Sorption and desorption of BrF3 on NaF: Studies on thermodynamics and kinetics

Article abstract of DOI:10.1016/j.jfluchem.2019.03.003

Temperature dependence of bromine trifluoride vapor pressure over its adduct with sodium fluoride has been determined; the adduct normal dissociation temperature has been determined. Kinetics of sorption and desorption processes in BrF₃(gas)–NaF(solid) system have been studied. The equation was proposed to illustrate the link between forward reaction rate (К_а) described by Arrhenius equation and experimental value (K_i): K_i= K_а? (P_a/P_eq)^m, where Ki – observed reaction rate constant, K_a – forward reaction rate constant described by Arrhenius equation; P_a – partial pressure of BrF₃; P_eq – equilibrium pressure of BrF₃ above BrF₃?3NaF; m – factor characterizing the interaction area of BrF₃ with NaF. The application of this equation makes possible to determine the true reaction rate values and reaction activation energy in chemisorption terms; it also helps to calculate the degree BrF₃ sorption on NaF reaction at different temperatures and adsorbate pressures. The possibility of sorption-desorption separation of bromine trifluoride – uranium hexafluoride – iodine pentafluoride system with use of sodium fluoride was shown.

Full text of DOI:10.1016/j.jfluchem.2019.03.003

Accepted Manuscript
Title: Sorption and Desorption of BrF3 on NaF: Studies on
Thermodynamics and Kinetics
Authors: I.I. Zherin, A.I. Rudnikov, R.V. Ostvald, V.I.
Sobolev, G.N. Amelina
PII:
S0022-1139(19)30031-4
DOI:
Reference:
https://doi.org/10.1016/j.jfluchem.2019.03.003
FLUOR 9297
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Received date:
Revised date:
Accepted date:
21 January 2019
11 March 2019
12 March 2019
Please cite this article as: Zherin II, Rudnikov AI, Ostvald RV, Sobolev VI, Amelina GN,
Sorption and Desorption of BrF3 on NaF: Studies on Thermodynamics and Kinetics,
Journal of Fluorine Chemistry (2019), https://doi.org/10.1016/j.jfluchem.2019.03.003
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1
Sorption and Desorption of BrF3 on NaF: Studies on Thermodynamics and Kinetics
I.I. Zherinа, A.I. Rudnikovb, R.V. Ostvalda, V.I. Soboleva,*, G.N. Amelinaa
a National Research Tomsk Polytechnic University, 30 Lenin Avenue, 634050 Tomsk, Russia
b Joint Stock Company “Siberian Chemical Combine”, 1 Kurchatov Street, 636039 Seversk, Tomsk region,
Russia
Graphical abstract
Highlights
 It is possible to separate the IF5-BrF3-UF6 system into individual components by
sorption on NaF;
 The recommended sorption scheme is: separation of IF5 from BrF3 and UF6;
selective desorption of BrF3; desorption of UF6;
 Kinetics of BrF3 interaction with NaF is limited by the conditions of new
crystalline phase formation for BrF3∙3NaF;
 The observed thermal dissociation rate of BrF3∙3NaF under vacuum is acceptable
for industrial implementation.

2
ABSTRACT
Temperature dependence of bromine trifluoride vapor pressure over its adduct with sodium
fluoride has been determined; the adduct normal dissociation temperature has been determined.
Kinetics of sorption and desorption processes in BrF3(gas)–NaF(solid) system have been studied.
The equation was proposed to illustrate the link between forward reaction rate (Ка) described
by Arrhenius equation and experimental value (Ki):
Ki= Kа (Pa/Peq)m,
where Ki – observed reaction rate constant, Ka – forward reaction rate constant described by
Arrhenius equation; Pa – partial pressure of BrF3; Peq – equilibrium pressure of BrF3 above
BrF33NaF; m – factor characterizing the interaction area of BrF3 with NaF. The application of this
equation makes possible to determine the true reaction rate values and reaction activation energy
in chemisorption terms; it also helps to calculate the degree BrF3 sorption on NaF reaction at
different temperatures and adsorbate pressures.
The possibility of sorption-desorption separation of bromine trifluoride – uranium
hexafluoride – iodine pentafluoride system with use of sodium fluoride was shown.
Keywords: uranium hexafluoride; bromine trifluoride; iodine pentafluoride; sodium fluoride;
sorption; desorption.
1. Introduction
Bromine trifluoride compounds are one of the most interesting building blocks between
organic and inorganic chemistry. Recently we characterized some of them [1-6]; these works
describe the properties of Mx(BrF4)y compounds where M is alkali or alkali-earth metal.
Tetrafluorobromates of alkali and alkali-earth metals (TFB) can be considered as BrF3-based
complexes which are safer and more convenient for use than BrF3 [4-6]. It was shown that that
these compounds can be applied for various promising reactions, for example - urban mining
activities (noble metals extraction) [4] and for several reactions in organic chemistry [5, 6].
Another interesting field of initial halogen fluoride application (not tetrafluorobromate
complex) is a removal of solid uranium-containing products from different equipment using vapor
mixture BrF3 – IF7 [7]. It results in formation of UF6–BrF3–IF5 system, which needs further
separation to return the halogen fluorides to the regeneration cycle, and UF6 – to the isotope
separation process.
In the study [7] we presented the results of our studies of liquid – solid, liquid – vapor, and
solid – liquid – vapor phase equilibria in the triple system UF6–BrF3–IF5 and the corresponding
binary systems. Possibility of their separation into highly-pure individual components by methods
based on phase changes (crystallization distillation, and rectification) has been shown. However,
these methods have a very significant drawback caused by the nature of the components in the
system UF6–IF5–BrF3: all of them are powerful oxidizers and due to this fact, their industrial use
is associated with high risk [8-12].
Alternative separation methods of the mentioned system include sorption and desorption;
these methods are based on the property differences of the corresponding adducts with alkaline
and alkaline-earth metal fluorides. For uranium hexafluoride, these aspects can be found in
numerous studies [13-20], and for BrF3 – in the research [21].
_________
Corresponding author. Tel.: +7 913 846 2548; e-mail: vsobolev1989@tpu.ru
Sodium fluoride is the most preferred sorbent among others [7] due to its non-hygroscopic
nature and reversibility of sorption-desorption processes, which is necessary for separation of the
system and regeneration of sorbent.
The study [22] investigates the possibility of formation of a complex compound between IF5

