Indi an Journal of Chemi stry
Vo l. 42A, January 2003, pp. 35-4 1
Phase diagram of thermodynamic non-equilibrium state of
MgO-B20 3-18%MgCb-H20 system at O°C
Li xia Zhu, Shiyang Gao* & Shuping Xia
Xi'an Branch, Inst itute of Salt Lakes, Academ ic Si nica, Xi'an, P.R.China,71 0043
Received 3 JUli e 2002 ; revised II SepTember 2002
The supersaturated soluti ons of MgO-B zOr I8%MgC lr HzO with different mole ratios of MgO:B 20 , ha ve been
prepared and kept at 0±0. 1°C. The crystalli zati on paths of bori c ac id , Mg-borates and Mg-ox yc hloride from the so luti ons
have been determined by chem ical analysis of the solution samples take n at a give n time interval during crystalli zati on. and
then the liquid-solid phase diagram of thermodynami c non-equilibrium state of MgO-B zO,- 18%MgC lz-HzO system at O°C
has been constructed. In thi s phase diagram, fi ve crystal li zati on fields appear, boric ac id (H, BO,), mca lli sterite
(MgO·3 B20 , ·7 .5H20), hungtsai te (MgO'2B 20 ,'9 HP), inde rite (2 MgO'3B 20 , ' 15HcO) and magnesium oxyc hl oride
(5 Mg(OH)2· MgCI2·8 H20) .
A few of the salt lakes on Qinghai-Xizang plateau are
abundant in borate minerals I . Oa Chaidan salt lake
belongs to mag nesium su lphate sub-type according to
Va li as hko's classification of salt lakes. As indicated in
th e previous work 2, the Mg-borate does not generally
crystalli ze out until th e dynamic max imum solubility
of Mg-borate, and mi ght be enriched in the
co ncen trated brine during solar evaporation of the
brine taken from Oa Chaidam salt lake. A dyn am ic
maximum solubility of Mg-borate in the co ncentrated
brine has bee n determined to be 7.5%MgB 4 0 7 , and
even hi gher' . Therefore, the concentrated brine
containing hi gh borate might be cons idered as MgOB ~ O, -MgCI2-H 2 0 supersaturated soluti on.
In order to understand both the formation condition
and crystalli zation reaction mechani sm of Mg-borate
hyd rates, and utilize the reso urces of the salt lake
brine in Qaidam Bas in , it is necessary to study the
phase diagram of thermodynamic non-equilibrium
state of MgO- B20 ,-MgCI 2-H20 system. The phase
diagra m of thermodynam ic non-equilibrium state of
MgO-B 2 0 r 2S%MgCITH 2 0 system at lODC has been
earlier reported 4 . In this paper, the liquid-solid phase
diagram of thermodynami c non-eq uilibrium of MgOB20 .1 -18%MgCh-H20 system at ODC has bee n
prese nted.
Materials and Methods
Both H, BO, and MgC I2'6H 20 are AR grade
chemicais produced by Xi' an Chemicals Factory and
MgO with soluble act ive were prepared by calcining
Mg(OH h AM gCO, '6H20 (A R grade produced by
Beijng Chemicals Factory) in an electric test furnace
at 600°C for three hours. At first, MgCh·6H 20 was
dissolved completely in a given amount of redi still ed
water ,and then H3 BO, and at last MgO were added to
th e solution, and stirred at 60 DC unti I H.1 BO.1 and
MgO were dissolved completely and then filtered.
The supersaturated soluti on of MgO-B ~ O.1 18%MgCh-H 20 with different mole rati os of
MgO:B 20 3=1:0, 2.5:1 , 1:1, 1:1.5 , 1:2, 1:2.5,1: 3, 1:5,
1:7 and 0: I were prepared as above. All these
supersaturated solutions were put in a glass co ntainer
with a closed cover and set in a water- alcohol
isothermal bath of O±O.I DC wi thout stirrin g. The
experiments of crystalli zati on kinetics have bee n
conducted by th e method described prev iously).
