Galvanizers Are Chasing 99.9% Fume Removal. Here’s the Engineering Behind It

Hot‑dip galvanizing and metal pickling generate concentrated acid fumes — notably HCl (hydrogen chloride), and often H₂SO₄ (sulfuric acid) or HNO₃ (nitric acid) — along with fine mists. These fumes are highly corrosive, toxic and tightly regulated (e.g. Indonesian Standards and international EPA limits). Modern plants routinely target >99% removal of HCl emissions to meet sub‑10–20 mg/Nm³ limits. In practice, engineers design to reduce HCl from hundreds of ppm (parts per million) to single‑digit ppm, often 99.5%–99.9% reduction.

Wet scrubber technologies compared

Venturi (Jet) Scrubbers: A venturi scrubber injects liquid into a narrow, high‑velocity throat to atomize the stream (i.e., break it into fine droplets that contact gas). They excel at removing both gases and fine particulates such as acid mists or metal oxide dust. A well‑designed venturi can achieve very high gas‑phase removal — a lab study reported 98.3% HCl removal at 500 ppm inlet HCl (60 m/s gas velocity, water head ~0.77 m) (www.researchgate.net). In practice single‑stage venturis typically remove ~95% of HCl (www.s-k.com), with efficiencies rising toward 98–99% at higher pressure drop or in multi‑stage systems. The trade‑off: very high pressure drops (often on the order of 10–80 kPa, i.e. ~4–32 in H₂O) and large fan power (dehnungsband.com) (www.researchgate.net). Liquid demand is also high (often >10–50 L per 1000 m³ gas). Venturis are compact, simple (no packing), and robust against plugging, making them a solid first stage if both acid gas and dust/mist are present.

Packed‑Bed (Countercurrent) Scrubbers: A packed tower is a vertical column filled with inert packing (saddles, rings, or structured media) over which scrubbing liquor trickles counter‑flow to the gas, providing large contact area for absorption. These can routinely achieve up to 99.9%+ removal of HCl (www.s-k.com) if designed with sufficient height and liquid flow. Because gas velocity is low and uniform, pressure drop is very low (typically <0.5 kPa per meter of height). Industry notes that “packed bed scrubbers are…more popular for the treatment of gas,” with efficient neutralization but low gas‑phase ΔP (dehnungsband.com). In practice, a packed tower is often a polishing stage (following a venturi) to push residual acid ppm down (www.s-k.com). Towers require careful liquid distribution to avoid channeling and include mist eliminators to prevent entrainment.

Spray Towers / Tower Scrubbers: Simpler spray towers (sparse packing or trays) serve lighter acid loads. They run at very low pressure drop but typically deliver moderate removal (<95%) unless oversized. Because removal of sub‑micron mists is limited without packing, these are best when space is ample and inlet concentrations are low.

Comparison: Venturis deliver high removal and tolerate particle‑laden or “hot” streams but at high energy, whereas packed towers deliver the highest gas‑phase removal with lower energy. Typical HCl systems often combine a venturi (first stage) and packed bed (second stage) (www.s-k.com): the venturi removes 90–95% (plus dust), and the packed tower polishes to meet stringent limits (www.s-k.com) (www.researchgate.net). A standalone packed tower can exceed 99% HCl removal, but requires sufficient tower height or high mass‑transfer coefficients to do so.

Performance metrics and energy balance

Removal efficiency: Venturis typically achieve 90–98% acid removal, including a measured 98.3% at lab scale (www.researchgate.net). Packed towers can reach ≥99% (up to 99.9%+) by extending packing height or liquid flow (www.s-k.com). Laboratory work confirms that adding caustic to the scrubbing liquor boosts capture: one test showed HCl capture rising from 87.8% (with plain water) to 92.5% with only 0.005 N NaOH (www.researchgate.net).

