Contamination of gold plating bath, removing contaminants

A. Hi Ngoi. There is a good discussion on that subject on this thread, but in case you miss it ... In gold plating, a gold strike not only improves adhesion, but it also serves as a garbage collector, trapping those contaminants in a weaker, less expensive, more disposable solution rather than letting them contaminate your concentrated and expensive gold plating solution.

Luck & Regards,

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A. Mr Hendrickson,

The nickel present in your solution will more than likely be the complex: nickel tetracyanide. This is a rather stable complex and I have had similar problems with my silver plating bath in the past. I have been quoted a figure of 30 g/liter as the top spec limit for this complex and achieved a 100% pure silver deposit at this level of nickel tetracyanide (an orange coloured complex).I know from experience of no safe or effective way of removing, or 'breaking', this complex or even whether 'dummying' your bath at LOW current density would be effective. As such my advice would be to RENEW this bath if practical or economical if you are indeed reaching this critical threshold of 30 g/l. I hope this helps.

Good luck!

A. As you have no free cyanide in an gold electrolyte, you won't get an Nickel-cyanide complex. One way for removal is to use an cation-ion exchanger, which will remove the nickel cation , but not the Gold-cyanide complex which is an anion. Your supplier of your bath or any other supplier of gold electrolytes should be able to provide you with special cation ion exchanger for this job.

A. We used to do a lot of lead frame plating with pure gold during the 1970's. The gold bath was a so called neutral bath that operated at pH 6-7 and 160 deg F. No free cyanide but the gold was added as PGC. When the sum of the concentrations of nickel, iron and copper totaled 50 ppm we would treat the bath with a stoichiometric amount of sulfide. I remember that it was a polysulfide and would stink quite a bit. But it did precipitate out all of the impurities and allowed us to continue to use the bath till the specific gravity got too high.

The polysulfide was available from ACR which is now part of Technic. You should check with them and see if it is still being sold.

A. Have you considered asking the supplier of your gold plating chemistry?

A. Graciela,
Best way is to control and stop impurities entering gold plating bath.
Organic impurities: Organic breakdown products of additives,together with drag-in of wetting agents from previous cycles(cleaning/acid dip/plating baths)
Use a strike/flash bath with minimum gold/ltr prior to main bath.This also will prevent impurities entering to the main bath
Removal: Activated carbon treatment- Heat the solution 60 degree> Add activated carbon(1-2 grm/lt) > Stir > Filter within 45 minutes (use vacuum filter)

Metallic impurities:Lead is the main enemy.
Most of other metals like Copper,Nickel,silver etc will co deposit with gold and will get removed though the characteristic of the deposit will change.

Atomic Absorption Spectrometer ⇦ this on eBay or Amazon [affil links] is the best equipment to detect in ppm level of these impurities.

The best approach to the problem is to prevent impurities entering the bath.
In case of severe metallic contamination it will be more economical to dump the bath and make fresh bath.

A. You did not state the electrolyte so I will assume it is citric or phosphoric. In either case the gold is probably present as potassium gold cyanide which is stable down to pH 3.1. or 3.2. When the gold is plated out of the complex the cyanide (in that acid medium) forms a long chain organic polymer which lowers the electrical plating efficiency. At some point this polymer must be carbon treated out using sulfur free powdered activated carbon.

This should be the only excuse for organics to be there.

Any other organics or any inorganics (in a pure soft gold solution) must be dragged in with sloppy procedures.

A. Use AA machine (HPLC machine) to check PPM of metallic contamination

A. You cannot!! Bite the bullet, send it in for refining. When you start back up, use a gold strike as a "sewer" to catch the copper so that your expensive plating solution only "sees" gold and stays pure.

A. Hello Chiou. I think you should start with plating a simple Hull Cell panel with each bath to try to get a good profile on what is going on.

Regards,

A. Hi Vasanth.
Pat Mentone has said on this thread that precipitation of contaminants with polysulfide just might be worth trying, but Trevor Crichton has noted on other threads the common sense precaution that analysis must always precede any attempts at purification. On the other hand, Robert Probert says forget it, it's just not practical to remove copper from a gold plating bath.

TK Mohan and other readers say here and elsewhere on the site, and are supported by similar suggestions in the literature, that the correct resolution is to do everything possible to prevent contamination, including using a gold strike bath first as a disposable 'garbage collection' to help keep contaminants out of your plating bath.

General advice from me from posting a quarter million Q&A's on this site for 3 decades, is that begging strangers to help us isn't nearly as effective towards getting a conversation started/restarted as starting the conversation yourself. For example, although you don't know how to remove copper, it seems that you probably do know some of the answers to Duho Rhee's other questions, and could help him and other readers, while getting the conversation restarted for your own needs, by addressing the portions of his question that you can answer. Good luck!

Regards,