1 Introduction

As a great agricultural country, the demand for potash has increased rapidly in China, but potash has been in short supply for many years and requires significant imports from abroad (Gui et al., 2014). Potassium chloride is the major type of potash, which accounts for about 96% of the total amount. However, chloride-free fertilizers such as potassium sulfate account for only about 4% of the total (Chen et al., 2010; Wang et al., 2014). Potassium chloride can meet the basic demand of most plants for potassium, but it is not suitable for some saline alkali soils and fear-chlorine crops. Compared with potassium chloride, potassium sulfate and chloride-free compound fertilizer are more applicable.

There are both sulfate and chloride brines in Kunteyi Salt Lake, Qinghai province, China, in which the salinity is 329.0 g/L, the relative density is 1.217, and the pH is 6.20. This indicates that the lake belongs to the highly mineralized brine resources rich in potassium and magnesium salts. Using the Kunteyi Salt Lake brine as an example, potassium ores can be obtained from solar ponds and the intermediate mineral shoenite can be produced

(Chen et al., 1998). Further, potassium sulfate can be produced by two-times-conversion of potassium chloride and shoenite. However, magnesium sulfate resources have not been fully utilized in the mother solution for this process (Qin et al., 2016). This not only accounts for the destruction of the salt lake resource structure, but poses a threat to the protection of species and ecology.

Ammonium sulfate is widely used in agriculture as a nitrogen fertilizer, and is also the raw material of traditional potassium sulfate production. It can be produced via a by-product of coking, caprolactam and other industries. Ammonium sulfate also has the characteristics of low price, stable physicochemical properties, widespread sources and resistant to absorb moisture and agglomerate, which can be suitable for the raw material of compound fertilizer production (Zhou et al., 2013).

The process of chloride-free potassium fertilizer production using the carnallite decomposition product of potassium chloride and ammonium sulfate as raw materials has been reported (Zhao et al., 2018; Li et al., 2019a). Similarly, shoenite is also a mineral intermediate that has large contents of potassium and magnesium resources. Based on this, a new process for the production of potassium sulfate and N-Mg compound fertilizer via the

Production of Potash and N-Mg Compound Fertilizer via

Mineral Shoenite from Kunteyi Salt Lake: Phase Diagrams of

Quaternary System (NH4)2SO4-MgSO4-K2SO4-H2O in the

Isothermal Evaporation and Crystallization Process

LI Cheng, CHEN Xueqing, GUO Hongfei, ZHOU Xue and CAO Jilin*

Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, College of Chemical

Engineering and Technology, Hebei University of Technology, Tianjin 300130, China

Abstract: Based on the requirement of the new technology for producing potassium sulfate and N-Mg compound fertilizer,

boussingaultite, by the reaction of the mineral shoenite from Kunteyi Salt Lake, Qinghai province, and the industrial by-

product ammonium sulfate, the solubilities of the quaternary system (NH4)2SO4-MgSO4-K2SO4-H2O at 25.0ºC in the

isothermal evaporation and crystallization process were measured using the isothermal evaporation method, and the

corresponding phase diagrams were plotted. According to the diagram, this system contains six saturation points and six

solid phase fields of crystallization, which correspond to (K1-m,(NH4)m)2SO4, (NH4)2SO4·MgSO4·6H2O,

K2SO4·MgSO4·6H2O, MgSO4·6H2O, (K1-n,(NH4)n)2SO4·MgSO4·6H2O and MgSO4·7H2O, respectively. By analyzing and

calculating the isothermal evaporation and dissolution phase diagram of this quaternary system at 25.0ºC, K2SO4 and (NH4)

2SO4·MgSO4·6H2O can be separated via K2SO4·MgSO4·6H2O and (NH4)2SO4 as raw materials. Theoretical calculations

about the proposed process were carried out and verified by experiment, which indicated that the yield of potassium sulfate

was improved and the magnesium resources were fully utilized.

