The advantages of the magnetite anode for chlorate manufacture stem from its resistance to corrosion and wear. Owing to its dimensional stability the cell anode cathode distance is kept practically constant during electrolysis 81. A loss of 5 mm on the external diameter of 60 mm anodes after 4 years continuous operation is reported 81. This loss represents the very small cell voltage increase of 0.01 V under normal operating conditions 81. In addition temperature has little effect on the wear rate, permitting high temperature operation (70°) and consequently improved C.E. 80 81; graphite anodes can only stand 40° and undergo excessive consumption 80. Chemical, as opposed to electrochemical, formation of chlorate and minimisation of cooling water requirements are additional advantages of high temperature operation in chlorate cells 81.
The limitations of the magnetite anode arise from difficulties in fabrication, non-machinability, fragility and low conductivity, 20 times poorer than graphite 82 34 77.
Japan Carlit feel that the disadvantages of the magnetite anodes are more than offset by the dimensional stability of the anodes and the high operating temperature of the cells 82. They claim the following operating conditions for their magnetite cells: cell voltage 3.2-3.5 V, final chlorate concentration 400-450 g/1, a C.E. of 83-85 % andanelectricpowerconsumptionof 6500-7000 kWh/tonne of NaClO3 81.
Figure 3 shows chlorine overvoltage on pure magnetite in 5.3 M NaCl at 25°.
The Japan Carlit anodes are prepared from hematite 81 which is smelted in an electric furnace and then cast into hollow cylinders with one end closed. The inner surface is then coated with a layer of electrodeposited copper. This procedure seems to have been adopted by previous manufacturers78 79.
Other methods have however been claimed for anode manufacture83 93. Among the more interesting of these is a sintering technique77 91 92 which enables the production of various anode shapes 77.
Attempts have been made to improve the physical properties of magnetite anodes by addition of other oxides 86 89 91 93 96. It has been found that anode strength is greatly increased by the addition of CuO 97, whereas it has been suggested that the presence of A12O3 and SiO2 weakens the anodes 89. Addition of up to 10% of metal oxides or SiO2 and up to 1 % V2O5 have been found to improve the anodes with respect to oxidation resistance when sintering 91. Nagai et al.94 96, have carried out a systematic examination of the effects of various oxide additives on magnetite anodes. The Fe3O4-SiO2-Na2O system was found to have equal conductivity and erosion resistance but much higher strength 94. Some anode variations involving magnetite have been claimed in the literature 98 103. Among the most recent of these is a lead or lead alloy 98 into which a piece of solid magnetite is embedded. Earlier, a magnetite anode into which a piece of noble metal was embedded was reported 99. The use of magnetite on Pt for protection against anodic attack in chlorine production is a further modification 100 101 and Japanese workers have recently been investigating the deposition of platinum on magnetite for use as the anode in perchlorate production 102 103.
The non-applicability of the magnetite anode for perchlorate production is attributed to the much lower oxygen overvoltage 104 on magnetite in chlorate solution. Use of the magnetite anode, besides the commercial chlorate application, has been proposed elsewhere. Electrodeposition of metals from ammonium metal complexes, electrodeposition of chromium 105, electrolysis of sewage 106, and the decomposition of fatty acids 107 108 have all been investigated using the magnetite anode.
Chlorate production remains however as the single commercial application of the anodes. Japan Carlit seem optimistic about the continued use of the anodes and, with present work showing that resistance to corrosion and wear can be coupled with increased strength, there may still yet be a place for the magnetite anode in the electrochemical manufacturing industry.
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