Properties of Fluoride in wet Phosphoric Acid Processes

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Accepted Manuscript Title: Properties of Fluoride in wet Phosphoric Acid Processes: Fluorosilicic acid in an aqueous solution of H2 SiF6 -H2 0 at temperatures ranging from 298.15 K to 353.15 K Author: Mohamed EL Guendouzi Ahmed Rifai Mourad Skafi PII: DOI: Reference:

S0378-3812(15)00126-0 http://dx.doi.org/doi:10.1016/j.fluid.2015.03.014 FLUID 10484

To appear in:

Fluid Phase Equilibria

Received date: Revised date: Accepted date:

10-2-2015 2-3-2015 8-3-2015

Please cite this article as: Mohamed EL Guendouzi, Ahmed Rifai, Mourad Skafi, Properties of Fluoride in wet Phosphoric Acid Processes: Fluorosilicic acid in an aqueous solution of H2SiF6-H20 at temperatures ranging from 298.15K to 353.15K, Fluid Phase Equilibria http://dx.doi.org/10.1016/j.fluid.2015.03.014 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.

Properties of Fluoride in wet Phosphoric Acid Processes: Fluorosilicic acid in an aqueous solution of H2SiF6-H20 at temperatures ranging from 298.15 K to 353.15 K

Mohamed EL Guendouzi*, Ahmed Rifai, Mourad Skafi Laboratoire de Chimie Physique, URAC17 Faculty of Sciences Ben M'Sik, University of HassanII- Casablanca, B.P 7955, Casablanca, Morocco. *To home corresponding: e-mail: [email protected], [email protected]

Highlights    

Water activities of aqueous solutions of fluorosilicic acid were measured at various temperatures from (298.15 to 353.15) K. The osmotic and activity coefficients of H2SiF6–H2O were determined at various temperatures. A construct model based on ion interaction was developed to calculate the excess thermodynamic properties. The proposed data help in understanding the role of fluorosilicic acid in the wet production process of phosphoric acid.

Abstract The properties of fluoride in wet phosphoric acid processes were of interest us to understand the processes and roles that fluorosilicic acid plays in phosphoric acid solutions. The literature data indicate that some deficiencies exist in the thermodynamic properties of hexafluorosilicic acid aqueous solutions. In this investigation, the water activity and osmotic coefficients of H2SiF6(aq) were determined in the temperature range from T= 298.15 to 353.15 K. The measurements of the water activity were performed at molalities from 0.10 to 3.00 mol kg-1 of H2SiF6(aq) using the hygrometric method. The modelling approach based on the Pitzer model was developed to determine the thermodynamic properties. From these measurements, the ion interaction parameters in binary solution were evaluated at different temperatures ranging from 298.15 K to 353.15 K and were used to predict the solute activity coefficients. The development of a model for solution

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behaviour was also employed to determine the excess molal Gibbs energy, enthalpy, entropy and heat capacity of the H2SiF6 aqueous solution within the temperature range from 298.15 K to 353.15 K.

Keywords: Fluorosilicic acid, water activity, osmotic coefficient, ion interaction parameters, excess thermodynamic properties. 1. Introduction The production of phosphoric acid is directly related to world phosphate fertilizer consumption, which continues to increase [1]. Phosphoric acid for use in fertilizer applications is mainly produced by wet processes. In these processes, phosphate ore is digested by a mixture of sulphuric and phosphoric acids and large amounts of hydrated calcium sulphate are precipitated as a by-product. The phosphate ore is mainly fluoroapatite (Ca10(PO4)6F2), with some additional compounds, such as calcite, quarts, and clay. Fluoroapatite contains approximately 4% fluoride [2]. During its digestion by phosphoric acid, fluoride is released as hydrogen fluoride, which reacts with the silica present in the ore or added as clay, to form a fluorosilicic acid solution in the phosphoric acid. Some of the fluorosilicic acid precipitates during the production process with sodium or potassium ions or as more complex compounds [3]. The remaining fluorosilicic acid in the phosphoric acid partly decomposes as SiF4 and HF. The residual fluoride in solution is distributed between the phosphoric acid and the byproduct during its precipitation [4]. During the formation of the acid and fluorosilicate, as its concentration increases, solid-based compounds, such as Na2SiF6, K2SiF6, CaSiF6, and MgSiF6, are formed in turn by inducing the problem of fouling. Reactors, pumps, and pipes are clogged by the solids, which causes disruptions

