# Alternative reductants for foam control during vitrification of high-iron High Level Waste (HLW) feeds

J.C. Rigby, D.R. Dixon, J. Kloužek, R. Pokorný, P.B.J. Thompson, A.

Scrimshire, A.A. Kruger, A.M.T. Bell, P.A. Bingham

► To cite this version:

J.C. Rigby, D.R. Dixon, J. Kloužek, R. Pokorný, P.B.J. Thompson, et al. Alternative reductants for foam control during vitrification of high-iron High Level Waste (HLW) feeds. Journal of Non-Crystalline Solids, 2023, 608, pp.122240. 10.1016/j.jnoncrysol.2023.122240. hal-05094730

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# Alternative reductants for foam control during vitrification of high-iron High Level Waste (HLW) feeds

J.C. Rigby<sup>a, *</sup>, D.R. Dixon<sup>b</sup>, J. Kloužek<sup>c,d</sup>, R. Pokorný<sup>c,d</sup>, P.B.J. Thompson<sup>e</sup>, A. Scrimshire<sup>a</sup>, A.A. Kruger<sup>f</sup>, A.M.T. Bell<sup>a</sup>, P.A. Bingham<sup>a</sup>

## ARTICLE INFO

### Keywords:

- Cold cap

- Vitrification

- Redox

- Multivalent species

- Secondary foaming

## ABSTRACT

Foaming during vitrification of radioactive waste in Joule-Heated Ceramic Melters (JHCM) is exacerbated by trapping of evolving gases, such as CO₂, NOₓ and O₂, beneath a viscous reaction layer. Foaming restricts heat transfer during melting. Sucrose is employed as the baseline additive at the Hanford site in Washington State, USA to reduce foaming. Alternative carbon-based reductant additives were explored in simulated, inactive Hanford high-iron HLW-NG-Fe2 feeds, for both their effect on foaming and to give insight to the behaviour of multivalent species in glass melts under different redox conditions. Graphite, coke (93% C), formic acid and HEDTA additives were compared with sucrose, and a feed with no additive. Graphite and coke additions proved most effective in reducing the maximum foam volume by 51 ± 3% and 54 ± 2%, respectively, compared with 24 ± 5% for sucrose. Lower foaming could result in more efficient vitrification in JHCMs. Reductants also affected redox ratios in the multivalent species present in the feed. The order of reduction, Mn³⁺/Mn²⁺ > Cr⁶⁺/Cr³⁺ > Ce³⁺/Ce⁴⁺ > Fe³⁺/Fe²⁺ was as predicted on the basis of their redox potentials. There is less reduction overall, particularly in the Fe³⁺ → Fe²⁺, than predicted by the calculations, attributed to the oxygenated atmosphere of the experiments.

## 1. Introduction

The Hanford site in Washington State, USA, is home to ~200,000 m³ of radioactive legacy waste, historically stored in 177 carbon-steel tanks underground [1–3]. The Waste Treatment & Immobilization Plant (WTP) located at Hanford is currently under construction and commissioning. The tank waste will be partitioned into low-activity waste (LAW) and high-activity fractions which will then be vitrified in Joule-Heated Ceramic Melters (JHCM) [4–6] in separate LAW and high-level waste (HLW) vitrification facilities, respectively. The plan for the first phase of treatment is named Direct Feed LAW (DFLAW). Once LAW sustained operations are underway efforts will turn to HLW vitrification. The waste will be mixed with glass forming chemicals (GFCs) and fed to the melter. After feeding the melter, the liquid slurry lands on

top of the melt pool creating a feed reaction layer called the “cold cap” [7]. At the bottom of the cold cap, glass-forming melt produced in the later stages of the conversion process can trap gases that are evolving during the feed-to-glass reactions, causing foaming. Foaming can insulate the melt pool, reducing heat transfer to the reacting material [8–10] and thereby reducing the efficiency of the process. In extreme cases, foaming can pose a risk to melter components [11].

Feeds that are rich in nitrates generally cause excessive foaming, as the nitrates release gases in the 600–800 °C temperature range, the same range as when the viscous glass-forming melt is evolving [12,13]. The foam that forms in this temperature range is referred to as primary foam [4,5,14] and is caused by the evolution of NOₓ and CO₂ from batch nitrates, organics, and carbonates, as well as smaller contributions from O₂ and SO₂. At higher temperatures, the NOₓ and CO₂ evolutions are

<footnote><sup>a</sup> Materials and Engineering Research Institute, College of Business, Technology and Engineering, Sheffield Hallam University, City Campus, Howard Street, Sheffield S1 1WB, United Kingdom</footnote>

<footnote><sup>b</sup> Pacific Northwest National Laboratory, Richland, WA 99354, United States</footnote>

<footnote><sup>c</sup> University of Chemistry and Technology Prague, Technicka 5/1905, 166 28 Prague 6, Czechia</footnote>

<footnote><sup>d</sup> Institute of Rock Structure and Mechanics of the Czech Academy of Sciences, V Holesovickach 41, 182 09 Prague 8, Czechia</footnote>

<footnote><sup>e</sup> XMaS UK CRG Beamline, ESRF - The European Synchrotron, 71, avenue des Martyrs, 38000 Grenoble, France</footnote>

<footnote><sup>f</sup> U.S. Department of Energy, Office of River Protection, Richland, WA 99352, United States</footnote>

<footnote>* Corresponding author.
E-mail address: p.a.bingham@shu.ac.uk (J.C. Rigby).</footnote>
completed, while the contributions from O₂ and SO₂ increase, as these gases are usually released up to the melting temperature of the glass, ~1150 °C [2,4] provoked by redox reactions [12,13,15]. Foaming at higher temperatures, once primary foam has collapsed, is referred to as *secondary foaming* [5,8,9,12]. Compared to LAW feeds, secondary foaming is much more prevalent in vitrification of HLW streams that are rich in multivalent species such as Fe₂O₃, Cr₂O₃ and MnO₂ [12,15,16].

The high-iron inactive simulated feed, HLW-NG-Fe2, Table 1, developed by Vitreous State Laboratory of The Catholic University of America (VSL), Washington DC [11], is of particular interest for exploring the foaming behaviour of HLW feeds as it foams up to 10 times its original volume during heating [2,17]. This was examined by pellet expansion tests where a feed pellet was heated to 1150 °C at 10 °C min⁻¹ and photographs are taken at temperature intervals to track the volume expansion [6,7].

Sucrose is the baseline reductant additive at the Hanford site. This choice, for balancing the C/N ratios in feeds was selected based on experience at the defense Waste Processing Facility (DWPF), SC, West Valley Demonstration Project, NY and Hanford Low-Activity Waste feed research [12,18–20]. In the early evaluation of different reductants on the Hanford site waste glasses, particularly using carboxylic acids, the effect on redox behaviour of mixed multivalent species in feeds was not well understood [21]. In 1995, Seymour suggested that fine tuning of the interrelationship of the complex redox chemistry of the cold cap and melt pool should be pursued [21]. Previous studies relating directly to Hanford LAW compositions have explored the effects of sucrose and formic acid on the redox behaviour of waste glasses [5,19,22,23]. A study varying the carbonate and nitrate content of the high-iron HLW-NG-Fe2 feed concluded that the previous algorithms used to determine the amount of sucrose to add to high-nitrate LAW feeds needs to be modified for HLW feeds high in multivalent species such as Fe to avoid overreduction, and further testing is required to better define the relationship between sources of carbon [24]. Experiments studying coke and other reductant additions to industrial soda-lime-silica type glass batches to promote sulphate decomposition (thereby helping decrease foam generation) have also measured iron redox previously [25–27]. Two studies reported reduction in Fe and melt oxygen partial pressure in commercial glasses using graphite addition [27,28]. Beyond the suggestion of its use in 1995 [29], the effects of HEDTA on glass redox has not been explored in commercial glasses or waste glasses, despite it being present in some Hanford waste streams [30–33].

