Introduction, methods of washing solids

Solid-liquid separation often has to be followed by washing of the separated solids in order to  

1 Remove the last traces of the mother liquor from which the sohds have just been separated and which is chemically unacceptable in the product solids or has some undesirable constituents such as dissolved species (e.g. salts) or ultrafine insoluble material. 

2 Recover the last traces of the mother liquor (or a dissolved/insoluble constituent) from the solids because it is a valuable product. 

3 Clean the solids before disposal, for environmental reasons. 

Any of the above can, of course, be single-stage or multi-stage and, for the latter, co-current or countercurrent. Methods 2 and 3 are similar in that both are based on diluting the solids in the wash liquid except that method 2 starts with a solid mass (filter cake or sediment) and method 3 starts with the solids 

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