WO2010139589A1

WO2010139589A1 - Process for producing mechanical pulp

Info

Publication number: WO2010139589A1
Application number: PCT/EP2010/057288
Authority: WO
Inventors: Pia Hellström; Pär NILSSON; Karin Susanne Maria Walter; Patrik Johansson; Eva Linnea Elisabeth Wackerberg; Magnus Lars Paulsson
Original assignee: Akzo Nobel Chemicals International B.V.
Priority date: 2009-06-02
Filing date: 2010-05-27
Publication date: 2010-12-09
Also published as: EP2438235A1

Abstract

The present invention relates to a process for preparing a high-yield pulp comprising a) treating a lignocellulose containing material chemically by means of an oxidising system comprising at least one non-enzymatic oxidant and an activator and optionally an alkali at a pH from about 2 to about 6.5; and b) treating the lignocellulose containing material mechanically in at least one mechanical treatment stage for a time sufficient to produce a high-yield pulp, wherein the lignocellulose containing material is chemically treated prior to and/or during at least one mechanical treatment stage, wherein the temperature of the lignocellulosic material before, during and/or after at least one mechanical treatment stage of the process ranges from about 45°C to about 200°C for a period of time ranging from about 1 second to about 5 hours and wherein oxidant is present: wherein oxidant is present: (i) after addition of alkali provided alkali is added to the oxidising system and/or (ii) after said at least one mechanical treatment stage.

Description

Process for producing mechanical pulp

The present invention relates to a process for producing a high-yield pulp from a lignocellulose containing material.

Background of the invention

In a time when global wood and energy prices are rising, efficient processes for producing and utilizing pulp and paper products are of great importance. Mechanical and chemimechanical pulping make efficient use of available virgin fibre resources since they are high-yield manufacturing processes. The production of mechanical and chemimechanical pulps is an efficient way of using the world's natural resources since the yield of these manufacturing processes is high and the environmental impact is relatively low. Mechanical and chemimechanical pulping constitute about 25% of the total virgin fibre production in the world. However, mechanical pulping processes involve high energy consumption. WO2007/064287 A1 describes a process of producing high-yield pulp while reducing the energy consumption without substantially reducing the fibre length or strength properties of the pulp. However, it has been observed that optical properties including brightness of the produced pulp are not always at a satisfactory level.

The optical properties of high-yield pulps are quality parameters that are becoming increasingly important since brightness and whiteness requirements for inter alia improved newsprint and many uncoated and coated magazine paper grades, e.g. SC (Super-Calendered Paper), LWC (Low-Weight Coated Paper), MWC (Medium-Weight Coated Paper) have been raised in recent years. The brightness depends on the relationship between the light absorption coefficient (k) and the light scattering coefficient (s). High values of the ratio s/k are required to attain high brightness values. The s-value for pulp can only vary between about 15 and 60 m²/kg whereas the k-value can vary by several powers of ten (Pauler, N. (2002): Paper Optics, AB Lorentzen & Wettre, Kista, Sweden). A low light absorption value is therefore needed to obtain a high brightness value It is also known that high unbleached pulp brightness is important for reaching high bleached pulp brightness. It is therefore important that the unbleached brightness of mechanical and chemimechanical pulps is as high as possible prior to bleaching.

It is thus an object of the present invention to provide a process of producing a high-yield pulp in an energy-efficient way while improving or better preserving the brightness of the pulp. A further concern of the present invention is to provide such process without substantially reducing the fibre length or strength properties of the produced pulp. It is a further object to provide a process of producing a high-yield pulp which has a suitable end pH for papermaking, for example for neutral and alkaline papermaking with the benefits of lowering the furnish cost as well as reducing scaling and corrosion of equipment used in the processing of the pulp.

