CA1145106A - Procedure for improving properties of mechanical wood pulps - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Mechanical pulps in the form of RMP or, more pre-ferred, TMP, of improved properties suitable for substitu-tion for chemical pulp, especially in newsprint furnish, is obtained in a substantially pollution-free process by subjecting the mechanical pulp to chemical treatment using sodium sulphite solution. This treatment results in increased wet stretch and stress-strain properties while retaining high drainage and avoiding substantial yield loss.

Description

11~5106 PROCEDURE FOR IMPROVING PROPERTIES OF
MECHANICAL WOOD PULPS
The present invention is directed to the treatment of mechanical wood pulps to improve their properties so as to 5 render the same useful for substitution for chemical pulps.
The term "mechanical pulp" as used herein has its normal meaning in the art and refers to the product of dis-ruption of a woody substance by mechanical action to yield a product consisting mainly of liberated and separated single 10 woody fi~xes and their fragments and which is suitable for use in the manufacture of paper.
The term "fibre" as used herein also has its normal meaning in the art and refers to individual plant cells which make up the woody material and whicll, in softwoods, are known 15 botanically as parenchyma cells and tracheids. These fibres inherently have average diameters generally beIow 0.05 mm and in the case of wood species commonly used in pulp formation and paper making, such as, spruce, balsam, pine, aspen and poplar, considerably below 0.05 mm.
Traditionally newsprint has been manufactured from a furnish consisting of about three parts of groundwood pulp and one part of chemical pulp. Groundwood pulp is the cheap-est component of the furnish and contributes several desirable properties to the sheet, including paper opacity and ink 25 acceptance during printing.
The chemical pulp component of the furnish is usually manufactured by either the well-known kraft process or sulphite process in yields ranging from about 45 to about 65%. Chemical pulps are expensive to produce, make heavy ~o demands on wood resources and entail formidable pollution problems.
Chemical pulps are used in newsprint furnishes since they impart properties to thq furnish which improve its run-ability.
"Runability" refers to that combination of properties which allows the wet web to be transported at high speed through the forming, pressing and drying sections of the paper making machine and allows the dry sheet to be reeled and printed with not more than an acceptable number of breaks.

