In SX–EW tankhouses, oxygen bubbles fling sulfuric acid into the air as fine mist. The fix that consistently drives exposures down isn’t a thicker blanket of floats — it’s a few dozen milligrams per liter of a specialty surfactant.
The physics of a nickel electrowinning (EW) cell are unforgiving. As oxygen rises and bursts, it entrains concentrated sulfuric acid and ejects it as fine aerosol — “acid mist” — across the tankhouse (www.911metallurgist.com). Even trace amounts irritate and caustically burn skin, eyes, and lungs, accelerate corrosion, and fall under tight exposure limits: OSHA PEL ≈1 mg/m³ and ACGIH TLV ≈0.2 mg/m³ (mg/m³ = milligrams per cubic meter). Strong inorganic acid mists such as concentrated H₂SO₄ are also classified as human carcinogens (laryngeal cancer) (echa.europa.eu).
Trying to “blow it away” is costly. One analysis estimated a 200‑cell tankhouse would need ≈100,000 m³/min (cubic meters per minute) — that’s 3.5×10⁶ cfm — of fresh air to hold mist below 0.8 mg/m³ (www.911metallurgist.com). The energy penalty alone runs to ~20% of the plant’s electrical power (www.911metallurgist.com). In-cell source controls, not brute-force ventilation, are the lever that works.
Acid mist formation and limits
In SX–EW tankhouses, evolving oxygen bubbles entrain concentrated sulfuric acid and eject it as fine mist upon bursting (www.911metallurgist.com). The health and corrosion risks are significant, and regulators enforce strict limits (OSHA PEL ≈1 mg/m³, ACGIH TLV ≈0.2 mg/m³), with “strong inorganic acid mists” classed as human carcinogens (laryngeal cancer) by EU authorities (echa.europa.eu).
Ventilation alone is onerous: a 200‑cell tankhouse would need ≈100,000 m³/min (3.5×10⁶ cfm) of fresh air to keep acid mist under 0.8 mg/m³ — consuming ~20% of plant power (www.911metallurgist.com; www.911metallurgist.com).
Floating ball covers: benefits and limits
Rigid inert floats — typically 20–30 mm polypropylene or polyethylene spheres — shrink the exposed electrolyte area and intercept droplets, functioning as a surface barrier (patents.google.com). In tests on copper cells, buoyant HDPE balls achieved ~3.4% better mist capture than polypropylene balls of the same size, due to greater buoyancy and surface coverage (researchonline.jcu.edu.au). Even a single‑layer array yields measurable reduction (publications often report combined “barrier” performance rather than a standalone %).
But there’s a mechanical catch: rising oxygen pushes lightweight floats radially outward, leaving the electrode gap — where most mist originates — partially uncovered (patents.google.com). Floats rarely achieve 100% coverage. Balls accumulate acid film and require cleaning or replacement, and very high packing fractions hinder maintenance such as cathode removal. Buoyancy matters: lower-density HDPE sits higher and intercepts more mist than heavier PP, as observed in the same study (researchonline.jcu.edu.au).
Foam or blanket barriers: safety trade‑offs
Loose or fixed “blankets” — shredded polystyrene or PE foam blocks — create multi‑layer barriers with many capture surfaces and some insulation (patents.google.com). Yet continuous foam layers can trap evolved hydrogen/oxygen under the cover, creating an explosive mixture; industry sources warn covers must be perforated or segmented to vent gases (patents.google.com).
Foams degrade under acid attack and UV light, so they require replenishment. Because of these hazards, fully sealing blankets are less common; loose “crushed foam” layers are reported but need careful monitoring (patents.google.com). Other covers include rigid hoods/caps with ducting. Overall, physical barriers reduce mist by limiting exposed surface, but effectiveness is inherently limited (<≈30% further reduction beyond aerated conditions), and studies recommend pairing barriers with chemical suppressants for best results (researchonline.jcu.edu.au).
Surfactant additives: interface physics
Chemical mist suppressants — specialty surfactants added at low concentrations (typically tens of mg/L) — change bubble‑burst behavior. They lower surface tension and form a viscous surfactant film at the bubble cap; a thin “skin” collects at the free surface, so the cap thins slowly, dissipates kinetic energy, and produces far fewer, larger droplets that fall back rather than suspend as aerosol (1library.net; 1library.net).
