Homegrown Oxidizers
Entry posted by WSM
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Homegrown Oxidizers Part Four
WSM
A fellow enthusiast has offered this method:
When is a batch finished?
For maximum anode life, one should end a batch run when the potassium chloride concentration drops to about 10% by weight. Without expensive analytical equipment the easiest way to infer the concentration is by measuring the specific gravity, which falls as the chloride is converted to the chlorate that largely drops out of solution. This can be done by carefully weighing a known volume of liquid in a small graduated cylinder. More convenient, however, is measuring the s.g. (specific gravity) with a battery acid hygrometer (or a more-expensive scientific one). You need the type with a large floating bulb with numerical values on the float. Any other type is useless for our purpose.
This process is most conveniently done at 20°C (68°F). Lange’s Handbook has tables of solubilities for salts, including potassium chloride and potassium chlorate. It also has specific gravities. If we ignore the solubility of the chlorate as a first approximation, the specific gravity of a 10% solution of KCl at 20°C is found to be 1.0633. That for a 24% solution is 1.1623. Now, extrapolating (using other table values) for the s.g. at the solubility limit (25.4% KCl), we get a value of 1.1727.
Thus, we would easily know the concentration of pure KCl solution based on a density measurement. We do have some potassium chlorate in our solution (after the first run). Lange’s Handbook tells us the solubility of KClO3 at 20°C is 7.4 grams per 100 grams water. That’s a 6.89% solution. Using the same process as above yields 1.0435 for its s.g. at that solubility limit.
The above values are fairly accurate for the two pure salts separately in water. Now here’s where we have to make some assumptions for our actual solution. We assume the solubility of potassium chlorate in a KCl solution is about the same as that in pure water and further, that the actual specific gravity is given by adding the chloride’s specific gravity to the chlorate solution’s specific gravity minus 1. So we have 1.1727 + 0.0435 = 1.2162. Now, the mutual solubilities of the salts are somewhat less than in pure water, so it’s probably closer to 1.20. This is for a solution saturated with both salts. This is approximately the density of the second (and succeeding) batch starting solution after re-saturation with KCl.
Finally, the density of the stopping point solution, if 10% KCl is desired, will be: 1.0633 + 0.0435 = 1.1068, or 1.10, rounded down with the same assumption as above.
After all this complication, the final procedure is fairly simple. Our starting specific gravity will be near 1.20 (for the second and later batches saturated with chlorate; less for the first batch starting with pure potassium chloride). Its actual value may be less, but it doesn’t really matter. The ending specific gravity at 10% KCl will be about 1.10, so to ascertain the end point of a batch run, a sample from the cell is taken and cooled in an ice bath to 20°C (ignore the bit of crystallization which takes place). The supernatant liquid is checked for specific gravity, aiming for about 1.10. The solutions can be easily measured with enough accuracy for our purposes, and the actual stopping value doesn’t have to be very precise unless you’re trying to accurately measure your electrical current efficiency.
A few more thoughts about oxidizer harvesting
Before we continue, another fellow enthusiast shares a method he uses of harvesting chlorate crystals from his bucket cell:
Harvesting the crystals may be done with a set of 100 micron screens (available from Duda Diesel at www.dudadiesel.com/) made to fit a 5 gallon bucket. The screens are strong and have held from 5 to 10 pounds of wet product without failing. They should be rinsed clean after each use. The mass of crystals is left in the cell bucket, and the warm liquor is poured off carefully through a screen into a catch bucket, not disturbing the crystals. The screen is then switched to a third bucket for removing and washing the crystals.
This allows the rinsing (of the raw chlorate crystals) to be done without cooling down the mother liquor (which, at this point, will take up renewal salt quickly, with some stirring). Two things can be accomplished at the same time: The cold water rinse can be allowed to sit and drain while stirring the recharged bucket. Then back to a cold wash when the first quantity of cold water has gone through.
A harvesting setup using multiple buckets and bucket screens.
The description immediately above is useful for operating a single bucket cell fairly consistently and is capable of producing hundreds of pounds of oxidizer a year, with diligent effort.
Turning the Product into a “Pyro Ready” Oxidizer
We’ve harvested the crystals, but how do we prepare them for use in pyrotechnics? Once the chlorate crystals are rinsed with chilled, pure water in a vacuum filtration setup, they need to be dried. Drying can be as simple as spreading the crystals in a half inch layer on a cookie sheet and letting them dry in an oven set to 150°F (66°C) for an hour or so. For thick layers of crystals (up to 1.5”), use up to 250°F for a lot longer and with occasional stirring to break up the lumps. Another option is to leave the crystals on a polymer screen, tilted up slightly on one edge (for airflow) with a fan gently blowing across them until they’re fully dry. This may take a day or longer.