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: ' ' ,' ' ' ~ ' - ' ~,, 11~5106 In effect, runability is a measure of the efficiency with which the paper passes through the paper machine and printing pr~ess.
Despite all the aforementioned disadvantages assoc-5 iated with the use of chemical pulps, they are generallyemployed in making newsprint because runability is the key to paper making machine and press-room efficiency, which in turn is the key to profitability.
In accordance with this invention, there is provided 10 a process for the formation of an improved mechanical pulp which is suitable as a replacement for chemical pulps in many applications, including newsprint furnish.
The process of this invention results in an increase in the elongation to rupture (hereinafter known as "wet 15 stretch") and an improvement in the stress-strain properties of the wet web formed from the pulp, while simultaneously maintaining rapid drainage. We have discovered a hitherto unknown phenomenon that high wet stretch and high stress-strain characteristics, in combination with rapid drainage, 20 are fundamental pulp properties which improve the runability of a newsprint furnish.
The fibre-to-fibre bonding within a dry paper sheet formed from the pulp produced by the process of the invention is improved, thereby resulting in the desirable properties of 25 increased tensile and burst strengths and increased sheet densit~.
One important feature of this invention is that there is formed a modified mechanical pulp which can be used as a substitute, in whole or in part, for chemical pulp in 30 many of its applications and which result~ ~rom a proaedure which does not produce more than insignificant quantities of polluting effluents, in complete contrast to chemical pulp-ing procedures, where large quantities of polluting effluents must be handled.
-35 The process of the invention comprises two steps, ~amely (a) subjecting particulated cellulosic fibrou~ material to mechanical action to form a pulp consisting mainly of single fibres and fragments thereof, and (b) chemical reac-~ .
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11~51~6 tion of the pulp with a soluble salt of sulfurous acid under certain precise elevated temperature and pressure conditions as detailed below.
The present invention is applicable to 5 refiner pulps. "Refiner pulps" are a class of mechanical pulps formed by passing particulated cellulosic fibrous material, usually wood chips, through a small gap between two ribbed parallel plates rotating with respect to each other (known as a disc refiner). The procedure may be effected at atmospheric pressure, the product being known as "refiner mechanical pulp" (RMP), or under pressure, typically about 1 to 2 atmospheres greater than atmospheric pressure, and at elevated temperature, such as, about 120C, the product being known as "thermomechanical pulp" (TMP). The refining 15 process usually is effected in two stages. In the first stage, the fibres are separated and liberated and in the second stage, additional refining energy is supplied to increase the fibre flexibility and conformability, fibrilla-tion and bonding. Usually about half the overall refining energy of about 100 to about 120 horsepower-days per ton is applied to the fibre-li~eration stage.
The two individual steps comprising the process of the invention are discussed separately below.
Step (a) Preparation of the Mechanical Pulp As noted above, refiner pulps are usually produced in two steps, namely, fibre separation by mechanical action and refining by mechanical action. The character of the re-finer pulp and its response to subsequent treatment in the process o the invention depends on the conditions that pre-vail at the moment of fibre separation in a disc refiner.
A wood fibre consists essentially of a cell wall, whose outer surface is made up of cellulose-rich fibrillar layPrs known as the Sl and S2 layers. In wood, the space between the fibres known as the middle lamellae, is filled with a lignin-rich material.
The process of the invention requires that, in the initial liberation of the fibre from the wood in a disc refiner, the fracture occurs mainly in the Sl and S2 layers, thus exposing the cellulose-rich fibrillar material which is the source of the fibrillation characteristic of a good mech-a~ical pulp Since this fibre morphology is established at the moment of fibre liberation, it is necessary that the process of fibre liberation proceed largely to completion.
Therefore, the product of the initial mechanical fibre separa-tion step of the process of the invention must consist mainly of single wood fibres, which inherently have average diameters less than 0.05 mm. More than the minimum energy to accomplish this separation may be applied, but is unnecessary.
It is well known that, in thermomechanical pulping, if the refining temperature exceeds the thermal softening point of lignin, fibre separation occurs in the middle lamellae to yield a smooth fibre with a lignin-rich surface.
This fibre is difficult or impossible to fibrillate by further refining and is generally unsuitable for use as a mechanical pulp. Hence the initial fibre separation step in this invention is effected at a temperature below the thermal softening point of lignin. The latter temperature i5 variable with the wood species, duration of heating and re-fining conditions, but is generally below about 150C.
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Attempts have been made to decrease the energy required for fibre separation and improve pulp quality by a chemical softening of the wood prior to refining. Such a process, using sulphite as the treating chemical, is disclosed in U.S.
Patent No. 4,116,758. The products of the latter process are smoo~h walled fibres sh~wing little tendency to ~ibrillatlon, similar to those described above resulting from refining above the lignin so~tening temperature, and are unsuitable for use as a mechanical pulp in this invention.
It is within the scope of this invention, however, to add the chemicals required in the chemical treatment step to the wood chips prior to their entering the disc re~iner, pro-vided that the temperature and time of contact is such that no substantial reaction occurs and no significant chemical softening of the chips results. The disc refiner acts as an efficient mixer o~ the pulp and chemicals at the high con-sistency normally encountered.
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~1~51~$i s It is also within the scope of the invention to subject the wood chips, prior to refining, to steam under pressure at a temperature below the thermal softening temper~-ture of the lignin, typically below about 140C in accordance 5 with conventional industrial practice in TMP manufacture A suitable product of fibre separation is obtainable simply by following the first stage refining procedures well known to the art, for the Production of a good thermo-mechanical pulp (TMP). This lO is usually accomplished by presteaming wood chips, usually at a temperature of about 120~ to about 135~C and l to 2 atmospheres pressure for 2 to lO minutes, then passing the presteamed wood chips, which have not been softened by chemical action, through a disc refiner at a temperature ~elow 15 the thermal softening temperature of the lignin, and applying sufficient refining energy to yield a mechanical wood pulp consisting mostly of single fibres and their fragments, such fibres and fragments being predominantly below 0.05 mm in average diameter. This operation is generally effected at a consistency of about lO to about 40% by weight, usually about 25 to 30% by weight. The fibre separation may be effected at atmospheric pressure, if desired, but the results attainable in subsequent processing are in general inferior to those attainable from the product of a pressurized refiner.
Once the mechanical pulp is obtained by fibre separation, the ~ulp may be subjected to fur~her refining action in a disc refiner, following the usual practice of the in-dustry for the seCOnd stage refining of a mechanical wood pulp. It is well known that the quality of a mechanical wood pulp can be improved by increased refining but at a cost of slower drainage and increased energy demand.
The consistencies employed in the application of the refining step may be varied over the range normally employed in the second stage refining of a mechanical wood pulp, ~ut the properties of the product depend to some extent on the refin-ing consistency chosen. Higher consistencies over about 20%
by weight yield products with higher wet stretch while lower consistencies tend to produce pulps with higher strength.

