METHOD FOR THE PURIFICATION OF SILICON

Abstract: The invention concerns a method for purifying silicon from boron by treating crude silicon with molten slag. Silicon is contacted with a slag, the slag-forming components of which consist of solid chlorine compounds and at least one compound selected from the group consisting of oxides, hydroxides, carbonates of alkali and/or alkaline earth metals as well as silica. Chlorides of alkali and/or alkaline earth metals can be used as solid chlorine compound.

Description: METHOD FOR THE PURIFICATION OF SILICON The present invention relates to a method for the purification of silicon. More particularly, the inven¬ tion comprises a method for purifying metallurgical si¬ licon for the production of raw materials for the semi- conductor and solar cell industry. To make the electric energy produced by solar cells economically competitive, the silicon from which the so¬ lar cells are made must be relatively inexpensive. In the production of silicon for electronics components, purification by gaseous phase is used. This technique is far too expensive for the production of solar cells. How¬ ever, since the purity requirements placed on solar cell silicon are not as high as for silicon for electronic uses, simpler purification techniques can be used. U.S. patent specification 4,304,763 discloses a multiple step method for purifying metallurgical sili¬ con. Finely divided silicon is leached with acid, whereupon it is melted and subjected to vacuum evapo¬ ration at 1500°-1700°C. If there is a high content of contaminating boron, molten silicon is contacted with a molten slag which may consist of a mixture of CaF ~ , CaO and Si0 2 . Solar cell silicon is defined by its property of converting light energy with an efficiency of at least 10%. Foreign elements affect the efficiency different¬ ly. Producers of solar cell silicon have especially wished for a boron reduction to 1-5 pp w in the start¬ ing material, and this has not been possible with the prior art techniques referred to above. The present invention as defined by the claims indicates an improved method for the purification of silicon from boron by treating crude silicon with a molten slag. This method gives a higher degree of purification and utilises a less poisonous melt than the above-mentioned method. A further advantage is that the amount of slag-forming agents employed is considerably lower than in prior art methods. For ex- ample, the above-mentioned U.S. patent uses an amount of slag which is twice as large as the amount of sili¬ con that is being processed. Solid chlorine compounds are used as the active slag-forming component. As solid chlorine compounds, chlorides of alkali and/or alkaline earth metals may be used, such as sodium, potassium or lithium chloride, or calcium, magnesium, barium or strontium chloride. Other conceivable solid chlorine compounds are hypo- chlorites, chlorates or perchlorates of alkali and/or alkaline earth metals. Other constituents of the slag which may be pre¬ sent are oxides of alkali and/or alkaline earth metals, and silica. Further constituents that can be used are oxide-forming carbonates and hydroxides of alkali and alkaline earth metals. The slag-forming agent has essentially three functions. It constitutes the extraction phase, i.e. some impurities pass from the silicon phase into the slag phase. It eliminates losses in the form of heat as well as losses in the form of silicon, the latter being caused by splashes from the silicon surface and oxidation thereof. The active component binds boron in the form of volatile boron chlorides. The slag is oxidising in the electrochemical sense, which increases the valency of boron from 0 to +11I, thus facilitating the formation of B(III)chlorides. The method is preferably carried out such that silicon is melted and heated to 1500-1600°C in a melt- ing furnace, whereupon the slag-forming agent is added. The particle size of the slag-forming agent is not critical, but less fumes and less dust are obtained if coarser materials are used. A suitable particle size lies within the range 1-3 mm. The slag- forming agent may be added in one or several runs. After the slag has been added, the melt is again heat- 5 ed to 1500-1600°C. Normally, no longer contact time is required. After the desired number of slag treat¬ ments- has been carried out, the silicon is teemed, crushed and allowed to cool and then analysed. Thus, the silicon need not first be finely ground and mixed 1.0 with the slag-forming agent and then heated, as indi¬ cated by other prior art methods. The method is there¬ fore suitable for purifying liquid metallurgical sili¬ con directly after a reduction furnace. The slag treatment may take place in a conven- 15 tional melting furnace, for example in an arc furnace or an induction furnace. The furnace is important to the process in so far as it increases the temperature of the material to be processed to a suitable process¬ ing temperature. 0 The temperature is an important parameter of the purification process. High temperature means high puri¬ fication, whereas low temperature gives no purification. A suitable processing temperature is from 1410°C, pre¬ ferred temperatures lying between 1500°C and 1600°C. 5 The higher the temperature, the higher will be the pu¬ rification efficiency, the upper limit being set by production engineering factors. The quantity ratio between slag and crude silicon based on weight may be 0.1-2, preferably 0.5-1. The com- 0 position of the slag suitably is 0.1-50% by weight of solid chlorine compounds, 0-50% by weight of at least one compound selected from the group consisting of oxides, carbonates and hydroxides of alkali and/or alkaline earth metals, and 0-80% by weight of silica. Preferred 5 values are 10-30% by weight of solid chlorine compounds, 10-30% by weight of at least one compound selected from the group consisting of oxides, carbonates and hydroxides of alkali and/or alkaline earth metals, and 40-70% by weight of silica. The values most pre¬ ferred are 20, 20 and 60% by weight, respectively. After addition of the slag-forming agent and heating for the desired period of time, the melt in the furnace consists of two phases, a lower silicon phase and an upper slag phase. Thus, the slag is a top slag. To be able to separate the silicon from the slag, it may be necessary to allow the slag to cool somewhat to cause it to set. The silicon obtained by the method according to the invention has a far higher degree of purification than silicon purified by prior art techniques employ¬ ing slag purification. The boron content, in particu- lar, can be lowered from about 16 ppmw to about 5 ppmw. It has also been found that a considerable purifi¬ cation of the carbon content is achieved. The purifica¬ tion of carbon is especially advantageous if performed in an arc furnace. In an induction furnace, silicon is contaminated by carbon from the graphite lining. How¬ ever, in order to achieve an especially high degree of purification for the remaining impurities, it is neces¬ sary to couple the method according to the invention with a known acid and/or vacuum treatment. The invention will now be described in more detail in the following Examples. Example 1 Equipment: Arc furnace with injection equipment in the form of a graphite lance. hot ladle was filled with about 1700 kg commer¬ cial silicon. The ladle was heated in the arc furnace for about 50 min, after which the temperature was about 1600°C. Then about 1/3 of CaCl 2 , and after that 1/3 of a mixture of CaO and SiO_ were charged. The procedure was repeated 2 times until all slag-forming agent, in all 1400 kg, had been added. The slag-forming agent consisted of 275 kg CaCl 2 , 250 kg CaO, and 875 kg SiO p . After the melt had been further heated for about 30 min, the ladle was allowed to cool until the slag had set to such an extent that silicon could be teemed without slag admixture. The silicon charged was analysed both before and after purification. The result is shown in Table 1. Silicon charged 13 0.26 0.60 0.12 0.01 0.08 Silicon teemed 6 0.37 0.07 0.05 0.01 0.01 It appears from the Table that the boron content is reduced from 13 ppmw in commercial silicon to 6 ppmw in purified silicon. Example 2 Equipment: Induction furnace with graphite crucible. 300 g silicon are melted and heated to 1500-1600°C. One third of the slag-forming agent which consisted of 50 g CaCl 2 , 50 g CaO and 175 g Si0 2 , was added. After that, heating to 1500-1600°C was again carried out, and the melt was held for about 30 min. Two fur- ther slag admixtures were carried out in the same man¬ ner. The finished silicon was teemed, allowed to cool and analysed. The result is shown in Table II. It appears from the Table that the boron content is reduced from 17 to 5 ppmw ("ppmw" means parts per million based on weight).

