Transport Properties Of Charged Membranes In The Treatment Of Salt Lake Brines

Membrane separation technology has been successfully applied in various fields and proved to be environment-friendly and energy-saving, with strong economical significance. The utilization of membrane processes in the exploitation of salt lake resources would solve a series of problems encountered in the salt lake industry. The research focused on the transport properties of two types of charged membranes: nanofiltration (NF) and ion-exchange membranes (IEM), in order to explore the potentials of them in the separation and extraction of alkali and alkaline earth metals from salt lake brines. The following aspects were covered in the thesis.Firstly, the applicability of NF for extracting lithium chloride from diluted bitterns was preliminarily studied. The overall separation performance of the tested DL membrane was unsatisfactory. DL exhibited high rejection of SO₄^2-(>96%), but poor rejection of boron (<46%). Due to the membrane is negatively charged, Donnan effect of the fixed charges helps the transfer of counter ions, especially the divalent Mg²⁺. The separation performance was also affected by the contradictory mutual influences of Donnan exclusion and dielectric exclusion. As a result, the rejection of magnesium was only 61～67%. Therefore, although lithium transported across the membrane under reverse concentration gradient, the separation efficiency was low. Moreover, due to the hindered nature (hence limited driving force for ions) of NF membranes, the salt fluxes were very low under conditions of low convection flow. It can be concluded that good separation of both co-ions and counter ions cannot be achieved by a single type of the singly charged NF membranes.Secondly, the separation mechanisms of ion-exchange membranes between counter ions were elaborated. Experimental results proved that the partitioning of ions at the solution-membrane interface was governed by kinetics as apposed to equilibrium. Therefore, the Donnan equilibrium based solution-diffusion model predicted too low separation factors for electrodialysis (ED) runs, whereas, the kinetic solution-diffusion model's errors were only of experimental origins. The interfacial kinetics was accounted by energy barriers of the permselective layers for different ions, the interfacial overvoltages, as well as current density. Based on Saracco's theory of energy barriers of the permselective layers of CEMs, the influence of overvoltage was introduced into the formula of the separation factor, by utilizing the Butler-Volmer equations. As a result, a more generalized model consisting of both equilibrium and kinetic factors was proposed, with the influences of modified layers,concentrations and composition of the solutions, operating current, as well as hydrodynamics briefly discussed. For certain operating conditions, the generalized model was simplified into a quasi-kinetic model (QKM):α_d^m=k₁j_e^k2exp(-k₃j_e). The QKM model showed better accuracy than both the kinetic model (KM) and the combined model (COM) by Saracco. Thus, the QKM model can serve as a substitute for both KM and COM.In addition, special attention was paid to the transport characteristics of different components and some engineering problems in ED. The multicomponent competitive transport of counter ions showed complicated concentration and composition dependence. For salt lake brines, ED exhibits very low efficiencies for removal of trace divalent ions or extraction of trace univalent ions. The economical efficiency of ED for recovering trace monovalent salts is also poor when extremely high ratios of divalent ions are present. Due to the condition of electroneutrality, the transport of cations and anions through IEMs showed coupling characteristics, which reduced the fluxes of univalent salts. In the diluate feed of ED, boron exists in the order of B₄O₅(OH)₄^2->B(OH)₄^->B(OH)₃>B₃O₃(OH)₄^->B₅O₆(OH)₄^-,in terms of boron molar fraction. As opposed to Gao's generalization for bitterns, the average alkalinity-to-boron ratio (∑z i Ci TB) in the diluate feed is -0.548, instead of -0.5. In AEMs, the monovalent polyborate species have very low mobilities, which results in the accumulative effects of boron pollution to membranes. In the startup stage of electrodialysis of bittern diluates, the concentration of univalent salts in the concentrate feed should not be lower than 50 mmol L in order to prevent local water splitting and scaling. Continuous material flow of ED operation can be simulated by the outcome of laboratory batch operations, the method can serve as a simplified and empirical means for providing a basis for scale-up decisions. -1The more rigid means of predicting the performance of ED should be building a mass transfer model for treating brines, which is based on the Maxwell-Stefan (M-S) equations. This irreversible thermodynamical approach entails extensive experimentation and challenging programming, which is not the objective of the present work. Thus, the thesis only gives a brief theoretical description of the M-S model in the appendix. Further work will cover such efforts.