CA1177607A - Chemical treatment of mechanical wood pulp - Google Patents
Chemical treatment of mechanical wood pulpInfo
- Publication number
- CA1177607A CA1177607A CA000404442A CA404442A CA1177607A CA 1177607 A CA1177607 A CA 1177607A CA 000404442 A CA000404442 A CA 000404442A CA 404442 A CA404442 A CA 404442A CA 1177607 A CA1177607 A CA 1177607A
- Authority
- CA
- Canada
- Prior art keywords
- reject fraction
- range
- fraction
- reject
- pulp
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
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Classifications
-
- D—TEXTILES; PAPER
- D21—PAPER-MAKING; PRODUCTION OF CELLULOSE
- D21C—PRODUCTION OF CELLULOSE BY REMOVING NON-CELLULOSE SUBSTANCES FROM CELLULOSE-CONTAINING MATERIALS; REGENERATION OF PULPING LIQUORS; APPARATUS THEREFOR
- D21C3/00—Pulping cellulose-containing materials
- D21C3/04—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides
- D21C3/06—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides sulfur dioxide; sulfurous acid; bisulfites sulfites
- D21C3/12—Pulping cellulose-containing materials with acids, acid salts or acid anhydrides sulfur dioxide; sulfurous acid; bisulfites sulfites sodium bisulfite
Abstract
ABSTRACT OF THE DISCLOSURE
A method of improving the strength properties of mechanical wood pulp is disclosed. The method reduces the shive content and makes the shive content in the reject fraction particularly susceptible to being broken up. In the method, the pulp is separated into a reject fraction and an accept fraction, the reject fraction containing a higher portion of shives and long fiber material, an aqueous solution of sodium sulfite is applied to the reject fraction, the solution having a pH in the range of about 2.5 to 13. The reject fraction is cooked at a temperature in the range of about 100 to 160°C for a period of time in the range of about 2 to 120 minutes such that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced.
The reject fraction is then refined and recombined with the accept fraction.
A method of improving the strength properties of mechanical wood pulp is disclosed. The method reduces the shive content and makes the shive content in the reject fraction particularly susceptible to being broken up. In the method, the pulp is separated into a reject fraction and an accept fraction, the reject fraction containing a higher portion of shives and long fiber material, an aqueous solution of sodium sulfite is applied to the reject fraction, the solution having a pH in the range of about 2.5 to 13. The reject fraction is cooked at a temperature in the range of about 100 to 160°C for a period of time in the range of about 2 to 120 minutes such that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced.
The reject fraction is then refined and recombined with the accept fraction.
Description
1 17760~
CHEMICAL TREATMENT OF MEC~NICAL WOOD PULP
The present invention relates to mechanical wood pulp.
More specifically the present invention relates to a chemical treatment process for increasing the strength properties of mechanical wood pulp.
There are two basic types of wood pulp, mechanical pulp where the wood is mechanically defibrated, that is, reduced to fibrous form, and chemical pulp where wood chips are chemically treated to achieve defibratiQn. Mechanical pulps have a higher yield and are cheaper to produce than chemical pulps, but chemical pulp is considered a higher grade of pulp. In the past it has been the practice to mix a certain quantity of chemical pulp with mechanical pulp to produce a satisfactory newsprint furnish.
There are a number of methods of making mechanical pulps.
Groundwood pulp is a type of mechanical pulp made by grinding wood against a rotating grinding wheel. The pulp is generally referred to as stone groundwood (SGW). In another method, wood i-s first cut into small chips and the chips defibrated or pulped in one or more disc type refiners to make what is called refiner - mechanical pulp (RMP) In yet another method chips are de-fibrated in a disc refiner at elevated pressure and at temperatures above 100C, and here the resulting product is called thermo-mechanical pulp (TMP). TMP is commonly used in the production of newsprint as it has superior physical strength properties for a mechanical pulp enabling the percentage of chemical pulp to be kept to a minimum. However, in some forms of newsprint it is preferred to add a proportion of SGW because this is cheaper to produce than TMP and increases the opacity of the resulting sheet of newsprint. Many types of newsprint furnish have a mixture of chemical pulp with both TMP and SGW.
Pulping processes in general have as their aim the defiberization of woody material into individual fibers, which are sufficiently flexible to bond well to each other. In this 1 17760~
regard, mechanical pulping processcs are not entirely effective since they tend to produce numerous fiber bundles or shives, which cause linting and runability problems in the paper sheet.
In addition, longer fibers liberated during mechanical pulping may be left in a relatively stiff and nonconformable condition and thus do not bond well to each other. Thermomechanical pulps contain particularly large proportions of long and relatively stiff fibers.
These problems have long been recognized and it has been common practice in the industry to screen out the shives and longer fiber material as a reject fraction, using a metal screen with holes or slots. The remaining pulp is referred to as the accept fraction. The reject fraction is then subjected to further refining to break up the shives and develop some flexi-bility in the long fibers. This reject refining process requires considerable energy and also tends to decrease the fiber length which impairs the sheet strength, particularly tear strength.
It is known that the properties of mechanical pulp can be improved by treatment of the wood with sodium sulfite solution prior to pulping. The effect is believed to be at least partly due to sulfonation of the lignin in the wood It has also been shown that post-treatment of mechanical pulp with sulfite, after refining, can increase the tensile strength It has now been surprisingly found that an equivalent strength increase can be obtained by applying a sulfite post-treatment simply to the reject fraction instead of to the whole pulp. The reason for this wou]d appear to be because the sulfite treatment has a much larger effect on the strength of the reject fraction than on the strength of the whole pulp.
The advantage of applying the sulfite treatment only to the reject fraction is that much less chemical is required, approximately one-quarter of that required to treat the whole pulp. Treatment of the reject fraction with sulfite prior to 1 17760~
refining and recombining with the accept fraction, produces an increase in tensile strength and burst index at least equal to the increase achieved by treating the whole pulp with sulfite.
Furthermore, the energy to refine a sulfite treated reject frac-tion is only about one quarter of that required for the untreated reject fraction to achieve the same tensile strength, Sulfite treatment applied only to the reject fraction does not increase the freeness or drainage rate of the pulp, unlike the situation where the whole pulp is treated.
