Production of silicon from magnesium silicide

Article abstract of DOI:10.1134/S1070427213040060

Kinetic methods and thermogravimetry were used to study the oxidation process of magnesium silicide in air in the temperature range 300-1000 C. The reaction products were identified by X-ray phase analysis. It was found that the reaction occurs in the tem

Full text of DOI:10.1134/S1070427213040060

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 4, pp. 493−497. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © V.M. Borshchev, A.N. D’yachenko, A.D. Kiselev, R.I. Kraidenko, 2013, published in Zhurnal Prikladnoi Khimii, 2013, Vol. 86, No. 4,
pp. 530−534.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Production of Silicon from Magnesium Silicide
V. M. Borshchev, A. N. D’yachenko, A. D. Kiselev, and R. I. Kraidenko
Tomsk National Research Polytechnic University, Tomsk, Russia
e-mail: atom@tpu.ru
Received September 17, 2012
Abstract—Kinetic methods and thermogravimetry were used to study the oxidation process of magnesium silicide
in air in the temperature range 300–1000°C. The reaction products were identified by X-ray phase analysis. It was
found that the reaction occurs in the temperature range 510–710°C to give silicon and magnesium oxide. With the
temperature increasing further, silicon is oxidized to silicon dioxide.
DOI: 10.1134/S1070427213040060
The conventional techniques for manufacture of
silicon for solar power engineering include a number
of stages in which silicon-containing raw materials
are purified to remove interfering impurities and then
elementary silicon is obtained [1, 2].
The reduction product is the three-component system
composed of silicon, magnesium oxide, and magnesium
silicide Mg Si:
2
2
Mg + SiO = 2MgO + Si,
(1)
(2)
(3)
2
4
Mg + SiO = 2MgO + Mg Si,
Despite being widely used and well developed,
methods for production of polycrystalline silicon
from metallurgical silicon are technically complex,
require substantial capital investment for setting-up
production plants, and gross maintenance expenditure
in manufacture of commercial products.
2
2
2Mg + Si = Mg2Si.
To remove magnesium silicide, it is suggested to
subject magnesiothermy products to thermal calcination
in air. The separation is based on the reaction of
magnesium silicide with oxygen to give magnesium
oxide and silicon. In this way, it is possible to provide
the maximum recovery of silicon from silicon dioxide
and to preclude any loss of silicon.
Alternative technologies, such as production of
silicon from special-purity quartz by its reduction and
subsequent additional purification occupy a rather small
part of the market; however, the share of polycrystalline
silicon produced by these techniques steadily grows
annually [3].
The goal of the present study was to examine the
interaction of magnesium silicide with oxygen and to
identify reaction products.
A method is known for production of silicon by
magnesiothermic reduction of silicon dioxide, followed
by separation of reaction products [4]. Silicon and
magnesium oxide are separated by acid leaching
with hydrochloric acid. Together with dissolution of
magnesium oxide, there occurs decomposition of the
Thermodynamic calculations of the decomposition of
magnesium silicide were performed for the temperature
range 298–900 K [5] (Table 1). The equilibria of
chemical reactions (4)–(7) were found by the Temkin–
Svwartzman method:
silicide to give silane (SiH ) vapor. Interacting with
water contained in hydrochloric acid, silane self-ignites
4
Mg Si + O = 2MgO + Si,
(4)
(5)
(6)
(7)
2
2
Mg Si + 2O = 2Mg SiO ,
2
2
2
4
with explosive blasts. In the end, SiO is again formed.
2
Mg Si + 2O = 2MgO + MgSiO ,
A loss of this kind is inadmissible in reduction of the
pure and rather expensive oxide.
2
2
3
Mg2Si + 2O2 = 2MgO + SiO2.
4
93

494
BORSHCHEV et al.
