SMB is selective for gold, this does not mean it will not reduce other metals from solution, for instance, it can be a reducing agent for other metals, or salts, like copper, example reducing CuCl2 to insoluble CuCl...
A soup of metals has all kinds of reactions going on, this is one reason we recover the gold and get it as pure as possible before we even put it into a solution for a refining process. In a fairly pure solution of gold chloride the sulfur dioxide gas is selective for gold, it will precipitate gold atoms (giving back electrons to the gold ions in solution, the copper if minimal will stay in solution, as a copper chloride ion, the less metals in solution the less chance of reduction of these metals or inclusion in the gold through drag down. Dirty solutions (full of base metals), can produce an impure gold precipitant (full of base metals and base metal salts, black to dark brown precipitated gold, and salts of other metals...
Dirty solution (full of base metals) can make it hard to recover gold,these base metals can form nitrates, which can make it hard to remove (free nitric acid) with evaporation as the nitrates form salts which resume to nitric acid when an acid is added, which will reoxidize gold putting it back into solution... Here is where we use several evaporation steps adding some acid between evaporations the HCl acid converts base metal nitrate salts to HNO3 which can be converted to gases through the next evaporation step...
These nitrate salts and even the free nitric acid cannot be removed with Urea. Urea reacts with the gas not the acid or nitrate salts in solution... Urea Use is a potential danger, basically little or of no benefit in the removal of free nitric acid from your gold chloride solutions.
Dirty solutions (base metal rich), can form other salts, PbCl2, PbSO4, SnCl2... These can contaminate gold, reduce gold, or even lock your gold up in solution as a colloid which will not precipitate....
SMB Na2S2O5 which produces SO2 gas (the reducing agent), it can also form sulfates which can also form sulfate base metal salts like PbSO4... Dirty solutions---can make things difficult and give dirty gold, or can even lock gold up in solution...
Different reducing agents (selective for gold) can reduce different base metals or salts, this is one way to get gold pure is to refine fairly pure gold with a different reducing agent the second time you refine the gold, leaving the base metals or their salts in solution with the other reducing agent in the second refining process...
When we dissolve gold in acid we remove electrons from gold atoms the gold is oxidized (loss of electrons) which form gold ions in solution, In the process the acids are reduced (gain electrons), nitric reduced to -->ionic salts of nitrates or gases, HCl reduced -->ionic salts of metal Chlorides, and gases.
To precipitate the gold we have to give the gold ions back the missing electrons which reduce the gold salt back to gold metal as clumps of gold metal atoms joined to make visible gold powder...
In order to reduce the gold we have to remove the oxidizing agent, (free nitric acid or nitrates salts in the case of aqua regia) the oxidizing agent will continue to oxidize the gold (dissolve and redissolve the gold keeping it from being reduced to atoms by keeping it or making it back into gold ions by removing electrons)....
Limiting oxidizing agent use (using nitric or other oxidizers wisely), and evaporation methods (see Hoke's), and Harold's gold button trick are excellent tools for nitric removal. for newbies and those who get heavy-handed in their use of nitric acid, a sulfamic acid is a good tool...
Platinum group, in my opinion, are difficult, concentration plays a major role, someone with more experience with the PGM's can help you here.
Stibnut, recovery and refining of precious metals may seem simple on the surface, but the devil is in the details, the only way to learn these details is with study and experimenting with what you learn through study...
Many members get this back-asswards they believe they will learn by experimenting with things they know nothing about, to learn from their mistakes and by asking questions, a foolish and dangerous practice when dealing with chemistry and reactive metals....
You may learn a tiny tidbit from a question, but you gain true understanding through study and experimentation, I can only touch a small point my answer, that may or may not answer your question) that will leave you with many more unanswered questions, but you can get the whole picture through your own research...
