Advanced Materials Research
ISSN: 1662-8985, Vols. 834-836, pp 519-522
doi:10.4028/www.scientific.net/AMR.834-836.519
© 2014 Trans Tech Publications, Switzerland
Online: 2013-10-31
The ternary system phase equilibrium of KCl-NH4Cl-H2O at 80˚C
Bin Zhao1,a, Guoyue Geng1,b, Jianxin Chen 2,c*, Shaofeng Zhang2,d,Yinhui Li2,e
and Aidang Lu2,f
1
School of Chemical Engineering, Hebei University of Technology, Tianjin 300130, China;
2
School of Ocean Science and Engineering. Hebei University of Technology, Tianjin 300130,
China
a
[email protected],[email protected],c*[email protected],[email protected],
e
[email protected], f [email protected]
Key words: phase equilibrium; potassium chloride; ammonium chloride; phase diagram
Abstract. The ternary system of solid-liquid phase equilibrium relation of KCl-NH4Cl-H2O was
studied at 80 ˚C. The solubility data of KCl and NH4Cl in water were obtained by isothermal
equilibrium method, and the phase diagram of the system was plotted. The result showed that this
system exist six phase regions: i.e. the unsaturated solution region, pure KCl , pure NH4Cl, the solid
solution base of KCl, the solid solution base of NH4Cl and their common crystallization region
with the solid solution base of KCl and the solid solution base of NH4Cl crystallization regions.
Some crystallization area will be changed with the various temperature, According to the phase
diagram, it can be calculated the quantity of evaporated water in the crystallization process of
KCl-NH4Cl-H2O and controlled the change of concentration in the evaporation process preferably.
Introduction
The salt-water system is a complex reaction systems in nature (ocean, salt lakes) and inorganic
chemical industry (inorganic salts production)[1].In order to understand the characteristic of the
phase transformation (crystallization, evaporation, cooling, dissolution, separation, etc.) of the
salt-water system, it is important to know the solubility data and phase equilibrium rule of the salts
under the equilibrium state. It can be seen that these rules have important application values in the
chemical production process [2].
In recent years, many scholars have carried out studies on the phase equilibrium of the
potassium-containing salt-water system [3-5] as well as a large number of experiments to
explore different water-salt system with K+[6-9]. Some literatures[10] showed that when
the K+ and NH4+ coexist, the solution can easily form a solid solution owing to the similarity of
K+ and NH4+, which causes great difficulty for the purification of the potassium salt. For
the KCl-NH4Cl-H2O system, Ren [11] , Zhao [12] performed an experiment on the stable phase
equilibrium of KCl-NH4Cl-H2O system respectively at 25 ˚C and 35 ˚C, and their phase diagrams
at two temperatures are similar. They, both have the similar KCl and NH4Cl single salt zone as well
as the presence of discontinuous solid solutions (K,NH4)Cl, (NH4,K)Cl and the co-crystallization
region of two solid solutions[13]. Farelo [14] further obtained the experimental solubility and the
miscibility of KCl, NH4Cl at 25 ˚C to 60˚C under saturated condition.
In our work, a new double chamber and double tube evaporator [15] was applied to improve the
evaporation process, it need to know the rule of phase changes in the higher temperature, the
solubility and equilibrium phase diagram of this system at 80 ˚C need to be further determined.
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Research in Materials and Manufacturing Technologies
Experimental Section
Reagents and Instruments. Potassium chloride, ammonium chloride (analytical reagent grade
chemicals, Tianjin Damao Chemical Reagent Factory); water (double-distilled water); 501-A type
super thermostat (Shanghai Laboratory Instrument Works Co., Ltd.), crystallizer, electronic balance
(Shanghai Precision & Scientific Instrument Co., Ltd.).
Experimental process. The phase equilibrium was performed in a jacked crystallizer heated by
circulating water bath within ± 0.1 ˚C. The phase equilibrium time determination: after stirring for
10 hours, its sample was taken out for the analysis of its liquid phase composition every 0.5 hours,
when the compositions did not change, it meant this system achieved equilibrium state. The reaction
time was determined on 12 hours.
The equilibrium compositions of the liquid phase in the system were determined by a chemical
analysis method, while the morphology and compositions of the solid phase in the equilibrium state
were determined by a "wet slag method"[16].
Results and Discussion
The solubility measurement and drawing of phase diagram. The isothermal dissolution
equilibrium method was used for measurement of the solubility of KCl-NH4Cl-H2O ternary
salt-water system at 80 ˚C, and the wet slag method was used for determine of solid phase
composition, its detailed data were shown in Table 1.
