Arun S. Wagh

Stabilization of Rocky Flats Pu-contaminated ash within chemically bonded phosphate ceramics

Abstract A feasibility study was conducted on the use of chemically bonded phosphate ceramics for stabilization of combustion residue of high transuranic (TRU) wastes. Using a matrix of magnesium potassium phosphate formed by the room-temperature reaction of MgO and KH 2 PO 4 solution, we made waste forms that contained 5 wt% Pu to satisfy the requirements of the Waste Isolation Pilot Plant. The waste forms were ceramics whose compression strength was twice that of conventional cement grout and whose connected porosity was ≈50% that of cement grout. Both surrogate and actual waste forms displayed high leaching resistance for both hazardous metals and Pu. Hydrogen generation resulting from the radiolytic decomposition of water and organic compounds present in the waste form did not appear to be a significant issue. Pu was present as PuO 2 that was physically microencapsulated in the matrix. In the process, pyrophoricity was removed and leaching resistance was enhanced. The high leaching resistance was due to the very low solubility of PuO 2 coupled with superior microencapsulation. As a result, the waste forms satisfied the current Safeguard Termination Limit requirement for storage of TRU combustion residues.

Chemically bonded phosphate ceramics for low‐level mixed‐waste stabilization

Abstract Novel chemically bonded phosphate ceramics are being developed and fabricated for low‐temperature stabilization and solidification of mixed‐waste streams that are not amenable to conventional high‐temperature stabilization processes because volatiles, such as heavy‐metal chlorides and fluorides, and/or pyrophorics are present in the wastes. Phosphates of Mg, Mg‐Na, and Zr are being developed as candidate matrix materials. In this paper, we present the fabrication procedures for phosphate waste forms with surrogate compositions of three typical mixed‐waste streams, namely ash, cement sludges, and salts. This study was focused, but not limited to, magnesium phosphate‐ash wastestream final waste form. The performance of the final waste forms, such as compression strength, leachability of the contaminants, and durability in aqueous environments were conducted. In addition, parametric studies have been conducted to establish the optimal ash waste loading in the magnesium phosphate binder system. Based...

Phosphate ceramic process for macroencapsulation and stabilization of low-level debris wastes

Abstract Across the DOE complex, large quantities of contaminated debris and irradiated lead bricks have been accumulated for disposal. Under the US Environmental Protection Agency’s Alternative Treatment Standards, the preferred method of disposal of these wastes is macroencapsulation. Chemically bonded phosphate ceramic (CBPC) is a novel binder that was developed at Argonne National Laboratory to stabilize and solidify various low-level mixed wastes. This binder is extremely strong, dense, and impervious to water. In this investigation, CBPC has been used to demonstrate macroencapsulation of various contaminated debris wastes, including cryofractured debris, lead bricks, lead-lined plastic gloves, and mercury-contaminated crushed glass. This paper describes the fabrication of the waste forms, as well as the results of various characterizations performed on the waste forms. The results show that the simple and low-cost CBPC is an excellent material system for the macroencapsulation of debris wastes.

Chapter 2 – Chemically Bonded Phosphate Ceramics

Publisher Summary This chapter discusses phosphate-bonded ceramics, and cements and phosphate-bonded dental cements. The chapter also provides an overview of magnesium phosphate ceramics. Unlike silicophosphate cements, magnesium phosphates are highly crystalline, and hence they may be appropriately called room-temperature-setting ceramics rather than cements. Various magnesium phosphate-based ceramics have been developed for use in structural materials. These include magnesium ammonium phosphate ceramic grout for rapid repair of roads in cold regions, and for repair of industrial floors and airport runways; and magnesium potassium phosphate ceramics for stabilization and solidification of low-level radioactive and hazardous wastes. The chapter also explores the generalization of formation of chemically bonded phosphate ceramics (CBPCs). When a partially neutralized phosphoric acid solution is reacted with a metal oxide, a ceramic formed with a reaction product MxBy(PO4)(x+y)/3, where M stands for a metal, and B can be hydrogen (H) or another metal such as aluminum (Al). The phosphoric acid in these reactions is partially neutralized by dilution or by reaction of an oxide of B.

Experimental study on cesium immobilization in struvite structures

Ceramicrete, a chemically bonded phosphate ceramic, was developed for nuclear waste immobilization and nuclear radiation shielding. Ceramicrete products are fabricated by an acid-base reaction between magnesium oxide and mono potassium phosphate that has a struvite-K mineral structure. In this study, we demonstrate that this crystalline structure is ideal for incorporating radioactive Cs into a Ceramicrete matrix. This is accomplished by partially replacing K by Cs in the struvite-K structure, thus forming struvite-(K, Cs) mineral. X-ray diffraction and thermo-gravimetric analyses are used to confirm such a replacement. The resulting product is non-leachable and stable at high temperatures, and hence it is an ideal matrix for immobilizing Cs found in high-activity nuclear waste streams. The product can also be used for immobilizing secondary waste streams generated during glass vitrification of spent fuel, or the method described in this article can be used as a pretreatment method during glass vitrification of high level radioactive waste streams. Furthermore, it suggests a method of producing safe commercial radioactive Cs sources.

