C1-C4 hydrocarbon release in the preparation of SiF4 through Na2SiF6 pyrolysis

Article abstract of DOI:10.1134/S0020168511120089

Hydrocarbon impurities in silicon tetrafluoride originate from the sodium hexafluorosilicate used as a precursor for the preparation of SiF₄ through the thermal decomposition of Na₂SiF₆. We have studied the fraction of C₁-C₄ hydrocarbons released as a function of temperature during the thermal decomposition of sodium hexafluorosilicate. The lighter hydrocarbons-methane, ethane, and ethylene-are shown to be released at lower temperatures. One source of hydrocarbon impurities in sodium hexafluorosilicate is the sodium carbonate used to prepare it.

Full text of DOI:10.1134/S0020168511120089

ISSN 0020ꢀ1685, Inorganic Materials, 2012, Vol. 48, No. 1, pp. 7–9. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.A. Krylov, T.G. Sorochkina, A.D. Bulanov, A.Yu. Lashkov, 2012, published in Neorganicheskie Materialy, 2012, Vol. 48, No. 1, pp. 11–13.
C –C Hydrocarbon Release in the Preparation of SiF
1
4
4
through Na SiF Pyrolysis
V. A. Krylov , T. G. Sorochkina , A. D. Bulanov , and A. Yu. Lashkov
2
a
6
a,b
a
a
a
Institute of Chemistry of HighꢀPurity Substances, Russian Academy of Sciences,
ul. Tropinina 49, Nizhni Novgorod, 603950 Russia
Lobachevsky State University, pr. Gagarina 23, Nizhni Novgorod, 603950 Russia
b
eꢀmail: bulanov@ihps.nnov.ru
Received February 14, 2011
Abstract—Hydrocarbon impurities in silicon tetrafluoride originate from the sodium hexafluorosilicate used
as a precursor for the preparation of SiF4 through the thermal decomposition of Na SiF6. We have studied the
2
fraction of C –C hydrocarbons released as a function of temperature during the thermal decomposition of
1
4
sodium hexafluorosilicate. The lighter hydrocarbons—methane, ethane, and ethylene—are shown to be
released at lower temperatures. One source of hydrocarbon impurities in sodium hexafluorosilicate is the
sodium carbonate used to prepare it.
DOI: 10.1134/S0020168511120089
INTRODUCTION
The addition of a stoichiometric amount of Na CO
2
3
leads to Na SiF6 precipitation [9]:
2
Silicon tetrafluoride is a starting material for silicon
isotope separation and the preparation of isotopically
and chemically pure silicon. To create a physically reaꢀ
2
H SiF + Na CO
2
→
Na SiF + H O + CO . (3)
2 6 2 2
2
6
3
Therefore, one possible source of hydrocarbon impuꢀ
sonable mass standard based on highly enriched 2 Si
8
,
rities in sodium hexafluorosilicate is Na CO3
.
2
The objectives of this work were to ascertain
one needs highꢀpurity silicon containing less than
at % carbon [1]. In the preparation of highꢀpurity
Si by a hydride process, the starting silicon tetrafluoꢀ
ride is the most likely source of carbonꢀcontaining
impurities in the resultant silicon [2]. According to gas
chromatography data, the content of С –С hydrocarꢀ
4
×
–
6
whether С –С4 hydrocarbons pass from sodium
1
0
1
2
8
hexafluorosilicate to silicon tetrafluoride, to study the
dynamics of this process, to examine the possibility of
minimizing the concentration of С –С4 hydrocarbons
1
in the silicon tetrafluoride, and to check whether
hydrocarbon impurities may be present in sodium carꢀ
bonate.
1
4
bon impurities in the starting silicon tetrafluoride is as
–5
–3
high as
2
×
10 to 3 10 vol % [2]. In view of this, an
×
important issue is to find out the origin of hydrocarbon
impurities in silicon tetrafluoride.
EXPERIMENTAL
Silicon tetrafluoride can be prepared through the
thermal decomposition of sodium hexafluorosilicate
We used pureꢀgrade sodium hexafluorosilicate
Purity Standard TU 6ꢀ09ꢀ05807960ꢀ114ꢀ94). The
hydrocarbon release in the course of sodium hexafluꢀ
(
[
3–5]:
Na SiF6
2
→
2NaF + SiF4.
(1) orosilicate decomposition was investigated in the temꢀ
perature range from 315 to 710 С. To this end, we
°
According to data in the literature, the thermal
determined the amounts of silicon tetrafluoride and
hydrocarbons released during successive heating to
decomposition of sodium hexafluorosilicate begins at
50 [6]. The SiF4 production process is run at temꢀ
peratures from 540 to 710 with constant removal of
4
°
С
3
15, 415, 480, 540, and 710 С. The hydrocarbons were
°
°
С
determined by a gasꢀchromatographic procedure
described elsewhere [10], with detection limits in the
the forming silicon tetrafluoride vapor from the reacꢀ
tion zone [7]. The likely source of hydrocarbon impuꢀ
rities in the resultant tetrafluoride is the starting
–6
range (2–6) 10 vol %.
×
The hydrocarbon release in the course of sodium
hexafluorosilicate decomposition was studied experiꢀ
mentally using an apparatus schematized in Fig. 1.
The experimental procedure was as follows:
Sodium hexafluorosilicate (70 g) was placed in a stainꢀ
Na SiF6
.
2
A major raw material for the preparation of sodium
hexafluorosilicate is fluorosilicic acid. It forms when
silicon tetrafluoride resulting from phosphoric fertilꢀ
izer processing is dissolved in water [8, 9]:
lessꢀsteel flanged cylinder (
1) equipped with two valves
3
SiF + 2H O
4
→
2H SiF + SiO .
6
(2)
(2, 3
). The cylinder was mounted in a resistanceꢀ
2
2
2
7

