Rheology of pb-zn post-flotation wastes in the sierra de cartagena-la unión (se spain)"

Laburpena

The degree of saturation has a significant influence on the geotechnical stability of tailings dams. In these structures, water saturation remains high throughout the year—even in semiarid areas—due to the density, particle size, and hydrogeological properties of the materials controlling capillary rise, which ranges 1–20 m. Such tailings, depending on its dewatering state, exhibit non-Newtonian behaviour, characterized by a shear thinning, yield stress, and thixotropic response. This thesis explains the rheological pattern of the Pb-Zn mine slurries prepared from the materials sampled in the tailings dams, which underwent static liquefaction and flow failure (El Descargador, Peña del Águila) or minor liquefaction processes (La Peraleja) in the Sierra de Cartagena – La Unión (SE Spain). The research also contains a flow behaviour comparison of materials collected in original strata with those previously liquefacted forming sand dykes. Additionally, the study includes the influence of other physicochemical characteristics of the post-flotation mine wastes—particle size distribution, density, particle shape, elemental and mineralogical composition—on its viscoelastic properties as mineral suspensions. The three tailings dams under review are off-valley facilities, with a progressive construction following the upstream method. Emissions to air, water, and land make these structures a sturdy hazard to the environment. Most of the specimens were collected in El Descargador tailings dam (9 original strata sampled from an existing trial pit, and 13 samples along a line of sand dykes from a failure surface occurred on 14/10/1963). Sample collected in Peña del Águila deposit was sieved in the laboratory to aim most of the rheometric tests over the fraction lower than 50 µm. Conversely, the sample from La Peraleja tailings dam was a liquefied sediment resulted from an elutriation process across the bedded tailings. Rheometric research can be summarized in five categories: 121 previous tests, 278 tunning or calibration operations, 939 flow tests, 105 transient tests, and 85 dynamic tests. Previous tests refer to the initial, non-systematic experimentation, where tentative tests exceeded the successful ones, mainly dedicated to compare the available sensors. Rheometer calibration includes the daily procedures—inertia and friction calibration, rotational mapping, and verification tests—executed before each sequence of experiments. Flow tests, with an average of 81% successful experiments, were the bulk of the work, because they produced important parameters, such as overshoot stress or yield stress. Furthermore, 53% of the transient tests failed, and 100% of the dynamic tests were successfully accomplished. An overall of 309 slurries were prepared along the study. The results confirmed that vane geometry was the best fixture for flow tests, and parallel-plate geometry was a plausible alternative for certain transient and dynamic tests. A wide-gap vane sensor allowed to record smooth rheograms of slurries with particle finesses of up to 750 µm. Herschel-Bulkley model was the best approach to fit the stress-rate response of the slurries. A viscoelastic sprectrum of the dispersion could be generated in the range 102–10–3 Hz. Exponential curves of yield-stress versus concentration revealed the easiness to yield of materials from the sand dykes, even at higher concentrations than the original strata. They also presented longer time-effects and shear-thinning behaviour. The results disclosed that an increase in pH and EC favours the stability of the tailings. Field phenomena, such as static and dynamic liquefaction, can be addressed by flow and dynamic tests, respectively. In fact, a flow-slide model that used rheological results as input data, predicted coherent values of maximum velocity and maximum run-out distance for El Descargador flow failure. Finally, the critical yield stress of the slurry can be assimilated to the concept of residual shear stress in soil mechanics, what opens the door to use these studies to complement triaxial testing in the solution of stability problems. http://repositorio.bib.upct.es/dspace/