WO2023236185A1

WO2023236185A1 - Method for treating paper or board mill sludge for reuse

Info

Publication number: WO2023236185A1
Application number: PCT/CN2022/098119
Authority: WO
Inventors: Peng Zhou; Sari KRAPU; Suhua WU
Original assignee: Kemira Oyj; Kemira (Asia) Co. Ltd.
Priority date: 2022-06-10
Filing date: 2022-06-10
Publication date: 2023-12-14

Abstract

Here is provided method relating to treating waste, more specifically sludges removed from paper or board production, such as recycled fibre (RCF) production. The disclosure relates to a method for treating said sludge in order to improve reusability thereof, particularly, as conditioned sludge in said paper or board production processes. Herein is also provided uses in a paper or board mill of said conditioned sludge obtained by said method.

Description

METHOD FOR TREATING PAPER OR BOARD MILL SLUDGE FOR REUSE

TECHNICAL FIELD

The present disclosure generally relates to treating sludges removed from paper or board production, such as recycled fibre (RCF) production. The disclosure relates a method for treating said sludge in order to improve reusability thereof, particularly, though not exclusively as conditioned sludge in said paper or board production processes. Herein is also provided uses in a paper or board mill of said conditioned sludge obtained by said method.

BACKGROUND

Papermaking processes involve large amount of water and aqueous media. Water is employed as carrier and matrix to aid forming the pulp into continuously moving wet mat of fibre. The modern processes aim at minimizing both fresh-water consumption and production of aqueous effluents, and wastewaters to be treated i.e. sludges. However, aqueous effluents are constantly removed from paper or board production. With said aqueous effluents, a variety water of soluble and/or water immersible substances and solids are removed as well.

Substances and solids not retained in the cellulosic fibre web comprise organic, inorganic and microbiological material. They originate from the fibre source, either virgin or recycled, from additives fed to the process and from the process itself, namely side reactions and microbial growth in this nutrient rich aqueous environment. They are not bound to the cellulosic fibre web mainly because of their size, but also due to their shape or charge.

Sludge disposal induces costs and environmental burden. Even though some of these substances and solids would be valuable if returned to the process, reusing aqueous effluents or sludges as such has been proven problematic.

Recycling untreated sludge back to the papermaking may lead to technical challenges in the machine runnability, such as to deteriorated drainage and retention, bringing bacteria (back) to the system and/or worsening of wet end. Inorganic micro-particles and fines typically cause poor drainage leading to speed decline, vacuum load and vapor consumption rise. Further, when micro-particles and fines cannot retain in the cellulosic fibre web, the system becomes dirty.

Further, the finished paper/board product obtained thereof may have deteriorated quality, such as weaker strength caused by inorganic content from the recycled sludge impeding the inter-fibre bonding. In the products, this can also be seen as dusting and fluffing and as need for more dry strength agent.

The recycle processes for paper and board fibres have been in use for decades. Paper and board waste, often referred to as wastepaper/wasteboard, recovered (waste and scarp) paper/paperboard, or paper/board for recycling, are efficiently collected from industry, offices, wholesale, retailers, and consumers, and returned to paper and board mills for reuse through repulping. However, the supply and demand do not always meet. After the implementation of import ban on solid waste in 2017 in China, annual import quota for wastepaper companies had been reduced gradually until it was eliminated by 2021. There has been a huge shortage of raw material for RCF containerboard since then. There is a need to minimise or even eliminate the disadvantages existing in the prior art.

There is a need to reduce wastewater/solids and treatment thereof in a paper or board mills.

There is a need to recover for recycling any solids not retained in the paper or board product and which end up in aqueous effluents.

There is a need to reduce the need for wastepaper/wasteboard supply and replace it with solids recoverable form paper and board mill sludges.

SUMMARY

The appended claims define the scope of protection. Any examples and technical descriptions of processes and/or methods in the description and/or drawings not covered by the claims are presented not as embodiments of the invention but as background art or examples useful for understanding the invention.

According to a first example aspect there is provided a method for producing conditioned sludge. The method comprises obtaining a sludge from a paper or board mill and treating said sludge with addition of at least a first cationic polymer to obtain a conditioned sludge.

The present method has shown to provide advantages for both the paper and/or board-making process, and additionally to the wastewater treatment. Surprisingly, solid material from the sludges can be recovered, and recycled back to the papermaking processes as flocs in size and shape suitable to be retained in the cellulosic fibre webs. The embodiments of the method have proven to provide further advantages for the papermaking process and products thereof.

According to a second example aspect there is provided a use of conditioned sludge obtained by a method of the present invention in a paper or board mill. The use provides advantages for both the papermaking process and products thereof, as well as to the wastewater treatment.

Different non-binding example aspects and embodiments have been illustrated in the foregoing. The embodiments in the foregoing are used merely to explain selected aspects or steps that may be utilized in different implementations. Some embodiments may be presented only with reference to certain example aspects. It should be appreciated that corresponding embodiments may apply to other example aspects as well.

BRIEF DESCRIPTION OF THE FIGURES

Some example embodiments will be described with reference to the accompanying figures, in which:

Fig. 1 schematically shows schematically a conventional process;

Fig. 2 schematically shows schematically a process according to an example embodiment;

Fig. 3 schematically shows schematically a process according to another example embodiment.

