This invention relates to the making of chemi-mechanical pulp from hardwood for wood-containing printing paper, especially wood-containing fine paper.

Pine paper traditionally is manufactured from a mixture of short-fibred and long-fibred chemical pulp. In order to improve the opacity of the paper and to increase its bulk, conventionally a certain amount of bleached groundwood pulp was mixed into the stock. The term wood- -containing fine paper is used when the amount of ech- anical pulp exceeds 10%.

In recent years a new type of pulp, peroxide-bleached chemi-mechanical pulp of hardwood, so-called CTMP _H , has implied an interesting alternative as a replacement of both the groundwood pulp and the chemical hardwood pulp at the manufacture of wood-containing printing papers. Especially asp and poplar have been used there¬ for.

For this type of paper very high requirements are made on the properties of the CTMPττ-pulp, because the paper in addition to high ISO-brightness and good opacity preferably also should have high bulk and simultaneously high stremgth and low roughness. For coated papers also low porosity is required. It is especially important, that these papers have a uniform surface structure. For being able to use CTMP _H -pulp in wood-containing fine paper, it must be refined to a drainage resistance of 100-200 ml CSF, and it must be possible to bleach it to a brightness of 82-85$ ISO, which is a very high brightness for a mechanical pulp. The energy consumpt- ion at the refining thereby is relatively high, and so is the chemical consumption at the bleaching. This applies particularly to dark hardwood with high density and various types of eucalyptus. An extensive equipment

in the form of means for drainage and washing is re¬ quired for the bleaching.

The present invention implies, that this equipment can be simplified and the consumption of chemicals and electric energy be reduced without deteriorating the pulp quality.

According to the invention, the hardwood in the form of chips is impregnated in one or two steps at a temper¬ ature below 80°C with 15-40 kg NaOH and 0-30 kg Na ₂ S0, per ton pulp. Thereafter a refining under pressure is carried out in such a way, that the temperature of the chips prior to the refining does not exceed 8θ°C for a period longer than 10 seconds. Subsequent bleaching of the pulp is carried out with peroxide in one or two steps, and additionally a minimum of 15 kg NaOH is charged so that the total amount of NaOH charged is 30- 80 kg per ton pulp. Thereafter refining of the pulp is carried out in a second step.

The invention implies that the brightness and light-scatt- eriiϊg coe ficient of the pulp is maximized and at the same time the energy consumption at the refining is minimized.

It is generally known that a high alkali charge during the impregnation step prior to the refining increases the light absorption coefficient and decreases the light-scattering coefficient of the pulp, i.e. decreases the brightness, but improves the strength and reduces the shives content.

According to the invention, the strength of the pulp is determined by the total of the alkali charge during the impregnation and peroxide bleaching steps.

According to the invention, the alkali charge during the impregnation step shall be determined only with regard being paid to the shives content and fibre distribution number and disregarding the pulp_-_strength obtained by

the alkali charge during the bleaching step. As a result, the reduction in brightness and light scattering coeffic¬ ient can be minimized. The refining in the second step is carried out with partially washed pulp, i.e. pulp con- taining residual alkali and residual peroxide from the bleaching, in order to minimize the energy demand at the refining and to be able to utilize the refining step as a third bleaching step carried out at a very high temper¬ ature, whereby it shall be possible to obtain highest brightness with a certain peroxide charge. The total peroxide charge at the bleaching shall be 30-50 kg peroxide per ton pulp, preferably 30-40 kg for light wood types and 40-50 kg for dark ones.

The characterizing features of the invention become app- arent from the attached claims. The invention is describ¬ ed in the following with reference to an embodiment there¬ of and experiment results shown in the accompanying Figures, of which

Fig. 1 shows the ISO-brightness after peroxide bleaching as a function of the NaOH-charge during the impregnation step, and

Fig. 2 shows the tensile strength of the pulp as a funct¬ ion of the total NaOH-charge.

The pretreatment of the hardwood in the form of chips of eucalyptus saligna is carried out by washing, atmos¬ pheric steaming and so-called PREX-impregnation with a NaOH/Na _p SO,-solution with pH 13. By the impregnation, 20 kg NaOH and 20 kg Na- _j SO., per ton are supplied to the chips. The chemicals are allowed to react for 30 minutes with the chips which hold 50 C.

