launder

A launder is a channel or trough, typically constructed from wood, steel, or concrete, that transports powdered ore or ore slurry between processing stages in a mill or concentrator. In historical placer and hard-rock operations, launders received crushed material from stamp mills, jaw crushers, or similar devices and directed it toward classification, gravity separation, or further grinding. The term survives in modern metallurgical practice, though the equipment itself has evolved considerably.

Launders work by gravity flow or gentle incline. A typical ore launder might be 30 to 60 centimeters wide and several meters long, with a slope of 5 to 15 degrees to maintain steady throughput without excessive turbulence. The bottom profile is often V-shaped or U-shaped to minimize material holdup and ensure even distribution. In wet circuits, water acts as a carrier medium; in dry circuits, material moves along the trough bed. Feed regularity depends on consistent upstream discharge and a slope steep enough to overcome friction but shallow enough to prevent settling or material bypass.

Common variants include cross-launders (which split flow to multiple destinations), curved launders (used to change direction without sharp angles that cause wear and spillage), and launders fitted with internal baffles or riffle plates to enhance stratification during gravity concentration. The choice of material matters: wood launders are cheap and absorb shock but wear quickly; steel launders resist impact but can corrode in acidic slurries; rubber-lined or composite troughs extend life in abrasive circuits.

Wear and performance problems are endemic. Abrasive ore particles, especially coarse quartz or magnetite, pit and groove the wearing surface within weeks or months. Blockages occur when material is too fine, moisture content too high, or slope too shallow. Spillage at discharge points wastes ore and creates environmental hazards. Operators monitor launder condition visually and by measuring throughput; worn sections are typically patched or replaced rather than rebuilt entirely.

The word itself derives from the French lavandière (laundering woman) and the Latin lavare (to wash), reflecting the original function of washing ore. Today the term persists even when launders are dry, a linguistic remnant of the wet-gravity era. In integrated mill design, launders are often oversized deliberately to act as temporary buffers between equipment with different cycle times, reducing upstream pressure and downstream starving.