D. R. Click
Westinghouse Savannah River Company
Aiken, SC 29808

Sludge Batch 3 feed from Tank 7 sludge is a complex mixture containing not only metal oxides and hydrous oxides but also significant amounts of oxalate, coal, sand and monosodium titanate. This complex mixture makes the analysis of a particular species in the sludge more challenging. Herein, are details of experiments that were performed with non-radioactive simulant sludge to determine if Loss on Ignition (LOI) would be a suitable technique for measuring the amount of coal present in simulant sludge.

The data and experimental observations support the following conclusion:

• LOI may not be the best method suitable for measuring the amount of coal in sludge due to other solids, which are collected on the mesh filter that also have volatile components.
• The size of particles in the simulant sludge must vary significantly as various amounts of total solids were collected on the mesh filter in addition to the coal and sand spiked into the sludge.

Total Inorganic Carbon (TIC) and Total Organic Carbon (TOC) analyses or Thermogravimetric Analysis (TGA) may be better techniques for measuring the total amount of carbon in the Sludge Batch 3 feed by more effectively measuring the amount of coal present in the feed.

For the LOI experiments six standard samples were made up. Approximately 30 g of sludge was added to a 250 mL wide-mouth bottle and then coal was added to the sludge in 25% of the coal/sludge ratio (1.3mg coal/1g sludge) in Tank 7 sludge waste. Sand was then added to the sample (110 wt % relative to coal) and the sample was diluted to 250 mL. This process was repeated three more times with approximately 30 g of sludge and coal was added to each sample in 50%, 100% and 200% of the coal/sludge ratio expected in tank 7 sludge waste, respectively. Two additional samples were made up, one sample had only ~30 g of sludge diluted to 250 mL and the other had coal (25% of the expected coal to sludge ration in Tank 7 waste) and sand diluted to 250 mL. The samples were then filtered through a mesh filter having 37-micron openings into a 1 L bioassay bottle and the solids that collected on the mesh filter were rinsed repeatedly. The solids collected were dried for two hours at 100 ºC they were cooled, weighed and then heated to 1100ºC for twenty minutes. The LOI was determined from the difference in mass. Please see Table II for more details.

Keywords: Analytical Methods, Process Control, Sludge Characterization

Sludge Batch 3 feed from Tank 7 sludge, as mentioned, is a complex mixture containing not only metal oxides and hydrous oxides but also significant amounts of oxalate, coal, sand and monosodium titanate. This complex mixture makes the analysis of a particular species in the sludge more challenging. Of particular importance, is the determination of the coal content in the Sludge Batch 3 feed. Determination of the coal content is important because burning hydrocarbon can increase the potential of the Defense Waste Processing Facility (DWPF) melt. If the melt becomes to reducing, it can precipitate metals in the melter and short-circuit the melter electrodes causing premature melter failure. LOI is a technique commonly employed to measure the volatile organic compounds (VOC) and carbon content of coal by comparison to the other ash-forming materials present in coal. Herein, are details of experiments that were performed with non-radioactive simulant sludge to determine if LOI experiments would be a suitable technique for measuring the amount of coal present in simulant sludge and subsequently the coal in Tank 7 sludge waste.

The experimental plan had the following objectives:

1. Compare the amount of total solids collected on the mesh by filtration of the standard samples through a mesh filter.
2. Compare the LOI versus the amount of coal added to the 250 mL standard samples.
3. Measure the wt % lost for standard coal samples to determine if weight loss is similar for varying amounts of coal.

For this section of the experiment, 15 standard samples having the following composition were made up and then filtered using a mesh filter.

The average amount of solids from the sludge (total solids – g of coal added – g of sand added = total solids from sludge) collected on the mesh was 0.0661g. The standard deviation is 0.0122g (18.45%). These data suggest that the sludge simulant may vary in homogeneity and that a larger size mesh may be more suitable to only capture the larger particles present in the sludge such as coal and sand.

Six standard samples were made up having the following composition.

Graph II illustrates the LOI vs total solids collected on the mesh filter. The linear data suggests there is consistent weight loss from the sludge samples spiked with coal and sand as the mass of total solids increases.

Graph II.

Coal samples containing roughly the same amount of coal that was spiked into 250 mL standard samples were made up to determine if the LOI from coal varied as the weight of coal increased. Graph I. is used to illustrate a linear relationship between the weight of coal and the LOI. There is >10% difference in LOI upon increasing the amount of coal by a factor of four.

LOI analysis may be a useful way to evaluate the amount of coal present in sludge but its use is conditional. One condition is that particles in the sludge must be smaller and fairly uniform in size compared to that of the coal and sand in the sludge. Another condition is finding the perfect size of mesh that captures sand and coal but allows passage of other solids present in the sludge. Finally, if the mesh filter traps other volatile components of the sludge matrix, another analytical technique in addition to LOI may need to be used for the determination of coal present in the sample. These experiments have shown that the amount of total solids collected by a mesh filter can vary by more than 18%. The data also suggest that there is a 10% decrease in the LOI as the mass of coal increased by a factor of four. It was also shown that that there is a linear relationship between the LOI and total solids collected. It is recommended that TIC/TOC or TGA be used with or instead of LOI to determine the coal concentration of Sludge Batch 3 feed.