Why Heap Leach Bleed Water Is Hard to Clarify
Heap leach bleed is a punishing feed containing suspended fines (clay, silica, and sub-100 µm ore particles), colloidal material below 10 µm, residual leach reagents (cyanide degradation products for gold operations; sulfuric acid and dissolved metals for copper), and intermittent organic load from process upsets. TSS routinely swings from under 200 mg/L to over 2,000 mg/L on a single shift, and temperatures in heap circuits can range between 5 °C and 30 °C seasonally. Conventional gravity settling underperforms on this feed because colloidal clays below 10 µm have settling velocities of 1 m/h or less, and reagent residues further retard sedimentation by stabilizing the colloid. Dissolved air flotation (DAF) is the most commonly used flotation technology in mineral processing (per MDPI 2020, Processes Vol. 8, p. 383), and continuous-process optimization of DAF for industrial mineral-contaminated water has been validated in peer-reviewed work (DUT 2020, doi:10.51415/10321/3182). For a heap leach bleed duty specifically, the combination of high colloidal clay, variable TSS, and reagent-laden water is where DAF outperforms a lamella or circular clarifier.
How DAF Works on Mining Feedwater
A DAF clarifier attaches micro-bubbles to destabilized particles to float the agglomerate to the surface. A sidestream of clarified effluent — typically 20–30% of throughput — is pressurized at 4–6 bar in a saturator with dissolved air, then released through a needle-valve or eductor into the flotation cell. The pressure drop nucleates a cloud of micro-bubbles in the 10–100 µm range; the engineering target for colloidal-clay capture is the 10–30 µm band (per general high-rate DAF design practice). These bubble-particle agglomerates rise into a stable surface layer — the white-water blanket — that is skimmed by a mechanical or hydraulic scraper into a float hopper, while the clarified underflow exits the cell bottom.
Edzwald's listed DAF advantages — rapid output, high loading rate, and low hydraulic retention time (per MDPI 2020) — translate directly into mining operating benefits. A short HRT of 3–5 minutes means the cell absorbs bleed flow surges without washing out the float layer. A high hydraulic loading rate keeps the cell footprint small relative to a settling basin. Rapid output ensures the clarified underflow reaches the recycle tank or polishing filter within minutes, which is critical when the same water is being re-irrigated onto the pad at high volumetric rates. The DAF cell can be operated alone or combined with other processes for primary, secondary, or tertiary duty (per MDPI 2020); for heap leach bleed, it serves as primary clarification, with downstream polishing optional. For equipment selection, a ZSQ series DAF system sized to the bleed flow and peak TSS is the typical configuration.
Recommended DAF Design Parameters for Heap Leach Bleed
The spec set below defines the working envelope for a heap leach bleed duty. These parameters are tightened for colloidal clay and variable TSS rather than generic potable-water DAF standards.
| Parameter | Design range for heap leach bleed | Notes |
|---|---|---|
| Micro-bubble size | 10–30 µm | Targets colloidal clays below 10 µm and fine ore particles up to ~50 µm |
| Saturator pressure | 4–6 bar (60–90 psi) | At 5 bar and 20 °C, roughly 60 mg/L of air dissolves in water — the design anchor |
| Recycle ratio | 20–30% of throughput | Higher than potable DAF (typically 6–12%) because bleed TSS is highly variable and colloidal |
| Air-to-solids (A/S) ratio | 0.02–0.06 g air / g TSS | Ensures sufficient bubble flux during peak solids events |
| Hydraulic loading rate | 10–25 m/h | Higher end tolerated on bleed feeds due to short HRT |
| Contact zone residence time | 3–5 minutes | Long enough for bubble-particle attachment, short enough to absorb surges |
| Float sludge DS | 3–6% dry solids | Thick float simplifies downstream dewatering |
The recycle ratio is critical. Potable-water DAF often runs at 6–12% recycle because raw-water TSS is low and stable. Heap leach bleed swings by an order of magnitude, and the colloidal fraction demands more bubble surface area per unit of solids. A 20–30% recycle guarantees sufficient bubble flux during a peak event; under-sizing the recycle is the most common cause of DAF underperformance on mining feeds. These parameter choices are consistent with the MDPI 2020 framing of DAF as the dominant flotation technology in mineral processing, applied specifically to a heap-leach-bleed envelope.
Coagulant and Flocculant Pairing for Clay- and Reagent-Laden Bleed
DAF requires destabilized particles because bubble attachment does not work on a stable colloid. Coagulant and flocculant chemistry is part of the DAF configuration, not an accessory (per the pre- and post-coagulation configuration framing in MDPI 2020). The standard pairing for clay- and reagent-laden bleed is a ferric-based coagulant ahead of the cell, followed by a high-molecular-weight flocculant in a small flocculation zone upstream of the saturator recycle injection.
