Effect of multi-layer feeding of sized green pelletson bed permeability, structure and properties of fired pellets in straight grate induration furnace

Abstract

Green pellets while indurating in a straight grate induration furnace, pass through different thermal treatments, namely drying, preheating, heating and cooling. The firing of the pellet bed is downdraft using Corex gas and cooling is updraft using ambient air. Coke breeze is used as solid fuel to supplies the necessary heat for uniform heating of the pellet. The physicochemical conditions prevailing in the indurating furnace, i.e., feeding rate, bed height, pellet size, position of the pellets in the bed, temperature, partial pressure of oxygen, amount of fuel, etc. have bearings on the phases and structures of the fired pellets across the core to the shell. The time difference between the reduction and oxidation of the pellet depends on the size of the pellet as well as position of the pellet in the bed. The pellet size as well as the position of the pellet in the furnace has a marked effect on the formation of different phases and microstructure. Usually, mixed pellets of different sizes varying from 8 to 16 mm pellets are fired in duration furnace. The study was aimed at to optimize the pellet bed with different size range of pellets put in layers to make uniform firing and cooling of the pellets throughout the bed to achieve desired micro-structure and properties. The green pellets were divided into three sizes as A − coarse (–16 + 12.5 mm), B − medium (–12.5+10 mm) and C − small (–10 + 8 mm). The bed permeability of pellet plant induration furnace was 91.7JPU (Japanese Permeability Unit). Except permeability of pellet bed with single layer of smaller size pellets (C-C-C), the bed permeability with three layers, two layers & single layer of different size pellets was similar or greater than the bed permeability of pellet plant. From the results of basket trials conducted at pellet plant, it was found that the layer wise pellets like B-A-C, CA-B, AB-C and B-B-B resulted in higher Tumbler Index (TI) and Cold Crushing Strength (CCS) compared to other different size pellets as layer due to better slag bonding and lower pellet porosity. These pellets were exposed to optimum firing temperature as well as the cooling process with the presence of sufficient oxygen for the conversion of magnetite to hematite.