Huan Zhou*,Jingjing Tang,Jian Guo,Yaping Dai,Guangbi Li,Bo Yan

College of Chemical Engineering and Materials Science,Tianjin Key Laboratory of Marine Resources and Chemistry,Tianjin University of Science and Technology,Tianjin 300457,China

Keywords:Phase diagram Brine Salt-forming Nonequilibrium state Comprehensive utilization

ABSTRACT The comprehensive utilization and environment-friendliness of processes for recovering fresh water or valuable salt from seawater,salt-lakes,or mineral deposits are of utmost importance for sustainable development.One primitive sustainable process for recovering salt from sodium-sulfate-type brine in Yuncheng salt lake had been considered one of the greatest inventions of ancient China,however,the replaced process of mass extraction of single Na2SO4 in recent years,has reduced a large amount of residual brine.In this research,relying on the salt-forming diagram in the non-equilibrium state,the technical secrets of ancient salt processes were uncovered,and a new comprehensive utilization system was proposed and tested experimentally.The new system includes a vacuum salt-making process and a normal pressure kieserite process,which can gradually eliminate the existed waste liquid and aid in the sustainable developmentofthe Yuncheng salt-lake.The continuous experiment of salt-making process running stably in the double salt region without double salt formation,which proves the feasibility of salt-forming diagram applied in industrial process.Thus salt-forming diagram would be extremely valuable to industry process design and control,especially,the treatment of concentrated brine.

1.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 nonequilibrium 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 supportfortwo-step crystalnucleation and show thatsolvation 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 saltformation in the non-equilibrium state was experimentally studied[6–10],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 sodiumsulfate(Na2SO4)type,the specialsalt-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 Na2SO4was replaced.However,accumulation of the lake's single product of Na2SO4overthe next50 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 nonequilibrium state,the ancient technical secrets were uncovered,and one comprehensive utilization system for the joint production of industrial NaCl and monohydrate MgSO4was developed,producing no waste liquid and consuming the residual bittern.

Table 1 The composition of Yuncheng salt-lake

2.Background

2.1.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 salt-formation,the concepts and principle of salt-forming regions including primary region,extreme region,and conditional region are theoretically proposed[3].A summary is summarized as below.

Salt-formation means that one solid is built from the liquid phase by primary nucleation or crystal growth.The essential driving force for the salt-formation is the difference value between the chemical potential of solute in the liquid phase and solid phase.The different driving forces needed for primary homogeneous nucleation of different salts lead to the difference of salt-forming region changed from its solubility region.Since the primary nucleation is further distinguished into homogeneous and heterogeneous nucleation,the salt-forming region can thus be distinguished into primary,extreme and conditional regions.

Primary region(PR)is where the homogeneous nucleation for the considered salt could occur within the isothermal concentration process.The boundary of PR for two species salts is where the critical phase equilibria exist between the liquid species and the nucleus in critical size of each salt.

Extreme region(ER)is where the considered solid could stably exist and its mass amount could be increased during the isothermal concentration process either by nucleation or crystal growth.The boundary of ER is where the heterogeneous nucleation for second salt could occur while the first solid salt existed.

Conditional region(CR)is the overlap region of extreme regions for two species salts.The species of salt-formation in CR maybe one,or another,or together,which depends on the species of crystal seed.

2.2.Brine composition

Yuncheng salt-lake had produced sodium chloride before the 1960s,but then,the product is Na2SO4.The brine composition of Yuncheng salt-lake[13]from the earliest record of the 1950s to the present is summarized in Table 1 and plotted on the phase diagram of the Na+,Mg2+//Cl-,SO42-–H2O quaternary system[14]at 298.15 K,as shown in Fig.1.The formulas of mass percent converted to J?necke index are shown in Table 1.

The compositions located in region ‘A’comprise the earliestrecord of the surface brine in salt-lake before the 1950s and the underground brine at present.‘B’is feed brine to produce mirabilite(Na2SO4·10H2O)which is mixed by ‘A’and ‘C’in recent years.‘C’is waste bittern produced from the mass production of Na2SO4.Fig.1 indicates that:

Fig.1.Brine composition in Yuncheng salt-lake(phase diagram at25°C).‘’mine brine atpresent;‘’earliestrecord ofsalt-lake brine;‘’waste bittern ofcurrentproduction;‘●’feed brine of mirabilite production.

