CA1094263A - Method of producing ultra high yield pulp - Google Patents

Abstract

- ABSTRACT -An ultra high yield chemimechanical pulping process is provided in which softwood chips are cooked, in a liquid phase or a vapour phase, with acid sulphite having a low pH. The treated chips are subsequently refined in an atmospheric and/or thermomechanical refining system yielding pulps of high strength and brightness.

Description

10~ ~Zfi3 The present invention relates to a method of pulping and more particularly, relates to an ultra high yield pulping method.
Known in the art are several pulping methods which are generally classified as refiner mechanical pulping (RMP), chemimechanical pulping (CMP) and thermomechanical pulping (TMP). One aspect of the present invention is directed to chemimechanical pulping while in a further aspect, it is directed to chemithermomechanical pulping~(CTMP).
Chemimechanical pulping includes many different processes such as, for example, the bisulphite,neutral sulphite and soda processes. Generally, each of the processes is used for producing specific types of pulps; each process suffers from its own disadvantage.
The neutral sulphite process is generally used for producing pulps from hardwood, which pulps have a good stiffness making them suitable for corrugated material or the like. It is generally not used to produce pulps for use in printing grade paper products.
The bisulphite process is a relatively recent one and produces pulps suitable for use in newsprint, fine papers and tissues. Basically, the bisulphite process employs a cook-ing liquor having a pH of 4 to 6; the wood is cooked to a temp-erature of 150 to 170C. for a period generally in the range of between one and two hours and lower when the cook is carried out in the vapour phase. The process is an energy intensive one and the pulp produced has good bonding properties giving print-ing grade papers with reasonably acceptable opacity.
According to a broad aspect of the present invention, in a pulping method which includes the steps of subjecting soft-wood in chip form to a chemical treatment and subsequently q ~_, 7 ,~3 ~0~4Z6;3 refining the treated chips, there is provided the improvement wherein the treatment step comprises treating the chips with acid sulphite having a pH of between about 1.5 and about 2.5.
In a further aspect of the present invention, there is provided a pulping method comprising the steps of supplying softwood chips, treating said wood chips with acid sulphite having a pH of between 1.5 and 2.5, washing said wood chips, and subjecting the washed wood chips to a thermomechanical refining operation.
The softwood chips employed in the pulping operation according to the method of the present invention, are uiilized in the form of conventional pressed wood chips. Preferably waferized or flaked wood chips are employed for more rapid liquor penetration. As used herein, the term "chips" is generic to any of the above types.
The chips, prior to undergoing the chemical treat-ment step, undergo a conventional steam pretreatment to remove air from the chips. This step is conventional in the art and need not be discussed herein.
In another aspect of the present invention, the chips are submerged in the acid sulphite solution and the chemical treatment is carried out in the liquid phase by cooking the mixture for a desired time period as will be discussed in greater detail hereinafter, In a further aspect of the present invention the chips are submerged in the acid sulphite solution and completely impreg-nated, excess liquor is then drained off and the chemical treat-ment step is carried out in the vapour phase.
In the aspect wherein the chips are treated in the liquid phase, conventional treatment equipment may be employed.

