Solvent Extraction Process for Refining Cobalt and Nickel from a “Bulk Hydroxide Precipitate” Obtained by Bioleaching of Sulfidic Mine Tailings

A flowsheet with different solvent extraction (SX) units was tested to purify cobalt, nickel, copper, and zinc from a so-called "bulk hydroxide precipitate" (BHP) from bioleaching of sulfidic tailings. A complex pregnant leaching solution was obtained by sulfuric acid leaching of the BHP, dissolving target (>99.5%) and secondary elements (including silicon). A solvent extraction step with the chelating extractant LIX 984 was integrated into conventional Co–Ni SX refining, separating copper selectively (>99%). Subsequently, di(2-ethylhexyl)phosphoric acid (D2EHPA) followed by phosphinic acid (Cyanex 272) SX enabled impurity removal and cobalt separation (77%) from nickel and magnesium, respectively. Silica gel was observed in the aqueous raffinate after the Cyanex 272 SX. Dissolved silica involvement in crud/precipitate/gel formation depended on its speciation (polymerization and surface groups) during SX (pH and saponification). Interference by silica was mitigated until cobalt recovery. However, nickel could be separated with the carboxylic Versatic Acid 10 from magnesium only after silica gel filtration, as crud/precipitate formed during SX at pH 7. Advanced ultrasound-assisted antisolvent crystallization gave pure monohydrated CoSO4 and NiSO4.