Inside the tankhouse: how mines tame acid mist

A copper or gold electrowinning tankhouse is a mist machine. As oxygen bubbles rise and rupture at the electrolyte surface, they fling droplets of strong sulfuric acid into the air, creating a fine aerosol (trea.com) (www.911metallurgist.com). A large plant (~200 cells) can see emissions on the order of 10²–10³ mg/min per cell under high current conditions (www.911metallurgist.com) (www.911metallurgist.com).

The aerosol is more than a nuisance. It irritates skin, eyes, and the respiratory tract, and it eats equipment and building structures (trea.com) (www.911metallurgist.com). OSHA/NIOSH peg sulfuric acid mist at 1 mg/m³ TWA (time‑weighted average, an eight‑hour occupational exposure limit) (www.cdc.gov), pushing operators toward aggressive control at the source.

Ventilation alone won’t cut it. One estimate puts the dilution airflow at around 100,000 m³/min (www.911metallurgist.com), or fans drawing power equal to ~20% of electrowinning consumption (www.911metallurgist.com) — expensive and inefficient.

Acid mist formation mechanism

Electrowinning (using electric current to plate dissolved metal from an acidic sulfate electrolyte) produces oxygen at the anode. “The evolution of oxygen gas forms bubbles which entrain strong acid electrolyte, carrying it into the air above the electrowinning tank in the form of a fine mist or spray when the bubbles break” (trea.com).

Every bubble burst ejects droplets; higher temperatures and higher current densities elevate mist generation, while increased electrolyte viscosity (for example from surfactant additives) reduces it (researchonline.jcu.edu.au) (researchonline.jcu.edu.au). One study attributed over 90% of mist variation to electrolyte temperature and use of a surfactant additive (FC‑1100) (researchonline.jcu.edu.au). The control lever is clear: manage bubble size and the surface activity of the electrolyte.

Ventilation versus source suppression

Because dilution requires huge air volumes and power — on the order of 100,000 m³/min and ~20% of electrowinning load in one analysis (www.911metallurgist.com) (www.911metallurgist.com) — plants focus on local suppression. The tactic: stop droplet creation at the surface with physical barriers, chemical additives, or both.

Floating covers and spheres

Floating balls (“poly balls” or “mist balls”) are the tankhouse workhorse. A buoyant layer intercepts erupting bubbles and splashes. Two performance drivers matter: surface coverage and buoyancy/height above the electrolyte. Spherical floats deliver more coverage than flat mats; one study found single‑layer spheres outperforming flat sheets or irregular shapes (researchonline.jcu.edu.au).

Material matters too. In the same experiment, HDPE spheres (lower density, higher buoyancy) outperformed polypropylene spheres by about 3.4% in acid mist reduction (researchonline.jcu.edu.au). In practice, well‑packed spheres or floats covering 60–80% of the surface can cut emissions significantly (on the order of tens of percent), though exact results depend on coverage and cell geometry. Maintenance is light — floats resist acid but degrade over months and need periodic replacement — and gaps (including at perimeters) mean full sealing is unlikely.

Continuous floating blankets or foam mats offer near full‑surface coverage, again blocking bubble‑burst energy. If sealed completely they trap gases, so venting gaps are needed, which limits effectiveness. Public data on blanket performance are scarce; theoretically a well‑sealed foam layer could nearly eliminate mist, but in practice operators mix blankets or balls with loose coverage (e.g., 50–70%) to allow venting.

• Passive surface coverage intercepts bubbles; effectiveness rises with area coverage and buoyancy.
• Spherical HDPE balls provided ~3–4% better reduction than PP in tests (researchonline.jcu.edu.au).
• A single layer of spheres was more effective than flat layers due to a higher coverage factor (researchonline.jcu.edu.au).
• Physical covers alone typically reduce mist by a moderate amount (tens of percent); they require minimal chemical handling and do not address mist forming under the cover.

Surfactant-based mist suppression

Specialty chemical additives — chiefly surfactants — change the surface physics. By reducing surface tension, they promote a stable foam layer that blankets the electrolyte so rising bubbles collapse under foam rather than ejecting droplets. As one patent notes, stabilized foam “works to suppress acid mist by blanketing the breaking gas bubbles under a foam layer” (www.freepatentsonline.com).

Formulations range from fluorinated surfactants such as 3M’s FC‑1100 to non‑fluorinated options, including plant‑derived saponins now in development (trea.com) (patents.google.com). At low ppm levels, the effect is dramatic: about 10 ppm of active surfactant in the electrolyte can “reduce the measured level of sulfuric acid mist in the tank house by as much as 50%” (patents.google.com). Reduction tends to level off above ~30 ppm for FC‑1100 (researchonline.jcu.edu.au).