Key words: industrial application, N-Mg compound fertilizer, shoenite, salt lake brine, solubility, phase diagram, Qinghai

province

Citation: Li et al., 2021. Production of Potash and N-Mg Compound Fertilizer via Mineral Shoenite from Kunteyi Salt Lake: Phase Diagrams of

Quaternary System (NH4)2SO4-MgSO4-K2SO4-H2O in the Isothermal Evaporation and Crystallization Process. Acta Geologica Sinica (English

Edition), 95(3): 1016–1023. DOI: 10.1111/1755-6724.14409

* Corresponding author. E-mail: caojilin@hebut.edu.cn

Acta Geologica Sinica (English Edition), 2021, 95(3): 1016–1023

© 2021 Geological Society of China

http://www.geojournals.cn/dzxbcn/ch/index.aspx; https://onlinelibrary.wiley.com/journal/17556724

Acta Geologica Sinica (English Edition), 2021, 95(3): 1016–1023 1017

mineral resources of shoenite and industrial by-product ammonium sulfate is proposed. The quaternary system (NH4)2SO4-MgSO4-K2SO4-H2O is the theoretical basis for the technological process. The isothermal dissolution phase diagram has already been reported (Xue et al., 2016). Therefore, our aim is to determine the isothermal evaporation phase diagram in the isothermal evaporation and crystallization process. On the basis of these two diagrams, the design and calculation of the proposed process is proposed. 2 Experimental Process 2.1 Materials

Purity of the chemicals (NH4)2SO4, K2SO4, MgSO4· 7H2O and other analytical reagents provided by Tianjin Third Reagent Plant is not less than 99%. Details are presented in Table 1. The water for experiments was twice-distilled with conductivity <5 μS∙cm-1. 2.2 Experimental method

This experiment adopted the isothermal evaporation method in the isothermal evaporation and crystallization process. On the basis of the invariant point solubilities seen in the ternary evaporation phase diagram at 25.0ºC, the salts and deionized water were mixed in a specific proportion in a polyethylene container. After the salts completely dissolved, the containers were placed in a thermostatic evaporator for isothermal evaporation, in which the relative humidity and temperature were controlled in the range of 30%–40% and 25 ± 0.1ºC to approximate the climate of the Qaidam Basin. During this experiment, the precipitated solid phase was observed periodically. Once enough solid phases crystallized out, the wet solid phase was separated from the solution using a spoon. The obtained wet solid was filtered using a Büchner funnel and then washed with ethanol. After air-drying, XRD analysis was performed. Meanwhile, a specific amount of liquid phase was taken from the solution and diluted in a 250-mL volumetric flask for chemical analysis of the composition. Then the remainder of the solution was further evaporated and analyzed. These same procedures were repeated until the solutions were completely evaporated. 2.3 Analytical approach

The ammonium ion concentration was determined using the formaldehyde reaction-titration method (Cai, 2010). The sulfate ion concentration was determined using the barium sulfate precipitation method (Li et al., 2008). The magnesium ion concentration was titrated by the titration

of the ethylenediaminetetraacetic acid (EDTA) standard solution (Gong et al., 2009). The potassium ion concentration was calculated by the subtraction method by subtracting the concentration of magnesium and ammonium ions from the total amount of sulfate ion. The water content was calculated by subtraction (Dou et al., 2015; Gong et al., 2015; Dou et al., 2016; Wang et al., 2017; Li et al., 2019b; Li et al., 2020). The solid phase identification was done using the wet solid method and analysis of X-ray diffraction (XRD) at ambient temperature and atmospheric pressure. XRD patterns were taken with a Rigaku D/MaxII-2500VB2+/PC X-ray diffractometer with Cu Kα radiation (λ = 0.15406 nm), and a scan mode with a speed of 12° min-1. The operating conditions of the X-ray diffractometer were 40 kV and 40 mA.

The uncertainty of the liquid phase composition in weight fraction is approximately 0.02. The uncertainty is caused possibly by uncertainties in sampling procedure and titration process. 3 Results and Discussion 3.1 Solubilities and phase diagrams of the system (NH4)2

SO4-MgSO4-K2SO4-H2O in the isothermal evaporation and crystallization process at 25.0ºC

The solubilities of the system (NH4)2SO4-MgSO4-K2SO4-H2O in the isothermal evaporation and crystallization process at 25.0ºC are given in Table 2. On the basis of Table 2, the corresponding phase diagram and water-phase diagram of this system are plotted in Figs. 1 and 3, respectively. Fig. 2 contains the part enlargement of the phase diagram. Fig. 4 contains the part enlargement of the water-phase diagram.