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By adjustments in refining consistency, the desired balance between wet stretch and strength for a particular application can be achieved. For most applications, it is preferred to carry out the refining step at consistencie~ between about 1% and about 35~ by weight.
An alternative method of manufacture of refiner mechanical pulps is to supply all of the energy needed to separate and to refine the fibres in a single step. In this method, the material passes between the refiner plates only once and is known to the art as single stage refiner mechanical pulp. In the process of this invention, either single stage or multistage refining may be used.
The amount of energy applied in the refining step may be varied according to the desired properties of the product and the intended end use. The degree of refining to which the pulp is subjected is usually controlled by the freeness of the refined pulp. For most applications, this freeness should fall within the range of about 50 to about 700 C.S.F. For example, boxboard stock is typically of higher freeness than magazine grade paper stock. For newsprint application, it is preferred to re ine to a freeness in the range about 100 to about 400 C.S.F.
STEP (b) Chemical Reaction .
After the required physical form of the wood fibre is obtained in Step (a), the chemical nature of the fibre is modified by reaction with an aqueous solution of a soluble salt of sulphurous acid, usually sodium s~llphite.
Other soluble salts of sulfurou9 acid may be used, such as, potassium sulfite and ammonium sulphite but these materials are le~s preferred.
The reaction is effected at temperatures above about 110C under a superatmospheric pressure for a time sufficient to yield a chemically-treated refiner mechanical wood pulp capable of forming a paper web having improved wet stretch and stress-strain properties and exhibiting rapid drainage, but for a time insufficient to cause substantial dissolution of lignin with consequent loss of yield and gener-ation of polluting effluents. The exact nature of the , .

114SlV6 chemical reactions involved in the chemical treatment effected in this invention are not fully understood, but are thought to involve sulphonation and deacetylation.
During the reaction, the pH of the solution drops and alkali is consumed. It is essential to the process of the present invention that sufficient alkali be present in the chemical charge to prevent a pH drop below 3 during treatment, otherwise there is a risk of damaging the fibres through hydrolytic action with consequent loss of strength. The exact amount of alkali required varies according to the acetyl content of the wood supply and cannot be specified exactly, but is readily established by experimentation.
The alkali requirement may be met entirely with sodium sulphite. However, since only half of the sodium of sodium sulphite is available for neutralization, it is usually more economical to meet part of the alkali require-ments by additions of sodium hydroxide or sQdium carbonate.
The pH of the mixture, however, is preferably kept below about 12 because hemicelluloses are dissolved from wood fibre by higher pH's, with consequent loss in yield.
In a preferred embodiment of the invention, the amount of sodium sulphite used in the chemical treatment is in the range of about 4% to about 15% by weight based on the mechanical wood pulp resulting from step (a), although lower concentrations down to about 1% by weight may be used with reduced beneficial effect, with the provision that the residual sulphite content of 'he mixture, as measured iodi-metrically, does not fall substantially to zero before termination of the reaction. Below 1% by weight of sodium sulphite,improvements are too small to justify the expense of treatment. Similarly improvements are observed with chemical charges up to about 25% by weight of the pulp, but the additional cost is not justified by the small additional improvement. Generally, therefore, a chemical charge of between about 1% and about 25~ by weight, preferably between about 4% and about 15% by weight, of the mechanical pulp, is used. The chemical charge preferably has a pH between about .:

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7 and about 12, and contains sodium sulphite and sufficient alkali to maintain a pH greater than 3 throughout the reaction.
The reactions of sulphite with wood are known to 5 consist of a large number of different reactions, whose rates are dependent on reaction conditions, particularly pH and temperature. The present state of our knowledge of this complex subject has been summarized by G. Gellerstedt in Svensk Papperstidning nr. 16, 1976, p. 537 to 543.
10 It has been established that the reactions necessary for the application of the process of the invention and the results attained thereby are those that proceed at pH's greater than 3 and preferably over 7, and at temperatures over about 110C, and preferably over about 130C. Other 15 reactions of woody substances wit~ sulphite which proceed at lower pH's and at temperatures below lQ0C are known, such as those described by H.J. Kvisgaard in Norsk Skogindustri lq, no. 4, 1965, p.155-163. Such reactions, however, are not effective to produce an improvement in wet ~20 and dry properties and in fibre flexibility and consolidation, such as is contemplated in this invention.
We have found that the maximum improvement, namely, maximum increase in wet stretch, maximum improvement in stress-strain and maximum increase in strength characteristics is 25 ob~ained from the process of the invention when the mechanical pulp from step (a) with added chemical is heated at about 160C
for 30 minutes. As with any other chemical reaction, the tem-perature aan be lowered if the reaction time i9 increased.
Below about 120C, reaction time becomes impractically long~
and below 110C, the required reactions effectively cease.
Similarly, the reaction temperature can be increased if the reaction time is shortened. The practical upper limit of temperature appears to be about 200C with reaction times of 1 to 2 minutes. We prefer not to opexate under these extreme conditions because the precise control of conditions and reaction times needed to achieve an optimum product are difficult to secure.
It is also possible to operate at shorter or longer ,-:114S106 than the optimum reaction times to produce a less than optimum but still useful result. If the reaction time is shorter than optimum, the improvements in wet stretch, stress-strain and strength properties are less than may be otherwise obtained by operating under optimum conditions. If the reaction time is too long, substantial dissolution of the lignin from the pulp, in the treating chemical occurs, with consequent loss of yield and formation o~ polluting effluent. While the process i5 still operable to produce property improvements under these condi-tions some of the advantages of wood economy and low pollution are lost and generally are avoided.
The chemical treatment is operable over a time-temperature range from about 110C for about 12 hours to about 200C for about 1 minute. It is understood that an increase in temperature must be accompanied ~y a concomit-tant decrease in reaction time. For example, the process is not operable at a temperature of 200C for 12 hours.
To derive maximum benefits from the chemical treatment step, it is preferred to operate in the more limited range of about 130C for about 2 hours to about 180C for about 15 minutes.
Because of uncertainties in specifying the exact upper limits of the chemical treatment step in terms of time and temperature, it is considered more useful and precise to specify the upper limit in terms of the effect of the chemical treatment on pulp yield therefrom. Reaction conditions which decrease the yield, based on mechanical wood pulp, below about 85% are outside the scope of our process, since wood losses and the pollution capability of ~he spent ~ aqueous phase become significant and intolerable beyond this limit. It is preferred to select maximum reaction conditions such that the yield of treated pulp is greater than about 90%.
The exact conditions requirea vary with wood species, chemical charge and consistency, but will fall within the limits of 35 time and temperature as defined above, and are easily estab-lished by experimentation.
The chemical reaction which is effected in step (b) on the mechanical wood pulp resulting from step (a) is ~uite 11~5 distinct from the methods used iIl the pulping of woody sub-stances with sulphite or bisulphite to form chemical pulp. In sulphite pulping, heat and chemical are supplied to the woody material in chip form (i.e., fibre bundles) by circulating hot cooking liquor through a bed of the woody material. With the mechanical pulp produced in step (a) of the process of the invention, the resistance to flow of liquor is so great that circulation of li~uor therethrough is impractical. In con-sequence, all of the chemical required to effect the reaction of step (b) must be incorporated in the pulp when it enters the reactor. It is advantageous to incorporate the chemical in solution in a volume of water which can be totally absorbed by the pulp. In practice this means that the consistency after chemical addition normally should be above about 15~
by weight. Consistencies below about 50~ by weight are pre-ferred because it is easier to secure uniform mixing o~
chemical and pulp below that level. The consistency range of about 15% to about 50~ by weight, therefore, is preferred for reasons of convenience, but the operability of the process is not limited by consistency.
The chemical treatment step in the process of the invention is also distinguished from chemical pulping process- !
es in that ~he process of the invention cannot be oonducted practically in a batch process,such as is used in chemical pulping. This is because the thermal insulating properties of the mechanical pulp are so high that a large pulp mass cannot be heated to reaction temperature by conduction in a reasonable length of time. The chemical treatment may be carried out batch-wise using dielectric or microwave heating techniques but 30 such methods are expensive. It is preferred to carry out the chemical reaction step in an apparatus wherein pulp is continuously raised to reaction temperature and introduced into one end of a reaction vessel of such size as to provide the desired reaction duration, while treated pulp is 35 removed simultaneously from the other end.
The invention is illustrated by the following Examples:

