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Provisional Data sheet Jan/1998
Following a request for assistance in stripping gold economically from copper wire that had been used for wiring up articles, I began by looking up guidelines for the stripping of gold in the general literature advice area. The suggestions mainly used strong cyanide electrolytically, or with additions of peroxide, which were unstable and dangerous in that they boiled over.
I then looked at how gold was recovered from ore etc. Here much lower concentrations of cyanide are used and aeration or controlled additions of peroxide were added.
Metal stripping requires a suitable solvent for the metal to be dissolved, and an oxidizing agent to supply the electrons to make the dissolution happen.
In the case of gold plating the proprietary approach is to use cyanide (as the solvent) in conjunction with organic oxidants, usually based on metanitrobenzoic acids, which are expensive, and although this is not a problem when reclaiming parts, for racks, wire, scrap etc this is unnecessarily wasteful.
I decided to try a number of experiments using the lower concentrations of cyanide similar to that used in gold recovery.
I found that electroplated gold dissolved quite readily in aerated solutions of cyanide below 10 g/l of potassium cyanide but very slowly above this limit.
(Sodium cyanide produces similar results to Potassium cyanide. 10 g/l of Potassium cyanide has an equivalent cyanide concentration as 7.7 g/l of sodium cyanide.)
Comparing concentrations of potassium cyanide it was found that at
1 g/l KCN stripping rate was slow
2 g/l fastest
5 g/l medium fast
10 g/l no strip
Oxidation with hydrogen peroxide was experimented with to find the optimum required to get stripping with minimum attack on the nickel underneath with a 5 g/l KCN solution.
0.5 ml/l stripped with no attack on bright nickel
1.0 ml/l was faster but stained bright nickel
2.0 ml/l faster but some etching of bright nickel
3.0 ml/l but slower with slight patchy attack on nickel
(Also with 10 g/l KCN and 2 ml/l of hydrogen peroxide stripped OK)
Thought best to use 0.5- 2 ml/litre of hydrogen peroxide in 5 g/l KCN
STRIPPING PRACTICE
In practice it is suggested that a plastic tank be set up with gentle well distributed air agitation.
Ideally have plastic mesh above a number of air bubblers to keep the work in oxygenated solution.
This helps with the supply of oxygen and importantly gives mixing and keeps fresh cyanide at the stripping surface, as with the low concentrations of cyanide used, the surface film soon dissipates which slows or stops stripping.
A fish tank agitator and bubblers from your local aquarium supplier is ideal as this gives fine air distribution or use a normal air coil with a large number of small holes.
Solution Makeup
Use either 5 g/l of Potassium Cyanide
i.e. (25 ml/l of STOCK SOLUTION 200g/l see under Maintenance
OR 4 g/l Sodium Cyanide
Stripping
Turn on agitation.
Add 0.5-1 ml/l of Hydrogen peroxide 50% or equiv. Don’t use an excess.
Strip
Turn off agitation when not stripping to minimize cyanide oxidation.
NB.Stripping will occur without the addition of hydrogen peroxide but with air agitation, or with hydrogen peroxide and no air, but best results are with both initially. Usually only air is then needed.
Maintenance
Make up a 200 g/l STOCK SOLUTION of cyanide (either potassium or sodium.)
Label and store correctly
To add 2g/l of cyanide add 10 ml/l of STOCK SOLUTION 200g/l.
Each 1 gram of gold dissolved will require approx. 0.66-. 75 gram Potassium cyanide
i.e. about 3.5 ml of STOCK SOLUTION
When stripping stops due to lack of cyanide add 10 ml/l of STOCK SOLUTION
If no stripping then, add 0.25 ml/l of Hydrogen Peroxide.
IT'S REALLY THAT SIMPLE
As there is only a low concentration of cyanide in the STRIP this will be consumed relatively quickly if there is a large surface area as with wire waste and so regular additions will need to be made to endeavour to maintain free potassium cyanide at between 1 and 5g/l.
It would be best if initially free cyanide analysis could be done to determine usage rate in relation to the quantity of wire or parts until the process is understood and operators get to understand that this is different to the normal commercial approach, where high concentrations mean that you can get away with little skill and maintenance, but these processes are costly.
If stripping is no longer practical, make up a new solution and reclaim the gold.
This is also very effective for stripping of parts. Remove as soon as all gold is removed.
THIS IS PRACTICALLY ALL PROFIT
NOTES
Keeping cyanide concentration low and particularly peroxide low will minimize the amount of copper and nickel that are also dissolved simplifying and minimizing reclaim costs.(although some copper cyanide accelerates the stripping rate). Usually peroxide need only be added for the initial stripping there after just use air.
If a lot of copper is exposed don’t add peroxide as this accelerates attack on copper. Preferably remove copper overnight or the cyanide will be used up and solution may become cloudy indicating no free cyanide in which case add 20 ml/l of stock solution which will clear the solution.
Turn off agitation when not stripping and only add peroxide (if required) when stripping to minimize cyanide usage.
Also occasionally move parts particularly with tight wire to make sure that fresh solution is able to get to the exposed areas.
Make small regular additions of cyanide
ANALYSIS
Free cyanide
Take a 10 ml sample add 50 ml cold water
Add 5 ml of 10% potassium iodide solution
Titrate with 0.1N silver nitrate until the first permanent cloudy precipitate
Titre x 1.3 = g/l free potassium cyanide
Maintain between 1 and 5 g/l
Above 10 g/l of free Potassium Cyanide stripping rate is much slower
Geoffrey Whitelaw
gpwhitel@gmail.com
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(addendum)
Gold stripping test at a Plating shop
The test started with 375 ml of 200g/l KCN to 15 litres water = 5g/l + 10 ml H2O2
I made maintenance additions of 30, 300, 100,100 ml of KCN while stripping about 16 Kg of wire etc.
Approx 800 ml of KCN = 160g was consumed which should have correspond to over 200g gold theoretically.
Some would have been consumed by oxidation and some with copper dissolution though.
In future would keep H2O2 down to a minimum as this increased attack on copper and when I left wire in overnight all the cyanide was consumed. It would be better with more even agitation as if I didn't move the solution and keep it aerated there were some spots where the gold didn't dissolve, so I have learned quite a bit. Thanks.
Stripping speed was about 30 minutes to take the majority off at room temperature.
I tried one piece of wire at approx 50 °C and it stripped in 5 minutes.
Stripping speed is about the same at this gold concentration as at the beginning which surprised me as most processes slow down but that is because the oxidant is consumed. Here air is the main oxidant.
Stripping of various parts, done after stripping all the wire, showed no dulling or attack on the bright nickel.
It is not saturated with gold and still stripping well, but guess from a practical point it probably has enough gold dissolved.
Hope you can find a use for this information.
Geoffrey Whitelaw
115 Esplanade East
Port Melbourne 3207
Vic. Australia
+61(0) 438717868
gpwhitel@gmail.com