Filter cake

What is a filter cake?

When a slurry is pushed or pulled through a filter cloth or other medium, the liquid passes through and the solids are held back. As filtration continues, the solids themselves build into a layer that does most of the filtering. That layer is the filter cake. When filtration finishes, the cake is usually blown with air or squeezed to remove more water, then discharged.

Cake moisture

The moisture left in the cake determines how dry the product is. For concentrates, moisture adds shipping weight and cost and must stay below safe limits for sea transport; see dewatering. For filtered tailings, drier cake is easier to stack and more stable.

Water stays in a cake in the pores between particles, held by capillary forces, and as films on particle surfaces. The finer the particles and the smaller the pores, the harder it is to remove.

What affects filter cake performance

• Particle size distribution. Fine particles and clays form dense, low-permeability cakes that filter slowly and hold water.
• Cake thickness and formation. Uneven or cracked cakes let air bypass during drying.
• Filter type and cycle. Pressure filters generally achieve lower moisture than vacuum filters; cycle times and pressures matter.
• Slurry chemistry. Flocculation, residual flotation reagents and froth or air carried over from flotation affect how the cake forms.
• Filter aids. Some operations use chemical dewatering aids, often surfactants, which lower the surface tension of retained water so more can be removed.

Frequently asked questions