Case Studies : Control of Acid Mist

The Problem
A Metal Casting and Extrusion company had exceeded acceptable airbourne acid mist and gas levels.

Irregular aluminium alloy extrusions and castings were manually cleaned in an acid bath on a regular basis.

Acid mist from the open bath, containing a mixture of hydrofluoric (HF) and sulphuric acids, generated airborne acid mist and gas at concentrations consistently greater than the occupational exposure limits.

The Process – Acid Etch and Cleaning
Through a sequential application of various control measures the airborne levels were reduced to the degree that respiratory protection was no longer required. Fugitive emissions to adjacent departments ceased to be a regular problem.

Stage 1: Addition of croffles (plastic balls) to the surface reduces the surface area and hence the evaporation rate of the HF.

Stage 2: Installation of local exhaust ventilation along rear edge of bath in the form of a vertical slot captured the greater proportion of the airborne gas and mist.

Stage 3: Addition of a flange along the top and ends of the extraction slot further improved the effectiveness of the system.

Stage 4: Increase in the fan capacity to further reduce concentrations when the workload increased and the published limit was reduced.

The Occupational Hygiene Unit monitored the airborne concentrations at each stage and provided the design parameters for the control systems.

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