Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/26—Electrolytic production, recovery or refining of metals by electrolysis of melts of titanium, zirconium, hafnium, tantalum or vanadium
  • C25C3/28—Electrolytic production, recovery or refining of metals by electrolysis of melts of titanium, zirconium, hafnium, tantalum or vanadium of titanium
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
  • C25B1/00—Electrolytic production of inorganic compounds or non-metals
  • C25B1/01—Products
  • C25B1/33—Silicon
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
  • C25C3/34—Electrolytic production, recovery or refining of metals by electrolysis of melts of metals not provided for in groups C25C3/02 - C25C3/32
• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
  • C25C7/00—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells
  • C25C7/005—Constructional parts, or assemblies thereof, of cells; Servicing or operating of cells of cells for the electrolysis of melts

Definitions

• Tho present invention relates to a method for electrolytic production and refining of metals having a high melting point at above about 1000 0 C, particularly silicon.
• US patent No. 3,254,010 there is disclosed another method for refining impure silicon or germanium where a current is passed between a cathode and an anode through a molten salt electrolyte containing a fluoride, where the anode is made from impure silicon or germanium or alloys of impure silicon or germanium with more noble metals than silicon or germanium to deport on the cathode refined silicon or germanium. Also in this process solid refined silicon or solid refined germanium are deposited on the cathode.
• the electrolyte is preferably cryolite.
• the present invention thus relates to an electrolytic method for production and of refining of metals having a melting point above about 1000 0 C, particularly silicon, said method being characterized in that it:
• (a) provides to a first electrolytic cell, an upper molten electrolyte layer comprising a first oxide-based electrolyte containing an oxide of the metal to be refined, wherein the first electrolyte is in a molten state and has a melting point below the operating temperature of the process, an anode positioned in the upper molten electrolytic layer, and a lower molten alloy layer comprising an alloy of the metal to be refined and at least one metal more noble than the metal to be refined, said alloy constituting a cathode in the first electrolytic cell, said first electrolyte having a density less than the density of the alloy;
• (c) passes a direct current through the anode to the cathode for reducing the metal oxide to produce an alloy having a higher concentration of the metal to be refined;
• (e) provides to the second electrolytic cell an upper molten metal layer comprising a metal of the same metal as the metal to be refined, said upper molten metal layer constituting a cathode, and an intermediate molten electrolyte layer comprising a second oxide-based electrolyte containing an oxide of the metal to be refined, where the second electrolyte is in a molten stale and has a melting point below the operating temperature of the process, said second electrolyte having a density between the density of the upper molten metal layer and lower molten alloy layer; and
• (f) passes a direct electric current through the anode to the cathode of the 5 second electrolytic cell whereby the metal to be refined moves from the anode alloy to the upper molten metal layer.
• the first cell produces an alloy from the raw material and the second cell refines the alloy to produce a metal.
• direct current passes through the anode, the first electrolyte and the cathode alloy to produce an alloy having a higher concentration of the metal to be refined in the alloy layer from the raw material.
• direct current passes through the anode alloy, the second electrolyte and the metal to refine the alloy to the metal.
• the two cells can also be operated independent of one another.
• the method of the present invention can be defined as a two-step process.
• the first step is producing an alloy from raw material in one electrolytic cell; and the second step is refining an alloy to make a metal.
• the alloy is preferably transferred from the first electrolytic cell to the second 0 electrolytic cell in fluid state, but the alloy may also be tapped from the first electrolytic cell, solidified and supplied to the second electrolytic cell in solid state.
• (a) provides to a first electrolytic cell, an upper molten electrolyte layer comprising a first oxide-based electrolyte containing an oxide of the first metal wherein the first electrolyte is in a molten state and has a melting point below the operating temperature of the process, an anode positioned in the upper molten electrolytic layer, and a lower molten alloy layer comprising an alloy of the first metal and the second metal, wherein the second metal is more noble than the first metal, said alloy constituting a cathode in the first electrolytic cell, said first electrolyte having a density less than the density of the alloy;
• (c) passes a direct current from the anode to the cathode alloy to in the first electrolytic cell to produce an alloy having a higher concentration of the first metal.
