Acid Fume Scrubbers for Electroplating and PCB Plants

In short: Plating, pickling and PCB etching lines release acid mist and gases such as hydrochloric acid, sulfuric acid mist, nitric acid fumes, chromic acid mist and ammonia. An FRP packed-bed scrubber removes them by passing the exhaust up through wet packing while a scrubbing liquid flows down. Water or a dilute caustic solution handles most acid fumes, and a dilute acid solution handles ammonia. A mist eliminator at the top stops droplets from leaving with the air, and an FRP fan moves the air through the system. To size a scrubber, a supplier needs the air flow, the pollutants and their levels, the inlet temperature and the outlet limit. FIBROVX builds FRP scrubbers, fans and ducting to order.

Where the fumes come from

A plating or PCB plant has many open tanks. Each tank gives off vapor or fine mist, especially when it is heated, agitated with air or used for electrolysis. Gas bubbles from the electrodes burst at the surface and throw tiny droplets of the bath into the air.

Typical sources include:

These fumes harm workers, corrode the building and the equipment, and are often regulated at the stack. A local exhaust system captures them at each tank and sends them to a scrubber.

Why scrubbers in these plants are FRP

The scrubber, ducts and fan in a plating plant carry wet acid air every hour the line runs. Carbon steel corrodes quickly in this service, and even stainless steel can suffer in chloride-rich air from HCl pickling or cupric chloride etching.

FRP (fiber reinforced plastic) suits this duty because:

• It resists acid and alkaline fumes. The resin is chosen for the chemicals in the exhaust. A corrosion barrier on the inside faces the wet gas.
• It is light. Towers and ducts can be installed on roofs and mezzanines without heavy steelwork.
• It can be formed to fit. Towers, transitions, hoods and ducts are made to the plant layout.

The resin, liner and wall thickness are selected for each project. FIBROVX does not publish generic chemical or performance ratings; these are confirmed with the quotation.

How a packed-bed scrubber works

The US EPA's chromium plating rule gives a simple definition. A packed-bed scrubber is an add-on control device with a single or double packed bed. Droplets impinge on the packing media, and the packed section is followed by a mist eliminator to remove water carried up from the bed.

In practice the scrubber has these parts, from bottom to top:

1. Sump. Holds the scrubbing liquid. A recirculation pump draws from it.
2. Gas inlet. Dirty air enters above the liquid level.
3. Packed bed. Random or structured plastic packing gives a large wet surface. Gas flows up and liquid flows down. This counter-current contact is where gases dissolve and react.
4. Liquid distributor. Spray nozzles or a trough spread the liquid evenly over the packing.
5. Mist eliminator. Chevron blades or a mesh pad catch droplets before the air leaves.
6. Outlet. Clean air goes to the fan and the stack.

Soluble gases such as HCl and ammonia dissolve readily in water. Adding a reagent to the liquid keeps absorbing them as the liquid is reused. Mists such as chromic acid and sulfuric acid are droplets, not gases, so the packing and the mist eliminator must capture them by impact.

Choosing the scrubbing liquid

The scrubbing liquid depends on what you remove:

A dosing system with a pH probe keeps the liquid at the right strength. Spent liquid is bled off to the plant's wastewater treatment. Your process engineer or the supplier sets the liquid chemistry for each system.

Separate systems for separate fumes

One common mistake is to send every tank into one duct and one scrubber. Acid and alkaline fumes react in the duct and form salts. Cyanide baths must never share a system with acids, because acid can release hydrogen cyanide gas. Chromic acid mist is often kept separate, so the recovered liquid can go back to the chromium tank.

A typical plant groups its exhaust as follows:

• Acid line: HCl, sulfuric and other acid tanks to one scrubber with caustic dosing.
• Alkaline line: cleaners and ammonia sources to a scrubber with acid dosing.
• Chromium line: chromium tanks to a dedicated scrubber or mesh-pad system.
• Cyanide line: kept fully apart from acid fumes.

US rules for chromium plating

In the US, chromium electroplating and anodizing tanks fall under the EPA's National Emission Standards for Hazardous Air Pollutants, 40 CFR Part 63 Subpart N. The rule sets limits in two ways:

• Concentration at the stack. For example, existing open surface hard chromium plating tanks at large facilities must not exceed 0.011 mg of total chromium per dry standard cubic meter of ventilation air. Other limits apply to other tank types and sizes.
• Surface tension of the bath. Where a chemical fume suppressant is used, the bath's surface tension must not exceed 40 dynes/cm measured by a stalagmometer, or 33 dynes/cm by a tensiometer. Lower surface tension means fewer droplets are thrown into the air.

The rule also lists work practices for control devices. For packed-bed scrubbers and composite mesh pads, these include regular visual inspections and washdowns, and checks of the ductwork for leaks.

