WO2025016539A1

WO2025016539A1 - Process for producing mechanical wood pulp

Info

Publication number: WO2025016539A1
Application number: PCT/EP2023/069938
Authority: WO
Inventors: Sven Hildebrandt; Sabine Weiguny; Sabine FRISCHHUT; Stefan Erren; David Diminique SCHWEINFURTH; Ralf Boehling
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 2023-07-18
Filing date: 2023-07-18
Publication date: 2025-01-23
Anticipated expiration: 2026-01-18

Abstract

A process for producing mechanical wood pulp is provided, wherein the process comprises a step of grinding optionally pretreated wood chips in a refiner unit at an operating temperature of at least 155°C, a bleaching formulation is added prior to and/or during the grinding step, and the bleaching formulation comprises (A) a salt of the dithionous acid H₂S₂O₄, and (B) a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO^- group and a PO₃ ^- group.

Description

Process for producing mechanical wood pulp

Description

The present invention relates to a process for producing mechanical wood pulp in a refiner unit using a bleaching formulation, to a mechanical wood pulp, obtainable by said process, to said bleaching formulation and use thereof to increase the run-time of a refiner unit in a process of producing mechanical wood pulp and to the use of a salt comprising a polyanionic component as an anti-scaling agent in a process for producing mechanical wood pulp.

Background of the invention

Mechanical wood pulps and processes for manufacturing thereof are well-known. There are different processes in use, where wood is mechanically converted into pulp for the paper industry, usually by subjecting raw material in the form of wood chips to a mechanical, a chemical and/or thermal treatment, which may be combined and carried out either separately or at least partially simultaneously.

For example, thermomechanical pulp (TMP) is produced with thermal pretreatment in steam, followed by refining in a pressurized system. Thermo refiner mechanical pulp (TRMP) is a variation of thermomechanical pulp, wherein the chips are preheated under pressure, and refining is carried out at atmospheric pressure.

Chemimechanical pulp (CMP) is produced in a refiner, but the chips are pretreated with chemicals before refining, wherein the wood may be softened by dissolving some of the lignin. Chemi-thermomechanical pulp (CTMP) is similar to CMP, but the refining step is made under elevated pressures and less chemicals are used. Bleached chemi-thermomechanical pulp (BCTMP) also includes a bleaching step, after the refining step.

The purpose of refining is to modify the morphology and surface characteristics of the fibers to match the requirements of the papermaking process and the desired properties of the finished paper product. The process conditions in the refining process are usually very harsh. A temperature of up to 200°C or even higher and a pressure of up to 20 bar may occur within the refiner, especially between the segments of the refiner filling. Therefore, reducing energy consumption is an important parameter of the process.

Applying bleaching agents during refining may be of advantage in view of energy consumption as well as brightness characteristics, as described in WO 2015/144455 A1. Disclosed is a process for preparing bleached mechanical wood pulp comprising at least two refining steps of a ATMP variant (Advanced TMP by Andritz), including a single grinding disk refiner, wherein a composition of sodium dithionite and sodium carbonate is added to the second step applying higher refining conditions (2300 rpm / 5.2 bar) than the first step (1800 rpm / 2.4 bar). However, the use of dithionite as a bleaching agent usually leads to the formation of sulfite and sulfate under refiner conditions. Due to the presence of calcium ions in the water used in the papermaking process corresponding calcium salts (CaSC>4 and CaSCh, each including any crystal water-containing salt or adduct thereof) of low solubility are formed as deposits on the refiner equipment which cannot be removed under operational mode. Thus, the refiner equipment has to be cleaned, and/or segments have to be replaced on a regular basis.

In order to prevent such deposits and to mask Ca ions or other heavy metal cations which may cause discoloration, chelating agents, like ethylene diaminetetraacetic acid (EDTA), diethylenetriaminepentaacetic acid (DTPA), or other polyanionic components, like sodium tripolyphosphate, are suggested to be added.

WO 92/20855 A1 discloses a process for producing chemi-thermomechanical pulp by preheating wood chips pre-impregnated with a liquid of pH 6 containing 30 kg of sodium sulfite, 10 kg of BOROL (12% aqueous solution of sodium borohydride) and 3 kg of DTPA per tonne of chips at a temperature of 165°C for 1 minute, followed by refining while essentially maintaining pressure and temperature of the preheating step. The process is described to achieve less energy consumption and similar brightness, compared to conventional refining at 130°C.

WO 2008/129048 A1 discloses a process for producing bleached wood particles, wherein a bleaching composition of sodium dithionite, sodium sulfite, sodium carbonate and sodium tripolyphoshate was added to a refiner, followed by removing the wood particles and processing into pale to white wood-base materials.

However, chelating agents are required to be added in a stochiometric amount which is not desired, as many of the conventional chelating agents cannot easily be eliminated from the effluent streams under standard conditions; for example, DTPA is not biodegradable. Thus, reducing the amount of any additive preventing Ca-containing deposits within the refiner equipment may be of advantage.

DE 19950941 A1 discloses the use of a polymer of acrylic acid, polymerized in the presence of water/isopropanol, as an anti-scaling agent in a refiner at 120°C in an amount of 50 ppm by weight, based on the total weight of wood chips.

Accordingly, there is still a need to provide a technology that improves the process for preparing wood pulp at high refining temperatures with respect to economic aspects, like process efficiency and/or minimizing operational costs of the whole papermaking process.

It is therefore an object of the invention to provide a process for producing mechanical wood pulp which considers economical and/or environmental aspects. The process should improve the process efficiency at high refining temperature due to a longer run-time and, thus, less disruptions in the papermaking process, allow for less effluent contaminants, less cleaning efforts and less equipment costs, and allow for a targeted regulation of paper product. A further object is to provide a bleaching formulation to be suitably used to improve the process of producing mechanical wood pulp with respect to the economical and/or environmental aspects mentioned herein-before.

