Method and apparatus for removing colloidal suspensions from a liquid

Abstract

A system and method for removing colloidal suspensions or particles from a liquid, in particular tall oil soap particles from black liquor of a wood pulping process. Black liquid is moved along a flow path from an upstream to a downstream zone. First and second electrical conductors are respectively located in the upstream and downstream zones and means are provided for applying opposite electrical potentials to these conductors. For removal of anionic colloids, the first conductor is made positive and the second conductor negative. Thus, colloidal particles in the liquid having a relative potential opposite to the potential of the first conductor are attracted towards the first conductor as the liquid flows through the upstream zone, and some of the ions attached to some of these particles are stripped off by the electrical field, leaving these particles with a potential of the opposite polarity from particles not so affected. The stripped colloidal particles attract other colloidal particles the charge of which has not been changed by stripping, thereby commencing the formation of agglomerates. As a result of the continuous pressure from the flow, and of air introduced into the stream and a small amount of hydrogen and oxygen resulting from electrolysis of the water present, these agglomerates or globules are 'washed off' the conductors into a separation vessel wherein there results a separation in the phases. Improved agglomeration is achieved by also applying pulsating potentials to the conductors and by insulating both conductors so that, in addition to the electric field between them, there are separate electric fields between each of them and ground.