Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing fused bath
Patent
1997-02-25
1999-02-23
Valentine, Donald R.
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing fused bath
205372, 205410, 204244, 204245, C25C 300, C25C 306, C25C 308, C25C 334
Patent
active
058739939
DESCRIPTION:
BRIEF SUMMARY
This application is a section 371 application of PCT/NO95/00092 filed Jun. 2, 1995.
BACKGROUND OF THE INVENTION
The present invention concerns a procedure for continuous and batch production in one or possibly more steps in one or more furnaces of silicon "metal" (Si), possibly silumin (AlSi alloys) and/or aluminium metal (Al) in the required ratio in a molten bath, preferably using feldspar or feldspar containing rocks dissolved in a fluoride, as well as process equipment for the implementation of the procedure.
Controlled production of high purity silicon by electrolysis using feldspar or species of rock containing feldspar dissolved in fluoride has been a problem up to now.
Continuous production of silicon and silumin has previously been described in ISBN 82-993110-0-4, which is the inventor's own publication. Minerals (species of rock) poor in iron such as feldspar ((Ca, Na, Ka)Al.sub.2-1 Si.sub.2-3 O), pegmatite, granite syenite or anorthosite can be used in a mixture with NaF or cryolite and electrolysed directly with an Al (Al--Si) cathode to produce pure Si (99%). The disadvantage of the method stated in relation to the present application is that electrolysis for the production of Si cannot take place in controlled fashion separately from aluminothermic reduction when Al is present. As the aluminothermic reduction is rapid, a lot of Al will be oxidised and used at the same time as current passes through the cell for the reduction of Si (IV). As a lot of Al is consumed, a lot of Al(III) must be recovered to form Al by electrolysis and, besides, a lot of silumin is formed. Today, this is not desirable because the Si market is much larger than the silumin market. Besides, electrolysis of Si on Al requires more energy with a Si-rich Al cathode surface because solid Si is formed at a process temperature of 1000.degree. C. (melting point (Si)=1410.degree. C.). Solid Si has semiconductor properties and, therefore, high electrical resistance. The Si particles which are formed are deposited mainly on the outside of the molten Al metal (in this case Si should be considered as the cathode instead of Al).
In ISBN 82-993110-0-4, it is further stated that Si crystals containing approximately 1% Al will crystallise on the Al cathode surface, in silumin and/or at the bottom. The Si crystals formed by electrolysis can be sucked, raked and/or filtered from the Al cathode. The disadvantage of so much (1%) Al being formed in the Si crystals is that it is difficult to remove this Al by known refinement methods. Since only small amounts of Si are observed formed on the surface and at the bottom, it is difficult to remove them with known technique.
The equipment in ISBN 82-993110-0-4, as sketched in FIG. 1, lacks detail and does not show how Si is separated from the silumin. Nor does it show how the electrolyte runs over into the bath in which the Al is produced. U.S. Pat. No. 3,022,233 describes the production of Si, a metal silicide, fluorocarbons and silicon tetrafluoride in one and the same step, but the quality of the Si and the temperature of the process are not stated. The starting materials are SiO.sub.2 dissolved in alkaline or alkaline earth fluorides or fluorides of rare earth metals. The cathode is made of metal.
In U.S. Pat. No. 3,405,043, just silicon is produced and it is important that the raw material (silica) is pure. The silica raw material is dissolved in cryolite. During electrolysis Si sticks to the cathode like an adhesive ball; the cathode must be removed and cleaned periodically. The anode and the cathode are fastened vertically beside one another.
SUMMARY OF THE INVENTION
The present invention concerns a procedure for continuous and batch production in one or possibly more steps in one or more furnaces of silicon metal (Si), possibly silumin (AlSi alloys) and/or aluminium metal (Al) in the required conditions in a melting bath, preferably using feldspar or species of rock containing feldspar dissolved in a fluoride. The procedure is characterised in that highly pure silicon metal is produ
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Stubergh, "Fremstilling av silisium og alumnium i en kontineurlig prosess ved bruk av norsk feltspat som r.ang.stoff", Oslo College of Engineering, Norway, ISBN 82-993110-0-4, pp. 1-31 (1993) (No Month).
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