| Dead-burned magnesite is an important component of the magnesia refractory raw material, study block magnesite calcining process kinetics in large shaft kiln with the temperature and pressure changes, for design large dead burned magnesia shaft kiln and process parameters settling important basis.Thermal decomposition of cubic magnesite blocks with the side length of 20 mm,30mm,40 mm and 50 mm was investigated by TG.The relation between block size(L=20-50 mm) and activation energy meets the formula:E=4.795×L+34.070(KJ/mol). At earlier stage, 3D mould with n=2 controlled by Anti-Jander’s formula is the most probable mechanism function. Kinetic equation is dα/dT =6.111×1010×β-1exp [(4.098-0.577×L)×T-1](1+α)2/3[(1+α)1/3-1]-1; In the middle stage, random nucleation and subsequent growth models with n=3(Code: AE3) controlled by Avrami-Erofeev formula is the most probable mechanism function. Kinetic equation is dα/dT=1.422×109×β-1exp[(4.098-0.577×L)×T-1](1-α)[-ln(1-α)]-2;In the later stage, random nucleation and subsequent growth models with n=3/2 controlled by Avrami-Erofeev formula is the most probable mechanism function. Kinetic equation is dα/dT=2.477×109×β-1exp[(4.098-0.577×L)×T-1](1-α)[-ln(1-α)]-2.Using high temperature furnace to calcined block magnesite, periclase particles densification behaviour was studied at early stage、middle stage and last stage of block magnesite sintering.Early stage: the densification degree of particle groups is increased. Pseudomorph of magnesite have important effects on the densification behaviour of particles, when the temperature within the range of 1150~1200 ℃, pseudomorph particles’ volume contract, leading the coordination numbers of particles and specific surface area reduce. Sintering densification is impeded and appearing slowing phenomenon slowing, which is a significant difference between block magnesite and powder magnesite sintering. under 1000 ~ 1110 ℃, Q=412.368 kJ·mol-1. under 1110~1230 ℃, Q=473.990 kJ·mol-1; under 1230~1300 ℃, Q=385.266 kJ·mol-1.Middle stage :The densification coefficient increased from 23.51% to 71.24% at 1400~1600℃. At the middle stage of the block magnesite sintering can be divided into two stages under the action of liquid, phase 1: the method of mass transfer is flowing, with temperature rise the sintering activation energy decreases and the rate densification increases rapidly; Phase 2: the method of mass transfer is "dissolving—precipitation", with the temperature increasing, chemical gradient on particles surface and the liquid phase mass transfer rate increase, the sintering activation energy is reduced and particles densify rapidly.Last stage :the grain boundary moves rapidly, the rate of porosity “collect—remove” and grain boundary moving to the center of curvature is rapid, which make the Particle densify quickly and grain grow obviously. At the period of 1600~1700 ℃, grains growth activation energy is 1.655×103 kJ·mol-1 and crystal is the main way of grain growth; At the period of 1700~1800℃, volume diffusion is key mechanism to control sintering densification, grains growth activation energy is 1.291 × 103 kJ·mol-1, the glomerocryst is main way of grain growth.Sintering process of block magnesite under deadweight pressure was simulated by hot-pressing sintering. The results show that at the stage of constant speed heating, periclase particles densify rapidly, due to the deadweight pressure increasing, particles rearranged fast, time shortened and temperature reduced. At the stage of constant temperature, the methods of densification are plastic flow and diffusion. At the stage of constant speed cooling, the higher deadweight pressure is, the greater elastic modulus is. Stress releasing produces larger elastic deformation, which causes relative density quick reducing. The densification and crystallinity of sample fired under 5MPa is the highest by analysis of microstructure and sintering process. |
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