I'm a big chemistry nerd and gold refining has long been on my radar, but I was kind of intimidated by the cost and risk involved. After a brief set of experiments in 2017, I set gold refining aside and didn't go back to it until about a month ago. Lately I've gotten back into it in a big way and have been devouring as much as I can on the subject while refining some karat and gold-filled scrap from eBay. I know I won't turn a profit from eBay purchases, but for now it's all about learning the process, while trying to keep losses as small as possible.
My first drops were kind of unimpressive - my first button of 5 g was pretty heavily contaminated and I had lost at least 2 g to a boil-over onto the patio after I added sulfamic too quickly, which was itself an attempt to correct excess nitric. Two of my subsequent refinings involved processing the paper towels soaked with sodium carbonate solution which were used to wipe up the spill, netting about 0.8 g. Then I started processing more batches of mostly karat scrap and learned the importance of washing with HCl after precipitation - that made a pretty big difference although there was still some discoloration and frostiness. I precipitated these drops with ferrous sulfate, figuring it would make sense to use that for the dirtier solutions for somewhat better separation at the expense of a little iron contamination, and then later combine the gold and do a second refining using SMB.
Yesterday I had time to do this second refining on a total of 42.8 g of impure gold from the previous batches. I cornflaked it and dissolved it in AR on a hot plate, then filtered, added a little sulfamic to deal with a slight excess of HNO3, diluted with distilled water (at which point a bunch of AgCl appeared), filtered again, and then added to a large beaker for the drop. I waited for it to cool to take advantage of the better particle size at low temps, then added an excess of SMB. The gold came crashing out quickly and had settled into a nice compact sediment within an hour, with a negative stannous test on the rest of the solution. I decanted the solution off and rinsed twice with hot HCl, which lightened the gold up considerably, then rinsed several times with distilled water, put it in a Buchner funnel and filtered, adding another HCl rinse followed by more water rinses.
I put it in a melt dish, melted with MAP gas, and poured into a round mold. Other than a little bit of spillage on the edge of the mold it went really well. Total weight is 41.738 g, just a little more than 1 g less than the original impure material, which could then have been less impure than I thought.
When I looked at it, I was stunned by how beautiful and pure it looked. Because of how impure my first attempts were, I was expecting to have to refine it again and was planning to use oxalic acid for the third drop. But I got such nice patterns around the edges when it froze, two vent pipes in the middle, and dazzling reflectance that I think it’s already highly pure. I think I’ll find somewhere to get it looked at by XRF and decide after that whether I want to go ahead with another refining or sell it as is.
I’m really glad GRF exists and has so much useful information. I certainly couldn’t have made anything like this without having spent several weeks looking through the forums absorbing all your insight!
My first drops were kind of unimpressive - my first button of 5 g was pretty heavily contaminated and I had lost at least 2 g to a boil-over onto the patio after I added sulfamic too quickly, which was itself an attempt to correct excess nitric. Two of my subsequent refinings involved processing the paper towels soaked with sodium carbonate solution which were used to wipe up the spill, netting about 0.8 g. Then I started processing more batches of mostly karat scrap and learned the importance of washing with HCl after precipitation - that made a pretty big difference although there was still some discoloration and frostiness. I precipitated these drops with ferrous sulfate, figuring it would make sense to use that for the dirtier solutions for somewhat better separation at the expense of a little iron contamination, and then later combine the gold and do a second refining using SMB.
Yesterday I had time to do this second refining on a total of 42.8 g of impure gold from the previous batches. I cornflaked it and dissolved it in AR on a hot plate, then filtered, added a little sulfamic to deal with a slight excess of HNO3, diluted with distilled water (at which point a bunch of AgCl appeared), filtered again, and then added to a large beaker for the drop. I waited for it to cool to take advantage of the better particle size at low temps, then added an excess of SMB. The gold came crashing out quickly and had settled into a nice compact sediment within an hour, with a negative stannous test on the rest of the solution. I decanted the solution off and rinsed twice with hot HCl, which lightened the gold up considerably, then rinsed several times with distilled water, put it in a Buchner funnel and filtered, adding another HCl rinse followed by more water rinses.
I put it in a melt dish, melted with MAP gas, and poured into a round mold. Other than a little bit of spillage on the edge of the mold it went really well. Total weight is 41.738 g, just a little more than 1 g less than the original impure material, which could then have been less impure than I thought.
When I looked at it, I was stunned by how beautiful and pure it looked. Because of how impure my first attempts were, I was expecting to have to refine it again and was planning to use oxalic acid for the third drop. But I got such nice patterns around the edges when it froze, two vent pipes in the middle, and dazzling reflectance that I think it’s already highly pure. I think I’ll find somewhere to get it looked at by XRF and decide after that whether I want to go ahead with another refining or sell it as is.
I’m really glad GRF exists and has so much useful information. I certainly couldn’t have made anything like this without having spent several weeks looking through the forums absorbing all your insight!