Preparation of metal oxide gel spheres with hexamethylenetetramine

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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 461-466

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Preparation of Metal Oxide Gel Spheres with Hexamethylenetetramine as an Ammonia Donor Paul A. Haas, W. Wilson Pitt, Jr., Sharon M. Robinson,' and Allen D. Ryon Chemical Technology Dlvision, Oak RMge National Laboratory, Oak Ridge, Tennessee 37830

Gel spheres containing U03, (Th,U)O, , A1203, Al,O,-ZrO,, and mixtures of various other metal oxides (including Fe, Zr, AI, and Si) have been produced by the internal gelation process by the same general flowsheet. I n the internal gelation process, hexamethylenetetramine (HMTA) decomposes to produce ammonia which precipitates hydrous metal oxides gels. The process conditions (metal molarities, HMTA/unneutralized metal ratio, urealmetal ratio, and gelation temperatures) are within the same ranges for all of the products. Selection of the feed compositions and process conditions can be guided by the general criteria described, but optimum conditions need to be determined empirically. General process requirements are discussed in detail, and products made under these conditions are described.

Introduction The decomposition of hexamethylenetetramine (HMTA) to convert solution drops into hydrous metal oxide gel spheres has been reported frequently. The most common applications have been in preparation of catalyst spheres (Hilfman, 1969; Michalho, 1973), ceramic shapes (Hayes and Sobel, 1974) or nuclear reactor fuels (van der Brugghen et al., 1970; Haas et al., 1980; Bischoff et al., 1974; Urbank and Dolezal, 1977). Most of these reports are recipes with little explanation of the process requirements. These gel sphere processes commonly require close duplication of the process conditions or recipes because the feed liquids and the gels are not at thermodynamic equilibrium. Nevertheless, the recipes and the results have many similarities. The objective of this paper is to discuss process conditions necessary to prepare gel spheres of different metal oxides or mixtures of oxides using HMTA. The general chemical flowsheets and process requirements described are based mainly on studies made at Oak Ridge National Laboratory (ORNL), but most processes referenced in open literature meet these requirements. Specific compositions made at ORNL using these process conditions are also given. Chemical Flowsheets For the gel sphere processes using HMTA, the solutions are prepared and formed into drops cold and then heated in an organic forming medium to promote decomposition of the HMTA into ammonia. These processes are commonly described as internal gelation since the controlling characteristic is the absence of mass transfer during gelation. The ammonia for the gelation reaction is generated homogeneously, which causes uniform precipitation of metal oxides without stresses or shell structures from concentration gradients. The heat transfer for small drops is more rapid than the chemical reactions, and the gelation times are only slightly dependent on the drop diameter. These characteristics of internal gelation become important advantages as the gel sphere diameter is increased. The overall reaction for the hydrolytic decomposition of HMTA in an acidic medium may be written (CH&N, + 6H20 4H+ s 4NH4++ 6HCHO (1) Hydrogen ions are neutralized in the decomposition of

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HMTA in the aqueous solution causing an increase in pH and thus can precipitate hydrous metal oxides or convert metal oxide sols into gels. Both the rate and the equilibrium for the HMTA decomposition (reaction 1) are temperature dependent. Feed solutions used to prepare spheres by internal gelation with HMTA should have (1) a long gelation time at a low temperature so that premature gelation does not occur in the feed system and (2) a short gelation time at a higher temperature so that drops are quickly solidified as gel spheres. The feed solution composition and the gelation temperature determine some gel properties. The metal concentrations should be as high as practical from solubility limits since low concentrations are likely to give soft gels, cracking, excessive shrinkage, or other undesirable results. Gels are produced from buffered solutions as ammonia is released by the decomposition of HMTA, causing the metal salts to precipitate as hydrous oxides. The overall reaction, given above, for decomposition of the HMTA is really a series of reversible steps which gives equilibrium pH values of 3 to 6 for most internal gelation process conditions. The HMTA is a weak base in aqueous solutions and can form salts as illustrated for nitrate salts HMTA H+ NOzHMTA-HNOB (2)

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The H+ and OH- concentrations are related by the equilibrium for the reaction HzO s OH- H+ (3)

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The precipitation of the hydrous metal oxides can be represented by M+" + nOHM(OH),+ MO,jy0.5nH20 (4)

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For most metals, gelation at low temperatures can be prevented or delayed by adding urea to complex the metal as follows M+" + x urea e M(urea),+" (5) The effect of reaction 5 is to reduce the M+" concentration so that a higher OH- concentration-that is, a higher pH-is required to form the gel according to reaction 4. The decomposition of the urea complex may be slow and may limit the gelation rate. Some metals form insoluble complexes with HMTA, and the more stable and soluble urea complex can prevent precipitation of the HMTA complexes. Although many other chemical reactions are possible, the most important gelation effects can be explained by reactions 1 through 5 or their equivalents. 0 1983 American Chemical Society

462

Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 3, 1983

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G E L A T I O N A N C AGING I N ORGANIC" 4- 5 0 T C I O O ' C

SINTERING H2

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1200 T O 1600'C

DENSE OXIDE

SPHERES

*TRICHLOROETHYLENE, PERCH LOROE T H Y L E h ? , MIYERAL C # L S , S I L I C O N E O I L S , OR Z-ETHYL-'-*EXANOL

Figure 1. Microsphere preparation by internal gelation.

The gelation of a drop is determined by a very complex combination of temperature-dependent equilibriums and reaction rates for reactions, 1,4, and 5 along with rates of heat transfer from the organic forming medium. Reactions represented in (2) and (3) are rapid and show much less temperature dependence than those in (11, (4), and ( 5 ) . These results were confirmed by measuring the pH and by observing color and viscosity changes. The reaction of HMTA with dilute HNO, or HC1 was followed by pH measurements to obtain information with respect to reactions 1and 2 without effeds from reactions 4 and 5. The pH changes from reaction 2 occur rapidly, even at 0 "C, and quite commonly give a feed pH that would precipitate the metals from nitrate or chloride salt solutions at room temperature. Precipitation is prevented by the combined effects of reaction 5 and the temperature dependence of reaction 4 at the low (usually near 0 " C )feed mixing temperature. The pH measurements for HMTA in dilute HNOBindicate that equilibrium pHs with respect to reaction 1 are approached in several days at 0 "C, several hours at 30 "C, and several minutes at 60 "C. Visible changes from clear to opaque drops can be observed in less than 2 s, indicating that reaction 4 is rapid at high temperatures. The pH measurements during formation of the same gels show changes over much longer time periods, which indicates effects from slower reactions such as (1) or (5). A schematic flowsheet (Figure 1)shows conditions used for several different products. Lower temperatures for the final step (sintering) can be selected to leave porous gels of large surface areas. The process operations on this flowsheet will be discussed in the following sections, and important process parameters will be defined. Feed Preparation Selection of feed compositions can be guided by general criteria even though the optimum conditions are generally

determined empirically as recipes. The acid deficiency, HMTA concentrations, and urea concentrations are best expressed as mole or equivalent ratios. The meaning of and selection of these ratios will be discussed in the order that would be logical for selection of a flowsheet. Metal Salt Compositions The desired product composition determines the metals present as salts or sols in the feed solution. For practical internal gelation, the metals should remain fluid as solutions or sols up to about pH 2 and should form hydrous gels at pH