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Stripping gold from aluminum without damage to the aluminum


An ongoing discussion beginning back in ...

May 9, 2009

Q. I work in the semiconductor equipment cleaning industry. We have been asked to remove gold from an aluminum vacuum pump impeller with minimal damage to the aluminum. The owner wants to re-use the impeller. The gold is not a plated coating. Abrasive removal is not suitable because it unevenly wears the substrate. Is there a stripping solution capable of dissolving the gold without etching the aluminum? The ones I know of have either high or low pH values and will attack aluminum.

Dave Watson
Cleaning company process engineer - Glendale, Arizona, USA

June 23, 2009

? I wonder why gold is coated on aluminum impeller! Are you sure it is gold and not TiN?

H.R. Prabhakara - Consultant
bangaloreplasmatek.com - Bangalore Karnataka India

June 24, 2009

I would be mildly amazed if it were possible as both acids and bases attack aluminum. As said before, I bet that it is TiN and not gold.

James Watts
- Navarre, Florida

June 25, 2009

Q. The pump is downstream from a gold sputter chamber.
It is not TiN. That is easy to remove from aluminum.


Dave Watson [returning]
- Glendale, Arizona, USA

July 31, 2009

A. The only chemical that does not harm Aluminium is Nitric acid. This acid will help you get rid of Gold from aluminium without damaging and you will also recover gold too.

Mustafa Gandhi
- Surat, Gujarat, India

July 31, 2009

A. Nitric does not remove thick gold, that takes cyanide or "aqua regia". Nitric will definitely attack high alloy aluminum like 2024 or 7075, with serious attack on weld joints.

James Watts
- Navarre, Florida

August 23, 2009

A. The gold can be removed anodically in strong sulfuric acid with little or no damage to the aluminum. Some alloys will possibly be pitted, but I have never had any problems with the alloys I have worked with. The original US patent on this is #2185858 - search for this in Google Patents. Although Al is not mentioned in the patent, gold can be stripped from most metals successfully using this method. The only problems I have had were with high nickel or cobalt alloys.

I would dilute concentrated sulfuric acid with 5%, by volume, distilled water (add the acid to the water!). Let it cool before you use it. Concentrated sulfuric will work, but a little water speeds up the stripping. The patent uses weaker sulfuric but, in my experience, that is more likely to pit. Most any metal can be used as the cathode, but I prefer mild steel. Just about any kind of wire can be used to suspend the part.

Start with zero voltage/amperage and slowly crank it up. At first, the amps will be high and the voltage will be quite low. As the gold strips, the amps will decrease and the voltage will increase. Keep turning it up to maintain the amperage. When the voltage is somewhere between 6 and 8, don't make any further adjustments. At some point, when all the gold is stripped, the amperage will automatically go to zero - this will tell you when it is finished.

With too high an amperage, the solution will get quite hot. When adjusting the rectifier, I wouldn't allow the solution temperature to go above about 110F. The hotter the solution, the more likely the Al will pit. Just about any volt/amp combination will work, but I would suggest trying the parameters that I have suggested, to reduce the possibility of pitting. Watch the temperature!

The gold dissolves only momentarily and then is reduced to gold powder. All the gold can be collected by first allowing it to settle to the bottom of the tank.

Chris Owen
- Nevada, Missouri, USA

September 2, 2009

A. Consider fine sand or glass bead blasting. Obviously this procedure will lead to an almost impossible recovery of the gold but it may salvage the impeller which seems to be of most importance.

Guillermo Marrufo
Monterrey, NL, Mexico

September 4, 2009

thumbs up sign You're right, Guillermo. I didn't think about blasting. I once did some experiments with steel shot peening to strip the plating so that the parts could be replated. The peening action flattened and stretched the plating laterally, the bond was broken, and the plating just completely fell off. Worked very well on every type of plating that I tried. Zero attack on the parts. The gold might be recovered by separating the shot magnetically.

Chris Owen
- Nevada, Missouri, USA

September 12, 2009

A. Hi, Chris,
The problem using steel shot is that it imparts a strong peening action which may distort thin sections. That's why I was suggesting sand or glass at the expense of gold. Best regards,

Guillermo Marrufo
Monterrey, NL, Mexico

June 6, 2018

A. Use Cyanide and a stripping agent to take gold from aluminum.
3 lbs of NaCN per 25 gallons, 115-120 °F

Cyanide will eventually attack the aluminum but the gold will dissolve well before.

Victor, David
- Knoxville, Tennessee, USA

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