Microstructure and Microchemistry of Waste Forms - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 205

FIGURE 9.3 Photomicrograph of electroplating waste with 8 wt% Pb(NO 3 ) 2 stabilized in
lime-fly ash (5300×) (inside void below × in Figure 9.2).
(Ca 5 (PO 4 ) 3 (F,Cl,OH)) and monazite A(PO 4 ) (where A = light rare earth minerals)
often naturally contain high levels of radioactive elements. These minerals have also
been made to crystallize from radioactive wastes by techniques such as sol-gel or
sintering. Eusden et al. 6 reported the microstructure and mineralogy of the waste
form generated from old mine tailings in Colorado containing Pb as the principal
contaminant. The treatment resulted in the precipitation of several phosphate phases.
The phosphate grains that crystallized were of very small size, in the nanometers
range, or were amorphous. Many of the reaction products occurred as rinds around
the original minerals.
Immobilization of high-level radioactive wastes using ceramics and glasses has
recently been reviewed. 7 The waste form can be completely glassy, or crystalline
(ceramic), or a mixture of the two. The ceramic phases are such that they can contain
most of the waste elements in solid solution, i.e., replacing some of the original
elements in the crystal structure of the ceramics. The microstructure of such waste
forms can be gainfully studied by comparing them with similar textures produced
in nature or laboratory. For example, waste vitrification processes produce a micro-
structure similar to glassy volcanic rocks, where both consist of a glassy matrix with
enclosed bubbles. The waste may be completely dissolved in the matrix or can be
enclosed by the glass. Similarly, for the ceramic waste forms, crystals will form
with triple junction contact, just like slowly cooled igneous rocks or slowly cooled
molten metals.
Raman 8 recently reported a study of the microstructure of a partially vitrified
glass-ceramic waste form containing simulated high-level radioactive waste pro-
duced by hot isostatic waste (1000°C, 138 MPa). The proportion of crystalline phases
varied in direct proportion to waste loading. Some relict phases remained depending
on the waste loading. The phases that are present or should be present can usually
be predicted from phase diagrams.
The use of pure sulfur as a waste form has been suggested. 9 Contaminants are
trapped between the crystals of sulfur. The microstructure of such waste forms is
similar to those of ceramic waste forms from nuclear wastes.