Resumen de Study and optimisation of copper bioleaching process for electronic waste valorisation
In the current economical context, the use of waste material with economic potential should be a priority. In this sense, the increasing production of electrical and electronic equipment waste (WEEE) makes these materials a potential source for valuable and scarce metals. For this reason, it is important to develop new metal recovery methodologies economically that are more profitable, sustainable and environmental friendly. A possible solution to this problem is to take advantage of the metabolic activity of certain microorganisms, mainly bacteria, to regenerate the responsible agents for the extraction of metals from the matrix in which they are contained once the useful life of them has ended. This process is known as bioleaching or biological leaching.
In this thesis, a study of this biotechnological process for metal recovery from WEEE has been carried out. Firstly, bioleaching to recover copper from low-grade chalcopyrite was studied to establish the bases of the methodology, already applied in the biomining field, as well as to check the feasibility of the technique in this field. Subsequently, bioleaching was extended to be applied in the field of the electronic waste, thus recovering metals from printed circuit boards (PCB) based on their high metal content and their limited availability in the nature. Given the interest of this process, not very studied in the field of the electronic waste, an adjustment of those parameters that allow optimizing the operation is necessary. For this reason, the effect of several parameters has been studied such as pH effect, PCB concentration or particle size, as well as the most appropriate system to perform the process (flasks, bioreactor or column).
After bioleaching, the extracted metals remain in the leaching solution, so a last step to obtain the metals in their metallic state again and separated from the initial matrix should be perform which closes the recovery cycle. The study to recover the bioleached copper has been carried out more superficially in this thesis, focusing on cementation as a simple and cheaper alternative to other more complex processes such as electrolysis.
In addition to the metals extraction through bioleaching, this thesis has been also focused on studying the limits of the technology due to the complex and varied composition of the waste, such as the toxic effect that bioleached metals could cause to the microorganisms involved in the process or the evaluation of possible substrate inhibition. The measurement of the biological activity may be the solution when there are limitations of quantifying biomass in systems where the formation of precipitates can be habitual, as in bioleaching occurred. For this reason, a microrespirometry-based procedure has been developed that allows to directly measure the oxygen consumption and, thus, the microbial activity at real time. In microrespirometry, the formation of precipitates does not interfere with the measurement which allows obtaining a reliable result of the microbial concentration.
Thus, after affirming the feasibility of bioleaching as a simpler, cheaper and environmental friendly alternative to traditional physical-chemical processes, this thesis establishes the most favourable conditions to obtain the greatest possible copper recovery through bioleaching. These bases are the previous phase to scale-up the technology to be implemented in an industrial environment.