Table.1 The phase equilibrium data of KCl-NH4Cl-H2O system at 80˚C
Composition of liquid phase/%(wt)
Composition of wet solid/%(wt)
NH4Cl
KCl
H2O
NH4Cl
KCl
H2O
39.43
0.00
60.57
100.00
0.00
0.00
39.36
1.09
59.55
98.93
0.00
1.07
38.75
1.88
59.37
97.12
0.00
2.88
37.33
4.01
58.66
96.69
0.00
3.31
35.41
5.83
58.76
94.15
0.00
5.85
35.15
6.31
58.54
88.86
1.64
9.50
34.34
7.72
57.94
85.06
3.91
11.03
33.56
9.28
57.16
81.56
8.17
10.27
32.73
10.56
56.71
70.57
14.45
14.98
32.73
10.56
56.71
21.66
71.30
7.04
31.33
11.06
57.61
20.71
72.35
6.94
28.20
12.36
59.44
17.12
75.18
7.70
26.35
13.44
60.21
15.40
79.77
4.83
24.39
14.60
61.01
13.44
77.94
8.62
22.87
16.05
61.08
13.30
80.09
6.61
21.54
17.12
61.34
11.77
82.33
5.90
20.57
18.26
61.17
10.02
84.16
5.82
19.01
19.62
61.37
8.46
84.53
7.01
16.92
21.20
61.88
7.40
88.13
4.47
15.28
22.56
62.16
6.57
86.55
6.88
13.77
23.89
62.34
5.77
88.20
6.03
10.49
26.21
63.30
2.72
91.86
5.42
7.52
28.43
64.05
0.92
95.43
3.65
Advanced Materials Research Vols. 834-836
521
6.35
29.14
64.51
0.00
96.18
3.82
3.67
31.05
65.28
0.00
97.75
2.25
1.72
32.32
65.96
0.00
98.02
1.98
0.85
33.02
66.13
0.00
99.08
0.92
0.00
33.34
66.66
0.00
100.00
0.00
The isosceles right triangle representation method was used to draw the phase diagram of
KCl-NH4Cl-H2O ternary salt-water system at 80 ˚C, (shown in Fig. 1).
C
100
90
G
80
70
NH4Cl/%(wt)
60
50
40
A
F
E
30
N
20
10
M
0
0
10
20
30 B 40
D
50
60
70
80
90
100
KCl/%(wt)
Fig.1 Phase diagram of KCl-NH4Cl-H2O ternary system at 80˚C
From Fig. 1, the phase diagram of KCl-NH4Cl-H2O is a complex system at 80 ˚C, which
contains five crystallization regions: there are the pure NH4C, pure KCl, the solid solution base of
NH4Cl and the solid solution base of KCl crystallization region. AF and BM are solubility curves of
pure NH4Cl and pure KCl respectively. The NH4Cl-based solid solution whose composition was
continuously changed from the C point of the pure NH4Cl to G of that had a solubility curve of FE;
The KCl-based solid solution whose composition was continuously changed from the D of the pure
KCl to N of that had a solubility curve of ME; in which, the E was their co-saturation point.
The compositions of five key points in phase diagram are: E (NH4Cl mass fraction was 32.73%,
KCl mass fraction was 10.56%); F (NH4Cl mass fraction was 35.41%, KCl mass fraction was
5.83%); M (NH4Cl mass fraction was 6.35%, KCl mass fraction was 29.14%); N (NH4Cl mass
fraction was 21.66%, KCl mass fraction was 71.30%); G (NH4Cl mass fraction was 70.57%, KCl
mass fraction was 14.45%);
Discussion. Due to the similar ionic structure of KCl and NH4Cl and other similarities between
them, they can easily form solid solution when they were crystallized from aqueous solution.
From the Fig. 2, the NH4Cl crystallization regions are significantly larger than the KCl
crystallization area. The solubility of NH4Cl is 39.43% at 80 ˚C, and the solubility of KCl is
33.34%, thus the salt that has smaller solubility will easier to be precipitated. So the solubility
relationship can be used for the separation of NH4Cl and KCl. If the mass fraction of NH4Cl can be
controlled within 6.35%, the KCl crystals can be almost precipitated, and if the mass fraction of
KCl can be controlled not more than 5.83%, the NH4Cl can be precipitated too. Similarly, in the
evaporation of the double chamber double tube pass evaporator, the amount of evaporated water at
a position can be calculated theoretically according to the phase diagram law, so as to control the
evaporation conditions to further reach the desired concentration of the solution.
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Research in Materials and Manufacturing Technologies
Generally, the larger crystallization region will result larger covered area of the whole
coordinate, and the solid phase is easier to be precipitated. When comparing with the phase diagram
of KCl-NH4Cl-H2O at 25 ˚C in literature [10], we found that the crystallization area of KCl increased
with the rising temperatures, while the crystallization area of NH4Cl decreased with the rising
temperatures. Therefore, relative low temperature is favorable to the precipitation of NH4Cl and
higher temperature is more conductive to the precipitation of KCl in experiments.
Conclusions.
According to research on phase equilibrium, the solubility and the phase diagram of the
KCl-NH4Cl-H2O ternary salt-water system were determined at 80 ˚C,The phase diagram contains
six phase regions, they are the unsaturated solution area and five crystallization regions: the pure
NH4Cl, pure KCl, the solid solution base of NH4Cl, the solid solution base of KCl crystallization
region, and their common crystallization region with the solid solution base of NH4Cl and the solid
solution base of KCl. Comparing with the phase diagrams at 25 ˚C and 35 ˚C, the crystallization
area of KCl will be increased, while the crystallization area of NH4Cl decreased with the rising
temperatures.
Acknowledgements
Thank the supports by National Natural Science Fund of China (21276063, 21302038, 51309074),
PCSIRT(IRT1059), Hebei province science and technology support program, and SRF for ROCS,
SEM, China.
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