The tendency for aggregation and disruption of aggregates of calcium oxalate dihydrate (CaOx) crystals in urinelike mother liquor was quantitatively studied in a two-stage mixed-suspension, mixed-productremoval crystallizer / Couette-flow aggregator operated in series. The degree of aggregation was obtained by comparing measured numbersize and volume-size crystal distributions in the crystallizer and aggregator outlets. Aggregation was a strong function of CaOx supersaturation and may have changed slightly with rotation speed in the Couette-flow aggregator. Rupture of the aggregates increased as the spindle rotation in the aggregator increased (presumably increasing turbulence). Average shear rates in these experiments were appreciably above estimated physiological levels. R. W. Hartel, B. E. Gottung, A. D. Randolph Department of Chemical EngineeringUniversity of Arizona Tucson, AZ 85721 G. W. Drach Department of Surgery Arizona Health Sciences Center SCOPECalcium oxalate is one of the main mineral constituents found in kidney stones from patients in developed countries with a high gross national product, presumably because of protein-rich diets. CaOx kidney stone formation in the United States represents a significant medical problem. Such CaOx stones examined ex vivo are often found to be polycrystalline aggregates of CaOx crystals. It is likely that aggregation mechanisms play a role in the genesis and retention of such stones. The purpose of this study was to quantitatively examine the formation of aggregates in a urinelike liquor to specifically study the effects of average shear rate (aggregator rotation rate) and CaOx supersaturation. CONCLUSIONS AND SIGNIFICANCEThe degree of aggregation in the experimental apparatus could be quantitatively expressed by several statistical measures, e.g., the decrease in crystal numbers through the aggregator (normalized by total number in the crystallizer), the increase in crystal numbers above 15 pm, and the increase in average particle volume. All three of these statistical measures of aggregation correlated strongly with aggregator rpm and [Ox'-] feed concentration. Maxima in aggregation vs. rotator speed curves coincided with the predicted onset of Taylor turbulence instabilities within the flow pattern. Increased shear rate up to the point of turbulence increased aggregation. Aggregation increased with increasing [Ox2-], indicating the formation of more stable particle clusters at higher supersaturations.These data should be useful to the urologist in constructing theoretical models of kidney stone formation. The experimental technique and data analysis should prove useful in the study of aggregation phenomena in other systems.

The importance of aggregation in calcium oxalate urolithiasis, although not fully understood, has long been postulated. Previous investigators of calcium oxalate crystal aggregation have applied static crystallization rather than continuous flow techniques to their studies. We describe the use of a Couette agglomerator in series with our previously reported continuous flow mixed suspension-mixed product removal crystallization system. We compared synthetic urine controls with 5 per cent volume-in-volume human urine additions from normal persons or patients with calcium oxalate stones. There was no significant difference in nucleation, linear crystal growth rate or total crystal mass between normal persons and those with stones. Control nucleation rate was significantly higher than in either human urine addition group. Comparison of aggregator particle size distributions revealed significant differences in aggregation among the control, normal and stone groups. We concluded that urine inhibitors to aggregation are somewhat deficient in patients with stones, resulting in the generation of larger particle masses or eventually stones.

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