Enhanced selectivity in the separation of nickel and cobalt from ammoniacal solutions

An ammoniacal solution containing nickel and cobalt dissolved as nickel-ammonia complexes and cobalt-ammonia complexes is treated with a material capable of providing free ammonia in the solution, such as gaseous ammonia or aqueous ammonia, in order to increase the proportion of higher nickel-ammonia complexes to lower nickel-ammonia complexes in solution until at least about 85% of the dissolved nickel is in the form of higher nickel-ammonia complexes, i.e., complexes in which the number of NH3 molecules is greater than 3. The attainment of this high concentration of higher nickel-ammonia complexes is readily determined by various analytical procedures such as, for example, free ammonia electrode measurements and spectrophotometer measurements. The solution is then treated with a sulfiding agent in an amount sufficient to selectively precipitate out the dissolved cobalt as cobalt sulfide. The resulting slurry is separated into a nickel-enriched liquid fraction and a cobalt-enriched solids fraction. Surprisingly, when the dissolved nickel is present as a higher ammonia complex, less tends to undesirably coprecipitate with the cobalt during sulfiding. The highly desirable result is a precipitate containing up to 50% more cobalt and 20% less nickel, and a mother liquor more enriched in nickel, than normally obtained in a conventional selective sulfiding.