Effects of grain size gradation on the porosity of packed heap leach beds

Fluid flow through packed ore/rock beds is among the critical processes that control the release of valuable metals as well as substances potentially harmful to the environment. The properties of fluid flow in porous media are associated with the structure of the pores through which the fluid flows which, in turn, is influenced by grain size gradation, grain shape and packing method. In this study, we investigated the effect of three types of grain size gradation on porosity and pore size distribution using the bulk density and the computed tomography (CT) scanning methods. It was generally observed with the uniformly graded grains that the porosity decreased as the mean grain size increased until a limit was reached. The porosities of the well graded grains were lower than those of the uniformly graded grains in the coarse size range, but there was no difference in the porosities between the two types of gradation in the fine size range. Furthermore, the influence of the packing method on the well graded grains was more pronounced than on the uniformly graded grains, implying relative ease of compaction of well graded grains. The proportion of fine grains in the gap graded grains influenced the porosities, which firstly decreased and then increased. The pore size distribution of the gap graded grains showed a high degree of heterogeneity compared to those of the uniformly graded and the well graded grains. The findings indicate that in ore/rock dumps fine particles account for the bulk of the porosity and possibly the bulk of any pore water content, suggesting that fine particles are likely to contribute most of the leachable substances.