Summary of the invention

It has now been found that a process of preparing mechanical wood pulp may be improved in that the run-time of a refiner unit, especially of the segments of the refiner filling, may be increased by using a specific bleaching formulation.

Therefore, in a first aspect, the invention relates to a process for producing mechanical wood pulp, the process comprising a) grinding wood chips in a refiner unit at an operating temperature of at least 155°C, and b) adding a bleaching formulation to the wood chips prior to step a) and/or during step a), wherein the wood chips are optionally pretreated, and the bleaching formulation comprises

(A) a salt of the dithionous acid H2S2O4, and

(B) a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a POa' group.

In a further aspect, the invention relates to a mechanical wood pulp, obtainable or obtained by a process, as defined in any aspect herein.

In a further aspect, the invention relates to a bleaching formulation comprising, based on the total weight of the bleaching formulation,

(A) 60 to 95 wt% of a salt of the dithionous acid H2S2O4,

(B) 0.01 to 5 wt% of a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a POa' group, and

(C) 5 to 40 wt% of an additive selected from a basic salt (C-1), a salt of disulfurous acid (H2S2O5) (C-2), a salt of sulfurous acid (H2SO3) (C-3), a complexing agent (C-4), a surfactant (C-5) and any combination thereof, preferably selected from an additive (C-1), (C-3) and/or (C- 4).

In a further aspect, the invention relates to the use of a bleaching formulation, as defined in any aspect herein, to increase the run-time of a refiner unit in a process of producing mechanical wood pulp.

In a further aspect, the invention relates to the use of a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a POa' group, (B), as an antiscaling agent in a process of producing wood pulp.

Detailed description of the invention The term “refiner unit”, as used herein, means a device for mechanical treating optionally pretreated wood chips into pulp by grinding.

The pressure in bar, as used herein, means the pressure in bar absolute.

The term “ethylenically unsaturated monomer”, as used herein, means a non-aromatic monoethylenically unsaturated monomer, wherein the C=C double bond is susceptible to radical polymerization.

The term "(meth)acryl" or similar terms, as used herein, encompasses acryl, methacryl and a combination thereof.

The term “any combination thereof”, as used herein, means two or more combinations thereof, either different kinds of one constituent or one group, i.e. , different subgroups, or different kinds of a list of constituents or groups.

As used herein, the indefinite article “a” comprises the singular but also the plural, i.e., an indefinite article in respect to a component of a composition means that the component is a single compound or a plurality of compounds. If not stated otherwise, the indefinite article “a” and the expression “at least one” are used synonymously.

Figure 1 shows a schematic overview of a micro-plant used in the Examples.

Figure 2 shows a SEM image (scanning electron microscopy) of the precipitate, obtained by Comparative Example 1.

Figure 3 shows a SEM image of the precipitate, obtained in Example 2.

Figure 4 shows a SEM image of the precipitate, obtained in Comparative Example 3.

Figure 5 shows a SEM image of the precipitate, obtained in Example 8.

The instant process describes a mechanical refining step, wherein optionally pre-treated wood chips are ground at an operating temperature of at least 155°C, and a bleaching formulation is added to the wood chips prior to step a) and/or during step a).

The wood chips used in the instant process may be derived from any plant source. The plant source may be any fibrous plant, which may be subjected to mechanical pulping, such as trees, including hardwood fibrous trees (non-coniferous woods) and softwood fibrous trees (coniferous woods). Examples of hardwood fibrous trees include aspen, eucalyptus, maple, birch, walnut, acacia, beech, poplar and any combination thereof. Examples of softwood fibrous plants include spruce, pine, cedar, fir and any combination thereof. Generally, wood chips, preferably of debarked coniferous or non-coniferous wood, used in the instant grinding step a) have a size of about (15-50) mm x (15-50) mm x about (6-12) mm.

The suitable size of wood chips to be treated in the grinding step a) may be obtained by any known mechanical step, for example by comminuting and washing and one or more pretreatment steps generally known in the art, which may be conducted in any order.

The pretreatment steps may include one or more of the following steps, for example steps i) and ii) or steps i) and iii): i) treating the wood chips with one or more chemicals, like sodium hydrogensulfite (NaHSCh), sodium sulfite (Na2SOs), and/or water, ii) heating the wood chips, for example in saturated steam environment; iii) delaminating the wood chips by (iii-1) exposing the wood chips to mechanical pressure and/or shearing forces and (iii-2) grinding those in a refiner unit, operating at a temperature less than the temperature of step a), preferably < 150°C.

The delaminating step iii) usually converted the wood chips into modified wood chips, comprising typically lose bundles of fibers, which may have a longitudinal dimension of from 5 cm to 0.3 cm and which generally have a substantially enlarged surface area compared to the wood chips after comminuting.

Step (iii-1) may be carried out, for example, in a screw press, which is generally be used to dewater and simultaneously pre-fiberize the wood chips. Step (iii-2) may be carried out in a refining unit at a temperature less than the operating temperature of the grinding step a), preferably < 150°C. Typically, pressure and energy consumption of step (iii-2) are lower than the corresponding parameters for the instant grinding step a).

Thus, preferred is a process for producing mechanical wood pulp, wherein the wood chips are pretreated comprising one or more of the steps of i) treating the wood chips with one or more chemicals, like sodium hydrogensulfite (NaHSCh), sodium sulfite (Na2SOs), and/or water, ii) preheating the wood chips, for example in saturated steam environment; iii) delaminating the wood chips by (iii-1) exposing the wood chips to mechanical pressure and/or shearing forces and (iii-2) grinding those in a refiner unit, operating at a temperature less than the temperature of step a), preferably < 150°C.

The optionally pretreated wood chips may be introduced into an inlet of the refiner unit and processed at an operating temperature of at least 155°C, usually at an atmosphere of steam.

