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Effects of acidic concentrations, temperatures and nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid solutions
Accepted Manuscript Title: Effects of acidic concentrations, temperatures and nitrates on the solubility of K2 SiF6 in phosphoric acid and nitric acid solutions Author: Heng Wang Rui Li Caimei Fan Junqiang Feng Shouliang Jiang Zhongmin Han PII: DOI: Reference:
S0022-1139(15)00211-0 http://dx.doi.org/doi:10.1016/j.jfluchem.2015.07.011 FLUOR 8612
To appear in:
FLUOR
Received date: Revised date: Accepted date:
10-3-2015 12-7-2015 15-7-2015
Please cite this article as: H. Wang, R. Li, C. Fan, J. Feng, S. Jiang, Z. Han, Effects of acidic concentrations, temperatures and nitrates on the solubility of K2 SiF6 in phosphoric acid and nitric acid solutions, Journal of Fluorine Chemistry (2015), http://dx.doi.org/10.1016/j.jfluchem.2015.07.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract - Pictogram
Graphical abstract
H2O 5% H3PO4
1.0
15% H3PO4
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1.2
25% H3PO4 0.8
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5% HNO3
10% HNO3
0.6
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15% HNO3
0.4
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Solubility of K 2SiF6 (g/100 mL solvent)
1.4
0.2 0.0 45
M
25
65
Ac
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T (C)
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*Graphical Abstract - Synopsis
Graphical Abstract Effects of acidic concentrations, temperatures and nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid solutions
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phosphoric acid solution but were rising in nitric acid solution.
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The solubility values of K2SiF6 increased with rising temperature, and were declining in
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*Highlights (for review)
Highlights · Solubility of K2SiF6 in phosphoric acid and nitric acid solutions was reported as a function of the acidic concentrations. · Solubility of K2SiF6 in different acidic solutions was measured at three temperatures of 25, 45, and 65 °C.
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· Effects of different nitrates on the solubility of K2SiF6 in acidic solutions were also discussed.
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*Manuscript Click here to view linked References
Effects of acidic concentrations, temperatures and nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid solutions Heng Wang1, Rui Li1, Caimei Fan1*, Junqiang Feng2, Shouliang Jiang2, Zhongmin Han2 1. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024,
ip t
China
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2. Shenzhen Batian Ecological Engineering Co., Ltd, Shenzhen 518057, China
*Corresponding author.
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Tel: +86-0351-6018193; Fax: +86-0351-6018554; E-mail: [email protected]
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ABSTRACT In this paper, the solubility values of potassium hexafluorosilicate (K2SiF6) were measured in phosphoric acid, nitric acid and mixed acidic aqueous solutions according to the different acidic concentrations and different temperatures of 25, 45, and 65 °C. The influence of different nitrates on the
ip t
solubility of K2SiF6 in mixed acidic aqueous solutions was also discussed. The results revealed that the solubility values of K2SiF6 increased with raising temperature, and were declining in phosphoric acid
cr
solution but were rising in nitric acid solution. The nitrates in acidic solutions have some influences on the solubility of K2SiF6. These obtained results would provide basic data for the fluorine removal from the
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production process of nitrophosphate fertilizer.
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Keywords: solubility; potassium hexafluorosilicate; phosphoric acid; nitric acid; nitrophosphate
1. Introduction
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Nitrophosphate is the general name of nitrogenous and phosphorus complex fertilizer by nitric acid decomposing phosphate rock. Current worldwide production of nitrophosphate is
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over 30000 kt/a. Tianji Brand nitrophosphate of China is a typical nitrogen and phosphorus
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fertilizer, total nutrient is 42%, the ratio of N/P2O5 is 2:1 [1]. The main steps of producing nitrophosphate based on the Odda process are as following: rock phosphate storage and handling, digestion of phosphate rock with nitric acid, crystallization and separation of calcium nitrate crystals from nitrophosphate acid, mother liquor neutralization with ammonia
[2].
Ac
gas, evaporation of slurry, granulation and drying and so on post-processing of the product
In the digestion process, nitric acid decomposing phosphate rock occurs according to the simplified reaction equation (1) [3, 4]: Ca5F(PO4)3 + 10HNO3 = 5Ca(NO3)2 + 3H3PO4 + HF
(1)
The exothermic digestion reactions make the temperature of digesting acidic mixture raise to about 65 °C. Hydrogen fluoride, evolved during the decomposition of phosphate rock,
2
Page 5 of 19
reacts with active silica coming into acid digestion liquid with the phosphate rock, to produce hydrofluosilicic acid according to the following equation [5]: 6HF + SiO2 = H2SiF6 + 2H2O
(2)
Then the acid digestion liquid contained phosphoric acid, nitric acid, calcium nitrate,
ip t
fluosilicic acid and other nitrates impurities such as magnesium nitrate, aluminum nitrate and iron (III) nitrate. The presence of H2SiF6 in the acid digestion liquid will cause a series of
cr
problems for subsequent processing processes, for example, the difficult separation of solid
us
and liquid, increased viscosity of the neutralized slurry and even the degeneration of dissolved phosphorus [2-5]. On the other hand, the fluorine in the fertilizer could cause the pollution to
an
ecosystems when the fertilizer was used. Therefore, the fluorine in the reaction system should be removed and reclaimed.
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In order to achieve the purpose of defluorination, researches generally design to introduce sodium ion or potassium ion in H2SiF6 aqueous solution, let them react and form the
ed
precipitation of hexafluorosilicates of alkaline metals like the equations (3) and (4) [6]: (3)
H2SiF6 + 2NaNO3 = Na2SiF6 + 2HNO3
(4)
ce pt
H2SiF6 + 2KNO3 = K2SiF6 + 2HNO3
Frayret et al. [7] have studied the solubilities of three hexafluorosilicates compounds of (NH4)2SiF6, K2SiF6 and Na2SiF6 in acidic solutions according to the concentration and the
Ac
nature of HF, HCl, H2SO4, and HNO3 aqueous solutions. The obtained results revealed that the solubility of K2SiF6 is lower than Na2SiF6 in water and nitric acid solutions. As we all know, potassium (K) is a beneficial element for plants. Hence, the addition of KNO3 in the acid digestion liquid can remove the fluorine and facilitate the production of nitrogen-phosphorus-potassium (NPK) compound fertilizer. However, up to now the solubility data of K2SiF6 is very lacking, only was measured in one single acidic species at 20 °C. And the effects of different acid medium, temperatures, and nitrates on the solubility of
3
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K2SiF6 are rarely researched. Therefore, in order to achieve effective defluorination, this work is carried out to study the variations of the solubility values of K2SiF6 in phosphoric acid and nitric acid solutions with different acidic concentrations at 25, 45, and 65 °C, and further to study the influence of
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different nitrates such as calcium nitrate, magnesium nitrate, aluminum nitrate, iron (III) nitrate on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions. The
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obtained results are meaningful for further understanding of the intrinsic attribute of K2SiF6
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dissolution in acidic solutions of phosphoric acid and nitric acid, and also can provide basic data for the fluorine removal from the production process of nitrophosphate fertilizer.
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2. Experimental 2.1. Materials and chemicals
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The details (CAS-No, purity, relative molar mass, and supplier) of all materials are listed in Table 1. All the chemicals were of analytical grade and were used as received without
Table 1
Chemical HNO3
7697-37-2
Purity (mass fraction) ≥65%
Relative molar mass 63.01
7664-38-2
≥85%
98.00
CAS No.
Ac
H3PO4
ce pt
All chemicals used in this work.
ed
further purification. Distilled water was used in all the experimental runs.