When a solid phase began to crystalli ze out from the
so lution, the liquid samples were taken for anal ysis at
a given time interval, mea nwhile their dens ity and p H
va lue were determinecl until the density remained
constant. Finally, the solid phase was separated.
was hed with I: I alcohol-glyco l (V/v) mi xed so lve nt,
th en abso lute alcohol, and at last with ether, and
stored in desiccator till constant weight was attained
at room temperature. The sol id phases were identi fi ed
by IR spectroscopy , X-ray powder diffraction,
thermal analysis and chemical analysis. The chemi ca l
analysis methods were as foliows: mag nes ium was
titrated with Na-EOTA soluti on in an alkaline
co ndition by add ing NH,' H20 + NH.jC! buller
solution , boron was titrated by a stand ard so luti on of
NaOH in the presence of mannitol and ch lorid e was
titrated by a standard soluti on of Hg(N0.1 )2 using
diphenylcarbazone as indi cator.
36
IN DI AN J C HEM, SEC A. JAN UA RY 2003
p H-m ete r (P H S-I OA mode l with a g lass el ectrode
and ca lo me l e lec trode) was co rrected usi ng a standard
so luti o n of 0 .05 M KHC s H 4 0 4 (P H=4.00 at 20°C),
0. 025 M in both KH 2P04 and N a2 HPO" (pH=6.88 at
20°C) a nd 0 .0 I M Na28 40 7 (PH=9. 88 at 20°C) .
200
400
600
800 11h
20
40
60
80 lid
0.8
Resu lts and Discussion
Th e crystallization kin etics of th e supersaturated
solution of MgO-B 20,l -J 8%MgClr H2 0 systelll at OOC
The ex perimental data of c rys ta lli zati o n kin etics are
presented in Tabl e I. All the c rystalli zati o n kineti c
c urves (Fi gs I and 2) sho w that th e co ncentrati o n o f
B2 0 3 o r MgO in so luti o n decreases ex po ne nti a ll y with
time until the de nsity o f th e so luti o n does no t ch ange
any mo re. The crys tall izatio n processes can be
d ivided into three stages: inducti o n peri od, grow th
and equilibrium pe riod . In th e first stage, the
co mposi ti o n of th e liquid ph ase is hardl y ch anged,
and it mean s th at no so lid phase separated o ut.
Therefore, during thi s peri od , the kinetic curve is
almost para ll e l to time ax is. It is no ti ced that the
supersaturated so luti o ns o f MgO :8 2 0 3 = I: I , I : 1.5, 1:2,
2.5: I and MgO-1 8% MgCl r HzO have a lo nger
crys ta ll izati o n induc ti o n peri od , w he reas th e o ther
supersaturated so luti o n w ith th e res t o f MgO :B2 0 3
mo le rati o has a sho rter c rystalli zati o n indu cti o n
peri od. The c rystalli zati o n kineti c curve o f
MgO '2 B20 r 18% MgC h -H 20 s upersaturated soluti o n
has two indu cti o n peri ods whi ch are parall e l to time
coo rdinate axis, a nd it sho ws that w he n th e first
crystalli zed so lid ph ase has e nded, th e c ry stalli zation
inducti o n peri od of the second so lid phase fo ll ows at
o nce. Th e supersaturated so luti o n of M gO ·2.5B 20 318%MgC b-H 2 0 also has two so lid ph ases cry stallized
o ut, but there are no cl ear two stages. The
supersaturated soluti o n with th e o th er d iffe rent
MgO :B 20 3 mo le rati o crys talli zed o ne so lid ph ase o ut.
Th e crysta llin e g rowth is the main process, whi ch
revea ls the reacti o n rate o f crystalli zati o n. Whe n the
crystalli zati o n reacti o n reaches the the rmody namic
equilibrium , the co ncentrati o n o f liquid ph ase will no
lo nger change. This co mpositi o n of the so luti o n is the
conce ntrati o n at the rmody na mi c equi librium state,
and it is called the so lubil ity o f the nnody na mic
equilibrium .
5
The fo ll o wing mathe matical mode ls were used :
Fig. I-CD"o.r t
c urves
of
MgO-13 "O:.- IS'/(,
MgCi "-H, O
supe rsaturated solut io n at DoC MgO:13 , O , : G 1:2. @ 1:2.5 . • 1:3 .
... 1:5, _ 1:7, t:;. 0: I
50
100
150 lid
I
0.15
0.6
~
-
~
-0E
0.10
'"
..5
,
0.4 :::::
•
J
U
0.05
0.2
'+
400
800
1200
----"--
160011h
Fig .2-C llp,IMgO)-t c urves
of
MgO-B,O,- IS% MgCi "- H"O
s upe rsa tu rated so luti o n at DoC MgO:B,O, : 0 2.5: I. ... I: I . •
I: 1. 5, t:;. 1:0
(ii ) Th e mode l o f mo no nuc lear
g row th (MB )
laye r co nt ro ll ed
-deldt=k(Co- C)4/\ C-Ccy.,y'
(ii i) The mode l of co nstant s urface area (MC)
- deldt=k(c-c,xY
w here k is the rate constant of crysta ll izati o n reac ti o n.
