Investigation on Microhardpan Texture using µ-CT and MLA

Abstract

The development of microhardpans on mining residues has major influence on the water infiltration. Hardpans on the blocky unsorted heaps at Peña del Hierro, Spain, at the headwaters of Rio Tinto, are 1– 2-mm thick and guide water over 100 m horizontally. We investigated some of these microhardpans in detail with scanning electron microscope (SEM), μ-computed tomography (μ-CT), EDXRF-microscope and MLA (mineral liberation analyses). From SEM, it was obvious that the uppermost 100 μm of the hardpan show a reduced porosity. Extremely fine pores could be observed, and porosity was below 15%. On the other hand, μ-CT showed a pronounced coarse pore system, mainly parallel to the hardpan surface below the top 100 μm. We interpret the rapid change in pore size as well as in alteration style as a well-pronounced capillary barrier, which is even emphasised towards the lower hardpan boundary where horizontal megapores developed. By combining MLA of the polished surface with μ-CT, a better attribution of densities to minerals could be performed, allowing one to highlight zones of mineral enrichment due to sedimentary transport in microchannels prior to formation of microhardpans. The combination of the different techniques provides a better understanding of the process of hardpan formation in such an environment and the potential remediation effects.