Fines generated by dynamic crack propagation, as in the blasting of rock

Fine particles (fines) generated by dynamic fracture methods like rock blasting is a liability from resources, production and environmental aspects. It is argued that common models for such fines generation are contradicted by carefully made blasting tests. The general aims of this project are to create a better understanding of how the fine and very fine particles are created under blasting conditions. One possible mechanism is that the macroscopic crack propagation process itself is a major source of fine material. Some models of brittle fragmentation predict fragment size distributions (FSD) that describe the sieving curves of rock quite well. This supports the underlying ideas that instability of fast propagating cracks plays a fundamental role for the FSD of the fines. Along the crack propagation paths this would leave behind a trace of small fragments generated by an inherently scale-invariant branching-merging mechanism. Some indirect evidence is given in support but direct evidence is missing. The scientific aims are to verify or refute the mechanism for crack generated fines by capturing images of branching at a crack tip, to find if there is critical velocity below which this mechanism ceases to contribute, further to compare the FSD with models and in the end to provide a scientific explanation of how and in what proportions these fines particle are generated. The testing will primarily use high speed photography of blasting of a cylindrical specimen confined in a closed container with soft padding, allowing a limited expansion of the broken parts but not a complete separation so that secondary fragmentation is minimized. The container has a window through which the specimen is illuminated and the cracking is viewed. The sieving of the fragments down to about 2 m will require special sieving methods. Three fine grained materials will be tested, a magnetic mortar used in previous model tests, and a marble/limestone and granite. Their mechanical, fracture roughness and Natural Breakage Characteristic (NBC) properties will be determined to find correlations with the statistical models parameters. The project will have 2 PhD students, one focusing on the practical aspects, the other on the theoretical aspects. It involves cooperation with researchers in Canada, Finland, Spain and Sweden. The results will probably also be of use for a) developing a more sustainable and economically viable comminution of rock, e.g. for mitigating dust generation in mining/quarrying, and b) as input to global circulation models regarding the emitted fraction of clay aerosols.

Fine particles (fines) generated by dynamic fracture methods like rock blasting is a liability from resources, production, and environmental aspects. The general aims of this project have been to create a better understanding of how the fine and ultra-fine particles are created under blasting conditions. A new test set-up for studying blast generated fines in rock cylinders has been developed. The dynamic fracturing is filmed while a confinement prevents the specimens disintegration and a premature dispersion of the fines. Cylinders of cement mortar and granodiorite were blasted. The dynamic images and post-mortem studies allow us to better understand how blast fracturing generates fines. The CT-scan, optical microscopy, and SEM studies reveal several mechanisms for branching-merging and other ways of fines creation at and below the grain size level. At this level most of them appear to depend on the polycrystalline character of the material rather than on the crack velocity. For the intermediate and fines size ranges the fragment size distributions mainly have the character predicted by branching-merging crack growth and laser sieving below the mechanical dry sieving limit extends this behaviour to about 2 m where breathing of the fines is injurious. From the primary crushed zone around the blast-hole, more material is removed from the granodiorite than from the mortar. When the compaction around the blast-hole and losses are considered, the amount of branching merging fines and crushing fines are about the same. In the granodiorite there is no compaction zone and the branching merging fines dominate. The numerical results cannot yet however with acceptable accuracy be extrapolated to full scale blasts. We can also model the concomitant fracturing and fragmentation reasonably well with a distinct element code. The simulation results indicate whether the fines are generated by branching-merging or crushing-shearing and this agrees with the physical observations. Our work accentuates the hypothesis that branching-merging type mechanisms make an important contribution to the creation of ultra-fines, fines, and intermediate size fragments. This doesnt mean that crushing fines are negligible in comparison. It means however, that an extrapolation of particle numbers down to harmful fragment sizes can probably be made with a slower progression than if all fines were created by crushing.