DISSOLUTION OF SODA-LIME, SILICA, AND HIGH-LEVEL WASTE GLASS BY STATIC AND SINGLE-PASS FLOW-THROUGH TESTS

A. Jiricka A. Helebrant Department of Glass and Ceramics Department of Glass and Ceramics Institute of Chemical Technology Institute of Chemical Technology Technicka 5, 166 28 Prague Technicka 5, 166 28 Prague Czech Republic Czech Republic ABSTRACT

Static and single-pass flow-through tests (SPFT) were conducted on soda-lime (float and SK1) glasses, silica glass, and a borosilicate (high-level waste high-chromium) glass at 85 and 50°C (static tests) and 90, 85, 60, and 30°C (flow-through tests). Tests were performed at pH 10, 6, 5.5, 4 and 2; some static tests were unbuffered. Solution pH in unbuffered static tests on soda-lime glass changed rapidly during the first 24 h, while in SPFT tests, the pH change was not significant. Borosilicate glass dissolved minimally near pH 4 and more rapidly with increasing and decreasing pH. Borosilicate glass dissolved much more incongruently in the initial dissolution stage than soda-lime glasses, but less so with time. The activation energy of the final dissolution rate based on the normalized Si release was 47.4±4 kJ/mol for SK1 glass, 55.4±11 kJ/mol for the float glass, and 52.4±13 kJ/mol for the borosilicate glass. INTRODUCTION

When glass is exposed to water, the process of glass dissolution begins by forming a thin diffusion layer at the interface. This initial stage is controlled by two simultaneous processes1,2: 1) interdiffusion of moveable ions (M+, H3O+), and 2) dissolution of the glass matrix. After the initial rapid release of alkali ions, the rate that the diffusion layer forms equals the rate that the glass matrix dissolves, and pseudo-congruent dissolution occurs.3

This paper is a part of a high-level waste (HLW) high-chromium glass corrosion study. A HLW high-chromium experimental glass is compared with an industrial float glass, soda-lime glass (SK1), and silica glass. Static and single-pass flow-through tests (SPFT) were used to determine the effects of temperature, pH, and glass surface area to solution volume ratio (S/V) on the corrosion of these glasses. EXPERIMENTAL

Float1 glass and C162 high-chromium glass were obtained from the manufacturer. Soda-lime glass (SK1) was melted in a platinum crucible at 1450°C and subsequently annealed. Glass compositions are given in Table I.

1 Glavunion, Teplice 2 Nuclear Research Institute - Rez

Glasses were crushed and sieved. The fraction between 0.3 and 0.5 mm, corresponding to the specific surface 110 cm2/g, was used for testing.

Table I: Chemical Compositions of Tested Glasses (target, wt%)

SiO

2

Na 2

O

CaO

Al 2

O3

B2O

3

K2O

Li2O

SO3

Cr 2

O3

Fe2O

3

MgO

TiO

2

Float 72.5 13.7 8.1 1.0 - 0.3 - 0.25 - 0.07 3.9 - C16 53.15 8.0 - 4.5 14.4 3.84 3.15 - 2.85 4.5 0.36 5.25 SK1 75.15 15.5 9.35 - - - - - - - - -

Glass grains were cleaned ultrasonically several times to remove fine particles adhering on the

glass surface, dried at 140°C, and stored in a desiccator. A flow-through apparatus consisting of a corrosion solution supply, peristaltic pump, water bath, thermometer, ismaprene tubing, and sampling bottles was used for SPFT tests. The flow rate was kept at 1 mL/min, allowing corrosion solutions to be used without buffer. The temperature ranged from 30 to 90°C. Most of the tests were conducted in an 8-hour period.

An oven or water bath with a shaker was used for static tests at 85 and 50°C. Solutions with pH 2 and pH 10 were prepared by dropping nitric acid or sodium hydroxide into distilled water, while pH was continuously measured by a pH-meter. A corrosion solution with pH 4 was prepared by dissolving 10.2 g of potassium hydrogenphthalate in 1 L of distilled water. Distilled water with pH adjusted by 0.01 M solutions of sodium hydroxide and nitric acid was used as a corrosion solution for tests with pH 5.5 and 6. Polypropylene vessels tightened with a screw cap were used for static tests.

