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Model for the decomposition of carnallite in aqueous solution
International Journal of Mineral Processing 139 (2015) 36–42
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Model for the decomposition of carnallite in aqueous solution Huaide Cheng a,⁎, Haizhou Ma a, Qingyu Hai a,b, Zhihong Zhang a, Liming Xu a, Guangfen Ran a a b
Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, No 18 Xinning Road, Xi'ning, Qinghai 810008, China University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 16 September 2014 Accepted 21 April 2015 Available online 23 April 2015 Keywords: Carnallite Decomposition ratio Temperature Water amount
a b s t r a c t Carnallite is a double salt (KCl·MgCl2·6H2O or KMgCl3·6H2O), which is a common source of potassium chloride (KCl). It is treated with a mother liquor containing magnesium chloride (MgCl2), KCl and sodium chloride (NaCl), and then KCl and NaCl precipitate in the mother liquor solution. The amount of water and the temperature are the two crucial factors for the decomposition of carnallite in aqueous solution. Because of the lack of research on the decomposition process of carnallite, the yield and quality of the KCl produced are limited. In this study, we develop a model for the decomposition of carnallite based on the multi-temperature quaternary diagram of the NaCl–KCl–MgCl2–H2O system and the principle of mass conservation for MgCl2. The model is verified using experimental and literature data for different types of carnallite: normal, Mg-rich, Na-rich, and (Mg and Na)-rich. The proposed model predicts the amount of water required for decomposition of carnallite and the yield of KCl very well for normal and Mg-rich carnallite (within 2%), for Na-rich and (Mg and Na)-rich carnallite (within 8%), and for ancient evaporite carnallite in Laos (within 2%). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Potassium chloride is an important chemical fertilizer in agriculture (Harper et al., 2012; Chen et al., 2013; Cheng et al., 2013). A source of KCl is the carnallite double salt, which has the composition KCl·MgCl2·6H2O or KMgCl3·6H2O, and is either obtained as the ore or recovered from brine that favors carnallite formation (Dave and Ghosh, 2006). Irrespective of its origin, carnallite is treated with decomposition mother liquor containing MgCl2, KCl, and NaCl, so that KCl and NaCl are precipitated in the equilibrium mother liquor. The driving force for decomposition is the difference in the concentration at the experimental temperature and the saturation of equilibrium mother liquor (Mohameed et al., 2002). The key factor is the amount of water used to dissolve MgCl2. This water causes the carnallite to decompose to its components. As is shown in Fig. 1(A), if the MgCl2 concentration is at or near the triple-saturation point (the point at which the solution is saturated with carnallite, NaCl, and KCl), the solubility of KCl is suppressed to the point where most of the KCl will precipitate (Liu et al., 2007). Moreover, the composition of the triple-saturation point changes with temperature (Fig. 1(B)). For maximum recovery of the components, the crystallizing mixture must be saturated with carnallite at its triple-saturation point. If the mixture is not saturated, then KCl will dissolve in the water. If the amount of water is not suitable or inadequate, carnallite will not decompose in the aqueous solution. Generally, the amount of water used to dissolve carnallite and the reaction ⁎ Corresponding author at: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, No 18 Xinning Road, Xi'ning, Qinghai 810008, China. E-mail address: [email protected] (H. Cheng).
http://dx.doi.org/10.1016/j.minpro.2015.04.007 0301-7516/© 2015 Elsevier B.V. All rights reserved.
temperature are two important factors that restrict KCl production. In the crystallization processes, the solid–liquid phase equilibrium is very important. Furthermore, owing to the lack of a systematic study about the decomposition process of carnallite in aqueous solution by the solid–liquid phase equilibrium method, the yield and quality of KCl produced are limited. Here, a functional model for the amount of water and the decomposition ratio of carnallite in aqueous solution is established and evaluated from 256.15 to 323.15 K in quaternary solid–liquid phase equilibrium systems based on the mass transfer equilibrium of MgCl2 between the precipitated salts and the ionic species remaining in solution (Ettouney, 2006; Demming et al., 2008; Ji et al., 2010). As discussed above, we found that the water amount and decomposition ratio of carnallite are related to the temperature and composition of the equilibrium mother liquor. The model for the effect of the system properties on the decomposition process of carnallite was established by the multi-temperature quaternary diagram data of the NaCl–KCl– MgCl2–H2O system. This model and the decomposition process of modern solar-pond carnallite in the Qarhan Salt Lakes and ancient evaporite carnallite in Laos are discussed. 2. Experimental 2.1. Materials Carnallite was taken from the drilling core of Vientiane potash deposits in Laos. Its composition (Table 1) indicates that the carnallite is pure. The content of water-insoluble material in the carnallite was less than 0.5%. Doubly-distilled water was used in all the experiments, as well as for the analytical measurements. Standard glassware was
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
37
Fig. 1. Quaternary diagram of NaCl–KCl–MgCl2–H2O system. (A) at 298.15 K. (B) Partially enlarged multi-temperature from 263.15 to 348.15 K. (•) triple-saturation point, (Sy) KCl, (Ha) NaCl, (Car) KCl·MgCl2·6H2O, and (Bis) MgCl2·6H2O.
used in all experiments. Whenever heating was required, a super constant temperature water-bath (HH-501, Shanghai Qiqian Electronic Technology Co. Ltd., China) was used. The solid phases were obtained by vacuum filter (SHZ-3 circulating water vacuum pump, Shanghai Yarong Biochemistry Factory, China) separation. 2.2. Analytical methods The ionic compositions of the liquid and solid phases were determined using well-established procedures. The Mg2+ ion concentration was determined by complexometric titration with EDTA standard solution with an accuracy of ± 0.5%. The K+ ion concentration was determined by gravimetric method with sodium tetraphenylborate with an accuracy of ±0.3%. The Cl− ion concentration was determined by Hg(NO3)2 titration with an accuracy of ±0.5%. The Na+ ion concentration was calculated by the ion-equilibrium subtraction method with an accuracy of ±1%. 2.3. Experimental procedures The decomposition process of carnallite in aqueous solution is an endothermic reaction, and was carried out in a super constant temperature water-bath. The reaction temperature was controlled at 293.15 K (ambient temperature). The speed of the agitator was 600 rpm. The reaction time was 45 min, and 2000 ml standard glassware was used as the reaction vessel. The solid-phase minerals and liquid-phase solution were separated by vacuum filtration after the end of decomposition. Because there is water adsorbed to the crystals/minerals in the products of the decomposition, it is necessary to remove this water and discuss the experimental results in this study. There are four methods for the removal of this solution: (1) Washing with an organic solvent (Moretto, 1988; García-Veigas et al., 2009) (e.g. ethanol,
Table 1 The composition of carnallite from the drilling core of Vientiane potash deposits in Laos. Ion concentration, wt.%
Substance components, wt.%
K
Mg
Cl
Na
KCl
NaCl
MgCl2
H2O
Water-insoluble material
8.34
5.35
46.84
15.35
15.90
39.02
20.96
23.76
0.36
ethanol–glycol, or acetone), yet the effect of salting-out in organic solvent with high salinity would occur (Xu et al., 1991). (2) Washing with doubly-distilled water (McCaffrey et al., 1987), but some of crystals/minerals would dissolve because mineral salts are soluble. (3) Obtaining large-grained crystals with 5 mm in diameter by crystallization, and then the water adsorbed to the crystal surfaces can be removed by tissues (Siemann and Schramm, 2000). However, the KCl particle size at the end of decomposition is generally very small (b 0.5 mm) (Mohameed et al., 2002; Li et al., 2009) and it is difficult to avoid fluid inclusion. (4) Removal with the evaporation of mother liquor method (Herrmann, 1980). This method has been applied to research the crystallization behavior of borate during the brine evaporation process (Gao and Li, 1982). The pure solid phase of precipitation is obtained through deducting the solution attached to the crystal/mineral surfaces. The ionic composition of the equilibrium mother liquid is constant at the desired temperature, and was accurately determined. Furthermore, the amount of water in the equilibrium mother liquid is known. To estimate fluid inclusion, method (4) was applied. The product crystals/minerals obtained from decomposition were placed in a drying oven on forced convection at 373.15 K for 4 h (Cheng et al., 2009). The amount of water adsorbed to the crystal/mineral surface was calculated based on the loss of weight with the drying method. Additionally, the mass of the product is always lower than the mass of the feed because of the viscosity of the brine and solution adsorbing to the surface of the container during the experiments. When the quantity of the solid phase in the product is weighed by electronic balance with an accuracy of ±0.01 g, the loss of mass of the system is the quantity of the liquid phase, which can be calculated by the mass balance of the feed and the product. The components of the liquid phase are constant at the desired temperature. Therefore, we can estimate the decomposition ratio of carnallite based on our experimental results. The decomposition ratio of carnallite in aqueous solution includes the KCl yield and the MgCl2 decomposition yield. The KCl yield is given by εKCl ¼
M KClðSÞ −MKClðLÞ 100%: MKClðSÞ
ð1Þ
The MgCl2 decomposition yield is given by εMgCl2 ¼
MMgCl2 ðLÞ MMgCl2 ðSÞ
100%:
ð2Þ
38
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
MKCl(S) and M MgCl2 ðSÞ are the weights of KCl and MgCl2 in the carnallite ore before decomposition, and MKCl(L) and MMgCl2 ðLÞ are the weights of KCl and MgCl2 in the equilibrium mother liquor at the end of decomposition. The weights of KCl and MgCl2 in the solution adsorbed to the crystal surface that can be gained by evaporation of mother liquor method are included in MKCl(L) and MMgCl2 ðLÞ , respectively.
