The increase of waste from electric and electronic equipment has pushed the research towards the development of high sustainability treatments for their exploitation. The end-of-life printed circuit boards (PCBs) represent one of the most significant waste in this class. The interest for these scraps is due to the high Cu and Zn concentrations, around 25% and 2% respectively. Biohydrometallurgical strategies are gaining increasing prominence, for the possibility to decrease the environmental costs. Nevertheless, these techniques show the main limit due to the low treated PCB amount, which makes unsustainable the further scale-up. To overcome this criticality, the present research introduces an innovative bioleaching process carried out by At. ferrooxidans. The developed technology allows to reach high PCB concentration thanks to a high efficiency two-step design, able to reduce the metal toxicity on the bacteria metabolism. The treatment uses the Fe3+ generated by bacterial oxidation, as oxidant, to leach Cu and Zn from PCBs. The possibility to overcome the solid concentration criticality is combined with high yield of 95% for Cu and Zn. The best selected conditions are: 30°C, pulp density of 5% (w/v), 10 g/L of Fe2+, time of treatment 11 days. A mathematical model to study the kinetic of the chemical reaction for the Cu leaching from PCBs, by Fe3+, is reported and an activation energy of 18-25 kJ/mol is estimated. The mechanism of the bioleaching process is studied in detail, integrating the previous model results with two differential equation to describe the bacteria growth and metabolism rate. The developed model is consistent with a R2 higher than 0.97. The results confirm the positive effect of the new innovative process. The determined mathematical model, suitable for the implementation at industrial scale, could be an important tool to predict the bioleaching mechanism. In the next step, the research is focused on the selective metal recovery from the leaching solution. The best identified conditions include: the Fe precipitation with NaOH, followed by the Cu cementation with Zn and a final Zn precipitation with oxalic acid. The metals show recovery efficiencies and purities higher than 95%. The experimental results are further enhanced by the carbon footprint assessment which proved the environmental advantage, compared to both the reference chemical treatment through Fe3+ and literature processes (hydrometallurgical and bioleaching approaches). The environmental impact assessment drives the optimization and improvement of the developed process in laboratory scale. It is used to identify the best conditions for the bioleaching process able to make the treatment actually sustainable. Overall, the environmental assessment identifies, as main criticalities to solve: the high energy demand of bioleaching process and the raw materials demand of the following recovery steps. These preliminary results have determined the choices in the further experiments. More in detail, the bioleaching process is improved by the increase of treated PCB concentration from 5% to 10% (w/v) maintaining a leaching efficiency higher than 95% for Cu and Zn. This technical improvement is translated into the halved of energy demand. Furthermore, the environmental load of recovery can be reduced by a double strategy: avoiding Zn recovery or substituting the hydrometallurgical techniques by electrochemical approaches. In the last chapter, a sustainable process, which uses the fungal strain Aspergillus niger for metal extraction from PCBs, is described. The best identified conditions are PCB addition after 14 days, Fe3+ as oxidant agent, and a pulp density of 2.5% (w/v). Extraction efficiencies of 60% and 40% for Cu and Zn, respectively, are achieved after 21 days of fermentation. The ecodesign of the process is further enhanced by using milk whey as substrate for the fungal growth and the consequent citric acid production.
La crescita dei rifiuti da apparecchiature elettriche ed elettroniche ha spinto la ricerca verso lo sviluppo di processi sostenibili per la loro valorizzazione. I circuiti stampati (CS) a fine vita rappresentano uno dei principali rifiuti di questa categoria. L’interesse per questi scarti è dovuto alla concentrazione di Cu e Zn, corrispondente a circa il 25% e 2% rispettivamente. L’interesse per le biotecnologie continua a crescere come possibile strategia per la loro valorizzazione. Tuttavia, il limite principale di questi approcci è la bassa concentrazione dei CS trattati, il che li rende poco applicabili a livello industriale. Per superare tale criticità, questa ricerca introduce un approccio biotecnologico innovativo condotto con At. ferrooxidans. Il processo sviluppato permette di aumentarne la concentrazione grazie ad un approccio in più fasi, in grado di ridurre la tossicità dei metalli sul metabolismo dei batteri. Il processo usa il Fe3+ prodotto dall’ossidazione batterica per lisciviare il Cu e lo Zn dai CS. Le condizioni migliori individuate sono: 30°C, concentrazione dei CS del 5% (m/v), 10 g/L di Fe2+, tempo di trattamento di 11 giorni. La cinetica della reazione chimica tra il Cu e il Fe3+ è stata studiata tramite un modello matematico ed è stata stimata un’energia di attivazione pari a 18-25 kJ/mol. Il meccanismo del processo biologico è stato studiato nel dettaglio integrando i risultati del precedente modello con due equazioni differenziali per descrivere il tasso di crescita e il metabolismo del batterio. Il modello sviluppato è consistente con un R2 superiore a 0.97. Inoltre, questo modello potrebbe essere utile per lo sviluppo di tale tecnologia a livello industriale. Nella fase successiva, la ricerca si è focalizzata sul recupero selettivo dei metalli dalla soluzione di lisciviazione. Le migliori condizioni operative individuate sono: precipitazione del Fe con NaOH, seguita dal processo di cementazione del Cu con lo Zn e il recupero finale dello Zn con acido ossalico. Le efficienze e purezze dei metalli recuperati sono superiori al 95%. I