Smelting is a method of melting ore, which is rock containing valuable metals, to purify out the contents. Silver smelting to extract pure silver from lead- and copper-based ores has been a practice since at least 2,000 BC. The metal was discovered in its natural state and used for jewelry as early as 4,000 BC. Processes for refining silver through smelting were only discovered after the smelting of gold and copper were first perfected, with gold being worked as far back as 6,000 BC.
Of the 12 original metals known to exist prior to the 18th century, silver was the one precious metal that was the most reactive, causing it to be rare in its pure form. Most ores used in smelting silver contain only very small concentrations of the metal, usually less than 1% of the total ore content. For this reason, silver smelting is often a beneficial byproduct of refining copper or lead, and it is a multi-stage process.
Copper-bearing ores that contain around 0.2% silver are first crushed and then smelted in a blister process that produces a waste product known as slime, which contains up to 20% silver content. The slime is then oxidized in an additional silver smelting furnace, which removes all ore components except silver, gold, and platinum metals. This material, known as dorZ, typically contains less than 1% gold and around 1% platinum, with the bulk of the content being silver. The dorZ is electrolytically treated in a silver-copper nitrate solution, using either the Moebius or Thum Balbach systems, each of which position the electrodes differently, and the resulting silver content is 99.9% to 99.99% pure.
Lead concentrates that contain silver are first roasted, which is a precursor state to silver smelting used to drive off sulfur compounds in the ore, and this produces lead bullion. The impurities in lead bullion include arsenic, tin, and silver, and this silver is then removed through the Parkes process, named after Alexander Parkes, a UK metallurgist who patented it in 1850. The Parkes process involves adding zinc to the liquid lead bullion, as silver is more likely to dissolve in zinc, so it migrates away from the lead. The zinc is then removed from the silver by vacuum retorting, a type of distillation. The remaining silver contains traces of lead and gold, which is treated through cupellation that oxidizes off the lead at a temperature of 1,450° Fahrenheit (788° Celsius).
Zinc concentrates that contain silver are also roasted and sulfuric acid is added to leach away the zinc. Slag fuming is then used to mix the remaining ore with coke and air to produce lead bullion in the silver smelting furnace. This lead bullion is then processed in the same way that lead ores are to produce silver, using the same type of smelting equipment.