Mother liquor is what remains after crystals have been separated from a crystalline slurry. This page refers to mother liquor only from crystallized whey and permeate (from whey or milk) concentrates used for lactose production. The whey and permeate is first evaporated and then pre-crystallized in tanks, after which it is passed through decanters for the removal of lactose crystals.
If the starting material is whey, the remaining mother liquor will typically consist of 1/3 amorphous lactose, 1/3 salts, and 1/3 whey proteins. Due to the content of lactose and salt, it is difficult to dry, but if a Multi-Stage Dryer MSD™ is selected and fines return is used (where also other dry materials like skim milk powder or soy protein are added to form a final mixture for animal feed purpose), it is possible.
Mother liquor from permeate from whey or milk contains only lactose and salts and is practically impossible to dry unless some carrier such proteins (soy) is added. It is usually disposed off to sewage plants or sold as animal feed in liquid form to be mixed with other ingredients to increase the feed value.