Treatment of silicon and ferrosilicon with slag.

0001The invention relates to a method for treating elemental silicon and ferrosilicon with slag, the slag being in the molten state.

0002It is known from DE-OS 27 06 175 to introduce silicon into a slag which, in addition to 2 to 30% by weight of calcium oxide, 5 to 35% by weight of magnesium oxide, 0.5 to 28% by weight of aluminum oxide and 45 to 70% by weight of silicon oxide contains at least 0.29% by weight of copper, based on the amount of silicon to be treated. According to this process, silicon fine dust, which is obtained when grinding lumpy silicon, could be recovered. Disadvantageously, the silicon obtained in this way could not be used as a raw material in the semiconductor sector, since even a few hundredths of a percent of copper in silicon are detrimental to this purpose.

0003The object of the invention was to find a process for treating silicon and ferrosilicon which, while preserving the advantages of the process known from 27 06 175, avoids the disadvantages described above. In particular, it was an object of the invention to find a slag composition in which even finely divided silicon or ferrosilicon can be obtained as a coherent liquid phase, which collects below the slag surface, which has no addition of copper and which nevertheless ensures a smooth melting process without turbulence.

0004It has now been found that the addition of copper can be dispensed with if a slag of the composition listed below contains K₂O (analytical).

0005The invention relates to a method for treating silicon or ferrosilicon by means of a slag liquefied by means of electrical resistance heating or kept in the molten state, which is characterized in that silicon or ferrosilicon is brought into contact with a slag which has the following analytical composition:<tables id="tabl0001" num="0001"><img file="EP0141125A2_D0001.tif" /></tables>

0006The above-mentioned totals are requirements that restrict the free choice of the individual components. The proportion of impurities due to raw materials is usually in the range from 0.1 to 1.5% by weight. All percentages by weight add up to 100 percent by weight for each special slag composition.

0007The following slag composition is preferred for melting fine-particle silicon with more than 97% silicon:<tables id="tabl0002" num="0002"><img file="EP0141125A2_D0002.tif" /></tables>

0008Before being used in the process according to the invention, the slags according to the invention can be present both as mixtures of their individual components and as premelted powders.

0009As ferrosilicon, in particular those with an iron content of 10 to 25% by weight are used. Silicon in particular has a purity content of at least 97% by weight.

0010The treatment of silicon or ferrosilicon is carried out according to methods known per se. For example, lumpy or fine-particle silicon or ferrosilicon, including fine silicon or ferrosilicon dust with particle sizes below 50 μm, can be introduced into the molten slag according to the invention. The lumpy or finely divided silicon or Ferrosilicon is converted into the molten state after it has been introduced into the slag, a liquid two-phase system being formed with an underlayer of molten silicon or ferrosilicon on which the slag melt floats.

0011The ratio of the amount of slag to the amount of silicon or ferrosilicon to be melted is advantageously 0.7 to 1.7: 1 part by weight.

0012Electrical resistance furnaces with stamped carbon lining and one or more graphite electrodes are preferably used as the device for carrying out the method. The graphite electrodes are immersed in the slag. The slag heated in this way is kept above the melting temperature of silicon or ferrosilicon. The temperature is typically 1500 to 1800 ° C., in particular 1600 to 1700 ° C.

0013Surprisingly, even without the addition of copper, the slag according to the invention has no emulsifying properties with respect to silicon or ferrosilicon. This prevents a higher silicon or ferrosilicon content from building up within the slag, which would lead to short circuits in the resistance heating system.

0014The slag treatment described can be used to recover, for example, very fine silicon dust, which is a waste product when grinding particulate silicon to a low-dust particle size, as a particulate product. In addition, the slag treatment according to the invention achieves a cleaning effect which relates in particular to the undesirable content of calcium and aluminum in silicon or ferrosilicon.

0015It is also within the scope of the invention to treat molten silicon or ferrosilicon, as is obtained, for example, from a reduction furnace, with slag according to the invention. To carry out this variant of the method, molten silicon or ferrosilicon, for example, is tapped from a reduction furnace in a furnace as described above. In this furnace there is the slag of the composition according to the invention, which was melted and kept warm by means of electrical resistance heating. After pouring into the furnace described, the liquid silicon or ferrosilicon collects below the slag layer and is freed from the contaminants aluminum and calcium by reaction with the slag. The silicon or ferrosilicon is then tapped into a preheated pan and poured onto ingots or plates.

0016Another variant of the method consists in tapping the molten silicon or ferrosilicon from the reduction furnace into a preheated pan and then covering it with slag according to the invention. The slag then changes to the molten state and forms a coherent protective layer over the liquid silicon or ferrosilicon tapping. The temperature required to keep the system fluid is maintained by resistance heating the slag. With these treatment methods, the cleaning effect already described above is also achieved.

0017In the latter process variants, a ratio of the amount of slag to the amount of treated silicon and ferrosilicon of 0.05 to 0.3: 1 parts by weight is expediently used.

0018The slags according to the invention also have the advantage of longer service lives than the slags known from DE-AS 27 06 175. While the previously known slags could be reused up to 20 times, it is possible with the slags according to the invention to carry out up to 35 taps. Surprisingly, an increase in yield of an average of 90% by weight of fine-particle silicon used is achieved by the above-mentioned known method to an average of 93% by weight using the method according to the invention.

0019The invention will now be explained in more detail by means of examples:

Examples 1 to 8

0020In each case 3.5 t of the slags referred to in Table 1 below were melted down in an electric resistance furnace with stamped carbon lining and graphite electrodes. Then 2.7 t of dust-like silicon were introduced into the melt. Quiet furnace operation was observed without slag or silicon being ejected from the melt. After 5 hours all solid components had melted. The temperature of the melt was 1650 ° C. Finally, the melted silicon melt was tapped. The yields and the cleaning action are shown in the table below.<tables id="tabl0003" num="0003"><img file="EP0141125A2_D0003.tif" /></tables>

Examples 9 to 14

0021The procedure according to Examples 1 to 8 was repeated, with the modification that ferrosilicon was used instead of elemental silicon.

0022The results are shown in Table 2 below.<tables id="tabl0004" num="0004"><img file="EP0141125A2_D0004.tif" /></tables>