Crud in Solvent Extraction: Where It Comes From and How to Get Rid of It
Crud is a stable emulsion of organic, aqueous and solids that collects at the interface. It reduces settler area, carries organic out with the aqueous, and eventually forces a shutdown. Treatment depends entirely on which of three types you have.
The three types
| Type | How it looks | Usual origin | Treatment |
|---|---|---|---|
| Solids-stabilised | Gritty, dense, breaks down when squeezed | Fines from leach or filtration, gypsum, unreacted ore | Improve upstream filtration; desludge mechanically; consider a coalescer |
| Silica crud | Gelatinous, voluminous, hard to filter | Silica in feed above ~50–100 ppm, pH shifts | Control pH, add a silica removal step, use a bleed and clay treatment |
| Degradation crud | Dark, waxy, smells, forms at low shear zones | Oxidation, hydrolysis, temperature, ageing extractant | Caustic wash, clay treatment, replace a portion of the organic |
Confirming the type quickly
Take a crud sample, dilute with kerosene and shake. Solids that settle out and can be filtered indicate solids-stabilised crud. If the crud disperses into a gel that will not filter, suspect silica. If the organic phase itself is dark and the crud smells of degradation products, it is chemical.
Prevention beats removal
Filtration. Most leach circuits that fight crud continuously are sending 200–1,000 ppm solids to the settlers. Tightening filtration to below 50 ppm often removes the problem entirely.
pH control. Silica polymerises when pH moves into the wrong band. In copper SX, running the extraction pH too high is a classic cause of silica crud.
Organic hygiene. Keep the organic cool, exclude air where possible, avoid unnecessary shear, and bleed a small percentage continuously rather than replacing large volumes after a crisis.
Cleanup sequence that works
Isolate the stage, drain and remove the pad mechanically, wash the settler, then treat the organic: caustic wash to remove degradation products, water wash to neutralise, and clay treatment if colour persists. Return the organic gradually and watch the interface for a week. If crud returns within days, the source upstream has not been fixed.
Send us the feed solids analysis, pH profile and a photo of the crud, and we will identify the type and the likely fix.