On 23rd November, 2018, Wenzhong Zhang successfully defended his doctoral dissertation in chemistry at the University of Helsinki. The title of the dissertation is “Metal(IV) Phosphate Based Functional Materials For Selectively Harvesting Rare-Earth Elements From Bauxite Residue”. Professor Freddy Kleitz, University of Vienna, served as the opponent, and Professor Gareth Law as the custos.
The press release from the University of Helsinki can be found here.
The research was supervised by Dr. Risto Koivula and the late Professor Risto Harjula. Wenzhong Zhang will obtain his Ph.D. degree in December 2018, which is completed in the framework of the European Training Network for Zero-Waste Valorisation of Bauxite Residue (REDMUD). The dissertation (summary part) is published also in electronic format (ISBN:978-951-51-4651-9). Wenzhong will now continue his research work as a post-doctoral researcher in the ion-exchange group of Dr. Risto Koivula, where he will work on the separation of Group 3 and f-elements by hybrid materials.
Inorganic metal(IV) phosphates with adjustable structural features, strong acid resistance and selective ion-exchange capabilities are an ideal class of materials to be applied in solid-liquid metal separation processes. The surface hydroxyl groups with ion-exchange capability also grant the metal(IV) phosphates with vast possibilities for further functionalisation and hybridisation.
Rare-earth elements (REEs), made up by scandium (Sc), yttrium and the entire lanthanide series, are irreplaceable enablers for the transition to a low-carbon economy. Pure fractions of REEs are, until today, vital components in high-tech applications such as electric motors. The ever-growing demands for REEs are restricted by limited mining productions and regulated cross-border trading.
Bauxite residue (BR), the waste generated from the industrial production of alumina, are stockpiled in enormous quantity across the globe. Certain types of BR are considered as exploitable REE reservoirs since they contain minable concentrations of REEs. To harvest the REEs contents from BR, the metals are typically leached by mineral acids before further separations. However, the recovery of REEs from the acidic leachates is challenging due to low concentrations of REEs and high concentrations of other interfering metals.
The dissertation summarised the design and application of various functional materials based on metal(IV) phosphate for separation processes relating to the recovery of REEs from BR leachates. Traditional inorganic titanium and zirconium phosphate materials with amorphous and layered crystalline structures were firstly tested for selective Sc separation. Although these materials preferentially exchange Sc3+ compared to all other investigated metal ions, the internal interlayer surfaces were unavailable to Sc3+ in the case of the crystalline materials. To fully utilise their ion-exchange sites, titanium phosphate moieties were functionalised onto the surfaces of mesoporous MCM-41 silica. Inspired by the structure of solvating extractant tri-n-butylphosphate (TBP), short n-alkyl chains were grafted onto the titanium phosphate grafts to mimic TBP structure. Batch tests confirmed the solvating extraction capability of the obtained solid material, with excellent performance for separating Sc3+ and lanthanide ions. Hybrid titanium di-n-butylphosphate coordination polymers were prepared and they exhibited exceptional intralanthanide separation capability. The selective uptake of smaller lanthanide ions was resulted from a transmetalation process, where the lanthanide ions substituted the framework titanium-oxo clusters. An almost quantitative separation between neodymium and dysprosium was demonstrated by simply controlling the solution pH in a batch system.
A multitude of characterisation methods were utilised to study and confirm the compositional and structural properties of the newly synthesised materials and their metal separation mechanisms. Functional materials assembled on the metal(IV) phosphate platform offer versatile functionalities and alterable metal uptake mechanisms that are suitable for hydrometallurgical separation of the REEs.
Publications by Wenzhong Zhang in the framework of his PhD:
- Zhang , R. Koivula, E. Wiikinkoski, J. Xu, S. Hietala , J. Lehto, R. Harjula, Efficient and Selective Recovery of Trace Scandium by Inorganic Titanium Phosphate Ion-Exchangers from Leachates of Waste Bauxite Residue, ACS Sustainable Chemistry & Engineering journal, 5, (4), 2017, 3103-3114, DOI: 10.1021/acssuschemeng.6b02870
- Zhang, D. Avdibegović, R. Koivula, T. Hatanpää, S. Hietala, M. Regadío, K. Binnemans, R. Harjula, Titanium alkylphosphate functionalised mesoporous silica for enhanced uptake of rare-earth ions, Journal of Materials Chemistry A, 2017, 10.1039/c7ta08127h
- Xu, R. Koivula, W. Zhang, E. Wiikinkoski, S. Hietala, R. Harjula, Separation of cobalt, neodymium and dysprosium using amorphous zirconium phosphate, 2017, DOI 10.1016/j.hydromet.2017.11.010
- Yagmurlu, W. Zhang, R. Koivula, C. Dittrich, B. Friedrich, Solid State Conversion of Scandium Phosphate to Oxide by Sodium Compounds, Proceedings of BR2018 Conference, 7-10 May 2018, Athens, Greece
- Yagmurlu, W. Zhang, D. Avdibegovic, M. Regadio, R. Koivula, C. Dittrich, K. Binnemans, B. Friedrich, Advances on Scandium recovery beyond state of the art, Proceedings of ALTA Conference 2018, Perth (Australia), 21-26 May 2018