A pickling plant is an industrial plant or part of a plant in which oxide layers, impurities or scale are removed from metallic materials using chemical processes. This is done using pickling solutions, which typically contain acids or alkalis. Pickling is used to prepare metallic surfaces for subsequent processing, such as coating, electroplating or painting, and ensures that the surfaces are metallically bright and free of residues.

In industrial water and wastewater treatment, the treatment of wastewater from pickling plants is of crucial importance, as this wastewater often contains heavy metals, acid residues and other pollutants that must be treated before being discharged into the environment.

Technical background of the pickling process

Pickling removes unwanted surface impurities from metals through contact with a chemical pickling solution. This process is mainly used in metal processing and surface technology to prepare metal surfaces for further processing steps or to increase corrosion resistance.

The chemical reactions in the pickling process vary depending on the material, the acid used and the aim of the pickling process. Frequently used pickling solutions are

• Hydrochloric acid (HCl): Particularly effective for removing scale and corrosion products from iron and steel.
• Sulphuric acid (H₂SO₄): Widely used for pickling steel, especially in steel production. It is often used in high concentrations as it effectively removes scale and oxides.
• Hydrofluoric acid (HF) and nitric acid (HNO₃): These acids are often used in combination, especially for pickling stainless steels and aluminum. They are able to remove particularly resistant oxide layers.
• Phosphoric acid (H₃PO₄): Used to passivate the surface and protect it from corrosion.

In addition to acids, alkalis such as sodium hydroxide (NaOH) can also be used for alkali pickling, especially for aluminum or zinc. In some cases, inhibitors are added to the pickling solutions to slow down the attack of the acid on the base metal and ensure the uniform removal of impurities.

Pickling process and systems

Depending on the application and the material to be treated, pickling processes can be carried out using different methods:

1. Immersion pickling: The workpiece is immersed in a pickling bath in which the chemical reaction takes place. This process is widely used in mass production, e.g. in the steel and aluminum industry. Complete immersion ensures that even complex geometries are fully treated.

2. Spray pickling: The pickling solution is sprayed onto the metal surface. This process is often used for large-area or continuously produced materials, such as sheet metal coils or pipes, in order to efficiently control contact with the pickling solution.

3. Electrochemical pickling: In this process, an electric current is used to accelerate the chemical reaction. It is mainly used for stainless steels as it ensures a uniform surface treatment.

Waste water from pickling plants and its treatment

The wastewater produced in pickling plants usually contains high concentrations of acids, heavy metals (such as iron, nickel, zinc, chromium), organic residues and dissolved oxides. The treatment of this wastewater is of crucial importance, as untreated wastewater must not be released into the environment.

Various physico-chemical processes are used to treat wastewater from pickling plants:

1. neutralization:

Waste pickling water often contains excess acids that need to be neutralized. This is done by adding alkalis such as sodium hydroxide or milk of lime (Ca(OH)₂) to bring the pH value to a neutral range. Precise pH control is crucial to ensure optimum conditions for the subsequent treatment stages.

2. precipitation and flocculation:

Precipitants such as iron(III) chloride or calcium-based precipitants are used to remove heavy metals and dissolved solids from the pickling wastewater. These cause the formation of insoluble compounds, which are then aggregated into larger flocs by flocculants such as polymers and subsequently separated by sedimentation or filtration.