FR2519961A1 - PROCESS FOR PURIFYING SILICON - Google Patents

Abstract

A.PURIFICATION PROCESS OF SILICON BY TREATMENT OF CRUDE SILICON WITH MOLTEN SLAG CONTAINING ALKALINE-EARTH OXIDES OR ALKALINE OXIDES. B. PROCESS CHARACTERIZED IN THAT CRUDE SILICON, IN CRUSHED FORM, IS MIXED WITH SLAG, OR THE COMPONENTS OF SLAG, BEFORE MELTING. C. THE INVENTION IS CONCERNED WITH THE PURIFICATION OF SILICON.

Description

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PROCESS FOR PURIFYING SILICON

  The invention relates to a method for purifying silicon, in particular for

  manufacture of solar grade silicon.

  Given the growing importance of solar cells for energy production, many tests have been carried out in recent times to purify silicon

  at least 95% purity, referred to as silicon

  raw material so as to make it suitable for

of solar cells.

  In order to purify the raw silicon, acids or mixtures of acids have been used, according to patents CH 567 435 and US Pat. No. 2,972,52, as purification agents, but determined impurities, such as, for example, boron or phosphorus, have

could be eliminated.

  DE-PS 1,022,806 and DE-OS 2,706,175 describe processes according to which the purification is carried out using a slag treatment, such as, for example, molten silicate; the purification action is limited however,

  in these processes, to calcium and aluminum elements.

  Even combining the acid and slag treatments, you can not get a

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  silicon which is suitable for obtaining solar cells Thus, for example, according to DE-OS 2 729 464, the raw silicon is brought into contact with a melt extraction before the acid treatment; however, this also does not eliminate elements such as, for example, boron or phosphorus. It is true that we know of processes whose purifying action extends to all undesirable matters; but these processes are too expensive and therefore not very cost-effective, or do not lead to the desired degree of purity. DE-OS 2 933 164, for example, describes a multi-step process where the extraction melting is followed by a treatment. with acid and under vacuum After several operations of fusion and resolidification, one does not always obtain

  not the degree of purity desired for boron.

  In the process according to German Offenlegungsschrift No. 2 944 975, the melt is reacted with barium carbonate to give a satisfactory degree of purity with respect to boron, but this process is very expensive and laborious. very high temperatures and the need to inject an oxidizing gas such as, for example, oxygen or water vapor, to obtain

a sufficient degree of purity.

  Accordingly, the object of the invention is to propose a process for purifying silicon which does not have the disadvantages mentioned, and which makes it possible to obtain, by a simple, easy and economical route, silicon having a purity suitable for the manufacture batteries

  solar energy, particularly at the boron level.

  For this purpose, the invention proposes a process for purifying silicon by treatment

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  raw silicon with molten slag containing alkaline earth oxides and / or alkali oxides, characterized in that the raw silicon, in milled form, is mixed with the slag or slag components, prior to melting.

One embodiment of the invention

  The invention is characterized in that the milled raw silicon is mixed with the entire slag or slag-forming components, also milled and then melted, or the milled raw silicon is mixed with a portion of the slag, or components forming the slag. slag, crushed, and that the mixture is added to

remaining slag melted.

  According to a preferred form of

  In this way, the milled raw silicon is mixed with about half of the slag or slag-forming components and then added to the other half of the slag.

slag melted.

  As the slag-forming products, compounds which can be converted into oxides under the stated conditions, such as, for example, the carbonates or hydroxides of the alkali or alkaline-earth metals, which are converted into oxides, can also be used in place of the oxides. by releasing carbon dioxide or water It is also possible to use mixtures of two

  or of several alkaline and / or alkaline compounds

earth.

As alkaline compounds

  preferably, the oxides of magnesium, calcium, strontium and / or barium, or the carbonates and / or hydroxides leading to these oxides. As alkaline compounds, the lithium and sodium oxides are used. and / or potassium, or the corresponding carbonates and / or hydroxides. It is advantageous to operate in the presence of a suitable flux such as, for example, magnesium and / or calcium fluorides, and / or in the presence of a diluent. For the alkaline earth oxides, silicon dioxide, and for alkaline oxides,

  aluminum oxide, and / or silicon dioxide.

  Preferably, the slag consists of 20 to 100% by weight of alkaline and / or alkaline earth oxides,

  % by weight of flux and 0 to 80% by weight of diluent.

  The particle size of the crushed ground silicon should preferably be between 10 micror and 1 mm and in particular between 40 and 100 microns. The grinding of the raw silicon metal can be carried out in suitable grinders of the ball-type centrifuge type. opposing jet disintegrator (particle size: about 10 microns), or by crushing (particle size: less than or equal to 1 mm) The crushed slag or slag components preferably have the same particle size spectrum as the As a result, it is also possible to grind the silicon

  in common with that of the slag or

forming the slag.

  The melting can be carried out in a suitable furnace, of the current type, such as a

  electric resistance furnace, or an induction furnace.

  The melting temperature is most often between 1400 and 16000 C, in particular approximately

15000 C.

  The ratio by weight of the raw silicon to the slag depends on the type and composition of the slag The weight ratio of raw silicon / oxide

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  alkaline or alkaline-earth metal is preferably

  1: 0.2 to 4 and in particular 1: 0.3 to 3.

