Disintegration of Northern Cape iron ores under reducing conditions

Abstract

Cracking occurs in the first step of gaseous reduction of hematite iron ore, to magnetite, and can lead to the formation of fine material, with deleterious effects on operation of shaft furnaces. To study this, samples of three ore types from the Northern Cape iron ore field in South Africa, and one blended ore from this region, were studied. The methods were high temperature microscopy (during reduction) and quantification of fines formation following reduction disintegration tests. The ore types do differ significantly with regards to their propensity to form fines. Although disintegration is clearly triggered by reduction, no direct correlation could be established between the degree of reduction and the amount of fines generated. Reduction disintegration increased with higher hydrogen percentages (.5%) in the reduction gas, and at higher temperatures (in the 500-700uC range). Disintegration of the samples decreased at temperatures .750uC. There was no correlation between the presence of gangue minerals and fines formation.

Characterization of Dust Generated during Mechanical Wear of Partially Reduced Iron Ore Pellets

“…This variation in the wear rate (degradation) of iron ore due to reduction has been reported by several researchers . Especially, the significant increase in the disintegration of iron ore in the temperature range of 400–600 °C is well known.…”

“…Especially, the significant increase in the disintegration of iron ore in the temperature range of 400–600 °C is well known. Usually, this is justified by a 15–25% volume expansion during the reduction of hematite to magnetite, which is associated with the structural transformation of a hexagonal hematite to a cubic magnetite . This expansion leads to internal stresses around the reduction region.…”

“…It can lead to higher heat losses and fuel rates . Hence, a lower production efficiency and unstable furnace operation can take place due to generation of dust in a BF process …”

“…Several researchers have investigated the mechanical wear/degradation of partially reduced iron ore, especially iron ore lump and sinter, at varied temperatures and using different compositions of the reduction gas. Generally, the ISO 4696‐1 standard is used to determine the reduction disintegration index (RDI) of iron ore (lump, sinter, and pellets) by isothermally reducing it at 500 °C for 60 min using a gas mixture of 20% CO, 20% CO 2 , 58% N 2 , and 2% H 2 .…”

“…It has been reported that the maximum wear/degradation of iron ore occurs in the temperature range of 400–600 °C, which is associated with the stress concentrations caused by the volume expansion during the reduction of hematite to magnetite . Furthermore, a higher mass percent of particles smaller than 0.5 mm is produced (≈0.8–3 times higher) due to the reduction degradation of iron ore pellets at 550 °C compared to lump ore and sinter .…”