Data from the measurement of black liquor properties are used to gain insight into and to optimize, the operation of the recovery boiler and evaporators. One important parameter in measuring the extent of reaction in pulping and optimizing black liquor processing in terms of viscosity, is the residual effective alkali, or REA. In North America, it is normally expressed as % Na2O/black liquor solids. Many mills have used old procedures developed by PAPTAC (J15P) and TAPPI (625 cm-85) based on acidimetric titration.  However, these methods overestimate the true concentration of alkali due to interference from dissolved organic compounds. The SCAN method (SCAN-N 33:94) provides a more accurate measure of the alkali content of a black liquor. In this method, potentiometric titration with 1M HCl to the first inflection point is used for black liquors with an initial pH of at least 11 and concentrations not exceeding 20% solids. If the first inflection point is not visible, the volume of HCl at a pH of 11.5 is used instead. A correction factor is applied to obtain the true value of the REA.

A more recent work [1] compares different methods for measuring REA in black liquor, and recommends measuring the alkali content with potentiometic titration at a dilution factor of 4 for strong black liquor, using 1M HCl to a pH inflection point between 11 and 11.5.  The measured REA is then corrected using a factor. The correction factor is obtained from the linear regression equation of measured REA versus true REA. The true REA is measured by a capillary electrophoresis method. If an auto-titrator is not available, or a distinct inflection point is not observed, the REA is calculated at a fixed pH end-point of 11.3. Again, a correction factor is used to obtain the true REA. The difference between the fixed end point of 11.5 in the SCAN method and 11.3 in the more recent work is that the latter uses a dilution factor of 4, whereas the SCAN method uses a dilution factor of 10 and adds carbonate to the black liquor sample.

Another issue that affects the expressed value of REA in black liquor is the measurement of its solids content. Because strong black liquor forms a skin when dried, it traps some residual water, affecting the accuracy of any solids measurement.  TAPPI method 650 om-89 dries the liquor sample in a standard oven at 105 C, using sand as a surface extender to ensure complete removal of water. Oven drying provides consistently reproducible results, but it has some drawbacks. Volatile organics such as methanol, turpentine and reduced sulfur compounds are also evaporated around 105 C. Although these compounds are not dried solids, they affect the liquor’s material balance. Unless oxygen is excluded during the drying process, the sulfide content of the black liquor is oxidized to thiosulfate, increasing the apparent dry solids by about 3-5% [2]. This error can be corrected when the sulfide content of the black liquor is known.

References:

1.    Improved Methods for Measuring Residual Effective Alkali in Kraft Black Liquors – Part 2: Comparison of Titration Methods, T. Radiotis, J.L. Sullivan, M. MacLeod, S. Syed and T. O’Hagan, FPInnovations, Paprican, TAPPI Engineering, Pulping and Environmental Conf. 2007.

2.    Black Liquor Properties, Chapter 3, W.J. Fredrick, Kraft Recovery Boilers, Ed., T.N., Adams, TAPPI Press, 1997.