| Summary: | The aim of this project was to investigate the possible dissolution, separation and
purification of tantalum and niobium in synthetic ((Ta/Nb)F5 and (Ta/Nb)2O5) and natural
(tantalite minerals) compounds using eco-friendly reagents which are easy to handle as
well as having benefits such as economical and scalable properties.
The possible separation of Ta or Nb was investigated using the pentafluoride
compounds due to their solubility in a wide variety of solvents and their possible
formation from the NH4Fâ¢HF flux fusion. Selective precipitation of the two metals was
initially investigated with different chelating or precipitating agents and good separations
were obtained using p-phenylenediamine as reagent. The experimental parameters
which were investigated include the influence of concentration and the pH effects on
metal recovery. Niobium recoveries in excess of 80% with only 4% tantalum were
obtained in the precipitate with the separation factor of 100(9) using a 50% mixture of
NbF5 and TaF5. The quantitative determination of Nb and Ta content in the supernatant
solution using ICP-OES indicated that more than 95% of the initial tantalum complex
remained in solution.
The presence of the pentoxides in the tantalite minerals prompted the investigation into
the separation of the metals in a synthetic mixture. It involved the dissolution and
subsequent separation of Nb and Ta using a variety of fluxing agents and solvent
extraction, selective precipitation and ion exchange techniques. The dissolution of a
50% of Nb2O5 and Ta2O5 was investigated with Na2HPO4/NaH2PO4â¢H2O mixture, KF,
NaF and NH4Fâ¢HF as fluxes in a flux fusion digestion technique. NH4Fâ¢HF was found to
be highly successful in the dissolution of the pentoxide mixture with recoveries of
100.4(9)% and 100(1)% for Nb and Ta respectively. In addition, only the NH4Fâ¢HF
fusion dissolution was found to support the subsequent separation techniques. The
application of the selective precipitation on the NH4Fâ¢HF digestion solutions indicated
23(5) and 73(3)% precipitation of Nb and Ta respectively. The separation of Nb and Ta using solvent extraction technique indicated that Ta is selectively extracted into the
methyl isobutyl ketone (MIKB) with recoveries of 101(1)% and 0.6(5)% for Ta and Nb.
The distribution ratio of tantalum (D(Ta)) was found to increase with an increase in H2SO4
concentration and was determined as 3.46 at [H2SO4] = 4.0 M. The separation on the
strongly basic Amberlite IRA-900 and a weak basic Dowex Marathon anion exchange
columns using HCl as an eluent indicate unfeasible separations with recoveries of 91.7
to 96.1% Nb and 52.4 to 73.4% Ta.
The dissolution and separation methods that were successfully developed and
evaluated using the synthetic (Ta/Nb)2O5 mixture were then evaluated on the two
tantalite mineral samples. The NH4Fâ¢HF fusion dissolution was successfully applied on
the tantalite ore samples and experimental conditions such as the fusion temperature
and flux concentration effects were investigated to optimise the procedure. Excellent
recoveries of 100.3(5) and 100.0(9)% were obtained for Nb and Ta in Sample A and
96.4(3) and 101.3(4)% for Nb and Ta in Sample C. The application of the selective
precipitation, solvent extraction and ion exchange separations produced results similar
to the separation of Nb and Ta in the pentoxide mixture. Different solvents were
evaluated for the selective extraction of Ta and a pure tantalum compound was isolated
using methyl isoamyl ketone (MIAK).
The magnetic removal of some of the impurities and the H2SO4 leaching of the NORMâs
preceded the dissolution of the rest of the minerals. TiO2 and Fe2O3 (57.4(9) and
65(2)%) were removed in the magnetic separation step and 60.77(1)% ThO2 and
62.4(1)% U3O8 were separated in the H2SO4 leaching method. Tantalum was selectively
extracted into the MIAK after its dissolution using NH4Fâ¢HF with D(Ta) of 6.21 at 4.0 M
H2SO4. Niobium was extracted from the mineral matrix using the weak Dowex Marathon
wba anion exchanger using HCl as an eluent. Nb2O5 and Ta2O5 of 96% purity were
obtained after precipitation with ammonia.
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