Measurement of S and Na distribution in impregnated chip by XRF

Hafizur Rahman1, Siwen An2, Börje Norlin3, Erik Persson4 and Per Engstrand5. Department of Chemical Engineering, FSCN1, 4, 5 & Department of Electronics, STC2, 3, Mid Sweden University, Sweden.

Summary

As there are increasing demands to replace plastics especially as packaging material with renewable, easy to recycle and compostable materials as those produced by industry, there is an increasing demand also to improve the fundamental scientific understanding of and paper manufacturing systems. High yield pulping (HYP) processes, such as CTMP, are increasingly interesting for packaging material as well as manufacturing of hygiene paper. The yield from wood chips to final fiber is about 90%-98% and due to that, the lignin (28% of coniferous wood) plays a key role when designing properties of packing materials. A key unit operation when producing CTMP is the pre-treatment of wood chips before defibration. In order to separate the wood to individual fibers with a minimum amount of electricity it is necessary to soften the lignin. The lignin is softened by means of a combination of sulphonation at high pH and elevated temperatures in the preheater and in the refiner, where the fiber separation occurs. As the size of wood chips is normally about 20 mm in length, 3-4 mm in thickness at the same time the fiber size is 20-40 μm in width with 1.5-5 mm in length, it is challenging to create a process technology that gives an even distribution across the wood chips of the sodium sulphite (Na2SO3) containing liquid used for impregnation. In order to improve the impregnation technology, it is valuable to measure the sulphonation degree on a detailed level. Our XRF imaging system using a collimated X-Ray source and an energy-dispersive X-Ray spectroscopy can make an image of sulphur (S) and sodium (Na) across wood chips or in individual fibers.

Introduction

Pulp production is based on chips and regardless of chemical pulping process (kraft or sulphite), or high- yield pulp (chemimechanical or semi-chemical pulp), the efficiency of the impregnation is crucial. Strategies to improve the impregnation have been developed as extended impregnation time for the as well as several other suggestions to solutions and it has always been very demanding to measure how well the suggested impregnation improvements are working [1-7]. Chip pre-treatment techniques utilizing the combinations of steaming combined with compressions screw treatment at elevated temperatures and lately modified chipping, maximizing the force in the fiber direction to open up the wood structure and increase the impregnation efficiency in the manufacture of chemical mechanical pulps [8-13]. Manufacturing of chemical pulp and chemical mechanical pulp (CTMP) is increasing significantly for packaging materials such as and as well as for other hygiene products. The decisive factor in developing the CTMP for middle layer in cardboard is to produce a fiber material that has the potential to contribute to a maximum bulk. The basic strategy for obtaining well-separated fibers at the minimum possible shives content is linked to the maximization of wood softening prior to defibration in the chip-refiner. The softening is accomplished 2- by sulfonating the lignin of the wood with sulphite (SO3 ) at high temperature (170-180°C) in preheating and refining. The challenge in both chemical and chemimechanical pulp manufacturing is to achieve even chemical distribution in the wood chips of sodium sulphide (Na2S) and sodium hydroxide (NaOH) in the kraft process and sodium sulphite (Na2SO3) and sodium hydroxide (NaOH) in the CTMP process. In order to obtain homogeneous fiber properties, it is important to study the distribution of sulphur and sodium in wood chips and individual fibers. Specifically, in the case of wood chip impregnation for the manufacturing of CTMP and NSSC (Neutral Sulphite Semi Chemical), it is difficult to obtain even sulphonation of the wood i.e. that each wood fiber should get about the same degree of sulphonation that all fibers get about the same level of softening before being defibrated in the chip refiner. Probably the inner parts of the tiles get a significantly lower degree of sulphonation than the outer parts. The studies available are based on analyzing individual chips by microtome cut layers of about 100 μm and then determining the local sulphonation degree (sulfur content on washed samples) and impregnation efficiency (total wavelength on unwanted samples). Sulphur concentrations are then determined by processing of wood samples by means of Schöniger combustion, after which the samples are dissolved in acid, oxidized so that all sulphur is present 2- as sulphate (SO4 ) ions whose content is then determined by ion chromatography. The procedure is that samples of chips and fiber material with different properties can be taken at different times and controlled with the micro X-ray technology. There are a wide variety of analytical techniques used to obtain the elemental distribution map: X-ray absorption spectroscopy [14], Scanning electron microscopy and energy-dispersive X-ray (SEM-EDX) spectroscopy [15] and X-ray fluorescence spectroscopy (XRF) [16-19]. At Mid Sweden University research center STC (Sensible Things That Communicate), a laboratory-scale setup utilizing X-ray fluorescence has been built to simultaneously measure Ca (3.7 keV) mapping in the coating of and the transmission image from a Cu target (8.0 keV) that placed behind the paper. This study showed that modern miniature X-ray has the potential to contribute to a solution of how to study improved impregnation through high-resolution measurements of sulphur and sodium distribution. The aim of our work is to investigate whether direct X-ray fluorescence method or XRF transmission method can be used to obtain sulphur and sodium distribution in samples from a few millimeters to micrometer in size. The suggested method uses a collimated X-ray source and an energy-dispersive X-ray spectroscopy. The sample is scanned to make an image of the content of the substances of interest. A specific challenge in this case is that the low energy fluorescence photons from sulphur (2.3 keV) and sodium (1.0 keV) are easily absorbed in air, resulting in a poor signal-to noise ratio, thus helium atmosphere are needed. The technique we are developing can become useful in mills to improve and control process efficiency, product properties and to find solutions to process problems in future. In addition, a more even distribution of the sulphonation can reduce specific energy demand in chip refining at certain shive content.