The reductants selected provide a wide range of carbon content values per gram of compound, Table 2 [29]. To our knowledge, little research has been published on reductant additions in Hanford HLW feeds, where, in particularly high-foaming feeds such as the HLW-NG-Fe2 feed [7], there is opportunity for increased processing

<caption>**Table 1**</caption>

HLW-NG-Fe2 composition developed by the Vitreous State Laboratory of The Catholic University of America (VSL) to immobilise the C-106/AY-102 high-iron tank waste, adapted from [11].

<table><caption>**Table 2**</caption><thead><tr><th>Raw Material</th><th>Batch g/100 g (glass)</th><th>Raw Material</th><th>Batch g/100 g (glass)</th></tr></thead><tbody><tr><td>Al(OH)<sub>3</sub></td><td>8.61</td><td>Ni(OH)<sub>2</sub></td><td>0.59</td></tr><tr><td>H<sub>3</sub>BO<sub>3</sub></td><td>0.56</td><td>FePO<sub>4</sub>·2H<sub>2</sub>O</td><td>1.71</td></tr><tr><td>Na<sub>2</sub>B<sub>4</sub>O<sub>7</sub>·10H<sub>2</sub>O</td><td>37.16</td><td>PbO</td><td>0.63</td></tr><tr><td>CaCO<sub>3</sub></td><td>0.94</td><td>Na<sub>2</sub>SiO<sub>3</sub></td><td>8.03</td></tr><tr><td>CeO<sub>2</sub></td><td>0.12</td><td>Na<sub>2</sub>SO<sub>4</sub></td><td>0.39</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub>·1.5H<sub>2</sub>O</td><td>0.30</td><td>SiO<sub>2</sub></td><td>37.33</td></tr><tr><td>Fe(OH)<sub>3</sub></td><td>20.54</td><td>SrCO<sub>3</sub></td><td>0.28</td></tr><tr><td>La(OH)<sub>3</sub></td><td>0.11</td><td>ZnO</td><td>0.03</td></tr><tr><td>Li<sub>2</sub>CO<sub>3</sub></td><td>3.87</td><td>Zr</td><td>1.57</td></tr><tr><td></td><td></td><td>(OH)<sub>4</sub>·0.654H<sub>2</sub>O</td><td></td></tr><tr><td>Mg(OH)<sub>2</sub></td><td>0.24</td><td>NaNO<sub>2</sub></td><td>0.01</td></tr><tr><td>MnO<sub>2</sub></td><td>3.98</td><td>NaNO<sub>3</sub></td><td>0.45</td></tr><tr><td>NaOH</td><td>0.81</td><td>H<sub>2</sub>C<sub>2</sub>O<sub>4</sub>·2H<sub>2</sub>O</td><td>0.06</td></tr><tr><td>Na<sub>2</sub>CO<sub>3</sub></td><td>4.04</td><td>Total</td><td>132.36</td></tr></tbody></table>

Reductants used for each of the feeds studied and the associated carbon g⁻¹ per compound [29].

<table><thead><tr><th>Feed Name</th><th>Reductant Carbon Source</th><th>Carbon content /g of reductant</th></tr></thead><tbody><tr><td>HLW-NG-Fe2-S</td><td>Sucrose (C<sub>12</sub>H<sub>22</sub>O<sub>11</sub>)</td><td>0.42</td></tr><tr><td>HLW-NG-Fe2-G</td><td>Graphite (C)</td><td>1.0</td></tr><tr><td>HLW-NG-Fe2-C</td><td>Coke Dust (93% C)</td><td>0.93</td></tr><tr><td>HLW-NG-Fe2-F</td><td>Formic Acid (CH<sub>2</sub>O<sub>2</sub>)</td><td>0.26</td></tr><tr><td>HLW-NG-Fe2-H</td><td>HEDTA* (C<sub>10</sub>H<sub>18</sub>N<sub>2</sub>O<sub>7</sub>)</td><td>0.43</td></tr></tbody></table>

efficiency. In addition, wider understanding of the effects of chemically different reductants in radioactive waste feed vitrification can also add to the pool of knowledge in the field. The aims of this study were thus to: (a) determine the effect of alternative reductants on the foaming behaviour of simulated HLW-NG-Fe2 feed; and (b) contribute to the pool of knowledge on the effects of alternative reductants in radioactive waste feed vitrification and glass melting. We also consider the behaviour of the multivalent species in the feed, comparing those to work by Schreiber [34–36] to understand the influence of different reductants on the redox behaviour of these species and the oxygen fugacity, fO₂, in the melt.

The coke used in the sample is ~93.3 wt% C according to x-ray fluorescence (XRF), with 3.1 wt% O, 1.1 wt% Si, 0.8 wt% Al, 0.6 wt% Fe, 0.4 wt% S, 0.3 wt% Ca, 0.1 wt% Mg and 0.1 wt% Ti, with trace amounts of P, Na, Cl, Ba, Sr, Mn, Cr, Ru and Ni. The mechanisms of foam expansion and collapse are determined by the quantity of gas evolution, the temperature of gas evolution and the viscosity of the melt at the temperatures of gas evolution [6,37,38]. The secondary foam, caused by SO₂ and O₂ evolution is less significant than the residual CO₂ contributing to the primary foam for most feeds. The HLW-NG-Fe2 feed has been described as exhibiting irregular foaming with multiple secondary peaks prior to foam collapse [6,7]. The HLW-NG-Fe2 feed has been shown to produce more oxygen than regular feeds, as a result of Fe³⁺ reducing to Fe²⁺ [7].

The order of reduction of multivalent species, M, in the same melt and temperature conditions has been derived by Schreiber and colleagues [35,39] and is correlated to assigned relative electron reduction potentials, E*<sub>M</sub> [39], given by:

$$-\log(fO_2) = \frac{4}{n}\log\left[\frac{M^{(m-n)+}}{M^{m+}}\right] + E_M^* \quad (1)$$

where n, is the number of transferred electrons in the redox reaction M(m-n)+ ↔ Mm⁺. The relative electron reduction potentials provide the order in which each multivalent species will reduce in a glass melt. For those in the HLW-NG-Fe2 glass, the order of Mn³⁺/Mn²⁺ > Cr⁶⁺/Cr³⁺ > Ce³⁺/Ce⁴⁺ > Fe³⁺/Fe²⁺ is expected based on the electromotive force series defined by Schreiber [39–44]. These electron reduction potentials have been used in many studies of glass melting to predict the redox behaviour, those of interest to this study are presented in Table 3 [36,41, 44,45]. The range of reductants explored in this study provide an

<caption>**Table 3**</caption>

Relative reduction potentials for redox states of key multivalent species in the HLW-NG-Fe2 feed for a borosilicate melt at 1150 °C [39].

<table><thead><tr><th>Element</th><th>Initial Oxidation State</th><th>Final Oxidation State</th><th>Reduction Potential</th></tr></thead><tbody><tr><td rowspan="2">Chromium</td><td>Cr<sup>3+</sup></td><td>Cr<sup>2+</sup></td><td>-1.38</td></tr><tr><td>Cr<sup>5+</sup></td><td>Cr<sup>3+</sup></td><td>-0.04</td></tr><tr><td rowspan="2">Cerium</td><td>Ce<sup>4+</sup></td><td>Ce<sup>3+</sup></td><td>-0.03</td></tr><tr><td>Fe<sup>2+</sup></td><td>Fe<sup>0</sup></td><td>-1.27</td></tr><tr><td rowspan="2">Iron</td><td>Fe<sup>3+</sup></td><td>Fe<sup>2+</sup></td><td>-0.68</td></tr><tr><td>Mn<sup>3+</sup></td><td>Mn<sup>2+</sup></td><td>0.33</td></tr><tr><td rowspan="2">Manganese</td><td>Ni<sup>2+</sup></td><td>Ni<sup>0</sup></td><td>-1.06</td></tr><tr><td>Ni<sup>3+</sup></td><td>Ni<sup>2+</sup></td><td>0.69</td></tr></tbody></table>

<footnote>* N'(-2 hydroxyethyl) ethylenediamine-NNN' triacetic acid.</footnote>
interesting case for exploring the redox behaviour of multiple multiva-
lent species in glasses melted under different redox conditions.