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in the production chain. The elimination of these contaminants is performed either by cleaning or by changing the reactors [5, 6]. The valuation and resolution of some of the problems encountered in the industrial production of phosphoric acid are of interest to us, such as dirt and derivatives that allow better production yields of phosphoric acid and the valuation of the derivatives. The knowledge of the physicalchemical properties of compounds and their mixing allows a better understanding of the mechanisms governing such complex thermodynamic equilibria. The presence of fluoride in phosphoric acid not only makes this acid more corrosive, but it also makes it unsuitable for fertilizer applications if the concentration is too high. Fluoride also causes precipitation of the compounds as chuckrovite. The removal of fluoride from the phosphoric acid is an industrial need, and control of the concentration of the fluoride to be released into the air is critical for the protection of the environment [4]. Moreover, phosphate fertilizer plants produce a particular type of effluent, characterised by its aggressiveness and its composition. This effluent is highly acidic and contains high amounts of both fluoride and phosphate [7, 8], in addition to variable contents of other species, mainly sodium, calcium, chloride and sulphate. The presence of fluoride at acidic pH makes this wastewater aggressive towards skin, laboratory glassware and various metallic materials [9]. The fluoride present in such wastewater is commonly of two forms: hydrofluoric acid (HF) and hexafluorosilicic acid (H2SiF6) [3, 4, 10]. During the acidulation of phosphate rock (fluorapatite containing 3–4% fluoride and 3–5% SiO2) to produce phosphoric acid or superphosphates (single and triple), fluoride is released as hydrofluoric acid, which in turn reacts with silica, forming the volatile gas silicon tetrafluoride (SiF4) and hydrosoluble hexafluorosilicic acid (H2SiF6). Both HF and H2SiF6 are partially carried in the wastewater from gaseous waste scrubbing and phosphogypsum transportation, which are performed during the processing of phosphate rock. The thermodynamic

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properties of fluorosilicic acid and/or the by-products generated during production are important, especially if the conditions of the study are similar to that of the production. Hexafluorosilicic acid and its sodium salts are the most commonly used compounds for drinking water supply fluoridation [11-13]. The pH of a water supply may be adjusted by various hydroxides to neutralize the hexafluorosilicate anion in potable water supplies. Upon dilution, the hexafluorosilicate anion releases fluoride ion and dissociates, producing soluble aquo-hydroxo and oxosilicates. Their speciation is complicated, with a number of polymeric species observed. The literature indicates that some deficiencies exist in our understanding of the speciation of hexafluorosilicic acid. The experimental thermodynamic data of the fluorosilicic acid aqueous solution are not available in the literature. Moreover, few studies have been performed for these electrolyte solutions in this field at different temperatures [14]. The main objectives of this study are: (1) determination of the water activity and osmotic coefficients of the binary solution of H2SiF6-H2O within the temperature range from T= 298.15 to 353.15 K on the basis of hygrometric measurements; (2) development of a thermodynamic model for the solution behaviour at different temperatures ranging from 298.15 to 353.15 K; and (3) evaluation of excess thermodynamic functions of the aqueous solution H2SiF6 (aq).

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2.

Met hods and materials The water activity was determined using the hygrometric method, which has been previously

described [15]. The apparatus used in this study solution is based on the measurement of the relative humidity over an aqueous solution containing non-volatile electrolytes. For the water activity measurements of these electrolytes at different temperatures ranging from 298.15 to 353.15 K, the apparatus was modified and adapted to these conditions at elevated temperatures (relatively) from 298.15 to 353.15 K [16]. The apparatus used is a hygrometer, in which a droplet of salt solution is maintained on a thin thread. The droplets of a reference solution of NaCl(aq) are deposited on the spider-thin thread by pulverization. This thread is kept tense over a perspex support, which is fixed to a cup containing the selected solution to be studied. The cup is then placed in a thermostatted box. The apparatus was held at a constant temperature, and at standard pressure P°. The temperature was controlled to within 0.1 K. The diameter measurement of the previously calibrated droplet allows for the determination of the water activity of aqueous solutions. The droplet diameter was measured by a microscope with an ocular equipped with a micrometric screw. The relative humidity was equivalent to the water activity aw in our experiments. The uncertainty in the water activity depends on the accuracy of the diameter measurements and is therefore less than  0.02 per cent for

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aw>0.97,  0.05 per cent for 0.97>aw>0.95,  0.09 per cent for 0.95>aw>0.90 and  0.2 per cent for aw < 0.90. The reference solutions of NaCl were prepared from crystalline material (extrapur-grade chemicals, mass fraction > 0.99) and deionized distilled water. The studied solutions were prepared by the dissolution of H2SiF6 at 34%, analytical grade compounds Riedel de-Haën (Sigma-Aldrich) and deionized distilled water (conductivity < 5 µS m-1) at 298.15 K. The purity of H2SiF6 at 34% is  99.98%; the density is d20=1.30 and the molar mass is 144.09g.mol-1. The molality is prepared at the considered value and its uncertainty is 0.01 mol kg-1.

