wash

In mining, washing is the mechanical separation of valuable minerals from lighter waste rock and clay using flowing water. The principle is simple: water carries away fine material and lighter particles while denser ore settles, or conversely, ore can be floated away depending on the mineral's density and the water velocity. This operation sits at the coarse end of mineral processing, typically performed on freshly broken ore before it enters finer crushing and concentration stages.

The most common wash setup is a trommel or rotary screen through which ore and water pass together. As the drum rotates, water jets spray onto the tumbling ore, breaking apart clay coatings and mud that cling to valuable particles. Material smaller than the screen aperture, along with suspended fines, exits with the water; larger rock either reports to waste or, if it still carries ore, moves to further processing. For gold and cassiterite operations, open-channel washing using simple sluices or riffles remains standard in alluvial mining, where gravity and water velocity do most of the work.

Wash water quality affects both recovery and cost. High sediment loads in the return water clog screens and require settling ponds or cyclones to recycle the water; in arid regions, water loss is a critical loss factor. In tropical climates with abundant rainfall, washing generates large volumes of slurry that must be managed to prevent acid mine drainage or heavy metal seepage into local waterways. The density and hardness of the ore determines wash intensity: friable oxide ore washes easily with gentle water flow, while hard fresh rock or clay-bound ore needs higher water pressure or mechanical scrubbing action.

Wash efficiency is measured by recovery versus dilution: how much valuable mineral reports to the product stream against how much waste rock it carries. A wash that removes 90 percent of clay but sends it with a tonne of barren rock is less valuable than one that removes 75 percent of clay while keeping the product clean. This trade-off is why wash is often staged, with a primary wash removing bulk waste and a secondary wash upgrading the concentrate.

The term comes from the literal action of water washing away material, and it is widely understood across English-speaking mining regions. In some contexts, operators use 'scrub', 'wet screen', or 'deslime' to refer to variants of the same process, depending on whether the emphasis is on mechanical abrasion, screening, or removal of fine clay respectively. Dry screening, by contrast, uses air and vibration instead of water and reports to different equipment and operating costs.