.Solder, high in tin and lead, this is something you do not want in solution with values.
These should both be eliminated as much as possible, (mechanical separation as much as possible), pretreatments of heated HCL is a good method (tin in solder dissolves into solution, breaking down the solder, copper or gold is not attacked by HCL alone, lead will fall to bottom as lead chloride white powder).
Tin in nitric makes a gel very hard to filter.
Tin will almost dissolve in concentrated HCl, but will lock up gold in solution and is still somewhat of gel and will give trouble filtering.
Lead is will dissolve in nitric acid slowly, but is only slightly soluble (slightly soluble in water).
Lead is insoluble as chloride, (almost insoluble in cold water, but is much more soluble in boiling hot water)
If you have fine gold in powders or flakes with very much solder (lead and tin in powders or solution (tin being the bigger problem, in my mind), you have not done the pretreatment methods very well (so you need to practice that more next time).
Now it will take more work to deal with the lead and tin now.
Incineration of these dry powders to red-hot, but not melting.
(If these powders are chlorides, or were treated with HCl, neutralize them first by washing in a sodium hydroxide solution, and rinse well to remove salts {gold in chloride salts are volatile when heated}).
Incineration, helps in several ways, burns off oils plastics or carbons, removes previous acids as gases, will oxidize base metals, (here tin being oxidized is important, as tin oxide dissolves better in HCl wash we will use later).
Many way's you can do this I will show one way:
After neutralizing powders with dilute NaOH (if needed to neutralize and remove chloride salt water), and rinsing powders.
Wash them from jar (using squirt water bottle into a white corning ware dish on a hot plate (solid cast iron heater plate), turn hot plate on low,
use pipette to remove the water but not picking up powders, allow powders to dry on low heat, after dry crush powders, I used a Pyrex pestle, raise heat slowly on dry powders (some powders may fuse and become syrup again, these we want to dry under the higher temperature but not so hot that they bubble and splash out of dish, after they calcine and dry again (acids driven off), we will re-crush them). Other powders or gold flakes may just dry and not fuse, raise heat to high, with lid on dish let heat on high till no more fumes evolve, remove lid with a hand held propane torch on low heat with the tip of the torch being careful not to blow out powder, as they heat you can bring in flame of torch closer, get powder glowing red hot, stirring with stainless steel (old knife for purpose), keeping these powders hot and stirred exposing them to air (oxygen to oxidize the base metals in this red heat), (keep powder red hot 30 minutes, if powders were sulfates heat red hot longer), put lid on dish and let cool,
when cool add HCL to cover powder, boil, put heat on low, add just a little water to dilute some, let powders settle, decant the hot HCL with pipette, but do not pick up powders, if HCL is very colored (HCl wash hot will remove most all the tin, and lead as will hot water rinses) repeat HCL wash, rinse with water several times until clear on decanting water, leave powder in dish on low to dry again (and incinerate again if you plan on using nitric to remove more base metals the HCl wash did not get), (the powders can now be washed in nitric acid if needed ) or if gold can be dissolved in dish, only reason to remove values from dish is for filtering solution into clean jar for precipitation of values.