To eliminate possible effects of sodium ions added to corrosion solution during the pH adjustment, initial corrosion solutions were used to calibrate zero values for chemical analysis. Concentrations of Na+ and Ca2+ ions released into the solution were obtained by flame photometry. The concentration of SiO2 released into the solution was obtained by spectrophotometry using the yellow form of silicomolibdic acid.

RESULTS

Experimental data were plotted using i-th element concentration (ci), and other variables defined in Table II: i-th element total mass release (Qi), normalized i-th element mass release (NLi), and normalized i-th element dissolution rate (NRi).

Table II: Definition of Variables(a)

Qit NLi

t NRit

cFS

t Qit

it t∆ ∆+ − c

fVS

NLit

iit t+ − ∆ NL NL

tit

it t− −∆

∆

(a) F is the flow rate, t the time, and ∆t the time interval between two subsequent samples, and fi the i-th element mass fraction in glass.

The following semiempirical equation was used for the SPFT normalized dissolution rate:

−−+

−=

ii

iii

tW

tNR

ττβ exp1exp (1)

where βi is the i-th component initial dissolution rate, Wi is the i-th component final dissolution rate, and τi is i-th component time constant. Glass dissolution is congruent if Wi has the same value for each component.

An integration of Equation (1) yields

( ) tWt

WNL ii

iiii +

−−−=

τβτ exp1 (2)

0.00

0.05

0.10

0.15

0.20

0.25

0 2 4 6 8 10Time

NL i

[g

/m2 ]

αtg(α)= Wi

tg(γ)= βi

τi

γ

3τi

Figure 1: NLSiO2

, Float, pH 2, 1 mL/min, 85°C

Equation (2) is equivalent to the semiempirical equation used by Helebrant and Tošnerová2. The graphical representation of βi, Wi, and τi is displayed in Figure 1, including the transition time (time needed for conversion from the initial stage to the final stage of dissolution), estimated as 3τi. Experimental data were fitted by Equation (2) to obtain the coefficient values. An example of data fitting is given in Figure 2. Figure 3 compares experimental data plotted as NRi(t) and values v

calculated from Equation (1). Time dependence NRi(t) indicates also congruent dissolution, which occurs when the NRi(t) values for each element are equal. The coefficient values calculated from SPFT tests are listed in Table III.

0.0

0.2

0.4

0.6

0.8

0 2 4 6 8

Time [h]

NL i

[g/m

2 ]

SiO2

Ca2+

Na+

Figure 2: NLi(t), SK1 Glass, 85°C, pH 6,

1 mL/min

0.0

0.1

0.2

0.3

0.4

0 2 4 6 8

Time [h]

NR

i [g/

m2 /h

]

SiO2

Ca2+

Na+

Figure 3: NRi(t), SK1 Glass, 85°C, pH 6,

1 mL/min In static tests, component releases are affected by both glass dissolution and precipitation of

secondary corrosion products from the solution. Static test data cannot be fitted by Equation (2). Instead, the initial dissolution rate in static tests was calculated as a slope between zero and the NLi value of the first experimental data-point. The first data point was obtained at 24 h for all glasses. The final dissolution rate was calculated as an NRi average for the last three data-points. Coefficients calculated from the static tests and estimated transition times are given in Table IV.

Table III: Initial Dissolution Rates, Final Dissolution Rates, and Transition Times for SPFT Tests

Float Glass SK1 C16 C16 SILICA S/V 22 000 m-1 22000 m-1 22 000 m-1 31000 m-1 22 000 m-1 T[°C] pH 2 pH 6 pH 10 pH 6 pH 2 pH 4 pH 5.5 pH 5.5 pH 2 pH 6 pH 10 Initial Dissolution Rate - βi [g/m2/h]

90 0.040 0.010 0.045 0.039 85 0.132 0.108 0.086 0.153 0.012 0.008 0.039 60 0.023 0.044 0.003 Si 30 0.007 0.001 90 0.287 0.065 0.200 0.130 85 0.140 0.148 0.161 0.701 60 0.041 0.081 0.002 Na 30 0.026 0.001 85 0.096 0.062 0.073 0.177 60 0.053 0.088 Ca 30 0.014