Material and Methods

Micro X-ray technology including a collimated X-ray source and a spectroscopic detector can be used to make an element-mapping image of sulphur and sodium across wood chips or in individual fibres. The basic physics behind XRF is that when a material that has been excited by being bombarded with high-energy X-rays, it might eject of one or more electrons from the atom. The removal of an electron makes the electronic structure of the atom unstable, and electrons in higher orbitals "fall" into the lower orbital to fill the hole left behind. In falling, energy is released in the form of a photon, the energy of which is equal to the energy difference of the two orbitals involved. Thus, the material emits radiation, which has energy characteristic of the atoms present. The presence of elements of interest can be detected by measuring the emitted X-rays photons. In the XRF image method, the sample is scanned to make an image of the content of the substances of interest. The setup is shown in Figure 1. The X-ray tube, Moxtek X-ray source (5 kV to 60 kV, 12W) is used as the radiation source in this study. The typical focal spot size of the tube is approximately 400 μm, which needs to be collimated by a pinhole. In order to obtain the scanning image of the individual fiber, the focal spot size of the beamline should be less than 20 μm to obtain a high-resolution image. 100 nm is a big challenge, we are aiming 10 um pinhole so far.The measurement is done by spot-scan imaging using 2-dimensional precision translation stages, each one has a 25 mm travel distance (Thorlabs). For the transmission measurements, very thin samples are required, approximately 10 μm. The experiment can be done in a beamline where energy can be scanned over the absorption edges of the elements to be studied. The measurement setup has been simulated using MCNP (Monte Carlo N- Particle) [20] to validate the system setup and to select the correct, geometry, shielding, filtering and atmosphere for the measurement. A filter might also need to be added to the X-ray source to make it nearly monoenergetic and to avoid emission of photons with energies close to the expected fluorescence.

Fig 1: Photograph of XRF measurement setup with a moveable Helium atmosphere Ti box.

Results and Discussion

The fluorescence radiation from lighter elements is of relatively low energy (long wavelength) and has low penetrating power, and is severely attenuated if the beam passes through the air. The helium, air and vacuum environment for measuring sodium and sulphur photons was modelled in MCNP. In this simulation, a 2 cm thickness of air totally absorb the Na signal and half absorb the S signal, resulting in a low signal in the XRF spectrum, as shown in Figure 2. The vacuum atmosphere removes the air absorption. This means in practice that most of the working parts of the instrument have to be located in a large vacuum chamber. The problems of maintaining moving parts in vacuum, pose major challenges for the design of the instrument. For less demanding applications, or when the sample is damaged by a vacuum, a helium chamber can be substituted, with some loss of intensities. Therefore, the solution in this study is to use a titanium (Ti) box flooded with helium (He) gas to minimise the absorption of fluorescence photons and to shield the scattered photons in the air that might disturb the measurement.

