An easy method of producing 4 nines silver even if base metals are in solution is to reduce the solution with sodium formate as follows
1 Filter solution to remove all ppt's
2 Raise pH to 1.5 using liquid caustic
raise the temp to 130 degrees F
3 Reduce the silver by adding a solution made of saturated sodium
formate which has been reduced to a pH of 4 using formic acid (filter
any precipitate)
4 The solution will turn brown from the reduction of the silver and will
turn a bright blue (if the silver nitrate was from sterling) when the
reaction is completed.
5 Test for completeness of the reaction by sampling the solution on a
glass rod and dripping a drop into chlorinated tap water. White ppt
means more silver is present. Add more formate solution until test
yields clear drops into chlorinated tap water.
Needless to say no chlorides in the make up chemistry water
Works extremely well on sterling silver scrap digested in 50% Nitric/Distilled water. The starting pH is critical as is the pH of the formate reducing solution.
I used the series as reference(as well as most of the videos you've made)! One of the first channels I subscribed to! Just didn't know it was you 
