Help me design the ULTIMATE perchlorate cell!
Entry posted by Swede
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The blog entries have been coming fast and furious. If you haven't seen the one from yesterday, well, it's still there.
I've come a ways, from a crummy, claptrap cell, to a decently performing system...
It works well, but... it's too small, and too labor-intensive. I want to execute an awesome homemade cell, one that probably replicates many of the features found industrially but on a smaller scale. I have some ideas, but I'd really like some outside the box thoughts from the membership. Once the design is completed, I'll execute it and snap some pics.
Materials: The tank will consist of sheet PVC, and make liberal use of PVC square and angle stock as strategically-placed doublers for strength along the seams. Fittings will be PVC, PTFE, and Kynar. Thickness... I'm leaning towards 5/8" (16mm) or 3/4" (19mm) as a compromise between strength, workability, and cost. The cost for enough 3/4" PVC sheet to build this system will probably be about $125, but the cell should last for many years even with hard usage.
Sources: The cheapest and I think friendliest supplier I've found: "Freckleface" PVC sheet. There are others, but they all seem to be both more expesive, AND they want to sell larger pieces. I don't want to mess with a 4' X 8' piece of 3/4" PVC sheet. For reference purposes, though, here are some more links... Professional plastics, The Plastic Depot, U.S. Plastics. The latter company, BTW, sells pre-made PVC plating tanks.
Volume: It is amazing how much volume you get from what mentally appears to be a small box. Picture a cube 12" X 12" X 12". The volume contained in that box is a whopping 28 liters! In reality, a 12" cubed tank will have an interior volume of 22 liters due to wall thickness. Still, with my production at 200 g/l, such a tank would yield 4.4 kilos before recharging, and at 60 amps, that would take maybe 5 days, give or take. Serious production.
Methodology: Such a tank will require recirculation for best efficiency. I picked up a peristaltic pump off eBay capable of 0 to 4 liters/minute, a serious industrial unit, definitely no toy. Here's what I'd like to explore, and why I think this tank will rock...
The area immediately surrounding the electrodes gets very hot, very quickly, and is also the zone where the chlorate ions are created. Due to the heat, the molecules remain dissolved and dissociated. As the liquor migrates, it cools, and the KClO3 crystals form. Being heavier than the brine, they fall to the bottom, where further growth occurs. I want to make use of the fact that hot electrolyte rises (quite strongly) being less dense, and falls as it cools, to create a separate partition for the electrodes, where the reaction takes place, and then transport that hot liquor to a cooling zone, either by gravity or by pump, where the crystals may be harvested without turning the power off, or shutting down the system. With a partition, I can also gather and safely vent chlorine and hydrogen gasses; thus, the electrolyte that flows into the "cool zone" will, worst case, smell of hypochlorite, rather than actively gassing off chlorine bubbles.
The system may either be partitioned internally, or externally. I am favoring external partition with an active pumping of the liquor from the hot side, to the collection vat. Picture this - the peristaltic pump draws liquor from the hot partition near the bottom, and this run to the pump intake is quite short, maybe 300 cm. The pump head sees hot, clear liquor, no crystals. The pump output tubing is routed through a recirculating water bath, a cooling plenum, where crystals will no doubt start to seed and grow. The tube is then exhausted into the much-cooler collection vat, and the crystals fall to the bottom. At the top of the collection vat, fresh, clear liquor flows by simple gravity to the hot cell, closing the loop. Sound good?
The "hot" cell, where the action occurs, will be much smaller than the collection vat. A portion of the top opening will be permanently closed (using PVC glue) and that section will contain an electrical lead to a permanently installed sheet Ti cathode, as well as ports to vent gasses, a thermometer port, a pH probe, and a sampling port if needed. The "lid" to the hot cell will be aggressively gasketed with expanded PTFE and will carry ONLY the anode. This lid will be a standard size and construction, so a second lid, c/w new anode, may be swapped quickly. The anode can be different, such as MMO vs platinized Niobium, chlorate vs perchlorate, etc.
The cooler collection vat will still have a decent seal, but it need not be so effective, perhaps with quick-acting clamps as opposed to bolts. It too will have a sampling port, ports for pH and temperature, and it would probably be a good idea to vent it as well via a "tee" joining it to the hot cell exhaust tubing. At the bottom of the collection vat will be a stopcock to drain it, or better yet, simply reverse the pump direction and draw depleted liquor from the cool vat into a recharge vat, where chloride ions may be reintroduced.
Query - might it be possible to plumb this beast so at some point, the liquor can take up KCl salt and dynamically replenish the chloride? Can you imagine how cool a self-sustaining system like that would be? Dump a 50 pound bag of KCl into a chamber, add water, turn on the pump, the electrodes follow hours later after the liquor is saturated, and start harvesting two days later. Watch the salt pile at the bottom of the KCl chamber diminish. When it is gone, dump in another sack! I love it!! This may be possible because the solubility of KClO3 is so much less than KCl, and we can strategically design the flow routing to make use of hot vs. cold liquor to do this.
I know that there are at least two or three other people out there (hopefully more) that find this sort of work exciting and interesting. It might be possible to simplify such a system so that no pump is required, the "cell" is monolithic (but separated internally) and heat alone will recirculate the liquor. This could be a practical and acheivable project for a dedicated amateur. This would certainly be a long way from a food storage bin and hot glue.
Disclaimer/Conclusion I know, I know, it is cheaper to buy the stuff. So what? I love the challenge that something like this provides. I am a dork, but an unashamed older one with the time, tools, and a bit of money to make this happen. I should have been a chemical engineer! PLEASE, if you like this stuff and have ANY thoughts, I don't care if you think they're stupid, I want to hear them... comment freely. Perchlorates Forever! 
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