Ferric chloride or ferric sulfate dosed at 20–80 mg/L provides charge neutralization on the negative faces of kaolinite and montmorillonite clays that dominate heap-leach fines. For cyanide-bearing bleed, ferric dosing also helps oxidize residual ferrous cyanide complexes. A high-molecular-weight anionic or non-ionic polyacrylamide flocculant at 0.5–3 mg/L then bridges the destabilized particles into flocs in the 100–1,000 µm size range, which captures bubbles reliably (per standard DAF design practice). A short HRT in the flocculation zone (1–3 minutes) is sufficient; over-polymerizing produces flocs too large to attach bubbles efficiently. Dosing is best handled on a dedicated automatic coagulant and flocculant dosing skid with flow-paced control, as manual dosing cannot track a heap-leach bleed that swings hour to hour.
Reuse or Discharge: What the Clarified Underflow Has to Meet
Clarified underflow must satisfy one of two end-use envelopes, with specifications differing between them. For recycle to the leach pad or stack, targets are TSS below 30 mg/L and turbidity below 50 NTU to avoid blinding drip emitters and maintain pad permeability. DAF underflow at the design parameters above typically meets this envelope without polishing. For surface discharge, the typical envelope is TSS below 30–50 mg/L (the MDPI 2020 source cites a 50 mg/L SOG benchmark as a comparable physio-chemical discharge limit) plus site-specific metals limits under the operating permit. Project engineers must confirm numeric values against current permits, as the source research does not specify them. When discharge is the target, DAF underflow usually requires polishing through a multi-media filter for downstream polishing to protect receiving-water body ecology and guard membrane or ion-exchange units.
The float sludge is the concentrated waste stream and must be dewatered before disposal or dry-stack return to the mine. A plate-and-frame filter press for float-sludge dewatering typically takes float sludge from 3–6% DS up to 30–40% DS cake, which is acceptable for dry-stacking in most mining jurisdictions. DAF alone is sufficient for recycle to the pad; DAF plus a multi-media filter is the typical configuration for surface discharge or process-water reuse into the mill.
DAF vs Lamella Clarifier for Heap Leach Bleed
Lamella clarifiers are often considered first due to capex familiarity, but DAF is superior for colloidal, variable-TSS bleed.
| Criterion | DAF | Lamella clarifier |
|---|---|---|
| Footprint at equivalent throughput | ~30–50% smaller (per general high-rate DAF design practice) | Larger; relies on plate area for equivalent rise rate |
| TSS removal on colloidal feed | >90% | ~60–80% — colloidal clays pass through |
| Sludge dryness | 3–6% DS float (thick, skimmable) | 1–2% DS underflow (thin, hard to dewater) |
| Hydraulic surge tolerance | Strong — short HRT absorbs swings (per Edzwald, MDPI 2020) | Weaker — surges disturb the lamella blanket |
| Capex | Higher unit cost, smaller footprint | Lower unit cost, larger civil works |
| Opex at scale | Lower — less polymer, smaller building, easier sludge handling | Higher — more polymer, more civil maintenance |
The colloidal-clay fraction and TSS swings make DAF the better technical fit for heap leach bleed. Edzwald's advantages — rapid output, high loading rate, and low hydraulic retention time (per MDPI 2020) — map directly onto surge-prone mining flows where a lamella would wash out during a peak event. A lamella is acceptable for low-clay, low-variability bleed where footprint and a thick float are not critical; a high-efficiency sedimentation tank configured as a lamella remains a reasonable choice for a pre-DAF roughing stage or a non-critical side stream. For the main bleed clarification duty, DAF wins on TSS removal, sludge dryness, and surge tolerance.
Frequently Asked Questions
What micro-bubble size should a DAF use for heap leach bleed?
Target 10–30 µm micro-bubbles, generated by a saturator operating at 4–6 bar. Bubbles in this band attach efficiently to colloidal clays below 10 µm and to fine ore particles up to ~50 µm; coarser bubbles above 50 µm rise too fast and skip the colloidal fraction.
What recycle ratio does a heap-leach DAF need?
Run the saturator recycle at 20–30% of throughput, rather than the 6–12% typical of potable DAF. Heap leach bleed TSS is highly variable and colloidal, and the higher recycle guarantees sufficient bubble flux during peak solids events. Under-sizing the recycle is the most common cause of DAF underperformance on mining feeds.
Which coagulant and flocculant pairing works on clay- and cyanide-laden bleed?
Dose ferric chloride or ferric sulfate at 20–80 mg/L ahead of the cell for charge neutralization on kaolinite and montmorillonite clays; on cyanide-bearing bleed, the ferric also helps oxidize residual ferrous cyanide complexes. Follow with a high-molecular-weight anionic or non-ionic polyacrylamide at 0.5–3 mg/L to build flocs in the 100–1,000 µm range for reliable bubble capture.
Can DAF underflow go straight back to the leach pad?
Yes, if the underflow holds TSS below 30 mg/L and turbidity below 50 NTU, which protects drip emitters and pad permeability. At the design parameters in the parameter table, DAF alone typically meets that envelope; surface discharge usually requires a downstream multi-media filter to reach receiving-water body targets and meet site-specific metals limits under the operating permit.
