Rheological characterization of tailing pastes from ore mining

Reliable measurement of the yield stress of iron ore tailings is decisive for two practical problems faced by the mining industry: defining the self-rescue zone in dam-break risk assessments and dimensioning the pressure gradient required to keep tailings flowing in pipelines. Yet, tailing pastes are exceptionally complex materials — elasto-viscoplastic, thixotropic and made of coarse and irregular particles over a wide size range — and a single rheological test rarely yields a confident value. This work compares three independent test types applied to tailings discharged from the Fundão dam failure, identifies the limits of each technique, and proposes a practical workflow combining them to obtain yield-stress values suitable for engineering use. Background Tailings dams remain the most common disposal route for the iron ore industry in Brazil, the world’s second largest ore producer. Their potential failure carries severe environmental, social and economic consequences, including loss of human life. Numerical simulations of post-failure tailings flow — central to delimiting the self-rescue zone — depend on accurate rheological data, particularly on the yield stress, τ0, which sets the minimum shear stress required to initiate flow. The same parameter also dictates the pumping demand for pipeline transport of these pastes, since flow is only sustained once the wall shear stress exceeds τ0. Despite this central role, the τ0 of iron ore tailings is notoriously difficult to quantify. Most published studies rely on a single technique, typically flow curves, without addressing the time dependency and slip effects that affect such materials. Materials and methods Two tailings samples were collected from material discharged following the 2018 Fundão dam failure in the Santarém stream (Bento Rodrigues, Minas Gerais, Brazil). The pastes used in the rheological tests were prepared in the laboratory from these samples. Geotechnical characterization revealed contrasting fines content. Sample 1 contained 8.42 percent clay and 53.48 percent silt, while sample 2 was predominantly sandy (82.46 percent sand). X-ray diffraction identified quartz, hematite and goethite, with iron oxides concentrated in the finer fractions. Tests were carried out on a TA Instruments AR 2000ex stress-controlled rheometer, using a vane-in-cup geometry (14 mm × 42 mm vane in a 15 mm cup) to minimize wall slip. Material was oven-dried at 105 °C, hydrated with distilled water at four water fractions (26.62, 28.85, 31.25 and 33.85 percent), and homogenized manually for five minutes. Below 26.62 percent water content, the required torque exceeded the rheometer’s working range. The tested range, however, is representative of in situ paste conditions. Three independent test types were applied at 25 °C: (1) shear-rate-imposing flow curves fitted with the Herschel–Bulkley (HB) model , for τ0 > τ0 (where τ0 is yield stress, κ is the consistency modulus and n is the power-law index) (2) strain-over-time measurements at fixed imposed stresses and (3) oscillatory tests measuring the storage (G’) and loss (G’’) moduli as functions of stress amplitude. Results Fig. 1 Shear stress as a function of shear rate for sample 1. Tests were conducted for a range of water fractions for an imposing shear rate from 0.001 1/s to 1,000 1/s at an interval of 600 s. Flow curves displayed the characteristic plateau of viscoplastic materials, with yield stresses decreasing as water fraction increased (Fig. 1). For sample 1, τ0 ranged from 87 Pa (w = 26.62 percent) to 18 Pa (w = 33.85 percent), and for sample 2, from 58 Pa to 13 Pa over the same range. HB parameters (Table 1) revealed a clear contrast. Sample 1 exhibited consistency moduli around 5 to 7 Pa·sn with strong shear-thinning (n = 0.44 to 0.58), while sample 2 was less consistent (κ ≈ 0.5 to 0.7) and closer to Newtonian (n = 0.79 to 0.91). The higher yield stresses of sample 1 are consistent with its higher fines content. Silt and clay fractions contain higher amounts of iron oxides — hematite (4.9 to 5.3 g/cm³) and goethite (3.3 to 4.3 g/cm³) — than the sand fraction, which is dominated by quartz (≈ 2.65 g/cm³). At the same moisture content, this produces a denser, more cohesive paste. Strain-over-time tests at fixed stresses corroborated the flow-curve results and exposed the material’s time dependency. For sample 1 at w = 33.85 percent, strain stabilized at a value near 1 for imposed stresses of 15 and 18 Pa (solid-like elastic response, of Kelvin–Voigt type), while 20 Pa produced continuous flow, placing τ0 between 18 and 20 Pa, in agreement with the flow curve (Fig. 2). For sample 1 at w = 28.85 percent, τ0 lay between 50 and 53 Pa, and for sample 2, between 7 and 10 Pa (w = 33.85 percent) and between 33 and 35 Pa (w = 28.85 percent). Several tests displayed apparently stable strain for tens of seconds followed by an abrupt rupture, a thixotropic signature typical of clay suspensions. Fig. 2 Strain over time for fixed stresses. Tests were conductedfor a range of fixed stresses for sample 1with a water fraction of 33.85 percent. Oscillatory tests confirmed the same order of magnitude. The crossover where G’’ surpasses G’ yielded τ0 ≈ 22 Pa for sample 1 at w = 33.85 percent and approximately 50 Pa at w = 28.85 percent, both consistent with the values from the previous tests. A test frequency of 0.1 Hz was low enough to suppress frequency dependence. Discussion The three techniques converged on consistent yield-stress values, but each carries trade-offs. Flow curves are fast and widely available, but require slow enough rate sweeps to reach steady state. Strain-over-time tests are the most reliable for thixotropic, elasto-viscoplastic materials as they allow rupture to be observed directly, but they are laborious, requiring multiple runs to bracket the bifurcation. Oscillatory tests give a fast first estimate, but are frequency-dependent and must be run at low enough frequency. A practical compromise for paste characterization is to obtain a candidate τ0 from a flow curve, then confirm it with two strain-over-time runs at stresses immediately below and above that value. Conclusions For tailings collected from the Fundão deposit, yield stresses determined at four water fractions ranged from 13 to 87 Pa, of the same order as values reported by Chácara and Oliveira for tailings from the same region (Quadrilátero Ferrífero). The higher τ0 measured in the finer-grained sample reflects its larger iron-oxide content.For complex, time-dependent pastes, no single rheometric test is fully sufficient: combining flow curves with strain-over-time confirmation provides the safest path to engineering-grade yield-stress values for dam-break simulations and pipeline transport design. n Selected references Ancey C (2007) Plasticity and geophysical flows: A review. J Non-Newtonian Fluid Mech 142:4–35 Chácara DM, Oliveira Filho WL (2021) Rheology of mine tailings deposits for dam break analyses. REM – Int Eng J 72. Coussot P, Nguyen QD, Huynh HT, Bonn D (2002) Avalanche behavior in yield stress fluids. Phys Rev Lett 88:175501 Pullum L, Boger DV, Sofra F (2018) Hydraulic mineral waste transport and storage. Annu Rev Fluid Mech 50:157–185 Tarcha BA, Forte BPP, Soares, EJ, Thompson RL (2015) Critical quantities on the yielding process of waxy crude oils. Rheol Acta 54:479–499

Rheological characterization of tailing pastes from ore mining

Authored by: Neemias Almeida Dias, Patrício José Moreira Pires and Edson José Soares

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