How can copper concentrate with a small amount of silver produce slimes with much more silver?

Copper concentrate produces silver-rich slimes through the process of electrolytic refining. During this procedure, blister copper—which contains nearly all the silver from the original concentrate—is cast into anodes and placed in an electrolytic cell.

As a direct current is applied, the copper and other soluble metals dissolve into the electrolyte. However, silver and other precious metals are insoluble and do not dissolve; instead, they fall to the bottom of the refining tank as a residue known as slimes. Because the bulk of the copper is removed into the solution while the silver remains behind in the solid residue, the concentration of silver in the slimes becomes significantly higher than it was in the original sulfide concentrate. For example, a concentrate with 0.2 percent silver can yield slimes containing 20 percent silver.12