| Strengthening the utilization of lean ores such as limonite is a requirement for the development of the iron and steel industry,and drying pretreatment is important for the smelting of the limonite pellets in the furnace.The hot air convection drying has good heating uniformity,but it has low drying rate and high energy consumption.Microwave is a unique dielectric heating method,which would lead to both high drying rate and energy saving,but the heating nonuniform in the materials existed limits its application.The hot air-assisted the microwave drying is a meaningful method,which can take advantage of each other's strengths.The drying behavior of limonite pellets by the microwave-hot air combined drying experimental system was examined in terms of the influence of microwave power,hot air temperature,additives and drying modes.The moisture migration characteristics of the limonite pellets(diameter:15 mm)at the hot air convection(100-200?)or microwave radiation(100-600 W)were investigated.For the hot air drying alone,the effective moisture diffusivities of the sample during the falling rate period ranged from 2.07×10-9to 4.78×10-9 m2/s,and the apparent activation energy was 12.3 k J/mol.For the microwave drying alone,the effective moisture diffusivities of the sample during the falling rate period ranged from 1.71×10-8to 1.23×10-7 m2/s,and that for the limonite after mixing with 0.6%Na2CO3 increased,which were between 5.84×10-8-1.37×10-7 m2/s.Also,the values for the apparent activation energy of the limonite and mixture were 37.4 W/g and 16.4 W/g,respectively.In addition,the diffluent inorganic salts such as KCl,Na Cl and Na2SO4could effectively improve the microwave drying characteristics.For the hot air assisted with microwave drying,the range of the effective moisture diffusivities of the limonite at hot air speed of 0.5 m/s was between 5.26×10-8and1.12×10-7 m2/s.Through the response surface principle,the significance order of the factors regarding to the drying time was microwave power,hot air temperature and hot air speed,and that for the energy consumption was hot air temperature,hot air speed and microwave power.The microwave power of 500 W,hot air temperature of 146? and hot air speed of 0.5 m/s was an optimal operation condition for the combined drying.The energy efficiency for the microwave drying was 10 times as much as that for the hot air convection.The energy consumption of the combined drying was between hot air drying and microwave drying.In the intermittent combined drying mode,the energy consumption decreased with increasing the microwave radiation time.The heating uniformity of the microwave drying mode was improved by the increasement of feed sources number,mode stirrer,and combined drying.Through the experimental approach,the uniformity coefficient of the surface temperature distribution for the pellet reduced by 3.82%-13.39%with increasing the feed sources from one to three.The uniformity coefficient of the combined drying mode decreased by 11.7%-12.05%compared to that of the microwave.Through the numerical simulation,the uniformity coefficient of the electric field distribution inside the pellet fell by 10%-41%as rising the number of feeds from one to three.The uniformity coefficient of the electric field distribution inside the pellet with the O-mode agitator reduced by 56.9%compared with that without the agitator.The data in this thesis can provide a reference of the design and operation for the iron ore drying pretreatment and industrial drying equipment. |
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