Emerging Technologies in Hazardous Waste Management V

Chapter 10

Iron-Enriched Basalt Waste Forms

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G. A. Reimann, J . D. Grandy, and G. L. Anderson Idaho National Engineering Laboratory, EG&G Idaho, Inc., P.O. Box 1625, Idaho Falls, ID 83415-2210

This paper reviews data on the iron-enriched basalt (IEB) waste form developed at the Idaho National Engineering Laboratory (INEL). IEB is a glass-ceramic waste form proposed for stabilization and immobilization of large volumes of low-level nuclear wastes for permanent disposal in an appropriate repository. IEB resembles natural basalt rock and is a product of melting a mixture of soil and retrieved wastes. Studies indicate that IEB will have a high tolerance for heterogeneous waste materials, including scrap metals, while maintaining the desired chemical and physical performance characteristics. Controlled cooling of molten IEB produces a glass– ceramic with good mechanical properties and high leach resistance. Production of IEB in a Joule-heated melter was difficult because of rapid electrode and refractory corrosion associated with high melt temperatures (1400 to >1600°C). Present studies include investigating the applicability of arc furnace technology to waste form production and determination of compositional limits for satisfactory waste form performance.

This paper summarizes results of research on the iron-enriched basalt waste form conducted at the Idaho National Engineering Laboratory during 1979-1982 (7,2) and from 1991 to present. This research investigated the applicability of thermal treatment to stabilize and immobilize between 227,000 and 340,000 m (8 to 12 million ft ) of wastes that were placed in "temporary" storage in the INEL's Radioactive Waste Management Complex (RWMC) since this facility was opened in 1952. (5) The figures given for the wastes include substantial quantities of overburden and underburden soils that may be contaminated and require treatment. Much of the material is low-level waste (LLW) contaminated with transuranic (TRU) elements from weapons-related work at the Rocky Flats Plant (RFP), and contains other substances defined as toxic and/or hazardous. Boxes and drums of these wastes were buried in pits and trenches between 1952 and 1970. After 1970 waste containers were stacked on asphalt pads, covered with soil for shielding, and protected with a membrane to 3

0097-6156/95/0607-0121$12.00/0 © 1995 American Chemical Society Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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EMERGING TECHNOLOGIES IN HAZARDOUS WASTE MANAGEMENT V

shed water. Some of the wastes have been stored beneath an air-supported canopy since 1976, which permits operations to proceed regardless of the weather. The heterogeneous wastes stored at the R W M C include wood, paper, cloth, plastics, concrete, metals, chemical sludges, and contaminated soil. Many of the buried waste containers were ruptured by heavy equipment when leveling the soil cover during pit closure, while other containers were breached due to deterioration during long-term burial. The extremely variable nature of the mix of wastes complicates conversion into a suitable waste form. The waste stacked on pads is better characterized and more homogeneous, these containers are in better condition, and less entrained soil is present. Thermal treatment is considered the best approach for most of the R W M C wastes as it would destroy the organic content, decompose the sludges, oxidize the metals, and generate a durable and versatile waste form. Interest in IEB as a waste form developed as a consequence of investigating the applicability of the slagging pyrolysis incinerator (SPI) for processing the R W M C wastes. (4) While applying the SPI to R W M C waste did not appear feasible, the slag produced by this furnace exhibited some very desirable characteristics. This aspect was pursued, and the IEB waste form evolved. (7) The IEB compositions described in this paper are the natural, unavoidable consequence of melting the mix of soil and waste encountered at the R W M C . A variety of thermal processing options are adaptable to the conversion of the R W M C wastes into stable waste forms. Previous research at the INEL used a Jouleheated melter for pilot-scale melts. (5) Carbon arc melting (6) and in situ vitrification (ISV) (7) were investigated also. This paper will concentrate on the IEB waste form. Other waste forms and the various thermal processing technologies will be covered in other papers presented at this Symposium. Natural Basalt Natural basalt is a fine-grained, dark-colored extrusive igneous rock that covers large areas of the earth's surface. Natural basalt is composed primarily of calcic plagioclase and monoclinic pyroxene, with magnetite, olivine, and certain other minerals often present. It is mechanically durable and very resistant to weathering, and it seems logical that a manufactured waste form of similar composition would be similarly durable and leach-resistant. Natural basalts have been remelted and transformed into stable glass-ceramics that exhibit good chemical durability, high mechanical strength, and good resistance to abrasion. (8) Examples of microstructures of basalts from the U . S. Northwest are given in Figure 1. Iron-Enriched Basalt Soil from the R W M C area consists mostly of loess and clay-dominated sediments deposited in old stream channels of the Big Lost River. The composition of R W M C soil was obtained by analyzing a blend of samples taken from several locations. (9) The results of this analysis are given in Table I as "A-100." A similar soil, "SDA lake bed soil," was excavated outside the R W M C boundary and used as fill and to cover material stacked on storage pads. The lake bed soil has a slightly higher silica and

Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995. Prineville, Oregon O L - olivine PL - plagioclase

Richland, Washington Px - pyroxene

Figure 1 - Examples of Natural Basalt Microstructures

ilmenite



124


alumina content and was used as an ingredient in most of our experimental work. Soil is a major component in the formulations used to produce IEB. Borosilicate glass (BSG) has been approved as a waste form for immobilization of high-level wastes, but converting several million cubic feet of L L W at the R W M C to the B S G composition would require a large amount of additives. While conversion to B S G would permit use of a low-temperature (1000-1200°C) melter, it would increase the waste form volume without improving the waste form properties, and it would require separation and removal of scrap metal that otherwise would accumulate unmelted in the low-temperature melter used for BSG. The temperatures used to produce IEB (above 1400°C, usually 1550-1650°C) could melt this scrap and hasten its oxidation while maintaining appropriate slag viscosity. The early studies showed that, in terms of leach resistance, the IEB waste form is at least as good as, and possibly better than, BSG. (1,10) Part of the present INEL technical mission is to generate sufficient data on IEB to justify its approval for use as a waste form. A range of compositions is expected to result from the variety of R W M C wastes to be treated. In the early INEL work, the "Series" system was devised to describe these wastes. The "Α-Series" describes waste form compositions that should result from mixing "Average" T R U waste with "average" R W M C soil. The T R U wastes include sludges, combustibles, and noncombustibles (including metals). Because of the variable nature of wastes and soils, the likelihood of encountering a waste or soil sample that corresponds to the average is remote, so the compositions given are reference compositions only. The "A-100" in Table I describes the composition of 100% R W M C soil with no waste, while "A-0" describes the composition of average R W M C waste with no soil. The "A-40" composition, 60% waste and 40% soil, corresponds most closely to the composition defined as nominal IEB and is the basis for our studies. Compositional ranges of some natural basalts are included in Table I for comparison.

Table I. Compositions of R W M C Soil, Wastes, Nominal IEB, and Natural Basalt Oxide Compounds (wt%) S i 0 A 1 0 FeO+Fe 0 CaO MgO N a 0 K 0 T i 0 Misc. 2

RWMC Soil (A-100) SDA Lake Bed Soil A-0 Hl-0 H2-0 S-0 P-0 N-0 M-0 IEB (A-40) Natural Basalt (77)

2

3

2

2

3

1.5 1.5

65.4 12.5 69.9 13.2

4.8 4.7

9.6 4.1

2.5 1.9

38.0 25.2 32.0 35.4 20.6 0.7 2.5

34.5 29.6 27.9 40.9 22.8 36.1 75.1

8.3 12.9 22.9 14.7 39.8 0.1 1.0

4.6 3.0 6.3 3.5 7.6 0.0 0.0

19.6 11-14

9.7 7-10

3.2 3.5 4-10 2-4

7.4 7.0 4.7 4.8 9.3 0.3 5.1

51.0 10.3 45-55 13-17

2

2

2.9 0.7 3.3 0.8

2.4 4.8 15.9 2.6 0.0 2.2 0.2 3.3 0.4 0.0 0.3 0.0 0.0 0.0 37.3 25.5 0.0 0.0 0.0 0.0 2.6 1-5