DETAILED DESCRIPTION

In the following description, like reference signs denote like elements or steps.

Generally, as used herein, “papermaking” refers to both papermaking and boardmaking. By analogy, paper mill, paper machine, paper product, and like, refer to both paper and board equally. In the context of the present disclosure, the sludge from a paper or board mill refers to aqueous medium comprising solids removed from the paper-making process to aqueous effluents. The water content is preferably decreased to limit the sludge volume. However, keeping the sludge as slurry provides easier handling and transport thereof.

As used herein, the term “primary sludge” refers to sludge obtained from primary wastewater treatment of the paper or board mill aqueous effluents. They follow typical wastewater treatments, where the solid material is removed in the primary phase. Typical process steps comprise settling, sedimentation and/or air flotation. The solids are recovered to be treated according to the present process and aqueous phase further treated in a secondary treatment. As used herein, “ash” refers to inorganic particles, the main component of primary sludge. Ash comes from recycled wastepaper, virgin fibre and fillers, such as ground calcium carbonate, precipitated calcium carbonate and/or clay.

In wastewater treatment, the secondary treatment comprises biotreatment, referring to decreasing the waste load by microbes consuming mostly carbohydrates still contained in the aqueous phase recovered from the primary treatment. The solids, i.e. the “secondary sludge” recovered from the secondary treatment comprise mostly microbial mass, typically bacteria.

The conditioned sludge as used herein comprises solid flocs formed by the cationic polymer (s) with solids in the sludge during the treatment of the present disclosure. It may be in the form of heterogenous mixture of aqueous medium with said flocs, or it may be more or less drained (i.e. at least part of the aqueous medium reduced or removed) in the form of moist or wet flocs. According to a specific embodiment where the conditioned sludge is subjected to separation, the conditioned sludge may be in a form of a thin or thick slurry, or as moist solids, where the water content is less than 20 %-wt.

In the present context the expression “flocculate” means that the solids present in the aqueous phase of the pulp interact with the cationic polymer forming “flocs” or “bundles” . The interaction may be based on physical entanglement of the solids and the polymer structure, wherein the solids are “trapped” or “caught” by the polymer structure, and/or the interaction may be based on chemical interactions, where the solids and/or fibres may be bound to cationic polymers e.g. by electrostatic forces. The association of the solids and/or fibres to flocs, elicited by the cationic polymer, makes it possible for the cellulosic fibres to carry the flocs forward in the manufacturing process, inhibit removal of solids from the process with the aqueous phase and finally enable its retention to the final web to be formed.

Without being bound to a theory, it is believed that the addition of a cationic polymer to sludge, at least some micro-stickies are entrapped into flocs and retained in the cellulosic fibre webs when returned to papermaking process. Further, loss of strength is reduced due to flocs retaining from the sludge at least some of the inorganic material, which would otherwise interrupt bonding between fibres.

In the present context it is understood that the cellulosic fibres in the pulp may originally be produced by any suitable pulping method, i.e. they may originate from chemical pulping, mechanical pulping or chemi-mechanical pulping. The cellulosic fibres may usually be wood-based fibres, but it is possible that at least some of them are non-wood-based fibres, e.g. cellulosic fibres originating from annual plants. The pulp according to the present invention usually comprises a significant amount of recycled fibres or fibres originating from broke. The degree of recycling of the cellulosic fibres may be high, so that the cellulosic fibres of the pulp comprise at least 50 weight-%, preferably at least 70 weight-%, more preferably 100 weight-%, of recycled fibre material, calculated from total amount of fibres, as dry. According to a preferred embodiment, the recycled cellulosic fibre material comprises at least 40 weight-%, preferably at least 50 weight-%, of fibre material originating from old corrugated containerboard (OCC) .

In the present method, a paper or board mill sludge is treated with at least one cationic polymer to improve the reuse of said sludge as conditioned sludge.

At the simplest, the method can be defined to comprise obtaining a sludge from a paper or board mill and treating said sludge with addition of at least a first cationic polymer to obtain a conditioned sludge.

The present experiments have shown the method being specifically effective for treating primary sludge and rendering it suitable for reuse in a paper or board mill. Hence, according to a preferred embodiment the sludge from a paper or board mill comprises primary sludge, or preferably, it is a primary sludge from a paper or board mill.

The primary sludge contains fibre, fine fibre, ash, paper-making chemicals, water-soluble residues, and other matter not attached to the fibre structure of the paper or board product formed. With the present treatment with at least one cationic polymer, said fines/ash/fibres are flocculated or aggregated to larger bundles which may be retained in the paper structure when the conditioned sludge is returned to the paper or board manufacturing process. An example of a primary sludge composition may be described by characteristics compiled to table 1.

Table 1. An example of a primary sludge composition.

However, the present method is suitable for treating secondary sludges as well. According to a specific embodiment, the present method is suitable for treating combined primary and secondary sludges.

