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.
4. Washing the Silver powder
Now, you are left with Silver powder and a royal blue Copper nitrate solution. This step is about separating the two. Not an easy task as it may sound. Take a piece of polyester cloth or just a regular cloth (an old white T shirt will do) or a large coffee filter to be used as your filter and wrap it around the moth of the receiving vessel. Make sure to leave enough of a dent in it to accommodate all of the Silver powder. See an example in the attached video. The reason for using a cloth, which obviously, will not catch all of the very fine powder, is speed. You will soon find that trying to filter through a fine filter paper proves to be very inefficient, and may take hours… just to catch those few grams in very fine powder. Pour off the bulk of the blue copper solution through the filter, leaving the bulk of the powder in the original vessel. Drain as much as possible. Now, boil some tap water in a kettle and cover the Silver powder, use plenty of water, at least double the powder hight, and mix vigorously for a minute. The hot water will leach out the Copper nitrate solution entrapped in the Silver powder. Once cooled and settled, drain the bulk of the solution to the filter and leave the bulk of the powder in the original vessel. Repeat that wash two more times, at least. After the last wash, just wash all of the powder into the filter and allow it to drain for few hours. Next, allow it to dry for several days or force dry it in a glass/porcelain pan on top of a heat source. You will find that it is next to impossible to remove ALL of the Copper solution with just 3-5 water washes, and that's OK, as the Copper contaminate now is probably less then 1% by mass. Good enough for the next electro-refining process. (Separate article)
As to the fine particles that did pass through the filter, just allow the solution to stand for about 24-36 hours to let silver powder settle completely. Siphon the bulk of the solution to your waste acid bucket and filter the last portion with the fine powder though a regular filter paper. I found that filtering the Silver powder that way, especially with more then 5-7 ounces of silver is the most practical way.
