Effect of structure of unfluxed burnt titanomagnetite pellets on strength under static compression

Abstract

Burnt pellets must retain their strength from the moment they are taken out of an induration machine until they are loaded into a blast furnace. One of the indicators of the burnt pellets’ strength is the compressive strength, i.e. the ultimate force. In experiments to determine compressive strength, the main type of fracture is occurrence and development of cracks that pass through the core center of pellets (where the maximum radial tensile stresses present) or near it. The paper presents the requirements for static compression strength imposed by blast furnace production to iron ore pellets. Using an optical and scanning electron microscope equipped with an energy-dispersive microanalyzer, we analyzed the relationship of structural components and pores in the core of burnt unfluxed iron ore titanomagnetite pellets with the ultimate force under static compression. By scanning electron microscopy and X-ray spectral microanalysis, it was established that the core of pellets is a multiphase material, and its main phases are titanomagnetite, magnetite, titanohematite, hematite and aluminosilicate binder. Optical microscopy made it possible to establish the microstructure of the pellet core, which has three types of microstructures: non-oxidized core (magnetite or titanomagnetite), partially oxidized core – around (magnetite or titanomagnetite) hematite grains (titanohematite) and oxidized core (hematite and titanohematite). The main factors for obtaining pellets with an ultimate force of more than 2.5 kN/pellet according to the requirements of blast furnace production are: the number of closed macropores and the number of large grains in the core. It is shown that with an increase in the number of closed macropores and the number of large grains in the core, the ultimate force is reduced from 3.5 kN to 0.87kN/pellet.