K2SiF6
16871-90-2
≥98.0%
220.26
NaOH
1310-73-2
≥96.0%
40.00
KNO3
7757-79-1
≥99.0%
101.10
Mg(NO3)2·6H2O
13446-18-9
≥99.0%
256.41
Al(NO3)3·9H2O
7784-27-2
≥99.0%
375.13
Fe(NO3)3·9H2O
7782-61-8
≥98.5%
404.00
Ca(NO3)2·4H2O
13477-34-4
≥99.0%
236.15
H2O
7732-18-5
-
18.015
Supplier Luoyang Chemical Reagent Tianjin Tian Li Chemical Reagents Factory Sinopharm Chemical Reagent Beijing Co., Ltd Sinopharm Chemical Reagent Beijing Co., Ltd Sinopharm Chemical Reagent Beijing Co., Ltd Tianjin Fu Chen Chemical Reagents Factory Tianjin Guangfu Fine Chemical Research Institute co., Ltd Tianjin Guangfu Fine Chemical Research Institute co., Ltd Tianjin Fu Chen Chemical Reagents Factory Taiyuan University of Technology
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2.2. Experimental procedures The solubility of potassium hexafluorosilicate (K2SiF6) was measured according to the different acidic species and concentrations of the solutions at different temperatures. Based on the composition of industrial acid digestion liquid, sets of tests were performed in phosphoric
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acid and nitric acid solutions with different acidic concentrations at 25, 45, and 65 °C. The first set of tests was carried out to measure the solubility of K2SiF6 in phosphoric acid
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solutions with different concentrations of 0%, 5%, 15%, 25%, respectively, but no nitric acid
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in solutions. The second set of tests was carried out to measure the solubility of K2SiF6 in nitric acid solutions with different concentrations of 0%, 5%, 10%, 15%, respectively, but no
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phosphoric acid in solutions. The third set of tests was performed with fixed nitric acid concentration of 5% and different phosphoric acid concentrations of 0%, 5%, 15% and 25%,
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respectively. The fourth set of tests was carried out with fixed phosphoric acid concentration of 15% and different nitric acid concentrations of 0%, 5%, 10% and 15%. To clarify the
ed
influence of different nitrates in acid digestion liquid of phosphate rock on the solubility of K2SiF6, the fifth set of tests was carried out with different nitrates concentrations in 15%
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phosphoric acid and 5% nitric acid mixtures at 25 °C. For each studied case, five parallel solutions have been prepared and analysed. In a typical experimental procedure, an excess amount of K2SiF6 was added in a
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quantitative bottle with 100 mL acidic solution, then the quantitative bottle sealed with parafilm was kept in a thermostated water bath at constant temperature of 25, 45, or 65 °C for one week to ensure the dissolution equilibrium of K2SiF6 in acidic solution. After that the undissolved residue was separated from the saturated solution, and washed with water and then dried at 105 °C overnight. The mass of dried residue was employed in solubility counting. The saturated solution was used for determining the concentrations of dissolved potassium ions (K+) and fluosilicic anions (SiF62-) in the saturated solution.
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2.3. Analytical methods Solubility could be calculated directly from the mass of dried residue [8]. Flame photometry analysis was performed on a flame photometer (flame photometer 410, Sherwood Scientific Ltd., Cambridge, UK), to measure potassium concentration in the saturated
ip t
solution. The concentration of SiF62- in the saturated filtrate was determined by acid-base titration
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method [9-11]. About 1 mL of the filtrate was mixed with about 50 mL of distilled water and
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a few drops of phenol phthalein (0.5%) were added to the mixture as indicator. The mixture was titrated slowly with 1.0 M NaOH solution until the color of the mixture turned to pink
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without fading at least 30 s. As consequence, the H3PO4 and HNO3 in the sample were neutralized at the first equivalence point. In order to prevent any hydrolysis reaction between
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NaOH and K2SiF6, the mixture should be kept in ice bath. After reaching the first equivalence point, about 300 mL of CO2-free water was added to the mixture and the whole mixture was
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heated until boiling point. Finally, K2SiF6 started to decompose according to the following reactions:
(5)
SiF4 + 4NaOH → 4NaF + H2SiO3 + H2O
(6)
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K2SiF6 → 2KF + SiF4
Titration continues until reached the equivalent point which would correspond to the
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concentration of SiF62- in the sample.
The dissociation of K2SiF6 in solutions is shown as follows: K2SiF6 (S) ↔ 2K+ (aq) + SiF62- (aq)
(7)
Ksp = [K+]2[SiF62-] = 4S3
(8)
Where Ksp is the solubility product and the square brackets denote concentration. S is the solubility value. In each case of the solution, the concentrations of K+ and SiF62- had been measured, and 6
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then the solubility values of K2SiF6 were calculated by equation (8). The given solubility value of K2SiF6 in each case was the average of solubility values calculated from the five parallel tests. The corresponding relative standard deviation (RSD) was calculated in the same way. The crystal structures of the K2SiF6 obtained in water were characterized by X-ray
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diffraction (XRD). Diffraction peaks of the crystalline phase were compared with those of the standard compounds reported in the JCPDS data base.
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3. Results and discussion
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3.1. The accuracy of measured solubility
Potassium hexafluorosilicate (K2SiF6) exists two crystalline forms: cubic and hexagonal
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[12-15]. After evaporation of water in a pure water solution of unsaturated K2SiF6 at 20 °C, an X-ray powder diffractogram demonstrates that the crystalline form of K2SiF6 powder obtained
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is the cubic form [7, 13]. Furthermore, the powder X-ray diffraction of K2SiF6 obtained from saturated solution at 353 K also validates that the solid phase is cubic form [16]. Figure 1
ed
shows the X-ray diffraction (XRD) patterns of K2SiF6 in water obtained at 25, 45, and 65 °C
ce pt
in our lab. It can be seen that all the detectable peaks of K2SiF6 obtained in water at different temperatures correspond well to pure cubic K2SiF6 (JCPDS No. 07-0217). Therefore we can
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deduce that K2SiF6 in aqueous solutions is present as the more stable cubic form.
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Intensity (a.u.)
K2SiF6 obtained in water at 65 C
K2SiF6 obtained in water at 45 C
30
ip t
620
622
440 531 600
511
422
331 420
400
311 222
40 50 60 2 Theta (degree)
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20
JCPDS No. 07-0217
70
80
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10
220
111
K2SiF6 obtained in water at 25 C
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Figure 1. XRD patterns of K2SiF6 obtained in water at 25, 45, and 65 °C.
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Meanwhile, in order to validate the analytical methods we used, the solubilities of K2SiF6 in pure water at 25, 45, and 65 °C and in nitric acid solutions with different acidic
ed
concentrations at 25 °C were determined firstly. The results are listed in Table 2 with the values found in the literature. Through unit conversion, it was found that the experimental
ce pt
data obtained in our lab and the corresponding literature values are very close. In water, because the dissolution process of K2SiF6 is endothermic in accordance with Le Chatelier's Principle and higher temperature is favorable for the dissolution of K2SiF6, so the solubility of
Ac
K2SiF6 increases with raising temperature. However, in nitric acid solutions, the deviations of the solubility of K2SiF6 can be attributed to the differences in temperature conditions and acidic concentrations. Hence the comparative results show that our experimental data is valid.
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Table 2 Comparisons between experimental solubility of K2SiF6 in water and nitric acid with those reported in the literature.
RSD (%)
Solutions
Temperature of solubility (°C)
RSD (%)
17.5 17.5 20 25 45 55 70 20 20 20
3.44
1.16 4.06 1.62
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Water Water Water Water 0.179 25 2.32 Water Water 0.262 45 3.15 Water Water 0.358 65 3.61 Water Water Nitric acid (5%) 0.496 25 2.05 Nitric acid (6.61%) Nitric acid (10%) 0.664 25 2.20 Nitric acid (11.80%) Nitric acid (15%) 0.698 25 3.48 Nitric acid (17.65%) RSD is calculated between the five measured solubility values of K2SiF6.