The invention The present invention relates to a process for preparing a high-yield pulp comprising a) treating a lignocellulose containing material chemically by means of an oxidising system comprising at least one non-enzymatic oxidant and an activator and optionally an alkali at a pH from about 2 to about 6.5; and b) treating the lignocellulose containing material mechanically in at least one mechanical treatment stage for a time sufficient to produce a high-yield pulp, wherein the lignocellulose containing material is chemically treated prior to and/or during at least one mechanical treatment stage, wherein oxidant is present: i) after addition of alkali provided alkali is added to the oxidising system and/or ii) after said at least one mechanical treatment stage. The present invention also relates to a process for preparing a high-yield pulp comprising a) treating a lignocellulose containing material chemically by means of an oxidising system comprising at least one non-enzymatic oxidant and an activator and optionally an alkali at a pH from about 2 to about 6.5; and b) treating the lignocellulose containing material mechanically in at least one mechanical treatment stage for a time sufficient to produce a high-yield pulp, wherein the lignocellulose containing material is chemically treated prior to and/or during at least one mechanical treatment stage, wherein the temperature of the lignocellulosic material before, during and/or after at least one mechanical treatment stage of the process ranges from about 45 ⁰C to about 200⁰C for a period of time ranging from about 1 second to about 5 hours and wherein oxidant is present: i) after addition of alkali provided alkali is added to the oxidising system and/or ii) after said at least one mechanical treatment stage. According to one embodiment, the temperature of the lignocellulosic material before, during and/or after at least one mechanical treatment stage ranges from about 45°C to about 180⁰C, for example from about 50⁰C to about 180°C or from about 55°C to about 180⁰C. According to one embodiment, the temperature of the lignocellulose containing material is maintained at the defined temperature range before, during, and/or after at least one mechanical treatment stage for from about 5 second to about 3 hours, for example from about 10 seconds to about 1 hour, or from about 1 minute to about 0.5 hours. According to one embodiment, the temperature of the lignocellulosic material is maintained in the defined temperature range after at least one mechanical treatment stage. According to one embodiment, the temperature of the lignocellulosic material is maintained during a period of time as defined herein after at least one mechanical treatment stage, for example after one or two mechanical treatment stages. According to one embodiment, oxidant is present after i) as defined above.

According to one embodiment, oxidant is present after ii) as defined above.

Since the alkali addition is optional and thereby i), one embodiment of the invention concerns presence of oxidant only after ii) as defined above

According to one embodiment, the non-enzymatic oxidant (calculated as 100%) is added in an amount from about 0.1 to about 5, for example from about 0.5 to about 3 or from about 0.5 to about 2 wt% based on the weight of the lignocellulose containing material

According to one embodiment, an activator (calculated as 100%) is added in an amount from about 0.0005 to about 0.05, for example from about 0.001 to about 0.025, or from about 0.001 to about 0.01 , or from about 0.0025 to about 0.01 wt% based on the weight of the lignocellulose containing material. An activator is a component which is able to activate the oxidant of the invention, i.e. to increase its oxidative function.

According to one embodiment, the weight ratio of activator to oxidant ranges from about 0.0001 to about 0.5, for example from about 0.0001 to about 0.01 , or from about 0.0005 to about 0.01. According to one embodiment, alkali is added in an amount from about 0.05 to about 3, from about 0.1 to about 2, for example from about 0.1 to about 1 , or from about

0.2 to about 0.8 wt% based on the weight of lignocellulose containing material.

According to one embodiment, at least about 5%, for example at least about 7%, or at least about 10%, or at least about 15% of the total amount of oxidant added is present after stage i) or ii). The amount of oxidant remaining after stage i) or ii) is also referred to herein as "residual oxidant", for example "residual hydrogen peroxide".

According to one embodiment, up to about 60%, for example up to about 40%, or up to about 25%, or up to about 10%, or up to about 5%, of the total amount of oxidant added is present after stage i) or ii).

According to one embodiment, oxidant is present after the first mechanical treatment stage, for example in a process comprising only one or two or three or more mechanical treatment stages.

According to one embodiment, oxidant is present after the last mechanical treatment stage in a series of two or three or more mechanical treatment stages.

According to one embodiment, oxidant is present after addition of alkali, for example prior to and/or during the first mechanical treatment stage. According to one embodiment, an activator is added prior to or during any mechanical treatment stage, either separately or simultaneously with a non-enzymatic oxidant. The activator may be added either before, simultaneously or after the addition of a non-enzymatic oxidant. This may be just before the addition of a non-enzymatic oxidant before a mechanical treatment stage such as a refiner, but may also be before e.g. a primary refiner whereas the non-enzymatic oxidant is added after the primary refiner but before a secondary refiner.

