From Christopher James's book "Alternative Photographic Processes", third edition, pag. 455-457:
"SAFE DISPOSAL OF POTASSIUM CYANIDE
Neutralizing Potassium Cyanide to a Non-Hazardous Potassium Cyanate
READ THIS FIRST: Never execute the neutralization of potassium cyanide without adequate safety protection. This means you should wear industrial grade nitrile gloves instead of medical grade, as they are thicker. Potassium cyanide is readily absorbed through breaks in the skin in the form of cuts and scratches. After use, rinse the gloves with hydrogen peroxide and discard. Always work in well ventilated spaces. Wear an apron, dual respirators, and eyewear.
Before you begin, read these directions twice. Have a back-up person present in the event that something goes wrong. I know this sounds scary, but if you pay attention to detail, and exercise proper safety techniques, this is an uncomplicated process.
Potassium cyanide (KCN) has an MSDS Health Level Hazard of 3 and a dangerously high pH of 11, depending upon concentration. It is a heavily regulated chemical. It can be detoxified most efficiently with hydrogen peroxide and will be converted to potassium cyanate (KOCN), with an MSDS Health Level Hazard of 1 and a water-like pH of 8 that is unregulated and safe.
KCN + H202 — KOCN + H20
You need a 3% hydrogen peroxide (H2O2) (the type found in the drugstore and supermarket and that is the same strength as that used for cuts and minor wounds).
50 ml of 3% hydrogen peroxide will neutralize 1 g of dry potassium cyanide.
To Neutralize Dry Potassium Cyanide: Put on eye protection, a respirator, and industrial nitrile gloves. Know exactly how many grams of dry potassium cyanide you are going to neutralize. Place the dry potassium cyanide in a non-breakable beaker. Dissolve the potassium cyanide in cool water first in a proportion of 1 g to 100 ml water. After it is dissolved in the water, oxidize with 3% hydrogen peroxide (drugstore grade is 3%), using at least 50 ml of hydrogen peroxide to every gram of potassium cyanide that you dissolved.
Place the open beaker in a well-ventilated and VERY SAFE place and let it stand overnight. DO NOT cap the container, as the peroxide may create a gas and that will result in pressure within the closed container. The following morning, dilute this now safe potassium cyanate solution with water at a proportion of 3-4 times the liquid volume you are neutralizing. Rinse everything with hydrogen peroxide.
An interesting side note: when you neutralize the potassium cyanide, it will turn a milky white. If you leave the neutralized KCN outside in the light, it will turn chocolate brown because of the silver in the fixer.
To neutralize a 1.2% potassium cyanide fixer solution (a standard dilution for wet plate collodion), put on eye protection, a respirator, and industrial nitrile gloves and place the old potassium cyanide fixer in an indestructible plastic bucket (a drywall compound bucket will work well). Be sure the total volume of the bucket will accommodate twice the volume of the fixer you are neutralizing. If you have an industrial chemical hood you are in luck. Use it. If not, I recommend going outside for this next step.
Slowly add 600 ml of 3% hydrogen peroxide to each liter (1000 ml) of used 1.2% potassium cyanide fixer. You have now converted a heavily regulated chemical to a non-hazardous, unregulated, potassium cyanate. You will notice that there is a visible reaction between the potassium cyanide and the hydrogen peroxide. This will subside in a relatively short time. Allow the solution to stand overnight in a very safe place out of reach of anyone who may be curious about it. DO NOT cap the container, as the peroxide may create a gas and that will result in pressure within a closed container. The next morning, saturate this now safe potassium cyanate solution with water at a rate of three to four times the liquid volume you are neutralizing.
Once again: for every 100 ml of 1.2% potassium cyanide (12 g to a liter of distilled water [1000ml] is the standard wet plate fixer concentration) you will use 60 ml of 3% hydrogen peroxide. This is the minimum dilution, but it is acceptable to use more hydrogen peroxide if you wish. It is always better to err. on the side of excess hydrogen peroxide when performing this task.
Side Note: Even though the cyanide has been oxidized to potassium cyanate, the wet plate fixer still contains silver that may require special disposal depending on state and local regulations. It would have to be treated the same as spent sodium thiosulphate fixer.
Potassium Cyanide & Sodium Thiosulphate Fixer Warning: There is a published formula that mixes sodium thiosulphate with a small amount of potassium cyanide. This fixer works well but cannot be neutralized in the same manner as described in this section. A mixed thiosulphate/potassium cyanide fixer is a different bag of worms when it comes to disposal! Hydrogen peroxide will not only oxidize the potassium cyanide, it also will react with the thiosulphate to oxidize it to sulphate. Therefore, the 600 ml of 3% hydrogen peroxide per liter of fixer does not apply and this ceases to be a simple disposal problem. Instead it becomes a complex analytical problem of (a) How much cyanide is present? (b) How much thiosulphate is present? (c) How do I detect the end point of the oxidation reaction so the cyanide and thiosulphate are consumed? Bottom line . . . don’t be complacent about using these instructions for neutralizing a hybrid fixer.
Neutralizing Waste Water After Using Potassium Cyanide Fixer
You will have wash water from fixed plates (the first two rinses) whether you are working on the road or in the lab. The potassium cyanide concentration will be very low in these rinse baths. Collect your waste water and then add about 300-400 ml of 3% hydrogen peroxide per liter of water and let stand overnight before discarding. After the first two or three rinses, the cyanide concentration should be so low that further treatment is not needed.
Make sure you clean everything that is touched by the potassium cyanide with hydrogen peroxide.
Silver Recovery from Neutralized Potassium Cyanide
Once you have successfully neutralized the potassium cyanide by converting it into an unregulated potassium cyanate, it is a simple matter to run it through a silver recovery system as you would any other fixer used in photography. Please adhere to any, and all, federal and state regulations regarding silver removal."