composition of carnallite (SC,M(α1, Q)). Therefore, the functional relationship of Eq. (6) can be described as f ðW Þp ¼ F SC;M ; LC;T ; δ :
The KCl yield and MgCl2 decomposition yield can be determined using Eqs. (1), (2), (5), and (6), respectively.
3. Modeling The various processes used for KCl production from carnallite depend upon the equilibria existing in the quaternary salt system at the desired temperature. When the carnallite is decomposed by adding an appropriate amount of water, MgCl2 goes into the solution of the added water but KCl crystallizes. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ LðEÞ;
ð3Þ
where L(E) is the triple-saturation equilibrium mother liquor at the desired temperature. However, if only a small amount of water is added, the carnallite cannot completely decompose in the aqueous solution. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ LðEÞ þ ð1−XÞKMgCl3 6H2 OðsÞ;
ð4Þ
X denotes the percent decomposition of the carnallite. In both cases, the process occurs at the interface between the solid and liquid phases, MgCl2 dissolves completely or incompletely into the liquid phase from the carnallite crystals, and KCl is crystallized. Thus, the relationship of mass conservation of MgCl2 between reactants and products is obtained from Eqs. (3) and (4): α1 Q ¼
δW þ Aα 1 Q 1−α 2 −β2 −γ 2
α 2 þ ð1−δÞα 1 Q
ð5Þ
where A is the 6H2O/MgCl2 molecular mass ratio in carnallite, which is a constant (1.1343). The 6H2O and MgCl2 in carnallite would enter into the solution during carnallite decomposition in aqueous solution. The concentration of MgCl 2 in solution can be obtained from Fig. 1(A) (Niu and Cheng, 2002). W represents the theoretical water amount used to decompose carnallite, which can be calculated based on Fig. 1(A) (Niu and Cheng, 2002). δ is the ratio of the actual water amount to the theoretical water amount: δ = Wact / Wtheo × 100%, where Wact is the actual water amount and Wtheo is the theoretical water amount. The left-hand side of Eq. (5) represents the mass of MgCl2 component of carnallite before decomposition. The first term on the righthand side of Eq. (5) represents the mass of the MgCl2 component of the triple-saturation equilibrium mother liquor at the end of decomposition. The second term on the right-hand side of Eq. (5) represents the mass of the MgCl2 component of the residual carnallite at the end of decomposition. This model can be used to investigate the conditions of the variant water amount used to decompose carnallite, and estimate the required values of the operating parameters to achieve a desired decomposition ratio of carnallite. Eq. (5) can be simplified as W¼
1−α 2 −β2 −γ2 A α 1 Q: − δ α2
ð6Þ
Analysis of Eq. (6) indicates that the water amount is intimately related to three factors: (1) the composition of the triple-saturation equilibrium mother liquid at the desired temperature (LC,T(α2, β2, γ2)); (2) the ratio of additional water amount (δ); and (3) the weight and
ð7Þ
εKCl ¼
δα β 1− 1 2 α 2 β1
ð8Þ
εMgCl2 ¼ δ
ð9Þ
The functional relationships of Eqs. (8) and (9) can be described as follows: f ðε KCl Þp ¼ R SC ; LC;T ; δ
ð10Þ
f εMgCl2 ¼ P ðδÞ:
ð11Þ
p
In addition, when carnallite is decomposed by adding an excessive amount of water, MgCl2 goes into solution but KCl is crystallized. Furthermore, KCl and NaCl partially dissolve because of the extra water. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ RðEÞ;
ð12Þ
where R(E) is the double-saturation equilibrium mother liquor (the solution is saturated with NaCl and KCl in the quaternary diagram of NaCl–KCl–MgCl2–H2O system) at the desired temperature. Therefore, the relationship of mass conservation for MgCl2 between reactants and products is obtained from Eq. (12). α1 Q ¼
W þ Aα 1 Q 1−α 3 −β3 −γ3
α3
ð13Þ
Eq. (13) can then be solved for W: W¼
1−α 3 −β3 −γ3 −A α 1 Q: α3
ð14Þ
Analysis of Eq. (14) indicates that the water amount is intimately related to two factors: (1) the composition of the double-saturation equilibrium mother liquor at the desired temperature (RC,T(α3, β3, γ3)); and (2) the weight and composition of carnallite (SC,M(α1, Q)). Thus, the functional relationship of Eq. (14) can be written as ∀
f ðW Þp ¼ F
∀
SC;M ; RC;T :
ð15Þ
In this case, the KCl yield and MgCl2 decomposition yield at the desired temperature can be calculated using Eqs. (1), (2), (12), and (13).
