Investigations On Burden Flow And Distribution Laws And Particle Segregation Behaviors During Charging Process Within Bell-less Blast Furnace

Currently, under the background of energy conservation and emission reduction in steel industry and upgrade of manufacturing industry, requiring to realize high efficiency and low carbon smelting as well as precise control for blast furnace operation, optimization and improvement on the blast furnace operation becomes one of the key issues. As the most flexible and most commonly used adjustment means among the four blast furnace operating patterns, top charging pattern determines burden particle distribution within blast furnace, and thus affects gas flow distribution in-furnace, playing an important role in promoting blast furnace smooth operation, improving gas utilization, reducing fuel ratio and so on. Currently, the widely used bell-less top blast furnace is mainly divided into serial hoppers type and parallel hoppers type bell-less top with huge difference, and burden flow and distribution rule on parallel hoppers type top is more complex, with insufficient researches. On the other hand, during charging process, burden not only flow and distributes as macroscopic burden flow, but also exists microscopic inter-particles segregation distribution among different size and different kinds of particles, researches on the latter part are insufficient for long time. Therefore, based on predecessors' researches, aiming at the charging process of serial hoppers type and parallel hoppers type bell-less top blast furnace, the paper applies the mechanism modeling method and discrete element simulation method respectively to the systematic study and analysis on the macroscopic burden particle flow and distribution laws and microscopic segregation behaviors, which provides reference basis and theoretical guidance for subsequent blast furnace design and selection and the operating practice. The main research contents and results are as follows.(1) Considering burden distribution difference in serial hoppers type and parallel hoppers type bell-less top, as well as in different type chutes, through analyzing the stress acting on burden flow, mathematical model of burden flow rate at flow control gate outlet, mathematical model of burden flow between flow control gate and rotating chute, three dimensional mathematical model of burden flow in semicircular section chute and rectangular section chute during multiring burden distribution process, mathematical model of burden flow trajectory and width in freezone, mathematical model of burden falling point and transient flow rate and mathematical model of burden profile are established, and through the 1:7 scale burden charging experiment, accuracy and reliability of the established mathematical models are verified. Among these, the burden flow mathematical model of the commonly used arc gate valve is built for the first time, and the impact point trajectory of burden flow on the semicircular section chute and rectangular section chute are calculated quantitatively respectively, of which the former is non-elliptic shape, the latter is ellipse. At the same time, the composite chute movement characteristics with horizontal circular rotation and tilting during multiring burden distribution process have also been considered, which can be used to calculate annular and spiral burden distribution process.(2) Using the developed comprehensive mathematical model, the influences of two kinds of main influential factors of furnace top equipment structure parameters and blast furnace production related parameters on the burden flow and distribution are calculated and analyzed respectively. The former mainly includes type of bell-less top, inner diameter of central throat tube, height of chute suspension point, chute tilting distance, chute length and shape of chute section, while the latter mainly includes burden kind, inversing hoppers pattern of parallel hoppers, opening of flow control gate, chute inclination angle, chute rotating speed, chute rotating direction, stockline height and gas flow rate. The results show that, circumferential segregation of burden falling point and transient flow rate on stock surface both exist in parallel hoppers charging process, height of chute suspension point, chute tilting distance, chute length, chute inclination angle and stockline height mainly affect burden falling point distance, having low influence on segregation degree of burden falling point and flow rate in parallel hoppers charging process. Reducing central throat tube diameter and increasing opening of flow control gate both can effectively reduce segregation degree of burden falling point and flow rate in parallel hoppers charging process. Compared with semicircle section chute, when using rectangular section chute, burden flow width is smaller and more concentrated, and the circumferential segregation degree of burden falling point radius and flow rate on stock surface are lower. The increase of chute rotating speed or gas velocity will not only increase overall burden falling point radius, but also deteriorate segregation degree of burden flow rate in parallel hoppers charging process. Altering inversing hoppers pattern and chute rotating direction will make burden falling point and flow rate circumferential distribution curves symmetrical about blast furnace center and 0°-180° line respectively with origin curves, and thus can compensate burden segregation degree in blast furnace to some extent.(3) Based on discrete element method, the discrete burden particle flow mathematical model is established, and the whole charging process from bunker to throat aiming at the actual 4350 m3 serial hoppers type and the 5500 m3 parallel hoppers type bell-less top blast furnace are simulated respectively, the microscopic particle segregation behaviors within each process are analyzed, and the reliability of the simulation model is verified by using measured results before blow-in stage of the serial hoppers type blast furnace. Studies show that, particle segregation phenomenon exists throughout the whole charging process, particle segregation exists in the burden layer thickness direction on the belt, and particles in the top hopper and bottom hopper of serial type and left hopper and right hopper of parallel type bell-less top also distribute nonuniformly. In discharging process, particles within the hopper flow as 'funnel flow', mean discharged particle size is small at early stage, then large at later stage, and becomes small at final stage with more little particles discharged, and the existence of feed cone can reduce particle size variation degree during serial hoppers charging process. Burden particles within the chute will deflect to some extent, and for parallel hoppers type blast furnace, particle flow can be different as chute locates in different orientation. In the throat, significant variation of mean particle size appears mainly in the radial and vertical direction, little segregation in circumferential direction, but for parallel hoppers type blast furnace, ununiform burden volume distribution also exists in circumference.