EP3981912A1

EP3981912A1 - Method of manufacturing a blend of fibers

Info

Publication number: EP3981912A1
Application number: EP20201409.8A
Authority: EP
Inventors: Nuno Miguel dos Santos; Peter Kuske; Jaroslav Stavik; Philippe Stephano; Markus Wiesinger; Christoph Storz
Original assignee: Austrocel Hallein GmbH
Current assignee: Austrocel Hallein GmbH
Priority date: 2020-10-12
Filing date: 2020-10-12
Publication date: 2022-04-13

Abstract

The invention relates to the manufacturing of a fiber blend. In order to provide a more even fiber blend, a first fiber of wood-pulp and an at least one second non-wood fiber are blended in a liquid medium. The invention also relates to a fiber blend.

Description

The invention refers to a method of manufacturing a blend of a first fiber which is a wood-pulp fiber and a second fiber which is a non-wood fiber and to a blend of such first and second fibers.
For the manufacture of cellulose derivatives and direct cellulose dissolution processes, lignocellulosic fibers having specific properties are needed. Such specific properties are depending on the respective method of use of the fiber and comprise defined properties like e. g. viscosity, α-cellulose content, ash content, metal ion content, fiber size distribution or brightness. Often, a blend of fibers is required to provide the required parameters.
In order to adjust the desired fiber properties, the prior art method comprises grinding separately pulp sheets of a first fiber made from wood pulp and of a second fiber made from non-wood fiber, e.g. cotton, followed by dry mixing of said first and second fiber. The resulting fiber blend shows an uneven distribution of fibers comprising clusters of the first and the second fiber, respectively.
It is therefore an object of the present invention to provide for a more evenly mixed blend of fibers.
This object is achieved by a method according to claim 1 and by a blend of fibers according to claim 8.
The method of manufacturing a fiber blend according to the invention comprises blending of a first fiber which is from wood-pulp and at least one second fiber which is a non-wood fiber, said first and second fiber having at least one different fiber property, wherein the step of blending said first fiber and said at least second fiber is conducted in a liquid medium.
Said blending is not conducted for the purpose of papermaking but is conducted in order to provide an improved raw material for the manufacture of e. g. cellulose derivatives or for cellulose direct dissolution, i. e. for cellulose products other than paper.
A wood-pulp fiber which is used as a first fiber according to the invention is a lignocellulosic fiber which is the result of a mechanical, chemo-mechanical or chemical pulping process by means of which lignocellulosic particles like e. g. wood chips from hard- or softwood, particles of annual plants like miscanthus or other lignocellulosic particles, e. g. bamboo chips are manufactured. Lignocellulosic particles are treated either mechanically by grinding or refining, chemo-mechanically by chemical treatment followed by the aforementioned mechanical treatment or by a chemical treatment, most commonly in liquid medium under acidic, neutral or alkaline conditions often in the presence of anorganic salts, e. g. sulfides, sulfites or sodium hydroxide, or organic salts, often at increased temperature and increased pressure. At the end of the chemical pulping process, the particles are discharged from the digester, thereby being at least partly disintegrated into fibers. Depending on the intensity of pulping treatment, pulp grades comprise mechanical and semi-mechanical as well as biomechanical, bio-chemical or fully chemical pulp grades. Well-known pulping methods comprise Kraft pulping, Kraft pulping with pre-hydrolysis, sulfite pulping and organsolv pulping. A wood-pulp fiber according to the invention is derived from a pulping process, the raw material for said pulping process including wood chips or other lignocellulosic particles like e. g. annual plant particles, bamboo particles or the like. The invention is not concerned with the raw material for pulping but only with the fiber resulting from a pulping process.
Selecting the pulping method is a first step towards defining the wood-pulp fiber properties. Downstream of the pulping operation, wood-pulp fiber properties are preferably further adjusted by at least one treatment step in liquid medium prior to drying the fiber. Said treatment steps comprise for example washing, homogenizing, fractioning, enzymatic treatment, bleaching, alkaline treatment, refining and milling. The aforementioned treatment steps may be conducted as a single step or in any sequence or combination that is deemed necessary to adjust fiber properties.
While some wood-pulp fiber properties like brightness, metal content or ash content can be adjusted by further treatment steps downstream of the pulping operation in order to comply with desired properties, other wood-pulp fiber properties like e. g. viscosity or fiber size distribution cannot be sufficiently improved because the desired properties are beyond the properties as defined by the raw material. While a single first fiber may be used in the method and blend according to the invention, often a mix of at least one first sort and one second sort of first fiber may be used as a first fiber for the manufacture of a blend of first and second fibers.
As the requirements for the manufacture of e. g. cellulose derivatives or for cellulose direct dissolution methods sometimes necessitate fiber properties which cannot be met fully by a wood-pulp fiber, the first fiber is blended with a second fiber which is a non-wood fiber usually comprising cellulose. Said non-wood fiber is preferably a cotton fiber, especially a cotton pulp or a cotton linters fiber. The cotton linters fiber may be used as a raw fiber or as a pulped fiber. The second fiber may also be a hemp fiber, a sisal fiber or an abaca fiber. The second fiber may be used as a raw fiber or as a pulped fiber. If waste textile is used as a source for the second fiber, the waste textile tissue will be shredded, cut or milled or the waste fiber tissue will in any other way be reduced to waste textile fiber.
