Effect of Gypsum Addition on the Mechanical and Microstructural Performance of Sulphide-Rich Cemented Paste Backfill

The present study investigates the effect of β-hemihydrate gypsum (HG) dosages on the mechanical and microstructural performance of cemented paste backfill (CPB) produced from sulphide-rich mine tailings using NaOH-activated slag (NAS) as the major binder. X-ray diffraction (XRD), scanning electron microscopy (SEM) and mercury intrusion porosimetry (MIP) analyses were carried out to elucidate the mineralogical composition and microstructure of NAS-HG-CPB samples. The results illustrate that the main hydration products of NAS-HG-CPB from sulphide-rich tailings are crystalline (CaSO4•2H2O and ettringite (AFt), 3CaO•Al2O3•3CaSO4•32H2O) and amorphous. The results also show that the 28 d unconfined compressive strength (UCS) of CPB with 30 wt. % HG replacing NAS increased by 52% compared to the UCS of CPB containing no HG, and both have stable long-stage (180 d) UCS (i.e., no strength loss). Excess HG addition (≧50 wt. %) reduced the early-stage (≦28 d) UCS of NAS-HG-CPB and led to unstable long-stage (180 d) UCS by the formation of secondary gypsum. The use of 30 wt. % HG replacing NAS in NAS-HG-CPB accelerates the hydration process of ground granulated blast furnace slag (GGBS) in the alkaline solution by forming ettringite (AFt), leading to the denser microstructure and improved mechanical performance in comparison with CPB containing no HG. The NAS-HG binder with low dosages of HG (≦30 wt. %) will be a promising binder for stabilising sulphide and non-sulphide tailings and CPB production.

Chemometric Optimisation of a Copper Sulphide Tailings Flocculation Process in the Presence of Clays

The presence of fine and ultra-fine gangue minerals in flotation plants can contribute to sub-optimal valuable ore recovery and incomplete water recycling from thickeners, with the performance of the latter equipment relying on adequate flocculation. In order to study the dependence of the flocculation process on the suspension-flocculant mixing conditions, a series of experiments—chosen using chemometric analysis—were carried out by varying mixing conditions, solid concentration, water salinity and flocculant dosage. To this purpose, two different tailings (both featuring coarse and fine content) were considered and a response surface methodology based on a Doehlert experimental design was used. The results suggest that the operational conditions to optimise the flocculated tailings settling rate and the suspended solids that report to a thickener overflow are not necessarily the same. This is a reasonable outcome, given that the settling rate depends on the coarse aggregates generated in the slurry, while the overflow solids content is governed both by either fine particle content (and its characteristics) or small aggregates. It is inferred that to maximise dewatering performance two stages should be involved—a separate treatment of the thickener overflow to remove fine content and thickening at optimal flocculant dosage to enhance this process.

Stabilisation of high-sulphide tailings with alkali activated fly ash – mechanical performance

Mine tailings could represent a step forward in terms of the quality of the aggregates that usually accepted in civil engineering applications, due to their specific weight and compressive strength. The Neves-Corvo copper mine produces approximately 3 million tons of tailings every year, which could supply several construction works, at least in the south of Portugal. Nevertheless, this industrial waste requires stabilisation, not only due to their high sulphur content, but also due to mechanical performance demands. This paper focus on the stabilisation, without previous thermal treatment, of the mine tailings from Neves-Corvo.

Oxygen consumption test to evaluate the diffusive flux into reactive tailings: interpretation and numerical assessment

The oxygen consumption (OC) test is conducted on sulphide tailings by measuring the decline of oxygen concentration in a closed headspace, at the top of a cylinder, as a result of diffusion and oxidation reactions. For a short-duration test, the measurements may be interpreted using a simplified analytical method based on modified Fick’s laws, which provides the combined value of the effective oxygen diffusion (De) and reaction rate (Kr) coefficients of the tailings. This lump value can be used to evaluate the steady-state oxygen flux entering the exposed sulphide tailings. In this paper, a numerical parametric study is performed to investigate the effect of test duration and headspace height, h, on results obtained from OC tests. The assessment also considers tailings with different values of De and Kr. The results indicate that the simplified interpretation method usually tends to underestimate the surface oxygen flux, in proportions that depend on the testing conditions. Results from this study can be used to estimate the relative precision of the oxygen flux for specific conditions, thus helping practitioners decide how to best interpret testing measurements for a given application.

Oxygen diffusion and consumption in low-sulphide tailings covers

Acid-generating tailings can be reclaimed using oxygen barrier covers in humid climates, such as in Quebec. A single-layer low-sulphide tailings cover is an attractive alternative to traditional water covers to maximize the impoundment’s storage capacity. Low-sulphide tailings can serve as a moisture-retaining material to limit oxygen diffusion, and their residual sulphides can consume the diffusive oxygen that still gets through the cover. A laboratory experiment using instrumented columns was initiated to confirm the efficiency of a low-sulphide tailings monolayer cover placed over acid-generating tailings to reduce the oxygen flux reaching the reactive tailings. Oxygen concentration profiles were measured over the thickness of the cover and oxygen fluxes were calculated to evaluate the effect of three parameters on these fluxes: water table level, cover sulphide content, and cover thickness. Oxygen fluxes at the bottom of the low-sulphide tailings cover layer, calculated using oxygen gradient and estimated effective diffusion coefficient and predicted using Vadose/W, were lower than 1.5 mol·m−2·year−1. These results suggest that a low-sulphide tailings cover with an elevated water table can reduce oxygen migration and potentially limit acid mine drainage generation.