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Smelting is a process of melting ore to purify precious metals. Silver smelting is often a byproduct of copper or lead refining and involves multi-step processes. Copper, lead, and zinc concentrates that contain silver are roasted and smelted to produce pure silver.
Smelting is a method of melting ore, which is rock containing precious metals, to purify the contents. Smelting of silver to extract pure silver from lead and copper ores has been practiced since at least 2,000 BC. The metal was discovered in its natural state and used for jewelry as early as 4,000 BC. The processes for refining silver through smelting were discovered only after the smelting of gold and copper had first been perfected, with gold being worked on as far back as 6,000 BC.
Of the 12 original metals known to exist before the 18th century, silver was the most reactive precious metal, making it rare in its pure form. Most ores used in silver smelting contain only very small concentrations of the metal, usually less than 18% of the total ore content. For this reason, silver smelting is often a beneficial byproduct of copper or lead refining and is a multi-step process.
Copper-bearing ores that contain about 0.2% silver are first crushed and then smelted in a blister process that produces a waste product known as a sludge, which contains up to 20% silver. The slime is then oxidized in a further silver smelting furnace, which removes all components of the ore except the metals of silver, gold and platinum. This material, known as dorZ, typically contains less than 1% gold and about 1% platinum, with the majority 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 positioning 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 of silver smelting used to strip the sulfur compounds in the ore, and this produces lead bullion. Impurities in lead ingots include arsenic, tin and silver, and this silver is then removed through the Parkes process, named after Alexander Parkes, a British metallurgist who patented it in 1850. The Parkes process involves adding zinc to the liquid lead ingot, 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 retort, a type of distillation. The remaining silver contains traces of lead and gold, which is treated by cupellation which oxidizes 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 remove the zinc. The slag burning is then used to mix the remaining ore with coke and air to produce lead ingots in the silver smelting furnace. This lead ingot is then processed in the same way lead ores are to make silver, using the same type of smelting equipment.
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