Résumé
The mining of non-ferrous metals produces the largest volume of metal-containing, extractive waste in Europe, and about 29% of all the waste produced in the EU-28. In the frame of the European project NEMO (Near-zero-waste recycling of low-grade sulfidic mining waste for critical-metal, mineral and construction raw-material production in a circular economy), new ways to valorise sulfidic tailings are being developed through the recovery of valuable metals and critical raw materials and the transformation of the residual in clean mineral fraction to be used for the mass production of cement, concrete and construction products. The first step of NEMO concept consists in removing the sulfides remaining in the tailings and extracting the residual metals using either enhanced bioleaching or alkaline autoclave conversion processes. This paper focuses on one of the project case study, the residues obtained at an operating heap leaching plant (Terrafame, Finland). This material still contains several sulfidic minerals (pyrrhotite, pyrite, sphalerite, pentlandite, violarite, chalcopyrite) and significant amounts of metals (Zn, Ni, Cu, Co, REEs). The study aimed at increasing metals leaching yield. Bioleaching tests were performed first in shake-flasks and then in stirred tank reactors (2L) at increasing solid concentration (from 3 to 10%) and at three temperatures using three microbial cultures growing at 42°C, 48°C, 55°C. The results show that Ni is released very quickly suggesting that part of Ni dissolved in the primary heap is re-precipitated and remains in the residues. In the contrary, Cu dissolution is much slower but the kinetics is substantially improved when the temperature is increased