CHEMICAL PULP

Technical field

The present invention relates to the field of paper making, in particular it relates to an energy efficient method of beating a chemical pulp.

Background

Wood is composed of cellulose, hemicellulose, lignin and extractives and is the most common raw material in papermaking. Typically, a paper making process includes debarking and chipping of logs followed by a chemical or mechanical pulping process. Chemical pulping can be an acidic process, such as the sulfite process or an alkaline process, such as the sulfate process (Kraft process). In the Kraft process, wood chips are cooked in a mix of sodium hydroxide and sodium sulfide. Prior to cooking, the chips are normally first wetted and preheated with steam. This step is followed by a step of impregnation of the chips with black and white liquor,

comprising sodium hydroxide and sodium sulfide. The chips are thereafter cooked in a digester for several hours, at a temperature between 130 °C to 180 °C, and during this process lignin and some hemicellulose is degraded and dissolved in the cooking liquid. The remaining cellulose fibers are blown or squeezed from the outlet of the digester through an airlock. The swift change in pressure results in a sudden expansion of the fibers, which leads to a further separation of the fibers. The resulting fiber suspension in water solution is called "brown stock" due to its brown color. The brown stock is thereafter washed in a series of brown stock washers, in order to remove degraded lignin and hemicellulose, as well as other contaminates such as spent cooking chemicals. The extracted liquid is called black liquor. Sodium and sulfur compounds from the black liquor is recovered and recycled in the process and the remains are burned to produce energy. In the sulfite process the extracted liquid is called red liquor, brown liquor or spent sulfite liquor. Lignosulfonate is a valuable product which can be recovered from the spent sulfite liquor. After the brown stock washing steps a clean pulp is obtained and depending of the intended use of the paper, the clean pulp can optionally be bleached. The pulp is next put through a mechanical rubbing action called beating, which increases the bonding ability of the fibers so that most of the strength properties of the produced paper will be improved. Different fillers and other additives is thereafter added to the beated pulp to produce a furnish. The additives can for example improve different characteristics of the paper and the filler can be added to reduce the amount of fibers in the paper which will reduce the costs. The furnish is finally fed or pumped into a paper machine. The paper machine typically consists of forming, press and dryer section. In the forming section the furnish is dewatered to form a paper web. After the forming section the web enters the press section, the dewatering continues by pressing the web under load. Finally the paper is dried in the dryer section, using steam

Summary of the present disclosure

Cellulose fibers present in a chemical pulp, such as a sulfate pulp or a sulfite pulp are typically straight, smooth, rounded and largely undamaged. These characteristics make the bonding between the fibers fairly weak. To be able to produce paper with high strength the contact area and thus the strength of the bonds between the fibers need to be increased. This is achieved by a process known as beating wherein the pulp is subjected to a mechanical rubbing action which flattens the fibers and unravels microfibrils. Thereby the surface area and the hydrogen bonding potential between fibers are greatly increased. Increased beating of pulp therefore generally increases the strength of the pulp (i.e. strength of a hand sheet made from the pulp) as well as the strength of a paper product generated from said pulp. However, the beating of the pulp is a highly energy demanding step and there is thus a need for improved energy efficient methods for making paper from a chemical pulp. In particular there is a need for improved energy efficient methods for beating of chemical pulps.

The present inventor has discovered that the energy required for beating a pulp, such that the strength of the pulp reaches a certain value, can be vastly reduced if lignosulfonate is present in the beater during the beating process. Therefore the energy needed for producing a paper product can be vastly reduced if lignosulfonate is added to the process such that the lingosulfonate is present in the beater.

Thus a first aspect of the invention relates to a method for production of a paper product comprising the following step:

a) providing a chemical pulp

b) adding lignosulfonate to the chemical pulp such that a chemical pulp comprising lignosulfonate is obtained c) beating the chemical pulp comprising lignosulfonate obtained in step b) such that a beated chemical pulp is obtained, and

d) producing a paper product from the beated chemical pulp obtained in step c)

A second aspect of the invention relates to a method of increasing the efficiency of a beating of a chemical pulp in an industrial process for production of a paper product comprising the following steps:

a) providing a chemical pulp

b) adding lignosulfonate to the chemical pulp such that a chemical pulp comprising lignosulfonate is obtained

c) beating the chemical pulp comprising lignosulfonate obtained in step b) such that a beated chemical pulp is obtained, and

d) optionally producing a paper product from the beated chemical pulp obtained in step c)

A third aspect of the invention relates to use of lignosulfonate for increasing the efficiency of a beating of a chemical pulp in an industrial process for production of a paper product.

