funnel box

A funnel box is a settling vessel used in ore dressing to separate fine mineral particles from the water that carries them. Crushed ore slurry enters the box, where heavier particles settle to a cone-shaped bottom while clear water overflows or drains from the top. The device is simple, passive, and does no work itself; it relies entirely on the weight difference between ore and water to perform separation.

The geometry is the critical part. The body widens from a narrow inlet to a broad settling area, then tapers to a discharge cone at the base, typically angled at 45 to 60 degrees. This shape allows settling time in the wide zone while avoiding dead corners where slurry can stall. Boxes are usually made of timber, steel, or concrete; older mill installations often used lined wood. Sizes vary from small laboratory units holding 10 to 20 liters up to industrial tanks handling several tons per hour of solids, depending on feed grade and desired recovery.

Efficiency depends on particle size and density contrast. Coarser particles settle quickly; material finer than 100 mesh (150 micrometers) may pass through to overflow unless residence time is long or the ore is unusually dense. Gold, lead, and copper ores work well. Light minerals and clay slow settling and cloud the discharge. The rate of feed, water temperature, and particle distribution all affect how cleanly separation occurs.

Common problems and placement

Funnel boxes clog when slurry thickens or clay content rises. Overflow ports block easily; regular flushing is necessary. The cone bottom can hold trapped water even after discharge, leading to dilution of the next batch. In circuit, they sit downstream of primary crushers and screens, before further concentration stages. They are often paired with subsequent classifiers or thickeners that handle finer material.

The name describes the shape, not the function. The device is sometimes called a settling box or trap box in older texts. It has largely been superseded by mechanical classifiers and cyclones in modern mills, but remains in use where low capital cost and simple maintenance matter more than precision, particularly in small-scale or artisanal operations.