Eect of melting temperature and time on iron valence and crystallization of iron phosphate glasses C.S. Ray * , X. Fang, M. Karabulut, G.K. Marasinghe, D.E. Day Department of Ceramic Engineering, Department of Physics and Graduate Center for Materials Research, University of Missouri-Rolla, Rolla, MO 65409, USA Received 12 March 1998; received in revised form 29 March 1999 Abstract The eect of melting temperature and time on iron valence, dissolution rate (DR) in deionized water, and crystal- lization of iron phosphate glasses was investigated using a 40Fe 2 O 3 ±60P 2 O 5 , mol%, batch composition. The concen- tration of Fe 2 ions in these glasses increased from 17% to 57% as melting temperature increased from 1150°C to 1450°C, but remained nearly constant at about 20% for melting times longer than 1 h at 1200°C. Measurements by dierential thermal analysis (DTA) combined with X-ray diraction (XRD) and thermogravimetric analysis (TGA) showed that these glasses crystallized to Fe 3 (P 2 O 7 ) 2 and Fe 4 (P 2 O 7 ) 3 when heated in nitrogen between 600°C and 820°C, but with continued heating in air at 820°C the Fe 3 (P 2 O 7 ) 2 changed to Fe(PO 4 ), which produced a weight gain in the sample associated with the oxidation of Fe 2 to Fe 3 ions. The DR (in deionized water) of these glasses was generally very low (10 9 g cm 2 min 1 ) and nearly independent of the relative concentration of Fe 2 or Fe 3 ions, but decreased with total iron content. Ó 1999 Elsevier Science B.V. All rights reserved. 1. Introduction Iron phosphate glasses are candidates [1±5] for immobilizing intermediate and high level nuclear wastes. Currently, the only glasses being used for such applications are the borosilicates [6±11]. However, borosilicate glasses may not be suitable [4,5] or cost eective for vitrifying certain types of high level nuclear wastes (HLW), particularly those containing signi®cant amounts of phos- phates, iron oxide, ¯uorine, and heavy metal ox- ides such as Bi 2 O 3 and UO 2 . These components generally have a low solubility in borosilicate glasses, which eectively limits waste loading to a low percentage such that the radioactive waste volume becomes too large [5]. There is also evidence [12] that P 2 O 5 causes liquid±liquid immiscibility (phase separation) in borosilicate melts. Phase separation in a glass can drastically reduce its chemical durability which is one of the properties of prime importance to whether a glass is suitable for vitrifying nuclear wastes. Besides avoiding the problem of phase separation, the waste loading can be increased considerably in iron phosphate glasses since they use the P 2 O 5 and Fe 2 O 3 contained in the waste. For a comparable waste loading, the volume of an iron phosphate wasteform would be only 25% of that for a borosilicate wasteform [5]. Furthermore, the amount of additional material required to produce the borosilicate wasteform such as SiO 2 ,B 2 O 3 , and alkalies, was 6 to 7 times more Journal of Non-Crystalline Solids 249 (1999) 1±16 www.elsevier.com/locate/jnoncrysol * Corresponding author. Tel.: +1-573 341 6432; fax: +1-573 341 2071; e-mail: csray@umr.edu 0022-3093/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 3 0 4 - X