Harold,
I think you knew that I would have to put in my 2 cents worth on this subject.
There are several ways to successfully refine karat golds. Let me preface this by saying that I truly believe that your method of inquartation and, finally, boiling down is the absolute best method for the members of this forum to use. The reason being is that, if they follow your instructions, this method will work each and every time. For this reason, there will be less frustration for the beginners. Also, since you had to satisfy your customers, I think that your choice of method was the best. However, since the gold I worked was already mine, I never inquarted or boiled down and never will. And, I would bet that our final products were equally pure. Also, I have very successfully used the "minimum amount of nitric" method for 30 years and I don't think you would put me into the category of a hobby refiner.
To everyone else, here's a little of my history and the history of this method:
(1) My first true experience in big time refining started in 1968, when I was hired as Senior Chemist for a new refinery in LA. At that time, this was the largest gold refinery West of the Mississippi. This same company was also the world's largest seller of precious metals plating systems. I was in charge of manufacturing the plating solutions, routinely analyzing and troubleshooting plating systems for our some 1500 customers, assaying refining lots (about 100 fire assays per day), and developing refining systems. Since we were using the refined gold to make plating salts, the necessary gold purity was 9999. At first, the company had big problems producing this purity. They were using AR, ferrous sulfate or SO2, and Wohlwill cells. They didn't want to inquart or boil down because of the time involved and the additional dissolving equipment that was needed.
For about the first six months, our gold was only pure enough to sell - about 9995. To make the plating salts, we had to buy expensive 9999 gold on the open market. Most of the gold we were refining (usually powder) came from processing electronic materials and the workers used premixed AR. I eventually discovered a unique chelating agent that would drop 9999 gold. It actually worked better with a lot of extra nitric in the AR and it chelated the base metals, kept them in solution, and prevented them from contaminating the precipitated gold. This eliminated any need for inquartation, boiling down, or Wohlwill cells. At that point, my method was adopted and was used to refine hundreds of thousands of ounces of 9999 gold, over a period of several years.
The chelating agent was expensive, however, and, it could have been eliminated or reduced, had I known about the "minimum nitric" (and, purifying in the melt) method at that time.
(2) I started my own refinery in about 1972 and used the chelating agent to drop the gold. In 1979, I went to work for a guy that used the "minimum nitric" method. He and David Fell were childhood friends and had worked together for the largest karat gold refiner in LA - where they learned the business. I don't know if Dave also used the method or if my boss had invented it. I do know that I have never heard of any other refiner using it and have never seen it in print. My boss used twelve 4 liter beakers (each with a cover and attached exhaust hose that took all the fumes to a manifold and then to a scrubber) to dissolve the gold and, in 1979-1980, we had so much karat gold scrap that the 12 beakers were always being used. We also sometimes used a 50 gallon glass-lined, steam-heated Pfaudler kettle, when we were over-burdened with too much scrap for the beakers.
His method was to first cover the karat gold jewelry (as is) with HCl and heat it. Then, he would add a splash of HNO3 from a gallon jug and let it react. This was repeated until an addition of HNO3 produced no reaction. At this point, either it all was dissolved or more HCl was needed. He would then add more HCl. If nothing happened, it was finished. If the HCl produced a reaction, he let it work until the reaction subsided and then added more nitric. He used the HCl and HNO3 separately, back and forth, until there was no reaction from an addition of either. He never estimated or measured either acid, yet never overshot the nitric. He had done this so many times, he just knew, by looking, when he had added the right amount. He dropped the gold with sodium sulfite and cleaned the gold with fluxes in a gas crucible furnace. If he couldn't get the gold to at least 9995 (maybe 5% of the lots), he used a big set of 6" rolls to roll the bar super thin and then quickly redissolved it in AR. From start to finish, the gold was always done in one day - usually 8 to 5.
Anything that didn't dissolve in the AR (maybe 5%) was accumulated and treated separately at a later date. He had a quirk that he didn't want anyone to rinse the yellow out of the filter papers. They were accumulated and burned and the yield was used to throw a party. Since all the gold was already bought and paid for, he didn't have to satisfy anyone as far as the yield was concerned. It was a fun place to work.
(3) When I owned my next refinery, I started estimating the amounts of acids needed. This was mainly done to make sure I had an excess of HCl to start with. That way, I didn't have to do the back-and-forth thing with the acids. When an addition of HNO3 produced no reaction, I knew it was finished. For karat gold, instead of using the customary 120 ml of HCl/ounce, I used 150 ml. I didn't worry about the nitric, since I was good enough by then to know when I had added exactly enough. I usually measured it out into a beaker, though, as a precaution against having a brain f**t and dumping in too much by adding it straight from the jug.
(4) To use the "minimum nitric" method, you should keep these things in mind:
(a) Any mixture of HCl and HNO3 will dissolve gold. You don't need 3 to 1 or 4 to 1 AR. As an example, for one certain type of application, I always used a 50% Nitric/45% water/5% HCl to dissolve the gold.
(b) When dissolving a quantity of metal, both the HCl and HNO3 are consumed at definite rates for each. In the case of gold, when only one of the acids has been totally used up, the dissolution of the gold will cease.
(c) For dissolving gold or gold alloys, I haven't pre-mixed aqua regia for the last 30 years and will never do it again. When you pre-mix it, it is very likely that you'll need to use urea or to boil-down, both of which I have a tremendous aversion for.
(5) I'm not going to go into details on using this method, since I have gone over it many times on this forum. All I will say is that it is simple and fast (especially when the alternative is boiling-down) and it works. Like Steve just said, you just have to adjust your mentality.