Chlorate crystals drying in a tray lined with plastic foam wrapping sheets.
The next step is to reduce them to powder. There are various means to accomplish this task, but the most common is ball milling. Also, small amounts of crystals can be milled in a high speed, rotary blade type coffee mill. This is perfectly safe if done without any type of fuel present.
However it’s accomplished, once the chlorate crystals are reduced to a fine powder, they should be stored in an air tight container (preferably with a desiccant pack to keep them free flowing) and stored in a cool, dry (and secure) place.
Setting up for Subsequent Runs
The used electrolyte is a better starting point than the salt solution we originally began with. The reason for this is that the used electrolyte is full of precursor ions as well as dissolved chlorate from previous runs. The advantage of this is that subsequent runs produce chlorate right away.
In storing used electrolyte, the best thing to do is keep it at room temperature in a covered container. A normal 5 gallon (20 liter) bucket with a secure lid will work, but any compatible, clean container will do.
To recharge the used electrolyte, several methods will work. One of the easiest is to simply add the potassium chloride nuggets or crystals to the solution and let them dissolve till they reach equilibrium (no more salt dissolves at room temperature). Once that’s accomplished (from a few hours to a couple days, depending on the temperature of the liquid), the recharged electrolyte is ready to use for the next run. It’s usually best to use the recharged solution in the cell without any un-dissolved salt crystals included (use just the liquid portion). This way, you know your starting concentration and can calculate the current efficiency when the run is finished.
Recharging the warm electrolyte by simply dissolving in more chloride crystals with either manual or mechanical stirring (for efficiency).
Improving Yields
As mentioned in an earlier section, higher temperatures increase the cell efficiency (and activity). However the greatest improvement comes with pH control. The pH is a number that represents the acidity or alkalinity of a solution. Neutral pH is represented by 7.0, lower pH numbers are acidic and higher pH numbers are alkaline. The further away from neutral, the stronger the acidity or alkalinity is.
Left uncontrolled, the chlorate cell runs moderately to strongly alkaline. The greatest current efficiency (CE) is achieved when the pH of the cell is at or near 6.8 (slightly acid, since neutral is 7.0), where the ideal precursor ratio of 2:1, HOCl:ClO- (hypochlorous acid to hypochlorite) strongly favors the production of chlorate ions with the minimum of electrons (6 per chlorate ion as opposed to 9 without pH control). The end result of good pH control is more chlorate faster while using less current!
The proper pH for peak current efficiency of chlorate cells.
How do we control the pH?
We add acid to the cell as it runs. Our acid of choice is hydrochloric acid (HCl or pool acid), which lowers the pH by adding atoms already present in our electrolyte. By using equipment and materials made to handle acids, and using careful assembly, this task can be greatly simplified.
Some have suggested using an “off the shelf” pH controller. Unfortunately, using an active pH controller (like those used for saltwater aquariums or in laboratories) is impractical because the “ionic soup” our electrolyte is composed of, poisons most of the common pH sensors available.
How much acid is required?
Fortunately, industry has determined the quantity of acid required for the given current consumed per time unit (in milliliters HCl per amp, per hour) to achieve the ideal pH for the best CE of the cells.
One number for a batch system like ours, as shared by Swede, is:
0.057 ml of concentrated (32%) HCl per ampere, per hour, with the HCl delivery cut by 33% after the chlorate crystals begin to form.
Now since our system consumes water as it runs, we find it convenient to dilute the acid. If we dilute the acid to one fourth the concentrated strength (from about 32% to 8%), we use four times the volume to get the required amount (i.e. 0.228 ml 8% acid per ampere per hour to start) to optimize our CE.
Note that the above recommendation is only an approximation. The actual usage in practice will depend on the individual operating conditions of your setup. Furthermore, it’s recommended to carefully calibrate your own pH control system to make it fail-safe, to avoid accidentally dropping the pH much below 6, where copious chlorine fumes will start to generate. Obviously, this is very undesirable.
How is the acid added? Carefully!
The best way to introduce the acid is slowly, low enough under the surface of the liquor that the chlorine stays in solution and does its work without escaping the cell. If acid is dumped on the surface of the electrolyte in the cell, the usual result is the release of toxic chlorine gas!
