Inside the thickener: how mines turn messy tailings into clear, reusable water

In mineral plants, tailings thickeners are the quiet workhorses—vast settling tanks that concentrate slurry solids and generate clear overflow water for recycling (fls.com). A central feedwell dissipates the energy of incoming slurry to reduce turbulence and promote uniform distribution, while rotating rake arms—sometimes with spikes—sweep settled sludge toward a central underflow discharge (sinoninetech.com) (sinoninetech.com).

The gains from modern flocculated operation are stark. A pair of 128 m diameter conventional thickeners (9.8 m deep) handling 5,500 tonnes/day achieved only about 0.01 t/m²·h solids flux—t/m²·h is tonnes per square meter of settling area per hour—whereas a high‑rate thickener can typically handle ~0.4 t/m²·h under similar conditions, roughly a 40× increase (researchgate.net) (researchgate.net). One gold mill lifted thickener underflow from 35% to 50% solids (wt%, weight percent) with a feedwell/dilution upgrade (flsmidth.com).

That clear overflow is the lifeblood of the circuit. In tailings dams, reclaim systems draw from a supernatant pond via floating pump barges or fixed decant towers (a riser with submerged intake collars) to return water to the plant by pumps or gravity through a buried conduit (tailings.info) (tailings.info). Clarity matters: recycle water often targets <100 mg/L suspended solids (mg/L is milligrams per liter), especially where it feeds sensitive circuits like SX/EW (solvent extraction/electrowinning) or CIP (carbon‑in‑pulp) (research.csiro.au). One mine now recycles ~80% of its process water after a thickener upgrade (flsmidth.com).

Thickener vessel and hydraulics

At heart is a cylindrical tank (often steel) with a conical or flat bottom sloped to a central discharge so settled solids (“cake”) collect near a cone or sump (sinoninetech.com) (sinoninetech.com). Conventional designs deliver feed via a bridge launder into a tall feedwell—often 5–10 m high—to minimize turbulence so even the slowest particles can settle (sinoninetech.com).

High‑rate and paste thickeners build on this with high molecular‑weight polymer flocculants and controlled feed dilution to achieve much higher solids fluxes per area (researchgate.net). These typically use shallow feedwells and higher flocculant doses, so controlled feed dilution upstream of flocculation is essential (researchgate.net) (researchgate.net). Deep‑cone “high‑density” units can push underflow to 60%+ solids for paste disposal, shrinking stored water and the footprint of tailings storage facilities (fls.com) (tailings.info).

Stable operation hinges on bed level and rake torque control to prevent short‑circuiting or “donutting” (a cleared central hole). Operators use torque and bottom probes to regulate feed and dilution, keeping underflow concentration and overflow clarity on target (researchgate.net). Retrofitting modern feedwells (e.g., E‑Volute™) has delivered measurable gains: one study saw a 46% reduction in water reporting to tailings and a ~40% cut in flocculant usage after a gold‑mine thickener upgrade (flsmidth.com).

Performance targets and case outcomes

Without flocculant, underflow solids often stagnate at ~30–40%; with optimized dosing, underflow commonly exceeds 50% (flsmidth.com). Overflows can be very clear—typically <100 mg/L suspended solids—enabling >90–95% recovery of feed water as recycle (recovery refers to water returned to process) (researchgate.net) (research.csiro.au).

The water‑quality dividend can ripple through the plant. After one thickener upgrade, recycled water was so clear that scale inhibitor costs dropped 35% and debris in concentrators fell—while also boosting water recovery and cutting polymer consumption (flsmidth.com). In that case, the mine now recycles ~80% of its process water (with minimal fresh make‑up) (flsmidth.com).

Reclaim systems on tailings dams

Once thickened, tailings flow to the impoundment. The decant pond provides a buffer of supernatant water for reuse, removed by pump barges (floating platforms carrying submerged pumps that can move with the pond) or by decant towers (fixed risers with collars set below the surface), with gravity conveyance via a buried conduit where applicable (tailings.info) (tailings.info).

Design capacity and redundancy must handle variability; guidance calls for decant systems sized to remove storm inflows over 2–4 weeks to protect freeboard (tailings.info). Reclaimed water quality remains critical; industry practice often targets <100 mg/L solids in recycle water, particularly when reporting to SX/EW or CIP (research.csiro.au).

What flocculants do in tailings

Synthetic polymer flocculants—typically high‑molecular‑weight polyacrylamide copolymers—provide inter‑particle links (bridging) that transform fine dispersions into fast‑settling flocs and enable dense underflows with a clean overflow (researchgate.net) (mdpi.com). Their arrival in the 1960s effectively revolutionized thickening, shrinking required area and accelerating separation compared to gravity settling alone (researchgate.net).