  After the complete melting of all the components, the molten silicon is drawn off or spilled. A prolonged residence time is not usually necessary. To obtain a particularly high degree of purity of the silicon, it can

  be advantageous to follow the process according to

  treatment with acid and / or vacuum, as described, for example, DE-OS

2 933 164.

  Unexpectedly, the process according to the invention makes it possible to obtain silicon with a degree of purity clearly greater than that of a silicon obtained according to a known slag treatment process. In particular, the boron content of the raw silicon can be lowered from about 40 ppm to about 3 to 4 ppm The method according to the invention thus provides, in a simple and economical way, silicon of a purity perfectly

  suitable for the manufacture of solar cells.

The invention will be better understood

using the examples below.

Example 1

  2 kg of crude silicon of a current composition were milled to a mean particle size of 40 microns; with 5.5 kg of a slag consisting of 41% by weight of magnesium oxide, 41% by weight of silicon dioxide and 18% by weight of calcium fluoride. They were mixed vigorously, and slowly added to kg of previously melted slag After subsequent purification with the acid, the following degree of purity was obtained: iron 0.003% by weight aluminum 0.0015% by weight calcium z 0.01% by weight boron 3 ppm

-

Example 2

  1 kg of crude silicon of current composition was milled to an average particle size less than or equal to 1 mm, vigorously mixed with 2.75 kg of a slag formed of 41% by weight of barium hydroxide, 41% by weight weight of silicon dioxide and 18% by weight of calcium fluoride, and slowly added to 2.75 kg of previously melted slag After subsequent purification with the acid, the following degree of purity was obtained: aluminum 0.01% by weight calcium weight / 0.01% by weight boron 2 ppm

Example 3

1 kg of raw silicon, of

  The current batch was milled, with a mean particle size of 40 microns, vigorously mixed with 2.8 kg of slag composed of 50% by weight of sodium hydroxide and 50% by weight of silicon dioxide, and slowly added. to 2.8 kg of a previously melted slag After subsequent purification with the acid, the following degree of purity was obtained: iron 0.02% by weight aluminum 0.01% by weight calcium t 0.01% by weight boron 1 ppm

Reference example.

  When the raw silicon used in Examples 1 to 3 is subjected to a slag treatment according to a known method, by adding the raw silicon to a slag previously

  melted, after treatment is obtained after the

  of, a product having the following degree of purity: iron 0.01% by weight aluminum 0.02% by weight calcium 0.01% by weight boron 36 ppm

Claims (14)

  Process for the purification of silicum by treatment of raw silicon with molten slag containing alkaline earth oxides and / or alkaline oxides, characterized in that the raw silicon, in ground form, is mixed with the slag, or the components of the slag, before the merger.   Process according to claim 1, characterized in that the milled raw silicon is mixed with all of the slag or slag-forming components, also milled and then melted, or the milled raw silicon is mixed with a portion of the slag, or slag forming components milled, and this mixture is added to the remainder of the slag. Process according to claim 2, characterized in that the milled raw silicon is mixed with about half of the slag or slag-forming components and is then added to   the other half of the slag melted. 4 Process according to any one   Claims 1 to 3, characterized in that,   the slag forming components are alkali metal oxides, carbonates and / or hydroxides and / or alkaline earth. Process according to any one   claims 1 to 4, characterized in that   310 one works in the presence of a fondant.   6 Process according to the claim, characterized in that, as a fluxing agent, it is used   magnesium fluoride or calcium fluoride. 19961 7 Process according to any one   claims 1 to 6, characterized in that   the operation is carried out in the presence of a diluent.   Process according to Claim 7, characterized in that silicon oxide is used as the diluent for the alkaline earth oxides, and aluminum oxide and / or aluminum oxide are used as the diluent for the alkaline oxides. dioxide of silicon. 9 Process according to any one   claims 1 to 8, characterized in that   the slag consists of 20 to 100% by weight of alkaline and / or alkaline earth oxides, 0 to 40%   by weight of flux and 0 to 80% by weight of diluent. Process according to any one   than claims 1 to 9, characterized in that   the alkaline earth oxides are magnesium oxide   sium, calcium oxide, strontium oxide and / or barium oxide. 11 Process according to any one   than claims 1 to 9, characterized in that   the alkaline oxides are lithium oxide,   sodium oxide and / or potassium oxide. 12 Process according to any one   that claims 1 to 11, characterized in that   that the particle dimensions of the raw silicon   milled is between 10 microns and t mm, preferably between 40 and 100 microns. 13 Process according to any one   than claims 1 to 12, characterized in that   slag, or slag-forming components, crushed, have the same particle size spectrum as raw silicon crushed. 14 Process according to any one   claims 1 to 13, characterized in that   that the fusion is carried out at a temperature of 1400 to 1600 O C. Process according to any one   claims 1 to 14, characterized in that   the ratio by weight of crude silicon / alkaline or alkaline earth oxide is of the order of 1: 0, 2 to 4, of preferably 1: 0.3 to 3.