Sustainable Stainless Steel – A Review on Acid Regeneration Systems for Application in Continuous Pickling Lines

Steels go through several processing steps to reach the desired state of delivery. Properties and quality of stainless steel are mainly determined by the quality of the product surface. In high temperature processes, such as annealing, rolling and forging, scale layers of 5 to 10 µm thickness consisting of oxides and oxide hydrates are formed on the steel surface. Furthermore, underneath the scale layer the chromium content of the alloy is depleted by diffusion effects, leading to reduced chemical stability. Thus, the chemical removal of both the scale layer and the chromium depleted layer in pickling processes is a fundamental step to achieve clean and homogeneous surfaces. Most of the steels are pickled at least once, usually several times. Strong mineral acids are commonly applied as pickling media. For stainless steel pickling, usually mixtures of nitric acid and hydrofluoric acid – so called mixed acids - are applied. Due to the chemical reaction, an enrichment of the pickling medium with detached scale particles, dissolved metal salts (bound acid), as well as a depletion of active acid (free acid) takes place. In this way, the pickling medium loses its effectiveness and must be replaced or refreshed. This causes significant emissions of acidic waste streams, which need to be treated to comply with regulations. The waste streams contain considerable amounts of dissolved heavy metals, nitrates, and free acid. Commonly, precipitation neutralization is applied; however, this leads to a loss of the dissolved valuables. Furthermore, significant amounts of neutralization sludge are generated, which need to be disposed of, as they constitute hazardous wastes. For both environmental and economic reasons the loss of valuables should be avoided. For this reason, pickling media are continuously regenerated in modern pickling lines. The focus may be on acid recovery only - this is referred to as partial regeneration - or on a combined acid and metal recovery - this is referred to as total regeneration.