It has been found that sulfite treatment of the reject fraction decreases the shive content of rejects and makes the remaining shives particularly susceptible to being broken up in the reject refining step.
After recombining the sulfite treated and refined reject fraction with the accept fraction, it has been found that there is a slight increase in brightness in the whole pulp.
Whereas it is known that sulfite treatment of a whole TMP may give a pulp yield of about 90%, it has been found that the same sulfite treatment applied only to the reject fraction produces a reject fraction yield of about 96%. When the reject fraction constitutes about 25% of the whole pulp, the yield based on whole pulp is equivalent to about 99%. Yields are in all cases calculated on the bone dry weight of pulp prior to chemical treatment.
Throughout the specification reference is made to an aqueous solution of sodium sulfite. Depending on the pH of the solution, the term sodium sulfite includes sodium bisulfite or a mixture of sodium sulfite and sodium bisulfite. The sulfur bound to the pulp after cooking the pulp with the sodium sulfite solution is believed to be present in the form of sulfonate and results are accordingly calculated as percent sulfonate by multiplying the measured percent sulfur contents by 2.5, 1 17760~
The present invention provides a method of improving the strength properties and reducing the shive content of mechani-cal wood pulp comprising the steps of: separating the pulp into a reject fraction and an accept fraction, the reject fraction con-taining a higher proportion of shives and long fiber material, . applying an aqueous solution of sodium sulfite to the reject frac-tion, the solution having a pH in the range of about 2,5 to 13, cooking the reject fraction at a temperature in the range of about 100 to 160C for a period of time in the range of about 2 to 120 minutes such that after cooking, the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6 based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced, refining the cooked reject fraction, and recombining the reject fraction with the accept fractionO
In preferred embodiments the mechanical wood pulp is RMP, TMP, SGW or a mixture of the latter two. In another embodi-; ment the aqueous solution of sodium sulfite has a pH in the range of about 5.5 to 9.5, the reject fraction is cooked at a tempera-; 20 ture in the range of about 130 to 155C for a period of time in the range of about 2 to 30 minutes In a preferred embodiment, the quantity of sulfonate bound to the reject fraction after cooking is in the range of about 1.1 to 1.4%. In another embodiment the quantity of sodium sulfite added to the reject fraction is preferably in the range of from about 4 to 50%, and the consistency of the reject fraction after application of the : sodium sulfite is preferably in the range of from about 10-50%.
In a preferred embodiment the yield of the reject fraction is not less than 95% and in another preferred embodiment, an additional step of pressing the long ~iber reject fraction occurs after separating the pulp and before applying the sodium sulfite such that the reject fraction has a consistency oi' about l 17760~
25% before application of the sodlum sulflte.
In drawings which illustrata the embodiments of the invention, Fig. 1 is a flow sheet showlng one embodlment of the process.
Fig. 2 is a graph comparing the shive content against refining energy for sulfite treated and untreated refined reject fraction.
Fig. 3 is a graph comparing the breaking length against refining energy for sulfite treated and untreated refined reject fraction.
Fig. 4 is a graph showing the breaking length against sulfonate bound to pulp.
Fig. 5 is a graph showing the energy against sulfonate bound to pulp.
Fig. 6 is a graph comparing the sulfonate bound to pulp against cooking time for different pH levels.
; Mechanical wood pulps whether SGW, RMP or TMP generally require a number of stages of refining before being suitable for newsprint furnish. In the case of SGW, after the grinding step there is generally a screening step wherein the reject portion which includes long fibers, shives and large debris, are separated and then passed through a further refining stage, or in some cases are discarded with the result that the yield of the SGW is reduced. In the case of TMP, the first refining step occurs in a disc refiner operating at increased temperature and pressure conditions. This may be followed by a second refining step, usually at atmospheric pressure. Following the refining steps there is a screening step wherein the accept fraction is separated from the reject fraction, the latter being further refined before being recombined with the accept fraction. TMP
generally has longer fibers than SGW but is more expensive to manufacture.
l 177607 The treating liquor may be sulfite, bisulfite or any mixture of the two depending upon the pH of the solution. The preferred sulfite is sodium sulfite primarily because of cost.
The pH of the solution may vary over the range of about 2,5 to 13. It has been found there is substantial sulfonation even at the low pH, particularly with longer cooking periods. However, there are operating problems at low pH due to equipment corrosion, and evolution of sulfur dioxide which creates an unpleasant working environment. It is preferred that the process not be operated below about pH 5.5.
As the pH of the solution increases, there is a con-tinuing increase in strength and density of the reject fraction.
However, at the higher pH there is a loss in brightness, and at pH levels above 10 the pulp becomes quite slippery and is harder to refine. It is preferred that the process not be operated above about pH 9.5.
In the examples referred to in the specification, the pH figures refer to the starting pH of the sulfite solution.
Because of acids liberated from the pulp, the pH drops to some extent during the cooking stage.
As illustrated in Fig. 1 the process of the present invention takes mechanical pulp and screens it to divide the accept fraction from the reject fraction. The reject fractio~
amounts typically to 10 - 30% of the whole pulp, depending on process variables, particularly the size of the holes or slots in the screens. The reject fraction is then pressed to increase the consistency of the pulp to a consistency in the range of approximately 10-50~, preferably 10 - 30%. Consistency refers to the relative portions of pulp and water or treating liquor.
10~ consistency means 10 parts by weight of pulp and 90 of water or liquor. The press is preferably a screw press. In some I 1 776~)7 ; instances it is not necessary to use a press provided the con-sistency is not too low, i e. not below about 10% A sodium sulfite solution is then applied to the reject fraction, pre-ferably by spraying in which case the consistency of the fraction is slightly reduced. The resulting reject fraction is then cooked at a temperature in the raDge of about 100 to 160C
for a period of time in the range of about 2 to 120 minutes.
This cooking step is carried out under increased pressure. The cooking step may be a vapor phase, liquid phase, or combination vapor and liquid phase. In a preferred embodiment the cooking temperature is in the range of about 140-155C and the period of time for cooking is about 5 to 20 minutes.
