Real-Time Electrical Conductivity Monitoring and Correlation with Sulfate Release and Acid Mine Drainage Potential from a Gold/Silver Paste Tailing Storage

Safe disposal of tailings as high-density thickened tailings or paste tailings can reduce the environmental risks of conventional tailings deposits, reduce water use, minimize tailings storage facility footprints, reduce the potential for acid mine drainage (AMD), and minimize risks of failure, among other advantages. In the dewatering process, the addition of flocculants is key to improving the sedimentation of the tailings and the formation of a compact paste. Despite the environmental and operational advantages of using paste tailings, it is not clear how the chemical nature of coagulants and flocculants could influence the discharge of toxic elements (salts and metals) from tailings after storage. In this study, we show the results of the real-time evaluation of the release of polluting runoffs from a paste tailings deposit. To do this, we analyzed paste tailing samples for AMD potential through static and kinetic tests and monitored the electrical conductivity and real-time pH, evaluating the correlation with the sulfate in the thickener and downstream from the tailings deposit. Tailing samples have low sulfur content (<2%) and low acid-generating potential. Moreover, there is no evidence of a significant positive correlation (Pearson’s coefficient r < 0.8) between the sulfate concentrations with the pH or EC. Thus, the chemical nature of the paste tailings prior to discharge has no direct impact on the release of sulfate-rich runoffs from the tailings after its storage. This indicates that the tailings paste at the evaluated site is chemically stable in the short term.

Dynamic response of thickened tailings in shaking table testing

In this study, an instrumented thickened tailings deposit model was designed, built and employed to evaluate the behaviour of layered thickened tailings to dynamic loading by using a shaking table equipment. The thickened tailings were deposited subsequently in three thin layers in a flexible laminar shear box mounted on top of the shaking table. Cyclic loading with a peak horizontal acceleration of 0.13 g and a frequency of 1 Hz was applied to the layered tailings deposit. Different types of sensors were placed to monitor the accelerations, displacements, volumetric water content and pore water pressures at the intermediate depth of each layer. Results indicated that the acceleration for the bottom and middle layers were similar to that of the base of the shaking table; but, this was not the case for the top layer. The measurements of vertical displacements indicated that all layers of thickened tailings experienced initially contraction and subsequently dilation during the shaking. The excess pore water pressure ratios were found to exceed unity through all layers of thickened tailings when the shaking ended. In other words, the results showed that the layered thickened tailings are susceptible to liquefaction under the considered testing conditions. It is also found that upward pore water migration to the top layer and downward pore water flow to the bottom layer occurred in the thickened tailings deposit. This water migration generated additional pore water pressure and also impacted the vertical displacement and liquefaction susceptibility of the thickened tailings material. The results of this study give a better understanding of the dynamic behaviour of thickened tailings, which is crucial for the safety of thickened tailings systems as well as sustainable mining.

Mineralogical Characterization, Resource Estimation and 3D Modeling of Sulfidic Tailings at the Neves Corvo Mine: An On-going Assessment

&lt;p&gt;Neves Corvo is an underground high-grade Cu-(Sn)-Zn mine, currently producing copper, zinc and lead concentrates. Copper production started in 1989, followed by tin production, between 1990 and 2001, and zinc / lead production started in 2006. The operation is owned by SOMINCOR, a subsidiary of Lundin Mining, with a maximum capacity of 2.6Mtpy for the copper processing plant and 1.0Mtpy (ongoing expansion to 5.6Mtpy) for the zinc processing plant.&lt;/p&gt;&lt;p&gt;The Neves Corvo VMS deposit is located in the Portuguese part of the world-class Iberian Pyrite Belt (IPB) and is composed of seven orebodies. The Neves, Corvo, Zambujal and Lombador orebodies are currently in production, whereas the Semblana and Monte Branco orebodies are relatively recent discoveries still under development and evaluation, and the Gra&amp;#231;a orebody has been already fully mined.&lt;/p&gt;&lt;p&gt;From 2010 till end of 2019, the mine has accumulated 7.3Mt of waste rock and 17Mt of thickened tailings. These mining residues are stored in Cerro do Lobo Tailings Management Facility (Cerro do Lobo TMF), which completes a volume of 47Mt since the beginning of the operation in 1989 (30Mt are slurry tailings).&lt;/p&gt;&lt;p&gt;The deposition method changed in 2010 from slurry subaquatic deposition to sub-aerial thickened tailings stack (vertical expansion) in co-deposition with potentially acid-generating (PAG) waste rock. The thickened tailings have an average of 63% solids. X-ray fluorescence analysis have shown copper and zinc grades variation in the waste rock between 0.3 and 0.9%, and 0.4% and 1.1%, respectively, and concentrations up to 0.3% and 0.4% of copper and zinc, respectively, in the tailings.&lt;/p&gt;&lt;p&gt;Mineralogically, the tailings consist mainly in pyrite, sphalerite, chalcopyrite, +/- arsenopyrite, +/- tetrahedrite-tennantite, gangue minerals such as quartz, phyllosilicates, carbonates and some oxides, and have a non-uniform particle size distribution ranging between 1 and 100 &amp;#181;m. The waste rock fraction is millimetric to centimetric in size, and is formed by the local host rocks, which include acid volcanic rocks, schists and graywackes, all of them containing variably significant disseminated sulfides, largely dominated by pyrite.&lt;/p&gt;&lt;p&gt;On-going research is being undertaken aiming to build a geometallurgical model for the Neves Corvo mine, ground on a huge database on the chemical and mineralogical composition, and particle size distribution of the mine tailings, coupled with (and calibrated by) new analytical and automated data acquired in a large set of carefully selected representative samples, in order to assess the potential recovery of base metals and their by-products out of these potentially valuable mine residues. The model construction and consequent resource estimation will be based on the daily monitoring of the tailings deposition at the disposal units, over the past 10 years (i.e., since the subaerial deposition has started at Neves Corvo), in terms of volume/tonnage, chemical and mineralogical compositions and physical characterization of the material.&lt;/p&gt;&lt;p&gt;This study is part of the work package 1 (WP1) of ETN&amp;#8211;SULTAN project (H2020) - European Training Network for the remediation and reprocessing of sulfidic mining waste sites. Publication supported by FCT- Project UID/GEO/50019/2019 - Instituto Dom Luiz.&lt;/p&gt;

Pore Connectivity and Dewatering Mechanism of Tailings Bed in Raking Deep-Cone Thickener Process

Paste and thickened tailings (PTT) technology can improve the utilization and management of tailings from processing plants. The pore size distribution (PSD) and microstructure evolution affected by the rake shear in thickening tailings beds are essential to produce a high-density tailings underflow. Continuous thickening and computed tomography (CT) scanning tests were conducted to study the PSD with and without shear. The pore morphology was studied to reveal the shearing-dewatering performance of the tailings bed. The results show that at a flocculant solution concentration of 0.01 wt % and a feed slurry concentration of 10 wt%, the underflow concentration with and without shear can reach 58.5 wt %and 55.8 wt %, respectively. The CT image reconstruction models demonstrated that the porosity of the sheared tailings bed increased with the bed height. When the bed height increased from 2.5 to 10 cm, the porosity increased from 35.1% to 41.9%, the pore fractal dimension increased from the range 1.8–1.95 to the range 2.1–2.15, and the pore quantity decreased by 21.39%. The average pore volume increased with increasing height by 13.93%, 16.57% and 12.07%. The pore structure became more complex with the bed height, and the connectivity between pores increased to form water-flow channels, which were beneficial to the drainage of sealed water.