Studies On Modeling And Optimization Of Iron-ore Pellets Induration Processes

The requirement of iron ore pellets is increasing which has displaced the dimishing natural rich iron ore as the main feedstock of metallurgical process.The iron and steel profits have been declined sharply in domestic and international market in recent years.The gloomy circumstance pushes the iron and steel enterprises transforming the extensive production into fine production which is particularly important to the high energy consumptive induration process. Based on the accurate process models, the improved control and optimization have been implemented by modern sintering enterprises to achieve the high efficiency, energe saving and emmission reduction.Based on the realistic grate-kiln systerm, modeling and optimizaion of sequential subprocesses of thermic iron ore pellets induration are discussed in this dissertation. The dissertation contains modeling and optimizaiton. In the first part, the whole process is divided into three subprocesses:green ball pelletizing, drying and oxidation and consolidation, due to the different kinetic and thermodynamic principles.In the second part, the energy extensive subprocesses- drying and oxidation and consolidation- are optimized by whole process optimization and segmented iterative optimization. The dissertation is organized as follows.1. The sequential addition of spherical agglomeration is proposed. In previous researches of induration process, the bed porosity is regarded as instant, and the parameters like porosity and permeability are not accurately calculated. For iron-ore pellet induration process, the green ball packing arrangement influences physical characteristics of pellet bed. Take the plastic deformation and size distribution of green balls into consideration, an improved numerical model for sphere packing process by using dropping and rolling rules which are derived from a realistic physical phenomenon is presented. The simulation is taken for different packing heights and size distributions and corresponding accurate porosity, permeability and surface area for normal distribution of sphere radii can be got by this model. The result shows that the model is efficient and well reflects realistic packing process, and the result are also applied in later models.2. The models of the dry and oxidation process of green ball as well as predicted models of dry and preheated pellets strength are proposed based on thermodynamic principles which depict the physicochemical reaction of drying and oxidation. The partial differential equations can well reflect the key parameters change with time and height of pellets bed, then with the historical data of realistic process,the parameters of model can be recognized, and comparison shows that the model is of efficiency and accuracy.3. The model of combustion and consolidation of preheated pellets is built together with the pellet strength prediction model. First, the temperature distribution is got by coal powder combustion simulation, then considering the ringing forming, the radius varying function is got by the data of realistic process of grate-kiln system. With the force analysis of pellets, the dynamic angle of repose can be calculated. The radius varying function and the corresponding dynamic angle of repose then substituted into the improved calculation of height of pellets, the height of pellets in each sub-kiln can be got. The curve of height of pellets in different feed rate is well simulated the real pellets movement and the mean residence time (MRT) is also close to the data of rotary kiln process. Based on these simulations, the predicted model of pellets final strength is proposed.4. Based on the kinetic model, the neural network ensemble is applied for hybrid modeling which can made compensation for the influence of uncertain or unconsidered factor. The Back propagation network is used as individual network, and the Bagging method is used for generating the training sample set. The simulation result shows that the hybrid model is more accurate and efficient.5. Research on optimization of induration process and proposed the segmented iterative optimization. Based on the former hybrid model, the optimal operation can be got by an improved NSGA-?algorithem. A new optimization based on shortest disctance between utopina point and Pareto front plane is proposed. The method is made the problem of global optimization transfer into the solution of shortest disctance between utopina point and Pareto front plane. The result proves the efficiency of the layered optimization.The whole work of dissertation is summed up, the future development and improvement of grate kiln systerm is also mentioned in the end.