While one second fiber may be sufficient, it is also possible that a first sort and a second sort of second fiber may be used, for example cotton linters fiber and cotton fiber from waste textile or two sorts of cotton linters, each sort showing different properties, e. g. different fiber length distribution, different brightness or different purity. Thus, the second fiber may be a fiber mix of at least two sorts of second fibers. The second fiber differs in its fiber properties from the first fiber. The second fiber which is often more expensive than the first fiber is preferably chosen to complement the properties or deficiencies of the first fiber in the fiber blend in order to provide a blend of fibers for further treatment that complies with the requirements of said further treatment.
Said blend of a first fiber and of a second fiber does not need to have a unimodal distribution of fiber properties. It is acceptable if the blend of fibers according to the invention shows e. g. a bimodal or a polymodal distribution of fiber properties. While the modal distribution of specific fiber properties is not of premier relevance for the blend of fibers according to the invention, even or homogenous distribution of the fibers of the first and the at least one second fiber as a result of blending is relevant for the quality of the blend of fibers. It is an advantage of the blend of fibers according to the invention that blending the fibers in liquid medium results in a homogenous blend of a first and at least one second fiber having different properties. The blending of a first and a second fiber according to the invention does not aim to provide a first and a second fiber having the same properties.
While conducting an immersion of fibers in liquid medium followed by blending and drying as a separate blending method is a viable option, it is energy consuming because not only immersing and blending but also drying of the blend of fibers has to be considered.
According to a preferred embodiment of the invention, blending of a first and a second fiber in liquid medium is conducted after a pulping operation and prior to drying of the first fiber. Thus, an energy consuming separate immersion, blending and drying operation of the first and second fiber is avoided. Addition of the second fiber may be conducted as an immersion into the suspension of the first fiber in liquid medium as soon as the first fiber is discharged from the digester. Addition of the second fiber to a suspension of the first fiber may alternatively be conducted at the latest immediately prior to drying the blend of fibers. The second fiber may be added to a suspension of the first fiber prior, during or after any stage of treatment between discharging the first type of fiber from the digester and prior to drying of the blend of fibers. The method according to the invention offers maximum flexibility for manufacturing the blend of the first fiber and the second fiber.
It is understood that a first sort of second fiber can be admixed to the suspension of the first fiber at a first station and that a second sort of second fiber can be added at a second station to the blend of the first fiber and the first sort of the second fiber immersed in liquid medium.
The liquid medium required for blending the first and the at least one second fiber is preferably aqueous. The liquid medium may contain chemicals like e. g. acid or alkaline components or salts or other additives required for pulping or treating cellulose containing fibers. The liquid medium may also contain residual pulping components from pulping like e. g. dissolved lignin components, dissolved hemicellulose or cellulose components, residual bark or impurities which could not be removed during the pulping process. The liquid medium may be adjusted to a broad range of temperature and even to raised pressure, e. g. during oxygen bleaching. The liquid medium may change after blending the at least one second fiber into a suspension of the first fiber and prior to drying the fiber blend, e. g. by removing cooking chemicals, residuals or impurities or by adding or removing bleaching chemicals.
According to a first alternative the at least one second fiber is added immediately after the wood-pulp fibers are released from the digester. At this stage, the wood-pulp fiber or first fiber is still immersed in a suspension comprising cooking liquid which contains cooking chemicals and residual wood components and impurities. This suspension is called brown stock. The at least one second fiber is immersed into the brown stock. The brown stock containing the first and the at least one second fiber is then washed in order to remove chemicals and residual organic material like dissolved lignin or cellulosic material or impurities. Immersing the at least one second fiber into the brown stock allows for a maximum blending effect as the first and the second fiber are subjected to one or a sequence or a combination of the treatments of the group comprising washing, homogenizing, sorting, fractioning, enzymatic treatment, bleaching, alkaline treatment, refining and milling prior to drying.
According to a second alternative, the at least one second fiber is immersed into the suspension of the first fiber in liquid medium at any stage after the brown stock stage, i. e. during washing the brown stock or at any one or a sequence or a combination of the treatments of the group comprising any stage of washing, homogenizing, sorting, fractioning, enzymatic treatment, bleaching, alkaline treatment, refining and milling prior to drying of the fiber blend allows for reducing the amount of fiber from which to remove the pulping chemicals and pulping residuals and impurities and further allows to subject the at least one second fiber to exactly those treatments which are considered necessary or advantageous in order to achieve a blend of fibers showing optimum properties for further processing, e. g. for the manufacture of regenerate fibers or direct cellulose dissolution. If, for example the washed first fiber suspension is subjected to a sequence of oxygen bleaching, alkaline extraction, ozone bleaching, washing and peroxide bleaching, the at least one second fiber may be added before, at or after each of the aforementioned sequence of bleaching, extraction or washing treatments.
