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Study on microwave carbon thermal reduction - ultrasonic leaching indium zinc slag

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Tutor: PengJinHui
School: Kunming University of Science and Technology
Course: Non-ferrous metallurgy
Keywords: microwave,Ultrasound,Kinetics,Simulation,Optimization ofprocess,Zinc,indium
CLC: TF813
Type: PhD thesis
Year:  2013
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Abstract:
The traditional hydrometallurgy procedures applied for Zinc production are roasting, leaching, purification and electro-deposition. However, the common zinc ore is generally associated with iron, which inevitably leads to the generation of zinc ferrite in the roasting process, meanwhile, Indium associated in zinc ore will also replace iron ion or interstitial manner in the zinc ferrite lattice to generate the Indium based zinc ferrite. Because of the strong stability zinc ferrite, it is hard to be treated by acid leaching in conventional hydrometallurgy process, Zinc and Indium are lost in form of Indium based zinc ferrite in the leaching slag. At the same time, the insoluble lead sulfate usually concentrated on the surface of zinc leaching residue, which would hinder the leaching reaction and lead to the lost of large amount of zinc and indium resource. Tradition methods used to deal with such leaching residue could be rotary kiln volatilization, dust capture and acid leaching. However, it should be noted that the fumes phase of rotary kiln dust is complex, which still contains a lot of smoke full of zinc ferrite and lead sulfate, thus, the recovery rate is still low.With the increasing demand on waste resource recycling, the present paper attempts to treat zinc leaching residue by combination of microwave and ultrasonic for the recovery of zinc and indium. First, the microwave heating with carbon is used to decompose the zinc ferrite in the slag, and then ultrasonic is used to enhance the leaching process to break the insoluble materials such as PbSO4and SiO2gangue composition on the surface of slag particles, some meaningful results are obtained.The dynamics on conventional and microwave heating carbon reduction of zinc ferrite are studied, at the conditions of conventional heating of950℃, C/ZnFe2O4of1:3, particle size of74-61μm and heating time of90min, the maximum reduction rate of zinc ferrite is85%; while at the conditions of microwave heating of850℃, C/ZnFe2O4of1:4, particle size of89-74μm, microwave power of1800W and heating time of60min. the maximum reduction rate is90%. The experimental result indicates that the microwave heating has increased only5%reduction rate, however, it not only reduces both the reaction temperature and the reaction time, but also alleviates the effect of particle size on reduction rate. The apparent activation energy of conventional heating is about11.47kJ/mol, the main control step is diffusion control; the apparent activation energy of microwave heating is about31.40kJ/mol, the main control step is chemical reaction control. The reason might be the characters such as penetrating and selectivity of microwave heating, which could optimize heat and mass transfer.In order to guide the ultrasonic enhancing leaching experiment, Matlab is applied for anticipation and simulation of characteristic value of the sound field and the cavitation effect, the following conclusion are obtained: ultrasonic frequency20KHz, sound intensity40w/cm2、Bubble radius of the initial equilibrium5×10-5m、environment stress1×10Pa% temperature70℃; and the force on the mineral particles appears a sine wave form with the past of time, and with mineral particles movement, the force also show irregular variations.The leaching kinetics on conventional and ultrasonic enhancement experiments is studied. At the conventional leaching conditions of sulfuric acid concentration of170g/L, particle size of61-53μm, solid to liquid ration of1:5, temperature of85℃, and leaching time of240min, the leaching rate of zinc and Indium are82%and80%, respectively. At the ultrasonic enhancing leaching conditions of ultrasonic power of260W, sulfuric acid concentration of140g/L, particle size of74-61μmm solid-liquid ratio of1:4, reaction temperature of75℃and reaction time180min, the leaching rate of zinc and Indium are both in excess of90%. Basing on the above results, the ultrasonic enhancing leaching can raise the leaching rate of zinc and indium for at least10%, the reaction temperature, the reaction time and the sulfuric acid consumption are also greatly reduced, moreover, the influence of particle size on the leaching rate is reduced as well. Based on the kinetics, the apparent activation energy of the conventional leaching on zinc and indium are of15.46kJ/mol and19.09kJ/mol, main control steps are diffusion control; the apparent activation energy of ultrasonic enhancing leaching on zinc and indium are33.90kJ/mol and46.70kJ/mol, the main control step is surface chemical reaction control; the kinetics equation of conventional leaching and ultrasonic enhancing leaching have been obtained.In order to get better experimental parameters, both conventional and unconventional experiments are designed using response surface methodology. Compared with conventional heating, the microwave heating can shorten the reaction time and lower the reaction temperature. The biggest zinc ferrite reduction rate of81.29%for conventional heating is obtained at reaction time of92min and reaction temperature of904℃, in contrast, the reaction time of microwave heating is reduced by23%, the reaction temperature is reduced for about114℃while the reduction of zinc ferrite rate is above85%; while in the leaching process, the model of the optimized zinc and Indium Leaching indicates a temperature reduction of17℃with the assistance of ultrasound enhancement, the zinc leaching rate increased from80.49%to91.22%, Indium Leaching rate increased from83.45%to92.99%.In all, aiming at the existing deficiencies of deal with high zinc ferrite and insoluble substance content residue process.The microwave heating with carbon is used to decompose the zinc ferrite in the residue, and then ultrasonic is used to enhance the leaching process to break the insoluble materials on the surface of slag particles at the first time.Then,using kinetics, computer simulation and response surface optimization to research the experimental process and mechanism, and the SEM-EDS, XRD and laser particle size analysis are used to proved strengthening mechanism of microwave and ultrasonic experiment. In conclusion, an effective way is provided for the recovery valuable metal from the unmanageable residue.
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