With energy costs rising rapidly, the need to reduce energy consumption is one of the big drivers behind current comminution research.
Comminution '14 will show how this might be achieved by innovations in grinding technology and circuits, and pre-concentration.
In deciding whether there is any hope for significant improvement in comminution energy efficiency, a keynote lecture from Prof. Tim Napier-Munn, former Director of the JKMRC, will consider the key technical and cultural impediments to progress, and speculate on how the innovation process may yet provide the long-sought paradigm change. The keynote will show that, despite intensive research, and early claims of high energy efficiencies, HPGRs have still only succeeded in niche applications in mineral processing after 30 years of trying. AG/SAG mills have dominated grinding because of their capital efficiency and operability rather than their energy efficiency. Stirred mills have been a genuine innovation in fine grinding but even they are prodigious consumers of specific energy. As Tim will show, we still do not know enough about the physics of the fracture of heterogeneous brittle materials such as mineral ores. Comminution science is really a branch of materials science, but materials scientists, unlike minerals engineers, are only interested in the fracture event itself (read ‘failure event’), not the nature of the products of fracture. The main job of comminution is mineral liberation, and recent research has taught us that liberation is a product of the texture of the ore and to only a limited extent the fracture mechanism.
So who is researching the fundamentals of breakage at the moment, with a view to enhancing mineral liberation? There was a lot of interest in liberation enhancement in the 1980s, and in 1993 I co-authored a paper,
published in Minerals Engineering, which considered the need to research more deeply the mechanisms of the breakage processes, particularly the promotion of intergranular fracture. In order to do this, control of crack propagation, and the nature and role of grain boundaries, were considered to be areas deserving most attention. My notes below refer to aspects of that paper.
High throughput comminution machines liberate minerals relatively inefficiently and inadequate liberation in itself leads to higher energy consumptions, as finer grinding has to be performed in order to achieve an adequate degree of liberation. This also leads to the generation of ultra-fine slimes particles, which may be lost in the downstream process. More research is needed into the mechanisms of the breakage process in these machines, particularly into the promotion of intergranular, rather than transgranular fracture. The inhomogeneity and non-perfect elasticity of rocks make crack generation and propagation studies in them difficult and tbese difficulties apply equally to ores which contain more than one mineral species. Despite these difficulties, fracture creation in such complex ores needs clearer definition as it must be recognised a priori that the generation of intergranular fractures is the key to successful liberation.
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| Stress concentration at a crack tip |
Brittle materials such as rock and glass break with little plastic deformation, deforming elastically to the instant of fracture. The tensile strength of such materials is much less than that predicted theoretically, which led
Griffith to postulate that the low observed strengths were due to the presence of small cracks or flaws, the extremities of these cracks acting as stress raisers . Griffith assumed that the theoretical stress was obtained at the end of the crack, even though the average stress was still far below the theoretical strength. Fracture, according to this concept, occurs when the stress at the ends of the cracks exceeds the theoretical stress, and when this occurs the crack is able to expand catastrophically. The failure of a brittle solid such as rock is therefore caused by the extension of Griffith cracks which are inherent in the material. The energy required to create new crack surface is supplied from the work done by external forces, by release of stored strain energy in the solid, or a combination of these two sources. Plastic deformation in the vicinity of the crack tip and the need to overcome the surface energy of new crack surface can ensure stable growth of a crack under external forces, but above a critical crack length, the stored potential energy becomes greater than the resistance to crack extension (i.e. surface energy) and the crack will accelerate (unstable propagation), and failure will be inevitable, after which excess stored strain energy will be released as heat. As the shock waves of the crack movement have to be dissipated through the material, the maximum theoretical speed of a crack tip is the speed of sound in that material, and it has been shown that if there is sufficient localised stress, the propagation speed of a crack can approach 40 % of the speed of sound.
All rocks contain inherent cracks, derived from many sources, due to the appreciable mechanical, thermal and chemical actions to which they have been subjected over many millions of years. There is also evidence to show that when rock is removed from the earth, particularly from depth, stress relief initiates nucleation of new, and growth of existing cracks, due to the release of the stored compressive strain energy, it being well known that quartz from depth is easier to crush than that from nearer the surface. New cracks are also produced in crushing and grinding, but as the particle size decreases the proportion of cracks above the critical crack length decreases, so that fine particles are characteristically more difficult to comminute.
Crystal boundaries are regions of misfit or disorder between crystals, and it would be expected that such areas should be relatively weak in relation to the ordered crystal lattices. Despite this they can act as barriers to propagation of cracks, in the same manner as grain boundaries in metals are known to impede the motion of dislocations along slip planes. That crystal boundaries act as inhibitors to crack propagation is evidenced by the greater fracture toughness of fine-grained compared with coarse-grained rocks. In most comminution operations, however, cracks appear to be little influenced by grain boundaries, the grains often being cleft across, producing a low degree of liberation. This has led many minerals engineers to conclude that boundaries are a source of strength. However, as has been shown, existing cracks, and others developed from flaws in the matrix, can propagate at very high speed when excess strain energy is put into the lattice. This is the case with all current comminution processes. Cracks moving at such high speeds therefore tend to ignore obstacles in their path, even though these may be paths of lower resistance. This is analogous to an axe moving at high speed through wood, tending to ignore knots and flaws, thus moving in a straight line. A saw moving much more slowly tends to have its blade deflected by such areas.
