THE SOLVENT-EXTRACTION OF NICKEL AND COBALT BY MIXTURES OF CARBOXYLIC-ACIDS AND PYRIDINECARBOXYLATE ESTERS

A series of fifteen pyridinecarboxylate esters has been synthesized wi th the structure C5H4N.CO.OR, in which the carboxylate group is situat ed at the 2-, 3-, or 4-position on the pyridine ring, and R = n-octyl, 2-ethylhexyl, 2-octyl, cyclooctyl, or cyclohexyl. The presence of the se compounds causes substantial synergistic shifts in the pH(50) value s for the extraction of nickel (up to about 2 pH units) and cobalt (up to about 1.5 pH units) by solutions of carboxylic acids in xylene. Th e extent of the synergistic shift depends on the nature of the carboxy lic acid, and increases in the order: n-octanoic < 2-ethylhexanoic < V ersatic 10 < 2-bromodecanoic < 3,5-diisopropylsalicylic acid (DIPSA). The synergistic shifts depend to a lesser extent on the identity of th e pyridinecarboxylate ester, except that they are smaller for the ster ically hindered 2-carboxylates than for their 3- and 4-isomers. Studie s using the methods of continuous variation and saturation loading ind icate that the metal complexes extracted by mixtures of pyridinecarbox ylate esters (L) and DIPSA (HA) have the stoichiometry MA(2)(HA)L in t he case of sterically hindered compounds such as 2-octyl 2-pyridinecar boxylate, and MA(2)L(2) for less hindered compounds, such as n-octyl 3 -pyridinecarboxylate (where M = Ni or Co). These stoichiometries were also confirmed by slope-analysis treatment of metal distribution data.