As seen in Fig. 1, this isothermal evaporation phase diagram consists of six crystallization fields: 1), CFP1P2DAC corresponds to (K1-m,(NH4)m)2SO4 with saturated solution; 2), FGP6P4P3P1F corresponds to MgSO4·(NH4)2SO4·6H2O with saturated solution; 3), P1P2P3P1 corresponds to (K1-n,(NH4)n)2SO4·MgSO4·6H2O with saturated solution; 4), DP2P3P4P5ED corresponds to K2SO4·MgSO4·6H2O with saturated solution; 5), P4P6P5P4 corresponds to MgSO4·6H2O with saturated solution; 6), GBEP5P6G corresponds to MgSO4·7H2O with saturated solution.

XRD patterns corresponding to the equilibrium solid phase of invariant points are seen in Fig. 5. By contrasting PDF standard cards one by one using software MDI Jade, it can be determined that point P1 corresponds to the coexistence of solids (K1-m,(NH4)m)2SO4, (K1-n,(NH4)n)2 SO4·MgSO4· 6H2O and MgSO4·(NH4)2SO4·6H2O with the saturated

Table 1 Source and purity of the reagents used

Compounds CAS number Source Mass fraction purity Purification method

(NH4)2SO4 7783-20-2 Tianjin Third Reagent Plant 0.99 none

K2SO4 7778-80-5 Tianjin Third Reagent Plant 0.99 none

MgSO4∙7H2O 10034-99-8 Tianjin Third Reagent Plant 0.99 none

HCHO 50-00-0 Tianjin Third Reagent Plant 0.99 none

BaCl2 10361-37-2 Tianjin Third Reagent Plant 0.99 none

NaOH 1310-73-2 Tianjin Third Reagent Plant 0.99 none

C10H16N2O8 60-00-4 Tianjin Third Reagent Plant 0.99 none

C20H14O4 77-09-8 Tianjin Third Reagent Plant 0.99 none

Li et al. / Phase Diagrams and Application in Industry of Mineral Shoenite 1018

solution. Point P2 corresponds to the coexistence of solids (K1-m,(NH4)m)2SO4, (K1-n,(NH4)n)2SO4·MgSO4·6H2O and K2SO4·MgSO4·6H2O with the saturated solution. Point P3 corresponds to the coexistence of solids (K1-n,(NH4)n)2

SO4·MgSO4·6H2O, K2SO4·MgSO4·6H2O and MgSO4·(NH4)2SO4·6H2O with the saturated solution. Point P4 corresponds to the coexistence of solids K2SO4·MgSO4· 6H2O, MgSO4·(NH4)2SO4·6H2O and MgSO4·6H2O with the saturated solution. Point P5 corresponds to the coexistence of solids K2SO4·MgSO4·6H2O, MgSO4·6H2O and MgSO4·7H2O with the saturated solution. Point P6 corresponds to the coexistence of solids MgSO4·(NH4)2

SO4·6H2O, MgSO4·6H2O and MgSO4·7H2O with the

saturated solution. The standard PDF cards of solid solutions (K1-n, (NH4)n)2

SO4·MgSO4·6H2O can not be retrieved. Fig. S1 shows the XRD patterns of equilibrium solid phase corresponding to two different saturated liquid points Q1 and Q2 in the crystallization area P1P2P3P1 of (K1-n,(NH4)n)2SO4·MgSO4· 6H2O, respectively. Fig. S2 shows the XRD patterns of points H and I on the saturated line. By contrasting the XRD patterns for these points, it can be determined that the peak positions and peak types of different points that correspond to (K1-n,(NH4)n)2SO4·MgSO4·6H2O have little difference.

Fig. 6 presents the comparison of the isothermal

Table 2 Solubilities in the isothermal evaporation and crystallization process of quaternary system (NH4)2 SO4-MgSO4-

K2SO4-H2O at 25.0 ºC

No.

