Reading Hoke's you will come to realize she is teaching to remove base metals before putting your gold into solution.
Keep reading Hoke's book until it is clear.
You need to treat your waste solution, you do not just throw them away as you stated.
Read in our safety section dealing with waste.
Your solutions could hold values depending on what is processed, so understanding how to test and recover values is another important skill to study.
Urea I would not put it in my solution, my opinion it is just good for fertilizer, and should not be used in refining.
Study the forum to learn to use only the minimum acid needed, this way dealing with it before precipitation is a breeze.
Study the forum for subjects like:
Using the minimum amount of nitric acid to dissolve gold.
Using a gold button, or leaving some undissolved gold, to insure nitric acid is used up.
Or sulfamic acid to remove excess nitric from aqua regia.
Aqua regia is not the only way to dissolve your gold powder or foils, hydrochloric acid HCl and sodium hypochlorite (NaClO) common household bleach, works very well and is a lot easier to deal with, as far as removing the oxidizer, in this case chlorine gas, but like any process or procedure you need to study it well to get a good understanding how it works and the dangers involved.
Hokes was talking about removing as much copper as possible before dissolving the gold, there will always be traces of metals left even after you remove as much as possible, it is only these very small traces that would be no problem, but then also the more base metal the bigger the problem is, and the higher the chance for failure or loss of values, so the more unwanted stuff you can remove from your gold in the earlier stages, the easier and better your gold will be refined.
Tin from solder will rob you of your gold if you do not deal with it properly.