Table 1. Values of the standard ΔH°, ΔS°, ΔG°, and log K for the reactions of thermal decomposition of magnesium silicide by
p
oxygen at temperatures of 298–900 K
Temperature, K
600
Mg Si + O = 2 MgO + Si
Parameter
2
98
400
500
700
800
900
2
2
∆
G°, kJ mol–1
∆
H°, kJ mol–1
Kp
–1062
–1041
–1020
–999
–978
–957
–936
–1124.6
–1124.76
–1124.97
–1125.18
–1125.39
1.12 × 1073
–1125.6
3.51 × 1062
–1125.81
2.36 × 1054
1.07 × 10185 1.12 × 10136 4.49 × 10106 1.13 × 1087
Mg Si + 2 O = 2 Mg SiO
2
2
2
4
∆
G°, kJ mol–1
∆
H°, kJ mol–1
Kp
202
230
255
278
299
319
337
96.28
95.65
94.81
93.97
93.13
92.29
91.16
5.64 × 10–36 8.41 × 10–31 2.09 × 10–27 5.98 × 10–25 4.67 × 10–23 1.56 × 10–21 2.87 × 10–20
Mg Si + 2 O = 2 MgO + MgSiO
2
2
3
∆
G°, kJ mol–1
∆
H°, kJ mol–1
Kp
–1003
–967
–932
–898
–864
–830
–797
–510.68
–511.23
–511.961
–512.69
–513.42
–514.15
–514.88
1.233 × 1069 1.212 × 1051 8.666 × 1035 1.16 × 1026 1.304 × 1019 9.054 × 1013 9.428 × 109
Mg Si + 2 O = 2 MgO + SiO
2
2
2
∆
G°, kJ mol–1
∆
H°, kJ mol–1
Kp
–1918
–1879
–1840
–2036.03
–1800
–2036.74
–1761
–1721
–1681
–2035.5
–2035.5
–2037.45
–2038.16
–2038.87
1.58 × 10334 3.43 × 10245 2.08 × 10192 6.73 × 10156 2.94 × 10131 2.77 × 10112 4.36 × 1097
The standard Gibbs energies are negative for all the
reactions and have large values in the whole temperature
range 298–900 K, i.e., these reactions may occur
spontaneouslyandtheirequilibriumisirreversiblyshifted
toward formation of reaction products. The reactions
of thermal decomposition of magnesium silicide are
exothermic and occur with a substantial release of
heat. With increasing temperature, the probability
of oxidation of magnesium silicide to silicon and
magnesium oxide becomes lower, with a simultaneous
decrease in the probability of side reactions in which
magnesium meta- and orthosilicate and silicon dioxide
are formed. However, the equilibrium constants of the
decomposition reactions remain large in the whole
variation range of the given parameter.
temperature range 20–1000°C. In the study, a set of three
runs was performed with a calorimetric reproducibility
of ±2% and gravimetric reproducibility of ±0.1 μg; the
reagents used were of analytically pure grade.
The kinetic experiment was performed by the
method of continuous weighing of a reacting weighed
portion with automatically recording of its mass. The
conversion was found from the gain in mass, resulting
from the formation of solid products in the course of
oxidation of the weighed portion. The temperature was
maintained constant to within ±2°. A 1-g portion of the
Mg Si powder in a platinum crucible was placed in
2
a reactor electrically heated to a prescribed temperature.
After the decomposition process was complete, which
was judged from the attainment of a constant mass,
the heating was switched off and the crucible was
removed from the reactor. The solid residue formed
upon the thermal decomposition of magnesium silicide,
constituted by silicon and undecomposed magnesium
silicide, was weighed on an analytical balance. To rule
out a methodological experimental error, five runs were
made at each temperature. The average deviation of the
experimental data was 3–5%. The reproducibility of the
EXPERIMENTAL
Differential scanning calorimetry and thermogravi-
metric and differential-thermal analyses were per-
formed in a flow of air on a SDT Q600 combined ana-
lyzer with software processing of thermal analysis data
(
Instruments Universal V4.2E). The heating was made
–
1
at a rate of 5 deg min in corundum crucibles in the
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PRODUCTION OF SILICON FROM MAGNESIUM SILICIDE
495
Q, W g–1
m, %
T, °C
2
θ, deg
Fig. 1. Thermogram of the process of Mg Si interaction
with atmospheric oxygen. (m) Mass, (Q) heat flux, and (T)
temperature.
