Thickener Underflow Density Explained
What thickener underflow density means, how it relates to percent solids and slurry density, why it matters for underflow pumping and tailings water recovery, and why target densities are always site-specific. A concept guide — not a thickener design model.
Definition
Thickener underflow density is the bulk slurry density of the thickened stream drawn from the bottom (apex) of a thickener or clarifier. A thickener separates a dilute feed slurry into a clarified liquid overflow and a much denser, higher-percent-solids underflow. The underflow density — usually reported as a slurry density (kg/m³ or SG) or as a percent solids by mass (Cw) — is the headline number that says how much water has been removed and how concentrated the product slurry is. Because density, percent solids, and specific gravity are tied together by the solids and liquid densities, an underflow density target and an underflow percent-solids target are two ways of stating the same thing.
Why it matters
Underflow density sets the duty for everything downstream. A denser underflow means less water carried forward, which lowers the volumetric load on underflow pumps and tailings or transfer pipelines and improves water recovery to the overflow for reuse. But push the density too high and the underflow becomes hard to pump, the yield stress climbs, lines block, and rakes can overload. Run it too dilute and you waste pumping capacity, lose water to the tailings stream, and under-utilise the thickener. The right target is a balance — and it is specific to the ore, the particle size, the flocculant, and the plant — so the density is monitored continuously and used as a control handle, not set once.
Formula
Units involved
- •ρ_underflow, ρ_liquid, ρ_solids — densities in kg/m³ (or SG, dimensionless)
- •Xs — solids mass fraction (0–1), i.e. Cw / 100
- •Q_underflow — underflow volumetric flow (m³/h)
- •ṁ_solids — solids mass flow (t/h or kg/s)
Concept diagram
Worked example
A thickener underflow is measured at 1500 kg/m³ (SG 1.50). The solids SG is 2.65 (2650 kg/m³) and the liquor is water (1000 kg/m³). What percent solids by mass does that density represent?
- 01ρ_underflow = 1500, ρ_solids = 2650, ρ_liquid = 1000 kg/m³
- 02Xs = 2650 × (1500 − 1000) / (1500 × (2650 − 1000))
- 03Xs = 2650 × 500 / (1500 × 1650) = 1,325,000 / 2,475,000
- 04Xs ≈ 0.535
About 53.5% solids by mass — a typical magnitude for a thickened mineral underflow, though the real target depends entirely on the ore and plant.
Common mistakes
- •Treating an underflow density target as transferable between plants — it is set by the specific ore, grind, flocculant, and equipment.
- •Confusing percent solids by mass (Cw) with by volume (Cv); underflow targets are almost always quoted by mass.
- •Using clean-water properties to size the underflow pump instead of the actual underflow density and rheology.
- •Assuming a higher density is always better — beyond a point the underflow stops being pumpable and rakes overload.
- •Reading density alone as a measure of dewatering performance without checking the overflow clarity and solids losses.
When to use the calculator
Use the Slurry Density Calculator to convert between a measured underflow density and percent solids (it accepts mass percent, volume percent, or g/L, and has a target-density mode). Use the Percent Solids Mass ↔ Volume Calculator when you need the Cw ↔ Cv basis conversion explicitly, and the Slurry Mass Balance Calculator to turn the underflow flow and density into solids and liquid mass flows for a water balance.