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topic 1588

HCN Concentration in Dilute Cyanide Solutions Used in Plating Process


Dear Sir:

I am looking into a problem that may have some impact to the plating industry in southern california with regard to the state risk management program (RMP) under the clean air act. According to the state regulation, the use of NaCN and KCN in the plating process may be covered by the state RMP if it is over 100 lbs. One of the concerns associated with the plating process is the potential for generating HCN vapor to the atmosphere. According to an RMPP Technical bulletin dated 10/13/94, a NaCN solution will contain roughly 16% HCN at a pH of 10. At pH 12 or above, the concentration of HCN in the NaCn solution will be less than 1%. I believe these data came from a 1993 DuPont paper which was based on "Data based on work by D. Milne, 1950, for dilute cyanide solutions at ambient temperature".

My questions are:

1. Is there more recent date and technical information and papers on the concentration of HCN in dilute cyanide solution other than Milne's data (1950)?

2. If the HCN is present in the cyanide plating solutions, in what form is it? Is HCN readily available for releasing to the atmosphere as a vapor from the cyanide solution? If HCN vapor is present at all time in the plating shop, then why the operators are not exposed to it?

Any responses to my questions are really appreciated.

phuoc le


The MFIS CD-ROM does show a few articles on the topic, so it's worth a computerized literature search. I don't think we're looking at a real complex situation here though. The baths require free cyanide (KCN or NaCN) in proportion to the metal content (this is known as 'the ratio'). At high pH (low H+) there is obviously very little HCN; at low pH (lots of H+) there is going to be a lot of HCN. HCN is a gas, so it comes out of solution. The operators are not exposed to it because cyanide baths are kept alkaline; make them acidic and the operators die.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey

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