cbarney522
Member
I want to pass on some recent success I have had with an abrasive method for recovering gold from gold-plated metalware.
On March 22-23, 2014, I cut up 3.75 lbs of gold plated metalware using a stud punch. These were gold-plated bowls, cups, vases, and trays. I wanted to experiment with an abrasive recovery method because my experiments with reverse electroplating have not been terribly successful on large items. I also wanted to evaluate how well this process might work on silver plated metalware.
Some of the items were brass, while others appeared to be stainless steel. I cut up the items into 1.5 inch diameter circles. I then tumbled them in a Thumler Model B tumbler with several pounds of angle cut ceramic cylinder media (aluminum oxide) and water for 16 hours. Not all of the gold was scoured off; several of the pieces had designs stamped into them, and the crevices still had gold in them. But I estimate overall that this process abraded off around 80% of the gold. I poured off the dirty gray liquid and sprayed everything repeatedly to rinse off the powder. I let the powder settle overnight in a 5-gallon bucket, poured off the clear water, and added hydrochloric acid to the powder to digest any aluminum, zinc, iron, or tin. I filtered what was left and cleaned it with fresh water. I then added nitric acid to the remaining powder to digest any residual copper or nickel. I poured off the pregnant liquid and then washed everything again with clean water. Finally, I applied aqua regia and let it react with any gold in the powder. There was a fair amount of ceramic abrasive dust that did not react, but was easily filtered out. After letting everything react, I poured the pregnant AR through a filter. I then evaporated the AR on a hot plate to drive off the nitric acid. I used a small cast iron pan filled with sand, with a 600ml glass beaker containing the AR. I evaporated it down to nearly a crystal, reconstituted it with hydrochloric acid, and repeated two more times. I then added distilled water and evaporated it down again one final time, and then reconstituted with more distilled water.
What was interesting was that gold started to precipitate out of solution on its own. I ended up with two large flaky ‘scabs’ of gold, plus a lot of gold floating on top. I filtered off the precipitated gold, which was mixed in with some unidentified crystals that refused to dissolve. I put these in a small beaker of AR to redigest. On the main solution I used sodium metabisulfite to precipitate the gold. It took about 4 hours to completely settle. I filtered off the mud, washed several times with distilled water, and placed into a small glass bowl to dry at 350 in the oven. I used a small paint brush to ensure that I get every last speck of gold into the crucible. I then melted using a MAPP gas torch. The final result was a nice looking gold nugget that weighs 0.7 grams. I had to soak it in sulfuric acid overnight to remove traces of flux.
I still have a small batch of AR to process. This was the precipitated gold that was mixed in with crystals, along with some more gold that I wiped from the inside of the rinse bucket. I guess that I’ll get another 0.3 grams of gold from it. I’ll report back when I’m finished.
But for now, if anybody is wondering, I managed to get a 0.7 gram gold nugget from 3.5 lbs of gold plated metalware. I know that in the U.S., gold plate has to be a minimum of 7 millionths of an inch thick, and on average, the metalware tends to be around 1/50 of an inch thick. Before processing, I estimated that the percentage of gold in my batch was 0.07% by volume, and 0.14% by weight. Therefore, I expected to extract around 2 grams of gold. I already know that I was only able to abrade 80% of the gold in the tumbler, and I estimate I probably lost 20% in the refining process. On a larger scale, and using a variety of different tumbling media, and with more careful refining, it may be possible to obtain half a gram per pound of gold plated metalware.
Update March 27, 2014 - I precipitated the last little batch of AR and re-melted the first nugget along with the new gold powder. I ended up with 0.75 grams altogether. I was expecting to be closer to 1 gram, but the scale doesn't lie. For other refiners out there, if you are especially careful with recovery, filtering, and refining, I would expect that you would be able to extract 1/4 gram per pound of gold plated metalware. On large scale of hundreds of pounds, you may be able to get 1/2 gram per pound.
On March 22-23, 2014, I cut up 3.75 lbs of gold plated metalware using a stud punch. These were gold-plated bowls, cups, vases, and trays. I wanted to experiment with an abrasive recovery method because my experiments with reverse electroplating have not been terribly successful on large items. I also wanted to evaluate how well this process might work on silver plated metalware.
Some of the items were brass, while others appeared to be stainless steel. I cut up the items into 1.5 inch diameter circles. I then tumbled them in a Thumler Model B tumbler with several pounds of angle cut ceramic cylinder media (aluminum oxide) and water for 16 hours. Not all of the gold was scoured off; several of the pieces had designs stamped into them, and the crevices still had gold in them. But I estimate overall that this process abraded off around 80% of the gold. I poured off the dirty gray liquid and sprayed everything repeatedly to rinse off the powder. I let the powder settle overnight in a 5-gallon bucket, poured off the clear water, and added hydrochloric acid to the powder to digest any aluminum, zinc, iron, or tin. I filtered what was left and cleaned it with fresh water. I then added nitric acid to the remaining powder to digest any residual copper or nickel. I poured off the pregnant liquid and then washed everything again with clean water. Finally, I applied aqua regia and let it react with any gold in the powder. There was a fair amount of ceramic abrasive dust that did not react, but was easily filtered out. After letting everything react, I poured the pregnant AR through a filter. I then evaporated the AR on a hot plate to drive off the nitric acid. I used a small cast iron pan filled with sand, with a 600ml glass beaker containing the AR. I evaporated it down to nearly a crystal, reconstituted it with hydrochloric acid, and repeated two more times. I then added distilled water and evaporated it down again one final time, and then reconstituted with more distilled water.
What was interesting was that gold started to precipitate out of solution on its own. I ended up with two large flaky ‘scabs’ of gold, plus a lot of gold floating on top. I filtered off the precipitated gold, which was mixed in with some unidentified crystals that refused to dissolve. I put these in a small beaker of AR to redigest. On the main solution I used sodium metabisulfite to precipitate the gold. It took about 4 hours to completely settle. I filtered off the mud, washed several times with distilled water, and placed into a small glass bowl to dry at 350 in the oven. I used a small paint brush to ensure that I get every last speck of gold into the crucible. I then melted using a MAPP gas torch. The final result was a nice looking gold nugget that weighs 0.7 grams. I had to soak it in sulfuric acid overnight to remove traces of flux.
I still have a small batch of AR to process. This was the precipitated gold that was mixed in with crystals, along with some more gold that I wiped from the inside of the rinse bucket. I guess that I’ll get another 0.3 grams of gold from it. I’ll report back when I’m finished.
But for now, if anybody is wondering, I managed to get a 0.7 gram gold nugget from 3.5 lbs of gold plated metalware. I know that in the U.S., gold plate has to be a minimum of 7 millionths of an inch thick, and on average, the metalware tends to be around 1/50 of an inch thick. Before processing, I estimated that the percentage of gold in my batch was 0.07% by volume, and 0.14% by weight. Therefore, I expected to extract around 2 grams of gold. I already know that I was only able to abrade 80% of the gold in the tumbler, and I estimate I probably lost 20% in the refining process. On a larger scale, and using a variety of different tumbling media, and with more careful refining, it may be possible to obtain half a gram per pound of gold plated metalware.
Update March 27, 2014 - I precipitated the last little batch of AR and re-melted the first nugget along with the new gold powder. I ended up with 0.75 grams altogether. I was expecting to be closer to 1 gram, but the scale doesn't lie. For other refiners out there, if you are especially careful with recovery, filtering, and refining, I would expect that you would be able to extract 1/4 gram per pound of gold plated metalware. On large scale of hundreds of pounds, you may be able to get 1/2 gram per pound.
