The effects of excluding air (oxygen) from a cobalt solvent extraction circuit were studied for lix 64n extractant. After solvent extraction of divalent cobalt from ammoniacal solutions with chelating extractants such as lix 64n, the loaded co-chelate is believed to oxidize to a stable trivalent complex that resists subsequent stripping with mineral acids. To improve stripping efficiency, cobalt was extracted and stripped in the absence of oxygen in pulsed, perforated-plate columns that were designed and constructed by the Bureau of Mines. When the ammoniacal cobalt(II) sulfate and lix 64n solutions were purged, blanketed, and pulsed with argon, cobalt extraction exceeded 99%, and stripping with 1 vol % h2so4 reached 98.5%. The stripped extractant was recycled to the extraction column with no loss of extraction efficiency in six passes through the circuit.