~45106 Example 1 This Example illustrates application of the process of the invention to commercial RMP mechanical pulps.
Screened stock to the mill, manufactured from 5 spruce chips and from jack pine chips in a Bauer 480 refiner, was mixed with 10~ by weight of sodium sulphite at pH 9 and heated at 15% consistency, at 145C for 1 hour.
The usual practice in mechanical pulping is to re-move latency prior to screening, cleaning and final use.
10 This procedure was explained by L.R.Beath et al in an article entitled "Latency in Mechanical Pulps", Pulp and Paper Magazine of Canada, vol. 67 (10) page T423 (1966). To correspond to this common practice, latency was removed from the pulps formed in this and subsequent Examples by treatment 15 at 90C for 15 minutes prior to testing.
The properties of the pulp are compared in the following Table I:
TABLE I
Jack Pine _ Spruce Untreated TreatedUntreated Treated Yield, ~ 1~0 ~3 lOa ~6 Freenes~,C.S.F. 180 210 185 175 Wet Stretch, % 4.3 4.6 4.1 5.1 Wet Tensile, N/m 44 48 61 70 25 Bulk, cm /g 3.39 2.68 3.12 2.38 Burst Factor 11 17 15 24 Brea~;ing Length, km 24~0 3200 3100 4400 Tear Factor 56 72 79 81 Jack pine is considered an inferior species because of its low strength. It will be seen from the results of the above Table I that by utilization of the process of the inven-tion, the properties of this inferior species can be improved to yield a pulp comparable in properties to spruce RMP. By the application of the process of the invention, spruce RMP yields a product comparable to untreated spruce TMP.
The results set forth in Table I show that, with both species, wet stretch and wet tensile strength ar~
improved.
These properties are essential to paper machine .
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rlmability. With both species, bulk is reduced markedly.
This is the result of more flexible fibres which consolidate better to yield a denser, better bonded sheet. The results of this can also be seen in the improved strength measure-ments, namely burst and breaking length.
The process of the invention is effective in in-creasing the tear factor of jack pine, a property in which this species normally is deficient.
Example 2 This Example illustrates the greatly superior pro-perties obtained by applying the process of the invention to a preferred mechanical pulp, spruce TMP.
Spruce chips were presteamed for 25 minutes at 35 psig and fed to a 1000 HP Sprout-Waldron 36 ICP pressurized refiner under the following conditions:
Throughput: 25.0 tons per day Discharge consistency: 25 to 30%
Specific energy: 45 horsepower days per ton The pulp from the pressurized refiner, consisting mainly of single fibres, was refined to 200 Canadian Standard Freeness in a 1500 HP Bauer 412, open discharge refiner, mixed with 10% by weight sodium sulphite at pH 9 and heated at 15% consistency at 145C for 1 hour. After latency re-~al, the properties set forth in the following Table II were 25 measured: -TABLE II
Untreated Treated Yield, % 100 94 Freeness, C.S.F. 198 179 30 Wet Stretch, % 4.7 10.1 Wet Tensile, N/m 81 72 Bulk, cm /g 2.7 1.9 Burst Factor 21 30 Breaking Length, m 3400 5400 35 Dry Stretch, ~ 1.8 2~3 The greatly improved product obtained by the appli-cation of the process of the invention by starting with a preferred mechanical pulp, namely TMP, rather than RMP, is e~ident by ~omparison of the results set forth in Table II