1-5

0.5

3.8 0.4 0.0 0.0 0.0 16.3

—

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With the accumulation of additional information describing the R W M C wastes, additional waste series were developed; (12) these are listed in the table also. Most of these wastes result from defense-related activities at the Rocky Flats Plant (RFP) and are stored in steel drums. H1-0 and H2-0 designations describe the RFP 741 and 742 sludges, respectively, with Portland Cement and Oil-Dri added to absorb free liquid. The S-0 series is organics (oils and solvents, RFP 743) immobilized by mixing with Micro-Cel Ε (CaSi0 ) and Oil-Dri. The P-0 series is chemical wastes (RFP 744) immobilized by mixing with Portland Cement and magnesia cement. The N-0 designation refers to evaporator salts (RFP 745) from solar drying of liquid wastes and consists mostly of sodium and potassium nitrates with limited amounts of other wastes and small amounts of Oil-Dri. The M-0 waste (RFP-480) is a variety of unleached scrap metals bagged in plastic and loaded into boxes or drums. In this work, waste form characterization assumes that all volatiles have evaporated, all combustibles have been burned, and only oxide residues remain.


3

Production of IEB The slagging pyrolysis incinerator, mentioned previously, was devised primarily for treating municipal waste. This concept was the first considered for treatment of the R W M C waste because its 200,000 kg/day (220 tons/day) capacity could be large enough to treat the waste on a scale necessary to complete the task within a reasonable time frame. (4) The SPI could achieve high temperatures (1650°C), and its offgas could reach 1450°C. These temperatures are substantially higher than those normally encountered when incinerating municipal waste. Supplemental fuel requirements depended on the amount of combustibles in the waste. The byproducts were metal, a basaltic slag, and the offgas. However, the SPI did not perform well on municipal waste unless the feed was fairly uniform; system upsets were common when variable feed compositions were used. The SPI was a shaft-type furnace (see Figure 2), and thus inherently vulnerable to bridging in the descending column of waste and freezing of the bath due to "off normal" conditions caused by feed material variations. Such problems usually must be corrected manually, which may be difficult, and which cannot be tolerated when melting R W M C wastes that require remote operation of equipment. However, the basaltic slag generated by the SPI furnace was of interest, as its composition was what would be expected when melting the R W M C waste and accompanying soil. The SPI slags were quenched in water to produce a frit. Examination of SPI slag samples disclosed bits of charcoal and beads of metal trapped in a glassy phase resembling obsidian, indicating that this slag was developed in a strongly reducing rather than in an oxidizing environment. At that time (ca. 1979), experience with incineration of radioactive waste was lacking and few data were available regarding behavior of radionuclides in a basaltic waste form. Much of the early INEL research on IEB was performed in fractional-liter crucibles, as no equipment was available to do pilot-scale studies. Through an interagency agreement, the U . S. Bureau of Mines (USBM) conducted melting tests in a one-ton tilt-pour electric arc furnace at the Albany Research Center. (6,13) While operation was inefficient, with high refractory wear and rapid electrode consumption, the furnace was able to achieve bath temperatures of 1500 to 1700°C and to convert