**2. Experimental methods**

2.1. Sample preparation

Feeds based on the composition in Table 1 were batched on a 2 d p.
scale with >99% purity raw materials. Chemicals were mixed with
deionised water with a target glass yield of 290 g L⁻¹ and stirred
continuously during batching. Reductants, Table 2, were added to the
feed slurries during mixing to ensure homogeneity. The feeds were then
dried at 105 °C for 24 h. Twenty grams of dried feeds were heated in a
refractory-lined induction furnace up to 600, 800, 1000 and 1150 °C at
10 °C min⁻¹ and quenched in air.

A preliminary study investigated 0, 0.5, 1, 2, 3, 4, 5 and 6 wt% of graphite additions to the HLW-NG-Fe2 feed, with graphite having the highest carbon content per gramme. Feeds were melted in a refractory lined induction furnace at 1150 °C. X-ray powder diffraction (XRD) was performed on a PANalytical X'Pert Pro-X-ray powder diffractometer, using Cu Kα X-rays (λ = 1.5406 Å) with a power setting of 40 mA / 40 kV. Samples were spun during data collection to improve the powder average. XRD scans were collected over the angular range 5 to 80 ° 2θ. From the positions of the Bragg reflections in the XRD scans crystalline phases were identified using the PANalytical HighScore Plus software and the International Centre for Diffraction Data (ICDD) Powder Diffraction File database [46]. The XRD patterns are given in Appendix A. A spinel phase was present in all samples. At 5 and 6 wt% graphite addition, a peak emerged ~45° 2θ which is close to the main peak in this angular range for metallic Fe (ICDD Powder Diffraction File ref. 6–696), at 44.674° 2θ, however, without further peaks to assign to the phase the presence of metallic iron cannot be confirmed. To avoid excessive crystallinity or precipitation of any metallic phases [19], 3 wt% additions of each reductant were chosen for this study.

## 2.2. Thermal analysis

For feed volume expansion tests (FETs), 13 mm diameter pellets of dried feed, pressed under 10 T for 3 min, were heated at 10 °C min⁻¹ to 1150 °C in an electric furnace with an observation port. Photographs were taken at ~100 °C intervals initially, and then at 20 °C intervals, once the pellets began to change shape, and analysed for the volume (assuming spherical symmetry) in Photoshop®. The volume was then normalised to the volume of the final glass, calculated using the pellet mass, density and loss on ignition. Averages of three runs were taken for each composition.

The release rate of evolved gases (EGA) was measured using a gas chromatograph and Agilent 6890/5973 N mass spectrometric detector. The equipment was connected to a silica tube inside a furnace containing each sample. The furnace heated the sample to 1150 °C at 10 °C min⁻¹ under He, flowing at 50 ml min⁻¹.

Thermogravimetric Analysis (TGA) was performed on a TA In-
struments SDT 650 DSC/TGA. Samples were heated in N₂, from 30 –
1150 °C and held at 1150 °C for 10 min. Processing of the spectra was
performed in the TRIOS® and Origin Pro® software packages.

2.3. X-ray fluorescence spectroscopy

For compositional analysis of the coke sample, the coke was mixed
thoroughly with cellulose binder at a ratio of 4:1 sample to binder. The
mixture was placed in an aluminium mould in the Retsch PP 40. The
pellet was pressed for 60 s hold time under 20 T. The pressed pellet was
then sent for XRF at Glass Technology services, Sheffield, UK. XRF was
performed on a Bruker S4 pioneer – 3 kW wavelength dispersive X-ray
fluorescence spectrometer with a rhodium X-ray tube.

2.4. X-ray diffraction and phase identification

X-ray diffraction was performed as described in Section 2.1, on the feed samples heated to 600, 800, 1000 and 1150 °C, and phase ID and refinement of XRD patterns carried out using the ICDD and Rietveld refinement functions in the HighScore Plus software [47,48]. Addition of 5 wt% Si to each ground sample was used to determine the amorphous content, and therefore the quantitative proportion of each phase,

2.5. X-ray absorption near-edge spectroscopy (XANES)

Samples of each of the feeds heat treated at 600, 800, 1000 and 1150 °C were analysed using X-ray Absorption Near-Edge Spectroscopy for the behaviour of Cr, Ce, and Mn during melting. Pressed pellets were prepared of powdered samples mixed with cellulose acetate at appropriate ratios to form sample 10 mm diameter, ~0.5 mm thick. K-edge spectra of Cr, and Mn were collected and the Ce L<sub>III</sub>-edge. The Ni K-edge was also collected but there was no observed change in the edge at the resolution attainable. It is assumed based on the lack of apparent change, and the reduction potential of Ni that the oxidation state does not change throughout melting [39]. All HLW-NG-Fe2, HLW-NG-Fe2-S and -G samples were measured on the BM28 XMaS beamline [49] at the European Synchrotron Radiation Facility (ESRF), France, and HLW-NG-Fe2-C, -F and -H samples were measured at the B18 XAS beamline [50] at the Diamond Light Source, UK. Both beamlines operated with a Si monochromator and an energy range greater than that required for all edges measured.

All spectra were collected in simultaneous fluorescence and trans-
mission mode. Only Mn was present in the samples in sufficient quan-
tities to obtain useful transmission data. Spectra were collected from
energies ~150 eV below the absorption edge and up to 250 eV beyond
the edge. Data intervals of 0.15 eV over the pre-edge features was
attainable at the BM28 beamline at ESRF and steps of 2 eV were used
before and after the edge. At the B18 beamline at Diamond, 0.3 eV steps
were used for the entire spectra. Up to 5 repeat measurements were
performed at the BM28 beamline at ESRF and up to 20 repeat mea-
surements were performed at the B18 beamline at Diamond, improving
signal to noise ratios. Reference materials for each likely oxidation state
were measured at each beamline, Cr₂O₃ (Cr³⁺), Na₂Cr₂O₇ (Cr⁶⁺), Mn-
foil, MnO (Mn²⁺), MnO₂ (Mn⁴⁺), Mn₂O₃ (Mn³⁺), CeO₂ (Ce⁴⁺),
Ce₂(C₂O₄)₃ (Ce³⁺). Spectra were processed with the Athena software
(0.9.26) [51] where scans were deglitched, normalised and merged. The
integrated self-absorption correction was applied to the fluorescence
spectra of the Mn K-edge. Measurements of redox states of multivalent
species were taken on quenched samples, under the assumption that no
changes to the redox states occurs during cooling. Studies show that
these measurements may vary by up to 20% when compared to
high-temperature measurements [43].

2.6. ⁵⁷Fe Mössbauer spectroscopy

For analysis of the redox state of the iron in the final glasses, samples
heated to 1150 °C for each of the feeds were powdered for room tem-
perature ⁵⁷Fe Mössbauer spectroscopy. The 14.4 keV γ-rays from decay
of a 25 mCi ⁵⁷Co source in a Rh matrix oscillated at constant accelera-
tion, were absorbed by the sample. ⁵⁷Fe Mössbauer spectra were
collected in the velocity range of ±12 mm s⁻¹ relative to α-Fe. Signal to
noise was improved with collection times of up to two weeks. The Recoil
software package was used to fit the data, and the assumption was made
that the recoil-free fraction ratio f(Fe³⁺/Fe²⁺) = 1.0 when obtaining
redox ratios from fitted component area ratios. Each of the spectra were
fitted with Lorentzian doublets and sextets for the paramagnetic and
magnetic contributions, respectively.
# 3. Results

## 3.1. Feed volume expansion testing

Feed expansion tests, illustrated in Fig. 1, show the range of foaming behaviours of the feeds with the 5 different reductants added at 3 wt%, compared with the original feed with no reductants. While adding sucrose has a reducing effect on the maximum foam volume, similar to that of adding HEDTA, graphite and coke have a significantly greater effect. The maximum normalised foam volume is linear with the carbon content per gram of reductant added, shown in Fig. 2. Table 4 shows the reduction in maximum foam volume, correlates reasonably well with the reductant equivalence per gram of compound.

