Homegrown Oxidizers Part Twelve
WSM

Making Sodium Chlorate

There are several advantages to making sodium chlorate over making potassium chlorate… and a few disadvantages.

Potassium chlor-alkali salts are less and less soluble as their oxygen level raises. Sodium chlor-alkali salts, on the other hand, are more and more soluble as their oxygen level raises. What does this mean?

When potassium chloride is converted to potassium chlorate in our cells, the chlorate, being less soluble than the chloride, eventually begins to drop out as solid crystals as its concentration increases. If converting potassium chlorate to potassium perchlorate, the perchlorate crystals drop out almost immediately, and are prone to foul the electrodes and minimize the current (a self-limiting process), not to mention, making it a tough chore to clean up and recover the product!

Sodium chlorate is more soluble than sodium chloride, so several industrial techniques that aren’t practical with the potassium salts, will work very well in the sodium system:

• The use of pumps for moving solutions
• Using a site glass to determine fluid levels
• Creating a highly concentrated solution of our end product
• Less crystal fouling of system components
  

to mention a few. On the negative side of the coin, sodium salts present as a contaminant are very strong color donors, which causes problems for colors and color stars (especially the cool colors; green, blue and purple) and other effects; so very good removal methods must be in place to purify our desired end product (potassium or other perchlorates). Sodium salts, as contaminants, also tend to increase moisture sensitivity of the final products, because (with few exceptions) of their hygroscopic or deliquescent nature.

All this in mind, using sodium salts to produce perchlorates or specialty chlorates is our best option for economy and simplicity (which explains why industry uses this method). If there were simpler or easier ways to do it, it would be the industrial norm; and if industry can do it on a large scale, we can do it on a small one, if we’re properly prepared.

Setting up a Cell

Fortunately, when we switch from potassium chlorate to a sodium chlorate system, we don’t have to completely re-invent the wheel. Most of the components of the potassium chlorate cell will work equally well for our sodium chlorate cell.
Let’s consider what our goals are. Since we want feedstock for our perchlorate system, our needs are:

• Large quantities of sodium chlorate (our starting electrolyte for the perchlorate system is 600 to 750 grams per liter [or 5 to 6.25 pounds per gallon] of sodium chlorate to water)
• A larger scale cell to produce lots of sodium chlorate
• Larger electrodes and a higher capacity power supply to feed power to them
• Monitoring sensors and meters to maintain proper cell parameters
• Adequate ventilation and accessories to accommodate this higher capacity
• A safe place to set this system up and run it
  

A Larger Cell

We’ve determined a larger capacity cell is needed for this part of our research.

In our enthusiasm for producing homegrown oxidizers, one of our earliest home built reaction tanks was built of large scale PVC pipe. It was originally made for potassium chlorate but never used, when we calculated the yield to be much more than we were prepared to deal with.

Our needs for sodium chlorate, on the other hand, are much higher, so we brought this big tank out of mothballs.


A large tank made of PVC pipe, flat plate and fittings

This tank is fabricated from 12” PVC pipe, schedule 40. The wall thickness is nearly ½” and the volume per running foot is about 5.75 gallons. Between the bottom of the tank and the upper side port, we estimate the working volume as about 7 gallons (or about 27 liters).

The rated temperature limits for this type of PVC is 60°C (or 140°F) for operation at the rated pressure. Since our system is designed to operate at one atmosphere (ambient pressure), we may be able to exceed these temperature limits, but we’re not planning to.

Our base plate and lid are ½” thick clear PVC, which is what we had on hand when we made this tank up. The standard grey or white PVC plate would have worked just as well and normally costs less than the clear PVC. The lid is fitted with a backing ring of PVC and a silicone tubing O-ring seal to help prevent salt creep at the lid edge when the cell is operating.


Power Supply

For our power supply, we’ve chosen one similar to the type we used for smaller potassium chlorate cells, but with higher current output.


A 5 Volt, 55 Amp DC power supply

We are using 12 AWG wires to feed a pair of bus bars supplying our cell so we’ve connected three parallel pairs of leads to the power supply to carry the higher current output without overheating the wires.

We plan to run the supply continuously while the cell is operating, so we’ve sized the electrodes to demand no more than 80% of the supply’s output capacity, or 44 Amps. The voltage range of this supply is adjustable from 4.20 to 5.84 volts DC.


Electrodes

The first step in designing our electrodes is to determine the length needed. We don’t anticipate crystals collecting in the bottom of the RC (reaction chamber), so our electrodes can reach deeper into the tank without concerns of crystals interfering with natural convection mixing or reducing current flow.

We want the leads to be long enough to go from about 2” (~50 mm) above the cell lid fittings, down to the top of the electrodes, leaving enough room for fluid flow at the bottom of the electrodes (we decided to leave a 2” gap between the bottom of the tank and our electrodes).

The titanium tubing leads are about 18” long, the copper fill rods (copper is more than 6 times as conductive as the solder alone) are designed to touch the inside bottom of the sealed titanium tube and extend about 2” above the top end. The length of the electrodes will add about 3.5” (~90 mm) to the bottom end of the titanium leads, for a projected total length of about 23.5” (or nearly 600 mm).

The first step was to determine the proper area of the anode to demand a peak current of 44 Amps. We cut a piece of surplus MMO anode stock 2 7/8” by 4”, and two pieces of CP titanium sheet metal, 0.050” thick about the same size. Several pieces of the same titanium sheet metal were cut and formed to create the cathode box to surround the anode, while leaving plenty of space for adequate fluid flow.


Electrode parts prepped for assembly

The titanium tubes had the ends faced on a lathe and the surfaces lightly scoured before forming to accept the electrodes. Titanium is prone to harden while being worked to where cracking or splitting will occur without first preparing the metal. We prepare the CP titanium by heating with a propane torch till it glows and then press till it “resists”, stop, re-heat as before and continue. The alternate heating and pressing cycles continue until the bottom ends of the tubes are pressed flat and sealed, where the electrodes will be spot welded.


Heating the CP titanium tube before pressing

To test the seal on the end, we quench the pressed end in a tin of water and blow through the top end of the tube. If bubbles come out of the sealed end, we repeat the heating and pressing until they don’t appear.

This procedure seems to work better with thicker walled tubing (± 1 mm wall, or more), and after the pressing, heating again and carefully tapping the tube end on an anvil with a ball peen hammer to seal the closed tube end usually stops the bubbles from coming through.

It’s hard to get the air bubbles to stop coming through the thinner pressed tubes. We’ve resorted to finally sealing the ends of thinner tubes with a row of spot welds before attaching the electrodes.

The sealing steps are important to prevent the incursion of electrolyte into the tube, where it can negatively affect the filler materials and shorten the electrode life.


Pressing to form and seal the flat end where the electrode will be spot welded


Alternate heating and pressing cycles till the end is sealed flat


The flat ends are buffed clean before spot welding the electrodes on


Sealed end of the flattened, thin wall tube with the end spot welded (no air comes through)

The thin walled titanium tube tends to overheat when spot welding, so we sandwiched the flattened end between the MMO electrode and MMO ribbon, in the case of the anode; and in the case of the cathode between a strip of 0.050” CP titanium and the boxed cathode.


The flattened end of the CP titanium tube sandwiched between the anode and a strip of MMO ribbon


Both electrodes showing the sandwiched titanium lead tubes

We still have to tin the copper fill rods and secure them in the titanium lead tubes with lead-free solder. Also we need to modify the PVDF compression fittings and mount them in the lid of the large PVC cell tank, along with numerous other ports.

In the next part we’ll finish our preparations and begin to run our sodium chlorate cell…