Grind or file the lump of alloy into a fine powder. Add conc. hydrochloric acid and boil until all the powder dissolves. Cool and add water. Silver chloride will separate as a white precipitate (all the other chlorides are soluble in water).
Filter it out, wash it a few times with cold water and take it to red heat in a crucible. AgCl decomposes at that temperature, releasing chlorine. Cool to room temperature and you are left with pure silver.