Journal of Physics and Chemistry of Solids 91 (2016) 152–157
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Journal of Physics and Chemistry of Solids
journal homepage: www.elsevier.com/locate/jpcs
Solid state reduction of chromium (VI) pollution for Al2O3–Cr metal
ceramics application
Hekai Zhu, Minghao Fang n, Zhaohui Huang n, Yangai Liu, Hao Tang, Xin Min, Xiaowen Wu
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials
Science and Technology, China University of Geosciences, Beijing 100083, PR China
art ic l e i nf o
a b s t r a c t
Article history:
Received 25 July 2015
Received in revised form
22 September 2015
Accepted 7 January 2016
Available online 8 January 2016
Reduction of chromium (VI) from Na2CrO4 through aluminothermic reaction and fabrication of metalceramic materials from the reduction products have been investigated in this study. Na2CrO4 could be
successfully reduced into micrometer-sized Cr particles in a flowing Ar atmosphere in presence of Al
powder. The conversion ratio of Na2CrO4 to metallic Cr attained 96.16% efficiency. Al2O3–Cr metalceramic with different Cr content (5 wt%, 10 wt%, 15 wt%, 20 wt%) were further prepared from the reduction product Al2O3–Cr composite powder, and aluminum oxide nanopowder via pressure-less sintering. The phase composition, microstructure and mechanical properties of metal-ceramic composites
were characterized to ensure the potential of the Al2O3–Cr composite powder to form ceramic materials.
The highest relative density and bending strength can reach 93.4% and 205 MP, respectively. The results
indicated that aluminothermic reduction of chromium (VI) for metal-ceramics application is a potential
approach to remove chromium (VI) pollutant from the environment.
& 2016 Elsevier Ltd. All rights reserved.
Keywords:
Chromium (VI)
Al2O3–Cr composites
Ceramic material
1. Introduction
The dual role of chromium as a beneficial and harmful metal
can be recognized in its wide application in electroplating, ore
refining, pigmentation and other numerous industrial processes,
whereas the metal’s harmful effects become prominent when industrial wastes containing chromium (VI) are treated improperly,
thereby posing a serious threat to the nature and eventually
causing harmful effects on human health [1]. One way to minimize
chromium (VI) pollution is reduction of chromium (VI) to chromium (III), because chromium (III) had been estimated to be much
less toxic than chromium (VI) and has limited hydroxide solubility
[2,3]. A large volume of prior research has been directed toward
the reduction of chromium (VI) to chromium (III) in aqueous
medium via chemical, electrochemical and biological treatment
methods, such as sulfur compounds reducing agents, iron electrodes electrocoagulation process and chromium (VI) reducing
bacteria [4]. Besides chromium (VI) reduction, numerous studies
have also been conducted on techniques for the removal of chromium (VI), such as using PVA-PEI magnetic microspheres [5],
humic acid coated on magnetite [6], NH2-mediated indium metal–
organic framework [7], and adsorbent of cross-linked polymer
n
Corresponding authors.
E-mail addresses: [email protected] (M. Fang),
[email protected] (Z. Huang).
http://dx.doi.org/10.1016/j.jpcs.2016.01.008
0022-3697/& 2016 Elsevier Ltd. All rights reserved.
(PDVB-IL) [8].