(i) Th e mode l of
g ro wth (MA )
po lyn uc lear
layer
con tro ll ed
Co is the ini tial co ncentrati o n, c is th e co nce ntrati o n of
com po ne nt in so luti o n at a g iver: time t during
c rysta ll izati o n,
Coo
is
the
CGncentratio n
of
the rmody nam ic equilibrium .
37
ZHU el al. : PHASE DIAGRAM OF MgO- B20 r I8% MgCI2-H20 SYSTEM
Table I- Chemical composition of MgO-B 20 r 18%MgC lr H20 supersaturated soluti on durin g crystalli za ti on of Mg-borates at O°C
MgO:8 ~ 0)
No
Mol:mol
Time
(h)
MgO
(w t%)
1:0
2.5:1
I
2
3
4
0
12.2
15 .0
50.0
0.230
0.191
0.163
0.090
I
2
3
4
0
544
760
1336
0.46
0.41
0.32
0.25
2
3
4
5
0.0
0.5
1.0
6.5
10 1.5
2
3
4
0
50
125
158
17.51
17.52
17.52
17.50
3.0 1
2.52
2. 10
1.20
0.33
0.29
0.29
0.26
17.55
17.56
17.56
17.57
5.70
5.09
4.07
3. 14
1.1 8
0.97
0.82
0.55
0.52
17.82
17.85
17.90
17.91
17.94
0.85
0.82
0.68
0.65
1.49
1.47
1.39
1.37
17.90
17.94
18. 10
18. 15
0
42
56
67
77
95
120
130
186
1.03
0.94
0.85
0.77
0.72
0.64
0.50
0.42
0.30
2.65
2.50
2.34
2. 14
1.94
1.67
1.28
1.10
0.71
2
3
4
5
6
7
8
9
0
41
46
48
49
50
52
63
80
1.01
0.98
0.96
0.87
0.74
0.54
0.36
0.32
0.24
2
3
4
5
6
7
8
9
10
11
12
13
0
3.2
6.2
14.2
22.2
27.7
28.7
29.0
29.25
31.25
35.75
57.75
12 1.75
0.90
0.82
0.79
0.75
0.69
0.63
0.58
0.53
0.46
0.41
0.3 1
0. 17
0.07
0:1
1: 1
1: 1.5
I
2
3
4
5
6
7
8
9
1:2
Liguid Ehase
Composition phase diagram index
MgO
8 20 )
MgC I2
B2O.1
(w t%)
(w t%)
(11101%)
(mol%)
I
1:2.5
Density
(kg' L- 1)
pH
Solid
MgC I2
(mol %)
iU8
96.99
97.48
97.90
98.80
1.0792
1.0786
1.0784
1.0778
8.18
8. 15
8.1 2
SA
SA
SA
2.38
2.12
2.1 4
2. 15
91.92
92.79
93.79
94.71
1.1619
1. 16 14
1.1 603
1.1594
8.40
8.29
8. 16
8.06
S8
S8
S8
8.30
6.92
5.90
4.03
3.81
91.70
93.08
94. 10
95.97
96 .1 9
1.1648
1.164 1
1.1628
1.1 6 19
'1. 16 15
2.51
2.79
2. 85
3.0 1
3. 12
SC
SC
SC
SC
9. 16
8.20
7.45
7. 13
9. 12
9.1 8
8.80
8.69
8 1.72
82.62
83.75
84. 18
1. 1863
1.1 863
1.1 84 1
I. 1833
7.8 1
7.82
7.60
7.58
SO
SO
SO
18.25
18.24
18. 16
17.73
17.48
17.34
17.12
16.65
16. 15
10.03
9.33
8.62
8.09
7.75
7. 13