Brief description of the figures

Figure 1 a shows the energy needed for beating of a pulp to a tensile strength of 100 Nm/g in the presence of 1 % lignosulfonate, 0.3 % lignosulfonate or in the absence of lignosulfonate (ref). The y-axis shows the beating energy in kWh/ton.

Figure 1 b shows the beating degree of a pulp beated to a tensile strength of 100 Nm/g in the presence of 1 % lignosulfonate, 0.3 % lignosulfonate or in the absence of lignosulfonate (ref). The y-axis shows the beating degree in degree Schopper- Riegler (°SR).

Figure 2 a shows the tensile strength of a pulp beated with a beating energy of 100 kWh/t in the presence of 1 % lignosulfonate, 0.3 % lignosulfonate or in the absence of lignosulfonate (ref). The y-axis shows the tensile strength in Nm/s.

Figure 2 b shows the compression strength of a pulp beated with a beating energy of 100 kWh/t in the presence of 1 % lignosulfonate, 0.3 % lignosulfonate or in the absence of lignosulfonate (ref). The y-axis shows the compression strength in Nm/s. Figure 3 shows the fiber length of the pulps after beating with different beating energy in the presence of 1 % lignosulfonate, 0.3 % lignosulfonate or in the absence of lignosulfonate (ref). The x-axis shows the beating energy in kWh/ton and the y-axis shows the fiber length in mm.

Detailed description

The present invention is based on the discovery that the energy required for the beating of a chemical pulp, such that the strength of the chemical pulp reaches a certain value, can be vastly reduced if lignosulfonate is present in the beater during the beating process. Therefore the energy needed for producing a paper product can be reduced if lignosulfonate is added to the process such that the lignosulfonate is present in the beater. Accordingly, a first aspect of the invention relates to a method for production of a paper product comprising the following step:

a) providing a chemical pulp

b) adding lignosulfonate to the chemical pulp such that a chemical pulp comprising lignosulfonate is obtained

c) beating the chemical pulp comprising lignosulfonate obtained in step b) such that a beated chemical pulp is obtained, and

d) producing a paper product from the beated chemical pulp obtained in step c)

In a second aspect the invention relates to a method of increasing the efficiency of a beating of a chemical pulp in an industrial process for production of a paper product comprising the following steps:

a) providing a chemical pulp

b) adding lignosulfonate to the chemical pulp such that a chemical pulp comprising lignosulfonate is obtained

c) beating the chemical pulp comprising lignosulfonate obtained in step b) such that a beated chemical pulp is obtained, and

d) optionally producing a paper product from the beated chemical pulp obtained in step c)

Exemplary embodiments of the first and second aspect are further described below. In the art, mechanical pounding and squeezing processes of cellulosic fibers in a pulp can some times be described as beating of a pulp and some times as refining of a pulp. The term beating in the present disclosure covers all these different kinds of mechanical pounding and squeezing processes of pulps. In the present disclosure, the expression "increasing the efficiency of a beating of a chemical pulp" means that less energy is needed in the beating step to produce a chemical pulp of a certain strength. The strength of the pulp can for example be measured as a tensile strength or a compression strength of a hand sheet produced from the pulp. In the present disclosure the expression "industrial process" means that the process in not a laboratory experiment but that a significant and industrially relevant amount of pulp is beated in the beating step, such as for example more than 500 kg/h such as more than 1 ton/h.