∀
εKCl ¼
1−
α 1 β3 α 3 β1
∀
εMgCl2 ¼ 1
ð16Þ ð17Þ
Thus, the functional relationship of Eqs. (16) and (17) can be described as follows: ∀
f ðεKCl Þp ¼ R
f
∀
∀
SC ; RC;T
εMgCl2 ¼ P ðBÞ: p
ð18Þ
ð19Þ
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
39
Table 2 The model parameters of f(W)p, f(εKCl)p, and f εMgCl2 p for a wide range of temperatures (256.15–323.15 K). T(K)
wt.% MgCl2α2
KClβ2
NaClγ2
f(εKCl)p = R(SC, LC,T, δ)
f εMgCl2
δ=1
δ=1
δb1
δ=1
δb1
1 1− 0:0615α β1 1 1− 0:0726α β1 0:0918α 1 1− β 1 1 1− 0:1050α β1 0:1162α 1 1− β 1 1 1− 0:1270α β1 1 1− 0:1296α β1 1 1− 0:1234α β1 1 1− 0:1404α β1 1 1− 0:1636α β1
1δ 1− 0:0615α β1 0:0726α 1 δ 1− β 1 1δ 1− 0:0918α β1 0:1050α 1 δ 1− β 1 1δ 1− 0:1162α β1 1δ 1− 0:1270α β1 1δ 1− 0:1296α β1 1δ 1− 0:1234α β1 0:1404α 1 δ 1− β 1 1δ 1− 0:1636α β1
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
256.15
24.86
1.53
1.90
1.7503α1Q
263.15
24.80
1.80
1.95
1.7467α1Q
273.15
25.05
2.30
1.90
1.6901α1Q
283.15
25.43
2.67
1.92
1.6176α1Q
288.15
25.39
2.95
1.90
1.6132α1Q
293.15
25.44
3.23
1.88
1.5957α1Q
298.15
25.85
3.35
1.80
1.5349α1Q
303.15
26.30
3.27
1.60
1.4828α1Q
313.15
26.70
3.75
1.65
1.4088α1Q
323.15
26.90
4.40
1.80
1.3527α1Q
δb1
α1 Q 2:8810− 1:1343 α1 Q δ α1 Q 2:8244− 1:1343 δ α1 Q 2:7519− 1:1343 δ α1 Q 2:7475− 1:1343 δ α1 Q 2:7300− 1:1343 δ α1 Q 2:6692− 1:1343 δ α1 Q 2:6171− 1:1343 δ α1 Q 2:5431− 1:1343 δ α1 Q 2:4870− 1:1343 δ 2:8846− 1:1343 δ
Generally, the functional model (Eqs. (7), (10), (11), (15), (18), and (19)) of the water amount, KCl yield, MgCl2 decomposition yield is attainable. The decomposition process of carnallite will be investigated at the desired temperature through these models. 4. Results and discussion In this study, the hypothesis is that the decomposition process of carnallite in aqueous solution either completely or incompletely occurs, and the reaction pressure is normal atmospheric pressure. A series of experiments were carried out at room temperature. To obtain the model parameters and demonstrate the model, the data for the composition of the triple-saturation equilibrium mother liquor should be obtained. Generally, the reaction temperature of decomposition is in the range from 280 to 300 K during the process of KCl production from carnallite. However, in a few areas, such as in winter at the Qarhan Salt Lakes in China, the reaction temperature may be drop to 268.15 K. Furthermore, in summer in Laos the reaction temperature may exceed 300 K. Because the literature data for the composition of the triplesaturation equilibrium mother liquor are available in various temperature ranges, we chose the temperature range from 256.15 to 323.15 K. However, if the carnallite is decomposed by adding superfluous water, the parameters of the predicated model for the water amount, KCl yield, and MgCl2 decomposition yield are under-estimated. In this case, the mean spherical approximation method is usually used to calculate the composition of the double-saturation equilibrium mother liquor in the quaternary system (NaCl–KCl–MgCl2–H2O) under the desired temperature and water amount conditions (Lu et al., 1993; López-Pérez et al., 2000; Yu et al., 2003; Ge and Wang, 2009; Memarnejad and Dehghani, 2012). The compositions of the triple-saturation equilibrium mother liquor (Zdanovskiy et al., 1975) at various temperatures are listed in Table 2. From Table 2, it can be seen that the solubility of KCl and MgCl2
1.9
W ¼ ð−0:0062T þ 3:3883Þα 1 Q:
εKCl ¼ 1−
KCl)P/R(S C,LC,T )
1.5 2
R = 0.9715
f(
f(W)P/F(S C,M,LC,T )
1.6
1.3
¼ P ðδÞ
ð20Þ
Analysis of Eq. (20) indicates that (1) W decreases as T increases, when α1 and Q are constant and (2) W increases as α1 increases when T and Q are constant. This suggests that (1) the water amount is a univariate function of the MgCl2 content of carnallite when the reaction temperature is constant and (2) the water amount is also a univariate function of temperature when the composition of carnallite is known. Thus, we can calculate the amount of water required to decompose carnallite at any temperature and any composition of carnallite. The model expressed in Eq. (10) indicates that there is a linear relationship between f(εKCl)p and R(SC, LC,T)δ, and the functional relationship was established as
(A)
1.7
p
increases with increasing temperature, while the solubility of NaCl shows a nonlinear variation with temperature. From the data collected in Table 2, the calculated values of the model parameters from 256.15 to 323.15 K using Eqs. (7), (10), and (11) are obtained (see Table 2). The functional relationships expressed in Eqs. (7) and (10) are shown in Fig. 2. The ratio of f(W)p/F(SC,M, LC,T, δ) shown in Fig. 2(A) decreases as the temperature increases, indicating that the amount of water required to decompose carnallite decreases with increasing temperature. In contrast, the ratio of f(εKCl)p/R(SC, LC,T, δ) shown in Fig. 2(B) increases with increasing temperature, indicating that the solubility of KCl in the balance mother liquor increases with increasing temperature. This is the principle that is used to improve the KCl yield from carnallite by the cold-decomposition method. The model expressed in Eq. (7) shows a linear relationship between f(W)p and F(SC,M, LC,T)δ, and the functional relationship was established as
1.8
1.4
f(W)p = F(SC,M, LC,T, δ)
0.19 0.17 0.15 0.13 0.11 0.09 0.07 0.05
α 1 ð0:0014T−0:3033Þ : β1
(B)
2
R = 0.9701
250 260 270 280 290 300 310 320 330
250 260 270 280 290 300 310 320 330
T(K)
T(K)
Fig. 2. Relationships between (A) f(W)p/F(QC,M, LC,T)δ and temperature, and (B) f(εKCl)p/R(QC, LC,T)δ and temperature.
ð21Þ
40
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
48
(A)
43
46
41
44
W(g)
W(g)
45
39 37
(B)
42 40
35
38 270
275
280
285
290
295
300
270
275
280
T(K)
290
295
300
290
295
300
T(K) 38
(C)
34
36
32
34
W(g)
W(g)
36
285
30 28
(D)
32 30
26
28 270
275
280
285
290
295
300
270
275
280
T(K)
285 T(K)
Fig. 3. Relationship between the amount of water (W) and temperature (T) for the decomposition of different types of carnallite: (A) normal, (B) Mg-rich, (C) Na-rich, and (D) (Mg and Na)-rich. Solid lines represent the model results and dashed lines represent the literature data (Wang, 2000).
Analysis of Eq. (21) indicates that (1) εKCl decreases as T increases when α1 and β1 are constant, (2) εKCl decreases as α1 increases when α1 and β1 are constant, and (3) εKCl increases as β1 increases when T and α1 are constant. It can be seen that (1) the KCl yield changes inversely with temperature when the composition of carnallite is constant, and a low reaction temperature is favorable for increasing the KCl yield, (2) the KCl yield is a bivariate function of the composition of carnallite when the reaction temperature is constant, and (3) the KCl yield is restricted to the KCl content of carnallite. Wang (2000) collected more than 2000 different types of carnallite samples from the Qarhan Salt Lakes in China, and calculated the amount
0.88
of water used to decompose carnallite and the KCl yield for various temperatures (273.15, 283.15, and 298.15 K). According to the average composition of these carnallite samples, there are four types of carnallite: normal, Mg-rich, Na-rich, and (Mg and Na)-rich. For the various types of carnallite, the predicted values of the model (Eq. (20)) were compared with the calculated values from the literature (Wang, 2000), as shown in Fig. 3. Fig. 3 shows the relationship between the water amount used to decompose the different types of carnallite (normal, Mg-rich, Na-rich, and (Mg and Na)-rich) and the temperature, and the model can be used to describe the actual decomposition process of the different types of carnallite, in particular the model describes the
(A)
0.84
KCl
KCl
0.86
0.82 0.8 0.78 270
275
280
285
290
295
0.82 0.8 0.78 0.76 0.74 0.72 0.7 0.68
300
(B)
270
275
280
T(K) 0.88
0.84
KCl
KCl
0.86
0.82 0.8 0.78 275
280
285 T(K)
290
295
300
290
295
300
T(K)
(C)
270
285
290
295
300
0.82 0.8 0.78 0.76 0.74 0.72 0.7 0.68
(D )
270
275
280
285 T(K)
Fig. 4. Relationship between f(εKCl)p and temperature (T) for the decomposition of different types of carnallite: (A) normal, (B) Mg-rich, (C) Na-rich, and (D) (Mg and Na)-rich. Solid lines represent the model results and dashed lines represent the literature data (Wang, 2000).
