Alan's
Tips

Be
patient

The single most important thing about making Chlorates or
Perchlorates is patience. Runtimes tend to be 1-2
weeks, as cells with passive cooling can't take much more than 1-2 Amp per 100
ml. Purification of Potassium Perchlorate will take
*at least* three crystalisation cycles to achive a purity sufficient for good pyrotechnic blues. This
takes several days of part-time work as you can only do 1-2 cycles a day
because of the cooling time. This also involves a large volume of water and evapourating the supernant liquid
to recycle what is dissolved in it will take an hour or two per cycle.

pH control isn't a big deal

You will get more efficiency benefit for less engineering
hassles by keeping the Chloride concentration up than maintaining the pH
precisely. To do pH control properly you will need a closed-loop system, you
can't just add some HCl each day or rely on a syringe
pump or similar metered dosage system. The pH will stabilize fairly alkaline
and the efficiency remain better than 50% as long as
the Chloride concentration is kept above 300 g/l.

Keep
Chloride/Chlorate concentration high

Not only will doing so keep your cell's electrochemical
efficiency and electrical conductivity high, it will minimise
wear on your anodes. The simplistic method is to run your cell only to 100g/l
of Sodium Chloride or Sodium Chlorate rather than to complete consumption of
the Chloride or Chlorate. Those last 100g/l will be at terriable
efficiency and at the cost of much wear on the anode.

A better but more complex method is to resaturate the liquor continously
as the cell runs. This may be achieved simply by extra solid input material
left undissolved on the bottom of the cell. It will
be slowly consumed as the cell runs, and will "salt-out" the product
and exchange places with it. This complicates taking out the product if you
don't run the cell long enough to consume most of the input chemical. However
you can simply decant the warm liquor and allow it to cool, taking whatever
product crystalises out then resaturate
by pouring it back into the cell. The stuff on the bottom of the cell is
replenished when it starts getting thin.

The yields will seem small at first, especially for
Sodium Chlorate cells because of high solubility of Sodium Chlorate, however
over time this is one of the easiest processing methods. It can be arranged
into a continous process, by pumping a quantity of
warm liquor into a crystalising chamber where it
cools and then decanting off the liquor where it is fed back into the cell to resaturate from material on the cell floor (or through a resaturator before injecting back into the cell).

Don't
waste your Platinum

If you have Platinum anodes available don't waste them in
Chlorate cells unless you are keeping them continously
resaturated to avoid wear on the anode. It is best to
keep Platinum for Perchlorate production, strong
low-Chloride solutions of Sodium Perchlorate are the
best feedstock for a Perchlorate cell with a Platinum
anode. Similar to the case with Chlorate cells continous
resaturation will help prevent anode ware and keep
the electrochemical efficiency back up.

Use
KCl if you only need KClO3

Despite its somewhat lower
electrochemical efficiency a Potasium Chlorate cell
is much more straight forward and skips the frustrating process of removing the
Sodium from the product. Like a Sodium Chlorate cell it can be continously resaturated with
Potassium Chloride which will keep the efficiency up, and Potassium Chlorate
will precipitate out as the reaction proceeds. If you are using MMO anodes in
most cases a simple recrystalisation to remove any
stray Chloride is all that will be needed.

You may leave an inch or two of KCl
crystals on the bottom of the cell, they will be
slowly replaced by KClO3 over time while keeping the Cl-
concentration and electrochemical efficiency high and anode wear low. Knowing
when to stop and process the crystals and replenish the KCl
can be a matter of guess work unless you have a good idea of the efficiency of
your cell. After a few cycles you will know when there is little Chloride left
and its time to process the crop. You can use Silver Nitrate to estimate the
Chloride concentration, if it is below 100 g/l it is time to do a cycle before
the anode starts to wear.

