Tammyjefferywilliam said:
What is the correct method? If you mean electrolysis then let me tell you I have tried to find out how to do it, but nobody seems to want to tell me what to do with the black powder that builds in the sulphuric acid and how to retrieve the gold. I was merely trying an experiment to find the best method for me.
Here's how I would probably do it, if I only had a gallon or less of solution:
(1) Using a face shield and gloves for all of the following, pour all the solution into a suitable settling container, not too wide and not too short, preferably glass. A beaker is a good shape. If any solids remain in the original stripping container, rinse them into another beaker, carefully with a lab squirt bottle - when you mix the rinse water with the sulfuric on the sludge, it will get hot and could splatter.
(2) Put something underneath the back of the beaker to make it tilt towards you - that way the powder will settle on the front bottom of the beaker. Make sure the spout on the beaker faces towards you. Let the black powder settle. Often, the solution itself will be black and that makes it hard to see when it's settled. Turn off the lights and try shining a bright flashlight through the backside of the beaker. In any case, I would assume that, after settling overnight, it's settled about as much as it's going to.
(3) Slowly, steadily, and carefully pour the settled solution back into the stripping vessel (cell), trying hard to pour off most everything except the solids. When the solids start pouring off a little, stop pouring. The solution is then reused in the cell (after making up for volume loss with fresh sulfuric. Any solids accidentally poured off will be picked up next time.
(4) Estimate the volume of solids (sludge) and, into a separate clean container, add about 6-8 times this volume of water. Transfer all the sludge into the water by pouring and rinsing with a squirt bottle. Add the sludge from the stripping vessel that you picked up in step(1) above.
(5) After completely cool, filter it all through the same paper. I usually let it settle first, somewhat, and then carefully pour off the solution into the filter. When all the solution is filtered, I transfer the sludge into the same paper. I would then rinse this, say, 3 times. After the cake is just damp, I would transfer everything, including the paper, into a clean beaker.
(6) At this point, the paper contains most all of the gold, probably some copper, maybe some silver, and likely some dirt. The sludge must ultimately be dissolved in aqua regia (AR) under a fume hood. An intermediate nitric acid leach could be used but usually, just going straight into AR will work fine. First, cover the sludge with HCL and heat to about 140-150F (first sign of fumes or steam). When hot, stir with plastic or glass stir rod to slurry it a bit and then add a very small amount of nitric. This addition depends on the amount of sludge. For a very small amount of sludge, I'm talking a few drops. For, say, 50-100ml of fairly well dewatered sludge, I would add 2 or 3ml of nitric. The solution will fizz and foam. When the reaction dies down, give it a stir and add a little more nitric. Repeat until a small addition gives no reaction. Stop adding nitric. The whole thing is visual.
(7) Estimate the final volume and add 3 volumes of tap water and a couple ml of sulfuric acid to it. Stir. Filter until crystal clear. Transfer clear solution to a very clean beaker and drop out the gold with SMB.
Chris
