alper,
Burning electronic scrap is very dangerous, with most materials there are better ways to process the material and do it more safely and efficiently.
Ash from burnt electronic materials is normally very alkaline, carbonates and borates, which will consume a lot of your acids making salts out of the acids, the other metals also make salts of metals in acid, most chlorides and nitrates are soluble, with metals like silver and lead and mercury being some of the exceptions.
Urea will not remove nitric acid and is basically useless when you use too much nitric acid in your aqua regia.
Urea will remove the byproducts of nitric acid from solution, such as nitrous acid HNO2 or NO2 gas that has decomposed into the solution, but any free nitric acid in solution when mixed with urea forms into a salt of urea nitrate, which if acidified will reform nitric acid again.
Too much SMB can also precipitate copper salts as well as itself oversaturating the solution and leaving insoluble salts.
The white salts are possibly several water-soluble salts.
Urea Nitrate (water-soluble), (will reform nitric acid with acids), (explosive if dried or heat strongly).
Silver chloride (insoluble in acids or water).
Lead chloride (very low water solubility in cold water, (much more soluble in boiling hot water).
Adding water should dilute and dissolve many of the salts, hot water can be used to separate lead and silver salts...
I would treat the solution for waste, adding a bar of copper metal to recover your gold and any valuable metals as black powders decanting liquid solution from the insoluble powders.
Save the valuable powders until you learn more.
Add iron to the remaining solution to recover copper from the solution as a reddish powder, decanting or carefully pouring off the liquid from the insoluble powders.
Raise the pH of the solution to around a pH of about 10 with either sodium hydroxide or a lime solution, letting the insoluble powders settle and decanting solution.
Bring the pH back to neutral (by adding acid), so more insoluble powders will settle from solution, decanting solution of saltwater.
Dry the waste (I roast it to red hot to make the hydroxides form oxides which are less water-soluble before disposal)...
Study dealing with waste, you will recover your gold or any values during the process and learn many principles that will help you understand recovery and refining better...
Forget using urea, study to learn better methods.
Study to learn safer and better or easier methods to recover gold from your material.
Basically, you need to begin with safety and studying better and safer methods.
Learn better methods to process your scrap and do it more safely.