Case Studies: Full-Scale Operations and Delivery Systems - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 339

up in either batch or continuous mode. In the batch mode, the batch size is limited
by the availability of large, rugged mixers of the pugmill type. For some waste
streams, a concrete mixer of the rotary type (used in transit mixers) can be used, in
which case batch size can be up to 7.6 m 3 (10 yd 3 ). However, this type of mixer is
limited to pourable, relatively low-viscosity slurries, not the sticky pastes that are
often encountered in waste treatment, especially for limited water content.
For batch systems, the mixing process normally takes 1 to 15 minutes, depending
on the mechanical system used, the size of the batch, the type of waste, and the
amounts and types of reagents being used. In continuous systems, residence time in
the mixer is generally 0.5 to 2.0 minutes, and intensive mixing is required. After
mixing is complete, the waste is removed from the mixer by either pumping (if
liquid) or screw conveying or dumping (if viscous or like soil). The treated waste
is then moved by pump or other conveyance device to an area where it can develop
its final physical and chemical properties. If the waste has been pretreated, the
hazardous components, usually heavy metals, have already been converted to a
relatively insoluble form. If it has not been pretreated, the metals are often immo-
bilized during the mixing process (unless immobilization depends upon a physical,
monolithic form). In this case, by the time the waste exits the mixer it has largely
developed its final chemical properties.
The hardening, or curing, process often takes place in a temporary container or
impoundment near the S/S plant, with the solid subsequently being conveyed to the
disposal site. Alternately, the treated waste can be conveyed directly into the disposal
site and solidified in its final location, regulations permitting. In this case, testing
must first confirm successful treatment prior to the deposited material being accepted
for permanent placement, since material not meeting regulatory and site requirements
would have to be excavated and retreated. In principle, solidifying in the disposal
site is preferred because it involves fewer steps in handling the waste and is less
expensive. Also, a waste that is still liquid or semi-solid can be poured into place
in the landfill with minimum void space, yielding minimal permeability and maxi-
mum landfill space utilization in the final product. Waste solidified directly in its
final resting place becomes, essentially, a monolithic form that exhibits minimal
permeability.
Pit Mixing Methods . Pit mixing is sometimes used as an ex situ method at
remedial sites, where part or all of the waste from the site is excavated and moved
to a pit rather than to a mechanical mixing arrangement. However, in most cases,
pit mixing is done at a fixed installation, which is discussed in Section 13.3.2.
Spray-On Methods . This variety of S/S system is used only in special situations
and consists simply of spraying a solution or slurry of the reagent onto the waste
passing underneath on a conveyor belt. The technique depends on the solution
thoroughly wetting the waste because the system does not positively mix the waste
and reagent. Thus, the waste particle size should be small and fairly uniform (unless
only surface treatment is required) and the layer of waste encountering the spray
should be thin. Some mixing action takes place naturally in downstream handling
steps, or can be deliberately induced by incorporating a static mixer or tumbling
action into the mechanical system. A popular example is spraying a phosphate
solution to stabilize contaminants remaining in municipal incinerator ash. 2 The ash