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Screening of pulp can be expressed as a process where solid contaminants are removed from a pulp slurry without an excessive loss of fibres of acceptable quality. The impurities originate from the feedstock, or they are generated during the transport, handling, storage, or pulping process. The most typical impurities include bark, fibre bundles or shives, knots, plastic, rubber, and sand. Moreover, there are also other objectives for screening, such as the improvement of the pulp quality, savings in bleaching chemical consumption, and protection of process equipment. In industrial applications, pulp screening is typically carried out with more than one screen for a more beneficial screening result. The pulp screening equipment can be divided into different types by the separation technique employed, such as atmospheric or pressurized screens, centrifugal screens, vibrating screens, etc. In addition, there are other types of related equipment that are common in the pulp screening process, such as knotters, refiners, and hydrocyclones. The main objective of this review is to discuss the advantages and disadvantages of modern screening equipment as well as introduce contemporary screening strategies for chemical pulp mills.

To be able to charge a correct amount of chlorine dioxide in the D0-stage, a pulp mill needs good control of the bleaching process and good sensors to keep the variability of the bleaching result on an acceptable level. It is also important to include the bleaching agent demand from the dissolved matter in the pulp slurry. If this is done correctly, over- or undercharging of bleaching agents can be reduced, which lead to lower bleaching chemical cost, lower polluting emissions and higher pulp quality. Our previous research has shown that the dissolved organic carryover from the O2-stage varies significantly when bleaching softwood kraft pulp. The present study investigated the corresponding impact in the case of bleaching of birch pulp. Different mill configurations and process conditions have been simulated in laboratory trials, including proceeding A-stage treatment, different degrees of washing before and between the stages, and a comparison of the effects of recycled and non-recycled wash filtrates. The results have confirmed the significant impact of the dissolved organic matter, and the knowledge which have been generated can be used to understand how measurement and control concepts can be developed to improve the pulp quality control and to decrease production cost.

Dissolved matter (DM) in the oxygen delignification (O2Del) stage affects the lignin and carbohydrate degradation. To understand this topic better, laboratory O2Del experiments were conducted with controlled addition of either unoxidized dissolved matter (UnoxDM) originating from the cooking stage or oxidized dissolved matter (OxDM) recycled from the post-O2Del washing. The presence of UnoxDM decreased the delignification, while the presence of OxDM showed the opposite effect, both compared to a well-washed pulp and at the same alkali charge level. However, both types of DM had a negative impact on the carbohydrate degradation. The distribution between these DMs will affect the resulting lignin degradation, because the filtrate accompanying the fibers into the O2Del stage is a mixture of UnoxDM and OxDM. It is proposed that the positive impact on the delignification by OxDM is due to the high carbonate ion concentration in the filtrate. Further, the high content of thiosulfate ions in the UnoxDM was one of the reasons for the lower delignification in its presence.

The presence of covalent bonds between lignin and polysaccharides was investigated in dissolving pulps made with one-stage and two-stage acidic sulfite pulping for 100% pine heartwood raw material. ...

The presence of dissolved matter in the pulp suspension in an oxygen (O2) delignification stage, consisting mainly of dissolved lignin, is normally considered to negatively affect the delignification rate due to the competing reactions between the fiber bound lignin and the lignin dissolved in the filtrate. Recirculated oxidized filtrate from the post-O2 washing is usually considered to be less harmful to the delignification efficiency than unoxidized dissolved matter originating from the cooking stage. To develop a better understanding of the mechanisms of the dissolved matter’s reactions and impact on the O2 stage performance, laboratory oxygen delignification experiments with varying levels of unoxidized and oxidized dissolved matter were conducted. The results showed that unoxidized dissolved matter had a negative impact on the delignification in the O2 stage, whereas oxidized dissolved matter actually had a positive effect. The delignification efficiency of the laboratory experiments thus depends on both the amount of dissolved matter and its origin. The pulp viscosity decreased with increasing dissolved matter content irrespective of its origin but at higher COD levels; however, the viscosity drop was larger for the unoxidized dissolved matter. In terms of selectivity, the oxidized filtrate had a similar impact as additional NaOH charge. Both types of filtrates consumed hydroxide and the final pH decreased with increasing dissolved matter content. The final pH was significantly lower in the unoxidized filtrate experiments at higher COD levels, indicating a high reactivity between the unoxidized dissolved matter and the oxygen in the reactor. Based on the results, further understanding is achieved about the relation between pre-O2 washing performance and process configuration in an actual mill case, as well as the impact of dissolved matter on delignification. The importance of efficient removal of harmful unoxidized dissolved matter is verified, but the results also suggest that the oxidized dissolved matter that is recirculated from post-O2 washing actually has a significant positive impact on the delignification and is not just a potential problem in case of carryover to the bleach plant. Subsequently, pulp washing efficiency is critical both pre- and post-O2.