126


TRU waste and additives

ι

ι

combustion

Molten slag to slag processing system

Figure 2 - Slagging Pyrolysis Incinerator System



10. REIMANN ET AL.


127

various simulated R W M C wastes into basaltic melts that were poured into 208-L (55gal) steel drums. This process appeared to have merit for conversion of mixed wastes into the IEB waste form; however, an arc furnace should not be used as an incinerator. The furnace had to be fed slowly when combustibles were charged, especially when significant polyethylene waste was present, to avoid generation of excessive soot and fumes that would produce unstable furnace operation. In order to develop melter data at the INEL and produce IEB on a scale more applicable to production, an 80-liter joule-heated melter (JHM) was designed, built, and operated. (5) The decision to investigate the J H M concept was based on its successful application to large-volume production of BSG, and on the ability of Penberthy's electromelter to produce 90 kg experimental pours of simulated R W M C waste compositions. (1,14) Figure 3 shows a schematic of the INEL melter. Molybdenum electrodes and chromia-alumina refractories were used in this furnace to better endure the high temperatures. Electrode oxidation was troublesome and refractory attrition was high. Operating experience obtained with several versions of this furnace enabled performance to be improved. The most noteworthy design improvements were watercooled refractory walls and incorporation of provisions to protect Mo electrodes from oxidation with a nitrogen blanket. While additional design changes and adjustments to operating procedures would have further improved melter durability, the J H M experience encouraged the INEL to have another look at arc melters. The arc furnace used by the U S B M for the work described previously has since been replaced with another unit that is more suitable for waste treatment. A n American Society of Mechanical Engineers (ASME) test series for vitrification of incinerated municipal waste was performed in this furnace in 1992. (75) The INEL has a new interagency agreement with the U S B M Albany Research Center to determine whether this arc furnace is suitable for treatment of the R W M C wastes on a larger scale. A test series was completed on simulated R W M C wastes in this furnace in July 1993 that entailed melting nearly 22,000 kg of material in five consecutive test days at feed rates of up to 700 kg/h and melt temperatures to 1850°C with no apparent equipment problems. (16) Other pilot-scale arc melting approaches for radioactively-contaminated waste are under investigation, including the Pacific Northwest Laboratories/Massachusetts Institute of Technology (PNL/MIT) DC arc furnace and the Mountain States Energy/RETECH centrifugal reactor. Schematics of these three melting units are shown in Figure 4. Details of the progress of research efforts are contained in papers published in this Symposium. Crystallization In I E B Improved physical properties and leach resistance may be obtained from crystallized IEB, as compared to IEB that is mostly vitreous, especially if the radionuclides and/or toxic metals can be incorporated into the structures of crystalline phases. Slow cooling of IEB melts in the 1300 to 700°C range produces a mixture of crystals and a small amount of residual glass phase. In terms of leach resistance, crystalline structures are superior to glass of the same composition. Glass is metastable by definition and is more vulnerable to leaching than crystals of the same composition. (7 7) The crystalline structures encountered in IEB are similar to those found in natural basalts, as may be observed in Figure 5; however, the redox conditions during melting,



Figure 3 - I N E L Joule-Heated Melter


I

1

t

ο ο

2 S 2

S

Ο

5?

M

Cil

H

2 ο

M

M

1

hts> 00

Tedder;Pohland; Emerging Technologies in Hazardous Waste Management V ACS Symposium Series; American Chemical Society: Washington, DC, 1995. USBM 3φ AC Arc Furnace

MSE/RETECH Centrifugal Reactor

Figure 4 - Schematics of Pilot-Scale, High-Temperature Waste Form Furnaces

PNL/MIT DC Arc Furnace

Concentric electrode


NO

I

5

a.

I

I

Ι

Ζ


Iron-Enriched Basalt

2

IEB with added T i 0 and Z r 0

Figure 5 - Comparison of Natural Basalt and IEB Microstructures

Craters of Moon Basalt


2

ο

10. REIMANN ET AL.

131


the presence of the additional iron, and other oxides introduced during the course of waste assimilation may alter the composition sufficiently to affect the microstructure. The concentration at which the additives may have a deleterious effect on the desired properties of IEB has not been established and is a question that must be addressed. Up to this point, the major observed effects of compositional shifts have been on the quantities of crystalline phases present. In the IEB compositions, iron spinels ( F e O F e 0 ) are usually the first to crystallize from the liquid because of the amount of iron present, followed by augitic pyroxene [Ca(MgFe Al)(SiAl) 0 ] and calcic plagioclase. With further temperature decline, the plagioclase becomes more sodic, that is, the Ca-dominated plagioclase (anorthite, C a A l S i 0 ) grades into an Na-dominated plagioclase (albite, N a A l S i 0 ) . One effect of the development of these crystals in IEB is to deplete the residual glass of iron, alkali, and alkali-earth oxides. The remaining glass thus becomes richer in silica and alumina and therefore becomes more leach resistant than the parent glass composition. The ratio of FeO to F e 0 (or F e to Fe ) has a major influence on which crystals may nucleate and grow from the liquid. Beall and Rittler (8) reported that highly oxidized basaltic melts would crystallize into very durable glass-ceramics when F e : F e < 1. Redox conditions encountered during the course of melting, and the compositional variations within limits defining IEB, will alter significantly the composition of crystalline phases that develop, as well as the composition of the residual glass phase. Augite and/or olivine will form more readily when F e is abundant. Fe becomes dominant in well-oxidized melts and anorthite may form before augite, or augite may not form at all due to the F e insufficiency. The three melter types shown in Figure 4 operate with radically different redox environments, with the PNL/MIT furnace being the most reducing and the Retech centrifugal reactor being the most oxidizing. The U S B M arc furnace will permit a range of redox conditions, depending on the amount of in-flow air permitted or whether the melt is oxygen-lanced, while the composition of the plasma gas in the Retech furnace may adjusted from reducing to pure oxygen. Feeding identical waste compositions into each furnace at the same rate and then subjecting them to identical cooling conditions may result in the formation of significantly different crystal species due to the different oxidation-reduction conditions. 2

