Sidebar by Ed Quillen
14 – July 1996 – Colorado Central Magazine
Why they used Salt to get Silver
Every account of the Salt Works mentions that rock salt was used
The process was apparently discovered by the Spanish, who began using it in Mexico and Bolivia in the 1600s.
Many rich deposits are silver sulfide, and the chemical bond is quite tight: heat it and beat it all you want, and you still don’t get metallic silver from silver sulfide. The tarnish on grandma’s old silver spoon is silver sulfide, and you know how hard it is to get that off.
Some desperate Spanish mining engineer decided to try crushing rich sulfide ore, then roasting it with rock salt — sodium chloride.
Under those circumstances, the chlorine from the salt replaces the sulfur that is attached to the silver. When things cool down, the sulfur in the silver ore has oxidized and turned into the kind of air pollution that produces acid rain. The sodium from the salt has likewise oxidized.
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The chlorine from the salt has bonded with the silver from the ore to form a silver salt. Silver salts are chemically unstable (so unstable that they respond to light, which is why they are used in photography), and thus relatively easy to extract the silver from.
Adding a strong alkali solution like sodium hydroxide (household lye) will produce salt, water, and silver.
To recap, this process consisted of crushing the silver ore (in a Spanish arrastra or an American stamp mill) and roasting it with rock salt. That converted a silver sulfide, hard to separate, into a silver chloride, easy to separate.
Sometimes, when the ore was especially rich, droplets of pure silver would form during roasting. The ore had to be pretty rich for this process to pay at all — it wasn’t very efficient, and a lot of the silver remained in the slag.
Improved smelting methods in the 1880s eliminated the need for rock salt, but the chemistry of silver refining remains the same — replace the sulfide with a chloride, and then break apart the chloride to extract the metal.
–Ed Quillen
