(142bb) Effect of Non-Cohesive Particulates On Yield Stress of Waste Sludge Simulants

Effect
of Non-cohesive Particulates on the Yield Stress of Waste Sludge Simulant

Ibraheem R. Muhammad and John P.
Kizito

North Carolina A&T State
University, Greensboro, NC 27411

The goal
of the study was to determine the effect of coarse, non-cohesive particles on a
mixture exhibiting a yield stress.  A simple, inexpensive, and easy to run
experimental approach, was used to estimate the yield stress based on the definition
that yield stress is the critical shear stress that must be applied before the
material begins to flow [1].  Yield stress is an important
parameter in nuclear waste processing, as the settled waste behaves as a
Bingham plastic [2-4].  Off bottom suspension of the
sludge is necessary and usually achieved using jet mixers.  Therefore, it is
important to know the yield stress of the material as to determine how much
force is needed from the jet to achieve suspension [5].  The sludge is usually composed
of particles of varying size and density, which can be simulated using kaolin
clay/water mixtures [6,
7]. 
The smaller particles (D  < 62.5 µm) are cohesive in behavior, while the
larger particles (D > 62.5 µm) are non-cohesive [3].  It such systems, it is not
well known how the non-cohesive particles affect the cohesion[4].  Before studying how the
mixtures behave in a tank once suspended, the rheology of the mixture must be
determined.  Therefore, the current study is used as an initial step in understanding
the rheology of such mixtures, but actual mixing studies are run.

            In
the current study, experimental tests were run using a capillary tube rheometer
and a manometer.  Figure 1 shows a schematic of the experimental apparatus that
was constructed.  Pressure was supplied from compressed air, which is tightly
controlled using a pressure regulator and the pressure difference through the
capillary is measured using the water manometer.  Such an apparatus is
inexpensive and easy to construct, while still able to provide accurate data.  It
is also advantageous because little to no calibration of the equipment is
needed compared to other devices.  The simulants used for testing included
kaolin clay/water mixtures and kaolin/sand/water mixtures.  The weight percent
of total solids (i.e. kaolin + sand) was varied.  The kaolin particles (D =
1.36 µm) represented the cohesive particles and the sand particles (100 µm <
D < 800 µm) represented the non-cohesive particles.  The yield stress was
estimated using a visual observation technique.  The yield stress was measured
using the pressure difference in the capillary required to make a sample
extrude from the capillary.  Such a technique is not uncommon and it rather
easy to implement [8,
9]. 
The results of the technique are compared to yield stress data using shear rate
and shear stress data.  Figure 2 displays the yield stress results of the
current study compared to that of other studies and it was shown that the data
was in good agreement.  The technique used can be applied as an initial means
of measurement for yield stress in not only nuclear waste facilities, but other
processes in which yield stress needs to be calculated as well. 

Figure 1:  Schematic of
experimental apparatus that was constructed for yield stress tests.

Figure 2: Comparisons of
yield stress data from current study and that of previous studies using
kaolin/water mixtures at different weight %.

References

1.         Macosko,
C.W., Rheology : principles, measurements, and applications. Advances in
interfacial engineering series. 1994, New York [etc.]: Wiley-VCH.

2.         Powell,
M.R., Onishi, Y., and Shekarriz, R., Research on jet mixing of settled
sludges in nuclear waste tanks at Hanford and other DOE sites: A historical
perspective. 1997, Pacific Northwest Laboratory: Richland, Washington. p.
90.

3.         Abulnaga,
B.E., Slurry Systems Handbook. 2002, New York, NY: McGraw-Hill.

4.         Wells,
B.E., Enderlin, C.W., Gauglitz, P. A., and Peterson, R.A., Assessment of Jet
Erosion for Potential Post-Retrieval K-Basin Settled Sludge. 2009, Pacific
Northwest National Laboratory: Richland, WA.

5.         Muhammad,
I.R. and J.P. Kizito, Evaluation of Pulse Jet Mixing Using a Scalar Quantity
and Shear Stress. ASME Early Career Technical Journal, 2011. 10: p.
45-51.

6.         Laxton,
P.B. and J.C. Berg, Relating clay yield stress to colloidal parameters.
Journal of Colloid and Interface Science, 2006. 296(2): p. 749-755.

7.         Rassat,
S.D., et al., Physical and Liquid Chemical Simulant Formulations for
Transuranic Waste in Hanford Single-Shell Tanks. 2003: Richland,
Washington.

8.         Poloski,
A.P., et al., Estimate of Hanford Waste Rheology and Settling Behavior.
2007, Pacific Northwest National Laboratory: Richland, WA.

9.         Gauglitz,
P.A. and J.T. Aikin, Waste behavior during horizontal extrusion: Effect of
waste strength for bentonite and kaolin/ludox simulants and strength estimates
for wastes from Hanford waste tanks 241-SY-103, AW-101, AN-103, and S-102.
1997, Pacific Northwest Laboratory: Richland, WA.