In short
- Different tanks release different fume, so treatment starts with a tank-by-tank survey.
- Chromium (VI) and nickel mists are respiratory sensitisers; HSE treats them as asthmagens.
- Nitric acid lines can release NOx, which needs a different approach from the acid mist itself.
Metal finishing lines combine heat, acids and large open tank surfaces. Local exhaust ventilation captures the fume at the tank; the scrubber decides what happens to it next. This article looks at the second half of that job: what each process sends down the duct, and how to treat it before it reaches the stack.
Why plating fume needs care
HSE identifies hexavalent chromium and nickel sulphate mists from electroplating as respiratory sensitisers that can cause occupational asthma. Once someone is sensitised, even low levels may trigger an attack. HSE research found that mist suppressants and local exhaust ventilation can both provide adequate control, provided they are properly implemented and maintained.
Once fume is captured, it has to be treated rather than simply moved outside. That is where a scrubber comes in.
What comes off each process
| Process | Main fume | UK workplace limit (EH40) | Treatment approach |
|---|---|---|---|
| Hard chrome plating | Chromic acid mist (chromium VI) | 0.01 mg/m³ long term (as Cr) | Mist eliminator and packed scrubber; recover mist where possible |
| Sulphuric acid anodising | Sulphuric acid mist | 0.05 mg/m³ long term (mist) | Mist elimination plus packed tower |
| Hydrochloric acid pickling | HCl gas and mist | 1 ppm long term, 5 ppm short term | Packed tower; water or dilute caustic |
| Nitric or mixed-acid pickling | Nitric acid mist and NOx | Nitric acid 1 ppm short term | Acid stage for mist, separate approach for NOx |
| Nickel plating | Nickel salt mist | See EH40 for nickel compounds | Mist elimination and scrubbing |
Limits in the table are workplace exposure limits for the people working at the line. Your emission limit at the stack comes from your permit and is a separate number.
Running a plating, anodising or pickling line?
Send the tank list and the extract flow. We will tell you which streams can share a scrubber.
Mist versus gas
Some processes produce a gas, such as HCl, which dissolves readily into scrubbing liquor. Others, such as chromic and sulphuric acid, produce mostly fine liquid droplets. Mist behaves more like a particle than a gas, so it needs good mist elimination and sometimes a dedicated mist stage ahead of the packed tower.
Designing for the wrong form is a common reason for visible plumes and failed stack tests.
The NOx problem on nitric lines
Nitric acid pickling, particularly on stainless steel, can release nitrogen oxides as well as acid mist. Nitrogen dioxide is the brown component of the plume. Nitric oxide is poorly soluble in water and does not scrub well on its own, so NOx usually needs a multi-stage design with an oxidising or reducing chemistry rather than a single caustic tower.
Engineer's note
Before sizing anything on a finishing line, we walk the tanks with the line manager and list each bath, its chemistry, its temperature and how it is agitated. Air-agitated and electrolytic tanks throw far more mist than still baths, and that survey decides which tanks can share a scrubber.
Keeping incompatible streams apart
Cyanide plating exhausts must never meet acid exhausts. Chromic acid streams are often kept separate so the chromium can be managed. A tank-by-tank review before any ductwork is designed is the simplest way to avoid expensive mistakes.
Our acid fume scrubbers are designed around those surveys, and our multi-stage scrubbers handle lines where acid mist and NOx have to be treated in sequence.
Need the full design guide? Our acid fume guide covers liquor and material choices for HCl, HF and nitric duties.
Get the acid fume guideCommon questions
Do mist suppressants remove the need for a scrubber?
They reduce mist at the tank surface and help with workplace exposure. Whether you also need a scrubber depends on your permit and what is still being extracted.
Can pickling and plating extracts share one scrubber?
Sometimes. Compatible acid streams can, but cyanide and some chromium streams should be kept apart. We review each tank before advising.
Why is there a brown plume from our nitric line?
A brown plume usually indicates nitrogen dioxide. It needs a NOx-specific stage; a standard acid scrubber will not remove it.