However, neither of these methods, viz. chromium (VI) reduction or chromium (VI) removal is completely safe and friendly to
the environment or biological systems. Additionally, various materials used to remove chromium (VI) incur extra cost and the
removal process rarely transforms chromium (VI) into some useful
end-product (say Cr0 metal). Chromium (VI) can exist in various
forms, such as CrO24−, HCrO4 or Cr2O72 − depending on both pH of
the medium and total chromium (VI) concentration, and above pH
7 only CrO24− ions exist in solution throughout the concentration
range [4,9,11]. Besides, Na2CrO4 is widely used as the simulated
pollutant of chromium (VI) in aqueous solutions, because it is the
common intermediate or end product of the ion exchange process
for removal and recovery of chromium (VI) form wastewater
[9,10], and reaction for sodium chromate production process from
chromite ore processing residue [11,12]. As a metal particulate
reinforced phase, metal Cr particles have been incorporated into
many ceramic matrices to improve the mechanical properties,
including Al2O3 [13], β-Sialon [14] and other ceramic composites. Ji
reported that incorporation of nanometer-sized metal Cr particles
into Al2O3 ceramic matrices had the beneficial effects in improving
the strength of the metal-ceramic composites [15]. Even if oxidation of Cr occurs during sintering process, Cr2O3 and Al2O3 are
completely miscible within the same crystal structures, which may
be helpful for achieving a good bonding between the metal and
oxide phases [16]. Moreover, Al2O3–Cr composite material have
H. Zhu et al. / Journal of Physics and Chemistry of Solids 91 (2016) 152–157
some practical application in industry such as interlayer of Al2O3–
Cr functionally graded material, which is used for reduction of
thermal stresses in alumina–heat resisting steel joints [17]. Thus,
reducing chromium (VI) to metal Cr powder and applying the
obtained Cr powder to metal-ceramic composites can provide a
new advantageous method for treating solid state chromium (VI)
pollution. Analyzed from the commercial cost, the industrial price
of Cr powder ( 80 μm) is about five times higher than the sum
price of Al powder and Na2CrO4 powder ( data from MOLBASE
company, Shanghai). Moreover, Al2O3–Cr composite powder can
be obtained through aluminothermic reaction as major raw material for Cr reinforced metal-ceramic materials, which reduces the
cost compared with directly using Al2O3 and Cr power as the raw
material. Therefore, the method of reducing Na2CrO4 to metal Cr
particles and obtaining Al2O3–Cr composite powder by aluminothermic reaction possesses certain economic value.
The main objective of the present study was to reduce the toxic
chromium (VI) from Na2CrO4 by aluminothermic reaction. Thermal
analysis of the reduction products, along with their phase composition and microstructural studies were done to obtain pure and
micrometer-sized metal Cr powder. The conversion ratio of
Na2CrO4 to metal Cr via reduction was studied by X-ray Fluorescence (XRF) spectrometer. The obtained Cr containing composite
powder was then used to prepare the metal-ceramic followed by
characterization of its microstructure and mechanical properties.
2. Experimental and methods
2.1. Aluminothermic reduction of Na2CrO4
Na2CrO4 was chosen as the simulated contaminant of chromium (VI). Na2CrO4 (AR grade, 45 μm) and Al powder (AR grade,
61 μm) was supplied by Beijing Chemical Co, China as the starting
material. The starting materials were mixed in their solid state
with 40% excess Al than Na2CrO4 by weight to compensate for any
loss of the former during sintering process. Afterward, the mixture
was subjected to ball milling in ethanol for 6 h, followed by drying
at room temperature. The dried samples were placed in a tube
furnace and treated under an identical sintering procedure in a
flowing Ar atmosphere (heating rate of ∼5 °C/min, holding at 900–
1300 °C for 3 h, respectively, cooling rate of ∼10 °C/min). The obtained reduction products were then mixed with equal volume
water, stirred for 2 h to remove any soluble Na2O or Na2CrO4, and
finally filtered to obtain the final product mixture.
2.2. The reduction products for preparation of Al2O3–Cr ceramic
The reduction product Al2O3–Cr composite powder was further
used as a source of Cr to prepare Al2O3–Cr ceramic composites.
Nanometer sized Al2O3 powder (99.9999%, 100–300 nm) was
purchased from Zibo Nuoda Chemistry Co, Shandong. The Al2O3–
Cr composite powder prepared at 1000 °C was mixed with a certain amount of nanometer Al2O3 powder to obtain Al2O3–Cr
composite powder with different Cr contents (5 wt%, 10 wt%,
15 wt%, 20 wt%). The mixture samples were ground to
3 mm 4 mm 40 mm green bars by die-pressing with the
maximum pressure of 40 MP. Subsequently, they were further
compacted by cold isostatic pressing at 200MP for 1 min. The
Al2O3–Cr ceramic bars were then sintered at 1600 °C for 4 h in a
flowing Ar atmosphere to prevent any oxidation of the Cr. Before
testing the mechanical properties, the sintered ceramic bars were
polished to a 3 mm diamond surface finish on the tensile surface
to remove any machining damage.
