Study On The Mechanism And Reaction Kinetics Of Ring Forming In The Grate-Rotary Kiln

Ring forming has become the chief problem of restraining the pellet ratary-kiln's direct motion. It has been the key to make this technology successful that we should find the best way to search the essence of ring forming and restrain it. This project was done with WISCO Ezhou Pellet Plant and mainly studied on the reaction mechanism and kinetics of ring forming from the standpoint of high-alumina lining refractory reacting with the mass in the kiln. Hereby, we could present effectual theoretical gist to slow up the ring forming.Firstly, the basic characteristics of the ring and lining refractory and their effects on ring forming were investigated. The results show that: the main mineral phase of the ring is hematite; the impurity in the ring is very little and congregates in grain boundaries and pores as glass phase, which is composed of Al₂O₃ and SiO₂ mainly and fewer alkali metal oxides such as CaO,K₂O,Na₂O. The ring's growth mainly depends on ferrum oxide's solid-phase sintering and reaction process, the impurity only promotes the reaction process a little. The lining refractory used in Ezhou Pellet Plant at present is mainly composed of corundum and mullite. The Fe₂O₃ which is the main component of the mass in the kiln solid solves into corundum severely and promotes the mullite's decomposition in high temperature. All the corrosion and solid state reaction is not the direct reason of the refractory's desquamation and breakage, but they realize the transition between the ring and refractory, and accelerate the ring growing on the surface of the refractory firmly.Secondly, the mechanism of actions between different kinds of refractory (including: corundum, mullite and alumine) and different components of the mass in the kiln were investigated. The results show that: the corundum refractory possesses the best erosion-resisting characteristic in the pellet kiln, so it is the most suitable to be applied as lining in this kiln. The bentonite and coal ash have the similar erosion mechanism that they all turn to glass phase in high temperature, then corrode into the inner of the refractory. the liquid phase changed from them impels other mass to adhere on the surface of the liner on one hand, on the other hand it accelerates the solid state reaction process. The hematite and corundum solid solve each other to form saturated limit solid solutions. On the influence of Fe₂O₃, mullite begins to decompose into Al₂O₃ and SiO₂ from 1350℃in atmosphere. After this, the Al₂O₃ reacts with Fe₂O₃ immediately, and some SiO₂ occurs vitrification transition.Finally, separately study on the kinetics of Fe₂O₃ reaction with corundum and mullite. Adopt static crucible to research on industrial Fe₂O₃'s macro corrosion behavior to corundum refractory. The results certify that this corrosion behavior is controlled by solid diffusion. Corrosion activation energy E = 525.5kJ·mol^-1, frequency factor A0 = 4.15×10¹⁵, the corrosion experiential formula is deduced as follow: d = 4.15×10¹⁵e^-525500/ (R T)t^1/2。According to SEM and EDS analysis, and the results of calculation, fitting by Origin, we have calculated the diffusion coefficient of Fe₂O₃ solving into corundum and made sure the relationship between diffusion coefficient and temperature. Above high temperature, this diffusion is controlled by intrinsic defect, activation energy Q = 929.2kJ/mol, frequency factor D0 = 4.61×10²⁰m²/s, the formula which describes the relationship between diffusion coefficient and temperature is that: D = 4.61×10²⁰exp(-9.292×10⁵/RT). Thermal analytical kinetics study was done for the decomposition reaction of mullite on the influence of Fe₂O₃. The results show that: this decomposition reaction mechanism is that G(α) =α1(integrating formula), f(α) = 1(differential formula). This reaction is controlled by one-dimension phase boundary reaction, activation energy E = 3234.7kJ/mol, frequency factor A = 7.3×10102min-1.