The present inventors have demonstrated that the addition of lignosulfonate to the beating step can decreases the energy consumed in the beating of a sulfate pulp, to a specific strength, with as much as 50 %. Accordingly, in one embodiment the addition of lignosulfonate added in step b) decreases the energy consumed in step c) with at least 10 % such as at least 20 % such as at least 35 % such as with about 50 %, in the beating of a pulp to a certain strength. It is surprising that addition of lignosulfonate can decreases the energy consumed in the beating with as much as 50 %, especially in the light of previous experiment where the effect of lignosulfonate as a beater additive in a beating of commercial hardwood unbleached Kraft pulp has been investigated in laboratory experiment. The conclusion of these experiments was that lignosulfonate need to be chemically modified by desulfonation and amination to accelerate the beating. Unfractionated unmodified lignosulfonate did not show any positive effect of the beating at all, and a low molecular fraction of the lignosulfonate even had a negative effect on the beating (Levon et al. 1986). It should be mentioned that Levon et al. did not measure the actual strength of the pulp, such as the tensile strength or the compression strength. Instead the acceleration effect on beating was measured as CSF (Canadian Standard Freeness). CSF is not a direct measure concerning the strength properties of pulp but was considered as an adequate estimation by Levon et al. CSF is in fact a measure of the beating degree of the pulp where a low CSF corresponds to a high beating degree. It is also known in the art that a low beating degree (i.e. a high CSF) facilitates the dewatering and drying. Thus a low beating degree is desirable to save energy in the papermaking process. Since Levon et al. approximate strength with beating degree it is not possible to determine from Levon et al. if the energy saved by a more efficient beating is compensated for the higher energy consumption in the drying and dewatering, due to the increased beating degree. Levon et al. is not at all addressing dewatering aspects or energy saving aspects in the paper making process. Since Levon et al report that unfractionated, lignosulfonate is ineffective, unless it is chemically modified by addition of aminogroups and preferably desulfonation, Levon et al. clearly teaches away from a method according to the present invention where a lignosulfonate obtained from a sulfite pulping process is added to the beating process without any further chemical modification of the lignosulfonate molecules.

Surprisingly the present inventors have demonstrated that this gives rise not only to a decreased energy consumption in the beating process, beating the pulp to a specific strength, but also to a decreased beating degree, at the specific strength, leading to reduced energy consumption also in the dewatering and drying process.

In the present disclosure, the term pulp strength or strength of pulp refers to the strength of a hand sheet (in the art commonly referred to as a paper hand sheet or pulp hand sheet) made from the pulp. The strength of a pulp is an important feature since the strength of the pulp correlates with quality parameters of the final paper products produced from the pulp. For example, increased pulp strength generally leads to increased paper strength. Furthermore, if the strength of the pulp is high it is possible to use less fiber (i.e. pulp) and more fillers (e.g. clay and calcium carbonate) to obtain a paper product of a certain quality. Since the cost of the pulp is higher than the cost of the fillers, the total cost of the paper product can be reduced. As mentioned above, the present inventor has also demonstrated that the beating degree is lower in a pulp beated in the presence of lignosulfonate compared to a pulp beated to the same strength in the absence of lignosulfonate. A lower beating degree makes it easier to dewater and/or dry the pulp which further saves energy in the process of production of a paper product. Since dewatering of a pulp can be a limiting step of the paper production, the lowered beating degree, mediated by beating in the presence of lignosulfonate, can increase the production capacity of a paper mill. Therefore, in one embodiment the method according to the present invention can be a method for decreasing the energy needed for dewatering and/or drying of a beated chemical pulp. In another embodiment the method according to the present invention can be a method of increasing the production capacity of a paper mill. In one embodiment the method according to the present invention is a method of decreasing the energy consumption in the production of a paper product of certain strength. In one embodiment the method according to the present invention is a method of decreasing the energy consumption in an industrial process for production of a paper product of certain strength. In one embodiment the method according to the present invention is a method of decreasing the energy consumption in a beating step, dewatering step and/or drying step in an industrial process for production of a paper product of certain strength. In one embodiment the method according to the present invention is a method of decreasing the energy consumption in a beating step and in at least a dewatering step and/or a drying step in an industrial process for production of a paper product of certain strength. In one embodiment the method is a method of increasing the strength of a paper product using certain energy consumption in the beating step.

The method according to the present invention is suitable both for high concentration (HC) beating and for low concentration beating (LC). HC beating is typically performed at a pulp concentration of about 25-30 % (w/w) and LC beating is typically performed at a pulp concentration of about 2-4% (w/w). LC-beating is very efficient for increasing the strength of the pulp and of the finished paper product and thus in a preferred embodiment the beating in step c) is a LC beating. Accordingly, in one embodiment the concentration of the pulp in step c) is between 1 and 5 % (w/w), such as between 2 and 4 % (w/w). In an alternative embodiment the beating in step c) is a HC beating and in one embodiment the concentration of the pulp in step c) is between 20 and 35 % (w/w), such as between 25 and 30 % (w/w).