153
2.3. Characterization
The phase compositions were recorded by X-ray diffraction
(XRD, D8 Advance diffractometer, Bruker Corporation, Germany)
with Cu Kα (λ ¼1.5406 Å) radiation. Elemental compositions of the
starting materials and the reduction products were evaluated by
X-ray Fluorescence (XRF) spectrometer (ZSX Primus IIWD, Rigaku
Corporation, Japan). The principle of method used in XRF equipment could be explained as following: after excitation by the
primary X-ray from the XRF equipment, different elements in the
samples would irradiate X-rays fluorescence that have specific
energy characteristics or wavelength characteristic. Next, the energy and amount of these specific X-rays fluorescence would be
measured and collected by detector system of the XRF equipment
and then converted into the type and quantitative of various elements in the samples. The microstructures of the composite
powder and ceramic composites were observed under scanning
electron microscope (SEM, JEOL JSM-IT300) along with energy
dispersive spectra analysis (EDS). The bulk density of the prepared
composites was measured by Archimedes’ method. The flexure
strength was evaluated using 3-point bend testing, which was
conducted on a universal testing machine with a cross-head speed
of 0.5 mm/min and a span of 20 mm.
2.4. Method for determination of reaction conversion.
According to the quantitative result of various elements in the
samples measured by X-ray Fluorescence (XRF) spectrometer, the
amount of chromium (VI) in Na2CrO4 and Cr0 in the Al2O3–Cr
could be calculated by the quantitative of Cr element together
with the mass of the samples, respectively. Finally, the conversion
ratio of Na2CrO4 to Cr could be determined by the proportion of
the amount of Cr0 to chromium (VI).
3. Results and discussions
3.1. Thermodynamic analyses of the reduction reaction
The reduction reaction between Al and Na2CrO4 can be represented by equation (1). At temperatures above the melting
point of Al, 660.5 °C and Na2CrO4, 792 °C, the powder materials
would be involved in thermite reaction in molten state and the
reaction product Na2O would decompose and volatilize at such
temperatures.
Na2CrO4(l)þ2Al(l) ¼Al2O3(s) þCr(s) þNa2O(g)↑
(1)
ΔG0298 ¼ 736.76 kJ/mole, ΔH0298 ¼ 764.41 kJ/mole
Eq. (1) exhibits negative ΔG0298 and ΔH0298 values through
thermodynamic calculation [18]. Thus, this reaction is thermodynamically favorable at the temperature above the melting point
of the starting materials.
3.2. Phase composition of the reduction products of Na2CrO4
Prior studies report that Al2O3 ceramic reinforced by metal Cr
particle can be prepared according to Eq. (2), where the thermite
reaction between Al and Cr2O3 was highly exothermic and exhibited an enthalpy of 541.0 kJ per mole of Al2O3 generated from
aluminothermic reduction of Cr2O3 [19]. Besides, the change in the
Gibbs free energy for Eq. (2) was also calculated. It is noteworthy
that any excess Al was capable of reacting with the reduced Cr to
produce different Al–Cr intermetallic compounds (Cr2Al, Cr5Al8,
Cr4Al9, CrAl4, Cr2Al11, and CrAl7) [20].
H. Zhu et al. / Journal of Physics and Chemistry of Solids 91 (2016) 152–157
Fig. 1. XRD patterns of the reduction products of excess 40% Al powder and
Na2CrO4 reacting at different temperatures from 900 °C to 1300 °C.
Cr2O3 þ 2Al-Al2O3 þ 2Cr
ΔG ¼ 469.7 KJ
Fig. 2. XRD patterns of reduction products Al2O3–Cr reacting at different temperatures from 1000 °C to 1300 °C.
(2)
The XRD patterns of the reduction products with excess 40% Al
powder and Na2CrO4 reacting at different temperatures from
900 °C to 1300 °C are shown in Fig. 1. As can be seen in Fig. 1, after
stirring and filtering, the reduction products obtained at 900 °C
primarily contained the starting materials and almost no new
phase appeared, indicating that Na2CrO4 did not react with Al
powder at 900 °C. When the reacting temperature was increased
to 1000 °C, the main phase of the products included Al2O3, Cr and
some incompletely reacting Al, representing that aluminothermic
reaction between Al and Na2CrO4 occurred. With further increase
in the reaction temperature to 1100 °C and 1200 °C, no Al and
Na2CrO4 phases could be identified and the reduction products
transformed into Al2O3 and Cr2Al intermetallic compounds. Cr2Al
intermetallic can degrade the mechanical properties of the ceramic and should be prevented from formation during high temperature sintering. With further increase in the reacting temperature to 1300 °C, the diffraction peaks of Al2O3 and Cr was
detected, indicating that Na2CrO4 was reduced completely to form
Al2O3, Cr according to the Eq. (1).