A variety of disk refiners or conical refiners are known in the field of mechanical refining. Details regarding the orientation of the opposed refining surfaces, and the pattern of bars, grooves or surface irregularities formed thereon may be selected according to known parameters. A refiner unit comprises typically a refiner filling of rotating and optionally fixed blades or preferably disks for grinding fibrous stock, and preferably consists of one or two metallic disks with a radial relief which are close together and form a gap therebetween. In a two-disk refiner, only one disk may turn or both disks turn, typically in that case in opposite directions. Usually, over-pressure is applied in the refiner unit.

The operating temperature in the refiner unit is at least 155°C, preferably at least 160°C or at least 170°C. Typically, the temperature in the refiner unit is at most 230°C. The operating temperature in the refiner unit is preferably of from 155 to 230°C, more preferably from 160 to 220°C.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the operating temperature is of from 155 to 230°C, preferably from 160 to 220°C.

The operating temperature in a refiner unit may be adjusted with a temperature control which usually corresponds to the internal temperature of the housing of the refiner unit. The operating temperature in a refiner unit, as used herein, means the adjusted internal temperature of the housing of the refiner unit.

The temperature which may occur within the refiner filling, especially within the refiner segments, may be determined by any suitable measuring device, for example, a temperature sensor, preferably connected to the segments of the refiner filling. This temperature may deviate and is usually higher than the temperature of the housing of the refiner unit. The temperature difference may be up to 30°C.

The pressure applied in the refiner unit is typically at least > 1 bar and may range up to 20 bar, preferably 2 to 15, more preferably 3 to 12 bar.

The disk speed of the refiner unit(s) of the instant grinding step a) or of subsequent refining/grinding steps may be selected in a suitable manner to obtain the desired properties of pulp, for example at least 1500 rpm for a double disk refiner or at least 1800 rpm for a single disk refiner.

The bleaching formulation may be added prior to or during the grinding step a), said bleaching formulation comprises

(A) a salt of the dithionous acid H2S2O4, and

The salt of the dithionous acid preferably comprises an alkali metal salt, preferably the lithium, the sodium or the potassium salt, or an alkaline earth metal salt, preferably the calcium or the magnesium salt, of dithionous acid or any combination thereof, including any crystal watercontaining salt or adduct thereof. More preferably, the salt of the dithionous acid is an alkali metal salt. Particular preference is given to sodium dithionite (Na₂S2C>4) or potassium dithionite (K2S2O4), especially sodium dithionite.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the salt of the dithionous acid (a) is selected from sodium dithionite, potassium dithionite and any combination thereof, preferably sodium dithionite.

The amount of the salt of the dithionous acid may be varied in a broad range. The bleaching formulation preferably comprises the salt of the dithionous acid in an amount of from 20 to 95 wt%, based on the total weight of the bleaching formulation, more preferably from 60 to 95 wt%.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the bleaching formulation comprises the salt of the dithionous acid (A) in an amount of from 20 to 95 wt%, based on the total weight of the bleaching formulation, preferably from 60 to 95 wt%.

The bleaching formulation preferably comprises the salt comprising a polyanionic component (B) in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation.

More preferably, the bleaching formulation comprises the salt comprising a polyanionic component (B) with COO' groups in an amount of from 0.01 to 1 wt%, based on the total weight of the bleaching formulation, especially from 0.01 to 0.95 wt%, in particular from 0.1 to 0.9 wt%.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the bleaching formulation comprises the salt comprising a polyanionic component (B) in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation.

Further preferred is a process for producing mechanical wood pulp, wherein the bleaching formulation comprises the salt comprising a polyanionic component (B) in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation, with the proviso that the amount of the salt comprising a polyanionic component (B) having COO' groups is of from 0.01 to 0.95 wt%.

The salt of dithionous acid (A) may also be employed by using a compound generating dithionous acid or a salt thereof. Examples of a compound generating dithionous acid or a salt thereof include thiourea dioxide, also called formamidine sulfinic acid (HN=C(NH2)SC>2H), in combination with lye like caustic soda lye, and a salt of sulfurous acid H2SO3 (hydrogensulfite) in combination with sodium tetraborohydride (NaBF ). A salt of sulfurous acid may be an alkali metal salt, for example the lithium, sodium or potassium salt, an alkaline earth metal salt, for example the calcium or magnesium salt, or any combination thereof, including any crystal water-containing salt or adduct thereof. Particular preference is given to the combination of sodium hydrogensulfite NaHSOs with sodium tetraborohydride.

Those compounds generating a salt of dithionous acid may be used to formulate the bleaching composition instead of a salt of dithionous acid (A).

Preferably, a salt of dithionous acid is used as salt (A).

The salt comprising a polyanionic component (B) is preferably water-soluble.

The term “water-soluble”, as used herein with respect to the salt comprising a polyanionic component (B), means at least 1 g/l under normal conditions (20°C, 1013 mbar), preferably 5 or 10 g/l.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the salt comprising a polyanionic component (B) is water-soluble.

It is further preferred to use a salt comprising a polyanionic component (B) with food contact approval to enable the paper product produced with the instant process to be used in packaging material with food contact.

The cation of the polyanionic component of salt (B) may be an alkali metal cation. The alkali metal cation may be lithium, sodium or potassium or any combination thereof, preferably sodium or potassium or a combination thereof, especially sodium.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the salt comprising a polyanionic component (B) is a sodium salt and/or a potassium salt, preferably a sodium salt.

The salt comprising a polyanionic component (B) may be a salt comprising a polyphosphate (B- 1), which may be cyclic, linear or branched. Preferably, the salt comprising a polyphosphate (B- 1) is a salt comprising a cyclo-polyphosphate (B-1a), a salt comprising a linear polyphosphate (B-1 b) or a combination thereof.