A crack moving slowly in a rock (stable propagation) would be expected to similarly exploit grain boundary weaknesses as paths of least resistance, particularly as cracks propagate along grain boundaries at low velocities. Evidence for stable crack propagation exploiting grain boundary weakness is the high degree of liberation found in secondary alluvial deposits. The minerals in these deposits have been released from the primary source by millions of years of chemical and mechanical weathering, the minimal strain energies extending cracks in a stable fashion, and breaking the rock eventually around the grain boundaries in a manner similar to the stress-corrosion cracking of metals. It is likely that the strain energy is sufficient only to move existing cracks rather than produce new ones, such that the final result is not only liberated, but also crack free, minerals. Good evidence for this is the high proportion of gem (flawless) diamonds which are found in placer deposits such as those in Namibia, transported by rivers many hundreds of kilometres from the primary source. An ideal comminution process would therefore impart only sufficient strain energy to create stable propagation of existing cracks to liberate minerals at grain boundaries, but would leave the material stress free after comminution, preventing nucleation of further major cracks in the matrix.
However conventional crushing and grinding machines comminute rocks by the application of massive successive stresses. When the rock breaks, the large amount of excess strain energy in the bulk of the lattice is dissipated as heat, and the excess energy causes cracks to propagate in an unstable fashion and accelerate rapidly. There is some evidence to show that mineral liberation can be improved using HPGR. The high compressive stresses, acting on a compacted bed of material, may effectively seal micro-cracks, therefore allowing plastic strain to occur, with the associated flow of dislocations, which could pile up at grain boundaries, coalesce and promote intergranular cracking. It is interesting to note that when the pressure is released, and the rock examined, many microcracks are observed, which substantially reduce the work index of the rock. In some eases, these cracks have been observed at the grain boundaries, and this has been exploited in some kimberlite processing plants to enhance the liberation of the diamonds without breaking the valuable larger stones.
The method simulating most closely the gentle mechanical action of alluvial formation is autogenous grinding, and there is evidence to show that liberation is improved over steel grinding. Several tests have shown that ores ground autogenously float faster and with better selectivity than if ground conventionally. The arguments often put forward to support enhanced liberation, such as abrasion of the matrix exposing the stronger mineral grains, are not, however, convincing. It seems more likely that slow crack propagation occurs, due to rapid unstable crack acceleration being inhibited by minimisation of the stored lattice strain energy. The cracks are thus able to exploit the inherent boundary weaknesses in finding paths of least resistance.
Returning to the analogy with placer deposits, what is needed is a process similar to physical weathering to weaken the grain boundaries, to facilitate the subsequent liberation. In the mid 80s work was carried out at Camborne School of Mines and the University of Birmingham on the use of prior treatment to weaken the rock, preferably at the crystal boundaries. It was hoped that heat treatment would promote grain boundary weakening due to differing thermal expansion rates of the different phases in neighbouring grains, which effectively "loosens" the crystals in the matrix. If this is so, then this effect could be enhanced in mineral assemblies, where near neighbours may have widely differing coefficients of thermal expansion. Heat treatment could, therefore, have potential for weakening rock prior to grinding.
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| Tin ore before (left) and after heat treatment |
A number of studies were directed at an evaluation of thermally-assisted liberation as a means to improve mineral recovery. At CSM we heated a hard-rock tin ore to 600°C, before quenching it in water. The differential expansion and contraction promoted significant intergranular cracking and a 55 % reduction in grinding resistance caused by the weakening of the rock matrix. An economic evaluation showed that only a 1% improvement in tin recovery would have made such treatment economically viable, as the increase in smelter revenue, together with the reduction in grinding energy, would offset the energy required to heat the ore. We were obviously excited with this finding, but the results of testwork to assess liberation improvements after conventional steel grinding were disappointing. It was suggested, however, that heat treatment could have potential as a prelude to the gentler action of autogenous grinding, the autogenous mill making use not only of the boundary weakening, but also of the weakened matrix; the gentler action also preventing the critical growth of the transgranular microfractures.
It is over 20 years since the paper was published, and its purpose was to stimulate debate. Maybe 20 years on it will? I have been out of this field for so long now that I may have missed much which has taken place, but I have a feeling not, as I see little evidence of research into liberation enhancement in recent journal papers, and there is no work in this area scheduled for presentation at
Comminution '14. So I have two questions:
