On-Line Determination of Major Ionic Species in Kraft Liquors by Short-Wavelength Near Infrared Spectroscopy, 2001 Pulping Conference Proceedings

M. Kester, T. Trung, D. Leclerc, J. Carver--We have developed and commercialised an on-line method for determining the concentration of major ionic species present in the kraft pulping liquors used during the pulp manufacturing process. Liquor samples are analysed for hydroxide, hydrosulfide, carbonate, and black-liquor solid concentrations, with the use of infrared absorbance measurements. These measurements were obtained from near infrared spectral changes that are produced by liquor components in the water combination bands found between 14,000 and 7000 cm -1 . An 8-mm flow cell was found be the optimal pathlength and was used for collecting all spectra. With the aid of a multivariate calibration technique, the contribution of each species to the perturbation of the water spectrum correlated directly with the concentration of that species, as determined from standard-method laboratory analyses. Validation runs were done with both synthetic and mill samples. Results show that the precision of the calibration is 0.4 g/L (as Na2O) for effective alkali (EA), and 1.0 g/L (as Na2O) for sodium sulfide and sodium carbonate. Process data obtained from two commissioned mill analysers show that the FT-NIR liquor analyser is accurate for EA within 0.55 g/L (as Na2O) for white liquor samples, 0.45 - 1.05 g/L (as Na2O) for circulation cooking zone black liquor samples, and 0.85 - 1.3 g/L (as Na2O) for extraction zone black liquor samples. The results are comparable to the accuracy obtained with the titration method. Twelve systems have been either implemented or commissioned at mill sites for monitoring white-liquor and residual EA for digester control and EA, carbonate and sulfide for recausticizing control.