The first and second alternatives of adding the at least one second fiber to brown stock or any further treatment stage allows to use second fibers which require some treatment in order to develop the desired or required fiber properties. Thus, a broad range of second fiber raw material including raw cotton or hemp fiber, e. g. cotton linters containing impurities, but also waste textile fiber containing recycling residuals or colored raw material may be used as treating the second fiber raw material does not require any extra installations but the existing devices for treating wood-pulp fibers can be used. This is considered an added essential advantage of blending first and second fibers in liquid medium if said blending is conducted at a pulp plant.
According to a third alternative, the at least one second fiber is immersed into the suspension of the first fiber in liquid medium prior to drying the blend of fibers. Immersing the at least second fiber prior to drying in the context of this invention means that the at least one second fiber is not subjected to any of the aforementioned treatments but is simply blended into the suspension of the first fiber in liquid medium and is then dried once the blending has resulted in a homogenous blend of fibers. This third alternative is chosen if the properties of the at least one second fiber shall remain unchanged or if a change of properties shall be kept at a minimum.
Blending of the first and second fibers does not necessarily require a separate mixing treatment. The turbulence occurring during transport of the fiber suspension provides a sufficient blending effect on the first and second fibers. However, it is of course possible and may in some instances even be required to install a blending device suited to stir the fiber suspension until it is homogenous. Suitable blending devices are a pump or a blender, preferably a rotating blender.
The blend of fibers may contain a minimum or maximum share of the first fiber and the complement of the blend of fibers is made up of the at least one second fiber. The share of the first fiber may range from 0,001 weight-% (hereinafter: wt.-%) to 99,999 wt.-%. The share of the at least one second fiber may range from 99,999 wt.-% to 0,001 wt.-%. For many purposes, the share of the at least one second fiber will be less than the share of the first fiber as the second fiber shall only make up for deficiencies of the first fiber. The share of the at least one second fiber may for example be 1 wt.-% to 50 wt.-%, preferably 3 wt.-% to 40 wt.-% or 5 wt.-% to 30 wt.-% with the complement of the fiber blend made up of the first fiber.
The blend of fibers prior to drying is a homogenous blend of fibers that does not contain clusters of fibers of the first fiber or clusters of fibers of the second fiber. Instead, an even blend of the first and the at least one second fiber is provided. Homogeneity of the blend of a first and a second fiber is tested by taking samples from a sheet of the fiber blend according to the invention and testing a parameter for homogeneity. Usually, results for all samples of a blend of fibers are at the level of the standard deviation of measurement for a specific parameter like e. g. viscosity, brightness or metal content. Details of testing are given hereinafter. It is a typical feature of the method and the blend of fibers according to the invention that each sample shows a homogenous distribution of the respective share of first and second fibers.
After mixing, the blend of fibers according to the invention is formed into a sheet and dried to e. g. a maximum of 20 wt.-% moisture content, preferably to a maximum of 10 wt.-% of moisture content. The sheet of blended fibers can be used as raw material for e. g. manufacture of cellulose derivatives or regenerates of direct cellulose dissolution. The blend of fibers according to the invention can be manufactured according to customer specification in order to provide the optimum raw material for the production of e.g. cellulose derivatives or regenerates of direct cellulose dissolution. The sheet formed from the blend of a first and at least one second fiber preferably consists of fiber only. Said sheet is not a paper sheet; it does not contain e.g. binder, filler, retention aids, coatings or other substances used in papermaking. The sheet is used as a simple medium for transport of the fibers and the sheet is designed to be disintegrated into a slurry again for the manufacture of e. g. cellulose derivatives or for cellulose direct dissolution methods.
Examples of a fiber blend according to the invention after adjusting the properties of the fiber blend according to a specification of a parameter or a plurality of parameters will be given hereinafter for some exemplary fiber properties. It is apparent that the same principle of adjusting the properties by blending a first and at least one second fiber with different fiber properties will apply to any parameter that is specified for the fiber blend. While in some cases exact parameters need to be achieved by the fiber blend, for most parameters of the fiber blend a range is given. The respective parameter or the range of parameter of the fiber blend may be achieved by a unimodal, bimodal or polymodal fiber blend. The respective parameter or range of parameter is relevant for the further processing of the fiber blend mainly in chemical processes like e.g. manufacturing cellulose derivatives or manufacturing regenerates of direct cellulose dissolution which rely on an even or homogenous fiber raw material that can be processed under the same reaction conditions always.