The conditions of treatment depend at least partially on the levels of sulfite applied to the reject fraction. The aim of the sulfite treatment is to ensure that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction. The preferred range of the bound sulfonate is about 1.1 to 1.4% for most softwood species; the range may vary somewhat depending on the particular type of wood being treated.
This preferred level maximizes strength properties of the resulting whole pulp, and minimizes yield losses and refining energy for the reject fraction. To achieve the required level of sulfonate bound to the reject fraction, about 4 to 50% sodium sulfite by weight of the bone dry reject fraction is applied to the pulp, in aqueous solution. The preferred application to achieve the preferred range of bound sulfonate is 10 to 20~
sodium sulfite. Tests have shown that the amount of sulfonate bound to thé pulp is not simply determined by the amount of sulfite applied. Variations occur depending on the reaction time, reaction temperature and the pulp consistency.
After the cooking step, the reject fraction is passed l 17760~
~ to a refiner, and lt ~9 found that the sulfite treatment sub-stantially decreases the refining energy required to develop tensile strength in the resulting whole pulp compared with the non-sulfite treated reject fraction. Test results show that to obtain a whole pulp with a breaking length oi 5000 m., only one-quarter of the refining energy is required for the sulfite treated reject fraction as compared to the untreated re~ect fraction.
The sulfite treatment of the reject fraction does not increase the drainage rate and in most instances the drainage rate is decreased. The related and more sensitive parameter of freeness has been found to decrease in all cases after sulfite treatment of the reject fraction of mechanical pulp.
After refining of the reject fraction, it is re-combined with the accept fraction to produce combined whole pulp.
This pulp has a substantially reduced shive content compared to mechanical pulps made by conventlonal methods.
Shives are fiber bundles and are known to cause :
problems in runability and linting of paper sheets. It is apparent that sulfite treatment of the reject fraction renders shives particularly amenable to being bro~en up in subsequent reject refining.
Another parameter affected by the sulfite treatment of the reject fraction is the paper sheet density. The sulfite treatment increases the sheet density which is beneficial because low density is associated with linting and other printing problems.
Furthermore, increased sheet density gives a greater yardage of paper on paper rolls which are made to constant diameter.
I 1 776a~
TABLE I
Pulp Sample No: 1 2 3 4 5 6 7 8 9 SULFITE TREATMENT
Consistency % 16 16 16 6 16 16 16 34 16 Sodium Sulfite applied % 10 10 7 12 12 12 12 12 12 Cooking Time mins. 30 30 30120 20 20 20 10 20 Cooking Temp. C 150 150160 130145 145145 145 145 Yield % 89.6 89.0 88,0 98.5 97.2 98.5 96.5 96.0 96.0 Sulfonate bound to pulp % 1.93 1.80 1.83 0.95 1.28 1.23 1.18 1.45 1.20 PULP PROPERTIES
~= .
Freeness ml original 122 85 398 585543 540602 91 93 treated 183 128507 565511 536576 92 96 Drainage, sec. original11.7 _ _4.5 4.04.2 4.1 17 _ treated 7.2 _ _ 4.54.4 4.24.4 17 _ Density, g/cc original .338 .363 .242 .271 .251 .274 .263 .357 373 treated .379 .413 .299 .302 .341 .323 .299 .392 .401 Burst Index original 1.62 1.80 0.53 1.33 .94 1.36 0.72 2.oo 2.20 kPa.m 2/g treated 1.76 2.00 0.71 1.90 2.06 2.08 1.07 2.40 2.40 Breaking length, original 3420 3800 1670 2740 2530 3030 1950 4220 4260 treated 3750 4200 1930 3310 3880 3970 2440 4620 4660 STFI - Total original _ _ _ 2298 624 2154 6252 _ treated _ _ _ 16 4 639 1924 6103 -1 177607.
Sulfite treatments were carried out and the results tabulated in Table 1. Details of the resulting pulps and their properties are shown therein.
Sample No: Material 1 Whole TMP, secondary (two refining stages~
CHEMICAL TREATMENT OF MEC~NICAL WOOD PULP
The present invention relates to mechanical wood pulp.
More specifically the present invention relates to a chemical treatment process for increasing the strength properties of mechanical wood pulp.
There are two basic types of wood pulp, mechanical pulp where the wood is mechanically defibrated, that is, reduced to fibrous form, and chemical pulp where wood chips are chemically treated to achieve defibratiQn. Mechanical pulps have a higher yield and are cheaper to produce than chemical pulps, but chemical pulp is considered a higher grade of pulp. In the past it has been the practice to mix a certain quantity of chemical pulp with mechanical pulp to produce a satisfactory newsprint furnish.
There are a number of methods of making mechanical pulps.
Groundwood pulp is a type of mechanical pulp made by grinding wood against a rotating grinding wheel. The pulp is generally referred to as stone groundwood (SGW). In another method, wood i-s first cut into small chips and the chips defibrated or pulped in one or more disc type refiners to make what is called refiner - mechanical pulp (RMP) In yet another method chips are de-fibrated in a disc refiner at elevated pressure and at temperatures above 100C, and here the resulting product is called thermo-mechanical pulp (TMP). TMP is commonly used in the production of newsprint as it has superior physical strength properties for a mechanical pulp enabling the percentage of chemical pulp to be kept to a minimum. However, in some forms of newsprint it is preferred to add a proportion of SGW because this is cheaper to produce than TMP and increases the opacity of the resulting sheet of newsprint. Many types of newsprint furnish have a mixture of chemical pulp with both TMP and SGW.
Pulping processes in general have as their aim the defiberization of woody material into individual fibers, which are sufficiently flexible to bond well to each other. In this 1 17760~
regard, mechanical pulping processcs are not entirely effective since they tend to produce numerous fiber bundles or shives, which cause linting and runability problems in the paper sheet.
In addition, longer fibers liberated during mechanical pulping may be left in a relatively stiff and nonconformable condition and thus do not bond well to each other. Thermomechanical pulps contain particularly large proportions of long and relatively stiff fibers.
These problems have long been recognized and it has been common practice in the industry to screen out the shives and longer fiber material as a reject fraction, using a metal screen with holes or slots. The remaining pulp is referred to as the accept fraction. The reject fraction is then subjected to further refining to break up the shives and develop some flexi-bility in the long fibers. This reject refining process requires considerable energy and also tends to decrease the fiber length which impairs the sheet strength, particularly tear strength.
