Patent Number: 043292488
Section: description

Six examples of the operation of the process according to the present invention are given below, together with certain modifications thereof. These examples relate to the immobilization of typical "high-Al" and "high-Fe" sludges possessing compositions as given in Table 3, Columns 1 and 2. Sludges possessing intermediate compositions, e.g. Table 3, Column 3 can be immobilized by treatments appropriately intermediate in nature between those described for Examples 1 and 2. EXAMPLE 1 (a) A "high-alumina" sludge characterized by a mixture of fission products and actinide elements with excess oxides of Al, Fe, Mn, Ni, U and Na, possessing the composition given in Table 3, Column 1, is mixed with about 30 percent of TiO.sub.2, ZrO.sub.2, CaO and SiO.sub.2, in proportions chosen so that when the mixture is heated, the added oxides combine with the sludge components to form a mineral assemblage consisting principally of hercyniterich spinel+perovskite+zirconolite+nepheline. The heat treatment is carried out under controlled redox conditions such that most of the iron and nearly all manganese and nickel is maintained in the divalent state. The mixture is heated at a temperature of 1200.degree. C. for several hours and simultaneously subjected to a confining pressure using the conventional technique known as hotpressing. Alternatively, the mixture may be formed and sintered at 1200.degree. C. under the appropriate redox conditions without the application of pressure. The resulting product is found to be a fine grained, mechanically strong rock composed of the above minerals in which the HLW fission products and actinides are effectively immobilized. Actual compositions of the minerals in a rock produced in this manner are given in Table 4. TABLE 4 ______________________________________ Compositions of coexisting mineral phases in high-alumina sludge (Table 3, Column 1) treated as described in Example 1(a). Nepheline Perovskite Zirconolite Hercynite ______________________________________ SiO.sub.2 41.5 -- -- -- TiO.sub.2 0.2 53.4 29.5 5.8 ZrO.sub.2 -- 0.7 37.8 0.3 UO.sub.2 -- 0.2 13.9 -- Al.sub.2 O.sub.3 35.9 2.4 1.1 48.2 Fe.sub.2 O.sub.3 -- -- -- -- FeO 0.8 2.7 4.1 37.4 MnO 0.2 1.7 0.9 7.2 NiO -- -- -- 0.4 CaO -- 39.6 12.3 -- Na.sub.2 O 21.5 0.3 0.4 -- Sum 100.1 101.0 100.0 99.4 ______________________________________ (b) In a modification of Example 1(a) above, the sludge is mixed with about 20-30 percent of the same oxides in proportions chosen to form a hercynite-rich spinel+zirconolite+nepheline mineral assemblage, and the mixture treated as above. A product physically similar to that of Example 1(a) is obtained with the fission products and actinides immobilized in the zirconolite phase. (c) A "high-alumina" sludge as described in Example 1(a), is pretreated by washing to reduce the sodium content, mixed with about 20-30 percent of TiO.sub.2, ZrO.sub.2 and CaO in proportions chosen to form a hercynite-rich spinel+perovskite+zirconolite mineral assemblage and the mixture treated as above. A product physically similar to that of Example 1(a) is obtained. (d) In a modification of Example 1(c) above, the sludge is mixed with about 20-30 percent of the same oxides in proportions chosen to form a hercynite-rich spinel+zirconolite mineral assemblage and the mixture treated as above. A product physically similar to that of Example 1(c) is obtained. EXAMPLE 2(a) A "high-iron" sludge, characterized by a mixture of fission products and actinide elements with excess oxides of Al, Fe, Mn, Ni, U and Na, possessing the composition given in Table 3, Column 2 is mixed with about 35 percent of TiO.sub.2, ZrO.sub.2, Al.sub.2 O.sub.3, CaO and SiO.sub.2 in proportions chosen so that when the mixture is heated, the added oxides combine with the sludge components to form a mineral assemblage consisting principally of ferrite spinel (Mn, Ni, Fe).sup.II Fe.sub.2.sup.III O.sub.4 +perovskite+zirconolite+nepheline. The heat treatment is carried out under controlled redox conditions such that most of the iron is in the trivalent state whilst most of the nickel and manganese are divalent. The mixture is heated at a temperature of 1200.degree. C. for several hours and simultaneously subjected to a confining pressure using the conventional technique known as hot-pressing. Alternatively, the mixture may be formed and sintered at 1200.degree. C. under the appropriate redox conditions without the application of pressure. The resulting product is found to be a fine grained, mechanically strong rock composed of the above minerals in which the HLW fission products and actinides are effectively immobilized. Actual compositions of the minerals in a rock produced in this manner are given in Table 5. TABLE 5 ______________________________________ Compositions of coexisting mineral phases in high-iron sludge (Table 3, Column 2) treated as described in Example 2(a). Nepheline Perovskite Zirconolite Ferrite Spinel ______________________________________ SiO.sub.2 40.6 -- -- -- TiO.sub.2 0.5 56.3 35.1 7.9 ZrO.sub.2 -- 0.6 25.2 -- UO.sub.2 -- 0.2 15.5 -- Al.sub.2 O.sub.3 34.4 0.1 0.4 8.1 Fe.sub.2 O.sub.3 5.0 3.9 7.8 43.5 FeO -- -- -- 20.7 MnO -- 1.0 1.8 9.0 NiO -- 0.2 -- 9.7 CaO -- 37.3 14.6 -- Na.sub.2 O 20.1 