Washing of Precipitates

in chemical technology, the extraction or the reduction in concentration of a substance dissolved in the liquid that is present in the pores of a precipitate. The term “precipitate” is understood to mean the aggregate of solid particles that contains liquid in the spaces (pores) between the particles and that is formed during the separation of suspensions in settling reservoirs, condensers, filters, centrifuges, liquid cyclones, or other separators. Water and organic solvents, among other things, are used as wash liquids.

The dilution method of washing precipitates involves the alternate mixing and separation of the precipitate and the wash liquid. Diffusion washing is effected by the transfer of the substance being washed out from the pore liquid to the wash liquid as a result of the difference in concentration that occurs when the precipitate comes in contact with the wash liquid. Filtration washing (displacement method) is carried out by the leaching of the wash liquid through the precipitate layer on a filter screen; the pore liquid is partially displaced and replaced by the wash liquid. The most effective washing is achieved by using multistage processes of combined filtration and grinding. Washing of precipitates makes it possible to remove harmful impurities or to extract valuable components that are contained in the pore liquid.