8
KRYLOV et al.
As the temperature was raised from 540 to 710 С,
°
Inflow
GC
2
the sodium hexafluorosilicate decomposed entirely to
give silicon tetrafluoride. The process was run in
another apparatus, where a reactor was connected to a
cylinder for SiF4 freezeꢀout with constant silicon tetꢀ
rafluoride sampling until gas evolution ceased.
6
1
3
5
The silicon tetrafluoride that resulted from comꢀ
plete sodium hexafluorosilicate decomposition at
7
10 С was analyzed for hydrocarbon impurities by gas
°
4
chromatography.
We also examined hydrocarbon release from
sodium carbonate (RF Standard GOST 83ꢀ79) during
heating in the temperature range from 25 to 300 С.
°
Fig. 1. Schematic of the apparatus used to study the hydroꢀ
The experimental procedure was similar to the above
procedure for assessing the hydrocarbon release in the
course of sodium hexafluorosilicate decomposition.
carbon release during thermal decomposition of Na SiF :
2
6
(1) Na SiF ꢀcontaining flanged cylinder, (2, 3) Hoffer
2 6
valves, (4) resistanceꢀheated tube furnace, (5) thermomeꢀ
ter, ( ) vacuum gage.
6
RESULTS AND DISCUSSION
heated tube furnace (
system of a gas chromatograph. After the inflow sysꢀ
tem and cylinder were pumped down, valve was
closed and the cylinder, containing Na SiF6, was
4) and connected to the inflow
From the experimentally determined SiF4 presꢀ
sure, the volume of the inflow system, and the free volꢀ
ume in the reactor, we calculated the amount of siliꢀ
2
2
con tetrafluoride (
cedure at temperatures of up to 540
Clapeyron equation. From gas chromatography data,
we calculated the amount of hydrocarbons ( , moles)
n
, moles) released in the above proꢀ
heated to 315
ture was 1 h. The temperature was measured by a highꢀ
temperature thermometer ( ). The pressure of the
released silicon tetrafluoride was monitored with a
vacuum gage ( ), with valve open. After holding at
15 , valve was opened, the cylinder was connected
to the inflow system of the gas chromatograph, and the
gas phase was analyzed. Next, valve was closed, and
the cylinder, containing Na SiF6, was heated to 415
°
С. The holding time at each temperaꢀ
°
С
using the
5
n
in the silicon tetrafluoride released at these temperaꢀ
tures. In addition, we calculated the amount of hydroꢀ
6
3
3
°
С
2
carbons (
at 710
hydrocarbon (
heating to a particular temperature in the range 315 to
10 . Figure 2 shows the temperature dependences
n
, moles) in the silicon tetrafluoride obtained
. We estimated the total amount of each
, moles), and the fraction released on
°
С
2
n
°
С,
2
where it was held for 1 h. After analysis, the silicon tetꢀ
rafluoride was removed from the inflow system of the
chromatograph by a roughing pump.
7
°
С
of the fraction released for the hydrocarbons in quesꢀ
tion. As seen, the fraction released increases with temꢀ
In a similar manner, we took samples of the formꢀ perature in all cases and depends on the hydrocarbon.
ing silicon tetrafluoride at 480 and 540
. After heatꢀ The lighter hydrocarbons—methane, ethane, and
ing to 540
, all of the silicon tetrafluoride formed was ethylene—are released at lower temperatures. At
removed from the sodiumꢀhexafluorosilicateꢀconꢀ 540 , the fraction released reaches unity for methane
°
С
°
С
°
С
taining cylinder by pumping. The inflow system of the and is 0.75 for ethane, 0.78 for ethylene, 0.36 for proꢀ
3
chromatograph had a volume of 92.4 cm , and the gas pane, 0.08 for propylene, 0.44 for butane, and 0.08 for
volume in the cylinder to the vacuum gage (
6
) was isobutane. The release of the С –С4 hydrocarbons
2
3
132.7 cm .
continues until complete sodium hexafluorosilicate
decomposition at 710 С.
°
We compared the percentages of hydrocarbons in
silicon tetrafluoride prepared via sodium hexafluoroꢀ
silicate decomposition after heat treatment at 300 [2]
Percentages of hydrocarbons in SiF prepared via Na SiF
decomposition after heat treatment at 540 and 300°C
4
2
6
and 540
It can be seen from the table that the removal of
hydrocarbons by heatꢀtreating Na SiF6 at 540
°
С (table).
Mole percent
Impurity
540
°
C
300
°
C
°
С
2
10–6
10–4
10–4
10–4
10–5
10–4
10–5
10–5
makes it possible to reduce the percentages of methane
and ethylene in the SiF4 by two orders of magnitude
and that of ethane by one order of magnitude, whereas
the concentrations of the heavier hydrocarbons, С3
CH4
<2
×
×
×
×
×
×
×
(1.4 ± 0.2)
×
×
×
×
×
×
×
C H
(3.2 ± 0.8)
(2.4 ± 0.6)
(2.5 ± 0.6)
(2.0 ± 0.5)
(1.1 ± 0.3)
(1.4 ± 0.4)
10–5 (3.4 ± 0.5)
10–5 (7.2 ± 0.1)
10–5
10–5
10–5
2
6
4
8
6
C H
2
and C , decrease by no more than 20%.
C H
3
(3.1 ± 0.6)
(2.2 ± 0.3)
(1.3 ± 0.3)
4
Removing the gas released during the preparation
C H
3
of silicon tetrafluoride from 70 g of Na SiF6, we
2
i
ꢀC H
4 10
obtained 36.7 g of SiF4, which corresponds to a 95%
product yield. The amount of carbon released in the
n
ꢀC H
10–5 (1.5 ± 0.3)
4
10
INORGANIC MATERIALS Vol. 48
No. 1
2012