3
and granular sodium fluoride and shows a dependence of iodine pentafluoride vapor pressure on
temperature (Eq. 1):
ln P (mmHg) = (18.60.1) – (454438)/T
(1)
It means that the vapor pressure of IF5 over its compound with NaF at 373.15 K is 81.1кРа.
This is slightly less than the saturated vapor pressure of IF5 (boiling point of IF5 is 373,65 К [24])
and indicates low sorption capacity of sodium fluoride towards IF5.
The interaction in the system UF6 – NaF has been studied rather completely [14-21]. Depending
on temperature and pressure of UF6, the following compounds can form following compounds:
UF6NaF UF62NaF UF63NaF; capacity of NaF granules by UF6 varies within 0.91.3 g UF6/g NaF.
The generalized equation of uranium hexafluoride vapor pressure for compounds UF62NaF and
UF63NaF looks as follows [7]:
lg P (mmHg) = 10.99 – 5130/T
(2)
Special attention should be paid to the system of bromine trifluoride – sodium fluoride. The
study [24] reports on BrF3∙2NaF compound which was obtained by keeping the sodium fluoride
powder in saturated bromine trifluoride vapor (at ambient temperature, for one-two weeks). Study
of this compound by thermal analysis showed that its complete decomposition occurred in the
temperature range of 503-573 K. The solid decomposition product was identified by powder X-
ray diffraction; according to this study this product was NaF. The activation energies of the thermal
decomposition of NaBrF4 and BrF3∙2NaF values reported in [24] are 884 kJ/mol and 808 kJ/mol
– for BrF3NaF and BrF32NaF, respectively. Since both values are essentially the same within the
uncertainty interval, and also due to the lack of powder X-ray diffraction data for BrF3∙2NaF, it is
currently not clear if this compound is a separate phase or simply a mixture of NaF and NaBrF4.
The separation of mixtures, that are formed by uranium compounds fluorination by BrF5 are
discussed in [26]. The UF6 is adsorbed by NaF powder at 373-448 К to separate the corresponding
UF6 from BrF5 and BrF3 (the bromine (III) fluoride is formed in situ). In this case the BrF3
adsorption degree is at minimum and BrF5 does not adsorbed at all. Elemental fluorine is used to
remove the traces of BrF3 from the sorbent and then the UF6 is desorbed at higher temperatures
(573-473 К).
Authors of the research [26] work used elemental analysis and powder X-ray diffraction to
study the reaction product obtained by interaction of NaBr with XeF2 and came to the conclusion
that the composition of the compound lied in the range of BrF3∙2,9NaF – BrF3∙3NaF, however, no
detailed structural report was given.
The study [21] reports on the kinetics of NaF powder interaction with gaseous BrF3 (2.0 кРа)
in the range of 348.15 to 438.15 К. It showed that in the specified temperature range a solid
compound with a general formula of BrF3∙nNaF was formed, where n varies from 3 to 3.4
depending on the bromine trifluoride partial pressure and the specific surface area of NaF. The
complete decomposition of the resulting compound occurs at the temperature of circa 573.15 К.
When studying the isotope exchange reaction using 18F isotope, it was found that relative fluorine
exchange was 19% and occurred due to a complexation reaction between BrF3 and NaF. The
author provides an equation of temperature dependence of BrF3 pressure on temperature (for the
temperature range of 446-476 К):
ln PmmHg = 3.76 – 4/3 × 104T
(3)
This equation seems to be incorrect since the numerical values obtained with this equation
conflict with BrF3 to NaF sorption results outlined in the work [21].
We can conclude that the known data cannot be applied for BrF3–UF6 system separation on

