Title

PHASE RELATIONS IN THE MAGNETITE CORE OF IRON ORE PELLETS

PHASE RELATIONS IN THE MAGNETITE CORE OF IRON ORE PELLETS

Authorship

Firth, Andrew Robert; Garden, John Frederick; Douglas, Jeffrey Donald

Firth, Andrew Robert

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Garden, John Frederick

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Douglas, Jeffrey Donald

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DOI

10.5151/2594-357x-13004

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Abstract

During the induration of pellets produced from magnetite concentrates, oxygen access to the core is restricted while slag formation is occurring. This situation is analogous to what happens in the core of pellets produced from hematite concentrates with added carbon, where significant amounts of secondary magnetite are formed from hematite being reduced. Individual green balls and compacts of ground magnetite ore, limestone flux, silica and carbon were fired at precisely controlled temperatures to determine what slag phases formed at that temperature, especially in the core. In some cases, more silica and lime was added than in real pellets to produce larger quantities of slag that were easier to characterise. Optical and scanning electron microscopy showed that the lack of oxygen in the core of the pellets results in the onset of calcium silicate slag formation at lower temperatures, around 1100 °C. The initial slag may not be fully molten, however, as the slag phase does not cover the particles until the temperature exceeds 1250 °C. The quicker onset of slag formation in the core often results in greater sintering than in the shell of the pellet, causing the core to pull away. As a result, low compressive strengths are obtained.

During the induration of pellets produced from magnetite concentrates, oxygen access to the core is restricted while slag formation is occurring. This situation is analogous to what happens in the core of pellets produced from hematite concentrates with added carbon, where significant amounts of secondary magnetite are formed from hematite being reduced. Individual green balls and compacts of ground magnetite ore, limestone flux, silica and carbon were fired at precisely controlled temperatures to determine what slag phases formed at that temperature, especially in the core. In some cases, more silica and lime was added than in real pellets to produce larger quantities of slag that were easier to characterise. Optical and scanning electron microscopy showed that the lack of oxygen in the core of the pellets results in the onset of calcium silicate slag formation at lower temperatures, around 1100 °C. The initial slag may not be fully molten, however, as the slag phase does not cover the particles until the temperature exceeds 1250 °C. The quicker onset of slag formation in the core often results in greater sintering than in the shell of the pellet, causing the core to pull away. As a result, low compressive strengths are obtained.

Keywords

Iron ore pelletising; Pellet mineralogy; Thermodynamics.

Iron ore pelletising; Pellet mineralogy; Thermodynamics.