Rare earth element recovery from monazite concentrate : process simulation, thermodynamic evaluation and pourbaix diagram characterization / Fatin Nasuha Binti Yahya

Faculty of Chemical and Process Engineering Technology

Thesis (Master of Science) -- Universiti Malaysia Pahang – 2024

Includes bibliographical references

Monazite concentrate consists mainly of light rare earth elements, which are lanthanum (La), neodymium (Nd), and cerium (Ce). Chemical beneficiation is a part of the separation method in the extraction of rare earth elements (REEs) from monazite concentrate. Chemical beneficiation can be modelled as a two-step process consisting of digestion and leaching. These processes involve the reactions between monazite concentrate and acid solutions. The limitation of producing high efficiency of leachate is due to a lack of understanding of the dissolution and precipitation of REEs from rare earth (RE) minerals. The first objective of this work is to study the effect of different temperatures (25–300°C) on the reactions of monazite concentrate, REPO4 (RE: La, Nd, and Ce) with sulfuric acid (H2SO4) by constructing Pourbaix diagrams using HSC Chemistry. The second objective is to study the effect of temperature (200–700°C) on chemical beneficiation by constructing Pourbaix diagrams based on experimental work by other researchers. The third objective is to simulate the beneficiation of monazite concentrate REPO4 (RE: La and Nd) reactions employing METSIM software and study the effect of different liquid-to-solid (L/S) ratios (0.5–8), acid types (H2SO4 and hydrochloric acid (HCl)), and mass flow rates of acids. A Pourbaix diagram can map the aqueous system's stability area, predict the potentials at which a given species will be oxidized or reduced, and identify the conditions under which different species will be in equilibrium. This will be useful in determining process conditions, such as those that favour dissolution and potential RE metals that may be present in the system. The results reveal that as the temperature increases, the stability area of complex compounds in the Pourbaix diagrams increases as well. The order of stability at elevated temperatures was determined by comparing the three REEs in this study: Ce > Nd > La. The influence of temperature on REE digestion reveals that the effectiveness of monazite ore digestion in the La, Na, and Ce Pourbaix diagram systems ranges from 200 to 700°C, from 300 to 500°C, and from 200 to 300°C, respectively. The results suggest that the best L/S ratio is 8:1 to recover the greatest quantity of REEs, with HCl outperforming H2SO4 as a solvent for REE recovery. The addition of sulfate ions from the leaching solvent (H2SO4) introduces soluble metal-sulfate complexes. The simulation findings reveal a good agreement on the results of REE percentage recovery.