4
sodium fluoride (by sorption-desorption methods).
The purpose of the present work is to study the equilibrium in BrF3–NaF system, kinetics of
sorption and thermal decomposition processes and provide the chemical thermodynamics and
kinetics basis for the interaction of NaF with BrF3 with practical implementation approaches.
2. Results and discussion
2.1. Studying the BrF3 vapor pressure above its compound with NaF
Study of the temperature dependence of BrF3 vapor pressure above its compound with a
composition of NaF:BrF3 = 3:1 (by moles) was performed to determine its thermal stability and
compare it with UF6 ∙ NaF compounds; at the same time the verification of Equation (3) published
in [22] becomes available. The measurements were performed in the temperature range from
373 К to 493 К, with a 5 К step; the results are shown in Fig. 1.
10
Experimental date
Calculated by the equation
8
6
4
2
0
2 2,1 2,2 2,3 2,4 2,5 2,6 2,7
-1
1000/T, K
Fig. 1. Results of measuring the BrF3 vapor pressure above its combination with NaF.
The analytical equation of the temperature dependence looks as follows:
lnP(Pa)= 34.97960.1890 – 12657.8879.1/T
(4)
The standard error was equal to 0.109 for the logarithm of the measured pressure values
with respect to those calculated by equation (4).
The absence of bends on the experimental data approximation line in coordinates 푙푛푃퐵푟퐹
–
3
1/Т (Fig. 1) shows the invariance of the heat of adsorption in the studied temperature range. The
differential adsorption heat determined by equation (4) is 105.60.7 kJ/mol, it indicates the
chemisorption nature of BrF3 interaction with NaF and is consistent with value ΔН=110.9 kJ/mol
that was obtained earlier [22]. The normal dissociation temperature of BrF33NaF determined by
extrapolation of the experimental data is 539.05 К. We can observe the satisfactory consistency of
the results while comparing this value to the graphical data [22] (the normal dissociation
temperature 553.15 К).
The performed experiments indicate the presence of the stable compound of BrF3 with
NaF (in a wide temperature range). It provides the thermodynamic prerequisites for separation
of the corresponding system of UF6-BrF3. For example, the adsorption heat of UF6 to NaF is
97.2 kJ/mol and the normal dissociation temperature of UF63NaF is equal to 636.15 К [15].
While assessing the possibility of sorption-desorption separation of BrF3-UF6-IF5 system,
the main attention should not be paid to absolute values of vapor pressure in individual

5
components, but to their ratio. Fig. 2 shows the dependence of lnP=f(1/T) function for the separated
components calculated by equations: for IF5 – (1), for UF6 – (2), for BrF3 – (4).
Fig. 2. Temperature dependence of equilibrium vapor pressures of UF6, BrF3, IF5 above their compounds
with NaF
We can see that vapor pressure of IF5 above NaF significantly exceeds the BrF3 pressure
and that one of UF6. Therefore, during the sorption of the components in BrF3–UF6–IF5 mixture
onto NaF at 373.15 К under reduced pressure, IF5 cannot be significantly adsorbed.
For UF6–BrF3 system the components dissociation pressure difference above NaF is not so
significant. Fig. 3 shows the ratio of the equilibrium pressures for BrF3 and UF6 above their
compounds with NaF for complete quantitative assessment of their potential separation by
sorption.
30
25
20
15
10
5
320 360 400 440 480 520 560
Temperature, K
Fig. 3. Dependence of equilibrium vapor pressure ratio for BrF3 and UF6 above their compounds
with NaF on temperature
We can see that the ratio of the equilibrium pressures 푃퐵푟퐹 /푃푈퐹 which may be called a
3
6
“relative volatility factor (훼)”, increases linearly with the increase of temperature i.e. the
efficiency of BrF3 and UF6 separation is improved. For example, at 373 К 훼 = 12.5, and at
533 К 훼 = 27.4.
The analytical dependence of the relative volatility factor on temperature looks as follows:
 = –20.5771626 + 0.0898∙Т,
(5)
where  – relative volatility factor; Т – temperature, К.
Thus, thermodynamics of separation processes in the BrF3–UF6–IF5 system (in the process
of
their
sorption-desorption)
is
favorable
at
the
temperatures
from
373 К to 533 К. However, the separation degree can be quantitatively characterized via experiment
only by comparison of the sorption-desorption rates; these rates can be determined by kinetic

6
characteristics of the processes (with attention to each component features).
2.2. Study of BrF3 interaction with NaF powder
The interaction in the heterogeneous system of NaF(solid) – BrF3(gas) is characterized by
several factors: interaction temperature, BrF3 pressure, sodium fluoride layer thickness,

dispersion, porosity, etc. Influence of an individual parameter on BrF3 interaction with NaF was
studied with the other parameters being constant.
Current value of NaF to BrF33NaF (а) conversion degree was calculated from observed
sample mass increase (in terms of conducted experiments). Equation (6) shows this value with
account for stoichiometric composition of formed compound BrF33NaF:
α = (mi – mo)/{mo∙[(Mc/3Mo) – 1)]} = NBrF3/3NNaF
(6)
where
mi - mass of sample to time ; mo – initial mass of sodium fluoride; Мо – molecular
weight of sodium fluoride; Мс – molecular weight of complex BrF33NaF; NBrF3 – number of moles
in BrF3; NNaF – number of moles in NaF.
2.2.1. Kinetics of forming the bromine trifluoride compound with sodium fluoride powder
It was found that in case of preliminary evaluation of particle size distribution influence on
kinetics of bromine trifluoride sorption at sodium fluoride (Fig. 4) the sorbent capacity in real time
dynamic experiment did not reach the values corresponding to gross composition of BrF33NaF
(static experiment).
a) temperature 373.15 К, BrF pressure 0.8 кРа
b) temperature 373.15 К, BrF3 pressure 2.7 кРа
Fig. 4. Dependence of NaF powder conversion degree on its particle size distribution and interaction
time – at BrF3 pressure 0.8 and 2.7 кРа
Powders with particle size distribution 0.05 to 0.1 mm have the highest dynamic capacity (at
bromine trifluoride pressure up to 0.8 кРа). Increase of adsorbate pressure cause the increase of
the process rate (at initial stage) for such powders; however, in terms of limited heat removal, it
results to compaction of powder and inhibits the further reaction. This effect occurs from
conversion degrees value of 0.1-0.2 (NBrF /3NUF6).
3
Powders with particle size distribution 0.5 to 1 mm show a decrease of dynamic capacity,
which is related to formation of dense layer (reaction products) on sorbent surface and achieving

