EFFECT OF GELATIN ON THE AGGLOMERATION OF FINE AGBR CRYSTALS IN DOUBLE-JET PRECIPITATION

The effect of gelatin on the agglomeration of fine AgBr crystals forme d during the early stages of double-jet precipitation was examined by turbidimetry. At high gelatin concentrations, the stable number of cry stals formed increases with increasing reactant addition rate, as pred icted by an earlier model. However, if the amount of gelatin is progre ssively reduced, deviations from this model are exhibited. The control ling parameter is the relative amount of gelatin available to the prec ipitating AgBr at the silver reactant introduction point, expressed by the gelatin-to-AgBr ratio, and is dependent on the silver reactant ad dition rate, the gelatin concentration in the bulk suspension, and the dilution rate of the silver reactant stream. When the gelatin-to-AgBr ratio is decreased below a critical level (50 g/mol at 40-degrees-C), coalescence occurs, and the number of stable crystals decreases as th e reactant addition rate increases. This mechanism explains the deviat ions from the earlier model and other previously reported studies. The coalescence mechanism is further supported by electron microscopy and x-ray diffraction. At intermediate gelatin-to-AgBr ratios (50-150 g/m ol at pBr 2.3, pH 4.5 and 40-degrees-C) flocculation occurs. Unlike co alescence, flocculation is reversible and does not interfere with the nucleation and growth of the AgBr crystals.