Oxidation of polycrystalline zinc selenide with atmospheric oxygen

Article abstract of DOI:10.1023/A:1013038111447

Oxidation of granulated polycrystalline zinc selenide (grain size 0.2-6.5 mm) with atmospheric oxygen at 430-700°C was studied. The optimal conditions for the oxidation were found. An installation for the oxidation of kilogram amounts of zinc selenide was

Full text of DOI:10.1023/A:1013038111447

Russian Journal of Applied Chemistry, Vol. 74, No. 7, 2001, pp. 1079 1081. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 7,
2001, pp. 1051 1053.
Original Russian Text Copyright
2001 by Khlopochkina, Gaivoronskii, Gavrishchuk, Elliev, Yashina.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Oxidation of Polycrystalline Zinc Selenide
with Atmospheric Oxygen
E. L. Khlopochkina, P. E. Gaivoronskii, E. M. Gavrishchuk,
Yu. E. Elliev, and E. V. Yashina
Lobachevskii State University, Nizhni Novgorod, Russia
Institute of Chemistry of High-Purity Substances, Russian of Academy of Sciences, Nizhni Novgorod, Russia
Received June 16, 2000; in final form, February 2001
Abstract Oxidation of granulated polycrystalline zinc selenide (grain size 0.2 6.5 mm) with atmospheric
oxygen at 430 700 C was studied. The optimal conditions for the oxidation were found. An installation
for the oxidation of kilogram amounts of zinc selenide was designed and fabricated.
Published data on the oxidation of zinc selenide as
single crystals, polished plates, and powders include
estimates of the oxidation rate at 300 950 C, com-
position of the reaction products, and suggested equa-
tions for the oxidation [1 5]. However, there are a
number of contradictions in the determination of com-
position of the nonvolatile oxidation products. For
instance, according to [1 3], zinc oxide is the only
nonvolatile product, but in [4] significant amounts
(up to 18%) of zinc selenite were found in the range
490 560 C along with zinc oxide. According to [5],
zinc selenite is the main product of the oxidation
at 300 C; as the temperature increases, its fraction
appreciably decreases as compared to zinc oxide.
Volatile oxidation products either were not inves-
tigated at all [1, 2, 5] or only their qualitative com-
position as a mixture of selenium and selenium di-
oxide [3, 4] was established. The effect of the grain
size of the initial zinc selenide on the rate of its oxida-
tion is still obscure, and probable oxidation mech-
anisms were not discussed.
conditions were far from equilibrium, the thermo-
dynamic analysis is important for the estimation of
the relative contents of products both in gas and con-
densed phases. The calculation was carried out at
various stoichiometric ratios of the initial reactants.
The calculated equilibrium concentrations of com-
ponents (in moles per total number of moles of atoms
in the initial substances) for the system zinc selenide
air in excess oxidant (molar ratio zinc selenide
oxygen in the experiments was 1 : 4.5) are given in
Fig. 1. Apparently, within the whole temperature
(a)
In this work the study was concerned with waste
polycrystalline zinc selenide of various grain size,
formed from production of optical parts for IR tech-
nique [6]. One of the ways of processing waste prod-
ucts is their oxidizing calcination. In this connection,
the aims of this work were to study the oxidation
kinetics of granulometric zinc selenide samples of
various grain size, to identify the products of the oxi-
dation at 430 700 C, and also to study the mech-
anism of the oxidation with atmospheric oxygen.
(b)
T, K
Fig. 1. Temperature dependence of the equilibrium com-
position of the system zinc selenide oxygen. (C) Equilibri-
um concentration [mol (mol of atoms) ]. Phase: (a) gas
EXPERIMENTAL
1
First we carried out a thermodynamic analysis of
the system zinc selenide air. Though the oxidation
and (b) condensed.
1070-4272/01/7407-1079$25.00 2001 MAIK Nauka/Interperiodica