As discussed above, the present inventor has demonstrated that the addition of lignosulfonate to the beating step can decreases the energy consumed in the beating of a sulfate pulp with as much as 50 % beating the pulp to a certain strength. Since the physical properties of the fibers in different types of chemical pulps are quite similar the method according to the present invention is suitable for any kind of chemical pulp. However, particularly suitable pulps according to the present invention are sulfate pulps and sulfite pulps. Therefore, in one embodiment the chemical pulp is a sulfate pulp or a sulfite pulp and in a preferred embodiment the chemical pulp is a sulfate pulp.

A particularly suitable lignosulfonate according to the present invention is a soft wood lignosulfonate prepared from spent sulfite liquor obtained in a sulfite pulping process. The present inventors have demonstrated that addition of such a lignosulfonate to the beating process can decrease the energy consumed in the beating with as much as 50 %, without any need for a fractionation or chemical modification of the lignosulfonate. Thus, in one embodiment the lignosulfonate is a soft wood lignosulfonate and in a preferred embodiment the lignosulfonate added in step b) has been prepared from spent sulfite liquor obtained in a sulfite pulping process. In one embodiment at least part of the lignosulfonate such as at least 50 % (w/w), such as at least 75% (w/w), such as at least 95 % (w/w), is added to step b) without any fractionation of the lignosulfonate and in another embodiment at least part such as at least 50 % (w/w), such as at least 75% (w/w), such as at least 95 % (w/w), of the lignosulfonate is added to step b) without any chemical modification of the

lignosulfonate molecules. However it is possible that the energy efficiency of the beating process can be even further improved if the lignosulfonate is of a certain molecular weight. For example it is possible that a high molecular weight faction in some cases can decrease the energy consumed in the beating more than if for example a non-fractionated lignosulfonate is added. Such high molecular fractions of lignosulfonate can for example be obtained by filtering the lignosulfonate through a 20 kDa membrane cut-off filter or a 30 kDa membrane cut-off filter, or through a 40 kDa membrane cut-off filter, or through a 50 kDa membrane cut-off filter. In most cases this is however less preferred given the good results of an unfractionated lignosulfonate and given the costs associated with fractionation of the lignosulfonate. The average molecular weight of an unfractionated lignosulfonate prepared from spent sulfite liquor is typically in the range 1 -100 kDa. Thus In one embodiment the average molecular weight of the lignosulfonate is 1 -100 kDa, such as 5-75 kDa, such as 10-60 KDa. It is also possible that introduction of chemical modifications in the lignosulfonate can increase the efficiency of the lignosulfonate as a beater additive but this is also less preferred considering the good results of an unmodified

lignosulfonate and considering the costs generally associated with introduction of chemical modifications. Preferably the lignosulfonate does not contain amino groups covalently bound the lingosulfonate molecules, In one embodiment at least 75 %, such as at least 90 % of the lignosulfonate molecules does not comprise amino groups. In one embodiment the nitrogen content of the lignosulfonat, i.e the content of nitrogen covalently bound to the lignosulfonate molecules, is below 0.5 % (w/w) such as below 0.2 % (w/w). such as below 0.1 % (w/w), such as below 0.05 % (w/w), such as below 0.01 % (w/w), such as below 0.005 % (w/w), such as below 0.001 % (w/w). Lignosulfonate molecules obtained in a sulfite pulping process generally only consist of sulfonated lignin and possibly carboxylic acid groups. Therefore, in a preferred embodiment at least 50 %, such as at least 75 %, such at least 90 % of the lignosulfonate molecules only consists of sulfonated lignin and does not comprise any other functional groups except possibly carboxylic acid groups. In one embodiment the lignosulfonate is sulfonated lignin according to the CAS number 8062-15-5.

