The production of usable copper metal requires a purification process to remove impurities found in the natural ore. Copper refining transforms raw mineral concentrates into a highly pure metal suitable for industrial and technological applications. The remarkable electrical conductivity of copper, which makes it indispensable for wiring and electronics, is only achieved when the metal reaches a purity exceeding \(99.99\%\). This high standard necessitates sophisticated, multi-stage metallurgical techniques to separate copper from other elements.
Preparing Copper Ore for Processing
The journey from mined rock to metal begins with increasing the concentration of copper minerals. The initial step is comminution, which involves crushing and grinding the ore into a fine powder. This process liberates the minuscule copper mineral particles from the surrounding, unwanted gangue rock.
Once the ore is finely ground, it is mixed with water and chemical reagents to create a slurry for mineral flotation. Flotation is a physical-chemical separation process that exploits the surface properties of the minerals. Chemical collectors, such as xanthates, are added to selectively coat the copper mineral particles, making them hydrophobic, or water-repellent.
The slurry is then agitated in large tanks while air is continuously blown through the mixture. The hydrophobic copper particles attach to the rising air bubbles, which are stabilized by frother chemicals. This creates a stable froth layer on the surface that is skimmed off, resulting in a copper concentrate typically containing \(20\%\) to \(30\%\) copper.
Smelting and Converting to Blister Copper
The concentrated copper sulfide material is subjected to pyrometallurgy, a high-heat process. The first stage, smelting, involves melting the concentrate in a furnace, often with a silica flux, at temperatures around \(1,200^\circ\text{C}\) to \(1,400^\circ\text{C}\). This separates the material into two liquid layers: a lighter slag layer containing most of the iron and silicates, and a heavier, molten copper iron sulfide layer known as matte. The matte is generally composed of \(50\%\) to \(70\%\) copper, with the remaining content being primarily iron and sulfur.
The molten copper matte is then transferred to a converter vessel for the second thermal stage, known as converting. Air or oxygen-enriched air is blown through the liquid matte, which rapidly oxidizes the remaining iron and sulfur. The iron combines with the added flux to form a final iron silicate slag, which is poured off.
The sulfur reacts with oxygen to form sulfur dioxide (\(\text{SO}_2\)) gas, which is captured. This oxidation continues until nearly all the iron and sulfur are removed, leaving behind a \(98.5\%\) to \(99.5\%\) pure product called blister copper. The name “blister copper” comes from the appearance of the metal as it solidifies, since the escaping \(\text{SO}_2\) gas creates large surface blisters and pits.
Achieving High Purity Through Electrorefining
To meet the purity requirements for electrical applications, blister copper must undergo a final, electrochemical purification step called electrorefining. This process takes place in large electrolytic cells filled with an electrolyte solution of copper sulfate and sulfuric acid. The impure blister copper is cast into thick anode plates, while thin starter sheets of pure copper or stainless steel blanks serve as the cathodes.
When a controlled direct current is applied across the cell, the impure copper atoms at the anode lose electrons and dissolve into the electrolyte as copper ions (\(\text{Cu}^{2+}\)). Simultaneously, the pure copper ions already present in the solution migrate toward the negatively charged cathode. There, they gain electrons and plate out as extremely pure metallic copper.
The process achieves its high separation efficiency by precisely controlling the voltage. At this low potential, less noble impurities like nickel and iron also dissolve from the anode but remain suspended in the electrolyte, as they are not easily reduced and deposited at the cathode. Conversely, more noble impurities, such as gold, silver, and platinum, do not dissolve at all and fall to the bottom of the cell, forming a valuable sludge known as anode slimes.
Chemical additives are introduced into the electrolyte to ensure a smooth, dense, and high-quality copper deposit on the cathode. After several days, the thick copper deposit is stripped from the cathode, yielding copper that is \(99.99\%\) pure, the industry standard for high-conductivity metal.
Hydrometallurgy as an Alternative Method
While pyrometallurgy is the standard for high-grade sulfide concentrates, hydrometallurgy is used for low-grade oxide ores or mining waste. This process uses chemical solvents rather than high heat for extraction. The first step is leaching, where a solvent, typically a dilute sulfuric acid solution, is poured over the crushed ore, dissolving the copper into a liquid solution called the pregnant leach solution.
The copper-rich solution then moves to the solvent extraction (SX) stage, which is necessary to separate the copper from other dissolved impurities. Here, the aqueous solution is mixed with a specific organic solvent containing a chemical that selectively bonds with the copper ions. The copper is essentially stripped from the initial leach solution and concentrated into a purified electrolyte.
The final stage is electrowinning (EW). In this process, the purified, concentrated copper solution is placed in an electrolytic cell with insoluble anodes. An electric current is passed through the solution, causing the copper ions to directly plate onto the cathodes as high-purity metal. This method offers a more energy-efficient route for certain types of copper deposits, bypassing the need for initial smelting entirely.