5.84
5.0 1
3.99
14.9 1
14.3 1
13.69
13.02
12. 16
10.8 1
9. 17
7. 89
5.49
75.06
76.36
77.69
78.89
80.09
82.06
85.29
87.10
90.52
1. 2034
1.20 10
1.1953
1.1 888
1.1 84 1
1.1772
1.1 597
1.1591
1.1490
7.2 1
6.95
6.88
6.78
6.79
6.92
6.9 1
6.90
6.9:
SE
SE
SE
SE
SE
SE
SE
SE
3.46
3.32
3. 15
2.85
2.50
1.74
1.00
0.83
0.69
17.90
17.93
17.94
18.10
18.34
18.65
19.02
19.08
19. 17
9.55
9.39
9.28
8.54
7.47
5.73
3.97
3.59
2.75
18.9 1
18.3 1
17.56
16.20
14.54
10.67
6.44
5.41
4.56
71.54
72.30
73.16
75 .26
77.99
83.60
89 .59
9 1.00
92.69
1.2 100
1.204 1
1.2026
1.1995
1.1 959
1.1 882
1.1 809
1.1 793
1.1780
6.55
6.32
6.32
6. 32
6.28
6.26
6.27
6.28
6.27
SF
SG
SG
SG
SG
SG
SG
SG
3.85
3.66
3.49
3.2 1
2.98
2.72
2.38
2.23
1.97
1.68
1.42
0.87
0.56
17.96
18.0 1
18. 14
18.30
18.45
18.60
18.63
18. 53
18.62
18. 75
18.79
18.82
15.68
8.36
7.78
7.55
7.27
6.80
6.3 1
5.86
5.44
4.83
4.41
3.36
1.99
0.98
20.77
20.06
19.26
17.94
16.86
15.6 1
14.00
13.36
12.03
10.43
9.05
5.83
4.6 1
70.87
72. 16
73. 19
74.79
76.34
78.08
80. 14
81.20
83.14
85. 16
87.59
92. 18
94.41
1.2059
1.2046
1.2032
1.2006
1.1 989
1.1962
1.1924
1.1 895
1.1 875
1. 1840
1.1 8 13
1.1742
1. 1564
6.34
6.39
6.44
6.45
6.26
6.23
6.23
6.26
6.23
6.23
6. 13
5.77
6.05
SH
SH
SI
SI
SI
SI
SI
SI
SI
SI
SI
SI
CO/lld
INDIAN J C HEM , SEC A, JANUARY 2003
18
Table I-Chemical composition of MgO-8 20 .d 8%MgCl z-H 20 supersaturated sol uti on during crystallization of Mg-borates at O°C-Collld
MgO:8 2O.1
Mol:mol
0
T ime
(h)
MgO
(wt %)
1:3
1
2
3
4
5
Density
(kg'L- 1)
pH
Sol id
MgCl z
(mol %)
7
0
2
42
172
398
614
974
0.86
0.84
0.75
0.62
0.50
0.41
0.33
4.25
4.i I
3.83
3.06
2.44
2.06
1.61
18.02
18.05
18.16
18.40
18.64
18.77
18.97
7.87
7.75
7.05
6.10
5. 11
4.30
3.56
22.48
21.91
20.81
17.40
14.41
12.49
10.03
69.65
70.34
72.14
76.50
80.48
83.21
86.41
1.2051
1.2048
1.2024
1.1954
1. 1902
1.1871
1.1833
5.30
5.32
5.34
5.43
5.48
5.54
5.57
SJ
SJ
SJ
SJ
SJ
SJ
I
2
3
4
5
6
7
0
4
24
34
58
li S
166
0.55
0.53
0.43
0.28
0. 18
0.10
0.06
4.62
4.36
3.49
2.40
1.68
1.19
0.9S
17.44
17.48
17.8 !
