Home produced Magnetite Anode using oxy-fuel cutting torch

Magnetite ((Fe3O4) is known by a variety of names. Black oxide of Iron, Magnetic oxide of Iron (it will stick to a magnet), Iron(III) oxide, Loadstone (an impure form occurring in nature and used as an Iron ore). It can be purchased from Ceramic supply stores as black Iron Oxide fairly cheaply. The Magnetite must be melted and casted into a shape that is suitable for use as an anode. Magnetite melts at around 1540C so it is not the simplest of tasks. It is possible to melt magnetite using an oxygen-fuel cutting torch. Welding torch will probable do as well. Many cutting torches use acetylene as the fuel and it has been found that it is difficult to use an oxy-acetylene flame as it is inclined to blow the powdered magnetite away when you are initially melting it. It is difficult/impossible to operate an oxy-acetylene torch at a low gas flow if you try to cure the problem by using a smaller flame.
A combination of LPG (liquid petroleum gas), ie. propane, butane etc and oxygen has been found to be OK. The torch must be used at a low setting at the beginning of the operation so as not to blow all of the powdered magnetite away. When you get the magnetite melted into granules or bigger pieces you can then turn up the gas flow (and therefor heat output) so as to start to form the anode proper. The anode casting is not liquid all at the same time it should be noted, you melt the magnetite and move along the anode to be, in a fashion not unlike welding. It is not very difficult to do.

The mold

The mold is simply a piece of Iron of similar dimensions to the anode you wish to make. A piece of mild steel pipe about 1.5 inch in dia. cut down lengthwise will suffice. You may like to weld two semicircle ends onto this so that it looks like a boat. The black Iron oxide is put into the boat and packed down with a piece of wood so that it is not fluffy. You may need to then put more powder in (and pack) to fill up the boat some more. The boat is held steady in a vise or similar device and the gas torch is applied.

First stage of melting

The gas torch should be use OXYGEN RICH. You should be able to tell that your torch is oxygen rich by looking at the nature of the flame. The torch, at this stage, must be used at a low gas flow rate or the flame will be inclined to blow all of the powdered magnetite away. The volume of the powdered magnetite will decrease a lot as it melts and gets more dense. You may be inclined to blow some of the powder away at the start but there is not much you can do about it. Once you get a little pool of molten magnetite started you can 'drag' in more of the powder, without blowing it away, by moving the torch in a circular motion and slowly moving down the boat. You won't have made an anode on the first 'pass' of the torch as this phase of the job is simply to get the powdered magnetite to melt together into lumps that will not be blown away when you turn up the gas flow (heat) in the next stage of the process.

Second stage of melting

The gas flow (heat) of the torch is turned up and melting is started again at one end of the boat. The flame should be oxygen rich. This time the anode proper should start to form and become a continuous piece of Magnetite as you move slowly up the boat. The flame should be kept close to the molten surface so as to make best use of the heat. If you do not get a continuous piece of material you simply repeat the melting operating again. The magnetite is inclined to bubble a lot not unlike as if it were boiling. This not a problem in the long term but it can make the magnetite hard to get into the shape you would like.

Third stage of melting

You now take the hot piece of magnetite that you have formed and turn it over (using a pair of pincher's). The underside will not have fused properly and will be a mass of un-melted, very rough magnetite. You simply melt again in order to fuse the magnetite together. When you have all of the anode surface melted a fused you are finished. You may like to melt selected parts of the anode in order to tidy it up a bit or shape it so that it has a fairly consistent cross section from top to bottom. The anode will not be a pretty sight but you can do the best you can. Keep the anode hot at the later stages so that it will not crack.

Annealing

If you simply walk away from the anode after it is make it will probably crack and break. The magnetite is very brittle and contracts a lot when it solidifies and cools. This caused it to crack. The anode must be annealed. This is done be simply getting the torch and heating up the whole anode to red heat. The anode should be already fairly hot from the previous stage. When the whole anode is red hot the boat is rapped in something that will stop it from cooling rapidly. Some fiber glass should suffice. Another better solution is to get an old element from a domestic cooker together with its controller. This is plugged in before you are finished making the anode so that it is red hot when you start to anneal the anode. The red hot anode + boat is simply placed on the red hot element and a cover of some sort put across the boat. The heat can be turned down over the course of a few hours in order to stop the anode from cracking up.

Using the anode

It should be noted that the internal structure of an anode made by the process above is very very porous. It is in fact a mass of bubbles which were caused by the gas from the cutting torch. This structure seems to make the annealing process less critical that if you had a similar piece of magnetite that was solid all through. The porous nature of the anode does not seem to be a problem to the anode overall, it probably increases it resistance a bit but not much. The porosity of the anode will not cause salts from the electrolytic cell to be conducted (by capillary action) up to the electrical connections because the bubbles do not seem to be joined, they are genuine bubbles!
Magnetite is used at a low current density, similar to carbon. Cell temperature is not a problem. You can simply make a connection to the magnetite by putting a piece of copper braid at one end and clamping with a 'crocodile' clip or even a clothes peg. Since the current density is not high the actual current drawn by the anode is not very large and therefor current connection are not very critical. You should not make your anode too long as they may be more inclined to crack and the higher resistance of magnetite may be a problem if you have a long anode. About 7 inches long is a good starting point.

Current efficiency

Although high current efficiency using Magnetite anodes have been reported in the literature, the current efficiency of the above homemade Magnetite anode was very poor. I do not know the reason for this.


The Magnetite anode was run for about one month. The efficiency was very low in a cell that
had no pH control.

A pH controlled cell was set up and run.
About 40 grams of K Chlorate was produced. The cell ran for 168 hours at
1.2 amps. This represents a current efficiency of about
28%. This is very low, don't forget pH control (not perfect though!) was
used.
The current density on the anode was about 45mA per cm^2,  voltage across
the cell was about 6 volts. (bench power supply).

It would seem that Magnetite anodes made by heating and melting Magnetite
bought from the ceramics store are not much good at making Chlorate.

The anode shows no corrosion. It does not conduct salts up itself to the
connections so they are easy to maintain. 
 It would make an interesting substrate for a Lead Dioxide anode, but
that's another story I suppose.
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