Esau Nisalile,
Urea is not a good idea, in my opinion, actually a very bad idea, as far as I am concerned, it can add contaminates, it can form ammonium nitrate (an explosive mix) which under some conditions could be very dangerous (especially with heated glassware being so close to our face, the urea can also form ammine in solutions the ammine compounds can complicate platinum group recovery (if involved).
Sulfamic acid will work better if needed, as its byproduct is sulfuric acid which has the benefit in solution (can help when evaporating to keep from forming salts if the process carried too far), (and will precipitate lead from solution as an insoluble lead sulfate), sulfamic acid can be found as a grout cleaner for tile grout (found in the flooring section of the hardware store near where I live).
Harold's trick of adding a button of gold to use up excess nitric acid, with heating, is a great way to remove excess HNO3 from solution.
Knowing How much acids to use for each gram or ounce of gold, and adding the nitric measured in small increments, using heat to drive the reaction and dissolve the gold, using up the added HNO3 before adding a bit more to insure excess nitric is consumed in the reaction, is the first step to not using too much nitric or having to eliminate it later in the process.
Leaving a little bit of undissolved gold behind in the reaction (works similar to Harold's method above), this little bit of gold can be added to the next refining process.
4 fl. Oz. of 32% HCl and 1 fl. Oz of 70% HNO3 will dissolve a troy ounce of gold (31.103 g Au).
Using the formula above from Hoke's book we can figure from this:
118.29 ml HCl + 29.57 ml HNO3 per Troy Oz of gold.
3.8 ml of HCl and 0.95 ml HNO3 per gram of gold
With this, lets say I have 10 grams of gold powders recovered and I wish to refine them, I can cover them with 38 ml of HCl (I may add just a bit more no harm), set this on a mild heat, then measure out 9.5 ml HNO3 in a clean vessel, using a pipette I can add only a few milliliters of HNO3 at a time, waiting for the reaction to complete before adding more, heating the solution also speeds the reaction, but also concentrates the acid, as it evaporates the water added (involved in acids and the water produced from the reaction), this is actually concentrating and evaporating my solution as I work, we just do not want to heat too hard, or we could vapor off the needed HCl in solution. We may not need all of the measured nitric to dissolve our gold.
At this point after the gold has reacted, we can leave a bit of the gold undissolved for the next batch, or add a gold button to consume any free HNO3 (if we used more than needed), we can continue to concentrate the solution through evaporation, getting the solution concentrated, (here we normally add a few drops of H2SO4) but we can add a little sulfamic acid for the same purpose, which will also work to eliminate free HNO3, after concentrating we can cool the solution down with three to four volumes of water, let it sit overnight to precipitate silver chloride and lead sulfate..., and filter the clear bright yellow decanted solution before precipitating the gold with our choice of chemical precipitant.
Although not all of these methods are needed to eliminate free HNO3, using a combination of them for me insures I do not have problems with precipitating the gold, because of excess nitric in solution.
A trick I use is the stannous chloride test of the gold solution before precipitating gold, getting a positive purple of Cassius reaction, lets me know the stannous chloride can reduce the gold to metal colloids, this is a good indicator free nitric acid has been eliminated from the gold chloride solution, and thus my chemical precipitant (such as sodium metabisulfite) should not have problems reducing the gold ionic salts back to elemental metal gold powders and precipitate my gold.
