Porosity of Iron-Ore Pellets at Different Stages of Roasting and Reduction

Abstract

Metallurgical properties are determined by the structure of pellets at the macrolevel as well as at the level of minerals and elements. The goal of this work is to analyze changes in the porosity of iron-ore pellets during their lifecycle, from pelletizer to full reduced state. The model used in this work relies on several assumptions: (1) the primary porosity of pellets is built in at crude nodulizing in which case the porous space is partially filled with water and partially with air (when the water evaporates, the space remains air-filled and the dimensions of pellets remain unchanged); (2) at high-temperature firing and baking the porous space volume changes due to the removal of gaseous products of chemical decarbonization, dewatering, oxidation, and baking (as a rule, the volume contraction at baking does not exceed 2–5%); (3) the reduction of pellets makes them more porous as a result of an increase in volume, so-called swelling, which occurs in parallel with the contraction in the volume of the mineral part of pellets at the reduction of iron oxides. Eventually, the reduced pellets are more porous than the oxidized pellets by 100–200% (the porosity of the former is 46–60%). A test data analysis has confirmed the performability of this model. The model can be used for calculating pellet porosity. It is proven that, during their lifecycle, the porosity of pellets undergoes predictable transformation in which case oxidative roasting can either increase or decrease porosity, depending on the furnace charge composition. The reduction of pellets always results in a two-to-threefold increase in porosity.