IF MAGNETITE IS BEING USED AS AN ANODE TO MAKE CHLORATE, pH CONTROL IS NECESSARY IN ORDER TO ACHIEVE A SENSIBLE CURRENT EFFICIENCE.
Industrial reports had stated that the wear rate
of the Anode is small, about 1mm per year. Current efficiency reported from the Japanese was good.
The Japanese used Magnetite up to 1971 (and probably longer) to produce Chlorate, it was also used in India. Cells were of course pH controlled.
The resistance of Magnetite is higher than Graphite (about 20 times, it has a resistivity of 1 to 3 by 10^-4 ohm m) but that is not a major problem.
Magnetite has a reverse Spinal structure which makes it conductive. It's formula can be more accurately described as FeO:Fe2O3. [FeO = Fe(II), Fe2O3 = Fe(III)]
A description of Spinal Structures can be seen here or local to this page here.
Home produced Magnetite anodes made by melting Magnetite using a welder and annealing (or using an oxy/gas cutting torch) are relatively easy to product. However the current efficiency in very low when used in a non pH controlled amateur cell (3%CE). A Magnetite anode made with an electric welder and annealing oven was run in a K Chlorate cell. The anode surface area was 60 cm squared. The current was 3.8 amps. Current density on anode was approx. 63mA per cm squared (very high). The pH was not controlled. Temperature was about 15 Centigrade. The current efficiency was HOPELESSLY low. A guesstimation of approx. 6%. Also 11 Volts was needed to drive in the 3.8 Amps. The connection on the Anode was Silver paint. Silver does not seem to suit Magnetite (unlike Lead Dioxide). A lot of Voltage was dropped across connection. Copper would be better. No visible wear on anode. Nickle cathodes used. Black mess due to Nickle.
Another cell was run at a much lower current density on the anode (17mA per square cm, 1 amp into cell) with no pH control. The
current efficiency was HOPELESSLY low at 3% (measured).
This same cell was run with pH control. A syringe pump was used to meter in 12% HCl to the cell at a rate of 0.3ml per hour, (thats 0.35ml 12% HCl per amp per hour). The pH remained in the 6.7 to 7.0 region. The current efficiency was measured to be 61% over a two day period. The Chloride concentration was high. The cell had Copper cathodes, ran at approx. 23C and was 0.52 liter in size. Current into cell was 0.85 amps, 14mA per square cm. The connection to the Anode consisted of Copper braid clamped onto the Anode using a plastic 'clothes peg' type clamp. Voltage accross cell was low and stable.
This cell produced Perchlorate when Chloride conc. decreased to a low level.
There are patents describing the production of Magnetite Anodes that describe melting Magnetite in an arc type furnace and then casting
the molten Magnetite into the required shape. The Magnetite because of its
brittle nature must then be carefully annealed for a minimum of 16 hours in a
furnace that is cooled slowly. The set up requires a large amount of power going to a simple electric arc furnace.
The Japanese used hollow Magnetite Anodes and coated the inside of the hollow with Copper for to use as a current connection to the Anode when in service. This give an even current distribution on the Anode and was necessary because of the somewhat high resistance of the Magnetite.
Magnetite anodes can be given a Copper coating at the top for to make a good electrical connection to them. A simple
Copper electroplating bath is used.
18.5 grams Sulphuric acid
78 grams Copper Sulphate Heptahydrate
500 grams water
Anode is pure Copper
Cathode is the piece to be coated
Voltage 2 to 5 volts (about 60mA per square cm on cathode is OK)
There is also the possibility of 'promoting' a Magnetite Anode. This is where a more active substance is attached to the surface of the Magnetite (Cobalt Oxide or possibly Lead Dioxide) thus making an Anode capable of both Chlorate and Perchlorate production. The Magnetite simply being used as a non corroding current carrying device. It never made it to commercial plant.
US Pat. No. 3,232,858 describes the process of melting, casting and annealing Magnetite Anodes. Magnetite melts at 1,595C.
US Pat. No. 3,294,667 describes a novel Anode that consists of Magnetite pieces embedded in a Lead matrix.
US Pat. No. 4,515,674 describes a sintered Magnetite Anode.
US Pat. No. 2,727,842 describes a method of converting Steel into Magnetite using steam in a furnace.
Other patents are described in the 'further reading section'.
Using magnetite Anodes for Chlorate production . Article from Electrochemical Technology 6, (1968) 402
Electrical Resistivity of Magnetite Anodes. Article from JES, 118, (1971) 1709.
Magnetite anode from Steel + Steam in a furnace. Magnetite from Steel and Steam
Magnetite anode from Iron coated on Ti. US Patent 3,850,701
Magnetite Anode made with a welder and annealing oven. Homemade Magnetite anode 1
Magnetite Anode made with an oxy-gas torch. Homemade Magnetite anode 2
See also "Industrial Electrochemical Processes"
Two German Patents using a sintering technique for making Magnetite Anodes are: DE 1,068,675 and DE 1,091,545.
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