Method for treating sludge of paper mill or paperboard mill to realize recycling

Abstract

Provided herein is a method of treating waste, more particularly sludge removed from paper or paperboard production, such as regenerated fiber (RCF) production. The present disclosure relates to a method of treating said sludge to improve its reusability, in particular as conditioning sludge in paper or paperboard production processes. Also provided herein is the use of the conditioned sludge obtained by the method in a paper mill or paperboard mill.

Description

Technical Field

[0001] The present disclosure generally relates to treating sludge removed from paper or paperboard production, such as recycled fiber (RCF) production. The present disclosure relates to a method for treating said sludge to improve its reusability, in particular but not limited to its reusability as conditioned sludge in said paper or paperboard production process. Also provided herein is the use of said conditioned sludge obtained by said method in a paper mill or paperboard mill. Background Art

[0002] The papermaking process involves large amounts of water and aqueous media. Water is used as a carrier and matrix to help form the pulp into a continuously moving mat of wet fibers. Modern processes are designed to minimize the consumption of fresh water and the generation of aqueous effluents and wastewater to be treated (i.e., sludge). However, during the production of paper or paperboard, aqueous effluents are continuously removed. Along with these aqueous effluents, a variety of water-soluble or water-miscible substances and solids are also removed.

[0003] Matter and solids that are not retained in the cellulosic fiber web include organic, inorganic and microbiological materials. They originate from the fiber source, whether virgin or recycled, from additives fed into the process, and from the process itself, i.e., side reactions and microbial growth that occur in this nutrient-rich aqueous environment. They are not bound to the cellulosic fiber web primarily because of size, but also because of shape or charge.

[0004] Sludge disposal carries costs and environmental burdens. Although some of these materials and solids would be valuable if they could be returned to the production process, reusing these aqueous effluents or sludges as is has proven to be problematic.

[0005] Recirculating untreated sludge back into the papermaking process can lead to technical challenges in paper machine operation, such as deteriorating drainage and retention performance, introducing bacteria (back) into the system, and / or deteriorating wet-end conditions. Inorganic particles and fines often lead to poor drainage, which in turn leads to reduced speed, increased vacuum loads, and increased steam consumption. In addition, when particles and fines cannot be retained by the cellulosic fiber web, the system becomes dirty.

[0006] Furthermore, the resulting finished paper / board may be of inferior quality, for example due to the inorganic content of the recycled sludge hindering the bonding between the fibres, resulting in a weakening of the strength. This can also be seen in the product through an increase in dust and fluff, and the need for more dry strength agents.

[0007] The process of recovering paper and paperboard fibers has been used for decades. Paper and paperboard waste, commonly referred to as waste paper / waste paperboard, and recovered (waste and trimmings) paper / paperboard, or paper / paperboard destined for recycling, is efficiently collected from industry, offices, wholesalers, retailers and consumers and sent back to paper and paperboard mills for reuse through repulping. However, supply and demand don’t always match up. After China’s solid waste import ban began in 2017, annual import quotas for wastepaper companies were gradually reduced until they were completely eliminated in 2021. Since then, there has been a huge shortage of raw materials for RCF containerboard.

[0008] It is necessary to reduce or even eliminate the disadvantages existing in the prior art as much as possible.

[0009] There is a need to reduce wastewater / solids and their disposal in paper or board mills.

[0010] Any solids that do not remain in the paper or paperboard product and end up in the aqueous effluent need to be recovered for reuse.

[0011] There is a need to reduce the reliance on waste paper / waste board supplies and replace them with solids that can be recovered from paper and board mill sludge. Summary of the invention

[0012] The attached claims define the scope of protection. In the specification and / or drawings, any examples and technical descriptions of processes and / or methods not covered by the claims are not presented as embodiments of the present invention, but are provided as background technology or examples to help understand the present invention.

[0013] According to a first exemplary aspect, a method for producing conditioned sludge is provided. The method comprises obtaining sludge from a paper mill or a board mill and treating the sludge by adding at least a first cationic polymer to obtain conditioned sludge.

[0014] The method of the invention has been shown to have advantages for paper and / or board manufacturing processes and also for wastewater treatment. Surprisingly, solid matter in the sludge can be recovered and recycled back into the papermaking process in the form of floccules of a size and shape suitable for retention in a cellulosic fiber web. Embodiments of the method have been shown to provide additional advantages for papermaking processes and their products.

[0015] According to a second exemplary aspect, there is provided the use of the conditioned sludge obtained by the method of the invention in a paper mill or board mill. Such use provides advantages for the papermaking process and its products, as well as for wastewater treatment.

[0016] The foregoing has shown different non-limiting exemplary aspects and embodiments. The foregoing embodiments are only used to explain the selected aspects or steps that may be used in different embodiments. The presentation of some embodiments may only be related to certain exemplary aspects. It should be understood that the corresponding embodiments may also be applicable to other exemplary aspects. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Some exemplary embodiments will be described with reference to the accompanying drawings, in which:

[0018] Figure 1 A conventional process is schematically shown;

[0019] Figure 2 schematically illustrates a process according to an exemplary embodiment;

[0020] Figure 3 Another process according to an exemplary embodiment is schematically illustrated. DETAILED DESCRIPTION OF THE INVENTION

[0022] In the following description, like reference numerals denote like elements or steps.

