Thermophysical properties of briquettes intended for use as an artificial bottom bed in iron ore pelletizing

The study aimed to examine the thermophysical properties of briquettes as an alternative to indurated iron ore pellets in the formation of an artificial bottom bed in horizontal-grate machines. The methodology included physical simulation of the drying process (circulation of a heat transfer agent at 150/300℃), dilatometric analysis, measurement of temperature profiles in briquettes, and mathematical modeling with the use of TOREX Sensible Indurating Machine software. The briquettes were made from oxidized ferruginous quartzite concentrate with organic and inorganic binders (cement; bentonite) under a load of 15 t (cylinders measuring Ø35×35 mm). It was established that excessive moisture during drying does not reduce the strength of briquettes (70.1 daN/briquette) due to their low porosity (10% as compared to 30% in pellets) that limits water absorption. The drying of briquettes was shown to proceed 30–40% slower due to their reduced specific surface area and permeability compared to iron ore pellets, thus requiring adjustments to the heating conditions. The thermal conductivity coefficient amounted to 0.12 W/(m·K). During firing, sintering occurs exclusively in the surface layers (depth of 2–3 mm) since the inner zones do not reach the sintering temperature threshold (>1000℃) due to the low thermal conductivity of the material. The integrity of the briquette is ensured by the strength of the indurated surface and the thermal stability of the binder at its core. The mathematical simulation performed for the horizontal-grate machine No. 3 at the Mikhailovsky Mining and Processing Plant showed that the replacement of a standard bed comprising indurated iron ore pellets having a temperature of 80℃ with briquettes having a temperature of 15℃ increases the capacity of this machine by 25% (up to 583 daN/pellet) and reduces the specific consumption of natural gas during iron ore pellet firing by 8.3% (up to 8.8 m3/t) while maintaining the quality of the main product – the strength index. The obtained results confirm the feasibility of using briquettes of all tested compositions as an artificial bottom bed in horizontal-grate machines, demonstrating their key thermophysical advantage – heat loss reduction in the grate zone.