Fig.2.Na2SO4 recovery process(phase diagrams at 0 °C and-5 °C).‘’mine brine at present;‘’earliest record of salt-lake brine;‘’waste bittern of current production.

(1)the primordial salt-lake is Na2SO4-type;

(2)the salt-lake composition was not significantly changed with the salt-making process for thousands of years[12];

(3)the large amount of waste bittern ‘C’has been accumulated with the recovering of single compound Na2SO4from the 1960s.

3.The Technique of Ancient Salt-making

3.1.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 Na2SO4but 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(Figs.2 and 3).

(1)Cooling the salt-lake brine in winter.As shown in the phase diagram(Fig.2)at temperature around-5 °C to 0 °C,the only product should be mirabilite but not salt(NaCl).

(2)Evaporating the salt-lake brine in summer.The phase diagram(Fig.3)at temperature around 25 °C to 35 °C,shows that the process can be roughly divided into three steps:①brine concentrated and mirabilite crystallized out until astrachanite(Na2SO4·MgSO4·4H2O) and mirabilite co-saturated, or astrachanite and thenardite (Na2SO4) co-saturated; ②astrachanite and thenardite precipitated simultaneously until NaClsaturated;and③astrachanite and NaClsolid precipitated together.

Fig.3.Process analysis of evaporation based on solubility diagram at 25°C.

Fig.4.Actual process of ancient salt.

(3)Further evaporating the residual brine ‘C’.As shown in Fig.3,the product in step④is still the mixture of astrachanite and NaCl.

3.2.The actual process of ancient salt-making

The conclusion based on the stable phase diagrams seems to be that it is impossible to recover pure salt from Yuncheng salt-lake under natural conditions.However,the actual ancient salt-making process included five steps[11],as shown in Figs.4 and 5,processes①to⑤are summarized as follows:

①Solar pond(I)for first step evaporation:raw brine is concentrated until sulfate is saturated.

②Solar pond(II)for second step evaporation:the sulfate salt precipitates from the saturated brine and forms the so-called “sulfate-bed.”

③Natural filtration:the mother liquid of the sulfate salt is filtered through the sulfate-bed and gathers in the outside trench.The filtered brine solution would normally contain a high concentration of NaCl.

④Solar pond(III)for third step evaporation:food-salt crystallizes out of the filtered brine.

⑤The mother liquid of the filtered food-salt is discharged into solar pond(I).

All steps were controlled by a skilled technician relying on experience.As such,these processes were repeated for thousands of years.

3.3.Uncovering the principles of ancient salt-making

The behaviors of stable and metastable solid–liquid phase equilibria were wisely and naturally used in the ancient salt-making process.Metastable data in the Na+,Mg2+//Cl-,SO42-–H2O system at 298.15 K[15],as defined by the pink curves in Fig.4,reveal the following:(1)there is no solid region for astrachanite;(2)the surface evaporation in the second solar pond forms the solids of mirabilite or thenardite and the liquid saturated with the metastable phase of astrachanite;(3)astrachanite fully precipitates during the long filtration process,and the final brine is already stable and saturated with NaCl.

The filtered brine is located on the NaCl region of the metastable diagram,and only the NaCl salt crystallizes out until epsom salt is saturated.

3.4.One primitive sustainable process

The residual brine of the third evaporation step of salt-making was discharged into the first evaporation step.This is very important to the efficiency of NaCl recovery and the sustainability of resource utilization.The compositions of the feed brine are located in the triangle area of NaCl–Ast–Na2SO4in Fig.4,and the solid products are just those three salts(NaCl and sulfate-bed comprised by astrachanite and Na2SO4).

Fig.5.Ancient salt-production processes in Yuncheng salt-lake.

The recycling of residual brine forms one sustainable process with the following advantages:(1)no residual brine is discharged;(2)all the chloride content is transformed into salt product;(3)all the sulfate and magnesium contents form the deposition of a sulfate-bed;and(4)the salt-making process is separated from the salt-lake,and the lake composition was not affected by the recovery process.Thus,the ancient salt-making method is a primitive sustainable process and a truly great invention of ancient China.However,solid waste of sulfatebed and very low productive efficiency are obvious disadvantages for the ancient method.