[0023] Generally, the term "papermaking" as used herein refers to both papermaking and boardmaking. By analogy, the terms "paper mill", "paper machine" and "paper product" apply equally to both paper and board. In the context of the present disclosure, sludge from a paper mill or board mill refers to an aqueous medium containing solids removed from the papermaking process into an aqueous effluent. It is preferred to reduce the water content to limit the volume of the sludge. However, keeping the sludge in the form of a slurry makes it easier to handle and transport.

[0024] As used herein, the term "primary sludge" refers to the sludge obtained from the primary wastewater treatment of aqueous effluent from a paper mill or board mill. They follow a typical wastewater treatment process, in which solid matter is removed in the primary stage. Typical process steps include settling, sedimentation and / or air flotation. The solids are recovered for treatment according to the process of the present invention, while the aqueous phase is further treated in a secondary treatment. As used herein, "ash" refers to inorganic particles, which are the main component of primary sludge. Ash comes from recycled waste paper, virgin fibers and fillers, such as calcium carbonate powder, precipitated calcium carbonate and / or clay.

[0025] In wastewater treatment, secondary treatment includes biological treatment, which refers to the reduction of waste loads by microbial consumption of carbohydrates still contained in the aqueous phase recovered from primary treatment. These solids recovered from secondary treatment, the "secondary sludge", mainly contain microbial mass, usually bacteria.

[0026] As used herein, conditioned sludge includes solid flocs formed by the cationic polymer and solids in the sludge during the treatment process of the present disclosure. It may be in the form of a heterogeneous mixture of aqueous medium and said flocs, or it may be more or less drained (i.e., at least a portion of the aqueous medium is reduced or removed) and in the form of moist or wet flocs. According to a specific embodiment in which the conditioned sludge is subjected to a separate treatment, the conditioned sludge may be in the form of a thin or thick slurry, or in the form of a moist solid, wherein the water content is less than 20%-wt.

[0027] In the context of the present disclosure, the expression "flocculation" refers to the interaction of solids present in the aqueous phase of the pulp with the cationic polymer to form "flocs" or "bundles". This interaction may be based on physical entanglement of the solids with the polymer structure, where the solids are "trapped" or "captured" by the polymer structure, and / or this interaction may be based on chemical interactions, where the solids and / or fibers may bind to the cationic polymer, for example, through electrostatic forces. The binding of solids and / or fibers to the flocs initiated by the cationic polymer enables the cellulosic fibers to carry the flocs forward during the manufacturing process, inhibiting the solids from being removed from the process with the aqueous phase and ultimately enabling them to be retained in the final paper web formed.

[0028] While not being bound by a particular theory, it is believed that when the cationic polymer is added to the sludge, at least some of the microstickies are encapsulated in the flocs and retained in the cellulosic fiber web when it is returned to the papermaking process. In addition, because the flocs retain at least some of the inorganic material from the sludge, strength loss is reduced, which would otherwise interfere with the bonding between the fibers.

[0029] In the context of the present disclosure, it is understood that the cellulose fibers in the pulp may initially be produced by any suitable pulping process, i.e. they may originate from chemical pulping, mechanical pulping or chemimechanical pulping. The cellulose fibers are typically wood-based fibers, but it is also possible that at least some of them are not wood-based fibers, such as cellulose fibers from annual plants. The pulp according to the invention typically contains a large amount of recycled fibers or fibers from broken paper. The recovery rate of the cellulose fibers may be so high that the cellulose fibers in the pulp contain at least 50% by weight, preferably at least 70% by weight, more preferably 100% by weight of recycled fiber material, calculated on a dry weight basis, based on the total weight of the fibers. According to a preferred embodiment, the recycled cellulose fiber material contains at least 40% by weight, preferably at least 50% by weight, of fiber material derived from old corrugated container board (OCC).

[0030] In the process of the present invention, paper or board mill sludge is treated with at least one cationic polymer in order to enhance the reuse of said sludge as conditioning sludge.

[0031] In the simplest case, the process can be defined as comprising taking sludge from a paper or board mill and treating said sludge by adding at least one cationic polymer to obtain a conditioned sludge.

[0032] The experimental results of the present invention show that this method is particularly effective in treating primary sludge and making it suitable for reuse in a paper mill or board mill. Therefore, according to a preferred embodiment, the sludge from a paper mill or board mill comprises primary sludge, or preferably, it is primary sludge from a paper mill or board mill.

[0033] Primary sludge contains fibres, fine fibres, ash, papermaking chemicals, water soluble residues and other materials not attached to the fibre structure of the formed paper or board product. By the treatment method of the present invention using at least one cationic polymer, the fines / ash / fibres will flocculate or aggregate into larger bundles which can be retained in the paper structure when the conditioned sludge is returned to the paper or board manufacturing process. An example of the composition of primary sludge can be described by the characteristics compiled in Table 1.

[0034] Table 1. An example of primary sludge composition.

[0035] Main parameters Primary sludge analyzed Total Solids 1.2–11％ Fixed solids (ash) 35–70％ Charge -1200–0 μeq / l Zeta potential -14–1mV Conductivity 2.6–4.7 mS / cm pH 5.9–7.4 Redox potential -140–14mV Fiber length <0.5mm (mainly fine materials <0.2mm)

[0036] However, the method of the invention is also applicable to the treatment of secondary sludge.According to a particular embodiment, the method of the invention is applicable to the treatment of a mixture of primary and secondary sludge.

