DD240730A1 - PROCESS FOR OBTAINING PURE SILICON POWDER - Google Patents

Abstract

The invention relates to a method for obtaining pure silicon from metallurgical silicon powders. It has particular importance for the production of base material for silicon nitride and carbide or as an additive for aluminum alloys. The aim of the invention is to obtain pure silicon from metallurgical silicon in a relatively simple way, in particular by reducing the energy-intensive processes. The object of the invention is to bring metallurgical Si powder through easily manageable process stages in a degree of purity, which allows further processing to pure base material for the production of silicon nitride and carbide and as alloy material for construction materials. Surprisingly, it has now been found that all the metallurgical silicon qualities produced by the usual methods can be increased in their purity according to the invention by activating the metallurgical silicon chemically or mechanically, washing them with water and mineral acid and subjecting them to a fluoride ion treatment. After careful washing with water remains after drying a silicon powder of high purity.

Description

Field of application of the invention

The invention relates to a method for obtaining pure silicon from metallurgical silicon powders. It has particular significance for the production of base material for silicon nitride and carbide or as an additive for aluminum alloys.

Characteristic of the known technical solutions

In addition to its importance to the semiconductor industry, pure metallic silicon powder has also become established as the starting material for the production of silicon carbide and nitride as well as an alloy material for corrosion-resistant solid aluminum materials over other silicon-containing starting materials. Technical silicon with a silicon content of less than 99 Ma ° / o is not suitable for these applications. Reiner silicon products are used on various u. a. from [Aulich, H. A .; Schulze, F.-W .; Grabmeier, J. G .; Chem. Ing. Techn. 56 (1984) 9, pp. 667-673].

For the production of semiconductor-grade silicon, the trichlorosilane hydrogen reduction process has become established. The silicon thus obtained is very pure but at the same time very expensive due to the technologically complex process.

In addition, methods for producing silicon by reduction or decomposition of silicon tetrachloride (SiCl ₄ ), dichlorosilane (SiH ₂ Cl ₂ ), monosilane (SiH ₄ ), silicon tetrafluoride (SiF ₄ ) or hexafluoride-silica (H ₂ SiF ₆ ) proposed.

In the context of the large-scale use of Si for solar cells, the efforts to cheaply produce the Si increased, in particular trying to circumvent the energy-intensive stages of the overall process.

For example, it was attempted to produce pure silicon directly from technical grade Si by using selected pure raw materials (carbonaceous silica pellets) and subsequent zone melting of the rods drawn from the melt of the technical Si obtained.

According to other methods, the interfering impurities from the melt are to be eliminated by refining with oxygen or chlorine gas, by addition of alkaline earth oxides, mixed melts with aluminum, targeted solidification and selective further processing of the material.

In the known so-called "Silgrain® method" (DE-OS 2,122,029) starts from coarse pieces of technical Si, which is leached with an aqueous FeCI ₃ solution with constant addition of chlorine and crushed. The entire process is by application of heat at about 110 ⁰ In this multistage process, in addition to the purified silicon, mainly hydrogen-containing exhaust gases containing up to 0.5% of the highly toxic phosphine are obtained, and the associated safety and environmental problems can only be satisfactorily resolved with considerable additional outlay.

All these methods have in common that either the achievable purity of the Si for further processing is too low or that the Si produced is too expensive.

A further disadvantage is that the silicon thus produced is too coarse for processing into ceramic materials (nitride, carbide) and therefore has to be ground, which leads to an additional contamination of up to 0.3%.

Object of the invention

The aim of the invention is to obtain pure silicon from metallurgical silicon in a relatively simple way, in particular by reducing the energy-intensive processes.

Presentation of the essence of the invention

The object of the invention is to convert metallurgical Si powder by easily manageable process stages in a degree of purity, which allows further processing to pure base material for the production of silicon nitride and carbide and as alloy material for construction materials.

Surprisingly, it has now been found that all metallurgical silicon qualities produced by the conventional processes can be increased in their purity according to the invention in that the metallurgical silicon is activated chemically or mechanically, washed with water and mineral acid and subjected to a fluoride ion treatment.

After thorough washing with water, a high purity silicon powder remains after drying.

The table shows some average analytical values of silicon products (grain fraction <100 μm) in% by weight.

FE Al Ca

0.03 0.1 0.15

An advantageous embodiment of the invention is that the activation of the metallurgical silicon takes place chemically by partial reaction with halogens, hydrogen halides or organohalides in the context of a corresponding silane synthesis, from which the activated powder is taken in the form of fly ash.

