Silver (Ag, from the Latin argentum) has been a prized commodity for millennia, valued for its lustrous appearance and unique properties. Historically used for jewelry and as a medium of exchange, the metal is now indispensable in modern technology. Its remarkable thermal and electrical conductivity makes it a material of choice for electronics, and its antimicrobial qualities find application in medical devices and water purification systems. This article explores the process required to transform raw ore buried deep within the earth into high-purity bullion ready for industrial and investment markets.
Sources of Silver Ore
The geological occurrence of silver dictates the complexity of its extraction, as the metal is rarely found in isolation. Only about 28% of global silver production originates from mines where silver is the primary metal sought. The vast majority, approximately 72%, is recovered as a byproduct from deposits mined principally for other base metals.
These are known as polymetallic ores, where silver is chemically bound or intimately mixed with sulfides of copper, lead, or zinc. Silver is also frequently found in association with gold, sometimes forming a natural alloy called electrum. The presence of these multiple elements means the initial mining operation often focuses on the economics of the base metal, with silver being a valuable secondary recovery.
Primary Mining Operations
The physical extraction of the silver-bearing ore relies on two distinct methods, chosen based on the depth and geometry of the deposit. Open-pit mining is utilized when the ore body is large, relatively low-grade, and located near the surface, typically less than one kilometer deep. This technique involves removing the overburden—the soil and rock covering the deposit—to expose the ore. The exposed ore is then excavated in a series of descending benches or steps using large machinery.
For higher-grade ores found in deep, narrow, or complex vein structures, underground mining is the preferred method. This involves sinking vertical shafts and digging horizontal drifts and tunnels to access the ore body directly. While underground mining is generally more expensive than its surface counterpart, it is necessary for accessing deposits that are uneconomical to reach otherwise. After blasting with explosives, the raw ore is collected and transported to the surface for processing.
Ore Processing and Concentration
Once the raw ore is brought to the surface, the first step is comminution, a process of crushing and grinding. Large crushers reduce the rock to smaller pieces, which are then fed into rotating mills that grind the material into a fine powder. This process significantly increases the surface area for subsequent chemical treatment and liberates the microscopic silver-bearing mineral particles from the surrounding host rock.
The most common separation technique for sulfide ores is froth flotation, which leverages the different surface properties of minerals. The fine ore powder is mixed with water and specialized chemical reagents, including collectors that adhere to the silver mineral particles, making them hydrophobic. Air is then pumped into the mixture, causing the silver-laden particles to attach to the bubbles and rise to the surface, forming a mineralized froth that is skimmed off.
For oxide ores or to recover silver from the flotation concentrate, chemical leaching, often using a sodium cyanide solution, is employed. In this process, the silver chemically dissolves into the solution, forming a silver-cyanide complex. This complex is then separated from the solid waste material, resulting in a silver-rich solution or high-grade concentrate.
Final Refining and Purity
The silver concentrate or the product from the leaching process must undergo final metallurgical steps to achieve marketable purity. Concentrates are typically smelted in a furnace to produce dore bars, which are a semi-pure alloy consisting mainly of silver and gold, along with residual base metals. These dore bars still contain impurities that must be removed.
The refining process for high-purity silver most commonly involves electrolysis, or electro-refining, which can produce silver crystals with a purity of 99.9% or higher. In this method, the dore bar is cast to form an anode, which is placed in an electrolytic cell containing a silver nitrate solution. When an electric current is passed through the cell, the silver at the anode dissolves and migrates through the solution as positively charged ions.
The silver ions then deposit onto a cathode as pure silver crystals, while less noble metals like copper also dissolve but remain in the solution. More valuable impurities, such as gold and platinum group metals, do not dissolve and fall to the bottom of the cell, forming an anode slime that is collected for separate processing. The final step involves melting the high-purity silver crystals and casting them into commercial bullion bars.