Manganese Electrowinning: Selenium, Diaphragm, Yield

Last updated: 2026-11-06

Manganese electrowinning deposits electrolytic manganese metal (EMM) from purified manganese sulfate electrolyte at 65–72% current efficiency and 6,300–7,200 kWh/t of DC energy. Three systems make those numbers possible: diaphragm cells isolating each cathode, an ammonium sulfate–buffered near-neutral catholyte, and precisely dosed selenium dioxide suppressing hydrogen evolution. This page breaks down the parameters, costs, and commissioning sequence behind the upstream leaching–purification–electrowinning flowsheet that feeds manganese and vanadium processing plants.

Key Takeaways

• Manganese electrowinning runs at 65–72% current efficiency because hydrogen evolution competes fiercely with metal deposition.
• Selenium dioxide at 0.02–0.05 g/L in the catholyte is the single most leveraged additive; it is dosed continuously, never in slugs.
• Diaphragm cells are non-negotiable: they keep catholyte near pH 7 while the anolyte sours to 25–40 g/L sulfuric acid.
• Direct-current consumption of 6,300–7,200 kWh/t makes electricity the dominant cost; power price drives plant siting.
• Most sub-70% current-efficiency events trace to selenium dosing drift, pH excursion, or a leaking diaphragm bag.

What Makes Manganese Electrowinning Different From Copper Electrowinning?

Process engineers arriving from the copper world routinely underestimate how hostile manganese chemistry is to the cathode. Five definitions frame the difference:

Manganese electrowinning is an aqueous electrometallurgical process that reduces Mn²⁺ ions from a purified manganese sulfate electrolyte onto cathode blanks, harvesting flake or powder metal that is subsequently crushed, sized, and packaged. The half-reaction Mn²⁺ + 2e⁻ → Mn has a standard potential of roughly −1.18 V versus SHE — nearly 1.5 V more negative than the Cu²⁺/Cu couple.

A diaphragm cell is an electrolytic cell in which a porous membrane bag encloses each cathode, physically separating the catholyte inside the bag from the anolyte outside it. Ionic conduction continues through the cloth, but convective mixing is blocked. Copper electrowinning — where a lab-scale copper electrowinning cell needs no internal barrier at all — cannot say the same; manganese cannot run without one.

The selenium dioxide additive is a catholyte dosing reagent that co-deposits trace selenium at the cathode surface, catalytically suppressing hydrogen evolution and refining the manganese deposit morphology. No comparable additive is required in copper tankhouses.

Current efficiency is the fraction of passed charge that actually deposits manganese rather than being consumed by hydrogen evolution, and it is the master variable of the plant’s economics. Where copper tankhouses speak comfortably of 90–98%, an EMM cellhouse celebrates 68%.

Anolyte is the liquor on the anode side of the diaphragm, which acidifies progressively as oxygen evolves and water is consumed at the anode. Managing that acid — and the MnO₂ sludge it carries — is a daily duty with no copper analogue.

The consequence of that −1.18 V potential is stark: at any ordinary acidity, hydrogen evolution is thermodynamically favored and kinetically easy. Three countermeasures define the flowsheet. First, the diaphragm holds the cathode compartment at near-neutral pH (6.5–7.2), where hydrogen overpotential is highest. Second, ammonium sulfate at 100–130 g/L buffers the catholyte against the OH⁻ generated at the cathode, preventing local caustic pockets that would redissolve manganese as manganous hydroxide. Third, selenium dioxide in the catholyte poisons hydrogen nucleation sites and promotes dense, adherent deposits. Everything else in the cellhouse — solution purification, temperature control, cycle times — exists to protect those three conditions.

One more contrast matters for equipment buyers. Copper electrowinning is documented exhaustively, including its manganese impurity behavior; if your interest is Mn as a contaminant in copper tankhouses rather than EMM production itself, see our page on manganese in copper electrowinning. The two topics share little operational DNA beyond the word “electrowinning.”

Which Electrolyte and Cell Parameters Govern Yield?

Yield in an EMM cellhouse is the product of deposition rate and current efficiency, and both are set by the catholyte chemistry and the electrical operating point summarized in Table 1.

Two interactions deserve emphasis. Raise Mn²⁺ toward the top of its band and deposition rate climbs, but the anode generates more MnO₂ sludge and cell voltage creeps up. Push current density above 450 A/m² and current efficiency typically decays because the cathode interfacial pH swings faster than the ammonium buffer can absorb it. The optimum is genuinely plant-specific, which is why every figure above carries the caveat “confirm with test work,” ideally in a lab-scale diaphragm electrolytic cell before a full cell is committed.

