Sequential separation of cobalt and lithium by sorption: Sorbent set selection

The sorption of cobalt and lithium was studied using low-cost natural (dolomite, diatomite, barite), commercial (13X zeolites and clinoptilolite) and synthesized (mesoporous carbon and NaY zeolite) sorbents. The pH of the medium was a key factor in the sorption process which can be modified due to the active chemical groups on the surface of the sorbents. Therefore, the doses of each sorbent were selected to avoid cobalt precipitation, working at pH below 8. Sorption with monometallic solution was carried out to select the best set of sorbents for subsequent selective separation. Only mesoporous activated carbon and dolomite allow to achieve significant cobalt removal, 85% and 65% respectively, with negligible lithium removal. 13X zeolite (high surface area and low Si/Al molar ratio) removed both metals without selectivity. Therefore, dolomite and mesoporous activated carbon were selected for the selective separation of cobalt, as they allow to achieve an adsorbed Co2+/Li+ weight ratio of 16 and 4, respectively. 13X was selected to remove lithium once cobalt was removed from the solution. The best separation was carried out by a two-sequential sorption stage, using dolomite in a first stage to separate cobalt and 13X zeolite in a second stage to remove lithium, reaching an almost complete removal of both metals. The pH control for working with high doses of dolomite was achieved by means of a solid phase pH buffer (solid buffer) composed by a mixture of the dolomite itself and non-activated mesoporous carbon (without sorption capacity) in a 1:4 weight ratio. This solid buffer allowed the pH of the medium to be controlled to around 6.5.