Bauxite-Based Synthetic
Refractory Raw Materials
Sophisticated Chinese-bauxite-based synthetic raw
materials classified as homogenized, property optimized
and converted are being developed.
Xiangchong Zhong
High-Temperature Ceramics Institute, Zhengzhou University, People’s Republic of China
C
hina has rich resources of refractory raw materials: bauxite, magnesite and
graphite are the three mainstays of universal renown. The reserves of refractory-grade
bauxite have been reported to be greater than one billion tons. The main deposits are
in Shanxi, Henan and Guizhou provinces. These deposits are different from Latin
American bauxites, because they are composed of alumina monohydrate—mostly
diaspore and some boehmite, associated with kaolinite or pyrophyllite. For the past
two decades and longer, the development of bauxite in China has essentially met the
fast-growing demand of high-temperature industries domestically. Moreover, their
export tonnage has considerably increased to 1.5 million tons per year.1
One of the main trends of technical development in China’s refractories industry
should be the development of a new generation of sophisticated synthetic refractory
raw materials with Chinese characteristics based on the country’s rich resources of
bauxite.2 The bauxite-based materials should partially or completely replace expensive
synthetic raw materials based on pure oxides and non-oxides, such as corundum,
ZrO2–mullite, spinel and SiAlON.
The bauxite-based materials would be used in manufacturing high-performance
refractory products for applications at crucial positions of high-temperature furnaces
and appliances. Their use would lead to significant increases in economic effectiveness.
Therefore, in this new century, China should strategically promote faster development
of bauxite-based synthetic raw materials, which include homogenized, property optimized and converted materials.1
Homogenized-Type Materials
In the homogenization process, the bauxite ores are finely ground. The mixes then are
adjusted to controlled composition, thoroughly blended and pelletized or briquetted.
The pellets or briquettes are subsequently fired at adequately high temperature to
obtain well-sintered materials with homogeneous and consistent composition, structure and properties. In some cases, to decrease impurity content, an appropriate
amount of coal gangue with low Fe2O3 and TiO2 content is incorporated. Four grades of
homogenized bauxite grogs with specific chemical composition, phase composition
and physical properties have been developed that contain 60, 70, 80 and 90% Al2O3
(Table 1).3 The chemical composition and physical properties of these four grades have
reached international standard levels.
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The higher-Al2O3 varieties (HBG80 and HBG90) are
corundum and corundum–mullite materials that have
been sintered at ~1500°C. Scanning electron
microscopy (SEM) of HBG90 shows that its skeleton
structure is composed of well-developed granular
corundum crystals (Fig. 1(a)). The higher-Al2O3 varieties exhibit good corrosion and erosion resistance
against slag or alkali at high temperature and can be
used in manufacturing arc furnace roof brick and
Al2O3–SiC brick and castable.
The lower-Al2O3 varieties (HBG60 and HBG70) belong
to the mullite series of high-alumina materials that are
sintered at 1650–1750°C. If reactive oxide is added as a
sintering aid, the sintering temperature can be lowered. SEM of HGB70 shows a continuous interlocking
network structure of well-developed prismatic mullite
crystals (Fig. 1(b)), which contribute to high hot
strength and improved thermal shock resistance. The
Rul temperature of HBG60 and HBG70 is 1530–1600°C,
which is higher than that of HBG90 (1445°C). HBG60
and HBG70 can be used in manufacturing low-creep
high-alumina brick and mullite-based brick and
castable for applications where hot strength and
spalling resistance are primary requirements.
(a)
(b)
Figure 1 SEM photographs of (a) HBG90 and
(b) HBG70.
Property-Optimized-Type Materials
Two technological routes have been undertaken to optimize high-temperature properties:
• Decrease of impurities by beneficiation and/or electrofusion (minus method); and
• Addition of an appropriate amount of beneficial oxide additives (plus method).