Why Is Selenium Dioxide Added to the Catholyte?

Without selenium, manganese deposits as a dark, friable, dendritic sponge saturated with occluded hydrogen, and manganese electrowinning collapses into the forties in current-efficiency terms. With it, the deposit is dense, silvery, and strippable. The mechanism is subtle: at the doses used, selenium does not plate as elemental metal in bulk. Instead, trace Se species reduce at the cathode and form thin MnSe/metallic-Se interlayers and surface sites that dramatically raise hydrogen overpotential and alter nucleation. The result is threefold — hydrogen evolution is suppressed (current efficiency rises), grain size refines (deposit density and adhesion improve), and dendrite growth is checked (shorts and bag punctures fall).

Dosage discipline matters more than the dose itself. Catholyte selenium is held at 0.02–0.05 g/L as Se, corresponding to a reagent consumption of roughly 0.5–1.5 kg of selenium dioxide per tonne of EMM, with the exact figure set by bleed rate and drag-out. The proven practice is continuous, metered addition proportional to current flow — a duty-cycle pump trimmed daily against catholyte assays. Slug dosing, however convenient, produces Se excursions that show up hours later as brittle cathodes.

Overdose has a real cost. Excess selenium co-deposits into the metal, pushing product Se content from a normal 20–60 ppm toward limits that crack during crushing or fail buyer specifications. Standard commercial EMM tolerates tens of ppm, but premium low-selenium product (typically <10 ppm Se, to be confirmed against the buyer’s spec sheet) cannot be made on the straight SeO₂ route; producers targeting that market switch to a sulfur dioxide–assisted deposition chemistry, trading current efficiency for spec compliance. For standard-grade campaigns, the selenium route is preferred because the 5–8 point current-efficiency gain repays reagent cost many times over; for contractually low-selenium product, the SO₂ route is preferred because no post-treatment can economically strip selenium from finished flake.

Selenium dioxide is also an occupational hazard that shapes cellhouse design. SeO₂ dust and fume are toxic — the same compound family used historically as a rodenticide — so dosing stations operate under local exhaust ventilation, operators wear respiratory protection during makeup, and dissolved selenium in spillage is treated as a controlled waste stream. These controls are budgeted from day one, not retrofitted.

How Do Diaphragm Cells Change the Cellhouse Layout?

The diaphragm bag converts a simple tankhouse into a two-liquor plant, and the layout of any manganese electrowinning cellhouse follows it. Each cathode sits inside a fabric bag (historically canvas, now typically polypropylene or polyester cloth) hung from a frame with its open top above bath level. Catholyte is fed into each bag and held a few centimeters higher than the cell liquor outside, so the net hydraulic gradient drives controlled permeation of fresh catholyte outward through the cloth — typically a slow, laminar seepage that delivers Mn²⁺ while keeping acid out of the cathode compartment.

Outside the bags, the anolyte is a different animal. As oxygen evolves at the Pb–Sn–Ag anodes, acidity builds to 25–40 g/L H₂SO₄ (typical range, confirm with test work) and insoluble MnO₂ precipitates as a voluminous sludge that must be bled, settled, and — in most flowsheets — re-reduced with SO₂ to recycle manganese back to the leach. That anolyte loop needs its own pumps, thickeners, and tankage beside the cells.

The layout consequences stack up quickly:

• Cathode blanks of 0.6–1.2 m² active area, with 20–40 blanks per cell, each individually bagged and sealed at the top rail.
• Bag-level differential liquid heads maintained by calibrated feed standpipes; a lost head on one bag silently starves its cathode.
• Diaphragm cloth life of 6–18 months depending on weave, temperature, and deposit practice, with bag inspection scheduled around harvest cycles.
• Acid mist capture over the anode compartments — local exhaust hoods or covered cells with mist suppressants — because 30 g/L acid at 40 °C puts a corrosive aerosol into the aisle air. For EMM service, woven polypropylene cloth is preferred because it resists 30 g/L acid at 40 °C and holds a stable head differential far better than canvas.
• Separate catholyte and anolyte distribution headers so the two liquors never share a pump or a sump.

Maintenance discipline follows the geometry. A bag leak does not announce itself with a voltage alarm; it announces itself weeks later with a soft cathode. The practical inspection habit is chemical, not electrical: periodic spot checks of catholyte acidity inside sampled bags catch cross-contamination far earlier than cell-voltage trending does.