According to a specific embodiment, a paper or board mill sludge is treated with a biocide prior to treating with at least one cationic polymer. Treating with a biocide inactivates or inhibits any enzymes present in the sludge, eliminates microbes or both. Prevention of microbial growth is important to avoid slime and/or biofilm formation. Microbial organisms may further negatively affect both the functioning of the chemistry of papermaking and/or the quality of the end product. For example, microbial organisms may produce organic acids, which lower the pH of the process that may successively lead to dissolution of calcium compounds and increased risk for formation of deposits. Presence of microorganisms may also lead to formation of large microbiological deposits, which spoil the quality of the final product and may cause runnability problems. Abundant growth of microorganisms in the paper or board production may further cause severe odour problems.

Furthermore, there may be a significant loss in yield of the manufacturing process when cellulose, hemicellulose or any other polysaccharides present in the pulp is destroyed by microbes. Loss thereof must be compensated by equivalent addition of fibre material, in order to maintain the same solids content for the pulp. The loss of polysaccharides may also decrease the strength of the produced paper or board, which has to be compensated by supplementary addition of other strength chemicals.

The present inventors have found that the dosage of the cationic polymer (s) may be surprisingly low in relation to the dry solids present in the sludges. Accordingly, in an embodiment of the present method and use, the dosage in the treating step of the first cationic polymer is 0.5 –10 kg/t of sludge dry solids, preferably 1 –5 kg/t of sludge dry solids, where t refers to ton, hence 1000 kg of sludge dry solids. Where the treatment comprises a combination of first and second cationic polymers, the dosage of each cationic polymer may be relatively lower than that of embodiments where the first cationic polymer is used alone.

The cationic polymer may be added in liquid form or in dry form, preferably in dry form as a particulate material. If the cationic polymer is in dry form, it is dissolved before its addition to the pulp. Irrespective if the polymer is in liquid form or dry form, it is usually diluted with water to a suitable dosing concentration before addition to the pulp.

Any cationic polymer used in the present invention may be obtained by any suitable polymerisation method for copolymerisation. A cationic polymer may be obtained by suspension polymerisation, such as solution polymerisation or gel polymerisation; dispersion polymerisation; or emulsion polymerisation. Preferably a cationic polymer is obtained by solution polymerisation or gel polymerisation.

The cationic polymer (s) may be obtained by copolymerisation comprising one or more cationic monomers. The cationic polymer is obtained by copolymerisation of at least 10 mol-%, preferably at least 15 mol-%, more preferably at least 20 mol-%of solely cationic monomer (s) . For example, the cationic polymer may be obtained by copolymerising 10 –50 mol-%, preferably 20 –40 mol-%, more preferably 15 –30 mol-%of cationic monomer (s) with another preferably neutral charge monomer (s) . Accordingly, the first cationic polymer has a charge from 10 to 50 mol-%, preferably from 20 to 40 mol-%, more preferably from 15 to 30 mol-%. Herein, 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 monomers is present in the polymerisation, the obtained cationic polymer is provided with good ability to associate with the solid material present in the sludge, e.g. through electrostatic forces, and at the same time its ability to interact also with the anionically charged cellulosic fine fibres present in the sludge is improved.

In some embodiments the cationic polymer may be obtained by copolymerization of a polyamide or acrylamide, cationic monomers and <1 mol-%, preferably <0.5 mol-%, more preferably <0.1 mol-%of anionic monomers. According to one preferred embodiment, the cationic copolymer is free of anionically charged structural units, i.e. the copolymerisation is performed in the absence of anionic monomers. The polymer thus preferably consists of structural units that originate from non-ionic monomers, i.e. amide (s) or acrylamide (s) , and from cationic monomers. Even in that case a minor amount of anionically charged groups may be formed to the polymer structure during polymer preparation, e.g. during drying.

According to an embodiment, the first cationic polymer has a standard viscosity at 25℃ from 2 to 5.5 mPa·s, preferably measured using Brookfield DVII T viscometer with UL adapter. Standard viscosity is measured at 0.1 weight-%polymer content in an aqueous 1 M NaCl solution, using Brookfield DVII T viscometer equipped with UL adapter, at 25 ℃. In general, the standard viscosity of the polymer gives an indication of the length and/or weight of the polymer chains of the polymer. It has been observed that when the standard viscosity SV of the cationic polymer is at from 2 to 5.5 mPa·s, the polymer is able to effectively associate with the solids present in the sludge (s) . It is assumed that the cationic polymer has an improved ability to tie anionic fine fibres to bundles or flocs large enough to be retained in the cellulosic fibre structure when recycled and added to the pulp.

According to an embodiment the first cationic polymer is selected from polyamines and/or polyacryl amides (PAMs) . Polyacryl amides are preferred polymers for the first cationic polymer.

By definition, polymers are formed of repeating units of monomers, and hence polymers may be defined by the monomers which through polymerisation or copolymerisation yield said polymers. According to one embodiment of the invention the cationic monomer (s) may be selected from 2- (dimethylamino) ethyl acrylate (ADAM) , [2- (acryloyloxy) ethyl] trimethylammonium chloride (ADAM-Cl) , 2- (dimethylamino) ethyl acrylate benzylchloride, 2- (dimethylamino) ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM) , [2- (methacryloyloxy) ethyl] trimethylammonium chloride (MADAM-Cl) , 2-dimethylaminoethyl methacrylate dimethylsulphate, [3- (acrylamido) propyl] trimethylammonium chloride (APTAC) , or [3- (methacrylamido) propyl] trimethylammonium chloride (MAPTAC) . Preferably the cationic monomer (s) may be selected from 2- (dimethylamino) ethyl acrylate (ADAM) , [2- (acryloyloxy) ethyl] trimethylammonium chloride (ADAM-Cl) , and [3- (acrylamido) propyl] trimethylammonium chloride (APTAC) .

