Homegrown Oxidizers
Entry posted by WSM
1444 views
Homegrown Oxidizers Part Three
WSM
The Power Supply
Our power supply (5Vdc, 30A)
The name plate, showing the input and output power
Since the power supply has a voltage adjust trimmer, we can adjust the output voltage slightly above or below the 5V rating. To save energy we adjust it downward as the cell needs less than 4 volts to operate. Since this supply has multiple positive and negative output terminals, combining the positive wires to the anode and likewise the negative wires to the cathode will allow us to carry higher current with smaller gauge wires. The same technique is applied when a modified computer power supply is used (gather all the +5V leads to a bus for the anode and gather all the negative or ground leads to a bus for the cathode) to power the cell.
A larger size and type of power supply with variable outputs.
This power supply has variable outputs and can supply 0-40V and 0-75A, but costs many times as much as the simple supply shown above. An additional feature is it’s capable of constant voltage (CV) or constant current (CC) operation. The CC operation allows for simple current efficiency (or CE) calculations which allow us to know how well our setup is performing (industry usually works at above 95% CE, where with even the most advanced amateur setup, we’re lucky to break the 90% barrier).
Setting the Cell up to Run
We’ve got all our various parts gathered together, so let’s start our cell. Before we begin, we need to decide about the details:
- Where to run the cell (Indoors or Outdoors)?
- How much electrolyte is needed (fluid levels?)
- Where do we want to place the electrodes?
- Vent sizing and placement
- When do I “Pull the Plug”?!
- Electrode Care and Storage
Where to run the cell
Indoors (not to mean any inhabited area, which is too risky), your system is protected from the elements and unwanted approach by those unfamiliar with the hazards present. A big downside of running the system indoors is the effects of chlorine on any metal (or living organism) nearby. A good (non-sparking) ventilation system is advised if running a system indoors.
We’ve normally elected to run our systems outdoors, for safety. That said, it’s still good to protect it from the elements and curious pets or people. A small lockable shed or other structure, tailor made for the purpose, is a good idea. The ideal structure should have a strong roof designed to shed any rain or contaminants but still allow proper ventilation to safely remove the hydrogen and any chlorine gas that may be generated. Protecting the components from direct sunlight is another function of a good roof. The walls of the structure can either be solid, screen material or a mixture. There are advantages and drawbacks to either one.
Fluid Levels
We have our cell but how much salt solution do we put in it? Most amateurs want the cell to be as full as possible, and this tends to cause (or encourage) more salt creep on the cell lid. Since the running cell generates heat and lots of very tiny hydrogen bubbles, a good friend and fellow enthusiast suggested keeping the fluid level well below the cell lid (up to 6” or 150mm below) to allow the space for those bubbles to pop, keeping any resulting spray away from the lid and minimize the salt creep problem. Though this will reduce the total volume of the cell liquor, the cell will run better overall.
Electrolyte preparation by simply dissolving the salt in distilled water
Electrode Placement
Where one places the electrodes is a matter of choice, but we tend to place them off-center in the cell to promote more fluid flow, as discussed earlier. This is especially important if your cell has anything like a dome shaped lid (even slightly dome shaped), where the highest point is where the hydrogen vent should be.
Don’t hesitate to experiment with electrode placement and see which spot is optimal for your system. The current usage and temperatures reached are a couple of the indicators showing how well the system is running based on electrode placement.
A “bucket cell” showing offset electrodes and a large barb fitting for the vent (upper left side of the adaptor)
Vent Size and Placement
We’ve already mentioned placement (the highest point in the cell), but what size vent is required? As the cell runs it breaks down water. The oxygen is pushed onto the salt component ions, but the hydrogen ions combine with each other in a covalent bond and bubble out of the liquid. The amount of hydrogen gas formed at the cathode is dependent on the surface area and current demand of the system (in other words, every situation with its myriad variables is different) but the need to safely vent that hydrogen is the same. Making the vent oversized is better than making it undersized, so go larger rather than smaller with vent tubing. Hydrogen is very explosive, and in venting it, make sure to avoid any source of sparks or ignition to prevent a dangerous hazard.
The material the vent is made of is not as critical as the cell structure itself, so polyethylene (PE), polypropylene (PP), or polyvinyl chloride (PVC) should work just fine. A piece of used garden hose (for example) attached to a barb fitting in the lid would work fine. Whether hard pipe or flexible tubing doesn’t matter, but it should rise straight up (or nearly so) so it won’t close off with collected condensed vapors and pressurize the cell (making the salt creep problem worse) as it will in a vertically coiled tube.
If chlorine gas is generated by your cell, a scrubber consisting of a glass or compatible polymer jar containing sodium hydroxide (lye) in line with the vent, will eliminate any escape of the gas by binding it safely as salt.
