Optimisation of the process parameters for upgrading tantalum and niobium oxides bearing a coltan ore from South Kivu (DRC)

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.