[0037] According to a particular embodiment, the paper mill or board mill sludge is treated with a biocide prior to treatment with at least one cationic polymer. The treatment with the biocide may inactivate or inhibit any enzymes present in the sludge, eliminate microorganisms or both. Preventing microbial growth is important to avoid the formation of slime and / or biofilm. Microorganisms may have further negative effects on the operation of the papermaking chemistry and / or the quality of the final product. For example, microorganisms may produce organic acids that lower the pH of the process, which in turn may lead to the dissolution of calcium compounds and increase the risk of deposit formation. The presence of microorganisms may also lead to the formation of large microbial deposits that undermine the quality of the final product and may cause operational problems. Extensive growth of microorganisms in paper or board production may also cause severe odor problems.

[0038] In addition, when cellulose, hemicellulose or other polysaccharides in the pulp are destroyed by microorganisms, there may be a significant loss in yield in the manufacturing process. This loss must be compensated by adding an equal amount of fibrous material to maintain the same solid content in the pulp. The loss of polysaccharides may also reduce the strength of the produced paper or board, which needs to be compensated by the additional addition of other strengthening chemicals.

[0039] The inventors of the present invention have found that the amount of cationic polymer used can be surprisingly low compared to the dry solids present in the sludge. Therefore, in one embodiment of the method and use of the present invention, the amount of the first cationic polymer used in the treatment step is 0.5-10 kg / t of sludge dry solids, preferably 1-5 kg / t of sludge dry solids, where "t" refers to tons, i.e. 1000 kg of sludge dry solids. When a combination of the first and second cationic polymers is included in the treatment process, the amount of each cationic polymer used can be lower than when only the first cationic polymer is used.

[0040] The cationic polymer can be added in liquid form or in dry form, preferably in dry form as a granular material. If the cationic polymer is in dry form, it is dissolved before being added to the pulp. Whether the polymer is in liquid or dry form, it is usually diluted with water to an appropriate dosage concentration before being added to the pulp.

[0041] Any cationic polymer used in the present invention can be obtained by any suitable copolymer polymerization method. Cationic polymers can be obtained by suspension polymerization, such as solution polymerization or gel polymerization; dispersion polymerization; or emulsion polymerization. Preferably, the cationic polymer is obtained by solution polymerization or gel polymerization.

[0042] The cationic polymer can be obtained by copolymerization of one or more cationic monomers. The cationic polymer is obtained by copolymerization of at least 10 mol-%, preferably at least 15 mol-%, more preferably at least 20 mol-% of pure cationic monomers. For example, the cationic polymer can be obtained by copolymerizing 10-50 mol-%, preferably 20-40 mol-%, more preferably 15-30 mol-% of a cationic monomer with another preferably neutrally charged monomer. Thus, the first cationic polymer has a charge of 10-50 mol-%, preferably 20-40 mol-%, more preferably 15-30 mol-%. Here, the charge value is relative to the amount of cationic monomer used in the polymerization. It has been observed that when at least 10 mol-% of cationic monomer is present in the polymerization, the obtained cationic polymer is provided with a good ability to bind to solid matter present in the sludge (e.g., by electrostatic forces), while its ability to interact with negatively charged cellulose fine material fibers present in the sludge is also improved.

[0043] In certain embodiments, the cationic polymer may be obtained by copolymerization of polyamide or acrylamide, cationic monomers and <1 mol-%, preferably <0.5 mol-%, more preferably <0.1 mol-% of anionic monomers. According to a preferred embodiment, the cationic copolymer does not contain negatively charged structural units, i.e. the copolymerization is carried out in the absence of anionic monomers. Therefore, the polymer preferably consists of structural units derived from nonionic monomers (i.e. amide or acrylamide) and cationic monomers. Even in that case, during the preparation of the polymer, for example during drying, a small amount of negatively charged groups may be formed in the polymer structure.

[0044] According to one embodiment, the first cationic polymer has a standard viscosity of 2-5.5 mPa·s at 25°C, which is preferably measured using a Brookfield DVII T-type viscometer equipped with a UL adapter. The standard viscosity is measured at 0.1 wt% polymer content in 1M NaCl aqueous solution at 25°C using a Brookfield DVII T-type viscometer equipped with a UL adapter. In general, the standard viscosity of the polymer provides an indication of the polymer chain length and / or weight of the polymer. It has been observed that when the standard viscosity SV of the cationic polymer is between 2-5.5 mPa·s, the polymer is able to effectively bind to solids in the sludge. We believe that the cationic polymer has an improved ability to bundle anionic fine material fibers into bundles or flocs that are large enough so that they can be retained in the cellulose fiber structure when recovered and added to the pulp.

[0045] According to one embodiment, the first cationic polymer is selected from polyamines and / or polyacrylamide (PAM). Polyacrylamide is a preferred polymer for the first cationic polymer.