7
its critical value, where chemical interaction rate is much higher than rate of bromine trifluoride
diffusion through layer of product.
For powders with particle size distribution 0.25-0.35 mm the significant diffusion inhibition
of the process begins at conversion degree over 0.6 (NBrF /3NUF6), which is consistent with law
3
on linear rate of product growth at the surface of sodium fluoride particles. Such behavior is typical
for macro-porous sorbents (average effective radius of sodium fluoride powder pores was
309.2405.6 nm).
That is why the powder fractions with particle size 0.250.35 mm and specific surface area
2322,65 m2/g were applied for the research purposes.
Series of experiments were conducted to determine the influence of pressure and temperature
on kinetics of bromine trifluoride interaction with sodium fluoride powder; the BrF3 pressure was
equal to 0.8, 2.0 and 2.8 кРа and temperature range was from 298.15 to 423.15 К. The change of
conversion degree over time (at different isobaric and isothermal conditions) is shown in Fig. S4-
S6) (see Supplementary Material). The obtained dependencies have typical shape for topo-
chemical reactions (caused by processes at the border of sorbent phase separation and reaction
products). All curves clearly show two stages of interaction process: “fast” – start section which
corresponds to sorbent conversion degree up to 0.4 and “slow” – end section, which correspond to
sorbent conversion degree after 0.6.
As can be seen from nature of change in dependence of reaction rate (W=а/) on depth
of the process (see Fig. 5), bromine trifluoride pressure and sorbent temperature have the most
significant effect on sodium fluoride conversion rate at the “fast” interaction stage in BrF3 – NaF
system.
Fig. 5. Dependence between the rate of BrF3 interaction with NaF powder and depth of the process
Presence of peak in dependences (Fig. 8) can be explained by Langmuir theory discussed in
Bretschneider work [27], who suggested that heterogeneous reaction takes place at the border of
solid product phase separation with initial sorbent (at active centers – planes, angles, defects in
crystal lattice structure), while being localized and determined by nature of active center growth
and phase separation surface. According to this hypothesis the BrF3 sorption is localized and
occurred at active centers only; at initial reaction stage product core appears in certain places,
which occupy the insignificant area of sodium fluoride granules. Then they grow and this results
to increased interaction surface and simultaneous appearance of new active centers (due to
appearing micro- and macro-defects of sorbent structure) and, consequently, to autocatalytic
acceleration of reaction (Fig. 8). Then, we can observe a decrease in reaction rate – due to reduced
inter-phase surface (BrF33NaF)/NaF caused by product occlusion in sorbent volume.
Increase of adsorbate pressure at constant temperature (Fig. 8) leads to accelerated
interaction at initial stage – due to higher BrF3 concentration at sorbent surface.
A similar effect is caused by increased sorbent temperature at constant pressure, which

8
accelerates the reaction towards formation of BrF33NaF compound; however, it also increases the
reverse reaction rate, which reduces the observed interaction rate.
For curve areas corresponding to the initial “fast” interaction stage, experimental data are
most closely approximated in coordinates of Spencer and Topley equation (“contracting sphere”
equation) [29] derived for linear movement of phase separation boundary (in depth of sorbent
grain) and describing the kinetic process area:
1-(1-)1/3= K,
(7)
As an example, Fig. 6 shows dependences in coordinates [1–(1–а)1/3] –  – for BrF3 pressure
of 2.8 кРа.
0.5
0.4
0.3
0.2
0.1
0
348.15 K
373.15 K
398.15 K
423.15 K
0
5
10
15
20
25 30
Time interval, min
Fig. 6. Changes of NaF conversion degree depending on interaction time in coordinates of
“contracting sphere” equation – at 2.8 кРа pressure.
Violation of the linear approximation indicates the reaction transition from one macro-
kinetic area to another. Inclination angle tangents of these dependencies were used to define the
values of observed reaction rate constants (К1, min-1) – for “fast” (а=0÷0.35) stages of BrF3
interaction with NaF powder (Table S1).
We can see that increase of BrF3 pressure at constant temperature leads to increase of value
(K1), and temperature increase at constant pressure – to its reduction. This phenomenon is
connected processes of the new crystalline phase formation; these processes include the
preliminary stage of BrF3 sorption at surface-active centers.
In order to account the gaseous reagent pressure influence, we assume that constant of
forward reaction rate (Ка) described by Arrhenius equation is linked to experimental (observed)
value (K1), by equation:
K1= Kа (Pa/Peq)m,
(8)
where K1 – observed reaction rate constant, Ka –forward reaction rate constant described by
Arrhenius equation; Pa – partial pressure of BrF3; Peq – equilibrium pressure of BrF3 above
BrF33NaF defined by equation (4); m – factor characterizing the interaction area of BrF3 with
NaF.
“Supersaturation” ratio (Pa/Peq) by Bretschneider [27] describes the system remoteness from
equilibrium state; its use in equation (8) allows the experimental reduction of observed reaction
rate constants to comparable values.
We used experimental dependence of observed rate constants on pressure in isothermal
conditions of coordinates ln(K1)–ln(Pa) to determine (m); m ratio values are shown in Table S2.
These data show that temperature has no practical effect on (m) value (correlation is almost
insignificant (r = 0.43) and it is determined by temperature fluctuations during experiments). We
can use the average value of factor m = 0.37 for calculation of forward reaction rate constant (Ka)

