Our starting material for this demonstration is probably the simplest and most common case of silver Recovery and Refining: A nitrate solution of silver and other metals.
The following process will describe how to separate the majority of metals contaminates from the Silver and preparing the Silver for the final refining process, of electrowinning in a Silver cell to produce 99.9 + Fine Silver. The Silver Nitrate (AgNO3) solution may be obtained by the different recovery/ refining processes such as Inquarting or dissolving Scrap Sterling Silver with dilute Nitric Acid and other sources of Silver bearing Scrap such as electronics. The following metals are the usual contaminates in most solutions: Copper (Cu), Nickel (Ni), Zinc (Zn), Lead (Pb), Palladium (Pd) and Platinum (Pt). The concentration of each metal is, of course dependent on the starting scrap material.
Abstract of the process:
1. Filtering. 2. Palladium removal with DMG (optional). 3. Cementing the Silver with Copper. 4. Washing the Silver powder. 5. Melting and pouring anodes.
First, we must make sure the Silver bearing solution is free of any solids such as: ceramics, organic material, glass, gold and platinum powders etc'… Filter the solution through filter paper into a clean and relatively graduated vessel. Wash the filter well with Distilled water to drain all of the Silver bearing solution.
2. Palladium removal with DMG (optional).
It is not widely known, that most silver and gold karat scrap is slightly contaminated with Platinum Group Metals (PGM) such as Platinum and Palladium in trace amounts, where the later one (Pd) is the most common.. Luckily, there's a rather simple way for removing Pd from the acidic nitrate solution by adding few drops of basic DMG (Dimethylglyoxime) solution and stirring well. The DMG will selectively and quantitatively remove any traces of Pd in a form of fluffy orange/yellow precipitate (looks like orange juice pulp) – Pd[HDMG]2. The reaction should not happen immediately, if it does, that means a high Pd content in the solution. Allow the solution to stand for at least 24 hours for a complete reaction. Of course, if there's no Pd in the solution, nothing will precipitate. If Nickel is also in the solution, it will appear for a short while as a pinkish/red solid that will soon re-dissolve upon stirring, as the Nickel-DMG complex is very soluble in acidic conditions. You will notice that the Palladium precipitate will not settle completely, but remains as a "cloud" at the bottom of the vessel. Given enough time, say… a week or so, at room temperature, some Platinum that may be present will also precipitate in similar manner as the Palladium does. The only difference is the color, which is blue. Filter the solution through a clean filter paper. Again, use a rather graduated receiving vessel. Try and keep the bulk of the Pd/Pt precipitates at the bottom of the vessel to be filtered last. I have found that gravity filtration works best in this case, where vacuum filtration fails miserably, the filter paper clogs immediately, rendering the process to a halt.