4.New Comprehensive Utilization System

4.1.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 cosaturated curve of Na2SO4·10H2O and MgSO4·7H2O.At temperatures from25 °C to 55 °C,both ‘B’and‘C’are located atthe astrachanite region.At temperatures from 75 °C to 100 °C,both ‘B’and ‘C’are located at the loeweite(6Na2SO4·7MgSO4·15H2O)region,and at 150 °C,both brines‘B’and ‘C’are located at the kieserite(MgSO4·H2O)region,and brine‘B’is near the loeweite region.

From those diagrams,the single salt or hydrates Na2SO4·10H2O,MgSO4·7H2O,and MgSO4·H2O can be produced at low temperatures below 0 °C or at high temperatures around 150 °C.Unfortunately,production at these temperatures is undesirable;e.g.,production at low temperature produces large amounts ofwaste liquid,and operation at high temperature requires high pressure,resulting in difficult solid–liquid separations and a high cost of operation.

4.2.Analysis based on salt-forming diagram

It is known from previous researches[9,10]thatsalt-forming behaviors in the evaporation process for the Na+,Mg2+//Cl-,SO42-–H2O system are largely different from the analysis results of the stable phase diagram.For example,the NaCl salt-forming region includes stable region and extreme region at 75°C[9],as shown in Fig.7,the curves D0–3and E0–3are the respective boundary for NaCl region in stable and non-equilibrium states.

Fig.6.Process planning on multiple temperature phase diagram.

Fig.7.Salt-forming diagram of Na+,Mg2+//Cl-,–H2O system for NaCl production at 75 °C.

The residual brine ‘C’is mostly located atthe extreme region ofNaCl,thus,it is possible to produce NaCl salt via multi-effect evaporation.During the evaporation process,the points of the liquid phase will move along the vector line of NaCl crystallization to the limited curve of E2–E3where kieserite begins to precipitate.Since the liquid around E2–3is enriched in MgSO4,and appears in the middle of the kieserite region.Therefore,MgSO4hydrate can be easily produced.

However,the extreme region of NaCl occupies the stable region of loeweite and kieserite.The experiments are necessary to verify the industrial feasibility of the salt-making process,in particular,the stability of the double salt in the long-time evaporation process.

4.3.Experiments

4.3.1.Materials and apparatus

Fig.8.Experimental apparatus.1—oil and vacuum jackets glass thermostat(feed tank);2—feed pump;3—oil and vacuum jackets glass evaporating crystallizer;4—water vapor condenser;5—water collector;6—buffer bottle;7—vacuum control valve;8—vacuum pump;9(a,b)—thermostatic oil bath;10—Heidolph stirrer.

Table 2 The isothermal boiling evaporation at temperature 75°C

4.3.1.1.Materials.Raw brine in regions “A”,“B”and “C”was taken from Yuncheng salt lake after being decolored with activated carbon.Other chemicals used were of analytical purity grade and obtained from Shanghai Aladdin Industrial Co.and these include MgSO4·7H2O,NaCl,and MgCl2·6H2O.Doubly deionized water was used to prepare the synthesized solution and chemical analysis.

4.3.1.2.Apparatus.Fig.8 shows the experimentalapparatus forcontinuous evaporative crystallization processes which consists of a thermal insulation tank for feed brine,feed pump,2 L double jacketed glass evaporating crystallizer(Chemglass)with a stirrer(Heidolph),watervaporcondenser with a watercollector,thermostatic oilheating bath(Huber,Unistat Petite Fleur),chemistry diaphragm pump(Vacuubrand,PC 610 NT),pressure–temperature controller,and online recorder system.

4.3.2.Vacuum boiling evaporation to produce salt

Batch and continuous vacuum boiling evaporation processes with isothermal and constant pressure were carried out respectively.The heating temperature differences(ΔT)for all the processes arefixed at 40°C.Solid and liquid samples were taken discontinuously by discharging the solid–liquid mixture through the bottom valve and separated via the isothermal filtering method[9].The wet solid and liquid sample compositions of Mg2+,Na+,Cl-,SO42-,and H2O were determined via quantitative chemical analysis[16].

Batch process for isothermalevaporation was carried outat 75°C for raw brine a0;the results are listed in Table 2 and plotted in Fig.9a.NaCl begins to precipitate atpointa1where the boiling pressure is 310 mbar.Continuing the evaporation process until the sulfate begins to precipitate at a7,the vapor pressure reduced to 18 kPa,the concentrated ratio of the liquid phase and the output ratio of NaCl were 46.6%and 75.2%,respectively.

