Kinetics of Mixed Low-Level Waste Incapsulation Using Iron Phosphate Chemically Bonded Cement
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Kinetics of Mixed Low-Level Waste Incapsulation Using Iron Phosphate Chemically Bonded Cement A.S. Aloy, T.I. Koltsova, E.N. Kovarskaya, M.Yu. Silin. RPA “V.G. Khlopin Radium Institute”, St. Petersburg, RF. ABSTRACT The iron phosphate cement (IPC) structure formation process has been studied by Mossbauer spectroscopy. The IPC is formed as a result of interaction of iron oxides with orthophosphoric acid and could be used as a matrix for immobilization of low-level radioactive waste. The structural formation process has been shown may be considered to consist of two stages. The first stage lasts to the moment of setting, when the diffusion process goes on in the liquid phase. At this stage the main fragments of the structural polymeric frame of the IPC are developed consisting of iron (+2) and (+3) phosphates. The rate of the oxides-with-phosphoricacid reaction as well as the time of hardening depends on the bivalent iron content. The second stage begins from the moment of setting when the diffusion becomes slower. At this stage the process is characterized by the negligible increase in the iron phosphates (+2), (+3) content and transformation of the previously formed phosphates. The nuclear gamma-resonance (NGR) parameters have been determined of FeH3(PO4)2⋅2.5H2O, forming in the hematite (Fe2O3) based IPC: isomeric shift (IS) = 0.46 mm/s, quadrupole splitting (QS) = 0.197 mm/s, FWHM =0.282 mm/s. INTRODUCTION The new method of low-level waste solidification in the form of iron-phosphate cement employs iron-phosphate binders, developing in the reaction of orthophosphoric acid with iron oxides at room temperature [1-4]. The properties of phosphate binders forming the cement matrix are governed by the properties and quantities of reaction products of iron oxides with phosphoric acid. The structure developing of the new iron orthophosphate formations is characterized by high mechanical strength. Variation in (+2) and (+3) iron oxides content in FexOy initial mixtures provides a way to control the rate of chemical reactions, optimize the time of mixing and therefore realize the optimal cement hardening conditions. The ferriphosphate cement obtained by tempering powdered Fe2O3 with phosphoric acid is known to harden at room temperature slowly [5]. The samples develop sufficient strength only in 10 days. The setting rate of the cement depends on the proportion of target constituents and phosphoric acid concentration. The lower is the acid concentration, the less the strength of the cement and the faster it sets. The final products hardened at 200С are X-ray amorphous, i.e. crystallite size in less than 50 Å. At mole ratio Fe2O3/P2O5 ≥ 1 the final products are intermediate orthophosphates. The ferrophosphate cement is obtained by reaction of FeO with phosphoric acid [5]. Ferrous oxide (+2) reacts with phosphoric acid actively with intense heat release. The main product of the reaction is bivalent iron phosphate. A lot of studies have been devoted to the kinetics of phosphoric acid reaction with iron oxides of different valen
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