Inside mining’s battle with SX “crud”: fines, filters, and fast paybacks

Crud—the industry’s catch‑all for stable, solid‑stabilized emulsions—builds up in SX settlers as a voluminous, low‑density mash of entrained organic and aqueous phases, fine solids, and even air (researchgate.net) (patents.justia.com). It often shows up as microdroplets of organic solvent suspended in the aqueous phase, with hydrophilic fine particles trapped at that interface (patents.justia.com).

The damage is tangible. Emulsions (“crud”) can trap 15–20% of the SX solvent volume (patents.justia.com), disrupt phase coalescence, widen dispersion bands, and carry over impurities to later stages—entraining aqueous into the organic phase (patents.justia.com) (researchgate.net). Left unchecked, crud drives solvent losses, contaminates electrowinning electrolyte, increases corrosion and power losses in EW cells, and forces unscheduled settler cleaning (patents.justia.com) (patents.justia.com). In practice, almost all SX plants see crud to some degree; on site it’s “gunk” or “lumps” (patents.justia.com).

Fine particulates and Pickering stabilization

The chief culprits are fine solids in the pregnant leach solution (PLS)—colloidal silica, alumino‑silicates (clays, micas), iron(III) hydroxides or jarosite, gypsum, and other leach residues. Typical crud particles are sub‑micron—often 0.1–1 μm—and one study notes a 0.37 μm scale in sampled material (researchgate.net). Chemistry mirrors mineralogy: solids are high in silica/alumina (~35–50% SiO₂ and ~10–20% Al₂O₃ in one copper SX plant) (researchgate.net).

These hydrophilic particles adsorb at the oil–water interface—a classic Pickering‑emulsion effect—lower interfacial tension, and lock organic/water microdroplets into stubborn gels that settle poorly (patents.justia.com) (freepatentsonline.com). Crystallites of gypsum or jarosite can “cement” organics at interfaces; even airborne dust or bugs can seed crud. Once formed, crud solids are typically 70–80% carbonaceous (organic extractant/diluent) by weight, with ash of Fe, Si, Al, etc. (freepatentsonline.com). Their gelatinous nature makes simple filtration or conventional centrifugation difficult (freepatentsonline.com).

PLS clarification and filtration targets

Because fines drive crud, prevention centers on removing them before solvent contact. Best practice is to clarify/filtrate the leachate to fewer than 10 mg/L total solids for copper SX circuits (researchgate.net) (researchgate.net). Even at 10 mg/L, sediment accumulates and must be removed; higher solids can force frequent shutdowns (researchgate.net). A rule of thumb: aim for <10 mg/L suspended solids in PLS for copper SX, versus up to 300 mg/L sometimes tolerated in uranium circuits (researchgate.net).

Plants get there with multi‑stage counter‑current decantation (CCD), sand or diatomaceous‑earth filters, and increasingly, fine membrane filtration. Coarse solids removal often employs high‑rate or lamella settlers; a compact option is a lamella clarifier (lamella settler) upstream, where iron and clay are flocculated back into a thickened underflow. Dual‑media filtration can polish overflows; many circuits deploy sand filters to drive TSS down from hundreds of mg/L to single digits.

For colloids, modern ultrafiltration can remove >90–95% of sub‑micron particles, delivering very low turbidity PLS; operators use modular ultrafiltration or broader membrane systems for that polishing step. Depth or bag filtration also features, with polishers such as a cartridge filter after CCD to target outcomes like <1 NTU turbidity or <5 mg/L TSS.

Regulatory and waste handling context

Indonesian regulations, while not SX‑specific, set effluent limits (e.g., on TSS, oil and grease) that indirectly pressure plants to remove solids and organics from process water. Spent organic solvents are often classified as B3 (hazardous) waste, so recovering rather than discarding them aligns with both environmental rules and cost savings.

Physical crud management and solvent recovery

Prevention reduces load, but crud still accumulates. Operators typically skim or vacuum the settler crud layer—often daily—and send it to a collection tank for treatment (flottweg.com).

Centrifugation with three‑phase machines (e.g., Flottweg Tricanters) is common. Crud feed is split into four streams: cleaned organic, an aqueous bleed, coarse solids, and an ultra‑fine “fourth phase” carried with the water (flottweg.com). By adjusting scroll speed, ultra‑fines are expelled with liquid, leaving a dewatered cake. In practice this can recover virtually all of the organic for reuse, with case data showing payback in under six months (flottweg.com).