Based on the above results, Al2O3–Cr composite powder was
successfully prepared at different temperatures from 1000 °C to
1300 °C with Na2CrO4 and corresponding amount Al powder, and
the XRD patterns of the samples are shown in Fig. 2. Fig. 2 also
shows that the diffraction peaks of Al2O3 and Cr mostly did not
undergo any change at different reaction temperatures.
3.3. Conversion ratio of Na2CrO4 to Cr
Fig. 3 shows the conversion ratio of Na2CrO4 to Cr at different
temperatures. Fig. 3 shows that the conversion ratio can reach
above 90.00% through the aluminothermic reaction, and the
maximum conversion amount could be 96.16% reacting at 1000 °C.
The result indicated that chromium (VI) in Na2CrO4 could be reduced to Cr0 in an appreciable amount, ignoring any loss of Cr
during the reaction process.
100
Conversion ratio (%)
154
98
96
94
92
90
1000
1100
1200
1300
Fig. 3. Conversion ratios of Na2CrO4 to metallic Cr reacting at different
temperatures.
Fig. 4. These two phases can easily be distinguished according to
the gray levels, morphology and test results of EDS. In Fig. 4a, the
metallic Cr particle appeared to be brighter than Al2O3, and exhibited a spherical shape. As shown in Fig. 4, the size of the Cr
metal particles was estimated to be 500 nm to 5 μm, which increased with the increase in the reaction temperature from
1000 °C to 1300 °C.
The Al2O3–Cr composite powder obtained from the reduction
of Na2CrO4 contained micrometer-sized Cr particles, which can
serve as an effective source of metal Cr to have potential applications in the syntheses of ceramic composites, coatings and catalysts materials containing Cr, Al2O3 [15,21,22]. To ensure the potential of the Al2O3–Cr composite powder to form ceramic materials, the properties of the latter were further studied
systematically.
3.4. Microstructure of the reduction products of Na2CrO4
3.5. Phase compositions and microstructure of ceramic materials
Back-scattered electron (BSE) micrographs of the reduction
products Al2O3–Cr obtained at different temperatures and the EDS
pattern of the Al2O3, Cr particles prepared at 1000 °C are shown in
The XRD patterns of the Al2O3–Cr ceramic composite samples
with different Cr contents prepared at 1600 °C are presented in
H. Zhu et al. / Journal of Physics and Chemistry of Solids 91 (2016) 152–157
155
Fig. 4. SEM micrographs of reduction products Al2O3–Cr reacting at different temperatures: (a) 1000 °C, (b) 1100 °C, (c) 1200 °C (d) 1300 °C and EDS pattern of the Al2O3, Cr
particles prepared at 1000 °C.
Fig. 5. The results showed that compared to the reduced composite
powder, α-Al2O3 and Cr were present as the main phase of the
ceramic materials and the sintering process did not bring any
major structural changes. Besides, with increase in the Cr content,
the intensity of Cr peaks were also increased.
Fig. 6 shows the BSE micrographs of the polished surface of
Al2O3–Cr ceramic composites prepared at 1600 °C with different Cr
contents ((a) 5 wt%, (b) 10 wt%, (c) 15 wt%, (d) 20 wt%). As it can be
seen, the polished surface comprised Al2O3 as the matrix phase
and Cr particle as the second phase. It could be observed that Cr
particles had a bimodal size distribution, where the larger Cr
particles ( 10 μm) coexisted with the smaller nanometer-sized
particles. Additionally, no large pores, which might degrade the
mechanical properties of the ceramic samples, were found to develop on the fractured surfaces.