18.24
18.51
18.67
18.78
5. 19
5.12
4.32
3.00
1.97
1.13
0.70
25.21
24 .1 3
20.22
14.78
10.82
7.93
6.6 1
69.60
70.75
75.46
82.22
87.21
90.94
92 .69
1.1958
1.1956
1.1904
1.1824
1. 1772
L I730
1.1712
4.14
4.36
4.50
4.79
5.06
5.17
5.20
SK
SK
SK
SK
SL
SL
0
0.182
0. 187
0.161
0. 147
0.1 36
0.128
2.20
2.20
l.94
1.68
1.56
1.39
17.71
17.78
17.82
17.87
17.95
17.92
2.04
2.08
1.83
1.70
1.58
1.50
14.23
14. 17
12.72
11.20
10.45
9.45
83 .73
83.75
85.45
87.10
87 .97
89.05
1.1749
1.1749
1.1734
1.1719
1.1716
1.1712
4.54
4.60
4.90
5.1 5
5.26
5.37
SM
SM
SM
SM
SM
()
1:5
Liguid Ehase
Composit ion phase diagram index
8 20)
MgO
MgCl z
8 20 .1
(wt %)
(w t %)
(mol %)
(mol %)
1:7
2
3
4
5
6
I
12
36
72
303
SA, S8 : 5Mg(OHh'MgCl2'8HzO; SC,SM: H.1 BO); SD,SE: 2MgO'3B zO)'15H 2O; SF,SH.SJ.SK:
MgO'213 20 y9H,0; SL: MgO·3B zO)·7.5H 20+ H)BO.1
The calculated method is the sa me as that of
literatures. The fi rst point where the so lid phasc
crysta lli zed out on the kinetic curves is considered as
the initial point and the experimental data were fitted
by computer. The selected standard of the calcu lated
va lue is th at the calculated error is no more than 5%,
Coo valuc is less than or approaches the concentration
of the last experimental point. The fitted kineti c
equatio ns have been obtained as following.
The supersaturated solutions a nd th e kinetic
eq uations are as foilows:
(cMgO-18 % MgCI2-H 20,
-dc/dl=0.1179
0.024 1 I )o n2 ;
2.5MgO· B 20 J -18 %M gC I2-H 20 ,
-dc/dt=0.044 I
(0. 1328-c)2/3
(c-0.05914)121 ;
MgO'B 2 0 r 18% MgCl r H 20 , -dc/dt =0.6089 (0.2506C)2/3 (c_0.1518) i80; an d MgO'28 20 J -18 % MgCb!-hO( I ),-dcldt =0.4016 (c-0.5954) 1.21.
The kinetic equations for MgO'28 20 y 9H 20
crystalli zed out from the supersaturated so lution of
MgO '28 20 3-l8 % MgCIz-H 20 system at O°C are:(the
crystalli zed so lid happened to collapse in the course
of crystallization, so that kinetic curve is divided into
two stage.):
The first stage: -dc/dt=0.5784 (0 .5884-c)2/3 (c0.3706)139;
the
second
stage:
-dc/dt=4.697 1
MgO' 38 zO)' 7.5H 1O; SG .SI:
(0. 1782-c)2/3
(c-0.1002)1.69;
MgO ·2.58 20 3! 8%MgC/z-H 20( I ), -dc/dt =0.6379 (0.6725- c)2/3 (c0.3481) 1.47; and MgO·2 .5B 2 0 r 18% MgCl z- H20(2),
-dc/dl =1.5400 (0 .2857-c)2/3 (c-0.029 1).154 .
The kinetic eq uati on for MgO·38 2 0 3·7.5 H20
crys tal lized o ut from the supersatu raled solu ti on of
MgO '3 B 20 r 18% MgCb- H20 System at O°C was (the
crystallizati o n kinetic curve can be divided into two
stages):
The first stage: -dc/dt=O.1213 (0 .7357 _C)2/3 (c0 .3789)213; the seco nd stage: -dc/dt=0.4463 (0.457 1C)4/3 (c_0. 18 19)262;
MgO'58 20 r 18%MgCb- HzO ,
-dc/dl =0. 1541 (0.7935-c)4/3 (c_0. 1215 )173; and
MgO·78 20 r 18% MgC/z- H20 ,
-dc/dt
= 141.9584
(0.37 13_C)2I3 (c-0.1982t
The results showed that when MgO:8 2 0 .1 in initial
solution is equal to 2.5: I , I : I , I :2, 1:2.5, 1: 3( I) and
1:7 respectively, the reaction mechanism of
crystallization belongs to polynuclear layer controlled
growth,
when
MgO :B 20 3= I :3(2)
and
1:5,
mononuclear layer controlled
growth.
When
MgO:B 20 3 = 0: 1 a nd I :2( I), the mechani sm of
crystallization are consistent w it h the model of
constant surface area.