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11~5106 above with those set forth in Table I in Example 1. The exceptional wet stretch of the pulp produced in this Example, in combination with the augmented strength-properties and high freeness, are the unique properties which permit use of the treated TMP as a replacement for low ~ield chemical pulps in newsprint nanufacture.
Example 3 This Example illustrates the necessity of employing a proper mechanical pulp as a starting material, i.e. a pulp consisting substantially of separated single fibres, in order to produce a useful product.
A sample of spruce chips was refined in a 12 inch Sprout-Waldron refiner at wide plate gap setting. The product, designated "A", consisted mainly of fibre bundles in the range of 0.05 to 0.1 mm in diameter. Product "B" was prepared from spruce chips in a Sprout-Waldron 36 ICP pressur-ized refiner and consisted essentially of separated single fibers, substantially free of particles with diameters greater than 0.05 mm.
Each pulp was then treated under twd conditions within the scope of this invention, namely 140C for 30 minutes at 1~ consistency with 4 wt.% sodium sulphite at ph ~ , and with 10 wt.% sodium sulphite at pH 9 The resulting products had the properties set forth in Table III below:
TABLE III
Product "A" Product "B"
Untreated 4% 10~ Untreated ~% 10%
Drainage time, sec. 0~57 0.56 0.76 1.32 0.95 1.16 30 Wet Stretch, % 2.6 2.9 2~7 9.7 7.9 7.9 Burst Factor 2 2 4 22 18 24 Breaking Length, m 500 500 700 3600 3050 3820 Tear Factor 17 16 13 105 108 96 Dry Stretch 0.6 0.6 0.6 1.9 2.5 2.9 The results of the above Table III show that a pro-duct consisting mainly of fibre bundles (i.e. product "A") is, for the purposes of paper making, virtually with~ut strength, and application of the chemical treatment step of this invention is incapable of developing useful paper making ., . ~ . . , - . . .

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properties.
Microscopic examination of product "A" after thechemical treatment showed that no dissociation of fiber bundles had occurred. These results are in agreement with 5 the common industrial experience that a stock containing any substantial amount of fibre bundles cannot be used in the manufacture of newsprint.
By contrast to the results obtained with product "A", product "B" is usable in paper making without the appli-cation of chemical treatment, but is greatly improved by theapplication of this invention. It will be seen from the results of Table III that treatment under the milder condi-tions, i.e. 4% sodium sulphite, greatly improves wet stretch and drainage, without beneficial effect on dry strength.
Treatment with more chemical (i.e. 10% sodium sulphite) improves wet stretch and dry strength with less beneficial effect on drainage. The chemically-treated pulp exhibited much faster draining than a pulp in which comparable proper-ties are developed by refining alone. The results set forth in this Example illustrate the flexibility of the process of the invention in modifying the properties of a mechanical pulp (i.e. product "~") according to the requirements for a particular application.
Example 4 This Example illustrates the effect of temperature of chemical treatment on pulp properties.
Samples of commercial spruce RMP were heated with 10% sodium sulphite by weight at pH 9.0 and at a consistency of 15~ for one hour at 100C and 130C, giving the results reproduced in the following Table IV:

11~5106 Table IV
Untreated 100C 130C
Yield, % 100 100 96 Freeness, C.S.F. 134 134 144 5 Dxainage Time, sec. 1.62 1.39 1.52 Wet Stretch, ~ 4.6 4.6 5.5 Wet Tensile, N/m 75 73 79 B~lk, cm /g 2.93 2.92 2.50 Burst Factor 18 19 22 10 Breaking Length, m 3400 3400 4000 The results of the above Table IV show that no measurable improvement in pulp properties is obtained by reac-tion at 100C, but useful improvements are produced at 130C.
Exam~le 5 This Example illustrates application of the inven-tion to southern pine TMP.
A sample of screened stock from a commercial TMP
plant operating on southern pine wa~ treated at 20 % consis-tency with 10% sodium sulphite at pH 9 at 145C for 1 hour.
20 The properties of the pulp before and after treatment were as follows:
Table ~
Untreated Treated Yield, % lQn ~5 25Freeness, C.S.F. 70 74 Drainage, sec. 1.57 2.Q0 Wet Stretch, % 6.7 8.1 Wet Tensile, N/m 55 59 Wet Caliper, mm Q,34 0. 3a 30 Bulk, cm /g 2.96 2.36 Burst Factor 13 20 Breaking Length, m 2600 3800 Dry Stretch 1.6 2.0 Tear Factor 5~ 71 The results set forth in the above Table V con-~irm the well known fact that mechanical pulps derived from southern pine are much inferior to spruce pulps, and also show that application of the process of the invention im- -proves the quality of southern pine TMP in every measured 40 property. The improvement in tear is particularly valuable.

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Example 6 This Example illustrates the use of a pulp prepared by the process of the invention to replace chemical pulp completely in a groundwood based newsprint furnish on a high speed paper machine.
Approximately 50 tons of pulp at 225 to 250 C.S.F.
were prepared in a Jylha, 5 Mw/1500 rpm tandem TMP system pressurized to 1 to 1.5 atmospheres. Specific refining energy was 75 horsepower-days per ton. This pulp, mixed with sodium sulphite at pH 9 r was heated at 150C in a Pandia continuous digester for a retention time estimated at 30 minutes. The chemically-treated pulp after centricleaning was used to replace a mixture of semi~leached kraft and lcw yield sulphite in the furnish to a commercial paper machine running at 2400 ft./min. The mechanical component of this furnish was about 70% groundwood, 30% TMP. This substitution had no adverse effect on paper machine operation through the duration of a 7-hour trial.
The recor~ of this trial are reproduced in the following Table VI:

~1~51()6 Table VI
Before Durin~ After Furnish Groundwood (%) 57 50 58 TMP (%) 25 22 26 Semibleached kraft (%) 9 0 8 Unbleached sulphite (%j 9 0 8 Treated TMP (%) O 2g 0 Paper Machine Conditions Wire speed (m/min) 693 694 695 Reel speed (m/min) 724 724 724 Headbox pressure (kPa) 63.2 63.8 63.3 Slice opening (mm) 10 10 10 Supercalender speed (m/min) 700 700 700 Linear pressure (K N/m) 190 190 190 Paper Characteristics 2 Basis weight (g/m ) 48.1 49.8 48.7 Bulk (cc/g) 1.56 1.57 1.56 Tensile Index M.D. (Nm/g) 44.5 41.9 41.2 Stretch M.D. (%) 2 1.05 1.13 1.11 Tear C.D. (mN m ~g) 222 196 203 Burst Index (kPa m /g) 1.43 1.31 1.32 Roughness 1 kg T.S. (m/min) 86 101 87 W.S. 97 104 92 Brightness I.S.O. (%) 60.9 59.2 61.2 Opacity I.S.O. (%) 93.9 93.6 93.6 Porosity (m/min) 224 213 216 Caliper uncalendered ~m 120 127 123 calendered ~m 75 78 76 The results of the above Table VI illustrate that a treated mechanical pulp formed by the procedure of this invention may be used satisfactorily to replace chemical pulp in groundwood-based newsprint, so that the problems of pollution and excessive consumption of wood and energy attendant the production of chemical pulps can be eliminated from newsprint manu~acture. As far as the applicants are a~are, such an accomplishment has not been heretofore reported.
In summary of this disclosure, the present invention 40 provides a process of treatment of wood chips to form a chemically-treated mechanical pulp of superior properties which is useful as a replacement for chemical pulp in many applications, including newsprint. Modifications are possible within the scope of this invention.
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Claims (10)