[0046] By definition, a polymer is formed by repeating units of a monomer, and therefore a polymer can be defined by the monomers that produce the polymer by polymerization or copolymerization. According to one embodiment of the present invention, the cationic monomer can be selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethylsulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulfate, [3-(acryloylamino)propyl]trimethylammonium chloride (APTAC) or [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC). Preferably, the cationic monomer may be selected from 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl) and [3-(acrylamido)propyl]trimethylammonium chloride (APTAC).

[0047] According to another embodiment of the present invention, the cationic polymer may be selected from polyamines comprising dimethylamine and epichlorohydrin monomers, wherein the dimethylamine provides the cationic charge.

[0048] According to a preferred embodiment, the treatment in the method and use of the present invention comprises the addition of a first cationic polymer and the addition of at least one second cationic polymer. Including the addition of two cationic polymers in the method makes the treatment process easier to control and adapt to the specific characteristics of the sludge at hand. For example, the viscosity, charge and dosage of the first and second cationic polymers can be selected to provide the desired flocculation.

[0049] The inventors of the present invention have found that it is most beneficial to add the first and second cationic polymers in sequence. The preferred order of addition is to add 1) the first cationic polymer first, and then add 2) the second cationic polymer. This process allows the first cationic polymer to balance the charge for a period of time, and then the solids are flocculated by adding the second cationic polymer.

[0050] It has been found that it is advantageous for the second cationic polymer to have a standard viscosity of 3-5.5 mPa·s at 25° C., which standard viscosity is preferably measured like the first cationic polymer, i.e., using a Brookfield DVIIT viscometer equipped with a UL adapter. The second cationic polymer preferably has a higher standard viscosity than the first cationic polymer. This applies correspondingly to the molecular weight. Therefore, according to a preferred embodiment, the molecular weight of the second cationic polymer is higher than the molecular weight of the first cationic polymer.

[0051] According to one embodiment, the charge of the second cationic polymer is higher than the charge of the first cationic polymer. It has been found experimentally that this provides controlled flocculation and optimized cationic polymer dosage. The dosage of the second cationic polymer in the treatment step may be at least 0.5 kg / t sludge dry solids, such as 0.5-7 kg / t, preferably 1-5 kg / t sludge dry solids.

[0052] According to the chemical principle, the second cationic polymer can be selected from cationic polyacrylamide. More specifically, the second cationic polymer can include a monomer selected from the following: 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethylsulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulfate, [3-(acryloylamino)propyl]trimethylammonium chloride (APTAC) or [3-(methacryloylamino)propyl]trimethylammonium chloride (MAPTAC).

[0053] For example, both the first and second cationic polymers may be copolymerized from polyacrylamide monomers and cationic monomers, provided that the amount of the cationic monomer in the second cationic polymer is higher than the amount of the cationic monomer in the first cationic polymer.

[0054] The conditioned sludge obtained from the treatment according to the method of the invention can be recycled back to the papermaking process. The conditioned sludge can be added as is, or alternatively, it can be subjected to further physical or chemical treatment steps. Adding the conditioned sludge as is without further treatment steps is useful because it does not require specific separation equipment nor maintenance thereof. Furthermore, the stock can be conveniently mixed with the paper pulp at the addition location.

[0055] A typical physical step is separation. Therefore, the conditioned sludge can be separated using at least one or more separation techniques for heterogeneous mixtures. The separation method for the conditioned sludge can be selected as one or more of filtration, centrifugation, sedimentation, screening and / or decantation. Preferred separation methods include screening and / or centrifugation. The purpose of separation is to collect solids from the conditioned sludge as a slurry. Preferably, at least some of the aqueous phase is removed from the solids. Along with the aqueous phase, some chemicals and / or microorganisms can be discarded so that they are not returned to the papermaking process.

[0056] From the separation, the flocs containing the cationic polymers bound to the solids can be recovered and recycled back to the papermaking, back to the formation of the fiber web. By separating, some chemicals and / or excess water can be removed. In particular, it is beneficial to remove the smallest particles and stickies along with the excess water.

[0057] The main advantage obtained by separation is the reduction of sludge volume. However, even if most of the solids flocculate with the addition of cationic polymer, some small particles still remain suspended in the water phase. These small particles will not remain in the fibrous web produced from the pulp and therefore will not affect the fibrous web. In embodiments where at least a portion of the water phase is separated from the conditioned sludge and discarded, these small particles are removed and do not interfere with the process to which the conditioned sludge is added. Another impurity that can be reduced by separation and discarding of the water phase is stickies, which tend to remain dispersed or dissolved in the water phase.

[0058] According to a preferred embodiment, the conditioning sludge can be reused in a paper or board mill, wherein the method further comprises adding the conditioning sludge to the pulp. The conditioning sludge returns some of the fine fibers, ash and other solids removed with the aqueous effluent to the pulp, but when these solids interact with the cationic polymer after treatment according to the method of the present disclosure, they behave in the pulp similarly to other larger particles in the process and are therefore retained in the fiber web and contribute to the products formed therefrom.

[0059] In the method or use of the present invention, the location for adding the conditioning sludge can be selected. Thus, the conditioning sludge can be added to the pulp at at least one location in the paper mill or board mill, which location can be selected from the pulper, discharge tower, mixing tank, machine chest and / or flushing pump, preferably added to the mixing tank or flushing pump, most preferably added to the mixing tank.

