Copper refining transforms impure copper (about 99% pure) into cathodes that are 99.99% pure, primarily through a process called electrorefining. The method works by dissolving copper from a dirty slab into a chemical bath and replating it, atom by atom, onto a clean sheet, leaving impurities behind. This is how the vast majority of the world’s high-purity copper is produced.
Two Routes to Refined Copper
The refining method depends on the type of ore the copper came from. Sulfide ores, which account for the bulk of global copper production, follow a pyrometallurgical route: the ore is mined, crushed, concentrated through flotation, dried, smelted in a furnace, and then converted into impure copper anodes. Those anodes then go through electrolytic refining as the final purification step.
Oxide ores and lower-grade sulfide ores take a different path called solvent extraction and electrowinning (SX-EW). In this hydrometallurgical approach, the ore is leached with acid to dissolve the copper into solution. That solution is purified and concentrated through solvent extraction, then the copper is plated out in electrowinning cells. The end product is still a high-purity cathode, but the copper never passes through a furnace. Both routes produce copper pure enough for electrical wiring and electronics.
How Electrorefining Works
Electrorefining is the core of the traditional process. It starts with anode slabs: thick plates of 99% pure copper cast from molten metal. Each slab is roughly three feet wide, three-and-a-half feet tall, two inches thick, and weighs about 750 pounds, with handles molded into the top for lifting.
These anodes are lowered into large tanks filled with an electrolyte solution of copper sulfate and sulfuric acid. Industrial electrolytes typically contain 5 to 90 grams per liter of dissolved copper and 20 to 100 grams per liter of sulfuric acid, heated to around 60°C (140°F). Thin starter sheets of pure copper, or increasingly stainless steel blanks, hang in the tank as cathodes.
When electric current flows through the tank at a density of about 200 amps per square meter, copper atoms at the anode lose electrons and dissolve into the solution as positively charged ions. Those ions migrate through the liquid and deposit onto the cathode, building up a layer of extremely pure copper. The current is selective: copper plates out cleanly, while other metals either drop to the bottom of the tank as sludge or stay dissolved in the solution.
After about 14 days, the anodes have largely dissolved away. The cathodes, now weighing around 375 pounds each, are lifted out and rinsed with water to stop any further reaction. What remains is 99.99% pure copper, ready to be melted and formed into wire, tubing, sheet, or rod.
What Happens to the Impurities
The sludge that collects at the bottom of the refining tanks, called anode slime, is far from waste. It contains a concentrated mix of valuable elements: gold, silver, platinum, palladium, selenium, and tellurium. These metals don’t dissolve easily under the electrical conditions used for copper, so they settle out and accumulate over time.
Recovering these byproducts is a significant source of revenue for copper refineries. The slime is typically processed through high-temperature smelting to produce a precious metal alloy rich in gold and silver. That alloy is then further separated through oxidation (to remove lead) and additional electrolysis to isolate pure gold, pure silver, and platinum-group metals individually. Selenium and tellurium, both critical for solar cells and semiconductor manufacturing, are recovered through separate hydrometallurgical steps. For some refineries, the value of these byproducts meaningfully offsets the cost of the refining operation itself.
Purity Standards for Finished Copper
Not all “pure” copper is equal. The global benchmark is London Metal Exchange (LME) Grade A copper, which requires 99.99% minimum purity. The LME sets strict limits on individual contaminants: arsenic and lead must each be below 0.0005% (5 parts per million), and the combined total of arsenic, cadmium, chromium, manganese, phosphorus, and antimony cannot exceed 0.0015%. Copper that meets these specifications trades at the standard market price. Copper that falls short trades at a discount and is limited to less demanding applications.
These tight tolerances matter because even tiny amounts of certain impurities degrade copper’s electrical conductivity. A few extra parts per million of arsenic or antimony can measurably reduce the performance of copper wire, which is why electronics and power transmission rely on fully refined cathode copper.
The SX-EW Process for Oxide Ores
When copper comes from oxide ores or low-grade deposits, smelting isn’t economical. Instead, the ore is leached, often in large open heaps sprayed with dilute sulfuric acid. The acid dissolves copper out of the rock over weeks or months, producing a weak copper-bearing solution called pregnant leach solution.
That solution passes through a solvent extraction circuit, where an organic chemical selectively grabs the copper ions and transfers them into a much more concentrated, cleaner solution. This concentrated solution then enters electrowinning cells, which work similarly to electrorefining tanks but with one key difference: instead of dissolving a copper anode, the process pulls copper directly from the liquid. Lead alloy anodes provide the electrical connection, and pure copper plates onto the cathodes.
SX-EW produces cathodes that meet the same 99.99% purity standard as electrorefining. The trade-off is that it doesn’t generate precious-metal-rich anode slime, since those metals were never concentrated in the first place. It also captures less selenium than the traditional smelting route, which has implications for the global supply of that element.
Environmental Considerations
Copper refining generates several waste streams that require management. Smelting operations produce sulfur dioxide gas, which modern plants capture and convert into sulfuric acid (conveniently recycled back into the leaching or electrolyte circuits). The electrolyte solution gradually accumulates dissolved impurities like nickel and arsenic that must be periodically bled off and treated.
Wastewater containing dissolved copper and other heavy metals is treated through a combination of chemical precipitation, membrane filtration, ion exchange, and electrochemical methods. Nanofiltration has gained traction as a treatment option because of its relatively low energy consumption and ability to approach zero liquid discharge. The electrowinning route produces its own residues, including organic crud from the solvent extraction step and cell sludge from the plating tanks, both of which require proper disposal or recycling.