Final Dissolution Rate - Wi [g/m2/h] 90 0.039 0.002 0.018 0.007 85 0.016 0.008 0.041 0.011 0.006 0.003 0.021 60 0.003 0.006 0.001 Si 30 0.001 0.0005 90 0.074 0.012 0.031 0.017 85 0.019 0.015 0.054 0.032 60 0.005 0.008 0.001 Na 30 0.002 0.0001 85 0.028 0.026 0.046 0.030 60 0.004 0.008 Ca 30 0.002

Transition Time 3τi [h] 90 3.600 1.932 2.496 1.605 85 1.935 2.499 1.926 1.927 1.527 60 2.376 3.060 1.608 Si 30 2.382 1.611 90 2.727 1.986 2.082 4.959 85 4.761 1.875 60 2.844 4.980 4.224 Na

30 2.382 2.190 85 4.029 2.499 3.177 3.75 60 2.265 4.010 Ca 30 2.961

DISCUSSION

Table III and IV can be used to compare the initial dissolution rate, final dissolution rate, and transition time for various conditions. Table V provides a general comparison of βi and Wi coefficients calculated from SPFT tests. In the initial dissolution stage, the SK1 glass is the fastest dissolving glass, followed by the float/C16 glass. In the final dissolution stage, the C16 glass shows the highest dissolution rates, indicating possible low durability in long-term corrosion tests.

Table IV: Initial Dissolution Rates, Final Dissolution Rates, and Transition Times for Static Tests

S/V [1/m] 300 300 300 300 300 300 300 T [°C] 85 °C 85 °C 85 °C 50 °C 50 °C 50 °C 85 °C

Duration 45 days 45 days 45 days 45 days 45 days 45 days 18 days

Si Na Ca Si Na Ca Si

Initial Dissolution Rate - [g/m2/h] - SK1 Silica pH 2 0.0100 0.0290 0.0180 0.0023 0.0090 0.0061 0.0010 pH10 0.0230 0.0380 0.0170 0.0017 0.0027 0.0023 0.0020

Final Dissolution Rate - [g/m2/h] - SK1 Silica pH 2 0.0002 0.0015 0.0005 0.0001 0.0002 0.0001 0.0001 pH10 0.0012 0.0046 0.0006 0.0002 0.0003 0.0003 0.0009

Transition Time [h] - SK1 Silica pH 2 360 360 456 168 360 360 144 pH10 312 408 312 168 360 168 144

Table V: General Comparison of SPFT Dissolution Rates

Initial Dissolution Rate (βi) Final Dissolution Rate (Wi) Si SK1>float>C16>Silica C16>SK1>float Na SK1>C16>float C16>SK1>float Ca SK1>float SK1>float

The ratio between the final and the initial dissolution rates equals approximately 10 for

float/SK1 glass, approximately 5 for C16 glass, and almost 2 for silica glass. The ratio between the initial dissolution rate of sodium and silica is highest for C16 glass, where it equals approximately 6, than for float glass, where it equals approximately 1.8, and SK1 glass, where it equals approximately 1.6.

This shows that the highest difference between the initial/final dissolution stage occurs for soda-lime glass and is smaller for borosilicate glass. The dissolution is more incongruent for borosilicate glass than for the soda-lime glass; however, the ratio between the initial dissolution rate of sodium and silica decreases strongly with time for the borosilicate glass (C16), but shows only a small difference for the soda-lime glass (float/SK1). In static tests, the initial and final dissolution rates are highest for Na and lowest for Si. The dissolution remained incongruent during the whole experimental run, just like in SPFT tests.

Unlike dissolution rates, the transition time does not show any obvious dependence on experimental conditions and does not seem to be affected by the element released. The average transition time is approximately 3 h for all glasses except silica, where it equals 2 h (3.36 h for SK1, 2.88 h for float, 2.67 h for C16, and 1.88 h for silica glass). In static tests, the transition time for SK1 glass ranges from 168 to 456 h and equals 144 h for silica glass. No correlation between transition time and experimental conditions was found.

