Forming Ceramics Through Vacuum Hot Extrusion
During the last few decades, a number of new forming methods have been put into use in the ceramic industry. Some of these, such as hot isostatic pressing and tape casting, evolved from previous technologies used in ceramic manufacturing. Others, such as sol gel processing, are extensions of processes used in other industries. While these developments have been very useful, for the most part they have been more evolutionary than revolutionary.
Recently, however, a new forming technology has been developed that promises to dramatically change how some ceramic products are manufactured. The process, called Ceramext,* uses vacuum hot extrusion to produce ceramic products in virtually a one-step process.
Extrusion, a plastic forming process, has been used in the ceramic industry for a very long time. In most cases, the process relies on the plasticity imparted to the ceramic body by wet clay. Sometimes other non-clay materials are used to impart plasticity to non-plastic materials, such as high alumina, mullite or silicon carbide bodies. In all of these cases, the extrusion process takes place at or near room temperature.
Injection molding, another type of extrusion process, also relies on plasticity. In this method, plasticity is imparted by heating a thermoplastic polymer in the body and forcing the plastic material into a die. However, the temperatures are still quite low compared to ceramic firing temperatures.
At its most basic level, vacuum hot extrusion is like these other extrusion processes in that a plastic material is forced under pressure through a die to form a shape. The difference is that in vacuum hot extrusion, the plasticity is generated by the formation of a viscous liquid phase in the body at elevated temperatures. As a result, many separate steps often required with conventional forming processes are not needed with vacuum hot extrusion. Additionally, this new forming technique also enables the use of a very wide variety of raw materials, including those that would otherwise be considered waste materials.
Low-Cost ManufacturingThe vacuum hot extrusion process was invented by mining geologist Ross Guenther—but the concept is not entirely new. Mother Nature has been performing hot extrusion underground for eons, moving magma around and then spewing it onto the earth’s surface. In vacuum hot extrusion, raw material is heated in a vacuum extruder until enough liquid phase is formed to develop the plasticity needed for extrusion. The use of vacuum in the extruder prevents air from being trapped in the material as it is coalesced under pressure. The plastic, pore-free material is then forced through a die at high temperature (1100-1200 degrees F).
As with conventional ceramic extrusion processes, subsequent plastic forming steps, such as pressing, can be performed on the extruded material to form a final shape. The product can also be glazed, if desired, before being subjected to a controlled cooling phase. Unlike other conventional forming processes, however, many additional separate steps—such as wet extrusion and pressing (or, alternatively, spray drying and dry pressing), drying and firing—are not required. If a glaze is used, it is matured in the controlled cooling step. The primary heating step has low heat losses and directly heats the raw material to processing temperature, which tends to be lower than that required in conventional ceramic firing. And since subsequent forming or glazing steps require very little, if any, extra energy, the process is very energy efficient.
The capital investment for vacuum hot extrusion is also expected to be lower than that required for an automated, conventional ceramic plant. Additionally, the extruders can be relatively small, enabling manufacturing to be modular and flexible. As a result, variations in business levels can be more easily accommodated.
Perhaps the biggest economic benefit to ceramic manufacturers, however, is the process’ ability to use waste raw materials—which otherwise cannot be used for conventional ceramic manufacturing—as the sole component of a ceramic body. The only requirement is that the composition must allow the development of enough viscous liquid at elevated temperatures to provide the plasticity needed for extrusion. High strength, very low porosity ceramic bodies have been made from materials such as coal fly ash, lignite fly ash, bio-mass fly ash, mine tailings, waste clay materials, schist-containing materials and overburdens. Even materials that do not develop adequate high temperature plasticity by themselves can be processed by adding small amounts of inexpensive fluxing agents. In most cases, the waste material is already a fine powder, so grinding is not needed.
Very large amounts of such waste materials are generated each year. In the U.S., for example, over 100 million of tons of coal, lignite and biomass fly ash are produced annually, and only a small fraction can be used, primarily as additions to cement products. The balance must be disposed of in landfills at a cost of several dollars to tens of dollars per ton, so companies generating these waste products are eager to find alternate uses. Fly ash can currently be obtained from several suppliers at a minimal cost, and the possibility also exists to obtain other waste materials at low or no cost from power plants or mines that currently have to pay to have waste materials hauled to a landfill. In many areas, local tax incentives and grants are also available for recycling, since the ecological benefit of reducing the amount of waste sent to landfills is so important.
Overall, direct manufacturing costs using vacuum hot extrusion are expected to be 30-35% lower than with conventional processing.
Technical ConsiderationsVacuum hot extrusion produces materials with attractive physical properties. Modulus of rupture values in excess of 10,000 psi (70 MPa) have been measured, and very low water absorption values (virtually 0%) can also be attained. As a result, products manufactured using vacuum hot extrusion exhibit high strength and superior freeze-thaw resistance.
The process is best suited for manufacturing products such as ceramic floor and wall tile, ceramic roof tile, brick and similar products, as well as tableware and cookware. Examples of some of the materials processed by this method are shown in Figure 1. Other materials that have been successfully used include wood ash, red clay, mine tailings, California power plant coal bottom ash, and Colorado coal fly ash. The process is not suitable for manufacturing materials that have no high temperature plasticity, such as advanced structural ceramics or electronic ceramics.
Several technical issues must be considered in using this process. Dies made of conventional metallic materials soften and severely oxidize at the high temperatures used in vacuum hot extrusion and are thus not suitable. Die abrasion by the material being extruded can also be a problem. However, a number of robust metallic and ceramic die materials are available that can be successfully used in this process.
Additionally, as with all ceramic manufacturing, the raw materials need to be reasonably consistent. Some of the raw materials of interest, such as coal, lignite and biomass fly ash, often contain residual carbon that must be removed during the preheating of the raw material. Standard chemical testing procedures can be used to identify such components, as well as to pinpoint waste material sources that might not be desirable due to variations in chemistry. In most cases, however, waste materials are quite consistent and do not require additional processing before being used in ceramics produced through vacuum hot extrusion.
Finally, body color is strongly influenced by oxidation/reduction conditions during processing but can be controlled by subsequent oxidation of components, such as iron oxide, that might undergo reduction during vacuum processing at elevated temperatures. Colors routinely attained in tests include adobe, brown, reddish brown, light gray, gray and black.
CommercializationThe vacuum hot extrusion process has been successfully demonstrated on a laboratory basis, and the physical properties of the resulting materials have been evaluated. Basic process variables, such as extrusion temperature and pressure, have been studied and are understood, and a wide variety of coal and other fly ash materials have been successfully converted into ceramic bodies. The next step toward full commercialization will be pilot plant production of ceramic products in enough volume to engineer full-scale production facilities. Licensees are also being sought to manufacture the vacuum hot extrusion equipment, supply the fly ash and other waste materials, and manufacture ceramic products using the patented vacuum hot extrusion method.
With the commercialization of this new technology, ceramic manufacturers will have a revolutionary new tool for producing low cost, high performance products—while also benefiting the environment.