Mechanism And Process Control Technology On The Separation And Purification Of Rare Earths, Thorium And Fluorine Without Saponification

Bastnaesite is the main type of rare earth mining in the world. Output from bastnaesite typically contains0.2-0.3wt%thorium and8-10wt%fluorine in addition to the rare earth. During the common process of bastnasite treatment, however, some environmental issues still remains. One is the discharge of fluorine and radioactive thorium via waste liquid or waste solid. The loss of fluorine from the bastnaesite damages the economy of the process and, more seriously, the environment. Hence, discovering a method to efficiently recover cerium, thorium, and fluorine from bastnaesite is a critical issue in rare earths metallurgy. A new eco-friendly process for the separation and the recovery of thorium and fluorine from bastnaesite treatment was developed and put into industrial application. Solvent extraction of Ce(IV), Th(IV) and F(I) using HEH(EHP)(P507) from a sulfuric acid medium is studied by investigating extraction dependence, with the extraction mechanism determined using slope analysis. The elements distribution were studied during the coordination scrubbing of F(I), the reductively stripping of Ce(IV) and Th(IV) stripping. The new, clean process consists of oxidation roasting, leaching with sulfuric acid, and then recovering the rare earths, fluorine, and thorium using HEH(EHP), and prevents pollution caused by the dumping of thorium and fluorine as hazardous industrial wastes. After oxidation, virtually all RE(III), Ce(IV), Th(IV), and F(I) can be leached out with sulfuric acid. The F(I) coordinates with Ce(IV) and Th(IV) to form [CeFx]4-x and [ThFx]4-x complexes respectively, which can be easily extracted into an organic phase with HEH(EHP) and separated from RE(III) for the negligible extraction of RE(III). The F(I) loaded into the organic phase was scrubbed using Al(III) as the coordination reagent, and recovered as cryolite. After the coordination scrubbing of F(I), the Ce(IV) was reductively stripped using H2O2and HCl while the Th(IV) remained in the organic phase, then recovered as CeO2with a purity of99.95%. In the final step, the Th(IV) was stripped with H2SO4and recovered as ThO2with a purity of99.5%.Normally, the separation factors for different rare earth ions show minimal differences owing to their similar valence and ionic radii. However, this similarity in chemical properties generally makes it difficult to separate and purify individual metals from lanthanides, and the separation of rare earths is perhaps the most difficult task in analytical chemistry. Solvent extraction processes have been well established and have been widely used in industry, and in these processes organophosphorus acids are commonly used as the extractants. The extraction mechanism of rare earths by the organophosphorus acids can be expressed as ions exchange process with hydrogen ion. Thus, conventional acidic extractants, such as HEH(EHP) used widely in rare earth hydrometallurgy, always need to be saponified by NH3H2O, NaOH, or Ca(OH)2to facilitate the cation exchange of rare earths. However, this method undoubtedly results in waste water containing NH4+, Ca2+and Na+, which causes discharge of ammonium nitrogen pollutants and increasing salt concentration. A cleaner extraction process is always more desirable to reduce environmental pollution generated by industry, and for rare earth production it is necessary to look for an appropriate non-saponification separation method to replace the existing system. A new environmentally friendly process for rare earth separation was developed and put into industrial application. A novel ideal for rare earth extraction and separation in hydrochloric acid medium with the mixture of HEH(EHP) and Cyanex272which does not need to be saponified with ammonium was put forward. Extraction mechanism and the behavior of RE(Ⅲ) in the systems were studied. A technique for controlling equilibrium acidity, RE concentration gradient and synergistic extraction achieves ammonia-free emissions during rare earth separation while obtaining an organic phase with high rare earth loading. A pilot test separating Gd and Tb in a HEH(EHP)-HCl system is conducted using the proposed equilibrium acidity control technology, and without saponification, to verify the process and show that the method obtains ammonia-free emissions using an industrial separation process.