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

According to a preferred embodiment, the treating in the present method and use comprises the addition of the first cationic polymer, an addition of at least one second cationic polymer. Including two cationic polymer additions to the method, the treatment is easier to control and adapt to specific characteristics of the sludge at hand. For example, viscosities, charges, and dosages of first and second cationic polymers can be chosen to provide desired flocculation.

The present inventors have found most beneficial to add the first and the second cationic polymer in sequence. The preferred order for additions is adding 1) the first cationic polymer and 2) the second cationic polymer. This procedure allows the first cationic polymer to level the charges for a period of time, before the solids are flocculated with the addition of the second cationic polymer.

It has been found beneficial that the second cationic polymer has a standard viscosity at 25℃ from 3 to 5.5 mPa·s, preferably measured like the first cationic polymer, i.e. using Brookfield DVII T viscometer with UL adapter. Compared to the first cationic polymer the second cationic polymer has preferably a higher standard viscosity. This applies accordingly to the molecular weight. Hence, according to a preferred embodiment, the second cationic polymer has a molecular weight higher than that of the first cationic polymer.

According to an embodiment, the second cationic polymer has a charge which is higher than that of said first cationic polymer. This has been found experimentally to provide controlled flocculation and optimised cationic polymer dosages. The dosage of the second cationic polymer in the treating step may be at least 0.5 kg/t of sludge dry solids, such as 0.5 –7 kg/t, preferably 1 –5 kg/t of sludge dry solids.

As per chemistry, the second cationic polymer may be selected from cationic polyacrylamides. More specifically, the second cationic polymer may comprise monomers selected from 2- (dimethylamino) ethyl acrylate (ADAM) , [2- (acryloyloxy) ethyl] trimethylammonium chloride (ADAM-Cl) , 2- (dimethylamino) ethyl acrylate benzylchloride, 2- (dimethylamino) ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM) , [2- (methacryloyloxy) ethyl] trimethylammonium chloride (MADAM-Cl) , 2-dimethylaminoethyl methacrylate dimethylsulphate, [3- (acrylamido) propyl] trimethylammonium chloride (APTAC) , or [3- (methacrylamido) propyl] trimethylammonium chloride (MAPTAC) .

For example, the first and second cationic polymers may both be copolymerised from a polyacrylamide monomer and a cationic monomer with proviso that the amount of said cationic monomers is higher in the second cationic polymers relative to the amount of said cationic monomers in the first cationic polymer.

The conditioned sludge obtained from the treatment according to the present method, may be recycled back to the papermaking process. The conditioned sludge may be added as such, or alternatively it may be subjected to a further physical or chemical process step. Adding the conditioned sludge as such, without further process steps is useful as it requires no specific separation equipment nor maintenance thereof. Further, slurries are conveniently mixed to the pulp at the site of addition.

A typical physical step is separation. Accordingly, the conditioned sludge may subjected to a separation using at least one or more separation techniques for heterogenous mixtures. Separation method (s) for the conditioned sludge may be selected to be one or more of filtration, centrifugation, sedimentation, screening and/or decantation. Preferred separation methods comprise screening and/or centrifugation. The aim of the separation is to collect the solids from the conditioned sludge as a slurry. Preferably at least some of the aqueous phase is removed from the solids. With the aqueous phase, some chemicals and/or microbes may be discarded whereby they are not returned to the papermaking process.

From separation, the flocs comprising the cationic polymer (s) associated with solids may be recovered and recycled back to the papermaking, to fibre web forming. By separation, some chemicals and/or excess water may be removed. Especially, removing smallest particles and stickies with the excess water is beneficial.

The major advantage gained by separation is the reduction of the sludge volume. However, even though with the addition of the cationic polymer (s) the majority of solids flocculated, some small particles still remain suspended in the aqueous phase. These small particles will not be retained in the fibre web produced from the pulp and hence they do not contribute thereto. In embodiments, where at least a part of the aqueous phase is separated and discarded from the conditioned sludge, these small particles are removed and will not disturb the process to which the conditioned sludge is added. Another type of impurities which can be reduced by aqueous phase separation and discarding, are stickies which tend to stay dispersed or dissolved to the aqueous phase.

According to a preferred embodiment, the conditioned sludge may be reused in a paper or board mill, wherein the method further comprises adding the conditioned sludge to pulp. The conditioned sludge returns to the pulp some fine fibres, ashes and other solids removed with aqueous effluents but as said solids after treatment according to the present disclosure interact with the cationic polymer (s) , they behave in the pulp like other larger particles in the process, and hence are retained in the fibre web and contribute to the product formed thereof.

