Homegrown Oxidizers Part Thirteen
WSM

As we continued our cell preparations by filling the titanium tubes spot-welded to our electrodes, we encountered a major setback. The thin-wall titanium tubing was seriously compromised by the spot welding process and leaked out some of the lead-free solder through the spot welds as we were trying to fill them with the copper rods and solder.

When things cooled down, we held the electrodes in our hands, and with very little force, could break them off the titanium tubes. We did some research and realized the current demand didn’t require leads as large as 1/2”, so we redesigned our titanium leads to use 3/8” diameter, thick walled titanium tubing with 1/4” diameter solid copper rod fillers. We estimate the 1/4” diameter copper rods can carry 95 Amps minimum, and since our anticipated current load is less than half that, we’re confident the smaller leads will operate without problems.

Reworking the Electrodes


Thinner wall tube on the left and thicker wall, but smaller diameter, tube on the right

We sized the length of the new lead tubes to be the same as the length of the failed thin-wall tubes. We began with cutting the ends and truing them on a lathe. Next we buff the ends and start the heating and pressing operations until the electrode ends are sealed flat (which was more successful than with the thin-wall tube).


The flattened and sealed ends of the new lead tubes

After the ends are sealed (no bubbles while blowing through the tube with the closed end immersed in water), we buff off the scale for a better spot weld on the electrodes.


The scale is buffed off preparatory to spot welding the electrodes to the leads

Now we spot weld the new leads to the pre-formed electrodes.


The lead tubes are mounted opposite of each other to allow a proper gap between the fittings holding them in the lid (otherwise the lid fittings would be too crowded)

The electrodes in their working position (without insulating spacers yet)

To finish the electrodes, we opt for a system we tried before, but this time we add a twist; because the copper fill rods are so large inside the titanium tube, we’ve decided to omit the solder fill. With only about 0.020” gap between the OD of the copper rod and the ID of the titanium tube, plus the large current carrying capability of the copper rod, we decide to try the setup without the solder fill between them. Also, we are locking the copper rods between the sealed ends of the titanium lead tubes and the brass bolts at the top end of the leads.

The next step is to thread the top end of the titanium lead tubes using a special tap designed to be used with titanium. Afterward a bottoming tap finishes the threads. We opt to use 5/16” course threaded brass bolts, 3/4” long, with a pair of brass washers to clamp cable terminal lugs on power leads to the electrode tubing leads.

The tricky part of this setup is to get the copper fill rods the exact length needed to tightly press its end into the bottom of the titanium lead tube and the other end tight enough against the bottom of the brass bolt to keep the lug and washers held firmly, to avoid high contact resistance and undue heating.

If the lugs are loose at all, the top of the electrode tube will overheat, possibly compromising the PVDF polymer compression fittings holding the lead tubes.


Threads cut in the top end of the titanium lead tubes with special taps

The brass bolt and washers clamping a cable terminal lug

We managed to get the copper rod length close enough to do the job without any apparent problems. Now we need to get the inert spacers between the electrodes to stabilize them and prevent electrical shorting.

Populating the Cell Lid

At this point we carefully measure the gap between the electrodes and plan the distance the leads need to be from each other for mounting in the lid of the cell. We calculate the gap to be 2.090” center to center for the proper spacing of our electrode assemblies. The gap between the electrodes averages about 7/16” - 1/2” (11-12.7 mm) and the anode dimensions are 2.875” by 4” (~73 by 101 mm) which will draw about 44 Amps of current.

After transferring this measurement to the lid, we drill holes with a 3/4” step-less taper drill and tap the holes with a 1/2" NPT (National Pipe Taper) tap so we can use 1/2" fittings to hold our various connections and sensors firmly in place.

We drill and tap several more holes for a vent and other needed connections. We use three wraps of common plumber’s Teflon tape on the threads of our fittings to make a gas and liquid tight seal with our threaded fittings.

For the vent, we were considering using a flexible tube or hose, but opted to use 1/2” rigid PVC pipe instead. With a larger system we anticipate a larger gas output, and suppose bigger is better in this case. We’ll use a PVC “Tee” fitting on the top of the vent tube to prevent dust and rain from getting into the vent, but allow the free output of gasses. As the system runs, we’ll alter the venting setup as needed.

Several ports are drilled, tapped and plugged in the cell lid. The 1/2” PVC vent tube is in the foreground

We mount the electrodes in the modified PVDF compression fittings in the lid and check that our clearance from the cell bottom is adequate, and tighten the fittings.

The mounted electrodes tend to bounce against each other as we move the cell lid, so we decide to add spacers to hold everything firmly in place and prevent electrical shorting of them.

Clear Teflon spacers between the electrodes and covering some of the cathode spanner bars

We elect to use short, stiff pieces of clear Teflon tubing, wedging them tightly between the electrodes and covering a couple of the bars spanning between the cathode plates.

We still need to finish plumbing our cell and add some sensors, plus prepare for future additions and improvements to our setup. Also our power supply needs a few modifications to make it suitable for use outdoors (where we decided to set our system up).


In the next part we’ll add our electrolyte, power up the cell and begin our first run making sodium chlorate.