Recovery of Rare Earths and Precious Metals from WEEE (Waste Electrical and Electronic Equipment) by acid leaching and immobilized agents

Today there is an increasing need for rare earths (REs) due to their usage in numerous high-technology applications such as: optical, electronics, ceramics and nuclear. Currently each EU citizen produces about 17 kg of waste electrical and electronic equipment (WEEE) per year. These wastes are rich in precious and strategic metals and, in many cases, are characterized by higher REs contents than those of natural minerals. Accordingly, recycling can be considered a valuable opportunity: this perspective is known as "urban mining”. For these reasons, the study of an efficient REs recovery from WEEE can lead to undeniable socio-economic and environmental benefits. The present work performed under the project “E-WASTE - Il ciclo intelligente” (ID 40511448, financed by Lombardia Region) focused on the recovery of REs and precious metals from WEEE, namely magnetic components, mobile phones and Printed Circuits Boards (PCBs). In particular one of the final aim of E-WASTE project was the realization of a pilot plant for WEEE treatment carried out through the partnership of two large companies (Amsa and Stena), a medium-sized enterprise (Consorzio Remedia), three SMEs (Seval, Tecnochimica, and, Gaser), two research organizations (Politecnico di Milano and Cefriel) and two public administrations (Comune di Milano e Comune di San Donato Milanese). During the PhD activity the plant was designed and projected with the industrial partners, basing on experience, known processes and research activity. Different unit operations are quite well known, while some steps are completely innovative. The research activity deeply described in the present work was focused on the study of innovative processes aiming to the REs recovery from WEEE. From a technological point of view, recycling of end-of-life WEEE and recovering of metals therein contained can be divided into three major steps: disassembly, upgrading and refining. For the last step, different methods have been proposed. In particular, hydrometallurgical method has been reported as one of the most interesting because it is generally applicable to very different compositions and allows the same processing steps of the REs extraction from primary ores. On the other hand, some disadvantages of this process are: the number of process steps, the consumption of large amounts of chemicals and the generation of huge amounts of waste water. Some of the reported disadvantages are related to the step of metal ions removal from aqueous solution. Recently, the adsorption method is obtaining more and more attention because of its advantages with respect to liquid-liquid extraction such as: high recovery efficiency, short extraction time, high enrichment factor, low cost, and low consumption of organic solvents. In particular, the use of clays and activated carbon as solid-phase has some outstanding advantages, such as low cost, good mechanical properties, good acid tolerance, convenient solid-liquid separation and excellent reusability. The present research activity has been focused on the deeply investigation of these solid sorbents, evaluating different solid matrices and different suitable modifications with the final aim of removing the metal ions and of recovering them if they have high added value. The experimental activity was divided in two main parts. The first one concerns the study and the comprehension of the interaction mechanisms. Since the involved systems are indeed very complex (e.g. in a magnet from a hard-disk there are 16 different ions and the amount of REs is very low), the study was carried out on model systems. The second part is mainly focused on the industrial application: different parameters (as the reusability of the solids, as the complexity of real solutions and as preliminary reasoning about the sustainability of the global process) are considered. As a first step, two natural smectite mineral clays (STx and SWy) were studied for lanthanum (chosen as representing element of REs family) adsorption and release in order to: 1) verify the clays sorption capability, 2) investigate the sorption mechanisms and 3) optimize the experimental parameters, such as contact time and pH. Since the global efficiencies of lanthanum recovery were around 30-35%, modified clays were also synthesized by intercalating different polyamines having coordinating properties. N-(methoxy-polyethylene glycol) ethylene diamine showed an improvement in the global efficiency (43%). Its intercalation mechanism was deeply investigated, varying operating parameters such as polymer concentration and pH. Optimal intercalation conditions for lanthanum recovery purpose were: 90 mM of initial polymer concentration and pH of about 11, corresponding to a not-protonated condition of the coordinating amine groups. In order to further increase the global efficiency of the process, different polyamines were tested in these optimal conditions. In particular two linear ethylene amines with different length of chain, a branched ethylene amine and an ethylenediiminodipropylamine were intercalated in the clay STx. In every case the adsorption and release behaviours towards lanthanum were studied. The results obtained with these polyamines pointed out huge increase in the lanthanum recovery global efficiency, achieving values around 85% (99.9% in uptake, 84% in release). Then a different solid matrix was considered: activated carbon was tested as sorbent material. The mechanism mainly involves surface adsorption, allowing lanthanum recovery efficiencies around 50%. Also in this case the modification of the solid with a linear ethylene amine leads to a huge increase in the lanthanum recovery global efficiency, achieving values around 90% (99.9% in uptake, 90% in release). The adsorbed and released metal ions were calculated by ICP-OES while the amounts of intercalated polyamines were estimated by COD analysis of the residual amounts in solution and deduced by difference. All the solid materials were characterized by FT-IR, TG-DTG and XRD. In addition, pristine clays have been also investigated by XPS measurements. The second main part of the present work focused on the final application; the reusability of the evaluated sorbent materials was first studied, proposing a regenerating system able to work for at least four recovery cycles. Then more complex (lanthanum and copper, lanthanum and nickel) and real solutions coming from different leached electronic scraps (mainly hard disk magnet and mobile phones) were tested for adsorption onto the studied materials. The obtained results (80% in uptake and release to be optimized) are the basis for the implementation of the pilot plant by December 2015, where the solid sorbents work as purifying materials with the perspective of recovering. A preliminary analysis about the sustainability of the global WEEE treatment was also carried out showing the great environmental benefits of the proposed process.