Sulfite pulping of pine is well-known to be a risky process because problematic lignin conden-sation reactions can occur resulting in poor pulp quality. However, sulfite pulping of pine is interest ...

While carryover of dissolved lignin between stages in the pulp mill fiber line is a well-known problem, it is still typically seen only as a minor disturbance factor or bias in the control of oxygen (O2) delignification and bleaching stages. The present study, however, reveals that it plays a larger role than anticipated, and that it should be properly analyzed in order to correctly control the process stages. This is especially important for the O2 and D0 stages as the lignin content is still high in these positions. The results of the study show that dissolved lignin carried over between stages may have a significant impact on the bleaching chemical consumption and, indirectly, on the pulp quality. Mill investigations have shown very large variations in the dissolved lignin content in the pulp before the oxygen delignification stage and before the D0 stage that have significantly influenced the bleaching chemical demand and, subsequently, the degree of delignification. In order to develop a better understanding of the mechanisms of the dissolved lignin’s reactions, laboratory O2 and D0 experiments with controlled levels of dissolved lignin were conducted. It was anticipated that a better feedforward control could be achieved using an online dissolved lignin measurement, and results from mill trials are presented. Chlorine dioxide laboratory experiments using different levels of carryover (i.e., different dissolved lignin contents) were conducted. It was concluded that the filtrate kappa number provides a relevant measure of the bleach demand due to the dissolved lignin and that, subsequently, the combined fiber and filtrate kappa number provides an appropriate measure for optimum feedforward control of the stages. Mill results support these findings, which show that the chemical consumption is reduced significantly using the total kappa number. The post-D or post-DE kappa number feedback control can most probably be eliminated by using this technology.

A laboratory study has been carried out on the utilization of magnesium oxide (MgO) as alkali in an oxygen delignification stage after acid sulfite pulping of softwood. The first objective was to reconfirm results from previous researchers to better understand the process mechanism concerning optimum MgO charge with respect to kappa number reduction. The second objective was to determine if for example the ionic strength could be an important variable in such a stage. It was found that the MgO charge should not be too high as this resulted in reduced degree of delignification which was unexpected. The results also suggested that the ionic strength in the liquor could be an important parameter in the oxygen delignification stage and that it should not be too high. Thus, ionic strength effects seemed to be present already at 0.02 mol/l which is below the normal ion concentration in a pulp mill. The results indicated that there are at least two chemical activities that are competing in an MgO based oxygen delignification stage. The alkalinity is one and a higher alkalinity improves the degree of delignification. The other seemed to be the ionic strength and if this is too high the degree of delignification will go down.

Kraft and sulfite pulp mills use several consecutive process stages for pulp production. However, usually only one key pulp parameter is used for process control and that is the lignin content in the fibers, typically expressed as the kappa number. Even so, to improve process efficiency, more variables need to be monitored. To do that, a new sensor was developed, the dissolved lignin transmitter (DLT), along with a new control concept. The DLT measures the dissolved lignin content in the pulp slurry using a unique principle based on optical measurements. The device can measure the dissolved lignin inline at low consistency and at medium consistency. The sensor has two major applications: 1) improving the efficiency in washing stages and 2) optimizing chemical charges. Results from several mill trials have shown that the contribution from dissolved lignin in the filtrate portion of the pulp is up to 30% of the total bleach load, i.e., fiber and filtrate kappa number combined into the bleach plant. Hence, chemical savings can be achieved taking this component into account compared to only measuring the washed fiber kappa number. Application: The results of this study can help mills understand how to better control the pulping stages, which might lead to significant economic savings and better pollution control.