Batch process for constant pressure evaporation was carried out at 10 kPa for raw brine b0(same with a0),and the results are listed in Table 3 and plotted in Fig.9a.When NaCl begins to precipitate,the boiling temperature is 53.2°C.Until sulfate starts to precipitate at b8,the temperature has risen to 61.3°C,where the liquid phase is concentrated to 45.1%,and an 84.1%output of NaCl was achieved.

In order to confirm the stability when the operation runs in the double salt region,the long-time and stable processes of evaporative crystallization were carried out for 10 h with the conditions of 18 kPa and 75 °C and 10 kPa and 61 °C respectively.In the continuous processes,the pressure was constant,and the boiling temperature was controlled by adjusting feed rate.The results show that the pure NaCl crystallized in the whole processes.It suggests that the use of vacuum boiling evaporation for the salt-making process can stably run in a double salt(loeweite)region and MgSO4·H2O regions without the precipitation of double salts.

4.3.3.Normal pressure evaporation to produce magnesium sulfate monohydrate

The mother liquor c0produced from the salt-making process via the vacuum boiling evaporation described in Section 4.3.2 was stored in a thermostat.The liquid was evaporated under normal pressure(～101.3 ± 0.2)kPa and with a heating temperature difference of 40°C.The results are listed in Table 4 and shown in Fig.9b.In the batch process,the boiling temperature is from 107.7 °C to 116.3 °C.The one solid region for kieserite(MgSO4·H2O)is from c1to c4with corresponding temperatures from 107.7 to 115.4°C.From c0to c4,the volume of the liquid phase is concentrated only to 80.2%,and the water evaporated ratio is only 13.6%;however,the output ratio of MgSO4·H2O reaches 72.2%.This means that MgSO4·H2O precipitates mainly because of the temperature increase rather than evaporation.

Fig.9.a.Liquid points in the evaporation process under vacuum and at atmospheric pressure.The background phase diagram is at75°C.b.Liquid points in the evaporation process under atmospheric pressure.The background phase diagram is at 100°C.

Table 3 Boiling evaporation at constant pressure of 10.0 kPa

Table 4 Boiling evaporation at normal pressure of 101.3 kPa

Fig.10.The crystal shape of kieserite produced from brine via normal pressure evaporation.

In a continuous process for 6 h,the boiling temperature was controlled to be(115± 0.2)°C by adjusting feed rate.Kieserite could form spherical crystals and with an average particle size of 170 μm,as depicted in Fig.10.The solid–liquid mixture could be easily separated by filtration.With alcohol washing,the purity of monohydrate MgSO4is higher than 98%and chlorine content is less than 0.1%.

4.4.Comprehensive utilization system

From these experimental results,a sustainable system for brine comprehensive utilization can be designed.The main parameters for the utilization of brine B0are shown in Table 5.As Fig.11 shows,the integrated system includes four sub-processes:

(1)mixing the raw brine A0with waste brine C0to form the feed brine B0,which is located on the diagonal line of NaCl–MgSO4;

(2)mixing the feed brine B0with the recycle brine C3to form the solid–liquid mixture C1;

(3)evaporating brine C1via multi-effect evaporation to produce salt and the mother liquid C2;

(4)evaporating brine C2at high temperatures to produce kieserite and to recycle brine C3.

In this process,all the components of raw brine are transformed to the target product.The evaporation amounts account for 88%and 12%, respectively,of the salt-making and kieserite-making processes;meanwhile,the recycle brine C3is also necessary.

Table 5 The main parameters for the integrate process of brine sustainable utilization

Fig.11.The integrated processes for Yuncheng salt-lake resource utilization.The background phase diagram is at 100 °C,the background salt-forming region for NaCl is at 75 °C.

Based on the comprehensive utilization system,the waste bittern ‘C’mixed with raw brine ‘A’can be arranged to produce salt and kieserite,and no residual brine or waste solid.Thus,it is hopeful that the waste bittern ‘C’will be gradually eliminated.

5.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 Na2SO4recovery 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 processand a normal pressure kieserite process.The experiments confirm that the salt-making process can stably run in the double salt region without double salt formation and show that the kieserite-salt process can produce crystalline monohydrate MgSO4,which can be easily separated and purified.

The evaporative crystallization process stably runs in the extreme region ofNaCl,which proves the feasibility ofsalt-forming regions in industrial application.Thus the salt-forming diagram would be extremely valuable to industry process design and control,especially,the treatment of concentrated brine.