A novel approach for Ta and Nb extraction consisting of the pre-treatment of a coltan-bearing ore with an ammonium bifluoride sub-molten salt and subsequent acid leaching has been studied. The effects from ore granulometry, ammonium bifluoride (ABF) to ore mass ratio, temperature and duration of fluorination on the degree of Ta and Nb extraction were examined. The ABF to ore ratio and process temperature were found to have the most pronounced impact on extraction efficiency. The following optimal process conditions were determined: ore granulometric fraction (−75 + 45 µm), ABF-ore (5/1), fluorination temperature (200 °C) and fluorination time (2.5 h). Maintaining these parameters enabled about 94% of Ta and 95% of Nb to be brought into solution during the sulfuric-acid-leaching stage. A comparison of the proposed method with previously reported studies suggests that due to the effects of mechanical agitation and the recirculation of the HF-containing gaseous phase back into the process, the dosage rate of ABF at the fluorination stage could be reduced significantly without sacrificing the overall recovery of Ta and Nb. In such a way, the approach could offer added environmental benefits since release of fluoride-containing effluents into the environment could be limited.

The contact details of the author Alidor Shikika have been updated

Current processes for tantalum (Ta) and niobium (Nb) leaching are based on fluoride media, which pose significant environmental challenges. This work focuses on upgrading Ta and Nb oxides from coltan ore retrieved in South Kivu (Democratic Republic of the Congo) mines. The overall process includes alkali roasting of coltan ore with potassium carbonate, water leaching, precipitation of Ta and Nb from the solution using guanidine carbonate and calcination of the precipitate. The effect of roasting process parameters was studied using the Response Surface Method based on the Box–Behnken design. The optimal alkali roasting parameters were found to be: temperature of 950°C, particle size of −150 + 106 µm, roasting time of 140 min and a mass ratio of K ₂ CO ₃ to ore of 1.32/1. Results from the optimised design showed that only 46.6% of Ta and 69.8% of Nb could be recovered from the ore. The overall design optimisation study showed that the mass ratio of K ₂ CO ₃ to ore has a high influence on Ta and Nb recoveries. Hence, a mass ratio was increased up to 1.98/1 to improve these recoveries, with other parameters kept at their optimal levels, resulting in recoveries of 64.92% for Ta and 78.43% for Nb. The leached Ta and Nb were recovered from the pregnant leaching solution by precipitation with guanidine carbonate, followed by calcination of the precipitates. The results indicate that a Ta-Nb mixed oxides concentrate of about 77% was obtained from a coltan ore containing 34% of the Ta-Nb mixed oxides. This process is a promising one regarding the low cost and environmentally friendly characteristics of the process. The residual metals can still be recycled by further extraction in a multiple-extraction process. Despite the relatively modest grade obtained, this technique can be a practical solution to address the conformity of the coltan industry to the mining law in Democratic Republic of Congo, which forbids any exportation of raw ore materials.