According to one embodiment, the mechanical treatment may be performed in one or two or three or more mechanical treatment stages. Typically, the mechanical treatment may be performed in two stages or more including a reject mechanical treatment stage where up to 60 wt% of the lignocellulose containing material may be passed through. The mechanical treatment stages usually are performed by passing the lignocellulose containing material through grinders and/or refiners. However, other mechanical treatments may also be performed in equipments as, e.g. plug screws (e.g. impressafiner), roller mills (e.g. Szego mill), double shaft extruders (Bi-Vis screw extruder), the reciprocating apparatus, RT Fiberizer™, dispersers or in any combinations thereof.

According to one embodiment, the oxidising system is applied to the lignocellulose containing material at one or several stages before and/or during mechanical treatment. According to one embodiment, the oxidising system is applied as an inter-stage treatment between two mechanical treatment stages. According to one embodiment, the oxidising system is applied to at least one reject refining stage.

According to one embodiment, the non-enzymatic oxidant is selected from inorganic peroxy compounds such as hydrogen peroxide or hydrogen peroxide generating compounds such as salts of percarbonate, perborate, peroxysulphate, peroxyphosphate, peroxysilicate or corresponding weak acids. According to one embodiment, the non-enzymatic oxidant is selected from organic peroxy compounds such as peroxy carboxylic acids, e.g. peracetic acid and perbenzoic acid.

According to one embodiment, the oxidising system comprises halogen containing oxidants such as chlorine dioxide, chlorite, hypochlorite, and chloro sodium salt of cyanuric acid. According to one embodiment, the oxidising system comprises oxygen, ozone and/or nitrogen oxides such as NO or NO₂. According to one embodiment, the oxidizing system comprises combinations of different oxidants, which can be either added or re-used from the process steps which generate the non-enzymatic oxidants. According to one embodiment, the activator is selected from metal ions, e.g. transition metal ions such as ions of Fe, Mn, Co, Cu, W or Mo; TAED (Tetra Acetyl Ethylene Diamine); cyanamide, or combinations thereof. According to one embodiment, transition metal ions such as Fe ions may be used, for example in the form of acids or salts such as cupper sulphate, iron sulphate, or complexes with common organic or inorganic compounds.

According to one embodiment, ultraviolet radiation or other radiation is applied to the non-enzymatic oxidant or to the lignocellulose containing material being treated with the non-enzymatic oxidant.

According to one embodiment, equipment for monitoring the amount of oxidant, for example hydrogen peroxide at various points, for example after stages i) and/or ii) in the process is employed. Such equipment could be used to safeguard that oxidant is present in certain amounts after certain stages in the process in order to optimize energy reduction and optical properties of the produced pulp. An example of instrument suitable for this purpose is BTG's in-line residual hydrogen peroxide analyzer RPA-5000. According to one embodiment, the oxidant is substantially free from ozone which can be advantageous due to the fact that ozone does not provide a sufficient pulp yield due to low selectivity and is usually a more expensive alternative. By the term "substantially free from ozone" is meant that the oxidising system comprises less than 5 wt%, for example less than 2 wt% or less than 1 wt% ozone (calculated as 100%) based on the total weight of the oxidising system.

According to one embodiment, the oxidant is substantially free from chlorine dioxide. By the term "substantially free from chlorine dioxide" is meant that the oxidising system comprises less than 5 wt%, or less than 2 wt% or less than 1 wt% chlorine dioxide (calculated as 100%) based on the total weight of oxidising system. According to one embodiment, the pH at which the chemical treatment is performed ranges from about 2.5 to about 6, for example from about 2.5 to about 5.5 or from about 3 to about 5.5 such as from about 3 to about 4. According to one embodiment, the pH is from about 3.5 to about 5

According to one embodiment, the lignocellulose containing material is not chemically treated before stage a) and/or between stages a) and b) at a pH from about 7 to about 14, for example from about 8 to about 14 or from about 9 to about 14, e.g. from about 10 to about 14 or from about 10.5 to about 14 or from about 1 1 to about 14, or from about 11.5 to about 14.

