Low Current Efficiency in Electrowinning: The Five Losses You Can Actually Measure
Current efficiency (CE) is the fraction of current that actually deposits metal. Losses show up as acid mist, gas evolution, re-dissolution and short circuits. Each has a different signature, so measure before changing the circuit.
The five losses
| Loss | Signature | Typical magnitude | Action |
|---|---|---|---|
| Short circuits | Cold cathode edges, nodules, uneven deposit | 1–5% | Improve cathode alignment, edge strips, spacing |
| Iron / impurity cycling | CE falls as Fe rises; redissolution at cathode | 2–10% | Purge iron upstream; control Fe < 2 g/L |
| Poor mass transfer | CE falls at low flow; rough deposit | 2–8% | Raise circulation, check sparger layout |
| Re-dissolution | CE falls when acid rises or current stops | 1–5% | Control acid, avoid open-circuit stands |
| Gas crossover / mist | Acid mist, low CE with normal deposit quality | 1–3% | Check diaphragm, anolyte level, mist suppressant |
How to measure CE properly
Weigh cathodes before and after a fixed period, and calculate deposited metal against theoretical from current × time × electrochemical equivalent. A single cell measurement tells you little; measure at least three cells and compare. A spread between cells points to shorts or flow distribution, not chemistry.
Impurity effects are non-linear
Iron is the classic culprit: it cycles between Fe²⁺ and Fe³⁺ at the anode and cathode, consuming current without depositing. Small increases have little effect until a threshold, then CE drops sharply. If CE fell suddenly and the feed has not changed, look for iron ingress from MOC corrosion or upstream steel equipment.
Operating windows
Current density, acid concentration, temperature and circulation interact. Pushing current density to raise output often lowers CE more than it gains in kilograms, so total daily production can fall. Verify with a short trial at two current densities before committing.
Send cell voltage, CE, feed and spent electrolyte analyses, and we will identify the dominant loss and the cathode/flow configuration to fix it.