Claims: CLAIMS 1. A method for purifying silicon from boron, by treating crude silicon with molten slag, c h a r a c ¬ t e r i s e d in that silicon is contacted with a slag in which one of the slag-forming components consists of a solid chlorine compound. 2. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that molten silicon is contacted with the slag. 3. A method as claimed in claim 1, c h a r a c - t e r i s e d in that solid chlorine compounds and at least one compound selected from the group consisting of oxides, carbonates and hydroxides of alkali and/or alkaline earth metals as well as silica are used as slag-forming components. 4. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d 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 con¬ sisting of oxides, carbonates and hydroxides of alkali and/or alkaline earth metals, as well as 0-80% by weight of silica. 5. A method as claimed in claim 1, c h a r a c ¬ t e r i s e d in that the ratio between the amount of slag and the amount of crude silicon, based on weight, is 0.1-2. 6. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d in that chlorides of alkali and/or alka¬ line earth metals are used as solid chlorine compounds. 7. A method as claimed in claim 6, c h a r a c - t e r i s e d in that CaCl- in an amount of 10-30% by weight of the slag is used as chloride. 8. A method as claimed in claim 3, c h a r a c ¬ t e r i s e d in that CaO is used as oxide-containing compound. 9. A method as claimed in claim 2, c h a r a c ¬ t e r i s e d in that the treatment temperature is from 1410°C, preferably above 1500°C.

Inventors: AAHLUND GOERAN, ANDERSSON OLAUS, JOHANSSON INGEMAR, MELLSTROEM ROLF, AAHLUND, GOERAN, ANDERSSON, OLAUS, JOHANSSON, INGEMAR, MELLSTROEM, ROLF
Applicants: KEMANORD IND AB, KEMANORD INDUSTRIKEMI AB
Application year: 1988
Main classification: C01B33
IPC codes: C01B33