The effect is more nuanced than tension alone. Heating that reduces surface tension can actually increase mist: cutting tension 55→43 mN/m raised mist ~7‑fold (from ~7 to 47 mg/m³). In contrast, adding 30 ppm of a fluorosurfactant (Dowfax/FC‑1100) cut tension to ~44 mN/m and decreased mist from ~47 to ~7 mg/m³ — an ~85% reduction (1library.net; 1library.net).
Dosing, formulations, and control
Widely used suppressants include fluorinated anionic surfactants (e.g., perfluoroalkane sulfonates), polymeric glycol ethers, and newer bio‑based saponins or mixed blends. Adding FC‑1100 (a C₈ fluorinated surfactant) at ~5–30 mg/L produces substantial reduction; benefits plateau beyond ~30 mg/L. Al Shakarji et al. (2013) reported acid mist levels plateau near ~30 ppm: below this, each increment sharply cuts mist, and adding 30 ppm FC‑1100 yielded a roughly 7× reduction (researchonline.jcu.edu.au). One recent patent describes a 5–30 mg/L additive blending a C₂–C₆ carboxylic acid, diphenyl oxide disulfonate, and a plant saponin extract for mist control.
In practice, low‑level dosing is easier when metered with an accurate dosing pump, given the tens‑of‑mg/L operating window cited above.
Effects on deposition quality
Potent surfactants can influence cathode deposition if overdosed. Studies note aggressive organics may cause cathode roughness or partial strip corrosion (www.researchgate.net). For example, sodium lauryl sulfate or Tergitol showed good suppression but can induce copper dissolution, so dosages must be optimized (www.researchgate.net). In nickel systems, which are very acidic, operators evaluate any additive’s impact on nickel plating fidelity and impurity codeposition and use minimal effective concentrations.
Efficacy: chemicals vs. covers
In controlled tests, chemical suppressants far outperformed physical covers. Chemical alone (30 mg/L FC‑1100) cut mist ~85% from baseline, while single‑layer float covers offered only modest reductions (HDPE spheres ~3–5% better than PP) (www.911metallurgist.com; researchonline.jcu.edu.au). Combining both methods is synergistic: adding 30 ppm FC‑1100 to a cell already using floating barriers improved overall mist reduction by ~29% over the chemical alone (researchonline.jcu.edu.au).
The takeaway echoed in multiple measurements: surfactants address aerosol formation at the source; balls/blankets intercept residual droplets.
Costs, adoption, and PFAS shift
Floats/blankets are one‑time capital with maintenance and access trade‑offs; chemicals are a recurring cost but integrate easily. Many operators default to polypropylene balls because they require no chemistry changes; chemical additives are a “process adjustment” that needs trialing. With tightening health and regulatory pressure, advanced suppressants are becoming standard in large electrowinning shops. Notably, legacy PFAS‑based surfactants are being phased out for environmental/health reasons, replaced by short‑chain or eco‑friendly alternatives (trea.com). Plants often procure mining‑grade reagents via chemicals for mining applications or broader portfolios such as a complete range of chemicals for mining, water, wastewater and oil and gas.
Regulatory benchmarks and monitoring
Indonesian and international rules typically mandate mist capture or control via exposure limits; compliance commonly means holding tankhouse air below ~0.5–1 mg/m³. Benchmarks like the Canadian study imply that physical means alone struggle to meet sub‑mg targets, whereas surfactants routinely reach that floor. Untreated cells can emit tens of mg/m³ in the breathing zone; a well‑suppressed cell emits only a few mg/m³ (1library.net; 1library.net).
In practice, plants monitor mist in mg/m³ with and without additives and adjust dosing accordingly. Published data indicate that 20–30 mg/L fluorosurfactant plus a standard floating‑ball cover routinely pushes acid mist into the single‑digit mg/m³ range, whereas either measure alone would leave tens of mg/m³ (1library.net; researchonline.jcu.edu.au) — and can slash fresh‑air demand by hundreds of thousands of cubic feet per minute.
Methodology sources
Industry studies and patents of electrowinning processes (patents.google.com; patents.google.com), peer‑reviewed hydrometallurgy research (researchonline.jcu.edu.au; 1library.net; 1library.net), and environmental/health guidance on acid mist (www.911metallurgist.com; echa.europa.eu) provide the performance data and mechanistic understanding summarized here.