The dissolution rate of glasses and minerals is significantly affected by the pH. The dissolution rate is minimal in a neutral pH range4,5,6. As Figure 4 shows, βSi and WSi for silica glass show an increase in initial and final dissolution rate at pH>6 and slightly decrease at pH<6. C16 glass shows a sharp minimum in both initial and final dissolution rates near pH 4. Surprisingly, the initial dissolution rate for float glass does not show a significant change with pH, and the final dissolution rate shows similar dependence on pH as the silica glass. Float/SK1 glasses show incongruent dissolution in both the initial and final dissolution stages. Ca release is

always higher than Si release and the NR(Na)/NR(Si) ratio does not change with temperature and pH.

0.001

0.01

0.1

1

0 2 4 6 8 10 12pH

β i, W

i [g

/m2 /h

]

Float- (Si)Float- (Na)C16- (Si)C16- (Na)Silica- (Si)Float-W(Si)Float-W(Na)C16-W(Si)C16-W(Na)Silica-W(Si)

β

β

β

β

β

Figure 4: βi, Wi=f(pH) for Float, C16, and Silica Glass

Some static and dynamic tests were conducted without buffered solutions in nearly neutral pH

to simulate corrosion behavior in conditions that are likely to occur in a real environment. Figure 5 shows nearly constant pH in unbuffered SPFT tests. Figure 6 shows the results from static unbuffered tests on SK1 model glass. These tests showed a rapid increase of pH during the first 24 hours causing an increasing dissolution rate that was equal to the dissolution rate at pH 10.

0.00

0.10

0.20

0.30

0.40

0.50

0 2 4 6 8Time [h]

NL(

SiO

2) [

g/m

2 ]

0.0

2.0

4.0

6.0

8.0

10.0

12.0

pHpH 2

pH 6

pH 10

Figure 5: NLSiO2

(t), Float Glass, 85 °C, pH 2, 6, 10, 1 mL/min

0.0

0.5

1.0

1.5

2.0

2.5

0 100 200 300 400 500

Time [h]

NL

(SiO

2) [

g/m

2 ]

0

2

4

6

8

10

12

pH

pH 10

pH 2

pH 6Unbuffered

Figure 6: NLSiO2(t), SK1, 85 °C, pH 2, 6, 10,

Static Test

The Arrhenius equation,

NR NRE

RTi iia=

−

0 exp (3)

where Ea is activation energy [kJ/mol], R is the gas constant [J/mol/K], and T is temperature

[K] was used to determine the activation energy of the dissolution rate. Activation energies for glass dissolution were determined by several authors, and show a dependence on glass composition and experimental conditions. G. Leturq7 found a characteristic activation energy of 60 ± 5 kJ/mol for several aluminosilicate glasses (approx. 50% SiO2, 14% Al2O3, 18% CaO), and

he suggests that the resulting activation energy is given by the breakdown of Si-O and/or Al-O bonds and depends on the nature of glass modifiers. Perrera and Doremus8 calculated Ea of the dissolution rate for several industrial glasses and concluded that the surface structure of glasses is the most important parameter determining the rate of dissolution. They found that soda-lime glasses dissolve faster because the open structure of the hydrated layers allows the water molecules easier access to the silicate network. Table VI provides a short overview of these results.

Table VI: Activation Energies Found in the Literature, Ea [kJ/mol]

Reference Material Solution T range [°C] Ea [kJ/mol] G. Leturq et al.7 Aluminosilicate glass deionized water 200-90 60 ± 5 Perera, Doremus8 Corning 015 soda-lime buffered pH 7 140-43 84 Pyrex borosilicate buffered pH 7 130-50 54 PPG float glass buffered pH 7 153-50 68 Bird et al.9 Quartz deionized water 155-121 51-55 Rimstidt, Barnes10 Silica deionized water 300-25 60.9-64.9 Perera, Doremus11 Pittsburgh Plate Glass pH 4 193-114 79 Pittsburgh Plate Glass pH 7 90-50 66 Pittsburgh Plate Glass pH 9 197-50 60

Activation energies, calculated from the final dissolution rate of silicon (WSi), are given in

Table VII. SK1 glass shows the lowest Ea, followed by C16 and float glass. Activation energies, calculated for three temperatures, are lower than values found in the literature, mainly because of values measured at 30°C.

