Sodium Perchlorate

Sodium Perchlorate is not used in pyrotechnics due to the fact that it is hygroscopic. It has three hydrates containing 0, 1 and 2 water molecules. It is used to make Ammonium or Potassium Perchlorate and other Perchlorate's by double decomposition.
There are two routes you can take when making Sodium Perchlorate.

1
You can start with Sodium Chloride and let the cell run and run until the cell turns from a Chlorate cell into a Perchlorate cell. You can top up the cell with either water or NaCl solution when the cell is a Chlorate cell. This is what is done in US patent 3,493,478. A 0.3% solution of Methylene Blue can be very useful here, as you can tell when your cell has become a Perchlorate cell by testing a sample of the cell with the Methylene Blue solution. When it has turned into a Perchlorate cell you can run the cell for the appropriate amount of time in order to convert the Chlorate (+ the 10% Chloride) into Perchlorate. It is generally agreed that Perchlorate will start to form when the NaCl in at a concentration of about 10% in the cell. You need a Lead Dioxide anode. Using Platinum anodes is not advisable for this method as there wear rate will be excessive when the Chloride concentration gets low and when the Chlorate concentration gets low.

2
The other route is to start off with solid Sodium Chlorate and dissolve it in water to make a 50%(wt) solution (that's approx. 720g Chlorate per litre of solution) of Chlorate and run it for the amount of time recommended in the run time section or until the Chlorate concentration is low.

Anode materials
The Anode materials for making Perchlorate are more limited than Chlorate making.
Lead Dioxide, Platinum, Manganese Dioxide(high wear, US Patent No. 4,072,586) and possibly some types of MMO.
Graphite will NOT make Perchlorate without hugh amounts of wear (totally impractical). It can be used in cells that contain a diaphram according to US Pat. 1,279,593.
Cathode materials
Cathode materials can be SS, Graphite, mild Steel, Nickel and Titanium. Phosphor Bronze has been used by industry.

When mild steel and Nickel is used, Chromate's are desirable to stop the Cathode from being attacked by the Hypochlorous acid in the cell electrolyte. (0.5 to 5 grams/Liter). Chromates also increase current efficiency by stopping reduction at the cathode (the conversion of Chlorate and Hypochlorite to Chloride).
Chromate's are not compatible with Lead Dioxide. Use Titanium as a cathode if you can get it.
Sodium Fluoride or a Persulphate may also be used to stop Chlorate or Hypochlorite being converted (reduced) back into Chloride at the Cathode.

When using Platinum as an Anode it's wear rate will increase as the Chlorate concentration decreases, and below 50g/l it may be excessive, high temperatures also increase it's wear rate. Wear rates from manufacturers have been reported as 3 to 6 grams Pt per ton Sodium Perchlorate.
The concentration of Chlorate should be kept above 100g/l if a high current efficiency is desirable. (The Perchlorates. Schumacher J.C. P86)
The temperature of the cell is usually maintained at about 30 to 60oC. The current density on the Anode is usually much greater than with Sodium Chlorate manufacture, and high current density's do not decrease current efficiency. Typical Anode current densities are from 150 to 400mA per square cm for Lead Dioxide anodes and 200 to 500mA per square cm for Platinum. Most commercial plants use water circulation through coils in the cell for to remove heat generated in the cell to stop it from overheating. The amateur can keep the cell cool by putting it sitting in a large tub of water or by keeping the current per volume low at about one amp per 100ml of solution.
The voltage across a Perchlorate cell is higher than a Chlorate cell and will be in the range of about 4 to 6.5 volts.
The cost of electrical power for to make one Kg of Na Perchlorate from Na Chlorate is about two KWh (two "unit's" of power) which is approx. 18 US cents.

To start your cell about 700grams Na Chlorate are dissolved in some water and more water added to make one liter (A solution of Sodium Chlorate containing 700g/l has a density of 1.428 and is 49%(wt) Sodium Chlorate, see Graphs and table's in the Sodium Chlorate section).
Dichromate, Fluoride (Na, K, or Hydrofluoric acid) or some Persulphate (Na, or K) or both F and the Persulphate is added to the cell for to improve the current efficiency. About 2 g/l Sodium fluoride or about 2g/l of K Persulphate is OK. If using Dichromate use 2 to 4 grams per liter.
The current is turned on and the cell is run for the appropriate time as outlined in the run time section. The product is then either turned into K or Ammonium Perchlorate (see relevant section) or solid Sodium Perchlorate is extracted and the mother liquor is returned to the cell for the next run of the cell.
All mother liquor should always be returned to the cell as it will be rich in Chlorate and Perchlorate because of their high solubility. Note that Platinum Anodes will corrode if used to reduce the Chlorate concentration to a very low value. You need Lead Dioxide if you wish to run a cell from Chloride to Chlorate to Perchlorate to Low Chlorate concentration, without stopping.

There is a good description of a Perchlorate cell using a Platinum wire anode at the bottom of this page by GarageChemist. In his cell you must use Dichromates to stop Chloride from forming in the cell and you must start the cell with zero (thats 0.00%), Chloride in the electrolyte to avoid Pt anode erosion.
SMALL amounts of Chloride in the cell are a Pt anode killer.

Since Sodium Perchlorate has a number of hydrates this will need to be noted if yields are being calculated. Reduce all product to the anhydrous state. Which hydrate forms depends on the solution concentration and temperature as your product crystallizes.

Run time for a Sodium Perchlorate cell
Separating out Sodium Perchlorate
GarageChemist Perchlorate cell using Pt and Dichromate additive
US patent No 3,493,478. Making Perchlorate from Chloride using Fluorine additive.
US patent No 2,813,825. Making Perchlorate from Chlorate using Persulphate additive.
JAE 1971. Large scale production of Perchlorates directly from Sodium Chloride
Article from Encyclopedia of electro chemistry regarding Perchlorate manufacture.
Why run time tables for Chloride to Perchlorate are not a good idea

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