| In addition to various pollutants,e-waste contains many recyclable metals,such as large quantities of copper and high value gold.Its grade is much higher than primary minerals.Extraction and recovery of these valuable metals can not only save limited natural mineral resources,but also obtain considerable economic and social benefits.It is of great practical significance to carry out the research on the microbiological leaching of copper and gold from e-waste due to the economic and environment-friendly advantages of microbiological treatment of e-waste.In this paper,the recovered waste circuit board was used as the electronic waste material to be processed,and the preserved Thiobacillus ferrooxidans and the isolated Chromobacterium violaceum were used as experimental strains.The bioleaching of metals from waste circuit board by Thiobacillus ferrooxidans and Chromobacterium violaceum were processed.Two-step bioleaching method was finally employed to extract copper and gold from waste circuit board,and the leaching step of gold was optimized by response surface method.The main research contents and conclusions of this paper are as follows:?1?Study on leaching process of metal from waste circuit board by Thiobacillus ferrooxidans.This study were conducted by the activation of bacteria and acclimation culture in the medium containing waste circuit board particles,these mthods was to improve oxidation activity of bacteria to Fe2+and the tolerance activity to waste circuit board.The results show that after activation and domestication,Thiobacillus ferrooxidans has the ability to oxidate 98.9%of ferrous oxide,and could withstand 15g/L of waste circuit board particles in the medium.With the optimized conditions are32°C,initial pH 2.0,initial Fe2+concentration 44.3 g/L,metal addition 5g/L,particle size 0.075-0.147mm,and the rotate speed 130 rpm,bacteria have the best leaching capacity and could extracte 99%copper from waste circuit board,while gold is basically not leached.?2?The investigation of metal bioleaching from waste circuit board by hydrogen cyanide bacteria—screening of bacteria.The bacteria producing hydrocyanic acid in soil were isolated and screened,and a purple strain with the ability to produce hydrocyanic acid was obtained.The strain was Gram-negative bacteria.The electron microscope result shows that the strain has short rod shape and obtuse circle at both ends.According to the 16s rDNA sequence of the strain and the constructed phylogenetic tree,it was found that the strain was closely related to the purple Chromobacterium violaceum,and the strain was identified to belong to the purple Chromobacterium violaceum by biochemical analysis.The conditions for the production of hydrocyanic acid from Chromobacterium violaceum were optimized.It is confirmed that the highest cyanide yield can reach 36.9mg/L after cultured in a pH10.0 medium containing 4 g/L glycine,5.0×10-33 mol/L MgSO4 and 2.00×10-22 mol/L K2HPO4.?3?The investigation of metal bioleaching from waste circuit board by hydrogen cyanide bacteria—studying of bioleaching.The bacteria were domesticated by culturing purple bacteria in the medium containing waste circuit board particles and increasing the pH value of the medium to make the bacteria more tolerant to waste circuit board and adapt to higher pH environment.After domestication,the bacterium was able to tolerate the waste circuit board particles of 20 g/L and adapt to the growth environment of pH range of 7-11.At the initial pH value 10.0,glycine 4g/L,metal addition 6g/L,particle size 0.075-0.147mm,MgSO4 4.0×10-3mol/L and K2HPO41.0×10-2mol/L,temperature 30?and the 130 rpm rotating speed of shaking bed,the leaching effect of this bacteria on metal in waste circuit board was the best.The leaching rate of copper can reach 45.43%,and the maximum leaching rate of gold is38.68%.?4?Study on two-step bioleaching process for leaching metals from waste circuit board.After the investigation of the single-step leaching of metals in the waste circuit board by bacteria,the two-step bioleaching method was further used to extract copper and gold from the waste circuit board.The first step was to treat the waste circuit board with Thiobacillus ferrooxidans,and leaching rate of copper was 99%.In the second step,the leaching of Chromobacterium violaceum was optimized by response surface methodology.Based on the single factor experiment,a four-factor-three-level experiment were designed,and leaching rate of gold was used as response value and developed by Box-Behnken central composite design method of Design-Expert.The results of two regression analysis show the best combination.The optimum leaching conditions are as follows:pH value 10.5,glycine 4.02 g/L,magnesium sulfate5.67×10-3 g/mol,culture temperature 31.0°C.Under this condition,the gold leaching rate can reach 72.58%.Through the research of this thesis,a strain of Chromobacterium violaceum producing hydrocyanic acid was isolated and screened,which provided a good strain resource for the treatment of waste circuit board by microbial method.In the process of microbial treatment of waste circuit board,when the bacteria are single-step leaching metal in the waste circuit board,the comprehensive leaching efficiency of the metal is low especially the gold,and the electron is processed by the two-step bioleaching method.After the waste is used in the second step of leaching to optimize the leaching conditions by response surface method,the leaching efficiency of gold is greatly improved,reaching 72.58%.Compared with the single-step direct treatment of e-waste,the two-step bioleaching method is more suitable for the comprehensive extraction of metals in e-waste,and has a guiding role in the future leaching of metals in e-waste by microbial methods. |
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