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.


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.


2.1 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.

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