Engineering Evaluation Case: A Hot-Dip Galvanizing Fume Scrubber Accumulates Zinc-Ammonium Chloride Salt Deposits
Hot-dip galvanizing exhaust can be misleading if it is described only as:
“acid fume.”
Flux-related emissions can contain chloride-bearing aerosol and particulate originating from zinc-ammonium-chloride-type flux systems.
Once captured in a wet scrubber, these materials enter the recirculating liquid and can form a significant salt load.
The tower then faces both:
- fume capture;
- salt deposition.
Project Situation
Consider an exhaust system above a galvanizing kettle or flux-related process.
The packed wet scrubber uses corrosion-resistant plastic internals.
Initially:
- pressure drop is low;
- fume removal is good.
Later, operators find:
- white or gray crystalline deposits;
- blocked spray or distributor openings;
- solids on the demister;
- increasing conductivity.
The tower is not necessarily suffering from polymer corrosion.
The captured flux material may be accumulating.
Fume Can Be Aerosol Rather Than Only Gas
Some emissions from hot flux processes are carried as:
- fine droplets;
- condensed salts;
- particulate.
This means removal mechanisms can include both:
- gas absorption;
- aerosol capture.
More packing height is not automatically the most important solution.
Chloride Salts Enter the Recirculating Liquid
Once captured, the material contributes dissolved and suspended salts.
If the liquid is reused continuously, concentration rises unless removed by:
- blowdown;
- purge.
Water evaporation can concentrate the solution further.
Cooling Can Create Deposits
Hot exhaust cools rapidly in the scrubber.
Species that were volatile or carried at high temperature may:
- condense;
- crystallize.
The location of cooling can therefore determine where deposits appear.
The lower packing or inlet zone may carry a disproportionate fouling load.
Fine Packing Can Become a Salt Trap
Small packing provides excellent initial contact.
In salt-forming service, it can also create many bridging points.
The design should consider whether a more open structure provides a better balance between:
- removal;
- run length.
Distributor Maintenance Is Critical
If concentrated salt solution is recirculated, small outlets can develop crystalline growth.
Partial blockage produces:
- poor wetting;
- local dry areas.
The resulting performance problem may be blamed on the packing even though the distributor is the first component failing.
Demister Deposits Can Affect Fan Protection
Captured chloride-containing droplets may leave salt residue in the demister.
If carryover continues downstream, salts can attack or foul:
- duct;
- fan.
Therefore, separator drainage and washing are important parts of the treatment system.
Conductivity Can Help Track Accumulation
A rising conductivity trend can indicate increasing dissolved-salt loading.
Useful operating information includes:
- conductivity;
- blowdown rate;
- makeup water;
- differential pressure;
- liquid temperature.
Do Not Assume Every Deposit Is Flux Salt
Galvanizing exhaust may contain other:
- zinc-containing material;
- dust;
- corrosion products.
Deposit analysis can confirm the dominant mechanism before changing tower internals.
Cleaning Method Should Match the Deposit
Water-soluble salts may respond to flushing.
Harder mixed deposits can require more aggressive cleaning.
The cleaning procedure must remain compatible with:
- packing;
- FRP;
- demister;
- gaskets.
Engineering Takeaway
A galvanizing wet scrubber can become a salt-concentration system even when the packing material itself is perfectly resistant.