Lignosulfonate is a product which can be recovered from the spent sulfite liquor. Typically isolation of lignosulfonate from spent sulfite liquor involves the step of removing water from the spent sulfite liquor by evaporation. Thereby thick spent liquor having a dry matter content of about 60-70 % can be obtained. In the industry, this lignosulfonate fraction can be used directly either as fuel in a sulfite pulping process or it can be sold as a valuable product. Alternatively the lignosulfonate can further be spray dried to obtain a dry lignosulfonate product. The present inventors have realized that it is beneficial if the lignosulfonate is added to the beating step in a liquid form since this will facilitate mixing of the lignosulfonate with the pulp. This will in turn further increase the efficiency of the beating. Therefore, in one embodiment the lignosulfonate added in step b) is thick spent liquor which has been obtained by elimination of at least part of the water present in the spent sulfite liquor. In another embodiment the elimination of at least part of the water present in the spent sulfite liquor has been achieved through evaporation. In a preferred embodiment the dry matter content of the lignosulfonate added in step b) is in a range 20-60 %, such as 30-50 %.

The present inventor has further realized that in some cases high levels of sugars or sugar acids in the lignosulfonate might have a negative effect on the beating. Sugars can be removed from the lignosulfonate by subjecting the spent sulfite liquor to fermentation. Therefore, in one embodiment the method further comprises a step of fermenting the spent sulfite liquor to decrease the amount of sugars in the

lignosulfonate. In one embodiment the amount of sugars and/or sugar acids in the lignosulfonate added in step b) is below 10 % (w/w) such as below 5 % (w/w). In one embodiment the amounts of sugar acids introduced to the chemical pulp during step a), b) and/or c) is less than 90 g/ ton beaten pulp.

The most common lignosulfonates obtainable from a sulfite process is sodium lignosulfonate, magnesium lignosulfonate ammonium lignosulfonate and/or calcium lignosulfonate. For economical reasons it is preferred to use any of these

lignosulfonates. Thus, in one embodiment the lignosulfonate is selected from sodium lignosulfonate, magnesium lignosulfonate, ammonium lignosulfonate and/or calcium lignosulfonate. In a preferred embodiment the lignosulfonate is sodium lignosulfonate.

The present inventor has demonstrated that addition of 1 % lignosulfonate to a beating process of a chemical pulp can decrease the energy consumed in the beating step with about 50 %. However the effect of addition of lignosulfonate is evident at much lower concentrations. Since lignosulfonate is a colored polymer, it can some times be preferred to add lower amounts of lignosulfonate to the pulp. For example if a high amount of lignosulfonate is added in a process of producing white paper, more bleach chemicals will be needed. However in most cases it is preferred that the concentration of lignosulfonate is above 0.05% (w/w). Thus in one

embodiment the concentration of lignosulfonate in step c) is at least 0.05 % (w/w) such as at least 0.1 % (w/w), such as at least 0.3 % (w/w). In one embodiment the concentration of lignosulfonate is between 0.05 to 5 % (w/w), such as 0.1 to 3 % (w/w), such as 0.3 % to 1 % (w/w). For clarity reasons it should be pointed out that the concentration of lignosulfonate mentioned in the present disclosure is in relation to the amount of pulp. For example the expression "addition of 1 % (w/w)

lignosulfonate" in the present disclosure means that 10 kg lignosulfonate is added to 1 ton of pulp. In the beating step the concentration of pulp is usually about 2 to 4 %. This means that if the concentration of lignosulfonate is 1 % (w/w) in relation to the amount of pulp, and the beating process is performed at a pulp concentration of 4 % (w/w), the actual concentration of lignosulfonate in the beating step is 0.04 % (w/w).

The lignosulfonate can be added to the process at any step upstream of the beaters provided that the lignosulfonate will not be removed from the process by for example a washing step prior to the beating. Therefore, in one embodiment, at least part of the lignosulfonate is added to a clean pulp in a step prior to the beating step but after a brown stock washing step. However, it is preferred that the lignosulfonate is added in a step close to the beating, for example directly to the beater. Addition of lignosulfonate directly to the beater facilitates a good mixing of the lignosulfonate with the pulp which further increases the efficiency of the beating. Therefore, in one embodiment at least part, such as at least 50 %, such as at least 75 %, such as at least 90 % of the lignosulfonate is added directly to the beater in a pulp and/or paper mill. Typically about 10 ton pulp / hour is beated in an industrial beating process in a pulp and/or paper mill. In one embodiment at least 100 kg pulp such as at least 1 ton pulp, such as at least 10 ton pulp is beated in step c). In one embodiment at least 100 kg pulp/hour such as at least 1 ton/ hour such as at least 10 ton/hour is beated in step c). The beating step can be performed in any pulp-beater or pulp-refiner known to the skilled person. Examples of such beaters and refiners include disc refiners and cone refiners. Thus, in one embodiment the beating in step c) is performed in a disc refiner and/or cone refiner and in one embodiment the lignosulfonate is added directly to a disc refiner and/or a cone refiner.