The process may be conducted on either a batch or continuous basis. Preferably, when conducted in the liquid phase, the chips are submerged in the acid sulphite solution and the chemical treatment is carried out by cooking the chips to a final tempera-ture of at least 100C; preferably to a temperature between 110C. and 130C. The total time-to-temperature period of the cook is at least 20 minutes; preferably 20 minutes to 90 minutes with the temperature of the cook being linearly programmed to reach a maximum at the end of the treatment period.
In the vapour phase, the impregnation of the chips proceeds at a faster pace and accordingly the treatment is carried out by submerging the chips in the acid sulphite solution, draining off the excess solution once the chips are completely impregnated;and,the chemical treatment is carried out in the vapour phase by cooking the chips to a final temperature of at least 110C; preferably to a temperature between 110C~ and 130 C. The total time-to-temperature period of the cook is at least 10 minutes; preferably 10 minutes to 45 minutes with the temperature of the cook being linearly programmed to reach a maximum at the end of the treatment period.
In a preferred embodiment, the chips are presteamed to remove air from the chips prior to undergoing the chemical pretreatment step in an acid sulphite liquor solution; the liquor to chip ratio being preferably 5:1 and the pH of the solution is between 1.5 and 2Ø
It is understood that, when emPloying the acid sulphite process above described, the temperature and time conditions of the cook are interrelated and may be at somewhat above or below the figures stated depending largely on the species of the soft~
wood chips, their form, i.e. waferized, flaked, etc., and, the make-up of the acid sulphite solution. At low tempçrature conditions the possibility of condensation reactions are desirably avoided. At higher temperatures the wood lignin may become a "brittle" substance resulting in undesirable pulp properties. The present process, which renders the lignin viscoelastic at the stated temperatures, has been found to produce pulps of superior properties.
Employing the acid sulphite process above described, it has been found that the softening temperature of the wood chips is depressed by 10 to 20 degrees Celsius. This enables one to carry out the refining step at the lower specific energy application yielding pulps of superior strength.
The term "ultra high yield" is employed to designate those pulping processes which have a y~eld of post refined pulp in excess of 90 per cent. Generally, the yield of the present invention is in the range of between 90 and 95 per cent.
Conventional chemimechanical pulps have generally had the advantage of approximately 30 per cent higher strength properties than refined mechanical pulps when compared at the same specific energy. In other words, when compared on an equivalent strength basis, refined mechanical pulps require 25 per cent greater energy to produce pulp having the same strength properties. However, chemimechanical pulps do have a slightly lower opacity of about 2 to 3 per cent and have a yield in the range of about 2 per cent lower than refined mechanical pulps. They do, however, exhibit greater fiber integrity than refined mechanical pulps.
The pulps produced according to the method of the present invention do have a higher opacity at a given breaking length than the bisulphite pulps. A further advantage of the method of the present invention is that the treated wood chips 10942~3 may be refined in an atmospheric discharge refiner and, the resulting pulp has better bonding properties.
The pulps produced according to the CTMP process also have significant advantages over those obtained in the prior art. Generally, when pulps produced by the TMP and CMP processes are compared, at the same specific energy, the chemimechanical pulps have a better bonding which leads to a resultant increase in properties dependent thereon - i.e.
burst strength, breaking length, etc. On the other hand, thermomechanical pulps have a longer fiber length and thus have better properties in respect of tear, etc. In both cases, the difference is in the order of 40 per cent.
Pulps produced by the CTMP process, according to the present invention, overcome several of the disadvantages associated with the CMP and TMP processes. The pulps pro-duced by the CTMP process have a higher breaking length and -tear strength than pulps produced by the CMP and TMP processes.
Furthermore, they produce a high bulk pulp with substantially no rejects. Stil~ further, the brightness of the pulp, as produced by the CTMP process, is such that for many uses it does not require the use of a brightening agent.
Still further, the rejects are a significant factor ln the pulping operation. . Generally, RMP pulps have re~ects in the 1 to 3 per cent range; CMP in the 0.3 per cent range ~nd TMP in the 0.6 per cent range. Pulps produced ac~ording to the CTMP process outlined above, have rejects in the 0.1 per cent range which are negligible.
It is believed that the invention will be understood by reference to the following Examples:

.