Composition of liquid phase

(wt%)

Composition of liquid phase

(g/100g dry salt)

Composition of wet solid phase

(g/100g dry salt) Equilibrium

solid phase

Point in phase

diagram MgSO4 (NH4)2SO4 K2SO4 MgSO4 (NH4)2SO4 K2SO4 H2O MgSO4 (NH4)2SO4 K2SO4

1 0.30 42.73 1.56 0.67 95.83 3.49 124.29 S + MN

2 0.26 40.98 2.53 0.58 93.64 5.77 128.51 S + MN

3 0.28 42.40 4.20 0.60 90.44 8.95 113.29 S + MN

4 0.27 42.34 5.96 0.57 87.17 12.27 105.85 S + MN

5 0.31 38.14 8.22 0.66 81.73 17.61 114.27 S + MN

6 0.24 37.29 10.77 0.49 77.21 22.30 107.06 S + MN

7 0.93 22.18 8.78 2.92 69.55 27.53 213.52 S + MN

8 1.35 19.87 9.59 4.37 64.49 31.14 224.59 30.07 36.63 33.30 S + MN J1, J2

9 2.18 23.61 13.39 5.57 60.25 34.17 155.25 S + MN

10 2.23 14.78 11.37 7.85 52.09 40.06 252.40 S +MN + M P1

11 2.84 13.07 12.74 9.91 45.62 44.47 249.02 S + M

12 3.22 9.77 11.15 13.36 40.47 46.18 314.23 28.72 19.39 51.88 S + M K1, K2

13 4.68 8.72 11.87 18.51 34.52 46.97 295.80 S + M

14 5.46 7.29 12.34 21.75 29.05 49.20 298.59 S + M

15 5.57 5.78 12.04 23.81 24.70 51.49 327.56 S + M

16 7.42 3.54 13.24 30.66 14.64 54.70 313.25 38.30 12.65 49.05 S + M L1, L2

17 8.87 2.36 11.93 38.31 10.19 51.50 331.70 S + M + MK P2

18 12.99 1.72 13.58 45.91 6.09 48.00 253.35 S + MK

19 3.35 11.38 8.93 14.16 48.09 37.75 322.72 M + MN H

20 4.13 10.77 8.46 17.67 46.13 36.20 328.11 43.86 41.19 14.95 M + MN I1, I2

21 5.66 7.96 7.68 26.57 37.38 36.05 369.71 M + MN

22 6.14 7.92 8.04 27.76 35.84 36.40 352.43 M + MN

23 7.21 5.11 7.07 37.18 26.37 36.45 415.91 M + MN

24 10.57 5.13 8.73 43.26 20.99 35.75 309.35 M + MN

25 11.58 4.04 8.73 47.56 16.61 35.83 310.66 45.77 23.37 30.86 M + MN U1, U2

26 10.89 3.05 7.89 49.87 13.98 36.15 357.97 M + MN

27 12.83 2.48 8.79 53.23 10.30 36.47 314.83 M + MN

28 13.89 1.28 8.89 57.74 5.30 36.95 315.73 M + MN + MK P3

29 10.05 2.29 10.79 43.45 9.90 46.66 332.26 M + MK

30 12.57 1.14 9.98 53.06 4.80 42.14 322.26 M + MK

31 16.57 1.19 6.25 69.02 4.96 26.02 316.50 47.26 7.39 45.34 MN + MK V1, V2

32 18.36 1.03 5.31 74.32 4.16 21.51 304.83 MN + MK

33 24.56 1.14 5.10 79.74 3.71 16.55 224.66 MN + MK

34 29.12 1.10 4.49 83.90 3.16 12.94 188.16 MN + MK + M6 P4

35 28.76 0.66 3.98 86.13 1.96 11.90 199.44 MK + M6

36 26.57 0.47 3.28 87.65 1.54 10.81 229.89 MK + M6

37 28.41 0.52 2.00 91.86 1.67 6.47 223.28 MK + M6 + M7 P5

38 28.20 1.74 4.56 81.73 5.05 13.23 189.82 M6 + MN

39 28.75 1.84 3.42 84.54 5.41 10.05 194.06 88.84 8.86 2.30 M6 + MN W1, W2

40 28.30 1.96 1.16 90.06 6.25 3.70 218.24 89.86 9.36 0.78 M6 + MN Y1, Y2

41 28.24 1.89 0.40 92.51 6.19 1.30 227.58 M6 + MN + M7 P6

42 28.99 0.70 1.40 93.26 2.24 4.49 221.67 96.18 1.28 2.54 M6 + M7 Z1, Z2

43 26.02 0.34 5.27 82.26 1.08 16.66 216.16 MK + M7

44 28.53 0.25 4.79 85.00 0.73 14.27 197.96 MK + M7

45 4.60 8.90 9.13 20.32 39.35 40.33 341.94 M Q1

46 7.71 5.67 8.98 34.49 25.35 40.16 347.47 M Q2

47 11.82 0.00 12.13 49.35 0.00 50.65 317.54 S + MK D

48 23.75 0.00 6.82 77.69 0.00 22.31 227.12 MK + M7 E