## 3.2. Gas evolution

The following decomposition reactions for the reductants assume that oxygen is available such that all C evolves as CO₂ and all N as NO,

$$C_{12}H_{22}O_{11} (\text{Sucrose}) + 6O_2 \rightarrow 12CO_2 + 11H_2O \quad (2)$$

$$C (\text{Graphite}) + O_2 \rightarrow CO_2 \quad (3)$$

$$0.93C (\text{Coke}) + 0.93O_2 \rightarrow 0.93CO_2 \quad (4)$$

$$CH_2O_2 (\text{Formic Acid}) + 1/2 O_2 \rightarrow CO_2 + H_2O \quad (5)$$

$$C_{10}H_{18}N_2O_7 (\text{HEDTA}) + 12O_2 \rightarrow 10CO_2 + 9H_2O + 2NO \quad (6)$$

These equations are used in this section to compare the measured gases evolved by EGA with the expected gas evolution. The derivative of the measured mass changes during heating by TGA of sucrose, graphite, coke and HEDTA, in isolation, are given in the top graph in Fig. 3, providing the temperatures of gases evolved. No measurements were taken on pure formic acid due to potential damage to the equipment.

Most gas evolution, therefore, decomposition, of the sucrose and HEDTA samples occurs prior to 500 °C. Minimal gas evolution from coke or graphite was detected within the temperature range measured. The same experiment was performed on the HLW-NG-Fe2 feed with the added reductants, the bottom graph in Fig. 3. Gas evolution peaks in the range 200–600 °C are similar for the HLW-NG-Fe2 and HLW-NG-Fe2-G and C feeds. Variation in behaviour prior to 600 °C in the HLW-NG-Fe2-S, F and H feeds can be attributed to decomposition reactions, in Eqs. (2), (5) and (6). In the bottom graph in Fig. 3, the HLW-NG-Fe2-G feed has a mass loss event starting ~1050 °C, which absent in the above graph. This indicates that the graphite decomposes prior to 1150 °C in the glass melt, but does not decompose in this temperature range in air.

The early gas evolution peaks, <200 °C, given by TGA are likely

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Normalised volume of pellets during Feed Expansion Test with different reductant and raw material feeds.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Maximum normalised volume of pellets during Feed Expansion Test with carbon content per gram of reductant added.</figcaption></figure>

<table><caption>**Table 4**
Foaming behaviour of feeds during melting from FETs and TGA.</caption><thead><tr><th>Feed</th><th>C g<sup>-1</sup> reductant</th><th>Normalised Maximum Foam Volume</th><th>Temperature of foam onset (°C)</th><th>Temperature of foam collapse* (°C)</th></tr></thead><tbody><tr><td>HLW-NG-Fe2</td><td>-</td><td>10.0 ± 2.1</td><td>674 ± 10</td><td>774 ± 10</td></tr><tr><td>HLW-NG-Fe2-S</td><td>0.42</td><td>7.6 ± 1.7</td><td>674 ± 8</td><td>834 ± 9</td></tr><tr><td>HLW-NG-Fe2-G</td><td>1.0</td><td>4.9 ± 0.7</td><td>642 ± 2</td><td>762 ± 1</td></tr><tr><td>HLW-NG-Fe2-C</td><td>0.93</td><td>4.6 ± 0.3</td><td>680 ± 1</td><td>780 ± 1</td></tr><tr><td>HLW-NG-Fe2-F</td><td>0.26</td><td>9.4 ± 1.7</td><td>661 ± 2</td><td>781 ± 2</td></tr><tr><td>HLW-NG-Fe2-H</td><td>0.43</td><td>7.9 ± 0.7</td><td>680 ± 1</td><td>880 ± 1</td></tr></tbody></table>

physical and chemical H₂O evolution which not captured by EGA in Fig. 4. The 200–600 °C range covers both chemical dehydration and most of the CO₂ evolution, Fig. 4. By EGA, most of the CO₂ evolution for all feeds occurs in a large peak centred around 500 °C, which contributes only small mass loss events by TGA. The larger mass-loss peaks around 250 °C are therefore attributed to H₂O evolution. CO₂ evolution begins at lower temperatures for the HLW-NG-Fe2-S, -F and -H feeds, explaining some variance in the mass loss events in this temperature region by TGA, Fig. 3. The HLW-NG-Fe2-H feed has significant CO evolution in the 600–900 °C showing insufficient oxygen available to continue CO₂ evolution. CO₂ and CO evolution continue almost up to melt temperature at significant quantities in the HLW-NG-Fe2-G and C feeds. The evolution, or retention, of SO₂ is a consequence of the redox state of the melt and oxygen partial pressure [52].

Given the assumption made in Eqs. (2-6), that all C evolves as CO₂, we expect negative evolution of O₂ from the reductant added feeds. No O₂ is observed in EGA for these feeds, Fig. 4. Calculation of the maximum CO₂ evolved, under the assumption that all C in each

<footnote>* Foam collapse taken as T<sub>FB</sub> = T<sub>FM</sub>, temperature of foam collapse equal to temperature of foam maximum.</footnote>
<figure><img src="image_6.png" /><figcaption>Fig. 3. Derivative of the thermogravimetric analysis (TGA) of some of the reductants (t) and feeds with added reductants (bottom) between room temperature and 1150 °C.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 4. Evolved CO (top left), CO₂ (top right), O₂ (bottom left) and SO₂ (bottom right) of all feeds during heating to 1150 °C in 50 ml min⁻¹ He.</figcaption></figure>
reductant evolves as CO₂, is given in Table 5 using the stoichiometry of the raw materials used during batching, compared with the final glass oxides, assuming all C evolved as CO₂, e.g. Na₂CO₃ → Na₂O + CO₂. This is then compared with the evolved O₂, CO₂ and CO for each of the feeds measured by EGA, which highlights a negative O₂ balance, being the difference between the expected and measured, for all of the feeds.

Evolution of CO rather than CO₂, is a consequence of the negative O₂ balance, C + ½ O₂ → CO occurring rather than formation of CO₂, leads to a less negative oxygen balance. The final O₂ balance in Table 5, takes this calculation into account for evolved CO in each of the feeds. For the HLW-NG-Fe2 feed the quantity of C evolved as CO₂ is approximately as expected. For the HLW-NG-Fe2-S and -F there is a deficit in the total C evolved as either CO or CO₂ and the expected value added to the feed, this is even greater for the HLW-NG-Fe2-G and -C feeds. In these instances, it is possible that some C is retained in the final glass sample, unreacted. For -G and -C feeds this is supported by the analysis of decomposition of the reductants in Fig. 3. For the HLW-NG-Fe2-H feed, there is significantly more evolved CO and CO₂ than possible by the calculations of the amount of C added to the feed. All samples were created, and subject to EGA, under the same conditions, so the excess carbon remains unexplained, and potentially down to error in the data collection or air in-leakage in the system.

### 3.3. Phase evolution with temperature

Proportions of crystal phases forming during melting and the corresponding amorphous glass proportions, Fig. 5, were found by refining the XRD patterns to identified phases, comparing the intensities to the internal Si standard. The rate of dissolution of silica is similar amongst all of the feeds. The rate of Fe₂O₃ dissolution is slowest in the HLW-NG-Fe2 feed, all other feeds show less Fe₂O₃ at 800 and 1000 °C. By 1000 °C, no Fe₂O₃ remains in the HLW-NG-Fe2-C, -G and -H feeds. In HLW-NG-Fe2-S, there is some Fe₂O₃ remaining at 1150 °C. There is a spike in spinel phase in the HLW-NG-Fe2-C feed at 1000 °C.

For the Fe₃O₄-like spinel phases identified in each pattern the refined lattice parameters are given in Table 6. The refined lattice constants indicate potential deviation from stoichiometric Fe₃O₄, a = b = c of 8.394

<caption>**Table 5**</caption>

Expected and observed CO, CO₂ and O₂ evolution from calculations of gas balance.