2
Fig. 2. XPA spectra of products formed in interaction of
Mg Si with atmospheric oxygen at a temperature of 650°C. (I)
2
Intensity and (2θ) Bragg angle.
experiments indicates that the temperature dependence
of the decomposition rate, obtained in the study, is
reliable and can be used to find kinetic parameters of the
decomposition process.
to 600°C. During the following 10 min, the rise in
conversion becomes slower and then it linearly varies
until the end of a run, slowly growing by, on average,
5
%. An increase in the decomposition temperature
The X-ray phase analysis was made on a DRON-3M
instrument with a copper anticathode (current 25 mA,
voltage 35 kV). The results of processing of the X-ray
data were compared with the NBSCR database for the
crystal structures of inorganic compounds.
from 520 to 600°C results in a significant rise in the
conversion of magnesium silicide. At a temperature of
6
00°C, the conversion reaches a value of 70% in 15 min.
The kinetic data on the interaction of magnesium
silicide with oxygen were mathematically processed by
the contracting-sphere equation (correlation coefficients
According to gravimetric analysis data (Fig. 1),
heating of a weighed portion containing magnesium
silicide results in the onset at 510°C of an exothermic
reaction between magnesium silicide and atmospheric
oxygen to give silicon and magnesium oxide. At 710°C,
the already formed silicon is oxidized to silicon dioxide:
0
.95–0.98):
α = 1 – [1 – 6.311τ exp(–58253±158.3/RT)]3,
where α is the conversion (fraction units).
Mg Si + O → 2MgO + Si,
2
2
The activation energy of the process was
58.25 kJ mol . The process occurs under kinetic control,
Si + O → SiO .
–1
2
2
An X-ray diffraction pattern of the product formed in
α, %
thermal decomposition of Mg Si in air at a temperature
2
of 650°C demonstrated that its main components are
silicon and magnesium oxide. No other phases were
found (Fig. 2).
The study of the oxidation of magnesium silicide in
the temperature range 520–600°C and the mathematical
processing of kinetic data demonstrated that the
conversion of magnesium silicide nonlinearly varies
with time (Fig. 3). The conversion grows to the greatest
extent during the first 3 min after the beginning of the
process for al of the temperatures studied. During this
interval of time, the conversion was 30 to 60 wt %,
respectively, at temperatures in the range from 520
τ, s
Fig. 3. Conversion α of magnesium(II) silicide vs. time τ for
the oxidation process. T (°C): (1) 520, (2) 560, (3) 580, and
(4) 600.
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496
BORSHCHEV et al.
with the rate of the overall process governed by the true
kinetics of the chemical reaction on the surface and little
dependent on the diffusion conditions. The process can
be intensified by raising its temperature.
oxidation to silicon dioxide. The largest yield of silicon
relative to the theoretical value (98.11%) was reached
at a temperature of 660°C and experiment duration of
30 min.
To determine the extent to which magnesium silicide
reacts with oxygen, we performed a set of experiments
on thermal decomposition under static (in an electrically
heated furnace) and dynamic (in a hindered-settling
reactor) conditions.
Decomposition of magnesium silicide under
dynamic conditions. The hindered-falling reactor is
a vertically mounted cylinder with a diameter of 40 mm
and a heated reaction zone of 300 mm. The reactor is
electrically heated to a prescribed temperature. A screw
feeder and a pipe for discharge of gases from the reactor
are mounted in its upper part. The reactor has in its
bottom part a receiving bin for decomposition products
and a pipe for delivery of oxygen into the reactor.
Decomposition of magnesium silicide under static
conditions. A 2-g portion of magnesium silicide was
placed in a ceramic crucible, with its layer uniformly
distributed over the surface so that its thickness was
1
–2 mm. Then the crucible was placed in the furnace
To determine the optimal parameters of the
magnesium silicide decomposition process, we
performed experiments at temperatures of 660 and
720°C. Data on the yield of silicon in relation to the
variable technological parameters (temperature range
electrically heated to a prescribed temperature. After
the run duration ended, the crucible with decomposition
products was removed from the furnace.
