Near-infrared spectroscopy technology has attracted more and more people's attention and recognition because of its rapid rise. It has a particularly prominent role in analysis and detection, and has also achieved widespread success in control. This paper introduces the application of near infrared spectroscopy technology in the pulp and paper industry on the basis of the development status of near infrared light technology and the development of the pulp and paper industry. 1. Development status of near infrared spectroscopy In recent years, near-infrared spectroscopy (Near-Infrared Spectroscopy—NIR), as a rapidly emerging spectral analysis technology, has played an increasingly important role in the field of analysis and testing. At the 1997 Pittcon Conference on Analytical Chemistry and Applied Spectroscopy (Pittcon'97), there were 178 articles on near infrared spectroscopy. NIR reflection spectroscopy is still an important topic of this conference. It can be seen that, because there is basically no need to deal with the sample analysis, and it does not damage and consume the sample, and it has no environmental pollution, NIR spectroscopy is being recognized by more and more analysts as a rapid analysis and testing instrument. And use, but also received attention in the field of industrial control. At the 1998 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy (Pittcon'98), the introduction of acousto-optic tunable filters (AOTF) and near-infrared probes have been successfully used in near-infrared online process monitoring instruments for monitoring Polymer viscosity, acid number, additives, degree of polymerization, etc. Since the 1990s, NIR has basically matured as a modern testing technology, and its application fields have expanded rapidly. From the initial food industry, it has quickly penetrated into the petroleum, chemical, environmental and biochemical industries. In terms of process control, product quality analysis and testing It has played a prominent role and created huge economic benefits, and has achieved wide success in process control. 2. The application of near infrared spectroscopy in the pulp and paper industry At present, the output and quality of pulp and paper are increasing all over the world. It is not enough to rely on providing high-quality fiber raw materials and improving the pulp and paper technology to promote production. Some new process analysis instruments and sensors must also be developed and used. With the development of near infrared spectroscopy technology and spectral data processing software, it provides a new way for the development of new process analysis instruments. The application of NIR introduced in the pulp and paper process described below, although the vast majority of applications are still limited to the laboratory, but the future development trend must be on-site analysis and measurement and control, to achieve the transformation from the laboratory to the production site. 2.1 Detection of moisture content in paper coatings In the range of 400 to 1100 nm, the transmission mode is used to analyze the moisture content in the paint mixture. Although the reproducibility of the measurement is affected by the presence of suspended particles in the paint, this problem can be avoided for continuous flowing paint, enabling online measurement. 2.2 Determine the content of softwood in pulp In the range of 1100 ~ 2500nm, using the reflection mode, take two samples for calibration laboratory measurement values ​​and online measurement values. Although the error of the online measurement is greater than expected, the measurement results still indicate that NIR can be used to determine the content of softwood in the pulp. 2.3 Measuring the Kappa number of mixed wood pulp In reflection mode, 27 samples were taken at 2180nm for calibration curve, 9 samples were used as test set, and the measurement error was 5.6%. Although the measurement results indicate that NIR may be used to measure the Kappa value of mixed wood pulp, it remains to be determined whether the measurement error can be accepted by industrial processes. 2.4 Measure the Kno (logarithm of Kappa number) of the pulp in the spray line of the cooking pot In the range of 1100 ～ 2500nm, the reflection mode is adopted to measure the Kno of the pulp. A linear regression at 1672nm was performed with a measurement error of 1.0%. Due to the influence of wavelength fluctuations, 1436nm was introduced to perform multiple linear regression to reduce the measurement error to 0.5%. 2.5 Measuring the moisture of the paper sheet and the weight of the cardboard Using the reflection mode, the characteristic of water at 1940nm is used to absorb and measure the moisture in the paper sheet; the change in the weight of the cardboard is obvious in the range of 2100 ～ 2500nm, and a linear regression at 2346nm is performed. The results show that NIR is sensitive to the moisture of the paper sheet and the weight of the cardboard. Due to the non-uniformity of the sample itself, the area of ​​the sample should be large during the measurement scan to take the average. 2.6 Monitoring the resin layer of the paper sheet Using the reflection mode, the samples were analyzed in the range of 1100 to 2500nm. As a result, it was found that the uncoated paper had no characteristic absorption at 1688nm, 1766nm and 2160nm, so it could avoid the influence of the paper. The calibration curve at 2160nm has an error of 0.7%. 2.7 Monitoring the Kappa value of the pulp cotton linters mixture Using the reflection mode, the sample is analyzed in the range of 1100 to 2500nm, and a linear regression at 1680nm is performed. The error is 1.2 kappa values, and the effective range of calibration is 3.4 to 33.9 kappa values. This result is applicable to both dry and wet pulp cotton linters. 2.8 Detect the content of silicone layer on tissue paper Eight tissue samples with a silicone concentration of 0.69% to 5.67% were scanned in the range of 1100 to 2500 nm, and a linear regression at 1744 nm was performed with an error of 0.2%. 2.9 Detection of wax and phenol resin content in wood fiber Using the reflection mode, in the range of 1100-2500nm, the wood fiber samples with wax concentration of 0.3% -2.4% and wood fiber samples with phenol resin concentration of 1.3% -4.3% were scanned. The phenol resin is calibrated at 2158nm with an error of 0.4%; the wax is calibrated at 1728nm with an error of 0.1%. 2.10 Detection of lignin content in dry wood pulp Using reflection mode, the wood pulp with a lignin concentration of 5.7% to 33.6% is scanned in the range of 1100 to 2500 nm. Multiple linear regression was performed at 2172nm and 1556nm with an error of 1%. 2.11 Detection of wax and phenol formaldehyde resin content in wood fiber Using reflection mode, scan wood fiber, pure wax, lyophilized phenol formaldehyde resin and wood fiber containing wax and phenol formaldehyde resin in the range of 1100-2500nm, found that the wax in the fiber is at 2250nm, 2310nm and 1725nm Characteristic absorption, while the resin has characteristic absorption at 1980nm. 2.12 Detection of water absorbent content in fluff pulp Using the reflection mode, the fluff pulp samples with a water-absorbing agent concentration of 0.00% to 44.9% are scanned in the range of 1100 to 2500 nm, and calibrated at 1752 nm with an error of 6%. 2.13 Detection of the coating layer of coated paper In the range of 1100 to 2500nm, for coated paper with a coating concentration of 0 to 12%, the NIR reflection spectrum was measured with an optical fiber and calibrated at 2316nm with an error of 0.1%. However, the influence of inorganic substances in the paper sheet cannot be ruled out. 3. Outlook It can be seen from the above that NIR spectroscopy is widely used in the pulp and paper industry and has a very good application prospect. In particular, the use of near-infrared spectroscopy for the determination of lignin content or Buber value in pulp has become a hot topic in the field of analysis and testing in the international pulp and paper industry in recent years. However, the issues to be noted are: (1) For different pulp types, the curves and equations must be recalibrated; (2) The main component of the pulp is carbohydrates, which are hygroscopic, so when measuring the spectrum, they should be in the same surrounding environment Carry out the next step to reduce moisture interference, thereby reducing errors; (3) Considering the complexity of pulp composition, near infrared spectroscopy is used for quantitative analysis of pulp. It is best to use derivative spectroscopy and multiple regression methods with multiple characteristic absorption peaks to determine pulp Lignin content.
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Application of Near Infrared Spectroscopy in Pulp and Paper Industry