The present invention relates to an absorbent chemi- thermomechanical pulp and to a method of manufacturing the same.

Hitherto, it has only been possible to apply the pro¬ cess of defibering chips with a low energy input subse¬ quent to preheating the chips under high pressure and high temperature (150-170°C), the so-called Asplund process, within the board manufacturing industry, since the pulp resulting from this process is dark in colour and cannot be bleached at reasonable chemical consump¬ tions. Furthermore, the "fibres become coated with a lignin skin and are therefore stiff and rigid, which results in poorer strength and absorption properties. Consequently, it has only been possible to produce chemithermomechanical pulp (CTMP) of high brightness and good absorbency by preheating and refining at a temperature of at most 140°C. High brightness is espe¬ cially important when producing tissue pulp.

The object of the present invention is to provide a chemithermomechanical pulp which exhibits a low resin content, an extremely high long-fibre content, an extremely low short-fibre content, and an extremely low shive content. Such pulps are particularly suited for the manufacture of fluff ahd tissue. The extremely low shives content is of special importance when producing tissue pulp. The extremely high long-fiber content with the corresponding high freenes is of special importance when producing fluff pulp.

A further object of the invention is to provide a novel method for the manufacture of absorbent chemithermo-

mechanical pulps at low energy inputs.

The invention thus relates to an absorbent chemither¬ momechanical pulp produced from lignocellulosic mate- rial at a wood yield above 88%, a resin content beneath 0.15%, calculated on the amount of resin which can be extracted in dichloromethane, a high long-fibre con¬ tent, a low short-fibre content and a low shives con¬ tent, the pulp being characterized in that when frac- tionating the pulp according to Bauer McNett, the long- fibre content is above 70%, preferably above 75% of fibres retained on a wire gauze of size 28 mesh and the short-fibre content is beneath 10%, preferably beneath 8%, of fibres which pass through a wire gauze of size 200 mesh according to Bauer McNett; and in that the shive content is lower than 3%, preferably lower than 2%, measured according to Sommerville.

The pulp should have such brightness that it can be bleached at a reasonable consumption of bleaching chemicals to a brightness of at least 65 % ISO, pre¬ ferably 70%. Alternatively the pulp may have been beached to such brightness.

This pulp is particularly well suited for the manufac¬ ture of fluff and tissue.

When the pulp is a fluff pulp it is preferably refined to a freeness of 740 ml at the lowest especially 750 ml at the lowest and suitably 760 ml CSF at the lowest.

Such a pulp does not need to be bleached and may have a brightness of at least 45 % ISO.

When the pulp is a tissue pulp it has suitably a

brightness of at least 65 % ISO, preferably above 70 %. The tissue pulp does not need to have as high a freenes as the fluff pulp. Suitably it is refined to a freenes of 650 ml CSF at the lowest.

The problem with manufacturing pulp suitable for fluff and tissue by means of a chemithermomechanical method lies in the desired combination of high freeness, high long-fibre content, low shive content and high bright- ness. An increase in temperature when preheating will favour the reduction in shive content but, at the same time, impair brightness.

It has now surprisingly been found that a chemither- momechanical pulp having the desired properties can be produced by

a) impregnating the chips with sodium sulphite, sodium dithionate, alkaline peroxide or the like, with an addition of a complex builder; b) preheating the chips; c) defibering the chips to pulp in a refiner at sub¬ stantially the same pressure and temperature as those employed in the preheating process; and d) washing and dewatering the pulp to, e.g., a con¬ sistency of 25-50%, wherein, in accordance with the invention, impregnation and preheating of the chips is effected in one and the same vessel over a combined treatment time of at most 2 minutes, particularly at most 1 minute, preferably at most 0.5 minute; and a) using a warm impregnating liquid having a tempera¬ ture of at least 100°C, suitably at least 130°C and preferably having essentially the same temperature as that of the preheating process;

b) preheating the chips at a temperature of 150-175°C, preferably 160-170°C; and c) carrying out the defibering process with an energy input which is at most half of the energy input re- quired for defibering to the same shive content in a similar refiner when preheating and defibering are performed at 135°C.