This thesis forms part of a project with the objective of developing and implementing a novel, wood-based, process for the industrial production of cellulose textile fibres. This new process should not only be cost effective but also have far less environmental impact then current processes. Natural and man-made fibres are usually plagued with problems (e.g. economic and environmental) and are unsuitable in meeting growing demands. The focus of this thesis was therefore to investigate the dissolution of cellulose derived from various pulps in novel aqueous solvent systems. It was shown that cellulose could be dissolved in a NaOH/H2O solvent at low temperatures (

This paper reports a new, successful, utilization of NIR FT Raman spectroscopy for determinining the polymorphic transformation of native cellulose I into the allomorph cellulose II quantitatively. A rapid prediction of the transformation order is made possible by applying multivariate linear regression to the FT Raman spectral data of alkali-treated cellulose pulps. Simultaneously, changes in the crystallinity of cellulose I of these pulps were followed with respect to the lattice conversion process. The application of both multivariate quantification methods to the FT Raman spectra of the alkali-treated pulps yields a corrected polymorphic transformation order and enables the quantitative description to be made of the cellulose lattice conversion process as a system consisting of three participating forms of cellulose: crystalline cellulose I, amorphous cellulose and cellulose II.

Chemical pulp fibres from wood are commonly used in products associated with packaging as well as with printing and writing. The prevalent way of liberating fibres is by subjecting wood chips to Kraft cooking. This process has a history of almost 130 years and should be both well described and well established. However, new products and new applications that use fibres as an important renewable resource make it all the more important that the properties of fibres be controllable. The properties of wood fibres are influenced by their carbohydrate composition which, in turn, is dependent on the cooking conditions used. This thesis studies the degradation and dissolution of the different carbohydrates during Kraft cooking and summarizes the results in kinetic expressions. Industrial wood chips from Norway spruce (Picea abies) were cooked at a high liquor-to-wood ratio in an autoclave digester at varying concentrations of hydroxide ions, hydrogen sulphide ions and sodium ions as well as varying temperatures. The pulps were analysed for carbohydrate composition, kappa number, content of hexenuronic acid and the pure cellulose viscosity, i.e. only the cellulose content in the pulp sample was used for calculating the viscosity. Kraft cooking of Eucalyptus urophylla and Eucalyptus grandis was also studied, using industrial liquor-to-wood ratios, to examine the relationship between hexenuronic acids and the amount of xylan in the pulp samples. For Kraft cooking of Norway spruce it was found that an increase in the concentration of hydroxide ions increased the rate of dissolution of the carbohydrates and the degradation of the cellulose degree of polymerization (DP). However, measured at a kappa number of 30, it is seen that a low hydroxide ion concentration can lower the carbohydrate yield and the pure cellulose viscosity dramatically. The hydroxide ion concentration not only affects the rate of dissolution but also the amount of xylan that reacts in the slower, final phase. Both cellulose and hemicelluloses were found to be affected by the hydrogen sulphide ion concentration. The dissolution of cellulose and hemicelluloses at varying sodium ion concentrations was found to be affected in different directions. The effect of sodium ion concentration on the DP was found to be dependent on the method of evaluation. The pulp viscosity was found to be affected twice as much by the sodium ion concentration than the pure cellulose viscosity was. For Kraft cooking of hardwood it was found that a high xylan yield not always is synonymous with a high hexenuronic acids content.

Carboxymethylcellulose (CMC) was prepared from nine commercial dissolving pulps with different origins, viscosities and S18-values. The rate of the etherification reaction and the final DS were studied with respect to the influence of pulp composition, pulp viscosity and S18-value, and with respect to the sodium hydroxide and sodium monochloro-acetate charges. The possible influence on the final DS-value of the CMC of the washing procedure after the etherification was also investigated.

