Application of Pourbaix Diagrams in the Hydrometallurgical Processing of Bastnasite

Open Access
Author:
Al-nafai, Isehaq
Graduate Program:
Materials Science and Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
March 24, 2015
Committee Members:
  • Kwadwo Asare Osseo Asare, Thesis Advisor
Keywords:
  • Bastnasite
  • Hydrometallurgical Processing
  • Pourbaix Diagrams
  • Eh-pH diagrams
  • Thermodynamic data estimation
  • Rare Earths
Abstract:
The hydrometallurgical processing of bastnasite was studied by constructing Pourbaix (potential vs. pH) diagrams at room temperature using HSC Chemistry 5.0 software. Different systems were considered to understand the overall behaviors of bastnasite and its species in aqueous systems. Most of the thermodynamic data were taken from HSC database, others were collected from literature and some were estimated. The standard Gibbs free energy of formation of bastnasite, REFCO3, were estimated using four different estimation methods. Each method shows its applicability, and the obtained average values were -379.9 kcal/mol for CeFCO3, -382.3 kcal/mol for LaFCO3, -379.8 kcal/mol for NdFCO3 and -381.3 kcal/mol for PrFCO3. RE-F-CO3-H2O systems show the stability regions of bastnasite which are located in nearly neutral to alkaline media (pH ~ 6.5-11). The RE-F-CO3-(SO4)-(Cl)-(NO3)-H2O systems were considered to display the decomposition behaviors of bastnasite when treated by concentrated acid solutions. Furthermore, the decomposition behavior of bastnasite by alkaline solutions can be explained by these systems. In treatment with sulfuric acid, CeFCO3 decomposes at pH ~ 6.2 when {SO4-2} = 1 m while other REFCO3 decomposes at pH ~ 8. On the other hand, hydrochloric and nitric acids decompose bastnasite at pH ~ 2. The alkaline decomposition of REFCO3 is feasible at pH ~ 11. In the investigation of recovery and recycling of rare earths by precipitation, the systems RE-C2O4-H2O were considered which show the large stability regions of RE oxalate hydrates over the pH range (-2 – 11). All these trends, which revealed by these systems, were related to the hydrometallurgical processing of bastnasite.