Microwave-enhanced Carbon-thermal Reduction Of Low-grade Pyrolusite

Manganese is an important resource of our country.The rich manganese ore,which accounting for only 5%of the total national manganese ore to identify resource reserves,is becoming increasingly scarce after years of mining and consumption.This has seriously constraint the production and sustainable development of manganese product in China.What is more,the rich reserves of pyrolusite with low grade between 20-30%has not been exploited,due to the questions of high reduction cost,serious environmental pollution,and so on.Microwave heating,which is clean and can decrease the processing temperature and shorten the processing time,is an effective method to solve the technical bottlenecks of reduction of low-grade pyrolusite.In this thesis,we aimed to realize the microwave heating carbothermal reduction of low-grade pyrolusite.Using simulation method,we determined the key factor that affects the product homogeneity of microwave calcination when measuring the temperature by thermal couple.We measured the microwave absorbing properties of the material by perturbation theory of microwave resonant cavity.With XRD,SEM,EDS,etc.we analyzed the phase,microstructure,and element distribution of the reduced product.For carbothermal reduction of low-grade pyrolusite of both microwave heating and conventional heating,we established the process mechanism,investigated the dynamic process,and optimized the process parameters.At last,we extended this carbothermal reduction method to the fabrication of nano-porous MnxOy powder.The main research details and results are as following:By using perturbation theory of microwave resonant cavity we studied the change of dielectric constant of soft manganese ore powder between room temperature and 1000?.We also studied the room temperature microwave spectroscopy of soft manganese ore powder with different particle size,of different carbonaceous reducing agent and soft manganese ore mixed with different carbonaceous reducing agent,the dielectric constant change of soft manganese ore at different temperature.Finally we studied the microwave heating process of material with different microwave absorbing performance under different microwave power.The results show that:the dielectric constant of soft manganese ore powder is relatively stable and the heating rate is constant below 300?.Above 300? the dielectric constant and heating rate increase gradually.Decreasing the particle size can efficiently could enhance the microwave absorbing performance obviously.The carbonaceous reducing agent and soft manganese ore mixture has well microwave absorbing performance that can satisfy the fast microwave heating requirement.The increase of microwave power or the microwave absorbing performance of material can not only remarkably shorten the time heating to target temperature,but also largely reduce the energy consumption.Combined with XRD,SEM and EDS,we focused research on the phase and micro structure change of product under different carbothermal reduction temperature for both microwave heating and conventional heating.The results show that:for microwave heating,MnO₂ in pyrolusite was reduced to low charge nano-porous Mn_xO_y due to deoxidation.The phase transition process is .The MnO transition temperature?500??is 300? lower than that reported by transitional heating.Above 650?,Fe₂O₃ in the pyrolusite started transforming to dense Fe₃O₄.Above 750?,the reduced MnO reacts with SiO₂ and forms MnSiO₄ at the interface.For transitional heating,the reduction process of MnO₂ and Fe₂O₃ in the pyrolusite is similar with that of microwave heating.The difference is that reduced MnO,Fe₃O₄,and MnSiO₄ was obtained at 750?,800? and 850?,respectively.The dynamic research of carbothermal reduction of low-grade pyrolusite under microwave heating and traditional heating was performed.The results show that under 750?microwave heating,when the granularity of pulverized coal and pyrolusite is about 61?m,the ratio of them is 1:10,the holding time is 50 minutes,the reduction ratio of manganese in the pyrolusite reached 96.72%.While under traditional heating,it should be hold at 900?,which is 150? higher than that of microwave heating.According to dynamic analysis,both of the microwave and traditional heating are determined by interface chemical reaction.The corresponding apparent activation energy of microwave heating is 34.66%lower than that of traditional heating,which are 39.61kJ/mol and 60.62kJ/mol,respectively.We studied the process parameter of carbothermal reduction of pyrolusite under microwave and traditional heating by response surface method.We found that when heated by microwave,the ratio of the pulverized coal and pyrolusite was 1.1:10,reduction temperature was 750?,holding time was 25 minutes,the manganese reduction rate of 92.54%can be achieved.For the traditional heating,however,the temperature was as high as 860?to achieve the same reduction rate.We investigated the carbide reduction of soft manganese ore as well as the comparison with reduction of pure MnO₂,and found that the change of phase and microstructure of MnO₂ in both these two materials are identical under carbide reduction,and the MnO₂ can translate to nano-porous structure.When using microwave heating,the transition temperature of MnO₂ to MnO in pure MnO₂ is 50?lower than that in soft manganese ore.By creatively using carbide reduction method,we prepared the phase,grain size and porosity controllable nano-porous MnxOy.The studies of this thesis provide new idea to make use of the low-grade pyrolusite economically and reasonably,which also has potential theoretical and practical meaning for exploiting other low-grade ore.