The insolubles from the aqua regia reaction contain all of the Silver that was in the lot as Silver Chloride. The method I prefer to separate the Silver Chloride from the other insolubles is to pour the entire solution through a 200 micron screen which catches the larger insolubles and allows the lions share of the fine Silver Chloride to pass through along with your pregnant aqua regia. These screens are typically for paint straining but are available from
US Plastics. They last a long time and are not expensive. This step only adds seconds to the process as the coarse filtration provided by the 200 mesh screen allows the acid and chlorides to flow through quickly. A quick rinse of what remains on the screen and you are ready for the next step.
What passed through the screen is now chilled with ice. This step will yield a cleaner gold in the first drop because the Silver Chloride is slightly soluble in the acid but in cold acid it is exponentially less soluble. The Silver Chloride is filtered out and the Gold in solution is dropped using your preferred precipitant. If the customer wants to be paid for Silver the chlorides are easily reduced using an iron frying pan. A 1 kilo stone removal lot will typically yield less than 2 1/2 ounces of Silver metal which is why the frying pan method works well for individual small lots.
What you are left with now is what did not pass through the screen. It contains some chunks of gold which have been encrusted with Silver Chloride to the point they will not dissolve in aqua regia. So the next step involves getting the Silver Chloride crusts to dissolve leaving exposed gold alloy.
There are 2 ways this can be done. One way is to dissolve the chlorides in a heated beaker with ammonium hydroxide. I hate this method for 2 reasons. One is the fumes, which because of the use of hydrochloric acid in the process causes dense white clouds. Plus ammonia in it's own right is quite offensive to the nose. However it will dissolve the Silver Chloride which can be recovered from the solution by adding Hydrochloric Acid. The other reason I dislike ammonia and Silver in a refinery is the ability of the mixture to form explosive compounds if it remains alkaline for too long. There will not be a lot of Silver in the ammonia (as Silver Nitrate) and refiners tend to let it sit around. Potentially a big problem which I like to avoid. Refiners have a tendency to leave liquids in the shop and I do not encourage that with liquids that have a tendency to explode.
The other way, which I consider the best method, is to tumble the remaining insolubles in a sodium thiosulfate water slurry. This does not need to be done in a hood as there is no fuming to deal with. A starting size lot of about 1 kilo of scrap will be handled in about 500 ml of water and a 1 measuring cup full of sodium thiosulfate. I prefer the crystals of thiosulfate often sold as prismatic rice as they look like tiny rice sized prisms of the thiosulfate. The water, thiosulfate, and insolubles are placed in a wide mouth plastic jar and put on a jar tumbler for about 15 minutes. Usually 1 treatment will suffice.
The jar is emptied through a screen like used for the chlorides above and rinsed well. What emerges will be the diamonds and gemstones and any platinum findings that may have been in the lot, and gold which is again yellow in color as it is no longer Silver Chloride encrusted. These solids are dried and need to be sorted.
The larger pieces of gold and Platinum can be picked out manually with a tweezer and the smaller remaining crumbs are easily separated with diamond sorting screens. Usually the gold is melted into a button and settled on an XRF result rather than dissolving it in aqua regia and repeating the process.
If the diamonds and gems appear a bit dirty it is because you did not sufficiently rinse off the thiosulfate liquid from tumbling. A small beaker full of aqua regia that can hold the stones is boiled for a few minutes in the hood and the stones will emerge shining.