In gold and copper tailings (often clay‑bearing), the right polymer can lift underflow solids by tens of percentage points and capture ultrafines. Laboratory work on siliceous tailings shows that polymers forming low‑porosity, large‑diameter flocs deliver the highest settling velocities (mdpi.com). Conversely, clay‑rich copper tailings settle very slowly even with polymers, underscoring the importance of appropriate flocculation strategy (mdpi.com). Well‑flocculated networks can also yield more fluid underflows, easing pumpability and deposition (qualitative behavior as reported).

Flocculant dosing has a true optimum. Too little leaves fines dispersed; too much can “over‑flocculate,” saturating particle surfaces so flocs break into bulky, low‑density aggregates that lower underflow concentration. Bench and continuous tests report underflow density peaking near an optimum (e.g., ~40 g/t in one study), with higher doses loosening solids and reducing bed density (iwaponline.com) (iwaponline.com) (researchgate.net). In practice, polymer solutions are carefully diluted—often 0.1–0.5%—and introduced via dedicated mixers for uniform contact (researchgate.net). Many plants rely on accurate chemical dosing hardware such as a dosing pump when metering flocculants.

Overflow carryover of residual polymer is typically negligible—most adsorbs on solids—and is generally compatible with downstream circuits, although published work notes that tolerance depends on the specific flotation or hydrometallurgical process (research.csiro.au). Where ultrafine colloids or extreme pH dominate, a two‑stage strategy can help: first a coagulant (e.g., aluminum or iron salts, or a cationic polymer) to destabilize particles, followed by a polymer to grow flocs (researchgate.net) (research.csiro.au). In such cases, plants may source specialty coagulants alongside polymers.

Selecting flocculant for a given ore

Characterize the slurry. Particle size distribution, mineralogy (including clays), pulp pH and ionic strength, and residual reagents matter. Fine fractions <20 µm—especially <1 µm clays/quartz—and clay type (kaolinite vs. smectite) strongly influence dose and the need for pretreatment. Zeta potential trends indicate surface charge qualitatively.

Shortlist polymers and doses. Mineral plants typically screen high‑MW water‑soluble acrylamide‑based copolymers. Base‑metal tailings usually run high‑MW anionic polyacrylamides (acrylamide–acrylate) because most oxides/silicates are negatively charged at neutral to alkaline pH; cationic polymers or aluminum coagulants are sometimes added first if solids carry strong negative charge (research.csiro.au) (fls.com).

Run laboratory jar tests. Agitate tailings and dose candidates across, say, 10–100 g/t, then observe settling rate, underflow solids, and supernatant turbidity. The best polymer delivers the fastest settling and clearest water at the lowest dose; one study reached a clear overflow and ~50% underflow at 30 g/t with a specific polymer (mdpi.com). Inspect floc size/structure to ensure large, robust aggregates (mdpi.com) (mdpi.com), and match mixing intensity to feedwell shear.

Validate at pilot scale. Bench tests miss full‑scale hydrodynamics, so many sites run pilot thickeners to verify polymer performance, addition points (single vs. distributed dosing), dilution volumes, and rake/bed interactions. One program found an optimum near ~40 g/t (iwaponline.com), with results informing feedwell design and dose control (tailings.info) (researchgate.net).

Account for water chemistry. High salinity or multivalent ions can change polymer coil dimensions and adsorption behavior; pH and temperature affect solubility and activity. Always test under actual plant liquor conditions (research.csiro.au). In continuous operation, optimize cost by dosing to a clarity target—e.g., <100 mg/L TSS (total suspended solids)—using feedback control (an online turbidity meter) to maintain the minimum effective dose. Upgrading hydraulics alone has cut polymer usage by ~40% per tonne in a gold case (flsmidth.com).

Design, chemistry, and the bottom line

Get the vessel geometry, feed dilution, and polymer program aligned, and a thickener will routinely deliver dense underflow (often 40–55% solids; >60% for deep‑cone paste) and a clean overflow suitable for recycle. From there, robust dam reclaim—barge or tower—closes the loop. For the polymer program itself, precise metering via a dosing pump and sourcing appropriate flocculants and, where needed, coagulants support steady, low‑turbidity operation consistent with the studies and case results cited above.

Industry studies and technical reviews backstop these figures: an SNC‑Lavalin analysis notes <0.01 t/m²·h for non‑flocculated tailings versus ~0.4 t/m²·h with flocculation (researchgate.net); FLS case work shows underflow rising from 35% to 50% solids and a near‑halving of water lost to tailings post‑retrofit (flsmidth.com); and laboratory research ties floc microstructure—dense, low‑porosity aggregates—to faster sedimentation (mdpi.com). Tailings management references emphasize decant capacity and overflow control for safe, effective reclaim (tailings.info) (tailings.info). (Numeric citations in the source paper refer to listed references; relevant line segments are denoted “†Lx–Ly.”)