Primary production of elements
Electrowinning methods and apparatus are suitable for producing elemental deposits of high quality, purity, and volume. Respective cathodes are used during electrowinning for bearing the elemental product, segregating impurities, dissolving morphologically undesirable material, and augmenting productivity. Silicon suitable for use in photovoltaic devices may be electrodeposited in solid form from silicon dioxide dissolved in a molten salt.
1. A method of electrowinning an element from a compound, comprising:
providing a liquid electrolyte in which the compound is dissolved;
providing a first cathode in electrical contact with the electrolyte;
providing an anode in electrical contact with the electrolyte;
extracting electrons from the anode while providing electrons to the first cathode, thereby depositing a solid material including an impurity from the electrolyte onto the first cathode, thereby depleting the electrolyte of the impurity;
stopping deposition onto the first cathode;
providing a second cathode in electrical contact with the electrolyte; and
after stopping deposition onto the first cathode, extracting electrons from the anode while providing electrons to the second cathode, thereby depositing a solid product, at least 99% of which is the element, from the depleted electrolyte onto the second cathode, wherein, during deposition of the solid product onto the second cathode, a membrane interposed between the anode and the electrolyte conveys anions from the electrolyte to the anode and further comprising electrically isolating the anode while extracting electrons from the second cathode while providing electrons to a counter cathode in contact with the liquid electrolyte, thereby electrodissolving a portion of the deposited solid product from the second cathode and plating solid material comprising the element onto the counter cathode.
2. The method of claim 1 further comprising:
stopping deposition onto the first cathode before depositing the solid product onto the second cathode; and
applying an electrical potential between the first cathode and the anode so that matter is neither dissolved from nor deposited onto the first cathode after stopping deposition onto the first cathode.
3. The method of claim 1 further comprising removing the first cathode from the electrolyte before depositing the solid product onto the second cathode.
4. The method of claim 1 wherein the element is one of niobium, copper, tantalum, neodymium and praseodymium.
5. The method of claim 1 wherein the element is silicon.
6. The method of claim 1 wherein
the solid material including an impurity is deposited onto the first cathode over a surface having a composition,
the solid product is deposited onto the second cathode over a surface having a composition, and
the composition of the surface of the second cathode differs from the composition of the surface of the first cathode.
7. The method of claim 1 wherein the element constitutes less than 50% of the first cathode at a surface over which the solid material including an impurity is deposited.
8. The method of claim 1 wherein a membrane interposed between the anode and the electrolyte conveys anions from the electrolyte to the anode during respective depositions onto the first and second cathodes.
9. The method of claim 1 further comprising, after depositing the solid product onto the second cathode:
stopping deposition onto the second cathode;
dissolving an increment of the compound in the electrolyte; and
resuming deposition of solid material including an impurity onto the first cathode.
10. The method of claim 1 wherein the element constitutes at least 99.99% of the solid product deposited onto the second cathode.
11. The method of claim 1 wherein the electrolyte is contained in a conductive vessel and further comprising applying an electrical potential between the anode and the vessel during deposition onto the second cathode.
12. The method of claim 5 wherein
the electrolyte comprises at least two metal halides constituting at least 60% by weight of the liquid electrolyte and silicon dioxide, and
the anode is separated from the electrolyte by a membrane capable of conducting oxygen anions.
13. The method of claim 1 wherein extracting electrons from the anode while providing electrons to the first cathode is accomplished by a DC constant-current source connecting the anode and the first cathode in an exterior circuit.
14. The method of claim 1 wherein extracting electrons from the anode while providing electrons to the first cathode are accomplished by a DC constant-voltage source connecting the anode and the first cathode in an exterior circuit.
15. The method of claim 10 wherein at least about 90% of the compound dissolved in the electrolyte is electrolyzed in depositing a solid product onto the product cathode.
16. The method of claim 1 wherein a first current density at the first cathode during deposition thereon is no greater than 25% of a second current density at the second cathode during deposition thereon.
17. The method of claim 1 wherein the first cathode comprises a spine and vanes extending from the spine into the liquid electrolyte.
18. The method of claim 10 wherein the solid product forms an epitaxial deposit.