9
by ratio (8).
Table S3 shows that BrF3 pressure at constant temperature has no significant effect on
reaction rate constant (Ka), thus allowing the use of Arrhenius equation to describe the temperature
dependence Ka.
Independence of reaction rate constant (forming of BrF33NaF compound) on adsorbate
pressure indirectly confirms the chemisorption nature of this interaction.
Numerical values of apparent activation energy (Ea) (for bromine trifluoride sorption at NaF
powder) and pre-exponential factor (ko) in Arrhenius equation were determined from dependence
ln(Ka)–1/T (Fig. 7 ) and shown in Table S4. Mutual superposition of all isobaric dependencies and
their good linear approximation confirms the validity of applied ratio (8).
-5.0
Pressure BrF3:
0.8 kPa
2.0 kPa
2.8 kPa
-6.0
-7.0
-8.0
2.2
2.4
2.6
2.8
3
3.2
1000/Т, К-1
Fig. 7. Dependencies of BrF3 and NaF interaction rate constant on reciprocal temperature (“fast” stage of
the process with NaF conversion degree а=0÷0.35)
These experiments allowed to derive an equation that describes the process of forming a
complex of bromine trifluoride with sodium fluoride powder in the kinetic interaction range:
1-(1-а)1/3= Ка(Рa/Рeq)0.37
,
(9)
where
Ka=9.451.66exp(313970/T).
(10)
Equation (9) satisfactorily describes the experimental data till conversion degrees a=0.35,
and is valid in the temperature range 323.15-423.15 K and pressures of bromine trifluoride 6 -21
mmHg. It means that this equation (9) makes possible to calculate the reaction degrees for different
temperatures and adsorbate pressures in corresponding ranges of pressure and temperatures.
A generalized kinetic equation of Erofeev [29] for topo-chemical reactions was used to
process the experimental data in the next reaction degree range limited by diffusion of the
adsorbate through the layer of reaction products, and to derive the generalized dependency
describing the interaction of bromine trifluoride with powdered sodium fluoride in a wider range
of conversion degrees a:
 = 1 – exp(–Kn)
(11)
where K – is the constant that determines the reaction rate; n – constant that determines the nature
of the process (n < 1 – diffusion process; n > 1 – kinetic process).
Table S5 presents the observed values of constants (K2) for the complex formation reaction,
determined from characteristic curves [lg(–ln(1–а)]–lg() of equation (11), and constants (n).
The closeness of n values to the unity, according to Sakovich [30] indicates the negligible
impact of the diffusion resistance on the process of trifluoride bromine interaction with sorbent
until the values a=0.6. In addition, there is no functional dependence of n on the temperature; all

10
this allows taking n=1 for further calculations. With such (n) value the constant (K) is essentially
identical to the observed reaction rate constant (K2), since constant (K) in equation (11) is related
to the rate constant (k) by the Sakovich ratio [30]:
k = n∙K1/n
(12)
At the same time, we can see that the bromine trifluoride pressure increase at constant
temperature increases the observed rate constant (K2) of complex formation reaction; the increase
of temperature at constant pressure leads to the decrease of this constant (K2). Therefore, the
equation (8) was also used to account the pressure influence and clarify the experimental values
of (K2) in the direction of formation of BrF33NaF compounds. The average ratio value (m) for the
corresponding range of the experimental kinetic curves was determined from isothermal
dependencies of ln(K2) – ln(Pa) and it is equal to 0.37. The calculation results for Ka are given in
Table S6.
The numerical values of the apparent activation energy (Ea) of bromine trifluoride
adsorption process on the powder of sodium fluoride and the values of pre-exponential factor (ko)
in the Arrhenius equation were determined from the lines approximating graphs of ln(Ka) –1/T;
Table S7 shows these values. We should mention, that all isobaric dependencies ln(Kа)–1/T have
good mutual superposition and linear approximation as it was for the case of NaF conversion
degree range а=0÷0.35 (see figure 10).
Processing of experimental data by the generalized equation (11) for topo-chemical
reactions allowed to make an equation (13) which describes fractional conversion of sodium
fluoride in the areas of kinetic curves after the initial period of reaction acceleration (Fig. 7):
ln(1-a) = –Ka (Pa/Peq.)0.37
(13)
(14)
where
Ka=16.99  4.08exp(–2980±21/T), min-1 .
Equation (14) describes the interaction of bromine trifluoride with sodium fluoride powder
in the temperature range 323.15-423.15 К and pressure of BrF3 to 2.8 кРа adequately for BrF3 and
NaF interaction process at conversion degrees a=0.20.65.
Thus, the study of temperature and pressure influence on the interaction of bromine trifluoride
with coarse-pored sodium fluoride powder showed that the kinetics of interaction at small degrees of
conversion is limited by the crystallization processes and passes the stage of autocatalytic acceleration
over the entire range of pressures and temperatures. After passing the maximum and till degrees of
conversion a=0.6 the process is characterized by a linear increase of the solid reaction products.
Further on the process grades into the deep-diffusion range. It is not of any interest by itself,
since the kinetic data for the reaction that takes place in this range do not contain information about
the reactivity of the substance.
This research taking into account the information in [14-22] on the interaction of uranium
hexafluoride with NaF, suggest that joint sorption of UF6 and BrF3 will take place in the temperature
range of 323.15-373.15 К. Temperature increase above 398.15 К results in the preferential extraction
of uranium hexafluoride gas mixture, but the required degree of separation at the sorption stage can be
only achieved in the temperature range 443.15-473.15 К; that is far from optimal in terms of complex
formation in NaF–UF6 system.
Thus, complete separation of UF6 and BrF3 into components is not possible at the sorption stage.
For this reason, the kinetics of decomposition of bromine trifluoride and sodium fluoride complex was
studied.
2.2.2. Thermal decomposition of the bromine trifluoride and sodium fluoride complex
The kinetics of decomposition of BrF33NaF complex was studied at temperature 453,15;
474.15; 498.15; 523.15 К and pressure less than 53 Ра. To eliminate the influence of the degree of