The influence of the solvent system used during the CMC manufacturing on the characteristics of CMC in different solutions was studied by producing CMC in ethanol, isopropanol or a mixture thereof. This paper shows that the solvent system used has an impact on the substituent pattern and this is especially the case during the mercerisation. When isopropanol is used in comparison with ethanol the distribution of the substituents along the cellulose backbone becomes more uneven and the amount of tri-substituted glucose units increases as well as the number the amount of substituents on the C-6 position. This affects the behaviour of CMC when dissolved in different electrolyte solutions.

The depolymerisation of laboratory-prepared kraft pulps from birch, eucalyptus and reed canary grass during acid hydrolysis was studied. The intention was to study especially the influence of xylan content on the levelling-off degree of polymerisation (LODP) and on the dissolution of carbohydrates during the acid hydrolysis. The xylan content in the pulps was varied by prehydrolysis prior to the kraft pulping or by alkali-extraction of the bleached pulps, and the levelling-off degree of polymerisation was compared with the amount of xylan left in the pulps at LODP. The dimensions of the original fibers in the pulps and of the fiber fragments after hydrolysis were also measured. It was found that the fiber fragments after hydrolysis were longer in the pulps containing a higher amount of xylan. Xylan thus appears to prevent degradation during acid hydrolysis, both on the fiber level and on the individual cellulose fibril level.

Controlling the reduction in molecular weight of the cellulose chains is essential in the production of carboxymethyl cellulose (CMC). Such a reduction can be achieved by the addition of cobalt during the process of cobalt(II) ions, which act as a catalyst for oxidative cleavage, and the influence thereof has been studied under a variety of conditions. This study has resulted in a model that summarises the effects of the added amount of cobalt, the time for the cobalt reaction, the temperature in the mercerisation stage of the CMC-manufacturing process and finally the effect of the temperature in the etherification stage. It is shown that it is important for cobalt to be present during the mercerisation stage in order to achieve the desired viscosity.

Four different spruce sulphite pulp samples were used for the preparation of carboxymethylcellulose (CMC). The characteristics of the unreacted fibre and particle residuals obtained in the CMC-preparation were used to establish a correlation between the hemicellulose in the pulp and the intrinsic viscosity, i.e.,␣cellulose chain length and the occurence of unreacted residuals. It was shown that the residual particles in the CMC consisted of fibres, fibre fractions and gel particles of different degrees of substitution. The results suggested that pulps with long cellulose chains, i.e., pulps with high intrinsic viscosities, resulted in particles that were more substituted and more swollen. These pulps also resulted in more substituted hemicelluloses in the CMC and more substituted residuals. It was also suggested that galactoglucomannan in the cellulose pulps is favourable for the swelling which results in more substituted hemicelluloses in the CMC and more swollen residuals. The amount of residuals was influenced mainly by the characteristics of the cellulose in the pulp. It is therefore believed that a combination of high viscosity and a suitable combination of hemicelluloses is the most favourable way of eliminating the occurrence of undissolved residuals in CMC.

The amount of disordered material in two types of hardwood kraft pulps was estimated by determining the weight loss at the point where the levelling-off degree of polymerisation (LODP) was reached. The pulps used were commercial pulps viz (1)one conventional birch kraft and (2)one mixed hardwood (MHW) kraft pulp that had been prehydrolysed prior to cooking. The results indicated that the hemicellulose xylan is closely associated with the cellulose in commercial birch pulps. It is therefore only possible to use LODP as a measure of the crystallite length of hardwood cellulose in highly purified pulps, such as prehydrolysed kraft pulp. A model explaining the LODP-results is proposed.

An industrial calcium sulfite pulp was fractionated in a hydrocyclone to four fractions that differed in dimensions and composition due to differences in density. The intention was to investigate whether the fibre dimensions had any influence on the properties of carboxymethyl cellulose (CMC) produced from the fractions and especially how the properties of the unreacted material differed. It was surprisingly found that the fraction containing thin-walled fibres gave CMC and dissolved residuals in the CMC-solution that had the lowest degree of substitution (DS). It was therefore believed that the thin-walled fibres were collapsed and more closely bound in the fibre network after drying of the pulp and that this impeded the chemical diffusion in the subsequent CMC-process, i.e. the diffusion of the CMC-chemicals into the cell wall was slower. There was thus a correlation between thinner fibres and a lower degree of substitution for CMC made from such fibres. It was also found that tick-walled fibres had a higher degree of substitution than the thin-walled fibres but that the highest degree of substitution was obtained if a mixture of thin- and thick-walled fibres were used.

