This invention relates to the making of mechanical pulp from lignocellulose-containing material, intended for coated paper with low grammage, so-called LWC-paper (-light weight eoδtad), magazine paper or similar paper qualities.

For this type of paper the properties of the pulp have to meet very high requirements, because the paper must have high density, low roughness, low porosity and high strength. A uniform surface structure of these papers is of special importance.

Paper of this type normally includes both chemical and mechanical pulp. The traditional mechanical pulp component has been groundwood pulp. As an alternative thereto,thermo- mechanical pulp (TMP) has been used in decent years, but with limited success. Several examples have proved that the employment of TMP as mechanical pulp component, even at low freeness values, has resulted in unevennesses in the surface structure of the paper. This in its turn has given rise to poor coating and thereby unacceptable printability. These problems could be avoided only in those cases when the paper manufacturer had taken special steps for modifying or eliminating the negative effects of the long fibre fraction in the thermomechanical pulp. This long fibre fraction, namely, includes some long, rigid and unworked fibres, which have a negative effect on the paper surface structure.

Since recently, chemically modified TMP (CTMP) has constituted an attractive alternative to TMP as mechanical pulp component in LWC-paper and similar qualities.CTMP improves the strength and binding properties and, besides, implies that the problems with long fibre fractions can be reduced. This is due to the fact, that the. ild chemical pretreatment substant¬ ially improves the defibering capability of the wood material and the swelling tendency of the fibres in the long fibre

fraction of the pulp. These changes bring about a lower shives content and an improved flexibility and ductility of the long fibres in CTMP compared to TMP.

TMP and CTMP for use in LWC-paper and the like usually are manufactured by refining in one or several steps and subsequent screening and bleaching. The energy consumption is relatively high, and a considerable equipment for dewatering and washing is required.

The present invention renders it possible that the equipment can be simplified and the energy consumption be reduced at the same time as the pulp quality is maintained or even improved.

The present invention implies in principle, that the refining is carried out in two steps with intermediate bleaching step. After impregnation and preheating of the raw material in the form of wood chips a refining is carried out in a double-disc refiner, i.e. a refiner with two counter-rotating refining discs. Thereafter a bleaching of the pulp at high concentration is carried out. After the bleaching a second refining at high concentration is carried out in a disc-

-refiner of single-disc type, i.e. with --one stationary and one rotating disc. First thereafter the pulp is subjected to screening. The invention implies that the development of the light-scattering coefficient can be maximized in the first refining step. It is generally known that a double-disc refiner yields a higher light-scattering coefficient than a single-disc refiner. Due to the arrangement of the bleaching step, the pulp is easy-dewatered, so that the high pulp concentration required for the bleaching can be obtained with a simple dewatering equipment. The energy consumption, further, can be reduced in that the refining of the second step is carried out on pulp already bleached. The shives content of the pulp can be minimized by using single-disc refiners for the final refining.

The invention is described in the following with reference to an embodiment thereof.

The raw material in the form of wood chips is pretreated by washing, chemical impregnation and preheating in a conventional manner. As impregnation chemicals preferably Na-SO, or Na-SO-, + NaHSO, with pH-range 6-12 are used. The temperature of the material is increased by the pre- -heating to 105-l ¹ 15 ^O C.

The material pretreated in this way is subjected to refining under pressure in a double-disc refiner. The refining in this first step yields a pulp with a freeness value according to CSF of 150-300. The pulp, thus, is relatively easy- -dewatered. The pulp, therefore, can be pumped to the bleaching step where it is dewatered to desired concentration 30-45 . The equipment for dewatering thereby can be simple and, thus, imply low investment costs. Thereafter the chemicals required for the bleaching are added. The bleaching preferably is carried out with peroxide or dithioni to an ISO-brightness > 70.

After the bleaching the pulp is washed and thereafter subject to the second refining, which is carried out at high concen¬ tration, 25- %, and driven to a freeness value according to CSF of 50-150.

Due to the refining being carried out after the bleaching, the energy consumption for the refining is reduced. The bleaching, thus, has rendered the pulp easier to be processed to desired quality. The second refining preferably is carried out at overpressure in a single-disc refiner equipped with a device for accurate gap adjustment and gap control, whereby it is possible to minimize the shives content of the pulp and simultaneously, within certain limits, to control the shortening of the fibre length to the desired mean fibre length of the pulp at desired energy consumption.

After the second refining step, the pulp is screened and cleaned according to conventional methods before the pulp

is ready for its transfer to a paper mill for the makimg of LWC-paper or similar paper qualities.

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