ZHU el al.: PHASE DIAGRAM OF MgO-B 20 r I8% MgC lr H20 SYSTEM
39
Table 2-Chemical composi ti on of solid phases
MgO:B 20 3
MgO
(wt%)
1: 0
2.5 : 1
1: 1
1: 1.5
I :2( I)
1:2(2)
1:2.5( 1)
1:2.5(2)
1:3
1: 5( I)
1:5(2)
l:7
0:1
37.38
37.50
14.22
14.29
10.53
11.90
10.47
11.60
10.52
10.2 1
6.80
ComQosition
B20 3
MgCI2
(w t%)
(wt%)
17.52
17.49
36.57
36.27
53.76
41.37
53.02
40.22
53.61
54.42
56.32
55.02
56.28
Mol rati o
MgO: B20, : MgC I2 : H2O
H2O
(wt%)
45.10
45.01
49.21
49.44
35.71
46.73
36.51
48. 18
35.87
35.37
43.68
38. 18
43.72
5.05
5.07
2.00
2.00
1.00
1.00
1.00
1.00
1.00
1.00
3.00
2.94
2.95
2.03
2.93
2.00
2.94
3.07
1.00
13.6
13.6
15.52
15.46
7.58
8.87
7.79
9.28
7.62
7.74
3.00
1.00
3.00
Formula
5Mg(OH h· MgCl 2·8H2O
5Mg(OH)2· MgC I2·8H20
2MgO·3B 20 y 15H2O
2MgO·3B 20 y 15HP
MgO· 3B20 ..-7 .5H 2O
MgO·2B 2Or 9HP
MgO ·3B,O,·7.5 H2O
MgO·2B 20 3·9H 2O
MgO·3B 2Gy 75H 2O
MgO·3B 2Or7.5H 2O
H3B0 3
H3B03+M gO ·3B203·7.5H20
H, B0 3
Th e crystallized out solid phase
During cry sta lli zati on of all the supersaturated
so lutions at ODC, there appear five different solid
phases,
H J B0 3 ,
MgO·3 B 20 3 ·7.5H 2 0,
MgO·2B 20 3 ·9H 20, 2MgO·3B 20 3 · 15H 20 (inderite)
and 5Mg(OHh·MgCI 2·8 H2 0 in which H 3 B03 mi ght
crystallize
out
from
B 20 r 18%MgCIz- H20 ,
MgO·7B20 3-18%MgCIz-H20
and
MgO·5B 20 J 18%MgCIz- H20
supersaturated
soluti ons;
MgO·3B 20 3 ·7.5 H20 from MgO·nB 2 0 J- 18%MgCbH20 supersaturated so luti ons where n=7 ,5,3,2. 5 and 2
respectively ; MgO·2B 20 3 ·9H 20 from MgO ·2B 20 r
18%MgCIz-H 20 and MgO·2.5B 20 r 18%MgCb- H20
supersaturated
so luti o ns;
2 MgO ·3B 20 J·15H 20
(inderite) from both MgO· B 20 3-18 %MgC I2-H20 and
MgO· I.5 B20 r 18 %MgCIc-H 20
supersaturated
so lu tions and 5Mg(OH)2· MgCI 2·8 H20 from both
MgO-18 %MgCh-H 20 and 2.5MgO·B 20 r 18%MgCI 2H20 supersaturated so luti ons.
The chemical analysi s results and ph ase
composition of all the so li d phase crystallized out
from the supersaturated so lu tio ns are li sted in Table 2.
The representative resu lts of X-ray power diffraction
shown in Fig. 3, IR spectroscopy shown in Fig. 4, and
thermal analytical res ults shown in Fig. 5 are in
agreement with the data in li terature 6 .
Th e phase diagram of thermodynamic 1101/ eqllilibrium slale of MgO-B 20r18%MgCl r H20
system at ODC -The crystallization path
The crystallization path of MgO-B 20 J-18 %MgChH2 0 supersaturated solutions with different mo ie
rati os MgO:B20 J = 1:0,2. 5:1 , 1:1 , 1:1 .5, 1:2. 5, 1:3,
I :5, 1:7 and 0 : I respectively are show n in Fig 6. It can
~Q)
\)
c:
I'll
+'
+'
E
III
c:
...ro
+'
3800
Fig . 3-FT-JR Spectru m of solid phase (I , 5Mg(O Hh-MgCI 2·
8H20; 2, 2MgO·3B 20, ·15H 20: 3. MgO·3B 20 3·7.5 H20 and
4, MgO·2B 20 3·9 H20)
be seen that when MgO:B 20 3 in inti al so luti o ns are
I : 1.5 , 1:2 and I :3, the cry stalli zed solid did not chan ge
from beginning to the end , in other words, MgO:B 2 0 ,
mo le rati o in solutions remains co nstant, the point of
co mpos iti o n of the soluti on moves close to MgCI 2
corner ill the eq uilateral triangie. When MgO:B 20 J is
1:2 and I :2 .5 , the crystallization paths can be clearl y
divided in to two stages, the crystal lization so lid in the
first period is MgO ·3 B 20 3 ·7 .5 H2 0 , whereas the
crys talli zation solid 111 th e seco nd period IS
MgO·2B 20 3 ·9 H20 .