It is known that the properties of mechanical pulp can be improved by treatment of the wood with sodium sulfite solution prior to pulping. The effect is believed to be at least partly due to sulfonation of the lignin in the wood It has also been shown that post-treatment of mechanical pulp with sulfite, after refining, can increase the tensile strength It has now been surprisingly found that an equivalent strength increase can be obtained by applying a sulfite post-treatment simply to the reject fraction instead of to the whole pulp. The reason for this wou]d appear to be because the sulfite treatment has a much larger effect on the strength of the reject fraction than on the strength of the whole pulp.
The advantage of applying the sulfite treatment only to the reject fraction is that much less chemical is required, approximately one-quarter of that required to treat the whole pulp. Treatment of the reject fraction with sulfite prior to 1 17760~
refining and recombining with the accept fraction, produces an increase in tensile strength and burst index at least equal to the increase achieved by treating the whole pulp with sulfite.
Furthermore, the energy to refine a sulfite treated reject frac-tion is only about one quarter of that required for the untreated reject fraction to achieve the same tensile strength, Sulfite treatment applied only to the reject fraction does not increase the freeness or drainage rate of the pulp, unlike the situation where the whole pulp is treated.
It has been found that sulfite treatment of the reject fraction decreases the shive content of rejects and makes the remaining shives particularly susceptible to being broken up in the reject refining step.
After recombining the sulfite treated and refined reject fraction with the accept fraction, it has been found that there is a slight increase in brightness in the whole pulp.
Whereas it is known that sulfite treatment of a whole TMP may give a pulp yield of about 90%, it has been found that the same sulfite treatment applied only to the reject fraction produces a reject fraction yield of about 96%. When the reject fraction constitutes about 25% of the whole pulp, the yield based on whole pulp is equivalent to about 99%. Yields are in all cases calculated on the bone dry weight of pulp prior to chemical treatment.
Throughout the specification reference is made to an aqueous solution of sodium sulfite. Depending on the pH of the solution, the term sodium sulfite includes sodium bisulfite or a mixture of sodium sulfite and sodium bisulfite. The sulfur bound to the pulp after cooking the pulp with the sodium sulfite solution is believed to be present in the form of sulfonate and results are accordingly calculated as percent sulfonate by multiplying the measured percent sulfur contents by 2.5, 1 17760~
The present invention provides a method of improving the strength properties and reducing the shive content of mechani-cal wood pulp comprising the steps of: separating the pulp into a reject fraction and an accept fraction, the reject fraction con-taining a higher proportion of shives and long fiber material, . applying an aqueous solution of sodium sulfite to the reject frac-tion, the solution having a pH in the range of about 2,5 to 13, cooking the reject fraction at a temperature in the range of about 100 to 160C for a period of time in the range of about 2 to 120 minutes such that after cooking, the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6 based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced, refining the cooked reject fraction, and recombining the reject fraction with the accept fractionO
In preferred embodiments the mechanical wood pulp is RMP, TMP, SGW or a mixture of the latter two. In another embodi-; ment the aqueous solution of sodium sulfite has a pH in the range of about 5.5 to 9.5, the reject fraction is cooked at a tempera-; 20 ture in the range of about 130 to 155C for a period of time in the range of about 2 to 30 minutes In a preferred embodiment, the quantity of sulfonate bound to the reject fraction after cooking is in the range of about 1.1 to 1.4%. In another embodiment the quantity of sodium sulfite added to the reject fraction is preferably in the range of from about 4 to 50%, and the consistency of the reject fraction after application of the : sodium sulfite is preferably in the range of from about 10-50%.
In a preferred embodiment the yield of the reject fraction is not less than 95% and in another preferred embodiment, an additional step of pressing the long ~iber reject fraction occurs after separating the pulp and before applying the sodium sulfite such that the reject fraction has a consistency oi' about l 17760~
25% before application of the sodlum sulflte.
In drawings which illustrata the embodiments of the invention, Fig. 1 is a flow sheet showlng one embodlment of the process.
Fig. 2 is a graph comparing the shive content against refining energy for sulfite treated and untreated refined reject fraction.
Fig. 3 is a graph comparing the breaking length against refining energy for sulfite treated and untreated refined reject fraction.
Fig. 4 is a graph showing the breaking length against sulfonate bound to pulp.
Fig. 5 is a graph showing the energy against sulfonate bound to pulp.
Fig. 6 is a graph comparing the sulfonate bound to pulp against cooking time for different pH levels.
; Mechanical wood pulps whether SGW, RMP or TMP generally require a number of stages of refining before being suitable for newsprint furnish. In the case of SGW, after the grinding step there is generally a screening step wherein the reject portion which includes long fibers, shives and large debris, are separated and then passed through a further refining stage, or in some cases are discarded with the result that the yield of the SGW is reduced. In the case of TMP, the first refining step occurs in a disc refiner operating at increased temperature and pressure conditions. This may be followed by a second refining step, usually at atmospheric pressure. Following the refining steps there is a screening step wherein the accept fraction is separated from the reject fraction, the latter being further refined before being recombined with the accept fraction. TMP
generally has longer fibers than SGW but is more expensive to manufacture.
l 177607 The treating liquor may be sulfite, bisulfite or any mixture of the two depending upon the pH of the solution. The preferred sulfite is sodium sulfite primarily because of cost.
The pH of the solution may vary over the range of about 2,5 to 13. It has been found there is substantial sulfonation even at the low pH, particularly with longer cooking periods. However, there are operating problems at low pH due to equipment corrosion, and evolution of sulfur dioxide which creates an unpleasant working environment. It is preferred that the process not be operated below about pH 5.5.
As the pH of the solution increases, there is a con-tinuing increase in strength and density of the reject fraction.
However, at the higher pH there is a loss in brightness, and at pH levels above 10 the pulp becomes quite slippery and is harder to refine. It is preferred that the process not be operated above about pH 9.5.
In the examples referred to in the specification, the pH figures refer to the starting pH of the sulfite solution.
