High Chrome Refractory Characterization: Part I. Impact of Melt Reduction/Oxidation on the Corrosion Mechanism

Abstract

High Cr 2 O 3 containing Monofrax TM K-3 is a robust refractory that is used in the fiberglass industry and used in radioactive waste glass melters worldwide. Monofrax TM K-3 is tolerant of transition metal oxides but contains highly reduced solid solutions of spinels, that is, (Mg,Fe 2+ )(Al,Cr) 2 O 3 . Conversely, many of the waste feeds being processed are highly oxidizing. The K-3 refractory corrosion was tested in sealed crucibles starting with slurried melter feed instead of prereacted glass called for by ASTM C621. Testing the refractory coupon during the feed-to-glass conversion exposes the refractory to the oxidizing and reducing species being released during vitrification, for example, NO 3 À , NO 2 À , CO 2 , CO, O 2 . Corrosion rates measured in highly oxidizing (high nitrate) feeds § were~1.8-2.8 times higher than those determined using prereacted glass or reduced feeds. ¶ Confirmatory corrosion rates were measured on Monofrax TM K-3 coupons immersed in oxidizing feed in a 1/100th-scale HLW pilot-scale melter. Corrosion is heterogeneous or incongruent as Ni and Fe in the waste glass exchange with Mg and Al in the refractory. An insoluble NiFe 2 O 4 spinel corrosion product is formed that can build up a protective layer along the refractory walls or spall and settle to the melter floor depending on melt pool convection/agitation.

High Chrome Refractory Characterization: Part II. Accumulation of Spinel Corrosion Deposits in Radioactive Waste Glass Melters

“…1 As determined by previous researchers, the Monofrax TM K-3 refractory surface becomes depleted in Al 2 O 3 , MgO, and Cr 2 O 3 leaving a porous outer corrosion layer that allows reaction with Fe 2 O 3 and NiO from the waste glass. 1 As determined by previous researchers, the Monofrax TM K-3 refractory surface becomes depleted in Al 2 O 3 , MgO, and Cr 2 O 3 leaving a porous outer corrosion layer that allows reaction with Fe 2 O 3 and NiO from the waste glass.…”

“…Currently the waste sludges are slurry-fed, also known as liquid-fed. 1,36 Micrographs of the K-3 refractory taken from the LSFM after 2 years of operation clearly showed a Ni-Fe-rich reaction layer. This layer was viscous and appeared to remain on the wall of the melter refractory protecting it from further corrosion.…”

“…Calcine feeding was abandoned in favor of wet slurry feeding which simultaneously controls dusting of radioactive and hazardous constituents which are inhalation hazards. 1 Additional Monofrax TM K-3 corrosion studies were recently conducted at the PNNL. 34 The existing data for slurry-fed melters are summarized in Table I and in Reference [35].…”

“…The LSFM processed only reduced DWPF formic acid flow sheet feeds which produced reduced glasses with SRNL131 glass and SRNL165 glass with an average waste composition. Since the DWPF was operating an oxidizing flow sheet for initial operations compared to the reduced flow sheets tested at the pilot scale, the following studies were performed: 1-8. see Part I 1 9. analyze the composition and phase identification of the DWPF bottom deposits 10. calculate spinel accumulation rates on the floor of the DWPF melter for oxidizing and reducing flow sheets from measured corrosion rates in crucible studies and pilot scale melters 11. verify the calculational method with the depth of the deposits rodded after 1.75 years of DWPF operation. 36 At the time the LSFM Monofrax TM K-3 refractory corrosion was being evaluated, the Carborundum Company identified the Monofrax TM K-3 corrosion mechanism as being a strong function of the nickel oxide and iron oxide in the glass, where Ni and Fe in the glass exchange for Mg and Al in the refractory forming Cr-Ni-Fe spinel as a reaction product.…”

“…1 The feeds that were vitrified in the SRS M-Area facility and in the WVNS HLW melter were also highly oxidizing. Currently, reduced feeds are being processed in the DWPF, but in 1994 during DWPF nonradioactive start-up, the feeds were very oxidizing.…”