DE1144010B - Process for the simultaneous production of sodium-aluminum double fluorides and silicon or aluminum-silicon alloys - Google Patents

Description

It is known by reacting sodium silicofluoride with aluminum, a sodium-aluminum double fluoride and silicon or an aluminum-silicon alloy manufactured according to the following equation:

3 Na ₂ SiF ₆ + (4 + x) Al -> - 6NaF + 4 AlF ₃ + 3 Si (+ xAl)

For this purpose, a mixture of sodium silicofluoride and aluminum or sodium silicofluoride alone is introduced into an aluminum melt. Care must be taken by excluding atmospheric moisture and thorough mixing by stirring that the sodium silicofluoride does not decompose and any silicon tetrafluoride that may have split off does not escape. The reaction * 5 produces a liquid aluminum-silicon alloy and a molten salt with a _{composition similar to chiolite (3AlF 3} * 5NaF). Chiolite can be used as a flux or refining agent in the aluminum industry; However, cryolite with a slightly different composition (AlF ₃ · 3NaF) is more sought-after. The practical implementation of this process is made difficult by the necessary intensive stirring and the exclusion of any moisture.

Another method is to combine the sodium fluoride silicofluoride and aluminum into a fluoride melt entered and mixed there by vigorous stirring and reacted, wherein again all moisture should be excluded. It has already been suggested that fluoride melt be used add so much sodium fluoride that cryolite is obtained instead of chiolite.

It is also known to cause an intimate mixture of salt and aluminum powder to react. In all cases, however, the reaction is carried out in the liquid phase, with aluminum always in the Excess is present.

The invention now relates to a process for the simultaneous production of sodium-aluminum double fluorides and silicon or aluminum-silicon alloys, in which the known reaction between Sodium fluosilicate and aluminum in a simpler way and in particular without stirring and without Exclusion of air and moisture carried out in open vessels under atmospheric conditions can be.

According to the invention, an intimate, powdery, densely packed mixture of sodium silicofluoride, Sodium fluoride and aluminum reacted in a solid state by heating. The sodium fluoride content the reaction mixture is the ge method of simultaneous preparation

of sodium-aluminum double fluorides

and silicon or aluminum-silicon

Alloys

Applicant:

Aluminum Industry Shares Society
Chippis (Switzerland)

Representative: Dr. K. Schwarzhans

and Dipl.-Chem. Dr. phil. E. Jung, patent attorneys,

Munich 19, Romanplatz 10

Claimed priority:
Switzerland of December 10, 1959 (No. 81 692)

Dr. Chem. Remi Alexis Chaperon, Zurich (Switzerland), has been named as the inventor

to the desired composition of the resulting double fluoride, while the aluminum content is at least sufficient to reduce the silicon. The mixture is heated so high that the reaction occurs, but the reaction product is not melted. Only after the reaction does this become separated by melting into salt and metal.

The reaction does not take place in a melt flow as in the known processes. There is no need to stir; it is sufficient if the reactants are present in an intimate, powdery, densely packed mixture. Air and its moisture do not have to be excluded. The reaction can be carried out in an ordinary melting furnace or in a crucible. The reaction mixture can, for. B. be heated by resistance, induction or electrode heating or by an aluminothermic mixture. The reaction begins at temperatures above 600 ^° C. and, with the development of heat, covers the entire powder mixture,

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but without this being melted down. A smoke development through escaping, from Silicon tetrafluoride resulting from decomposition of the sodium silicofluoride does not or only takes place to a very small extent. The higher the sodium fluoride content of the mixture, the lower it is is. The decomposition of the sodium silicofluoride is suppressed by the presence of sodium fluoride, τ ··, .. · ".

The solid reaction product is separated into a salt and a metal layer by melting. The required temperature depends on the composition of the resulting double fluoride. In the case of cryolite, around 1000 ^° C. are necessary. If there is less or more NaF, the temperature is reduced according to the AlF ₃ -NaF state diagram. Depending on the requirements, the melting can be carried out directly after the conversion reaction in the same vessel or later in a special furnace. To produce an aluminum-silicon alloy, the broken reaction product can also be introduced into an aluminum melt heated to the required temperature.