The undissolved fibre and gel residuals that had not completely reacted to form fully dissolved carboxymethyl cellulose (CMC) ID the production of CMC were studied to clarify the reactivity of wood components ID the pulp. The undissolved residuals, the pulp and the CMC were therefore analysed on the fibre level, the cell-wall level and the chemical composition level. The results may be interpreted as indicating that the presence of undissolved residuals ID the CMC was not due to any chemical difference. The undissolved residuals were shown to consist mainly of swollen cell wall parts and some whole wood cells, mainly thick-walled compression wood and summerwood cells. They react more slowly ID the mercerisation and etherification, probably because of a greater diffusion resistance due to their larger dimensions or to a more dense structure. These cells are assumed to be less accessible for chemical penetration, but they may also contain supramolecular structures that slow down the CMC reaction.

Microfibrillated celluloses (MFCs), produced by various pre-treatments of a fully bleached birch kraft pulp, were evaluated as strength enhancers in test sheets representing the middle ply of paperboard. The furnish consisted of hydrogen peroxide bleached high temperature spruce chemithermomechanical pulp (HT-CTMP), MFC and a retention system containing cationic starch and an anionic silica sol. The MFC was prepared via a mechanical treatment in a colloid mill after pretreatment with Fenton's reagent, monocomponent endoglucanase or acidic hydrogen peroxide. Addition of 5% MFC, produced with Fenton pre-treatment, resulted in improved HT-CTMP properties with respect to increased tensile index (similar to 35%), z-directional strength (similar to 50%), tensile stiffness index (similar to 25%) compared to HT-CTMP test sheets prepared without MFC addition. The strength improvement was linearly correlated to the density of the tests sheet, to the surface area (BET) and to the surface charge of the enzymatic or chemically pre-treated MFCs.

The strength of the adhesion between the paper and the drying Yankee cylinder is of great importance with respect to the final properties of a tissue paper product. Therefore, the effects of a few potentially important pulp properties have been evaluated in laboratory experiments. Four highly different kraft pulps were used, and the adhesion strength was measured by means of the force required when scraping off a paper from a metal surface with a specifically designed knife mounted on a moving cart. The adhesion strength was observed to increase with increasing grammage and increasing degree of beating of the pulp. It was also found that pulps containing more fines, or with higher hemicellulose content, gave rise to higher adhesion strength.

A fully bleached birch kraft pulp was treated with acidic hydrogen peroxide in the presence of ferrous ions (Fenton's reagent) and thereafter treated mechanically in a colloid mill to produce a product containing microfibrillated cellulose (MFC). The produced MFC products were chemically and morphologically characterized and compared with MFC products produced without pretreatment as well as with enzymatic hydrolysis. Fenton treatment resulted in an increase in total charge and number of carbonyl groups while the intrinsic viscosity decreased. The Fenton treated pulps were easier to process mechanically i.e. they reached a higher specific surface area at a given mechanical treatment time and the MFC produced had a stable water-fibre suspension for at least 8 weeks compared to enzymatic pretreated pulps and pulps not subjected to any pretreatment. © 2014 Springer Science+Business Media Dordrecht. CODEN: CELLE

Carboxymethyl cellulose (CMC) is produced commercially in a two-stage process consisting of a mercerization stage followed by an etherification stage. In this work, extended mercerization stages were used when producing CMC from a spruce dissolving pulp. Near infra-red (NIR) Fourier transform (FT) Raman spectroscopy was used to analyse the molecular structures of the CMC and the gel fractions formed in the CMC preparation. Three different CMC groups were obtained, representing backbone structures of cellulose I, cellulose II, and amorphous cellulose. By applying principal component analyses (PCA) to the spectral data, two CMC classes were identified with different degrees of substitution (DS). Thus, a low degree of substitution was obtained in the CMC if the alkaline concentration in the mercerization stage was only 9.0%, and the backbone structure was cellulose I or II. However, if the alkaline concentration was higher (18.25% or 27.5%), then the degree of substitution in the CMC was also higher, and the backbone structure was more amorphous.