Solubility values found in the literature (g/100 mL) 0.120 [9] 0.121* [12] 0.117* [7] 0.177 [9] 0.268 [9] 0.322 [9] 0.420 [9] 0.504* [7] 0.654* [7] 0.736* [7]
ip t
Temperature of solubility (°C)
cr
Solutions
Measured solubility (g/100 mL)
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* means converting the molarity (mol/L) of literature data to weight-based solubility values (g/100 mL).
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3.2. Solubility in phosphoric acid and nitric acid solutions
The single phosphoric acid or nitric acid aqueous solutions with different acidic
ce pt
concentrations were firstly prepared to study the solubility of K2SiF6 at 25, 45, and 65 °C, and the trends of the variation were shown in Figure 2. The detailed solubility values of K2SiF6 in phosphoric acid and nitric acid solutions are listed in Table 3, the corresponding relative
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standard deviation (RSD) is also provided. The lower RSD values indicate that the solubility values have excellent repeatability. From Figure 2, it can be noted that whether in phosphoric acid or nitric acid solutions the solubility of K2SiF6 increases obviously with raising temperature. This phenomenon further confirmed that K2SiF6 is easier to dissolve in solutions at higher temperature. Figure 2(a) and Figure 2(b) show that the solubility of K2SiF6 at the corresponding temperatures has a decrease tendency in phosphoric acid solutions with the increasing acidic concentration, but an increase significantly in nitric acid solutions. This is associated with the nature of acidic 9
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species in solutions. The same variation of the solubility constant of K2SiF6 in nitric acid solutions at 20 °C was also obtained by literature [7]. Furthermore, an interesting phenomenon should be noted that the solubility values of K2SiF6 in nitric acid solutions are always higher than in phosphoric acid solutions no matter
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what the acidic concentration is. This can be explained by differences of the activity coefficients associated to each dissolved species in solutions. According to the dissociation of
2
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K2SiF6 in acidic media (equation 7), the activity coefficients can be defined as follows:
2
K ap K SiF62 f k2 K f SiF 2 SiF62 f K SiF62
(9)
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6
Where Kap is the activity product and the braces indicate activity. The square brackets
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denote concentration, f is the activity coefficient, and f± is the mean activity coefficient. As one of the thermodynamic equilibrium constants, the activity product of K2SiF6 keeps
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constant at a fixed temperature. For a strong acid, nitric acid can completely dissociated in
ed
solutions, leading to the increase of [H+] concentration, and then decrease the stability of K2SiF6 and the mean activity coefficient. However, phosphoric acid is a tribasic acid and the
ce pt
dissociation constants of phosphoric acid are pKa1 = 2.148, pKa2 = 7.199, and pKa3 = 12.30 at 25 °C [17]. According to this, the dissociation of phosphoric acid is step-wise and each dissociation gives up an H+ and a phosphate anion, respectively. Due to the presence of high
Ac
charge density ions, all the dissociated phosphate anions of H2PO4-, HPO42-, and PO43- exhibit strong salting-out effect [18]. Therefore the combined effects of the relative low [H+] concentration and the strong salting-out effect of phosphate anions increase the mean activity coefficient, resulting in the difficulty of K2SiF6 to dissolve in phosphoric acid solutions. With increasing concentration of phosphoric acid, these two effects would become more apparent at relatively low temperature. As shown in Figure 2(a), the solubility of K2SiF6 decreases directly with the increasing phosphoric acid concentration and the K2SiF6 is almost insoluble in 25% phosphoric acid solutions at 25 °C. 10
Page 13 of 19
0.4
0.2
0.0
5 10 15 20 H3PO4 concentration (wt %)
65 °C 45 °C 25 °C
1.0 0.8 0.6 0.4 0.2 0.0
0
(b)
1.2
ip t
65 °C 45 °C 25 °C
1.4
25
0
5 10 15 HNO3 concentration (wt %)
cr
(a)
Solubility (g per 100 mL solvent)
Solubility (g per 100 mL solvent)
0.6
20
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Figure 2. Variation of the solubility values of K2SiF6 with the acidic concentration in the solution at 25, 45,
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and 65 °C: (a) phosphoric acid, (b) nitric acid.
Table 3
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Experimental results for the solubility of K2SiF6 in H3PO4 or HNO3 solutions at 25, 45, and 65 °C. Solubility of K2SiF6 (g per 100 mL solvent)
(HNO3)wt% 25 °C
RSD (%)
45 °C
RSD (%)
65 °C
RSD (%)
0.179
2.32
0.262
3.15
0.358
3.61
0.156
3.16
0.292
3.64
0.414
1.46
0.042
2.43
0.204
3.44
0.364
2.54
0
0.010
4.49
0.110
3.49
0.167
2.89
5
0.496
2.05
0.682
2.39
1.173
2.88
10
0.664
2.20
0.866
2.85
1.225
3.42
15
0.698
3.48
0.899
4.53
1.251
4.36
0
5
0
15
0
25 0 0
Ac
0
ce pt
0
ed
(H3PO4)wt%
3.3. Solubility in phosphoric acid and nitric acid mixed solutions
Table 4 presents the experimental results for the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25, 45, and 65 °C. Figure 3 displays the variations of solubility values of K2SiF6 with different acidic concentration in phosphoric acid and nitric acid solutions. Apparently, the solubility values in any mixed acid solutions increase with the raising temperature. 11
Page 14 of 19
From Figure 3(a), it can be found that all the solubility values of K2SiF6 decrease with the increasing phosphoric acid concentration in mixed acid solutions under 5% nitric acid concentration at 25, 45, and 65 °C. The phenomena can be ascribed to the strong salting-out effect of phosphate anions and the influence of phosphoric acid dissociation. It should be
ip t
noted that at the same temperature, K2SiF6 is more soluble in the mixed acid solutions containing 5% HNO3 than that in only H3PO4 aqueous solutions, indicating the presence of
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nitric acid in phosphoric acid solutions is beneficial for the dissolution of K2SiF6, and this
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effect is particularly evident for the low concentration phosphoric acid in the mixed acid solutions. The K2SiF6 is still almost insoluble in 25% phosphoric acid and 5% nitric acid
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solutions at 25 °C.
As shown in Figure 3(b), although the mixed acid solutions contain 15% phosphoric acid,
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the solubility values still increase with nitric acid concentration at 25, 45, and 65 °C. But the addition of phosphoric acid decreases the solubility of K2SiF6 at the corresponding
(a)
5% HNO3 at 45 C
1.2
0% HNO3 at 45 C
1.0
5% HNO3 at 25 C
0.8
0% HNO3 at 25 C
0.6 0.4 0.2 0.0
0% HNO3 at 65 C
ce pt
1.4
0
5 10 15 20 H3PO4 concentration (wt %)
25
1.6 Solubility (g per 100 mL solvent)
ed
5% HNO3 at 65 C
1.6
Ac
Solubility (g per 100 mL solvent)
temperature and nitric acid concentration.
15% H3PO4 at 65 C
(b)
1.4
0% H3PO4 at 65 C 15% H3PO4 at 45 C
1.2
0% H3PO4 at 45 C
1.0
15% H3PO4 at 25 C
0.8
0% H3PO4 at 25 C
0.6 0.4 0.2 0.0
0
5
10 15 20 25 HNO3 concentration (wt %)
30
35
Figure 3. (a) Variation of solubility values of K2SiF6 with phosphoric acid concentration in mixed acid solutions containing 0% and 5% HNO3 at 25, 45, and 65 °C. (b) Variation of solubility values of K2SiF6 with nitric acid concentration in mixed acid solutions containing 0% and 15% H3PO4 at 25, 45, and 65 °C.
