| Chadamu basin is the largest potassium fertilizer production base in China, where the throughput of KCl accounts for 90% of the total output. However, with large-scale development of the potassium resources, the traditional manufacturing process still faces challenges, i.e., the rate of natural evaporation of the brine is slow, the metallogenic time is long, and large volumes of valuable water are lost to atmosphere resulting in the shortage of production water supply.The highly concentrated brines were concentrated with direct contact membrane distillation (DCMD), and the feasibilities replacing the evaporation pond with DCMD process were studied in the thesis also. At the same time, mineral recovery and simultaneously producing pure water were carried out using MD-crystallization process. Based on the above objectives, studies were performed as follow:the selection of the membrane for membrane distillation, the effects of salt concentration on MD performance, phase equilibrium thermodynamics of brine system, and the development of coupling membrane distillation with crystallizer.(1) Membrane distillation is one of the attractive technologies for treating high salinity brines. In this study, four hydrophobic membranes (PTFE) with similar nominal pore size (0.2μm) but different porosity and thickness were compared in treating 2.0mol/L NaCl aqueous solution. Results showed that Changqi membrane, a non-supported membrane, has the highest flux (66.76kg/m2·h) and the highest energy efficiency (46.5%) at feed inlet temperature of 70℃, which was largely due to its higher porosity (90.8%) and lower thickness (39μm). Porosity and active layer thickness were identified as the main factors affecting permeate flux and energy efficiency in MD. At the same time, the effect of feed temperature on global mass transfer coefficient was related to membrane thickness, especiall for Changqi membrane with non-supported, energy efficiency gradually declined as feed temperature was increased, which is due to lower membrane thickness; Permeate flux of DCMD using 2.0mol/L aqueous NaCl solution was 31.1% lower than that of deionized water, which can be attributed to the decrease of water activity. In addition, permeate flux declined gradually as salt concentration increased. Moreover, it showed a distinct difference in terms of permeate flux at salt concentration of 2.0-4.0 mol/L between KC1, NaCl and MgC12, which was correlated to different thermodynamic properties in electrolytes solution at high salinities.(2) Highly concentrated NaCl, KCl, MgCl2 and MgSO4 solutions were treated using DCMD. The effects of salt concentration (1.0-4.0 mol/L) and circulation velocity (0.1-0.5 m/s), as well as thermodynamic and physical properties of the salt solutions on permeate flux were investigated. The permeate flux decreases with increasing salt concentration, which follows the order of KC1> NaCl> MgSO4> MgCl2 at salt concentration higher than 1.0 mol/L. The discrepancies of the permeate fluxes are mainly associated with the water activities in the salt solutions, which are dependent on the interactions of ion-ion, ion-solvent in solutions. Water activities of NaCl, KC1, MgCl2 and MgSO4 solutions could be accurately predicted with MSE model embedded in the OLI platform. A reversed flux occurs in the DCMD of highly concentrated MgC12 solution (> 2.5 mol/L) at the tested conditions, which is due to the combined effect of lower water activity and higher viscosity of the fluid. Moreover, the drastic increase of viscosity of MgSO4 and MgCl2 solutions at higher salt concentrations would also have a notable adverse impact on permeate flux. Under these circumstances, change of hydrodynamics, e.g. increase of circulation velocity would be a great help to improve the heat transfer and then the flux. DCMD is able to treat highly concentrated brines even in nearly saturated conditions. However, the temperature difference across the membrane should be controlled within a certain range, and it was 40 to 50℃,40 to 45℃ and 25 to 35℃ for NaCl, KC1, and MgSO4, respectively in this study. Otherwise, a dilution of the brine solution due to reversed transfer of vapor from cold side to hot side and/or a deposition of salt on membrane surface will appear, leading to failure of producing purified water or contamination of the distillate and partially loss of membrane hydrophobicity.(3) Phase equilibrium thermodynamics for K+, Na+, Mg2+//Cl", SO42--H2O quinary system at 15℃,35℃ and 50℃ were studied using dissolved brine from low grade potassium mineral in Mahai Salt Lake in Qinghai province as feed. Mineralization regularity, enrichment behaviors and precipitation rules of salts were analyzed during isothermal evaporation process of the dissolved brine by the metastable phase diagram of K+, Na+, Mg2+//Cl-,SO42--H2O quinary system; Amounts of salts precipitated and water evaporated brine were calculated, and the compositions of mother liquor in each crystalhne pond at terminal point were also given by the phase diagram also. Results showed that there were differences between precipitation rules of salts and crystalline phase zone at different evaporating temperature:In comparison with the metastable phase diagram of K+, Na+, Mg2+//Cl", SO42--H2O quinary system at 15℃, the crystallization region of schonite (including leonite) at 35℃ is the smaller, the crystallization regions of glasserite parallelly moved toward KCl region which made the KC1 region decreasing and the Na2SO4 region increasing with the temperature rising from 15 to 35℃, the sodium containing and water containing decrease conseuently. The evaporation process of the dissolved brine may be mainly divided into three crystallization ponds:NaCl depositing pond, potassium mixed salt pond and carnallite crystallization pond. And the precipitation rate of NaCl is higher than 93% at different temperature in NaCl depositing pond; with the increasement of evaporating temperature of brine, the kainite was deposited in the potassium mixed salt pond, whose content is 73.9%at 50℃; And the content of carnallite is 86.7%in the carnallite crystallization pond at 50℃.This work will provide theoretical foundation for the comprehensive utilization of dissolved brine in Mahai salt lake brine in Qinghai province.(4) Membrane distillation crystallization (MDC) of a highly concentrated solution of KC1, MgSO4 and KCl-MgCl2-H2O was investigated for recovery crystalline products and pure water using batch operation mode; the effects of the ways of cooling on supersaturation of solution, morphology, crystal size distribution were studied. It was found that the concurrent production of pure water and salt crystal from highly concentrated salt solution with membrane distillation-crystallization hybrid desalination technology. The highly concentrated KCl-MgCl2-H2O solution could be concentrated with DCMD. However, the temperature difference across membrane must be controlled carefully. Otherwise, a dilution of the brine solution due to reversed transfer of vapor from cold side to hot side and/or a deposition of salt on membrane surface will appear, leading to failure of producing purified water or contamination of the distillate and partially loss of membrane hydrophobicity. KCl crystalline products with shape and tidy surface and concentrated particle size distribution are prepared by nature cooling mode; and maximum degree of supercooling generating under nature cooling is 3.6℃, which is lower than 7.0℃ under quick cooling mode, resulting in effective control of rate of nucleation and crystal growth of KC1. In comparison with quick cooling mode, the maximum degree of supercooling generating under nature cooling for KCl-MgCl2-H2O solution is 1.0℃, which is lower than 2.2℃ under quick cooling mode, being beneficial for nucleation and crystal growth of KC1.Depending on all above studies and experiments, the membrane for membrane distillation of highly concentrated brine was selected; the recoveries of pour water and minerals from Salt Lake brine were realized by combination MD with crystallization. And it was found that membrane characteristics and thermodynamic properties of the brine were one of the key factors affecting MD performance. The study may lay a good foundation for developing an environmentally friendly and new generation of emissions-free technologies. |
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