• the raw material is any conventional source of metal oxide containing the metal to be refined, or the first metal, for example, quartz for silicon or rutile for titanium.
• the refining method of the present invention can use alloy made from a different process than the first step of the present invention.
• the method to electrolytically refine the alloy to the metal in accordance with the present invention is characterized in that it:
• (a) provides to a second electrolyfic cell an upper molten metal layer comprising a metal of the same metal as the metal to be refined, said upper molten metal layer constituting a cathode, a lower molten alloy layer comprising an alloy of the metal to be refined and at least one metal more noble than the metal to be refined said lower layer constituting an anode, and an intermediate molten electrolyte layer comprising a second oxide-based electrolyte containing an oxide of the metal to be refined where the second electrolyte is in molten state and has a melting point below the operating temperature of the process, said second electrolyte having a density between the density of the upper molten metal layer and lower molten alloy layer; and
• (b) passes a direct electric current from the anode alloy through the second electrolyte to the cathode whereby the metal to be refined is moved from the alloy and deposited in molten state at the cathode.
• the metal to be produced and refined is, in addition to silicon, titanium and scandium.
• both the alloy as well as a less pure metal of the metal to be refined can be added to the alloy layer.
• metallurgical grade silicon can be added to the alloy layer, thereby becoming refined.
• One of the unique aspects of the present invention is that a variety of raw material can be used in the first cell. Normal carbothermic production of metal puts constraints on the type of raw material used and introduces into the metal impurities especially through the carbon source. Any particulate form of raw material can be added to the first cell and the impurities from the carbon source are eliminated since no carbon source is necessary. This means that the alloy can be purer than conventional alloys and assists in the refining process of the present invention.
• the alloy used in the refining need not be the alloy made in accordance with the present invention.
• the alloy layer can comprise an alloy of the metal to be refined and a metal or metals more noble than the metal to be refined, called the second metal, or the second metal, alone.
• the alloy itself will form as the metal to be refined or the first metal moves into the alloy layer.
• the lower molten alloy layer comprising the alloy of the metal to be refined or the first metal and at least one metal more noble than the metal to be refined or the second metal must have a composition that meets the following requirements:
• the lower molten alloy layer may for example consist of Si-Cu alloy, FeSi alloy or Cu-Fe-Si alloy. These alloys have melting points well below the melting point of silicon and accordingly also below Lhe melting temperature of the first and second electrolyte.
• the first oxide-based electrolyte must have a composition that meets the following requirements:
• the main constiLuents of the oxide-base electrolyte must be less noble than the metal to be refined;
• - must contain an oxide of the metal to be refined, for example, SiO 2 for silicon.
• the second oxide-based electrolyte must have a composition that meets the requirements of the first oxide-based electrolyte, and it must have a density at the operating temperature which is greater than the density of the metal to be refined.
• the oxide-based electrolytes further have the advantages that oxides are nontoxic and have low vapour pressures. Another advantage is that used oxide- based electrolytes are non-toxic and do not have to be deposited as special waste. The non-toxic nature of the electrolytes is true except for those which contain barium oxide, because barium oxide is considered toxic.
• the following oxide based electrolytes are suitable: .
• CaO-SiO 2 preferably containing 40-75 wt % SiO 2
• halides particularly alkali and alkaline earth fluorides, may be 5 added to the oxide-based electrolytes in order to modify the viscosity, density, melting point and electric conductivity of the electrolytes.
• the amount of halides added to the oxide-based electrolytes is preferably below 20 wt % and more preferably below 7 wt %.
• the oxide-based electrolytes should have a density 0 above about 2.57 g/cm 3 which is the density of molten silicon at the melting point of silicon, and below about 3.37 g/cm 3 if 75% FeSi is used as alloy and below about 5.5 g/cm 3 if 50% FeSi is used as alloy.
• the oxide- based electrolytes must have a melting point close to or below the melting point of silicon which is 1414 0 C.