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
0.87
(A)
1
0.86
0.98
0.85 KCl
MgCl2
1.02
0.96
0.83
0.92
0.82
0.9
(B)
0.84
0.94
0.81 0.9
Fig. 5. Relationships between (A) f ε MgCl2
41
p
0.95 1 1.05 ratio of added water
1.1
0.9
0.95 1 1.05 ratio of added water
1.1
and the ratio of added water, and (B) f(εKCl)p and the ratio of added water for carnallite (Laos) decomposition at 293.15 K. Solid lines represent
the model results and dashed lines represent the experimental data in this study.
normal-type carnallite very well. However, the values of the model are slightly higher than the values of the calculated results for Mg-rich carnallite, and the difference is 2% to 3%. The values of the model are less than the calculated values for Na-rich and (Mg and Na)-rich carnallite, and the difference is about 8%. There is an important reason why about 10% extra water (free-water) was added compared to the theoretical water amount according to Wang's practical experience (Wang, 2000). The values of the free-water are included in the water amount used to decompose the carnallite. Moreover, the content of NaCl in the decomposition products for Na-rich and (Mg and Na)-rich carnallite is higher than normal-type carnallite, and only a small amount of NaCl is involved in the dissolution of the recrystallization process. As shown in Fig. 4, the KCl yield is related to the decomposition temperature for all the different types of carnallite (normal, Mg-rich, Na-rich, and (Mg and Na)-rich). The predicted values of the model (Eq. (21)) can be used to describe the actual decomposition process of various types of carnallite, and the model predicts the values of normal and Mg-rich carnallite very well. However, the values of the model are less than the calculated values for Na-rich and (Mg and Na)-rich carnallite, and the difference is about 2%. The reason is the same as that for the presence of 10% free-water in the decomposition products. The effect of the amount of water on the decomposition ratio of carnallite from ancient evaporite in Laos under isothermal conditions was also investigated, and the predicted model and experimental results are compared. The reaction temperature of 293.15 K was chosen for the isothermal temperature, and is consistent with the ambient temperature. This temperature was chosen because if the temperature is too high then evaporation of the balanced mother liquor occurs, and if the temperature is too low then the salting-out effect of the balanced mother liquor occurs, and both limit the discussion of the experimental results. Because the composition of carnallite in Laos is known (Table 1), the theoretical water amount required to decompose carnallite at 293.15 K can be calculated. From Eq. (20), the predicted amount of water is 98.77 g for 300 g carnallite. The ratios of added water (actual added water amount/predicted water amount) were chosen as 0.91, 0.94, 0.97, 1, 1.03, 1.06, and 1.09, and the experimental process and data analysis were treated using the description in Section 2.3. The relationship between the decomposition ratio and the temperature (293.15 K) was obtained by balance calculations of the experimental results, and the predicated model and the decomposition process of ancient evaporite–carnallite in Laos are discussed. The analytical results are shown in Fig. 5. As shown in Fig. 5, the model established in this research (Eqs. (8), (9), (16), and (17)) is consistent with the experimental results. Fig. 5 shows that the model describes the actual decomposition process of ancient evaporite–carnallite very well. The difference between the experimental results and the values predicted by the model is about 2%. This difference is probably due to systematic errors from the mass balance of feed and product.
5. Conclusions During the decomposition of carnallite in aqueous solution, the functional models for water amount f(W)p/f ∀(W)p, KCl yield f(εKCl)p/ ∀ f ∀(εKCl)p, and MgCl2 decomposition yield f εMgCl2 p / f εMgCl2 p from 256.15 to 323.15 K in the quaternary systems NaCl–KCl–MgCl2–H2O were investigated using the principle of mass conservation of MgCl2. The results showed that (1) the linear functional relationships, that is, f(W)p = F(SC,M, LC,T, δ), f(εKCl)p = R(SC, LC,T, δ), and f εMgCl2 p ¼ P ðδÞ, exist when the decomposition process completely/incompletely occurs and (2) the non-linear functional relationships, that is ∀ f ∀(W)p = F∀(SC,M, RC,T), f ∀(εKCl)p = R∀(SC, RC,T), and f εMgCl2 p ¼ P ðBÞ, exist when the decomposition process excessively occurs. Moreover, the water amount required to completely decompose carnallite linearly changes (W = (− 0.0062T + 3.3883)α1Q), and the KCl yield linearly Þ ). The model was verified by changes ( ε KCl ¼ 1− α 1 ð0:0014T−0:3033 β 1
experiment data and literature data. This indicated that the model corresponds well with the natural decomposition process of modern solar-pond carnallite in Qarhan Salt Lakes and ancient evaporite carnallite in Laos. This research provides the theoretical basis for the cold decomposition process to produce KCl from carnallite.
Nomenclature W amount of water used to decompose carnallite (g) Q mass of carnallite (g) T temperature (K) p pressure (Pa) α1 MgCl2 concentration in carnallite (wt.%) β1 KCl concentration in carnallite (wt.%) γ1 NaCl concentration in carnallite (wt.%) α2 MgCl2 concentration at the triple-saturation point (wt.%) β2 KCl concentration at the triple-saturation point (wt.%) γ2 NaCl concentration at the triple-saturation point (wt.%) α3 MgCl2 concentration at the double-saturation equilibrium mother liquor (wt.%) β3 KCl concentration at the double-saturation equilibrium mother liquor (wt.%) γ3 NaCl concentration at the double-saturation equilibrium mother liquor (wt.%) Acknowledgments The authors thank the West Light Foundation of CAS (2011-180) and the National Program on Key Basic Research Project of China (973 Program) (2011CB403004) for financial support.
42
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Liu, C., Ji, Y., Bai, Y., Cheng, F., Lu, X., 2007. Formation of porous crystals by coupling of dissolution and nucleation process in fractional crystallization. Fluid Phase Equilib. 261 (1), 300–305. López-Pérez, G., González-Arjona, D., Molero, M., 2000. Estimation of activity coefficients at different temperatures by using the mean spherical approximation. J. Electroanal. Chem. 480 (1), 9–17. Lu, J., Yu, Y., Li, Y., 1993. Modification and application of the mean spherical approximation method. Fluid Phase Equilib. 85, 81–100. McCaffrey, M.A., Lazar, B., Holland, H.D., 1987. The evaporation path of seawater and the coprecipitation of Br− and K+ with halite. J. Sediment. Petrol. 57 (5), 928–937. Memarnejad, M., Dehghani, M.R., 2012. A modified mean spherical approximation model for prediction of mean ionic activity coefficient in mixed electrolyte solution at different temperatures. Fluid Phase Equilib. 322, 97–104. Mohameed, H.A., Abu-Jdayil, B., Al-Khateeb, M., 2002. Effect of cooling rate on unseeded batch crystallization of KCl. Chem. Eng. Process. 41 (4), 297–302. Moretto, R., 1988. Observations on the incorporation of trace element in halite of Oligocene salt beds, Bourg-En-Bresse basin, France. Geochim. Cosmochim. Acta 52 (12), 2809–2814. Niu, Z., Cheng, F., 2002. Phase Diagram of Salt-Water System and Its Application. Tianjing University Press. Siemann, M.G., Schramm, M., 2000. Thermodynamic modeling of the Br partition between aqueous and halite. Geochim. Cosmochim. Acta 64 (10), 1681–1693. Wang, S., 2000. Influence of carnallite types on the production process of potassium chloride by cold decomposition-flotation. Sea-Lake Salt Chem. Ind. 29 (5), 1–4. Xu, Q., Luo, R., Kang, H., Li, X., Jiang, F., 1991. Salt effect of vapor–liquid equilibrium in system of formic acid-acetic acid-water containing magnesium chloride. Chem. J. Chin. Univ. 12, 799–802. Yu, Y., Gao, G., Daridon, J.L., Lagourette, B., 2003. Prediction of solid–liquid equilibria in mixed electrolyte aqueous solution by the modified mean spherical approximation. Fluid Phase Equilib. 206 (1), 205–214. Zdanovskiy, A.B., et al., 1975. The Solubility Directory of Experimental Data for Multicomponent Water–Salt System. Leningrad Branch Press, Moscow.