In the known experiment with sodium silicofluoride, sodium fluoride and aluminum powder in excess a silicon yield of only 86% could be achieved. With the method according to the invention On the other hand, the silicon yield reaches 95% and more without difficulty, even in large-scale technical operations.

The aluminum can be used as powder or grit. It is preferable to bet a little more one than the stoichiometric amount that is necessary to reduce the silicon, so that the silicon already in status nascendi in an aluminum-silicon alloy is bound. It can be separated better from the molten salt in this way, and moreover the silicon yield is increased.

The mixture of sodium silicofluoride, sodium fluoride and aluminum powder is advantageously added a little more clay. It becomes the Melting point of the reaction product lowered so that the separation into salt and alloy more quickly he follows. If so much clay is added that the resulting cryolite is saturated with it, then there is nor the risk of alumina from working in kilns lined with alumina stones is removed from the oven lining. When the resulting cryolite for the production of aluminum is used by fused-salt electrolysis, the added alumina does not interfere, as there is Alumina is added. The reaction mixture consists, for. B. from 50 parts of sodium silicofluoride, 25 parts of sodium fluoride and 25 parts of aluminum to which up to 15% alumina are added can.

In a particular embodiment of the process according to the invention, the implementation and the subsequent melting to separate the reaction product or just the latter Operation in an operating electrolysis furnace for the production of aluminum from alumina carried out in a fluoride melt. The process can thus be carried out in an aluminum smelter on one 65 ■ multiple ways in existing facilities; the electrolysis of the aluminum is taking place not bothered. With the appropriate composition of the starting mixture, an aluminum-silicon alloy is created and cryolite, as the latter is already in the electrolysis furnace. Both can periodically exhausted from the oven.

For example, it becomes a homogeneous mixture of 50% sodium silicofluoride, 25% sodium fluoride and 25% aluminum grits in portions, each 2 kg wrapped in an aluminum foil, in the melt flow of an aluminum electrolysis furnace of 40,000 A with pre-burned anodes, namely 300 to 400 kg in 24 hours. The packets, which are preferably preheated to a temperature below the reaction temperature, are immersed covered in the river with a river crust, in the interior of which the conversion reaction takes place. The packets only disintegrate after the reaction has ended, with the reaction products melting and separating. The resulting aluminum-silicon alloy gets into the electrodeposited aluminum and the cryolite mixes with it furnace flow also consisting of cryolite. In this way, they could without disturbing the oven 7500 kg of cryolite can be obtained in 4-week operation. The silicon yield as an aluminum-silicon alloy was 95%

The reaction mixture can also be filled into paper sacks. Burns when placed in the furnace flow the paper sack immediately, and the mixture is enveloped in an oven river crust without disintegrating. The reaction mixture can also be used in briquetted form. The important thing is that it doesn't simply poured loosely onto the furnace flow.

It is also possible to proceed in such a way that the reaction mixture passes through in a separate container slow heating brought to reaction and the comminuted reaction product, preferably still in warm state, is entered in the aluminum electrolysis furnace, where the separation in metal and salt takes place.

Claims (5)

PATENT CLAIMS: 1. Process for the simultaneous production of sodium-aluminum double fluorides, in particular cryolite, and silicon or aluminum-silicon alloys by reacting sodium silicofluoride, aluminum and sodium fluoride in an amount that is adapted to the desired composition of the resulting double fluoride, thereby known draws that an intimate, densely packed powder mixture of the starting materials is caused to react in a solid state with heating and then the reaction product formed is melted for the purpose of separation into a metal and a salt phase. 2. The method according to claim 1, characterized in that the mixture of sodium silicofluoride, Sodium fluoride and aluminum or alumina is added. 3. The method according to claim 1, characterized in that the reaction product for separation in an operating electrolysis furnace for the production of aluminum from alumina in a fluoride melt is entered. 4. The method according to claim I _5, characterized in that the reaction mixture in a burning 5 6 or fusible shell wrapped in the furnace temperature below the reaction temperature The flow of an aluminum electrolysis furnace is preheated. becomes, in which first the conversion reaction runs off without disintegration of the mixture and subsequently considered publications: the mixture is separated by melting. 5 U.S. Patent No. 1464,625; 5. The method according to claim 4, characterized in "Journal of Inorganic Chemistry", 171 (1928), draws that the reaction mixture on a p. 259 to 260. 309 510ß27 2.63