Fig 2: Simulation spectra at Helium, Air and Vacuum

Due to a practical issue (helium chamber is not ready), a special salt, called Seltin, were measured at 8 kV in the air. The sample was placed on the Al plate. This mineral salt product containing 50% less sodium than normal sea salt, which is the dangerous part of regular salt. The nutritional content of Seltin salt is shown in Table 1. Both elements of interest, Na and S, exist in Seltin salt. This experiment was performed in the air, thus, signal-to-noise ratio is crucial when measuring light elements. A Cu sleeve is mounted at the end of the X-ray tube to shield the detector from both scattered radiation and primary radiation from the X-ray source. A 0.05 mm Ni filter was used as the primary source filter. The main function of the filter is to block energies that interfere with the fluorescence lines to be measured, thus resulting in lower background noise.

Nutritional value per 100 g Seltin NaCl 50 g KCl 40 g MgSO4 10 g I 0.005 g Table 1: Nutritional value per 100 g

The XRF spectrum of Seltin salt is shown in Figure 3. Since the electronic energy levels for each element are different, the energy of X-ray fluorescence peak can be correlated to a specific element. The spectrum resolved Na, Mg, S, Cl, and K from the Seltin salt as expected. The L-lines of I is not presented in the spectrum due to the low concentration of I. The content of Na in Seltin salt is higher than elements content in the sample, however, the Na peak is relatively low due to the air absorption of light elements. The air absorption issue should be greatly improved when the experiment is performed in a helium chamber. The Ar peaks present in the spectra at 2.96 keV and 3.19 keV are due to the normal atmosphere content of Ar since no argon is expected in the salt. The peaks come from Cu, Al, Ti, Cr, Fe and Ni, materials present in the detector and in the source housing are presented as well. In the real case, the Na and S concentrations in wood chips and individual fibers are much lower than in Seltin salt, thus helium or vacuum chamber is required.

Cl K

Ar Fe Ni

K&Ca S

Al

Cu Cr Ti

Mg Na

Fig 3: XRF spectrum of Seltin salt, 8 kV, 30 minutes. In electron microscopy scanning measurements, the spatial resolution of the XRF image is limited by the spot size of the source and the scanning step size. To calculate the width of the projection spot obtained when imaging the point source with an idea pinhole camera by considering the geometry of the situation. The projection spot is proportional to the primary X-ray beam width, but it is inversely proportional to the distance between the pinhole and the sample surface. It should be noted that when using a pinhole in XRF measurement, exposure time is proportionally increasing with the pinhole size decreasing. In order to verify the feasibility of the setup for light elements with small sample size, a preliminary line scanning measurement for a 60 μm Al foil was performed in the air. The Al foil was made in the sandwich structure between a Cu plate and a Ti plate, as shown in Figure 5 (Left). The Al atom has fluorescence peaks at 1.48 keV (Kα) and 1.56 keV (Kβ). These two peaks cannot be distinguished by the spectrometer since the spectrometer has 125 eV full widths at half maximum (FWHM) resolution at 5.9 keV, thus it will merge together into one peak in the output spectrum. A line scanning measurement with 80 μm step size is performed to obtain the element distribution of Al, Cu and Ti. 30 points are scanned in total. For each point, the measurement time is 20 minutes. XRF spectrum of Al foil, 15 kV, 20 minutes, 100 μm pinhole. The characteristic Al, Cu and Ti photon counts in all measured spectra are integrated, respectively. The elements distribution map of Al, Cu and Ti is shown in Figure. 5(Right). It shows an agreement with the sample structure. Due to the air absorption of Al, the order of magnitude of the photon counts from Ti and Cu is much higher than Al. Figure 6 presents the Gaussian fit of the Al signal. Because the projection spot in the experiments is affected by the pinhole size and the geometry, for a 60 μm Al foil, the FWHM is 221.8 μm in the XRF image.

Fig. 5: (Left) Photo of the Al sample; (Right) Elements distribution map of Al, Cu and Ti.

Fig. 6: Gaussian fit of the Al signal.