Swede came up with an elegant solution to this problem. He plumbed a tube from the lid of the cell to plunge deep into the electrolyte and cross drilled it with many small holes, well below the liquid surface. As the acid is added to the cell in this dispersal tube, it slowly diffuses into the electrolyte which helps keep the chlorine in solution, where it benefits the chlorate producing system, instead of producing a noxious cloud.
One example of an acid dispersal tube.
Enter our pH control system!
Our first thoughts of a system to add dilute acid to the cell involved an accurate timer controlling a pump to move the acid from its reservoir to the dispersal tube. Our friend Swede noticed a problem with this scheme, where the fluids would retract in the tubing between doses and cause inaccuracies in the amount of acid delivered.
His simple and elegant solution to this dilemma is to place the acid reservoir at a higher level than the cell and let the additional head pressure gravity feed the acid injection, rather than pump it. A minimum acid reservoir height of one or two feet (±1/2 meter) above the cell is usually adequate to supply enough pressure for such a system, and more is stronger.
A timer, PTFE solenoid valve and PTFE needle valve with PVDF fittings for our gravity-fed pH control system.
Parts needed for our system include (acid compatible):
- Liquid tight, vented reservoir with a fill port and drain, including a shutoff valve
- Tubing and fittings
- An accurate timer system to control doses
- A solenoid valve for acid delivery and shutoff
- A needle valve for fine tuning the dose volume
- The fabricated acid dispersal tube running inside the cell
The timer is the heart of this system. An ideal timer is sold by Auber Instruments on eBay and is fully programmable and accurate to less than one second per day. It has a range from 0.01 seconds to 9999 days. To test the setup we programmed it for quarter hour dosing for 5 seconds. As seen in the photo below, the lower number is set for 14 minutes and 55 seconds (5 seconds short of fifteen minutes), and when that time runs out, the internal relay switches on for 5 seconds, then resets the cycle and continues the next timed interval.
For actual use with the cell, the timer is programmed for hourly injections of dilute acid, and the needle valve is adjusted to deliver the required volume, determined by testing (between the timer interval and needle valve setting, running water into a graduated cylinder) until the correct dose is achieved.
Our timer module, shown below (mounted in a PVC electrical box), is wired to feed power to the duplex outlet through a solid state relay. The timer’s internal relay switches on 9Vdc (from an auxiliary terminal on the timer module itself) to the control side of the solid state relay, switching on the 120Vac power to the outlet, which in turn opens the solenoid valve (powered by the 12Vdc adapter shown, plugged into the outlet), allowing the acid to feed into the dispersal tube in the cell.
The “dry test” of a gravity fed pH control system set for 15 minute intervals.
The tubing shown in the photo above is a rubber material designed to be used with peristaltic pumps and is listed as compatible with dilute HCl (hydrochloric acid). The acid reservoir tank is a modified laboratory pipette cleaner made of cross-linked polyethylene (XLPE), listed as impervious to HCl.
The acid reservoir shown below was fabricated from PVC pipe and fittings by a fellow enthusiast, and is also compatible with HCl. If building your own from PVC, be sure it’s liquid tight (leaking even small amounts of dilute acid can amount to a serious hazard over time).
A partially completed PVC acid reservoir.
The cutoff valve is mounted, but the fill port and vent fitting (toward the top) haven’t been installed yet. This is a closed system, so by venting the upper part of the reservoir to the cell, any vacuum created (when the fluid level drops) is broken and the acid will freely flow when the valves are open. By including two more tapped holes (to the top and bottom sides of the reservoir) a compatible and clear sight glass can be added to monitor the fluid level inside.
Testing the pH of the electrolyte
This can be a tricky proposition since our electrolyte is a very potent bleach mixture. If using standard pH paper, the paper tends to bleach almost instantly. However, if we dip and quickly observe the color before it disappears, we can get a close estimate of the pH level of our cell.
Some experimenters like to use the relatively inexpensive electronic pH pens available, but they don’t last long in practice, since the element is poisoned by the cell liquor. If it’s tried, the best approach would be to quickly test and then rinse the element in distilled water and store the pen tip in 3M (3 times molar) potassium chloride storage solution, for the best chance of survival for a pH pen. Never store pH sensors in water (which will eventually ruin them); store the sensor bulb in 3M KCl solution, either purchased or made up by the user.
Greater efficiency and production
In the next article, we’ll describe continuous chlorate cells and things to consider when setting them up and running them.
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