Patent Number: 041644794
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT These and other objects of the invention for an improvement in the method of suppressing fluoride and chloride volatility during the calcination of aqueous nuclear fuel reprocessing solutions containing zirconium fluoride, chloride and other values by adding calcium nitrate to the solution in an amount sufficient to establish a calcium to fluoride mole ratio of at least 0.55 wherein the calcium nitrate acts to suppress fluoride volatility during calcination, the improvement wherein aluminum nitrate is added to the solution before the calcium nitrate in an amount sufficient to make the aluminum to fluoride mole ratio from about 0.27 to 0.40, whereby the formation of gelatinous solids in the waste due to the presence of calcium nitrate is substantially reduced and the volatility of the chloride during calcination of the solution is suppressed. This invention is particularly suited for suppressing fluoride and chloride volatility while reducing the gelatinous solids formed by the addition of calcium nitrate to aqueous waste solutions such as the first cycle zirconium-fluoride waste resulting from the reprocessing of zirconium fuels at the Idaho Chemical Processing Plant (ICPP) and to the blend of first cycle waste with second cycle waste from zirconium fuel reprocessing. It is also suitable for improving the calcinability of any aqueous solution containing zirconium, fluoride and chloride compounds. In Table I below are given the nominal compositions of the two waste solutions. TABLE I ______________________________________ COMPOSITION OF WASTES Concentrations 1st Cycle Zirconium- 2nd Cycle Constituent Fluoride Waste Waste ______________________________________ H.sup.+ 2.3 M 1.3 M Zr 0.38 M Al 0.64 M 0.66 M Cr 1.6 g/l Sn 0.39 g/l B 1.7 g/l 0.14 g/l Na 59.5 g/l K 9.6 g/l Fe 1.2 g/l Mn 1350 ppm NH 0.035 M Hg.sup.4 0.99 g/l NO.sub.3.sup.- 2.8 M 6.0 M F.sup.- 3.0 M 0.0065 M PO.sub.4.sup.-3 2.7 g/l SO.sub.4.sup.-2 0.063 M Cl trace 1700 ppm ______________________________________ Calcium nitrate is added to the first cycle zirconium-fluoride waste solution in an amount sufficient to make the calcium to fluoride mole ratio at least 0.55 to provide adequate suppression of the fluoride volatility. Although this is sufficient for first cycle waste, a mole ratio of at least 0.6, preferably 0.7, is necessary when the blend of wastes is calcined. This is required to prevent nodules forming on the fluidized-bed material and ultimately causing a collapsed bed. These nodules are believed to be due to sodium in the second cycle waste. It will be noted in Table I, that both waste solutions contain aluminum, the first cycle waste having a normal aluminum to fluoride mole ratio of about 0.21 while the blend has a ratio of about 0.28. The amount of aluminum to be added to first cycle waste must be an amount sufficient to establish a mole ratio of aluminum to fluoride from about 0.27 to about 0.40. Although the 0.27 ratio is preferred, increased aluminum content was found to have no deleterious effects. Although the blend of first cycle and second cycle wastes, contains sufficient aluminum to establish an aluminum to fluoride mole ratio of 0.28, this is insufficient to provide adequate chloride volatility suppression for reasons unknown. However, when sufficient aluminum is added to establish an aluminum to fluoride mole ratio from about 0.32 to about 0.4, with 0.32 being preferred, the volatility of the chloride present in the blend was substantially reduced. The calcium and aluminum are generally added to the waste solutions as nitrates because of solubility and compatibility with the compounds already present, although any compound, which is soluble in the solution and compatible with the ions already present, would be suitable. The reasons for the effect of the increased aluminum to fluoride ratio on reducing the amount of gelatinous solids formed by the addition of calcium nitrate and on the suppression of chloride volatility are unknown. The following examples are given to show the operability of the method of the invention and are not to be taken as limiting the scope of the invention as defined by the claims appended hereto. EXAMPLE I To demonstrate the effect of the addition of calcium and aluminum on the amount of gelatinous solids formed in the first cycle waste and in the blend, experiments were run in which varying amounts of ions were added to the wastes. In Table II the rate of filtration of solids after calcium nitrate or aluminum nitrate plus calcium nitrate had been added to the wastes is used as a measure of the gelatinous nature of the residue -- the less the filtering time, the less the gelatinous nature of the solid. In each case 30 ml of