There is also a natural twist. A licorice root extract (rich in saponins) has been shown in a patent filing to foam and synergize with fluorosurfactants; combining licorice powder with 2 ppm of FC‑1100 provided more suppression than 12 ppm FC‑1100 alone (trea.com).

Dosage, foam, and performance

Key quantitative markers from studies and patents:

• ≈50% mist reduction at ~10 ppm active surfactant (patents.google.com).
• Substantial suppression at ~20–30 ppm; diminishing returns beyond ~30 ppm for FC‑1100 (researchonline.jcu.edu.au).
• Combining FC‑1100 with floating spheres gave ~29% further reduction versus FC‑1100 alone (researchonline.jcu.edu.au).

Typical programs keep additive levels in the 10–30 mg/L (ppm) range; the stable foam blanket is the working layer that captures breaking bubbles (www.freepatentsonline.com) (www.freepatentsonline.com). For dosing control, mines commonly rely on accurate metering equipment; maintaining target ppm is straightforward with a dosing pump integrated to the cell feed.

Comparative control strategies

In isolation, chemical suppressants typically beat passive covers. As a benchmark, ~10 ppm of surfactant delivered ~50% reduction in measured mist (patents.google.com), while a single layer of floating pellets tends to achieve a few tens of percent with performance tied to coverage and buoyancy (researchonline.jcu.edu.au).

The most effective setup is combined: reductions are multiplicative. Adding floating spheres to a surfactant‑dosed tankhouse produced an additional ~28–30% cut relative to surfactant alone (researchonline.jcu.edu.au). Practically, if 10 ppm FC‑1100 halves the mist, coverage by floats can trim the remainder by roughly another third, pushing total suppression toward ~65–80%.

Other tradeoffs matter:

• Coverage dependency: physical covers scale with % area covered; chemicals act uniformly over the surface.
• Operational cost: floats are a one‑time buy with infrequent replacement; chemicals incur continuous, low‑dosage consumption.
• Maintenance: floats need periodic cleaning/replacement; chemicals need dosing equipment, monitoring of pH/foam, and handling/disposal logistics.
• Environmental/regulatory: fluorinated surfactants are effective but raise environmental concerns; newer regulations and company policies may favor non‑fluorinated alternatives (biosurfactants, sulfonates) despite slightly lower performance. Physical covers have no chemical footprint.

Field practice and regulatory context

Public field data are sparse, but the pattern is consistent. A patent report notes that 10 ppm FC‑1100 halved the measured tankhouse mist (patents.google.com). Other published sources suggest combined controls can drive concentrations well below 1 mg/m³ — the OSHA/NIOSH limit — often with only modest additional ventilation (www.cdc.gov). In Indonesia and elsewhere, layered programs are typical: local ventilation, full‑coverage floats or covers, and anti‑mist chemicals.

Trendlines point to refining dosages and “greener” suppressants. Research by Reza Al‑Shakarji and colleagues quantified how cell temperature and additives such as FC‑1100 drive mist formation and control (researchonline.jcu.edu.au) (researchonline.jcu.edu.au). Best practice is converging on a composite strategy: cover as much surface as practical and run a few tens of ppm of surfactant, which achieves maximal reduction beyond what either method can do alone (researchonline.jcu.edu.au) (patents.google.com).

Procurement sits squarely in mining chemicals. Anti‑mist surfactants, including fluorinated and non‑fluorinated packages, are part of chemicals for mining applications, and on the operations side they are paired with surface covers tuned for coverage and buoyancy.

Sources and cited data

Peer‑reviewed studies and industry reports underpin the comparisons. Al‑Shakarji et al. (2013) document the comparative effectiveness of floats vs. surfactant, and the diminishing returns above ~30 ppm for FC‑1100 (researchonline.jcu.edu.au) (researchonline.jcu.edu.au). Industry literature and patents record that 3M’s FC‑1100 has long been a standard mist suppressor achieving ~50% reduction at ~10 ppm (patents.google.com). U.S. guidelines treat sulfuric acid mist as an occupational hazard at 1 mg/m³ TWA (www.cdc.gov). While absolute mg/m³ results vary by plant, the relative performance is consistent across sources: floats ≈ tens of percent reduction; surfactants ≈ 50% or more at typical dosages; combined ≈ 80%+ with additive effects (researchonline.jcu.edu.au) (patents.google.com).