Patent Number: 056132409
Section: description

DETAILED DESCRIPTION OF THE INVENTION The invention is based upon the discovery that sodalite can be produced from salt occluded zeolites by the use of heat or heat and pressure in the presence of glass contrary to prior teachings in the art. More specifically, it has been found that providing glass in the amount of about 5% to about 10% by weight and the presence of salt occluded zeolite while heating the material to a temperature of about 1000.degree. K. produces a material which, when tested by x-ray diffraction techniques, is sodalite. Because sodalite will absorb less waste salt than a corresponding amount of zeolite, it is required for the full appreciation of the method to provide excess amount of zeolite in the mixture prior to heating to accommodate the diminished capacity of sodalite to absorb the radionuclides. This prevents the resultant product from leaving a large amount of radioactive material not occluded by the sodalite. More specifically, zeolite in powder or pellet form may be initially dried by heating in a series of four steps to 800.degree. K. and flowing nitrogen or under a vacuum. This process removed nearly all the water from the zeolite and the zeolite was thereafter stored in an inert atmosphere such as in a glove box. In the protective atmosphere or in a glove box, the dry zeolite powder or pellets was loaded into a quartz test tube. The simulated waste salt was loaded into another quartz tube. The waste salt may be comprised of the following: ______________________________________ KI 0.3% NdCl.sub.3 1.04% LaCl.sub.3 1.06% CeCl.sub.3 0.74% YCl.sub.3 0.13% LiCl 32.9% NaCl 5.97% SrCl.sub.2 0.59% BaCl.sub.2 1.43% KCl 44.83% CsCl 3.73% ______________________________________ After the salt and zeolite are heated to about 700.degree. K. the salt is poured into the tube containing the zeolite and allowed to stand for 24 hours. In an ion exchange process, sufficient product chlorides are concentrated in the zeolite relative to the remainder of the salt. After the ion exchange, most of the excess salt is removed from the zeolite surface even though some of the free salt remains present. Thereafter, the salt loaded zeolite is combined with additional (up to 2 times) dehydrated zeolite in an alumina crucible. Because sodalite can occlude approximately 1/3 the volume of salt that a zeolite can occlude, generally twice the amount of occluded zeolite is added. In any event, enough dehydrated zeolite is added to reduce the total salt level to about 12wt % or less. Glass is added to this mixture in the range of between about 5% by weight to about 10% by weight of the zeolite and salt. Two hot pressing processes have been developed. In the one process, the zeolite powders/pellets are first converted to sodalite powders/pellets by heating to 1000.degree. K. for 24 hours or so. The sodalite powders/pellets are then densified using hot pressing at temperatures around 1200.degree. K. and 20-28 MPa. In a high pressure process, the zeolite powders and salt mixture is converted to sodalite directly during hot pressing at a temperature of 1000.degree. K. and pressures around 120 MPa. In a low pressure process, prior to hot pressing the zeolite and salt mixture is coverted to sodalite. If the glass is in frit form, the mixture is stirred and heated to 1000.degree. K. and held at that temperature for about 25 hours. After cooling, x-ray diffraction shows only sodalite. The sodalite powder with the occluded radionuclides is added to a graphite die and is initially cold pressed at 40 MPa. The cold pressed material is then heated to 1200 K. using a 20 K. per minute ramp rate and held at 28 MPa for approximately 30 minutes at maximum temperature. It is believed that a minimum pressure of 20 MPa will suffice. The measured gross pellet densities were between 2.1 and 2.4 grams per cubic centimeters (cc). Theoretical density of chlorosodalite is 2.31 grams per cc. In some cases, the salt loaded zeolite pellets were ground prior to conversion to the sodalite. When pellets were converted directly to sodalite, the preferred glass was aluminum 0.35 wt. %, calcium 13.1 wt %, sodium 7.6 wt %, magnesium 0.3 wt %, silicon 20.2 wt %, strontium 0.1 wt %, boron 6.7 wt %, potassium 0.06 wt %, zirconium 0.1 wt % with the balance oxygen. This glass was the only glass tested which provided full conversion of the zeolite pellets to sodalite. However, when the pellets were ground, a variety of glasses were useful to convert all of the zeolite to sodalite. Other glasses useful had the following compositions. ______________________________________ Best Others Worst ______________________________________ Al 0.35% 5.1 3.3 4.0 Ca 13.1% 9.61 7.9 0.37 Na 7.6% 4.9 2.4 4.1 Mg 0.3% 0.26 0.2 0.03 Si 22.2% 25.7 28.2 23 Sr 0.1% 0.06 6.8 0.8 B 6.7% 4.3 3.0 3.7 K 0.06% 0.66 1.0 0.17 Zr 0.06% balance O2 Ba 0.1 19.8 Balance O.sub.2 Zr.sub.x 0.35 ______________________________________ Another method of preparing the salt occluded sodalite is to dehydrate zeolite as stated above and to combine the dehydrated zeolite with a simulated waste salt of up to about 12% by weight or less and about 5 to about 10 weight % by glass. These materials were combined into a crucible and stirred for a short period of time on the order of less than one minute or about 10-30 seconds and then heated to about 1000.degree. K. and held at that temperature for about 24 hours. After cooling, x-ray diffraction showed only features consistent with sodalite. In order to produce sodalite of near theoretical density which is important for leach testing, the material has to be hot pressed as previously described. When zeolite is heated without the presence of glass, a mixture of nepheline and salt results and sodalite is not a major product. Nepheline has poor leach resistance and is not satisfactory for storing radioactive materials. However, when glass is added as described, then sodalite is the major product and is a significant improvement in leach testing compared to nepheline. Table 1 shows a comparison of normalized release rates for sodalite and nepheline using a salt such as that described above as a substitute for the radioactive chloride salt generally produced in the IFR process. TABLE 1 ______________________________________ Normalized Release Rates (g/m.sup.2 day)) Element Sodalite Nepheline ______________________________________ Cs 1.2 132 Sr 0.01 3.3 Ba 0.01 25 Na 0.4 6 K 0.6 9.4 Li 2.3 6.7 ______________________________________ In a high pressure process, the mixture of salt occluded zeolite, additional zeolite and glass is added to a graphite die and is initially cold pressed to 40 MPa. The cold pressed material is then heated to 1000.degree. K. with a ramp rate of 20 K. per minute. After the temperature is at least 700.degree. K., a pressure of about 120 MPa is applied. The pressure is maintained at 1000.degree. K. until densification is complete. The typical length of time required for a sample about 2.5 cm in diameter and about 0.3 cm thick is less than 30 minutes. In a twenty-eight day leach test, the sodalite prepared from and in accordance with the high pressure process set forth above provided the result set forth in Table 2. TABLE 2 ______________________________________ Normalized Release Rate Element 28 Day 90.degree. C. Test ______________________________________ Al 0.16 Ba 0.88 B 1.26 Ca 0.85 Cs 0.58 K 1.0 Li 0.83 Na 0.58 Si 0.23 Sr 1.22 Ce 0.013 Nd 0.009 La 0.009 Y .about.0 ______________________________________ Both Table 1 and Table 2 show results with deionized water maintained at 90.degree. C. It is preferred that a borosilicate glass is used and that it is present as glass frit. Moreover, while zeolites in general may be useful, the preferred zeolite is zeolite A and zeolite X otherwise known as faujasite. Mixtures of zeolite A and zeolite X are also useful. While there has been disclosed what is considered to be the preferred embodiment of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.