Block caving is becoming an increasingly important mining method for rock masses which were previously considered to be too strong, at greater than usual depth or as an extension of open pit mining. The behaviour of the rock mass is affected by the strength of the intact rock, the orientation, continuity, spacing and strength of the discontinuities and the in-situ stress. Within a specific rock mass targeted for block caving there may be several different geotechnical domains with different characteristics.

Microwave energy has previously been shown to have a major influence on the comminution behaviour of minerals and ores. Significant reductions in strength have been observed for microwave-treated ores. Other workers have reported increases in liberation after treatment. However, the majority of the work has been carried out at energy inputs too high for economic implementation. Whilst it was thought that the weakening and enhanced liberation was due to differential expansion of the heated constituent phases resulting in increased predominance of inter-granular fracture, the exact mechanisms have been poorly understood. Due to difficulties in measuring events inside a microwave cavity and inside the material being irradiated, it was suggested that numerical modelling could be used to simulate a simplified system in order to determine the underlying mechanisms. The model was used to examine the development of stresses as heat was applied to certain mineral phases. No heat was applied directly to the matrix component of the simulated ore. Given sufficient energy input, the stresses would exceed the strength of the material. It was found that the shear stresses in particular were likely to be highest at the edges of the grain boundaries of 2-D circular heated particles inside an unheated (microwave-transparent) matrix. This explained the increased occurrence of inter-granular fracture which has led to observations of enhanced liberation. It was also discovered that weakening is facilitated at very high microwave power densities, due to the increased magnitude of expansion and subsequent forces generated. The overall energy balance can be made favourable by using microwave exposure times of less than 0.1 seconds. Shorter exposure times result in less time for conduction to occur from the heated phase into the unheated phase, and temperature gradients are maximised leading to elevated shear stresses and increased likelihood of fracture.

Dans cette thèse, nous développons le modèle phénoménologique optimisé de lessivage chimique in situ (ISL) de l'uranium par l'injection d'acide sulfurique, en prenant en compte la précipitation des espèces non-solubles telles que le gypse, qui réduisent la récupération de l'uranium. Le modèle proposé décrit le transport de masse avec des réactions chimiques hétérogènes entre le liquide et les roches solides, qui mènent à la dissolution des oxydes d'uranium et à la récupération de l'uranium sous forme liquide. Ce modèle comprend à la fois des réactions utiles, qui décrivent la dissolution de divers types d'oxydes d'uranium, et les réactions néfastes qui conduisent à la précipitation des sédiments solides (gypse), dont les flocons couvrent la surface de canaux poreux et réduisent l'efficacité des réactions utiles. Parmi les résultats qualitatifs, nous avons révélé l'existence d'un taux critique de sédimentation de gypse, en dessous duquel la récupération ultime de l'uranium est complète. En revanche, elle tend à une valeur limite inférieure à 100% lorsque le taux de sédimentation est supérieur à la valeur critique. Ce taux de récupération limite dépend de divers paramètres du processus. La théorie et la méthodologie développées dans ce travail peuvent être facilement étendues et appliquées aux autres types de minerais qui sont récupérés par la méthode de lessivage in situ, et autres types de solvant / In the present thesis we develop the optimized phenomenological model of in-situ chemical leaching (ISL) of uranium by the injection of sulfuric acid, with special account for the precipitation of non-soluble species as gypsum, which reduces the uranium recovery. The suggested model describes the mass transport with heterogeneous chemical reactions between liquid and solid rocks, leading to dissolve uranium oxides and recover uranium in liquid form. It includes both useful reactions, describing the dissolution of various kinds of uranium oxides, and detrimental reactions, leading to the precipitation of solid sediments (gypsum), whose flakes cover the surface of porous channels and reduce the efficiency of useful reactions. Among the qualitative results we revealed the existence of a critical rate of gypsum sedimentation, below which the ultimate uranium recovery is complete. In contrast, it tends to a limit value lower than 100% when the sedimentation rate is higher than the critical value. This limit recovery depends on various parameters of the process. The theory and the methodology developed in this work can be easily extended and applied on other type of ores that are recovered by in-situ leaching method and other types of solvents