Copper (Cu) is a reddish-orange metal prized for its exceptional thermal and electrical conductivity, second only to silver among pure metals. This unique combination of properties makes it an indispensable material for modern infrastructure, particularly in electrical wiring, electronics, and power transmission systems. Extracting usable copper from its naturally occurring ore involves a complex, multi-stage industrial process necessary to achieve the high commercial purity required for these applications. The overall method chosen depends primarily on the mineral type, with different routes used for sulfide-based and oxide-based ores.
Preparing the Ore for Extraction
Physical preparation begins with crushing the large pieces of ore, often using multiple stages of crushers, to reduce the material to a manageable size. This material then undergoes grinding, or milling, where it is pulverized into a fine powder, often to a fineness where 60% to 80% of the particles are smaller than 200 mesh. This fine particle size is necessary to physically separate the copper mineral from the surrounding non-valuable rock, known as gangue.
Once the ore is finely ground and mixed with water to create a slurry, the concentration process begins, typically through froth flotation. Chemical reagents, such as collectors like potassium ethylxanthate, are added to selectively make the copper minerals hydrophobic, or water-repellent. Air is then injected into the slurry, causing the hydrophobic copper particles to attach to the bubbles and rise to the surface, forming a mineral-rich froth. This froth, which can contain 20% to 30% copper, is skimmed off and dried to create a high-grade copper concentrate, while the gangue minerals sink to the bottom.
High-Temperature Extraction (Pyrometallurgy)
Copper concentrate, typically a sulfide ore, is primarily processed using high-temperature extraction (pyrometallurgy). The initial stage is smelting, where the concentrate is heated in a furnace, such as a flash furnace, to temperatures often exceeding 1,200°C.
This extreme heat melts the material, causing a chemical reaction that removes a large portion of the iron and sulfur impurities. The process separates the molten material into two distinct layers: a dense, copper-rich layer called copper matte (containing approximately 58% to 60% copper) and a lighter, waste layer called slag. The lighter slag, composed mainly of iron silicates, floats on the surface and is skimmed off for disposal or further processing.
The copper matte is then transferred to a converter vessel where air or oxygen is blown through the molten material to complete the chemical transformation. This blowing step rapidly oxidizes the remaining iron and sulfur. The sulfur escapes as sulfur dioxide gas, which is typically captured and converted into sulfuric acid as a valuable byproduct. The result is an impure copper product known as blister copper, which is typically 98% to 99.5% pure.
Chemical Extraction (Hydrometallurgy)
An alternative method, hydrometallurgy, is used predominantly for lower-grade oxide ores and mining waste. This solution-based technique avoids the high temperatures of smelting and involves dissolving the copper from the ore using chemical solutions. The first step is leaching, where a weak acid solution, most commonly sulfuric acid, is trickled over a large heap of crushed ore. The acid reacts with the copper minerals, dissolving the copper and forming an aqueous solution of copper sulfate.
This copper-containing solution, referred to as the pregnant leach solution, is then sent through solvent extraction (SX). In this stage, copper ions are selectively transferred from the weak acid solution into a specialized organic solvent. The organic solvent is then separated from the remaining aqueous solution, which can be recycled for further leaching. The copper is subsequently stripped from the organic solvent using a stronger acid solution, creating a purified, highly concentrated electrolyte.
The final step in this chemical route is electrowinning (EW). The concentrated copper electrolyte is introduced into a cell where an electric current is passed between an inert anode and a cathode. The current causes the pure copper ions in the solution to be electrochemically reduced and deposited directly onto the cathode as solid metal. The copper produced by this SX/EW method is already highly pure, often rivaling the purity of electro-refined copper from the pyrometallurgical route.
Final Purification and Refining
Regardless of whether the copper was produced as blister copper from pyrometallurgy or as a highly pure product from electrowinning, the final commercial requirement for electrical applications demands ultra-high purity. This final purification is achieved through a process called electro-refining. The impure copper, typically cast into large plates called anodes, is submerged in an electrolyte solution of copper sulfate and sulfuric acid.
Thin sheets of highly pure copper or stainless steel act as the cathodes. When a direct electric current is applied, the copper atoms from the impure anode dissolve into the electrolyte as copper ions. These copper ions then migrate across the solution and plate out exclusively onto the pure cathode. Impurities fall to the bottom of the cell as an anode slime, while others remain dissolved in the solution. This controlled electrochemical separation produces copper cathodes with an exceptional purity of 99.99%, which is necessary for maximizing electrical conductivity.