Because of acids liberated from the pulp, the pH drops to some extent during the cooking stage.
As illustrated in Fig. 1 the process of the present invention takes mechanical pulp and screens it to divide the accept fraction from the reject fraction. The reject fractio~
amounts typically to 10 - 30% of the whole pulp, depending on process variables, particularly the size of the holes or slots in the screens. The reject fraction is then pressed to increase the consistency of the pulp to a consistency in the range of approximately 10-50~, preferably 10 - 30%. Consistency refers to the relative portions of pulp and water or treating liquor.
10~ consistency means 10 parts by weight of pulp and 90 of water or liquor. The press is preferably a screw press. In some I 1 776~)7 ; instances it is not necessary to use a press provided the con-sistency is not too low, i e. not below about 10% A sodium sulfite solution is then applied to the reject fraction, pre-ferably by spraying in which case the consistency of the fraction is slightly reduced. The resulting reject fraction is then cooked at a temperature in the raDge of about 100 to 160C
for a period of time in the range of about 2 to 120 minutes.
This cooking step is carried out under increased pressure. The cooking step may be a vapor phase, liquid phase, or combination vapor and liquid phase. In a preferred embodiment the cooking temperature is in the range of about 140-155C and the period of time for cooking is about 5 to 20 minutes.
The conditions of treatment depend at least partially on the levels of sulfite applied to the reject fraction. The aim of the sulfite treatment is to ensure that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction. The preferred range of the bound sulfonate is about 1.1 to 1.4% for most softwood species; the range may vary somewhat depending on the particular type of wood being treated.
This preferred level maximizes strength properties of the resulting whole pulp, and minimizes yield losses and refining energy for the reject fraction. To achieve the required level of sulfonate bound to the reject fraction, about 4 to 50% sodium sulfite by weight of the bone dry reject fraction is applied to the pulp, in aqueous solution. The preferred application to achieve the preferred range of bound sulfonate is 10 to 20~
sodium sulfite. Tests have shown that the amount of sulfonate bound to thé pulp is not simply determined by the amount of sulfite applied. Variations occur depending on the reaction time, reaction temperature and the pulp consistency.
After the cooking step, the reject fraction is passed l 17760~
~ to a refiner, and lt ~9 found that the sulfite treatment sub-stantially decreases the refining energy required to develop tensile strength in the resulting whole pulp compared with the non-sulfite treated reject fraction. Test results show that to obtain a whole pulp with a breaking length oi 5000 m., only one-quarter of the refining energy is required for the sulfite treated reject fraction as compared to the untreated re~ect fraction.
The sulfite treatment of the reject fraction does not increase the drainage rate and in most instances the drainage rate is decreased. The related and more sensitive parameter of freeness has been found to decrease in all cases after sulfite treatment of the reject fraction of mechanical pulp.
After refining of the reject fraction, it is re-combined with the accept fraction to produce combined whole pulp.
This pulp has a substantially reduced shive content compared to mechanical pulps made by conventlonal methods.
Shives are fiber bundles and are known to cause :
problems in runability and linting of paper sheets. It is apparent that sulfite treatment of the reject fraction renders shives particularly amenable to being bro~en up in subsequent reject refining.
Another parameter affected by the sulfite treatment of the reject fraction is the paper sheet density. The sulfite treatment increases the sheet density which is beneficial because low density is associated with linting and other printing problems.
Furthermore, increased sheet density gives a greater yardage of paper on paper rolls which are made to constant diameter.
I 1 776a~
TABLE I
Pulp Sample No: 1 2 3 4 5 6 7 8 9 SULFITE TREATMENT
Consistency % 16 16 16 6 16 16 16 34 16 Sodium Sulfite applied % 10 10 7 12 12 12 12 12 12 Cooking Time mins. 30 30 30120 20 20 20 10 20 Cooking Temp. C 150 150160 130145 145145 145 145 Yield % 89.6 89.0 88,0 98.5 97.2 98.5 96.5 96.0 96.0 Sulfonate bound to pulp % 1.93 1.80 1.83 0.95 1.28 1.23 1.18 1.45 1.20 PULP PROPERTIES
~= .
Freeness ml original 122 85 398 585543 540602 91 93 treated 183 128507 565511 536576 92 96 Drainage, sec. original11.7 _ _4.5 4.04.2 4.1 17 _ treated 7.2 _ _ 4.54.4 4.24.4 17 _ Density, g/cc original .338 .363 .242 .271 .251 .274 .263 .357 373 treated .379 .413 .299 .302 .341 .323 .299 .392 .401 Burst Index original 1.62 1.80 0.53 1.33 .94 1.36 0.72 2.oo 2.20 kPa.m 2/g treated 1.76 2.00 0.71 1.90 2.06 2.08 1.07 2.40 2.40 Breaking length, original 3420 3800 1670 2740 2530 3030 1950 4220 4260 treated 3750 4200 1930 3310 3880 3970 2440 4620 4660 STFI - Total original _ _ _ 2298 624 2154 6252 _ treated _ _ _ 16 4 639 1924 6103 -1 177607.
Sulfite treatments were carried out and the results tabulated in Table 1. Details of the resulting pulps and their properties are shown therein.
Sample No: Material 1 Whole TMP, secondary (two refining stages~
2 Whole TMP, secondary (two refining stages)
3 Whole TMP, primary (one refining stage)
4 TMP Centrisorter reject fraction TMP Centrisorter reject fraction 6 TMP Centrisorter reject fraction 7 Groundwood Screen reject fraction Table II collates the breaking length and burst index figures for the whole pulps and for the rejects. Clearly the sulfite treatment has a much larger effect on the rejects than on the whole pulps, showing that the process is more efficient if carried out on the rejects only.
In samples 8 and 9 whole TMPs were made by recombining a Centrisorter accept fraction with the treated reject fraction in the proportion 75% accept fraction to 25% reject fraction. The reject fraction was either refined, or sulfite treated and then refined. Refining was carried out at 10% pulp consistency using a Sprout Waldron laboratory refiner, the refining energy was adjusted so that the freeness of the final pulps was kept essentially constant.
~1~ l 17760 C~
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1 17760~
Details of the chemical treatments carried out and the properties OI the resulting pulps are given in Table III.
