In the production process at ThyssenKrupp Nirosta, the scale on the surface of the annealed stainless steel is removed in a pickling process using a mixture of hydrofluoric and nitric acid. Over time, the pickling liquor becomes less effective and the “spent acid” has to be replaced regularly with fresh acid. Previously, the spent acid was neutralized using lime milk, the resultant sludge compacted and sent to landfill, and the waste water fed to the sewer system. In the innovative process developed by plant construction company Andritz AG, metals and regenerated mixed acid are now recovered in a thermally driven process and fed back to the pickling process. “That turns the line into a closed circuit,” says Gunther Hartmann, responsible project manager at ThyssenKrupp Nirosta. “The new recycling system allows us to make substantial savings, which are urgently needed in the current situation.”

This method of acid recovery delivers advantages in many areas: Lower volumes of waste water with reduced nitrogen content mean lower pollution. Less acid and sludge needs to be transported, and the spent acid regeneration system also significantly reduces lime consumption. Moreover, acid recycling helps maintain consistent quality in the pickling process.