SEPARATION OF TANTALUM AND NIOBIUM FROM NIGERIAN TANTALITE USING AQUEOUS BIPHASE SYSTEM

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Department of Engineering

ABSTRACT
The study was carried out to determine the separation of tantalum (Ta), niobium (Nb) and other metal ions using aqueous biphase system (ABS). The metal ions were produced from alkali fusion and dissolution of local tantalite ores procured from Plateau, Nasarawa and Niger states. The tantalite ore samples were crushed, ground and ball-milled for six hours in a laboratory ball mill and sieved through a 200 sieve mesh (74 μm). The elemental/oxide composition was determined using energy dispersive X-ray florescence (EDXRF) and Xray florescence (XRF). From the elemental analysis results, the Ta content was highest, about 29 %, for the ore from Plateau state; as such all experiments were done using this ore. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also carried out on the tantalite ore and its residues after leaching with mineral acids. A number of leaching processes were carried out using mineral acids (H2SO4 and HF) varying the molar concentration of the HF used. Fifty grams of the -74 μm ball-milled sample was leached with 150 g (≈ 81.6 ml) of 10.8M H2SO4 acid at an acid/ore wt. ratio of 3:1 and also with 81.6 ml a mixture of 15M HF and 2M HF acid (1:1 ratio) in order to improve on the dissolution of Ta and Nb. The residues and leached liquors were analyzed by XRF. Results showed that the mineral acid leaching was not effective and therefore alkali fusion method was employed. Two g of the ground tantalite ore was weighed and mixed with 10 g of potassium hydroxide (KOH), which was earlier crushed in a ceramic hand mortar and pestle. Another 2 g of tantalite ore was weighed and mixed with 10 g of crushed sodium hydroxide. The fusion experiments were performed in an electric furnace using ceramic crucibles at 400oC for one hour. After the reaction, the fused mass was cooled and subsequently leached with de-ionized water at room temperature. The resulting leached solution known as the leached liquor and the residues were analyzed for Ta and Nb by XRF and inductively coupled plasma – atomic emission spectroscopy (ICP – AES). The leached liquor was the stock solution used in the aqueous biphase separation experiments. Polyethylene glycol (PEG) of various molecular weights (PEG3350, PEG4000, PEG6000, PEG8000, PEG7000-9000, PEG10,000, PEG12,000 and PEG20,000) and three salts- Na2SO4, (NH4)2SO4 and K2CO3 were used to form the aqueous biphase systems (ABS) at PEG to salt ratio of 2:1. The ABS formed was subsequently mixed with 1ml each of the leached liquor and shaken vigorously to facilitate partitioning between the two phases formed. Results showed that Ta and Nb were extracted and separated using ABS, the calculated distribution coefficients (D) were from 0.1 to 1.0 for Ta and 0.1 to 1.1 for Nb. The separation factors for Ta and Nb being 1.83, 3.01, 2.41, 1.27, 1.94 and 0.93 for the various PEG molecular weights used. For a second set of extractions, results further showed that Ta and Nb were extracted in ABS with D from 0.1 to 0.5 for Ta and 1.0 to 1.2 for Nb with separation factors of 3.43, 4.15, 8.0, 10.7, 3.55, 2.62 and 2.87 for the PEGs used. Tantalum and niobium were effectively separated using ABS. The separation factor was highest, 10.7 for PEG of average molecular weight 7000-9000. Chapter one focuses on the general background of the study, it also examines the research problem statement, research aim and objectives, justification and the significance of the study. The scope of the study was also highlighted. Chapter two looks at separation processes and their roles. It further looks at the history of tantalum and niobium, their properties, markets for the metals and their uses; sources of ore and mining methods. The demand and supply trends and tantalum prices were examined. The chapter also looks at solvent extraction and separations of polyethylene glycols; aqueous biphase systems were examined and methods of separating tantalum and niobium were examined. Chapter three focuses on the materials and methods, showing general outline of the procedure with reference to preparation of feed materials and protocol for digesting samples, preparation of aqueous biphase systems and actual separation of tantalum and niobium in aqueous biphase systems. Chapter four presents the results of the analyses of the feed materials, the leaching process and alkali fusion; the ABS formation and actual separation of tantalum and niobium in ABS. chapter five is concerned with the general discussion of the various results obtained in the study. Chapter six concludes the investigation by bringing out numbered statements of facts proven in the work and also made some recommendations on ways to further improve the work.