The Importance of Rheology in Mining Processes

By Sul MulroyAug 26 2019

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Rheology encompasses the science of deformation and how matter flows over time based on the mechanical properties of the material and whether it behaves in a brittle, elastic, or viscous fashion. Rheology has been extensively applied to studies on rock mechanics and deformation, where over short timescales. For example, the lithosphere of the Earth displays brittle-elastic properties and fracture under stress, while over longer, geological, timescales, and at depth, lithospheric rocks are within the elastic-ductile regime.

Significance of Rheology to Mining

Such studies are vital when evaluating the geomechanics of underground mines where the stress of supporting overburden for sustained periods can generate creep or failure in mine-walls and pillars. Processes and mechanisms of stress transfer in these environments can be investigated with rheological models using parameters derived from experimental data. Key parameters include compliance (stiffness), ductility (plastic deformability), compressive strength (resistance to compression under an applied force), the severity of the applied force (expressed as deviatoric stress) and time.

The longer a pillar is forced to endure significant compressive forces, the weaker it will become. In addition to these parameters, other inherent material properties including the rock’s Poisson’s ratio, i.e. the extent to which a material expands in directions perpendicular to maximum principal stress, and its elastic modulus, i.e. resistance to elastic deformation, will affect any possible pillar failure and can indicate likelihood of collapse under certain conditions. Such studies are vital to prevent catastrophic incidents, ensure the safety of the workforce, and offer insight into long-term subsidence over historic mine-sites.

Other Applications of Rheology

Aside from hard-rock mechanics rheology has numerous other applications throughout various stages of mineral and hydrocarbon extraction.

Of particular current importance is the behavior of mine-waste when tailings dams collapse. The Brumadinho disaster on 25 January 2019, in which at least 248 people lost their lives when the Vale owned Brumadinho iron ore tailings dam failed, saw almost 12 million cubic meters of tailings slurry surge downslope causing significant contamination of the nearby river.

Traditional assessment of tailing storage facilities focus on the geomechanical stability of the dams themselves. Considering the thousands of millions of tonnes of mine waste produced annually and the devastating nature of dam failure, where both the natural environment and human life can be severely threatened, studies on the flow of tailings upon release are integral to determining the impact such failures can have when they occur.

The rheology of tailings flow, like the creep and failure of mine walls and failures, can be modeled numerically; however, the nature of the wastes can be complex. Their behavior following dam breach can, therefore, be characterized as highly-viscous debris flows or muddy floodwaters. Numerical models can account for this complexity with models for debris and sediment flows serving as an initial template.

Key parameters in such models include depth-averaged flow velocities, fluid depth, shear stress (stress parallel to a surface), the density of the flowing material and acceleration due to gravity. Spatial Geographical Information System (GIS) data, whereby the path of tailings from a dam breach can be mapped from its source, allows rheology types to be altered according to topography and land-surface. Such models can indicate the extent to which a breach might flow following collapse, aid in identifying high-risk areas in such an event and provide an idea as to the economic cost of breaching.

Assessing the Impact of Rheology

The rheology of mineral slurries can significantly affect the processing of ores. Studies on the impact of rheology during beneficiation processes such as grinding, slurry transport, and froth flotatdensityion have been conducted in order to understand particle-particle interactions. Such interactions are affected by particle size and chemistry of the suspension medium.

In froth flotation, these mediums include xanthates, thiophosphates or thiocarbamates with depressants, including sodium carboxymethylcellulose, also used to remove certain gangue phases. Slurry chemistry is, therefore complex, and the overall composition becomes even more complex with different ore types where certain mineral phases can alter rheological behavior as a result of a surface charge or particle aggregation. As froth flotation relies on small mineralized air bubbles which are easily broken on the froth surface, the behavior of the slurry is key to efficient metal recovery.

In hydrocarbon extraction fluids used in hydraulic fracturing (fracking) are viscous and understanding their rheology under certain pressures, shear stresses and temperatures is integral to selecting effective proppants (the solid particles suspended in the fluid). Prediction of fracture growth and their geometry relies on knowing how the fluid and proppant will behave within the well. A poor understanding of the fluid rheology will significantly impact oil production.

Conclusion

Rheology is an important feature of materials. In the Resources, Industries it plays a part in the mechanics, and stability of mines, in the recovery of metals and hydrocarbons to maximize the yield of a deposit or reservoir, and can be applied in risk management and mitigation when things go wrong.

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