Review of Technologies in the Recovery of Iron, Aluminium, Titanium and Rare Earth Elements from Bauxite Residue (Red Mud)

Bauxite residue (BR), also known as red mud (RM), is the main waste product of the Bayer process in producing alumina from bauxite ores and is currently being managed via stockpiling, storage within settling pond and landfilling. However, these approaches are disadvantageous in terms of potential risk for the environment and the processing of RM as raw material brings about its own challenges. In recent years, research has been focused on the reuse of RM and the recovery of metals to transform the waste into a more viable secondary source of raw materials. This paper will further explore and thoroughly review technologies and processing techniques that have been previously available as well as ones that are currently being developed. The scope of this paper will be focused on iron, aluminium, titanium and rare earths

Απομάκρυνση οξειδίων του σιδήρου απο τα κατάλοιπα της μεταλλουργικής επεξεργασίας του βωξίτη μέσω φρύξης σε καμινό μικροκυμάτων και μαγνητικό διαχωρισμό

Bauxite residue (BR), also known as red mud, is the reddish solid waste generated during the production of alumina from bauxite ores. In particular, the production of 1 metric ton of alumina creates between 0.9 and 1.5 metric tons of residue depending on the composition of initial bauxite ore.Due to its high alkalinity, with a pH range between 10 and 13, the handling of this waste is a challenge for the alumina industry. In addition, the massive volume of BR produced, which exceeds 150 million metric tons per year worldwide, is causing drastic scarcity of available storage areas. However, its chemical composition encompassing iron, alumina and titania oxides turn BR into an interesting resource for base and critical metal recovery, while a large-scale and bulk utilisation is rather conceivable using BR in construction materials, such as aggregates, cements, ceramics or inorganic polymers. In the last decades, the research was concentrated to find environmentally friendly and cost-effective methods to dispose of or utilize the bauxite residue.In this framework, the aim of this PhD was to recover iron from BR, developing two different reductive roasting processes followed by magnetic separation. The first part of the PhD was focused on the study of optimal process for reduction of the nonmagnetic iron phases found in BR (namely hematite and goethite), to magnetic ones such as magnetite, wüstite, and metallic iron. BR was mixed with a carbon source (metallurgical coke) as a reducing agent and roasted in a resistance-heated tube furnace. The magnetic iron phases in the roasting residue were fractionated in a second stage through wet magnetic separation, forming a valuable iron concentrate and leaving a nonmagnetic residue containing rare earth elements among other constituents. The BR-roasting process has been modeled using a thermochemical software (FactSage 6.4) to define process temperature, Carbon/Bauxite Residue mass ratio (C/BR), retention time, and