homogenized slurry is sucked through a sintered glass filter (that has never been used before) having a 14 micron porosity by a vacuum pressure of 17 inches of mercury. The results are given in Table II below. TABLE II ______________________________________ Effect of Calcium and Aluminum Concentrations On The -Gelatinous Nature And Amount Of Solids Formed In Zirconium And Fluoride-Containing Wastes Residue Ca to F Al to F (g from Mole Mole Filtering 30 ml Waste Ratio Ratio Time waste) ______________________________________ 1st Cycle 0.55 0.21 25 min 4.6 Zr-F Waste 1st Cycle 0.55 0.27 5 min 2.6 Zr-F Waste 10 sec 1st Cycle 0.55 0.40 1 min 0.55 Zr-F Waste 5 sec 3 Vol 1st 0.7 0.28 3 min 2.4 Cycle Zr-F Waste blended with 1 vol 2nd Cycle Waste 3 Vol 1st 0.7 0.32 45 sec 0.52 Cycle Zr-F Waste Blend with 1 vol 2nd Cycle Waste ______________________________________ It can be seen that the addition of a small amount of aluminum resulted in a substantial reduction of the amount of solids formed. EXAMPLE II The method for decreasing gelatinous solids in calciner feed was tested by runs in a 4-inch diameter, fluidized-bed, in-bed combustion, pilot plant calciner to determine how the methods affected fluoride and chloride volatility, calciner operability, and calcine properties in such a calciner. Table III shows that increasing the aluminum to fluoride mole ratio in first-cycle zirconium-fluoride waste from 0.21 to 0.40 prior to Ca(NO.sub.3).sub.2 addition had no adverse effect on fluoride volatility, calciner operability, and calcine properties. The attrition index is a measure of the hardness of bed particles -- the smaller the index, the softer the particles. Table III also shows that the use of Mg(NO.sub.3).sub.2 produces a bed particle that is probably too soft; a soft bed particle breaks easily into fines during fluidized-bed operation, and the production of too many fines would likely result in plugging and bridging in the calciner off-gas and transport systems. Thus, the use of Mg(NO.sub.3).sub.2 is not recommended for use in the calcination of first-cycle zirconium-fluoride waste. Table IV shows that increasing the aluminum to fluoride mole ratio from 0.28 to 0.32 in a blend of three volumes of first-cycle zirconium fluoride waste with one volume of second-cycle waste prior to calcium nitrate addition reduced chloride volatility, suppressed fluoride volatility satisfactorily, resulted in smooth calciner operation and produced a calcine of acceptable properties. TABLE III ______________________________________ Calcination Of First-Cycle Zirconium-Fluoride Waste In A 4-Inch Diameter, Fluidized-Bed, In-Bed -Combustion Calciner Run # FV4-lb FV4-2 FV4-3 ______________________________________ Run Duration (Hrs.) 58.7 40 40 Ca/F Mole Ratio 0.55 0.55 0 Mg/f Mole Ratio 0 0 0.55 Al/F Mole Ratio 0.21 0.40 0.21 Wt % Volatilized 0.6 0.1 0.2 from Calciner Calcination Temp. (.degree.C.) 500 500 500 Product to Fines 2.76 2.01 1.66 Ratio Density of Product 1.22 1.21 1.15 (g/cc) Density of Fines 0.54 0.77 0.57 11 (g/cc) Attrition Index of 28 16 4 the Final Bed (of the -32 +35 Mesh Fraction) (%) Calciner Operability No No No problems problems problems ______________________________________ TABLE IV ______________________________________ Calcination Of A Blend Of 3 Volumes First-Cycle Zirconium-Fluoride Waste With 1 Volume Second-Cycle Waste In Fluidized-Bed, In-Bed Combustion Calciner Run # 53 FV4-4 SBW 4-9 ______________________________________ Run Duration (Hrs.) 131 72 40 Calcination Temp. (..degree.C.) 500 500 500 Ca/F Mole Ratio 0.7 0.7 0.7 Al/F Mole Ratio 0.28 0.32 0.32 Wt % F Volatilized (a) 0.2 0.7 from Calciner Wt % Cl Retained in 70 92 92 Bed Plus Fines Product to Fines (a) 2.77 5.6 Ratio Density of Product (a) 1.58 1.68 (g/cc) Density of Fines 0.49 0.46 0.65 (g/cc) Attrition Index of 68 76 80 the Final Bed (of the -32 +35 Mesh Fraction) (%) Calciner Operability No No No problems problems problems Texture of Smooth Smooth Smooth calcine surface ______________________________________ Run 53 was made in a 12 inch diameter fluidized bed, in-bed combustion calciner Run FV4-4, SBW 4-9 - were made 4 inch diameter fluidized bed, in bed combustion calciner.? As can be seen from the preceding discussion and Examples, the addition of aluminum to the zirconium-fluoride waste and in the blend prior to adding calcium nitrate, not only reduces the volume of gelatinous solids formed by the calcium nitrate but also substantially decreases the volatility of the chloride in the blend. Reducing chloride volatility helps not only to reduce equipment corrosion but also reduces the possibility of corrosive gases escaping into and polluting the environment. Reducing volume of gelatinous solids reduces the potential for plugging the feed system to a calciner.