Fast and Continuous Analysis Method for Steel Pickling Solution Using Near-Infrared Spectroscopy

In the production of steel sheet, the surface of the steel is inevitably oxidized. Removal of oxide layers with acid pickling has been necessary to maintain quality. Monitoring acid concentration in the pickling solution is of primary importance in controlling the pickling process. The acid concentration of the pickling solution fluctuates dramatically in short periods because of the rapid pickling process. This makes it difficult to follow changes in the concentrations of the acid and other constituents in the pickling process with conventional chemical analyses. Currently, near-infrared spectroscopy (NIR) has been attracting attention as a potential continuous analytical technique to determine the concentration of chemical species. Therefore, the authors examined the performance of NIR for monitoring the concentration species in acid pickling solutions of the pickling process. NIR spectra in the region of 9500 cm–1 to 5000 cm–1, which covers the first stretching over-tone of OH bonds (6800 cm–1), were investigated. The peak shape of the OH bond in aqueous solutions is altered by the interaction of water molecules with dissolved species. The concentrations of total acid and iron in the solutions were estimated by multivariate analysis using the NIR spectra of the solutions. The parameters of the multivariate analysis were optimized to improve the correlation between the analytical values obtained by the multivariate analysis of the NIR and those obtained by chemical analyses. The correlation coefficient reached 0.98, which is enough to apply the multivariate analysis to practical operations. The NIR analyzer enabled us to simultaneously measure more than two species in 1 minute. The applicability of the system to an actual pickling process was tested, and good results were obtained.