risultati sperimentali sono ulteriormente valorizzati dallo studio della carbon footprint, che ha dimostrato il vantaggio ambientale del nuovo approccio, confrontandolo con il trattamento chimico con Fe3+ e con processi riportati in letteratura (idrometallurgici e biotrattamenti). La valutazione dell’impatto ambientale ha guidato l’ottimizzazione del processo sviluppato in scala laboratorio. Grazie a questo metodo, sono state identificate le condizioni migliori affinché il processo biotecnologico sia realmente l’approccio più sostenibile. In generale, la valutazione ambientale ha individuato come principali problemi: la richiesta di energia nel processo di biolisciviazione e la quantità di materie prime nel recupero selettivo dei metalli. Questi risultati preliminari hanno determinato la scelta degli esperimenti successivi. Nel dettaglio, il processo biotecnologico è stato migliorato aumentando la concentrazione dei CS da 5% al 10% (m/v) mantenendo efficienze superiori al 95% per entrambi i metalli. Questo miglioramento ha permesso di dimezzare la richiesta di energia. Inoltre, il carico ambientale del processo di recupero può essere ridotto o escludendo il recupero dello Zn o sostituendo le tecniche idrometallurgiche con approcci elettrochimici. Nell’ultimo capitolo, si è preso in considerazione un processo biotecnologico alternativo, utilizzando il fungo A. niger per estrarre i metalli dai CS. Le condizioni migliori identificate includono l’aggiunta dei CS dopo 14 giorni, l’uso del Fe3+ e una concentrazione del substrato pari a 2.5% (m/v). Efficienze d’estrazione pari al 60% per il Cu e al 40% per lo Zn sono state ottenute dopo 21 giorni di fermentazione. Il design del processo è stato ulteriormente migliorato utilizzando il siero del latte come substrato per la crescita fungina e la produzione di acido citrico.
Biotechnologies for metal recovery from wastes / Becci, Alessandro. - (2021 May 28).
Biotechnologies for metal recovery from wastes
BECCI, ALESSANDRO
2021-05-28
Abstract
The increase of waste from electric and electronic equipment has pushed the research towards the development of high sustainability treatments for their exploitation. The end-of-life printed circuit boards (PCBs) represent one of the most significant waste in this class. The interest for these scraps is due to the high Cu and Zn concentrations, around 25% and 2% respectively. Biohydrometallurgical strategies are gaining increasing prominence, for the possibility to decrease the environmental costs. Nevertheless, these techniques show the main limit due to the low treated PCB amount, which makes unsustainable the further scale-up. To overcome this criticality, the present research introduces an innovative bioleaching process carried out by At. ferrooxidans. The developed technology allows to reach high PCB concentration thanks to a high efficiency two-step design, able to reduce the metal toxicity on the bacteria metabolism. The treatment uses the Fe3+ generated by bacterial oxidation, as oxidant, to leach Cu and Zn from PCBs. The possibility to overcome the solid concentration criticality is combined with high yield of 95% for Cu and Zn. The best selected conditions are: 30°C, pulp density of 5% (w/v), 10 g/L of Fe2+, time of treatment 11 days. A mathematical model to study the kinetic of the chemical reaction for the Cu leaching from PCBs, by Fe3+, is reported and an activation energy of 18-25 kJ/mol is estimated. The mechanism of the bioleaching process is studied in detail, integrating the previous model results with two differential equation to describe the bacteria growth and metabolism rate. The developed model is consistent with a R2 higher than 0.97. The results confirm the positive effect of the new innovative process. The determined mathematical model, suitable for the implementation at industrial scale, could be an important tool to predict the bioleaching mechanism. In the next step, the research is focused on the selective metal recovery from the leaching solution. The best identified conditions include: the Fe precipitation with NaOH, followed by the Cu cementation with Zn and a final Zn precipitation with oxalic acid. The metals show recovery efficiencies and purities higher than 95%. The experimental results are further enhanced by the carbon footprint assessment which proved the environmental advantage, compared to both the reference chemical treatment through Fe3+ and literature processes (hydrometallurgical and bioleaching approaches). The environmental impact assessment drives the optimization and improvement of the developed process in laboratory scale. It is used to identify the best conditions for the bioleaching process able to make the treatment actually sustainable. Overall, the environmental assessment identifies, as main criticalities to solve: the high energy demand of bioleaching process and the raw materials demand of the following recovery steps. These preliminary results have determined the choices in the further experiments. More in detail, the bioleaching process is improved by the increase of treated PCB concentration from 5% to 10% (w/v) maintaining a leaching efficiency higher than 95% for Cu and Zn. This technical improvement is translated into the halved of energy demand. Furthermore, the environmental load of recovery can be reduced by a double strategy: avoiding Zn recovery or substituting the hydrometallurgical techniques by electrochemical approaches. In the last chapter, a sustainable process, which uses the fungal strain Aspergillus niger for metal extraction from PCBs, is described. The best identified conditions are PCB addition after 14 days, Fe3+ as oxidant agent, and a pulp density of 2.5% (w/v). Extraction efficiencies of 60% and 40% for Cu and Zn, respectively, are achieved after 21 days of fermentation. The ecodesign of the process is further enhanced by using milk whey as substrate for the fungal growth and the consequent citric acid production.File | Dimensione | Formato | |
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