[Jewelry making - Article 00733] - Re: [Orchid] Separating gold and platinum

[Alden & Glenda Blood]

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Hello Andrew.

    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. 

Captain Blood
"Marlinespike Seamanship in Precious Metals"
mariner AT tgtel.com


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