Ceramic Industry

Waste Not, Want Not

August 1, 2010
Waste materials like mine tailings and coal fly ash can be used to produce quality ceramics.

Figure 1. Anhydrite in ceramics from fly ash material.

An economically sound and sustainable ceramic manufacturing technology has been developed that minimizes negative environmental impacts; conserves energy and natural resources; and is beneficial for job creation, communities, and consumers. Waste materials like mine tailings and coal fly ash, which would otherwise be deposited in landfills, can be used to produce quality ceramics. Several thousand square feet of tile (up to 1 ft2) that have been successfully produced in pilot plants using Ceramext(r) technology have been installed in commercial and residential buildings.

Porcelain-grade floor and wall tile has been produced with a range of earth tone colors.

Composition Patent

A U.S. composition patent for certain ceramics made from recycled waste materials was recently granted to Ceramext LLC. Tile made with this new technology have among the highest breaking strengths for commercial tiles and are essentially impermeable to water, even without glazing. Sulfur and many other contaminants can be rendered inert by locking them within a stable glassy matrix in the tile. The tile can also have the appearance of a variety of natural rock surfaces and can be polished or glazed if desired. Products made of this new composition can include floor, wall and roof tile; building cladding; brick and pavers; and other ceramic products.

The micro-fabric of this new composition provides for a high-strength tile while eliminating undesirable materials from the waste stream. The composition consists of some of the original partly melted fly ash or waste rock fragments (called clasts), glass melted from the clasts, and crystallites formed in the new glass. The strength of the material is increased by using pressure while the material is hot but below the melting point.

The unmelted clasts act much like the aggregate in concrete, with the newly formed glass as the cement binder holding the clasts together and the crystallites further reinforcing the glass itself. Since there is essentially a continuation from the clasts to the glass, there is no significant porosity to contribute to breakage, the release of contaminants from the composition, or the contamination of the composition by external liquids such as water.

Past attempts to make ceramics from waste materials used higher production temperatures that resulted in glass-like ceramics that were weaker than the products made with this technology. Unfortunately, sulfur is often released into the atmosphere as sulfur dioxide (SO2) at those higher production temperatures, adding to undesirable air pollution.

Using the new technology, sulfur in the waste materials typically oxidizes and combines with calcium oxide to form anhydrite (see Figure 1). Anhydrite cannot react with water to become softer, unstable gypsum since the stable glassy matrix prevents water from reaching the anhydrite. Almost all potential waste materials tested to date have at least trace amounts of sulfur, with some materials having several percent of sulfur that typically react to form anhydrite. The technology thus renders the sulfur in the waste materials inert by essentially locking the contaminant within a stable glassy matrix in the tile.

This new composition is also particularly helpful in making ceramic products using and stabilizing flue gas desulfurization materials originating from waste streams from coal-fired power plants. The U.S. Clean Air Act Amendments of 1990 significantly reduced permitted SO2 emissions into the atmosphere. The gypsum byproducts of this successful clean air effort eventually exceeded the demand for wallboard and other gypsum products and must now be disposed of in waste landfills.

The Ceramext technology has been used to make strong ceramic products from coal fly ash containing in excess of 20% gypsum. It has also been used to produce strong ceramic tile that use and render inert acid-forming sulfides contained in mine and quarry tailings.

Approximately 50 recycled feed materials have been tested to date.

Characteristics and Applications

Many natural rock-type appearances are possible, including rough, polished or honed. Glazing is optional, but can be useful with applications such as cool roof coatings. With a breaking strength of 500-1080 psi and a MOR of about 10,000 psi, the strength of the tile exceeds the strength of typical commercial Italian porcelain.

The tile is essentially impermeable (0.1-0.2% water absorption), and no sealants are required. The tile also features facial size control of ñ 0.03% (without grinding) for normal commercial installations, thereby allowing narrow grout joints and precise layouts. In addition, it includes up to 100% recycled content and is eligible for LEED credits.

The technology is expected to produce quality ceramic products suitable for marketing at competitive prices. Porcelain-grade floor and wall tile has been produced with a range of earth tone colors. Tile of various colors, including white, black, beige and subtle greens, can be produced with no additives; the tile color depends on the specific type of fly ash/mine waste being used. Mineral additives can produce many combinations of natural stone appearances and textures. Glazes in multiple colors can be applied, if desired, and relief designs can also be made.

Pavers and landscape stones made using this technology are essentially impervious to water and thus well-suited for outdoor use. They are very unlikely to absorb water that might cause them to freeze, crack or break. Roofing and siding tile produced with the technology are typically stronger and lighter than conventional cement and standard glazed ceramic roofing or siding tile. The tile also has desirable freeze-thaw qualities.

Future Potential

To date, approximately 50 recycled feed materials have been tested. The potential also exists to use the technology to make ceramic products with garbage ash and other toxic wastes. Initial studies indicate that the technology can enable certain potential contaminants such as arsenic and lead to be locked up and rendered inert within a stable glassy matrix in the tile.

For additional information, visit www.ceramext.com.

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