The term high-yield pulp may comprise e.g. mechanical pulp (MP), refiner mechanical pulp (RMP), pressurized refiner mechanical pulp (PRMP), thermomechanical pulp (TMP), thermomechanical chemical pulp (TMCP), high-temperature TMP (HT-TMP), RTS-TMP, Thermopulp, groundwood pulp (GW), stone groundwood pulp (SGW), pressure groundwood pulp (PGW), super pressure groundwood pulp (PGW-S), thermo groundwood pulp (TGW), thermo stone groundwood pulp (TSGW), chemimechanical pulp (CMP), chemirefinermechanical pulp (CRMP), chemithermomechanical pulp (CTMP), high-temperature CTMP (HT-CTMP), sulphite-modified thermomechanical pulp (SMTMP), reject CTMP (CTMP_R), groundwood CTMP (G-CTMP), semichemical pulp (SC), neutral sulphite semi chemical pulp (NSSC), high-yield sulphite pulp (HYS), biomechanical pulp (BRMP), pulps produced according to the OPCO process, explosion pulping process, Bi-Vis process, dilution water sulfonation process (DWS), sulfonated long fibres process (SLF), chemically treated long fibres process (CTLF), long fibre CMP process (LFCMP) or any modifications and combinations thereof.

According to one embodiment, the high-yield pulp is mechanical pulp, refiner mechanical pulp, groundwood pulp, chemimechanical pulp, semichemical pulp, thermomechanical and/or chemithermomechanical pulp. According to one embodiment, the high-yield pulp has a yield of at least about

60%, for example at least about 70%, or at least about 80%, or at least about 85%. According to one embodiment, the high-yield pulp has a yield of at least about 90% such as at least about 95%. The pulp may be a bleached or non-bleached pulp.

According to one embodiment, the lignocellulose containing material comprises non-defibrated wood. According to one embodiment, the lignocellulose containing material comprises mechanically treated lignocellulose containing material. The lignocellulose containing material may comprise e.g. wood logs, finely-divided raw materials, including woody materials, such as wood particles (e.g. in the form of wood chips, wood shavings, wood fibres and saw dust) and fibres of annual or perennial plants including non-wood. The woody raw material can be derived from hardwood or softwood species, such as birch, beech, aspen such as European aspen, alder, eucalyptus, maple, acacia, mixed tropical hardwood, pine such as loblolly pine, fir, hemlock, larch, spruce such as Black spruce or Norway spruce, and mixtures thereof. Non-wood plant raw material can be provided from e.g. straws of grain crops, reed canary grass, reeds, flax, hemp, kenaf, jute, ramie, sisal, abaca, coir, bamboo, bagasse or combinations thereof. According to one embodiment, the lignocellulose containing material comprises hardwood and/or softwood species.

According to one embodiment, the lignocellulose containing material is substantially free from non-wood. For example, the lignocellulose containing material contains less than 5 wt%, for example less than 1 wt% non-wood based on the total weight of lignocellulose containing material. According to one embodiment, the lignocellulose containing material is treated with the oxidising system for from about one second to about ten hours, for example from about five seconds to about five hours, or from about ten seconds to about three hours.

According to one embodiment, the lignocellulose containing material is chemically and/or mechanically treated at a temperature from about 30 to about 200⁰C, for example from about 50 to about 180⁰C, or from about 80 to about 180°C.

According to one embodiment, the lignocellulose containing material is preheated before the chemical treatment is initiated. Such pre-treatment may be performed at a temperature from about 30 to about 200⁰C, for example from about 50 to about 180°C, or from about 50 to about 140°C, for example for a period of time ranging from about 1 second to about 5 hours, such as from about 10 seconds to about 1 hour, or from about 1 minute to about 0.5 hour.

According to one embodiment, the oxidising system comprises an enhancer that facilitates/controls the oxidation depending on the amount thereof added. An enhancer may be selected from for example complexing agents, chelating agents and/or ligands. According to one embodiment, the enhancer is selected from nitrogen-containing polycarboxylic acids, nitrogen-containing polyphosphonic acids, nitrogen-containing polyalcohols, oxalic acid, oxalate, glycolate, ascorbic acid, citric acid, nitrilo acetate, gallic acid, fulvic acid, itaconic acid, haemoglobin, hydroxybenzenes, catecholates (e.g. 4,5- dimethoxycatechol, 4-methyl catechol), quinolines such as hydroxyquinoline (e.g. 8- hydroxyquinoline), dimethoxybenzoic acids, dihydroxybenzoic acids, dimethoxybenzylalcohols, pyridine, histidylglycine, phthalocyanine, acetonitrile, 18-crown- 6-ether, mercaptosuccinic acid, cyclohexadienes, polyoxomethalates, and combinations thereof.

According to one embodiment, the enhancer is selected from nitrogen-containing organic compounds, such as nitrogen-containing polycarboxylic acids, nitrogen- containing polyphosphonic acids, nitrogen-containing polyalcohols, and mixtures thereof. According to one embodiment, the enhancer is selected from diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and mixtures thereof.