As described above the present inventor has discovered that lignosulfonate can be used in the beating step in the beating of a chemical pulp to reduce the energy consumption in an industrial process for production of a paper product. More specifically the present inventor has shown that use of lignosulfonate in the beating process reduces the energy consumption in the beating process, beating a pulp to a specific strength. This also means that if the same amount of energy is used in the beating process, in the beating of a pulp in the presence of lingosulfonate as in the absence of lignosulfonate, a paper product of higher strength can be produce by addition of lignosulfonate in the beating process. Thus lignosulfonate can be used in the beating process to produce a paper product of increased strength at a certain energy consumption in the beating process. Furthermore lignosulfonate can be used in the beating process for reducing the energy consumption in a drying or dewatering step, subsequent of the beating process in an industrial process for production of a paper product.

Thus, a third aspect of the invention relates to use of lignosulfonate for increasing the efficiency of a beating of a chemical pulp in an industrial process for production of a paper product. In one embodiment the use of lignosulfonate is for reducing the energy consumption in an industrial process for production of a paper product. In one embodiment the use of lignosulfonate is for reducing energy consumption in the beating step. In one embodiment the use of lignosulfonate is for reducing energy consumption in a drying or dewatering step in an industrial process for production of a paper product. In one embodiment the use of lignosulfonate is for reducing energy consumption in a drying or dewatering step and in a beating step in an industrial process for production of a paper product.

A fourth aspect of the invention relates to use of lignosulfonate in a beating step in the beating of a chemical pulp for reducing the energy consumption in an industrial process for production of a paper product. A fifth aspect of the invention relates to use of lignosulfonate in a betating step in the beating of a chemical pulp for reducing energy consumption in a drying or dewatering step in an industrial process for production of a paper product.

A sixth aspect of the invention relates to use of lignosulfonate in a betating step in the beating of a chemical pulp for increasing the strength of a paper product produced from the beaten pulp.

The embodiments of the first and second aspect apply to the third, fourth, fifth and sixth aspects mutatis mutandis

Examples

A sulfate pulp from a paper mill having a kappa value of 52 and was beated in an Escher-Wyss cone refiner in the presence of 1 % or 0.3 % lignosulfonate or in the absence of lignosulfonate. The lignosulfonate was an unfractionated soft wood lignosulfonate prepared from spent sulfite liquor obtained in a sulfite pulping process (Domsjo fabriker AB, Sweden). The beated pulps were shaped in to sheets having a surface weight of 150g/m ² . The pulp sheets were analyzed for tensile strength and compression strength. The fiber length and the beating degree were also analyzed.

Example 1

The pulps were beated as described above to a tensile strength of 100 Nm/g. As can be seen in figure 1 a, the energy needed for beating a pulp to a tensile strength of 100 Nm/g was reduced with about 50 % by addition of 1 % lignosulfonate (w/w) and with about 20 % by addition of 0.3 % (w/w) lignosulfonate to the pulp. The beating degree was also decrease in the pulps beated in the presence of lignosulfonate, see figure 1 b. A low beating degree is desired since this can save energy in the preceding dewatering and drying steps.

Example 2

The pulps were beated as described above with a beating energy of 100 kWh/t. As can be seen in figure 2a and 2b, addition of 0.3 % or 1 % (w/w) lignosulfonate to the beating step increases the tensile strength (figure 2a) and compression strength (figure 2b) of the beated pulp. Example 3

The pulps were beated as described above and the fiber lengths of the pulps after beating with different beating energy were analyzed. As can be seen in figure 3, the fiber length is apparently unaffected by addition of lignosulfonate.

REFERENCES

Levon et al. "Acceleration of the beating of pulp by the use of modified lignosulfonate" Mokuzai Gakkaishi (1986), 32(12), 101 1 -16