Research On Microwave-enhanced Biomass Pyrolysis And Reduction Of Low-grade Pyrolusite

Manganese as an important national strategic resource,its application in the national economy and steel industry is irreplaceable.However,the industrial production of China's abundant low-grade pyrolusite?Mn%<30%?resources has been blocked by the tricky problems such as high process cost,low efficiency,high energy consumption and serious pollution.Compared with traditional heating,microwave heating can increase the reaction rate of the reduction process;meanwhile,biomass is the only green renewable carbon-containing energy source that can partially replace fossil fuels.Therefore,based on the urgency of currently developing environmentally friendly and cost-effective reduction technology for low-grade pyrolusite,this paper proposed a new process for microwave-enhanced biomass pyrolysis to reduce low-grade pyrolusite,using biomass instead of coal as a reducing agent and using microwave heating replacing conventional heating as a heat source,with the two environmentally friendly and green methods combined to reduce low-grade pyrolusite.The main findings were drawn as follows:?1?The reduction process of pyrolusite by biomass pyrolysis can be divided into four stages:dehydration stage,pre-pyrolysis stage,main reduction stage and slow reduction stage.The change in the dielectric properties of biomass-pyrolusite mixtures was mainly attributed to the crystal transformation of the amorphous Mn O₂ and the reduction reaction of pyrolusite caused by biomass pyrolysis.Biomass-pyrolusite mixtures endowed the excellent microwave-absorbing ability,having the minimum?_r'value high to 6.746?F/M?.It took only 11.0min for walnut shell-pyrolusite mixture to be heated from room temperature to 800?.The temperature rise curves of the five biomass-pyrolusite mixtures showed a lower temperature rise rate at the front and end,and a higher temperature rise rate at the middle,which the trend was attributed to the change in the dielectric properties of the mixtures with temperature.In addition,the reduction process of pyrolusite by biomass pyrolysis included the two stages:biomass pyrolysis to generate the reducing components,and the reducing components to reduce low-grade pyrolusite.?2?The manganese reduction ratio reached to 92.01%,under the conditions including 650? of reduction temperature,30 min of holding time,10:1.8 of M_ore/M_bio,and 1200 W of microwave power.Compared with conventional carbothermal reduction or microwave carbothermal reduction methods,the biomass pyrolysis reduction method under microwave heating rendered high manganese reduction efficiency and the reduction temperature decreased by 100?,which is expected to realize low-energy consumption production and efficient and clean utilization of low-grade pyrolusite.During the pyrolysis reduction process,the manganese oxides in the pyrolusite followed the principle of the step by step conversion:Mn O₂?Mn₂O₃?Mn₃O₄?Mn O.After microwave reduction,the peak intensities of the?100?,?110?,and?021?planes of the Mn O phase appeared at 2?=20.211,2?=21.557,and 2?=36.357,respectively,with smooth and clear diffraction peaks,indicating that the biomass reducing agent has efficiently converted Mn O₂ into Mn O under microwave heating;meanwhile,there was absent of Fe O phase detected,achieving the selective reduction of low-grade pyrolusite.The microwave reduced pyrolusite had a loose and porous structure,and many cracks and holes appeared on the surface,which is attributed to the selective heating characteristics of microwave heating and the reducing gas released by biomass pyrolysis.?3?Reaction kinetics analysis indicated that the reaction process of low-grade pyrolusite reducing by biomass pyrolysis under microwave heating was controlled by chemical reaction.Pyrolysis kinetics studies found that the Coats-Redfern first-order reaction kinetic equation fitted well with the thermogravimetric data of walnut shell-pyrolusite mixtures,with the degree of fit?R₂?exceeding 0.995,which the finding solidified the conclusion obtained from reaction kinetics analysis..Additionally,the activation energy of reducing low-grade pyrolusite by biomass pyrolysis was 5.62k J·mol^-116.69 k J·mol^-1,and the pre-factor was 0.0426 min^-10.515 min^-1.This thesis proposes a new process for the selective reduction of low-grade pyrolusite by biomass pyrolysis under microwave heating,which can provide a new direction for the low-energy consumption production and efficient and clean utilization of low-grade pyrolusite.