In the production line of steel sheet, surface of steel sheet is oxidized inevitably. So it is required to remove the oxide layer by acid pickling. It is very important for keeping the product quality stable to control the acid concentration in pickling process. With the increase in line speed of steel sheet, the acid concentration of pickling solution changes a lot in short time. Therefore, it makes difficult for a conventional chemical analysis to catch up the change of concentration.
Nowadays, NIR (Near-Infrared spectroscopy) has attracted attention as a rapid analytical method to determine the concentration of solutions. Therefore we carried out examination using NIR to measure the concentration for acid pickling solutions in steel-making process.
The NIR spectra in the region of 9500 cm^–1 to 5000 cm^–1 include OH bond of stretch first over-tone (6800 cm^–1) were used. The OH peak shape of aqueous solutions are altered by the interaction between water and other different species. Total acid and Fe concentration were calculated by multivariate analysis using the NIR spectra of the solution that contains those ion. Furthermore, the multivariate analysis conditions were optimized to improve the correlation between NIR analysis value and the chemical analysis values.
The correlation coefficient between NIR analysis values and chemical analysis values reached 0.98, which is good enough for operation. NIR analyzer enabled to measure multiple components at the same time in 1 minute. This system was also tested at an actual pickling line, and good correlations were obtained.