Pressure drop and energy: Venturis operate at very high ΔP (often 50–150 mbar or more). Packed towers run at very low ΔP (single‑digit mbar) because gas velocity is low (dehnungsband.com) (www.s-k.com). For perspective, [51] notes packed towers have “low gas‑phase pressure drop” (dehnungsband.com). The specific power requirement of a venturi can be 5–10× that of a packed tower for equivalent gas flow, which drives operating cost.

Liquid‑to‑gas (L/G) ratio: Venturi scrubbers usually require high L/G (e.g., 10–50 L of liquid per 1000 m³ gas) to form small droplets. Packed towers operate at lower L/G ratios (often 1–10 L per 1000 m³, depending on loading). Higher L/G increases removal but raises pumping needs and downstream handling.

Capital, space, and maintenance: Venturis have small footprints and simple internals but face abrasion and corrosion at the throat; packed towers are taller and need platforms, distributors, and periodic repacking or distributor maintenance.

Scrubbing liquor chemistry and selection

Caustic soda (NaOH) water solution is by far the most common for acid fumes, neutralizing via: HCl + NaOH → NaCl + H₂O; H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O; HNO₃ + NaOH → NaNO₃ + H₂O. These reactions convert acid gas to soluble salts. As Schutte & Koerting note, “it is very common to use aqueous NaOH… The reaction yields sodium chloride (NaCl)” (www.s-k.com). Sodium salts remain in solution (or precipitate if concentrated), allowing waste liquor to be discharged or treated as brine. Caustic is commonly recirculated in a closed loop with bleed and fresh NaOH to maintain alkalinity.

Alternative reagents include calcium hydroxide (slaked lime) and sodium carbonate. Lime is cheaper but poorly soluble (thick slurries) and produces CaCl₂ or CaSO₄ sludge. Sodium carbonate works for HCl but is less common industrially. HF (hydrogen fluoride) is a special case: NaOH + HF yields NaF, which has low solubility (1.4 g/100 mL at 20°C), so KOH is preferred (producing KF, highly soluble) (www.s-k.com). In summary, NaOH is used for HCl/H₂SO₄/HNO₃ fumes, while KOH is chosen if significant HF is expected (www.s-k.com).

Maintaining alkalinity and setpoints

Effective scrubbing requires maintaining sufficient alkalinity. As acid is absorbed, the scrubbing fluid becomes salt‑laden and less basic, so pH drops. Continuous monitoring of pH (or alkalinity) in the recirculating liquor is essential. Controls add make‑up NaOH to keep the solution mildly alkaline — often pH 8–10 for HCl; typically 1–5% NaOH by weight. If pH falls too low, acid “breakthrough” sharply increases. Experiments show even a trace of NaOH (0.005 N) improved HCl capture by ~5% (www.researchgate.net). In practice, scrubbers often operate with sodium hydroxide concentrations from a few g/L up to ~10% w/w. For example, cloth zinc‑plating lines often use ~1–2 % NaOH recirculating solution.

Stoichiometry is straightforward: each mole of HCl consumes one mole of NaOH. A rough example: if a galvanizing scrubber handles 50 kg HCl per hour, it must consume ~55 kg NaOH per hour. At 10% NaOH, this is ~550 L/h of make‑up liquor. This explains why large plants can use several tons of caustic per month. Multi‑stage systems may first wash gases with water or dilute acid to recover HCl as acid solution (www.s-k.com), saving NaOH cost.

Facilities that standardize chemical metering keep control tight; integrating accurate chemical dosing within the control loop can align with the approach described here, as equipment such as dosing pumps is designed for precise addition of caustic solutions.

Salt disposal and wastewater routing

The neutralization by‑product (e.g., NaCl solution) must be managed. Sometimes the spent liquor is [centrifuged/evaporated to recover salt](https://beta.co.id/en/blog/power-plants-are-chasing-zero-liquid-heres-the-gritty-chemistry-making-it-possible), or discharged to wastewater treatment. In Indonesia, resulting brine would be classified as non‑hazardous but subject to discharge limits, so design must consider waste management of the [spent scrubber liquor](https://beta.co.id/en/blog/galvanizers-dirtiest-stream-is-now-a-water-source-inside-the-plan-to-turn-scrubber). When routed to plant treatment, primary clarification hardware such as a clarifier can be part of the downstream plan for solids separation before further processing.

