SE460287B - PROCEDURE FOR PURIFICATION OF SILICONE FROM BORN - Google Patents

Description

io. 15. 20. _25. 30. 35. 460 287 2 um, potassium or lithium chloride or calcium, magnesium, barium or strontium chloride. Other possible solid chlorine compounds are hypochlorites, chlorates or perchlorates of alkali and / or alkaline earth metals. Other constituents that can be included in the slag are oxides of alkali and / or alkaline earth metals, as well as silica.

Other constituents that can be used are oxide-forming carbonates and hydroxides of alkali and alkaline earth metals.

The slag former mainly has three functions. It constitutes the extraction phase, ie. some pollutants pass from the silicon phase to the slag phase. It prevents losses in the form of heat and losses in the form of silicon. The latter arise by splashing from the silicon surface and oxidizing it.

The active component binds boron in the form of volatile boron chlorides. The slag is oxidizing in the electrochemical sense, which increases the valence of boron from 0 to + III. The formation of B (III) chlorides is thus facilitated.

The process is conveniently carried out so that silicon is melted and heated to 1500 "- 1600 ° C in a melting furnace. The slag former is then added. The grain size of the slag former is not significant, however, less smoke is obtained and dusting if coarser material is used. Suitable grain size is in the range 1-3 mm. The slag former can be added in one or more batches. After slag addition, the melt is heated again to 1500 ° - 1600 ° C. A longer contact time is usually not necessary. After the desired number of slag treatments, the silicon is dropped, crushed, allowed to cool and analyzed. Thus, the silicon does not first have to be finely ground and mixed with the slag former and then heated as other known methods state.

The process is therefore suitable for purifying liquid metallurgical silicon directly after a reduction furnace.

The slag treatment can take place in a melting furnace suitable for this purpose, such as, for example, in an arc furnace, or an induction furnace. The importance of the oven in the process is to raise the temperature of the material to be treated to the appropriate treatment temperature.

Temperature is an important parameter for the purification process. High temperature gives good purification, while low temperature, n ... .__ .. n .___._._..-.- «- ~« --- 10. 15. 20. 25. 30. 3 460 287 does not provide any 'purification. Suitable treatment temperature is from 1410 ° C, preferred temperature from 1500 ° C - 1600 ° C.

The upper limit is determined by production technical limitations, as from a purification point of view, the higher the temperature used, the better the effect.

The amount ratio of slag to silicon based on weight may be 0.1 to 2, preferably 0.5 to 1. from the group of oxides, carbonates and hydroxides of alkali and / or alkaline earth metals and O-80% by weight of silica.

Preferred values are 10-30% by weight of solid chlorine compounds, 10-30% by weight of at least one compound selected from the group of oxides, carbonates and hydroxides of alkali and / or alkaline earth metals and 40-70% by weight of silica. Most preferred values are 20, 20, 60% by weight, respectively.

After adding the slag former and heating for the desired time, the mass in the furnace consists of two phases, a lower silicon phase and an upper slag phase. The slag thus consists of a top slag. In order to be able to separate silicon from slag, it may be necessary to allow the slag to cool slightly to harden.

With the process according to the invention, silicon can be obtained which has a much higher degree of purification than silicon which has been purified according to known methods with impact branching. In particular, the boron content can be reduced from about 16 ppmw to about 5 ppmw.

It has also been found that a considerable purification of the carbon content is achieved. In particular, the advantage of purification regarding carbon in an arc furnace can be emphasized. In an induction furnace, silicon is contaminated with carbon from the graphite lining. However, in order to achieve a particularly high degree of purification with regard to other impurities, it is necessary to couple a known acid and / or vacuum treatment to the process according to the invention. The invention is now described in more detail with the aid of the following working examples: Example Equipment: Arc furnace with injection equipment in the form of 460 287 10. 15. 20. 25. 30. 35. a graphite lance.

A hot sideboard was filled with about 1700 kg of commercial silicon.

The sideboard was heated in the arc oven for about S0 min. The temperature was then about 1600 ° C. Then about 1/3 of CaCl2 was first charged and then 1/3 of a mixture of CaO and SiO2. The procedure was repeated 2 times until all slag formers, a total of 1400 kg, were added. The slag former consisted of 275 kg CaCl2, 250 kg Ca0 and 875 kg S102. The melt was heated for an additional 30 minutes. After heating, the ladle was allowed to cool until the slag solidified so much that the tapping of silicon could take place without slag mixing. Ingredient silicon was analyzed both before and after purification. The results are shown in Table 1.

Table 1: Contaminant content B Fe Al Ca Cl C ppmw%%%%% Incoming silicon 13 0.26 0.60 0.12 0.01 0.08 Outgoing silicon 6 0.37 0.07 0.05 0.01 0.01 The table shows that the boron content is reduced from 13 ppmw in commercial silicon to 6 ppmw in purified silicon.

ExemEel: 2 Equipment: Induction furnace with graphite crucible. 300 g of silicon are melted and heated to 1500-1600 ° C. A third of the slag former consisting of 50 g of CaCl2, 50 g of CaO and 175 g of SiO2 was added. Then it was heated again to 1500-1600 ° C and the melt was allowed to stand for about 30 minutes. Two more slag additives were made in the same way. The finished silica was dropped, allowed to cool and analyzed. The result is shown in Table 2. f-ñï '10. 460 287 Table 2: Contamination content "B Fe A1 eca cl c ppmw%%%%% Incoming silicon 17 0.31- 0.29 ø, o4 - 0.06 Outgoing diesel 5 0.42 0.66 0.08 0.09 0.14 The table shows that the boron content is reduced from 17 to 5 ppmw (by ppmw is meant "parts per million" calculated by weight).

Claims (8)

460 287 10. 15. 20. 25. 30. 6 Patent claims 1. l. A process for purifying silicon from boron, by treating raw silicon with molten slag, characterized in that silicon is contacted with a slag in which one of the slag-forming components consists of a solid chlorine compound. Oh 2. A method according to claim 1, characterized in that molten silicon is contacted with the slag. 3. Process according to claim 1, characterized in that solid chlorine compounds and at least one compound from the group of oxides, carbonates and hydroxides of alkali and / or alkaline earth metals and silica are used as slag-forming components. Process according to Claim 3, characterized in that the slag contains 0.1-50% by weight of solid chlorine compounds 0-50% by weight of at least one compound selected from the group of oxides, carbonates and hydroxides of alkali and / or alkaline earth metals, and O-80% by weight of silica. 5. A method according to claim 1, characterized in that the ratio between the amount of slag and the amount of silicon, based on weight, is 0.1 - 2. Process according to Claim 3, characterized in that chlorides of alkali and / or alkaline earth metals are used as solid chlorine compounds. 7.; Process according to Claim 6, characterized in that CaCl 2 is used as chloride in an amount of 10 to 30% by weight of the slag. 8. A process according to claim 3, characterized in that CaO is used as the oxide-containing compound. [Process according to claim 2, characterized in that the treatment temperature is from 140 ° C, preferably above 1500 ° C.