THE ROLE OF CALCIUM PHOSPHINO-POLYCARBOXYLATE COMPLEXATION IN INHIBITING BASO_4 PRECIPITATION FROM BRINE

摘要

The kinetics of BaSO_4 nucleation and its inhibition with phosphino-polycarboxylic acid (PPCA) has been studied. The induction period of BaSO_4 nucleation under a wide range of ionic strength and temperature has been measured through a turbidity method. The presence of Ca~(2+) does not show a significant effect on the induction period of BaSO_4 nucleation. The presence of PPCA increases the induction period of BaSO_4 nucleation while the extent of the prolongation depends on the solution pH and Ca~(2+) concentration. Only the dissociated fraction and the metal complexed fraction of PPCA are effective to inhibit BaSO4 nucleation while the protonated fraction has no significant effect at any conditions. In a low supersaturated solution, the calcium complexed fraction of PPCA is more effective than the dissociated fraction. While in a highly supersaturated solution, the calcium complexed fraction of PPCA is similarly effective as the dissociated fraction. Temperature has no significant effect on the efficiency of the calcium complexed fraction of PPCA and has only slight effect on that of the dissociated fraction of PPCA. The overall efficiency of PPCA to inhibit BaSO4 nucleation is proportional to the total concentration of Ca~(2+). Ca~(2+) enhances the overall efficiency of PPCA through decreasing the fraction of protonated PPCA and forming Ca-PPCA complexes. Ca~(2+) might play the role of a bridge connecting the crystal sites with exposed SO_4~(2-) and PPCA functional groups so that the adsorption of PPCA on the active sites of BaSO_4 crystal surfaces is enhanced.