11
the sorbent filling with bromine trifluoride on the desorption kinetics, sodium fluoride powder was
saturated at 373 К under the pressure of BrF3 equal to 2.8 kРа. Sorbent capacity in all cases was
1.0-1.1 g BrF3/g NaF. After the saturation the sample was heated to a predetermined temperature
and then bromine trifluoride was evacuated from the system.
IR spectrometric analysis of the desorbate and the XRD analysis of solid decomposition
products showed that desorption process of trifluoride bromine proceeds under the scheme:
BrF3∙3NaF(solid)  BrF3(gas) + 3NaF(solid)
(15)
The XRD pattern of the residue after the thermal decomposition is shown in Fig. S2 (see
Supplementary Material) and it shows that the only product is phase-pure NaF.
Fig. 8 shows the temperature dependence of conversion of BrF33NaF into NaF,
achieved at a residual pressure in the system  53 Ра.
1.0
0.8
0.6
0.4
373.15 K
473.15 K
0.2
498.15 K
523.15 K
0
0
40
80
Time interval, min
120
160
Fig. 8. Variation of conversion degree of the complex BrF33NaF into NaF at different
temperatures and residual pressure 53 Ра.
Kinetic parameters of BrF33NaF decomposition process were studied by equation (7); the
findings are presented in Fig. 9.
0.8
0.6
0.4
453.15 K
473.15 K
498.15 K
523.15 K
0.2
0
20
40
60
80
0
100
Time interval, min
Fig. 9. Variation of conversion degree of BrF33NaF into NaF depending in the time of interaction
in the coordinates of equation (7) at a residual pressure 53 Ра.
The values of BrF33NaF (K3, min –1) thermal dissociation constant for the “fast” process
stages, parameter values of Arrhenius equation (pre-exponential factor (ko) and the apparent
activation energy of the decomposition reaction (Ed) are presented in Table S8.
The equation (16) describes the kinetics of thermal dissociation of BrF33NaF in the
temperature range from 453 to 523 К:

12
1-(1-d)1/3= 7.46105 ехр(–8676.9/Т);
This dependence is true at the residual pressure in the system  53 Ра.
(16)
In case of BrF33NaF decomposition research at higher temperatures by the chosen method
the appropriate kinetic curves could not be obtained due to side interaction processes of desorbed
bromine trifluoride with installation materials especially with electromagnetic scales part.
Therefore, a serie of experiments was carried out to evaluate the apparent activation energy
(Ed) by indirect method based on half-reaction time (1/2) principle and to determine if BrF33NaF
dissociation process can be adequately described by equation (16) at higher temperatures and to
determine the temperature limits for reaction type transition from the kinetic to diffusion area.
Analytic expression of the activation energy in the considered process has the form:
Ed = R∙[T1∙T2/(T2 – T1)]∙ln(τ1(1/2)/τ2(1/2)
)
(17)
where T1, T2 – temperature values; τ1(1/2), τ2(1/2) – half-reaction time at temperature T1 and T2.
The studies were conducted under isothermal conditions in a temperature range from 453 К
to 593 К in increments of 20 К assuming constancy of value (Ed) within each increment; the
apparent activation energy values are presented in Table S9.
Up to temperature 553 К, the apparent activation energy satisfactorily coincides with the
same value (72.1 kJ/mol) calculated in the study of decomposition kinetics of BrF33NaF by
equation (7). When the process temperature rises above 553 К we can see a significant decrease
of (Ed ), indicating the occurrence pore-diffusion resistance in the product layer, which prevents
withdrawal of trifluoride bromine from the reaction zone (phase boundary of BrF33NaF/NaF).
Comparison of the experimental values (1/2) and calculated by equation (16) showed their
satisfactory coincidence that allows the use of the above equation for calculating the kinetics of
dissociation process of BrF33NaF till conversion degrees d=0.5 in the temperature range 453-
533 К.
Thus, thermal dissociation of BrF33NaF under vacuum at the rate suitable for use in
industrial practice starts at a temperature of 453 К, and the process takes place in the range defined
by the chemical conversion. Maximum BrF33NaF dissociation rate is observed in the temperature
range 523-543 К. At temperatures above 553 К and the conversion degree more than 50%, the
process is limited by diffusion of BrF3 in the layer of solid reaction products.
Considering the results in terms of separation of BrF3-UF6 system, we can conclude that
quantitative separation of the complex compounds of bromine trifluoride and uranium
hexafluoride with NaF is possible at their thermal dissociation stage in the temperature range from
453 К to 533 К.
2.2.3. Hydrolysis of the bromine trifluoride and sodium fluoride complex
The kinetics and thermodynamics of BrF33NaF hydrolysis process was not the separate aim
of this work but it was necessary to identify its hydrolysis products, because they can be
contaminants in real applications and during incorrect laboratory implementation. The sample of
BrF33NaF stayed in an open Teflon container overnight. The powder diffraction showed that the
hydrolysis product is a phase-pure NaHF2. Its powder diffraction pattern is shown in Fig. S3 (see
Supplementary Material). A low intensity peak at 2θ = 38.8° belongs to NaF and it was not
included into the Rietveld refinement due to instability problems.
2.3. Study of BrF3 interaction with pelleted NaF
In terms of the industrial implementation of the sorption-desorption separation method of