process atmosphere. Roasting process experiments with different ratios of C/BR (0.112 and 0.225) and temperatures (800 and 1100 °C), 4 h retention time, and, in the presence of N2 atmosphere, have proven almost the total conversion of hematite to iron magnetic phases (99 wt %). Subsequently, the magnetic separation process has been examined by means of a wet high-intensity magnetic separator, and the analyses have shown a marginal Fe enrichment in magnetic fraction in relation to the sinter.In the second part of the work a microwave-assisted heating was presented as a suitable method to transform hematite and goethite contained in bauxite residue into magnetite, wüstite and metallic iron, with a short processing time. The final target was the production of a sinter with strong magnetic properties, allowing the magnetic separation of Fe from the residue. The influence of microwave energy on the sample, the effect of irradiation time together with the carbon/bauxite residue mass ratio (C/BR) were the parameters that have been analyzed to optimize the process. Their optimized combination allowed to transform the 79 % of the iron present in the sinter into metallic iron. However, hercynite (FeAl2O4) was also formed and the presence of this mineralogical phase could be considered a possible drawback for its magnetic properties, since its presence in the magnetic fraction could reduce its purity. To avoid the formation of FeAl2O4, a combined soda sintering, and microwave reductive roasting process of bauxite residue was presented. In the first step, all the alumina phases in BR were transformed into sodium aluminates and leached out through alkali-leaching to recover alumina by adding sodium carbonate as flux to BR. Subsequently, the leaching residue have been mixed with carbon and roasted by using a microwave furnace at the optimum conditions. The iron oxide present in the sinter were converted into metallic iron (98 %). In addition, hercynite is not detected. The produced sinter is subjected to magnetic separation processes by using a wet high intensity magnetic separation to separate iron from the other elements.Tα Κατάλοιπα Βωξίτη (ΚΒ), γνωστά και ως ερυθρά ιλυς, είναι το στερεό παραπροϊόν που παράγεται κατά την εξαγωγή αλουμινας από βωξίτες. Συγκεκριμένα η παραγωγή ενός τόνου αλουμίνας παράγει από 0.9 εως 1.5 τόνους καταλοίπων βωξίτη, ανάλογα με τη χημική σύσταση του κατεργαζόμενου βωξίτη.Εξαιτίας της μεγάλης αλκαλικότητας, με ένα pH ανάμεσα στο 10 και 13, η διαχείριση αυτού του υλικού είναι προβληματική για τη βιομηχανία αλουμίνας. Επιπλέον, η μεγάλη ποσότητα ΚΒ που παράγεται κάθε χρόνο και η οποία ξεπερνά τους 150 εκ. τόνους παγκοσμίως, δημιουργεί ελλείψεις σε χώρος απόθεσης. Όμως η χημική σύσταση των ΚΒ, που περιέχουν οξείδια σιδήρου, αλουμινίου και τιτανίου, τα καθιστά αξιόλογους πόρους για την ανάκτηση βασικών και κρίσιμων μετάλλων ενώ παράλληλα μπορούν να χρησιμοποιηθούν και σε εφαρμογές δομικών υλικών όπως αδρανή, τσιμέντο, κεραμικά και ανόργανα πολυμερή. Τις τελευταίες δεκαετίες, η έρευνα έχει επικεντρωθεί στην εύρεση οικολογικών και ανταποδοτικών μεθόδων αξιοποίησης των ΚΒ.