Materials Research

EPA Grant Number: X83254101-1

Center: CEER at Alfred University

Investigator: Varner, James R.

Institution: Alfred University

Project Period: September 01, 2006 to February 28, 2008

Research Category: Congressionally Mandated Center

 

Description: 

Annually, more than 850 million tons of coal is used in the United States and the ash generated amounts to 90 million tons. Only about 17% of the fly ash is presently being used for construction purposes. Due to the shortage of disposal sites and tighter regulations, new viable ways of utilizing fly ash are needed. One of the major limitations for using fly ash in the cement industry is that it should contain a limited amount of carbon. Fly ashes are ideal precursors for the synthesis of SiAlON powder due to their small particle size, amorphous nature, and sufficient amount of carbon necessary for the reduction. Thus, these ashes could be used without any special preparations to produce very-low-cost SiAlON-based structural ceramics.

 

Objectives/Hypotheses:

Fly ash contains oxides of Si, Al, Fe and minor traces of oxides of other metals such as Na, K and C, etc. They are mainly amorphous materials, but crystalline products such as quartz, mullite, iron oxide, anhydrite, or feldspars, can be observed. These oxides can be reacted with suitable chemicals to convert the fly ash into useful materials like zeolites, glasses and glass ceramics. A review of recent literature suggests that fly ash from power plants can be used for the synthesis of SiAlON powder by carbothermal reduction and nitridation. The objective of this project is to prepare low-cost SiAlON powder from high-carbon-content fly ash, thereby reducing the waste going to land fill sites.

 

Approach:

The SiAlON powders will be synthesized by carbothermal reduction and nitridation of the fly ash powders collected from various power plants in the U.S. The synthesized SiAlON powder will be formed into shapes and sintered using a Centorr vacuum furnace over the temperature range of 1600-1800^oC for varying time of 30 min. to 5 hrs. in nitrogen atmosphere. Moreover, microwave sintering will be performed using a 2.45 GHz furnace to study the feasibility of energy savings. Polished sections of the samples will be used for microstructure characterization and XRD analysis. The same polished specimens will be used for measurements of Vickers hardness and indentation fracture toughness. Once trends are established, bar specimens will be used to measure fracture toughness using a standard technique.

 

Expected Results:

More than 25 years have been spent in developing SiAlON-based structural ceramics, and it is still a niche market. Cost remains a major barrier to the more widespread use of SiAlON based structural ceramics, and will do so until large-scale supplies of less-expensive raw materials are available. Varying quantities of SiO₂, Al₂O₃, CaO and C present in the fly ash could be used as a cost-effective means for synthesizing SiAlON powder. We estimate that this project has the potential to reduce the cost of SiAlON ceramic parts by 30 to 40%. The significant reduction in the manufacturing cost of SiAlON will find new markets for high-, medium- and low-temperature structural applications. The increase in the production rate of SiAlON ceramics provides a corresponding increase in the utilization of fly ash, resulting in significant reduction in the environmental and land/water pollution. We anticipate 500,000 pounds of fly ash can be consumed per year. The results of this research will partially help EPA in implementing the Clean Air Interstate Rule (CAIR, March 2005) in 2006.

 

Supplemental Keywords: fly ash, SiALON, chemical synthesis, sintering.