Study On The Effect Of Pre-treatment On Leaching Spent Auto-catalysts By Chlorination

The automobile exhaust catalysts is the most important metal’secondary resource of platinum group metals.So the research on the process of extracting platinum group metals has important practical significance.Because of its simple device,easy operation and short recovery time,the aqueous chlorination has been adopted widely as one of the most common methods recycling platinum group metal from the automobile exhaust catalysts.But the deficiency of this method is the high content of residual platinum group metals in leaching residue,which is up to 80-100g/t.So it can’t reach the abandoned standard.The research group did some relevant research on this problem in the early stage.On the basis of a large number of documents and some research in the previous research group,the paper study more comprehensively of this thesis about the influences of the low temperature oxidizing roasting,alkali roasting,reduction and oxidizing roasting combined with reduction pretreatment on the leaching ratio.Research shows that:By adopted the leaching system of HCl-H2SO4-NaClO3,under the conditions of liquid solid ratio L/S=5:l,the stirring speed was 350 rev/min and the leaching reaction time was 4h ect fixed parameters,the optimum leaching conditions for chlorination leaching of the exhaust catalysts after crushing and milling:sulfuricacid concentration is 4-5mol/L,hydrochloric acid concentration 3-4mol/L,sodium chlorate’s concentration 1.878mol/L,the dosage of sodium hypochlorite:15g and leaching temperature T=70 C.The optimum conditions for low temperature oxidizing roasting pre-treatment of automobile exhaust catalysts is roasting temperature 400 ℃ with3h roasting time.At this time,the leaching rate of automobile exhaust materials of Platinum and Palladium has greatly improved.The removal of organic matter is not sufficient under too low baking temperature or too short baking time.It may cause serious oxidation of platinum group metals when baking temperature is too high and the baking time is too long.Both are not conducive to the leaching of platinum group metals.In the pretreatment experiment of sodium borohydride reduction,the dosage of sodium borohydride 0.025mol and pH=9 are the best conditions for reduction pretreatment,which can maximize the leaching rate of Platinum,Palladium and Rhodium.Compared with the automobile exhaust catalysts,the reduction pretreatment of sodium borohydride can increase the leaching rate of Platinum,Palladium and Rhodium significantly.This shows that the platinum group metal or its surface being oxidized is an important factor which leads to the high residue of platinum group metals in the automobile exhaust catalytic chlorination leaching residue.The appropriate reduction pretreatment can increase the leaching rate of Platinum,Palladium and Rhodium,while excessive reduction may lead to the reduction of some base metal oxides,which is not conducive to the leaching of platinum,Palladium and Rhodium.The best compound pretreatment conditions for automobile exhaust catalysts is 200℃ roasting temperature and 0.02mol dosage of sodium borohydride.the leaching rates of Platinum,Palladium and Rhodium are respective 140.85%、99%、99.42%under these condition.Composite pretreatment has opened up the package of carbon and organic matter to the automobile exhaust catalysts to a certain extent,and also reduced some platinum group metal oxide to the form of monoplasmatic metal.So it promotes the leaching rate of Platinum,Palladium and Rhodium.