Some kind of titanium compound makes sense. I gave my anode a hydrochloric acid soak last year after mounting it to the titanium tubing- turned the acid a yellow color from dissolved iron, I believe. Might have to give the cathodes a soak.
I have a tube in the shape of the letter J with several small holes in the end. I used this shape because when my initial run was completed, a few days after I stopped adding acid, I noticed that the fluid level inside the tubing was well above the holes, meaning acid was still seeping into the cell.
I do believe you are correct, WSM, that my acid dosing rate is too high. Less K-chlorate seems to be forming at the bottom than I'd expect at this point and, after increasing the acid rate yesterday, there may be even less present today. I should add that my cell temperature is maintained at 60C, within about 0.3C by means of an internal temperature sensor controlling an external fan, so I've ruled out temperature increase as the cause of my crystals' disappearance.
The thing that's confusing me the most, however, is the apparent lack of change of the pH. When dosing the acid- even at a very low rate- the pH seems to settle at around 6. If I cut the rate in half, it's the same and if I stop adding acid altogether it looks to be about the same- even after a few days.. I must be reading my pH papers wrong or maybe they aren't very good. I got them this past Winter from ebay. When I dip them in the solution, the bottom part of the paper is bleached pretty much instantly, so I look above, where it has soaked up the paper a little bit and compare its color to the colors on the pH paper packaging.
Those who use pH control (HCl acid injections to lower the alkalinity) have reported that the amount of acid required diminishes as the cell runs. The pH tends to reach something of an equilibrium where further acid doses either aren't needed or the amount is greatly reduced.
Swede said something about dropping the acid injections to 2/3 the starting amounts when KClO3 crystals start to form. Since everyone's cell is different, I would suggest we use his recommendations as a starting point only, and fine tune our cell from there. Realize that the chlor-alkali cell is a complex and dynamic environment, with all the components in a state of constant flux.
This is what we want; we take a stable salt solution, add current and drive the reaction to build desired compounds. Lots of changes happen in the solution as we do so, all at the same time, and the complexity of it is the subject of Doctoral dissertations. The exact chemistry of what is in our cells at any given moment is highly dependent upon where in the process we are and for how long it has run (plus myriad other variables).
Optimizing our cell for the highest efficiency is tricky business, and adding acid with a light hand plus careful monitoring is called for. Even then, the cell's requirements change as it goes, so no hard, fast rules will do the job, but we need to watch it carefully and respond appropriately according to the changes we observe.
All this is not easy, but it is worth it; especially if you are scaling up the scope of your operation.
Edited by WSM, 28 August 2017 - 05:33 AM.