Mills that do not separate soap for tall oil production can have high soap content (around 2.5%) in their black liquor. The soap will increase the heating value of the black liquor, which may be advantageous if the boiler is not steam limited. Other beneficial effects include a reduction in chemical make-up, increased bed temperature and increased reduction efficiency. However, if soap is not removed it can adversely affect the washing and recovery operations. Burkeite (2Na₂SO₄. Na₂CO₃) and CaCO₃ scaling in the evaporators is greatly accelerated in the presence of soap. In the recovery boiler, blackouts have been associated with the entry of slugs of soap containing weak black liquor. To avoid this, it is recommended that the separated soap should be first acidified to crude tall oil. However, mills that do not have an acidification plant add soap to the strong or as-fired black liquor, even though this may reduce the liquor throughput. In this case, it is important to insure a homogenised mixture and a uniform flow to prevent unstable recovery boiler operation.

The viscosity of tall oil soap varies widely, depending on the soap composition and the entrained black liquor. However, the viscosity of soap is often an order of magnitude higher than that of black liquor. Addition of soap increases the viscosity of black liquor, but increasing the firing temperature by a few degrees can reduce the viscosity to acceptable levels. Another factor involved in increasing viscosity with high levels of soap addition may be the metal content of the soap. The Ca content of the soap is generally much higher than that of the black liquor. High levels of Ca and Mg promote the flocculation of lignin, and cause an increase in liquor viscosity. The addition of soap also reduces the swelling propensity of the liquor.