The first fiber has e. g. a first viscosity and the at least one second fiber has a second viscosity which is different from the first viscosity and the first fiber and the at least one second fiber are chosen to provide a fiber blend having a viscosity of 350 ml/g to 2500 ml/g. A fiber blend according to the invention can for example be adjusted to 350 ml/g to 450 ml/g or to 450 ml/g to 600 ml/g, or viscosity may be adapted to a viscosity of 600 ml/g to 800 ml/g, depending on the customer specification for a method of production of cellulose products. Viscosity may range e. g. from 600 ml/g to 700 ml/g for a specific production process of cellulose derivatives. For other products, viscosity might be adjusted within a range from 1.000 ml/g to 2.000 ml/g. Wood-pulp e. g. from a softwood sulfite pulping process which has been subjected to bleaching in order to remove residual lignin and most of the hemicellulose might provide a viscosity of ca. 800 ml/g. In order to provide a blend of fibers with a viscosity of 1.000 ml/g to 1.300 ml/g, at least one second fiber with a higher viscosity is added to provide for a viscosity of 1.000 ml/g to 1.300 ml/g. The at least one second fiber may comprise a smaller share of fibers having a viscosity of 2.800 ml/g or a larger share of fibers having a viscosity of 2.400 ml/g to 2.700 ml/g. The resulting blend will show a bi- or polymodal distribution of viscosity which can be allotted to the first and second fiber, respectively, but the average range of viscosity of the blend will be 1.000 ml/g to 1.300 ml/g. The desired range of viscosity can be achieved as described above by blending a first fiber with a second fiber having a much higher viscosity so that only a small share of second fiber is required. The desired range of viscosity can be achieved by blending a first fiber with a second fiber having a moderately higher viscosity, e. g. 800 ml/g vs. 1400 ml/g, so that a larger share of second fiber is required. The second fiber can thus be chosen freely according to fiber properties, cost and other relevant considerations.
Calculation of the properties of the blend of a first and at least a second fiber is done according to the rule of proportions as a first approach. For some parameters like e. g. viscosity, brightness or metal content the blending in liquid medium may have a washing effect which leads to a reduction of the viscosity, brightness or metal content compared to the value which has been calculated. The blending ration of the first and the at least one second fiber has to be adjusted accordingly.
The same consideration of the individual properties of the first and the at least one second fiber resulting in a larger or smaller share of the first or the second fiber as outlined-above apply to the adjustment of all parameters that might be specified for a fiber blend.
The first fiber has a first α-cellulose content and the at least one second fiber has a second α-cellulose content which is different from the first α-cellulose content and the first and the at least one second fiber are chosen to provide a fiber blend having an α-cellulose content of at least 85% based on the cellulose content of the blend of the first and the second fiber, preferably at least 89% or at least 95% based on the cellulose content of the blend of the first and the second fiber . According to a preferred embodiment of the invention, the α-cellulose content of the blend of the first and the second fiber is e.g. 99% based on the cellulose content of the blend of the first and the second fiber. The second fiber usually shows the higher α-cellulose content as cotton, sisal, abaca and hemp fiber raw material does contain almost no lignin and only a little hemicellulose.
The first fiber has a an average first fiber length, preferably between 0,5 mm and 2,5 mm and the at least one second fiber has an average second fiber length, preferably between 0,5 mm and 4,0 mm which is different from the first fiber length and the first and the at least one second fiber are chosen to provide a fiber blend having an average fiber length between 0,5 mm and 3,0 mm.
The first fiber has a first content of a metal oxide and the at least one second fiber has a second content of said metal oxide which is different from the first content of a metal oxide and the first and the at least one second fiber are chosen to provide a fiber blend having a specific content of the metal oxide.
The first fiber has a first metal ion content and the at least one second fiber has a second metal ion content which is different from the first metal ion content and the first and the at least one second fiber are chosen to provide a fiber blend having a specific metal ion content.
The first fiber has a first accessibility and the at least one second fiber has a second accessibility which is different from the first accessibility and the first and the at least one second fiber are chosen to provide a fiber blend having an accessibility which is different from the first and the second accessibility. Accessibility in the context of this invention refers to the dissolution of cellulose in a cold aqueous solution of 8 wt.-% caustic , i. e. NaOH based upon the weight of water. Fiber accessibility is then measured by gravimetric measurement of the amount of cellulose dissolved in said 8 wt.-% of caustic solution. With a blend according to the invention, an accessibility of 15% to 60% can be obtained. It should be noted that although for most other parameters, the rule of proportion applies closely, accessibility of the blend of the first and second fibers is often better by 5% or more, preferably by up to 8%, especially by up to 10% than indicated by the theoretical calculation based upon the rule of proportion.
Any feature of the method or the blend of fibers according to the invention may be combined to any other feature of the invention. Thus, it is apparent that even complex requirements for a blend of fibers can be met by using the method according to the invention.
Various embodiments will be given in order to explain aspects of the invention.

