Alkane-Insoluble Trialkylammonium Double Salts Involving the Dodecamolybdophosphate Anion. II. Effect of Amine Structure on Third-Phase Formation

In this paper, a rational basis is presented for the control of molybdenum “third phase” formation in tertiary amine extraction through appropriate selection of amine structure. The studies have been founded on the observation that third-phase formation in the amine-extraction of molybdenum is mainly caused by the formation of heteropolymolybdates, especially the dodecamolybdophosphate anion PMo₁₂O₄₀ ^3- (DMP). Eight trialkyl amine structures with straight and methyl - branched, 8- to 13-carbon chains have been examined with regard to the type of third phase formed and the maximum molarity of the dissolved salt (R₃NH)₃[DMP] that can be tolerated in the solvent before precipitation occurs (i.e., the DMP “tolerance”, a type of solubility measurement). In the experiments, a standard solvent consisting of 0.05 M tertiary amine (primarily as the trialkylammonium sulfate salt) and 0.15 M n-dodecanol in n-nonane was used. For all of the amines tested, third-phase formation first involved co-extraction of Mo(VI) and phosphate from 0.1 M H₂SO₄giving the dissolved salt (R3NH)₃[DMP] over extended equilibration times (days to weeks). With no NaCl added to the aqueous phase, liquid third phases were obtained except in the case of triisooc-tyl amine (TIOA), which gave solid (TIOAH)₃C[DMP]. In the systems containing added chloride, third-phase formation involved combination of (R3NH)3[DMP] with (R₃NH)Cl to give the solid double salts (R₃NH)₃[DMP]•3(R3NH)C1. The DMP tolerance of the resulting systems generally decreased with decreasing alkyl chain length, with the addition of methyl branches to a given chain, with decreasing amine concentration, and with addition of NaCl to the aqueous phase to give (R₃NH)C1 in the solvent via anion-exchange with (R₃NH)₂SO₄. Use of a tridecylalcohol - modified kerosene diluent under conditions more closely approximating process conditions generally led to higher DMP tolerances than obtained with dodecanol-modified nonane, though the effect of amine structure was similar, Amine purification procedures are discussed.