Σε αυτό το πλαίσιο ο σκοπός της παρούσης διδακτορικής διατριβής, είναι η ανάκτηση σιδήρου από τα ΚΒ μέσα από την ανάπτυξη δυο διαφορετικών διεργασιών αναγωγικής φρύξης των ΚΒ και μαγνητικού διαχωρισμού των παραγόμενων φρυγμάτων.Το πρώτο μέρος του PhD επικεντρώθηκε στη μελέτη της βέλτιστης διαδικασίας για τη αναγωγή των μη μαγνητικών φάσεων σιδήρου που συναντιόνται στα ΚΒ (συγκεκριμένα αιματίτης και γκαιτίτης), σε μαγνητικές φάσης όπως μαγνητίτης, βουστίτης και μεταλλικός σίδηρος. Τα ΚΒ αναμίχθηκαν με πηγή άνθρακα (μεταλλουργικό ΚΩΚ) ως αναγωγικό μέσο και φρυχθήκαν σε φούρνο ηλεκτρικής αντίστασης. Οι μαγνητικές φάσεις σιδήρου στο φρύγμα, διαχωρίστηκαν σε ένα δεύτερο στάδιο μέσω υγρού μαγνητικού διαχωρισμού, σχηματίζοντας ένα πολύτιμο συμπύκνωμα σιδήρου και ένα μη μαγνητικό κλάσμα που περιέχει στοιχεία σπάνιων γαιών μεταξύ άλλων συστατικών. Η διαδικασία φρύξης των ΚΒ έχει μοντελοποιηθεί χρησιμοποιώντας ένα θερμοχημικό λογισμικό πακέτο (FactSage 6.4) για τον καθορισμό της θερμοκρασίας της διαδικασίας, της αναλογίας μάζας άνθρακα / Καταλοίπων Βωξίτη (C/BR), του χρόνου παραμονής και την ατμόσφαιρα της διαδικασίας. Πειράματα φρύξης με διαφορετικές αναλογίες C/BR (0.112 και 0.225) και θερμοκρασίες (800 και 1100 °C), χρόνο παραμονής 4 ωρών και παρουσία ατμόσφαιρας Ν2, έχουν αποδείξει τη σχεδόν συνολική μετατροπή του σιδήρου σε μαγνητικές φάεις (99 % κ.β.). Στη συνέχεια, η διαδικασία μαγνητικού διαχωρισμού σε υγρό μαγνητικό διαχωριστή υψηλής έντασης κατέδειξε ένα οριακό εμπλουτισμό του σιδήρου στο μαγνητικό κλάσμα σε σχέση με το φρύγμα.Στο δεύτερο μέρος της εργασίας εξετάστηκε η χρήση φούρνου μικροκυμάτων ως κατάλληλη μέθοδος για τη μετατροπή του αιματίτη και του γκαίτη που περιέχεται σε κατάλοιπα βωξίτη σε μαγνητίτη, σίδηρο και μεταλλικό σίδηρο, με σύντομο χρόνο επεξεργασίας. Ο τελικός στόχος ήταν η παραγωγή φρυαγμάτων με ισχυρές μαγνητικές ιδιότητες, επιτρέποντας τον μαγνητικό διαχωρισμό του σιδήρου από το υπόλειμμα. Η επίδραση της ενέργειας μικροκυμάτων στο δείγμα, η επίδραση του χρόνου ακτινοβολίας μαζί με την αναλογία μάζας άνθρακα / ΚΒ (C/BR) ήταν οι παράμετροι που εξετάστηκαν για τη βελτιστοποίηση της διαδικασίας. Ο βελτιστοποιημένος συνδυασμός τους επέτρεψε να μετατραπεί το 79 % του σιδήρου που υπάρχει στα ΚΒ σε μεταλλικό σίδερο. Ωστόσο, σχηματίστηκε επίσης η μαγνητική φάση του ερκινίτη (FeAl2O4), η παρουσία της οποίας θα μπορούσε να θεωρηθεί μειονέκτημα καθώς θα μειώσει τη καθαρότητα του μαγνητικού κλάσματος.Για να αποφευχθεί ο σχηματισμός FeAl2O4, παρουσιάστηκε μια συνδυασμένη μέθοδος σύντηξης των ΚΒ με σόδας και αναγωγής σε φούρνο μικροκυμάτων. Στο πρώτο βήμα, όλες οι φάσεις αλουμίνας στα ΚΒ μετασχηματίστηκαν σε αργιλικά άλατα νατρίου και εκχυλίστηκαν μέσω αλκαλικής έκπλυσης. Στη συνέχεια, το υπόλειμμα της εκχύλισης αναμίχθηκε με άνθρακα και φρίχθηκε σε φούρνο μικροκυμάτων στις βέλτιστες συνθήκες. Τα οξείδιο του σιδήρου μετατράπηκαν σε μεταλλικό σίδηρο (98 %) στο φρύγμα. Επιπλέον, η φάση του ερκινίτη δεν εντοπίζεται πλέον. Το παραγόμενο φρυγμα υποβλήθηκε σε διεργασίες υγρου μαγνητικού διαχωρισμού