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Table 4 Experimental results for the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25, 45, and 65 °C. Solubility of K2SiF6 (g/100 mL solvent) (HNO3)wt% 25 °C
RSD (%)
45 °C
RSD (%)
65 °C
RSD (%)
1.173
2.88
0
5
0.496
2.05
0.682
2.39
5
5
0.419
1.62
0.574
4.84
15
5
0.180
2.83
0.358
3.75
25
5
0.004
4.67
0.118
2.86
15
0
0.042
2.43
0.204
15
5
0.180
3.89
0.358
15
10
0.332
2.69
0.428
15
15
0.361
3.25
0.490
ip t
(H3PO4)wt%
2.25
0.664
3.16
0.394
4.06
3.44
0.364
2.54
3.26
0.664
2.05
2.49
0.992
2.28
2.59
1.182
2.67
an
us
cr
0.836
M
3.4. The influence of different nitrates on the solubility of K2SiF6 in mixed acid solutions
As mentioned above, the industrial acid digestion liquid always contains phosphoric acid,
ed
nitric acid, calcium nitrate and other nitrate impurities such as magnesium nitrate, aluminum nitrate and iron (III) nitrate. Furthermore, as a potential precipitant for the fluoride removal,
ce pt
potassium nitrate will inevitably influence the solubility of K2SiF6. So it is necessary to study the influence of different nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions. Then the Ca2+, Mg2+, Al3+, Fe3+, and K+ nitrates were added into the acid
Ac
mixture containing 15% phosphoric acid and 5% nitric acid at 25 °C, respectively, and the solubility values of K2SiF6 in above solutions were measured and shown in Figure 4. As can be seen from Figure 4, the solubility of K2SiF6 showed different variations in the presence of different nitrates in mixed acid solutions. The behavior can be attributed to the charge of the cations and the number of ions furnished in the solution [19]. This kind of behavior has also been reported by Banat et al. [20] for propionic acid-water system. According to Figure 4 (a), the solubility values of K2SiF6 decrease directly with the increasing
13
Page 16 of 19
potassium nitrate concentration in acid mixed solutions, which is mainly attributed to the common ion effect of K+ [8]. And the addition of calcium nitrate in acid mixed solutions can significantly inhibit the solubility of K2SiF6, which is ascribed to the salting-out effect of large amount of calcium nitrate. However, the solubility values of K2SiF6 increase with the increase
ip t
of magnesium nitrate concentration in mixed acid solutions. The results demonstrate that magnesium nitrate can decrease the mean activity coefficients and promote the solubility
cr
values of K2SiF6, owing to the formation of MgSiF6 which is more soluble in the solution
us
than K2SiF6 [9, 10]. As for the trivalent cation salts Al(NO3)3 and Fe(NO3)3, with the increasing nitrates concentration, the solubility of K2SiF6 appears an increased firstly and then
an
declined trend. The phenomena occurred is mainly because the solubility equilibrium of systems containing salts of trivalent or bivalent cations is more complicated than systems
(a)
M
Mg(NO3)2
0.2
Solubility (g per 100 mL solvent)
0.6
ed
Al(NO3)3 Fe(NO3)3
0.4
KNO3
ce pt
Solubility (g per 100 mL solvent)
containing salts of univalent cations [21].
0.2
0.0
1 2 3 Nitrates concentration (wt %)
4
Ca(NO3)2
0.1
0.0
0
10 20 30 40 50 Calcium nitrates concentration (wt %)
60
Ac
0
(b)
Figure 4. The influence of different nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25 °C.
4. Conclusion Based on the experimental results, the following conclusions could be drawn: 1. The dissolution of K2SiF6 in solutions is endothermic and the solubility values of K2SiF6 always increase with the raising temperature whether in phosphoric acid, nitric acid or phosphoric acid and nitric acid mixed solutions. 14
Page 17 of 19
2. In phosphoric acid solutions, the solubility values of K2SiF6 appear an increased firstly and then declined trend with increasing phosphoric acid concentration at 45, and 65 °C, but decrease directly with phosphoric acid concentration at 25 °C. In nitric acid solutions, the solubility values of K2SiF6 increase with the increase of
ip t
nitric acid concentration at 25, 45, and 65 °C. 3. In phosphoric acid and nitric acid mixed solutions, the solubility values of K2SiF6
cr
increase with increasing nitric acid concentration at 25, 45, and 65 °C under 15%
us
phosphoric acid concentration. However, the solubility values of K2SiF6 appear a declined trend with the increasing phosphoric acid concentration under 5% nitric acid
an
concentration.
4. The solubility values of K2SiF6 increase with the increased magnesium nitrate
M
concentration in phosphoric acid and nitric acid mixed solutions, while the aluminum nitrate and iron (III) nitrate concentrations do not cause the obvious changes of
ed
K2SiF6 solubility. The presence of potassium nitrate and large amount of calcium
ce pt
nitrate in mixed acid solutions makes the solubility of K2SiF6 decrease.
Acknowledgements
The work described in this paper was substantially supported by a grant from the
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Shenzhen Batian Ecological Engineering Co., Ltd (Project no: 2013-0909).
References
[1] J.F. Fei, Engineering Technology of Nitrogenous Fertilizer. 24 (2003) 44-46. [2] I. Hussain, Procedia. Eng. 46 (2012) 172-177. [3] H.J. Jing, Freezing Nitrophosphate Production Technology, first ed., Chemical Industry Press, Beijing, 2002. [4] E.A. Abdel-aal, A.M. Amer, Miner. Eng. 8 (1995) 1221-1230.
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[5] Y.T. Zhang, Phosphate & Compound Fertilizer. 17 (2002) 36-37. [6] H.J. Jing, C.M. Fan, J.Q. Feng, X.H. Chang, Z.H. Wang, A.G. Wei, J.L. Liu, R. Li, H.S. Gao, Chinese Patent CN 103073344, 2013. [7] J. Frayret, A. Castetbon, G. Trouve, M. Potin-Gautier, Chem. Phys. Lett. 427 (2006)
ip t
356-364. [8] C. Rissom, H. Schmidt, W. Voigt, Cryst. Res. Technol. 43 (2008) 74-82.
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[9] R.H. Carter, Ind. Eng. Chem. 22 (1930) 886-887.
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[10] K.K. Worthington, M.M. Haring, Ind. Eng. Chem. 3 (1931) 7-9.
[11] M. Al-Harahsheh, M. Batiha, S. Kraishan, H. Al-Zoubi, Sep. Purif. Technol. 123 (2014)
an
190-199.
[12] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 56th ed., CRC Press, Cleveland,
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1975-1976.
[13] J.H. Loehlin, Acta Crystallogr. C 40 (1984) 570.
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[14] J.R. Hester, E.N. Maslen, N. Spadaccini, Acta Crystallogr. B 49 (1993) 967-973. [15] L. Kolditz, H. Preiss, Z. Anorg. Allg. Chem. 325 (1963) 245-251.
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[16] M. Skafi, M. El Guendouzi, MATEC Web of Conferences, 5 (2013) 04003. http://dx.doi.org/10.1051/matecconf/20130504003 [17] J. Bassett, R.C. Denney, G.H. Jeffery, J. Mendham, Vogel’s Textbook of Quantitative
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Inorganic Analysis, 4th ed., Longman Inc., New York, 1978. [18] T. Mourão, A.F.M. Cláudio, I. Boal-Palheiros, M.G. Freire, J.A. Coutinho, J. Chem. Thermodyn. 54 (2012) 398-405. [19] J. Dhanalakshmi, P.S.T. Sai, A.R. Balakrishnan, Fluid. Phase. Equilibr. 379 (2014) 112-119. [20] F. Banat, S. Al-Asheh, J. Simandl, Chem. Eng. Process. 42 (2003) 759-766. [21] A. Balaban, G. Kuranov, N. Smirnova, Fluid. Phase. Equilibr. 197 (2002) 717-728.