Why Does Current Efficiency Fall Below 70%?

Every manganese electrowinning operator carries a mental checklist for current-efficiency excursions. The seven causes below cover the overwhelming majority of sub-70% events; each is presented as symptom, cause, then remedy.

1. Selenium too low. Symptom: dark, spongy deposits and rising hydrogen bubble activity at the cathode; current efficiency drifts down over days. Cause: dosing pump wear, blocked line, or an assay drift below 0.02 g/L Se. Remedy: verify catholyte Se assay immediately, restore continuous dosing, and audit the pump calibration.
2. Selenium too high. Symptom: brittle flakes that crack in the crusher, elevated product Se, occasionally gassy deposits. Cause: slug addition or a double-stroke pump. Remedy: cut dosing to the assay target, hold production cycles shorter until product Se falls back into spec.
3. pH drift in the catholyte. Symptom: efficiency loss plus white Mn(OH)₂ haze above 7.2 or vigorous gassing below 6.5. Cause: buffer depletion, feed-strength error, or lime/ammonia make-up failure. Remedy: tighten (NH₄)₂SO₄ make-up control and re-center automatic pH trim on the catholyte header.
4. Mn²⁺ depletion. Symptom: falling deposition rate at constant current, lower efficiency, brownish catholyte. Cause: leach or reductive leach underdelivering relative to the cellhouse draw. Remedy: raise upstream Mn²⁺ generation, or trim current until the catholyte stabilizes at 12–18 g/L.
5. Ammonia/ammonium loss. Symptom: pH swinging faster than usual between trim actions; buffer titration falling below 100 g/L. Cause: drag-out, bleed losses, or inadequate (NH₄)₂SO₄ inventory control. Remedy: rebalance the bleed-and-feed and re-verify assay frequency.
6. Diaphragm breach. Symptom: one cell’s efficiency drops sharply while its voltage barely moves — the classic silent failure. Cause: torn bag, failed top seal, or cloth blinding reversal. Remedy: test suspect bags for acid ingress (spot acidity checks beat voltage trending here), re-bag, and re-balance the head differential.
7. Heavy-metal impurities (Zn, Fe, Ni, Co). Symptom: patchy deposits, localized burning, chronic efficiency shortfall of a few points. Cause: ppm-level impurities co-deposit or create micro-galvanic cells on the cathode surface; solution purification upstream has slipped. Remedy: audit purification — sulfide precipitation and cementation stages, applied with the same rigor seen in vanadium solvent extraction — and hold impurity targets at the levels proven in your test work.

Table 2 condenses the sensitivity picture for quick triage.

(All sensitivity magnitudes are indicative, typical range, to be confirmed by test work.)

What Does Manganese Electrowinning Cost per Tonne?

Electricity owns the cost structure. At a design point of 4.9 V cell voltage, 68% current efficiency, and a theoretical demand of about 975 Ah/kg for Mn²⁺ → Mn, the direct-current energy works out to roughly 7,000 kWh/t — squarely inside the 6,300–7,200 kWh/t industry band. At $0.06/kWh, power alone costs about $420/t of EMM, typically 55–65% of cash conversion cost (assumptions: DC metered at cell rectifier, tonne of saleable metal, plant-specific power price to be confirmed).

The worked sizing example shows how the numbers close. Take a 20,000 t/y EMM plant running 8,000 h/y: required production is 20,000 ÷ 8,000 = 2.5 t/h. At 7,000 kWh/t the DC load is 2.5 × 7,000 = 17,500 kWh ≈ 17.5 MW. With 4.9 V per cell and 12 kA per cell, each cell absorbs 4.9 × 12 = 58.8 kW, so 17,500 ÷ 58.8 ≈ 298 cells — install 300 for harvest and outage margin. The current check closes: 300 × 12 kA = 3.6 MA, and at 68% efficiency that deposits 3,600,000 Ah/h × (1.026 kg/kAh theoretical) × 0.68 ≈ 2.5 t/h. Cathode area follows: 12,000 A ÷ 400 A/m² = 30 m² of active cathode per cell, satisfied by 33–34 blanks of 0.9 m² — comfortably within the 20–40 blanks-per-cell convention.

