Refractories, fibers, aluminosilicate Chemische Eigenschaften,Einsatz,Produktion Methoden
Beschreibung
Aluminosilicate fibers (commonly called refractory ceramic
fibers (RCFs) in the United States) are amorphous fibers
belonging to a class of materials termed synthetic vitreous
fibers (SVFs), also termed man-made mineral fibers or manmade
vitreous fibers. This class of materials also includes glass
wool, rock (stone) wool, slag wool, mineral wool, and specialpurpose
glass fibers. Fibers can be classified in various ways,
such as natural versus synthetic, organic versus inorganic, and
crystalline versus amorphous. Several fiber taxonomies have
been proposed.
Aluminosilicate wools (ASWs) were first invented in the
early 1940s and commercialized in the 1950s in the United
States and somewhat later in other countries. Substantial
energy price increases beginning in the 1970s increased the
economic benefits of energy conservation and the market for
these fibers.
ASWs are SVFs produced by melting (at ～1925°C)
alumina, silica, and other inorganic oxides, and then blowing
or spinning these melts into fibers. These fibers can also be
produced by melting blends of calcined kaolin, alumina, and
silica. The bulk fibers produced by this process can be used
directly for some applications, but are more commonly converted
into other physical forms, including blanket, modules
(folded blanket capable of being installed rapidly in industrial
furnaces), paper, felt, board, vacuum formed parts, textiles, and
putties or pastes. Conversion to various physical forms takes place at locations where aluminosilicate fibers are produced,
facilities operated by converters (producers of intermediate
goods) or end users. Primary manufacturing facilities for
aluminosilicate fibers are located in North and South America,
Europe, and Asia. Conversion facilities and end users are
distributed throughout the industrialized world.
Chemische Eigenschaften
Refractory ceramic fibers are aluminum silicates with varying
amounts of metal oxides and other materials, depending
upon the raw materials.
The following is a guideline for the range of components by
percentage SiO2, 47–54%; Al2
O3, 35–51%; CaO, ＜1%;
MgO,＜1%; Na2O,＜1%; K2O,＜1%; Fe2O3, 0–1%; TiO2,
0–2%; ZrO2, 0–17%. Nominal fiber
diameter at manufacture is approximately 2–3 μm (26, 68a).
The chemistry of fiber formulations is constantly evolving
to meet market demands (16d).Irritation may occur due to
fibers on skin or in the eyes (16d).
Verwenden
ASWs have several desirable properties for use as hightemperature
insulating materials, including low thermal
conductivity, low heat storage (low volumetric heat capacity),
thermal shock resistance, lightweight, good corrosion resistance,
and ease of installation. Depending upon the fiber
composition, the maximum end-use temperature for ASWs can
be as high as 1430°C (2600°F). Because of this capability,
these fibers are also included in the class of high-temperature
insulating wools (HTIWs). Benefits of the use of ASW insulation
include reduced energy costs and reduced greenhouse
gas emissions. The energy savings can be substantial when
compared to conventional high-temperature insulation such as
insulating firebrick.
Applications and markets for ASWs are principally industrial
and vary by product form and country including furnace
linings and components in the cement, ceramic, chemical,
fertilizer, forging, foundry, glass, heat treating, nonferrous
metals, petrochemical, power generation (cogeneration), and
steel industries. ASWs are used for passive fire protection
applications where thin, lightweight materials are needed to
prevent flame penetration. ASWs are also used to a minor
degree in emission control applications such as heat shield
insulation, catalytic converter support mat, and filtration media
for air bag inflators. Though sometimes referred to in the
literature as a substitute for asbestos, aluminosilicate fibers are
not typically used in asbestos applications. Aluminosilicate
fibers are priced substantially higher than various types of
asbestos and have maximum end-use temperatures substantially
greater than those for asbestos (which vary depending
upon the product but are typically°850°C).
Vorbereitung Methode
Refractory ceramic fiber is produced from a mixture of sand
and alumina or kaolin. Metal oxides such as titanium and zinc
may be added, depending upon the final desired specifications
(16d, 26). The raw materials are transferred to the
furnace where the batch is melted. As the molten mix flows
from the furnace, it is fiberized as it passes a stream of air and
steam or falls on rotating disks. The fiber diameters are in the
range of 0.5–10 μm, and have lengths up to several centimeters
(69). Some raw fiber may be packaged in bulk for sale
or for use in products containing RCF. Alternatively, raw
fiber enters a chamber where it is sprayed with lubricating oil
and allowed to settle onto a moving conveyor. This material
moves through a needler, where opposing units of close-set
needles are forced through the RCF, interlocking the fibers
and forming a blanket. The blanket is conveyed through an
oven to burn off the lubricating oil, then trimmed to the
required dimensions, rolled, and boxed.
Secondary processes involve the use of bulk fiber or
blankets to produce additional products. These include vacuum-
formed products, folded modules, braids and ropes,
boards, and customer-specified shapes and use in catalytic
converters, metal reinforcements, heat shields, brake
pads, and airbags.
Fibers may undergo transformation from amorphous to
crystalline forms during use in high temperature applications.
Therefore, exposures to these so-called after-use fibers
may include mullite, cristobalite, or other crystalline
phases.
Carcinogenicity
In 2002, IARC (16d) classified
refractory ceramic fibers as “possibly carcinogenic to
humans” (Group 2B), based on inadequate evidence of
carcinogenicity in humans and sufficient evidence in experimental
animals. The American Conference of Governmental
Industrial Hygienists has designated RCF as a suspected
human carcinogen (A3).
Indulski et al. reported the numbers of occupational
diseases diagnosed from 1984 to 1994 among 600 Polish
workers employed in the manufacture of refractory ceramic
fibers. No cases of lung cancer were cited. Lung fibrosis and
silicosis were noted in four workers, ages 52–64 at diagnosis.
The duration of employment ranged from 24 to 37 years
preceding the manufacture of ceramic fibers; therefore, the
authors note that the conditions could be related to earlier
employment.
Environmental Fate
Numerous in vitro and in vivo studies have been conducted on
both natural and synthetic fibers to try to understand and
measure cytotoxicity, mutagenicity, and genotoxicity. Many of
these studies have proven inconclusive, so mechanism(s) of
action are still unclear. Other studies have indicated that
aluminosilicate fibers are less active biologically than various
forms of asbestos.
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