[0060] According to one embodiment, the location where the conditioning sludge is added is at the pulper. The conditioning sludge flows with the pulp through typical processing steps, such as screening, which is the most important step for removing large-sized impurities. Therefore, both large stickies and micro-stickies are at least partially removed during the normal cleaning process. Any large-sized impurities in the sludge are mainly removed by the screening process or broken up in the mechanical steps and thus removed in the normal process. Therefore, a separate purification of the conditioning sludge is not required. However, since the polymer is left in the pulp, any broken flocs may reform during the retention process, capturing inorganic and fine solids again.

[0061] Although addition to the pulper is performed early in the process, other embodiments allow addition to be performed later in the process, such as in a mixing tank or slurry pump. By adding later, a mechanical stage is avoided, which helps keep the flocs formed by the cationic polymer addition untangled and together. Addition to the mixing tank or slurry pump allows for better control of the drainage, retention and strength of the sludge for reuse than addition to the pulper. This also helps reduce energy consumption because recirculation of sludge components in the reslurry system required in early additions can be avoided.

[0062] Another advantage of adding conditioning sludge at a later stage of the process (e.g. to a mixing tank or flushing pump) is that it is easier to increase the sludge ratio in a controlled manner, resulting in reduced raw material costs. The improved sludge retention that can be obtained by adding it at a later stage closer to paper or board production is also desirable. By adding conditioning sludge at a later stage, the pre-formed microflocs help to remove moisture in the wire section. In addition, paper processing equipment surfaces benefit from reduced deposition through good fixation and retention.

[0063] According to a particular embodiment, conditioning sludge can be added to the pulp in significant amounts. Conditioning sludge can be added to the pulp in an amount of 1-10%-wt, preferably 2-8%-wt, more preferably 3-6%-wt, for example 5%-wt of the total pulp weight (dry weight / dry weight). Experiments of the present invention have shown that a surprisingly high proportion of the pulp can be replaced by conditioning sludge without affecting the production process or reducing the quality of the final product. Compared with pulp without sludge or with untreated sludge, it was even found that some product characteristics were improved using conditioning sludge produced according to the method and use of the present invention.

[0064] As another aspect of the present invention, there is provided herein the use of conditioning sludge in a paper mill or board mill, wherein the conditioning sludge is obtained by the method as described herein. The conditioning sludge can be used to replace at least a portion of the pulp, improve dewatering and retention, improve runnability, reduce solid waste and reduce chemical oxygen demand in the effluent, obtain a cleaner effluent in paper mill wastewater treatment, or improve the strength of the paper or board product. Preferably, two or more of the effects are obtained simultaneously by the use of the present invention.

[0065] As a specific embodiment, the use of conditioning sludge occurs in a RCF process, wherein the conditioning sludge is obtained by the method as described herein. The conditioning sludge can be used to replace at least a portion of the RCF pulp, improve drainage time in the RCF process, improve dewatering and retention rate in the RCF process, improve runnability in the RCF process, reduce emissions in the RCF process, reduce solid waste in the RCF process, obtain cleaner effluent in RCF board mill wastewater treatment, or improve the strength of RCF board products. Preferably, two or more of the effects are obtained simultaneously by the use of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0067] Certain embodiments are described in more detail in the following non-limiting drawings, in which

[0068] Figure 1 A conventional preparation process without sludge addition is shown.

[0069] Figure 2 A stock preparation process is shown including possible locations for regulating sludge addition according to some embodiments of the present invention, wherein the addition is performed at an early stage in the process.

[0070] Figure 3 A paper stock preparation process is shown wherein the addition of conditioning sludge is performed according to an embodiment of the present invention, wherein the addition is performed at a late stage of the process. DETAILED DESCRIPTION

[0071] Figure 1 A conventional pulp preparation process without sludge addition is shown. Schematically, the process can be presented starting from a pulper 100. The pulper produces pulp from cellulose fibers with the help of water and chemicals. If recycled material is used, it gently crushes the recycled paper or paperboard raw material into a pumpable pulp. Water is fed to the pulper via pipeline 401. The pulp is stored in a discharge tower 110. The following operations, namely screening 120 and classification 130, separate the fibers in the pulp according to size, especially fiber length. Next, the long fibers are filtered in a disc filter 140a and the short fibers are filtered in a disc filter 140b, and the aqueous effluent from the disc filter 140b can be discarded. After the hot disperser 150, the long fibers are stored in a storage tower 160a (the short fibers are stored in a storage tower 160b). Before the headbox 200, the pulp from the storage tower 160a / b is mixed in the mixing tank 170 with broke supplied from the broke storage tower 310 via the pipeline 301. The pulp from the machine chest 180 is mixed with the white water from the wire section and pumped to the headbox 200 by the flushing pump 190. Figure 1In FIG. 3 , the step after the headbox, in which the fiber mat is formed and the pulp is discharged, is depicted as unit 300 .

[0072] Figure 1 The black arrows in 400 indicate the flow of fiber stock through the stock making process, and the dashed lines represent the flow of water circulating therein, stored in water storage tanks 410 and 420, or removed from the stock making process via line 402 to wastewater treatment and / or sludge reuse.