<table><thead><tr><th>Feed</th><th>Calculated CO<sub>2</sub> mg kg<sup>-1</sup> glass</th><th>Calculated O<sub>2</sub> mg kg<sup>-1</sup> glass</th><th>CO<sub>2</sub> by EGA mg kg<sup>-1</sup> glass</th><th>CO by EGA mg kg<sup>-1</sup> glass</th><th>O<sub>2</sub> by EGA mg kg<sup>-1</sup> glass</th><th>O<sub>2</sub> balance mg kg<sup>-1</sup> glass</th></tr></thead><tbody><tr><td>HLW-NG-Fe2</td><td>45,400</td><td>772</td><td>45,300</td><td>0</td><td>6600</td><td>-5828</td></tr><tr><td>HLW-NG-Fe2-S</td><td>106,000</td><td>-66,500</td><td>76,900</td><td>1000</td><td>0</td><td>-66,100</td></tr><tr><td>HLW-NG-Fe2-G</td><td>189,900</td><td>-104,600</td><td>75,600</td><td>22,400</td><td>0</td><td>-96,500</td></tr><tr><td>HLW-NG-Fe2-C</td><td>179,800</td><td>-94,500</td><td>67,900</td><td>15,400</td><td>0</td><td>-88,900</td></tr><tr><td>HLW-NG-Fe2-F</td><td>82,800</td><td>-28,500</td><td>72,400</td><td>2900</td><td>0</td><td>-27,400</td></tr><tr><td>HLW-NG-Fe2-H</td><td>107,500</td><td>-76,900</td><td>86,600</td><td>52,700</td><td>0</td><td>-57,700</td></tr></tbody></table>

Å [53], likely a mixture of Fe, Mn, Ni and Cr bearing spinels. Spinel structures with a mixture of Mn, Cr and Ni have been detected in previous studies of the HLW-NG-Fe2 simulated glass composition [10,17].

### 3.4. ⁵⁷Fe Mössbauer spectroscopy

⁵⁷Fe Mössbauer spectroscopy was performed on each of the samples to analyse the redox state and coordination of iron. The ⁵⁷Fe Mössbauer spectra for HLW-NG-Fe2, HLW-NG-Fe2-S and -F are shown in Fig. 6. Doublets with a centre shift, CS (mm s⁻¹), greater than 0.90, and quadrupole splitting, QS (mm s⁻¹), greater than 2.0 were attributed to Fe²⁺ phases [54]. Fitted parameters of the magnetic phases were compared to literature for identification [55–58]. The results of the Lorentzian fits of the spectra are tabulated in Appendix C.

All ⁵⁷Fe Mössbauer spectra shown in Fig. 6 contain paramagnetic doublets with centre shift (CS) and quadrupole splitting (QS) values corresponding to Fe³⁺. At 600 °C and 800 °C only one paramagnetic phase is resolvable, however, there are consistently a minimum of 2 resolvable sites resolved at 1000 and 1150 °C. The two common sites between all three samples are tetrahedral paramagnetic Fe³⁺ sites with a CS between 0.25–0.27 mm s⁻¹ and QS of 0.75–0.79 mm s⁻¹ and 1.05–1.38 mm s⁻¹. The CS values of these sites are all within the range of tetrahedral Fe³⁺ [54] each sample has a site with relatively lower and higher QS, indicating a site where the electron cloud symmetry is distorted by local bonding, e.g. rather than Fe-O-Fe there are Fe-O-M bonds where M is an atom with a different electronegativity [59]. In the HLW-NG-Fe2-S sample at 1000 °C there is a ¹⁵Fe²⁺ phase contributing 2.4 ± 2% to the spectral area, which is only slightly above error. There is no Fe²⁺ phase detected in the 1150 °C sample.

Magnetically ordered dominant sextets at 600 and 800 °C belong to α-Fe₂O₃, with smaller contributions from Fe-bearing spinels with Fe³⁺ on the B site, and Fe, Mn or Ni likely on the A site [55]. The relative proportion of the spinel phases increases with decrease in the α-Fe₂O₃, in line with the phase ID by XRD in Section 3.3. At 1150 °C 24.8 ± 2% of the Fe in the HLW-NG-Fe2-S sample, and 14 ± 2% in the HLW-NG-Fe2-F sample, is in the α-Fe₂O₃ phase. There is a small amount of Fe₂O₃ detected in the HLW-NG-Fe2-S sample correlating with the XRD, Section 3.3.

⁵⁷Fe Mössbauer spectra for the HLW-NG-Fe2-G, -C and -H samples are grouped in Fig. 7. Fe²⁺ paramagnetic doublets emerge in all feeds with increasing heat treatment temperature. Samples quenched at 1150 °C show significant proportions of ¹⁵Fe²⁺, contributing 38.8 ± 2, 21.3 ± 2 and 11.8 ± 2% to the total spectral area for the HLW-NG-Fe2-G, -C and -H glasses, respectively. The reductants that had the greatest effect on foam reduction also had a reducing effect on the iron redox ratio in the paramagnetic phase, Fe³⁺/Feₜ, in the final glass, as shown in Fig. 8.

The transition from α-Fe₂O₃ dominating the magnetically ordered components to Fe₃O₄-like spinels [55], occurs by 800 °C in the HLW-NG-Fe2-G and -H feeds, earlier than all of the other feeds, and 1000 °C in the HLW-NG-Fe2-C feed. This order is in agreement with the XRD in Section 3.3. All of the magnetic sextets in the 1150 °C samples are attributed to magnetite, Fe₃O₄, or a similarly structured spinel phase.

The redox ratios in the 1150 °C samples were determined by the proportion of Fe in each of the two paramagnetic phases, assuming that the recoil-free fraction ratio f(Fe³⁺/Fe²⁺) = 1.0. This assumption may overestimate the Fe³⁺/Feₜ values [60]. The intermediate stages give indications of the temperatures at which the O₂ is evolved by these structural changes.

### 3.5. X-Ray absorption near edge spectroscopy

#### 3.5.1. Mn K-edge

Mn K-edge spectra were calibrated using the absorption edge position of the measured standards, Mn-foil, MnO, Mn₂O₃ and MnO₂ [61, 62], applying the shift in eV to the sample data collected. Qualitatively,
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Evolution of phases with temperature by Rietveld refinement of X-ray diffraction Patterns.</figcaption></figure>

<table><caption>**Table 6**
Refined lattice parameters for spinel phases identified in the final glasses.</caption><thead><tr><th>Feed</th><th>Phase wt% in glass</th><th>Phase ID</th><th>ICDD Ref</th><th>Lattice Constants (Å)</th></tr></thead><tbody><tr><td>HLW-NG-Fe2</td><td>1.7(04)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>01-076-1849</td><td>8.400(4)</td></tr><tr><td>HLW-NG-Fe2-S</td><td>3.52(2)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>01-088-0866</td><td>8.392(1)</td></tr><tr><td>HLW-NG-Fe2-G</td><td>3.49(2)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>04-009-4255</td><td>8.395(1)</td></tr><tr><td>HLW-NG-Fe2-C</td><td>4.15(3)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>00-019-0629</td><td>8.395(1)</td></tr><tr><td>HLW-NG-Fe2-F</td><td>1.98(2)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>04-008-4511</td><td>8.404(1)</td></tr><tr><td>HLW-NG-Fe2-H</td><td>3.70(3)</td><td>Fe<sub>3</sub>O<sub>4</sub></td><td>01-087-2334</td><td>8.398(1)</td></tr></tbody></table>

the absorption edge in Mn spectra shifts linearly with oxidation state, and this method has been used to obtain an estimate of Mn oxidation state in previous studies [7,61,63,64]. All Mn K-edge spectra are given in Appendix D. The first inflection point of each of the Mn K-edge spectra was plotted against a linear fit of the first inflection point of each of the standards and their known oxidation states, Fig. 9 (left) [7,65]. This fit was used to approximate an average Mn oxidation state for the stages of melting samples, Fig. 9 (right).

Temperature had the effect of reducing Mn⁴⁺ in all of the feeds, particularly above 800 °C. For HLW-NG-Fe2-G, -C and -H the main reduction occurred between 800 and 1000 °C. The HLW-NG-Fe2-S and -F feeds, similarly to the HLW-NG-Fe2 feed, reduced most significantly between 1000 and 1150 °C.

