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 cathode in electrical contact with the liquid electrolyte;
providing an anode separated from the liquid electrolyte by a membrane capable of conducting the ions from the electrolyte; and
executing a deposition-dissolution cycle comprising:
extracting electrons from the anode while providing electrons to the cathode during a first interval, thereby depositing a solid product, the element constituting at least 99% of the deposited solid product, onto the cathode, and electrically isolating the anode while extracting electrons from the cathode and providing electrons to a counter cathode in contact with the liquid electrolyte during a second interval, thereby electrodissolving a portion of the deposited solid product from the cathode and plating solid material comprising the element onto the counter cathode.
2. The method of claim 1 wherein dendrites are removed from the deposited solid product on the cathode during the second interval.
3. The method of claim 1 wherein the counter cathode is interposed between the cathode and the membrane.
4. The method of claim 1 further comprising executing an additional deposition-dissolution cycle.
5. The method of claim 1 further comprising removing the counter cathode from contact with the liquid electrolyte before executing the additional deposition-dissolution cycle.
6. The method of claim 1 wherein the deposition-dissolution cycle further comprises, after the second interval, reversing the polarity of the potential difference applied between the cathode and the counter cathode, thereby electrodissolving plated solid material from the counter cathode.
7. The method of claim 1 wherein the length of the first interval is 2 to 200 times the length of the second interval.