TABLE III
Sample No: 8 8 9 9 Reject Treatment A B A B
SULFITE TREATMENT
Consistency % - 34 - 16 Na2S03 on wood % - 12 _ 12 Time, mins. - 10 - 20 Temperature C - 145 - 145 Yield on pulp % - 96 - 96 Bound sulfonate % - 1.45 - 1.2 PULP PROPERTIES
Freeness, ml 91 92 93 96 Drainage time, sec. 17 17 Burst Index 2.0 2.4 2.2 2.4 kPa,m2/g Breaking length, m 4220 4620 4260 4660 117760~
In Table III, reject treatment A represents the reJect fraction being refined only, and B represents the reject fraction being sulfite treated and refined. From the data in Table III it can be shown that sulfite pretreatment of the reJect ~raGtion ; prior to refining gives an increase of 400 m in breaking length and a mean increase of 0.3 kPa.m2/g in burst index measured on the recombined pulp. These figures compare with increases of 330 and 0.17 where the sulfite treatment is applied to the whole TMP (Table II). This demonstrates that if the reject fraction of a TMP is treated with sulfite prior to refining and then recombined with the accepts the gain in breaking length and burst is at least equal to that which would have been obtained if the whole pulp had been treated with sulfite.
TMP -Centrisorter reject fraction was treated with 12%
sodium sulfite on wood at 6% consistency for 120 minutes at 130C
to give a pulp yield of 98.5% and a bound sulfonate content of 0.95%. The treated reject fraction was refined at varying levels of energy input in a Sprout Waldron 12" disc refiner and compared with reject fraction refined without the chemical treatment. The pulp properties are shown in Table IV.
,, 1 17760~
: TABLE IV
REJECT FRACTION REFINED WITHOUT SULFITE TREATMENT
Energy Input, hpd/t O 58 74 96 Freeness, ml Csf585 237161 78 Drainage Time, sec 4.56.0 7.7 10.9 Density, g/cm30,2710.359 0.3760.427 Breaking Length, m 2740 4940 5700 6400 STFI - Minishives16491667 1143 731 - Maxishives549 247 284 187 - Total Shives 2298 1914 1432 918 .
.
REJECT FRACTION REFINED AFTER SULFITE TREATMENT
. - Energy Input, hpd/t O 15 40,2 56.8 : Freeness, ml Csf565 350 170 85 : Drainage Time, sec. 4.3 5.2 10.2 19.4 Density, g/cm0.382 0.386 0.4580.499 Burst Index, kPa.m2/g1.9 3.0 4.0 4.2 Breaking Length, m 3310 5320 6780 7950 STFI - Minishives1229 822 572 319 - Maxishives455 163 76 29 - Tctal Shives 1684 985 648 348 I 177~0~
Figure 2 shows that at all levels of refining energy the sulfite treatment lowers the shive count substantially.
The shive reduction ratio, that is the ratio of shive level in the untreated reject fraction to that in the sulfite treated reject fraction, is shown in Table V. The reduction ratio increases dramatically with increasing refining energy, showing that the sulfite treatment renders shives particularly susceptible to being broken up in subsequent refining.
;
TABLE V
EFFECT OF SULFITE TREATMENT ON
SHIVE REDUCTION IN TMP RE~ECT FRACTION
STFI - Total Shives Refining Sulfite Reduction Energy UntreatedTreated - Ratio hpd/t (A) (B) (A/B) :
, 0 2300 --1680 1.4 . .
- 20 2250 900 2,5 ,,:
'-~ 50 1990 440 4.5 1600 220 7.3 Figure 3 shows that very much less refining energy is required to develop strength (breaking length) after sulfite treatment of the reject fraction. After sulfite treatment a ,~:
breaking length of 5000 m. is attained with approximately 25%
of the refining energy required without the treatment.
TMP Centrisorter reject fraction was treated with ` varying levels of sodium sulfite at 16% consistency for 20 minutes ~,J~ 30 at 140~C, and refined to 150 ml Canadian Standard Freeness.
Processing data and pulp properties are shown in Table VI.
1 17760~
TABLE VI
E~fect of % sodium sulfite (on bone dry pulp) on the properties of TMP reject fraction refined to 150 ml Canadian Standard Freeness Sodium Sulfite Burst Breaking Sulfonate Pulp Applied DensityIndex LengthEnergy bound to Yield % g/cckPa.m2/g mhpd/t pulp, % %
0 0.370 3.1 5700 76 0 100 4 0.445 3.6 6310 44 0.40 98.4 ; 8 0.460 3.6 6500 39 0.80 98.0 12 0.481 4.2 7000 40 0.94 97.5 16 0.506 4.3 7100 38 0.98 96.3 0.549 5.1 8435 34 1.20 90.0 Figure 4 shows that the breaking length increases continuously with increasing sulfonation but that the major effect occurs at levels of bound sulfonate above about 0.6~.
This illustrates the importance of sulfonating above the 0.6%
level, where it is believed significant sulfonation occurs within the fiber cell wall. The shape of the refining energy-bound sulfonate curve shown in Figure 5 also demonstrates the importance of sulfonating above the 0.6% level.
TMP Centrisorter reject fraction was treated with 16%
sodium sulfite on wood at 16% consistency and at 150C. The , effect of the starting pH on the sulfonation-time curves is shown in Figure 6. The results demonstrate that the rate of sulfonation increases over the pH range 2,5 - 9.5 but that there is substantial sulfonation at the low pH, particularly with longer reaction periods.
TMP Centrisorter reject fraction was treated with 12%
1 17760~
sodium sulfite at 16~ consistency for 20 minutes at 145C, and at different starting pH levels The results, shown in Table VII, indicate that there is a COntinUiDg - increase in strength as the pH increases above 9.5 However, this strength increase is accompanied by a loss in brightness.
Also at pHts much above 10 the pulp becomes slippery and hard to refine.