• a particular suitable oxide-based electrolyte for silicon is a CaO-SiC> 2 electrolyte containing 40-75% S1O 2 . This electrolyte has a density of between about 2.5 g/cm 3 and about 2.7 g/cm 3 and has a high solubility of Si-ions, low solubility of Si and low volatility at an operaling temperature above the melting point of silicon.
• the first and second electrolyte can have the same composition or they can be different.
• the second electrolyte must have a density in the molten state such that it forms the intermediate molten electrolyte layer and positions itself between the upper molten metal layer and the lower molten alloy layer.
• the first electrolyte is not so constrained.
• the first electrolyte must have a density in the molten state such that it floats on top of the lower molten alloy layer, i.e. has a density less than the molten alloy.
• the first electrolyte need not have a density in the molten state that is greater than metal in the molten state.
• Either the production of the alloy or the refining method of the present invention can be performed in suitable conventional vessels that have a heat resistant refractory lining such as alumina, magnesia silicon nitride, silicon carbide or graphite.
• a heat resistant refractory lining such as alumina, magnesia silicon nitride, silicon carbide or graphite.
• the side walls of the vessel may favourably be provided with conventional cooling systems, such as evaporation cooled elements in order to create a freeze lining on the inside of the side walls of the vessels.
• the method when the method entails simultaneously producing and refining where separate vessels are employed, they may be in fluid communication with each other, such as through a pipe in the side wall of both vessels.
• the port for the pipe in both side walls must be positioned below the level of the bottom molten alloy layer, in other words, the top of the molten alloy layer should be above the level of the ports for the pipe which provides fluid communication between the vessels.
• one vessel acts as the first electrolytic cell to produce the alloy and the other vessel acts as the second electrolytic cell for refining.
• a single vessel is used for simultaneously making the alloy and refining the metal, wherein the vessel has been divided into the first electrolytic cell and the second electrolytic cell and the two cells are in fluid communication with each other through the alloy layer.
• Such an arrangement 5 is shown in U.S. Patent No. 3,219,561 , issued November 23, 1965, the contents of which are incorporated herein by reference.
• the two electrolytes are separate from each other and do not contaminate each other.
• the anodes and the cathodes are connected to a direct current source in a conventional way in order to supply direct current for the method.
• the metal to be refined for example, silicon in the alloy enters the second oxide-based 5 electrolyte together with ions of any impurities in the alloy that is electrochemically less noble than silicon. Since silicon is the noblest element of the second electrolyte, silicon ions will be reduced at the cathode and will form molten pure silicon, which is collected in the molten silicon cathode. Thus impurities more noble than silicon are trapped in the alloy layer while 0 impurities less noble than silicon are trapped in the second electrolyte.
• the refining method of the present invention can be carried out both as a batch process and as a continuous process.
• the refining method is carried out as a batch process, alloy is added to the alloy layer continuously or intermittently. Eventually the electrolytes and the alloy will become too high in impurities. The process is then stopped and the electrolytes and the remaining part of the alloy are removed form the cell. New alloy and new oxide-based electrolytes are added together with a start cathode of the metal to be refined, whereafter electric current is again passed through the electrolytic cell.
• a first for production of the alloy and a second for refining the alloy from the second cell which is depleted of the metal to be refined, is intermittently tapped and added to the first electrolytic cell.
• the refining method of the present invention is carried out as a continuous process, there are arranged means for conlinuous or intermittent supply of alloy, means for continuous or intermittent removal of oxide-based electrolytes and means for continuous or intermittent supply of fresh oxide- based electrolytes. Finally there are arranged means for continuous or intermittent lapping of refined metal from the upper molten metal layer.
• the reason for removal of alloy is that the alloy will, during electrolysis get an increased content of impurity elements more noble than the metal to be refined. Also, during electrolysis the electrolytes will get an increased content of elements less noble than the metal to be refined, and to reduce this content of impurity elements, part of the electrolytes are removed and may after purification be returned to the electrolyte layers in the cell or be deposited.
• the method for both making the alloy and refining the metal can be carried out as either a batch or a continuous process.
• the present invention it is thus provided a simple cost effective method for obtaining a pure form of metals, especially, silicon.
