Journal of Electrochemical Technology. 6, (1968) page 402
Electrolytic Manufacture of Sodium Chlorate using Magnetite Anodes.
T. Matsumura, R. Itai, M. Shibuya, and G. Ishi.
The Japan Carlite Company, Limited, Shibukawa-shi, Gunma-ken, Japan.
Magnetite anodes and cells now in operation in a commercial Chlorate plant are briefly described. Chlorine overvoltage on magnetite is mentioned. Laboratory data were employed to optimise the operation conditions of the commercial cells. Optimum pH was 6.75. Current efficiency increased with increasing temperature. An improvement in ventilation conditions was also effective. As a result, a current efficiency of 83 - 85% and an electrical power consumption of 5900 to 6300 kwhr/ton of NaClO3 produced were achieved at a final Chlorate concentration of 400 to 450g/litre. Some features of Magnetite anode cells for Chlorate production are discussed.
In Japan, magnetite anodes were used in a Chlorate plant built in 1934 at Shibukawa, 80 miles north of Tokyo, because of the limited supply of graphite at the time. At present, 63% of the annual Chlorate production of the plant in derived from the magnetite anode cells.
The Magnetite anodes used in commercial cells are made of hematite, smelted in an electric furnace and cast into a hollow cylinder with one end closed, the inner surface being coated with a layer of electrodeposited copper. Their dimensions are 60mm outside Dia., 7mm thick, and 800mm long and their weight is approx. 5kg. About 90% by weight of cast magnetite consists of columnar crystallites with a Fe(III)/Fe(II) atomic ratio of 1.9 to 2.0. The remainder consists of glass matrices composed of silica, alumina, lime, alkali metal oxides, etc.
The Chlorine overvoltage of magnetite electrodes was determined by the conventional method. Specimens of 16 x 90mm with a thickness of 9mm were prepared from specially cast magnetite with a purity of 98% and a Fe(III)/Fe(II) atomic ratio of 2.00. The electrolyte was 5.3 molar NaCl saturated with Chlorine at one atmosphere. Electrode potentials were measured by reference to a saturated calomel electrode (SCE) at 25C. (ie. add about 0.23 volts to get the voltage that appears across the experimental cell)
Chlorate cell operation
Figure 2 (schematic not shown) shows a commercial 6000 amp cell, 1800 mm wide, 1750mm long, and 910mm high. In each cell are placed 192 electrodes, 16 cathode units, and a cooling coil. The anode to cathode distance is 10mm. The cell wall is made of steel, yet has not shown any sign of failure for 30 years. The gas phase is ventilated through a slate cover with six ventilation pipes, and the static pressure over the electrolyte surface was kept about 50mm H2O.
Feed solution from the brine purification system contain 250 to 270g/l NaCl, 30 to 50g/l Sodium Chlorate, and 2g/l Sodium Dichromate. Final electrolyte compositions are 120 to 130g/l Chloride and 400 to 450g/l Chlorate
Effects of Operating Variables
The effects of performance of solution pH, bath temperature, and ventilation conditions were studied with both experimental and commercial Chlorate cells.
In order to find out the optimum solution pH, the steady state current efficiency was measured using an experimental cell with a miniature magnetite anode in which the pH was measured every 10 minutes and maintained at desired levels between 5.5 and 7.25 by intermittently adding normal hydrochloric acid solutions. Figure 4 shows that a maximum efficiency is reached at a pH of 6.75, and the curve falls rapidly on either side of the maximum. |
The effects of pH on the current loss due to Chlorine evolution (escaping out of cell I presume) and the steady state hypochlorite concentration are given in Fig 5, which indicates that the aforesaid optimum pH is also accompanied by a minimum current loss/wastage due to small amounts of Chlorine gas getting out of cell at the optimum pH and also the hypochlorite has not risen from its low value so that you will not get Chlorate being made from hypochlorite by electricity at the anode surface as this is considered a non ideal reaction as far as current efficiency is concerned.|
|This table shows the effect of pH control on current efficiency and acid consumption|
|Quality of |
||Current efficiency, %||35% HCl consumed|
kg/ton - NaClO3
| Medium ||6.0 to 7.8||74.4||87.5|
| Good ||6.7 to 6.8||84.5||62.5|
Bath temperature also effects cell performance. A similar experimental cell and technique as before were employed to determine the effects of temperature on current efficiency, hypochlorite concentration, and acid consumption. Results shown in figures 6 and 7 indicate that the current efficiency increases linearly with temperature, whereas the acid consumption decreases.
This result in consistent with the concept that the rate of chemical Chlorate formation is favoured at high temperatures. The current efficiency increased from 83% at 30C to 88% at 80C, not a spectacular increase. REM: You will only get this improvement in a pH controlled cell, IMHO. However too high a temperature may cause a shorter cell life, excessive electrolyte evaporation, and so on. Hence an operation temperature of 60 to 70C was finally chosen for the commercial cell.
The space between the electrolyte surface and the lid of the cell, and the gas velocity in the tubes that carry away the waste gasses was studied for there effects on current efficiency. It was found that the optimum space between the electrolyte surface and the lid was 100mm and that the optimum gas velocity in the ventilation tubes was 1 meter per sec. Theses conditions, when optimised, give a current efficiency improvement of 5%.
Some features of Magnetite anode cells
Since Magnetite anodes are characterised by their diminished rate of wear, the anode to cathode spacing is kept practically constant during electrolysis. For example, the outer diameter of a magnetite anode, initially measuring 60mm, shower a decrease of 5mm after 4 years of service. This extra distance of 5 mm between anode and cathode give an increased voltage across the cell of 0.01 volts, which is very small.
Moreover, since their rate of wear is scarcely affected by temperature, magnetite anode cells permit high temperature operation that accelerates the chemical Chlorate formation (the bulk reactions) rather than electrochemical reactions (the surface reaction at the anode that converts hypochlorite directly into Chlorate) which are considered wasteful of electricity. The higher temperature operation lowers cooling water requirements.
In conclusion, by carefully selecting the major operating variables as mentioned above, we could efficiently operate the Chlorate cells with Magnetite anodes at a cell voltage of 3.2 to 3.5 volts, a final Chlorate concentration of 400 to 450g/l, a current efficiency of 83 to 85%, and an electric power requirement of 5900 to 6300kwhr per ton Na Chlorate produced. (if power was 6 cents per unit that's about $360 per ton for power cost).
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