WSRC-RP-2001-00806
Results of Tank 30H Sample Analysis after CARGOâ Addition
W. R. Wilmarth, J. T. Mills, V. H. Dukes, and R. C. Sullivan
Westinghouse Savannah River Company
Aiken, SC 29808
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Keywords Aluminosilicate, Silicon, 3H Evaporator
Background
Due to cracks in the cooling coils in the 3H Evaporator Drop Tank (Tank 30H) and thus the inability to cool Tank 30H, the evaporator was not running efficiently due to reaching the high temperature limit. The Materials Technology Section of SRTC identified a product to flow through the cooling coils that would seal the cracks. This sealant material was Cargoâ Brand Metallic Seal-Upâ . This sealant contains cellulose fiber in a caustic solution. The sealant also contains significant amounts of silicon and is 5 M in sodium silicate. As a result of the silicon concentrations, SRTC performed tests to determine if aluminosilicates would form when the sealant was heated with simulated waste solutions. Indeed, the sealant was shown to readily react and produce aluminosilicate.
Therefore, Ketusky examined the effect of the addition of several gallons of the sealant would have on the 3H Evaporator system. The evaluation utilized criteria established for Tank 40H Decants. These criteria center on not altering either the soluble silicon concentration or the model amount of aluminosilicate solids formed during evaporation from the historical maximum running average for the evaporator system. Ketusky’s conclusions showed minimal effects of the 3H Evaporator chemistry. This report describes the analytical results from two samples taken after some 3H operational time following successful deployment of the sealant.
Experimental
SRTC has received two samples from Tank 30H. The samples were placed into the shielded cells facility at SRTC and filtered using protocols established for Tank 40H samples., Aqueous samples were submitted for analysis by Inductively Coupled Plasma – Emission Spectroscopy (ICP-ES). Due to the high hydroxide level of the samples solids could not be collected because of filter failure. Therefore, the silicon level reported is soluble silicon concentration not the total silicon concentration. The total silicon concentration is probably slightly higher.
Results and Discussion
Table 1 lists some of the analytical data obtained from measurements of the samples received at SRTC on June 21, 2001 and were processed in the shielded cell on June 22, 2001. The two samples labeled 2H-433 and 2H-435 had densities of 1.43 g/mL and 1.41 g/ml, respectively. The samples were a dip sample and a variable depth sample taken at 120" from the bottom of the tank. The aluminum concentration of the 2H-433 sample was 1.21 M and the sodium concentration was 12.1 M. Both of these concentrations for 2H-433 were higher than 2H-435. Therefore, SRTC assumed that the 2H-433 sample was the dip sample.
Table 1. Analytical Results of Tank 30H Samples
|
2H-433 |
2H-435 |
||
|
Density |
g/mL |
1.43 |
1.41 |
|
OH |
M |
6.3 |
6.6 |
|
Na |
M |
12.1 |
11.1 |
|
Al |
M |
1.21 |
1.1 |
There was difficulty with the silicon analysis protocol. The method was established for low hydroxide wastes such as sludge washing decants. The method suffers at very high hydroxide levels such as the levels measured for these samples. The cellulose nitrate filters used in the method are attacked by the high caustic concentrations. Generally, the liquid phase will pass through the filter. However, when one attempts to handle the filter its integrity fails. In these analyses, the filters were not recoverable. Therefore, the total silicon concentrations are probably slightly higher than the liquid phase concentration. Table 2 contains the measured silicon concentrations (mg/L) for the as-received sample and the filtrates from 0.45 m m and 0.02 m m filtration steps. The silicon concentrations in sample 2H-435 were slightly higher than 25 mg/L with the exception of the filtrate from the 0.02 m m filter. The technician reported filter breakthrough; therefore, the average was calculated without that analysis. For sample 2H-435, the silicon concentrations for all samples were between 25.6 mg/L and 25.8 mg/L.
The historical maximum silicon concentration for the 3H Evaporator system is 22.75 mg/L. Ketusky estimated the resulting silicon concentration would be 24.75 mg/L. This estimate agrees with the sample results for 2H-433 but is significantly lower than the results from the sample 2H-435. The silicon concentration of 35.1 mg/L is well above the silicon concentration criterion for 3H Evaporator operation. If this sample (2H-435) is the dip sample, then it appears the silicon from the discharge of the sealant flow-through loop has not mixed with the contents of the tank. If this sample was taken from the 120" level, then this level of silison is potentially being fed to the evaporator.
Table 2. Silicon Analysis of Tank 30H Samples
|
2H-435 Filtrate (mg/L) |
2H-433 Filtrate (mg/L) |
|
|
As-Received |
25.6 |
32.9 |
|
0.2 |
25.8 |
31.6 |
|
0.02 |
72.8 |
40.8 |
|
Average |
25.7 |
35.1 |
Likewise, Ketusky estimated the amount of aluminosilicate solids that would form when processing one liter of waste with a 3X evaporation. We had developed a simple computer spreadsheet to model this evaporation. The program calculates the Ksp according to Equation 1 and the soluble silicon according to Equation 2. The soluble silicon assumes that silicon is the limiting reagent and does not significantly affect the aluminum concentration.
Equation 1 Ksp = 1.5 x 10-4 M2 * [ OH ]3/I1.6667
Equation 2 Soluble [ Si ] = Ksp * 28,000 mg/mole / [ Al ]
Where [Si] is the silicon concentration in mg/liter and [Al] is the aluminum molar (M) concentration.
Equation 3 estimates the amount of sodium aluminosilicate that will precipitate after the 3 x concentration by evaporation. Equation 3 calculates the insoluble silicon concentration at a given concentration factor, normalizes this concentration to the starting volume, and converts to equivalent aluminosilicate quantity. Equation 3 calculates the mass of sodium aluminosilicate produced by evaporating 1 liter of waste.
Equation 3 MassNaAlSi = [(Sitot – Sisol/Sitot*Sistart]/0.16 g Si/g NaAlSi/ 1000 mg/g
Note: NaAlSi is an abbreviation for sodium aluminosilicate.
The maximum historical production of aluminosilicate solids for the 3H Evaporator system was 0.131 g per liter of waste. Ketusky estimated the sealant would raise the amount of solids only very slightly to 0.137 g/L processed. If one uses the concentrations of sodium, aluminum and hydroxide from Table 1 and the average silicon concentrations from Table 2, one estimates 0.104 g for sample 2H-433 and 0.178 g for sample 2H-435. The estimate of solids formation for the 2H-433 sample would indicate a reduction in the aluminosilicate formation potential. However, the data for the 2H-435 sample would indicate a significant increase in solids formation potential beyond the formation criteria.
Conclusions
A commercial leak sealant, CarGoâ Brand Seal-Upâ , has been used to contain leaks in the cooling coils of the 3H Evaporator drop tank, Tank 30H. This sealant contains significant amounts of silicate in the form of a caustic sodium silicate solution. Because of the potential for aluminosilicate formation and accumulation in 3H Evaporator pot, an evaluation was performed prior to the addition. This evaluation by Ketusky predicted minimal impact on the evaporator system.
SRTC was requested to analyze dip and variable depth samples taken from Tank 30H after a successful deployment of the cooling coil sealant. The results of these analyses indicate higher silicon concentrations in one sample (2H-435) above the predicted and more than 120 % of the historical maximum silicon concentration for the 3H Evaporator system. Moreover, aluminosilicate formation estimates for this sample (2H-435) exceed prediction and the 110% criteria established in the evaluation.
References