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

A series of fifteen pyridinecarboxylate esters has been synthesized with the structure C5H4N.CO.OR, in which the carboxylate group is situated 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 these compounds causes substantial synergistic shifts in the pH(50) values 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. The extent of the synergistic shift depends on the nature of the carboxylic acid, and increases in the order: n-octanoic < 2-ethylhexanoic < Versatic 10 < 2-bromodecanoic < 3,5-diisopropylsalicylic acid (DIPSA). The synergistic shifts depend to a lesser extent on the identity of the pyridinecarboxylate ester, except that they are smaller for the sterically hindered 2-carboxylates than for their 3- and 4-isomers. Studies using the methods of continuous variation and saturation loading indicate that the metal complexes extracted by mixtures of pyridinecarboxylate esters (L) and DIPSA (HA) have the stoichiometry MA(2)(HA)L in the case of sterically hindered compounds such as 2-octyl 2-pyridinecarboxylate, 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.