0.3 0.2 -- Sum 100.6 100.0 100.6 99.4 ______________________________________ (b) In a modification of Example 2(a) above, the sludge is mixed with about 20-35 percent of the same oxides in proportions chosen to form a ferrite spinel+zirconolite+nepheline mineral assemblage, and the mixture treated as above. A product physically similar to that of Example 2(a) is obtained with the fission products and actinides immobilized in the zirconolite phase. (c) A "high-iron" sludge as described in Example 2(a) is pretreated by washing to reduce the sodium content, mixed with about 20-35 percent of TiO.sub.2, ZrO.sub.2 and CaO in proportions chosen to form a ferrite spinel+perovskite+zirconolite mineral assemblage, and the mixture treated as above. A product physically similar to that of Example 2(a) is obtained. (d) In a modification of Example 2(c) above, the sludge is mixed with about 20-35 percent of the same oxides in proportions chosen to form a ferrite spinel+zirconolite mineral assemblage and the mixture treated as above. A product physically similar to that of Example 2(c) is obtained. EXAMPLE 3 This example is similar to Example 1(a) except that (i) about 40 percent of mixed oxides (TiO.sub.2 +ZrO.sub.2 +CaO+SiO.sub.2) are added to the sludge and (ii) a larger relative proportion of TiO.sub.2 is added than in Example 1(a). Under these conditions, the synthetic rock is found to contain a pseudobrookite-type solid solution (Al.sub.2 TiO.sub.5 --FeTi.sub.2 O.sub.5) in addition to the minerals mentioned in Example 1(a). In compositions richer in alumina than that given in Table 3, Column 1, a separate Al.sub.2 O.sub.3 phase (corundum) may also occur. EXAMPLE 4 The same procedure is followed as in Example 3, except that the added oxides contain some BaO. The mineral assemblage produced is similar to that in Example 3 except that a hollandite-type solid solution (BaAl.sub.2 Ti.sub.6 O.sub.16 --Ba(Fe, Ni, Mn, Ti).sub.2 Ti.sub.6 O.sub.16) is also produced in the synthetic rock. EXAMPLE 5 This example is similar to Example 2(a) except that (i) about 40 percent of mixed oxides (TiO.sub.2 +ZrO.sub.2 +CaO+SiO.sub.2 +Al.sub.2 O.sub.3) are added to the sludge and (ii) a larger relative proportion of TiO.sub.2 is added than in Example 2(a). Under these conditions, the synthetic rock is found to contain ilmenite (FeTiO.sub.3).+-.pseudo-brookite solid solution (FeTi.sub.2 O.sub.5 --Al.sub.2 TiO.sub.5) in addition to the minerals mentioned in Example 2(a). EXAMPLE 6 This example is similar to Example 5, except that the added oxides contain some BaO. The mineral assemblage produced is similar to that in Example 5 except that a complex davidite-type mineral Ba(Al, Fe.sup.III).sub.2 --Fe.sub.8.sup.II Ti.sub.13 O.sub.38 is also produced in the synthetic rock. Under some conditions, a hollandite-type phase Ba(Al,Fe.sup.III,Ni, Mn,--Fe.sup.II, Ti).sub.2 Ti.sub.6 O.sub.16 may also be produced. The above examples lead to the production of strong, stable synthetic rocks in which fission products and actinide elements are immobilized in a mineral assemblage as was described in the prior patent specification. That specification, described the great stability of titanate-based synthetic rocks to leaching and alteration in diverse geological and geochemical environments. The modified synthetic rock compositions described herein, characterized by much higher abundances of Al, Fe, Mn, Ni, U and Na than were considered in the prior specification share the preceding characteristics. The method of immobilizing HLW sludges described herein is greatly superior to the conventional technology of immobilizing the sludges by dissolving them in borosilicate glasses. Firstly, as shown in the prior patent specification, titanate-based synthetic rocks are enormously more stable toward leaching and decomposition than borosilicate glasses. Secondly, in most US defence HLW sludges, the proportion of fission products and actinide elements to "introduced" Al, Fe, Mn, Ni and Na oxides is very small, mostly between 0.5 and 5 percent. Thus, in most cases, it is only necessary to introduce from 20 to 40 percent of additional inert oxides (e.g. TiO.sub.2 +ZrO.sub.2 +CaO+SiO.sub.2) in order to form the desired mineral assemblage. Of course, it would be possible to introduce more than 40 percent of additional inert oxide components if found especially desirable for specific purposes. However, in most cases, this would not be necessary. Accordingly, it is possible to produce synthetic rocks containing 60-80 percent of sludge in the form of stable minerals. In contrast, it is not possible to incorporate readily more than 30 percent of sludge in borosilicate glasses. Moreover, because of the much higher density of synthetic rock (.about.4.5 g/cm.sup.3) compared to borosilicate glass (.about.3.0 g/cm.sup.3), a correspondingly higher weight of sludge can be incorporated in a given volume of rock as compared to glass. This results in considerable economic advantages when HLW sludges are incorporated in synthetic titanate rock. It will be appreciated by persons skilled in the art that many modifications and variations may be made to the specific embodiments described herein without departing from the spirit and scope of the present invention as broadly described herein.