Interaction of datolite concentrate with ammonium hydrodifluoride

Article abstract of DOI:10.1134/S0036023610080012

The interaction of datolite concentrate with ammonium hydrodifluoride were studied. Under heating, ammonium hydrodifluoride fluorinates datolite to form complex fluoroammonium salts of boron, silicon, and impurity metals. The activation energy and rate constants were calculated for the reaction of datolite concentrate with ammonium hydrodifluoride in the range 100-150°C. The composition of the vola-tiles formed upon the heating of the fluorinated concentrate was studied, the temperature parameters of the evolution of boron and silicon to the gas phase were determined, and a process for separating these elements was proposed. Pleiades Publishing, Ltd., 2010.

Full text of DOI:10.1134/S0036023610080012

ISSN 0036ꢀ0236, Russian Journal of Inorganic Chemistry, 2010, Vol. 55, No. 8, pp. 1163–1165. © Pleiades Publishing, Ltd., 2010.
Original Russian Text © G.F. Krysenko, M.A. Medkov, D.G. Epov, 2010, published in Zhurnal Neorganicheskoi Khimii, 2010, Vol. 55, No. 8, pp. 1235–1238.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
Interaction of Datolite Concentrate
with Ammonium Hydrodifluoride
G. F. Krysenko, M. A. Medkov, and D. G. Epov
Institute of Chemistry, Far East Division, Russian Academy of Sciences,
pr. Stoletiya Vladivostoka 159, Vladivostok, 690022 Russia
Received February 12, 2009
Abstract—The interaction of datolite concentrate with ammonium hydrodifluoride were studied. Under
heating, ammonium hydrodifluoride fluorinates datolite to form complex fluoroammonium salts of boron,
silicon, and impurity metals. The activation energy and rate constants were calculated for the reaction of
datolite concentrate with ammonium hydrodifluoride in the range 100–150°C. The composition of the volaꢀ
tiles formed upon the heating of the fluorinated concentrate was studied, the temperature parameters of the
evolution of boron and silicon to the gas phase were determined, and a process for separating these elements
was proposed.
DOI: 10.1134/S0036023610080012
Boron and its compounds are widely used in engiꢀ nium hydrodifluoride of pure for analysis grade was
neering for manufacturing highꢀtemperature alloys used as the fluorinating agent.
and heatꢀresistant devices, in electronics, and other
fields. Datolite is of practical value for manufacturing
commercial boron products.
Experiments were carried out on a setup comprisꢀ
ing a Teflon reaction beaker with a hermetically screw
top equipped with an inlet for supplying air from a
Currently, datolite concentrates are processed by microcompressor (at a rate of 0.15 L/h) and a gas outꢀ
concentrated sulfuric acid followed by crystallization let pipe ending with an adapter sunk into a receiver
of boric acid [1]. However, the use of sulfuric acid sugꢀ filled with 0.1 N H SO4 and Methyl Red indicator. The
2
gests the use of highꢀcost acidꢀresistant apparatus and reaction beaker was placed in an electric furnace
the organization of sulfuric acid production facilities where temperature was set with a VRTꢀ2 highꢀpreciꢀ
in the locality where datolite concentrates are proꢀ sion temperature controller. The sample size was 10 g.
cessed; the results are a considerable complexity and The course of the reaction was monitored by powder
higher costs of the process and considerable amounts Xꢀray diffraction of residues and by measuring the
of waste borogypsum.
amount of ammonia evolved.
A process is known for processing boronꢀcontainꢀ
The volatiles composition was derived from the
ing materials via decomposition of natural borates by Xꢀray diffraction data of sublimates collected upon
oxalic acid under heating [2].
heating of fluorinated concentrates in an isothermal
conditions at 300 and 360 . These experiments were
carried out in platinum test tubes partially inserted into
the electrical furnace with an extended Teflon pipe used
to condense volatiles. Here, the sample size was 30 g.
°
С
It was of interest to study whether an acid reagent
that represents a crystalline fluorinating agent,
namely, ammonium hydrodifluoride, is useful for
decomposition of borosilicate concentrates.
Chemical analysis for ammonia was carried out as
the back titration with a NaOH solution of the subliꢀ
mate collected in the sulfuric acid solution.
Xꢀray diffraction patterns were recorded on a Dꢀ8
ADVANCE automated diffractometer with sample
This work is concerned with the interaction of
datolite concentrate with ammonium hydrodifluoride
in the view of its stripping and finding ways to separate
boron and silicon in this processing of boronꢀcontainꢀ
ing materials.
spinning using Cu
K radiation. Xꢀray diffraction analꢀ
α
ysis was carried out using the EVA search program
with the powder data file (PDFꢀ2).
EXPERIMENTAL
We chose to study a datolite concentrate batch conꢀ
Kinetic studies of the interaction of datolite conꢀ
taining (wt %): B O , 18.3; SiO , 35.5; CaO, 34.5; centrate with NH HF2 were carried out on 10ꢀg samꢀ
2
3
2
4
Fe O , 2.1; Al O , 0.7; CO , 2.6; H O, 5.0; and Mg ples in an isothermal conditions at 100, 110, 120, 130,
2
3
2
3
≤
2
2
and Mn cations,
0.2. The bulk density of the dry conꢀ 140, and 150 С; the amount of ammonia evolved in
°
3
centrate was 1400 and 1680 kg/m for the uncomꢀ certain periods of time was determined. Sampling
pacted and compacted product, respectively. Ammoꢀ repeated every 5–15 min. The reaction mixture was
1163