The cyclo-polyphosphate may be a cyclo-polyphosphate of formula (PC>3')P, wherein p is a number of 5 to 8, preferably 6 to 8. The salt of the polyanionic component (B-1a) may be a mixture of salts of cyclo-polyphosphates with various numbers of ring members or a single salt of a defined structure. A preferred salt comprising a polyanionic component (B-1a) is a salt comprising a cyclo-polyphosphate of formula (PO3')_P, wherein p is 6.

The linear polyphosphate may be a linear polyphosphate of formula (PC>3')q ■ O²', wherein q is a number of 10 to 35, preferably 10 to 30. Preferably, the salt of the polyanionic component (B- 1 b) is a mixture of salts of linear polyphosphate with various chain lengths. Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the salt comprising a polyanionic component (B) comprises a cyclopolyphosphate of formula (POS P, a linear polyphosphate of formula (PC>3')q ■ O²' or a combination thereof; p is a real number in the range of > 5 to < 8, preferably > 6 to < 8; and q is a real number in the range of > 10 to < 35, preferably > 10 to < 30.

The above formulae are a general definition of the polyphosphate. These polyphosphates are generally a mixture of polyphosphates of the type indicated having different chain lengths. For the indices, therefore, average values are calculated which may differ from an integer. Hence, the indices have been kept as real numbers, which take into account such possibilities.

The salt comprising a polyphosphate (B-1) is usually used as a mixture of polyphosphates having a distribution of various chain lengths. Alternatively, the salt comprising a polyphosphate (B-1) may also be applied in purified form, comprising essentially one salt of a specific length. The salt comprising a polyphosphate (B-1) is commercially available or may be produced, usually as a mixture, for example, by thermal condensation of acid orthophosphates, as described, for example, in Winnacker-Kuchler, 4^th ed., 2, 237-247.

The salt comprising a polyphosphate (B-1) is usually a solid component, at 23°C and 1 bar.

Any mixture of salts comprising a polyphosphate (B-1) may be used.

The salt comprising a polyanionic component (B) may be a salt comprising a polycarboxylate (B-2). The salt comprising a polycarboxylate (B-2) comprises as polyanionic component a polymer comprising a unit derived from an ethylenically unsaturated carboxylate.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the polyanionic component (B) comprises a polymer comprising a unit derived from an ethylenically unsaturated carboxylate.

The ethylenically unsaturated carboxylate may be derived from a Cs-Ce-monocarboxylic acid or a C4-C6-dicarboxylic acid.

Examples of an ethylenically unsaturated carboxylic acid include Cs-Ce-monocarboxylic acids, like (meth)acrylic acid, crotonic acid, 2-ethylpropenoic acid, 2-propylpropenoic acid, 2- (meth)acryloxyacetic acid, as well as C4-C6-dicarboxylic acids, like maleic acid, succinic acid, itaconic acid, mesaconic acid, citraconic acid and fumaric acid.

A preferred Cs-Ce-monocarboxylic acid is (meth)acrylic acid, especially acrylic acid.

A preferred C4-C6-dicarboxylic acid is maleic acid, itaconic acid or succinic acid. Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the ethylenically unsaturated carboxylate is derived from a C3-C6- monocarboxylic acids, a C4-C6-dicarboxylic acid or a combination thereof, preferably (meth)acrylic acid, maleic acid, itaconic acid and any combination thereof, more preferably acrylic acid, maleic acid, itaconic acid or a combination thereof.

The polymer comprising a unit derived from an ethylenically unsaturated carboxylate may comprise the ethylenically unsaturated carboxylate partially as free acid, generally in such amount that the pH value of the homo- or copolymer is at least 5 up to 12.

The polymer comprising a unit derived from an ethylenically unsaturated carboxylate may be a homopolymer comprising one unit derived from an ethylenically unsaturated carboxylate or may be a copolymer comprising two or more units derived from an ethylenically unsaturated carboxylate.

The polymer comprising a unit derived from an ethylenically unsaturated carboxylate may comprise one or more units derived from an ethylenically unsaturated comonomer M, which is different from a unit derived from the ethylenically unsaturated carboxylate, for example, a vinyl monomer (M1), an allyl monomer (M2) or a (meth)acrylic monomer (M3), which monomer M3 is different from (meth)acrylic acid or (meth)acrylate (salt of (meth)acrylic acid).

Examples of a vinyl monomer M1 include vinyl sulfonic acid or a salt thereof; styrene-4-sulfonic acid or a salt thereof and a C2-C8-monoolefin, for example, ethylene, propylene, 2- methylpropene, 1-butene, diisobutylene (2,4,4-trimethyl-1-pentene), 1-hexene or 1-octene.

Examples of an allyl monomer M2 include allyl sulfonic acid or a salt thereof, and a monoallylether of a polyalkylene glycol with 2 to 50 ethylene glycol units and allyl alcohol.

Examples of a (meth)acrylic monomer M3 include, for example:

- a hydroxy-C2-C4-alkyl ester of (meth)acrylic acid, preferably of acrylic acid; for example, hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate or hydroxybutyl (meth)acrylate;

- a Ci-C4-alkyl-ester of (meth)acrylic acid, preferably of acrylic acid; for example, ethyl (meth)acrylate, n-propyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, secbutyl (meth)acrylate, isobutyl (meth)acrylate;

- an ethylenically unsaturated (meth)acrylamide, for example, (meth)acrylamide, and C1-C4- alkyl (meth)amide, such as N-methyl (meth)acrylamide, N-ethyl (meth)acrylamide, N-propyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N-butyl (meth)acrylamide and 2- acrylamido-2-methylpropane sulfonic acid (AMPS); and

- an Ci-C4-alkylpolyethylene glycol (meth)acrylate with 2 to 50 ethylene glycol units.