49 0.55 41.45 0.00 1.31 98.69 0.00 138.10 S + MN F

50 23.51 2.08 0 91.87 8.13 0.00 290.78 MN + M7 G

Acta Geologica Sinica (English Edition), 2021, 95(3): 1016–1023 1019

dissolution equilibrium phase diagram (Xue et al., 2016) and the isothermal evaporation phase diagram of the quaternary system (NH4)2SO4-MgSO4-K2SO4-H2O at 25.0ºC. The black lines represent the boundary lines of the crystallization field, which correspond to the isothermal dissolution equilibrium phase diagram. The red represents the boundary lines of the crystalline field, which

corresponds to the isothermal evaporation phase diagram. Compared with the dissolution equilibrium, the crystallized zones of MgSO4·6H2O are produced, while the crystallization zones of (NH4)2SO4 and K2SO4 disappeared in the isothermal evaporation phase diagram. The area of MgSO4·7H2O and K2SO4·MgSO4·6H2O decreased significantly, and the area of MgSO4·(NH4)2

SO4·6H2O increased. 3.2 Application of the phase diagrams of quaternary system (NH4)2SO4-MgSO4-K2SO4-H2O at 25.0ºC

By analyzing the invariant points and the crystallization fields of the isothermal dissolution equilibrium phase

Fig. 1. Phase diagram of the system (NH4)2SO4-MgSO4-

K2SO4-H2O in the isothermal evaporation and crystallization

process at 25.0ºC. Points A, B and C represent pure K2SO4 MgSO4 and (NH4)2SO4, respec-

tively.

Fig. 2. The partial enlarged phase diagram of the system

(NH4)2SO4-MgSO4-K2SO4-H2O in the isothermal evapora-

tion and crystallization process at 25.0ºC. Points B and C represent pure MgSO4 and (NH4)2SO4, respectively.

Fig. 3. Water-phase diagram of the system (NH4)2SO4-

MgSO4-K2SO4-H2O in the isothermal evaporation and crys-

tallization process at 25.0ºC.

Fig. 4. The partial enlarged phase diagram of Fig. 3.

Li et al. / Phase Diagrams and Application in Industry of Mineral Shoenite 1020

diagram (Xue et al., 2016) and the isothermal evaporation phase diagram, the process to produce K2SO4 and MgSO4·(NH4)2SO4·6H2O was developed using K2SO4·MgSO4· 6H2O and (NH4)2SO4 as raw materials. On the basis of the linear and lever rules, the liquid phase composition, wet solid composition and pure solid phase composition of the reactions were determined and verified by experiments. As shown in Fig. 7, the black lines represent the isothermal

dissolution equilibrium phase diagram and the red represents the isothermal evaporation phase diagram. The dotted lines form the technological process lines and the triangular points represent the corresponding process points. Fig. 8 displays the technological flowchart of the process, which has been analyzed as discussed below.

When a specific amount of (NH4)2SO4 (point C) was added to K2SO4·MgSO4·6H2O (point MK), the mixture composition point would move from MK to C along MKC in Fig. 6. As the point was located on the line MKT of the

Fig. 5. The XRD characterization of the equilibrium solid phase of invariant point.

