FI71779B - FRAMEWORK FOR MECHANICAL REFINING - Google Patents

Description

II. ANNOUNCEMENT n <7 n g lbJ (11) utlAggnincsskrift (i <t y c (45) Patent granted

Patent rol / ciat Zd CC 1037 (51) Kv.lk. * / Lnt.Cl. ‘D 21 B 1/16 FINLAND —FINLAND (21) Patent application - Patentansökning 80265 ^ (22) Application date - Ansökningsdag 22.08.8 0

(23) Starting date - Giltighetsdag 22.08.SO

(41) Made public - Blivit offentlig] g 06.8l

National Board of Patents and Registration ^ Date of submission and publication. -

Patent- oeh registerstyrelsen 'Ansökan utlagd och utl.skriften publicerad 31-10-86 (86) Kv. application - Int. ansökan

(32) (33) (31) Privilege requested - Begärd priority I 2.7S

USA (US) 10 ^ * 668 (71) Q.N.S. Paper Company Limited, 12 ^ 5 Sherbrooke Street West, Montreal, PQ, Canada (CA) (/ 2) Adrian John Barnet, St. Catharines, Ontario, Alan C. Shaw, St. Catharines, Ontario, Charles Donald Logan, St. Catharines, Ontario, Canada (CA) (7 ^ 0 Oy Koi ster Ab (5 ^) Method for making pulp - Förfarande för framstä11 Ning av mekanisk raffinörmassa

The present invention is directed to the manufacture of a pulp with improved properties that can be used to replace chemical pulp.

The term "mechanical pulp" as used herein has the usual meaning in the art and refers to a product obtained by mechanically decomposing a wood material, consisting mainly of released and separated individual wood fibers and parts thereof, and suitable for papermaking.

The term "fiber" as used herein also has a conventional meaning in the art and refers to the individual plant cells that make up the wood material and are known botanically for conifers as parenchyma cells and tracheids. The diameters of these fibers are generally less than 0.05 mm and, in the case of wood species commonly used for pulp and paper production, such as spruce, spruce, pine, wound and poplar, considerably less than 0.05 mm.

2 71779

The term "grit" refers to a mechanical pulp formed by passing a reduced cellulosic fibrous material, usually wood chips, through a small gap between two parallel surfaces with ribs, the surfaces rotating relative to each other (known as a disc grinder). The procedure can be performed at normal atmospheric pressure, the product known as "tread" (RMP), or using pressure, usually about 1-2 atmospheres above normal pressure and at elevated temperatures, such as 120 ° C, where the product is known as "hot tread" (TMP). ). Grinding is usually performed in two steps. In the first stage, the fibers are released and separated, and in the second stage, additional grinding energy is used to improve the flexibility, uniformity, fibrillation and binding of the fibers. Usually about half of the total energy of grinding per about 1800-2000 kWh / t is used in the fiber separation step.

Because mechanical pulps can be produced in yields above 95% with minimal contamination problems, this is a powerful stimulus to increase their use in papermaking. In general, however, it is not possible to move a sheet made of mechanical pulp alone at high speed through the forming, pressing, drying and winding sections of a papermaking machine without excessive interruptions. Chemical pulp is usually added to paper raw materials to improve its processability in the machine. Newsprint is usually made from raw materials consisting of about three parts groundwood or other mechanical pulp and one part chemical pulp.

The term "processability" refers to a combination of properties that allows a wet sheet to be transferred at a higher speed through the forming, pressing, and drying sections of a paper machine and allows the dry web to be wound and printed with only an acceptable number of breaks. In fact, processability is a measure of the efficiency with which paper passes through a paper machine and a printing press.

The chemical pulp mixture is usually prepared by the sulfite process in yields in the range of 3,71779 to about 45-65%. Chemical pulp is expensive, requires a lot of wood raw materials and causes significant pollution problems.

As mentioned above, mechanical pulp is obtained in yields of more than 95% with minimal contamination problems.

Despite all the drawbacks associated with the use of chemical pulp, it is commonly used in the manufacture of newsprint because handling is crucial in the papermaking machine and because of the efficiency of the printing hall, which in turn is critical to economy.