Conclusion

In this work, we have developed an XRF imaging system using a collimated X-Ray source and energy- dispersive X-Ray spectroscopy to make an image of light element. The XRF spectrum of Seltin salt resolved Na and S peak. We conclude that it is possible to perform a direct X-ray fluorescence method to obtain sulphur and sodium distribution in samples from a few millimeter to micrometer in size in helium gas. This method can become useful in the to improve process efficiency, product properties and to find solutions to process problems. A more even distribution of the sulphonation can reduce specific energy demand in chip refining. Research on how sulphonation influence shives content indicate an electric energy reduction potential of 200 kWh/bdt corresponding to an energy saving potential of 200 GWh/year, as Swedish CTMP- production is about 1Mt/y. The technology can be validated using XRF transmission method or monochromatic radiation from synchrotron facilities in the further work.

Acknowledgements

This research work is collaborating with FSCN (Fiber Science Communication Network) and STC (Sensors That Communicate) at Mid Sweden University, Sweden, funded by the European Union regional fund, Sundsvall kommun and Familjen Kamprads Stiftelse at EcoMat (Ecofriendly sustainable strong material) project.

References

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Measurement of S and Na distribution in impregnated wood chip by XRF Hafizur Rahman PhD student, Mid Sweden University, Sweden Replace plastics especially as Packaging material with renewable

Lower costs in the manufacture of Tissue and Packaging Paper . Improve energy efficiency and quality stability in defibration process.

. To improve and control process efficiency, product properties and to find solutions to process problem.

. Leading to a reduced environmental foot print. Strategies to improve Impregnation . The efficiency of the impregnation is crucial . The impregnation extended impregnation time . Chip pre‐treatment techniques : ‐Utilizing the combinations of steaming with compressions screw ‐Lately modified chipping, maximizing the force . The basic strategy for obtaining well‐separated fibers at the minimum possible shives content

The challenge is to achieve even chemical distribution in

wood chips of sodium sulphite (Na2SO3) and sodium hydroxide (NaOH) at CTMP For Tissue and Packaging materials: Characterization of impregnation depth into fibers during the production process High Yield Pulp (HYP): Wood to individual fiber: Challenge to create . Yield from wood chip to final . To minimum yield loss‐ process technology: fiber is ~90‐98% soften the lignin . Size of wood chips and fibers . The lignin (28% coniferous . The lignin is softened‐ wood) plays a key role combination of . It gives on even sulphonation at high pH and distribution across wood . A key unit operation‐ enough temp in pre heating chips of Na2SO3 : The pre treatment of wood sulphonation reaction chips before defibration. . In the refiner where the fiber separation occurs. Improve impregnation technology‐ on fiber level can measure the sulphonation degree

. Development of XRF technique . Set up geometrical position . Check the feasibility

Set up Pilot Lab at Pulp Industry

. Select the sample . Energy and time . Validation the results What is XRF X‐ray fluorescence is the emission of characteristic "secondary" X‐rays from a material that has been excited by being bombarded with high‐energy X‐rays . It might eject of one or more electrons from the atom. . The removal of an electron makes electronic structure of the atom unstable. . Energy is released in the form of a photon. Thus, the material emits radiation, which has energy characteristic of the atoms present. XRF Measurement Setup TI Shield Box for XRF

Courtesy: Per Engstrand & Börje Norlin XRF Measurement‐Spectrometer

Spectrometer ae sample Paper

Slit X‐ray Source

Courtesy: Börje Norlin Simulation spectra at Helium, Air and Vacuum Transmission image of Na and S at Helium and Air Pinhole from 100‐50‐10µm XRF setup validation by Seltin® Salt

Seltin® 10µm Pinhole 10000

1000 Count 100 Na Nutritional value per Total 100 g Saltin Salt 10

1 NaCl 50 g 00.511.522.533.544.5 KCl 40 g Energy [keV] MgSO4 10 g I 0.005 g Blank (He) He Air XRF Setup Validation by Kraft pulp‐He gas

100000

10000 Blank measurement Pulp sample at He 1000 Pulp sample without He S Count

100 Total

10

1 012345678 Energy [keV]

Acknowledgement: Financial Support: Siwen An Project e2CMP & EcoMat Börje Norlin Erik Persson & Per Engstrand

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