Mechanisms, kinetics and pathways in the early-stage crystallisation of Ca-REE-F-bearing carbonates under ambient and hydrothermal conditions

Abstract

The aim of this study is to investigate the reaction pathways, mechanisms, kinetics, and energetics of the REE-CaF2-CaCO3 system crystallization reactions. It also seeks to understand the role of different REEs in the process. Chapter 1 provides the context and outlines the knowledge gaps addressed by this thesis. The findings acquired are presented in Chapters 2, 3 and 4: Chapter 2 investigates the effect of different REEs (La, Nd, Dy), used both individually and in combination, on CaCO3 crystallisation from solution at ambient temperature. The process was studied in situ and in real time. The resultant CaCO3 morphologies as well as the role of the peculiar ionic potentials of the REEs used in affecting its kinetics and mechanisms of crystallisation were discussed. In Chapter 3 the main aims were to study the kinetics, mechanisms and energetics of (La)-, (Ce)- and (Nd)-fluocerite crystallisation and to demonstrate experimentally that it can play the role of a precursor of bastnäsite, one of the most important minerals for REEs extraction worldwide. The REE-fluocerites were obtained by reacting fluorite powder (CaF2) with an REE-bearing solution at temperatures ranging from 30 to 90 ºC. They were then reacted with a CO3-rich solution at low hydrothermal temperatures (50-200 ºC). This study arose from the fact that fluocerite is commonly found in REE deposits associated with bastnäsite. Although its role in bastnäsite genesis was previously hypothesized, it had never been demonstrated. 9 Chapter 4 studies the early-stage crystallization and nanophase formation of La, Ce, Pr, and Nd carbonates. These were synthesized through homogeneous crystallization at temperatures ranging from 5 to 80 °C. The study investigated how different parameters, such as temperature, solution concentration, phase stability, and REE ionic potentials, affect the multi-step crystallisation pathways of REE carbonates and nanophase formation. The crystallization processes were followed in situ and in real time using UV-Vis spectroscopy and synchrotron-based wide-angle X-ray scattering (WAXS). Chapter 5 outlines the findings from Chapters 2-4 and discusses their potential applications. It also discusses potential future work arising from specific findings or experiments in this PhD thesis.