Preferably, the (meth)acrylic monomers have a water-solubility of at least 50 g/l, in particular at least 80 g/l or at least 100 g/l under normal conditions (20°C, 1013 mbar) in deionized water. The polymer comprising a unit derived from an ethylenically unsaturated carboxylate may also be modified by functional units, derived from the polymerization process in aqueous isopropanol, as described in DE 19950941 A1 or WO 2004/099092 A1.

Preferably, the copolymer comprising the ethylenically unsaturated carboxylate contains the ethylenically unsaturated carboxylate in an amount of at least 50 wt%, based on the total weight of the monomers, more preferably of at least 60 wt%, especially of at least 60 wt%.

The quantity for monomers of a polymer is based, unless otherwise explicitly stated, on 100 weight parts of all of the monomers. The term “based on the total weight of the monomers”, as used herein with respect to a copolymer, means all of the monomers used to form the copolymer.

The salt comprising a polycarboxylate (B-2) generally has a weight-average molecular weight of from 1000 to 70000 g/mol, preferably 1000 to 50000, as usually determined by gel-permeation chromatography (GPC) at room temperature using elution media.

The salt comprising a polycarboxylate (B-2) may be commercially available or may be obtained by conventional methods known in the art, for example, by solution polymerization in a free- radical solution polymerization in water or water/alcohol mixture like water/isopropanol, as described for example, by M.M. Basuni et al., KGK, 07-8, 2016 (www.kgb-rubberpoint.de), DE 19950941 A1 or US 4709091.

The salt comprising a polycarboxylate (B-2) may be applied to the bleaching formulation in solid form or as an aqueous solution, typically having a suitable solids content of 20 to 60 wt%. The water content of such solutions may be determined according to DIN ISO 3733 (2003).

Any mixture of two or more salts comprising a polycarboxylate (B-2) may be used.

Also, any mixture of a salt comprising a polyphosphate (B-1) and a salt comprising a polycarboxylate (B-2) may be used.

Preferred is a process for producing mechanical wood pulp, wherein the salt comprising a polyanionic component (B) comprises a polyphosphate (B-1), selected from a cyclo-polyphosphate (B-1a) of formula (POS , a linear polyphosphate (B-1b) of formula (PO3')_q ■ O²' and any combination thereof; p is a real number in the range of > 5 to < 8, preferably > 6 to < 8; and q is a real number in the range of > 10 to < 35, preferably > 10 to < 30; or a polymer (B-2) comprising a unit derived from an ethylenically unsaturated carboxylate, or a combination of a polyphosphate (B1) and a polymer (B-2). Preferably, the salt comprising an anionic component (B), like (B-1) or (B-2), has a pH value of at least 5 up to 12, more preferably at least 5.5 to 10. The pH value is usually determined according to DIN 19268 (2021), preferably using 1% dry substance in deionized water.

Accordingly, in a preferred aspect the invention relates to a process for producing mechanical wood pulp, wherein the salt comprising a polyanionic component (B) has a pH of from 5 to 12, preferably from 5.5 to 10, preferably determined according to DIN 19268 (2021).

The bleaching formulation may further comprise one or more additives (C). Examples of an additive (C) include a basic salt (C-1), a salt of disulfurous acid (H2S2O5) (C-2), a salt of sulfurous acid (H2SO3) (C-3), a complexing agent (C-4) including a further anti-scaling agent (C- 4a) and a surfactant (C-5).

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the bleaching formulation comprises

(C) an additive selected from a basic salt (C-1), a salt of disulfurous acid (H2S2O5) (C-2), a salt of sulfurous acid (H2SO3) (C-3), a complexing agent (C-4), a surfactant (C-5) and any combination thereof.

Examples of a basic salt (C-1) include a carbonate or a hydrogencarbonate of an alkali metal, preferably lithium carbonate, sodium carbonate or potassium carbonate, and a carbonate or a hydrogencarbonate of an alkaline earth metal, preferably calcium carbonate or magnesium carbonate, including any crystal water-containing salt or adduct thereof. Particular preference is given to sodium carbonate.

Examples of a salt of disulfurous acid (C-2) include an alkali metal salt, preferably the lithium, sodium or potassium salt, and an alkaline earth metal salt, preferably the calcium or magnesium salt.

Examples of a salt of sulfurous acid (C-3) include an alkali metal salt, preferably the lithium, sodium or potassium salt, and an alkaline earth metal salt, preferably the calcium or magnesium salt, including any crystal water-containing salt or adduct thereof. Particular preference is given to sodium sulfite.

Examples of a complexing agent (C-4) include EDTA, DTPA as well as an anti-scaling agent (C- 4a), which is different from a salt comprising a polyanionic component (B). An anti-scaling agent (C-4a) may be a triphosphate salt, like a sodium or potassium salt of tripolyphosphate or trimetaphosphate.

Examples of a surfactant (C-5) include an anionic, a cationic, a non-ionic surfactant or a glucose-containing surfactant. The bleaching formulation may comprise one or more additives (C) in an amount of from 0 to 40 wt%, based on the total weight of the bleaching formulation.

Preferably, the bleaching formulation comprises, based on the total weight of the bleaching formulation,

60 to 95 wt% of a salt of the dithionous acid (A),

0.01 to 5 wt% of a salt comprising a polyanionic component (B), and

0 to 40 wt% of one or more additives (C).

Typically, the bleaching formulation comprises an additive (C-4), preferably an additive (C-4a), in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation.

Typically, the bleaching formulation comprises an additive (C-5) in an amount of from 0.1 to 10 wt%, based on the total weight of the bleaching formulation, preferably from 1 to 5 wt%.

Preferably, the bleaching formulation comprises an additive (C-1) in an amount such that the additive, preferably a carbonate or hydrogen carbonate of an alkali metal, act as an acid buffer.

In a preferred embodiment, the bleaching formulation comprises, based on the total weight of the bleaching formulation,

60 to 95 wt% of a salt of dithionous acid (A), preferably sodium dithionite, potassium dithionite or a combination thereof,

0.01 to 5 wt% of a salt comprising a polyanionic component (B), preferably 0.1 to 3 wt%, and 5 to 40 wt% of one or more additives (C), preferably selected from additives (C-1), (C-3) and/or (C-4).