Contents lists available at ScienceDirect
International Journal of Mineral Processing journal homepage: www.elsevier.com/locate/ijminpro
Model for the decomposition of carnallite in aqueous solution Huaide Cheng a,⁎, Haizhou Ma a, Qingyu Hai a,b, Zhihong Zhang a, Liming Xu a, Guangfen Ran a a b
Key Laboratory of Salt Lake Resources and Chemistry, Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, No 18 Xinning Road, Xi'ning, Qinghai 810008, China University of Chinese Academy of Sciences, No. 19A Yuquan Road, Beijing 100049, China
a r t i c l e
i n f o
Article history: Received 13 November 2013 Received in revised form 16 September 2014 Accepted 21 April 2015 Available online 23 April 2015 Keywords: Carnallite Decomposition ratio Temperature Water amount
a b s t r a c t Carnallite is a double salt (KCl·MgCl2·6H2O or KMgCl3·6H2O), which is a common source of potassium chloride (KCl). It is treated with a mother liquor containing magnesium chloride (MgCl2), KCl and sodium chloride (NaCl), and then KCl and NaCl precipitate in the mother liquor solution. The amount of water and the temperature are the two crucial factors for the decomposition of carnallite in aqueous solution. Because of the lack of research on the decomposition process of carnallite, the yield and quality of the KCl produced are limited. In this study, we develop a model for the decomposition of carnallite based on the multi-temperature quaternary diagram of the NaCl–KCl–MgCl2–H2O system and the principle of mass conservation for MgCl2. The model is verified using experimental and literature data for different types of carnallite: normal, Mg-rich, Na-rich, and (Mg and Na)-rich. The proposed model predicts the amount of water required for decomposition of carnallite and the yield of KCl very well for normal and Mg-rich carnallite (within 2%), for Na-rich and (Mg and Na)-rich carnallite (within 8%), and for ancient evaporite carnallite in Laos (within 2%). © 2015 Elsevier B.V. All rights reserved.
1. Introduction Potassium chloride is an important chemical fertilizer in agriculture (Harper et al., 2012; Chen et al., 2013; Cheng et al., 2013). A source of KCl is the carnallite double salt, which has the composition KCl·MgCl2·6H2O or KMgCl3·6H2O, and is either obtained as the ore or recovered from brine that favors carnallite formation (Dave and Ghosh, 2006). Irrespective of its origin, carnallite is treated with decomposition mother liquor containing MgCl2, KCl, and NaCl, so that KCl and NaCl are precipitated in the equilibrium mother liquor. The driving force for decomposition is the difference in the concentration at the experimental temperature and the saturation of equilibrium mother liquor (Mohameed et al., 2002). The key factor is the amount of water used to dissolve MgCl2. This water causes the carnallite to decompose to its components. As is shown in Fig. 1(A), if the MgCl2 concentration is at or near the triple-saturation point (the point at which the solution is saturated with carnallite, NaCl, and KCl), the solubility of KCl is suppressed to the point where most of the KCl will precipitate (Liu et al., 2007). Moreover, the composition of the triple-saturation point changes with temperature (Fig. 1(B)). For maximum recovery of the components, the crystallizing mixture must be saturated with carnallite at its triple-saturation point. If the mixture is not saturated, then KCl will dissolve in the water. If the amount of water is not suitable or inadequate, carnallite will not decompose in the aqueous solution. Generally, the amount of water used to dissolve carnallite and the reaction ⁎ Corresponding author at: Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, No 18 Xinning Road, Xi'ning, Qinghai 810008, China. E-mail address: [email protected] (H. Cheng).
http://dx.doi.org/10.1016/j.minpro.2015.04.007 0301-7516/© 2015 Elsevier B.V. All rights reserved.
temperature are two important factors that restrict KCl production. In the crystallization processes, the solid–liquid phase equilibrium is very important. Furthermore, owing to the lack of a systematic study about the decomposition process of carnallite in aqueous solution by the solid–liquid phase equilibrium method, the yield and quality of KCl produced are limited. Here, a functional model for the amount of water and the decomposition ratio of carnallite in aqueous solution is established and evaluated from 256.15 to 323.15 K in quaternary solid–liquid phase equilibrium systems based on the mass transfer equilibrium of MgCl2 between the precipitated salts and the ionic species remaining in solution (Ettouney, 2006; Demming et al., 2008; Ji et al., 2010). As discussed above, we found that the water amount and decomposition ratio of carnallite are related to the temperature and composition of the equilibrium mother liquor. The model for the effect of the system properties on the decomposition process of carnallite was established by the multi-temperature quaternary diagram data of the NaCl–KCl– MgCl2–H2O system. This model and the decomposition process of modern solar-pond carnallite in the Qarhan Salt Lakes and ancient evaporite carnallite in Laos are discussed. 2. Experimental 2.1. Materials Carnallite was taken from the drilling core of Vientiane potash deposits in Laos. Its composition (Table 1) indicates that the carnallite is pure. The content of water-insoluble material in the carnallite was less than 0.5%. Doubly-distilled water was used in all the experiments, as well as for the analytical measurements. Standard glassware was
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
37
Fig. 1. Quaternary diagram of NaCl–KCl–MgCl2–H2O system. (A) at 298.15 K. (B) Partially enlarged multi-temperature from 263.15 to 348.15 K. (•) triple-saturation point, (Sy) KCl, (Ha) NaCl, (Car) KCl·MgCl2·6H2O, and (Bis) MgCl2·6H2O.