Optimization of Microwave Reductive Roasting Process of Bauxite Residue

In this study, microwave-assisted heating is presented as a suitable method to transform the hematite and goethite contained in bauxite residue into magnetite, wüstite, and metallic iron, with a short processing time. The final target was the production of a sinter with strong magnetic properties, allowing the magnetic separation of Fe from the residue. The influence of microwave energy on the sample, the effect of irradiation time, and the carbon/bauxite residue mass ratio (C/BR) were the parameters that have been analyzed to optimize the process. Their optimized combination allowed transforming 79% of the iron present in the sinter into metallic iron. However, hercynite was also formed, and the presence of this mineralogical phase could be considered a possible drawback for its magnetic properties

La costruzione collegiale di prove di verifica. Come valutare prodotti e processi nella formazione dei futuri insegnanti

Lo studio contribuisce alla riflessione sulla formazione iniziale degli insegnanti per poter predisporre interventi che tengano effettivamente conto delle loro esigenze e competenze professionali. Vengono considerate le attività svolte nell’ambito dei corsi TFA (Tirocinio Formativo Attivo) realizzati nell’Università Sapienza di Roma nell’anno 2012-13. In particolare la partecipazione al modulo “Verifiche, misurazioni e valutazioni”, in cui è stato richiesto ai corsisti TFA di progettare e realizzare, delle prove di verifica relative alla propria classe di concorso. I corsisti sono stati suddivisi in gruppi di lavoro e hanno utilizzato una Piattaforma Web 2.0 dedicata. Sono stati analizzati i seguenti elementi: grado di cooperazione e collegialità con cui i tirocinanti hanno progettato e realizzato le prove di verifica; la coerenza tra quanto dichiarato nella prova d’esame finale e le prove realizzate nei gruppi di lavoro. Il campione è costituito da 231 corsisti, di cui 100 (43,3%) di area scientifica e 131 (56,7%) di area umanistica. Sono stati formati in tutto 58 gruppi di lavoro. Circa l’80% dei gruppi ha costruito prove di tipo misto, con quesiti a risposta chiusa e di tipo criteriale, secondo quanto previsto nelle lezioni del modulo. La coerenza tra le prove costruite dai gruppi e le dichiarazioni individuali durante l’esame finale è stata valutata da una giuria di esperti circa i seguenti aspetti: presenza obiettivi, definizione strumenti e definizione scale di punteggio. La maggioranza dei corsisti ha riportato un giudizio positivo in tutti e tre gli aspetti, soprattutto per la definizione degli strumenti (81%). Non risulta determinante l’aver ricoperto il ruolo di coordinatore del gruppo. I corsisti dell’area umanistica risultano migliori per obiettivi e definizione scale di punteggio; quelli di area scientifica leggermente migliori per gli strumenti.The study contributes to reflection on initial teacher training in order to prepare interventions that actually take into account their needs and professional skills. The activities in the TFA (Tirocinio Formativo Attivo; Active Training Internship) courses carried out at the Sapienza University of Rome in 2012-13 are considered. Participation in the "Verification, Measurement and Evaluation" module is analyzed, TFA trainees have been required to design and carry out verification tests related to their teaching class. The participants were divided into working groups and have used a collaborative Web 2.0 platform. The following elements were analyzed: Degree of co-operation and collegiality with which trainees have designed and carried out verification tests; the consistency between what the trainees stated in the final exam and the tests carried out by the working groups. The sample consists of 231 coursers, of which 100 (43.3%) of the scientific area and 131 (56.7%) of humanities. 58 working groups were formed. About 80% of the groups have built-up mixed-type tests, with closed-ended and open-ended questions, as provided in the module lessons. Consistency between tests and individual statements during the final exam was evaluated by a jury of experts about the following aspects: defining goals, instruments definition, scoring scales. Most of the students got a positive evaluation in all three aspects, especially for instruments definition (81%). It does not seem crucial to have been the coordinator of the group. The students of the humanities are better for goals and scoring scales; Those of the scientific area slightly better for the instruments

I bisogni espressi dai docenti in formazione

Analisi dei bisogni formativi espressi attraverso la partecipazione a forum on- line (piattaforma moodle) dai docenti-allievi in formazione che hanno frequentato il TFA (tirocinio Formativo Attivo) organizzato dalla Sapienza nell'a.a. 2012-1

A Combined Soda Sintering and Microwave Reductive Roasting Process of Bauxite Residue for Iron Recovery

In this study an integrated process is presented as a suitable method to transform Fe3+ oxides present in bauxite residue into magnetic oxides and metallic iron through a microwave roasting reduction, avoiding the formation of hercynite (FeAl2O4). In the first step, all the alumina phases were transformed into sodium aluminates by adding sodium carbonate as a flux to BR and then leached out through alkali-leaching to recover alumina. Subsequently, the leaching residue was mixed with carbon and roasted by using a microwave furnace at the optimum conditions. The iron oxide present in the sinter was converted into metallic iron (98%). In addition, hercynite was not detected. The produced cinder was subjected to a wet high intensity magnetic separation process to separate iron from the other elements