Good information Dr. Poe, thank you.
I would like to add my version, they way I see this.
Dirty solutions like base metals from electronics, hold gold in solution, either as fine powders or colloids, tin from solder, aluminum from capacitors can be a major problem, as it makes a gel that does not completely dissolve in solution, and if more than just traces, makes filtering virtually impossible, now add copper to this solution and any gold will try and cement out on copper and tin in solution, also forming colloids in this solution.
Add another problem from the circuit boards (on top of the many different types of base metals), like traces of oils, other chemicals, epoxy coatings, conductive greases, dirt and carbonous materials on circuit boards just really complicates a bad situation.
Colloids are of both polarity and repel each other helping to keep each other held in solution (not joining together and precipitating), holding values in this gel, that will not filter so making it very hard to deal with without you losing values, (and not even know that you are throwing away your gold you work so hard to find).
Since gold locked in this dirty solution can be reduced to metal, (valuable gold not precipitating, and not dissolved, but locked up in a suspended gel), stannous chloride will not test for gold as a metal, the gold has to be dissolved into solution for the stannous test to work, so you may test a solution and not get a reaction and assume your solution is barren and loose a good portion of your gold.
Depending on the state of your solution filtering can even be difficult using the Charmin plug, or other methods.
If you have not paid attention to learn how to process your materials to keep from forming this mess, here is one way that it might be dealt with.
Example: a gel solution of colloids that will not filter, with powders at bottom of vessel, (colloids are worse problem if solution is not highly acidic in a solution of metal salts)
Use funnel with Charmin plug, if toilet paper will not filter well, fiberglass insulation or even ceramic fiber wool can be used, decant the liquid from powders to filter liquid portion with Charmin plug, save powders to be incinerated, the liquid can be put in a boiling vessel, and concentrated sulfuric acid added (heat and high acid content helps to break colloids), boiling this solution will vapor off water and the more volatile acids, like nitric and then hydrochloric acid, as solution removes these other acids and solution concentrates in acidity the gold will begin to precipitate from solution, the base metals will become metal sulfate salts in solution if reaction is driven to completion, and colloids are broken.
Juan Manuel has written several posts on dealing with colloids, reading them is of great benefit.
Depending on conditions you may not have to complete full detail of above process to break the colloid.
Incineration has been covered many times, and Harold has explained these benefits in many posts reading them is very valuable.
Basically pre-treating your materials for processing, separating material types, mechanically separating base metals and other materials as much as possible, removing solder by mechanical and or chemical means, incineration of materials to remove carbons oils and trash, incineration can also assist in oxidizing base metals and making some like tin easier to wash out in pretreatment washing (base metal leaching steps). And removing base metals as much as possible before putting your values into solution, and then also refining a second time if needed to re-crystallize your metals before melting.
Learning better methods to process this scrap can save you money and a heck of a lot of troubles.
It takes many more hours dealing with a problem you created, when your trying to save time by not spending time on pretreatment of the materials.
