I stumbled upon this oldie (but goodie) from a 2007 post by GoldSilverPro, while searching chelating agents:
and in a follow up by GSP:
Worth repeating. This post is already long enough so I'll save my comments/questions for later.
goldsilverpro said:In 1969, I was head chemist for a large refinery in L.A. Our Wohlwill Cell (gold purification cell) solution was often too contaminated with base metals and required frequent solution changes, in order to maintain high purity gold. I started experimenting with ways to extend the life of the solution. We had a well stocked lab and had quite an assortment of chelating agents. Most were variations of EDTA. Chelate comes from the Greek word Chelos, which means "claw". It grips metals in several places, like a claw, and holds them in solution. I figured that, if I added EDTA to the solution, it would chelate the base metals and prevent them from plating out with the gold.
When I added the specific type of EDTA to asample of cell solution, all of the gold dropped out. Surprise, surprise. For about a hour, I was very disappointed. I really thought it would work. Then, it dawned on me that I might have a new precipitating agent for gold. I started doing lots of experiments and maximized the process. We melted several batches of gold dropped from heavily contaminated aqua regia, under pristine conditions, and sent off samples to be analyzed for all impurities. To my amazement, none were under 99.99% pure. Some approached 99.999%, believe it or not. Some of these aqua regia solutions were intentionally contaminated with all of the platinum group. Still, the gold was above 99.99% pure, with only one drop. I'm not saying that the company always shipped 99.99% gold, because they could introduce impurities during the melt. I am saying that the powder was most always 99.99%.
None of the company experts could figure out why it worked. Later, I concluded that formaldehyde, a common gold precipitant, was released when the EDTA product neutralized the aqua regia. You could smell it. The base metals were tightly held into the solution and didn't come out with the gold. I never could figure out what happened with the silver and platinum group but, somehow, they were held into the solution. The company pretty much shut down the Wohlwill operation and went with my invention for everything. We called it EZ-Drop. I was a hero based on a failed experiment. The company was running about 10,000 ozs/month and ran the process for several years, until they were bought out by a big conglomerate.
When I left, I continued using it until about 1980, although I always kept some around for gold containing platinum group. From the late '80's though the mid '90's, when I owned my last refinery, I had no need for EZ-drop. I specialized in recovering 80/20, gold/nickel braze from military and commercial jet engine parts. Another reason I stopped using this product was that chelating agent wastes, with their tightly bound heavy metals, are very difficult to waste treat. The EPA hates them. Also, the EDTA product was expensive. In 1969, it cost about $650 per drum, of which, under optimum conditions , you could drop about 3000 ozs of gold.
and in a follow up by GSP:
goldsilverpro said:Use Dow's Versonal 120 to drop the gold and you'll always get 9999. It's an EDTA derivative that chelates the base metals and prevents their precipitation. It's my invention, by the way. After I saw it worked, it took a couple of months to perfect it. I'm including it as a chapter in my book.
Worth repeating. This post is already long enough so I'll save my comments/questions for later.
