Pickling acid recycling

Regeneration of acids and the recovery of metals from spent acids solutions after the stainless steel pickling process. Reducing the emission of nitrates in the waste water. Avoiding the need for land-filling of acid-metal residues in special landfills. Producing reusable by-products.

During the pickling operations stainless steel in a form of long or flat products is immerged in various mixtures of acids that remove needless and impure surface of oxides created during the prior productions processes. Conventional technologies installed after the process allow only for recovery of acids that did not react. Remaining waste from the process contains several hazardous and harmful constituents therefore currently is usually neutralised and disposed. Due to the costs of disposal and hazardousness, remaining waste is considered to be a significant environmental and economical issue. UGITECH uses large quantities of nitric and hydrofluoric acids in the pickling process of austenitic stainless steels and nitric acid in the passivation of ferritic and martensitic stainless steels. During the process, part of the acids is consumed. The acids baths are therefore refreshed from time to time with new acids. The spent acids need to be discharged. The spent discharged solution is neutralised, however causes unwanted discharge of the nitrates in the waste water. Beside the presence of the nitrates, the heavy metals entrapped in the acid waste need to be neutralised, which results in the production of a heavy metal sludge made by metals hydroxides and acids salts. There are about 7 million tones of stainless steel produced annually in Europe. A substantial part of this quantity is subjected to further processes in order to achieve better surface quality of semi-finished or finished products (e.g. through picking operations). Similarly, pickling is used in jewellery making, after fluxing or soldering, in order to remove any scale and contaminants from the jewellery metal, which is often sterling silver, copper, or gold. Spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralisation. The lime neutralisation process raises the pH of the spent acid and makes heavy metals in the sludge less likely to leach into the environment. Unfortunately, due to the costs of disposal and hazardousness, remaining waste is considered to be a significant environmental and economical issue. The nitric acid based mixtures are traditionally used with consequent problems caused by this acid such as nitrogen oxides emissions into the atmosphere and nitrates into water. When these baths contain more than 50-60 g/l of dissolved metals, the acids lose their efficiency and become difficult to manage. Therefore the pickling tanks must be totally or partially discharged and refilled with a fresh solution that restores the acids concentration and reduces the metals concentration. The discharged acid baths are neutralised with lime in order to obtain a sludge which can be discharged and transported. This sludge contains metals, mainly iron, chrome and nickel, as hydroxides while the fluorides become calcium salts. The sludge must be disposed in proper dumps due to its toxicity. The biggest problem is the presence of nitric acids. This part of the waste treatment requires special attention since the high solubility of salts in nitric acids. This chemical phenomenon makes it impossible to remove in totality the nitrates in the discharged water. Only recently, in big installations where volumes to dispose are high, some techniques such as diffusion dialysis or acid retardation processes are used in order to recover the free acids in the waste and hence reducing the Nitric emissions in the waste water and neutralisation process. Some techniques are known for treatment of steel industry wastes, such as electro-dialysis, pyrolysis, cryogenic separation systems, solvent extraction. However all these techniques have not lead to any real acceptance and market penetration in the industry due to the high installation and management costs, their complexity and limited efficiencies. The need for UGITECH to move towards an environmentally sustainable production with the final target to operate a zero discharge process, has led to the proposed process innovation.
Project ID: 
5 795
Start date: 
Project Duration: 
Project costs: 
3 800 000.00€
Technological Area: 
Environmental Engineering / Technology
Market Area: 
Speciality metals (including processes for working with metals)

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