If you followed the steps to refining gold, you may have wondered if the silver refining process is the same. The refining process removes impurities from the silver, including other metals, until the silver purity reaches its target millesimal fineness.
While we have covered silver as it relates to magnetism, did you know that the configuration of its electrons reflects all electromagnetic radiation of wavelengths shorter than 3000 angstroms? This covers all visible wavelengths, so all visible light is reflected by silver, giving it a white look.
What is the Process for Refining Silver?
There are several processes to refine silver, including pyrometallurgical and electrochemical methods. Some of the steps are like gold refining, and depending on the feed material, a refinery may perform these in different sequences and may perform some more than once.
Pyrometallurgical Methods for Refining Silver
Pyrometallurgical processes use heat and smelting operations to concentrate or extract precious metals. Pyrometallurgy includes calcination, oxidation roasting, cupellation, reactive gas refining, and fusion melting.
While the exact steps vary depending on what impurities are present in the silver, the first step is usually the same.
Ore, scrap silver, or Doré bar (lower purity silver often produced on-site at the mine) is melted in a crucible. A dip sample is taken from it to measure the purity of its silver content. The steps after that may be determined by the impurities present in the silver, and these processes may be repeated as needed.
- Calcining is a process that burns off organic compounds, impurities, and volatile materials in the ore. Calcination involves heating ore in the presence of limited oxygen. The ore is heated below its melting point inside a cylindrical structure called a calciner.
- Roasting changes the chemical composition of silver by transforming silver dioxide into native silver. While this is technically not a part of the refining process, it is a vital step for processing ore and concentrates. Roasting is like calcination but involves heating ore in the presence of excess oxygen. The ore is heated below its melting point and treats the ore with scorching air.
- Cupellation aims to separate base metals from noble metals. It is the process of treating ores and alloys under extremely hot temperatures to separate noble metals like silver from base metals like lead and zinc. Cupellation takes advantage of the fact that noble metals do not oxidize or react chemically, like base metals. When gold and silver are subjected to very high temperatures, they remain apart, and base metals react, forming compounds and slags.
- The Miller Process removes silver and base metals from gold by piping chlorine gas through the molten metallic mixture. Silver chloride and base metals rise to the top and are skimmed from the molten gold. Silver chloride may then be processed with zinc, sodium hydroxide, and dextrose reduction.
- Fusion melting is a process that alloys silver and platinum group metals with lead. This process uses zinc which is later removed from the slag. The remaining high-grade silver will still need further refining to meet a target purity of 99.9% fine.
Electrochemical Methods for Refining Silver
Electrolysis uses an electric current, a silver anode, and a low-acidity electrolyte to deposit pure silver crystals on a silver cathode. There are two primary methods for electrorefining silver and variations of both exist: the Moebius method and the Balbach-Thum method.
The Moebius and Balbach-Thum Methods for Refining Silver
In both methods, unrefined silver is cast as an anode which is processed in the refining cell. The used anode matter can be recast as another anode and refined again, so there is no waste.
As the electrolyte removes base metals from the anode, impurities grow in concentration, and the electrolyte is removed to be processed and make room for fresh electrolytes in the cell.
If the anode is managed with care, electrolysis can produce high-quality silver. Some variations employ a bagged anode compartment to separate the contaminants from the growing crystal structure.
What is the Difference Between the Moebius and Balbach-Thum Methods?
In the Moebius method, electrodes are disposed of vertically; in the Balbach-Thum method, electrodes are disposed of horizontally. The Moebius method is often chosen since it has a smaller footprint, and its silver is easier to remove from the cell.
How do These Processes Work Together?
The specific process for refining silver depends on the impurities in the ore. There are processes and sequences for refining specific metals from silver, including copper, lead, and zinc.