Since the late 1980s, bauxite-based electrofused corundum prepared using the minus
method and sintered spinel prepared using the plus method have been developed. The former has been used to manufacture Al2O3–SiC–C castables for blast furnace troughs with service performance of >150,000 tons molten iron throughput per campaign. The latter has been
used to prepare Al2O3–spinel–C brick and Al2O3–spinel castable for ladle linings with service
lives of >100 heats.
During the past three years, our laboratories and industry have collaborated to develop
bauxite-based, fused corundum–mullite–ZrO2 and corundum–spinel–ZrO2 materials using
bauxite, magnesite and zircon as starting materials. The technology initiated is summarized as
“two stages of reduction smelting and one stage of oxidation refining.”
In the first stage of reduction smelting, SiO2 and other impurities in the bauxite melt are
mostly removed at high temperature by addition of carbon and mill scale. In the second stage
of reduction smelting, SiO2 in zircon added to the melt at higher temperature is almost completely eliminated, also by appropriate addition of carbon and mill scale.
In the oxidation refining stage, the melt is refined to eliminate residual carbon and carbide to
ensure completion of reactions involved and to promote homogeneity of the melt. In case of
corundum–spinel–zircon, light-calcined magnesite is added at the oxidation refining stage for
spinel formation. In case of mullite–ZrO2, there is no need for the second stage of reduction smelting.The chemical composition and physical properties of bauxite-based, fused corundum–ZrO2,
mullite–ZrO2 and corundum–spinel–ZrO24 (Table 2) show that their purity and density are at the
same levels as their counterparts prepared from pure oxides.
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SEM of bauxite-based, fused corundum–ZrO24
shows that columnar crystals of ZrO2 are interlaced
into the interstices of the skeleton structure of welldeveloped granular corundum crystals (Fig. 2).
Figure 2 SEM photograph of bauxite-based
corundum–ZrO2.
Bauxite-based, fused corundum–ZrO2 and mullite–ZrO2 have been used in manufacturing
Al2O3–ZrO2–C slide plates with improved hot
strength that have been used in ladles and tundishes with satisfactory performance. Bauxite-based
corundum–spinel–ZrO2 can be used to prepare
Al2O3–MgO–ZrO2 brick and castable with improved
corrosion resistance for applications in ladle and
secondary refining vessels.
Converted-Type Materials
Converted-type materials include bauxite-based SiAlON and AlON and their composites with
Al2O3. They are prepared from bauxite using a reduction–nitridation method at appropriately
high temperature (1450–1550°C), which converts Al2O3 and SiO2 in the bauxite to SiAlON and
AlON. At the same time, most of the impurities can be reduced or converted to beneficial
nitride (e.g., TiN).
The process of synthesizing SiAlON based on bauxite using silicon, aluminum and carbon as
reducing–nitriding agents and the effects of TiO2 and Fe2O3 on reduction–nitridation of bauxite have been studied. SiO2 in the bauxite is reduced to SiO or elemental silicon, which react
with N2 to form Si3N4 or Si2N2O. At higher temperatures, the SiO or elemental silicon react with
Al2O3 to form SiAlON.5–7 TiO2 and Fe2O3 are beneficial in promoting SiAlON formation at high
temperature. Excess TiO2 can be converted
to TiN, which is highly refractory with good
(a)
abrasive resistance.8
The chemical composition and physical
properties of bauxite-based SiAlON (β and
O') and AlON developed in our laboratories
(Table 3) show that the N2 contents of
SiAlON and AlON thus prepared are relatively high (20–25% for SiAlON and 6–10%
for AlON).9,10
XRD patterns show that the amount of
SiAlON and AlON formed is estimated to be
>90% with a minor amount of corundum.
SEM photographs (Figs. 3(a) and (b)) show
that SiAlON and AlON crystals are well
developed.