used in all experiments. Whenever heating was required, a super constant temperature water-bath (HH-501, Shanghai Qiqian Electronic Technology Co. Ltd., China) was used. The solid phases were obtained by vacuum filter (SHZ-3 circulating water vacuum pump, Shanghai Yarong Biochemistry Factory, China) separation. 2.2. Analytical methods The ionic compositions of the liquid and solid phases were determined using well-established procedures. The Mg2+ ion concentration was determined by complexometric titration with EDTA standard solution with an accuracy of ± 0.5%. The K+ ion concentration was determined by gravimetric method with sodium tetraphenylborate with an accuracy of ±0.3%. The Cl− ion concentration was determined by Hg(NO3)2 titration with an accuracy of ±0.5%. The Na+ ion concentration was calculated by the ion-equilibrium subtraction method with an accuracy of ±1%. 2.3. Experimental procedures The decomposition process of carnallite in aqueous solution is an endothermic reaction, and was carried out in a super constant temperature water-bath. The reaction temperature was controlled at 293.15 K (ambient temperature). The speed of the agitator was 600 rpm. The reaction time was 45 min, and 2000 ml standard glassware was used as the reaction vessel. The solid-phase minerals and liquid-phase solution were separated by vacuum filtration after the end of decomposition. Because there is water adsorbed to the crystals/minerals in the products of the decomposition, it is necessary to remove this water and discuss the experimental results in this study. There are four methods for the removal of this solution: (1) Washing with an organic solvent (Moretto, 1988; García-Veigas et al., 2009) (e.g. ethanol,
Table 1 The composition of carnallite from the drilling core of Vientiane potash deposits in Laos. Ion concentration, wt.%
Substance components, wt.%
K
Mg
Cl
Na
KCl
NaCl
MgCl2
H2O
Water-insoluble material
8.34
5.35
46.84
15.35
15.90
39.02
20.96
23.76
0.36
ethanol–glycol, or acetone), yet the effect of salting-out in organic solvent with high salinity would occur (Xu et al., 1991). (2) Washing with doubly-distilled water (McCaffrey et al., 1987), but some of crystals/minerals would dissolve because mineral salts are soluble. (3) Obtaining large-grained crystals with 5 mm in diameter by crystallization, and then the water adsorbed to the crystal surfaces can be removed by tissues (Siemann and Schramm, 2000). However, the KCl particle size at the end of decomposition is generally very small (b 0.5 mm) (Mohameed et al., 2002; Li et al., 2009) and it is difficult to avoid fluid inclusion. (4) Removal with the evaporation of mother liquor method (Herrmann, 1980). This method has been applied to research the crystallization behavior of borate during the brine evaporation process (Gao and Li, 1982). The pure solid phase of precipitation is obtained through deducting the solution attached to the crystal/mineral surfaces. The ionic composition of the equilibrium mother liquid is constant at the desired temperature, and was accurately determined. Furthermore, the amount of water in the equilibrium mother liquid is known. To estimate fluid inclusion, method (4) was applied. The product crystals/minerals obtained from decomposition were placed in a drying oven on forced convection at 373.15 K for 4 h (Cheng et al., 2009). The amount of water adsorbed to the crystal/mineral surface was calculated based on the loss of weight with the drying method. Additionally, the mass of the product is always lower than the mass of the feed because of the viscosity of the brine and solution adsorbing to the surface of the container during the experiments. When the quantity of the solid phase in the product is weighed by electronic balance with an accuracy of ±0.01 g, the loss of mass of the system is the quantity of the liquid phase, which can be calculated by the mass balance of the feed and the product. The components of the liquid phase are constant at the desired temperature. Therefore, we can estimate the decomposition ratio of carnallite based on our experimental results. The decomposition ratio of carnallite in aqueous solution includes the KCl yield and the MgCl2 decomposition yield. The KCl yield is given by εKCl ¼
M KClðSÞ −MKClðLÞ 100%: MKClðSÞ
ð1Þ
The MgCl2 decomposition yield is given by εMgCl2 ¼
MMgCl2 ðLÞ MMgCl2 ðSÞ
100%:
ð2Þ
38
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
MKCl(S) and M MgCl2 ðSÞ are the weights of KCl and MgCl2 in the carnallite ore before decomposition, and MKCl(L) and MMgCl2 ðLÞ are the weights of KCl and MgCl2 in the equilibrium mother liquor at the end of decomposition. The weights of KCl and MgCl2 in the solution adsorbed to the crystal surface that can be gained by evaporation of mother liquor method are included in MKCl(L) and MMgCl2 ðLÞ , respectively.
composition of carnallite (SC,M(α1, Q)). Therefore, the functional relationship of Eq. (6) can be described as f ðW Þp ¼ F SC;M ; LC;T ; δ :
The KCl yield and MgCl2 decomposition yield can be determined using Eqs. (1), (2), (5), and (6), respectively.
3. Modeling The various processes used for KCl production from carnallite depend upon the equilibria existing in the quaternary salt system at the desired temperature. When the carnallite is decomposed by adding an appropriate amount of water, MgCl2 goes into the solution of the added water but KCl crystallizes. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ LðEÞ;
ð3Þ
where L(E) is the triple-saturation equilibrium mother liquor at the desired temperature. However, if only a small amount of water is added, the carnallite cannot completely decompose in the aqueous solution. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ LðEÞ þ ð1−XÞKMgCl3 6H2 OðsÞ;
ð4Þ
X denotes the percent decomposition of the carnallite. In both cases, the process occurs at the interface between the solid and liquid phases, MgCl2 dissolves completely or incompletely into the liquid phase from the carnallite crystals, and KCl is crystallized. Thus, the relationship of mass conservation of MgCl2 between reactants and products is obtained from Eqs. (3) and (4): α1 Q ¼
δW þ Aα 1 Q 1−α 2 −β2 −γ 2
α 2 þ ð1−δÞα 1 Q
ð5Þ
where A is the 6H2O/MgCl2 molecular mass ratio in carnallite, which is a constant (1.1343). The 6H2O and MgCl2 in carnallite would enter into the solution during carnallite decomposition in aqueous solution. The concentration of MgCl 2 in solution can be obtained from Fig. 1(A) (Niu and Cheng, 2002). W represents the theoretical water amount used to decompose carnallite, which can be calculated based on Fig. 1(A) (Niu and Cheng, 2002). δ is the ratio of the actual water amount to the theoretical water amount: δ = Wact / Wtheo × 100%, where Wact is the actual water amount and Wtheo is the theoretical water amount. The left-hand side of Eq. (5) represents the mass of MgCl2 component of carnallite before decomposition. The first term on the righthand side of Eq. (5) represents the mass of the MgCl2 component of the triple-saturation equilibrium mother liquor at the end of decomposition. The second term on the right-hand side of Eq. (5) represents the mass of the MgCl2 component of the residual carnallite at the end of decomposition. This model can be used to investigate the conditions of the variant water amount used to decompose carnallite, and estimate the required values of the operating parameters to achieve a desired decomposition ratio of carnallite. Eq. (5) can be simplified as W¼
1−α 2 −β2 −γ2 A α 1 Q: − δ α2
ð6Þ
Analysis of Eq. (6) indicates that the water amount is intimately related to three factors: (1) the composition of the triple-saturation equilibrium mother liquid at the desired temperature (LC,T(α2, β2, γ2)); (2) the ratio of additional water amount (δ); and (3) the weight and
ð7Þ
εKCl ¼
δα β 1− 1 2 α 2 β1
ð8Þ
εMgCl2 ¼ δ
ð9Þ
The functional relationships of Eqs. (8) and (9) can be described as follows: f ðε KCl Þp ¼ R SC ; LC;T ; δ
ð10Þ
f εMgCl2 ¼ P ðδÞ:
ð11Þ
p
In addition, when carnallite is decomposed by adding an excessive amount of water, MgCl2 goes into solution but KCl is crystallized. Furthermore, KCl and NaCl partially dissolve because of the extra water. The process occurs in a suspension and can be represented as T;P
KMgCl3 6H2 OðsÞ þ NaClðsÞ þ H2 OðlÞ → KClðsÞ þ NaClðsÞ þ RðEÞ;
ð12Þ
where R(E) is the double-saturation equilibrium mother liquor (the solution is saturated with NaCl and KCl in the quaternary diagram of NaCl–KCl–MgCl2–H2O system) at the desired temperature. Therefore, the relationship of mass conservation for MgCl2 between reactants and products is obtained from Eq. (12). α1 Q ¼
W þ Aα 1 Q 1−α 3 −β3 −γ3
α3
ð13Þ
Eq. (13) can then be solved for W: W¼
1−α 3 −β3 −γ3 −A α 1 Q: α3
ð14Þ
Analysis of Eq. (14) indicates that the water amount is intimately related to two factors: (1) the composition of the double-saturation equilibrium mother liquor at the desired temperature (RC,T(α3, β3, γ3)); and (2) the weight and composition of carnallite (SC,M(α1, Q)). Thus, the functional relationship of Eq. (14) can be written as ∀
f ðW Þp ¼ F
∀
SC;M ; RC;T :
ð15Þ
In this case, the KCl yield and MgCl2 decomposition yield at the desired temperature can be calculated using Eqs. (1), (2), (12), and (13).
