Mixed Metal Oxide Anodes (MMO)

The MMO (Formally known under the trade name of DSA, Dimensionally Stable Anode) was invented by Henry Beer. The term is generally used to describe any Anode having a Valve metal substrate coated with Oxides of Noble metals. The Noble metal Oxides alloy with the valve metal Oxide, usually Titanium Oxide and the coating is conductive. They are the Anode of choice for all Chlorate manufacture nowadays. They will not make Perchlorate, though this is debatable (US 4267025).
There are a large amount of combinations of Oxides, processes and substrates. The original MMO (DSA) was a Ti substrate with a Ru Oxide coating. The Ru Oxide alloyed with the Ti Oxide giving a new Anode surface material. They can be made by thermal decomposition of compounds that have been applied to the substrate. There are a large number of patents on the subject. The new Anodes had the advantage of lasting a long time, the Anode/Cathode spacing did not require adjustment (Dimensionally Stable) and they had a low over potential for Chlorine evolution in salt solution which meant that cells now ran at a lower voltage giving huge power savings.
MMO Anodes are available to purchase from corrosion control companies, pool chlorination companies, sewage treatment companies and others. They have impressive life spans when used as corrosion control Anodes at current densities that are lower (in Brackish water) that what you would use an MMO at in a Chlorate cell.
The coatings on corrosion control Anodes will be geared toward corrosion control as opposed to Chlorate making but it would be fair to assume that they will have long lifetimes in the Chlorate cell. In fact the higher the salt concentration the longer the life. The MMO is cheap if you can purchase in small amounts. Minimum orders may apply. There is a good article at the bottom of this page from Platinum Metals Reviews if the reader is interested in the development of the MMO anode.
Producer info on MMO (local to this page)

An Australian manufacturer of MMO is: nmtelectrodes.com

Pool Chlorinator link I

Pool Chlorinator link II
Also: http://www.vchlorin.com

http://www.directpoolsupplies.com.au/prod646.htm Supplier of Sal Chlor Anode assembly product . A pic. of this Anode is here. It may not be an MMO anode, but a bipolar Pt based anode.
Quote:
The Salchlor anode uses precious metals (not listed on their website) on a titanium substrate. Read the thread at www.sciencemadness.org/talk/viewthread.php?tid=5050&page=2
[Pool Chlorination anode]

Pool Chlorination devices are often dumped when in fact the actual Anodes and Cathodes of the device are OK. They may have salted up or perhaps the power supply failed. They can be had from companys supplying Chlorinators as scrap. An example from Xenoid is shown below.

[Pool Chlorination anode]

It is difficult to find as much info on MMO as on the other Anodes regarding current densities and temperatures used in Chlorate cell.
Current density used in Chlorate cells are in the region of 250mA/square cm. Temperature in industry (pH controlled cells), is in the region of 70 to 80C. There is a Thesis in the further reading section regarding MMO in modern Chlorate cells.
MMO is very rugged. It can be used at high temperatures which favour Chemical Chlorate formation (in pH controll cells). They can also be used to take the Chloride level low (50 grams per liter say) without undue damage to the anode. This is an advantage if trying to take solid Sodium Chlorate out of solution.
The anti corrosion anodes are designed for use in very low Chloride situations. They (AFAIK) are made from Iridium and Ta Oxide mix. It may be OK to use them in Chlorate cells with low Chloride content without undue erosion but CE will obviously suffer. They are used at low current densities when being used as corrosion control anodes.

When the Anode in installed in the cell it should be surrounded sensibly with cathode(s) so that the current is fairly evenly distributed on the Anode. If you place just one Cathode in the cell the current on the Anode will be greater on the side close to the Cathode than it will be on the side away from the Cathode. This will give a greater current density on the side close to the Cathode which may have implications for Anode erosion if you are running the Anode at high current densities.
The current carrying ability of the wire type Anodes is high as it has a Copper core. If a very long wire Anode is being used be sure that the top of the Anode is capable of carrying the current that is being fed to the Anode.

The wire Anode from the corrosion control companies is composed of a Copper core surrounded by Titanium that has the black MMO coating. If this Anode is placed into the cell as it is, the bottom of the Anode will have the Copper exposed. This Copper will corrode which may damage the Anode. It will contaminate the product with Copper. This problem can be solved by installing the Anode into the cell in a U shape. Both ends of the Anode are taken outside the cell. Power (+) can be applied to both ends.
If you must use the Anode as a straight rod the end must be sealed. A good way to do this is to drill a hole in the Copper for a depth of about 4mm. The hole should be close to the Diameter of the Copper. It will be difficult to drill out all the Copper. Nitric acid is then put into the hole to corrode all of the Copper for about 4mm. You may have to warm the wire to hurry it up. Epoxy or resin is them put into the hole and the Titanium tube (which has been left intact) is crimped a small amount.
It may be easier to simply install the Anode in a cell and use for a time until the Copper at the bottom is eroded away. The Anode can then be washed and sealed. The electrolyte is them discarded.

MMO in Action

The wire anode (above link) was used in a cell in an attempt to make Perchlorate. Current density on anode was 120mA/cm squared in a 0.5 liter cell containing 250grams Na Chlorate. 2 grams NaF where added to cell. Chlorate was twice recrystallized to give a low Chloride concentration. The cell was run for approx. 25 days at one amp. No Perchlorate formed. The Chloride concentration (as guesstimated with a visual Silver Nitrate test) did not seem to diminish.
When the anode was examined after the run it was noticed that the black coating on the anode had been removed in places and where it was not removed it could be simply rubbed off with a cloth. There was still an active coating on the anode though as it continued to emit gas when put into a Chlorate cell. There was no noticeable rise in cell voltage.

According to www.corrpro.com MMO should not be used in electrolytes containing Barium or Cadmium.
According to Encyclopedia of Chemical Processing and Design, Vol 51, 1995 page146, Barium is a poison to Noble Metal or Noble Metal Oxide anodes and may even reduce there lifetimes.
According to US Patent 7,250,144 (July 2007) Fluorine additive can damage Chlorate anodes (probably MMO, since it is a modern patent).

There are a hugh variety of Patents for MMO (DSA) anodes using TiO2 + Nobel metal Oxides. Nobel metal Oxides can be obtained from some photographic supply stores, eg http://www.artcraftchemicals.com

US Patent 4267025 depicts an MMO anode made from Ti, Platinum Group Oxides + Tin Oxide that is suitable for Perchlorate production.

There's plenty of further reading regarding MMO Anodes in the Platinum Metals review journal.
There is an article depicting the development of the MMO Anode in PMR 1998 (42) 2 (Journal article at above link).
It is also available here (local copy, 994Kb PDF)
There is an article in PMR Volume 52 Issue 3 July 2008 Pages 177-185 describing the use of MMO for Pool Chlorination and water disinfecting.

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