In a further preferred embodiment, the bleaching formulation comprises, based on the total weight of the bleaching formulation,

0.01 to 5 wt% of a salt comprising a polyanionic component (B), preferably 0.1 to 3 wt%,

1 to 10 wt% of an additive (C-1),

1 to 25 wt% of an additive (C-3), and

0.01 to 5 wt% of an additive (C-4), especially an additive (C-4a), and wherein the sum of all components is 100 wt%.

The bleaching formulation may be used in the form of a solid bleaching formulation or in form of a solution or suspension. Typically, the bleaching formulation is used in the instant process in the form of a solution or suspension, preferably in the form of a solution. A suitable solvent or dispersing medium dissolves or disperses the bleaching formulation without its active ingredient or ingredients, in particular component (A), being rendered inactive or greatly reduced in their activity by decomposition or otherwise.

Examples are water, a water-containing solvent or a dispersing medium, for example, a mixture of water and a protic or aprotic organic solvent, for example an alcohol, like methanol, ethanol, n-propanol or n-butanol, an ether, like diethylether, or a ketone, like acetone or methylethylketone.

Water is a preferred solvent or dispersing medium. Accordingly, the bleaching formulation is used in the instant process in the form of an aqueous solution, preferably in the form of a solution in water.

The concentration of the bleaching formulation in a solution or a suspension is generally of from 1 to 30 wt%, preferably from 5 to 20 wt%, based on the total weight of the solution or dispersion.

Accordingly, in a preferred aspect, the invention relates to a process for producing mechanical wood pulp, wherein the bleaching formulation is applied as an aqueous solution or suspension, preferably having a concentration of from 1 to 30 wt%, based on the total weight of the solution or suspension.

More preferred is a process for producing mechanical wood pulp, wherein the bleaching formulation is applied as an aqueous solution having a concentration of from 1 to 30 wt%, based on the total weight of the solution or suspension, especially from 5 to 20 wt%.

The aqueous solution or suspension of the bleaching formulation has typically a pH value of at least 5 up to 11, preferably at least 5.5 to 10.

The solution or suspension of the bleaching formulation is preferably used and prepared as fresh as possible or alternatively kept in a substantially complete absence of oxidizing media, for example, atmospheric oxygen.

The bleaching formulation is usually prepared by mixing the components in any conventional known manner. The solution or suspension of the bleaching formulation may be prepared by dissolving or dispersing the components, separately or as a mixture, in the solvent or dispersing medium.

The amount of the component (A), per kilogram of wood chips to be treated, for example wood chips or pre-treated wood chips, is in the range from 1 to 50 grams, preferably in the range from 5 to 20 grams.

The bleaching formulation in form of an aqueous solution or suspension, preferably a solution in water, is added prior to grinding step a) and/or during grinding step a). The bleaching formulation in form of an aqueous solution or suspension, preferably a solution in water, may be metered directly into the refiner unit of step a). Alternatively or additionally, the bleaching formulation is usually metered into the supply line, wherein the optionally pretreated wood chips are transferred to the refiner unit, preferably in the flow direction of said wood chips, shortly upstream of the refiner unit.

The bleaching formulation may also be metered in pretreatment step (iii), preferably in pretreatment step (iii-2) or in the corresponding supply lines of said units.

Thus, preferred is a process for producing mechanical wood pulp, wherein the bleaching formulation is added to a pretreatment step of grinding the woodchips in a refiner unit operating at a temperature less than the temperature of step a), preferably < 150°C.

Further preferred is a process for producing mechanical wood pulp, wherein the bleaching formulation is added to a pretreatment step of grinding the woodchips in a refiner unit operating at a temperature less than the temperature of step a), preferably < 150°C, and/or to the grinding step a).

Further preferred is a process for producing mechanical wood pulp, wherein the bleaching formulation is added to a pretreatment step of grinding the woodchips in a refiner unit operating at a temperature less than the temperature of step a), preferably < 150°C, and to the grinding step a).

The stock obtained in the refiner unit in the instant grinding step a) may be processed in any suitable way, for example, by subjecting to one or more further refining steps and/or to reductive or oxidative bleaching. Bleaching agents and bleaching processes relating to the production of mechanical wood pulp are known in the art and may be conducted, for example, in a bleaching tower.

Oxidative bleaching may be carried out with peroxides, for example, hydrogen peroxide or an alkali metal peroxide, usually in the presence of NaOH or sodium silicate and at elevated temperatures. Reductive bleaching may be carried out with a dithionite, for example, sodium dithionite, sodium sulfite, sodium hydrogensulfite or formamidine sulfinic acid.

The mechanical wood pulp, obtainable by the instant process, may be further processed into paper, preferably tissue, newsprint paper, magazine paper or paper for cardboard, typically using the known papermaking processes.

The mechanical wood pulp, obtainable by the instant process, may also be further processed into a light-colored wood-base material, preferably a HDF or MDF wood-base material, wherein bleached mechanical wood pulp is resonated, optionally under addition of a white pigment, and compression molded into a wood-base material. In a further aspect, the invention relates to a mechanical wood pulp, obtainable or obtained by a process, as defined in any aspect herein.

The bleached mechanical wood pulp, obtainable or obtained by a process, as defined in any aspect herein, may be used for producing a paper or a wood-base material.

In a further aspect the invention relates to a process for producing paper, which comprises mechanical wood pulp, obtainable or obtained by a process, as defined in any aspect herein.

In a further aspect the invention relates to a process for producing light-colored wood-base material, which comprises mechanical wood pulp, obtainable or obtained by a process, as defined in any aspect herein, which is resonated, optionally under addition of a white pigment, and compression molded into a wood-base material.

