Zhang, M; Jiang, S; Shi, J; Zhao, G; Yin, G; Zhang, Yi; Shi, Q; Liu, Z
Silver, as an active component, was mainly used for many oxidation reaction catalysts, especially as industrial catalysts for ethylene oxidation to ethylene oxide and methanol oxidation to formaldehyde for decades. Silver of the waste methanol oxidation catalyst was recycled by the electrolytic process after deironing with oxalic acid. Silver of the waste ethylene oxidation catalyst was covered by wet process, representative processes for silver dissolution include sulfuric acid dissolution, nitric acid dissolution, sodium sulfite dissolution, sodium thiosulfate dissolution, thiourea dissolution and alkali dissolution methods, and HNO(3)method was usually used while silver content was high in the waste silver catalyst.Either from the viewpoint of decreasing nitric acid consumption or from the NO(x) emission pollutant, it was beneficial to choose dilute HNO(3) to dissolve silver in wet one.The methods to obtain single silver from silver ion currently include reduction method (embodying the wets and dries), melting -screening -decarbonizin, hydropyrolysis and adsorption ones, the reduction one was often selected, and sodium borohydride, hydrogen peroxide, ferrous sulfate and Fe, Zn, and Al and aldehydes, alcohols, hydrazine hydrate and ascorbic acid were used as reductants respectively.Nitrate was first reduced to intermediate HNO(2) in the reaction HNO(3) with metals, and the types and compositions of products changed with the HNO(3) concentrations and the kind of metals.It should be believed that the optimum conditions of dissolving silver with HNO(3) were as follows: reaction temperature 85 degree C, reaction and constant temperature time 70 min,the amount of HNO(3) 1.2 times of theory value and the ratio of solid to liquid 1:4.The treatment courses of NO(3) exhaust included alkali adsorption, molecular sieve adsorption, catalytic reduction and dilute HNO(3) adsoportion.And finally it should be pointed out that the better way to obtain the highly pure single silver from the waste silver catalyst was silver dissolved in nitric acid first, then silver ions precipitated with chloride ion,and last silver chloride reduced by iron.The proposal concerning recovering single silver from the waste silver catalyst was to raise the recovery rate of silver.