According to one embodiment, the enhancer is selected from compounds based on other aminopolycarboxylic acids, polyphosphates or polyphosphonic acids, hydroxycarboxylates, hydrocarboxylic acids, dithiocarbamate, iminodisuccinic acid, [S, S^']- etylenediaminedisuccinic acid.

According to one embodiment, the enhancer is selected from dihydroxybenzene (e.g. hydroquinone), trihydroxybenzene, dihydroxybenzoic acid (e.g. 3,4- dihydroxybenzoic acid, 2,3-dihydroxybenzoic acid), 3,4-dimethoxybenzylalcohol, 3,4- dimethoxybenzoic acid, 3,4-dimethoxy toluene, 1 ,3-cyclohexadiene, polyoxomethalates. According to one embodiment, the oxidising system comprises as an enhancer also at least one enzyme.

According to one embodiment, an enhancer is added prior to or during any mechanical treatment stage, either separately or simultaneously with a non-enzymatic oxidant and optionally an activator. The enhancer may thus be added either before, simultaneously or after the addition of a non-enzymatic oxidant. This may be just before the addition of the non-enzymatic oxidant, for example before a mechanical treatment stage such as a refiner, but may also be before e.g. a primary refiner whereas the non- enzymatic oxidant is added after the primary refiner but before a secondary refiner. According to one embodiment, an enhancer (calculated as 100%) is added in an amount from about 0.01 to about 2, for example from about 0.05 to about 0.5, or from about 0.05 to about 0.3 wt% based on the weight of the lignocellulose containing material.

According to one embodiment, the weight ratio of enhancer to oxidant ranges from about 0.001 to about 2, for example from about 0.01 to about 1 , or from about 0.025 to about 1.

The following examples will illustrate how the described invention may be performed without limiting the scope of it.

All parts and percentages refer to part and percent by bone dry weight of lignocellulose containing material, if not otherwise stated. The chemicals are calculated as 100%. Example 1

Norway spruce {Picea abies) wood was used for the production of thermomechanical pulp (TMP). The wood logs were debarked and chipped and washed prior to pre-steaming (90⁰C, 10 minutes retention time), pre-heating (125°C, 2 minutes retention time) and refining operations. A two-stage refining setup was used and the energy input was varied in the second refining stage to obtain pulps with different freeness (refining) levels. A single disc 20" pressurized refiner (Sunds Defibrator OVP- MEC with 5811 refining plates run at 1500 rpm, housing pressure of 140 kPa) was used in the first refining stage and a single disc 20" atmospheric refiner (Sunds Defibrator ROP-20 with 5811 refining plates run at 1500 rpm) in the second refining stage. The energy input in the primary refiner was about 550 kWh/bone dry metric ton. The chemicals (hydrogen peroxide, iron sulphate, ethylenediaminetetraacetic acid) were added to the primary refiner. The pulp from the primary refiner was then stored for about 30 minutes at about 60⁰C before further mechanical treatment in the atmospheric refiner. All trials were run at constant conditions which mean that the variation in specific energy consumption and pulp and paper properties is a result of the chemicals added during the trials.

A TMP reference was produced without addition of chemicals. The pulp is denoted TMPREF in table 1. A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.015 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.4 wt% H₂O₂ and 0.34 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry wood to the primary refiner. The pH of the resulting pulp was 3.0. The pulp is denoted TMPAP1 in table 1.

A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.014 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.4 wt% H₂O₂ and 0.13 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry wood to the primary refiner. The pH of the resulting pulp was 3.2. The pulp is denoted TMPAP2 in table 1.

A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.0044 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.3 wt% H₂O₂ and 0.22 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry wood to the primary refiner. The pH of the resulting pulp was 3.1. The pulp is denoted TMPAP3 in table 1.

A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.0044 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.3 wt% H₂O₂ and 0.22 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry wood to the primary refiner. The pH of the resulting pulp was 3.2. The pulp is denoted TMPAP4 in table 1. A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.014 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.4 wt% H₂O₂, 0.13 wt% sulphuric acid (H₂SO₄) and 0.10 wt% ethylenediaminetetraacetic acid (EDTA) based on the weight of bone dry wood to the primary refiner. The pH of the resulting pulp was 3.4. The pulp is denoted TMPAPEDTA in table 1.