Workers are covered separately. OSHA's chromium (VI) standard, 29 CFR 1910.1026, sets a permissible exposure limit of 5 µg/m³ as an 8-hour time-weighted average, with an action level of 2.5 µg/m³. For hydrogen chloride, OSHA's air contaminants table in 29 CFR 1910.1000 sets a ceiling of 5 ppm.

Local and state rules vary, and many countries set their own stack limits. The scrubber must be designed to the limit that applies at your site. A supplier cannot pick it for you.

Hoods, ducts and the fan

A scrubber can only clean the air that reaches it. Capture at the tank matters as much as the scrubber itself:

• Hoods. Lateral slot hoods along the tank edge are common on plating lines. Push-pull systems help on wide tanks.
• Air flow. The exhaust rate per tank depends on the tank size, the bath, its temperature and the hood design. Your ventilation engineer sets it, often with guidance from the ACGIH Industrial Ventilation manual.
• Ducts. FRP ducts resist the wet acid air. Slope them toward drain points so condensate does not pool.
• Dampers. Balancing dampers let each branch draw its share.
• Fan. An FRP fan is usually placed after the scrubber, so it handles cleaner, wet air and keeps the ducts under negative pressure. Leaks then pull air in, rather than pushing fumes out.

Operation and maintenance

A scrubber works well only if it is looked after. Plan for:

• Liquid control. Check the pH and the bleed-off rate. Low dosing lets fumes slip through.
• Pressure drop. A rising pressure drop across the packing or mist eliminator means fouling or salt build-up.
• Washdowns. Wash the packing and mist eliminator on a schedule.
• Spray nozzles. Clear blocked nozzles. Dry patches in the bed reduce removal.
• Inspection. Look inside the tower and ducts for liner damage, especially near the inlet, where hot, dry and wet zones meet.
• Records. Keep logs of checks, especially where a permit requires them.

Common problems and their causes

Most scrubber complaints trace back to a few causes:

• Fumes at the tank, not at the stack. The hood or exhaust rate is too small, or a damper is set wrong. The scrubber cannot fix poor capture.
• Visible mist at the stack. The mist eliminator is fouled, damaged or missing, or the gas velocity is too high for it.
• Odor of acid downwind. The dosing has failed or the pH probe has drifted, so the liquid is saturated.
• White deposits in ducts. Acid and alkaline fumes are meeting and forming salts. Separate the lines.
• Rising fan power. Packing is fouled or blocked, so the pressure drop is higher than designed.
• Liner damage near the inlet. Hot, dry gas hits a wet zone. A quench spray or a better inlet design solves this.

When a line is extended, check the scrubber again. Adding tanks to an existing system without rechecking the air flow and liquid rate is a common reason for poor results.

What to send for a quote

To size an FRP acid fume scrubber, send:

• Air flow rate, per tank and in total, in m³/h or cfm.
• Pollutants and their expected inlet concentrations.
• Inlet temperature and humidity.
• Required outlet concentration or local emission limit.
• Tank list, with baths, sizes and temperatures, if the hoods are in scope.
• Layout, with available space and height for the tower and fan.
• Scope: tower only, or with fan, pump, dosing, ducts and stack.
• Utilities: water supply, drain and electrical supply (voltage and frequency).

FIBROVX makes FRP scrubbers, FRP fans and ducting to order. Browse the air pollution control range or send your data for a quotation. Dimensions, resin, test data and drawings are confirmed in writing for each item quoted.

Related: Industries we serve; Custom FRP Manufacturing: Sizes, Resins, Colors, Molds and OEM Explained; FRP in Semiconductor Fabs: Exhaust Ducts, Scrubbers and Tanks.

FAQ

Which scrubbing liquid removes HCl fumes from a plating line?

Water or a dilute caustic (NaOH) solution. HCl is very soluble, and caustic keeps the liquid absorbing as it is recirculated.

How is ammonia from PCB alkaline etching scrubbed?

With a dilute acid scrubbing solution, which neutralizes the ammonia. Keep ammonia exhaust separate from acid exhaust where possible.

Why does a packed-bed scrubber need a mist eliminator?

Chromic and sulfuric acid mists are droplets, not gases. The packing and the mist eliminator capture them by impact, and the eliminator stops scrubbing liquid from leaving with the air.

What data does a supplier need to size a plating scrubber?

The air flow, the pollutants and their levels, the inlet temperature and the outlet limit, plus the tank list and the duct layout.

Sources

• US EPA, 40 CFR 63.341, Definitions (packed-bed scrubber), Subpart N, chromium electroplating and anodizing: https://www.law.cornell.edu/cfr/text/40/63.341
• US EPA, 40 CFR 63.342, Standards (emission limits, surface tension, work practices): https://www.law.cornell.edu/cfr/text/40/63.342
• OSHA, 29 CFR 1910.1026, Chromium (VI): https://www.law.cornell.edu/cfr/text/29/1910.1026
• OSHA, 29 CFR 1910.1000, Air contaminants: https://www.law.cornell.edu/cfr/text/29/1910.1000