Microwave-enhanced Recovery Of Mercury, Selenium, And Lead From Lead-zinc Smelting Acid Sludge

Acid mud containing elements such as mercury,selenium and lead produced by lead-zinc smelting,which is a kind of hazardous waste.It is of great significance to carry out the research on the recovery of mercury,selenium and lead in acid mud for the secondary resource recovery and prevention and control of pollution in lead-zinc smelting industry.In view of the three problems existing in the current acid mud treatment technology,such as unclear reaction process principle,high treatment cost and low recovery rate of valuable metals,based on the analysis of the thermal decomposition kinetics of acid mud with simple component and the principle of microwave heating to promote the pyrolysis of mercury compounds,two acid mud treatment processes were designed to treat lead-zinc smelting acid mud with complex components,they are wet pre-oxidation process for selenium extraction and pre-calcification process for selenium fixation and mercury removal,the influencing factors and process principle were discussed.The research contents and conclusions are as follows:This paper studied the thermal decomposition kinetics of acid mud by using model free method,model method and master curve method,the results show that the thermal desorption process of mercury compounds in acid mud accords with different diffusion models under different conversion rates.When the conversion is 0.1-0.2,the pyrolysis process of acid mud is controlled by the Parabolic law model;when the conversion is0.2-0.5,the pyrolysis process is controlled by the Jander model;when the conversion is over 0.5,the pyrolysis process is controlled by the Parabolic law model.On the basis of revealing the principle of pyrolysis process of acid mud,by comparing the mercury removal effect of microwave heating and conventional heating on acid mud containing mercury,it is found that microwave heating can selectively heat mercury compounds and enhance their pyrolysis.Under the microwave-assisted condition,the pre-oxidation process for selenium extraction and pre-calcification process for selenium fixation and mercury removal were proposed.In the process of extracting selenium from acid mud with complex components by pre-oxidation,through comparing the oxidation effect of different oxidants on acid mud,it is concluded that potassium permanganate is an ideal oxidant,which can effectively oxidize and leach selenium compounds from acid mud.Adding sodium hydroxide to selenium leaching solution can get mercury oxide precipitation,which is mixed with mercury leaching residue and dried,the mercury recovery rate reached 98.5%.Selenium was reduced by sulfur dioxide into the filtrate containing selenium,and the purpose of efficient separation and recovery of selenium and mercury was realized.Under the optimum conditions,the leaching rate of selenium can reach 96%.A large amount of lead in the roasting slag can be returned to the smelting system for recovery.The pre-calcification process for selenium fixation and mercury removal is to add calcium oxide to the acid mud containing mercury,in the condition of microwave roasting,selenium is converted into calcium selenite and lead selenite,which are left in the roasting slag.Mercury is volatilized separately,and then the roasting slag is added with sulfuric acid to get selenite,which is reduced by sulfur dioxide gas to get selenium.Through the study of roasting temperature,roasting time and calcium oxide addition,in the optimal conditions,the selenium fixation rate can reach 99.6% and mercury removal rate can reach 99.7%,which saves 45% roasting time compared with the conventional roasting process,a large amount of lead in the roasted slag can be returned to the smelting system for recovery.This process can save energy and efficiently recover mercury,selenium and lead from acid mud.By calculating the economic benefits of the two processes,the net benefit of pre-calcification process is the highest.To sum up,based on the thermodynamics and kinetics of pyrolysis of acid mud with simple component,this paper explored the principle of microwave-enhanced pyrolysis of mercury compounds in acid mud,so as to design two microwave-assisted processes for acid mud with complex component,the three problems existing in the current treatment technology of acid mud are solved,and the efficient separation and recovery of mercury,selenium and lead and the high value utilization of acid mud are realized.