13
UF6–BrF3–IF5 gas mixtures, the study of interaction of the separated mixture components with the
industrial sorbents is of practical interest. Therefore, interaction of bromine trifluoride with
pelleted sodium fluoride manufactured by technology [32] was studied. The extra data about this
part of the research are presented in Supplementary Materials section.
The results verification of IF5-BrF3-UF6 mixture separation by sorption-desorption on
pelleted NaF using newly constructed experimental installation showed that it is possible to
separate the corresponding mixture into individual components with high efficiency. The obtained
IF5 and BrF3 are suitable for reuse.
3. Conclusion
The reaction of bromine trifluoride with sodium fluoride is reversible:
BrF3(gas) + 3NaF(solid) ⇄ BrF33NaF(solid)
.
The temperature dependence of the vapor pressure of BrF3 above its compound BrF33NaF
in the temperature range 383-498 К is described by the equation:
lnP(Pa)= 34.97960.1890 – 12657.8879.10/T
Adsorption differential heat of BrF3 on NaF, equal to 105.60.7 kJ/mol, indicates the chemo-
sorption nature of interaction. Normal dissociation temperature of BrF33NaF is 539.05.65 К, which
is consistent with the reported data.
Kinetics of interaction of BrF3 with coarse-pored sorbents (powder and pelted sodium
fluoride) at small conversion degrees is limited by the conditions of new crystalline phase
formation for BrF33NaF and passes the autocatalytic acceleration stage in the entire range of the
investigated pressures and temperatures. After passing the maximum, and up to conversion degrees
50-60%, the process is characterized by the linear pattern of solid film growth. The average value
of the apparent activation energy of the process for this section of the kinetic curves was 24.4
kJ/mol. Further on, we can see the process transition in the diffusion range.
It was established that interaction rate between BrF3 and NaF depends on supersaturation in
the system, defined by partial pressure and trifluoride bromine ratio to its equilibrium pressure
above the compound (Pa/Peq)m. We propose the following equation to illustrate the link between
forward reaction rate (Ка) described by Arrhenius equation and experimental value (Ki):
Ki= Kа (Pa/Peq)m,
where Ki – observed reaction rate constant, Ka –forward reaction rate constant described by
Arrhenius equation; Pa – partial pressure of BrF3; Peq – equilibrium pressure of BrF3 above
BrF33NaF; m – factor characterizing the interaction area of BrF3 with NaF. The application of
this equation makes possible to determine the true reaction rate values and reaction activation
energy in chemisorption terms; it also helps to calculate the degree BrF3 sorption on NaF reaction
at different temperatures and adsorbate pressures.
Thermal dissociation of BrF33NaF under vacuum at temperatures of 453-523 К at
conversion degrees more than 10% is complicated by diffusion resistance, however, its rate is
acceptable for industrial implementation. The average value of the apparent activation energy of
thermal decomposition of BrF33NaF for samples of different morphology amounted to 72.1
kJ/mol, which is consistent with the similar value (808 kJ/mol) defined in [25].
Analysis of experimental and literature data on thermodynamics and kinetics of interaction
of IF5, BrF3, UF6 with NaF, experiments on pilot installation show the possibility to separate the
IF5-BrF3-UF6 system into individual components by gas sorption methods using the scheme:
separation of iodine pentafluoride from BrF3 and UF6 at the sorption stage at a temperature of 363-
383 К, and reduced total pressure in the system; selective desorption of BrF3 at temperature 453-
523 К and pressure 1300 Ра; desorption of UF6 at 623-673 К and pressure of less than 53 Ра.
We made several attempts to determine the crystal structure of BrF33NaF compound,
but all of them were unsuccessful. The BrF33NaF in case of direct contact with even a flame-
dried quartz capillary leads to sample decomposition with the formation of NaHF2 Na2SiF6

