced in Example 1 were used to remove boron and phosphorous from molten silicon. Molten metallurgical grade silicon containing 47 ppmw boron, 9 ppmw phosphorous and 0.25 % by weight of iron was treated with the slags A, B and C produced in Example 1. The weight ratio of slag to silicon was 2,6 for all runs. The content of phosphorous, boron and iron in the used slags and in the treated silicon were analysed. The results are shown in Table 2. Table 2 Pppmw B ppmw Fe % by weight Slag in slag In Si in slag in Si JnSi A 2.5* 3 25.5 8.6 0.4 B 2.5* 4 25.1 8.5 0.4 C 2.5* 4 25.7 8,5 0.4 detection timit for P Jn s)aa; 2.5 Domw (ICP) From the results in Table 2 it can be seen that a very good boron removal was obtained for all three sfags and that the phosphorous content in the treated silicon was reduced from 9 to about 4 ppmw. WE CLAIM: 1. A method for producing a low phosphorous calcium-silicate based slag comprising the steps of treating a molten calcium-silicate based slag in a vessel with a molten ferrosilicon alloy; whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy, and removing from the vessel the calcium-silicate based slag with a phosphorus content of less than 3 ppmw. 2. The method as claimed in claim 1 wherein a molten layer of the ferrosilicon alloy is provided in the vessel whereafter a Si02 source and a CaO source are supplied to the top of the layer of ferrosilicon alloy to provide a layer of molten calcium-silicate based slag, whereby phosphorous in the calcium-silicate based slag is transferred to the ferrosilicon alloy, and removing from the vessel the calcium-silicate based slag with a phosphorous content of less than 3 ppmw. 3. The method as claimed in claim 1, wherein the molten ferrosilicon alloys is produced in situ in the vessel by adding Fe203 and Si together with the calcium-silicate based slag. 4. The method as claimed in claim 1, wherein the ferrosilicon alloy contains up to 30% by weight of silicon, the reminder except for normal impurities, being iron. 5. The method as claimed in claim 4, wherein the ferrosilicon alloy contains 10 to 20% by weight of silicon. 6. The method as claimed in claim 1, wherein the calcium-silicate based slag supplied to the vessel is calcium-silicate based slag which has been used for slag treatment of molten silicon. 7. The method as claimed in claim 1, wherein a reducing and/or an inert gas is supplied to the vessel in order to stir the layers of molten ferrosilicon and liquid calcium-silicate based slag. 8. The method as claimed in claim 7, wherein carbon monoxide and hydrogen are supplied as reducing gases. 9. The method as claimed in claim 7, wherein argon and nitrogen are supplied as inert gases. 10. A low phosphorous calcium- silicate based slag produced by the method claimed in any one of the preceding claims.

Documents:

2613-chenp-2004 abstract-duplicate.pdf

2613-chenp-2004 abstract.pdf

2613-chenp-2004 claims-duplicate.pdf

2613-chenp-2004 claims.pdf

2613-chenp-2004 correspondence-others.pdf

2613-chenp-2004 correspondence-po.pdf

2613-chenp-2004 description(complete)-duplicate.pdf

2613-chenp-2004 description(complete).pdf

2613-chenp-2004 form-1.pdf

2613-chenp-2004 form-18.pdf

2613-chenp-2004 form-26.pdf

2613-chenp-2004 form-3.pdf

2613-chenp-2004 form-5.pdf

2613-chenp-2004 pct search report.pdf

2613-chenp-2004 pct.pdf

2613-chenp-2004 petition.pdf