The complex builder used in the impregnating process may, for instance, be DTPA, which contributes to an improvement in pulp brightness.

The pulp may e) be refined to a brightness above 65 % ISO, preferably above 70 %. To accomplish this at a reasonable consumption of bleaching chemicals the brightness after refining has to be at least 45 % ISO, preferably at least 50 %. Such bleaching should prefer¬ ably be performed when the pulp is a tissue pulp.

In order to obtain a pulp of sufficient brightness, it is essential that preheating at the aforesaid high temperature is not permitted to proceed over a period of time of as long a duration as the standard prehea¬ ting time of about 3 minutes used when producing chemi- mechanical pulp of CTMP type. In order to enable the preheating time to be lowered to at most 2 minutes, preferably at most 1 minute, it is necessary to use an impregnating solution which is heated to a temperature of at least 100°C, particularly at least 130°C and preferably substantially to the same temperature as that used in the preheater. Furthermore, no impregna¬ ting liquid shall be removed between the impregnating and preheating steps. Consequently, impregnation is effected in the same vessel as that in which the chips are preheated, and at the same pressure and suitably at

the same temperature or only a slightly lower tempera¬ ture. The brightness of the pulp is sustained because of the very short stay time at the high temperature, so that an excessively large quantity of bleaching che i- cals, ssuch as peroxide, will not be required in the following bleaching step. Furthermore, the wood yield obtained in this way is almost equal to the wood yield obtained when preheating the chips conventionally at 130-140°C. In addition, when refining to a freeness slightly above 750 ml CSF, the energy input required for the defibering process is reduced from about 600 kWh/tonne at 130°C to less than 300 kWh/tonne at 170°C. These values have been obtained in a pilot plant. Com ersial values may differ from those obtained at pilot level. The relative differences between the levels for shives content, brightness and energy input obtained in the pilot plant at conventional temperature and at the temperature according to the invention, respectively, should, however, remain in a commercial plant.

The inventive method suitably includes the conventional steaming, impregnating, preheating, defibering, wash¬ ing, screening, washing, possibly bleaching, washing and drying stages. Whereas a conventional impregnating process is carried out with cold liquid in a vessel other than the preheating process, which is carried out over a period of about 3 minutes and at a temperature of about 130°C, and in which process impregnating liquid is removed between the impregnating stage and the preheating stage, the impregnating and preheating processes of the inventive method are combined in one and the same vessel and are carried out at the same pressure and substantially the same temperature 100- 175°C, 150-175°C respectively, over a combined time

period of at most 2 minutes, suitably at most 1 minute and preferably at most 0.5 minute.

Because preheating is effected at high temperature, the refining process requires less energy. A low energy input will normally result in high freeness and high shive content. A surprising characteristic of the present invention is that at low energy inputs, success is achieved in combining high freeness with low shive content. Low energy input would otherwise result in a high shive content.

When applying the inventive method in tests on a laboratory seal, a freeness of above 780 ml CSF was achieved with an acceptable shive content. In some instances, a freeness of above 800 ml was achieved. This can be compared with a freeness of about 650-750 ml CSF in the normal production of CTMP-fluff.

The pulp is washed subsequent to the refining process, suitably under pressure and at high temperature, pre¬ ferably while excluding air from the system and in immediate connection with the refining stage. The pulp is dewatered to a consistency of e.g. 25-50%. Possible bleaching is then carried out with peroxide or other bleaching chemical. If desired, the pulp can again be washed, after the bleaching process.

When producing fluff, defibering is carried out to a freeness of 740 ml at the lowest, suitably of 750 at the lowest, preferably of 780 ml CSF at the lowest. When producing tissue pulp the refining may be carried out to a freenes of 650 ml CSF at the lowest.

When applying the inventive method, it is possible to produce pulp with a wood yield above 88%, preferably above 90%, a resin content of less than 0.15%, calculated on the amount of resin that can be extracted in dichloromethane, and a brightness above 65% ISO after bleaching.