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S0022-1139(15)00211-0 http://dx.doi.org/doi:10.1016/j.jfluchem.2015.07.011 FLUOR 8612
To appear in:
FLUOR
Received date: Revised date: Accepted date:
10-3-2015 12-7-2015 15-7-2015
Please cite this article as: H. Wang, R. Li, C. Fan, J. Feng, S. Jiang, Z. Han, Effects of acidic concentrations, temperatures and nitrates on the solubility of K2 SiF6 in phosphoric acid and nitric acid solutions, Journal of Fluorine Chemistry (2015), http://dx.doi.org/10.1016/j.jfluchem.2015.07.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
*Graphical Abstract - Pictogram
Graphical abstract
H2O 5% H3PO4
1.0
15% H3PO4
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1.2
25% H3PO4 0.8
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5% HNO3
10% HNO3
0.6
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15% HNO3
0.4
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Solubility of K 2SiF6 (g/100 mL solvent)
1.4
0.2 0.0 45
M
25
65
Ac
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T (C)
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*Graphical Abstract - Synopsis
Graphical Abstract Effects of acidic concentrations, temperatures and nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid solutions
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an
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phosphoric acid solution but were rising in nitric acid solution.
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The solubility values of K2SiF6 increased with rising temperature, and were declining in
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*Highlights (for review)
Highlights · Solubility of K2SiF6 in phosphoric acid and nitric acid solutions was reported as a function of the acidic concentrations. · Solubility of K2SiF6 in different acidic solutions was measured at three temperatures of 25, 45, and 65 °C.
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ed
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an
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cr
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· Effects of different nitrates on the solubility of K2SiF6 in acidic solutions were also discussed.
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*Manuscript Click here to view linked References
Effects of acidic concentrations, temperatures and nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid solutions Heng Wang1, Rui Li1, Caimei Fan1*, Junqiang Feng2, Shouliang Jiang2, Zhongmin Han2 1. College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024,
ip t
China
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2. Shenzhen Batian Ecological Engineering Co., Ltd, Shenzhen 518057, China
*Corresponding author.
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Tel: +86-0351-6018193; Fax: +86-0351-6018554; E-mail: [email protected]
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ABSTRACT In this paper, the solubility values of potassium hexafluorosilicate (K2SiF6) were measured in phosphoric acid, nitric acid and mixed acidic aqueous solutions according to the different acidic concentrations and different temperatures of 25, 45, and 65 °C. The influence of different nitrates on the
ip t
solubility of K2SiF6 in mixed acidic aqueous solutions was also discussed. The results revealed that the solubility values of K2SiF6 increased with raising temperature, and were declining in phosphoric acid
cr
solution but were rising in nitric acid solution. The nitrates in acidic solutions have some influences on the solubility of K2SiF6. These obtained results would provide basic data for the fluorine removal from the
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production process of nitrophosphate fertilizer.
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Keywords: solubility; potassium hexafluorosilicate; phosphoric acid; nitric acid; nitrophosphate
1. Introduction
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Nitrophosphate is the general name of nitrogenous and phosphorus complex fertilizer by nitric acid decomposing phosphate rock. Current worldwide production of nitrophosphate is
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over 30000 kt/a. Tianji Brand nitrophosphate of China is a typical nitrogen and phosphorus
ce pt
fertilizer, total nutrient is 42%, the ratio of N/P2O5 is 2:1 [1]. The main steps of producing nitrophosphate based on the Odda process are as following: rock phosphate storage and handling, digestion of phosphate rock with nitric acid, crystallization and separation of calcium nitrate crystals from nitrophosphate acid, mother liquor neutralization with ammonia
[2].
Ac
gas, evaporation of slurry, granulation and drying and so on post-processing of the product
In the digestion process, nitric acid decomposing phosphate rock occurs according to the simplified reaction equation (1) [3, 4]: Ca5F(PO4)3 + 10HNO3 = 5Ca(NO3)2 + 3H3PO4 + HF
(1)
The exothermic digestion reactions make the temperature of digesting acidic mixture raise to about 65 °C. Hydrogen fluoride, evolved during the decomposition of phosphate rock,
2
Page 5 of 19
reacts with active silica coming into acid digestion liquid with the phosphate rock, to produce hydrofluosilicic acid according to the following equation [5]: 6HF + SiO2 = H2SiF6 + 2H2O
(2)
Then the acid digestion liquid contained phosphoric acid, nitric acid, calcium nitrate,
ip t
fluosilicic acid and other nitrates impurities such as magnesium nitrate, aluminum nitrate and iron (III) nitrate. The presence of H2SiF6 in the acid digestion liquid will cause a series of
cr
problems for subsequent processing processes, for example, the difficult separation of solid
us
and liquid, increased viscosity of the neutralized slurry and even the degeneration of dissolved phosphorus [2-5]. On the other hand, the fluorine in the fertilizer could cause the pollution to
an
ecosystems when the fertilizer was used. Therefore, the fluorine in the reaction system should be removed and reclaimed.
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In order to achieve the purpose of defluorination, researches generally design to introduce sodium ion or potassium ion in H2SiF6 aqueous solution, let them react and form the
ed
precipitation of hexafluorosilicates of alkaline metals like the equations (3) and (4) [6]: (3)
H2SiF6 + 2NaNO3 = Na2SiF6 + 2HNO3
(4)
ce pt
H2SiF6 + 2KNO3 = K2SiF6 + 2HNO3
Frayret et al. [7] have studied the solubilities of three hexafluorosilicates compounds of (NH4)2SiF6, K2SiF6 and Na2SiF6 in acidic solutions according to the concentration and the
Ac
nature of HF, HCl, H2SO4, and HNO3 aqueous solutions. The obtained results revealed that the solubility of K2SiF6 is lower than Na2SiF6 in water and nitric acid solutions. As we all know, potassium (K) is a beneficial element for plants. Hence, the addition of KNO3 in the acid digestion liquid can remove the fluorine and facilitate the production of nitrogen-phosphorus-potassium (NPK) compound fertilizer. However, up to now the solubility data of K2SiF6 is very lacking, only was measured in one single acidic species at 20 °C. And the effects of different acid medium, temperatures, and nitrates on the solubility of
3
Page 6 of 19
K2SiF6 are rarely researched. Therefore, in order to achieve effective defluorination, this work is carried out to study the variations of the solubility values of K2SiF6 in phosphoric acid and nitric acid solutions with different acidic concentrations at 25, 45, and 65 °C, and further to study the influence of
ip t
different nitrates such as calcium nitrate, magnesium nitrate, aluminum nitrate, iron (III) nitrate on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions. The
cr
obtained results are meaningful for further understanding of the intrinsic attribute of K2SiF6
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dissolution in acidic solutions of phosphoric acid and nitric acid, and also can provide basic data for the fluorine removal from the production process of nitrophosphate fertilizer.
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2. Experimental 2.1. Materials and chemicals
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The details (CAS-No, purity, relative molar mass, and supplier) of all materials are listed in Table 1. All the chemicals were of analytical grade and were used as received without
Table 1
Chemical HNO3
7697-37-2
Purity (mass fraction) ≥65%
Relative molar mass 63.01
7664-38-2
≥85%
98.00
CAS No.
Ac
H3PO4
ce pt
All chemicals used in this work.
ed
further purification. Distilled water was used in all the experimental runs.
