酸性チオシアン酸アンモニウム溶液による銀の溶解

抄録

Exit gas from a coke oven is treated to eliminate sulfur and cyanate with aqueous ammonia, and the waste solution from this process includes about 11% NH4SCN and about 9%(NH4) 2S203. The solution, however, is only disposed of after combustion. Fundamental researches were made on the utilization of the solution in dissolving metals or metal oxides.
The effects of concentration of thiocyanate, quantities of oxidizing agents, pH, and temperature on the dissolution of silver were studied.
The following have been concluded:
1. Fe (III) and oxygen gas are necessary to dissolve silver. The pH value of the solution must be below 3 for keeping a sufficient concentration of Fe (IlI).
2. Dissolution rate of silver is proportional to the concentration of thiocyanate at lower concentrations ofthiocyanate, but, at higher concentrations, it becomes constant independently of the concentration.
3. When the concentration of thiocyanate is high enough to make the dissolution rate of silver independent of the concentration, the dissolution rate increases almost proportionally to the increase in the concentration of Fe (III).
4. When the dissolution rate is almost proportional to the concentration of Fe (III), the rate-determining step is the diffusion of Fe (III) ions to the surface of silver particles.
5. When the rate is proportional to the concentration of thiocyanate, the rate-determining step is the chemical reaction of thiocyanate with silver on anodic sites on the surface of silver particles.
6. Overall reactions are considered as follows:
Ag+Fe (SCN) _b ^(b-3)-+a·SCN^--Ag (SCN) _a ^(a-1)-+Fe (SCN) _c ^(c-2)-+ (b-c)·SCN^-Fe (SCN) _c ^(c-2)-+ (b-c)·SCN^-+H⁺+1/4 O₂-Fe (SCN) _b ^(b-3)-+1/2H₂O