[0073] As mentioned above, the conditioning sludge can be reused in a paper or board mill, wherein the method further comprises adding the conditioning sludge to the pulp. Figure 2 1 depicts an embodiment in which conditioning sludge 500 is added to the pulp at an early stage. According to one embodiment, the addition location of the conditioning sludge is at the pulper 100 via line 501, or at the discharge tower 110 via line 502, or a combination thereof. This embodiment enables both large and micro stickies to be at least partially removed during the normal cleaning process, mainly at the screen 120. Therefore, a separate purification of the conditioning sludge is not required.

[0074] Figure 3 , an embodiment in which conditioning sludge is added to the pulp at a later stage of pulping is depicted. Other embodiments provide for addition at a later stage of the process, such as at the mixing tank 170 via line 511, at the machine chest 180 via line 512, at the flushing pump 190 via line 513, or any combination thereof. Although Figure 3 Addition to all of these locations is depicted, but according to various embodiments, one, two, or more addition locations may be employed depending on conditioning sludge characteristics and / or process control. Advantages associated with late addition of conditioning sludge include avoiding mechanical stages, providing improved control, reuse of sludge in terms of drainability, retention, and strength, and further reduction in energy consumption.

[0075] Example

[0076] Example 1: Product Strength 1

[0077] The effect of primary sludge (from paper mill wastewater treatment plant, WWTP) on paper products was tested. A paper mill that recycles primary sludge by adding it back into the papermaking process was used in the study. The paper mill mainly produces low-grade liner boards from OCC as raw material. Primary sludge from the WWTP of this paper mill was collected for forming handsheets. Pulps were prepared from OCC liner samples using specific dilution water. Handsheets were formed using pulps with or without the addition of primary sludge to the paper stock. According to ISO 5269-2:2004 Pulps–Preparation of laboratory sheets for physical testing–Part 2: method to prepare hand sheets.

[0078] The pH of the primary sludge was 5.93, the solid content was 8.75% by weight, and the ash content (ash in the solid) was 60.8% by weight. A dilution water consisting of 70% by weight calcium acetate, 20% by weight sodium sulfate, and 10% by weight sodium bicarbonate was prepared. The conductivity was adjusted to 4 mS / cm and the pH was adjusted to 7. The dry matter content of the OCC liner sample was 94% by weight, the ash content was 15% by weight, and the gram weight was 120 g / m 2 . OCC plates of 2 cm x 2 cm size were soaked in 85°C dilution water for 5 minutes. Pulp (2.5 wt%) was thermally decomposed using a Lorentzen & Wettre Pulp Disintegrator at 30000 rpm for 10 minutes, after which the pulp was diluted to 1.25 wt% consistency. The pulp was cooled to room temperature and the pH was adjusted to 7.

[0079] 100 g of sludge was provided in a beaker. The sludge was rapidly mixed using a 317 stirrer / timer (Triton). The polymer was added to the sludge and then mixed. In the case of treatment with a combination of polymers, they were added sequentially with stirring in between. The polymer flocculated the solids in the sludge, thereby providing a conditioned sludge.

[0080] Four sets of handsheets were made according to standard method ISO 5269-2:1998. One set was made from a mixture containing pulp and 50 kg of primary sludge (untreated) per ton of pulp, based on the weight of dry solids of the stock and sludge. One set of handsheets was made from a pulp containing the pulp and 50 kg of sludge treated with polymer A per ton of pulp, based on the weight of dry solids of the stock and sludge. The sludge treated with polymer A contained 2.0 kg of polymer A / ton of sludge, based on the weight of dry solids of sludge and polymer A. Another set of handsheets was made from a pulp containing the pulp and 50 kg of sludge treated with polymer B per ton of pulp, based on the weight of dry solids of the stock and sludge. The sludge treated with polymer B contained 5 kg of polymer B / ton of sludge, based on the weight of dry solids of sludge and polymer B. The fourth set of handsheets was made from a mixture containing pulp and 50 kg of sludge treated sequentially with a combination of polymers A and B, based on dry solids weight. The sludge treated with a combination of polymers A and B consisted of primary sludge treated with 2.0 kg of polymer A and then 3.0 kg of polymer B per ton of sludge, based on the dry solids weight of polymer A solution and polymer B solution and sludge. Polymer A was a medium molecular weight cationic polyacrylamide with a charge of 20 mol% and a standard viscosity of 3.5 mPa·s. Polymer B was a high molecular weight cationic polyacrylamide with a charge of 40 mol% and a standard viscosity of 4.15 mPa·s.

[0081] The effect of sludge addition on the folding endurance (an indicator of strength) of handsheets was measured according to ISO 5626:1993 Paper-Determination of folding endurance. Table 2 below shows the test results obtained from the addition of 50 kg of treated sludge and untreated sludge per ton. All weights are dry solid weights based on the dry solid weight of the paper stock (t=1000 kg).

[0082] Table 2: Folding strength

[0083]

[0084] Reference 1 shows the folding strength of paper without sludge, i.e. the paper is made of pulp only. Reference 2 shows the folding strength of paper containing untreated sludge. Examples 3, 4 and 5 show the folding strength of paper containing polymer-treated sludge according to the present invention. Comparison of Reference 1 and Reference 2 shows that the addition of sludge to pulp has an adverse effect on the strength of paper made from the resulting pulp. Treating the sludge with a single polymer and then adding the treated sludge to the pulp can reduce the loss in folding strength of the resulting paper. The combined use of two polymers in the sludge treatment further reduces the loss in paper strength compared to the treatment using only one polymer.

