My First Try

The Cell

As a proof-of-concept run I built a small cell using a PE
screw top jar, about 500 ml in volume. Two high density graphite electrodes
fitted with threaded brass contacts were screwed through the lid and a 12.5 mm
PVC vent tube was glued into the lid to discharge the gasses produced safely out
the window. Additionally a 25 ml syringe was fitted through the lid to allow
topping up durring operation.

The Run

The cell was charged with about 400 ml of saturated Potassium
Chloride solution and the level noted. A high current DC supply was connected to
the electrodes and the voltage adjusted to deliver 3 Amps to the cell. Although
the supply was capable of delivering up to 25 A it can do so only at higher
voltages continously because it uses a linear regulator that overheats at high
voltage drops and modest current.

The electrolyte temperature stabilised at about 60-70 C, at
such a high temperature the anode wore down fairly quickly. Within two days the
liquid was black with extremely fine graphite particles in suspension.

The evapouration of the electrolyte was slower than I
expected. The vertical placement of the vent pipe acted like a reflux condenser
limiting the loss rate to less than 10 ml per day. The volume was made up as
required with additional saturated KCl solution.

After about five days the solution began to wick up through
the threads of the electrode connections and evapourate, forming a crust of
salts that grew larger and larger. There was also visible crystalisation in thin
plates settling on the bottom. This crust forming process eventually destroyed
the brass anode connection and a large part of the gator clip lead, the
electrode dropped to the bottom and the cell failed.

The cell failed after approximately 1005 Ah. At the expected
efficiency that would have converted most of the Potassium Chloride in the
electrolyte. The cell was cold when I discovered it, most of the graphite had
settled out, mixed with an 40 mm thick crust of crystals in the bottom of the
jar.

Processing The Electrolyte

The greenish and brown contaminated crusting around the anode
was disposed of, but the clean crystals formed on the electrodes themselves were
scraped off and added to the raw electrolyte. The electrolyte was diluted to 1
litre and boiled to destroy the hypochlorite and reduce the volume to about 800
ml, then half was filtered off and further boiled down to 300 ml then allowed to
cool. The remaining unfiltered liquid and sludge from the filter was also
allowed to cool in another container for later processing.

Approximately 70 grams of flaky crystals fell out of solution
on cooling and were filtered off. Once dry I puzzled over how to purify these
crystals. Looking at the solubility curve for Potassium Chloride I figured that
if I assume they were 100% Potassium Chloride and dissolved them in 100 ml per
30 g then cooled the solution to room temperature nothing would precipitate. I
tested this experimentally dissolving 30 g of pure Potassium Chloride in 100 ml
of water by heating, nothing precipated on cooling.

So in went the 70 g of raw crystals into 200 ml or so of
water. A quick boil and into the fridge to cool. 15 minutes later I had what I
would later measure as 55 g of clearly monoclinic crystaline product. The
remaining liquid was boiled down a little and added to the unprocessed sludge
container so as not to loose any material.

But was it really Potassium Chlorate, and was it contaminated
with too much Sodium to be useful?

Tests

Your most basic of tests to start with; will it burn
something? A few grams of Lancaster Chlorate Red #3
was prepared. It worked perfectly.

A truely beautiful red. Very pure, no significant Sodium
contamination, but a blue would be a better test. The yellow/white in the
picture is mainly due to overload of the camera, the illuminated smoke shows the
true flame colour. Of course there is some incadescent carbon in the flame that
makes it more orange to the eye than the actual majority of the radiated
light.

A small amount of H3 was prepared. Even
just screened together it burnt very rapidly with a violet/white light. It sure
appears to be Potassium Chlorate.

Lessons and Comments

Make the electrical connections to your electrodes outside
the cell, protected from creeping crystals with some kind of sealant. I was
rather concerned with the contamination of my electrolyte with an unknown amount
of Copper ions from the dissolved Brass anode connection. Copper is a powerful
sensitiser for Chlorate, there are many recorded
accidents with Copper and Chlorate, quite a few involving Copper heat exchangers
in Chlorate factory evapourators. I believe I was able to minimise and remove
the Copper contamination, but it is something to consider if you try this
yourself.

There is much Chlorate left in the mother liquor. I hope to
extract, or at least recycle it back into the next run. On cooling much of it
crystalised out on top of the settled anode mud. The fine graphite particles
probably acting as nucleation sites for crystalisation. Some larger crystals
were removed and studied, they were somewhat imperfect flat rhomic prisms often
including graphite particles in their defects. Some looked more like hexagon
prisms, like the one in the photo. No doubt the influence of the high Chloride
concentration on the growing process also contributed to the non text-book
monoclinic crystal shape.

The erosion of the anode was fairly severe, about half its
volume was lost. If it had run the full month for complete conversion I doubt
there would have been much left of the anode. The rate of erosion would only
have increased once the Chloride level dropped to virtually nothing and
Perchlorate production started. It might be reusable as a cathode, or perhaps an
anode at low temperatures in the chlorate cell. The remaining brass material
would need to be removed and the electrode cleaned carefully though.

Better anode material is definately required for the cell.
I've investigated MMO/Ti and Pt/Nb substrate electrodes from a supplier in
India, I expect to get a quote soon. I'll probably faint when I read it
though...

The graphite cathode and its connections remained pristine,
protected by its polarity in the cell, a quick wash and it was ready for reuse.
However other tests have shown there is a slight graphite erosion even from the
cathode, so I think I will persue using Titanium as the cathode material in my
future cells.

I've purchased some Titanium rods on eBay to use as cathodes and anode connection
and support structures. A friend in the US contributed a length of Pt/Ir wire
which I've yet to test.

Another observation for future reference is surface erosion
of the cell container. It is only PE and was probably etched by the hot moist
chlorine. The anode side of the container was the worst effected, appearing like
HF etched glass used in privacy screens. PP or a fluropolymer would probably be
a better material for a long-life cell. The solution when first filtered appears
yellow/green from elemental Chlorine in solution, boiling helps drive its
reaction with the cathodic Hydroxide to form additional Chlorate and its
evapouration out of the water.

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© Alan Yates VK2ZAY alany(at)ay(dot)com(dot)au