The chemicals and solved substance are thereafter pressed out of the chips, which are pressure refined with

2 1000-1200 kWh/ton at a steam pressure of 3,5 kg/cm in the refiner house to a dewatering number measured as 350 ml CSF. The energy input at this refining should

be 800-1400 k h/ton

The pulp is diluted and dewatered to 35$ pulp concentrat¬ ion and thereafter mixed with complex former, prefer¬ ably EDTA, at pH 5 _a thereafter diluted and again press- ed to 3 $ dry matter content.

The pulp is thereafter peroxide bleached in two steps, the first one at 15-18$ pulp concentration with residual peroxide from the washing following after the second bleaching step. A^ er the first bleaching step, which is carried out at 70 C for 30 minutes, the pulp is again washed and pressed to 35$ dry matter content, at which content the fresh bleaching chemicals are mixed into the pulp. The pulp is bleached for 120 minutes at 70°C with 0 kg peroxide, 25 kg NaOH and 30 kg sodium silicate Na _p SiO-, per ton pulp, whereafter the pulp is washed by dilution with 4,5 parts washing water and pressing to 35$ dry matter content. The pulp is refined in a pressurized disc refiner,together with reject from the screen room, with 600 kWh/ton, with 3-5 kg/cm steam pressure and simultaneous addition of 5 kg peroxide per ton pulp to the refiner inlet. The energy input at this refining should be 400-800 kWh/ton. The pulp is latency treated at 3.5$ pulp concentration and 70 C, screened in pressurized screens in two steps and thereafter pumped to storage before being used in a paper mill.

After the different reaction steps, the pulp has the properties as follows:

After the first refinin step

After the second bleachin step

Different NaOH-charges during the impregnation step result in different ISO-brightness of the pulp after the peroxide bleaching.

A lower NaOH-charge implies, that it is possible to obtain a higher final ISO-brightness of the pulp. In Fig. 1 is shown how the brigltness varies with the NaOH- -charge during the impregnation step at three different peroxide charges.

Pulp I was manufactured from chips, which had been impregnated with 25 kg NaOH per ton pulp. With a peroxide charge of 45 kg per ton pulp it is then possible to obtain a brightness of above 80$ ISO.

Pulp II was manufactured from chips impregnated with 50 kg NaOH per ton pulp.In spite of bleaching with a peroxide charge as high as 45 kg per ton pulp, it is not possible to increase the brightness higher than to about 77$ ISO.

At the embodiment described above the original NaOH- -charge was 20 kg per ton pulp. The brightness could there be increased by peroxide bleaching to 82.5$ ISO.

A lower NaOH-charge at the chip impregnation, however, results in a decrease of the strength properties of the pulp. It was found, however, that the pulp strength within certain limits depends only on the total NaOH-charge during the impregnation and peroxide bleaching steps. During the impregnation step, therefore, NaOH is charged in an amount of 15-40 kg per ton pulp in order to ensure the necessary shives content and fibre distribution number. Additional NaOH in an amount of at least 15 kg per ton pulp to a total amount of 30-80 kg per ton pulp is charg¬ ed first in connection with the peroxide bleaching after the first refining. As regards the NaOH-charge can be said, that heavy and dark wood types, such as eucalyptus, require more alkali for the development of their strength than wood types, which are Oiighter and brighter, such as asp and poplar. Against this background, the NaOH-charge for the heavier and darker wood types should be 20-40 kg and for the lighter and brighter ones 15-25 kg per ton pulp. The total NaOH-charge should be 40-80 and, respectively, 30-50 kg per ton pulp.

In Fig. 2 is shown how the tensile strength of the pulp changes with the total amount of NaOH charged and peroxide bleaching with 4 kg peroxide per ton pulp. Freeness 125 ml CSF.

It is, thus, possible at the peroxide bleaching by means of an additional NaOH-charge to increase the tensile

strength of the pulp I to a level very close to the tensile strength of pulp II, i.e. about 45 Nm/g.

One prerequisite for being able to increase the strength properties of the pulp in this way, hovever, is that the original NaOH-charge during the impregnation step is at least 15 kg NaOH per ton pulp, see above.

The invention, of course, is not restricted to the em¬ bodiments described above, but can be varied within the scope of the invention idea.