Processing parameters and kinetics of phosphate-based dechlorination for electrorefiner salt waste

Abstract

Pyroprocessing generates a salt waste stream of fission products dissolved in alkali chloride salts. Iron phosphate (FePs) glasses have been identified as candidate materials due to the high waste loading, good chemical durability, and lower processing temperatures. Phosphate-based dechlorination has been used to produce intermediate phosphate products before the addition of iron to create the chemically durable final waste form. Using this method, mixing salt with a phosphoric acid and heating to T < 600˚C leads to the evolution of HCl(g) and H2O(g) with concurrent formation of an alkali metaphosphate glass. Dechlorination in air environments was achieved at temperatures as low as 300°C in air, but full dechlorination was achieved in both air and argon between 500-600°C.This work aimed to reduce the thermal demands of the phosphate-based dechlorination process by investigating the structural evolution of dechlorinated phosphate products at previously investigated terminal temperatures and evaluating the effect of processing parameters on dechlorination at 300°C. Studies were conducted to investigate the relationship between dechlorination, structural evolution, and reaction kinetics under varying heating rates in both air and argon atmospheres. Through Raman analysis, it was found that at T ≥ 300°C, in quasi-static conditions, metaphosphate Q1 and Q2 units are formed in both air and argon. Following these findings, materials were processed to 300˚C under low gas flow conditions (0.1-0.2 LPM) at heating rates of 1, 5, and 10°C min-1. Dechlorination behavior and structural evolution were characterized using ICP-MS and Raman analysis. Under these low flow conditions, a quasi-static environment was still present, with the removal of off-gassed byproducts limited by diffusion rather than forceful convection from the surface of the melt. Therefore, as dechlorination proceeded with increasing temperature, water vapor accumulated over the melt, creating a saturated local environment that suppressed reactions leading to dechlorination. In the argon atmosphere, the conditions were comparable to those in previous studies, resulting in similar residual Cl contents in the intermediate product (~6-9 wt% compared to ~6-7 wt% previously). However, samples processed in air experienced a more water-rich environment in the current study, leading to significantly higher residual Cl content than previous studies having ~0-2 wt%, compared to this study, ~8-9 wt%. Despite these quasi-static conditions, slower heating rates were found to enhance structural evolution in both air and argon atmospheres. Structural differences were observed in Raman spectra, with more bands associated with Q2 units prominent in samples processed at lower heating rates. However, the influence of the humid environment was apparent, as bands associated with H3PO4/Q0 units were present in all the samples. The humidity appeared to be greater in the air environment as Q2 units that were present in argon at all heating rates were only observed for air at the lower heating rates. A higher flow rate (2 LPM) was analyzed as the quasi-static conditions initially limited off-gas collection. Analysis was conducted by analyzing the overall conversion of Cl for both air and argon. It was determined that argon had a higher conversion of Cl than air and samples processed with 1°C min-1 heating rates had a higher overall conversion. Ea from 100-300°C for the removal of Cl was quantitatively analyzed using the KAS integral isoconversional method for samples processed in both air and argon, although further testing is necessary to further elucidate Ea. It was determined that between 100-200°C and 200-300°C the behavior of Ea for air and argon were dissimilar. For air samples, it was suggested that for 100-200°C O2 may decrease the energy barrier necessary to dechlorinate, but from 200-300°C Ea sharply increases, indicating that Ea may be affected by a change in reaction mechanism potentially caused by a change in surface mediated processes. However, for argon environments Ea appeared to be thermally driven as the energy barrier decreases as the heating profile was followed. Ultimately, reducing the thermal and atmospheric processing requirements could improve efficiency and advance the technological readiness of the FeP waste form.