Stabilization and Solidification of Metal-Laden Wastes by Compaction and Magnesium Phosphate-Based Binder

ABSTRACT Bench-scale and full-scale investigations of waste stabilization and volume reduction were conducted using spiked soil and ash wastes containing heavy metals such as Cd, Cr, Pb, Ni, and Hg. The waste streams were stabilized and solidified using chemically bonded phosphate ceramic (CBPC) binder, and then compacted by either uniaxial or harmonic press for volume reduction. The physical properties of the final waste forms were determined by measuring volume reduction, density, porosity, and compressive strength. The leachability of heavy metals in the final waste forms was determined by a toxicity characteristic leaching procedure (TCLP) test and a 90-day immersion test (ANS 16.1). The structural composition and nature of waste forms were determined by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. CBPC binder and compaction can achieve 80-wt % waste loading and 39-47% reduction in waste volume. Compressive strength of final waste forms ranged from 1500 to 2000 psi. TCLP testing of waste forms showed that all heavy metals except Hg passed the TCLP limits using the phosphate-based binder. When Na2S was added to the binder, the waste forms also passed TCLP limits for Hg. Long-term leachability resistance of the final waste forms was achieved for all metals in both soil and ash wastes, and the leachability index was ~14. XRD patterns of waste forms indicated vermiculite in the ash waste was chemically incorporated into the CBPC matrix. SEM showed that waste forms are layered when compacted by uniaxial press and are homogeneous when compacted by harmonic press.

Chapter 9 – Magnesium Phosphate Ceramics

Publisher Summary This chapter discusses magnesium phosphate ceramics. Magnesium phosphate ceramics are the most developed and studied CBPCs and also have found several commercial applications. Mg phosphate-based materials are finding application as quick-setting cements for repair of roads, industrial floors, and runways, and for stabilization and solidification of low-level radioactive and hazardous waste streams. The chapter summarizes various magnesium phosphate ceramics that have been produced using either H3PO4 or an acid phosphate. These acid phosphates are produced by neutralizing H3PO4 partially with NH4, Al, or K ions. Because of the increased use of CBPCs during the last decade, some manufacturers are now producing these acid phosphates and supply them commercially. One common characteristic of these ceramics is that a significant amount of MgO is unreacted and left behind in the final product. Thus, one finds a large amount of MgO, and to a small extent, brucite [Mg(OH)2] in the ceramic. There are several other Mg-based phosphate ceramics or cements that use different acid phosphates or salts of magnesium. This chapter concludes that Magnesium titanates have also been used to form cements.

Chapter 1 – Introduction to Chemically Bonded Ceramics

Publisher Summary This chapter provides an introduction to chemically bonded ceramics and hydraulic cements that are the two major classes of inorganic solids. Ceramics are formed by compaction of powders and their subsequent fusion at high to very high temperatures, ranging anywhere from ∼700° to 2000°C. Once fused, the resulting ceramics are hard and dense, and exhibit very good corrosion resistance. Ceramics are made dense unless their application requires some degree of porosity. Ceramics tolerate very high temperatures, and are corrosion resistant in a wide range of pH, while cements are made for use at ambient temperatures and are affected by high temperature, as well as acidic environment. Compared to cements, ceramics are more expensive; thus, cement is produced in high volume while ceramics, except few products such as bricks, are specialty products. Hydraulic cements are another class of technologically important materials. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed this way set into a hard mass that has sufficient compression strength and can be used as structural materials. Their structure is generally noncrystalline. Hydraulic cements are excellent examples of accelerated chemical bonding. Hydrogen bonds are formed in these materials by chemical reaction when water is added to the powders. These bonds are distinct from the bonds in ceramics, in which high temperature interparticle diffusion leads to consolidation of powders.

Chapter 11 – Aluminum Phosphate Ceramics

Publisher Summary This chapter discusses aluminum phosphate ceramics. High alumina ceramics are preferred materials for a number of reasons. Their strength is valuable for high-load bearing applications, and they are resistant to corrosion in high temperature environments such as steam and CO atmospheres. Alumina ceramics are also well known for their low electrical and thermal conductivity. Therefore, they are the most useful materials in refractory bricks and electrical insulating components. Because of their technological importance, their low-temperature processing by chemical bonding has considerable technological significance. Alumina ceramics consist of particles, whose surfaces are coated with berlinite (AlPO4), a crystalline orthophosphate. It forms by boiling a mixture of alumina and phosphoric acid and then pressing the dried powder. When the mixture was boiled, some reaction of alumina must have occurred that produced an intermediate phase of AlH3(PO4)2·H2O, which transformed into a berlinite bonding phase upon curing. The chapter concludes that conversion of alumina needed to form a berlinite bonded alumina ceramic is very small, and therefore, berlinite bonding is possible even when the solubility of alumina is very small.

Chapter 14 – Chemically Bonded Phosphate Ceramic Matrix Composites

Publisher Summary This chapter deals with the chemically bonded phosphate ceramic matrix composites. Sintered ceramics have been in use as structural products since the beginning of human culture. Ceramics are also modern technological materials, especially in high temperature applications, and hence ceramic science is an active field of research even today. Sintering of these ceramics, however, is energy intensive and expensive, when large sizes are sintered. The alternative is chemical bonding. Moreover, ceramics exhibit superior mechanical properties compared to cement. Ceramics are far more stable in acidic and high temperature environments. The chapter provides information where various other waste streams can be incorporated in Ceramicrete to produce useful ceramic matrix composites. This chapter also discusses case studies on swarfs and red mud. The most common metal swarfs are iron-based and produced by the machine tool and automobile industries.