Recovery of metals in a double-stage continuous bioreactor for acidic bioleaching of printed circuit boards (PCBs)

Abstract

Many studies are now focusing on bioleaching methods to recover metals from WEEE. The efficiency of this process is highly dependent on microorganisms but also on the solid-liquid-gas mass transfer, which is controlled by the reactor design. In this study, bioleaching of comminuted spent printed circuit boards (PCBs) was performed in a stirred tank reactor operated in batch mode and in a double-stage continuous bioreactor. The metal dissolution kinetics were compared. The first stage of the continuous bioreactor was a bubble column in which a BRGM-KCC acidophilic consortium comprising Leptospirillum ferriphilum and Sulfobacillus benefaciens was used to oxidise Fe(II) into Fe(III). The resulting liquor was used to leach out metals contained in PCBs in the second stage of the bioreactor with mechanical stirring. The use of two distinct stages allowed the bacteria to adapt gradually to the PCBs and reach high dissolution yields, i.e. 96% Cu, 73% Ni, 85% Zn and 93% Co when 1% (w/v) PCB scraps were added into the bioleaching reactor, with a hydraulic residence time of 48 h. By using the double-stage bioreactor, the concentration of PCB scraps could be increased up to 1.8% (w/v) without reducing bioleaching performance. Biomass concentration in the second stage and adaptation of the microorganisms to the toxicity of the metals were sufficient for only the second stage to be used. Under these conditions, the dissolution kinetics were stable, even when iron was provided only by the comminuted PCBs.