The present invention provides a method of making an improved pulp suitable for replacing chemical pulp in a variety of applications, including newsprint raw materials.

Thanks to the present invention, the elongation at break (wet elongation) is increased and the stress-strain properties of the wet web formed from the pulp are improved while maintaining rapid dewatering. It has now been observed a hitherto unknown phenomenon that high wet elongation and good stress-strain characteristics when wet, together with rapid dewatering, are fundamental properties that improve the flow of newsprint in a paper machine.

The fiber-fiber bond in a dry sheet of paper formed from a pulp made by the method of the invention is improved, thereby increasing the tensile and tear strength and the density of the sheet.

An important feature of the present invention is the production of a pulp that can be used to replace all or part of the chemical pulp in many of its applications, resulting in a process that produces only insignificant amounts of polluting emissions, which is completely different from chemical pulp production processes that require large amounts of polluting emissions. . The total energy required in grinding to form a pulp with a predetermined quality level is also reduced compared to a conventional milling process.

The method of the invention is characterized in that it comprises the steps of: 4 71779 (a) subjecting a non-chemically softened chip to mechanical action in a disc mill at a temperature below the thermal softening point of lignin to obtain a mechanical wood pulp consisting essentially of discrete wood fibers and parts thereof; (b) treating the pulp at an elevated temperature above about 100 ° C under an overpressure with an aqueous solution of a soluble salt of sulfuric acid containing sufficient alkali to maintain a pH greater than about 3 during the treatment at a temperature and for a time such that the pulp is chemically reacted to obtain a treated pulp capable of forming a paper web having increased wet strength and improved stretch deformation properties while maintaining rapid dewatering, the treatment being performed at a temperature and for a time sufficient to give a treated pulp yield of greater than about 85% by weight; and (c) chemically the treated pulp is subjected to the treatment of the name meka in a disc mill to improve the quality of the pulp and obtain a pulp with a Canadian Standard freeness number of about 50 to 700.

The types of cellulosic fibrous material and processing conditions required to produce a useful mechanical pulp are well known in the art. For example, it is well known that most hardwood grades cannot be ground to obtain a mechanical pulp of sufficient strength. The application of the invention is limited to a pulp made of wood species or other cellulosic fibrous materials considered suitable in the industry for the production of a mechanical pulp. The invention is further presented with particular reference to wood species.

The three steps that make up the method according to the invention are presented separately below:

Step (a) Separation of fibers

The wood fiber consists mainly of a cell wall, the outer surface of which in turn consists of fibrous layers rich in cellulose, known as S1 and S2 layers. In wood 5,71779, the space between the fibers, known as the middle lamella, is filled with a lignin-rich material.

The material according to the invention requires that, in the initial release of the fibers from the wood in the disc mill, the detachment takes place mainly in the S1 and S2 layers, thereby exposing a fibrous material rich in cellulose, which gives good mechanical pulp fibrillation properties. Since this fiber morphology is formed at the time of fiber release, it is essential that the fiber release process continue substantially to completion. Thus, in the initial mechanical separation step of the process according to the invention, the product obtained must consist mainly of individual wood fibers with an average diameter of less than 0.05 mm. More than the necessary amount of energy can be used in this separation step, but it is unnecessary.

It is well known that in the manufacture of a hot mill, if the temperature used exceeds the temperature of the lignin, the separation of the fibers takes place in a central lamella, resulting in a smooth fiber with a surface rich in lignin. This fiber is difficult or impossible to further fibrillate and is generally unsuitable for use as a mechanical pulp. Thus, the initial fiber separation step of this invention is performed at a temperature below the softening temperature of lignin. This latter temperature varies with different types of wood and depends on heating and rubbing conditions, but is generally below about 150 ° C.

Efforts have been made to reduce the amount of energy required to separate the fibers and to improve the quality of the pulp by chemically softening the wood before grinding. Such a process using sulfite as a treatment chemical is disclosed in U.S. Patent No. 4,116,758. The products of this process are smooth-walled fibers with a low tendency to fibrillate, corresponding to the above fibers obtained at a temperature above the softening point of lignin, and are not suitable for use in this process. as a mechanical pulp according to the invention.