Around that power bill, the remaining cost lines are smaller but real (all indicative; confirm with current vendor pricing):

• Selenium dioxide: 0.5–1.5 kg/t SeO₂ consumption; at typical reagent pricing this lands in the low tens of dollars per tonne — a rounding error against its efficiency contribution.
• Sulfur dioxide for MnO₂ reduction: the anode loop exports roughly 30–50 kg of MnO₂ solids per tonne of EMM; reductive leaching that sludge with SO₂ consumes on the order of 20–40 kg SO₂/t EMM, stoichiometry plus excess (typical range, confirm with test work).
• Ammonium sulfate makeup: buffer drag-out and bleed losses drive continuous (NH₄)₂SO₄ purchases; small per tonne but a logistics line worth contracting early.
• Stripping labor: manganese is stripped manually or semi-mechanically every 24–48 h; labor and crane time per harvest cycle is a genuine cost line at scale.
• Depreciation and diaphragm consumables: cell furniture, bags (6–18 month life), anode refurbishment, and rectifier maintenance amortize into the fixed block.

No public cost table exists for EMM, so every figure above is presented as an indicative build-up with stated assumptions, to be replaced by site-specific quotations during feasibility.

How Do You Commission a New EMM Cellhouse in Seven Steps?

Commissioning a manganese electrowinning cellhouse is a chemistry ramp, not a switch-on. The sequence below reflects the order in which variables are actually stabilized:

1. Water and electrolyte make-up. Fill, verify water quality (low chloride, low hardness), dissolve ammonium sulfate, and build the full catholyte inventory to 12–18 g/L Mn²⁺ / 100–130 g/L (NH₄)₂SO₄, ideally using solution produced by the upstream leach circuit.
2. Diaphragm installation and leak testing. Hang and seal every bag, then pressure/level-test the head differential before any current flows — a bag repaired after energization costs a harvest cycle.
3. Anode polarization and MnO₂ pre-film. Run the anodes at low current until a uniform MnO₂ layer develops; a well-formed pre-film moderates anode corrosion and sludge generation for the following months.
4. Low-current-density acclimatization, 24–72 h. Operate at a fraction of design current density while chemistry equilibrates; watch pH and temperature before chasing rate.
5. Selenium dosing and pH closed-loop control. Start continuous SeO₂ metering, tune the dose against catholyte assays, and lock automatic pH trim into its dead-band.
6. Stepwise current ramp to design. Raise current cell-by-cell toward 400 A/m², holding efficiency readings at each step; a cell that will not hold efficiency at part load will not hold it at full load either.
7. Cathode cycle and stripping regime. Set the 24–48 h harvest cycle, commission stripping lines and blank preparation, then log per-cell efficiency daily until the plant demonstrates its steady-state band.

Teams that pilot chemistry in a lab-scale diaphragm electrolytic cell before mechanical completion routinely skip weeks of on-line tuning. Bench data on Se response and pH sensitivity cannot be improvised in a 300-cell building.

What Do Running Cellhouses Look Like in Practice?

Three anonymized manganese electrowinning snapshots illustrate the patterns above (indicative, synthetic-typical basis; details anonymized and not attributable to any producer).

A 15,000 t/y cellhouse struggled for a year with current efficiency pinned at 61–63%. Catholyte assays revealed Se swinging between 0.01 and 0.07 g/L on an intermittent manual dosing round. Converting to a duty-cycled metering pump trimmed daily against assays stabilized Se at 0.03–0.04 g/L; efficiency settled at 69–70% and DC energy fell from 7,600 to about 6,900 kWh/t — roughly $42/t of electricity saved.

A 25,000 t/y plant experienced a slow efficiency slide across one particular block of 40 cells while the rest of the tankhouse held 68%. Cell voltages were uniform; nothing tripped. Spot acidity checks inside individual bags found three leaking bags with catholyte pH down near 4. Re-bagging the block restored the block’s efficiency within two cycles — the voltage trend had shown nothing, the bag acidity test showed everything.

A specialty producer needed contract EMM below 10 ppm Se for welding-wire applications. On the SeO₂ route the product floor sat at 20–25 ppm no matter how the dose was trimmed. The plant converted one line to a sulfur dioxide–assisted deposition practice, accepting a 4–5 point efficiency penalty (65% vs 69–70%) and a modest energy increase to around 7,300 kWh/t, and met the spec consistently. The economics worked because the low-selenium product carried a price premium well beyond the added power cost.

FAQ

Why is a diaphragm mandatory in manganese electrowinning?