The site of the addition of the conditioned sludge may be selected in the present method or use. Accordingly, the conditioned sludge may be added to the pulp to at least one site in the paper or board mill may be selected from a pulper, a dump tower, a mixing chest, a machine chest, and/or a fan pump, preferably to a mixing chest or fan pump, most preferably to a mixing chest.

According to an embodiment, the site of addition of the conditioned sludge is at pulper. The conditioned sludge flows along with the pulp through typical process steps, such as screening, which is the most important step for removing big-size impurities. Thereby both macro-and micro-stickies are at least partly removed in the normal cleaning process. Any big-size impurities in sludge are mainly removed by the screening process or broken down in the mechanical steps and are thereby removed in the normal process. Hence, no separate purification of the conditioned sludge is needed. Nevertheless, as the polymers stay in the pulp any broken down flocs may be reformed in retention, again entrapping inorganic and fine solids.

As addition to pulper takes place at an early stage of the process, and other embodiments provide the addition at a later stage of the process, such as at a mixing chest or fan pump. With a later addition, mechanical stages are avoided which contributes to keeping the flocs formed by cationic polymer addition untangled and together. Compared to addition to the pulper, addition to mixing chest or fan pump provides improved control the sludge reuse for drainage, retention, and strength. It also helps to reduce energy consumption as recirculation of the sludge components in the re-pulping system needed in the early addition can be avoided.

A further advantage of the addition of conditioned sludge at a later stage of the process, such as to a mixing chest or fan pump, is easier increase of the sludge ratio in a controlled manner thereby leading to reduced raw material costs. Improved sludge retention obtainable through addition at a later stage, that is closer to paper or board production, is also desired. With addition of the conditioned sludge at a later stage, the pre-formed micro-flocs contribute to water removal in the wire section. In addition, the paper process equipment surfaces benefit from reduced deposition through good fixation and retention.

According to a specific embodiment, the conditioned sludge may be added to the pulp in a significant amount. The conditioned sludge may be added to the pulp in an amount of 1 – 10 %-wt, preferably 2 –8 %-wt, more preferably 3 –6 %-wt, such as 5 %-wt of the total pulp weight (dry weight/dry weight) . The present experiments have shown that surprisingly high proportion of pulp may be replaced with the conditioned sludge without disturbance in the process or deterioration of the end product quality. Compared to pulps where no sludge was used or untreated sludge was used, some product characteristics were even found to be improving with conditioned sludge produced according to the present method and use.

As another aspect of the present invention, herein is provided a use of conditioned sludge in a paper or board mill, wherein the conditioned sludge is obtained by a method as here described. The conditioned sludge may be used at least for replacing a part of pulp, for improving dewatering and retention, for improving runnability, for decreasing solid waste and decreasing chemical oxygen demand in effluent, for obtaining a cleaner effluent in a paper mill waste-water treatment, or for improving paper or board product strength. Preferably two or more of said effects are obtained by the present use simultaneously.

As a specific embodiment, the use of conditioned sludge takes place in a RCF process, wherein the conditioned sludge is obtained by a method as here described. The conditioned sludge may be used at least for for replacing a part of RCF pulp, for improving drainage time in RCF process, for improving dewatering and retention in RCF process, for improving runnability in RCF process, for decreasing emissions from RCF process, for decreasing solid waste from RCF process, for obtaining a cleaner effluent in a RCF board mill waste-water treatment, or for improving RCF board product strength. Preferably two or more of said effects are obtained by the present use simultaneously.

Some embodiments are described more closely in the following schematical non-limiting figures, where

Figure 1 shows a conventional preparation process without sludge additions.

Figure 2 shows a stock preparation process with possible sites for conditioned sludge additions according to embodiments of the present invention, where said addition takes place at early stages of the process; and

Figure 3 shows a stock preparation process where conditioned sludge additions are conducted according to embodiment (s) of the present invention, where said addition takes place at late stages of the process.

Figure 1 shows a conventional stock preparation process without sludge additions. Schematically the process may be presented beginning from a pulper 100. Pulper produces pulp from cellulose fibres with aid of water and chemicals. In case recycled material is used, it gently slushes the recycled paper or board raw material into a pumpable pulp. Water is fed to the pulper via line 401. The pulp is stored in dump tower 110. The next operations, screening 120 and fractionation 130, separate the fibres in the pulp by size, especially the fibre length. The long fibres are next filtered in disc filter 140a and the short fibres in disc filter 140b wherefrom aqueous effluent may be discarded. After heat disperger 150, the long fibres are stored in storage tower 160a (and short fibres in storage tower 160b) . Before the head box 200, the pulp from storage tower 160 a/b is mixed in mixing chest 170 with broke fed from broke storage tower 310 via line 301. The pulp from machine chest 180 is mixed with white water from wire section and pumped with a fan pump 190 to the head box 200. In figure 1, the steps after the head box, where the fibre mat is formed and pulp is drained are depicted as unit 300.

The black arrows in Figure 1 indicate the fibre stock flow through the stock preparation process and the dash lines indicate aqueous flows recycled within, stored in

water storage tanks

410 and 420, or removed from the stock preparation process to the wastewater treatment and/or sludge reuse via line 402.