Design parameters and construction materials

Material selection: Scrubbers and ductwork must resist strong acids and alkalis. Common materials include PVC/PP, FRP (fiberglass‑reinforced plastic), stainless steel (for NaOH durability), or acid‑resistant alloys. Seal flanges and pumps must withstand caustic and salt. Plants also keep auxiliary spares and accessories close at hand; specifying supporting equipment for water treatment helps maintain uptime across the recirculation loop.

Mist elimination: All wet scrubbers need droplet separators to prevent caustic aerosol carryover. Mesh or vane pack demisters atop packed towers and secondary cyclonic separators on venturi exhausts are standard. Effective demisters prevent fence‑line corrosion and NaOH loss.

Liquid/gas contact and distribution: Venturis inherently mix well; packed towers require good liquid distributors (spray headers or shower nozzles) and quality packing to avoid channeling or dry spots that degrade performance.

Scrubber sizing: Engineers use mass‑transfer design methods (e.g., transfer‑unit or HTU, height of a transfer unit) to size packs. Designs ensure an L/G sufficient to meet the required normality flux; packed columns are sized so that 98–99% of acid is captured in the available height.

Operating conditions and instrumentation

Gas velocities: Industry practice and cited studies suggest high gas velocities (up to a few m/s) in venturis, and much lower velocities (≪1 m/s) in towers. A study of an HCl venturi found optimal removal at ~60 m/s throat velocity (www.researchgate.net). By contrast, a packed tower example (sulfuric mist study) used gas 10–30 L/s in a 6″ tube and saw removal from ~77% to 86% as flow (and turbulence) rose (www.researchgate.net), indicating packed columns should keep superficial gas velocity in a range (often 0.5–2 m/s) to balance contact time vs. pressure drop.

Instrumentation: Include pH or conductivity probes in the liquor loop, acid gas analyzers on inlet/outlet (e.g., extractive FTIR or titration), and flow/pressure gauges. Alarm and interlocks for low pH, high outlet acid, or flooded column are prudent. Pressure drop across the stages is often measured in kPa (versus only a few Pa in ambient gas filters), so plant fans must be sized accordingly.

Measured outcomes and case examples

Implementing a high‑efficiency wet scrubber yields quantifiable emission reductions and compliance results. A dual‑stage system (venturi + packed) can typically reduce HCl emissions by >99.5%, bringing outlet HCl to <5 ppm (www.s-k.com) (www.researchgate.net). In one case study of a titanium tetrachloride loading operation, a custom venturi + packed scrubber system achieved near‑complete HCl removal under intermittent flow (torch-air.com).

Statistics: While plant‑specific, modern installations reduce acid fume concentrations from hundreds of mg/Nm³ down to the low ppt (µg/Nm³) range. For liquor consumption, a design study might note: for 10,000 Nm³/h gas with 500 mg/Nm³ HCl, a 5% NaOH scrubber would consume ~15 kg NaOH per hour to neutralize the acid. Such figures guide operating and reagent cost estimates.

Takeaway on system architecture

For galvanizing and plating fumes, wet scrubbers using caustic liquor are the proven solution. Venturi scrubbers offer rugged, high‑throughput removal of acids and particulates (95–98%+ efficiency, www.s-k.com) but at the cost of large pressure drop and energy. Countercurrent packed towers excel at achieving the highest gas‑phase removal (≥99.9%, www.s-k.com) with much lower ΔP. The common architecture pairs a venturi first stage with a packed polishing stage, sized for peak acid load, and built from corrosion‑resistant materials. Crucially, the scrubbing liquor (typically NaOH) must be carefully controlled so that essentially all acid vapor encounters sufficient OH⁻ to neutralize it. Data‑backed design — using reaction stoichiometry and reported efficiencies (www.researchgate.net) (www.researchgate.net) — keeps the system compliant while optimizing reagent use.