Graphite
is a b*tch

Ohmic losses in the
graphite and at its connection point with the wiring will give you lots of
grief. Even with Silver epoxy or plating the heating at the graphite/Cu
interface will often destroy the electrode mounting assembly. The heating
inside graphite anodes above the solution tends to dry the fine spray of
electrolyte, causing crusting and creeping of electrolyte up to the electrical
connections and speeding their destruction. The crust of chlorate-heavy salts
will rapidly react with the binders in some graphite rods, causing them to
break off and fall into the solution ruining the cell.

Combined with the slow errosion
of graphite into the liquor and the huge pain it is to filter it out you will
probably rapidly loose your patience with graphite and start looking for
alternatives in your Chlorate cells. That said, graphite is cheap and easily
obtained and is all you need for making Chlorate.

Use
good quality labware

Destruction of residual Chlorate requires lowering the pH
significantly. In poor-quality stainless pots this will contaminate the liquor
with Iron Chloride, in fact even the best quality pots don't like the nasty
liquor much and it is best to use borosilicate glassware for handling liquor
that hasn't been processed and neutralised. The Iron
Chloride itself isn't a big deal, you can selectively remove it during crystalisation, but once you add Sodium or Potassium
Hydroxide to raise the pH back towards neutral Hydrated Iron Oxide/Hydroxide
will be precipitated in an extremely fine form that is near impossible to
filter out. The Iron Hydrates will stain your product and be quite difficult to
remove.

Iron Hydrates can be removed from Potassium Perchlorate by dissolving it with Hydrochloric Acid. Do
this either by washing in a filter with HCl or by
adding HCl to a recrystalisation
cycle until the Iron Hydrate floc dissolves
completely leaving a clear greenish yellow solution. This will not decompose
Potassium Perchlorate which can be recrystalised leaving the Iron Chloride in the acidic
solution. The solid Perchlorate can then be washed
clean of Iron then further purified by more cycles of recrystalisation
during which any residual acidity is neutralised with
a Hydroxide.

Obviously this can't be attempted with Chlorates as the
low pH will cause decomposition of the Chlorate. Coagulation of the Iron into
its Hydroxide/Hydrate by taking the pH very alkaline and boiling may allow
filtering out of a large part of the Iron contamination from Chlorate liquors
before crystalisation.

Precipitation
of Potassium Perchlorate is exothermic

When you add Potassium Chloride solution to the Sodium Perchlorate liquor to precipitate Potassium Perchlorate ensure at least one of the solutions is cold.
The reaction is exothermic, and if both solutions are near boiling the energy
release may very well cause a local boiling event and spray you with extremely
hot crystal slurry. You get a better yield if the solution is cold anyway, so I
recommend cooling both solutions first. Stiring
briskly is a good idea, it dissipates the heat and
ensures a complete reaction. Thermal and density effects can cause seperation of the reactants into layers that will react
only very slowly at the interface if you just tip the Chloride solution into
the Perchlorate one and leave it, especially if the
Chloride one is warm - you will get a creamy precipitation instantly that will
float because of the heat generated, then slowly sink, seperating
from the Perchlorate depleted liquor and leaving
Chloride rich liquor on top.

Some
plastics are soluble in the liquor

Polyethene in particular will
partially dissolve into the hot caustic solution rich in Chlorine/Oxygen
radicals. You may find if you use PE that a slimy deposit of it will form on
the cathode reducing the conductivity of the cell somewhat. Dissolved material
will also come out as a floc when the pH is lowered
during liquor processing. This can be especially difficult to filter out as it
blocks filters very easily.

Some filter materials (Cellulose) also partially dissolve
into the raw cell liquor, only to precipitate Viscose when the pH is lowered.
Viscose floc also easily blocks filter membranes,
frustrating attempts to filter it out.

PE /is/ a usable material for a cell, it won't fail
completely, but its side effects are annoying. Chlorinated polymers are much
more stable, common PVC is an excellent choice, as is Polypropene.
Stainless steel is usable, but must be bonded to the Cathode to protect it.

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