However, the present invention involves the addition of a chemical required in the next processing step to the chips prior to their transfer to the disc mill, provided that the temperature and contact time are such that no substantial reaction occurs or significant chemical softening of the chips.

The disc grinder acts as an efficient mixer of pulp and chemicals at the high consistencies commonly used.

Also within the scope of the invention is the steam treatment of chips using pressure prior to milling at a temperature below the softening point of lignin, typically below about 140 ° C, according to a procedure conventionally used in industry to produce hot milling.

According to the present invention, the product suitable for further processing from step (a) can be obtained simply according to the first step of the grinding method known in the art in the preparation of a good mechanical pulp. This usually involves pre-evaporating the wood fibers, usually at a temperature of about 120-135 ° C and 1-2 atmospheric pressure for 2-10 minutes, transferring the pre-vaporized chips that have not softened by chemical treatment to a disc mill at a temperature below the softening point of lignin and using sufficient energy to obtain a mechanical pulp consisting mainly of individual fibers and parts thereof, the diameter of these fibers and parts thereof generally being less than 0.05 mm on average. This step is generally carried out at a consistency of about 10 to 40% by weight, in particular 25 to 30% by weight.

Step (b) Chemical reaction

After achieving the desired physical form of the wood fibers in step (a), the chemical nature of the fibers is changed by reaction with a soluble salt of sulfuric acid, usually an aqueous solution of sodium sulfite. The reaction is carried out at a temperature above about 110 ° C under an overpressure time sufficient to form a chemically treated mechanical pulp capable of forming a paper web with improved wet strength and stress deformation properties and rapid dewatering but not sufficient to cause lignin. substantial dissolution, which would lead to a reduction in yield and the formation of polluting emissions. The exact nature of the chemical reactions involved in the chemical treatment used in this invention is not known, but sulfonation of li 71779 7 is believed to be related to them.

During the reaction, the pH of the solution decreases and the alkali is consumed. It is essential for the process of the present invention that there is sufficient alkali in the chemical charge to prevent the pH from dropping below three during treatment to prevent damage to the fibers due to the hydrolytic effect, which would result in a decrease in strength. The exact amount of alkali required varies according to the acetyl content of the wood raw material and cannot be determined precisely, but it is easily detectable by experiments.

The alkali requirement can be met by sodium sulfite alone. However, since only half of the sodium in sodium sulfite is available for neutralization, it is usually more economical to satisfy part of the alkali requirement by adding sodium hydroxide or sodium carbonate. However, the pH of the mixture is preferably kept below about 12 because the hemicelluloses dissolve from the wood fibers at higher pH values as the yield decreases correspondingly.

In a preferred embodiment of the invention, the amount of sodium sulfite used in the chemical treatment is in the range of about 4-15% by weight based on the mechanical pulp from step (a), although lower concentrations, up to about 1% by weight, may be used with reduced beneficial effect provided that the mixture has a residual sulfite content. iodometrically measured does not substantially approach zero before the reaction is complete. When the amount of sodium sulfite is less than 1% by weight, the improvements are too small compared to the treatment cost. Similar improvements are observed with the use of chemicals up to about 25% by weight of the mechanical pulp, but the additional costs are not justified on the basis of the small improvement achieved. The chemical charge is thus generally about 1 to 25% by weight, preferably about 4 to 15% by weight based on the mechanical pulp. The chemical charge preferably has a pH of about 7-12 and contains sodium sulfite and sufficient alkali to maintain the pH above 3 throughout the reaction.

Sulfite reactions with wood are known to involve a large number of different reactions, the rates of which depend on the reaction conditions, in particular pH and temperature. Our current 71779 knowledge of this complex event is summarized in G. Gellerstedt, Svensk Papperstidning No. 16, 1976, pages 537-543. It has been found that the reactions necessary for the application of the process of the invention and the results associated therewith proceed at pH values above 3 and preferably above 7 and at temperatures above about 110 ° C and preferably above about 130 ° C. . Other reactions of wood materials with sulfite pathways that proceed at lower pH values and temperatures below 100 ° C and are described in H.J. Kvisgaard, Norsk Skogindustri 19, No. 4, 1965, pages 155-163. However, these reactions are not effective in improving wet and dry properties, fibers or woodiness, bonding them together, or the performance requirements achieved by the present invention.