These results indicate that bauxite can be
effectively converted by reduction–nitridation at high temperature to SiAlON and
AlON of relatively high purity, which can be
used to prepare high-performance oxide
and non-oxide composite refractories.
Progress in this direction has been made
with promising prospects. The incorporation of an appropriate amount of β-SiAlON
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(b)
Figure 3 SEM photographs of bauxite-based (a) SiAlON and
(b) AlON.
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into carbon-bonded brick and alumina-based castables leads to appreciable improvement in
thermal shock resistance and, in some cases, increase in hot MOR and slag resistance.
The chemical composition and physical properties of low-carbon Al2O3–SiAlON slide plate, in
which bauxite-based β-SiAlON is added in substitution for carbon and Al2O3 in the formulation
(Table 4), has been compared with conventional Al2O3–C slide plate. The comparison shows
that carbon content is decreased by 3%, density and strength are almost identical, and residual strength ratio after thermal shock is increased from 30–40% to 45–55%. This type of lowcarbon Al2O3–SiAlON composite slide plate has been trial-used with satisfactory performance
comparable with conventional materials and has good prospects in applications for continuous casting of clean steel.11
Promising Future
The above-mentioned bauxite-based synthetic raw materials classified into homogenized,
property optimized and converted types are in the development stage. They are innovations
with Chinese characteristics that aim at better comprehensive utilization of Chinese bauxite
resources and at upgrading of Chinese bauxite product quality. The composition, microstructure and properties of these raw materials are at the same level as corresponding international
materials and similar synthetics based on pure oxide. However, their production cost is considerably less, which means significant economic effectiveness. Therefore, Chinese industry looks
forward optimistically to their future development. ■
References
1X. Zhong,“Outlook on the Development of Synthetic Refractory Raw Materials Based on Natural
Resources in China,” China’s Refractories, 1, 3–7 (2000).
2X. Zhong,“Thoughts
on Strategic Development of China’s Refractories Industry”; pp. 1–7 in Proceedings of
4th International Symposium on Refractories (Dalian, China, 2003); China’s Refractories, 2, 3–8 (2003).
3Z. Yang, F. Ye and X. Zhong,“Study on Synthesizing Bauxite-Based Mullite by Sintering Method”; private
communication, 2004.
4T. Ge, Y. Liang
and X. Zhong,“Properties and Microstructure of Bauxite-Based Fused Corundum–Zirconia
and Mullite–Zirconia Materials,” Refractories (Chin.), (2005).
5X. Hou and X Zhong,“Process of Synthesizing Bauxite-Based β-SiAlON by Aluminum Thermal
Reduction–Nitridation,” Refractories (Chin.), 4, 230–33, 237.
6X. Hou and X. Zhong “Process of Synthesizing Bauxite-Based β-SiAlON by Aluminum Thermal
Reduction–Nitridation,” Refractories (Chin.), (2005).
7H. Sun, Z. Liu and X. Zhong,“Research on Phase Transitions in Synthesizing β–SiAlON from Low-Grade
Bauxite,“ Refractories (Chin.), 38 [5] 305–11, 323 (2004).
8S. Li, H. Sun
and X. Zhong,“Effect of TiO2 Addition on Reduction–Nitridation Process of Coal Gangue,”
Refractories (Chin.), (2005).
9H. Zhang and X. Zhong,“Preparation and Microstructure of Bauxite-Based SiAlON by
Reduction–Nitridation”; pp. 437–40 in UNITECR ‘03 Proceedings, Osaka, Japan, 2003.
10H. Zhang, Z. Liu
and X. Zhong,“Thermodynamic Study and Reduction–Nitridation Synthesis of O'-SiAlON
from Coal Gangue,” J. Inorg. Mater., 5, 1129–37 (2004).
11X. Zhong,“Innovative
Development of High-Performance New Refractory Materials,” Refractories (Chin.),
1, 1–5 (2005).
©The American Ceramic Society
American Ceramic Society Bulletin
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December 2005
9104