Fig. 6. Comparison of the isothermal dissolution phase dia-

gram (Xue et al., 2016) and isothermal evaporation phase

diagram of the quaternary system (NH4)2SO4-MgSO4-K2SO4

-H2O at 25.0 ºC. Points A, B and C represent pure K2SO4, MgSO4 and (NH4)2SO4, respec-

tively.

Fig. 7. Phase diagram analysis of process application. Points A, B and C represent pure K2SO4, MgSO4 and (NH4)2SO4, respec-

tively.

Acta Geologica Sinica (English Edition), 2021, 95(3): 1016–1023 1021

crystallization field of K2SO4, K2SO4 would crystallize out of solution. On the basis of the lever rule and the phase diagram analysis, (NH4)2SO4 and H2O were added to KCl at a certain ratio to ensure that the mixture composition point was located on the intersection point M1 of line AN with line CMK, so that a relatively larger amount of K2SO4 could crystallize out. Therefore, point M1 was selected as the operating point. After stirring for 3 h in the three-necked flask at 25.0ºC, the composition of the liquid phase (point N) and wet solid phase (point O) were measured and results are shown in Table 3. After filtration and separation, the composition of the liquid phase reached point N. In order to separate MgSO4·(NH4)2SO4·6H2O from the mother solution, this solution corresponding to point N was recycled.

As shown in Fig. 7, the above mother solution (point N) fell into the crystallization field of K2SO4·MgSO4·6H2O of the isothermal evaporation phase diagram. Then a specific amount of water was evaporated isothermally at 25.0ºC so that the liquid phase composition reached point P. Meanwhile, the solid phase K2SO4·MgSO4·6H2O was precipitated, which corresponds to point MK. After filtration and separation, a certain amount of (NH4)2SO4 was added to the mother solution (point P) again so that the total composition of the system fell into the crystallization field of MgSO4·(NH4)2SO4·6H2O. According to the lever rule, once point M3 was selected as the operating point, the yield of the solid phase MgSO4·(NH4)2SO4·6H2O was maximized. After stirring for 3 h at 25.0ºC, the liquid phase (point Q) and wet solid phase (point R) compositions were determined, as shown in Table 3. Thus, the liquid phase composition reached point Q, and the corresponding solid phase composition reached point MN.

After filtration and separation, a certain amount of K2SO4·MgSO4·6H2O was added to the mother solution (point Q) so that the total composition of the system fell into the crystallization field of K2SO4. When the intersection point M4 of line NA and QMK was selected as the operating point, the liquid phase composition could return to the point N. The liquid phase (point N) and wet solid phase (point S) compositions were determined, as shown in Table 3. The process can achieve a cycle by

repeating the above steps. Employing 100 g of dry salt as the starting material, the

phase diagram calculation of the proposed process was carried out based on the material conservation law, the lever rule and the linear rule. The appropriate ratio of raw materials and amount of water added or evaporated for the four processes were determined and the calculation results are presented in Table 4. 4 Conclusions

The solubilities of the system (NH4)2SO4-MgSO4-K2SO4-H2O in the isothermal evaporation and crystallization process at 25.0ºC were measured and the corresponding dry salt phase diagram and water-phase diagram were plotted. As shown in the diagrams, there are six saturation points and six crystallization fields, which correspond to (K1-m,(NH4)m)2SO4, (NH4)2SO4·MgSO4· 6H2O, K2SO4·MgSO4·6H2O, MgSO4·6H2O, (K1-n,(NH4)n)2

SO4·MgSO4·6H2O and MgSO4·7H2O, respectively. By analyzing the invariant points and the crystallization

fields of the isothermal dissolution equilibrium phase diagram and isothermal evaporation phase diagram, the separation process to produce potassium sulfate and N-Mg compound fertilizer, boussingaultite, was proposed via the mineral shoenite and the industrial by-product ammonium sulfate. In the resulting process, the mother solution was completely recycled, for which the yield of potassium sulfate was increased to 91.62% and the magnesium resources were fully utilized. Meanwhile, the appropriate ratio of raw materials and amount of water added or evaporated for the four processes were determined.

Simple theoretical calculations for the proposed process

Fig. 8. Technological flowchart of K2SO4 and N-Mg compound fertilizers produc-

tion via K2SO4·MgSO4·6H2O and (NH4)2SO4.

