What chemicals or processes are used to Strip gold plating?

A.

A. A straight KCN solution heated will strip Au from substrates. The Orostrip from Technic has an accelerator for KCN solutions.

Another thin film supply house on the East coast had a similar product but I forget the name of the company (it has been years since I've dealt w/ this).

Another way to strip Au is to use a heated solution of KI:I:H20. This chews Au quickly but is very corrosive to many metals (and you can see the Iodine fumes rise off the heated surface).

A. Every gold process supplier sells a gold stripper. If you put it on chances are you'll occasionally have to take it off too!

I have used LeaRonal Super Strip 100 for years, but the Orostrip works good too.

Most of the strippers are based on a solution of sodium or potassium cyanide and an organic oxidizer like meta-nitro benzoic acid.

A. Enthone-Omi has a product named Circuitprep NX to which you have to add KCN.

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A. I would expect that your fume is something other that HCN, but in any case, you should use a pump to agitate instead of air. I limit air sparging to rinse tanks and tanks with very dilute chemistry. Air sparging always increases emissions.

A. Hello, Doug. In a sufficiently alkaline cyanide solution, HCN is just not going to be present either in the solution or in an emission because HCN is an acidic form of KCN. While it is true that CO2 can behave as an acid, it is neutralized by the alkaline species in your solution long before the solution becomes acidic enough to allow the HCN to exist. Since it doesn't form in the solution (except perhaps as a very short-lived intermediate), it is not a potential emission as a vapor. All you need to do to ensure that you don't evolve HCN is to check your pH at regular intervals. Also, KCN will not have an emission because it does not form molecular vapors from aqueous solutions. What you do have is ammonia [on eBay or Amazon]. ammonia is generated from the natural hydrolysis (reaction with water) of all solutions that contain free (non-complexed) cyanide. Basically, cyanide reacts (breaks down) with water and ultimately forms ammonium and formate ions slowly over time. The reaction rate for cyanide hydrolysis is only a function of time and temperature. Some of the ammonium ions in solution (NH4+) will further react to form an emission of ammonia (NH3) gas. The amount of this emission is related to the pH, the temperature, and air agitation. Increasing any of those 3 things will increase the emission, but the reaction still happens with no additional heat and no sparging. In general, air agitation increases emissions for any chemical that emits a vapor though not all emissions are harmful (example- water vapor). Evaporation does concentrate the solids in the mixture, but the water holds an amazing amount of KCN before it will become saturated and settle out on the bottom. Part of your chemical solution control should be to monitor and maintain the solution level for this reason. I hope this helps to clarify things.

Jon, Thank you very for your response and insight. I have a regulator wanting me to provide emissions of cyanide compounds from our strip tanks. He directed me to use the evaporation equations in AP-42. I was beside myself knowing that solids do not evaporate with water from an open tank, unless agitated and carried away as bursting bubbles, yet he insists we provide cyanide compound emissions.

Our sparging is experimental to keep the chemistry mixed and the appearance of HCN was not expected, especially since pH is maintained above 12.

It is always nice to confirm with others to ensure I was not missing something. Thanks again.
Doug

A. Hi Doug
Jon has covered the situation really well. Just a couple of practical suggestions re stripping is that you do need some oxidising potential to strip. Suggest you use an aquarium type bubbler to aerate, not vigorous air, and/or alternatively add 0.2-0.5 ml per liter of hydrogen peroxide well diluted.

A. Hi Tom. So nice to hear from you! Between a general diminishing of our industry and father time claiming some of our mutual friends, it is starting to get a bit lonely for this old-timer :-)

Most gold strippers are cyanide-based but there are some cyanide-free ones. Gold Touch [a finishing.com supporting advertiser] offers an electrolytic one you could look into at goldtouchinc.com/product/tivastrip-free-quart/

Stay well and stay in touch. old friend :-)

Luck & Regards,

A. I developed a simple process for stripping gold where you use a very low concentration of K or NaCN (2-5g/l) and gentle air agitation. The procedure is available in the finishing.com on-line library at
https://www.finishing.com/library/whitelaw/goldStripping.html