K2SiF6
16871-90-2
≥98.0%
220.26
NaOH
1310-73-2
≥96.0%
40.00
KNO3
7757-79-1
≥99.0%
101.10
Mg(NO3)2·6H2O
13446-18-9
≥99.0%
256.41
Al(NO3)3·9H2O
7784-27-2
≥99.0%
375.13
Fe(NO3)3·9H2O
7782-61-8
≥98.5%
404.00
Ca(NO3)2·4H2O
13477-34-4
≥99.0%
236.15
H2O
7732-18-5
-
18.015
Supplier Luoyang Chemical Reagent Tianjin Tian Li Chemical Reagents Factory Sinopharm Chemical Reagent Beijing Co., Ltd Sinopharm Chemical Reagent Beijing Co., Ltd Sinopharm Chemical Reagent Beijing Co., Ltd Tianjin Fu Chen Chemical Reagents Factory Tianjin Guangfu Fine Chemical Research Institute co., Ltd Tianjin Guangfu Fine Chemical Research Institute co., Ltd Tianjin Fu Chen Chemical Reagents Factory Taiyuan University of Technology
4
Page 7 of 19
2.2. Experimental procedures The solubility of potassium hexafluorosilicate (K2SiF6) was measured according to the different acidic species and concentrations of the solutions at different temperatures. Based on the composition of industrial acid digestion liquid, sets of tests were performed in phosphoric
ip t
acid and nitric acid solutions with different acidic concentrations at 25, 45, and 65 °C. The first set of tests was carried out to measure the solubility of K2SiF6 in phosphoric acid
cr
solutions with different concentrations of 0%, 5%, 15%, 25%, respectively, but no nitric acid
us
in solutions. The second set of tests was carried out to measure the solubility of K2SiF6 in nitric acid solutions with different concentrations of 0%, 5%, 10%, 15%, respectively, but no
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phosphoric acid in solutions. The third set of tests was performed with fixed nitric acid concentration of 5% and different phosphoric acid concentrations of 0%, 5%, 15% and 25%,
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respectively. The fourth set of tests was carried out with fixed phosphoric acid concentration of 15% and different nitric acid concentrations of 0%, 5%, 10% and 15%. To clarify the
ed
influence of different nitrates in acid digestion liquid of phosphate rock on the solubility of K2SiF6, the fifth set of tests was carried out with different nitrates concentrations in 15%
ce pt
phosphoric acid and 5% nitric acid mixtures at 25 °C. For each studied case, five parallel solutions have been prepared and analysed. In a typical experimental procedure, an excess amount of K2SiF6 was added in a
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quantitative bottle with 100 mL acidic solution, then the quantitative bottle sealed with parafilm was kept in a thermostated water bath at constant temperature of 25, 45, or 65 °C for one week to ensure the dissolution equilibrium of K2SiF6 in acidic solution. After that the undissolved residue was separated from the saturated solution, and washed with water and then dried at 105 °C overnight. The mass of dried residue was employed in solubility counting. The saturated solution was used for determining the concentrations of dissolved potassium ions (K+) and fluosilicic anions (SiF62-) in the saturated solution.
5
Page 8 of 19
2.3. Analytical methods Solubility could be calculated directly from the mass of dried residue [8]. Flame photometry analysis was performed on a flame photometer (flame photometer 410, Sherwood Scientific Ltd., Cambridge, UK), to measure potassium concentration in the saturated
ip t
solution. The concentration of SiF62- in the saturated filtrate was determined by acid-base titration
cr
method [9-11]. About 1 mL of the filtrate was mixed with about 50 mL of distilled water and
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a few drops of phenol phthalein (0.5%) were added to the mixture as indicator. The mixture was titrated slowly with 1.0 M NaOH solution until the color of the mixture turned to pink
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without fading at least 30 s. As consequence, the H3PO4 and HNO3 in the sample were neutralized at the first equivalence point. In order to prevent any hydrolysis reaction between
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NaOH and K2SiF6, the mixture should be kept in ice bath. After reaching the first equivalence point, about 300 mL of CO2-free water was added to the mixture and the whole mixture was
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heated until boiling point. Finally, K2SiF6 started to decompose according to the following reactions:
(5)
SiF4 + 4NaOH → 4NaF + H2SiO3 + H2O
(6)
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K2SiF6 → 2KF + SiF4
Titration continues until reached the equivalent point which would correspond to the
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concentration of SiF62- in the sample.
The dissociation of K2SiF6 in solutions is shown as follows: K2SiF6 (S) ↔ 2K+ (aq) + SiF62- (aq)
(7)
Ksp = [K+]2[SiF62-] = 4S3
(8)
Where Ksp is the solubility product and the square brackets denote concentration. S is the solubility value. In each case of the solution, the concentrations of K+ and SiF62- had been measured, and 6
Page 9 of 19
then the solubility values of K2SiF6 were calculated by equation (8). The given solubility value of K2SiF6 in each case was the average of solubility values calculated from the five parallel tests. The corresponding relative standard deviation (RSD) was calculated in the same way. The crystal structures of the K2SiF6 obtained in water were characterized by X-ray
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diffraction (XRD). Diffraction peaks of the crystalline phase were compared with those of the standard compounds reported in the JCPDS data base.
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3. Results and discussion
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3.1. The accuracy of measured solubility
Potassium hexafluorosilicate (K2SiF6) exists two crystalline forms: cubic and hexagonal
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[12-15]. After evaporation of water in a pure water solution of unsaturated K2SiF6 at 20 °C, an X-ray powder diffractogram demonstrates that the crystalline form of K2SiF6 powder obtained
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is the cubic form [7, 13]. Furthermore, the powder X-ray diffraction of K2SiF6 obtained from saturated solution at 353 K also validates that the solid phase is cubic form [16]. Figure 1
ed
shows the X-ray diffraction (XRD) patterns of K2SiF6 in water obtained at 25, 45, and 65 °C
ce pt
in our lab. It can be seen that all the detectable peaks of K2SiF6 obtained in water at different temperatures correspond well to pure cubic K2SiF6 (JCPDS No. 07-0217). Therefore we can
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deduce that K2SiF6 in aqueous solutions is present as the more stable cubic form.
7
Page 10 of 19
Intensity (a.u.)
K2SiF6 obtained in water at 65 C
K2SiF6 obtained in water at 45 C
30
ip t
620
622
440 531 600
511
422
331 420
400
311 222
40 50 60 2 Theta (degree)
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20
JCPDS No. 07-0217
70
80
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10
220
111
K2SiF6 obtained in water at 25 C
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Figure 1. XRD patterns of K2SiF6 obtained in water at 25, 45, and 65 °C.
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Meanwhile, in order to validate the analytical methods we used, the solubilities of K2SiF6 in pure water at 25, 45, and 65 °C and in nitric acid solutions with different acidic
ed
concentrations at 25 °C were determined firstly. The results are listed in Table 2 with the values found in the literature. Through unit conversion, it was found that the experimental
ce pt
data obtained in our lab and the corresponding literature values are very close. In water, because the dissolution process of K2SiF6 is endothermic in accordance with Le Chatelier's Principle and higher temperature is favorable for the dissolution of K2SiF6, so the solubility of
Ac
K2SiF6 increases with raising temperature. However, in nitric acid solutions, the deviations of the solubility of K2SiF6 can be attributed to the differences in temperature conditions and acidic concentrations. Hence the comparative results show that our experimental data is valid.
8
Page 11 of 19
Table 2 Comparisons between experimental solubility of K2SiF6 in water and nitric acid with those reported in the literature.
RSD (%)
Solutions
Temperature of solubility (°C)
RSD (%)
17.5 17.5 20 25 45 55 70 20 20 20
3.44
1.16 4.06 1.62
an
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Water Water Water Water 0.179 25 2.32 Water Water 0.262 45 3.15 Water Water 0.358 65 3.61 Water Water Nitric acid (5%) 0.496 25 2.05 Nitric acid (6.61%) Nitric acid (10%) 0.664 25 2.20 Nitric acid (11.80%) Nitric acid (15%) 0.698 25 3.48 Nitric acid (17.65%) RSD is calculated between the five measured solubility values of K2SiF6.