1164
KRYSENKO et al.
NH HF2, which enhances their fluorination. Ammoꢀ
nium fluorometalates in accordance with their physiꢀ
α
, %
4
100
cochemical properties provide the solubility of the
products and a possibility of separating mixtures in
solutions and in solids by means of sublimation. For
boron, two fluoroborates are known, namely, NH BF
f
80
60
40
20
0
e
d
c
b
4
4
and NH BF4 NH4F, as well as numerous oxoꢀ and
⋅
4
hydroxofluoroborates. Under heating, the NH BF
4
4
decomposition pressure reaches the atmospheric presꢀ
sure at 354 С [3].
°
Powder Xꢀray diffraction shows that the interaction
а
of datolite concentrate with NH HF2 yields complex
4
fluoroammonium salts of boron, silicon, and impurity
metals (aluminum and iron) and calcium fluoride; a
makeꢀup fluorinated product contains a fluoroammoꢀ
nium salt of boron in an Xꢀray amorphous form, which
0
20
40
60
80
100
, min
τ
crystallizes in NH BF4 upon standing. Thus, the interꢀ
4
Extent of ammonia evolution (
α
) vs. time (
τ
) during the
action of datolite with NH HF2 may be described by
the equation
4
fluorination of datolite concentrate by ammonium
hydrodifluoride at ( ) 100, ( ) 110, ( ) 120, ( ) 130,
) 140, and ( ) 150°C.
а
b
c
d
CaBSiO (OH) + 6NH HF = CaF + NH BF
4
(e
f
4
4
2
(NH ) SiF + 3NH + 5H O.
2
4
+
4 2 6 3 2
This equation demonstrates that ammonia is
evolved into the gas phase during the reaction. The
amount of evolved ammonia is helpful to study the
kinetics and gain more data on the process.
prepared immediately before an experiment. The
timezero was the moment when the sample was
mounted in the furnace preheated to a set temperaꢀ
ture.
The reaction of a datolite concentrate and NH HF
4
2
Thermogravimetric experiments were performed
on a Qꢀ1000 derivatograph using platinum crucibles at
a heating rate of 10 K/min and sample sizes of 100–
taken in the ratio 1 : 6 (mol/mol) was studied in an isoꢀ
thermal conditions at 100, 110, 120, 130, 140, and
1
50
°
С
. When datolite concentrate is decomposed at
200 mg.
temperatures below 120
°
С, we noticed that the fluoriꢀ
nated product was always wet; above this temperature,
the product was a readily pulverizable dry cake.
The figure displays the extent of ammonium evoluꢀ
tion versus time during the reaction of datolite conꢀ
centrate with ammonium hydrodifluoride at the
aboveꢀspecified temperatures.
RESULTS AND DISCUSSION
The physicochemical properties of ammonium
hydrodifluoride are responsible for its specific behavꢀ
ior in the action on oxides, consisting of the complexꢀ
At 100 С, the fluorination of datolite concentrate
°
ing ability of NH HF2. All transition elements and
4
by ammonium hydrodifluoride occurs at a linear rate
and far more slowly than at other temperatures: in
many nontransition elements form ammonium fluꢀ
oroꢀ or oxofluorometalates when reacting with
1
.5 h, the extent of the reaction is as low as 18% of the
theory. At 110 or above, the reaction rate is noticeꢀ
°
С
Values of the order of reaction (n) and rate constants (K) for ably higher, most likely, because of the reaction occurꢀ
the fluorination of datolite concentrate by ammonium hyꢀ ring in the melt produced by the melting of the fluoriꢀ
drodifluoride at various temperatures
nating agent at 126
at 109 С (as the reaction is accompanied by ammonia
°
С or an NH F + NH HF eutectic
4 4 2
°
Activation enꢀ
ergy, kJ/mol
min–1
evolution, the NH F + NH HF eutectic can form).
T
,
°
C
n
K
,
4
4
2
Temperature rising to 150
°
С
considerably improves
the characteristics of the process: in 1.5 h, up to 80%
of the datolite concentrate is reacted.
The experimental ammonia evolution data were
used to calculate the activation energies, the order of
100
110
120
130
140
150
0.99
2.04
2.16
2.22
2.02
2.00
0.00128
0.02335
0.03239
0.04357
0.06244
0.07015
reaction (
n), and rate constants (
K) of the reaction of
datolite concentrate with NH HF2 at 100, 110, 120,
4
42.6
130, 140, and 150 С. The calculations employed the
°
generalized Kolmogorov–Erofeev equation [4]. The
results of the calculations are displayed in the table.
Inspection of the variation of rate constants showed
that at 100 С the interaction of datolite concentrate
°
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010

