In pine kraft softwood mills tall oil soap is a valuable by-product as well as a toxin in the wastewater treatment system. Thus it is necessary to properly manage foam and soap within the mill. Managing this issue requires a number of steps including the handling and deaeration of foam, separation of soap from liquor, and the separation of liquor from soap. Once the soap is separated from the system, it should be kept separate and not remixed with the liquor.

Foam is first seen in the washer filtrate system. Foam breakers called Skumslakers are installed either directly on the filtrate tank or on a foam tower to break the bubble-bath-like soap bubbles.

The air is released to a dilute NCG system and the broken soap is further deaerated in a concentrator. Filtrate then stored in a weak liquor tank will build up a soap layer through natural processes, which may be removed by manual skimming or continuous skimming.

Manual skimming requires the addition of nozzles up and down the side of the tank at different elevations to allow decanting of soap at different levels. Continuous skimming utilizes a floating soap intake that rests on the soap-liquor interface. The use of swivel joints in the soap outlet piping allows movement of the float with the change in liquor level.

Soap in black liquor is at its lowest solubility midway through an evaporator set. Here the soap is allowed to decant from the liquor in a soap skim tank with a continuous rake mechanism skimming the soap from the top. The tank size may be reduced by adding air to the liquor to improve the flotation of the soap. The level in the skim tank is maintained by the use of an external adjustable weir standpipe.

Depending upon the care taken in a number of the recovery steps above, the separated soap may contain a large amount of black liquor. The black liquor not only costs money to ship to the tall oil processing plant but also represents a handling issue at the tall oil processing plant. Black liquor can affect the equipment used to separate the acidulated soap from the brine as well as increase the TRS emissions from the plant. A Lundberg Associates' soap separator is designed to remove at least 50% of the liquor from the soap. The mechanism utilizes rotating tines to provide a path for the lower viscosity liquor to separate from the soap.