Refractories, fibers, aluminosilicate

Functions

Refractories perform four basic functions:
(i) they act as a thermal barrier between a hot medium (e.g., flue gases, liquid metal, molten slags, and molten salts) and the wall of the containing vessel;
(ii) they insure a strong physical protection, preventing the erosion of walls by the circulating hot medium;
(iii) they represent a chemical protective barrier against corrosion;
(iv) they act as thermal insulation, insuring heat retention.

Description

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.

Chemical Properties

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).

Uses

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).

Production Methods

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.

Definition

Amorphous man-made fibers produced from the melting and blowing or spinning of calcined kaolin clay or a combination of alumina (Al2O3) and silica (SiO2). Oxides such as zirconia, ferric oxide, titanium oxide, magnesium oxide, calcium oxide and alkalies may also be added. Approximate percentages (by weight) of components follow:alumina, 20-80%; silica, 20-80%; and other oxides in lesser amounts.

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.

Toxicity evaluation

Aluminosilicate fibers are white fibrous solids, soluble to a degree in human lung fluid (see below). The usual physicochemical parameters relevant to fate and transport (e.g., solubility, vapor pressure, octanol–water partition coefficient, and Henry’s law constant) are not applicable or relevant; vapor pressure, octanol–water partition coefficient, and Henry’s law constant are exceedingly low and not measurable. Fibers are capable of being transported in the air and are removed by gravitational settling. The Member State Support Document submitted to the European Chemicals Agency in favor of listing aluminosilicate fibers as a substance of very high concern (SVHC) notes that environmental fate and hazard data were not relevant.