How Long to Run the Cell
Most beginners to electrochemistry face this question. In the changes going on in an active cell, we start with high chlorides and transition to low chlorides, high precursors and chlorate.
The process efficiency is not a straight line but an arc with a sharp drop off at a certain point [during the run]. Many beginners want to run the cell to completion, but converting the last bits of chloride uses much more power than the earlier parts of the run, and efficiency drops off greatly (diminishing returns).
Practice has shown us that when the initial chloride level (maybe up to 16% chloride in solution) drops much below 10% KCl, the efficiency drops dramatically, and rather than use a lot more power to gain slightly more product; we stop the run, harvest the chlorate, resaturate the depleted liquor with chloride and start another run.
Lacking the ability to simply or cheaply measure the chloride level in our liquor, we use the “rule of Thumb” and keep an eye on the current draw of our system. When the amperage demand of our cell (using the power supply above and without pH control) draws roughly half of the highest current level observed, we end the run, harvest the crystals and set up to run the next batch. We don’t worry about squeezing every bit of dissolved chlorate out of the depleted liquor because that chlorate and the precursor ions with it will help the cell to yield more chlorate much more quickly in subsequent runs, after replenishing it with fresh chloride.
Electrode Care and Storage
Our electrodes are chemically and electrically tough… as long as the power is on. Once the current stops flowing, the cathodic protection ceases and at this point it’s advisable to remove and wash the electrodes with fresh water, air dry and store them till the next run.
Doing so will protect the working surfaces and lengthen the life of the electrodes. Other things can negatively affect the life of the electrodes, too. One reason we recommend using deionized or distilled water (or rain water, filtered to remove particles) to make up our electrolyte is that several dissolved minerals in tap water can either cover the working surfaces or attack the titanium substrate of the MMO anodes, shortening their life and efficiency. This is especially true if your tap water is fluoridated (fluorine ions can etch and destroy titanium fairly quickly and can dramatically reduce the electrode life). There are other elements in tap water that can cause problems too, depending on the water sources and quality in your area.
Running the Cell
Due to the nature of these cells, it’s best to run them without interruption. Once set up and running properly, they do their work most effectively when we leave them alone. Even just taking a look might reward you with a whiff of chlorine, so avoid doing that and maintain good ventilation where your cell is run.
If your area is prone to power outages, an automatic power transfer system (ATS) might need to be considered for the health and life of your electrodes (let alone other vital services you depend on).
A “bucket cell” powered by a modified computer power supply with a digital ammeter added
Harvesting the Crystals
Now that our cell is loaded with chlorate crystals, how do we recover them from the liquor without ruining our clothing or leaving our hands stinking of bleach? Wearing rubber or nitrile gloves helps (along with other common PPE’s or Personal Protective Equipment, such as goggles and face shield, rubber or vinyl apron, etc.). Even with such protective equipment it’s not advisable to reach into the cell liquor (essentially bleach) to recover the crystals.
A friend uses an inexpensive aquarium fishnet to remove the crystals from his system. If the crystals are caked together (a common occurrence if the crystals sit for much time in the solution), a length of clean PVC pipe can be used to break them up, with or without a small square piece of flat PVC sheet glued on the end for a type of scraper or spatula.
Purifying the Product
Now that we’ve removed our wet crystals from the cell, we need to prepare them for actual use. The first step is removing the residual liquor. This is most effectively done with a process called vacuum filtration, with the side benefit of making washing any impurities off the crystals a simple task.
Some examples of a vacuum filtration setup (many variations of this are possible)
After the mother liquor is separated from the crystals we wash the crystals with chilled deionized water (to minimize dissolving our chlorate) in the same setup. The depleted liquor and wash water is recharged with salt and then added to the cell for another run. If further purification is required, the crystals can be dissolved in pure water and recrystallized. The moist crystals then are dried, powdered and stored for later use.
How to Measure Total Yield
To measure the yield we first weigh the dry crystals from our run. Next we determine the volume of the remaining liquor. Once we have the volume, we measure the temperature of the liquor. Compare the volume and temperature to the “Relative Solubilities” chart (published in part two of this series) and calculate the amount of dissolved chlorate and add that amount to our dry weight and we have a close approximation of the total yield of the run.
Bibliography, Continued
“Chloric Acid and Chlorates” section of Encyclopedia of Chemical Technology by Kirk-Othmer, published by Wiley and Sons, Inc., 4th and 5th Editions plus others.
“Effects of Electrolyte Impurities in Chlorate Cells” by Richard A. Kus, Senior Research Chemist for ELTECH Systems Corporation at the 16th Annual Chlorine/Chlorate Seminar.
-
2
3 Comments
Recommended Comments