[0085] Example 2: Product Strength 2

[0086] In this example, the same sludge as above was used and treated in the same manner. The resulting pulp was prepared into hand sheets. The effect of sludge addition on the bursting index (another indicator of strength) of the hand sheets was measured according to ISO 2759:2001 Board-Determination of bursting strength. As shown in Table 3 below, treating the sludge with a single polymer and then incorporating the treated sludge into the pulp reduced the bursting strength loss of the resulting paper compared to the paper treated with the raw sludge.

[0087] Table 3: Rupture Index

[0088]

[0089] Example 3: Drainage during papermaking

[0090] The effect of primary sludge on drainage on a paper machine was investigated using a dynamic drainage analyzer (DDA).

[0091] 100 g of sludge was provided in a beaker. The sludge was rapidly mixed using a 317 type agitator / timer (Triton). The first polymer was added to the sludge and then mixed for 10 seconds. After the second polymer was added, the sludge was rapidly mixed again for 30 seconds. The polymer flocculated the solids in the sludge and provided a conditioned sludge.

[0092] The sludge was primary sludge from a paper mill WWTP that mainly used OCC as raw material to produce duplex board and kraft board. The primary sludge used in the experiment had a pH of 6.7, a solid content of 3.9 wt%, and an ash content (ash in solid) of 60.8 wt%.

[0093] As described in detail in Example 1, pulp was prepared from OCC liner. About 304 g of pulp was provided to the reaction vessel of DDA. The consistency of the pulp was about 1.25 wt%. The pulp was subjected to rapid mixing at about 500 rpm. Then about 18 g of the above-mentioned regulated sludge (i.e., sludge treated with polymer) was transferred to the reaction vessel according to the sludge recycling rate (50 kg dry sludge / ton of dry pulp). The pulp was rapidly mixed with the sludge for another 20 seconds, and then about 475 g of water was added to the reaction vessel (making the total volume of the paper stock in the reaction vessel about 800 g). Then the diluted paper stock was subjected to rapid mixing at about 1000 rpm for another 15 seconds. Then the paper stock containing sludge was treated with a retention procedure (retention polymer was added in an amount of 0.25 kg / t, rapid mixing for 5 seconds, and then silica sol was added in an amount of 0.4 kg / t, and rapid mixing for another 10 seconds). Then the paper stock in the reaction vessel was discharged through a 0.25 mm net under a vacuum of about 200 mPa. After drainage is completed, record the drainage time (the value automatically calculated and displayed).

[0094] The results are shown in Table 4.

[0095] Table 4: Drainage time

[0096]

[0097] Reference 11 shows the results without any sludge addition. Reference 12 shows that the addition of untreated sludge worsened the drainage. Examples 13-15 clearly show a reduction in drainage time compared to the reference sample, indicating that the treatment of the sludge by a single polymer leads to much better drainage. In the sludge treatment, the combined use of two polymers significantly improves drainage compared to treatment with one polymer. Surprisingly, all the results obtained with the conditioned sludge according to the present invention (polymer A, polymer B and a combination of polymers A and B) provide better drainage, i.e. shorter drainage time, than reference sample 11 without any sludge addition.

[0098] Example 4: Ash Retention in Papermaking

[0099] The effect of primary sludge on paper machine ash retention was investigated using a dynamic drainage jug (DDJ). The DDJ is designed to simulate shear forces and analyze the dynamic retention properties of pulp.

[0100] About 190 g of pulp (prepared as described in Example 1) was provided to the DDJ container. The pulp was subjected to stirring at 500 rpm. About 10 g of conditioned sludge (the same sludge treated in the same manner as in Example 3) was subsequently transferred to the container according to the sludge reuse rate (50 kg dry sludge / ton of dry pulp). The pulp was stirred with the sludge for another 20 seconds, and then about 300 g of water was added to the reaction vessel (making the total volume of the paper stock in the container about 500 g). The diluted paper stock was then subjected to rapid mixing at about 1000 rpm for another 15 seconds. The paper stock containing the sludge was then treated with a retention procedure (retention polymer was added in an amount of 0.25 kg / t, rapid mixing for 5 seconds, and then silica sol was added in an amount of 0.4 kg / t, and rapid mixing for another 10 seconds). The paper stock in the container was then discharged under gravity through a 60 mesh net. The first 100 g of filtrate was collected to determine the ash concentration. The ash in the pulp before DDJ filtration was also measured. The ash retention rate was calculated by (ash concentration in pulp - ash concentration in wire water) / ash concentration in pulp. The results are provided in Table 5.

[0101] Table 5: Ash retention rate

[0102]

[0103] Sludge contains many small particles, inorganic ash and fines, which are difficult to retain. After adding untreated sludge, a small decrease in ash retention was observed. The above data show that treating the sludge with polymers significantly increased the ash retention, while this phenomenon was not observed in the absence of treatment. Compared with the treatment using one polymer alone, the combined treatment with two polymers further improved the ash retention. As in Example 3, all results obtained with the conditioned sludge according to the present invention (polymer A, polymer B and the combination of polymers A and B) provided higher ash retention than reference sample 16 in which no sludge was added to the papermaking process.