INDIAN J CI-IEM, SEC A, JANUARY 2003
40
IIgCI.
'"0.
~
>. +'
C
II>
24
+'
C
32
B~,L-
Fig. 4--XRD patterns of solid phase ( I, SMg(OH)2· MgCI 2·8H20:
2. 2MgO'3B 20 ,' ISH 20: 3, MgO·2B 20,.9 H20 and 4, MgO'3BP, '
7.5 H20)
TGI%
100
0
100
-6
80
80
60
40
0
200
-12
400/"C
400 /"c
0
100
100
4
4
-6
85
2
2
200
400 / "C
80
60
70
0
60
-12
0
40
100
200
300 / "C
Fig. S- T he TG. DTG and DSC of so lid phase
( I, SMg(OH )"'MgCI"'8H 20: 2, 2M gO·3B 20 ,·7.SH 20: 3. MgO'
3B 20 ,' ISH "O and 4. MgO'2B 20 y 9H 20)
The phase diagralll of thermodynamic 11 0 1/.equilibrium state
The ph ase diagram of stabl e equilibrium so lubility
Na+, K+, Mg 2+//Cr, SO/ -H 20 system at 25°C has
been given by Vant'hoff. N.S. Kurnak has presented
partly th e so lar phase diagram of the same sys te m by
evaporation of sa lt lake brine which is si milar to
concentrated sea-water .The whole metastable phase
diagram of the same syste m has been completed with
the method of isothermal evaporation by Jin Zuomei 8 .
It is obviously noticed that there exists the
supersaturated area between the stable equilibrium
solubility phase diagram and the metastable phase
diagram. In order to understand the supersaturated
____~____~~____~____~~__~W~
J2
24
16
8
Fig. 6- The crystalli zation paths of MgO- B"O,- 18%MgCi 2-H"O
supersaturated solutio n with different mo le rati o at O°C
MgO:B 20 3 : 0 2.S :I , 01:1 I:I.S , @ 1:2 . • 1:2.S. 6. 1:3. A I:S. 0
1:7
solubility phe nomena, it is necess ary to study the
liquid-solid phase diagram in supersaturated area.
Based on the crystallization paths of th e
supersaturated solutions with different MgO:8 20 }
mole ratios, the liquid-so lid phase di agram of
thermodynamic non-equilibrium state of MgO-8 20 ,l8 % MgCh-H 2 0 system at O°C has been drawn in an
equilateral tri ang le (Fig. 7) . The three corners of th e
triangle represent component MgO , 8 20 3 a nd MgCl 2
respective ly.
The ten crystallization paths of the supersaturated
so luti on of MgO-8 20 3-l 8% MgCI 2- H20 sys tem with
differe nt MgO:8 2 0 } mole rati os have been shown in
eq ui lateral triangle.