Fig. 1: shows sampling of test samples from the blend of first and second fiber according to the invention

Table 1 below indicates that the second fiber may be added and blended with the first fiber at any stage of treatment of the first fiber from brown stock until prior to drying. The blend of first and second fibers is dried without any further components like e. g. binder, filler, retention aid or other additives. It is not meant for paper production but as a raw material for the manufacture of e. g. cellulose derivatives.

Table 1 Adding of Second Fiber

	Example 1	Example 2	Example 3	Example 4
1^st fiber type	Softwood sulfite pulp	Softwood sulfite pulp	Softwood sulfite pulp	Softwood sulfite pulp
2^nd fiber type a)	Cotton linters	Cotton linters	Cotton linters	Cotton linters
2^nd fiber type b)				Textile waste cotton fiber
blended into
Brown stock	X
Blended during bleaching		X
Blended prior to drying			X
2^nd fiber type a) blended into brown stock				X
2^nd fiber type b) blended in prior to drying				X

The first fiber used in this embodiment is a wood fiber which is a result of a pulping process. Softwood is pulped, e. g. by sulfite pulping, to yield wood fiber as a first fiber. Sulfite pulping uses sulfite containing cooking liquor which is added to a digester together with wood chips. Wood chips and cooking liquor are heated and the cooking liquor dissolves the lignin in the wood chips at least partly. The pulped wood chips and the cooking liquor containing the dissolved lignin are discharged from the digester as brown stock. Immediately downstream of the digester, the wood chips are then defiberized and the cooking liquor containing the dissolved lignin is removed in a washing process. Defiberizing and washing occurs without intermediate drying of the first fiber. After washing, the first fiber is bleached in a single stage or, preferably, in a multi-stage process, also without any intermediate drying. Once the bleaching is completed, the first fiber is dried for the first time. Bleaching may preferably comprise a chlorine-free bleaching sequence, e. g. an oxygen treatment in alkaline condition, an alkaline extraction stage, an ozone stage in acid condition, a washing stage and an alkaline peroxide stage.
The second fiber which is a cotton fiber, a sisal fiber, an abaca fiber or a hemp fiber and which may be e.g. raw or pulped cotton linters or shredded waste textile fiber from cotton or hemp textiles is usually delivered in the form of a sheet or a reel. Prior to blending with the first fiber, said sheet or reel of the second fiber is then disintegrated in a device for disintegrating pulp by adding the sheet or reel to water and stirring until a homogenous slurry with a consistency of e.g. 20 wt.-% of second fiber is obtained. Disintegration usually takes 10 minutes to 30 minutes at ambient temperature. The second fiber slurry is then preferably stored in a high-consistency storage tank.
The blending of the first and the second fiber can be done by weight or volumetrically. As the first and the second fiber are usually available in liquid medium as a slurry, and as the consistency of each slurry is known or can be controlled, volumetric blending is preferred. Devices for analyzing the consistency of a slurry are available as optical consistency analyzers or as consistency analyzers based upon detection of shearing force. Consistency analyzers are known as blade consistency analyzers, microwave consistency analyzers, optical consistency analyzers or rotating consistency analyzers. Based upon information about the consistency of at least two slurries to be blended volumetrically, metering devices for metering the required volume of slurry of the first and the second fiber to be mixed are known, e.g. pumps pumping exact volumes of a slurry. The second fiber slurry is blended with the first fiber slurry preferably by a blending device, e. g. a blender or stirrer or a blending pump, thereby inducing shear force to blend the first and the second fiber slurry. The blending takes usually up to 15 minutes but can be done in 1 to 3 minutes if consistency is higher than 10%. Slurries with a consistency of e. g. 5% or less can be blended within seconds, maximum in one minute. No blending device is required if the second fiber is blended with the first fiber at a stage where treatment of the first fiber requires application of shear forces, e. g. for washing or during the addition of bleaching chemicals or the removal of chemicals or dissolved material during extraction stages.
