PAM in Mining Tailings Treatment: Dosage, Flocculation & Settling

The Tailings Treatment Challenge

Mining operations generate tailings — the fine-particle residue after ore concentration — at rates of millions of tonnes per year. These slurries contain mineral fines (typically 80% passing 75 µm), process water with residual reagents, and dissolved salts from ore and process chemistry. Settling this material efficiently determines the water recovery rate, the stability of tailings storage facilities, and compliance with environmental discharge limits.

Polyacrylamide is the dominant flocculant technology across the mining industry, applied in thickeners, clarifiers, and tailings dams.

Mineral-Specific PAM Selection

Coal Washery Effluent

Coal fines (d₅₀ typically 5–30 µm) carry a mixed surface charge depending on the carbon-to-ash ratio. High-carbon fine coal particles tend to be hydrophobic and weakly negatively charged; clay minerals in the ash fraction carry strong negative charge.

Recommended PAM type: Anionic PAM, MW 15–22 million Da, hydrolysis degree 20–30% Optimal pH range: 6.5–8.5 Dosage: 1–3 ppm (coarse coal thickener), 2–5 ppm (fine coal centrifuge feed) Performance target: Thickener underflow >65% solids, overflow turbidity <50 NTU

The high ionic strength of coal process water (TDS typically 2,000–8,000 mg/L from dissolved salts and flotation reagents) partially screens electrostatic interactions. Ultra-high MW anionic PAM compensates by extending bridging contact distance.

Gold Tailings (Cyanide Circuit)

Gold CIL/CIP tailings slurries contain fine sulfide minerals, siliceous gangue, and residual cyanide (pH typically 10–11). The strongly alkaline pH and cyanide matrix present a challenging environment for PAM.

Recommended PAM type: Anionic PAM, MW 18–22 million Da, hydrolysis degree 25–35% Optimal pH range: 9–11 (PAM is stable, but requires higher dosage due to charge screening by Na⁺ from NaOH addition) Dosage: 0.5–2 ppm (thickener, after lime adjustment) Coagulant pairing: Lime addition to pH 10.5–11 precipitates heavy metals before PAM addition; improves PAM efficiency by 30–50%

⚠️ In cyanide-containing circuits, all PAM handling must comply with site safety procedures for cyanide exposure.

Copper and Base Metal Tailings (Flotation Circuit)

Copper flotation tailings contain sulfide mineral fines (chalcopyrite, pyrite), siliceous gangue, and frother/collector residues. The xanthate and dithiophosphate collectors used in flotation can interfere with PAM adsorption.

Recommended PAM type: Anionic PAM or nonionic PAM (MW 10–18 million Da) Dosage: 0.5–2 ppm (final tailings thickener), 1–3 ppm (cyclone overflow clarifier) Pre-treatment: If frother residue is high (>50 ppm MIBC equivalent), add antifoam before PAM to prevent air entrainment reducing floc settling rate

Nonionic PAM (VIT-N8) performs particularly well in copper flotation circuits because collector residues do not interfere with its hydrogen-bond adsorption mechanism.

Floc Size Control

Floc size is the primary determinant of settling rate. The relationship follows Stokes' Law:

Settling velocity ∝ (floc diameter)² × (Δρ) / viscosity

For a typical tailings slurry at 20°C:

Floc diameter 0.5 mm → settling rate ~3 cm/min
Floc diameter 2 mm → settling rate ~50 cm/min
Floc diameter 5 mm → settling rate ~300 cm/min

Achieving target floc size:

Floc Size Target	PAM Dosage Strategy	Mixing Energy
Large flocs (>3 mm) for gravity thickeners	Lower dosage (0.5–1 ppm), dilute PAM solution (0.05%)	Gentle addition at thickener feedwell
Medium flocs (1–3 mm) for high-rate thickeners	Standard dosage (1–2 ppm), 0.1% solution	Moderate turbulence at addition point
Small, dense flocs for centrifuge feed	Higher dosage (2–3 ppm), 0.2% solution	Inline mixer before centrifuge

Over-shearing after floc formation reduces floc size and settling rate. Once PAM flocs form, minimize pump passes and use low-shear transfer pumps.

Dosage Optimization Protocol

Jar test at 5 concentrations: 0.5, 1.0, 1.5, 2.0, 3.0 ppm
Measure at 2, 5, and 10 minutes: turbidity of supernatant (NTU), settled volume (%)
Plot turbidity vs. dosage: identify the dosage that achieves target NTU with minimum PAM (flattening of the curve)
Validate at pH extremes: Test at pH ±0.5 from operating pH; PAM efficiency can shift 20–30% across one pH unit in high-ionic-strength slurries
Test with fresh vs. recycled process water: Recycled water often carries residual PAM, reducing the required fresh dosage by 10–20%

Water Chemistry Effects

Calcium and magnesium ions (hardness): Ca²⁺ and Mg²⁺ at 50–500 mg/L improve anionic PAM performance by acting as cation bridges between the anionic polymer and negatively charged mineral surfaces. Soft water (TDS <200 mg/L) reduces anionic PAM efficiency; supplementing with 50–100 mg/L Ca²⁺ restores performance.

pH sensitivity: Anionic PAM hydrolysis degree increases slowly above pH 10, generating more charge but also reducing MW over time. For sustained pH >11 circuits, use nonionic PAM or select a low-hydrolysis anionic grade (15–20% hydrolysis).

Temperature: PAM solution viscosity decreases at elevated temperature, reducing adsorption time. In hot process water (>35°C), increase dosage by 10–15% to compensate.

Summary

Anionic PAM (VIT-A12 to VIT-A18) is the standard choice for gold and copper tailings thickening, delivering settling rates 5–10× better than unassisted gravity settling. Nonionic PAM (VIT-N8) handles high-ionic-strength or flotation reagent-loaded circuits where electrostatic mechanisms are suppressed. Dosage optimization through jar testing, combined with coagulant pre-treatment in high-turbidity circuits, achieves underflow densities of 60–70% solids and overflow turbidities below 20 NTU in well-operated tailings thickening systems.