[0104] Various embodiments have been presented.It should be understood that in this document, the words include, comprise, and contain are each used as open-ended expressions and are not intended to be exclusive.

[0105] The foregoing description has provided, by way of non-limiting examples of specific implementations and embodiments, a complete and informative description of the best mode presently contemplated by the inventor for carrying out the invention. However, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing embodiments, but may be implemented in other embodiments using equivalent means or in different combinations of embodiments without departing from the features of the invention.

[0106] In addition, some features of the exemplary embodiments disclosed above can be advantageously used without corresponding use of other features. Therefore, the above description should be considered as merely an illustration of the principle of the present invention, rather than a limitation thereof. Therefore, the scope of the present invention is limited only by the attached patent claims.

Claims

1. A method comprising the following steps: Obtain sludge from paper or board mills; The sludge is treated by adding at least a first cationic polymer to obtain a conditioned sludge.

2. The method according to claim 1, wherein the charge of the first cationic polymer is 10-50 mol-%, preferably 20-40 mol-%, more preferably 15-30 mol-%.

3. The method according to any one of the preceding claims, wherein the first cationic polymer has a viscosity of 2-5.5 mPa·s at 25°C, preferably measured using a Brookfield DVIIT viscometer equipped with a UL adapter.

4. The method according to any one of the preceding claims, wherein the first cationic polymer is selected from polyamines and / or polyacrylamides (PAMs), preferably polyacrylamides.

5. The method of claim 4, wherein the first cationic polymer comprises a monomer selected from the group consisting of 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethylsulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulfate, [3-(acrylamido)propyl]trimethylammonium chloride (APTAC), or [3-(methacrylamido)propyl]trimethylammonium chloride (MAPTAC).

6. The method of claim 4, wherein the first cationic polymer is selected from polyamines comprising dimethylamine and epichlorohydrin monomers.

7. The method according to any one of the preceding claims, wherein the first cationic polymer is used in the treatment step in an amount of 0.5-10 kg / t, more preferably 1-5 kg / t dry solids of sludge.

8. The method according to any one of the preceding claims, wherein the treatment comprises adding at least one second cationic polymer.

9. The method according to claim 8, wherein the first and second cationic polymers are added sequentially, preferably in the order of 1) the first cationic polymer first and then 2) the second cationic polymer.

10. The method according to claim 8 or 9, wherein the second cationic polymer has a viscosity of 3-5.5 mPa·s at 25°C, preferably measured using a Brookfield DVIIT viscometer equipped with a UL adapter.

11. The method of any one of claims 8, 9 or 10, wherein the molecular weight of the second cationic polymer is higher than the molecular weight of the first cationic polymer.

12. The method of any one of claims 8-11, wherein the charge of the second cationic polymer is higher than the charge of the first cationic polymer.

13. The method according to any one of claims 8 to 12, wherein the second cationic polymer is selected from cationic polyacrylamides.

14. The method of any one of claims 8-13, wherein the second cationic polymer comprises a monomer selected from the group consisting of 2-(dimethylamino)ethyl acrylate (ADAM), [2-(acryloyloxy)ethyl]trimethylammonium chloride (ADAM-Cl), 2-(dimethylamino)ethyl acrylate benzyl chloride, 2-(dimethylamino)ethyl acrylate dimethylsulfate, 2-dimethylaminoethyl methacrylate (MADAM), [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MADAM-Cl), 2-dimethylaminoethyl methacrylate dimethylsulfate, [3-(acrylamido)propyl]trimethylammonium chloride (APTAC), or [3-(methacrylamido)propyl]trimethylammonium chloride (MAPTAC).

15. The method according to any one of claims 8 to 14, wherein the amount of the second cationic polymer used in the treatment step is 0.5-7 kg / t, preferably 1-5 kg / t dry solids of sludge.

16. The method according to any one of the preceding claims, wherein the conditioned sludge is separated using at least one or more separation techniques for heterogeneous mixtures, preferably one or more of filtration, centrifugation, sedimentation, screening and / or decantation, preferably screening and / or centrifugation.

17. The method according to any one of the preceding claims, wherein the paper or board mill sludge is treated with a biocide before being treated with at least one cationic polymer.

18. The method according to any one of the preceding claims, wherein the conditioning sludge is reused in a paper or board mill, wherein the method further comprises adding the conditioning sludge to paper pulp.

19. The method according to claim 18, wherein the conditioned sludge is added to the pulp at at least one location in a paper or board mill selected from a pulper, a discharge tower, a mixing tank, a machine chest and / or a flushing pump, preferably to a mixing tank or a flushing pump, most preferably to a mixing tank.

20. Use of conditioned sludge in a paper or board mill, wherein the conditioned sludge is obtained by the method according to any one of claims 1 to 19.

21. The use according to claim 20, wherein the conditioned sludge is added to the pulp at at least one location in a paper mill or board mill, the location being selected from a pulper, a discharge tower, a mixing tank, a machine chest and / or a flushing pump, preferably added to a mixing tank or a flushing pump, more preferably added to a mixing tank.

22. Use according to claim 21, wherein the conditioning sludge is added to the pulp in an amount of 1-10%-wt, preferably 2-8%-wt, more preferably 3-6%-wt, such as 5%-wt, based on the total pulp weight.

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