Year

2020

Degree Name

Doctor of Philosophy

Department

School Mechanical, Materials and Mechatronics Engineering

Abstract

To reduce the emission of the carbon dioxide (CO2) in New Zealand (NZ), it is proposed to perform direct reduction (DR) of NZ titanomagnetite ironsand pellets using H2 gas. However, there is limited pre-existing knowledge on the DR behaviour of NZ titanomagnetite ironsand pellets in hydrogen. This thesis addresses this through an experimental investigation of the H2 gas-based reduction of Ar-sintered and pre-oxidised pellets, and an analytical kinetic model is developed to describe the reduction rate.

The reduction rate of both types of pellets is found to increase with increasing temperature, H2 gas flow rate, and H2 gas concentration. For both pellets reduced at 1443 K above the critical flow rate, the maximum reduction degree reaches ~97% with a similar reduction rate. However, in the lower temperature range (< 1143 K), pre-oxidised pellets are reduced much faster than Ar-sintered ones.

During reduction of both types of pellets, any TTH present is rapidly reduced first. After this step, TTM is then reduced to FeO, with Ti becoming enriched in the remaining unreduced TTM. FeO is further reduced to metallic Fe, which makes up to ~90% reduction degree. Eventually Ti-enrichment of the TTM leads to a change in the reduction pathway and it instead directly converts to metallic Fe and FeTiO3. Above ~90% reduction degree, reduction of the remaining Fe-Ti-O phases occurs (leading to the formation of TiO2 or (pseudobrookite) PSB/ferro-PSB).

At the pellet-scale, both types of pellets present a single interface shrinking core phenomenon at higher temperatures. Metallic Fe is generated from pellet surface with a reaction interface moving inwards. However, at lower temperatures this pellet-scale interface becomes less defined in the pellets. Instead, grain-scale reaction fronts are observed.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.