1~942fi31 100 kg of OD of steam purged black spruce chips in a waferized form were pretreated with an acid sulphite solution consisting of 5.0 per cent total SO2, 3.46 per cent free SO2, and 1.54 per cent combined SO2. The pH
of the solution was 1.7 and the reaction temperature was linearly programmed to reach a maximum of 110C. in 90 minutes. The thus treated chips were subsequently washed and refined in an open discharge refiner at a specific energy level of 70 HPD/ODT. Table I compares the proper-ties of the treated refined chips with those of refined untreated chips.
It will be noted that yields considerably in excess of 90 per cent were obtained when the chips were treated with the acid sulphite liquor and heated to a temperature of 11~0C.
with a time-to-temperature of 90 minutes. Examination of the chips showed that uniform impregnation had taken place.
The ultra high yield pulps showed values of bulk and scattering coefficient comparable to those of the control refiner mechanical pulp and furthermore, were significantly stronger and contained less reject material than the control RNP, even when produced with 25 per cent less specific refin-ing energy.

In this Example, a pilot plant was employed and reference may be had to the article by Evans, J.C.W., "New Pilot Plant Offers Extensive Facilities for Chip Refining", Pulp and Paper, 146 (October 1975).

A cold solution containing 16.3 kg of sodium 1 0 9 4 2fi3 TABLE I

Treated Untreated Y.ield % 93 - 7 95 Specific energy HPD/ODT 70 95 Freeness ml 208 120 Basis weight gm/sq. m 59.5 57.4 Bulk cc/gm 2. 61 2.92 Burst factor 28.8 17.1 Tear factor 68 58 Breaking length m 4482 3552 Stretch % 2.16 1.85 Toughness index g cm 557 375 Wet web work to rupture at 19% solids 2.90 2.30 gm cm/cm Brightness 8-457 ws 53.2 56.2 Visual efficiency 10-FMY/C WS 64.2 68.1 Tappi opacity PC 10-FMY/C 94.1 95.6 Scattering coefficient sq cm/gm 1-681 528 674 Absorption coefficient sq cm/gm 1-681 20.62 19.77 Re~ects % Somerville 0.85 o.88 Classification R14 8.4 o. g 4/28 22.6 ls . o 28/48 26.5 29.
48/loo 13.8 16.1 loo/200 7.5 8.9 P-200 21.2 30.0 -~0942t;3 bisulphite in 341 litres of water was pumped into the pre-cooled accumulator; sulphur dioxide (6.8 kg) was introduced into the bisulphite solution through a bubbling tube inserted through the top. The concentrations of total, free and com-bined sulphur dioxide were 4.86, 3.42 and 1.44 per cent respectively.
Table II gives a description of the pulping condi-tions in Example 2.
The chips were charged into a digester which had been preheated with steam for 10 minutes at 170 kPa. After sealing the digester, the chips were purged three times with saturated steam at a pressure of 239 kPa for periods of 3 minutes. The liquor was then pumped into the digester to give a liquor to wood ratio of 5:1 and was indirectly heated to 110C. over a 90 minute period. Upon reaching 110C., the liquor was blown. Portions of the cooked chips were refined at various levels of specific refining energy in the thermomechanical refining system and in the open discharge refiner.
In addition, control thermomechanical and open discharge refiner mechanical pulps were also prepared from untreated chips through a range of specific refining energies.
As may be seen, there werè employed four distinct pulping operations - RMP (refined untreated waferized chips i~ an atmospheric discharge refiner), CMP (refined cooked waferized chips in an atmospheric discharge refiner~, TMP
(untreated waferized chips in a thermomechanical refining system~ and CTMP (refined cooked waferized chips in a thermomechanical refining system). Table III gives character-istic properties of the pulps obtained in this example.

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TABLE III
Properties of pulps refined at 7 MJ/kg Pulp Breaking Bulk Tear Specific Somerville Elrepho Type length Index scattering rejects brightness coefficient (km) (cm3/g) (mN.m2/g) (cm2/g) (%) (%) RMP3.3 2.9 5.6 580 2.2 59 CMP4.6 2.6 6.8 530 0.2 63 TMP3.4 3.0 9.7 560 o.6 61 CTMP5.8 3.3 ll.0 480 0.1 63 For a typical operating specific energy of 7 MJ/kg, the strength properties of CMP, as measured by breaking length and tear index, are about 40 and 20 per cent higher respect~vely than those of RMP. Conversely, CMP pulps of equivalent strength to those of RMP would require about 20 per cent less specific energy and based on freeness measurements, would drain considerably faster. As aforementioned, pulps of the CTMP process possess several significant advantages. Their properties are compared with those of other high yield pulps at a freeness of 300 mm as shown in Table IV. In this respect, the bisulphite, neutral sulphite, sulphite and soda figures are those obtained from previous tests.