3


+2

2

2

2

6

8

3

+2

2

+2

8

+3

3

+3

+2

+2

Uranium and Transuranics in I E B When small amounts of uranium or TRUs were added to IEB, these oxides dissolved into the melt as would any of the other oxide components. When these melts cooled, crystals of uranium or plutonium oxide were found in the residual glass phase. Without the presence of compounds to form suitable host crystals or solid solutions, the U and Pu oxides precipitated independently of the crystalline phases that normally form in cooling basalt, and were incorporated only i f they became trapped within crystals that grew around them. Precipitated oxides of uranium and plutonium scattered throughout a glass phase would be more vulnerable to leaching than those incorporated into a leach-resistant crystalline phase. When sufficient zirconia (Zr0 ) was added to IEB, crystals of zirconia and/or zircon (ZrSi0 ) developed that incorporated uranium into their crystalline structures as solid solutions. (7) Zirconia 2

4


132


and zircon are seldom encountered in natural basalts because Z r 0 is insufficiently concentrated to form a crystalline phase. 2

Development of IEB4 Titania (Ti0 ) is normally present in basalts in minor amounts. The R W M C soil contains about 0.7 wt% T i 0 . Adding T i 0 to that already present in IEB to raise the total to about 5%, and adding a similar quantity of Z r 0 , will produce crystals of zirconolite (CaZrTi 0 ) as the melt cools. (18) These crystals will be in addition to those that normally form in basalt. Natural zirconolite crystals have demonstrated a very high durability as well as a high capacity for U and Th. These properties should increase the leach resistance of the durable IEB waste form by immobilizing the TRUs within leach-resistant crystals. IEB modified with additions of Z r 0 and T i 0 has been designated IEB4, in reference to the addition of these Group IVB elements. Efforts are presently underway to determine whether IEB4 will immobilize the TRUs in L L W in a manner analogous to the way the various formulations of Synroc immobilize TRUs in high-level waste. Zirconolite is formed with more difficulty in oxidized IEB4 melts with a high iron content because the titanium becomes tied up in the hematite and/or iron spinels. Pseudobrookite ( T i F e 0 ) may develop also, which has a small capacity (4 to 6%) for Z r 0 . Formation of these compounds depletes T i 0 and Z r 0 from the residual glass phase from which zirconolite must form. Synroc requires hot pressing equipment while IEB4 develops zirconolite from a cooling basaltic liquid. Preliminary work on IEB4 was done using lanthanides as surrogates for the actinides. These studies are described in detail in another paper in this Symposium. 2

2

2


2

2

7

2

2

2

2

2

5

2

2

Conclusions IEB is a waste form that is the unavoidable consequence of thermal treatment of toxic, hazardous, and/or radioactive INEL wastes that are mixed with soil. It closely resembles natural basalt in both composition and properties and thus would be a very durable material in which to stabilize and immobilize TRUs for time periods measured on a geologic scale. Adding small quantities (-5% each, or less) of T i 0 and Z r 0 enables a new suite of crystals to develop, so a very durable waste form may now contain TRUs within even more durable crystals. The attributes of IEB have become well known; however, development of suitable industrial practices for treatment of large volumes of alpha L L W have lagged behind waste form development. Studies are now underway to develop and demonstrate techniques suitable for treatment of large waste quantities, and to determine i f adjustments of these techniques are necessary in order to produce the desirable waste form properties. 2

2

Acknowledgments This work was supported by the U.S. Department of Energy, Office of Environmental Restoration and Waste Management, under DOE Idaho Operations Office Contract DEAC07-76ID01570.