As described, the conditioned sludge may be reused in a paper or board mill, wherein the method further comprises adding the conditioned sludge to the pulp. Embodiments, where the conditioned sludge 500 is added to the pulp in early stage (s) is depicted in Figure 2. According to an embodiment, the site of addition of the conditioned sludge is at pulper 100, via line 501, or at dump tower 110 via line 502 or a combination thereof. This embodiment enables both macro-and micro-stickies to be at least partly removed in the normal cleaning process (mainly at screening 120) . Thereby separate purification of the conditioned sludge is not needed.

Embodiments, where the conditioned sludge is added to the pulp in later stages of pulping are depicted in Figure 3. Other embodiments provide the addition at a later stage (s) of the process, such as at a mixing chest 170 via line 511, machine chest 180 via line 512, fan pump 190 via line 513, or any combination thereof. Even though the Figure 3 depicts addition to all these sites, according to different embodiments, one, two or more addition sites may be employed dependent on the conditioned sludge characteristics and/or process controls. Advantages related to later addition of the conditioned sludge include avoiding mechanical stages provision of improved control the sludge reuse for drainage, retention, and strength, and further reduction of energy consumption.

EXAMPLES

Example 1: Product strength 1

The effect of a primary sludge (from a papermill wastewater treatment plant, WWTP) has on a paper sheet product was tested. A paper mill that recycles primary sludge by adding it back into papermaking process was used in the investigation. This paper mill mainly uses OCC as raw material to produce low grade liner board. Primary sludge from the WWTP of this paper mill was collected for the purpose of forming hand-sheets. Pulp was prepared from an OCC liner board sample using a specific dilution water. Hand-sheets were formed using the pulp with and without the primary sludge being added to the stock. The hand-sheets were created in accordance with ISO 5269-2: 2004 Pulps –Preparation of laboratory sheets for physical testing –Part 2:

method.

The primary sludge had pH of 5.93, a solids content of 8.75 weight-%, ash content (ash in solids) of 60.8 weight-%. The dilution water was prepared consisting of 70 weight-%calcium acetate, 20 weight-%sodium sulphate, and 10 weight-%sodium bicarbonate. The conductivity was adjusted to 4 mS/cm and the pH to 7. The OCC liner board sample had a dry content of 94 weight-%, ash content of 15 weight-%, a grammage of 120 g/m ².2 cm x 2 cm sized pieces of the OCC board were soaked in 85℃ dilution water for 5 minutes. The pulp (2.5 weight-%) was hot-disintegrated with a Lorentzen &Wettre Pulp Disintegrator at 30000 rpm for 10 minutes, after which the pulp was diluted to 1.25 weight-%consistency. The pulp was left to cool down to room temperature, and the pH was adjusted to 7.

A beaker was provided with 100 g sludge. The sludge was subjected to rapid mixing using Type 317 Stirrer/Timer (Triton) . A polymer was added into the sludge, followed by mixing. In case of treating with a combination of polymer, they were added sequentially with a stirring in between. The polymer (s) flocculated solids in the sludge thereby providing a conditioned sludge.

Four sets of hand-sheets were created following standard method ISO 5269-2: 1998. One set was created from mixture that contained the pulp and 50 kg of the primary sludge (untreated) per ton of pulp, based on the dry solids weight of the stock and the sludge. One of the sets of hand-sheets was created from pulp that contained the pulp and 50 kg of polymer A-treated sludge per ton of pulp, based on the dry solids weight of the stock and the sludge. The polymer A-treated sludge contained 2.0 kg of polymer A per ton of sludge, based on the dry solids weight of the sludge and polymer A. Another set of hand- sheets was created from pulp that contained the pulp and 50 kg of polymer B-treated sludge per ton of pulp, based on the dry solids weight of the stock and the sludge. The polymer B-treated sludge contained 5 kg of polymer B per ton of sludge, based on the dry solids weight of the sludge and polymer B. The fourth set of hand-sheets was created from mixture containing the pulp and 50 kg of sludge treated with combination of polymers A and B-treated in sequence, based on the dry solids weight. The sludge treated with combination of polymers A and B consisted of the primary sludge treated with 2.0 kg of polymer A followed by treating with 3.0 kg of polymer B per ton of sludge, based on the dry solids weight of the polymer A and polymer B solutions and the sludge. The polymer A was a medium molecular weight cationic polyacrylamide having a charge of 20 mole-%, a standard viscosity of 3.5 mPa·s. The polymer B was a high molecular weight cationic polyacrylamide having a charge of 40 mole-%and a standard viscosity of 4.15 mPa·s.

The effect that the addition of sludge had on the folding endurance (an indicator of strength) of hand-sheets was measured according to ISO 5626: 1993 Paper –Determination of folding endurance. Table 2 below shows the tests results achieved from the 50 kg per ton additions of treated sludge and untreated sludge. All weights are dry solids weight based on the dry solids weight of the stock (t = 1000 kg) .