3. Results and Discussion 3.1. Water activity and the osmotic coefficient Binary aqueous solutions of fluorosilicic acid were studied using the hygrometric method in the temperature range from 298.15 to 353.15 K. The measurement of the water activities of H2SiF6(aq) were performed at molalities from 0.10 to 3.00 mol kg-1 and from 298.15 to 353.15 K. The experimental data are presented in Table 1 and Fig. 1. According to Fig. 1, the behaviour of the water activity depends on the molality and temperature and decreases with increasing concentration. This behaviour is due to the reduction of the vapour partial pressure of water relative to pure water, which causes an increase in the number of molecules of free water from the liquid state to the vapour state. The water activity increases as the temperature increases from 298.15 to 353.15 K as follows aw353.15K > aw 333.15K > aw 313.15K > aw 298.15K. In the solution, increasing molality causes a deviation from ideality. The water molecules under the influence of ion-dipole interactions are divided into two types. Bound water molecules are directly subjected to the electric fields of ions, and free water molecules are not affected by these fields. Therefore, when the concentration increases, the number of free water molecules decreases and the 6

proportion of water molecules passing to the vapour state reduces along with the corresponding water activity. Using the obtained experimental results of the water activity, the osmotic coefficients of the water were evaluated by Eq. (1),

 

1000 ln aw . M w  i mi

(1)

i

where i is the number of ions released by dissociation, mi is the molality of solute I, and Mw is the molar mass of water. Generally, the release of fluoride, as a chemical gas in the form of SiF4 and HF, can be absorbed in water to give a fluorosilicic acid solution [1, 4]. The activities of the volatile SiF4 and HF compounds in the liquid phase are both assumed to be negligible compared to the activity of SiF62- at a molar F/Si ratio of six, which has to be maintained in the liquid phase if a release maximal fluoride is pursued. The fluoride is therefore assumed to only be present as SiF62- in the liquid phase. Fluorosilicic acid is a strong acid and is assumed to be completely dissociated into SiF62-and H+ in pure water [1]. Thus, except under very acidic conditions, the dominant form in solution is the hexafluorosilicate ion. Therefore, the equilibrium reaction is generally considered to be similar to the form given in Eq. (2). From the equilibrium, the system was considered a 1-2 charge type.

(2) The osmotic coefficient data sets are determined directly from the hygrometric measurements and used in Eq.(1) (with 1-2 charge type) of the fluorosilicic acid solutions in the temperature range from 298.15 to 353.15 K (see Table 1 and Fig. 2 ).

3.2. Binary parameters and activity coefficient

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The ion interaction model [17] has been successful in describing the osmotic coefficient results. The osmotic coefficients of 1-2 electrolyte solutions are given by Eqs. (3-6)

 1 za zc f  (2 a c /)mB  2m2 [ ( a c )3 / 2 /] C ,

(3)

where f is the long-range electrostatic term, defined as

f



  A  I 1 / 2 /(1  1 .2 I 1 / 2 ) .

The second virial coefficient

(4)

is given by



 BMX   0    1 exp  I 1 2

with =2



(5)

A is the Debye-Hückel coefficient (defined as a dimensionless quantity) for the osmotic coefficient and (0), (1), and C are the ion interaction parameters, which are functions of the temperature and pressure. For the single–electrolyte solution of anion “a” and cation “c”, Za and Zc are the charge numbers of the anion and cation, respectively, and a and c are the total number of anions and cations per mole of the electrolyte produced by dissociation of one molecule, respectively. The ionic strength I is given by the usual definition.

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I  (1 / 2) mi zi

(6)

i

The corresponding values of the ion parameters  ( 0) ,  (1) and C of the pure electrolytes H2SiF6 (aq) were obtained from Pitzer’s expressions by the fits of the experimental osmotic coefficients. The uncertainty of the parameters is calculated for each temperature, and the standard deviation of the fit () is also given in Table 2. Equations 7-10 give the expressions for the activity coefficients  as

ln  za zc f  (2 a c /)m B [2( a c )3 / 2 /] m 2C  ,

(7)

where the long-range electrostatic term, f, is given by

f   A [I 1 / 2 /(11.2I 1 / 2 )(2 / 1.2)ln( 11 .2I 1 / 2 )]

(8)

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is defined as.









