Galvanic industries

Metal-finishing operations can include degreasing, pickling, electroplating, galvanizing and black oxide coating. Not every facility uses all of these processes. The appropriate treatment of process water and wastewater depends on the bath chemistry, contaminants and water volumes.

Degreasing

Before a thin metal coating is applied, the workpiece must be properly prepared to ensure good adhesion. Oil, grease, dust, metal particles and other residues must be removed from the surface.

Contamination can reduce the effectiveness of a degreasing bath. Bath-maintenance methods are selected for the chemicals and contaminants present. Filtration removes suspended particles. Suitable ion exchangers may remove selected dissolved ions. Ultrafiltration can remove suspended particles and colloids; it does not generally remove dissolved salts.

Pickling

Pickling uses acid to remove corrosion products and scale from a metal surface. The workpiece is then rinsed to remove remaining acid. The rinse water requires treatment appropriate to its acidity and dissolved contaminants.

Electroplating and galvanizing

Process baths and treatment systems may produce sludge containing metal hydroxides and other compounds. The composition and value of the sludge depend on the metals and process used. Metal-bearing streams may be suitable for recovery, including electrolysis, depending on their composition.

Cyanide compounds, where used, and dissolved metals require specific treatment and controlled disposal. Neutralization alone does not establish that the wastewater is suitable for discharge.

Black oxide coating (blackening)

The process described here is chemical blackening: a workpiece is immersed in an alkaline oxidizing bath to form an oxide layer on the metal surface. It is distinct from mechanical burnishing.

The surface is cleaned and rinsed before blackening. A protective oil or another suitable sealant is applied afterwards to improve corrosion resistance. The oxide layer provides a base for this protective finish.

Wastewater treatment must account for the bath constituents, including sodium nitrite where present, and may require neutralization and treatment of other contaminants.

Rinsing

Rinsing between production steps can generate substantial volumes of wastewater containing bath chemicals, acids, alkalis, dissolved metals and suspended solids. Treatment must be selected according to the contaminants present and the applicable discharge requirements.