The powder containing magnesium silicide
decomposition products (magnesium oxide, silicon,
silicon oxide) was dissolved in hydrochloric acid. The
solution was filtered and the solid residue was dissolved
in hydrofluoric acid to remove the silicon oxide formed.
The remaining precipitate (elementary silicon) was
washed with twice-distilled water, filtered-off, and dried
at 200°C.
6
60–720°C, amount of powder repeatedly passed
through the reactor) were obtained.
After a single passage of the starting powder
through the reactor, the content of silicon in the product
was 34.8% at 660°C and 44.3% at 720°C. After the
powder was passed through the reactor three times,
the contents of silicon in the product were 73.9% and
8
9.6%, respectively. The content of silicon dioxide in
The thus obtained data on the yield of silicon are
listed in Table 2 in relation to the variable technological
parameters (temperature range 600–720°C and run
duration 10–30 min).
the product obtained at 72°C exceeded that or 600°C,
which is due to the intensification of the process of
silicon oxidation to silicon dioxide.
The thermal decomposition products were subjected
to an X-ray phase analysis. It was found that the
products are mostly composed of silicon and magnesium
oxide. The decomposition reaction occurs at 720°C
simultaneously with the oxidation of silicon, which is
indicated by the high content of silicon dioxide in the
samples under study.
At a temperature of 600°C, the highest yield of
silicon relative to the theoretical value was 72.61% after
3
0 min of heating in air. During the following 1.5 h,
the oxidation of magnesium silicide did not stop, with
the maximum yield of silicon at this temperature being
9
8%. At 720°C, the maximum yield (97.87%) was
reached during the first 10 min of a run, with the content
of elementary silicon decreasing during the following
Magnesiothermic reduction of silicon dioxide
(Fig. 4) yields a three-component system composed
of magnesium oxide, magnesium silicide, and silicon.
Magnesium silicide is removed via the oxidation stage,
in whose process silicon and magnesium oxide (both
solid) are formed. Silicon and magnesium oxide can be
separated by acid leaching. Magnesium chloride can be
used to recover metallic magnesium [6].
2
0 min due to the intensification of the process of its
Table 2. Yield of silicon in decomposition of magnesium
silicide in air under static conditions
Yield of Si relative to the theoretical value, %,
Run
at indicated temperature, °C
duration,
min
6
00
660
720
1
2
3
0
0
0
60.35
66.98
72.61
83.12
92.20
98.10
94.87
93.21
91.67
CONCLUSIONS
(
1) The oxidation of magnesium silicide by
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PRODUCTION OF SILICON FROM MAGNESIUM SILICIDE
497
with atmospheric oxygen) under static or dynamic
conditions. The optimal temperature for the process
in which silicon is obtained from magnesium silicide
under static and dynamic conditions is 600°C.
Mg
Reduction
Mg, Si, Mg Si
SiO2
2
(4) A process scheme is suggested for production of
silicon via the stage of magnesium silicide oxidation.
This scheme will make it possible to raise the yield of
the finished product and to preclude loss of pure silicon
in the form of silanes in the acid leaching stage.
↓
Thermal decomposition
O →
2
of Mg Si
2
MgO, Si
↓
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HCl →
Acid leaching
1
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solution
Si
MgCl2
↓
MgCl2
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Fig. 4. Scheme of the process for magnesiothermic production
of silicon from silicon dioxide, including the stage of thermal
decomposition of magnesium silicide.
3
. Grinberg, E.E., Naumov, A.V., and Suponenko, A.N., Vestn.
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atmospheric oxygen starts at 510°C to give magnesium
oxide and silicon, with silicon oxidized to silicon dioxide
at a temperature of 710°C.
4
5
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(
2) The activation energy of the process is
8.25 kJ mol in the temperature range 520–600°C.
3) Magnesium silicide can be decomposed (oxidized
6
. Lebedev, O.A., Proizvodstvo magniya elektrolizom
–1
5
(Manufacture of Magnesium by Electrolysis), Moscow:
(
Metallurgiya, 1988.
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