### 3.5.2. Cr K-edge and Ce L<sub>iii</sub> edge

Ce L<sub>iii</sub> and Cr K edges spectra were calibrated using measured standards, Cr₂O₃ and Na₂Cr₂O₇, for Cr [66–68] and CeO₂ and Ce₂(C₂O₄)₃ for Ce [69–71] and referenced literature values, applying the energy shift to each collected spectrum. All Cr K-edge and Ce L<sub>iii</sub>-edge spectra are given in Appendix E and F, respectively. A linear combination fitting (LCF) function was used to fit standard data [66–68] to each spectrum to determine the proportion of the element in each oxidation state, Fig. 10. Examples of LCF are given in Appendix G.

Some Cr oxidises from the Cr³⁺ raw material by 600 °C in the HLW-NG-Fe2 and HLW-NG-Fe2-S and -G feeds. Further oxidation in some feeds at 800 °C occurs, before reduction back to all Cr³⁺ by 1000 °C. Ce in all of the feeds reduces with temperature. As with Mn, they reduce in the order HLW-NG-Fe2-H, -G and -C, followed by -S by 1000 °C, and finally by HLW-NG-Fe2 and HLW-NG-Fe2-F at 1150 °C.

### 3.5.3. Summary of redox behaviour of multivalent species

The expected O₂ evolution due to the net change in redox from the feed to glass was calculated for each feed. Fig. 11 shows the expected O₂ evolution based on redox changes for each of the temperature stages measured, 600, 800, 1000 and 1150 °C, while Fig. 12 shows the total O₂ evolution by changes in redox throughout melting for each of the species measured. ⁵⁷Fe Mössbauer was used for iron redox, and XANES for chromium, manganese and cerium redox.

## 4. Discussion

### 4.1. Effects of reductants on foaming

The feed volume expansion results, Fig. 1, show the most effective additive for reduction of foam during melting. The order of reduction in maximum foam volume, was as follows,

Original < Formic Acid < HEDTA < Sucrose < Graphite < Coke

For HLW-NG-Fe2, HLW-NG-Fe2-S, -H, and -F, the order of maximum foam reduction correlates with the carbon content per gram of reductant added. Peak foam reduction with addition of carbon to the feed is expected [72]. At peak foam, coke was slightly more effective than graphite, however graphite had a greater effect on the foam reduction above peak foaming temperature. The mechanism of primary foam suppression is presently unclear. The primary foam peak is sensitive to both temperature and quantities of CO and CO₂ evolution, in the case of both coke and graphite additions, the EGA curves of these gases resembles the original HLW-NG-Fe2 feed. Both the viscosity of the melt, and particle size of reductant additions will influence also the kinetics of foaming and foam reduction [29,73,74], therefore further investigation of these reductants on the foam volume would be necessary for optimisation of the process.

Beyond the maximum primary foaming peak, there was considerable
<figure><img src="image_9.png" /><figcaption>Fig. 6. $^{57}$Fe Mössbauer spectra for the HLW-NG-Fe2, HLW-NG-Fe2-S and -F feeds heated to 600, 800, 1000 and 1150 °C.</figcaption></figure>

foaming up to melt temperature in the HLW-NG-Fe2, HLW-NG-Fe2-S, -F and -H feeds which was suppressed in the HLW-NG-Fe2-G and -C feeds to some extent. Both of these substances entirely suppress $O_2$ evolution, however the other reductants also have this effect. The main contributions to this secondary foaming, based on EGA, are residual CO and $CO_2$ trapped in the melt and released in small quantities with increasing temperature along with smaller contributions from $SO_2$ and $O_2$. The continuous release of $CO_2$ above maximum foaming temperatures appears to correlate with reduced secondary foam.

## 4.2. Redox changes during melting

The iron redox state is often used as the measure of redox in a glass batch, and to quantify the oxygen partial pressure [18,75,76], which is a reasonable approximation for waste streams, or industrial glass batches and melts that contain high levels of iron, and insignificant amounts of other multivalent oxides. In work by Goldman [77], even when there is no detected ferrous iron in the glasses, there is oxygen evolution associated with other redox species such as chromium, cerium and manganese. Other multivalent species, Mn, Cr, Ni and Ce, have been investigated for a range of properties in commercial glass melting [78–82] and in radioactive waste streams [3,83,84].

### 4.2.1. Iron redox

For the feeds studied, the rate of dissolution of $Fe_2O_3$ varied during melting. The slowest rate of dissolution of $Fe_2O_3$ is in the HLW-

NG-Fe2 feed, as predicted in the absence of additional reducing species [85]. The spinel structures are similar throughout the feeds, bearing most resemblance to magnetite, $Fe_3O_4$, $a = b = c = 8.394$, but are likely to also contain Mn, Cr and Ni, Table 5. Oxygen contributions of the spinel formation from haematite was considered negligible to the total balance of $O_2$ [12]. This evolution from $Fe_2O_3$ to $Fe_3O_4$-like spinel structure was also observed by $^{57}$Fe Mössbauer spectroscopy Section 3.4.

By $^{57}$Fe Mössbauer spectroscopy we observe a change in the redox state of iron in the paramagnetic phase in the HLW-NG-Fe2-S, -G, -C and -H samples. In the HLW-NG-Fe2-S sample the paramagnetic iron is reduced at 1000 °C and re-oxidises, in the -G, -C and -H samples there remains considerable $Fe^{2+}$ in the final glasses. A reduction to a minimum of $Fe^{3+}/Fe_T = 0.5$ was observed previously in the HLW-NG-Fe2 feed at temperatures of 850–1150 °C by XANES [7], there is no apparent reduction in the Fe by $^{57}$Fe Mössbauer spectrometry in the present study. While no studies have been found to report the iron redox ratio of Fe in glasses with coke additions, Ryan et al. [29] suggested that coke would have a greater affect than some reductants at higher temperatures, above the temperatures of glass-forming and foaming, and Kloužek et al. [52] reported an increase in refining efficiency in a commercial glass batch with coke addition of 0.02 wt% and a reducing effect on the glass.

Where the iron redox of feeds containing HEDTA has been measured previously, those feeds have also contained sucrose [31,86,87], therefore separating the effectiveness on reduction of multivalent species is not possible. In the present study 3 wt% HEDTA is shown to have a significant effect, reducing $Fe^{3+}/Fe_T$ to 0.83. The low-temperature
<figure><img src="image_10.png" /><figcaption>Fig. 7. $^{57}$Fe Mössbauer spectra for the HLW-NG-Fe2-G, -C and -H feeds heated to 600, 800, 1000 and 1150 °C.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Fig. 8. Maximum normalised foam volume with estimated iron redox state by $^{57}$Fe Mössbauer spectroscopy for the final glass samples.</figcaption></figure>

decomposition of sucrose and formic acid, by the TGA observed in Fig. 3, would suggest that the early elimination of nitrates, or decomposition of the raw material [29,88] precludes any efficient high-temperature reduction of multivalent species [89]. However, previous studies have

shown highly reduced glasses with varying sucrose additions [18,19,23] and ferrous iron content, up to $Fe^{3+}/Fe_{T} = 0$ [16] and 0.4 [90] for formic acid additions, to waste feed compositions.

From calculations of the $O_2$ contribution from a redox state change in Fe, by Eq. (7),

$$Fe_2O_3 \leftrightarrow 2FeO + \frac{1}{2}O_2, \qquad (7)$$

it has been determined that the HLW-NG-Fe2-G feed has the highest net $O_2$ contribution. HLW-NG-Fe2, HLW-NG-Fe2-S and -F feeds have no net change of redox state in the paramagnetic phase, and therefore no net $O_2$ contribution by this mechanism. These contributions to $O_2$ evolution begin to explain the highly negative $O_2$ balance for the reductant added feeds discussed in 3.2.