Solubility values found in the literature (g/100 mL) 0.120 [9] 0.121* [12] 0.117* [7] 0.177 [9] 0.268 [9] 0.322 [9] 0.420 [9] 0.504* [7] 0.654* [7] 0.736* [7]
ip t
Temperature of solubility (°C)
cr
Solutions
Measured solubility (g/100 mL)
M
* means converting the molarity (mol/L) of literature data to weight-based solubility values (g/100 mL).
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3.2. Solubility in phosphoric acid and nitric acid solutions
The single phosphoric acid or nitric acid aqueous solutions with different acidic
ce pt
concentrations were firstly prepared to study the solubility of K2SiF6 at 25, 45, and 65 °C, and the trends of the variation were shown in Figure 2. The detailed solubility values of K2SiF6 in phosphoric acid and nitric acid solutions are listed in Table 3, the corresponding relative
Ac
standard deviation (RSD) is also provided. The lower RSD values indicate that the solubility values have excellent repeatability. From Figure 2, it can be noted that whether in phosphoric acid or nitric acid solutions the solubility of K2SiF6 increases obviously with raising temperature. This phenomenon further confirmed that K2SiF6 is easier to dissolve in solutions at higher temperature. Figure 2(a) and Figure 2(b) show that the solubility of K2SiF6 at the corresponding temperatures has a decrease tendency in phosphoric acid solutions with the increasing acidic concentration, but an increase significantly in nitric acid solutions. This is associated with the nature of acidic 9
Page 12 of 19
species in solutions. The same variation of the solubility constant of K2SiF6 in nitric acid solutions at 20 °C was also obtained by literature [7]. Furthermore, an interesting phenomenon should be noted that the solubility values of K2SiF6 in nitric acid solutions are always higher than in phosphoric acid solutions no matter
ip t
what the acidic concentration is. This can be explained by differences of the activity coefficients associated to each dissolved species in solutions. According to the dissociation of
2
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K2SiF6 in acidic media (equation 7), the activity coefficients can be defined as follows:
2
K ap K SiF62 f k2 K f SiF 2 SiF62 f K SiF62
(9)
us
6
Where Kap is the activity product and the braces indicate activity. The square brackets
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denote concentration, f is the activity coefficient, and f± is the mean activity coefficient. As one of the thermodynamic equilibrium constants, the activity product of K2SiF6 keeps
M
constant at a fixed temperature. For a strong acid, nitric acid can completely dissociated in
ed
solutions, leading to the increase of [H+] concentration, and then decrease the stability of K2SiF6 and the mean activity coefficient. However, phosphoric acid is a tribasic acid and the
ce pt
dissociation constants of phosphoric acid are pKa1 = 2.148, pKa2 = 7.199, and pKa3 = 12.30 at 25 °C [17]. According to this, the dissociation of phosphoric acid is step-wise and each dissociation gives up an H+ and a phosphate anion, respectively. Due to the presence of high
Ac
charge density ions, all the dissociated phosphate anions of H2PO4-, HPO42-, and PO43- exhibit strong salting-out effect [18]. Therefore the combined effects of the relative low [H+] concentration and the strong salting-out effect of phosphate anions increase the mean activity coefficient, resulting in the difficulty of K2SiF6 to dissolve in phosphoric acid solutions. With increasing concentration of phosphoric acid, these two effects would become more apparent at relatively low temperature. As shown in Figure 2(a), the solubility of K2SiF6 decreases directly with the increasing phosphoric acid concentration and the K2SiF6 is almost insoluble in 25% phosphoric acid solutions at 25 °C. 10
Page 13 of 19
0.4
0.2
0.0
5 10 15 20 H3PO4 concentration (wt %)
65 °C 45 °C 25 °C
1.0 0.8 0.6 0.4 0.2 0.0
0
(b)
1.2
ip t
65 °C 45 °C 25 °C
1.4
25
0
5 10 15 HNO3 concentration (wt %)
cr
(a)
Solubility (g per 100 mL solvent)
Solubility (g per 100 mL solvent)
0.6
20
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Figure 2. Variation of the solubility values of K2SiF6 with the acidic concentration in the solution at 25, 45,
an
and 65 °C: (a) phosphoric acid, (b) nitric acid.
Table 3
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Experimental results for the solubility of K2SiF6 in H3PO4 or HNO3 solutions at 25, 45, and 65 °C. Solubility of K2SiF6 (g per 100 mL solvent)
(HNO3)wt% 25 °C
RSD (%)
45 °C
RSD (%)
65 °C
RSD (%)
0.179
2.32
0.262
3.15
0.358
3.61
0.156
3.16
0.292
3.64
0.414
1.46
0.042
2.43
0.204
3.44
0.364
2.54
0
0.010
4.49
0.110
3.49
0.167
2.89
5
0.496
2.05
0.682
2.39
1.173
2.88
10
0.664
2.20
0.866
2.85
1.225
3.42
15
0.698
3.48
0.899
4.53
1.251
4.36
0
5
0
15
0
25 0 0
Ac
0
ce pt
0
ed
(H3PO4)wt%
3.3. Solubility in phosphoric acid and nitric acid mixed solutions
Table 4 presents the experimental results for the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25, 45, and 65 °C. Figure 3 displays the variations of solubility values of K2SiF6 with different acidic concentration in phosphoric acid and nitric acid solutions. Apparently, the solubility values in any mixed acid solutions increase with the raising temperature. 11
Page 14 of 19
From Figure 3(a), it can be found that all the solubility values of K2SiF6 decrease with the increasing phosphoric acid concentration in mixed acid solutions under 5% nitric acid concentration at 25, 45, and 65 °C. The phenomena can be ascribed to the strong salting-out effect of phosphate anions and the influence of phosphoric acid dissociation. It should be
ip t
noted that at the same temperature, K2SiF6 is more soluble in the mixed acid solutions containing 5% HNO3 than that in only H3PO4 aqueous solutions, indicating the presence of
cr
nitric acid in phosphoric acid solutions is beneficial for the dissolution of K2SiF6, and this
us
effect is particularly evident for the low concentration phosphoric acid in the mixed acid solutions. The K2SiF6 is still almost insoluble in 25% phosphoric acid and 5% nitric acid
an
solutions at 25 °C.
As shown in Figure 3(b), although the mixed acid solutions contain 15% phosphoric acid,
M
the solubility values still increase with nitric acid concentration at 25, 45, and 65 °C. But the addition of phosphoric acid decreases the solubility of K2SiF6 at the corresponding
(a)
5% HNO3 at 45 C
1.2
0% HNO3 at 45 C
1.0
5% HNO3 at 25 C
0.8
0% HNO3 at 25 C
0.6 0.4 0.2 0.0
0% HNO3 at 65 C
ce pt
1.4
0
5 10 15 20 H3PO4 concentration (wt %)
25
1.6 Solubility (g per 100 mL solvent)
ed
5% HNO3 at 65 C
1.6
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Solubility (g per 100 mL solvent)
temperature and nitric acid concentration.
15% H3PO4 at 65 C
(b)
1.4
0% H3PO4 at 65 C 15% H3PO4 at 45 C
1.2
0% H3PO4 at 45 C
1.0
15% H3PO4 at 25 C
0.8
0% H3PO4 at 25 C
0.6 0.4 0.2 0.0
0
5
10 15 20 25 HNO3 concentration (wt %)
30
35
Figure 3. (a) Variation of solubility values of K2SiF6 with phosphoric acid concentration in mixed acid solutions containing 0% and 5% HNO3 at 25, 45, and 65 °C. (b) Variation of solubility values of K2SiF6 with nitric acid concentration in mixed acid solutions containing 0% and 15% H3PO4 at 25, 45, and 65 °C.
