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Molten Salt Electrolysis for Sustainable Metals Extraction and Materials Processing 11 2.4. Applications of Conventional Molten Salt Electrolysis There are many possible applications of fused salt electrowinning and electrorefining for metals extraction and materials processing. A number of typical examples for the applications to metals extraction and materials processing are given in this review. 2.4.1. Molten Salt Electrolysis for Aluminium Production (1) The Hall-Heroult Process All primary aluminium is worldwide extracted by electrolysis from Al2O3 dissolved in molten cryolite-based fluoride electrolytes, typically consisting of Na3AlF6, AlF3, CaF2, and Al2O3, in the HH process [1]. The HHC, as shown in figure 4, consists of a steel shell lined with refractory insulating bricks covered with Si3N4-bonded SiC sidewall lining materials, a pool of molten aluminium on carbon blocks as the cathode at the bottom of the cell, and a carbon block immersed in the electrolyte from the top of the cell as the anode. Molten aluminium is produced at the cathode while the carbon anode is electrochemically oxidised and, thus, consumed during electrolysis to produce 70-90 % CO2, with the rest being CO. The overall cell reaction is given as follows: 2Al2O3 + 3C = 4Al + 3CO2(g) (4) The reversible cell voltage is 1.19 V at a typical electrolysis temperature of 960 °C. To compensate for a sum of the resistances contributed by the electrolyte, the anode and cathode materials and their contact resistances, as well as the anodic overvoltage, HH cells operate at cell voltages of 4.0-4.6 V. There are two types of the carbon anodes, i.e., prebaked anodes (figure 4a) and Soderberg anodes (figure 4b) [1]. The prebaked anodes are large carbon blocks that must be replaced as they are consumed according to Reaction 4. The Soderberg anode is continuously formed by addition of a carbon/pitch paste to a mould held above the molten electrolyte in the cell with the paste being added at the rate of carbon consumption in the cell. The whole mass moves slowly down towards the melt and is baked in situ with the evolution of polluting gases. The density and electrical conductivity is also lower than that of the pre-baked anodes. The three most characteristic features of the HHC are the consumable carbon anode, the liquid aluminium cathode, and the frozen bath lining. For the latter, heat must conduct through the cell sidewalls to stabilize the frozen bath on the sidewall linings. Thus, the heat produced in the cell and the heat losses must be balanced to maintain stable bath temperature and frozen bath lining. As a result, the energy consumption of the typical HHC is 12.9-13.5 kWh per kg Al and the energy efficiency is as low as 40-45 %, which means that a major part of the electrical energy input is released as heat and, essentially, wasted [12].PDF Image | Molten salt electrolysis for sustainable metals extraction
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