14
(see Fig. S1, Supplementary Material). This reactivity may in our opinion be based either on
the presence of small amounts of HF in BrF33NaF, or on residual traces of water within the
quartz capillary, also forming HF. This would react with SiO2 under formation of SiF4 and
H2O. The H2O formed this way would again hydrolyze the BrF33NaF producing HF, and the
cycle would start again. That is why the question of BrF33NaF structure is still open and we
are working on it.
The BrF33NaF thermal decomposition products were characterized by X-ray diffraction
and classic chemical analysis. The powder X-Ray diffraction analysis (see Fig. S2, Supplementary
Material) showed that the only product of its thermal decomposition is NaF.
4. Experimental
Starting materials
Bromine trifluoride was synthesized directly from elements [8]. The measured saturated
vapors pressure and phase transition temperatures of BrF3 corresponded to the values given in [8].
Content of the target substance was not less than 99.9% wt. HF was removed by continuous
vacuum distillation at 283 К. The content of impurities was beyond the detection limits of infrared
spectrometric analysis.
Powder of sodium fluoride obtained by fluorinating the sodium carbonate with gaseous
fluorine, had the following characteristics: specific surface – 2.75 m2/g; average effective pore
radius – 309.2 nm; pore volume – (2.322.65), cm3/g; average particle size – (0.250.35) mm. The
content of the ground substance in sodium fluoride was at the level of 9997 % wt., the impurity
contents are given in Table 1.
The pelleted sodium fluoride provided by JSC “Siberian Chemical Combine” was obtained
through vacuum heat treatment of sodium hydro-fluoride according to the method described in
[26] and had the following characteristics: specific surface – 3.77 m2/g; static strength – 33.7
kgf/cm2; porosity – 55.1%.
Immediately before the experiments, the sorbent was treated with fluorine at a
temperature of 423-473 К, followed by activation at 573-593 К under vacuum.
The BrF3∙3NaF compound was synthesized by NaF powder saturation by BrF3 vapors at
348.15 К and 373.15 К within 72 hours. The direct gravimetric measurements showed the
correspondence with composition to BrF3nNaF, where n = 2.970.25. To ensure the product
composition the product sample was hydrolyzed, and concentration of bromine, fluorine and
sodium was determined using the method described in [34]. The sodium was identified by
gravimetric perchlorate method; the bromine was determined by direct differential-potentiometric
titration by AgNO3 with two Ag electrodes. The fluorine concentration was determined in fluoride
anion form by potentiometric titration with thorium (IV) nitrate and fluoride-selective electrode.
The analysis results showed that the corresponding adduct composition is equal to BrF3nNaF,
where n = 2.980.06.
The BrF3∙3NaF compound is a white crystalline solid which is hydrolyzed by air moisture
and covered by bromine vapors during this process (see the XRD pattern of hydrolysis product in
Supplementary Materials).
The phase composition of BrF33NaF compound and its related processes (thermal
decomposition, hydrolysis) products were studied with diffractometer XRD – 6000 at CuKά
radiation. Analysis of the phase composition was performed using PDF 4+ databases, and the full-
profile analysis program POWDER CELL 2.4.
The particulate composition was determined by sieve analysis of sodium fluoride powder
based on the scattering results of 1000 g of NaF.
IR spectra of bromine trifluoride were registered with the spectrophotometer “Perkin-Elmer”

15
model 16 PC in the gas cell made of nickel with optical path length 12 cm and windows of AgCl
with the range of transmission within 0.625 microns.
The specific surface area was measured by the Brunauer – Emmett – Teller method in
accordance with governmental methodic.
The average effective radius and pore volume were determined by gas chromatography with
chromatograph Agilent 7890A (Agilent Technologies, USA) using nitrogen-helium mixture.
Static strength of sodium fluoride pellets was determined in accordance with governmental
methodic. The testing machine MIP 100-2” with the strength measurement range from 0.1 to 10
MPa, and relative error ±4% was used.
Description of the experimental installation
Experimental installation (Fig. 10) was constructed to study the dependence of the vapor
pressure of BrF3 over its compounds with sodium fluoride by static method and to investigate the
kinetics of complex BrF3∙3NaF formation and its thermal dissociation by continuous gravimetry
in isothermal and isobaric conditions. The installation consists of following units: the study of
thermodynamic characteristics; kinetic studies; gas purification and vacuuming.
The unit for study of thermodynamic characteristics consisted of containers (1) and (2) of
known capacity 174 and 183 mL, respectively, which were placed in an air bath (3), which
provided the desired temperature in the containers in the range 343-523 К to an accuracy of 0.5
K. Container (2) was provided with an additional oven (4), which allowed calcination of sorbent
directly into the container both in fluorine and under vacuum at temperatures up to 623 К.
Container (5) with BrF3 was in liquid thermostat (6), which provided the required saturated vapors
pressure of the adsorbed substance. To measure the system pressure, differential pressure gauges
(7) were used; their zero chambers were constantly pumped out by a vacuum pump (7a). The
residual pressure in the zero chambers of the differential pressure gauges (7) was measured by an
absolute pressure sensor PMT-4 (8), completed with thermocouple vacuum gauge VT-2AP (9).
The unit of kinetic studies was made as a closed loop consisting of a circulation piston pump (10),
flow meter (11) (completed with recording device type KSU-2, fine adjustment valve (12) and orifice
plate (13)), container (5) and reactor (14) equipped with a heater, cooling jacket made of nickel tube
45 mm in diameter and 500 mm long.

16
Fig. 10. The experimental installation for research of the interaction in BrF3(gas) – NaF(solid) system
For the continuous recording of the sample mass change the reactor (13) was equipped with
scales with electromagnetic plunger system (15), completed with the secondary recording device
(15a). The reactor and scaled mounted on it were in air bath (16) with maintained temperature 323-
383 К. Piston pump (10) ensured circulation of vapors of BrF3 or IF5 in the reactor (14) with a linear
velocity up to 100 cm/min.
The vacuum-pumping system consisted of the absorption column (17) filled with chemical
absorber, filter (18), refrigerated trap (19) and vacuum pump (20), and ensured a residual pressure
in the system to 0.7 Pa.
In order to prevent vapor condensation in the devices, the meters, valves and communications
were placed in the air baths (3) (14) (21). The parts of communications located outside the air
thermostats were equipped with individual heaters and thermal insulation. To eliminate the
equipment corrosion influence on the empirical data, nickel and copper were selected as the main
structural materials, except for valves. To passivate equipment before the experiments and to “flush”
it before the plant opening, the plant filling with fluorine and dry nitrogen was provided.
Acknowledgments
We thank the Russian Governmental Program ‘Nauka’ N 4.3967.2017/PCh and Tomsk
Polytechnic University (TPU) Program on Competitiveness Enhancement (CE) for support.
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Table 1. The contents of the main impurities in sodium fluoride powder
Element
% wt.
Al
210-2
C
Fe
1.510-4
Ni
110-5
Cr
110-4
K
Acidity
none
2.510-3
3.510-3