Silver is a precious metal that has been highly valued throughout history for its beautiful white metallic color, its usefulness in jewelry, silverware, and currency. While silver can be found as nuggets or flakes in its pure form, most of the world’s silver supply comes from extracting it from silver-bearing ores found in rock deposits underground. Extracting silver from ore is a multi-step process that involves crushing, grinding, chemical leaching, smelting, and purification. This article will walk through the key steps for removing silver from rock and producing pure silver.
Where is silver found in rocks?
Silver is often found alloyed with other metals like gold, copper, lead, and zinc within specific minerals located in silver-bearing ore deposits. The most common silver-bearing mineral is galena (PbS), which can contain 0.1-0.2% silver. Other deposits may contain argentite (Ag2S), pyrargyrite (Ag3SbS3), stephanite (Ag5SbS4), polybasite (Ag6Sb2S7), or native silver (Ag). Significant silver deposits have been found across the Americas, Australia, Europe, and Asia.
How is silver-bearing ore extracted from rocks?
The first step in extracting silver from rocks involves mining the silver-bearing ore. Underground mining techniques like blasting and digging shafts are used to reach ore deposits deep below the Earth’s surface. Open-pit mining may be utilized when the ore body is close enough to the surface. Once ore has been blasted or dug out, it is transported to a processing facility for crushing and grinding.
Crushing and grinding silver ore
At the processing facility, the silver-bearing ore goes through a series of crushing and grinding processes to break down the rock into small particles and expose the silver minerals. Jaw crushers, cone crushers, and impact crushers are used for primary crushing. Then the ore is ground in ball mills or rod mills to create a fine “slurry” that will undergo chemical processing. Grinding helps liberate the silver minerals for separation and concentration.
How is silver extracted from crushed and ground ore?
There are several techniques used to extract silver from crushed ore, including cyanide leaching, thiourea leaching, thiosulfate leaching, and chlorination. Here are some of the most common methods:
Cyanide leaching
Cyanide leaching is considered the standard process for silver extraction. When crushed ore is piled into heaps or placed in tanks, a cyanide solution is sprayed or dripped on top and percolates through the heap leaching valuable metals. The cyanide complexes with silver ions forming a stable soluble complex that can be recovered from the leachate solution.
Thiourea leaching
Thiourea is a less toxic alternative to cyanide for leaching silver. The thiourea forms a complex with silver ions that can be extracted into solution. Similar to cyanide, thiourea leaching can be used in heap, vat, or agitation tank leaching processes.
Thiosulfate leaching
Ammoniacal thiosulfate leaching solutions can also solubilize silver from ore. This process is more selective toward silver extraction than cyanide, but can be slower reacting. Oxygen must be added to maintain thiosulfate stability.
Chlorination
Roasting crushed ore with chloride salts like sodium chloride or calcium chloride allows the silver to be converted to silver chloride, which can then be smelted into pure silver metal. This method helps isolate silver from copper and lead minerals.
How is leached silver recovered and purified?
Once the silver has been leached into solution, there are several ways it can be recovered and purified into the high-purity silver metal required for markets:
Precipitation
One recovery method is precipitating the silver out of solution using zinc dust replacement or chemical reagents like sodium sulfide or hyposulfite. The silver precipitate is then filtered, dried, and smelted.
Ion exchange
Passing the silver-bearing solution through an ion exchange column allows the silver ions to be exchanged for hydrogen ions. The silver is later recovered by stripping the ions off the column.
Solvent extraction
Organic solvents that selectively complex with silver can extract the silver ions out of solution. After separation from the aqueous leachate, the silver is recovered from the organic phase.
Electrowinning
Applying an electrical current to the leachate causes the silver to plate onto a cathode from which it can be removed. This is one of the most common methods to recover pure silver metal.
Crystallization
Evaporating the leachate solution allows purified silver crystals to precipitate out, which can be filtered, dried, and melted into high-purity silver bars called doré.
What are the environmental concerns with silver extraction?
While great care must be taken, silver mining and extraction can be done responsibly and sustainably. Here are some of the main environmental concerns:
Cyanide use
Cyanide is extremely toxic and tailings with residual cyanide must be safely contained. New low-cyanide or non-cyanide leaching processes are being tested to reduce environmental risks.
Metal-laden waste
Mine waste and tailings often contain other metals like lead and arsenic that must be stabilized to avoid leaching into groundwater. Proper disposal in tailings ponds with runoff containment is critical.
Water use
Mining operations require large volumes of water for leaching solutions. This can deplete groundwater if not carefully managed. Recycling and conservation strategies help minimize freshwater needs.
Land impacts
Surface mining can lead to vegetation clearing and land disturbances. Reclaiming land and re-vegetating sites after mining helps restore biodiversity and soil health.
Energy consumption
Fossil fuel-powered equipment used in mining and transport produces greenhouse gas emissions. Improving energy efficiency and switching to renewable power for operations can reduce environmental footprints.
Conclusion
Extracting silver from ore is a complex endeavor that involves a series of processes including mining, crushing, chemical leaching, precipitation, smelting, and purification. While cyanide leaching has traditionally been used, less toxic alternatives are being developed to reduce environmental risks. With responsible practices for waste containment, water use, energy sourcing, and land rehabilitation, silver mining can be done sustainably. Continued innovation and technology improvements will allow silver production to keep pace with global demand while minimizing its environmental footprint.
