The quality of stainless steel products is determined by defined surface conditions. During processing, complex structures of tightly adhering oxide layers - referred to as scale - form on the material surface. Additionally, chromium diffuses from the base metal alloy into the scale layers. Both reactions affect the chemical resistance of stainless steel products.
To remove scale and the chromium depleted metal layer from the base metal alloy, a combination of mechanical and chemical processes is applied. Chemical treatment with strong inorganic acids is referred to as pickling. In most cases, pickling of chromium-nickel stainless steels takes place with mixtures of HNO3 and HF–. During pickling, the amount of free acid decreases, while the amount of dissolved metal ions increases in the pickling solution. To keep the pickling rate as high as possible, metal ion and free acid concentration should be kept in a certain range. In large rolling mills, continuous regeneration of spent pickling solutions is state of the art. Two general technical approaches for acid regeneration and metal recovery can be distinguished: Total regeneration allows the recovery of both, free acid and metals in form of metal oxides. This can be achieved through a thermal process referred as pyrohydrolysis, or processes combining different treatment steps. Partial regeneration allows the recovery of the still active components of the pickling solutions that have not reacted with the stainless steel surface. In partial regeneration, an acidic byproduct is generated that contains the majority of the dissolved metals from the original pickling solution. To meet the statutory limits for wastewater, this byproduct needs to be neutralized with alkaline solutions. This treatment leads to the formation of the so called neutralization sludge. The sludge - containing, e.g. iron, chromium and nickel hydroxide - must be deposed of. Thus, valuable metals are irrecoverably lost.
To cope with this, options for the recovery of valuable metals from neutralization sludge were investigated. These techniques include...
Session: M3 - Separation of Complex Systems
Day: 23 October 2019
Time: 09:00 - 10:15 h