Decantation and settling also work: allowing crud to stand in tanks or ponds for 12–24 hours lets lighter organic coalesce and float off. Patents note pond holding alone can recover approximately 80–85%—“approximately 83%”—of the organic phase, and mixing crud with fresh organic to make the organic the continuous phase boosts separation on settling and decanting (patents.justia.com). Reported recovery rises to ~95% with further treatment (patents.justia.com).

Filter presses or belt filters may handle crud slurries after flocculation, but gelatinous, low‑density solids throttle throughput (freepatentsonline.com). Pretreatment with filter aids—diatomite or activated carbon—can help solidify mass. Continuous decanters that rely on density alone underperform because crud is too light; specialized cyclones or disc‑stack centrifuges are sometimes used first to coalesce phases (freepatentsonline.com).

When volumes are small, drying and incineration are possible, but they forfeit solvent and may conflict with waste rules. Most operations prioritize recovering and recycling organics. Across methods, the aim is an organic stream as free of solids as possible—ideally under 100 ppm solids—so it can be reused or reconditioned. Sites commonly target >90–95% solvent recovery from crud (patents.justia.com).

Chemical breaks and aggregation aids

Adjusting pH and using acid buffers can destabilize emulsions: because many extractants are acidic, raising aqueous pH or washing with water can break crud, while strong strip liquor can dissolve metal hydroxides that bind it. In related phosphoric/oil systems, heating and adding acids releases an organic phase (freepatentsonline.com). In copper SX, operators may wash collected crud with process strip solution or hot water to reduce viscosity and pull organics out.

Coagulants and flocculants aggregate sub‑micron fines so phases separate more readily. Plants dose inorganic coagulants or water‑soluble polymers; newer patents describe “water‑swellable polymeric aggregating agents” for solid‑stabilized emulsions (patents.google.com) (patents.google.com). Where colloidal silica dominates, a neutral‑pH floc stage drops out most solids; some sites add bentonite or clays to “dry out” crud—though this wastes solvent. Chemical feeds are typically metered with a dosing pump, and programs draw on available coagulants and flocculants.

Demulsifiers mirror petroleum practice: small additions of polar solvents (e.g., butanol) or surfactants are used in oilfields; analogously in SX, circuits may add a heavy paraffinic “de‑emulsifier” or use dilute caustic washes. “Diluent washing”—introducing fresh diluent oil to make the organic the continuous phase and let water drops coalesce—is a reported technique (researchgate.net). Temperature helps too: careful heating to 40–50 °C reduces viscosity and aids coalescence, but excessive heat can damage extractants or increase crud by evaporating diluent (researchgate.net).

Hydraulics, housekeeping, and recycle

Operating discipline matters. Mixer–settler hydraulics should avoid excessive mixing intensity, recycled entrainment, or aeration; a thin interfacial crud layer can actually improve phase disengagement, but excess causes carryover (researchgate.net). Operational monitoring—such as automated settler suction—is emphasized.

Routine settler cleanup is standard: skim the crud layer daily into a dedicated tank using vacuum or overflow skimmers set near the interface. Treat collected crud via pond settling and/or a dedicated three‑phase centrifuge, aiming for >90% organic recovery; pond‑settle plus a centrifuge is reported around ~95% (patents.justia.com). Recycle the cleaned organic back to extraction (flottweg.com); send the aqueous fraction (with ultra‑fines) to neutralization or re‑mineralization; dispose of solids in line with environmental rules.

Economics and performance indicators

Combining fine‑solids removal with active crud recovery reduces fresh solvent consumption—often 10–20% of circulating inventory—curbs unplanned shutdowns, and supports effluent compliance. Example outcomes cited include cutting settler downtime by >80% when PLS solids drop from 50 mg/L to <10 mg/L, and slashing organic losses by an order of magnitude with a dedicated crud centrifuge (moving from hundreds of kilograms per month to negligible), with tricanter payback reported at under six months (flottweg.com) (patents.justia.com). Cleaner EW electrolyte improves current efficiency, further reinforcing the case for upstream PLS polishing and disciplined crud management.

Sources and technical references

Authoritative hydrometallurgy texts and patents (e.g., Codelco US Patent 5,334,317) and industry studies (patents.justia.com) (researchgate.net) (researchgate.net) detail causes and cures of SX crud, including recommended solids limits and organic recovery rates (researchgate.net) (patents.justia.com). Supplemental field guidance from equipment suppliers and metallurgical reviews underscores the measurable benefits of filtration and crud treatment in practice (flottweg.com) (researchgate.net).