Incineration is a good pre-step for oils and carbonous materials.
(Calculate the amount of base metals and gold or other valuable metal when you can to determine how much acids are needed, if you are working in situations where you cannot calculate the amounts, use small additions, with heat, leaving some material left un-dissolved after solution is concentrated can insure you have used up the acid added).
GSP and other forum members, have worked out the numbers of how much nitric acid to use to dissolve silver or base metals, there are a few variations but they are all fairly close, the one I use is:
1.2ml 70% HNO3 + 1.2ml per gram of silver
(Copper takes 3.4 times as much)
4.15ml 70% HNO3 +4.15ml H2O
(one gram of copper will cement 3.4ml of silver from AgNO3 solution)
Hoke’s book gives us the amount of aqua regia to dissolve a troy ounce of gold, as one fluid ounce of 70% HNO3 and four fluid ounces of 32% HCl
From this we get:
118ml 32% HCl + 29.5ml 70%HNO3 to dissolve a troy ounce of gold.
3.8ml 32%HCl + 0.95ml 70%HNO3 to dissolve each gram of gold.
(Calculate the amount of base metals and gold when you can to determine how much acids are needed)
Remove silver and base metals from your gold before dissolving it.
As stated earlier HCl in excess cause no problems, and you can measure out amount needed and add it to the gold,
(Heating now can also let you see if you may have some base metals that did not get completely dissolved, or were left from previous treatment, silver chloride would be white copper would color acid green in this case I would raise the heat, lower it, but keeping it very hot let gold settle, siphon off HCl and replace it with clean HCl), HCl is fairly easy to get and cheap enough, let any liquid you decant settle in a cooling jar just in case you carried some values with the decanted solutions (I use a suction tool and a pipette to move liquid).
The nitric would be measured out amount calculated to dissolve the amount of gold you have, I measure it out in a small clean vessel, adding it only in proportions, to the heated HCl and gold, at first I may give it a larger portion of acid, and slowing additions down as solution concentrates, pipette the nitric in a drops at the later stages.
(As solution gets more concentrated I will add a little H2SO4 in drops, this will help form insoluble lead sulfate later, and can help from mistakenly evaporating solution to salts, it also helps in the process later of vaporizing off unused HNO3 from solution).
I try to use heat (not boiling) to drive off water from the acids as the gold dissolves, and as I add my nitric acid, my goal is to have the solution concentrated as the gold dissolves, and when most all of the gold is dissolved my last drops of HNO3 are added to finish the job, and just a little heat to consume the HNO3 and finish dissolving the gold, as the acids concentrate they get stronger, (adding acid we are also adding water that is involved in the acid), there is also a point where boil-over is likely to possibly happen where the acids concentrate to a certain point (azeotrope) and try to come out of solution all at once boiling the solution over in a foam of gases trying to escape, so watching for this reaction and lowering heat as solution is concentrated (you will see small bubbles begin to form around the rim of acid at this point lower heat for gases to escape more slower, after you are past this point heat can be raised again), after almost all gold is dissolved the acid is concentrated and the last drop of nitric is added to finish dissolving the gold it only takes a little time to vapor off any unused HNO3, because we would not have more than a few drops,
Also do not keep adding HNO3 trying to dissolve materials that will not dissolve or if to hard leave them for the next batch (or if platinum or some other hard to dissolve metal save for another process, basically do not keep using nitric acid to try and dissolve the sand or other not easily dissolved metal, and ending up with excess acid.
Now the gold is dissolved or almost all of it, if done correctly you should end up with very little to no excess nitric in solution.
dealing with excess nitric before precipitating gold. (leaving a little undissolved gold for next time, can insure you used up all of the nitric), (another option add a gold button to use up excess nitric Harold's trick), (and then there is eliminating nitric with sulfamic acid GSP trick), (and the three evaporation method taught in Hoke's book), Personally I use a combination of several of these methods.
Now the solution is concentrated I concentrate a little further the solution changes from yellow to orange and becomes almost a red orange color, add just a little HCl (this is 68% water and 32% HCL) so the acid also dilutes the solution this water will need vaporized off before the acid concentrates and pushes out any remaining HNO3, so we do not need much HCL, just enough to wet solution a little, concentrate back to thick reddish orange syrup, and do this one more time again, this here will insure most all of the nitric is either used or evaporated off, here I will turn off heat on the concentrated solution and let solution cool a little, and settle anything that would not dissolve, when solution is clear, I use a suction tool and a pipette to remove the liquid from the undissolved material, to a clean vessel, which I add about four time the amount of water, I will also add a small splash of HCl, letting this sit overnight in the cold night air, covering the vessel with a watch-glass cover to let silver chloride precipitate and any lead salt.
If I suspect any unused nitric I will use a little Sulfamic acid (I use grout cleaner from home depot, I also only do this on my first refine), the Sulfamic not only will destroy any unused HNO3, but will also form sulfuric acid in the reaction helping to precipitate lead sulfate.
The after sitting over night or longer, solution is again decanted and filtered to a clean vessel, before using my reagent to precipitate the gold, this gold is then washed per Harold's washing procedure before being dissolved and refined again, the second refining is always much easier than the first, and improves the gold making it well worth the effort.
Keep the urea to fertilize your grass, a few prills can be saved to test for nitric in some situations, but basically urea in my opinion should be used as fertilizer not in refining gold, the ammonium compounds it can create in solutions could become dangerous in some situations, or have adverse reactions with PGMs if you are dealing with them in solution, Urea it is just not needed in my opinion.
