AbstractPractically all hydrometallurgical processes involve leaching of solids to dissolve valuable constituents. This usually is followed by a liquid‐solids separation before producing the final product. The liquid‐solids separation step requires careful investigation because:1.Soluble values must be recovered to a very high degree for economic purposes.2.Final liquor volume must be minimized to reduce capital and operating costs of later steps.3.Usually large amounts of gangue solids are associated with the digested pulp requiring care in minimizing wash volumes.4.The colloidal solids are strongly dispersed by retention times and pH conditions in leaching. This necessitates flocculation investigations to achieve desired results.5.Operation must be dependable under severe conditions of pH, abrasion, fast settling solids, temperature, etc. Equipment must have flexibility to allow efficient operation under even abnormal fluctuations.6.An appreciable percentage of initial investment is required for this portion of the plant.The two common liquid‐solids separation methods —countercurrent decantation and filtration with cake washing, are discussed. Investigation and correlation procedures for prediction of full scale results and design requirements are stressed. Emphasis is given to the influence of particle size distribution, temperature, pH, flocculation, repulping, solids concentration, and filter cake perme