It has now been found that the greatest improvement, namely the largest increase in wet strength, the greatest improvement in stress-strain properties, the largest improvement in tensile strength and the largest reduction in the energy requirement for the second step of the tritering process (step (c) below) is achieved in the process of step (a). the mechanical mass with the added chemicals is heated at about 160 ° C for 30 minutes. As with any other chemical reaction, the temperature can be lowered if the reaction time is extended.

Below about 120 ° C the reaction time becomes impractically long and below 110 ° C the required reactions are thoroughly interrupted. Similarly, the reaction temperature can be raised if the reaction time is shortened. The practical upper limit of the temperature turns out to be about 200 ° C, with reaction times of about 1-2 minutes, because it is difficult to carry out precise control of the conditions and reaction times required to obtain the optimal product.

It is also possible to use shorter or longer times than the optimal reaction time to obtain a product which is not quite optimal, but still usable. If the reaction time is shorter than the optimal reaction time, the improvements in wet elongation, stress-strain properties and strength, and energy requirements are less than they would be when delivered under optimum conditions. If the reaction time is too long, considerable dissolution of lignin from the pulp into the treatment chemicals occurs as the yield decreases and polluting emissions occur. Although the method can still be used to improve its properties under these conditions, some of the benefits of wood economy and low contamination are lost and generally avoided.

The chemical treatment can be performed in a time / temperature range of about 110 ° C / 12 h and about 200 ° C / 1 min. It should be noted that the increase in temperature must be accompanied by a corresponding reduction in reaction time. For example, the method cannot be used at 200 ° C for 12 h. To achieve maximum benefits in the chemical treatment step, it is preferred to operate in a more limited range between about 130 ° C for about 2 hours and 180 ° C for about 15 minutes.

As it is uncertain to determine precise upper limits for the chemical treatment step for time and temperature, it is considered more useful and accurate to set an upper limit for the efficiency of the chemical treatment according to the yield obtained therefrom. Reaction conditions that reduce the yield by weight to less than about 85% are outside the process of the invention because wood losses and the contamination capacity of the effluent phase increase significantly and become intolerable outside this range. It is desirable to select the maximum reaction conditions so that the yield of treated pulp is greater than about 90%. The exact conditions required will vary depending on the tree species, chemical input, and consistency, but are within the range described above and can be readily determined experimentally.

The chemical reaction carried out in step (b) on the mechanical pulp obtained from step (a) is completely different from the methods used in the treatment of wood material with sulphite or bisulphite to obtain chemical pulp. In sulphite cooking, heat and chemicals are supplied to the wood chips (e.g. fiber bundles) by circulating the hot cooking solution through a layer of wood material. The resistance of the mechanical mass formed in step (a) of the process of the present invention to the flow of liquid is so great that it is impractical to circulate the solution through it. Thus, at 1 ° 71779, all the chemicals required in reaction step (b) must be added to the pulp as it enters the reactor. It is preferred to add the chemicals dissolved in an amount of water that can be completely absorbed by the pulp. In practice, this means that the consistency after chemical addition must usually be greater than 15% by weight. Consistencies of less than 50% by weight are preferred because it is easier to achieve uniform mixing of chemicals and pulp below this level. A consistency range of about 15-50% by weight is thus recommended for convenience, but the thickness does not limit the applicability of the method.

The chemical treatment step according to the present invention also differs from the chemical fiberization processes in that the process according to the invention cannot be practically carried out as a batch process, such as in the production of a chemical pulp. This is due to the fact that the thermal insulation properties of the mechanical pulp are so good that a large amount of pulp cannot be heated to the reaction temperature by heat transfer within a reasonable time. Chemical treatment can be performed as a batch method using dielectric or microwave heating, but these methods are expensive. It is recommended to perform a chemical reaction step in an apparatus in which the pulp is continuously heated to the reaction temperature and fed to one end of a reaction vessel of a size such that the desired reaction time is obtained and the treated pulp is simultaneously removed from the other end.

