Copper electrorefining is the core process for producing high-purity cathode copper. Impurities dissolved in the electrolyte strongly affect current efficiency, cell voltage, anode passivation and final cathode quality. Based on electrochemical potential relative to copper, dissolved impurities are generally divided into four categories.
1. Elements with Significantly More Negative Potential than Copper: Zn, Fe, Sn, Pb, Co, Ni
Most of these metals dissolve from copper anodes into the sulfuric acid electrolyte. Lead and tin mostly precipitate as insoluble salts or oxides, while zinc, iron, cobalt and nickel continuously accumulate in the circulating electrolyte. This group of impurities consumes sulfuric acid and raises electrolyte resistivity.
Zinc dissolves into the electrolyte as zinc sulfate. Its much more negative electrode potential prevents zinc deposition on the cathode. It does not directly disrupt copper electrodeposition, yet rising zinc concentration increases electrolyte viscosity and density.
Iron exists in both ferrous and ferric states. Fe²⁺ can be oxidized to Fe³⁺ at the anode, lowering anode current efficiency. Fe²⁺ is also oxidized by dissolved oxygen in the solution. When Fe³⁺ migrates toward the cathode, it is reduced back to Fe²⁺. This redox cycle reduces cathode current efficiency and increases dissolved copper ion content.
Tin enters the electrolyte mainly as stannous ions. Sn²⁺ gradually oxidizes to stannic species. Stannous ions reduce soluble arsenate into poorly soluble arsenite and trigger arsenic precipitation. Colloidal stannic acid adsorbs arsenic and antimony. If fully settled, these colloids help reduce dissolved arsenic and antimony. However, colloidal particles sticking to cathode surfaces degrade cathode copper quality. When tin concentration exceeds 1 g/L, insufficient acidity or temperature drop can trigger massive precipitation of stannic acid (SnO₂·H₂O). Heavy tin contamination of cathodes occurs, and stannic acid may wrap the anodes, hindering anode dissolution and raising cell voltage.
In acidic electrolyte, lead sulfate can further oxidize into brown lead dioxide, forming PbSO₄, PbO or PbO₂ films on the anode surface. High lead levels increase resistance and push up cell voltage.
Nickel dissolves and may form nickel-copper sulfate or complex precipitates with silver, arsenic and selenium. With arsenic and antimony present, nickel combines with them to form nickel mica, a complex double oxide salt soluble in copper matrix. Nickel oxide and nickel mica form tightly attached anode slime films. Excessive buildup creates a hard crust on anodes, resulting in anode passivation and higher electrode potential.
2. Elements with Significantly More Positive Potential than Copper: Ag, Au, Platinum Group Metals
These noble metals do not dissolve during electrolysis. They remain in solid form and enter anode slime. This fraction can be recovered as valuable by-products from anode slime treatment.
3. Elements with Potential Close to Copper but Slightly Negative: Arsenic, Antimony, Bismuth
Arsenic, antimony and bismuth have electrode potentials near copper. Under normal electrolysis conditions, they rarely deposit directly onto the cathode. Part of these elements hydrolyze into solid oxides, while others build up in the circulating electrolyte.
Arsenic and antimony dissolve as trivalent ions. As(III) and Sb(III) readily hydrolyze into arsenite (AsO₃³⁻) and antimonite (SbO₃³⁻). Monovalent copper ions plus dissolved oxygen generate active oxygen, oxidizing part of the trivalent species into arsenate (AsO₄³⁻) and antimonate (SbO₄³⁻). Therefore, arsenic and antimony coexist in both trivalent and pentavalent forms inside the electrolyte.
Cross-valence compounds such as As₂O₃·Sb₂O₅ and Sb₂O₃·As₂O₅ form fine flocculent particles, usually less than 10 μm. These tiny particles settle slowly and float in the electrolyte, adsorbing colloids and other compounds to form floating anode slime. Floating slime limits arsenic and antimony accumulation in the solution, yet the fine particles mechanically adhere to cathode surfaces or get trapped inside copper grains, seriously reducing cathode copper quality.
4. Other Impurities: Oxygen, Sulfur, Selenium, Tellurium, Silicon
These elements exist in electrolyte as colloids, simple ions or complex compounds. They participate in side reactions, form precipitates, affect slime characteristics and may cause surface defects on cathode copper.
Recommended Electrolyte Concentration Control Limits
Stable impurity monitoring is essential for consistent copper cathode quality and stable tankhouse operation. Typical control ranges are listed below:
- Arsenic: 1–5 g/L, maximum ≤13 g/L
- Antimony: 0.2–0.5 g/L, maximum ≤0.6 g/L
- Bismuth: 0.01–0.3 g/L, maximum ≤0.5 g/L
- Chloride: ≤0.2 g/L
- Iron: ≤0.5 g/L
- Zinc: ≤80 g/L
Conclusion
Impurity management is one of the key controls for copper electrorefining. Different impurities present unique electrochemical behaviors. Some increase cell voltage and energy consumption; others form floating slime and contaminate cathode copper. Strict monitoring and periodic electrolyte purification help maintain stable production, high cathode purity and optimal current efficiency.