If a mixture at least two types of first fiber and/or second fiber shall be used for the blend of first and second fiber, a mix of the two types of first and/or second fiber can be prepared by blending a slurry of the first and the second type of fiber prior to blending the slurries of the first and the second fiber. Alternatively, each type of fiber can be disintegrated in water and be admixed separately to the slurry of the first fiber.
As shown in Table 1, Example 1 blending of the first and second fiber may occur during the brown stock stage prior to washing the first fiber. A blending device may be used but is not required as the first and the second fiber will undergo a washing stage which will be sufficient for a homogenous blending of the first and the second fiber. Blending the second fiber this early with the first fiber is preferred when the second fiber might be improved by washing and e. g. bleaching.
According to Table 1, Example 2, the second fiber is blended with the first fiber during the bleaching process which may comprise a one stage treatment or a multi-stage treatment. Blending may be conducted with the help of a blending device or blending can be done by using equipment which is available on site and which is used for blending the first fiber slurry with e.g. bleaching chemicals or which is used for a washing stage but which is also suited to blending the second fiber slurry to the first fiber slurry. If the second fiber upon blending passes the bleaching stages with the first fiber, either the amount of bleaching chemicals will have to be adjusted to the increased amount of fiber to be treated or, as the second fiber might not require much bleaching, the amount of bleaching chemicals may remain unchanged resulting in e. g. a lower brightness of the fiber blend or a higher viscosity of the fiber blend.
According to Example 3 of Table 1, the second fiber is admixed after the bleaching sequence prior to drying. In this case, a blending device, e. g. a rotor using shear force, is used and blending is done as described hereinbefore. Upon blending for ca. 3 minutes, the blend of the first and second fiber is dried in sheet form.
Example 4 of Table 1 uses two types a) (cotton linter fiber) and b) (waste textile fiber) of second fiber. While both types of second fiber are cotton fibers, type a) had different properties compared to type b). For each type of second fiber, a separate slurry may be prepared as described-above. Alternatively, one slurry of second fiber containing both types of second fiber may be prepared. Upon blending, the blend of the first and second fiber is dried in sheet form.
Based on Example 3, a specific blending of first and second fibers is described. The first fiber which is softwood sulfite pulp fiber and the second fiber which is cotton linters, are blended volumetrically in a ratio 2:1 based on the volume of the first fiber immediately prior to drying. The first fiber is e. g. available as a slurry with a consistency of 10 wt.-%, meaning the slurry contains 10 wt.-% of fiber in water. The second fiber is available as a slurry with a consistency of 20 wt.-%. Blending is conducted by a blending device as described-above. Blending is done within 5 minutes. The blend of the first and the second fiber is then dewatered and formed into sheets which are then dried to e.g. a moisture content of 12 wt.-% maximum. Said moisture content is calculated based upon the absolute dry fiber content; the absolute dry fiber remains after drying at 105 °C until constant weight is achieved.
In order to explain the homogenous character of the blend of the first and second fiber, sheets from the production have been analyzed and the results are presented in Table 2 and 3.
The following pulp testing methods including the preparation of test sheets were used to analyze and to characterize the first and the at least one second fiber and the blend of a first and at least one second fiber according to the invention:

