废银催化剂中银的回收技术进展

Advances in technology for Silver Recovery from the Waste Silver Catalysts

银可用作多种氧化反应催化剂的活性组分,尤其是作为乙烯氧化制环氧乙烷和甲醇氧化制甲醛的催化剂已工业化应用多年。甲醇氧化制甲醛废银催化剂采用草酸除铁后,主要采用电解工艺进行回收。目前废环氧乙烷银催化剂中银的回收采用湿法,银的溶解方法包括硫酸溶解法、硝酸溶解法、亚硫酸钠溶解法、硫代硫酸盐溶解法、硫脲溶解法和强碱溶解法,高含量废银催化剂常采用硝酸溶解法。无论是从降低硝酸的消耗量,还是从降低NOx的排放量上看,选用稀硝酸溶解银都是有利的。从银离子得到单质银的工艺包括还原法(溶液中还原和干态还原)、熔炼-筛选-脱碳法、热解法和吸附法,通常采用还原法。常用的还原剂有硼氢化钠、双氧水、硫酸亚铁及Fe、Zn、Al等无机还原剂和醛类、醇类、水合肼、抗坏血酸等有机还原剂。硝酸与金属反应时,首先被还原成中间体HNO2,反应产物的种类及组成随硝酸浓度和金属种类而发生变化。有研究认为,硝酸溶解废银催化剂中银的最佳工艺条件为:反应温度85℃,反应及恒温时间70min,酸量为理论耗量的1.2倍,固液比1:4。NOx尾气的处理主要有碱吸收法、分子筛吸附法、催化还原法和稀硝酸吸收法等。硝酸溶解-NaCl沉淀-Fe粉还原法是将废银催化剂中的银回收为单质银的较好工艺。今后努力的方向是进一步提高银回收率。

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 HNO3 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 NOx emission pollutant ,it was beneficial to choose dilute HNO3 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 HNO2 in the reaction HNO3 with metals,and the types and compositions of products changed with the HNO3 concentrations and the kind of metals.It should be believed that the optimum conditions of dissolving silver with HNO3 were as follows： reaction temperature 85℃,reaction and constant temperature time 70 min,the amount of HNO3 1.2 times of theory value and the ratio of solid to liquid 1：4.The treatment courses of NOx exhaust included alkali adsorption,molecular sieve adsorption,catalytic reduction and dilute HNO3 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.