Asbestos, Silica, and Metals


Asbestos in Furnace Issues

Chrysotile asbestos has been used for many years as a thermal insulating material because of its low cost, low density in insulating blankets or as-packed, and its property of absorbing heat during its endothermic decomposition at high temperatures. For example, asbestos was used as insulation on early space capsules effectively absorbing heat on re-entry.

The main crystalline form of asbestos used in the United States is fiberform chrysotile asbestos. This material was used with some refractory bricks as adhered in a “pad” to one side of the brick surface forming a “gap” upon thermal decomposition of the pad – thus providing an expansion “allowance”. These pads were used on refractory bricks for open hearth steel furnace roofs. In many cases, alternative non-asbestos expansion methods were used to include adhered cardboard pads and steel “cases” around the bricks.

It is important to note that refractory bricks that are “fired” or sintered cannot contain chrysotile asbestos in their compositions sine the firing temperatures are well above the chrysotile decomposition temperature.

Because of the high temperatures in most industrial furnaces, the chrysotile fibers substantially decompose as the bricks slowly wear away. Most furnace linings are designed to last at least one year, so there is ample time and sufficient temperature for their total decomposition to take place. Therefore, there was no possibility for worker exposure to fiberform material during furnace demolition and reconstruction.

The decomposition temperature is very important in evaluating any residual risk in demolition of furnace linings. A classic reference gives this decomposition as 680 degrees Celsius or 1256 degrees Fahrenheit (S.S. Chissick, Asbestos: Properties, Applications and Hazards). This temperature is from a single series of laboratory experiments using rapid heating, but some older references indicate the onset of decomposition at lower temperatures. Lower temperature decomposition implies much lower or negligible risk of fiberform materials being released on furnace tear-outs (deconstruction).

Research by Brickexpert.org. confirms the onset of decomposition below 680 degrees Celsius and provides a kinetic model showing the decomposition versus time for the mineral at various temperatures. Electron diffraction studies have conformed the nature of the decomposition product.

Thus, the furnace operating conditions can be analyzed to show that negligible risk of exposure existed at the time of furnace deconstruction.

Transmission electron microscopy (TEM) micrograph of chrysotile fiber relic after thermal decomposition into non-fiberform protoenstatite. There was no electron diffraction pattern for chrysotile.

Silica

Crystalline silica in the form of quartz is a well-known component of many ceramic mixes in processing of traditional ceramic products (building bricks, certain refractory bricks, and porcelain products/toilets). It is a measurable component of most clay due to geologic processes such as the kaolinization of granite – leaving a quartz residue in the kaolin deposit. It is an intentional additive in the form of “Potter’s flint” in whiteware products/toilets serving as a filler and a framework to prevent excessive deformation in firing. It is a nemesis in some respects due to its relationship to cooling cracks in the firing process as different forms of silica have different densities leading to volume changes when changes are experienced. In some cases, residual stress within porcelain ceramics/toilets leads to spontaneous failures sometimes years after the date of manufacture. In fact, toilet breakage when used can be due to residua stress within a defectively manufactured toilet.

The fact that the low temperature form of crystalline silica, quartz,“converts” to higher temperature forms with other crystal structures on heating is well known, and these new forms are known as tridymite and cristobalite.

Exposure to crystalline silica in fired ceramic products is via a primary route of inhalation of dust from dry saw cutting.

The inhalation of “free silica” (quartz, tridymite, or cristobalite) can produce fibrotic lung disease known as silicosis. Free silica is a distinct particle or phase that exists as the mineral or compound SiO2. Ceramic materials and products also contain “combined silica”, i.e. silica in combination with other species (forming different minerals) or as a vitrified (glassy) phase.

Respirable particles (smaller than 10 microns in diameter) can be inhaled into the conducting airways of the lungs and the gas exchange regions of the lung.

In summary, forms of crystalline silica in fired ceramic products are “locked in” such that there is no pathway for effects on humans. Dry masonry saw cutting provides possible exposures of respirable crystalline silica. Non-crystalline silica, i.e. as glass or combined silica in vitreous products, is not considered an air inhalation hazard except as a nuisance dust.

Crystalline silica can be present in excessive concentrations in porcelain ceramics – such as toilets – leading to their unexpected breakage or spontaneous failure even years after manufacture through a process called “delayed dunting”. Epidemiological research continues on silica effects with the most famous cohort of British Pottery Workers where a large portion of the population were smokers during the exposure periods.

Porcelain ceramic/toilet microstructure with body (upper) and glaze (lower). The white particles are crystalline silica/cristobalite) surrounded by a vitreous matrix. Round pores in the body are typical of porcelain products.

Chromium and Metals

People are intrinsically afraid of ingestion, inhalation, and dermal contact of metals – in many cases thinking they “cause” cancer. Yet many multivitamins contain trace quantities of metals considered essential for life.

Chromium is one metal of special note as its hexavalent (+6) species is a known carcinogen.. Chromium is used in many products throughout our modern life, and in most cases the metal is alloyed or combined within the products such that there is little or no chance for human exposures. There are notable exceptions:

  • Soluble forms of chromium – even in the benign trivalent (+3) state can oxidize in the environment when the pH (acidity/basicity) is increased to highly basic states.
  • Thermal vaporization of chromium occurs at high temperatures, and the onset of measurable vaporization varies with the form of chromium. For example, chromium oxide as Cr2O3 exhibits an onset of vaporization at about 1000 degrees Celsius – while other forms such as chrmmium bearing spinel minerals only begin significant volatilization about about 1150 degrees Celsius.

Most transition metals to include iron (Fe) and manganese (Mn) exhibit similar volatilization behavior.

There are environmental emissions considerations from furnaces with these transition metals. In some cases, the heavy metals are found near the vicinity of smokestacks settling from aerosols because of their high density.

Lead (Pb) is the most well-known toxic metal. Lead was used in many low valued ceramics – including dinnerware – because of its “fluxing” potential. This allowed producers with rudimentary kilns to produce ceramics including glazed ones, a consequence found in products from “third-world” countries.

Detection of soluble metals is relatively straightforward. In general, acid extractions (liquids) are used. The more challenging aspect is determining where the metals are located within the ceramic product from a mineralogy standpoint to provide cause and effect relationships.

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