Extraction of Nb and Ta from a coltan ore from South Kivu in the DRC by alkaline roasting – thermodynamic and kinetic aspects

Abstract

An alkaline roasting of a coltan bearing ore from the Numbi deposit (South Kivu, the DRC) was investigated with particular focus on the extraction kinetics of Ta and Nb. Isothermal roasting experiments were carried out to study the effects from KOH to ore mass ratio, particle size and roasting temperature on the degree of Ta and Nb recoveries. The roasting efficiency was found to be affected by all the parameters being studied with their optimal values being found as following: particle size range +75–45 µm, KOH to ore ratio 3/1, roasting temperature 550 °C and roasting duration of 1 h. Under these conditions, 80% of Ta and 87% of Nb were recovered from the ore. The kinetic data analysis by different kinetic models indicated that the caustic potash roasting of the Numbi coltan ore is governed by a chemical reaction at particle surface. The apparent activation energies for Ta and Nb were estimated as 40.9 and 46.1 kJ.mol-1 respectively. The obtained results demonstrated that it is possible to bring Ta and Nb into solution without using hydrofluoric acid and deliver a pregnant leach solution (PLS) suitable for downstream treatment.