Technical Bulletin No. 0068: An Evaluation of Analytical Procedures for the Determination of Low Levels of Residual Sodium Sulfide in Oxidized Black Liquor (AQTB)

The need for improved sulfide measurement procedures in highly oxidized black liquor for defining the performance of black liquor oxidation processes as a means of controlling emissions arising from the flue gas direct contact evaporator in the kraft recovery furnace system has existed for some time. Ill defined relationships between identified emissions contributed by the contact evaporator and residual sulfides in the black liquor charged to the contact evaporator as measured by currently used methods have been largely responsible for inability to either define the degree of black liquor oxidation accomplished or assess the maximum benefits of its use in reduced sulfur emission control. This technical bulletin prepared by Dr. Robert P. Fisher, National Council Research Chemist, contains an extensive review of the procedures that have been used for the analysis of sulfide in black liquor and their utility in the measurement of low residual sulfide levels in highly oxidized black liquor. The review focuses on the applicability and limitations of potentiometric methods used for direct measurement of low levels of sulfide in black liquor, as well as indirect methods which call for the gasification and collection of the residual sulfur in a trap solution where it can be measured in the absence of interfering substances present in the liquor.