~ TABLE VII
., EFFECT OF pH ON THE TREATMENT OF TMP
- REJECT FRACTION WITH SODIUM SULFITE
Untreated Control Treated with Sodium Sulfite . . .
pH 5 9.5 11.7 13.0 ,~. CS Freeness, ml. 150 150 150 150 !1l Density, g/cc 0.380 0.479 0.528 0.536 Burst Index, kPa.m2/g3.20 4.09 4.35 4.50 $ Breaking Length, m 5800 7430 7879 7950 I Brightness % 48 50 44 39 . .
:;
In samples 8 and 9 whole TMPs were made by recombining a Centrisorter accept fraction with the treated reject fraction in the proportion 75% accept fraction to 25% reject fraction. The reject fraction was either refined, or sulfite treated and then refined. Refining was carried out at 10% pulp consistency using a Sprout Waldron laboratory refiner, the refining energy was adjusted so that the freeness of the final pulps was kept essentially constant.
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1 17760~
Details of the chemical treatments carried out and the properties OI the resulting pulps are given in Table III.
TABLE III
Sample No: 8 8 9 9 Reject Treatment A B A B
SULFITE TREATMENT
Consistency % - 34 - 16 Na2S03 on wood % - 12 _ 12 Time, mins. - 10 - 20 Temperature C - 145 - 145 Yield on pulp % - 96 - 96 Bound sulfonate % - 1.45 - 1.2 PULP PROPERTIES
Freeness, ml 91 92 93 96 Drainage time, sec. 17 17 Burst Index 2.0 2.4 2.2 2.4 kPa,m2/g Breaking length, m 4220 4620 4260 4660 117760~
In Table III, reject treatment A represents the reJect fraction being refined only, and B represents the reject fraction being sulfite treated and refined. From the data in Table III it can be shown that sulfite pretreatment of the reJect ~raGtion ; prior to refining gives an increase of 400 m in breaking length and a mean increase of 0.3 kPa.m2/g in burst index measured on the recombined pulp. These figures compare with increases of 330 and 0.17 where the sulfite treatment is applied to the whole TMP (Table II). This demonstrates that if the reject fraction of a TMP is treated with sulfite prior to refining and then recombined with the accepts the gain in breaking length and burst is at least equal to that which would have been obtained if the whole pulp had been treated with sulfite.
TMP -Centrisorter reject fraction was treated with 12%
sodium sulfite on wood at 6% consistency for 120 minutes at 130C
to give a pulp yield of 98.5% and a bound sulfonate content of 0.95%. The treated reject fraction was refined at varying levels of energy input in a Sprout Waldron 12" disc refiner and compared with reject fraction refined without the chemical treatment. The pulp properties are shown in Table IV.
,, 1 17760~
: TABLE IV
REJECT FRACTION REFINED WITHOUT SULFITE TREATMENT
Energy Input, hpd/t O 58 74 96 Freeness, ml Csf585 237161 78 Drainage Time, sec 4.56.0 7.7 10.9 Density, g/cm30,2710.359 0.3760.427 Breaking Length, m 2740 4940 5700 6400 STFI - Minishives16491667 1143 731 - Maxishives549 247 284 187 - Total Shives 2298 1914 1432 918 .
.
REJECT FRACTION REFINED AFTER SULFITE TREATMENT
. - Energy Input, hpd/t O 15 40,2 56.8 : Freeness, ml Csf565 350 170 85 : Drainage Time, sec. 4.3 5.2 10.2 19.4 Density, g/cm0.382 0.386 0.4580.499 Burst Index, kPa.m2/g1.9 3.0 4.0 4.2 Breaking Length, m 3310 5320 6780 7950 STFI - Minishives1229 822 572 319 - Maxishives455 163 76 29 - Tctal Shives 1684 985 648 348 I 177~0~
Figure 2 shows that at all levels of refining energy the sulfite treatment lowers the shive count substantially.
The shive reduction ratio, that is the ratio of shive level in the untreated reject fraction to that in the sulfite treated reject fraction, is shown in Table V. The reduction ratio increases dramatically with increasing refining energy, showing that the sulfite treatment renders shives particularly susceptible to being broken up in subsequent refining.
;
TABLE V
EFFECT OF SULFITE TREATMENT ON
SHIVE REDUCTION IN TMP RE~ECT FRACTION
STFI - Total Shives Refining Sulfite Reduction Energy UntreatedTreated - Ratio hpd/t (A) (B) (A/B) :
, 0 2300 --1680 1.4 . .
- 20 2250 900 2,5 ,,:
'-~ 50 1990 440 4.5 1600 220 7.3 Figure 3 shows that very much less refining energy is required to develop strength (breaking length) after sulfite treatment of the reject fraction. After sulfite treatment a ,~:
breaking length of 5000 m. is attained with approximately 25%
of the refining energy required without the treatment.
TMP Centrisorter reject fraction was treated with ` varying levels of sodium sulfite at 16% consistency for 20 minutes ~,J~ 30 at 140~C, and refined to 150 ml Canadian Standard Freeness.
Processing data and pulp properties are shown in Table VI.
1 17760~
TABLE VI
E~fect of % sodium sulfite (on bone dry pulp) on the properties of TMP reject fraction refined to 150 ml Canadian Standard Freeness Sodium Sulfite Burst Breaking Sulfonate Pulp Applied DensityIndex LengthEnergy bound to Yield % g/cckPa.m2/g mhpd/t pulp, % %
0 0.370 3.1 5700 76 0 100 4 0.445 3.6 6310 44 0.40 98.4 ; 8 0.460 3.6 6500 39 0.80 98.0 12 0.481 4.2 7000 40 0.94 97.5 16 0.506 4.3 7100 38 0.98 96.3 0.549 5.1 8435 34 1.20 90.0 Figure 4 shows that the breaking length increases continuously with increasing sulfonation but that the major effect occurs at levels of bound sulfonate above about 0.6~.
This illustrates the importance of sulfonating above the 0.6%
level, where it is believed significant sulfonation occurs within the fiber cell wall. The shape of the refining energy-bound sulfonate curve shown in Figure 5 also demonstrates the importance of sulfonating above the 0.6% level.
TMP Centrisorter reject fraction was treated with 16%
sodium sulfite on wood at 16% consistency and at 150C. The , effect of the starting pH on the sulfonation-time curves is shown in Figure 6. The results demonstrate that the rate of sulfonation increases over the pH range 2,5 - 9.5 but that there is substantial sulfonation at the low pH, particularly with longer reaction periods.