This study investigates how drying affects the quality of bamboo kraft pulps. Two bamboo pulps, cooked to different kappa numbers (i.e. 10 and 26) and D(EOP)D bleached to approximately the same brightness, were used to examine the relationship between cooking, drying, refining, and pulp/fiber properties. Drying of the two pulps caused, as expected, a loss in tensile and burst strength while the tear index was improved. The bleached high kappa pulp required less energy to reach a certain Schopper Riegler value and exhibited greater strength properties than the low kappa number pulp. These differences were also maintained after drying. Results showed that the properties of the pulp before drying determined the final strength potential of the pulp after drying. Thus, kraft cooking of bamboo to high kappa number prior to bleaching gave pulps with improved response to refining and pulp strength properties, which in turn influenced the properties of the dried pulps.

Carboxymethyl cellulose (CMC) is produced commercially in a two stage process consisting of a mercerization stage in which the pulp is treated with alkali in a water alcohol solution and a second etherification stage whereby monochloro-acetic acid is added to the pulp slurry. In this study, the influence of the conditions of an extended mercerization stage was evaluated on the etherification stage concerning the degree of substitution (DS) and the filterability of the resulting CMC. The parameters studied were: (1) the ratio of cellulose I and cellulose II in the original pulp, (2) the concentration of alkali, (3) the temperature and (4) the retention time in the mercerization stage. The DS results indicate that the NaOH concentration in the mercerization stage is the most important among the parameters studied. When the NaOH concentration in the mercerization stage was high (27.5%), cellulose II showed a lower reactivity than cellulose I with respect to the DS obtained in the resulting CMC. The results from the filtration ability of CMC water solutions are interpreted that the amount of cellulose II in the original pulp and the temperature has a negative influence, while the NaOH concentration in the mercerization stage has a positive influence on the filtration ability. Retention time between 1 h–48 h in the mercerization stage had no effect on the DS or the filtration value. The filtration ability was assumed to be highly influenced by the presence of poorly reacted cellulose segments. The CMC samples with the lowest filtration ability at a given DS can be assumed to have the highest degree of unevenly substituted segments.

In this project different aqueous based solvent systems were investigated for the dissolution of steam exploded (STEX) dissolving pulps prior to a coagulation step for preparation of textile fibers. The pulp samples studied could, to a large degree, be dissolved in a NaOH/urea/thiourea system, but there are always residuals, irrespective of the conditions. The dissolution process appeared to be kinetically controlled, as prolonged retention time decreased the amount of residuals. A pre-cooled NaOH/urea/thiourea system, and a retention time of 60 h generated the lowest amount (6%) of residuals of the STEX pretreated dissolving pulp studied. Some fundamental results are described and a model for dissolution of dissolving pulp fibers is proposed.

Representative families of models for the chemical kinetics of kraft pulping taken from the literature spanning the last three decades are compared using data obtained from a series of autoclave cooks and an instrumented laboratory circulation digester. The differences within families is slight, and they reasonably explain the data with fitted parameters, and more crucially, even when using original parameters. However the Purdue model arguably fits the measured data better for the more modern cooking schemes, and has the advantage that it describes the kinetics of hemicelluloses such as arabinoxylan which are needed for for pulp property predictions.

A recently proposed model for kraft delignification kinetics is used to generate optimum operating conditions for the production of kraft pulp. In contrast to previous models, the optimal profiles for temperature, [OH] and [HS] support the current operating practice in mill digesters. The optimisation results suggest coupling as high a [HS] as possible with a low [OH] but not below 0.2 M, at a low temperature for as long a cooking time as practical. It is also shown that the optimal [OH] conditions depends on whether the [HS] or the sulfidity is kept constant. These optimisation findings are summarised in a set of updated recommended procedures for industrial digester operation in order to maximise yield and selectivity.