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Literature Cited

1. Flinn, J. E.; Henslee, S. P.; Kelsey, P. V.; Malik, R. K.; McCormack, M. D.; Owen, D. E.; Reimann, G. Α.; Schuetz, S. T.; Seymour, W. C.; Tallman, R. L.; Welch, J. M. Annual Report on TRU Waste Form Studies with Special Refere to Iron-Enriched Basalt: 1980; EG&G-FM-5366, EG&G Idaho, Inc., Idaho Falls, ID, 1981. 2. Reimann, G. Α.; Grandy, J. D.; Eddy, T. L.; Anderson, G. L. In Nuclear and Hazardous Waste Management Spectrum '92 (Proc.); American Nuclear Soci Inc., La Grange Park, IL, 1992; Vol. 2, pp. 1083-1088. 3. Arrenholz D. Α.; Knight, J. L. A Brief Analysis and Description of Transuranic Wastes in the Subsurface Disposal Area of the Radioactive Waste Mana Complex at the INEL; EGG-WTD-9438, EG&G Idaho, Inc., Idaho Falls, ID, 1991. 4. Cox, N. D.; Nelson, D. C.; Burgus, W. F.; Richardson, L. S.; Hootman, Η. E.; Stouky, R. J.; Nebeker, R. L.; Thompson, T. K. Figure of Merit Analysis for TRU Waste Processing Facility at INEL; TREE-1293, EG&G Idaho, Inc., Idaho Falls, ID, 1978. 5. Reimann, G. Α.; Welch, J. M. Electromelt Furnace Evaluation; EGG-FM-5556, EG&G Idaho, Inc., Idaho Falls, ID, 1981. 6. Oden, L. L.; Johnson, Ε. Α.; Sanker, P. E. In Nuclear and Chemical Waste Management; Pergamon Press Ltd., New York, NY, 1983; Vol. 4; pp. 239-244. 7. Callow, R. Α.; Thompson, L. E.; Weidner, J. R.; Loehr, C. Α.; McGrail, B. P.; Bates, S. O. In-Situ Vitrification Application to Buried Waste: Final Report of Intermediate Field Tests at Idaho National Engineering Laboratory; EGG-WTD-

9807, EG&G Idaho, Inc., Idaho Falls, ID, 1991. 8. Beall, G. H.; Rittler, H. L. Ceram. Bull., 1976, 55, pp. 579-582. 9. Reimann, G. Α.; Grandy, J. D.; Eddy, T. L.; Anderson, G. L. Summary of INEL Research on the Iron-Enriched Basalt Waste Form; EGG-WTD-10056, EG&G Idaho, Inc., Idaho Falls, ID, 1992. 10. Hayward, P. J. In Radioactive Waste Forms for the Future; Lutze, W.; Ewing, R. C. Eds.; Elsevier Science Publishing Co., Inc., New York, NY, 1988; p. 471. 11. Basalts; Ragland, P. C.; Rogers, J. J. W.; Eds.; Van Nostrand Reinhold Co., Inc., New York, NY, 1984. 12. Bates, S. O. Definition and Composition of Standard Wastestreams for Evalua of Buried Waste Integrated Demonstration Treatment Technologies; EGG-W 10660, EG&G Idaho, Inc., Idaho Falls, ID, 1993. 13. Nafziger, R. H.; Oden, L. L. In Materials Research Symposium Proceedings, Brookin, D. G., Ed.; Elsevier Publishing Co., Inc., New York, NY, 1983; Vol.15, pp. 639-646. 14. Flinn, J. E.; Kelsey, P. V.; Tallman, R. L.; Henslee, S. P.; Seymour, W. C. IronRich Basalt-Type Waste Forms for Transuranic and Low-Level Was Containment: Evaluation of Electromelt Castings; EGG-FM-5241, EG&G Idaho, Inc., Idaho Falls, ID, 1980. 15. O'Connor, W. K.; Oden, L. L.; Turner, P. C. In Process Mineralogy XII; Petruk, W.; Rule, A. R., Eds; The Minerals, Metals, & Materials Society, Warrendale, OH, 1994; pp. 17-37.


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16. Oden, L. L.; O'Connor, W. K.; Turner, P. C.; Soelberg, N. R.; Anderson, G. L. Baseline Tests for Arc Melter Vitrification of INEL Buried Wastes; EGG-WT 10981, EG&G Idaho Inc., Idaho Falls, ID, 1993; Vol. 1. 17. Roy, R. Jour. Amer. Ceram. Soc., 1977; 60, p. 359. 18. Conley, J. G.; Kelsey, P. V.; Miley, D. V. In Advances in Ceramics; G. G. Wicks and W. A. Ross, Eds.; The American Ceramic Society, Columbus, OH, 1984; Vol. 8, pp. 302-309. RECEIVED March 14, 1995


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