C –C HYDROCARBON RELEASE IN THE PREPARATION OF SiF
9
1
4
4
We have studied the fraction of hydrocarbons
released as a function of temperature during the therꢀ
mal decomposition of sodium hexafluorosilicate. The
lighter hydrocarbons—methane, ethane, and ethylꢀ
ene—are shown to be released at lower temperatures.
1
0
0
0
0
.0
.8
.6
.4
.2
0
CH4
C H
3
6
Removing hydrocarbons by pumping at temperaꢀ
tures of up to 540 С during the thermal decomposition
°
of sodium hexafluorosilicate, we reduced the concenꢀ
tration of ethylene, a regulated impurity in the purifiꢀ
cation of SiF4 and SiН4 by fractional distillation, by a
factor of 30. This enables the purity of the resultant silꢀ
icon tetrafluoride to be markedly improved.
One source of hydrocarbon impurities in sodium
hexafluorosilicate is the sodium carbonate used to preꢀ
pare it.
1
0
0
0
0
.0
.8
.6
.4
.2
0
C H
2
6
C H
3
8
REFERENCES
1
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.8
.6
.4
.2
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4
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4
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6
i
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°
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6
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sponds to 8.1
×
–5
×
10 wt % carbon in the Na SiF6
.
2
2
6
Heating sodium carbonate to 300
°
С
, we observed a
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release of hydrocarbon impurities similar to those
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8
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sodium hexafluorosilicate used as a precursor for the
preparation of SiF4 through the thermal decomposiꢀ
1
1
4
Impurities in Silicon Tetrafluoride, J. Anal. Chem., 2005,
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.
2
INORGANIC MATERIALS Vol. 48
No. 1 2012