Many mixtures or multiphase systems often need to be purified or depurate in order to obtain products, as free as possible, from impurities. For example, caustic soda and salt are treated in evaporators, an important number of food products are dehydrated, some ceramic and colorants are filtered, while sugars and fruit juices may be centrifuged. Mixtures and multiphase fluids previously mentioned are varied in the materials processing industries and include solutions, suspensions, dispersions, and solid blends. For example, magnetic separation is a separation technique encompassed by a unit operation of great importance in food engineering: the cleaning of raw materials. Several separation techniques listed in the previous section are used to separate finely divided solids within a bicomponent system. Diffusional operations related to heat and/or mass transfer are based on principles out of the scope of particle technology and are widely addressed in classical chemical engineering texts.