Step (c) Processing

In the third and final step (c) of the process according to the invention, the product obtained from step (b) is ground in a disc mill according to a conventional method used in industry for grinding the second stage of mechanical pulp. The results of this second grinding differ from those obtained using conventional mechanical pulp because the use of steps (a) and (b) of this invention provides the pulp with the required physical and chemical configuration to use the additional grinding energy efficiently and economically. It is well known that the quality of mechanical pulp can be improved by additional grinding, but at the expense of slower dewatering and increased energy consumption. The product obtained from step n 71779 (b) can be ground to a corresponding quality level using significantly less energy and achieving faster dewatering than using the mechanical pulp from step (a) which has not undergone the treatment of step (b). These results are shown graphically in Figure 1, where the quality of the pulp is shown in terms of grinding power in two cases. The measure used for pulp quality was the tensile breaking strength of the wet web measured at 5% wet elongation to eliminate the effects of pulp latency energy. Similar curves are obtained using other similar mass quality measures, such as break length or burst index.

Point A in Figure 1 determines the state of the pulp after step (a) of the method according to the invention. Point B represents the same mass after step (b). Line B-C shows the properties of the pulps obtained from step (b) using different amounts of grinding energy in step (c) of the process according to the invention. Line A-D represents the properties of the pulp obtained by grinding directly the product of step (a) without using step (b). The significant effect of the chemical treatment of step (b) to improve the dewatering of the ground pulp, as measured by the Canadian Freeness Test (C.S.F.), to improve the strength of the pulp and reduce the energy requirement when using the grinding of step (c) is clear from the graph in Figure 1.

The densities used in step (c) can be varied in the range normally used in the second stage grinding of mechanical pulp, but the properties of the product depend to some extent on the grinding density chosen. About 20% higher consistencies give products with higher wet elongation, while lower densities give masses with higher strength. By adjusting the grinding consistency, the desired balance between wet elongation and strength can be achieved for each application. In most applications, it is preferred to perform the grinding step (c) at consistencies of about 1-35% by weight.

The amount of energy used in step (c) can be varied according to the desired properties and intended use of the product. The grinding applied to the product is usually adjusted by the freeness number of the finished pulp. In most applications, this freeness number should be in the range of about 50-700 C.S.F. For example, the freeness number of a boxboard material is typically higher than that of magazine printing paper. For newsprint, it is recommended to grind in the range of about 100-400 C.S.F. to the freeness number in step (c).

The invention is illustrated by the following examples.

Example 1 2

The spruce chips were pre-steamed for 25 minutes at 2.5 kp / cm and fed to a 1,000 HP Sprout-Waldron 26 ICP grinder under the following conditions:

Feed: 2.0 tons per day

Depreciation rate: 25-30%

Specific energy: 800 kwh per tonne

The pulp obtained from the pressurized refiner, which consisted mainly of individual fibers and had almost no particles larger than 0.05 mm in diameter, was divided into three parts. One portion was mixed with 10% by weight sodium sulfite at pH 9 and heated at 18% consistency at 90 ° C for one hour. The second portion was mixed with 10% by weight of sodium sulfite at pH 7 and the mixture was heated at 18% consistency and 160 ° C at 5.25 kp / cm for one hour. The third part was untreated. Each part was then further ground in a 30 cm Sprout-Waldron open-end grinder at a consistency of 18% with a specific energy supply of 1,120 kWh per tonne. A total of 1,920 kWh per tonne of grinding energy was then used in all these pulps.

A common practice in mechanical defibering is to remove latency before screening, cleaning, and final use. This procedure is described in L.R. Beath, M.T. Neill & F.A. In "Latency in Mechanical Pulps", Pulp and Paper Magazine of Canada 67 (10) T423 (1966). According to this general industrial procedure, latency was removed from the present pulps by treatment at 90 ° C for 15 minutes before testing. The properties of these masses are shown in Table I below.

