Valorization of solid waste from oil refining and biodiesel industries for the biorecovery of rare earth elements

Two types of solid waste generated in large amounts in oil refining and biocombustible industries, i.e., spent cracking catalyst and unprofitable raw glycerin were concomitantly valorized. Experiments for process optimization were carried out in two successive steps. First, a 24full-factorial experimental design was adopted to estimate the effects of four variables and, their interaction, on the rare earth metal biorecovery from spent cracking catalyst using Yarrowia lipolyticacell-free fermented medium (growth as expenses of raw glycerin as the main carbon source): pulp density, bioleaching temperature, contact time, and raw glycerin concentration in microbial culture. Next, tests were executed to maximize the biorecovery, considering the previous results in the experimental design. The optimized process conditions were found to be pulp density, 0.4% (m/v); bioleaching temperature, 50 °C; contact time, 45 min; and bioleaching production using 56 g/L of biodiesel-derived glycerin in the mineral medium. In such process conditions, up to 90% of La, Nd, and Ce were recovered. All of the biomining tests were carried out in 500-mL Erlenmeyer flasks and contained 100 mL of the cell-free fermented medium. Thermally or chemically pretreated catalysts or raw glycerin was not used in this study and the yeast strain was not previously adapted to the contaminants (organics or metals). Other goals were to reduce the process stages, seeking economic viability for future larger-scale (scale-up) studies.