Here is the recipe/method if you want to use Nitric Acid To Purify Silver:

First; The Dangers of Nitric Acid

1. Place the silver in a beaker or Pyrex container.
2. Add 3 parts distilled water to 1 part nitric acid.
3. Place on stove or hot plate and simmer until all of the metals are gone.
4. Take un-iodized table salt and drop into water and nitric acid solution. You should find a white substance that looks like cottage cheese. This substance is silver chloride. When the salt stays in the bottom of the solution and the silver chloride stops falling out, then all the silver should be broken down. 5
5. Add 10 parts tap water.
6. Filter off the silver chloride through a plastic funnel or strainer lined with a filter.
7. Rinse the filter containing silver chloride with water.
8. Dry the filter. If you let the filter dry in the sunlight, the silver chloride will turn a silver color.
9. Burn the filter by putting it in a clay crucible and lighting it with a match.
10. Covered the burn filter and silver with soda ash.
11. Place the crucible in a furnace or oven and heat to a temperature of 2100 degrees F. When the silver looks like honey and lays flat without any lumps in it; it is ready.
12. Quickly pour into mold or let it set in the crucible until the silver has cooled.
13. Remove the silver and wash it with soap and water.
14. Add ½ cup of lime juice to the leftover nitric acid and water solution to neutralize it and dispose of it.