anands138 said:
Hey everyone,
I have a scrap gold bar of 10 kg which contains 40% silver and 60% gold. purity 14k.
I would like to know the best process to refine the scrap gold bar and recover both the silver and gold from it.
Thank you
Anand
The karat of this is higher in gold than plumb 14K (58.33%). I've never seen jewelry that runs higher than plumb and have rarely seen it that is plumb. It's usually less than plumb and is often from 1/2 to 1 karat lower. The mix you describe would be green gold. Most often, along with high silver, green gold contains a little copper and/or zinc.
Assuming your figures are correct, here's the process. This is only a general description. The details can be found elsewhere on this forum and in the Hoke book.
As it sits, the gold won't dissolve in aqua regia because the silver is greater than 10%. The silver won't dissolve in nitric because the gold is greater than 25%. Therefore, the best way would be to melt it with an additional 14kg of silver (inquart it). This would reduce the gold to 25%. Then, shot it and dissolve the silver completely with 50/50 nitric acid. The gold residue is purified by separating it from the nitric solution, dissolving it in aqua regia, and dropping the gold with a sulfite (usually). The silver is collected from the nitric solution by cementing it on pure copper slabs or bus bar. With proper rinsing, the silver should be about 99% pure. The silver can then be further purified by melting and casting it into proper sized bars and running it through a silver cell. If the silver will be re-used for inquartation of future lots, purification in a silver cell isn't necessary.
To dissolve the 18kg of silver, it will take about 22 liters of 70% nitric acid and 22 liters of distilled water. To dissolve 6kg of gold, it will take about 23 liters of hydrochloric and 5 liters of 70% nitric. These acid figures aren't exact and are dependent on such things as your setup and the acid strengths. It will take about 6kg of SMB or sodium sulfite to drop the gold. To purify the 18kg of silver, it would take about 8-10 days, 24/7, in a 5 gallon Thum silver cell. You would need a fair sized crucible furnace, molds, tongs, furnace gloves, and other furnace equipment. You would need dissolving containers - probably stainless for the nitric and glass for the aqua regia (I would use 4000ml beakers in 5 liter pyroceram Corning Ware dishes). There are a lot of other incidentals you will need, like hot plates an a stainless shotting container. And, of course, you would need a good fume hood, furnace exhaust, knowledge and expertise. Are you sure you're up to this?