TMP Centrisorter reject fraction was treated with 12%
1 17760~
sodium sulfite at 16~ consistency for 20 minutes at 145C, and at different starting pH levels The results, shown in Table VII, indicate that there is a COntinUiDg - increase in strength as the pH increases above 9.5 However, this strength increase is accompanied by a loss in brightness.
Also at pHts much above 10 the pulp becomes slippery and hard to refine.
~ TABLE VII
., EFFECT OF pH ON THE TREATMENT OF TMP
- REJECT FRACTION WITH SODIUM SULFITE
Untreated Control Treated with Sodium Sulfite . . .
pH 5 9.5 11.7 13.0 ,~. CS Freeness, ml. 150 150 150 150 !1l Density, g/cc 0.380 0.479 0.528 0.536 Burst Index, kPa.m2/g3.20 4.09 4.35 4.50 $ Breaking Length, m 5800 7430 7879 7950 I Brightness % 48 50 44 39 . .
:;
Claims (21)
1. A method of improving the strength properties of mechanical wood pulp which has not been treated with sodium sulfite comprising the steps of:
separating the pulp into a reject fraction and an accept fraction, the reject fraction containing a higher portion of shives and long fiber material, applying an aqueous solution of sodium sulfite to the reject fraction, the solution having a pH in the range of about 2.5 to 13, cooking the reject fraction at a temperature in the range of about 100 to 160°C for a period of time in the range of about 2 to 120 minutes such that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced, refining the cooked reject fraction, and recombining the reject fraction with the accept fraction.
separating the pulp into a reject fraction and an accept fraction, the reject fraction containing a higher portion of shives and long fiber material, applying an aqueous solution of sodium sulfite to the reject fraction, the solution having a pH in the range of about 2.5 to 13, cooking the reject fraction at a temperature in the range of about 100 to 160°C for a period of time in the range of about 2 to 120 minutes such that after cooking the quantity of sulfonate bound to the reject fraction is in the range of about 0.6 to 1.6% based on the bone dry weight of the reject fraction, and the drainage time of the reject fraction is not reduced, refining the cooked reject fraction, and recombining the reject fraction with the accept fraction.
2. The method according to claim 1 wherein the aqueous solution of sodium sulfite has a pH in the range of about 5.5 to 9.5.
3. The method according to claim 1 wherein the reject fraction is cooked at a temperature in the range of about 130°
to 155°C.
to 155°C.
4. The method according to claim 1 wherein the reject fraction is cooked for a period of time in the range of about 2 to 30 minutes.
5. The method according to claim 1 wherein the aqueous solution of sodium sulfite has a pH in the range of about 5.5 to 9.5, the reject fraction is cooked at a temperature in the range of about 130° to 155°C for a period of time in the range of about 2 to 30 minutes
6. The method according to claim 1 or claim 5 wherein the quantity of sodium sulfite added to the reject fraction is in the range of from about 4 to 50% by weight of the bone dry reject fraction.
7. The method according to claim 1 or claim 5 wherein the quantity of sulfonate bound to the reject fraction after cooking is in the range of about 1.1 to 1.4%.
8. The method according to claim 1 or claim 5 wherein the quantity of sodium sulfite added to the reject fraction is in the range of from about 10 to 20% by weight of the bone dry reject fraction.
9. The method according to claim 1 or claim 5 wherein the aqueous solution of sodium sulfite is sprayed onto the reject fraction, and the consistency of the reject fraction after spraying is in the range of about 10 to 50%.
10. The method according to claim 1 or claim 5 wherein the aqueous solution of sodium sulfite is sprayed onto the reject fraction, and the consistency of the reject fraction after spraying is in the range of about 20-30%.
11. The method according to claim 1 or claim 5 wherein the reject fraction is cooked for a period of time for about 5 to 20 minutes.
12. The method according to claim 1 or claim 5 wherein the reject fraction is cooked above atmospheric pressure.
13. The method according to claim 1 or claim 5 wherein the cooking temperature is in the range of about 140 to 155°C.
14. The method according to claim 1 or claim 5 wherein after the cooking step, the drainage time of the reject portion increases.
15. The method according to claim 1 or claim 5 wherein the yield is not less than 95% based on the bone dry weight of the reject fraction.
16. The method according to claim 1 or claim 5 wherein the mechanical wood pulp is stone groundwood (SGW).
17. The method according to claim 1 or claim 5 wherein the mechanical wood pulp is refiner mechanical pulp (RMP).
18. The method according to claim 1 or claim 5 wherein the mechanical wood pulp is thermomechanical pulp (TMP).
19. The method according to claim 1 or claim 5 wherein the mechanical wood pulp is a mixture of thermomechanical pulp (TMP) and stone groundwood (SGW).
20. The method according to claim 1 or claim 5 wherein the refining step is carried out in a disc refiner.
21. The method according to claim 1 or claim 5 including the additional step of pressing the reject fraction after separating the pulp and before applying the sodium sulfite solution such that the reject fraction has a consistency in the range of about 20 to 30% before application of the sodium sulfite solution.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US27229181A | 1981-06-10 | 1981-06-10 | |
| US272,291 | 1981-06-10 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1177607A true CA1177607A (en) | 1984-11-13 |
Family
ID=23039193
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000404442A Expired CA1177607A (en) | 1981-06-10 | 1982-06-03 | Chemical treatment of mechanical wood pulp |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1177607A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4708771A (en) * | 1984-12-31 | 1987-11-24 | Bear Island Paper Company | Two stage process for sulfonating mechanical pulp fibers |
| US5503710A (en) * | 1995-05-31 | 1996-04-02 | Macmillan Bloedel Limited | Duplex linerboard formed from old corrugated containers |
-
1982
- 1982-06-03 CA CA000404442A patent/CA1177607A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4708771A (en) * | 1984-12-31 | 1987-11-24 | Bear Island Paper Company | Two stage process for sulfonating mechanical pulp fibers |
| US5503710A (en) * | 1995-05-31 | 1996-04-02 | Macmillan Bloedel Limited | Duplex linerboard formed from old corrugated containers |
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