∀
εKCl ¼
1−
α 1 β3 α 3 β1
∀
εMgCl2 ¼ 1
ð16Þ ð17Þ
Thus, the functional relationship of Eqs. (16) and (17) can be described as follows: ∀
f ðεKCl Þp ¼ R
f
∀
∀
SC ; RC;T
εMgCl2 ¼ P ðBÞ: p
ð18Þ
ð19Þ
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
39
Table 2 The model parameters of f(W)p, f(εKCl)p, and f εMgCl2 p for a wide range of temperatures (256.15–323.15 K). T(K)
wt.% MgCl2α2
KClβ2
NaClγ2
f(εKCl)p = R(SC, LC,T, δ)
f εMgCl2
δ=1
δ=1
δb1
δ=1
δb1
1 1− 0:0615α β1 1 1− 0:0726α β1 0:0918α 1 1− β 1 1 1− 0:1050α β1 0:1162α 1 1− β 1 1 1− 0:1270α β1 1 1− 0:1296α β1 1 1− 0:1234α β1 1 1− 0:1404α β1 1 1− 0:1636α β1
1δ 1− 0:0615α β1 0:0726α 1 δ 1− β 1 1δ 1− 0:0918α β1 0:1050α 1 δ 1− β 1 1δ 1− 0:1162α β1 1δ 1− 0:1270α β1 1δ 1− 0:1296α β1 1δ 1− 0:1234α β1 0:1404α 1 δ 1− β 1 1δ 1− 0:1636α β1
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
1
δ
256.15
24.86
1.53
1.90
1.7503α1Q
263.15
24.80
1.80
1.95
1.7467α1Q
273.15
25.05
2.30
1.90
1.6901α1Q
283.15
25.43
2.67
1.92
1.6176α1Q
288.15
25.39
2.95
1.90
1.6132α1Q
293.15
25.44
3.23
1.88
1.5957α1Q
298.15
25.85
3.35
1.80
1.5349α1Q
303.15
26.30
3.27
1.60
1.4828α1Q
313.15
26.70
3.75
1.65
1.4088α1Q
323.15
26.90
4.40
1.80
1.3527α1Q
δb1
α1 Q 2:8810− 1:1343 α1 Q δ α1 Q 2:8244− 1:1343 δ α1 Q 2:7519− 1:1343 δ α1 Q 2:7475− 1:1343 δ α1 Q 2:7300− 1:1343 δ α1 Q 2:6692− 1:1343 δ α1 Q 2:6171− 1:1343 δ α1 Q 2:5431− 1:1343 δ α1 Q 2:4870− 1:1343 δ 2:8846− 1:1343 δ
Generally, the functional model (Eqs. (7), (10), (11), (15), (18), and (19)) of the water amount, KCl yield, MgCl2 decomposition yield is attainable. The decomposition process of carnallite will be investigated at the desired temperature through these models. 4. Results and discussion In this study, the hypothesis is that the decomposition process of carnallite in aqueous solution either completely or incompletely occurs, and the reaction pressure is normal atmospheric pressure. A series of experiments were carried out at room temperature. To obtain the model parameters and demonstrate the model, the data for the composition of the triple-saturation equilibrium mother liquor should be obtained. Generally, the reaction temperature of decomposition is in the range from 280 to 300 K during the process of KCl production from carnallite. However, in a few areas, such as in winter at the Qarhan Salt Lakes in China, the reaction temperature may be drop to 268.15 K. Furthermore, in summer in Laos the reaction temperature may exceed 300 K. Because the literature data for the composition of the triplesaturation equilibrium mother liquor are available in various temperature ranges, we chose the temperature range from 256.15 to 323.15 K. However, if the carnallite is decomposed by adding superfluous water, the parameters of the predicated model for the water amount, KCl yield, and MgCl2 decomposition yield are under-estimated. In this case, the mean spherical approximation method is usually used to calculate the composition of the double-saturation equilibrium mother liquor in the quaternary system (NaCl–KCl–MgCl2–H2O) under the desired temperature and water amount conditions (Lu et al., 1993; López-Pérez et al., 2000; Yu et al., 2003; Ge and Wang, 2009; Memarnejad and Dehghani, 2012). The compositions of the triple-saturation equilibrium mother liquor (Zdanovskiy et al., 1975) at various temperatures are listed in Table 2. From Table 2, it can be seen that the solubility of KCl and MgCl2
1.9
W ¼ ð−0:0062T þ 3:3883Þα 1 Q:
εKCl ¼ 1−
KCl)P/R(S C,LC,T )
1.5 2
R = 0.9715
f(
f(W)P/F(S C,M,LC,T )
1.6
1.3
¼ P ðδÞ
ð20Þ
Analysis of Eq. (20) indicates that (1) W decreases as T increases, when α1 and Q are constant and (2) W increases as α1 increases when T and Q are constant. This suggests that (1) the water amount is a univariate function of the MgCl2 content of carnallite when the reaction temperature is constant and (2) the water amount is also a univariate function of temperature when the composition of carnallite is known. Thus, we can calculate the amount of water required to decompose carnallite at any temperature and any composition of carnallite. The model expressed in Eq. (10) indicates that there is a linear relationship between f(εKCl)p and R(SC, LC,T)δ, and the functional relationship was established as
(A)
1.7
p
increases with increasing temperature, while the solubility of NaCl shows a nonlinear variation with temperature. From the data collected in Table 2, the calculated values of the model parameters from 256.15 to 323.15 K using Eqs. (7), (10), and (11) are obtained (see Table 2). The functional relationships expressed in Eqs. (7) and (10) are shown in Fig. 2. The ratio of f(W)p/F(SC,M, LC,T, δ) shown in Fig. 2(A) decreases as the temperature increases, indicating that the amount of water required to decompose carnallite decreases with increasing temperature. In contrast, the ratio of f(εKCl)p/R(SC, LC,T, δ) shown in Fig. 2(B) increases with increasing temperature, indicating that the solubility of KCl in the balance mother liquor increases with increasing temperature. This is the principle that is used to improve the KCl yield from carnallite by the cold-decomposition method. The model expressed in Eq. (7) shows a linear relationship between f(W)p and F(SC,M, LC,T)δ, and the functional relationship was established as
1.8
1.4
f(W)p = F(SC,M, LC,T, δ)
0.19 0.17 0.15 0.13 0.11 0.09 0.07 0.05
α 1 ð0:0014T−0:3033Þ : β1
(B)
2
R = 0.9701
250 260 270 280 290 300 310 320 330
250 260 270 280 290 300 310 320 330
T(K)
T(K)
Fig. 2. Relationships between (A) f(W)p/F(QC,M, LC,T)δ and temperature, and (B) f(εKCl)p/R(QC, LC,T)δ and temperature.
ð21Þ
40
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
48
(A)
43
46
41
44
W(g)
W(g)
45
39 37
(B)
42 40
35
38 270
275
280
285
290
295
300
270
275
280
T(K)
290
295
300
290
295
300
T(K) 38
(C)
34
36
32
34
W(g)
W(g)
36
285
30 28
(D)
32 30
26
28 270
275
280
285
290
295
300
270
275
280
T(K)
285 T(K)
Fig. 3. Relationship between the amount of water (W) and temperature (T) for the decomposition of different types of carnallite: (A) normal, (B) Mg-rich, (C) Na-rich, and (D) (Mg and Na)-rich. Solid lines represent the model results and dashed lines represent the literature data (Wang, 2000).