Table 1 : The ISO brightness, remaining hydrogen peroxide and the energy reduction of the produced pulps

The energy reduction is given relative to the energy consumption of the TMPREF. 2n.a. = Not applicable

In table 1 above, the brightness as a function of residual hydrogen peroxide (% of added H₂O₂) after refining of reference and chemically treated pulps is shown. The pulps have a freeness value of approximately 100 ml CSF. The energy reduction is given relative to the energy consumption of the TMP reference (TMPREF).

It is evident from the results presented in table 1 that it is possible to significantly lower the energy consumption by applying hydrogen peroxide and an activator at an acidic pH (TMPAP series). It is also evident that it is beneficial to have residual hydrogen peroxide after refining in order to improve the brightness of acid hydrogen peroxide- treated pulps. For example, about 3.8 ISO brightness units can be gained if the residual hydrogen peroxide is increased from 20.6 to 46.5% (cf. TMPAP3 and TMPAP4 in table 1 ). It is also beneficial to add EDTA which is illustrated by comparing the trial series TMPAPEDTA with TMPAP1 and TMPAP2. Thus, the process according to the invention i.e., to have an oxidant present after refining of pulps treated with at least one non- enzymatic oxidant and an activator and optionally an alkali at a pH from about 2 to about 6.5, makes it possible to produce pulps with improved optical properties. Example 2

Norway spruce (Picea abies) wood was used for the production of thermomechanical pulp (TMP). The wood logs were debarked and chipped and washed prior to pre-heating and refining operations. A 20" pressurized refiner (model OVP-MEC run at 1500 rpm) was used to produce a high-freeness pulp (about 540 ml CSF). The energy input in the refiner was about 1160 kWh/bone dry metric ton. The activator was then added to the defibrated pulp followed by the addition of the oxidant. A mixer (Moulinex Type 843) was used to thoroughly mix in the chemicals in the pulp at room temperature. The mixing time was 30 seconds and the pulp consistency 25%. The pulp was then transferred to a plastic bag that was sealed and placed in a water bath (90⁰C) for 30, 35 or 40 minutes (see below). Three sets of experiments were performed;

A TMP reference was produced without addition of chemicals. The reaction time at 90⁰C was 35 minutes. The pH of the resulting pulp was 5.5. The pulp is denoted TMPREF1.

A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.005 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.0 wt% H₂O₂ and 0.13 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry pulp. The reaction time at 90⁰C was 40 minutes. The pH of the resulting pulp was 3.2. The residual H₂O₂ content was 0.08 wt% based on the lignocellulose containing material. The pulp is denoted TMPX.

A TMP using acid hydrogen peroxide (H₂O₂) was produced by adding 0.005 wt% iron (added as iron sulphate, FeSO₄ x 7 H₂O), 1.5 wt% H₂O₂ and 0.12 wt% sulphuric acid (H₂SO₄) based on the weight of bone dry pulp. The reaction time at 90⁰C was 30 minutes. The pH of the resulting pulp was 3.3. The residual H₂O₂ content was 0.5 wt% based on the lignocellulose containing material. The pulp is denoted TMPY.

The three pulps above (TMPREF1 , TMPX, TMPY) were then divided into two portions, diluted to a pulp consistency of 4%, transferred to a plastic bag that was sealed and placed in a water bath (80⁰C) for 120 minutes in order to simulate a latency treatment of the pulp. Additional chemicals, sodium hydroxide (NaOH) and diethylenetriaminepentaacetic acid (DTPA), were added in some cases prior to the 120 minutes heat-treatment. Two different conditions were employed; A) Addition of 0.2 wt% DTPA based on the weight of bone dry pulp prior to heat-treatment at 80°C for 120 minutes.

B) Addition of 0.2 wt% DTPA based on the weight of bone dry pulp and NaOH to adjust the pH to about 6 prior to heat-treatment at 80°C for 120 minutes. The TMPREF1 pulp was not pH-adjusted since the pH was close to 6. The amount of chemicals added and pH, residual H₂O₂ and brightness before and after heat-treatment, as well as brightness of the reference and chemically treated pulps are given in table 2 below. Table 2 also shows the brightness of the same pulps after a heat- treatment at 80⁰C for 120 minutes. TMPX was also heat-treated at 80⁰C for 120 minutes without addition of DTPA or alkali (condition NO in table 2).

