Decomposition of waste cyanide solution is accomplished by
oxidation. A frequently used process utilizes bleach as the
oxidant. I prefer hydrogen peroxide as the oxidant in a process
recommended by Solvay Interox.
It may be illegal to process cyanide waste by individuals in your
local area. It is advisable for you to check it out with the local
waste disposal authorities.
A word regarding lye (sodium hydroxide, NaOH), which is used to keep
cyanide solutions basic. The dangers of lye do not appear to have
received nearly as much attention as have the corrosive acids. Lye
will eat a hole in your skin before you feel the pain. It is
extremely reactive, especially with fats and proteins from which we
are all made. Be very careful not to spill any of the material on
yourself when opening a can of “Red Devil” lye (sold in grocery
stores and hardware stores). Protective clothing is a must. Lye is
very soluble in water, but paradoxically can be difficult to
dissolve. The trick is to add it to the water in small portions
with stirring and cooling as necessary. A large amount of heat is
liberated when lye dissolves in water. Do not allow the material to
accumulate in the bottom of the solution vessel. If you do, it will
glaze over and be very difficult to dissolve completely. This is
especially true of the pellet form that reagent sodium hydroxide is
supplied in. Lye is hygroscopic. Keep the container closed when
not in actual use.
The decomposition of cyanide solutions using bleach (5 to 6%
solution of sodium hypochlorite, NaClO) is described in detail in
the “Midas Guide to Plating.” This publication is available from
your supplier or from Midas. The Solvay hydrogen peroxide process
is much cleaner than the bleach process. The hydrogen peroxide
process eliminates the concern with production of chlorine and the
formation of toxic cyanogen chloride (ClCN) which occurs in the
bleach process. The final product in the peroxide process is
ammonium carbonate (NH4CO3).
The cyanide solution should be stripped of values regardless of the
decomposition process used. This can be done by using activated
charcoal (crushed up Bar-B-Que charcoal will not work), basic
anionic ion exchange resin, electrolytic plating (winning) or
chemically reducing with zinc (cementation). Removal of metals
allows recovery of values of course, but it is also advisable
because the metal-cyanide complexes present take much longer than
free cyanide to break down. The peroxide process is as follows: A
liter, or less, of the basic cyanide solution is diluted with five
to six times its volume with water. The pH is adjusted with 10% lye
solution to a value of at least 10 but no greater than 11. Four to
five ml. of a one percent cupric sulfate solution per liter of the
diluted cyanide solution is added with stirring. The copper acts as
a catalyst. Three percent hydrogen peroxide is added with stirring
in 200 ml. portions per gram of cyanide present. Let the mixture
stand 2 to 3 hours. Test for residual peroxide by adding a couple
of drops of the reaction mixture to 5 ml. of 3% potassium iodide
solution. Add a couple drops of a saturated starch solution and mix.
A blue color indicates the presence of peroxide. The cyanide
decomposition is complete if hydrogen peroxide is still present
after another 3 hours. Add more hydrogen peroxide as before if the
test for peroxide is negative.
A number of readers will see the similarity between this process and
bombing. The difference, of course, is the much higher
concentration of sodium cyanide and the use of 30% hydrogen peroxide
in bombing. The increased concentrations in bombing result in an
increase in reaction rate to the point of a near explosion.
While I am at it, let me correct a mistake in the literature that I
have seen in several places. It is apparently a typographical
error. I don’t know if it started with Ammen’s book or not, but it
has been perpetuated in several other places. Silver cyanide (Ksp =
1.2 X 10 to the -16) is AgCN not Ag(CN)2. The silver cyanide
complex is the anion Ag(CN)2(1-). I base this on no less an
authority than Cotton, et.al., “Advanced Inorganic Chemistry”, 6th
Edition, p. 1083.
"Marlinespike Seamanship in Precious Metals"