Experimental studies on the generation and evolution of mineral porosity during fluid-mediated mineral replacement reactions

Muhammet Kartal

The porosity in minerals contributes to enhanced permeability for fluid flow in natural systems and engineering processes. Porosity can be created by fluid-mediated mineral replacement reactions. Such reaction-induced porosity can evolve with time, yet the mechanisms and kinetics of porosity creation and evolution remain poorly understood. This thesis presents experimental investigations on the creation and evolution of mineral porosity in two model replacement reactions, i.e., the replacement of calcite by gypsum and anhydrite with a positive volume change and the replacement of pentlandite by violarite and millerite with a negative volume change. These replacement reactions were conducted under mildly acidic hydrothermal conditions for up to 18 months, and the mineralogy, microstructure and porosity of the reaction products were quantitatively analysed by powder X-ray diffraction, (ultra) small-angle neutron scattering, high resolution scanning electron microscopy, focused-ion beam scanning electron microscopy, and X-ray micro-tomography. The results showed that porosity creation and evolution are highly dependent on mineral systems and reaction conditions. In the calcite-gypsum-anhydrite mineral system, the experiments at 25-60 °C produced intragranular nanopores in gypsum replacing calcite. Because of the positive volume change, gypsum overgrowth also occurred on the grain surface, and the gypsum in the overgrowth region contained intergranular micropores. Porosity coarsening was rapid (a few weeks) in the replacement region, leading to the formation of micro-voids in the core of gypsum grains. The replacement reaction was sensitive to temperature. When the experiments were conducted at a higher temperature of 220 °C, anhydrite was formed instead of gypsum. Porosity evolution in anhydrite was different when compared to gypsum at lower temperatures. In the pentlandite-violarite-millerite mineral system, only replacement occurred, likely because the negative volume change does not require overgrowth for additional space. The replacement was sensitive to temperature and solution pH. The experiments conducted at 125 °C and pH 4 produced permeable nanopores leading to the complete replacement of pentlandite; these nanopores coarsened slowly during the 17 months of experiment and occurred preferentially near the grain surface. However, in experiments conducted at 125 °C and pH 5, violarite became impermeable in partially replaced grains due to hematite precipitation in the pore space, blocking the fluid flow. At a higher temperature of 220 °C and pH 4, the formation of millerite in addition to violarite resulted in faster porosity coarsening and formed micropores within 4 weeks. Fundamentally, these complex porosity creation and evolution phenomena observed in the two model mineral replacement reactions are controlled by the interplay between dissolution, precipitation, epitaxial nucleation, and Ostwald ripening processes which are all sensitive to reaction conditions. This understanding should generally be applicable to other mineral replacement reactions. Finally, a case study of the application of porosity control was presented. The leaching of chalcopyrite is often kinetically inhibited by surface passivation layers, which are formed by the replacement of chalcopyrite during leaching. Common passivation layers are elemental sulphur and jarosite. Our leaching experimental results showed that surface sulphur could be removed by adding sulphur-dissolving solvent tetrachloroethylene (TCE) into the sulfuric acid leaching solution. The removal of surface sulphur significantly improved the leaching rate by almost 6 times compared with TCE-free leaching. At the later stage of leaching, chalcopyrite was replaced by potassium jarosite. The jarosite shell did not passivate TCE-free leaching due to its porous structure. However, the jarosite shell became nearly impermeable in TCE-assisted leaching because elemental sulphur filled the pores in the jarosite. This case study suggests that chalcopyrite leaching can be significantly enhanced by either removing the surface passivating layer or by controlling the porosity and permeability of the surface layers formed on the chalcopyrite surface.

Experimental studies on the generation and evolution of mineral porosity during fluid-mediated mineral replacement reactions

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