Analysis of Eq. (21) indicates that (1) εKCl decreases as T increases when α1 and β1 are constant, (2) εKCl decreases as α1 increases when α1 and β1 are constant, and (3) εKCl increases as β1 increases when T and α1 are constant. It can be seen that (1) the KCl yield changes inversely with temperature when the composition of carnallite is constant, and a low reaction temperature is favorable for increasing the KCl yield, (2) the KCl yield is a bivariate function of the composition of carnallite when the reaction temperature is constant, and (3) the KCl yield is restricted to the KCl content of carnallite. Wang (2000) collected more than 2000 different types of carnallite samples from the Qarhan Salt Lakes in China, and calculated the amount
0.88
of water used to decompose carnallite and the KCl yield for various temperatures (273.15, 283.15, and 298.15 K). According to the average composition of these carnallite samples, there are four types of carnallite: normal, Mg-rich, Na-rich, and (Mg and Na)-rich. For the various types of carnallite, the predicted values of the model (Eq. (20)) were compared with the calculated values from the literature (Wang, 2000), as shown in Fig. 3. Fig. 3 shows the relationship between the water amount used to decompose the different types of carnallite (normal, Mg-rich, Na-rich, and (Mg and Na)-rich) and the temperature, and the model can be used to describe the actual decomposition process of the different types of carnallite, in particular the model describes the
(A)
0.84
KCl
KCl
0.86
0.82 0.8 0.78 270
275
280
285
290
295
0.82 0.8 0.78 0.76 0.74 0.72 0.7 0.68
300
(B)
270
275
280
T(K) 0.88
0.84
KCl
KCl
0.86
0.82 0.8 0.78 275
280
285 T(K)
290
295
300
290
295
300
T(K)
(C)
270
285
290
295
300
0.82 0.8 0.78 0.76 0.74 0.72 0.7 0.68
(D )
270
275
280
285 T(K)
Fig. 4. Relationship between f(εKCl)p and temperature (T) for the decomposition of different types of carnallite: (A) normal, (B) Mg-rich, (C) Na-rich, and (D) (Mg and Na)-rich. Solid lines represent the model results and dashed lines represent the literature data (Wang, 2000).
H. Cheng et al. / International Journal of Mineral Processing 139 (2015) 36–42
0.87
(A)
1
0.86
0.98
0.85 KCl
MgCl2
1.02
0.96
0.83
0.92
0.82
0.9
(B)
0.84
0.94
0.81 0.9
Fig. 5. Relationships between (A) f ε MgCl2
41
p
0.95 1 1.05 ratio of added water
1.1
0.9
0.95 1 1.05 ratio of added water
1.1
and the ratio of added water, and (B) f(εKCl)p and the ratio of added water for carnallite (Laos) decomposition at 293.15 K. Solid lines represent
the model results and dashed lines represent the experimental data in this study.
normal-type carnallite very well. However, the values of the model are slightly higher than the values of the calculated results for Mg-rich carnallite, and the difference is 2% to 3%. The values of the model are less than the calculated values for Na-rich and (Mg and Na)-rich carnallite, and the difference is about 8%. There is an important reason why about 10% extra water (free-water) was added compared to the theoretical water amount according to Wang's practical experience (Wang, 2000). The values of the free-water are included in the water amount used to decompose the carnallite. Moreover, the content of NaCl in the decomposition products for Na-rich and (Mg and Na)-rich carnallite is higher than normal-type carnallite, and only a small amount of NaCl is involved in the dissolution of the recrystallization process. As shown in Fig. 4, the KCl yield is related to the decomposition temperature for all the different types of carnallite (normal, Mg-rich, Na-rich, and (Mg and Na)-rich). The predicted values of the model (Eq. (21)) can be used to describe the actual decomposition process of various types of carnallite, and the model predicts the values of normal and Mg-rich carnallite very well. However, the values of the model are less than the calculated values for Na-rich and (Mg and Na)-rich carnallite, and the difference is about 2%. The reason is the same as that for the presence of 10% free-water in the decomposition products. The effect of the amount of water on the decomposition ratio of carnallite from ancient evaporite in Laos under isothermal conditions was also investigated, and the predicted model and experimental results are compared. The reaction temperature of 293.15 K was chosen for the isothermal temperature, and is consistent with the ambient temperature. This temperature was chosen because if the temperature is too high then evaporation of the balanced mother liquor occurs, and if the temperature is too low then the salting-out effect of the balanced mother liquor occurs, and both limit the discussion of the experimental results. Because the composition of carnallite in Laos is known (Table 1), the theoretical water amount required to decompose carnallite at 293.15 K can be calculated. From Eq. (20), the predicted amount of water is 98.77 g for 300 g carnallite. The ratios of added water (actual added water amount/predicted water amount) were chosen as 0.91, 0.94, 0.97, 1, 1.03, 1.06, and 1.09, and the experimental process and data analysis were treated using the description in Section 2.3. The relationship between the decomposition ratio and the temperature (293.15 K) was obtained by balance calculations of the experimental results, and the predicated model and the decomposition process of ancient evaporite–carnallite in Laos are discussed. The analytical results are shown in Fig. 5. As shown in Fig. 5, the model established in this research (Eqs. (8), (9), (16), and (17)) is consistent with the experimental results. Fig. 5 shows that the model describes the actual decomposition process of ancient evaporite–carnallite very well. The difference between the experimental results and the values predicted by the model is about 2%. This difference is probably due to systematic errors from the mass balance of feed and product.
5. Conclusions During the decomposition of carnallite in aqueous solution, the functional models for water amount f(W)p/f ∀(W)p, KCl yield f(εKCl)p/ ∀ f ∀(εKCl)p, and MgCl2 decomposition yield f εMgCl2 p / f εMgCl2 p from 256.15 to 323.15 K in the quaternary systems NaCl–KCl–MgCl2–H2O were investigated using the principle of mass conservation of MgCl2. The results showed that (1) the linear functional relationships, that is, f(W)p = F(SC,M, LC,T, δ), f(εKCl)p = R(SC, LC,T, δ), and f εMgCl2 p ¼ P ðδÞ, exist when the decomposition process completely/incompletely occurs and (2) the non-linear functional relationships, that is ∀ f ∀(W)p = F∀(SC,M, RC,T), f ∀(εKCl)p = R∀(SC, RC,T), and f εMgCl2 p ¼ P ðBÞ, exist when the decomposition process excessively occurs. Moreover, the water amount required to completely decompose carnallite linearly changes (W = (− 0.0062T + 3.3883)α1Q), and the KCl yield linearly Þ ). The model was verified by changes ( ε KCl ¼ 1− α 1 ð0:0014T−0:3033 β 1
experiment data and literature data. This indicated that the model corresponds well with the natural decomposition process of modern solar-pond carnallite in Qarhan Salt Lakes and ancient evaporite carnallite in Laos. This research provides the theoretical basis for the cold decomposition process to produce KCl from carnallite.
Nomenclature W amount of water used to decompose carnallite (g) Q mass of carnallite (g) T temperature (K) p pressure (Pa) α1 MgCl2 concentration in carnallite (wt.%) β1 KCl concentration in carnallite (wt.%) γ1 NaCl concentration in carnallite (wt.%) α2 MgCl2 concentration at the triple-saturation point (wt.%) β2 KCl concentration at the triple-saturation point (wt.%) γ2 NaCl concentration at the triple-saturation point (wt.%) α3 MgCl2 concentration at the double-saturation equilibrium mother liquor (wt.%) β3 KCl concentration at the double-saturation equilibrium mother liquor (wt.%) γ3 NaCl concentration at the double-saturation equilibrium mother liquor (wt.%) Acknowledgments The authors thank the West Light Foundation of CAS (2011-180) and the National Program on Key Basic Research Project of China (973 Program) (2011CB403004) for financial support.
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