Extraction, purification et séparation du tantale et du niobium contenus dans le coltan du Kivu (RDC)

Current industrial processes of the purification and separation of tantalum (Ta) and niobium (Nb) bearing columbite-tantalite (coltan) ores rely exclusively on hydrofluoric acid leaching and solvent extraction using MIBK. However, due to the harmful effect of HF, more environmentally friendly processes deserve to be developed. This thesis aims to develop a technological concept for purification and separation of Ta and Nb from the South Kivu coltan ore without using hydrofluoric acid at any stage of the process.
Two primary coltan ores samples from South Kivu (DRC) were collected in two artisanal mining zones (ZEA), namely Numbi (Kalehe territory) and Lulingu (Shabunda territory). Chemical and mineralogical characterizations being done revealed distinguished differences in both samples, in terms of the target metals contents, ratios and the carrier phases.
Two processes were investigated, based namely on alkaline process and fluorination approach coupled with sulfuric acid leaching. The metallurgical performance during the alkaline treatment differed somehow for both samples but were all ultimately acceptable in terms of Nb and Ta recoveries. The kinetic and thermodynamic aspects of alkali roasting of the Numbi coltan ore and the optimization of alkali roasting parameters for the Lulingu sample were studied. The fluorination process demonstrated similar performance to that of the hydrofluoric acid leaching process. Furthermore, the developed method witness a reduced the consumption of the fluorinating agent and more important less harmful effects of the generated gases. The resulting leachate has low acidity, which provides an opportunity to separate Ta and Nb using selective extraction.
A novel route for treatment of the alkaline leachates was investigated to separate Ta and Nb. This involved almost complete precipitation of both metals by calcium hydroxide, followed by oxalic acid leaching of the precipitate. The Ta and Nb purification using Aliquat® 336 based extractant were successfully carried out but attempts to separate these metals during stripping stage have not led to satisfactory results.
The purification and separation of Ta and Nb from fluoride leachates was carried out using OCL, MIBK, Aliquat® 336. The Aliquat® 336 extractant proved to be advantageous over the rest ones, in terms of technical and environmental point of view. This part of the thesis demonstrated the possibility of Ta and Nb separation using Aliquat® 336 to produce high-purity Ta and Nb oxides in powder form. XRF, XRD and SEM-EDS inspection of the obtained powders shed light on their intrinsic characteristics in light of their utilisation.