ArticleIntegrated system of comprehensive utilizing the concentrated brine of Yuncheng salt-lake basing on salt-forming diagram☆
Graphical Abstract
Integrated system of comprehensive utilizing the concentrated brine of Yuncheng salt-lake was development relying on salt forming diagram. The background phase diagram is at 100 °C, the background of salt-forming region (green color region) for NaCl is at 75 °C.
Introduction
Bulk processes such as seawater desalination and valuable salt recovering from seawater, salt lakes and mineral deposits, produce large amounts of concentrated brine enriched in soluble ions of Na+, K+, Mg2 +, Li+, Cl−, SO42 −, etc. For sustainability, the comprehensive utilization and environmentally friendly processes are of utmost importance.
The formation of crystalline solids from solution is fundamental to many natural and industrial processes [1]. Salt-water system phase diagrams expressing aqueous salt solubility and solid–liquid phase equilibrium behaviors for a multicomponent salt-water system is the most common tool for process analysis, design, and integration [2]. However, industrial processes are often operated at compulsive non-equilibrium stable or dynamic state in high evaporation rates and at the changing temperature or pressure. Salt-forming behavior in some case is even more complex and difficult to be predicted accurately by either the solubility diagram or the metastable diagram [3]. On the mechanism research, Wallace et al. [4] using computer simulations provide support for two-step crystal nucleation and show that solvation plays a key role in this process [1]; On thermodynamics research, W. Voigt [5] points out the future work should be directed to improve accuracy of solubility data in multi-component solutions combined with modeling and to consider kinetics. The complex behavior of salt-formation in the non-equilibrium state was experimentally studied [6], [7], [8], [9], [10], and the concepts of salt-forming region on the bases of solution thermodynamics and the knowledge of crystallization theories were proposed [3]. The effective use of salt-forming region needs more industrial practices.
Yuncheng salt-lake is located in the middle inland of China, neighboring closely the ancient capital cities of Xi'an and Luoyang; it has been the main salt-producing area in China for thousands of years and has played an important role in the history of Chinese civilization [11], [12]. Since salt-lake brine is not the sodium chloride (NaCl)-type, but is the sodium sulfate (Na2SO4) type, the special salt-making process has been considered one of the greatest inventions of ancient China. This salt-making process persisted for thousands of years until the 1960s, when the mass extraction of Na2SO4 was replaced. However, accumulation of the lake's single product of Na2SO4 over the next 50 years has resulted in a significant change in the salt-lake composition and environment. Thus, the development of comprehensive utilization and environmentally friendly technology is urgently needed.
In this study, relying on the salt-forming phase diagram in non-equilibrium state, the ancient technical secrets were uncovered, and one comprehensive utilization system for the joint production of industrial NaCl and monohydrate MgSO4 was developed, producing no waste liquid and consuming the residual bittern.
Section snippets
Salt-forming diagram
The salt-water system phase diagram normally includes solubility diagram and metastable phase diagram. Solubility diagram, also known as phase equilibrium diagram, which corresponds to a stable state with absolute minimum free energy of the system, has a rigorous thermodynamic mechanism. The metastable phase diagram presenting the metastable phase regions, but until now it is no rigorous thermodynamic mechanism to theoretically express. In order to present the complex behaviors of
Process analysis based on the phase diagram
How can food-salt be produced under the natural conditions of Yuncheng salt-lake, which is rich in Na2SO4 but not NaCl? Why brine composition were not obvious change with the salt recovery after thousands of years? However, the conclusion of ‘no possible to produce pure salt (NaCl)’ can be obtained via phase diagram analysis at a temperature range from − 5 to 35 °C (Fig. 2, Fig. 3).
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Cooling the salt-lake brine in winter. As shown in the phase diagram (Fig. 2) at temperature around − 5 °C to 0 °C, the
Process planning on solubility phase diagram
To develop one sustainable and optimized process for the utilization of actual salt-lake resources, we plotted the brine compositions of ‘A’, ‘B,’ and ‘C’ on the phase diagram at multiple temperatures from − 5 °C to 150 °C as shown in Fig. 6: (a) 55 °C, (b) 75 °C, (c) 100 °C, and (d) 150 °C.
At temperatures from − 5 to 0 °C (Fig. 2), brine ‘C’ is near the co-saturated curve of Na2SO4·10H2O and MgSO4·7H2O. At temperatures from 25 °C to 55 °C, both ‘B’ and ‘C’ are located at the astrachanite region. At
Conclusions
Based on the salt-forming phase diagram in the non-equilibrium state, the technical secrets of Yuncheng salt-making in ancient China were revealed. In order to utilize the large amount of residual brine produced in the Na2SO4 recovery process and to eliminate its environmental impact, one system of comprehensive utilization was proposed and experimentally tested.
This integrated system includes a vacuum salt-making process and a normal pressure kieserite process. The experiments confirm that the
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Supported by the National Natural Science Foundation of China (U1407204), the Yangtze Scholarsand Research Team in university of Ministry of Education of China (IRT_17R81), the Innovative Research Team of Tianjin Municipal Education Commission (TD12-5004), and the Foundation of Tianjin Key Laboratory of Marine Resources and Chemistry (201602).