Pore size characterization of Cretaceous ironstone by low-field nuclear magnetic resonance technique

Abstract

The microstructure of Cretaceous ironstone samples of Yewa within Eastern Dahomey Basin in Southwest Nigeria, has been investigated using low-field nuclear magnetic resonance and conventional porosity measurements. The aim was to evaluate the porosity and pore structure for heat flow consideration in blast furnace or rotary kiln. A total of eighteen iron core samples were analyzed using: X-ray fluorescence (XRF) spectroscopy to determine the percentage composition of iron content; water imbibition porosity (WIP) from dry and saturated sample weights; and low-field nuclear magnetic resonance (NMR) measurements at full water saturation for pore structure and pore size distribution. The average percentage by weight of iron content in the samples is 55 %. Porosity and water content from NMR and WIP shows high degree correlation of 79 % and 97 %, respectively with average porosities from both procedures given as 22.06 pu. and 21.96 pu., respectively. The NMR-T₂ distributions depict multimodal pore system with high degree of connectivity. The transverse relaxation time ranges have been classified into three regions: micropores, 0.3162–0.5012 ms, mesopores, 19.95–84.28 ms and macropores, 199.5–2117 ms, respectively. Using an established average surface relaxivity value (338 µms⁻¹) for haematite, the logarithmic mean of pore radius was estimated from the pore size distributions with values of 0.3098 µm, 33.57 µm and 733.3 µm, for micropores, mesopores and macropores, respectively. The strong pore connectivity demonstrated by the NMR-T₂ distribution, porosity values, and pore geometry all provide strong evidence that the iron ore samples from the study site will maximize heat and gas flow, improve reducibility and increase process efficiency in a rotary kiln or blast furnace. These findings provide valuable insights into the petrophysical characteristics of Yewa iron ores, thereby enhancing our understanding of its potential applications in the mining and ore processing industries.