INTERACTION OF DATOLITE CONCENTRATE
1165
with ammonium hydrodifluoride proceeds in the difꢀ sure at this temperature recover 98.4% of silicon to the
fusion area and one order of magnitude more slowly gas phase.
than in melt. In the melt of the fluorinating agent, the
On the other hand, the separation of waterꢀsoluble
interaction proceeds in the kinetic area and at a higher
fluoroammonium salts of boron and silicon may be
rate. The activation energy of the reaction of datolite
accomplished via the precipitation of silica by ammoꢀ
concentrate with NH HF was calculated from the
4
2
nia from a saturated solution of (NH ) SiF sublimates
4
2
6
results of the experiments carried out at 110–150°С.
[
6], which in addition will yield finely divided silica
powders. Indeed, our experiments showed that, when
the filtrate obtained after separating ammoniaꢀprecipꢀ
itated silica was concentrated on a water bath and then
subjected to powder Xꢀray diffraction, boron
Comparing the kinetic parameters of the fluorinaꢀ
tion of datolite concentrate by ammonium hydrodiꢀ
fluoride with those of the fluorination of quartz (which
presents in the concentrates), we showed that crystalꢀ
line silica restrains the development of reactions of
remained in the solution in the form of NH BF4.
4
datolite concentrate with NH HF2, because the rate
constants of quartz fluorination by ammonium
4
To summarize the results of our studies, the fluoriꢀ
nation of datolite concentrate by ammonium hydrodꢀ
hydrodifluoride are far lower than the rate constants of ifluoride at 100–150
the datolite fluorination by ammonium hydrodifluoꢀ of boron and silicon and calcium fluoride; the reaction
ride. For example, at 120 , the rate constant of occurs in the diffusion area until the fluorinating agent
quartz fluorination by ammonium hydrodifluoride is melts and in the kinetic area in the melt. The presence
°
С yields fluoroammonium salts
°
С
–1
0
.003289 min [5], which is one order of magnitude of crystalline silica in the concentrate retards the
lower than the rate constant of datolite fluorination by development of the reaction of datolite concentrate
ammonium hydrodifluoride at this temperature. The with ammonium hydrodifluoride. The calculated actiꢀ
moderate value of the activation energy of the reaction vation energy of the reaction of datolite concentrate
of datolite concentrate with ammonium hydrodifluoꢀ with NH HF , equal to 42.6 kJ/mol, indicates that
4 2
ammonium hydrodifluoride would easily strip datolite
concentrate.
ride signifies that this fluorinating agent would easily
decompose datolite concentrate.
We have found that 99.3% of the silicon and boron
is expelled to the gas phase upon heating ammonium
hydrodifluorideꢀfluorinated datolite concentrate to
Thermogravimetry shows that the fluorinated
product losses weight in two steps, 58 and 30 wt % at
3
08 and 360
°
С, respectively. Powder Xꢀray diffraction
3
90 С. To separate fluoroammonium salts of silicon
°
of the residues obtained upon heating of (NH ) SiF ,
4
2
6
and boron, we have proposed to precipitate silica via
the ammonia hydrolysis of a solution of these salts to
obtain finely divided silica powders and a solution of
NH BF4, fluorinated datolite concentrate, and a
4
(
NH ) SiF : NH BF = 7 : 3 model mixture, shows
4 2 6 4 4
that at 308
°
С (NH ) SiF sublimes and at 360 С
°
4
2
6
ammonium tetrafluoroborate NH BF4
.
4
NH BF is evolved to the gas phase.
4
4
The last mentioned results were supported by the
identification of condensation products. The volatiles
composition was determined by heating the fluoriꢀ
nated concentrate in an isothermal conditions at 300
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1
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°
densed in a cooled Teflon pipe. The condensed prodꢀ
uct in the first case was ammonium hexafluorosilicate
and in the second NH BF as identified by powder
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G. G. KatsꢀDavid, Khim. Prom–st, No. 6, 355 (1980).
4
4
3
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(1988).
Xꢀray diffraction.
The experiments showed that heating of fluoriꢀ
nated datolite concentrate up to 390 and 1ꢀh expoꢀ
15
4
. V. V. Boldyrev, Investigation Methods in the Kinetics of
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Univ., Tomsk, 1958) [in Russian].
°
С
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The different volatilities of fluoroammonium salts
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showed that heating of the mixture of fluoroammoꢀ
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(1994).
6
. G. F. Krysenko, E. I. Mel’nichenko, and D. G. Epov,
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RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 55 No. 8 2010