3.6. Mechanical properties of ceramic materials
Assuming the density of aluminum oxide
ρ1 ¼ 3.97 g/cm3 and
chromium ρ2 ¼7.19 g/cm3, the theoretical densities of the metalceramic materials were calculated, and the values obtained were
4.057 g/cm3, 4.138 g/cm3, 4.216 g/cm3 and 4.290 g/cm3 for the
ceramic composition Al2O3–Cr with different Cr contents 5 wt%,
10 wt%, 15 wt%, 20 wt% respectively. Fig. 7 shows the densification
behavior of Al2O3–Cr ceramic composites as a function of the mass
content of metallic Cr particles. The metal-ceramic samples with
mass content of 15% Cr had the highest relative density of 93.4%
among all the samples. The relative low density of the ceramic
samples was primarily attributed to (i) an uneven chromium
particle size distribution after fabrication at 1600 °C, and (ii) the
nanometer-sized Al2O3 particles have high surface energy and
chemical activity, which led them to aggregate easily. The relative
density of the Al2O3–Cr ceramic samples initially increased with
the increase of Cr content, which is attributed to the diffusion of Cr
particle along the Al2O3 grain boundaries that promote removing
the pores during sintering process. The relative density reached
maximum values with content of 15 wt% Cr and then gradually
decreased. This sintering behavior is closely related to the
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H. Zhu et al. / Journal of Physics and Chemistry of Solids 91 (2016) 152–157
Relative Density (%)
93.5
93.0
92.5
92.0
91.5
91.0
90.5
90.0
5
10
15
20
Content of Cr metal ( wt. %)
Fig. 5. XRD patterns of Al2O3–Cr ceramic composites prepared at 1600 °C with
different Cr content: (a) 5 wt% (b) 10 wt%; (c) 15 wt%; (d) 20 wt%.
presence of fine Cr particles and the formation of large agglomeration of Cr particle dispersion [23–25]. At higher Cr content, the
agglomeration of Cr particles became more difficult to break,
which is detrimental to the densification of Al2O3–Cr ceramic
composites.
It is well known that there are two main factors in principle
responsible for improving the bending strength of ceramics material according to the Griffith theory. One is to reduce the size of
preexisting defects or micro-cracks in the ceramic material, and
the other is to increase the energy barrier for crack propagation
Fig. 7. The relative densities of Al2O3–Cr ceramic composite as a function of the
mass fraction of metal Cr.
[26]. Therefore, one of the most effective methods to improve the
bending strength of ceramic composites materials is decreasing
the grain size.
In addition to the above two factors, the mechanical properties
of Al2O3–Cr ceramic composites can be controlled by many critical
factors, such as the size of Al2O3 and Cr particles, composition of
powder mixtures, including the mass fraction of Cr and the
amount of Cr2O3 as the oxidation product of obtained Cr particles.
The mechanical properties of Al2O3–Cr ceramic composites with
different Cr contents are shown in Fig. 8. The bending strength of
Al2O3–Cr ceramic composite with different Cr contents are 185 MP
Fig. 6. SEM micrograph of the polished surface of the prepared Al2O3–Cr ceramic composites with different Cr content: (a) 5 wt% (b) 10 wt%; (c) 15 wt%; (d) 20 wt%.
Bending strength (MPa)
H. Zhu et al. / Journal of Physics and Chemistry of Solids 91 (2016) 152–157
157
reduction of chromium (VI) for metal-ceramic application is a
potential way to treat solid state products coming from the removal and recovery of chromium (VI) from wastewater or chromite ore processing residue.
205
200
Acknowledgments
195
This work was financially supported by the National Natural
Science Foundation of China (NSFC Grant no. 51172216) and the
Fundamental Research Funds for the Central Universities (Grant
no. 2652013051).
190
185
References
5
10
15
20
Content of Cr metal ( wt %)
Fig. 8. Bending strength of the prepared Al2O3–Cr ceramic composite with different Cr content.
(5 wt%), 193 MP (10 wt%), 205 MP (15 wt%), 197 MP (20 wt%), respectively. As it can be seen, the bending strength initially increased and then declined with the increase in Cr content. The
maximum value of bending strength (205 MP) was obtained when
the mass content of Cr reached 15 wt%.
4. Conclusions
In summary, the reduction of chromium (VI) from Na2CrO4 was
successfully achieved by aluminothermic reaction and the obtained Al2O3–Cr composite powder was applied to prepare Cr reinforced metal-ceramic materials. The results indicated that
Na2CrO4 can be completely reduced to micrometer-sized Cr particles at 1000 °C in a flowing Ar atmosphere. The conversion ratio
of Na2CrO4 to metallic Cr through reduction reached 96.16%, and
the size of chromium particles tend to increase with increase in
the reaction temperature. The reduction product Al2O3–Cr composite powder was further applied to prepare metal-ceramic materials with different Cr content. The microstructure and mechanical properties of the metal-ceramic materials were measured, which showed that the relative density and bending
strength reached the highest values when the Cr content was
15 wt%. The above results showed that the aluminothermic
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