STUDY OF CAUSES AND PREVENTION OF HARD CAKE FORMATION DURING OUT-OF-PILE CIRCULATION OF AQUEOUS THORIUM OXIDE SLURRIES

< = B 9 8 9 = = ; < eposits formed on surfaces of the containing system during circulation of aqueous ThO/sub 2/ slurries at high temperature. Although entire surfaces of systems were coated with 1/4 to 1/8-in.- thick thoria deposits, cake formation was usually localized, forming preferentially in the pump impeller. Frequently, impeller deposits accelerated bearing failure. Cake formation was usually accompanied by formation of equal dense spherical aggregates 5 to 60 mu in diameter which remained suspended in the slurry. Caking and sphere forming phenomena, as investigated by tests of slurry properties in the laboratory and in a 30-gpm slurry loop, are described, as are methods for preventing caking. Thoria cakes and spheres are believed to form by reagglomeration of small particles or fragments worn or chipped off the larger thoria particles composing the initial slurry. It is thought that small fragments are forced over each other by the shear stress into positions of minimum energy where they are held together by van der Waals' forces. Thoria solubility is not believed to be an appreciable factor in cske or sphere formation. The extent of chemical bonding in the deposits is not clear. Greatest cake-forming tendency was noted in thoria prepared by calcination of Th(C/sub 2/O/sub 4/)/sub 2/ at 800 deg C, though cakes did occur with other ThO/ sub 2/ preparations. Deposits formed from oxide batches that contained particles more resistant to degradation were softer, apparently because there were fewer fine fragments. A deposition preference between different metal surfaces as well as between differert velocity sections was noted. Cake formation by fresh batches of thoria, that is, slurry in which the initial oxide particles were not made finer by previous circulation, is prevented by any factor which reduces particle comminution. Cake formation is avoided if particles are too resistant to degradation or if they degrade rapidly to intermediate-size particles. Intermediate-size particles, 0.1 to 0.5 mu , apparently do not yield fine fragments readily because of the reduced shear forces which are acting on smaller particles. Improved particle integrity can be accomplished by the method of oxide preparation, for example, the use of high calcining temperature or long digestion time. Cake formation by fresh oxides is also prevented by the addition of small quantities of surface-active electrolytes to the slurry. Effective electrolytes tested included CrO/sub 3/, NsAlO/sub 2/, Na/sub 2/SiO/sub 3/, and MoO/sub 3/. The electrolytes are thought to be effective by means of reducing particle fragmentation. Prevention of cske formation by slurries already containing particles in the caking-size range (e.g., a slurry composed of spheres produced during circulation) is more difficult, because agglomeration of the fine particles must be prevented. Lithium sulfate tended to reduce cake formation but was not completely effective. Other electrolyte additives tested (CrO/sub 3/ and NsAlO/sub 2/) failed to hinder cake formation during ThC/sub 2/ sphere circulation. (These spheres were "loop produced" in a previous slurry circulation experiment.) However, after extended circulation during which partial dispersion of the spheres took place, cake formation was almost absent. It is postulated that the very fine particles wearing off the spheres gradually agglomerate into particles which are too large to form cskes but so small that fine particles or fragments are not worn off readily. Autoclaving of fresh thoria in water or dilute acids did not strengthen the particles sufficiently to prevent cake formation. Cake formation did not occur when the loop circulation temperature was reduced to 45 deg C. Extremely low thoria concentration did not deter cake formation. The integrity of loop-produced spheres was greatly improved by a combination of chemical and thermal treatment but was not improved sufficiently to prevent cake formation. Cake deposited in a ThO/sub 2/- circulation system is partially destroyed by thermal cycling and can be remove