3. Cementing the Silver with Copper.
Now, that our solution is free from all of the starting materials contaminates and PGM's, we are ready to recover the Silver metal from the nitrate solution in a form of powder.
Following the Reactivity Series, we will find that Copper is just above Silver (and PGM's).
In simple words, Copper is more eager to give up its electrons (go into solution/oxidized) and Silver/PGM's are more eager to gain back their electrons (Go out of solution/reduced).
Any metal that is above Copper in the Reactivity Series will stay in solution.
Once introducing a solid Copper slab (could be a piece of pipe, a bus bar or rod), an immediate reaction will occur. Silver crystals will start growing on the surface of the Copper coating it with ever thickening layer until it collapse under its own weight and falls down to the bottom, thereby exposing new copper surface to react with more Silver ions.
That's why, a very efficient way to cement Silver, is to hang the copper slab about half way through the solution height and that is why you were suggested before to use a relatively graduated vessel.
Leave the Copper hanging in solution over night. When done, mix thoroughly for a minute and allow settling for few minutes.
Test for left over silver in the solution by mixing 3 drops of the solution with one drop of HCl (Hydrochloric acid) in a cavity plate or a test tube.
If it becomes somewhat cloudy after about 30 seconds, that means there's still silver in the solution and more copper is needed to cement the rest of the silver.
If there's no more silver in the solution, all you will notice is a momentarily yellow color flash in the solution with no cloudiness.
What happens in this reaction?
We start with a solution of Silver nitrate salt – AgNO3, the atoms of the copper give away their electrons to the Silver cations, therefore Reducing them back to Silver metal and take their place in the ionic bond with the nitrate anions.
As you can see, the nitrate anion (NO3-) plays no role in this reaction, so we can also call it a 'spectator ion'.
2 AgNO3 (aq) + Cu (s) = 2 Ag (s) + Cu(NO3)2 (aq)
This equation shows that each copper atom will reduce 2 Silver ions.
From that, we can calculate that each Gram of Copper will reduce 3.39 Grams of Silver.
In case you did not remove Pd/Pt before, they will also cement onto the copper. They tend to cement after the majority of the Silver, but that's not always the case as they may also co-cement with the silver.
They cement as black powder. Sometimes adheres to the copper rather tightly.
PGM's will be separated from the Silver in the following refining process.