At the cathode potentials required to reduce Mn²⁺ (standard potential near −1.18 V), hydrogen evolution is aggressive at any meaningful acidity. The diaphragm lets the plant hold the cathode compartment at near-neutral pH where hydrogen overpotential is highest, while the anode side acidifies freely to 25–40 g/L sulfuric acid. Without the barrier, acid mixes to the cathode, hydrogen dominates, and deposition stops being economically viable.

How much selenium dioxide is consumed per tonne of EMM?

Catholyte selenium is held at 0.02–0.05 g/L, which corresponds to roughly 0.5–1.5 kg of SeO₂ reagent per tonne of metal deposited, depending on bleed rate and drag-out (typical range, confirm with test work). Consumption is small in dollar terms but outsized in effect: a few grams per cubic meter of catholyte swing current efficiency by several points, which is why dosing is continuous and assay-controlled rather than batch-adjusted.

What selenium content can EMM buyers expect?

Standard selenium-route EMM typically carries 20–60 ppm Se in the metal. Low-selenium contract product below 10 ppm generally requires abandoning the SeO₂ additive in favor of sulfur dioxide–assisted deposition, accepting lower current efficiency. Because selenium is dissolved into the deposit itself, no post-deposition treatment economically removes it — the specification must be met at the cathode, not afterward.

How long do diaphragm bags last?

Practical bag life runs 6–18 months depending on cloth type, temperature, deposit adhesion practice, and handling discipline during stripping (typical range, confirm with site practice). Failure mode is usually tear or seal failure rather than cloth wear-through. Because a leaking bag degrades only its own cathode quietly, plants survive on scheduled inspection plus reactive re-bagging of cells flagged by catholyte acidity spot checks.

What current efficiency should a well-run EMM cellhouse achieve?

A stable, well-dosed plant holds 65–72%, with 68–70% a realistic sustained target (typical range, confirm with test work). Readings above 73% usually indicate a measurement or metal-accounting issue rather than genuine chemistry. Excursions of two or three points are routine and traceable to dosing, pH, or purity causes; sustained operation below 65% indicates a structural problem worth root-causing rather than trimming.

How is the manganese dioxide anode sludge handled?

The Pb–Sn–Ag anodes oxidize part of the Mn²⁺ to insoluble MnO₂, which accumulates as sludge in the anolyte at roughly 30–50 kg per tonne of EMM. Plants bleed the anolyte, settle or filter the solids, and re-dissolve the manganese by reductive leaching with SO₂, returning the liquor to the main leach. The sludge loop is therefore a recycle asset, not merely a waste line, and its availability directly supports catholyte Mn²⁺ balance.

Can lab-scale testing really predict full-scale performance?

Yes, with defined limits. A properly instrumented lab-scale diaphragm electrolytic cell reproduces catholyte chemistry, Se response, and pH sensitivity faithfully because those effects scale with chemistry, not geometry. What it cannot predict is thermal balance, mist behavior, and mechanical bag life at scale. The accepted practice is to fix chemistry and dosing strategy on the bench, then reserve the full-scale trial for mechanical and logistical confirmation.

Conclusion

Manganese electrowinning rewards narrow discipline. The chemistry tolerates no acidity at the cathode, no selenium drift, no buffer starvation, and no bag leaks — and it pays back that discipline with 65–72% current efficiency and metal that meets the tightest commercial specifications. Get the diaphragm system, the selenium dioxide loop, and the pH buffer right, and everything else in the flowsheet from agitated leaching tank design through purification to harvest becomes predictable. If you are sizing an EMM cellhouse or converting a line to low-selenium product, request a quote with your target tonnage and power price, and our process team will size the cells, cathode area, and dosing system against your specific electrolyte.

References

1. U.S. Geological Survey, Mineral Commodity Summaries — Manganese. https://www.usgs.gov/
2. Society for Mining, Metallurgy & Exploration, Mineral Processing and Extractive Metallurgy Handbook (SME), sections on electrometallurgy of manganese.
3. TMS / MetSoc of CIM conference proceedings, papers on electrolytic manganese metal cellhouse practice and selenium addition chemistry.
4. China Manganese Industry (中国锰业), selected articles on EMM electrolyte composition and current efficiency — citation as general industry literature, no public URL.
5. Electrometallurgy (湿法冶金/电冶金) journal literature on diaphragm cell design and anolyte management — citation as general industry literature, no public URL.
6. CSIRO process publications on manganese hydrometallurgy and electrolytic metal deposition. https://www.csiro.au/
7. ASTM-free industry practice notes on EMM product specification ranges, compiled by the authors from operating experience — no public URL.

Changelog

• 2026-11-06: Initial publication.