T-Cell Construction, V - Home Stretch
Entry posted by Swede
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If you haven't been reading this blog, you may want to go back a few days to put it all into context. The entries are coming fast and furious! I've had a few days off from work, so I was able to put some focused effort into this beast.
The placement of the electrodes really had me stymied. The cathode is necessarily massive... two 25mm wide titanium strips had to enter a 4" cylinder, AND it had to be air and water-tight. Ditto for the anode, although its shank was much smaller. In addition, the anode needed to be swappable, without ruining the precious E-Cell lid, upon which much work was lavished.
Before I had decided how I wanted to get the anode in place, I simply bored an accurate 1.375" hole in the lid. This will accept CPVC (or any suitable plastic) turned to fit that bore, with a viton o-ring. I really didn't think it through, and the result now is much more complex than it needed to be, but I think I have it knocked.
The Cathode: Two large Titanium plates were TIG-welded onto two shanks which were bent 90 degrees.
The undisturbed ring surrounding the cathodes is a separate piece, and will form a good surface that will press against the EC o-ring. It looked good on the bench, but I knew gluing it all in would present a real challenge. Previously, I had been using too little cement. This time, I wanted that cement oozing from every pore... those shanks needed to be permanent, and well-sealed.
Meticulous preparation paid off, and the gluing went well, if a bit messy.
I thought about welding a cross-bar of Ti between the two plates, but decided to connect the two externally. The tabs were drilled 1/4" for 316 SS hardware. One of the tabs is longer than the other... in that one, I drilled two holes. The unoccupied hole will accept the lead from the power supply.
On to the anode. The good part here is that I can experiment a bit, with no real loss if the system fails. I decided to make a slotted insert that will carry the anode shank. The first step was to bore a 3/4" hole in the already made plug. Next, rather than use CPVC, I decided to try PET, polyethylene terepthalate. PET is the same stuff that plastic coke bottles are made of, and is available quite cheaply as bar stock. It machines beautifully, maybe even better than acetal. Offhand, I know of four plastics that are good to go in a perchlorate system:
- PET
- PTFE
- PVDF
- PVC
Of the four, PET is the cheapest, followed by PVC, PVDF (Kynar) and then PTFE. Anyway, a bar of PET was placed in the lathe, and a little plug was turned to fit the larger CPVC carrier plug. Now, how to slot the beast?
My bandsaw has a pretty heavy blade, and remarkeably, the "kerf" (the slot it cuts) was an almost perfect match for the thickness of the anode shank. Using the bandsaw, I created a slot in the PET plug, and a bit of sandpaper cleaned it up nicely. The anode shank just clears the edge, and the fit of the PET plug into the CPVC carrier was a tight, almost cork-like fit. It is close to watertight as-is, and for use, a few wraps of teflon tape will do a good job.
To put it al together, the anode is inserted from below, through the lid of the EC, not shown. The gray CPVC plug (with o-ring) goes on next, then the white PET plug.
Then, the whole assembly is pushed down into the EC lid. As I mentioned, it's silly and overly complex, but it'll work. If the slotted carrier idea works well, all I'll need to do is permanently glue the CPVC carrier into the lid, and the process will be much simpler.
http://www.5bears.com/perc/tc058.jpg
On the top of the lid, you can see a stub of pipe connector (yellow CPVC) that is glued in place, and into that will go a 3/4" piece of CPVC pipe, topped with a tubing connector. That will be the primary system vent. The final hole in the lid is a 1/4 NPT female thread for a PTFE bubbler tube, which will provide strong agitation to the EC. One final port to be drilled will be another threaded hole for a thermowell; a blind cavity which is immersed into, and surrounded by, the electrolyte. Another good place to measure temperature would be at the EC outlet... we are interested in the effluent temperature, with 80 degrees C. being a practical limit.
One good thing about this sort of construction... this pipe, and these fittings, are designed to hold substantial pressure, upwards of 160 PSI. Since this vessel is NOT pressurized, we can legitimately thin, machine, work the material, and be confident that it will still hold just fine.
The last picture shows the #4 copper cable which unifies the two Ti cathodes. Just about ready to rock! Unfortunately, my string of free days is at an end, and I'm off for three days for my airline, flying to Guatemala.
Thanks to all the members here who have provided good input, especially Tentacles, TheSidewinder, and a "secret" unnamed third member who is helping me extensively with raw materials acquisition, and other helpful advise.
Some final notes... the Tygon tubing in the pump seems to be holding up fine. It's been running in a closed loop for four days now and shows no sign of tiring. I think for actual runs, I will replace the tubing section that is getting mashed inside the pump, each and every time. Additionally, the entire system will be placed in a big mortar (concrete) mixing tray, and I'm also going to integrate a float switch. If the level drops too far, the entire system will automatically shut down.
I had also mentioned I was testing some other plastic samples in the incredibly noxious leftovers from previous chlorate runs. The three samples have been soaking for days now. One sample was a 2-part urethane plastic. The second was 3M DP270 potting resin. The third, baked and hardened "Sculpey" clay.
Remarkeably, of the three, the lowly Sculpey is kicking ass. The urethane is discoloring badly, and the DP270 is not doing well either. But the Sculpey - 100% intact! I can still see my thumbprint on the surface. This may be a real boon for home chlorate production. I can see Sculpey being used in a number of ways, and one that excites me is as a substrate for a PbO2 anode. Since Sculpey is primarily PVC powder, I guess it shouldn't surprise me. An interesting test would be to see if baked sculpey will cement to other PVC parts using regular PVC cement... THAT would be cool.
Future improvements: automated topoff, and pH control! And as I've mentioned, this system is big, complicated, and uses some fancy tools that not everyone has access to. It does not need to be this complicated. My first setup, in a food storage container, worked fine and was producing pounds and pounds of potassium chlorate, at the expense of a lot of fiddling and rebuilding of the system between runs. If you have an itch to try this fascinating process, go for it!
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