NCER Assistance Agreement Final Report Executive Summary

 

Date of Final Report:

EPA Agreement Number: X-83254101-1

Center: Center for Environmental and Energy Research (CEER)

Project Title: The Use of Fly Ash in the Production of SiAlON Based Structural Ceramics

Investigator(s): James R. Varner

Institution(s) of PI(s): Alfred University

Research Category: Congressionally Mandated Center

Project Period: September 1, 2007 – February 28, 2009

 

Description and Objective of Project:

Approximately 125 million tons of coal-combustion waste was produced in 2007. About 43% of coal-combustion waste was beneficially used, leaving 70 million tons that was disposed of. There is great potential to utilize fly ash and reduce the amount which is going to landfills.

SiAlON is an advanced structural ceramic having excellent thermal shock resistance, high strength, good high-temperature strength, is very hard, has good fracture toughness, low thermal expansion, high wear resistance, and good oxidation resistance. Furthermore, SiAlON is not wetted or corroded by non-ferrous metals. This has made SiAlON very beneficial in abrasion and wear technologies as well as in the chemical and metals industries. However, the use of SiAlON is hindered by the high cost of manufacturing SiAlON ceramics. Since fly ash is a cheap raw material, it is expected to help lower the cost of making SiAlON ceramics when coupled with a cheap method of synthesizing SiAlON powders, such as the carbothermal reduction and nitridation (CRN) process.

Fly ash, a coal-combustion product, is an ideal precursor for the synthesis of SiAlON ceramics by the CRN process. The particles have spherical morphology, which is ideal for ceramic processing. High reactivity is expected because the particles are small and are mostly amorphous. However, mixed success of converting fly ash into SiAlON is apparent in the current literature. This is primarily associated with the differences in chemistry, which depends on what type of coal was burned, where the coal was mined, what the reactor parameters were when the coal was burned, and any post-burning processes that may be implemented. The fly ashes are alumino-silicates with various amounts of alkali- and alkali-earth oxides. They also have various amounts of iron compounds and carbon. However, it has yet to be demonstrated how to fully account for the complexity of the chemistry.

The goal of this research was to develop an approach to fully account for the differences in chemistry of fly ashes from different sources that were burned at different power-generating facilities. Using an adequate approach, predictability and reproducibility were demonstrated and pave the way for use of fly ash for making technical and structural ceramics. SiAlON powders were synthesized from fly ashes originating from various sources. Sintering of compacts using SiAlON powders made from these fly ashes was also performed. In addition to determining the solid-solution values of the synthesized SiAlON powders and the sintered compacts by X-ray diffraction and lattice refinement, the mechanical properties of the sintered compacts were evaluated using analysis of variance (ANOVA) statistical analyses. The microstructures of the compacts were also evaluated.

 

 

Summary of Findings:

      Six fly ashes were obtained from three different companies, which were collected from different power-generating facilities. The chemistries were represented using the unity molecular formula (UMF) approach and are shown in Table I. Using this approach, the system can be represented using a ternary functionality diagram, which is shown in Figure 1. Iron (II) oxide is grouped in RO, but after processing is removed. Therefore, the shift in chemistry due to removal is also shown in the diagram by a primed lettering scheme corresponding to the fly ashes.

 

Table I. Chemistries and Collective Molecular Weight of Fly Ashes Used in this Study

 

            Figure 1. Ternary functionality diagram given in mole % to represent SiAlON formation regions and fly ash chemistries.

 

      After modifying the chemistry of the fly ashes to perform synthesis and sintering, X-ray diffraction of the synthesized powder and sintered specimens was performed. Lattice refinement was used to determine the solid-solution value of the resulting β-SiAlON according to the formula Si_6-zAl_zO_zN_8-z. These results are shown in Figure 2. The experimental values for the synthesized SiAlON powders match theoretical predictions well. There is a drop in z-value after sintering. This is expected because of the use of yttria as a sintering aid. A typical grain-boundary crystallization product in the Y₂O₃-SiAlON system is yttrium aluminum garnet (YAG). Therefore, it is expected to pull aluminum from the SiAlON, hence the lowering of the z-value. Although theoretical predictions of this phenomenon were not performed, the drop in z-value appears to be consistent and therefore likely to be predictable.

Figure 2. Solid-solution values for the synthesized SiAlON powders and sintered specimens as determined using lattice refinement.

 

      The mechanical properties were evaluated by using Vicker’s indentations. Hardness and crack-length measurements under different loading forces were measured. Analysis of Variance (ANOVA) was performed with a 99% confidence interval. The results are shown in Table II, which indicate that the null hypothesis was accepted in all instances and that there is no statistical evidence that the samples are different.

      A representative microstructure of the sintered specimens is shown in Figure 3. In the microstructure, there are five notable areas. There are dark, elongated β-SiAlON grains. The bulk of the sample appears to be a β-SiAlON/sintering aid mixture. There are β-SiAlON/reduced zirconia clusters as well as individual zirconia grains which are indicated by the brightest phase. The zirconia is a contaminant in the system that was introduced during a mixing procedure. Lastly, the sintered compacts were not fully dense and some micro-porosity exists. It is unclear if this is mechanical damage (pull-out) introduced during polishing of the specimens or just regions where the sintering aid did not penetrate. The micro-porosity is the darkest phase in the image.

 

 

 

 

 

 

 

 

Table II. Results of ANOVA Statistical Analyses on the Mechanical Property Metrics

 

 

 

      Figure 3. Representative microstructure of the sintered specimens.

 

 

Conclusions:

      A ternary functionality diagram has been constructed to provide a systematic approach for the synthesis of SiAlON from fly ash by the carbothermal reduction and nitridation process. Using this diagram, six fly ashes were tested for purposes of demonstrating reproducibility and predictability. The solid-solution values of the synthesized powders were consistent and matched theoretical predictions well. The z-value of the sintered compacts was lowered, but in a consistent manner. The microstructure and mechanical properties evaluated in this study were also consistent. Therefore, reproducibility and predictability have been demonstrated despite the initial differences in the fly ashes coming from different sources.

Publications/Presentations:

J.P. Kelly, J.R. Varner, W.M. Carty, and V.R. Amarakoon, “Novel Chemistry-Modification Approach for Synthesis of SiAlON from Fly Ash,” 8^th Pacific Rim Conference on Ceramic and Glass Technology, Vancouver, British Columbia, Canada, May 31-June 5, 2009. (Manuscript submitted)

 

J.P. Kelly, “The Use of Fly Ash in the Preparation of SiAlON Ceramics,” M.S. thesis, Alfred University, 2009. Advisor: J.R. Varner. Committee: W.M. Carty and V.R. Amarakoon.

 

 

Supplemental Key Words:

SiAlON, silicon aluminum oxy-nitride, fly ash, synthesis, carbothermal reduction and nitridation

 

 

Relevant Web Sites:

http://ceer.alfred.edu

http://www.acaa-usa.org