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Milling and Separation
Ores dug or blasted from mines are ground into a fine powder in a multistep milling process. The ores are repeatedly washed along the way to remove contaminants. Placer ore is screened to remove larger chunks of rock and then sent through a wet sluice process, which separates out the lighter gravel and sand components, leaving the heavier gold and other metals to wash through to an accumulation tank. Ores that contain sulfur or carbon are roasted while exposed to the air, which breaks down the offending substances and prepares the ore for the next step. Autoclaving uses pressurized steam within a sealed chamber to begin an oxidation reaction that burns away the sulfur. It requires less heat than traditional "roasting" while achieving the same result.
Leaching
Leaching is used to isolate gold, silver and other precious metals from their parent ores. After milling and washing, ore is placed on a large drained surface and sprayed with a chemical that will dissolve the desired metal. For gold, a sodium cyanide solution is used. Removing silver from zinc ores requires sulfuric acid. The chemicals dissolve the precious metal, which drains out through the bottom of the apparatus and is collected. The process may take several weeks for gold. Zinc is then added to the cyanide-gold solution, which causes the gold to fall out, as a byproduct of the reaction. The gold is collected, impurities are removed, and it is melted for pouring into ingots or molds.
Platinum and palladium ores are also subjected to a leaching process, this one involving aqua regia, which is a mixture of nitric and hydrochloric acids.
Electrolysis
Silver usually occurs in ore along with other metals, most often lead or copper. Silver is removed from copper ore by placing the ore in a electrochemical cell, which separates the copper from all of the other materials. The pure copper attaches itself to the negative terminal of the cell, while the impurities, including silver, fall to the bottom and form a grimy deposit. This "sludge" is then removed and super-heated, burning off everything except the precious metals. The metal is then formed into bars that are placed in their own electrochemical cell, which is filled with silver-copper nitrate. This process allows silver with purity greater than 99 percent to be produced.
Parkes Process
Silver is most often found in lead-bearing ores, and using the Parkes process can get it out. Lead ore that has been milled and washed is cooked, then melted. This makes a lead bullion that usually contains antimony, arsenic, tin and silver. Zinc is mixed into the molten lead, where it quickly reacts with silver (or gold) and forms a compound that floats to the top of the processing tank. This is skimmed off, and the mixture is placed in a low-temperature vacuum furnace, where the zinc is removed. The remaining lead-gold-silver residue is heated to a very high temperature of about 1,450 degrees Fahrenheit in an oxygen-rich environment. This "burns off" the lead, leaving only the silver and gold.