Table VII: Calculated Activation Energies Ea [kJ/mol]

Material Solution T range [°C] Ea [kJ/mol] Ea* [kJ/mol]

SK1 glass pH 6 85-50 - 49.7 SK1 glass, static test pH 2 85-50 - 43.1 SK1 glass, static test pH 10 85-50 - 49.3 C16 glass (31000m-1) distilled water 90-30 39.6 65.2 float glass pH 2 85-30 44.5 66.4 * Ea calculated from two highest temperatures

Both the initial and final dissolution rates for Float and C16 glass depend much more on

temperature between 90/85 and 60°C than between 60 and 30°C. Because of this observation, Ea was also calculated from data points measured at the two highest temperatures. Resulting values are also given in Table VII. These values are higher and more consistent with experimental results found in the literature, which leads to the conclusion that more experimental points, at higher temperatures, are needed to correctly determine dissolution activation energy. CONCLUSIONS

SPFT and static tests were used to measure the dissolution rates of soda-lime (SK1 and float) glasses, silica glass, and a borosilicate (high-level waste high-chromium) glass.

Dissolution remained incongruent for all glasses during the entire 8-h SPFT test period and 1080-h static test. The degree of incongruence was highest for borosilicate glass (C16), but decreased faster with time than by the soda-lime glasses.

The initial and final dissolution rates for borosilicate glass showed a sharp minimum near pH 4 and increased with both increasing and decreasing pH. Surprisingly, pH had little effect on the dissolution rate of soda-lime glasses.

The activation energy of the final dissolution rate based on the normalized Si release was 47.4±4 kJ/mol for SK1 glass, 55.4±11 kJ/mol for the float glass, and 52.4±13 kJ/mol for the borosilicate glass. REFERENCES 1M. A. Rana and R. W. Douglass, “The reaction between glass and water. Part I: Experimental Methods and observations,” Phys. Chem. Glasses, 2, 196-205, (1961). 2A. Helebrant and B. Tošnerová, “A mathematical model used to compare glass durability tests under different flow conditions,” Glass technology, 30, 220-223, (1989). 3R. H. Doremus, “Chemical durability of glass”; pp. 41-69 in Treatise on Material Science and Technology, Academic Press Orlando, FL, 17, (1979). 4A. Helebrant and A. Jiricka: “Modeling of glass corrosion under dynamic conditions,” pp. 80-87 in Fundamentals of Glass Science and Technology, Edited by S. Persson, Glafo, Växjö, (1997). 5J. J. Mazer, J. V. Walther, “Dissolution kinetics of silica glass as a function of pH between 40 and 85°C,”Journal of Non-Crystalline Solids, 170, 32-45, (1994). 6R. K. Iler, “The occurrence, dissolution, and deposition of Silica,” pp. 42-43 in The chemistry of silica, A Wiley-Interscience Publication, New York, 42-43, (1979). 7G. Leturq, G. Berger, T. Advocat, C. Fillet, O. Halgand, and E. Vernaz, “Chemical durability of aluminosilicate glasses containing low solubility chemical elements,” Mat. Res. Soc. Symp. Proc., 506, 199-206, (1998). 8G. Perera and R. H. Doremus, “Dissolution of silicate glasses in water at pH 7,” J. Am. Ceram. Soc., 74, 1269-74, (1991). 9G. Bird, J. Boon, and T. Stone, “Silica transport during steam injection into oil sands,” Chem. Geol. 54, 69-80, (1986). 10J. D. Rimstidt and H. L. Barnes, “The kinetics of silica-water reactions,” Geochim Cosmochim Acta, 44, 1683-1699, (1980). 11G. Perera and R. H. Doremus, “Dissolution rates of commercial Soda-Lime and Pyrex Borosilicate Glasses: Influence of Solution pH,” J. Am. Ceram. Soc., 74, 1554-58, (1991).

ACKNOWLEDGMENT

This study was supported by the Grant Agency of the Czech Republic under the project numbers GA CR 106/960/0560 and GA CR 106/99/1419, which is a part of the research program: “Chemistry and Technology of Materials,” No. CEZ: J19/98:223100002.