Table 3 Composition of technological process points

Point in

diagram

Composition (wt%) Composition (g/100g dry salt)

MgSO4 (NH4)2SO4 K2SO4 MgSO4 (NH4)2SO4 K2SO4 H2O

N 11.47 1.09 11.30 48.08 4.57 47.35 319.00

O 18.58 1.90 43.59 29.00 2.96 68.03 56.08

S 9.87 0.90 82.85 10.54 0.96 88.50 6.82

P 11.79 1.52 10.20 50.15 6.47 43.38 325.21

Q 0.92 24.68 8.40 2.70 72.59 24.71 194.13

R 23.14 36.44 3.88 36.45 57.43 6.12 57.57

Li et al. / Phase Diagrams and Application in Industry of Mineral Shoenite 1022

were accomplished, in which the composition of products was determined; the results provide guidance for process applications. Compared with the production process of potassium sulfate from two-times conversion, this new separation technology shows the advantages of effective utilization of potassium and magnesium resources. Acknowledgements

This project was supported by the National Natural Science Foundation of China (grant No. 21576066), the Natural Science Foundation of Hebei Province, China (No. B2017202268) and the Research Fund Program of Guangdong Provincial Key Lab of Green Chemical Product Technology (No. GC201816). Susan Turner (Brisbane) assisted with English language.

Manuscript received Apr. 19, 2019

accepted Sep. 15, 2019 associate EIC: LIU Lian

edited by Susan TURNER and FANG Xiang Supplementary data to this article can be found online at

http: //doi.org/10.1111/1755-6724.14409.

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Table 4 The phase diagram calculation results of K2SO4 and N-Mg compound fertilizers production via K2SO4·MgSO4·6H2O

and (NH4)2SO4

Changing process System Composition (g/100g dry salt) Dry salt amount

(g)

Amount

(g) MgSO4 (NH4)2SO4 K2SO4 H2O

MK+C+H2O→M1→N+A

MK 40.86 0.00 59.14 36.69 96.08 131.33

C 0.00 100.00 0.00 0.00 3.92 3.92

H2O 227.92

A Precipitated 0.00 0.00 100.00 0.00 17.50 17.50

Solution N 48.08 4.57 47.35 319.00 82.50 345.68

N-H2O→P+MK

N 48.08 4.57 47.35 319.00 82.50 345.68

H2O 49.81

MK Precipitated 40.86 0.00 59.14 36.69 19.04 26.03

Solution P 50.15 6.47 43.38 325.21 63.46 269.84

P+C+H2O→M3→Q+MN

P 50.15 6.47 43.38 325.21 63.46 269.84

C 0.00 100.00 0.00 0.00 110.24 110.24

H2O 39.52

MN Precipitated 47.67 52.33 0.00 42.81 60.34 86.17

Solution Q 2.70 72.59 24.71 194.13 113.36 333.43

Q+MK+H2O→M4→N+A

Q 2.70 72.59 24.71 194.13 113.36 333.43

MK 40.86 0.00 59.14 36.69 2040.48 2789.13

H2O 4586.89

A Precipitated 0.00 0.00 100.00 0.00 412.27 412.27

Solution N 48.08 4.57 47.35 319.00 1741.57 7297.18

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About the first author

LI Cheng, male, born in 1996 in Shanxi Province of China, is a Ph. D. candidate of Georg-August-Universität Göttingen. Now his research interest is chemical modification of cellulose. He was mainly engaged in research of phase equilibrium of water-salt system during his master’s period in Hebei University of Technology. E-mail: chengli96@foxmail.com.

About the corresponding author CAO Jilin, male, born in 1965, received his postdoctoral position from Tianjin University, and is now a professor at Hebei University of Technology. His current interests include the study of water-salt phase equibrium, FGD gypsum, potash feldspar and whisker material. As first or correspondent author, more than 120 academic papers have been published in important academic journals such as Fluid Phase Equilibria, The Journal of Chemical

Thermodynamics, Journal of Solution Chemistry, Journal of Physical Chemistry, Journal of Chemical Engineering of Universities, etc. E-mail: caojilin@hebut.edu.cn.