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Table I

Handling Not 90 ° C 160 ° C

Yield,% 100 98 93

Freeness number 194 197 80

Dewatering, seconds 0.83 0.93 2.16 Wet strength, N / m 62 63 75 Wet elongation,% 3.3 3.7 5.3 Wet thickness, mm 0.344 0.353 0.302

Fixed density 2.68 2.57 2.12

Outbreak 16 17 23

Breaking Length 3100 3300 5100 Using the same total grinding power, the untreated sample and the sample treated at 90 ° C were ground to substantially the same free-ness number, with only insignificant differences in wet and dry properties. In contrast, a sample treated at 160 ° C ground to a lower freeness number using the same power gave a greater than 60% increase in wet stretch and break length, as well as a significant increase in wet tensile strength and puncture. The dewatering rate was much higher than with untreated hot mill of the same quality.

Wet thickness and solid volume are measures of the ability of fibers to form a sheet of paper. The low values obtained with the pulp treated at 160 ° C denote flexible fibers which adhere well to each other to form a dense, uniform sheet.

Example 2

The hot mill prepared in a pressurized mill according to Example 1 was mixed with 10% sodium sulfite at pH 9.0 and the mixture was heated at 18% consistency at 160 ° C and 5.3 kp / cm 2 for one hour. The treated and untreated hot mill samples were then ground to the corresponding freeness numbers and the pulp properties were measured after removal of latency at 90 ° C for 15 minutes.

The energy consumption and corresponding mass properties are shown in Table II below.

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Table II

Untreated Treated Untreated Treated

Energy consumption, kWh / ton 2 255 1 800 1 800 1 468

Freeness 107 118 305 331

Dewatering, seconds 1.27 1.63 0.66 0.83 Wet tensile strength, N / m 58 77 59 76 Wet elongation,% 4.0 5.1 3.3 4.3 Wet thickness, mm 0.322 0.290 0.385 0.326

Fixed density 2.47 1.92 3.14 2.08

Outbreak 14 28 14 32

Breaking length 3200 5600 2800 5800

Yield,% 100 94 100 96 These values show that with the treatment according to the invention the energy requirement to achieve the desired freeness number and dewatering rate can be reduced by more than 20%. In addition, this energy saving involves significant improvements in wet-web properties, dry strength, and fiber bonding.

Example 3

The Pinus Rigida pine pulp from the first stage mill of a commercial newsprint mill was treated at 145 ° C for one hour with 10% by weight of sodium sulfite at pH 9. The resulting pulp was then milled at 340 and 680 kWh per ton of energy at 30 cm Sprout-Waldron. in the refiner.

The untreated mass was ground in the same manner. The properties of these products after the latency removal treatment at 90 ° C for 15 minutes are shown in Table III below.

Table III

Untreated Treated

Grinding energy, kWh / tonne 680 340 680

Freeness, C.S.F. 194 243 146

Dewatering, seconds 0.64 0.65 0.82 Wet tensile strength, N / m 36 42 46 Wet elongation,% 4.4 5.4 5.9

Thickness, mm 0.402 0.375 0.348

Fixed density 3.65 2.74 2.56 15 71779

Table III (cont'd) Untreated Treated

Rest period 9 19 20

Breaking length 2100 3400 3800

Tearing 67 92 89 These values illustrate the application of the method according to the invention to difficult wood species; The fibers of Pinus Rigida pine are stiffer and thicker than spruce and usually result in poor quality hot rubbing. However, by using the process according to the invention, a product of the same quality as 0 or better than that normally obtained and having the following additional advantages can be obtained: 1) Less energy is required to achieve a corresponding freeness number.

2) The pulp treated with the same energy feeds was ground to a lower freeness number with greatly improved both wet and dry properties. The burst and break length are approximately doubled.

3) Using half of the second stage energy input and a higher freeness number, the treated product is even better than that obtained from untreated hot milling.

4) The method according to the invention provides a significant improvement in the coalescence of the fibers, as shown by the decrease in wet thickness and solid density.

Example 4

Spruce chips were ground in a Bauer 420 open debit mill at a rate of 65 tons per day using an energy of 1,074 kWh / t. A portion of this pulp was mixed with 10% sodium sulfite at pH 9 and heated at 145 ° C at 3.5 kp / cm for one hour. Both treated and untreated pulp were further ground in a 30 cm Sprout-Waldron grinder at a consistency of 20%. The properties of the obtained pulp are shown in Table IV below. The latency was removed at 90 ° C over 15 minutes.