• Low cost alloys of the metal to be refined and a metal more noble than the metal to be refined can be used as the alloy.
• silicon alloys such as FeSi alloys and Cu-Si alloys can be used as alloy.
• Such alloys can be produced in accordance with the present invention or in any conventional manner using any conventional means.
• Figure 1 shows a schematic view of the refining method according to the invention
• Figure 2 shows a schematic view of the method for making the alloy and refining the metal according to the invention
• Figure 3 shows a schematic of a method for producing the alloy.
• FIG 1 there it is shown a schematic view of an electrolytic cell for carrying out the method of the present invention for refining of silicon.
• the electrolytic 5 cell comprises a vessel 1 having a refractory layer 2.
• a lower layer 3 of an alloy of silicon and a metal more noble than silicon such as a Cu-Si alloy that acts as an anode in the electrolytic cell.
• oxide-based electrolyte 4 having a density lower than the density of the anode alloy 3 and a higher density than i o molten silicon.
• a suitable electrolyte 4 is a mixture of 50 % by weight of CaO and 50 % by weight of SiC> 2
• the anode 4 and the cathode 5 are, via contacts 6 and 7 respectively, connected to a direct current source (not shown) for conducting current to the electrolytic cell.
• direct 15 current is passed through the electrolytic cell, silicon in the anode alloy 3 enters the oxide-based electrolyte 4 together with ions of any impurities in the anode alloy 3 that is electrochemicaliy less noble than silicon.
• silicon is the noblest element of the electrolyte 4 silicon ions will be reduced at the cathode 5 and will form molten pure silicon, which is collected in the molten 20 silicon cathode 5.
• impurities more noble than silicon are trapped in the anode layer 3 while impurities less noble than silicon are trapped in the electrolyte 4.
• Pure refined silicon is from time to time tapped from the molten cathode layer 5.
• Addilional solid or molten anode alloy or solid or molten unrefined grade of the metal to be reFined is continuously or intermittently 25 supplied to the molten anode layer 3 through an anode alloy supply channel 8.
• vessel 10 has refractory layer 1 1.
• Alloy layer 12 comprises the alloy and electrolyte layers 13 contains the second electrolyte and electrolyte layer 14 contains the first electrolyte.
• Layer 15 is pure metal and acts as cathode.
• Anode 16 and cathode 17 via conventional contacts are connected 5 to a direct current source, not shown.
• Wall 18 separates the two cells, the first electrolyte cell 19 and the second electrolytic cell 20.
• Alloy layer 12 flows between the two cells under wall 18.
• raw material e.g. quartz, S1O 2
• the metal to be refined such as silicon alloy is moved from the anode layer through the second electrolyte layer 13 to the pure metal layer 15.
• the alloy layer 12 fills the cells to a level above the lower edge of wall 18 and thereby separates Ihe two electrolytes of the two cells.
• the anode 16 is immersed in electrolyte layer 14 and cathode 5 17 is immersed in metal layer 15, but neither is in direct contact with alloy layer 12.
• the alloy layer 12 acts as a common electrode.
• the metal to be refined and elements more noble than the metal to be refined that are in the first electrolyte of electrolyte layer 14 precipitate at, and alloy with, the molten alloy.
• Anode 16 can be either inert or consumable, such as, baked carbon or graphite.
• electrolyte layer 31 had a composition of 55 wt. % CaO and 45 wt. % Si ⁇ 2 .
• Raw material of Si ⁇ 2 quartz, was added frequently to layer 31 to maintain the electrolyte composition and to provide a source of raw material to the process.
• a voltage of 4.5 V was applied between graphite anode 32 and cathode 33, to give a cathode current density of approximately 1 A/cm 2 .
• the cell temperature was held constant at 1650 0 C.
• the cell started with a liquid cathode 34 made of copper.
• the first metal is silicon and the second metal is copper in this cell.
• the copper cathode contained about 20 wl. % Si, giving a current efficiency of about 40%.
• the alloy was produced of SiCu.
• this cell started with pure second metal in the alloy layer and through the operation of the cell the alloy is formed in the alloy layer.

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