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for the formation of refiner pulp having improved properties, which comprises:
(a) subjecting wood chips which have not been softened by chemical action to mechanical action in a disc refiner at a temperature below the thermal softening temperature of lignin to cause the formation of a mechanical wood pulp con-sisting mainly of single fibre and fragments thereof, (b) subjecting said mechanical wood pulp to mechanical action in a disc refiner to improve the pulp quality of the same to provide a refined pulp having a Canadian Standard Freeness of about 50 to about 700; and (c) treating said refined pulp at an elevated tempera-ture above about 110°C and under a superatmospheric pressure with an aqueous solution of a soluble salt of sulfurous acid containing sufficient alkali to maintain a pH greater than about 3 during the treatment, said treatment being effected at a temperature and for a time to enable reaction with the pulp to occur and to produce a chemically-treated pulp capable of forming a paper web having increased wet stretch and improved stress-strain properties while rapid drainage is retained, said treatment being effected at a temperature and for a time insufficient to result in a treated pulp yield below about 85% by weight.

2. The process of claim 1 wherein steps (a) and (b) are effected in separate passes through said disc refiner.

3. The process of claim 1 wherein steps (a) and (b) are effected in a single pass through said disc refiner.

4. The process of claim 1 wherein said soluble salt of sulphurous acid is an aqueous sodium sulphite solution and said treatment is effected at a pulp consistency of about 15 to about 50% by weight and at an applied chemical charge of about 1 to about 25% by weight of sodium sulphite based on pulp.

5. The process of claim 1, 2 or 3 wherein said treatment is effected at a temperature of about 130°C for about 2 hours to about 180°C for about 15 minutes, the temperature and time of treatment being effected to maintain the yield above about 90% by weight.

6. The process of claim 4 wherein said treatment is effected at a temperature of about 130°C for about 2 hours to about 180°C for about 15 minutes, the temperature and time of treatment being effected to maintain the yield above about 90% by weight.

7. A process for the formation of refiner mechanical wood pulp capable of forming a wet paper sheet having in-creased wet stretch and increased stress-strain properties, which comprises:
(a) subjecting wood chips which have not been softened by chemical action to steaming at a temperature of about 120°
to about 135°C under about 1 to 2 atmospheres pressure;
(b) subjecting said steamed wood chips to mechanical action in a disc refiner at an elevated temperature below the thermal softening temperature of lignin, under a super-atmospheric pressure and at a consistency of about 10 to about 40% by weight to cause the formation of a mechanical wood pulp consisting mainly of single wood fibers and frag-ments thereof of average diameter less than 0.05 mm;
(c) subjecting the mechanical wood pulp to mechanical action in a disc refiner at a consistency of about 1 to about 35% by weight to refine the same and provide a refiner mechanical pulp having a Canadian Standard Freeness of about 50 to about 700 C.S.F.; and (d) treating said refined pulp with an aqueous sodium sulphite solution containing sufficient alkali to maintain the pH of the solution greater than about 3 during the treatment at a consistency of about 15 to about 50% by weight an applied chemical charge of about 4 to about 15% by weight of sodium sulphite based on pulp, and at a temperature of about 130°C for about 2 hours to about 180°C for about 15 minutes, the temperature and time of treatment being effected to achieve the maximum improvement in refiner pulp properties while the pulp yield from the chemical treatment is maintained above about 90% by weight.

8. The process of claim 7 wherein said aqueous sodium sulphite solution has a pH of about 9 to about 12.

9. The process of claim 7 wherein said aqueous sodium sulphite solution is added to said wood chips prior to the passage of the latter through said disc refiner, whereby said aqueous solution is intermixed with the fibre as they are formed.

10. The process of claim 7, 8 or 9, wherein said refining step is effected to provide a refiner mechanical pulp having a Canadian Standard Freeness of about 100 to about 400 C.S.F.