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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 31 3.3.2. Nanostructured Materials and Carbon Nanotubes During electro-deoxidation of metal oxides in calcium chloride using a carbon anode, it was noticed that a carbon sludge built up on the cathode. This was due to the carbon dioxide, evolving at the anode, either dissolving in the salt or reacting with the oxygen ions to form the carbonate ion. If the carbonate ion or dissolved carbon dioxide contacts the cathode the following reactions can occur: CO 2-+4e=C+3O2- (33) 3 or CO2 +4e=C+2O2- (34) The carbon product is in the form of carbon nanotubes and carbon nanoparticles [114]. Carbon nanoparticles can also be produced by a completely different route using graphite. If a graphite electrode is made the cathode in a bath of sodium or lithium chloride, the alkali atoms intercalate into the graphite and force nanotubes out from the surface of the graphite. It appears that this method produces far more carbon nanotubes than the decomposition of carbon dioxide or the carbonate ion. Although Faraday’s laws will allow the amount of alkali deposited at the cathode, it is difficult to calculate the amount of stress caused by these intercalated atoms to form the nanotubes. The results show that about 80 % of the graphite ends up as nanotubes and, given that graphite is relatively cheap, this may be an inexpensive method of producing multiwalled carbon nanotubes [115]. Another interesting observation, is that if liquid metallic particles, such as tin, are deposited on the surface of the graphite, the carbon nanotubes are filled with tin. This product may find application as the anode in the lithium ion battery as tin can hold considerably more lithium that the graphite intercalation compound that is used at present. Preliminary results are very positive. During electro-deoxidation of oxides, the final product size is, to a certain extent, dictated by the temperature of electro-deoxidation so that by working at lower temperatures, the reduced metal can be in the form of nanoparticles. TiC nanoparticles have been prepared by electro-deoxidation of the TiO2/C pellets in molten CaCl2-NaCl eutectic at 850 °C [91]. 3.4. Molten Salt Electrolysis for Processing Wastes Compared with pyrometallurgical methods, the products of fused salt electrorefinning are frequently pure metals which are produced in a non-polluting manner. Pyrometallurgical processes frequently use toxic gases, such as chlorine, the processes are noisy, and although, a pure metal might be produced, the other products are usually drosses and slags which have a low or negative commercial value. The very high energy input is necessary to produce primary aluminium. Industry will still require high grade aluminium for its products, a greater production of aluminium will have to be imported into certain countries and this aluminium will need to be recycled.PDF Image | Molten salt electrolysis for sustainable metals extraction
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