Mcnew32(Ag) said:
Tried to search for cyanide leaching through the search bar. If anyone can link me to a good thread for cyanide leaching of gold plated material I would appreciate it.
I recently sent this as a PM to a member. Before attempting this, make sure you have read up and understand how to work safely with cyanide. There are some good websites with this info.
You might read these threads first.
http://goldrefiningforum.com/phpBB3/search.php?keywords=plunger+zinc+cyanide&terms=all&author=goldsilverpro&sc=1&sf=all&sr=posts&sk=t&sd=d&st=0&ch=300&t=0&submit=Search
About all I can do is tell you how I do it. I always tumble the gold plated parts (pins, fingers, etc) in a portable cement mixer using sodium cyanide plus a small amount of 30-35% hydrogen peroxide. No other chemicals. It's very fast and no excess oxidizer will be present to cause problems in precipitation.
I prefer a steel mixer because it's easier to cover the opening. When tumbling, drops of the solution will occasionally fly out. To prevent this, I use 4 magnets, one on each corner, to hold a sheet of thin flexible plastic over the opening. I use 2 of the very strong neodymium magnets (from hard drives) and 2 weaker horseshoe Alnico magnets. When I want to open the mixer, I stop it so the neodymium magnets are at the top, remove the 2 Alnico magnets, and then fold the plastic up over the top, leaving the 2 neodymium magnets in place securing the plastic. This is much easier to do than to explain.
Using this process, I have stripped 6 drums of material in an 8 hour day, by myself.
(1) Add 1.5 to 2 gallons of hot (about 50-60C) tap water to mixer.
(2) Add parts to be stripped. You can strip quite a lot at one time - say, 5-10 kilos, or even more, depending on how bulky the parts are. The pile of parts added can stick up out of the water - everything will get wet as you tumble it. You'll have to experiment with the amount of parts you can run at one time.
(3) Add about 100-200g of Sodium Cyanide (NaCN) and tumble until dissolved - about 1 minute.
(4) Add about 50ml of the hydrogen peroxide (H2O2). Cover the mixer opening and tumble. The H2O2 will heat the solution even more due to its reaction with the cyanide.
(5) After about 2 minutes, stop the mixer when the strong magnets are on top, remove the lower weak magnets. and fold the plastic over the top. With plastic gloves on, lift a handful of parts, wash with a little water from a squirt bottle, and see if they are stripped. If you see white nickel and/or uniform pink copper color on all the parts, the stripping is complete. If there is still yellow gold present or if you see a black smut (gold that's dissolved and cemented back onto the copper) on the parts, add about 50g of NaCN + about 20ml H2O2, and run for about 3 more minutes. Repeat. If, after doing this a total of 3 times, there is still gold or smut present (very rare - maybe 1 out of 25 batches), start with a fresh solution on the same parts.
Note: Essentially all gold plating on electronic material has a layer of nickel plating under the gold. If the nickel is thick (or, if the base metal is magnetic towards a weak magnet), the stripping will likely be finished in 1 to 2 minutes. If the Ni is very thin, the abrasion caused be the tumbling can remove it and expose the copper. In that case, there is a battle between the gold wanting to dissolve and the dissolved gold wanting to cement back onto the copper. When copper is exposed, the stripping will go slower.
(6) When the parts are stripped, carefully tilt the mixer and collect the solution in a 5 gallon plastic bucket without pouring off any parts. Put 2 gal of clean water in the mixer, tumble for a minute or so to rinse the parts. The same rinse water can be used for 3 or 4 batches.
When finished, transfer all solutions and rinses into a suitable sized plastic container - 5 gal bucket, open top 55 gal drum, etc. Place the container high enough so you can siphon all of it imto another container. While dropping the gold, stirring is quite important. A prop type electric stirrer is nice, but the same results can be achieved with a homemade "plunger" stirrer. To tell how much zinc to use, you must be able to see the color of the gold/zinc powder. This requires a lifting motion while stirring the solution. A 5" dia. plastic disk mounted of the end of a plastic pipe will work great, with an up and down stroke. Go slow on the upstroke or you'll slosh the solution on you.
Precipitating the gold:
(1) Weigh out about 2g of 325 mesh "zinc dust" for each gram of expected gold. This is only an approximation - it may take more or less. Note that the zinc will also drop any silver or copper that has dissolved. Before adding, you must break up any zinc clumps that have formed. I do this by putting the zinc through a fine kitchen sieve.
(2) While stirring, add about half of the the weighed, de-clumped zinc. Stir for a minute or two. As you lift up the powder, look at the color. At first, the powder will be brown (mostly gold and/or copper). As you add more zinc, the color will become a uniform light gray color. This happens when all the gold and copper have precipitated and there is an excess of undissolved gray colored zinc. This is what you want to see. At that point, stop adding zinc and stir for 5-10 more minutes.
(3) Obtain a couple of 5 pound (or, so) zinc bars, drill a hole near the top, and hang them in the solution using some insulated house wire. Let the solution settle overnight. The purpose of the zinc bars is to prevent any gold re-dissolving.
(4) Siphon off the solution into a like sized container without disturbing any of the settled solids.
(5) Filter the solids and rinse them 4 or 5 times with hot water.
(6) Transfer the solids and the filter paper to an adequate leaching vessel (I use a 5 gal bucket). UNDER A FUME HOOD, cover the solids and paper with water. Add very small increments of nitric acid, stirring slowly in between additions. When an addition of nitric produces NO reaction, stop adding nitric.
(7) Filter, rinse well, dissolve in aqua regia, precipitate pure gold with SMB, filter, rinse well, dry, and melt.
Note: When dissolving the Au powder in aqua regia.
(1) Transfer filtered gold and paper to a large beaker.
(2) Cover with HCl. Heat to about 60C under a fume hood.
(3) Add a few ml of nitric. There should be an immediate reaction. When the reaction subsides, give it a short stir and add another small amount of nitric. Repeat until a small addition produces NO reaction. Stop adding nitric.