Pulp sheet preparation was conducted according to ISO 5269-1.
Viscosity measurement was conducted according to ISO 5351.
Brightness Measurement was conducted according to ISO 2470.
Consistency measurement was conducted according to ISO 4119.
Fiber length distribution analysis was conducted according to ISO 16065-2

Test samples were taken according to Fig. 1 in equal distances in the direction of production (7 samples of Table 2) and across the sheet made from the blend of first and second fiber (6 samples of Table 3). Thus, the samples of Table 2 and 3 provide information about the blend of first and second fibers across the sheet which is the final product of the method according to the invention.

Prior to blending, the first fiber has an average viscosity of 565 ml/g and the second fiber has a viscosity of 2150 ml/g. The first fiber has a brightness of 92,0 % ISO and the second fiber has a brightness of 76,5 % ISO prior to blending. The first fiber and the second fiber were blended in a ratio of 2 : 1 and the blending was done prior to drying with a blender.

Table 2 Brightness and Viscosity Measurement in Machine Direction

Viscosity (ml/g)	982	1001	995	993	992	990	1007
Brightness (% ISO)	85,0	85,0	85,2	85,0	84,9	85,2	85,3
	Num.1	Num.2	Num.3	Num.4	Num.5	Num.6	Num.7
Average
Viscosity	994	(ml/g)
Brightness	85,1	(% ISO)

Testing reveals that the samples show brightness and viscosity results which are closely within the target range for each parameter. The brightness test results vary from 84,9 % ISO to 85,3 % ISO, thus they are within the standard deviation for brightness measurement but only marginally lower than the theoretically calculated brightness of 85,6 % ISO. This can only be achieved if the same ratio of first and second fiber is contained in each sample. Thus, the test results confirm that a homogenous blend of first and second fibers has been achieved. For viscosity measurement, the test results are also within the standard deviation for the test method and also marginally lower than the theoretically calculated viscosity which could be expected according to the rule of proportion. The blend of first and second fibers is thus homogenous in the direction of production and across the sheet of the dried blend of fibers.

Table 3 Brightness and Viscosity Measurement in Cross Machine Direction

Viscosity (ml/g)	1001	994	991	988	1000	995
Brightness (% ISO)	85,0	85,2	85,1	85,0	85,0	85,0
	Num.1	Num.2	Num.3	Num.4	Num.5	Num.6
Average
viscosity	995	(ml/g)
Brightness	85,1	(% ISO)
Theoretical visco sity calcul ation			10 44 ml/g
Theoretical brigh tness calc ulation			85 ,6 % ISO

