The pickling of steel in acid is part of the steel galvanising process. Galvanising involves coating steel in zinc to provide an impervious barrier that protects steel products from corrosion. Prior to the zinc-coating stage, pickling with acids removes rust and scale from the surface of steel, providing the required level of material purity. The pickling process forms metallic salts that generate a final residue known as spent pickling acids (SPAs). SPAs must be disposed of because the efficiency of pickling decreases with the increasing content of dissolved metals, mostly iron and zinc, in the pickling bath. The production of one tonne of galvanized metal produces about 38 kilogrammes of SPA.
SPAs are typically treated using conventional processes that involve neutralisation of the acids and precipitation of metal residues in the form of insoluble hydroxides. The precipitates are filtered and sent to landfill. Potentially useful materials, such as the remaining acid and valuable metal ions, are lost. A technology for the recovery of metals from SPAs would be an environmentally-friendly alternative in the galvanising sector and could create highly-skilled jobs.
The LIFE-2-ACID project will demonstrate a new technology to selectively recover zinc and iron chloride from SPAs. The project will design, construct and operate a prototype pilot plant able to treat 200 litres/day of SPAs. The plant will have two units:
The recovered zinc will be reused as a raw material for galvanising. The process will also permit the recovery of iron chloride as a sub-product to be used as a reagent in wastewater treatment plants.
LIFE-2-ACID will contribute to the implementation, updating and development of EU environmental policy and legislation, especially the Industrial Emissions Directive. The proposed technology has the potential to be considered as a BAT (Best Available Technology) for the surface treatment and ferrous metal processing industries. By recovering raw materials and reducing waste, the project will also be in line with the EU Waste Framework Directive and the Circular Economy Action Plan. The LIFE-2-ACID approach could also reduce greenhouse gas emissions from the extraction and processing of minerals, their transportation and their management when they become waste.