A Study On Drying And Oxidation Mathematical Model Of The Ferrochrome Pellets In Grate Sintering Process

The sintering process of ferrochrome pellets is a series of sophisticated and closely related physicochemical phenomena.Based on a plane circulating roaster,a EulerianEulerian multi-scale mathematical model was developed in a two-fluid model using the finite volume method.Visual analysis was made for the multi-physical fields in the machine by scientific calculation.The drying process and coke-magnetite oxidation were established by un-reacted shrinking core model.Then the microscale reactions in pellets were coupled to the governing equations of gas-solid phase by the specific surface area.The pellets layer was treated as a porous medium to calculate the momentum damping,and the turbulent viscosity was determined by Realizable k-? turbulence model.The temperature of gas-solid phase was predicted accurately as the effect of the temperature and component content of pellets on specific heat was considered.The mathematical model was validated by exhaust gas temperature,the vapor concentration of the exhaust gas,pellets sintering temperature,and pellets density.The results showed that the furnace geometry had a great effect on the distribution of the multi-physical fields.There were a low-velocity subzone and a low-temperature subzone between different furnace zones.The heat transfer coefficient in pellets layer was affected by gas velocity more easily,and the maximum value was up to 323 W·m-2·K-1.The mass transfer coefficient was affected more easily by the gas temperature and the maximum value was 0.34 m·s-1.There were an over-moistened subzone in green pellets layer and a condensation subzone in sinter layer.The maximum moisture content was 0.102,7.4%higher than the initial content.Drying processes at different layer heights were different,which were mainly affected by gas temperature,wet bulb temperature,and wet bulb radius.In the decreasing drying process of the bottom layer,the drying rate got to increase as the temperature of wet bulb surface was higher than the boiling point of water.The drying rate got to decrease again when the wet bulb radius was smaller than 4.75 mm.At this point,the spallation of pellets was easy to occur.The priority of coke combustion was higher than the oxidation of magnetite.At the material outlet,the Cr2O3 content was 0.39,and the average temperature of pellets was 1680 K.The maximum sintering temperature in pellets layer was 1820 K,meeting the sintering demands.Temperature rising rate in the lower layer of green pellets layer was higher than the upper layer,and the maximum value was 532 K·min-1.In the analysis of the influence of process parameters,the results showed that exhaust gas temperature floating 20 K had a small effect on the sintering process.The velocity of grate had a great effect on the sintering process.0.305 m extra length was needed with the velocity increasing 0.01 m·s-1.The temperature of green pellets at the bottom layer begun to be lower than 1673 K from the velocity being higher than 0.0274 m·s-1.The grate velocity should be controlled in 0.0254?0.0274 m·s-1.In this way,the quality and the qualified sintering rate of green pellets can be guaranteed.