12
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Table 4 Experimental results for the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25, 45, and 65 °C. Solubility of K2SiF6 (g/100 mL solvent) (HNO3)wt% 25 °C
RSD (%)
45 °C
RSD (%)
65 °C
RSD (%)
1.173
2.88
0
5
0.496
2.05
0.682
2.39
5
5
0.419
1.62
0.574
4.84
15
5
0.180
2.83
0.358
3.75
25
5
0.004
4.67
0.118
2.86
15
0
0.042
2.43
0.204
15
5
0.180
3.89
0.358
15
10
0.332
2.69
0.428
15
15
0.361
3.25
0.490
ip t
(H3PO4)wt%
2.25
0.664
3.16
0.394
4.06
3.44
0.364
2.54
3.26
0.664
2.05
2.49
0.992
2.28
2.59
1.182
2.67
an
us
cr
0.836
M
3.4. The influence of different nitrates on the solubility of K2SiF6 in mixed acid solutions
As mentioned above, the industrial acid digestion liquid always contains phosphoric acid,
ed
nitric acid, calcium nitrate and other nitrate impurities such as magnesium nitrate, aluminum nitrate and iron (III) nitrate. Furthermore, as a potential precipitant for the fluoride removal,
ce pt
potassium nitrate will inevitably influence the solubility of K2SiF6. So it is necessary to study the influence of different nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions. Then the Ca2+, Mg2+, Al3+, Fe3+, and K+ nitrates were added into the acid
Ac
mixture containing 15% phosphoric acid and 5% nitric acid at 25 °C, respectively, and the solubility values of K2SiF6 in above solutions were measured and shown in Figure 4. As can be seen from Figure 4, the solubility of K2SiF6 showed different variations in the presence of different nitrates in mixed acid solutions. The behavior can be attributed to the charge of the cations and the number of ions furnished in the solution [19]. This kind of behavior has also been reported by Banat et al. [20] for propionic acid-water system. According to Figure 4 (a), the solubility values of K2SiF6 decrease directly with the increasing
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potassium nitrate concentration in acid mixed solutions, which is mainly attributed to the common ion effect of K+ [8]. And the addition of calcium nitrate in acid mixed solutions can significantly inhibit the solubility of K2SiF6, which is ascribed to the salting-out effect of large amount of calcium nitrate. However, the solubility values of K2SiF6 increase with the increase
ip t
of magnesium nitrate concentration in mixed acid solutions. The results demonstrate that magnesium nitrate can decrease the mean activity coefficients and promote the solubility
cr
values of K2SiF6, owing to the formation of MgSiF6 which is more soluble in the solution
us
than K2SiF6 [9, 10]. As for the trivalent cation salts Al(NO3)3 and Fe(NO3)3, with the increasing nitrates concentration, the solubility of K2SiF6 appears an increased firstly and then
an
declined trend. The phenomena occurred is mainly because the solubility equilibrium of systems containing salts of trivalent or bivalent cations is more complicated than systems
(a)
M
Mg(NO3)2
0.2
Solubility (g per 100 mL solvent)
0.6
ed
Al(NO3)3 Fe(NO3)3
0.4
KNO3
ce pt
Solubility (g per 100 mL solvent)
containing salts of univalent cations [21].
0.2
0.0
1 2 3 Nitrates concentration (wt %)
4
Ca(NO3)2
0.1
0.0
0
10 20 30 40 50 Calcium nitrates concentration (wt %)
60
Ac
0
(b)
Figure 4. The influence of different nitrates on the solubility of K2SiF6 in phosphoric acid and nitric acid mixed solutions at 25 °C.
4. Conclusion Based on the experimental results, the following conclusions could be drawn: 1. The dissolution of K2SiF6 in solutions is endothermic and the solubility values of K2SiF6 always increase with the raising temperature whether in phosphoric acid, nitric acid or phosphoric acid and nitric acid mixed solutions. 14
Page 17 of 19
2. In phosphoric acid solutions, the solubility values of K2SiF6 appear an increased firstly and then declined trend with increasing phosphoric acid concentration at 45, and 65 °C, but decrease directly with phosphoric acid concentration at 25 °C. In nitric acid solutions, the solubility values of K2SiF6 increase with the increase of
ip t
nitric acid concentration at 25, 45, and 65 °C. 3. In phosphoric acid and nitric acid mixed solutions, the solubility values of K2SiF6
cr
increase with increasing nitric acid concentration at 25, 45, and 65 °C under 15%
us
phosphoric acid concentration. However, the solubility values of K2SiF6 appear a declined trend with the increasing phosphoric acid concentration under 5% nitric acid
an
concentration.
4. The solubility values of K2SiF6 increase with the increased magnesium nitrate
M
concentration in phosphoric acid and nitric acid mixed solutions, while the aluminum nitrate and iron (III) nitrate concentrations do not cause the obvious changes of
ed
K2SiF6 solubility. The presence of potassium nitrate and large amount of calcium
ce pt
nitrate in mixed acid solutions makes the solubility of K2SiF6 decrease.
Acknowledgements
The work described in this paper was substantially supported by a grant from the
Ac
Shenzhen Batian Ecological Engineering Co., Ltd (Project no: 2013-0909).
References
[1] J.F. Fei, Engineering Technology of Nitrogenous Fertilizer. 24 (2003) 44-46. [2] I. Hussain, Procedia. Eng. 46 (2012) 172-177. [3] H.J. Jing, Freezing Nitrophosphate Production Technology, first ed., Chemical Industry Press, Beijing, 2002. [4] E.A. Abdel-aal, A.M. Amer, Miner. Eng. 8 (1995) 1221-1230.
15
Page 18 of 19
[5] Y.T. Zhang, Phosphate & Compound Fertilizer. 17 (2002) 36-37. [6] H.J. Jing, C.M. Fan, J.Q. Feng, X.H. Chang, Z.H. Wang, A.G. Wei, J.L. Liu, R. Li, H.S. Gao, Chinese Patent CN 103073344, 2013. [7] J. Frayret, A. Castetbon, G. Trouve, M. Potin-Gautier, Chem. Phys. Lett. 427 (2006)
ip t
356-364. [8] C. Rissom, H. Schmidt, W. Voigt, Cryst. Res. Technol. 43 (2008) 74-82.
cr
[9] R.H. Carter, Ind. Eng. Chem. 22 (1930) 886-887.
us
[10] K.K. Worthington, M.M. Haring, Ind. Eng. Chem. 3 (1931) 7-9.
[11] M. Al-Harahsheh, M. Batiha, S. Kraishan, H. Al-Zoubi, Sep. Purif. Technol. 123 (2014)
an
190-199.
[12] R.C. Weast (Ed.), Handbook of Chemistry and Physics, 56th ed., CRC Press, Cleveland,
M
1975-1976.
[13] J.H. Loehlin, Acta Crystallogr. C 40 (1984) 570.
ed
[14] J.R. Hester, E.N. Maslen, N. Spadaccini, Acta Crystallogr. B 49 (1993) 967-973. [15] L. Kolditz, H. Preiss, Z. Anorg. Allg. Chem. 325 (1963) 245-251.
ce pt
[16] M. Skafi, M. El Guendouzi, MATEC Web of Conferences, 5 (2013) 04003. http://dx.doi.org/10.1051/matecconf/20130504003 [17] J. Bassett, R.C. Denney, G.H. Jeffery, J. Mendham, Vogel’s Textbook of Quantitative
Ac
Inorganic Analysis, 4th ed., Longman Inc., New York, 1978. [18] T. Mourão, A.F.M. Cláudio, I. Boal-Palheiros, M.G. Freire, J.A. Coutinho, J. Chem. Thermodyn. 54 (2012) 398-405. [19] J. Dhanalakshmi, P.S.T. Sai, A.R. Balakrishnan, Fluid. Phase. Equilibr. 379 (2014) 112-119. [20] F. Banat, S. Al-Asheh, J. Simandl, Chem. Eng. Process. 42 (2003) 759-766. [21] A. Balaban, G. Kuranov, N. Smirnova, Fluid. Phase. Equilibr. 197 (2002) 717-728.
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