Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 125

Research and development of SPC-based solidification technologies was initially
investigated at Brookhaven National Laboratory (BNL), 15-21 which received a patent
for SPC microencapsulation of radioactive, hazardous, and mixed wastes 22 and for
Sulfur Polymer Stabilization/Solidification (SPSS). 23 Additional development of
SPC for the treatment of waste has been conducted by the Commission of the
European Communities, 24 Idaho National Environmental and Engineering Labora-
tory (INEEL), 25-27 Pacific Northwest National Laboratory, 13 Oak Ridge National
Laboratory (ORNL), 28-31 and the University of Georgia. 32
Polyethylene is a similar thermoplastic material that is heated to melt tempera-
ture, combined with waste to form a composite mixture, and then cooled to form a
solid, monolithic final waste form. The melt viscosity of SPC is significantly lower
(~25 mPa-s), however, making the processing of SPC easier to accomplish. For
example, SPC can be processed in a variety of heated mixing vessels, whereas
polyethylene requires processing in an extruder or thermokinetic mixer. A typical
processing system consisting of a heat-jacketed vertical cone mixer with a conical
screw that rotates on its axis and around the circumference of the vessel mixing is
shown in Figure 6.3.2. The relatively slow orbital mixing action provides thorough
mixing without entraining air in the liquid solution, which would result in a less-
desirable, low-density product. Other processing options include low- and high-shear
mixers and various types of heated pug mill mixers. Several modified heated pug
mills, with hollow mixing shafts that allow circulation of hot oil or steam, were
tested at INEEL. 26 Process temperatures range from 120 to 140°C to ensure proper
mixing without overheating the SPC, which can lead to increased viscosity and
breakdown of the SPC. Systems that allow processing under negative pressure are
preferable to ensure containment of off-gas and to reduce the odors associated with
molten sulfur.
FIGURE 6.3.2 Pilot-scale process equipment for Sulfur Polymer Stabilization/Solidification
(SPSS) treatment of mercury waste.