(A) 60 to 95 wt% of a salt of the dithionous acid H2S2O4,

Preferably, the bleaching formulation comprises the salt comprising a polyanionic component (B) in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation, with the proviso that the amount of the salt comprising a polyanionic component (B) having COO' groups is of from 0.01 to 0.95 wt%.

The addition of the bleaching agent is of advantage with respect to less deposits within the refiner unit resulting in an increased run-time of a refiner unit under the instant temperature condition. Especially, the addition of the salt comprising the polyanionic component (B) to the bleaching formulation as an anti-scaling agent is of advantage to increase the run-time of a refiner unit in a process of producing mechanical wood pulp, preferably as defined in any aspect herein.

Accordingly, in a further aspect, the invention relates to the use of a bleaching formulation, as defined in any one aspect herein, to increase the run-time of a refiner unit in a process of producing mechanical wood pulp, preferably as defined in any aspect herein. Preferably, the term “increasing the run-time of a refiner unit”, as used herein, means that the run-time is longer, if compared to a run-time of a refiner unit, operating at temperature lower than 155°C.

In a further aspect, the invention relates to the use of a salt comprising the polyanionic component (B) as an anti-scaling agent in a process of producing mechanical wood pulp, as defined in any aspect herein. in a further aspect, the invention relates to a process for increasing the run-time of a refiner unit to produce mechanical wood pulp, the process comprising a) grinding wood chips in a refiner unit at an operating temperature of at least 155°C, and b) adding a bleaching formulation to the wood chips prior to step a) and/or during step a), wherein the wood chips are optionally pretreated, and the bleaching formulation comprises

(A) a salt of the dithionous acid H2S2O4, and

(B) a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a PCs' group.

The preferences for the process for producing a mechanical wood pulp applies to the other objects of the invention as well.

The instant process provides economical as well as environmental advantages in a papermaking process. The instant process allows for an improved process efficiency due a longer run-time of the mechanical refining step, compared to other additives known in the art. Thus, the instant process enables a mechanical refining step in a reliable manner, with less disruptions caused by deposits of Ca-containing salts on the refiner equipment. Further, the amount of such additives, inhibiting Ca-containing precipitates, may be reduced leading to less optional contaminants in effluents.

The instant bleaching formulation allows for significantly reducing Ca-containing deposits on the refiner equipment, especially on the segments/disks of the refiner fillings, and/or influencing the crystal morphology thereof in a favorable way in that deposits and precipitates, resp., do generally not adhere strongly at the refiner equipment. This allows that major parts of the precipitates like calcium sulfate may be discharged with the pulp and may be incorporated in the paper product. As such Ca-containing salts are usually used as fillers in paper products they usually do not interfere the whole process and the final paper product.

Further, the instant bleaching formulation may use components which have already approval in food contact which is desired in paper products for such application.

As Ca-containing deposits in less amount and especially in a desired morphology occur, any cleaning step of the refiner equipment may also require less effort. Especially, the segments of the refiner fillings may be replaced less frequently leading to less operational and equipment costs.

All percent, ppm or comparable values refer to the weight with respect to the total weight of the respective composition except where otherwise indicated. The terms “% by weight” and “wt%” are used herein synonymously. All cited documents are incorporated herein by reference.

The following examples shall further illustrate the present invention without restricting the scope of this invention.

Scanning electron microscope (SEM) (Zeiss GeminiSEM 500): topographical reproduction with emission electrons (at 5 kV) after deposition with approximately 12 nm of Pt.

The following anti-scaling agents were applied:

1) Sokalan PA 25 CL (BASF): polyacrylic acid, sodium salt

2) Sodium hexametaphosphate P 69 L (ICL Advanced Additives); CAS 68915-31-1

Based on pH and Na content a mixture of linear polyphosphate of approximately 20 to 30 is assumed.

3) Prayphos SHMP 450 FG: sodium hexametaphosphate: (NaPO3)n.Na2O with n = 16 - 20 CAS 68915-31-1

4) Sodium hexametaphosphate (NaHMP): (NaPOs CAS 10124-56-8; Acros Organics

5) Sodium tripolyphosphate (NaTPP): NasPsO ; CAS 7758-29-4; Sigma-Aldrich

6) Sodium trimetaphosphate (NaTMP): (NaPOsh; CAS 7785-84-4; Sigma-Aldrich

The Examples were conducted in a micro-plant according to the scheme shown in figure 1. The influence of the anti-scaling agent was investigated separately on the precipitation of CaSO4 and CaSOs under temperature conditions simulating those in a refiner unit.

List of reference symbols

1 feed solution A

2 feed solution B

3, 4 pump

5 oil bath of respective temperature

6 mixing T-piece

7 tube reactor

8 cooling unit

9 pressure release unit

General procedure A feed solution A of Na2SC>4 (2.1 wt% in water) (1) or Na2SOs (2.1 wt% in water) (1) and a feed solution B of CaCh (2.2 wt% in water) (2) were fed by HPLC pumps (3, 4) through stainless steel tubing. The anti-scaling agents were dissolved in the Na2SC>4 or Na2SOs feed solution A in a concentration twice of the target concentration (e.g., 200 ppm in Na2SC>4 feed for final concentration of 100 ppm). The feed solutions were pumped with a rate of 100 g/h each. After passing through a preheat passage (oil bath (5) of respective temperature) the two feed solutions were mixed in a T-piece (6) and passed through a wound tube reactor (7) (I = 1 m) at the respective temperature. The tubing was subsequently air-cooled by a cooling unit (8), and the pressure of the hot aqueous solution released via a pressure release unit (9). The plant was run in each experiment until at least one of the two feed pumps stopped, when a pre-defined maximum pressure of 40 bar was exceeded. Run-time is referred to as the start of the pumps until forced stop of one of the pumps due to exceeding pressure.