Table 2: Folding

Ref 1 shows folding of the sheets without sludge, i.e. prepared from the pulp only. Ref 2 shows folding of sheets containing the untreated sludge. Examples 3, 4 and 5 show folding of the sheets containing polymer-treated sludge according to the present invention. Comparison of Ref 1 and Ref 2 indicates that adding sludge to the pulp adversely affects the strength of paper sheets made from the resulting pulp. Treating the sludge with a single polymer prior to incorporating the treated sludge into the pulp, results in reduced folding strength loss of the resulting sheets. The combined use of two polymers in the sludge treatment further reduced the loss of sheet strength as compared with treatment with one polymer alone.

Example 2: Product strength 2

In this example, the same sludge as above was used and treated in the same way. The resulting pulps were prepared into handsheets. The effect that the addition of sludge had on the bursting index (another indicator of strength) of handsheets was measured according to ISO 2759: 2001 Board –Determination of bursting strength. As shown in Table 3 below, treating the sludge with single polymer prior to incorporating the treated sludge into the pulp, results in reduced bursting strength loss of the resulting sheets as compared with the sheets treated with raw sludge.

Table 3: Bursting index

Example 3: Drainage in papermaking process

The effect that primary sludge has on the drainage on a paper machine was examined using a Dynamic Drainage Analyzer (DDA) .

A beaker was provided with 100 g sludge. The sludge was subjected to rapid mixing using Type 317 Stirrer/Timer (Triton) . A first polymer was added into the sludge, followed by mixing for 10 seconds. After addition of a second polymer, the sludge was once again subjected to rapid mixing for 30 seconds. Polymers flocculated solids in the sludge, and conditioned sludge was provided.

The sludge was primary sludge from WWTP of a paper mill mainly using OCC as raw material to produce duplex board and kraft liner board. The primary sludge used in experiments had pH of 6.7, a solids content of 3.9 weight-%, ash content (ash in solids) of 60.8 weight-%.

A pulp was prepared from an OCC liner board as described in detail in example 1. The reaction vessel of a DDA was provided with about 304 g pulp. The consistency of the pulp is about 1.25 weight-%. The pulp was subjected to rapid mixing of about 500 rpm. About 18 g of the above conditioned sludge (i.e. the sludge treated with polymers) was then transferred into the reaction vessel according to the sludge reuse ratio (50 kg dry sludge/ton dry pulp) . The pulp together with the sludge was subjected to rapid mixing for another 20 seconds followed by addition of about 475 g water into the reaction vessel (to make a total volume of the stock in the reaction vessel about 800 g) . The diluted stock was then subjected to rapid mixing of about 1000 rpm for another 15 seconds. The stock containing the sludge was then treated with retention program (adding retention polymer in an amount of 0.25 kg/t, rapid mixing for 5 seconds, followed by addition of silica sol in an amount of 0.4 kg/t, rapid mixing for another 10 seconds. The stock in the reaction vessel was then drained through a 0.25 mm wire under vacuum of about 200 mPa. After the drainage was completed, the drainage time (avalue which is automatically computed and shown) was then recorded.

Results are provided in Table 4.

Table 4: Drainage time

Ref 11 shows the result without any sludge addition. Ref 12 showed that addition of untreated sludge had deteriorated the drainage. Examples 13-15 clearly show the decrease in drainage time compared to the reference sample, indicating that sludge treatment by a single polymer resulted in much better drainage. The combined use of two polymers in the sludge treatment significantly improved the drainage as compared with treatment with one polymer. Surprisingly, all results obtained with the conditioned sludges (Polymer A, Polymer B and combination of polymers A and B) according to the present invention provided better drainage, i.e. shorter drainage times than the reference sample 11 without any sludge addition.

Example 4: Ash retention in papermaking process

The effect that primary sludge has on the ash retention of a paper machine was examined using a Dynamic Drainage Jar (DDJ) . DDJ is designed to simulate the shear force and analyze the dynamic retention properties of pulp.

The DDJ container was provided with about 190 g pulp (prepared as described in Example 1) . The pulp was subjected to agitation at 500 rpm. About 10 g of the conditioned sludge (the same sludge treated in the same way as in Example 3) was then transferred into the container according to the sludge reuse ratio (50 kg dry sludge/ton dry pulp) . The pulp together with the sludge was subjected to agitation for another 20 seconds followed by addition of about 300 g water into the reaction vessel (to make a total volume of the stock in the container about 500 g) . The diluted stock was then subjected to rapid mixing of about 1000 rpm for another 15 seconds. The stock containing the sludge was then treated with retention program (adding retention polymer in an amount of 0.25 kg/t, rapid mixing for 5 seconds, followed by addition of silica sol in an amount of 0.4 kg/t, rapid mixing for another 10 seconds. The stock in the container was then drained through a wire of 60 mesh under gravity. The first 100 g of the filtrate was collected to determine the ash concentration. Ash in the pulp before DDJ filtration was also measured. The ash retention was calculated by (ash concentration of pulp-ash concentration of wire water) /ash concentration of pulp. Results are provided in Table 5.

Table 5: Ash retention

The sludge contains a lot of small particles, inorganic ash and fines, which are difficult to retain. Small decrease in ash retention was seen from the addition of untreated sludge. These data above illustrate that sludge treatment with polymer significantly increased the ash retention that is otherwise observed with no treatment. The combined treatment with two polymers further improved the ash retention as compared with treatment with one polymer alone. As in example 3, all results obtained with the conditioned sludges (Polymer A, Polymer B and combination of polymers A and B) according to the present invention provided higher ash retention than the reference sample 16 without any sludge addition to the papermaking process.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include, and contain are each used as open-ended expressions with no intended exclusivity.