### 4.2.2. Manganese redox

In the present study, the manganese is approximately $Mn^{4+}$ at 600 °C in the HLW-NG-Fe2 feed, consistent with the previous results for which the $Mn^{4+}$ oxidation state was observed at 500 °C [7]. However, the same study of the feed quenched at 850 °C showed reduction to almost entirely $Mn^{2+}$, where the main reduction of Mn occurs between 1000 and 1150 °C in Fig. 9. Reduction of $Mn^{4+} \rightarrow Mn^{2+}$ follows, before any reduction of $Ce^{4+}$, at 800 °C, in the HLW-NG-Fe2, HLW-NG-Fe2-S and -H feeds. This is in agreement with the order predicted by the electromotive force series for the reduction potential of $Mn^{3+}/Mn^{2+}$ [39,91]. The $Mn^{4+}$ to $Mn^{2+}$ continues all the way to $Mn^{2+}$ in all feeds. Each of these reactions is preferential to the $Fe^{3+} \rightarrow Fe^{2+}$ reaction. Net contributions
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Average Mn oxidation states determined by as a linear function of the standards (left) and associated oxidation states for each of the samples at 600, 800, 1000 and 1150 °C (right).</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** Average Cr (left) and Ce (right) oxidation states determined by a LCF of the standards in different oxidation states for each of the samples at 600, 800, 1000 and 1150 °C.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 11.** Predicted evolved O₂ by redox reactions for each multivalent species in the feeds at 600, 800, 1000 and 1150 °C using XANES measurements for Mn, Cr and Ce and ⁵⁷Fe Mössbauer measurements for Fe.</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Predicted net evolved O₂ by redox reactions for each multivalent species in the feeds using XANES measurements for Mn, Cr and Ce and ⁵⁷Fe Mössbauer measurements for Fe.</figcaption></figure>

to O₂ evolution by reduction, or partial reduction of Mn⁴⁺ to Mn²⁺ were given by the following equation,

$$MnO_2 \leftrightarrow MnO + \frac{1}{2} O_2. \qquad (8)$$

### 4.2.3. Chromium redox

As predicted by Schreiber's work, discussed in Section 1, reduction of any oxidised Cr⁶⁺ to Cr³⁺ preceded the reduction of the other species. The only deviation from this behaviour is observed in the HLW-NG-Fe2-G feed, where there is a re-oxidation of ~7% to Cr⁶⁺ at 1150 °C, which is only slightly above the limit of uncertainty. Previous collection of the Cr K-edge in the HLW-NG-Fe2 feed was not reported along with Mn and Fe K-edges as there was too much noise in the data [7]. However, the study did observe a similar trend in the HLW-E-M09 simulated waste feed, mostly Cr³⁺ was observed at 500 °C, and then a large increase occurred at 700 °C to over 80% Cr⁶⁺, at higher temperatures, the Cr reduced again down to over 90% Cr³⁺ in the final glass [7].

For the HLW-NG-Fe2, HLW-NG-Fe2-S, -C, -H and -F feeds, since the oxidation state of Cr returns to the oxidation state of the raw material used during batching, there was no net contribution to O₂ evolution during melting, by the following equation,

$$2CrO_3 \leftrightarrow 3/2O_2 + Cr_2O_3 \qquad (9)$$

For the HLW-NG-Fe2-G feed there is a net negative contribution due to the reoxidation of some Cr³⁺ to Cr⁶⁺. At 1150 °C there is a small energetic barrier to overcome to oxidise Cr³⁺ to Cr⁶⁺ at -0.04, from the reduction potentials in Table 3. Oxidation may become energetically favourable for a short time, although Ce³⁺ would also be expected to oxidise to some Ce⁴⁺ preferentially, given the reduction potential of that reaction is -0.03. There is oxygen available in the melt and the atmosphere at these temperatures, evidenced by the continued evolution of CO, Fig. 4. It is stipulated here that with longer time at melt temperature the Cr⁶⁺ would reduce to entirely Cr³⁺ reaching equilibrium with the surrounding melt, although a longer dwell times would be needed to confirm this.

### 4.2.4. Cerium redox

Following the oxidation and reduction of Cr, the Ce⁴⁺ was most readily reduced to Ce³⁺ amongst all of the feeds. For all feeds, the proportion of reduced of Mn and Ce follows a very similar path with increasing temperature, particularly in the HLW-NG-Fe2-G and -C feeds. In the HLW-NG-Fe2 feed, the reduction to Ce³⁺ is incomplete by 1150 °C, where Mn⁴⁺ has fully reduced to Mn²⁺. Based on the reduction potentials and electromotive force series outlined in Section 1, we would expect the inverse to be true, completion of the Ce⁴⁺/Ce³⁺ reaction prior to completion of Mn⁴⁺/Mn²⁺ [36,91].

Ce was reduced significantly for HLW-NG-Fe2-G and -H feeds by 800 °C and further for all of the feeds by 1000 °C. Significant quantities of trivalent Ce³⁺ are often found in glasses made with CeO₂ because of its high reducibility [92–94], studies have shown that both increasing the melting temperature can increase the proportion of Ce³⁺ present in some glasses [81,93,94], as well as the choice of raw materials [93].

The contribution to O₂ evolution by a redox change in Ce is small given the relatively small amount of Ce oxide in the glass composition compared to Fe and Mn oxides, Table 1. The contribution was calculated using the following equation,

$$2CeO_2 \leftrightarrow Ce_2O_3 + \frac{1}{2} O_2 \qquad (10)$$

### 4.3. Oxygen fugacity

The apparent oxygen fugacity the glass melt, -log(fO₂), can be inferred using the final redox states of the multivalent elements by Schreiber's equation, in Section 1, and comparisons to literature [36,39, 95]. A relationship was derived by Schreiber and colleagues for imposed oxygen fugacity in a borosilicate melt (SRL-131) from the DWPF and resulting redox states of key species. This provided a useful comparison for estimated oxygen partial pressure in the complex borosilicate glass melt in this study [95]. Estimated -log(fO₂) in the glasses is provided in Table 7, using the final estimated redox states of all multivalent species, predominantly Fe as it has the most control over oxygen fugacity [7].

These values, while estimations, are in agreement with other studies of glasses bearing similar amounts of Fe₂O₃ [3,15,18,41,79]. The effect of reductants on the dissolved oxygen in the borosilicate glass melt, has not been explicitly studied, however it is expected that carbonaceous materials will reduce the partial pressure of oxygen in glass melts [97–99]. The reductants which had the most negative oxygen balance in Table 5, namely graphite, coke and HEDTA, also had the most negative effect on the apparent oxygen fugacity.

<caption>**Table 7**</caption>

Estimated -log(fO₂) at 1150 °C by Fe³⁺/Fe⁻ by ⁵⁷Fe Mössbauer spectroscopy, Cr K-edge, Ce L₃₄-edge and Mn K-edge XANES spectroscopy [95,96].

<table><thead><tr><th>Glass</th><th>% Reduced Fe (⁵⁷Fe Mössbauer)</th><th>% Reduced Cr (XANES)</th><th>% Reduced Ce (XANES)</th><th>% Reduced Mn (XANES)</th><th>Estimated-log(fO<sub>2</sub>) (log (kPa))</th></tr></thead><tbody><tr><td>HLW-NG-Fe2</td><td>0 ± 7</td><td>100.0 ± 0.3</td><td>90.5 ± 3.2</td><td>100 ± 12</td><td>2</td></tr><tr><td>HLW-NG-Fe2-S</td><td>0 ± 0.1</td><td>100.0 ± 0.1</td><td>100.0 ± 3.8</td><td>100 ± 16</td><td>2</td></tr><tr><td>HLW-NG-Fe2-G</td><td>53 ± 0.2</td><td>92.9 ± 1.1</td><td>100.0 ± 4.8</td><td>100 ± 16</td><td>8</td></tr><tr><td>HLW-NG-Fe2-C</td><td>35 ± 7</td><td>100.0 ± 0.3</td><td>100.0 ± 1.8</td><td>100 ± 17</td><td>7</td></tr><tr><td>HLW-NG-Fe2-F</td><td>0 ± 7</td><td>100.0 ± 0.2</td><td>100.0 ± 1.6</td><td>100 ± 14</td><td>2</td></tr><tr><td>HLW-NG-Fe2-H</td><td>17 ± 9</td><td>100.0 ± 0.8</td><td>100.0 ± 1.7</td><td>100 ± 13</td><td>5–6</td></tr></tbody></table>
## 4.4. Balance of gases

In the present study, O₂ was available during melting in the electric furnaces used for the heat treatments, and the EGA was performed in a He atmosphere, limiting the interpretation of the gas balance. Table 8 provides an estimated balance of O₂ based on the data available. As calculated in Table 5, the total O₂ expected includes the stoichiometric evolution of O₂ from the batched raw materials should all C evolve as CO₂, minus the equivalent O₂ saved by the proportion of C evolving as CO₂.