With
the
boundary o f
MgO : 8 20 )= I : I, the left side is bori c ac id or bormes,
th e ri ght side is magnesium oxych loride. The initial
concentratio n of 8 2 0 3 in MgO SB}0 3-18% MgCI2
supersaturated so lution is hi ghest. and it is six times
as hi gh as that of the equilibrium co nce ntration. The
borate supersaturation extent fo r MgO'S8 20 318 %MgC1 2 so lution is the g reates t and its
crystallizaion path is the lon ges t. The point A on th e
s ide lin e of MgCh and 8 2 0 3 gives the eq uilibrium
solubility of H 3 80 3 in the 18% MgC I2 -H 20 so luti on,
the points B and C both re presen t the co mpos iti on of
saturated solution with two S;1!tS: H ,80 3 and
MgO '2 8 20 y 9H 20 , MgO'28 20 y 9H 2 0 and SMg(OHh'
MgCI 2 '8 H20 respectively, which were determined by
the isothermal equilibrium meth od of the synthet ic
complex (Table 3). The thermodynamic nonequilibrium ph ase diagram co ns ists of five
crystallization field s, corresponding to 1-1 3 80 3 ,
ZHU et al. : PHASE DIAGRAM OF MgO-B 20 r I8% MgCl r H 20 SYSTEM
41
Table 3--Composition of thermqdy na1n ic equilibrium poi nt
"
Equlibrium
po int
MgO
(wt%)
Composition
B20 3
(wt%)
MgC I2
(w t%)
Phase diag ram index
MgO
MgCl 2
B2O.1
(mo l%)
(mo l%)
(mo l% )
0.52
17 .94
3.8 1
96.1 9
B
0.03
0.57
18.77
0.44
3.97
95.66
C
0 .3 1
0.46
18.85
3.75
3.05
93.20
D
0.09
17.50
1.20
A
B,O, L.....---;t;:------;;~---f::---~--~ MgO
Fig. 7- The The phase di agra m of thermodynami c no nequilibrium stable of MgO-B 20 r 18%MgC lr H20 at ODC
(I , H.1 BO.1; 2, MgO·3B 2 0 3 ·7.5H 20; 3, MgO·2B20.1·9 H20;
4, MgO·3B 20.1· 15HzO and 5, 5Mg(OHh· MgCl 2·8H 20 1
MgO·3B z0 3 ·7.5H zO,
MgO '2B z0 3 '9H zO,
2MgO'
3B z0 3 ' 15HzO (inderite) and 5Mg(OHh·MgCb·8H 2 0,
in whi ch the phase fi e ld of MgO·3B z0 3·7.5H zO is the
largest, whereas H 3 B0 3 the small est. This showed that
the supersaturation extent of H 3 B0 3 in 18%MgC lzH20 is the smallest and the supersaturation extent of
MgO'5 Bz0 3 in 18%MgCb- HzO is the highest.
It is clearly noticed from Fig. 7 that the phase
diagram of thermodynami c non-equilibrium state in
MgO-BzOr 18%MgCIz-H 20 system at O°C consis ts of
three parts. The first one ABCD is the thermodynamic
eq uil ibrium solubility, the curve AB represent s
eq uilibrium solubility of H3 80 3; curve BC MgO'2 8 z0 3 '9H zO,
and
curve
CD-
98 .80
5Mg(OHh·MgCIz·8H 20 . The second part is ex pressed
by dotted lines EFGHIJ which is called as metastable
solubility, and the curve EF represents metastable
solubility of H3 B0 3; curve GH- MgO'38 20 :1"
7.5H 2 0 , curve HI-2MgO '3B z0 3 ' 15H 2 0 (inderite),
and curve IJ- 5Mg(OH h· MgCIz·8H zO. The third
part between thermodynamic equilibrium sol ubility
and metastable solubility is the supersaturated area
which consists of five fields, corresponding to H3 80 3 ,
2MgO '
MgO·3B 2 0 3 ·7.5H 20 ,
MgO'28 20 3'9 HzO ,
38 z0 3' 15H zO
and
5Mg(OHh'MgCh'8H 20
respectively.
The liquid-solid phase diagram of thermodyn amic
non-equilibrium state of MgO-B20 3-18 %MgCb-H cO
system is different from the stable equilibrium
solub ility phase di ag ram of the same syste m by
G.Bagirov 9 in wh ich there are ten phase field s.
References
I Gao S Y, Cheng J Q & Zha ng M P, Adv Sci Chill Chelll . 4
( 1992 ) 163.
2 Gao S Y, & Li G Y, Cilelll J Chill Ullil'erity , 3 ( 1982) 14 1.
3 Gao S Y, Fu T J & Wang J Z, Chill J illorg.Cilelll . 1 ( 1985 ).
97.
4 Gao S Y, Yao Z L, & Xia S P, Acta Ch illi Sill (C hin ese). 52
( 1994), 10.
5 Gao S Y, Chen X A & Xia S P, Acta Chilli Sill (C hin ese) . .+8
(1990), 1049.
6 Li J, Ph.D. Dissertatioll , Lan zho u Universit y, 1994.
7 vant'hoff J H. V llte r SlIeil/lII gell der Biidllllgsverhaitllisse der
Ozeallischell- Sai zabiagenlllgell , IIsbesolldere dos stas.ljilrille
Saizes,(verlag Chemie G.M.B .H, Le ipzig) (19 12). 91.
8 J in Z M. Xiao X Z & Li ang S M, Acta Chilli Sill (C hinese). 3X
( 1980), 3 13.
9 Bagirov G, Sedeliniron G S & Rza-Zade P F. ZII. Neorg.Khilll.
10 ( 1965) 19 t 8.