The invention will now be described in more detail with reference to the following exemplifying embodiments thereof and with reference to the accompanying draw¬ ings, in which

Figure 1 illustrates' schematically a test plant used in the exemplifying embodiments; Figure 2 is a diagram showing shive content against energy input at defibering;

Figure 3 is a diagram showing energy at defibering against preheating temperature; Figure 4 is a diagram showing long-fibre content against energy input at defibering;

Figure 5 is a diagram showing short-fibre content against energy input at defibering;

Figure 6 is a diagram showing network strength against energy input; Figure 7 is a diagram showing peroxide consumption against original brightness after defibering; Figure 8 is a diagram showing brightness after defiber¬ ing against peroxide consumption; and Figure 9 is a diagram showing fibre length against energy input after defibering.

Figure 10 is a diagram showing the brightness obtained after defibering against preheating temperature; and Figure 11 is a diagram showing brightness after defi¬ bering against preheating temperature.

In order to study the possiblity of manufacturing fluff and tissue pulp in a high-temperature variant of a CTMP-process, there was used a test plant schematically illustrated in Figure 1. The plant was constructed so that the pulps could be washed in immediate connection with refining at high temperature.

The chips are introduced into the preheater 2 with the aid of the feed screw 1 and are impregnated at the preheater inlet. The preheated chips are then passed immediately to the refiner 3, where the chips are defibered while supplying water. When starting-up the plant, and when samples shall be taken immediately after the refining stage, the resultant pulp is passed to the cyclone 4 where samples can be taken in the direction of arrow 5. The connecting line to the cy¬ clone 4 is then disconnected and the blower line 6 connected instead, such as to thin the pulp to a con- sistency of about 3% during transportation to a vessel 7 equipped with a pump which functions as a mixer. The pulp is then pumped to a level vessel 8 which is con¬ nected directly to a screw press 9. The entire system, from impregnation to dewatering in the screw press, can be pressurized to 1 MPa.

Spruce sawmill chips were used in the tests. The chips were screened on two different screens, to remove excessively coarse chips and sawdust. The screens had a hole diameter of 35 mm and 8 mm respectively. The chips were impregnated with 50 kg sodium sulphite and 3 kg DTPA per tonne of chips in all tests, prior to the preheating, refining and washing stages.

Example

Chips were treated in the plant shown in Figure 1 at different temperatures during the preheating-refining process. The temperature was allowed to vary between 135 and 170°C. The impregnating liquid was subjected to a heat exchange and brought to the temperature level of the preheater. At each temperature level in the re¬ finer, the pulp was washed at a temperature of about 10°C beneath the preheated temperature and at a te - perature of about 90°C under atmospheric pressure. The stay time in the preheater was maintained as constant as possible over a period of about 1 minute.

Subsequent to impregnation with the same chemical input as that used for remaining pulps, a CTMP-pulp was produced in an OVP-20 (Open Vertical Preheater) at a preheating and refining temperature of 135°C, this pulp being used as a reference pulp.

The results of the tests carried out on the pulps are shown partly in Figures 2-9 and in the following Table. These show typical results obtained in this pilot plant for some of the parameters of interest for the inven¬ tion.

The following Table I shows some of the results ob¬ tained.

TABLE I

o

Tests were also carried out at laboratory level in a 10 litre digester. The chips were steamed at atmospheric pressure and then impregnated with a weak alkaline sul- phite solution before the pressurized steam treatment at high temperature.

500 g of spruce chips with a dry solids content of 48.1% were steamed at a temperature of 100°C over a period of 2 minutes. The impregnating solution con¬ tained 20 g/1 sodium sulphite and 3.2 g/1 DTPA and had a temperature of 100°C. The impregnation was carried out for 1 minute under a nitrogen pressure of 7 bar. After removal of excess impregnating solution the chips were heated to their respective heating temperatures as fast as possible. Condensate was drained while heating. The time at each temperature was varied. Thereafter the chips were cooled in cold water. These chips were then refined and tested for brightness.

The results obtained are shown in the following Table II and on the Figures 10 and 11.

Table II Analysis Data

Preheating Preheating Sample Brightness temperature °C time, min. K 21/90 % ISO

58.3

58.3

58.3

135 2 62.6

10 58.2

150 1/2 60.7

2 B. 60.0

10 B, 54.1

160 2 54.5 10 49.5

170 1/2 54.1

2 51.3

10 46.6