Tailings Management Optimization for Open-Pit Copper Mine in Northern Chile

Scenario

An open-pit copper mine in the Atacama Desert (Chile) faced escalating tailings storage facility (TSF) capacity constraints due to high clay content in crushed ore. To extend TSF life and reduce water consumption, engineers evaluated thickening efficiency of flocculated tailings slurry. Key constraints included high evaporation rates (>3000 mm/yr), ambient temperatures up to 35°C (reducing viscosity), and strict seismic safety requirements limiting slurry density gradients. Accurate settling prediction was critical to avoid under-thickening (excess water return) or over-flocculation (costly polymer use).

Given Data

• Acceleration due to gravity: 9.79 m/s² (adjusted for elevation ~3200 m ASL)
• Particle density: 3100 kg/m³ (copper sulfide + silicate gangue)
• Fluid density: 1025 kg/m³ (saline process water, TDS ≈ 12,000 ppm)
• Particle diameter: 120 µm = 0.00012 m (median size after grinding and flocculation)
• Dynamic viscosity: 0.00078 Pa·s (measured at 30°C)
• Drag coefficient (turbulent): 0.44 (validated via lab sedimentation tests on flocculated particles)

Calculation

Laminar velocity: [ v_s^{\text{laminar}} = \frac{(3100 - 1025)(9.79)(0.00012)^2}{18 \times 0.00078} = \frac{2075 \times 9.79 \times 1.44 \times 10^{-8}}{0.01404} \approx \mathbf{0.020841} \text{ m/s} ]

Turbulent velocity: [ v_s^{\text{turbulent}} = \sqrt{\frac{4 (3100 - 1025)(9.79)(0.00012)}{3 \times 0.44 \times 1025}} = \sqrt{\frac{4 \times 2075 \times 9.79 \times 1.2 \times 10^{-4}}{1353}} \approx \mathbf{0.077219} \text{ m/s} ]

Reynolds number using turbulent result: (Re = \frac{1025 \times 0.077219 \times 0.00012}{0.00078} \approx 12.2) → transitional; however, lab data showed clear inflection at Re ≈ 10–20 and best fit with turbulent formula. Stokes’ law overpredicted hindered settling by 25% in pilot tests.

Result and Decision

The turbulent settling velocity (0.0772 m/s) was selected as the design basis after cross-validation with 2-m column tests. This enabled optimization of flocculant dosage to achieve target underflow density (55% w/w) while maintaining overflow clarity. The revised thickener design reduced freshwater makeup by 18% annually and deferred TSF expansion by 4.3 years.

Lesson

In industrial slurries with flocculated or aggregated particles, empirical drag coefficients and turbulent formulations often outperform Stokes’ law—even at moderate Reynolds numbers—because particle shape, density heterogeneity, and interparticle forces dominate over viscous effects.