Stirred-tank and heap-bioleaching of shredder-light-fractions (SLF) by acidophilic bacteria

Abstract

End-of-life vehicles are usually crushed in large scaled shredders in order to increase the surface area for further processing. During the shredding process, the so-called shredder-light-fraction (SLF), containing mainly plastics and other synthetic polymers, is discharged. However, this fraction still contains up to 20% metals such as iron (Fe), copper (Cu), nickel (Ni) and zinc (Zn), making them an interesting substrate for bioleaching and source of high-value metal traces. To develop a bioleaching process for metal recovery from shredded fractions, two experimental approaches including a heap- and a stirred-tank reactor were tested. The acidophilic, iron- and sulfur-oxidizing bacteria Acidithiobacillus ferrooxidans and the iron-oxidizing bacteria Leptospirillum ferrooxidans were investigated as pure and co-culture for their ability to solubilize trace metals. A. ferrooxidans proofed to be the more suitable bioleaching organism resulting in leaching efficiencies of up to 100% for Zn and Ni in batch experiments. Furthermore, in the stirred-tank reactor using A. ferrooxidans about 100, 80 and 55% of Cu, Zn and Ni have been recovered from 50 g l−1 of the SLF, respectively. Bioleaching of 3000 g SLF in the heap-reactor resulted in a recovery of about 100% Zn, 60% Cu and ~ 55% Ni after 33 days without the need of external sulfuric acid addition for pH regulation. The high leaching efficiencies in both experimental setups confirmed the potential of bioleaching for a sustainable metal recovery from shredder residues (i.e. SLF) making it a profitable, eco-friendly alternative to conventional chemical leaching processes.