In another example, the metal oxide content has been adjusted. A bleached spruce sulfite pulp has a calcium oxide (CaO) content of 23 ppm. Said bleached spruce sulfite pulp is blended with cotton linters pulp with a ratio of 2 parts spruce sulfite pulp as a first fiber: 1 part cotton linters as a second fiber. Blending is done prior to drying. The cotton linters pulp prior to blending has a CaO content of 264 ppm. CaO measurement is done according to the ICP-OES standard. Theoretically, the blend should have shown a CaO content of 102 ppm. Upon testing the actual blend, a CaO content of 99 ppm was found. The reduced CaO content may be due to a washing effect after blending and dewatering the blend of first and second fiber for drying.
In a further example, the metal ion content has been adjusted. A bleached spruce sulfite pulp has an iron ion content (Fe content) of 0,68 ppm. Said bleached spruce sulfite pulp is blended with cotton linters pulp with a ratio of 2 parts spruce sulfite pulp as a first fiber: 1 part cotton linters as a second fiber. Blending is done prior to drying. The cotton linters pulp prior to blending has an Fe content of 5,35 ppm. Fe content is measured according to the ICP-OES standard. Theoretically, the Fe content of the blend should have been 2,21 ppm. Upon testing the actual blend, a Fe content of 1,81 ppm was found. The significantly reduced Fe content may be due to a washing effect after blending and dewatering the blend of first and second fiber for drying.
In both cases, metal oxide content and metal ion content, the additional reduction in metal oxide and metal ion content is a desirable result and is considered a significant advantage of the method and the blend of first and second fiber according to the invention. When calculating the ratio of first and second fiber of the blend, this effect can be considered by a factor which can be determined by a few tests as the factor appears to be dependent on the conditions of the specific site of blending.

Claims

Method of manufacturing a fiber blend comprising a first fiber which is a wood-pulp fiber and at least one second fiber which is a non-wood fiber, said first and second fiber having at least one different fiber property, wherein the step of blending said first fiber and said at least second fiber is conducted in a liquid medium.
Method according to claim 1 wherein the second fiber is chosen from a group which comprises cotton fiber, sisal fiber, abaca fiber and hemp fiber.
Method according to claim 1 or 2 wherein the blend of at least one first and at least one second fiber is dried after blending.
Method according to one of the claims 1-3 wherein the first fiber or the blend of fibers is treated by at least one or a sequence or a combination of treatments of the group of treatments comprising washing, homogenizing, sorting, fractioning, enzymatic treatment, bleaching, alkaline treatment, refining and milling.
Method according to one of the preceding claims 1-4 wherein the at least one second fiber is blended into a suspension of the first fiber prior, during or after conducting at least one or a sequence or a combination of treatments of the group of treatments comprising washing, homogenizing, sorting, fractioning, enzymatic treatment, bleaching, alkaline treatment, refining and milling.
Method according to one of the claims 1-4 wherein the at least one second fiber is blended into the brown stock of the first fiber.
Method according to one of the preceding claims 1-4 wherein the at least one second fiber is blended into the suspension of the first fiber prior to drying of the fiber blend.
Fiber blend of a first fiber which is a wood-pulp fiber and a second fiber which is a non-wood fiber, said fiber blend having a maximum moisture content of 20 wt.-%.
Fiber blend according to claim 8 wherein a share of the first fiber is within a range of 0,001 wt.-% and 99,999 wt.-% and the complement of the blend of fibers is made up of the at least one second fiber.
Fiber blend according to claim 8 wherein the first fiber has a first viscosity and the at least one second fiber has a second viscosity which is different from the first viscosity and the first fiber and the at least one second fiber are chosen to provide a fiber blend having a viscosity of 350 ml/g to 2500 ml/g.
Fiber blend according to claim 8 wherein the first fiber has a first α-cellulose content and the at least one second fiber has a second α-cellulose content which is different from the first α-cellulose content and the first and the at least one second fiber are chosen to provide a fiber blend having an α-cellulose content of at least 85% to at least 99%.
Fiber blend according to claim 8 wherein the first fiber has a first fiber length and the at least one second fiber has a second fiber length which is different from the first fiber length and the first and the at least one second fiber are chosen to provide a fiber blend having a fiber length which is different from the fiber length of the first and the second fiber.
Fiber blend according to claim 8 wherein the first fiber has a first metal oxide content and the at least one second fiber has a second metal oxide content which is different from the first metal content and the first and the at least one second fiber are chosen to provide a fiber blend having a metal oxide content which is different from the metal oxide content of the first and second fiber.
Fiber blend according to claim 8 wherein the first fiber has first fiber accessibility and the second fiber has a second fiber accessibility, the fiber accessibility of the first and the second fiber being different and the fiber blend has a fiber accessibility which is different from the first and the second fiber accessibility.
Fiber blend according to claim 8 wherein a first fiber comprising brown stock wood pulp fiber and/or at least partly bleached wood fiber and at least one second fiber, said first and second fibers having been blended in a liquid medium and dried to a fiber blend having a maximum moisture content of 20 wt.-%.