The feasibility of using microwave energy to dry and fire pre-cured geopolymers was experimentally demonstrated, and supported by analysis of published microwave dielectric data for geopolymers. Dielectric loss tangents and half power microwave absorption depths were calculated from published room temperature dielectric constant and loss values of various geopolymer compositions. The published data indicated that geopolymers would heat at room temperature with microwave energy. Several laboratory experiments were performed to test the heating behavior of sodium and potassium based geopolymer compositions. Experiments demonstrated more vigorous microwave heating with sodium geopolymers than with potassium geopolymers. Both compositions were dried in less than 10 minutes with pure microwave heating. Further heating with pure microwave energy resulted in non-uniform, rapid heating, or "thermal runaway", with localized melting of the geopolymer. Hybrid microwave heating with susceptors resulted in uniformly fired geopolymers, without melting. INTRODUCTIONInterest in geopolymers has accelerated in recent years due to their possible use in structural applications, dentistry, and hazardous waste stabilization, while maintaining a trivial environmental impact compared to traditional building materials. Many useful geopolymer compositions are fabricated by low temperature chemical reaction based curing processes 1 " 3 . Some recent research has been reported for using microwave energy to enhance curing and drying 4 ' 5 . Other research has focused on high temperature firing of geopolymers, which can develop stronger glass-ceramic materials 6 " 8 . In situations where heat is required, as in drying and firing, microwave energy provides an energy efficient alternative in place of conventional heating. Traditional radiant heating methods rely on thermal conduction to deliver heat throughout a material.Microwaves generate heat throughout the volume of the material, allowing faster, more uniform heating to occur. Volumetric microwave heating helps to overcome sluggish endothermic phase transitions, such as evaporation of water or decomposition of kaolin to metakaolin through the loss of hydroxides. Both drying and dehydration occur in the firing of geopolymers. In traditional heating, these reactions often require slow heating, as the endothermic reaction prevents heat from progressing into the product until the reaction completes first at the surface. Microwaves can generate heat throughout the part despite an endotherm 9 .When microwave energy is the sole source of heat, a material that absorbs microwave energy will heat volumetrically, but cool from the surface. This situation creates an "inverse temperature profile" in which the sample is warmer inside and cooler at the surface during heating. This is opposite of traditional radiant heating where the material will be cooler in the center. In some materials, the inverse temperature profile can lead to thermal runaway -where the hotter center heats better than the cooler sur...
that a production size MAT rotary calciner was not feasible, and a different approach was adapted. The concept of a microwave post heat section located in the upper portion of the cooler was devised and well received by the industrial lime company and an industry expert. Commercialization of this technology will require 1) continued pilot scale calcining demonstrations, 2) involvement of lime kiln companies, and 3) involvement of an industrial microwave equipment provider. Ceralink identified two builders of lime kilns whom could be approached to partner with for continued technology and equipment development. Eventually, Ceralink would plan to license and transfer the technology to the kiln builders once the technical aspects are finalized. The lime company could then implement the MAT post heating sections in new builds and pursue retrofit of existing lime kilns in order to improve efficiency. Retrofits would likely be attractive to lime manufacturers, as the purchase of a new lime kiln is on the order of a $30 million dollar investment, where as a MAT retrofit might be on the order of $1 million. Ceralink would receive a royalty from the kiln builder. Current scale-up designs indicate that four 100kW microwave generators (915 MHz frequency) will be required to provide sufficient energy to a chute measuring 6ft in diameter and 4 ft. long to finish calcining the limestone before the limestone is cooled. This is scale for a production size rotary calciner with 600 ton/day output. Ceralink has identified a microwave equipment supplier who could supply the required microwave equipment and engineering work to assist with design and integration.
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