It is a purpose of this study to make the mechanism for the reduction of an aqueous silver nitrate solution clear. At each run 1.50×10-4 m3 of the solution was loaded into a reducing gas bubbling batchreactor. The experiments were conducted for a range of temperatures from 333 to 353 K, SO2 partial pressures from 0.05 to 0.01 MPa, gas flow rates from 0.83 to 1.67×10-6 m3·s-1 and Ag concentrations from 0.01 to 0.05mol·m-3.
At a temperature of 353 K, no product was obtained when pH=1.0 regardless of the reaction time. In the pH range from 5.0 to 7.0, the yields of Ag and Ag2SO3 at 0.3 ks were 0 and 95 %, respectively, while the yields of Ag were 10 % at 1.8 ks for both pH's. On the other hand, the yields of Ag at pH=9.0 were higher than those when pH=5.0 and 7.0. Based on these experimental findings, a reaction model was assumed to be expressed as a consecutive reaction. Results of the calculated changes with time in the concentrations of reactant and products agreed well with the experimental results, in particular when pH=9.0.
In contrast to the above, when pH=11.0, only Ag was formed at 353 K, while Ag and Ag2SO3 were formed at 343 and 333 K. Apparent reaction rates showed 1 st order dependency upon Ag concentration. These experimental results were explained by a simplified model simlar to the above. In this case the calculated changes with time in concentrations of reactant and products agreed well with those observed. Activation energies for the individual reaction steps were 84, 349 kJ·mol-1. Consequently, the ratedetermining step of each reaction step was presumed to be controlled by the chemical reaction.
