Silver refining removes base metals and other precious metals from raw silver sources to achieve high purity for commercial applications. Because silver rarely occurs in a pure state, it must be processed to meet investment-grade standards, typically requiring a minimum fineness of 99.9% or higher. This purification is necessary whether the silver is sourced from mined ore concentrates or recycled materials, transforming an impure alloy into a product used in electronics, jewelry, or as a financial asset.
Sources of Silver Requiring Purification
Silver refining material comes from primary mining and secondary recycling. Most newly mined silver is a co-product of mining for base metals like lead, zinc, and copper. Silver-bearing ores are often mixed with these sulfides, and the silver is recovered during the concentration and smelting of these larger volume metals.
Secondary sources include recycled industrial scrap, old electronics, and used photographic film. These materials require chemical or thermal separation from non-metallic and base metal components. The resulting impure alloy, often called Doré metal, is a mix of silver, gold, and residual base metals.
Initial Separation Methods
Initial separation uses pyrometallurgical and chemical steps to remove the bulk of base metal impurities. The process typically begins with smelting and roasting, where silver-bearing concentrates are crushed and heated. Roasting converts silver sulfide into an easily processed form, while smelting melts the material to separate the valuable metal from waste rock, often producing a lead-silver bullion.
Cupellation
Cupellation is a historical and relevant method for separating silver from lead. This process involves heating the impure metal above 960°C in a special furnace with an oxidizing atmosphere. The heat causes base metals like lead, copper, and tin to oxidize. The resulting lead oxide, known as litharge, is absorbed into a porous refractory container or skimmed off the molten surface. The remaining alloy, the Doré metal, is significantly richer in silver and gold, but it is not yet pure enough for commercial standards.
Chemical Leaching
For lower-grade materials or recycled scrap, chemical leaching methods are sometimes employed to dissolve the silver selectively. A mixture of silver and base metals can be treated with nitric acid, which dissolves the silver and most base metals, leaving behind gold and other acid-resistant impurities. The dissolved silver is then precipitated out of the solution as a silver compound, which is later reduced back into a metallic form for further refining.
Achieving High Purity Through Electrolysis
Electrolytic refining is the final stage of purification, yielding silver with a purity of 99.9% or 99.99%. This electrochemical process separates silver from remaining metal contaminants, including gold and platinum group metals. The impure Doré metal is cast into plates to serve as the anode, the positive electrode, within an electrolytic cell.
These cells, often based on the Moebius or Balbach-Thum systems, contain an electrolyte solution, typically silver nitrate mixed with nitric acid. When current is applied, silver atoms at the anode dissolve into the electrolyte as positively charged silver ions. Base metal impurities like copper also dissolve, but noble metals like gold and platinum fall to the bottom of the cell as an anode slime.
The current drives the dissolved silver ions to migrate to the cathode, the negative electrode, often a thin sheet of pure silver. At the cathode, the silver ions gain electrons and deposit as pure silver crystals. The electrical potential is controlled to ensure that only silver is deposited, while dissolved base metal ions remain in the electrolyte solution.