Study On The Clean Leaching Process From Ag-contained Catalysts And The Regeneration Of The Leaching Reagent Ce(?)

Silver is one of important precious metals,with excellent thermal conductivity,conductivity and ductility,and excellent visible light reflectivity.Silver and its related products are applied widely in many fields such as electronics,photosensitive materials,chemical industry,medical equipment,aerospace materials,and scientific research.The Ag/a-Al2O3 catalysts are widely used in the production of ethylene glycol in the polyester industry,and in the production of ethylene oxide to produce ethylene oxide.After losing the activity the silver catalysts will need replace,so it is a very important silver secondary resource and has extremely high recycling value.At present,recovery of silver with nitric acid is mainly used in the industry.The process is simple,however there are still many disadvantages such as process is long,consumes large amounts of nitric acid and reducing agent,generates NOx waste gas,nitrate wastewater and neutralization slag,which has a large environmental impact.Therefore,it is particularly important to develop a green,waste-silver catalyst recycling process.The author's research group proposed a clean and environmentally friendly new technology that uses Ce(IV)solution as a leaching agent to recover silver from Ag/a-Al2O3 spent silver catalyst.In the leaching process no NOx gas is generated,and the leaching agent could be recycled by electrochemical reaction,it is expected to solve the problem of NOx gas polluting environment in the traditional leaching process.Aiming at the key issues such as leaching process,washing process and electrochemical regeneration in the new technology,this thesis systematically studied the process of leaching silver by Ce(IV)solution,the washing process of residue,and the Ce(NO3)3 electrochemical oxidation process.The effects of different reaction conditions on the leaching process,washing and electrochemical oxidation efficiency were investigated,providing the foundamental data for commercial practices.The main conclusions of this paper are as follows:(1)The leaching silver process from spent Ag/?-Al2O3 catalysts by ceric ammonium nitrate(CAN)solution was systematically studied.The results showed that increasing the reaction temperature,ceric ammonium nitrate concentration,nitric acid concentration and rotation speed were beneficial to improve the silver leaching rate.No gas was produced in the leaching process.As the concentration of nitric acid was greatly reduced,the corrosion of alumina carriers was reduced,no aluminum ions were detected in the leachate,which reduced the amount of impurities.The best leaching rate of silver is up to 99.9%.(2)The orthogonal studies were carried out,the results indicated that the main factors affecting the leaching of silver by CAN are:temperature>CAN concentration>HNO3 concentration>stirring speed.The optimal experiment conditions were obtained,the experiment was performed at 70?,with a lixiviant solution of 1.6 mo1·L-1 CAN and 0.1 mol·L-1 HNO3 and under stirring(60 r/min),the leaching rate was 99.8%after 1 h reaction and 99.9%after 2 h reaction.(3)The kinetic analysis of the leaching silver process was studied.The results showed that the leaching reaction can be described by an internal diffusion-controlled model,and the activation energy was calculated to be 38.83 kJ/mol.(4)The expansion experiment of new silver leaching process for Ce(IV)has been carried out,the residue washing process was systematicly studied.The results showed that,whether 5%nitric acid or deionized water was used,the silver content in the eluate was stable at 20 min,while the Ce content was still elevated at 30 min.Compared with pure water,washing with dilute nitric acid is beneficial to reduce the residual amount of Ce in the residue solid,but has no significant effect on the residual amount of silver.Increasing the flow rate of the washing solution and the washing temperature are beneficial to the washing of the Ce ions in the residual,and have little effect on the washing of silver.Dilute nitric acid with a volume fraction of 5%was used as a lotion,and after the washing 8 times under optimized conditions,the Ce content of residue was 123 g/t,the silver content was about 1000 g/t,and the relative leaching rate is about 99.4%.(5)A systematic study of the electrolysis oxidation Ce(NO3)3 has been carried out.The results showed that the anode current efficiency decreased with increasing anode current density and nitric acid concentration,while increasing the Ce(NO3)3 concentration and the electrolysis temperature were beneficial to increase the anode current efficiency.When the anode current density is 1500 A/m2,the electrolyte composition is 1.5 mol/L niobium nitrate and 1.0 mol/L nitric acid.The electrolysis were performed at 50?,the anode current efficiency can reach 98.5%after 3 h reaction.