Stabilization of Colloidal Silica Using Small Polyols Page: 4 of 22
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Introduction
The Hanford site in Washington has numerous tanks containing solidified mixed waste
material in the form of fractal aggregates. The highly aggregated nature of these materials
has made their viscosity too high to easily transfer these wastes through the available
pipelines. Thus preprocessing steps, such as dilution and comminution, will be necessary to
reduce the waste viscosity. We are looking for ways to stabilize these materials from further
aggregation, and to prevent reaggregation after comminution, that require very little addition
of chemicals, so that they may be transferred through pipes to other tanks or to processing
sites that will incorporate these wastes into borosilicate glass logs. We have developed a
model colloidal silica suspension that mimics some aspects of these waste materials and the
conditions of high pH and salt under which they are found. The tanks wastes are typically
highly basic, and are several molar in salt. ,
Standard methods of colloidal stabilization include charge stabilization and steric
stabilization. Charge stabilization requires that the colloids are far from their isoelectric
point, so they have a large surface charge density. If the solvent in which they are suspended
is at low ionic strength, the Debye layer will be very thick, and a significant Coulombic
barrier will have to be overcome before the colloids can collide and fall into a deep, attractive
van der Waals well. Clearly at the high salt concentration of the tank wastes, this is not a
practical approach to stabilization. Steric stabilization uses high polymers to create so-called
"hairy balls", which repel each other due to the large free energy of mixing of high polymers,
much of which is due to the entropy term.
We were quite surprised to discover that low molecular weight polyols can also be
surprisingly effective at stabilizing colloidal silica at high pH and multimolar salt
concentrations. We have explored the homologous series of polyols; ethylene glycol,
glycerin, erythritol, 1,3 propanediol, and 1,4 butanediol. These were added in various
concentrations to our silica suspension and the aggregation kinetics were monitored with
quasielastic light scattering (QELS). In these experiments we determined the time for a 200
nm radius aggregate to form, as a function of polyol concentration. We found that the time
required to form such large aggregates increases exponentially with the stabilizer
concentration, and that some polyols are much more effective than others. In general, the
more hydrophilic the polyol, the better it stabilized our system. Studies of the salt and pH2
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GULLEY, GERALD L. & MARTIN, JAMES E. Stabilization of Colloidal Silica Using Small Polyols, article, September 7, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc624264/m1/4/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.