 BMX   0   2  1  2 I 1 2 1  exp   2 I 1 2 1   2 I 1 2

and

is related to

by



3   CMX  CMX 2

(9)

(10)

Using ion interaction parameters  ( 0) ,  (1) and C  , the solute activity coefficients are calculated by Eq.(7) and given in Table 1. The predicted solute activity coefficients of H2SiF6 (aq) as a function of the ionic strength I1/2 and within the temperature range from 298.15 to 353.15 K are shown in Fig. 3. At the same temperature, Fig. 3 shows that the activity coefficients increase slightly at a dilute-moderate concentration and increase greatly in concentrated solutions of H2SiF6. At the same concentration, with increasing temperature from 298.15K to 353.15K, the activity coefficients decrease in the following order: γ353.15K < γ333.15K 0.97, ±0.05% for 0.97 >aw> 0.95, ±0.09% for 0.95 >aw> 0.90 and ±0.2% for 0.90>aw. Uncertainty of osmotic coefficient  is estimated to be, at most ±0,005. (N=13) number of experimental data points (each point is the average of 8-10 series of experiments).

Table 2. Ion interaction parameters (0), (1), C() of the binary system H2SiF6- H2O in the temperature range from 298.15 to 353.15 K. T (K)

(0)

(1)

C()



A

17

(kg mol −1)

(kg mol −1)

(kg2 mol −2)

298.15

0.4981 0.0029

4.027 0.066

-0.00006 0.000005

0.0034

0.3930

313.15

0.4837 0.0022

3.313 0.037

0.00124 0.00001

0.0031

0.4023

333.15

0.4986 0.0031

0.302 0.069

0.00003 0.000001

0.0019

0.4190

0.5089 0.0027

-4.443 0.018

0.00001 0.000003

0.0027

0.4384

353.15

- Expanded uncertainty of temperature is 0.1 K. The uncertainty of parameters is calculated for each temperature. -







2  (  obs   th ) N  1

N is number of experimental data points

Table 3. Least squares estimated coefficients

of H2SiF6-H2O within the temperature range from

298.15 to 353.15 K.

Ion interaction parameters β(0)

-11.820.09

6.060.001

-7.820.08

0.880.002

β(1)

-12.79 0.16

45.350.16

-113.862.81

-12.570.03

C()

1.370.03

-0.690.003

0.910.03

-0.090.0004

- Values of uncertainties were evaluated using error propagation.

Table 4. Calculated parameters

,

,

,

,

and

of H2SiF6-H2O at

temperatures ranging from 298.15 to 353.15 K. T (K) 298.15

0.0129  0.00001

-0.2113 0.0007

-0.00138 0.00002

-0.00015 0.00009

-0.0024 0.0013

0.00002 0.00001

313.15

0.0107 0.0001

-0.2468 0.0018

-0.00112 0.00003

-0.00015 0.00007

-0.0024 0.0008

0.00002 0.000003

333.15

0.0077

-0.2938

-0.00077

-0.00015

-0.0023

0.00002 18

353.15

0.0001

0.0029

0.00005

0.00004

0.0005

0.000003

0.0046 0.0001

-0.3406 0.0040

-0.00041 0.00006

-0.00015 0.00003

-0.0023 0.0003

0.00002 0.000001

Expanded uncertainty of temperature is 0.1 K. Values of uncertainties were evaluated using error propagation. Figures

1.00

0.95

0.90

0.85

a

w

0.80

0.75

0.70

0.65 0.00

0.50

1.00

1.50

2.00

2.50

3.00

m ( mol.kg-1 )

Fig. 1

3 .0 0

2 .5 0

2 .0 0



1 .5 0

1 .0 0

19

0 .5 0

0 .0 0 0 .0 0

0 .5 0

1 .0 0

1 .5 0

{ I(m o l k g

2 .0 0 -1

) }

1 /2

2 .5 0

3 .0 0

3 .5 0

Fig. 2

10. 00

8.00



6.00

4.00

2.00

0.00 0.00

0.50

1.00

1.50

2.00

2.50

3.00

{ I(mol.kg-1) }1/ 2

Fig. 3

20

Fig. 4

21