The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented in the foregoing, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

Furthermore, some of the features of the afore-disclosed example embodiments may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

A method comprising:

obtaining a sludge from a paper or board mill;

treating said sludge with addition of at least a first cationic polymer to obtain a conditioned sludge.
The method of claim 1, wherein the first cationic polymer has a charge from 10 to 50 mol-%, preferably from 20 to 40 mol-%, more preferably from 15 to 30 mol-%.
The method of any of preceding claims, wherein the first cationic polymer has a viscosity at 25℃ from 2 to 5.5 mPa·s, preferably measured using Brookfield DVII T viscometer with UL adapter.
The method of any of preceding claims, wherein the first cationic polymer is selected from polyamines and/or polyacryl amides (PAMs) , preferably from polyacryl amides.
The method of claim 4, wherein the first cationic polymer comprises monomers selected form 2- (dimethylamino) ethyl acrylate (ADAM) , [2- (acryloyloxy) ethyl] trimethylammonium chloride (ADAM-Cl) , 2- (dimethylamino) ethyl acrylate benzylchloride, 2- (dimethylamino) ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM) , [2- (methacryloyloxy) ethyl] trimethylammonium chloride (MADAM-Cl) , 2-dimethylaminoethyl methacrylate dimethylsulphate, [3- (acrylamido) propyl] trimethylammonium chloride (APTAC) , or [3- (methacrylamido) propyl] trimethylammonium chloride (MAPTAC) .
The method of claim 4, wherein the first cationic polymer is selected form polyamines comprising dimethylamine and epichlorohydrin monomers.
The method of any of preceding claims, wherein the dosage in the treating step of the first cationic polymer is 0.5 –10 kg/t, preferably 1 –5 kg/t of sludge dry solids.
The method of any of preceding claims, wherein said treating comprises addition of at least one second cationic polymer.
The method of claim 8, wherein the first and the second cationic polymer are added in sequence, preferably in order 1) first cationic polymer and 2) second cationic polymer.
The method of claim 8 or 9, wherein the second cationic polymer has a viscosity at 25℃ from 3 to 5.5 mPa·s, preferably measured using Brookfield DVII T viscometer with UL adapter.
The method of one of claims 8, 9 or 10, wherein the second cationic polymer has a molecular weight higher than that of the first cationic polymer.
The method of one of claims 8 –11, wherein the second cationic polymer has a charge which is higher than that of said first cationic polymer.
The method of one of claims 8 –12, wherein the second cationic polymer is selected from cationic polyacrylamides.
The method of one of claims 8 –13, wherein the second cationic polymer comprises monomers selected from 2- (dimethylamino) ethyl acrylate (ADAM) , [2- (acryloyloxy) ethyl] trimethylammonium chloride (ADAM-Cl) , 2- (dimethylamino) ethyl acrylate benzylchloride, 2- (dimethylamino) ethyl acrylate dimethylsulphate, 2-dimethylaminoethyl methacrylate (MADAM) , [2- (methacryloyloxy) ethyl] trimethylammonium chloride (MADAM-Cl) , 2-dimethylaminoethyl methacrylate dimethylsulphate, [3- (acrylamido) propyl] trimethylammonium chloride (APTAC) , or [3- (methacrylamido) propyl] trimethylammonium chloride (MAPTAC) .
The method of one of claims 8 –14, wherein the dosage of the second cationic polymer in the treating step is 0.5 –7 kg/t, preferably 1 –5 kg/t of sludge dry solids.
The method of any of preceding claims, wherein the conditioned sludge is subjected to a separation using at least one or more separation techniques for heterogenous mixtures, preferably one or more of filtration, centrifugation, sedimentation, screening and/or decantation, preferably to screening and/or centrifugation.
The method of any of preceding claims, wherein said a paper or board mill sludge is treated with a biocide prior to treating with at least one cationic polymer.
The method of any of preceding claims, wherein the conditioned sludge is reused in a paper or board mill, wherein the method further comprises adding the conditioned sludge to pulp.
The method of claim 18, wherein the conditioned sludge is added to pulp to at least one site in the paper or board mill selected from a pulper, a dump tower, a mixing chest, a machine chest, and/or a fan pump, preferably to a mixing chest or fan pump, most preferably to a mixing chest.
Use of conditioned sludge in a paper or board mill, wherein the conditioned sludge is obtained by a method of any of claims 1 –19.
The use of claim 20, wherein the conditioned sludge is added to pulp in paper or board mill to at least one site in the paper or board mill selected from a pulper, a dump tower, a mixing chest, a machine chest, and/or a fan pump, preferably to a mixing chest or fan pump, most preferably to a mixing chest.
The use of claim 21, wherein the conditioned sludge is added to pulp in an amount of 1 –10 %-wt, preferably 2 –8 %-wt, more preferably 3 –6 %-wt, such as 5 %-wt of the total pulp weight.