Electrorefining

Part of Electrochemistry

How electrolysis purifies crude metals by dissolving impure anodes and depositing pure metal at the cathode.

Why This Matters

Electrorefining is the definitive method for producing metals of the highest purity achievable by practical means. Where pyrometallurgy (smelting) reaches 98–99% purity, electrorefining achieves 99.99% (“four nines”) or better. This extra purity is essential for electrical conductors, electronic components, catalysts, and any application where trace impurities degrade performance.

The process is particularly important for copper, nickel, lead, tin, and cobalt — metals that are smelted from ores but require higher purity than smelting provides. In a rebuilding context, electrorefining unlocks the difference between copper wire that degrades performance at 99% purity and copper wire that matches modern standards at 99.99%.

The economics are also compelling: anode slime from copper refining regularly contains silver, gold, and platinum group metals worth more per kilogram than the copper itself. Electrorefining pays for its electricity while concentrating precious metals for extraction.

The Principle

Electrorefining uses the selectivity of electrochemical potentials. In a copper sulfate bath:

• At very low voltage (0.2–0.35 V), only copper dissolves at the anode and deposits at the cathode.
• Metals more noble than copper (Ag, Au, Pt) do not dissolve — they fall from the anode as slime.
• Metals less noble than copper (Fe, Ni, Zn, As) dissolve with copper from the anode, but do not deposit at the cathode under normal conditions — they accumulate in solution.

The result: the cathode accumulates only pure copper, while impurities are rejected either as slime (precious metals) or as accumulating solution contaminants (base metals).

This principle applies to any metal, not just copper. The electrolyte is chosen to dissolve the target metal selectively.

Metals Refined Electrolytically

Metal	Electrolyte	Operating Temperature	Key Application
Copper	CuSO₄ + H₂SO₄	60–65°C	Electrical wire
Nickel	NiSO₄ + NiCl₂ + H₃BO₃	60°C	Stainless steel, batteries
Lead	Pb(SiF₆)₂ + H₂SiF₆	Ambient	Batteries, shielding
Tin	Sn(BF₄)₂ or SnSO₄ + H₂SO₄	25°C	Soldering, tinplate
Cobalt	CoSO₄ + H₃BO₃	50°C	Batteries, superalloys
Silver	AgNO₃	Ambient	Jewelry, electronics
Gold	HAuCl₄ + HCl	Ambient	Electronics, jewelry

Cell Design for Electrorefining

A basic electrorefining cell uses alternating anodes and cathodes in a single tank:

Cathode — Anode — Cathode — Anode — Cathode

All anodes are connected to positive bus; all cathodes to negative bus. This maximizes productive electrode area per tank volume.

Key dimensions:

• Inter-electrode gap: 30–50 mm for copper refining (tighter than plating — better efficiency, but risk of short circuit from dendrites if maintained badly)
• Anode: ~15 kg cast slab for small operations; 350–400 kg commercial
• Cathode: Thin starting sheet of pure metal, or stainless steel blank for stripped-sheet method

Anti-short circuit: As anodes dissolve, anode slime accumulates in cloth bags draped around each anode. This prevents slime from bridging to the cathode, which would create a short circuit and locally reverse deposition.

Process Control

Current Density

Optimal current density is lower in refining than in plating:

• Copper electrorefining: 200–300 A/m²
• Higher current densities produce rough, nodular deposits and draw down noble impurity potential

Bath Temperature

Higher temperature: better conductivity, faster ionic diffusion, more uniform deposits. Copper refining operates at 60–65°C — warm but not boiling. Temperature control via heat exchangers.

Electrolyte Circulation

Pumping electrolyte through the cell ensures uniform composition across all electrode surfaces. Concentration gradients (depletion at cathode, buildup at anode) cause non-uniform deposit quality.

Bleed and Feed

As refining proceeds, base metal impurities (Fe, Ni, As) accumulate in the electrolyte. Above threshold concentrations, they begin to co-deposit on the cathode, degrading purity. Manage by:

• Bleed and feed: Withdraw a portion of spent electrolyte, process it to recover copper by cementation (iron scrap displaces copper from solution), and return purified electrolyte.
• Crystallization: Cool electrolyte to crystallize out nickel sulfate if nickel accumulation is high.

Cycle Time

Anodes are consumed at a rate determined by current density:

Anode weight dissolved (kg) = (current × time × atomic weight) / (Faraday’s constant × valence)

For copper (atomic weight 63.5, valence 2): At 300 A/m², 1 m² anode: 1 kg copper dissolved in 300×3,600×63.5/(96,485×2) = 3.57 hours

A 15 kg anode in a 0.5 m² cell dissolves in approximately 28 days. Commercial operations time anode cycles for 21 days, then strip cathodes and install fresh anodes and starting sheets.

Anode Slime Recovery

After anodes are consumed, anode slime bags contain:

• 15–30% copper (recycled to blister copper smelting)
• 100–5,000 g/ton silver
• 1–30 g/ton gold
• 0.5–5 g/ton platinum group metals
• Lead, bismuth, selenium, tellurium

Processing route:

1. Filter and wash slime to remove copper sulfate.
2. Leach with dilute H₂SO₄ + air — removes remaining copper and some base metals.
3. Roast to convert Se and Te to oxides.
4. Smelt the decopperized slime to produce Doré metal (silver-gold alloy).
5. Refine Doré by parting: dissolve in nitric acid (silver dissolves, gold doesn’t) or electrolytic parting (electrolysis of silver nitrate solution, gold falls as slime).

The precious metal content of anode slime often exceeds the value of the electrical power consumed in refining — making electrorefining a net-positive economic activity in any society that values silver and gold.