Table 2: The amount of chemicals added and pH, residual H₂O₂ and brightness before and after heat-treatment (80°C, 120 minutes)

¹n.a. = not applicable.

It is evident that it is possible to significantly increase the brightness when applying hydrogen peroxide and an activator at an acidic pH if hydrogen peroxide is present after the chemical treatment; TMPY (34% remaining H₂O₂) had about 2.1 ISO units higher brightness compared to TMPX (8% remaining H₂O₂). It is also beneficial to have hydrogen peroxide remaining after addition of alkali (condition B) to a chemically treated pulp; the brightness of TMPY was in this case about 3.4 ISO brightness units higher than TMPX and about 6.6 ISO brightness units higher than TMPX heat-treated at an acidic pH without any chemical addition (i.e., condition NO in table 2). The addition of a chelating agent will also improve the brightness regardless of pH value of the pulp or amount of remaining hydrogen peroxide (conditions A and B in table 2). Thus, the method according to the invention i.e., to have an oxidant present after addition of alkali to pulps treated with at least one non-enzymatic oxidant and an activator at a pH from about 2 to about 6.5, makes it possible to produce pulps with improved optical properties.

Claims

1. Process for preparing a high-yield pulp comprising a) treating a lignocellulose containing material chemically by means of an oxidising system comprising at least one non-enzymatic oxidant and an activator and optionally an alkali at a pH from about 2 to about 6.5; and b) treating the lignocellulose containing material mechanically in at least one mechanical treatment stage for a time sufficient to produce a high-yield pulp, wherein the lignocellulose containing material is chemically treated prior to and/or during at least one mechanical treatment stage, wherein the temperature of the lignocellulosic material before, during and/or after at least one mechanical treatment stage of the process ranges from about 45 ⁰C to about 200⁰C for a period of time ranging from about 1 second to about 5 hours and wherein oxidant is present: i) after addition of alkali provided alkali is added to the oxidising system and/or ii) after said at least one mechanical treatment stage.

2. Process according to claim 1 , wherein the lignocellulose containing material is not chemically treated at a pH from about 11.5 to about 14 between stages a) and b).

3. Process according to claim 1 or 2, wherein the oxidant is substantially free from ozone and/or chlorine dioxide.

4. Process according to any one of claims 1 to 3, wherein several mechanical treatment stages are comprised and oxidant is present after the first mechanical treatment stage.

5. Process according to any one of claims 1 to 4, wherein several mechanical treatment stages are comprised and oxidant is present after the last mechanical treatment stage.

6. Process according to any one of claims 1 to 5, wherein oxidant is present after alkali is added prior to and/or during a first mechanical treatment stage.

7. Process according to any one of claims 1 to 6, wherein the oxidant is selected from peroxy compounds, halogen containing oxidants, oxygen, nitrogen oxides or combinations thereof.

8. Process according to any one of claims 1 to 7, wherein the oxidant is hydrogen peroxide.

9. Process according to any one of claims 1 to 8, wherein the weight ratio of activator to oxidant ranges from about 0.0001 to about 0.5.

10. Process according to any one of claims 1 to 9, wherein the oxidising system further comprises an enhancer.

1 1. Process according to claim 10, wherein the weight ratio of enhancer to oxidant ranges from about 0.001 to about 2.

12. Process according to any one of claims 1 to 1 1 , wherein the oxidising system comprises an activator selected from metal ions, TAED, cyanamide, or combinations thereof.

13. Process according to any one of claims 1 to 12, wherein the activator is selected from transition metal ions.

14. Process according to any one of claims 1 to 13, wherein an alkali is added in an amount of from about 0.05 to about 5 wt% based on the weight of lignocellulose containing material.

15. Process according to any one of claims 1 to 14, wherein at least about 5% of the total amount of oxidant added is present after stage i) or ii).

16. Process according to any one of claims 1 to 15, wherein at least about 10% of the total amount of oxidant added is present after stage i) or ii).

17. Process according to any one of claims 1 to 16, wherein the oxidising system further comprises an enhancer selected from EDTA, DTPA, NTA or combinations thereof.

18. Process according to any one of claims 1 to 17, wherein the temperature of the lignocellulosic material ranges from about 45 ⁰C to about 200⁰C for a period of time ranging from about 1 second to about 5 hours after at least one mechanical treatment stage.