Oggigiorno c'è un crescente bisogno di terre rare a causa del loro utilizzo in numerose applicazioni ad elevata tecnologia. Attualmente ogni cittadino dell'UE produce circa 17 kg di apparecchiature elettriche ed elettroniche (RAEE) all'anno: tali rifiuti sono ricchi di metalli preziosi e strategici. Di conseguenza, il riciclo può essere considerato una preziosa opportunità. Questo punto di vista è conosciuto come "urban mining" e per queste ragioni lo studio di un recupero efficiente da RAEE può portare ad innegabili benefici socio-economici ed ambientali. Il presente lavoro di tesi è stato svolto nell'ambito del progetto "E-WASTE - Il ciclo intelligente" (ID 40511448, finanziato dalla Regione Lombardia) e si è incentrato sul recupero di metalli preziosi da RAEE. In particolare uno degli obiettivi finali del progetto E-WASTE è stato la realizzazione di un impianto pilota per il trattamento dei RAEE attraverso la collaborazione di due grandi società (Amsa e Stena), un'impresa di medie dimensioni (Consorzio Remedia), tre piccole/medie imprese (Seval, Tecnochimica, e, Gaser), due organizzazioni di ricerca (Politecnico di Milano e Cefriel) e due amministrazioni pubbliche (Comune di Milano e Comune di San Donato Milanese). Durante l'attività di dottorato l'impianto è stato progettato con i partner industriali. Diverse operazioni unitarie sono ben note, mentre alcuni passaggi sono del tutto innovativi. L'attività di ricerca dettagliatamente descritta nel presente lavoro si è concentrata sullo studio dei processi innovativi finalizzati al recupero di terre rare dai RAEE. Da un punto di vista tecnologico, il riciclo dei RAEE a fine vita ed il recupero dei metalli ivi contenuti possono essere suddivisi in tre fasi principali: lo smontaggio, la separazione e la raffinazione. Per l'ultima fase, sono stati proposti diversi metodi. In particolare, il metodo idrometallurgico è stato segnalato come uno dei più interessanti perché è generalmente applicabile a differenti composizioni e si basa sulle stesse fasi di lavorazione dell'estrazione da minerali primari. D'altra parte, alcuni svantaggi di questo processo sono: l’elevato numero di fasi di processo, il consumo di grandi quantità di prodotti chimici e la generazione di grandi quantità di acque reflue. Alcuni degli svantaggi qui riportati sono legati alla fase di rimozione degli ioni metallici dalla soluzione acquosa. Recentemente, l’adsorbimento si è imposto come una metodologia in crescente sviluppo a causa dei suoi vantaggi nei confronti dell’estrazione liquido-liquido: alta efficienza di recupero, breve tempo di estrazione, alto fattore di arricchimento, basso costo e basso consumo di solventi organici. In particolare, l'utilizzo di argille e carboni attivi come fasi solide presenta alcuni vantaggi eccezionali, come ad esempio basso costo, buone proprietà meccaniche, buona tolleranza agli ambienti acidi, facile separazione solido-liquido ed eccellente riutilizzabilità. Il presente lavoro di ricerca si è focalizzato sulla studio approfondito di questi adsorbenti solidi, valutando diverse matrici solide e diverse modificazioni, con l'obiettivo finale di rimuovere gli ioni metallici e di recuperarli se il loro valore aggiunto è elevato. L'attività sperimentale è stata suddivisa in due parti principali. La prima riguarda lo studio e la comprensione dei meccanismi di interazione. Poiché i sistemi coinvolti sono molto complessi (ad esempio in un magnete proveniente da un hard-disk ci sono 16 diversi ioni e la quantità di terre rare è molto bassa), lo studio è stato primariamente effettuato su sistemi modello. La seconda parte si è concentrata invece sulla applicazione industriale: diversi parametri (come la riusabilità dei solidi, la complessità delle soluzioni reali e la sostenibilità del processo globale) sono stati considerati.