TABLE IV
Mechanical properties of chemimechanical pulps at 300 C.S.F.
.. . . _ .. _ , . .......................... , _ Cook type Acid Bisulphite Neutral Sulphite Soda Sulphite Sulphite CTMP
Yield (%) 96 87 87 89 90 Tear index (mN-m2/g) ll.0 4.6 5.1 6.o 2.1 Breaking length (km) 6.9 6.o 9.2 5.0 2.2 Bulk (cm3/g)2.90 1.87 1.63 2.23 2.16 Printing opacity (%~ 92 90 NA NA 95 Re~ects (%) 0.2 2.4 3.9 12.5 NA
.

4Zfi3 The ultra high yield pulps prepared by refining with a sequestering agent (EDTA) require only half as much sodium hydrosulphite to achieve the same brightness as the corresponding mechanical pulps. The CMP is brightened with only 0.5 per cent sodium hydrosulphite on OD pulp to obtain an optimum brightness of 66.8 per cent whereas RMP needs twice the charge to obtain a corresponding brightness.
CTMP prepared with EDTA showed a similar optimum response at 0.5 per cent charge of sodium hydrosulphite on OD pulp.
In summary of the above, CMP pulps produced accord-ing to the acid sulphite method discussed above are approxi-mately 40 per cent stronger than corresponding RMP, drain more readily, have a brightness of approximately 4 points higher, have 16 per cent lower scattering coefficient, and have 1 to 3 points lower opacity at equivalent specific energy application. Conversely, at equivalent pulp strength, the specific refining energy requirement for pulps produced by the acid sulphite process is about 20 per cent lower than for RMP.
Furthermore, the pulps of the present invention are better bonded than corresponding thermomechanical pulps and have approximately 40 per cent higher burst index and breaking length. The CTMP process produces pulps of significantly higher strengths, both breaking length and tear, than RMP, CMP, or TMP, and are comparable to the strengths of semi-chemical pulps at about 70 per cent yield.
It will be understood that the above examples are illustrative only and that the invention is not limited thereto, but rather by the claims appended hereto.

Claims (6)

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

1. In a chemi-mechanical pulping method including the steps of subjecting softwood chips to a chemical pre-treatment and subsequently refining the pretreated chips, the improvement wherein said chemical pretreatment comprises:
(a) submerging the chips in an acid sulphite solution having a pH of between 1.5 and 2.5;
(b) heating the mixture of solution and chips to a final temperature of between 100°C to 130°C in a time to temperature period of between 20 and 90 minutes.

2. Method according to claim 1 in which the pH
of the acid sulphite solution is between 1.7 and about 2Ø

3. Method according to claim 2 in which the refining step comprises refining the pretreated chips in a thermo-mechanical refining system.

4. In a chemi-mechanical pulping method including the steps of subjecting softwood chips to a chemical pre-treatment and subsequently refining the pretreated chips, the improvement wherein said chemical pretreatment comprises:
(a) submerging the chips in an acid sulphite solution having a pH of between 1.5 and 2.5 allowing the solution to completely impregnate the chips;
(b) separating the impregnated chips from said solution;
(c) heating the impregnated chips in a vapour phrase to a final temperature of between 110°C to 130°C in a time to temperature period of between 10 and 45 minutes.

5. Method according to claim 4 in which the pH
of the acid sulphite solution is between 1.7 and about 2Ø

6. Method according to claim 5 in which the refining step comprises refining the pretreated chips in a thermo-mechanical refining system.