The O₂ evolution from the formation of spinel crystals from haematite is assumed negligible at this stage, as it is likely insignificant at small quantities of spinel [12]. The theoretical O₂ evolution from net changes in the redox states of the multivalent species, Fig. 11, is given in Table 8, to provide a total expected O₂ evolution. The value of expected O₂ evolution is negative for all reductant added feeds, so it is unsurprising that little to no oxygen evolves, and the apparent oxygen fugacity is more negative for the HLW-NG-Fe2-G, -C and -H feeds. More reduced species would be expected based on the oxygen deficit for the HLW-NG-Fe2-H. Other explanations for the negative O₂ balance include consumption from the atmosphere, which is a consideration particularly necessary for this study where the heat treatments and EGA were performed in different atmospheres, and the potential for remaining unreacted reductants in the final glass samples. The latter is supported by the TGA of the reductants and gas balance in Table 5, particularly for the HLW-NG-Fe2-G and C feeds, however there is no evidence of remaining C in the glass samples was by XRD in Section 3.3 or by XRF of the final glasses given in the supplemental material section H.

Based on net redox change from raw material to final glasses, Fig. 12, Mn reduction, in theory, contributes significantly more to O₂ evolution for all feeds, apart from the -G and -C feeds where Fe was heavily reduced. Fe is often most abundant multivalent species, however, the other species such as Mn, where they are in significant quantities, should not be overlooked when considering contributions to foaming. While many estimations were made in the process of calculations due to the complexity of the system, the behaviour of the multivalent species was not well-predicted using the current models and previous work [7,12,15, 44]. For example, we would expect a larger reduction in Fe, given the lack of oxygen availability, which was not supported by ⁵⁷Fe Mössbauer spectroscopy. However, according to ⁵⁷Fe Mössbauer spectroscopy, all but HLW-NG-Fe2-G and C are within the upper limit for Fe²⁺ content in Hanford waste glasses, Fe²⁺/Feₜ < 0.33 to prevent precipitation of metallic species [84,100].

## 5. Conclusions

Reductant additions to the HLW-NG-Fe2 feed at 3 wt% of graphite and coke (93% C) were most effective in reducing the maximum foam volume by 51 ± 3% in and 54 ± 2%, respectively. Graphite reduced the total foam over the temperature range most significantly, showing no secondary foam behaviour. This high-temperature behaviour differentiates graphite as a reductant from those used previously. Graphite and coke both had the most reducing effect on the Fe redox state, while HEDTA also reduced the Fe considerably with little effect on the foam volume. A change of the baseline reductant used at the Hanford site from sucrose to graphite or coke, for feeds similar to the HLW-NG-Fe2, could provide considerable improvements to the melting rate. In terms of processability, precipitation of spinels with the addition of reductants would be a limiting factor. However, this study shows a very similar level of spinel precipitation in the final glasses for coke and the baseline, sucrose, while coke has a much greater effect on foam reduction. Further optimisation of particle size and varying amounts of the reductants would be required prior to adoption of these methods.

The redox states of the multivalent species Fe, Mn, Cr and Ce, reduced according to the reduction potentials of each species and the reducing atmosphere in the melt by the additions of carbon-based raw

<table><caption>**Table 8**
Expected and observed O₂ evolution based on calculations of gas balance and O₂ evolution from redox reactions, compared with observed O₂ evolution by EGA.</caption><thead><tr><th>Sample</th><th>Expected O<sub>2</sub><br>evolution from all<br>batch gasses<sup>*</sup><br>mg kg<sup>-1</sup> glass</th><th>O<sub>2</sub> from<br>Redox<sup>**</sup><br>mg kg<sup>-1</sup> glass</th><th>Total Expected<br>O<sub>2</sub> Evolution<br>mg kg<sup>-1</sup> glass</th><th>O<sub>2</sub> evolved<br>by EGA<br>mg kg<sup>-1</sup> glass</th></tr></thead><tbody><tr><td>HLW-NG-Fe2</td><td>800</td><td>8000 ± 900</td><td>8800 ± 900</td><td>6600</td></tr><tr><td>HLW-NG-Fe2-S</td><td>-66,100</td><td>9500 ± 400</td><td>-56,600 ± 400</td><td>0</td></tr><tr><td>HLW-NG-Fe2-G</td><td>-96,500</td><td>18,200 ± 500</td><td>-78,300 ± 500</td><td>0</td></tr><tr><td>HLW-NG-Fe2-C</td><td>-88,900</td><td>14,100 ± 700</td><td>-74,800 ± 700</td><td>0</td></tr><tr><td>HLW-NG-Fe2-F</td><td>-27,400</td><td>8900 ± 700</td><td>-18,500 ± 700</td><td>0</td></tr><tr><td>HLW-NG-Fe2-H</td><td>-57,700</td><td>11,200 ± 900</td><td>-46,500 ± 900</td><td>0</td></tr></tbody></table>

materials. The overall contributions to O₂ evolution of each of the redox changes was devised as well as the total O₂ evolution from the feed raw materials, giving a negative O₂ balance for all reductant-added feeds, and leading to no O₂ evolution by EGA. The analysis techniques used for these calculations were performed in different atmospheres, therefore interpretation of the oxygen balance is qualitative. Evaluation of the redox states of multivalent species, and the inferred melt oxygen fugacity in the complex borosilicate glass with additions of a variety of reductant sources has not been studied previously. The calculations made of the O₂ balance, in particular, do not predict the reduction behaviour of the multivalent species in the melt. The general order of reducing species is predicted based on their reduction potentials, Mn³⁺/Mn²⁺ > Cr⁶⁺/Cr³⁺ > Ce³⁺/Ce⁴⁺ > Fe³⁺/Fe²⁺, however the final oxidation states, given the calculations of O₂ balance are not well predicted using the simple electromagnetic force series outlined in the introduction, with a variance of 7% in the expected reduction of Cr⁶⁺/Cr³⁺. Further models have been developed since, to particularly address redox reactions within the cold cap for US waste treatment programs (Hanford and Savannah River sites) [84]. The results of this work aim to provide further data to improve these model predictions for wastes high in multivalent species.

### CRediT authorship contribution statement

J.C. Rigby: Conceptualization, Investigation, Writing – original draft, Visualization, Project administration. D.R. Dixon: Conceptualization, Methodology, Investigation. J. Kloužek: Formal analysis, Investigation. R. Pokorný: Conceptualization, Methodology, Writing – review & editing. P.B.J. Thompson: Formal analysis, Investigation. A. Scrimshire: Formal analysis, Investigation. A.A. Kruger: Supervision, Funding acquisition, Writing – review & editing. A.M.T. Bell: Supervision, Writing – review & editing. P.A. Bingham: Supervision, Writing – review & editing, Funding acquisition.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data availability

Data will be made available on request.

<footnote>* including CO₂ → CO + ½ O₂.</footnote>

<footnote>** with XANES for Cr, Mn and Ce and ⁵⁷Fe Mössbauer for Fe.</footnote>
## Acknowledgements

The authors acknowledge with thanks funding support from both the US Department of Energy Office of River Protection and from Sheffield Hallam University. This research used resources at the B18 Core XAS Beamline at Diamond Light Source, UK. The authors gratefully acknowledge Veronica Celorrio, Giannantonio Cibin and all of the staff at the beamline for their support setting up the experiment. This research also used resources at the XMAs UK CRG Beamline, at the European Synchrotron Radiation Facility, Grenoble, France. The authors gratefully acknowledge the support from Laurence Bouchenoire and all of the staff at the beamline for their support setting up and running the experiment.

## Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnoncrysol.2023.122240.

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