1080
KHLOPOCHKINA et al.
with sodium thiosulfate, thiourea, and a mixture of
concentrated potassium iodide and hydrochloric
acid [7].
We found that the main nonvolatile product in the
temperature range under study is zinc oxide, with the
total content of zinc selenide, selenite, and selenate
being lower than 5%. The volatile product is a mix-
ture of selenium and its dioxide. To determine the
amounts of individual components, the volatile prod-
ucts were dissolved in distilled water, the solution was
filtered, and the precipitated selenium was dried and
weighed. We found that the proportion of selenium
and its dioxide in volatile products varies in the
course of reaction [8]. This allowed us to offer the
following equation for oxidation of zinc selenide:
, min
Fig. 2. Kinetic curves of zinc selenide oxidation: ( ) con-
version and ( ) time. Temperature ( C): (1) 700, (2) 600,
(3) 550, (4) 500, and (5) 430.
ZnSe + (0.5 + y)O2
ZnO + xSe + ySeO2,
In this equation factors x and y change in the course of
the process, but the sum of them is equal to unity [9].
The variation of the molar ratio selenium : selenium
dioxide in volatile oxidation products requires an in-
dependent study and was not considered in this work.
To estimate the kinetic features, we studied only the
nonvolatile oxidation products. The prevalence of zinc
oxide in these products allowed us to estimate the
degree of conversion of the initial zinc selenide by the
gravimetric analysis. The time dependence of the zinc
selenide conversion at 430 700 C is shown in Fig. 2.
Fig. 3. Variation with relative oxidation time
the zinc selenide conversion . Temperature ( C): (1) 550
and (2) 700.
/
of
range (500 1500 K) the main reaction product in the
gas phase is selenium dioxide. The content of the
other selenium species (Se, Se , Se , etc.) is lower
To reveal the kinetic features of the oxidation, we
examined the variation of the zinc selenide conversion
( ) with the relative oxidation time / ( is current
2
3
by several orders of magnitude. In the condensed
phase selenium dioxide exists only within a narrow
temperature range.
time and
is time of complete conversion). The
dependences for 0.6 1.5-mm particles at 550 and
700 C are shown in Fig. 3. When studying the kinet-
ics of processes in gas solid systems, in particular of
calcination of solid materials, it is common to take the
model of a spherical particle with unreacted nucleus
[10] as a basic model. According to this model, the
limiting stage is characterized by the dependence of
the conversion on the contact time, particle size, and
temperature. The comparison of theoretical curves and
our experimental data (Fig. 3) suggests that the oxida-
tion process under consideration is controlled by
internal diffusion [10].
The installation for oxidation consisted of an in-
clined quartz reactor (30 mm in diameter and 400 mm
in length) and a condenser for volatile oxidation prod-
ucts. When the reactor is heated in a tubular electric
furnace, a steady-state convection air stream arises.
The temperature in the oxidation zone was varied
from 430 to 700 C during experiments. The accuracy
of temperature determination did not exceed 3%. The
granulometric compositions of powders were deter-
mined by sieving. The particle size of zinc selenide
varied from 0.20 to 6.50 mm. Samples of powders
(18.00 0.01 g) were placed in quartz boats (60 25
15 mm). The composition of the nonvolatile products
remaining in the decomposition zone was determined
by X-ray phase analysis. The qualitative composition
of the volatile products condensed in the receiver was
determined by chemical methods based on reactions
The dependence of the time of 90% oxidation of
zinc selenide (i.e., the reciprocal of the oxidation rate)
on the particle size of the initial zinc selenide is
plotted in Fig. 4. Particle sizes correspond to the mean
values within the ranges (mm) 0.2 0.3, 0.3 0.4, 0.4
0.6, 0.6 1.5, 1.5 2.5, 2.5 3.5, 3.5 4.5, and 4.5 5.5.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 7 2001

OXIDATION OF POLYCRYSTALLINE ZINC SELENIDE
1081
an installation for the oxidation of kilogram amounts
(0.6 1.5 kg) of zinc selenide [8].
90, h
CONCLUSION
Zinc oxide was found to be the main nonvolatile
product of zinc selenide oxidation at 430 700 C.
The oxidation is apparently controlled by internal dif-
fusion. The temperature of 600 C and the particle size
of 1.5 4.5 mm are the optimal for the oxidation of
powdered polycrystalline zinc selenide by atmospheric
oxygen.
d, mm
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Fig. 4. Rate of zinc selenide oxidation as a function of its
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It is evident that the reaction rate grows considerably
with decreasing particle size (the right branch of the
curve), which also confirms the assumption on the
internal diffusion control of the process.
At the particle size decreased further (the left
branch of the curve), the oxidation rate decreases,
which seems to be due to the effect of another limit-
ing stage, namely, diffusion of oxygen through a bed
of the solid product [10]. This mechanism was dis-
cussed in the literature as applied to oxidation of FeS
in the industrial production of sulfuric acid [11].
2
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From our experimental data we plotted the tem-
perature dependence of the reaction time in the coor-
dinates log(1/ ) 1/T and determined the apparent ac-
1
tivation energy of 83 kJ mol for the range 430
700 C. It follows from the theory of diffusion kinetics
of gas solid phase interactions that the activation
energy of processes controlled by internal diffusion is
approximately a half of the activation energy of a
chemical reaction [10]. The activation energy for the
oxidation of polycrystalline plates of zinc selenide is
1
known to be 170 kJ mol [2]. In this case the activa-
tion barrier seems to be caused by the proper reaction
between zinc selenide and oxygen, i.e., the reaction is
kinetically controlled. The obtained apparent activa-
tion energy is approximately half of the value given in
[2], which supports the assumption on the internal
diffusion mode of the process.
The kinetic features allowed us to find optimal
conditions for the oxidation of polycrystalline zinc
selenide. This made it possible to design and fabricate
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 7 2001