The process according to the invention can be coupled particularly advantageously with the chlorosilane or alkylchlorosilane synthesis already used on an industrial scale by subjecting the very fine-grained activated flue dust produced there as a by-product to the further process steps. Thus, the metallurgical silicon used there can be practically almost completely and environmentally friendly converted into high-quality products.

A further advantageous embodiment of the invention is that the preliminary cleaning of the actual activation of the metallurgical silicon mechanically by a fine grinding to less than '/ ₀ of the starting grain size, but μηη carried out at least to particle sizes <40th

Likewise, cutting sludge and abrasive powder from the field of semiconductor silicon production, which are already mechanically activated in their resulting form, can be cleaned with correspondingly little effort.

Overall, there is an additional advantage when using the silicon powder produced by the process according to the invention, in particular for the production of ceramic materials. When using metallurgical Si purified according to the state of the art, it had to be assumed that the ceramics were produced from a clean, but very hard and coarse-grained product, which could only be subjected to coarse comminution and fine grinding after fracture had occurred. In this case, subsequent impurities occurred up to 0.3%. In contrast, the pure silicon powder produced according to the invention can be further processed without subsequent comminution and thus contamination can be excluded.

embodiment

In the following, the invention will be explained in more detail by exemplary embodiments.

example 1

1 kg of metallurgical Si (about 96% Si content) in powder form are reacted in a fluidized bed reactor with hydrogen chloride, wherein about 30% of the powder used and the resulting chlorides of impurity elements entrained in the gas stream and deposited in the downstream filter system.

The chemically activated powder removed from the filters is leached several times with 2-3 l of distilled water and then with half-concentrated hydrochloric acid. To remove oxide constituents, a treatment with dilute hydrofluoric acid is connected and the powder slurry is washed with water and dried.

This gives 206 g of pure silicon powder of the following purity:

Fe Al Ca

0.025 0.11 0.16

In the event that the grain size of the obtained activated Si powder should be too large for the later use, it is possible to insert an additional grinding step in the process before the fluoride ion treatment without reducing the purity of the final product.

The resulting pure silicon powder can now be supplied to various uses, for which some examples are given below.

- 100 g of powder are added with 3 g of MgO as a sintering aid and mixed thoroughly in a ball mill, wherein the MgO is homogeneously distributed on the Si grain. The grinding powder is used to form blanks by cold pressing and to treat them in nitrogen at 1400 ° C. for 6 h to obtain ceramic silicon nitride bodies with less than 2% free silicon and about 30% porosity.

- 84 g of powder are mixed with 36 g of carbon (soot) and reacted in an inert gas atmosphere at 1400 ⁰ C within 4 h to silicon carbide. The reaction cake is comminuted, ground to grain size by 1 μηη and fed to the ceramic process for further processing.

The powder is added to a melt of pure aluminum in such proportions that an aluminum-silicon alloy with 5-6% silicon is formed. The iron content of this alloy is not more than 0.1%. Such an alloy is very resistant to corrosion and can be used for food processing machines.

Example 2

1 kg of metallurgical silicon of grain size 0.3 mm is mechanically activated by grinding to particle sizes of about 5 μηη and stirred with heating. After 3 h stirring at 70-90 ⁰ C is decanted and the process is repeated again. The sediment is mixed with about 3 l of half-concentrated hydrochloric acid and stirred for 2 h at elevated temperature, then decanted. The process is repeated and then added with vigorous stirring in small portions about 50 g of sodium fluoride. After completion of the gas evolution was diluted with water, decanted, washed several times with distilled water, filtered and dried. The resulting 650 g of powder were fed to one of the uses mentioned in Example 1.

Claims (3)

Invention claim: 1. A process for the preparation of pure silicon powder of metallurgical silicon, characterized in that the metallurgical silicon activated chemically or mechanically, extracted with water and mineral acids and then subjected to a Fluoridionenbehandlung followed by water washing and drying. 2. The method according to item 1, characterized in that the chemical activation takes place by partial reaction with halogens, hydrogen halides or organohalides in the context of a corresponding silane synthesis, from which the activated powder is taken in the form of fly ash. 3. The method according to item 1, characterized in that the mechanical activation by fine grinding of the technical silicon to less than Vio of the starting grain size, at least on grain sizes <40μπη.