Process for preparing silicon from quartz and carbon in an electric furnace

INTERNATIONAL MINERALS & CHEMICAL LUXEMBOURG SA.

PROCESS FOR PREPARING SILENCE FROM QUARTZ AND CARBON IN AN ELECTRICAL OVEN

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&lt;V -2 „_r carbon dioxide. Granulated quartz melts. The resulting silicon carbide reacts with. fused quartz, to give rise to silicon, also forming carbon monoxide.

This invention relates to a process for the preparation of silicon from quartz and carbon in an electric furnace, a mixture of quartz and carbon agglomerates together with granulated quartz introduced in an electric furnace, and being the mixture is conducted to a reaction in which silicon carbide is also formed. Within the context of the invention, the term electric furnace refers primarily to low pot electric ovens, conventional in metallurgy. Covered within the meaning of the invention, quartz indicates all conventional vehicles of silicon dioxide, and more specifically sand, quartzites and the like.

The process, which forms the basis of the invention (German Patent Spec. 20 55 565, column 3, lines 4 to 19), is a development of a process employing a dissolute mixture and a layer made of quartz and carbon. This is without the prior formation of clusters. Due to a proportion of gaseous silicon monoxide, the process, which folds into a dissolute layer, results in a cyclic reduction and hence a new formation of silicon dioxide, a fact that increases energy consumption and reduces the yield. In contrast, the process on which the invention is based increases the yield, but the addition of granular quartz separately results in the fact that silicon dioxide is separated within the electric furnace from the reducing agent containing carbon, a characteristic that results in irreparable and inefficient furnace work. Agglomerates consist of the aforementioned components with carbon in coal mode and are produced by extrusion or the like. The agglomerates of this species are not, however, very stable. Pasty fusion of the charge may be observed in certain regions of the electric oven. It is not intended to achieve, nor is it possible to achieve, a differential process with a reac. controlled in the agglomerates, on the one hand, and with quartz * and 'i' '23Ksm * ia2ntcEu;

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of the granular layer, on the other hand. This is perfectly regular if the objective is * a stoichiometric reaction according to the equation SiOg + 2C -&gt; Si + 2C0 and if a clustered charge is employed for the purpose and the addition of granular quartz is not required. and the particle size of silicon dioxide being further specifically limited, as are the relative weights of the particle size fractions and the bulk weight of the agglomerated charge. This process also results in silicon monoxide, which tends to react with carbon monoxide, to produce viscous products. The circumstances are similar, if the filler consists of pellets or briquettes, of silicon dioxide shaped bodies finely divided into a homogeneous mixture with the required quantities. carbon-reducing agents.

It is the object of the invention, therefore, to read. The process, on which the invention is based, to enable a differential procedure to be carried out by carrying out a controlled reaction in the agglomerates on the one hand and with the quartz of the layer on the other hand without any disturbing formation. of silicon monoxide.

For this purpose, and according to the invention, the agglomerates are employed in the form of stable briquettes containing from 30 to 60% by weight of carbon, with preferred briquettes made from fine-grained quartz and coal. grain at a temperature of 3502 to 6502C, preferably 5θθ- ^The 60020; quartz of the briquettes are first reacted in the electric furnace with the carbon of the briquettes to give silicon carbide, also forming carbon monoxide; and granular quartz is fused; and the resulting silicon carbide reacts with fused quartz to give silicon, also forming carbon monoxide. According to a preferred embodiment of the invention, the reaction between the quartz of the briquettes and the carbon of the briquettes is carried out at a temperature of 16002 ° C and above while the reaction is in progress.

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between silicon carbide and fused quartz and conducted to

at a temperature of from 1,800 to 2,000 ° C. Generally, the beans employed have a weight of from 10 to 100 g, and preferably from 20 to 60 g. Granular quartz has a grain size of 3 to 12 mm. The circumstances under which the process is conducted can be specified in full detail if the briquettes are arranged in a maximum density packing layer, i.e. a maximum density spherical wrapping, and the granular quartz is briquettes.

Regarding the reduction of silicon dioxide, it can be thought that the formation of silicon monoxide can be completely and practically eliminated by giving the quartz a considerable excess of carbon to allow rapid and minute reduction of silicon. . This idea, however, is not practical, as excess carbon forms an alloy with silicon to give silicon carbide:

Si0₂ + 3C -&gt; SiG + 2C0

A knowledge of these affinities does not get you. see prior influence in solving the problems described hereinbefore. The invention is based on the resultant of the preceding equation, that is, that excess carbon forms an alloy with silicon to give silicon carbide. According to the invention, this phenomenon is used to divide the feed substances into two parts. One part consists of briquettes, which contain a high excess of carbon, as regards the reduction of silicon dioxide. heat; The other part is usually granular quartz, whose grains are of a size adapted to the free volume between the briquettes. There is no difficulty in working the electric furnace so that the briquette is converted to the silicon carbide in the first phase during the descent of the bed, while the silicon carbide reacts with the fused quartz in the second phase:

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2Si G + Si0_O-&gt; 3 Si + 200 ^The t

A first advantage is that high carbon excess inhibits the formation of silicon oxide in the brine because of the very close contact between the reactants, and since any silicon oxide that forms can react immediately with the existing carbon. A second advantage lies in the high specific gravity of silicon carbide which enables it to migrate deep into the lighter quartz molten mass such that the rate of reaction is greatly increased. 0 meaning of these relations, in the true operation, O as follows:

According to the equation »

Si0₂ + 20 -_;Si + 2G0, about 400 kg of carbon is required to reduce 1,000 kg of quartz if the losses are not taken into account. These mixtures are equivalent to the stoichiometric reaction. However, 666 kg of quartz may be reacted with this 400 kg of carbon to give silicon carbide. As a result, there are 334 kg of quartz left over, which will react with the silicon carbide, which has been produced in the oven in the meantime. This quartz should preferably be in small pieces of the order of 3 to 12 mm when the oven is loaded. If account is taken of any losses that may have arisen, the provision of car. bono should be increased by about 5 to 10%. Take 8% as a real example. Accordingly, 432 kg of carbon should be briquetted with 666 kg of sand. In percentage terms, this means that there is 39.3% carbon in the briquette. However, it has been found that the process can be further improved if the carbon is incorporated into the quartz / carbon briquettes in an amount greater than necessary for the formation of silicon carbide. According to the foregoing, this inhibits the formation of silicon oxide. The result is a replacement of the silicon dioxide portions in the briquette, which becomes richer in carbon. The use of granular quartz should be stressed. Good practical results have been found if the amounts of quartz are used about one half in the sieved and broken quartzite grain mode and the other half in the sand or fine quartzite mode in a mixture with the carbon vehicles. If losses are taken into account, this translates, for example, into a 5θθ kg briquette of araa or thin quartzite, with 432 kg of carbon, which thus has a content of 46% in the briquette.

There are thus numerous technical and economic advantages. Firstly, with the process according to the invention, about 5θ% of the material employed may be cheap river sands or practically unusable fine quartzite, re. thereby greatly reducing the costs of raw materials. Secondly, energy consumption is reduced by 25 to 30% with the use of a mixture of granular quartz and sand / carbon briquettes. The advantages of environmental protection can also be mentioned.

Briquettes of suitable stability in the indicated composition can best be produced by agglutination of the coal in a hot briquetting process. To this end, 25 + 3% carbon is required, which translates to 20 + 3% carbon in the briquette. This makes up precisely half of the carbon required, which should be between 40 and 50%. · Other carbon vehicles can be used for this purpose. 0 Petroleum coke is preferred for this purpose as it has few impurities.