Before we start, you should know that there is some dispute regarding a direct Aqua Regia (AR) refining process for high gold content alloys, main reason for that dispute is usually the silver content, which, react with AR and convert to Silver Chloride (AgCl) and somewhat retarding the process. Some refiners just don't like to deal with AgCl and will prefer Inquarting process to eliminate the silver (and base metals + PGM) all together prior to gold dissolution.
In this current gold refining process we will dissolve the high gold alloy directly in AR leaving a solid AgCl residue at the bottom, then separate the liquid gold solution (HAuCl4) by means of filtration.
Few things to consider before choosing this process for high gold content alloy. This process is applicable as long as silver content is below 10%. 22K gold jewelry usually holds 5% silver. Placer gold may also be a good candidate for this direct approach, but should be melted and assayed before hand.
BTW, you can use AR for lower gold content alloys with very little to no silver in it, but in that case, the high base metals presence in the solute may interfere later with the reduction stage so it is not very recommended to do so, especially to the novice.
Gold in Aqua Regia
Abstract of the process:
1. Increasing surface area. 2. Dissolution in hot Aqua Regia. 3. Silver Chloride removal. 4. Reduction of gold. 5. Gold powder cleanup. 6. Re-refining (optional – recommended). 7. Melting.
1. Increasing surface area.
Whether the feedstock is, either jewelry or nuggets, first thing first… We want to speed up the reaction as much as possible, for that we increase surface area of the metal to make sure as much of it will react with the reagents in the least amount of time.
You can do so by melting the feedstock and pouring it into a deep tank filled with cold water to form flakes and beads. You can roll it in a mill (like in the showcase video). If no choice is available, enter the feedstock as it is.
2. Dissolution in hot Aqua Regia.
Place the metal in a beaker and add Hydrochloric Acid (HCl 32%), add 4-5ml of HCl for every gram of metal and place on heat source set on low. Don't forget to cover the beaker with a watch glass (IMPORTANT) or other condensation apparatus.
Now, when worm, start adding HNO3 (Nitric Acid 69-72%) very slowly. Don't bother calculating how much acid you need to add, we will use a visual indication to know when addition is needed and when the reaction is over.
The solution will start bubbling and fizz from the reaction, the metals are being dissolved into the solution. At first, the solution will gain colors, usually yellow/green/red and stay clear. As the oxidizer (Nitric Acid) is consumed, the solution will turn darker and darker, notice when all of the bubbling & fizzing had seized. Now ramp up the heat to medium (but do not boil) and add a few more drops or ml of nitric acid, respectfully to the amount of material you are processing. You will notice the solution turns a bit clearer when fresh oxidizer (nitric) is added… Again, wait until all reaction has seized before the consecutive addition.
Repeat the nitric additions until no more metals are reacting upon additions of nitric. Make sure not to confuse and thinking that Silver Chloride powder is left over metal, or as a result you will add to much nitric acid. When dissolution is done, remove the beaker from heat source and allow the solution to cool back to room temperature. The solution will be now very dark, and very little to no free oxidizer (nitric) should be left in it.
3. Silver Chloride removal.
When the solution had cooled back to room temperature, add a few ice cubes to hasten the AgCl precipitation. You will immediately notice a color change from dark brown solution to light and bright yellow. Allow the solution to stand for at least 12 hours before filtering it. It will probably change color again by then to deep and dark yellow.
Filter this solution through filter paper or several coffee filters, either by gravity or vacuum. First pour off the majority of the liquid, keeping the solids to the end (they tend to clog up the filter) an wash well the filter to rain all of the gold solution. Make sure the receiving flask is clean.
Gold Solution with Silver Chloride
4. Reduction of gold.
This process is well described at other videos and articles in Gold-N-Scrap. Under the fume hood or out side, simply dissolve SMB (Sodium MetaBisulfite) in water and slowly add to the gold pregnant solution. Soon you will notice a brown precipitate, keep adding until the solution turns black and all of the rich yellow color is gone.
You could just as well add the SMB dry, but I have found that relatively concentrated gold solutions (above 2 ozt per liter) don't react well to dry SMB, producing a lot of heat and steam, things which render the reduction of the gold.
You will need to dissolve approx 1 gram of SMB powder for each gram of expected gold in solution.
When the reduction is complete, allow the gold powder (some in suspension) to settle for 1-3 hours. Later test the solution with Stannous Chloride to make sure all of the gold is out of solution. It may sometimes test positive for Pt/Pd, if so keep the solution for later recovery of these metals.
Tip: Heating any powder suspension or slurry speeds up settling time.
Using the same filter from the previous step, decant the bulk of the solution to the filter and make sure the gold power remains in the original beaker, try an pour as much as possible without disturbing the power.