- THE MAGAZINE
In today’s advanced ceramics industry, innovation is more important than ever in maintaining a competitive market position. Manufacturers must continue to develop higher quality, higher performance products while simultaneously reducing production costs. Achieving these goals often requires the development and implementation of innovative manufacturing technologies—especially in the firing process, where high energy requirements, long firing cycles and a lack of temperature uniformity often create production problems.
Over the past several years, industry suppliers have teamed up with ceramic manufacturers to develop new firing technologies that can significantly reduce firing cycles and energy consumption, increase product quality, and enhance overall productivity. The results of these collaborations are already proving beneficial in a number of manufacturing applications.
Hybrid Gas/Microwave FiringOne such innovation is a 1600 degrees C kiln that uses both gas and microwaves as a source of heat input. The kiln features a setting volume of 10 m3 and provides the ability to fire products with or without microwave power. Adding microwave power to conventional gas firing at the beginning of the firing cycle enables additional thermal energy to be transferred to the center of both the load setting and the individual products. This increases the speed at which the binders are burned out of the product while decreasing the need for excess air, which allows for shorter firing cycles and reduced fuel consumption. Additionally, the microwave energy reduces the risk of cracking by minimizing the temperature differential between the outer surfaces and center of the product. The improved temperature homogeneity also provides a corresponding improvement in the physical properties of the products being fired. Once all of the binders have been burned out of the product (typically around 600 degrees C), the microwave energy can be turned off, and the rest of the cycle can be completed with conventional gas firing.
The use of microwave energy requires precise control to ensure the safety of the environment and nearby personnel. In the new hybrid kiln, the metal casing, gas and air piping, magnetron and wave guides are all completely airtight, and the microwaves are contained within the kiln. Traps installed in all piping and sheaths ensure that none of the microwaves leak into the external environment. Additionally, the refractory materials inside the kiln have been carefully selected to ensure that they will not react with the microwaves. The kiln’s construction also ensures that it will not create electromagnetic disturbances in any measurement devices in the plant.
One of these new hybrid gas/ microwave kilns was recently commissioned for a plant in the eastern U.S. to make honeycomb ceramics for diesel engine catalytic converters. The kiln has been in operation since the beginning of 2002 and has enabled the manufacturer to dramatically reduce firing cycles without any deterioration of the product. While conventional firing processes require about 170 hours for this application, the hybrid kiln has successfully fired these products in less than 90 hours. Research to refine the technology is still ongoing, and it is anticipated that the hybrid kiln will eventually be able to fire the honeycomb ceramic products in 70 hours using just 1⁄3 of the amount of fuel required in a conventional kiln.
Although this installation demonstrates the advantages of the hybrid technology, not all ceramic products will benefit from the use of microwave-assisted firing. For the technology to be successful, the product to be fired or sintered must be compatible with the use of microwaves. High-alumina and zirconia-based products, as well as other ceramic materials that require an extremely long firing cycle, typically work well in this environment. For products with a short firing cycle and/or minimal binder content, the microwave technology is generally not very economical.
Ceramic manufacturers should also consider the cost of the electricity required to generate microwave energy. In Europe, for example, the cost of electrical power to produce 1 kilowatt is considerably higher than the gas alternative. Additionally, the initial cost of the hybrid equipment is typically much higher than conventional gas-fired equipment. However, in applications where the required firing speed and product uniformity cannot be achieved with conventional equipment, the benefits of the new hybrid technology can far outweigh the higher initial investment. Obtaining an accurate comparison of the cost versus benefits for a given product will require a thorough analysis.
Simultaneous Pressing and FiringAnother supplier/manufacturer partnership has led to the design and manufacture of a kiln-press for producing oxide products. This equipment combines two existing technologies for sintering and pressing.
The equipment is used to press and fire multi-layered low temperature co-fired ceramics (LTCCs). Products with an LTCC core are used in “mechatronic” systems (i.e., systems that integrate mechanical, electronic and computer technology) as sensors and control units for the automotive and chemical industry, such as ABS brakes and actuators. These new electrical systems provide superior mechanical, thermal and chemical resistance compared to the conventional alternative. Mechatronics are expected to have huge growth potential and are considered one of the key ceramic technologies of the 21st century.
The new kiln-press provides convective electric sintering at 1000 degrees C in a 1 m3 firing chamber. A convection system, rather than a radiation system, is used for the heat transfer to ensure excellent temperature homogeneity within the product. The equipment also provides constant pressing (maximum 5 bars) during the entire debinding phase (for a very strong withdrawal) and sintering phase, as well as perfect parallels across the pressing faces (a distance in excess of 1 m), producing structures free of any thermal dilation.
The maximum axial force of 50 kN, with a tolerance of ±250 N, is transmitted by two vertical pistons. The 1.1-m-tall chamber accepts charges of 200 multi-
layers arranged in two 1-m stacks (typically referred to as a “layer and setter sandwich”). The maximum layer size is 240 x 240 x 3 mm in the green state.
The kiln-press also features advanced thermal processing, pressing and data-processing supervision technologies that work together to treat the fumes created by the many organic binders in the sintering process, thereby preventing them from adversely affecting the firing atmosphere.
Roller Kiln for High-Organic-Content ProductsYet another successful collaboration has resulted in the installation of a new-generation, high-temperature (up to 1650 degrees C) roller kiln with extremely short calcining cycles, designed for products with a high organic content (e.g., polystyrene, sawdust, binders, etc.).
The kiln (shown above) is designed to provide both excellent combustion homogeneity and low energy consumption. These objectives are achieved through the use of several state-of-the-art features. For example, a recuperator used during the cooling phase of the firing cycle allows the product to be cooled and the combustion air to be pre-heated simultaneously. Energy consumption is reduced by the flow of preheated combustion air and by the reduction of airflow through the kiln (the product is cooled under radiation and not convection).
Additionally, infrared burners installed in front of the firing zone heat the product quickly and uniformly, ensuring the removal of any organic particles under optimum safety conditions and contributing to superior quality of the product being processed. This “pre-fire” zone is also linked to a second recuperator that recovers the energy released directly from the products (as opposed to the products of combustion) to assist in preheating the combustion air.
Prior to installation in the ceramic facility, the goal of the new roller kiln technology was to reduce the firing time to four hours compared to the conventional eight-hour firing cycle. However, the results were even better than expected—after just a few weeks of operation, the new kiln was operating successfully with a firing cycle of two hours cold to cold.