Phase Separation Dynamics And Phase-Forming Mechanism In Liquid-Liquid-Liquid Three Phases Extraction System

Three-liquid-phase extraction is a newly developed technology for extraction and separation processes.Three-liquid-phase system(TLPS),which is composed of organic oil phase,polymer-rich middle phase and salt-rich bottom phase,is the most reported TLPS in literatures.Investigation on phase separation dynamics and phase-forming mechanism of such TLPS is not only benefit for our scientific understanding in complex systems containing multiple liquid phases,but provides supporting data that are needed for design of appropriate extraction separators and formulation of operation rules for future industrial application of three-liquid phase separation technology.In this dissertation,dynamic phase forming behavior of three-liquid-phase system(TLPS)composed of organic oil phase,polymer-rich middle phase and salt-rich bottom phase was investigated in detail.With changing mass composition of phase-forming components,the rate-control process in formation of such a TLPS was recognized,as well as the effect of system composition on the phase-forming equilibrium time(tE)of TLPS.It found that dynamic forming process of TLPS was in fact a course of dispersive polymer and organic oil droplets aggregated and separated out respectively from continuous salt aqueous bottom phase.The formation rate of polymer-rich middle phase and its equilibrium volume determined the value of tE of TLPS.Formation rate of organic oil phase was controlled mainly by coalescence rate of dispersed oil droplets;however,the rate-determining process for formation of polymer middle phase may change from drop sedimentation to coalescence or codetermined by both,when mass composition of the TLPS changed along different operation lines.The phase separation dynamics of TLPS closely related to the composition of system.Therefore,a quantitative correlation of phase separation rate of TLPS with its physicochemical properties was given.Effect of phase-forming salt on the dynamic behavior of phase formation of TLPS was also studied.It found that the species of salts exhibited almost no effect on the separation rate of oil top phase and polymer middle phase from salt aqueous bottom phase;however separation between oil top phase and polymer middle phase became slow with the increase of salting-out ability of the ions present in the slats.Theoretical analysis was provided in detail.On one hand,the Van der Waals attraction potential energy among dispersed oil drops and the media of polymer-rich phase decreased,while the energy from steric repulsion increased with increasing the salting-out ability.On the other,the Van der Waals attraction potential energy among dispersed polymer-rich aqueous drops and the media of oil phase decreased,too.Therefore,both decrease in the formation rate of the oil top and polymer-rich middle phases resulted into decrease in the rate of separation between them.Based on a TLPS which was successful in simultaneous separation of lithium from boron and magnesium in salt lake brine,the effect of dynamic phase forming behavior of TLPS on the dynamic partition behavior of objectives was focused.Results showed that the partition rate of lithium into the oil phase and that of boron into the polymer-rich middle phase was mainly governed by the increase rate of volume of their target phases during phase formation process of TLPS.This was because that the mass of lithium or boron partitioning into their target phases was determined by the volume of the phase and the concentration of lithium or boron in it.The separations between oil and polymer-rich drops from the salt bottom phase finished very quickly,but the separation between oil and polymer-rich phase was slow.With time went on,the concentrations of lithium in oil drops in the media of polymer-rich phase and that of boron in polymer-rich drops in the media of oil phase were fixed,but the volume of oil and polymer-rich bulk phases were increased along with separation of the two phases.Therefore,the rate of lithium and boron partitioning into their target bulk phases was actually determined by the separation rate of oil and polymer-rich phases.Mechanism of dynamic partition behavior of targets in TLPS was explored from view of phase formation.Series molecular weights of polymers were employed to form TLPSs for extraction and separation of lithium from boron and magnesium in salt lake brine.It found that the rheological property of system continued to decrease with increasing the polymer molecular weight.But the effect of rheological property of system on the partition behavior of lithium and boron was not durative.This was attributed to the change in partitioning mechanism of lithium and boron.When the viscosity of polymer-rich middle phase was low,the partition of lithium and boron depended on separation of B-containing polymer-rich drops and Li-containing oil drops achieved by the two drops merging with their bulk phases respectively.By such"merging" mechanism the partition rate of lithium and boron was governed by the rheological property of system.Whilst,when viscosity of polymer-rich phase increased to a certain extent,separation of B-containing polymer-rich drops and Li-containing oil drops was completed by shrink of oil drops/films and expansion of oil bulk phase.Thus,the partition rate of lithium and boron was independent of the rheological property of system.