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Table IV

16 Untreated Treated

Second stage grinding capacity, kWh / ton 0 806 806 572

Freeness, C.S.F. 388 122 81 156

Dewatering, seconds 0.61 1.16 2.3 1.28 Wet web properties

Tensile strength, N / m 36 56 63 68

Elongation,% 3.2 4.3 5.6 4.7

Thickness, mm 0.454 0.336 0.287 0.311

dry Features

Fixed density 3.98 2.70 1.98 2.22

Rest period 8 16 23 25

Breaking length 1700 3300 4900 4700

Elongation 1.2 1.8 1.7 1.9

Tearing 61 66 52 69 Several points can be observed in this example. The grit (first column) obtained from the first grinder requires additional use of grinding power to obtain sufficient properties. Using a further 806 kWh / ton of grinding energy gives the greatly improved results shown in column 2. However, using the same amount of energy for the pulp treated according to the present invention, a product with a lower freeness number and much better wet and dry properties is obtained (column 3). Alternatively, using only 572 kWh / ton of additional grinding energy gives a product with a similar freeness number and dewatering rate, but with improved all wet and dry properties (column 4).

This shows that the method according to the invention can be applied to pulp as well as to hot pulp.

In summary, the present invention is directed to the production of an improved mechanical pulp that can be used as a substitute for chemical pulp. Modifications are possible within the scope of the invention.

Claims (10)

17 71779 A method for producing a pulp with improved properties, characterized in that it comprises the steps of: (a) subjecting a non-chemically softened chip to a mechanical action in a disc mill at a temperature below the thermal softening point of lignin to obtain mechanical wood pulp consisting mainly of discrete wood fibers; and parts thereof; (b) treating the pulp at an elevated temperature above about 110 ° C at an overpressure with an aqueous solution of a soluble salt of sulfuric acid containing sufficient alkali to maintain a pH greater than about 3 during the treatment at a temperature and for a time such that the reaction takes place with the chemically treated pulp. capable of forming a paper web having increased wet strength and improved stretch-deformation properties while maintaining rapid dewatering, the treatment being performed at a temperature and for a time sufficient to give a treated pulp yield of greater than about 85% by weight; and (c) chemically treated pulp subjecting a mechanical treatment in a disc mill to improve the quality of the pulp and obtain a pulp having a Canadian Standard freeness number of about 50 to 700. Process according to Claim 1, characterized in that the soluble salt of sulfuric acid is an aqueous solution of sodium sulphite, preferably having an initial pH of about 9 to 12, that treatment step (b) is carried out at a pulp consistency of about 4 to 15% by weight, and that the amount of chemicals used is about 1 to 25% by weight of sodium sulfite based on the amount of pulp. Process according to Claim 1 or 2, characterized in that an aqueous solution of the soluble salt of sulfuric acid is added to the chips before being transferred to the disc mill in step (a) so that the aqueous solution mixes with the fibers as they form. Process according to any one of claims 1 to 3, characterized in that the treatment step (b) is carried out between about 130 ° C for about 2 hours and about 180 ° C for about 15 minutes, the treatment temperature and time being adjusted to give more than about 90% by weight -% yield. Process according to one of Claims 1 to 4, characterized in that the chips are treated with steam at a temperature of about 120 to 135 ° C for about 1 2 atm. under pressure before step (a). Process according to one of Claims 1 to 5, characterized in that step (a) is carried out at a consistency of 10 to 40% by weight. Process according to one of Claims 1 to 6, characterized in that the mechanical pulp obtained from step (a) consists essentially of individual fibers and parts thereof with a diameter of less than about 0.05 mm. Process according to one of Claims 1 to 7, characterized in that step (a) is carried out at an elevated temperature below the thermal softening point of the lignin and at an overpressure. Process according to one of Claims 1 to 8, characterized in that step (c) is carried out at a consistency of about 1 to 35% by weight. Process according to any one of claims 1 to 9, characterized in that step (c) is carried out to obtain a pulp having a Canadian Standard freeness number of about 100 to 400 C.S.F. 19 71779