The effects of melting reactions on laboratory-scale waste vitrification
- PDF / 1,978,254 Bytes
- 13 Pages / 576 x 792 pts Page_size
- 97 Downloads / 185 Views
At the U.S. Department of Energy's Hanford Site, processes are being developed to vitrify waste generated during nuclear materials processing. One of the wastes slated for vitrification is known as neutralized current acid waste (NCAW). The batch chemistry of simulated NCAW was varied with oxidants and reductants. Untreated, formated, nitrated, or sugar-added samples were combined with frit to produce melter feed. Offgas measurements of the formated melter feed showed that formates decomposed at temperatures too low for participation in melt redox reactions. Sugar pyrolyzed and produced CO and H2 at temperatures exceeding 665 °C. For the sugar-added samples, the glass quenched from 1200 °C produced an Fe 2 + /SFe of 0.79. The measured iron redox ratios from the glasses made from untreated, formated, and nitrated wastes were essentially indistinguishable (0.0024 at 1000 °C and 0.032 at 1200 °C). However, the batch chemistry affected volume expansion and the reaction paths.
I. INTRODUCTION Historically, formic acid has been used as a reductant for simulated neutralized current acid waste (NCAW) melter feed to reduce nitrates, nitrites, and higher valent transition metals in the glass melt. Reduction causes transition metals to release gas in the cold cap and after incorporation into the glass. If not reduced at low temperatures, transition metals will release a large volume of gas as equilibrium is approached in the melt; gas release may cause foaming.1 Foaming can slow the melting rate by insulating the cold cap from the melt, which can cause process interruptions. However, overreduced melts must be avoided to preclude the separation of sulfides and elemental metals. 23 The electrochemical series of redox couples was examined at 1150 °C for a prototypical waste glass by Schreiber and Hockman.4 Basically, the partial pressure of oxygen in the melt determines the oxidation reduction potential for the redox species in the glass. For most conditions, the partial pressure represented by a specific Fe 2+ /Fe 3+ ratio can be used to estimate the ratio of the other redox species present in the glass. Thus, determination of the ferrous/ferric ratio provides a measure of the overall glass redox state. The practical range for the ferrous/ferric ratio in high-level waste glass lies between 0.1 and O.5.2-3 Iron redox was correlated with foaming susceptibility by Goldman et al.5'6 More recent investigations have modeled simulated nuclear waste redox7"9 as a
a) Author
to whom all correspondence should be sent. J. Mater. Res., Vol. 10, No. 8, Aug 1995
http://journals.cambridge.org
Downloaded: 21 Apr 2015
function of batch chemistry.10 The purpose of this investigation is to determine the effects of melting reactions on nuclear waste vitrification. The batch chemistry of a waste simulant was varied with oxidants and reductants, and the decomposition gases, volume expansion and iron redox were measured. Redox, foaming, and temperature effects in simulated nuclear waste glass are briefly reviewed. A comparison of nuclear waste v
Data Loading...
