ABSTRACT ABSTRACT: Size distributions of intact rocks, blasted rocks and dumped rocks from the same in-pit location were measured and plotted as log of cumulative number against size. The fractal dimension from the negative exponential plot increased with each energy increment, demonstrating an increase in fine particle fractions which may form a basis for improved rock classification. 1 INTRODUCTION An important need in mining - particularly where leaching processes are involved - is an accurate description of the discontinuity structure of a rock mass, and an understanding of the way in which this structure affects rock fragmentation during excavation. This was specifically identified by the U.S. National Committee for Rock Mechanics (1981) in their study of".. research requirements for resource recovery .... " There have been many attempts to develop models to predict the size distribution of excavated; usually blasted; rock (see forinstance Kuznetzov, 1973; Cunningham, 1983 and Danell and Leung, 1987). Most of these models have failed to allow for the presence of natural or imposed fractures present in the original rock mass. These can often have a dominant role in determining the rock fragment size after excavation (see forinstance Hagan, 1983; Yang and Rustan, 1983). An approach which incorporates the structural properties of the rock mass and their effect on fragmentation is outlined below. 2 BACKGROUND 2.1 Effect of block size on leach recovery Description of rock block size distributions have applications in heap, dump, and in-place leaching. Investigations by Dahlberg (1979) at Inspiration Mine, Arizona, have shown recovery of70% of the copper in 50 days from porphyry ore that was crushed to minus one inch (25mm). The same ore crushed to minus two inches (50mm) gave 60% metal recovery, while ore crushed to minus four inches (100mm) gave 47% recovery in the same period. The results were based on 10 foot (3m) long, 12-inch (0.3m) diameter column tests. In another test at Inspiration Mine a 30 foot (9m) high test pad containing 50,000 tons (44,000 tonnes) of ore crushed to minus four inches (100mm) was constructed. Drilling, sampling, and testing before and after leaching showed a metal recovery of 76%. After leaching, 2-inch (50mm) size rock pieces were examined for solution penetration. On average, a 2-inch (50mm) rock particle showed 0.59 inches (15mm) of leach solution penetration. This indicated that rock fragments larger than about 2 inches (50mm) would have their centers untouched by the leach solution. Both Dahlberg, (1979) and Fountain et al. (1983) have demonstrated relations between extraction rate and recovery of copper during leaching with rock block size, in both oxide and mixed sulfide and oxide ores at Inspiration Mine. Fountain et al. (1983) concluded that even though run-of-mine ore at Inspiration Mine is well broken 30% of fragments are greater than 4 inches (100mm) and there is still a substantial loss of recovery due to the coarse fraction. Thus, block size distribution is a critical factor controlling mineral recovery in leaching operations. It is apparent that if fragmentation could be improved, the percentage recovery would be increased.
ABSTRACT 1 ABSTRACT Intact and fractured rock samples were studied in the laboratory in order to understand more fully the mechanism of closure of fractures subjected to high confining stresses and the resultant effect on sample permeability. Confining stresses applied to the samples ranged from 3.5 to 20.5 MPa, and the closure of fractures was observed by monitoring changes in the hydraulic conductivity of the specimens. Test results suggest that some resealing may occur due to crushing and realignment of mineral grains along a fracture surface. The closure of fractures is dependent upon the strength of the rock mass, the physical nature of the fracture, and the fluid pressure present in the fracture. Flow rates through fractures obeyed the cubic law although the induced permeability resulting from the fractures was very sensitive to changes in aperture, and was affected by the matrix permeability of the rock mass and the roughness of the fractures. 2 INTRODUCTION The general approach to analysis of fluid flow through fractures has been to equate fluid flow through fractures with viscous, incompressible flow between smooth parallel plates (see Snow, 1965). By considering the flow to be laminar, and the plates to be horizontal, the flow velocity and hydraulic gradient relationship becomes: (mathematical equation)(available in full paper) 3 LABORATORY PROCEDURES The Hassler cell apparatus was used for testing the hydraulic conductivity of the cylindrical specimens. Specimen dimensions were kept as uniform as possible to reduce the size effects that have been observed by other researchers including Gale and Raven (1980). Thirty-four samples were prepared from intact NX sized core specimens which ranged in diameter from 500 mm to 510 mm. The length of the specimens varied between 950 mm and 1150 mm. Four different lithologies were tested: Berea Sandstone, Hartshorne Sandstone, basalt from the Umatilla member in the Columbia Plateau Series, and gneiss from the Orofino Metamorphic Series near Dworshak Dam, Idaho. All samples were tested using straight flow along a single horizontal fracture that ran along the longitudinal axis. Fractures were induced in the specimens by a modified Brazilian test procedure, in which triangular platens were used to ensure even loading and splitting. All fracture surfaces were mapped using a LVDT and contour plots were generated to help characterize fracture roughness, and nonplanarity. Samples were _first saturated and initially tested for their hydraulic response under loading, in order to determine as accurately as possible their matrix permeability. The samples were then fractured and the surfaces of the induced fractures were characterized. The samples were resaturated and then reloaded into the Hassler cell permeameter to test their fracture permeability. Precautions were taken in order to ensure that the fractures were as horizontal as possible before final emplacement of platens and sealing of the pressure chamber. Water was employed as the confining medium, with nitrogen gas used to apply both the confining and infection pressures. Flow rates through the samples were measured as a function of both confining and head pressures, during both loading and unloading cycles.