The feed solutions A and B were listed in Table 1 :

Table 1

The runtimes and the target concentrations of the anti-scaling agents of the Examples and Comparative Examples are listed in Tables 2 and 3.

In the Examples and Comparative Examples with a feed solution containing Na2SC>4 a precipitate of CaSC>4 was observed. In the Examples and Comparative Examples with a feed solution containing Na2SOs a precipitate of CaSCh was observed. The precipitates were observed on the mixing T-piece.

Table 2

Table 3

The data in Tables 2 and 3 demonstrate that the run-time of a refiner unit may be significantly increased using the bleaching formulation or the anti-scaling agent (B), resp., at a temperature of higher than 150°C.

The precipitates obtained in Examples 2 and 8 and Comparative Example 1 and 3 at the T- pieces of the individual run were examined by SEM. The addition of P69 in Example 2 caused smaller crystallites of CaSC>4, including any crystal water-containing salt or adduct thereof, which combine to globular structures (see fig. 3), compared to needle-like sheets of CaSC>4 obtained in Comparative Example 1 (see fig. 2). The addition of P69 in Example 8 caused more smaller and irregular crystallites of CaSCh, including any crystal water-containing salt or adduct thereof (see fig. 5), compared to larger sheet-like crystallites of CaSOs (see fig. 4).

Examples 10 to 14 and Comparative Examples 5 to 7 The Microwave screening tests were conducted in the microwave system (UltraCLAVE, Milestone Sri) under the following conditions: heating +9°C/min, cooling -3°C/min. The influence of the anti-scaling agent was investigated for the precipitation of CaSC>4 under temperature conditions simulating those in a refiner unit.

General procedure

Solutions of CaCh (2.2 wt% in water) and Na2SC>4 (2.1 wt% in water) were mixed to obtain a solution containing 4000 ppm CaO (Ca²⁺ ions arbitrarily calculated as calcium oxide content). The mixtures were subjected to microwave irradiation. In the microwave reactor, 40 bar nitrogen pressure was applied to maintain liquid state. The performance of anti-scaling agents (components 1 , 4, 5 and 6) was checked by titration of the supernatant solution for calcium ions (EDTA) after treatment for 1 min at 200°C under microwave irradiation, followed by cooling and standing for 24 hours at room temperature (ca 23°C). CaSC>4 precipitate was observed. The CaO concentration in solution is shown in Table 4.

Table 4

The CaO concentrations in solution are significantly lower, when sodium tripolyphosphate or sodium trimetaphosphate is used, indicating that a CaSO4 precipitate could not be effectively decreased.

Claims

1. A process for producing mechanical wood pulp, the process comprising a) grinding wood chips in a refiner unit at an operating temperature of at least 155°C, and b) adding a bleaching formulation to the wood chips prior to step a) and/or during step a), wherein the wood chips are optionally pretreated, and the bleaching formulation comprises

(A) a salt of the dithionous acid H2S2O4, and

(B) a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a POs' group.

2. The process according to claim 1, wherein the operating temperature is of from 155 to 230°C, preferably from 160 to 220°C.

3. The process according to claim 1 or 2, wherein the salt of the dithionous acid (A) is selected from sodium dithionite, potassium dithionite and a combination thereof, preferably sodium dithionite.

4. The process according to claim 1, 2 or 3, wherein the bleaching formulation comprises the salt of the dithionous acid (A) in an amount of from 20 to 95 wt%, based on the total weight of the bleaching formulation, preferably from 60 to 95 wt%.

5. The process according to any one of the preceding claims, wherein the bleaching formulation comprises the salt comprising a polyanionic component (B) in an amount of from 0.01 to 5 wt%, based on the total weight of the bleaching formulation.

6. The process according to any one of the preceding claims, wherein the salt comprising a polyanionic component (B) is water-soluble.

7. The process according to any one of the preceding claims, wherein the salt comprising a polyanionic component (B) is a sodium salt and/or a potassium salt, preferably a sodium salt.

8. The process according to any one of the preceding claims, wherein the salt comprising a polyanionic component (B) has a pH of from 5 to 12, preferably from 5.5 to 10.

9. The process according to any one of the preceding claims, wherein the bleaching formulation is used as an aqueous solution or suspension, preferably having a concentration of from 1 to 30 wt%, based on the total weight of the solution or suspension.

10. The process according to any one of the preceding claims, wherein the salt comprising a polyanionic component (B) comprises a cyclo-polyphosphate of formula (PO3')_P, a linear polyphosphate of formula (PC>3')q ■ O²' or a combination thereof; p is a real number in the range of > 5 to < 8, preferably > 6 to < 8; and q is a real number in the range of > 10 to < 35, preferably > 10 to < 30.

11. The process according to any one of the preceding claims, wherein the salt comprising a polyanionic component (B) comprises a polymer comprising a unit derived from an ethylenically unsaturated carboxylate.

12. The process according to claim 11 , wherein the ethylenically unsaturated carboxylate is derived from a Cs-Ce-monocarboxylic acids, a C4-C6-dicarboxylic acid or any combination thereof, preferably (meth)acrylic acid, maleic acid, itaconic acid and any combination thereof.

13. The process according to any one of the preceding claims, wherein the bleaching formulation comprises

14. A mechanical wood pulp, obtainable or obtained by a process, as defined in any one the claims 1 to 13.

15. A bleaching formulation comprising, based on the total weight of the bleaching formulation,

(A) 60 to 95 wt% of a salt of the dithionous acid H2S2O4,

(B) 0.01 to 5 wt% of a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a PCs' group, and

16. The use of a bleaching formulation, as defined in any one of claims 1 , 3 to 13 and 15, to increase the run-time of a refiner unit in a process of producing mechanical wood pulp.

17. The use of a salt comprising a polyanionic component having at least 5 anionic units, selected from a COO' group and a PCs' group, (B) as an anti-scaling agent in a process of producing wood pulp.