METHOD OF MAKING MECHANICAL PULP

Abstract: Mechanical pulp, for example thermomechanical or chemimechanical pulp, is manufactured from lignocellulose-containing material in piece shape with a length in the fibre direction of at least 200 mm. The thickness of the material pieces first is reduced, for example by cleaving, to a minimum dimension across the fibre direction of at maximum 50 mm. Thereafter the material pieces are impregnated with liquid, whereafter the material pieces are defibered by being pressed against a rotating defibering member, while liquid is being supplied.

Description: Method of making mechanical pulp This invention relates to a method of making mechanical, thermo echanical and chemi-mechanical pulp of ligno-cell- ulose containing material, such as softwood, hardwood or bamboo, with the puppose of being used in different paper and cardboard qualities etc. Heretofore such pulps were made by two methods different in principle, viz. by refining chips or grinding logs. At refining, the material is chipped to chips, which are refined to pulp, normally in a disc-refiner in one, two or sometimes three steps. Different types of pulp made according to this method normally are distinguished, viz. PIMP (refiner mechanical pulp), TMP (thermomechanical pulp), CMP (chemi-mechanical pulp) and CTMP (chemi-thermomechanic¬ al pulp), as listed below. RMP refining of untreated chips at atmospheric pressure TMP refining of steam-heated chips at overpressure CMP refining of chemical-treated chips at atmospheric pressure CTMP refining of chemical-treated chips in steam atmosphere at overpressure. The pulps have a high content of long fibres and good streng properties. The energy consumption, however, is high, about 2000 kWh/ton pulp. Owing to the high content of long, stiff fibres, the forming in the paper aking machine and the smoothness and optical properties of the paper at times can be unsatisfactory. At grinding, which is the older method, blocks, i.e. logs cut to a definite length, are ground by being pressed against a rotating grindstone while simultaneously water is supplied. Variants corresponding to those listed above are as follows. Groundwood grinding of untreated wood at atmospheric pressure Pressurized grinding of untreated wood at overpressure groundwood Chemigrund- grinding at atmospheric pressure of wood, wo ° which had been digested with chemicals. At the later variant the stock is impregnated with chemic¬ als and digested in an autoclave prior to grinding (US-PS 2 713 5^0). Groundwood pulps have a lower content of long fibres than refined pulps. For this reason, the strength properties are inferior, but forming, smoothness and optical properties are better than at refining. The energy consumption is considerably lower, about 1000 kWh/ton pulp. One serious disadvantage is that roundtimberhas to be used. Chemiground ood has good strength properties. Its disad¬ vantage, however, is that the heart of the log is not im¬ pregnated. The heart, therefore, remains dark at digestion, thereby reducing the ISO-brightness of the pulp and render¬ ing the pulp difficult to bleach. The method, therefore, among other reasons has not got a wide-spread use. The present invention has the object to provide a method, which eliminates the disadvantages of high energy consumption and of inferior strength and dependency on round timber at grinding. This and further objects are achieved by the method accord¬ ing to the present invention as defined in the attached claims. The accompanying block diagram shows in principle an embod¬ iment of the invention. The raw material enters the process at 1. The raw material can be in the form of logs, small pulpwood, slabs, edgings or other piece shape having the minimum length stated below. The raw material is washed clean at 2 by heavy water sprays which remove sand and other impurities. The raw material then is cross-cut at 3 » for example by cross-cutting saws, to lengths adapted for the defibering apparatus under 8 below. The minimum length in question is 200 mm, but pref¬ erably at least 500 mm. The cross-cut wood is cleaved at 4 to sticks, which have a minimum dimension across the fibre direction of at maximum 50 mm, preferably at maximum 25 mm. The sticks, thus, can have the form of ribs with a width, which may be of the same size as the dimension of the raw material in transverse direction, but the thickness of which does not exceed the dimension stated. The cleaving can be carried out by cleaving apparatuses of different type preferably of the type cleaving the wood along its fibre structure, so that the fibres to the greatest possible exten remain intact. The object of the cleaving is to facilitate the subsequent impregnation. When the raw material, for example, consists of thin slabs or edgings, the cleaving, of course, can be passed over. The sticks then are steamed at 5 with steam of atmospheric pressure or low pressure for removing the air, which impedes the impregnation. The sticks then are impregnated at 6 with liquid, which may be water, possibly containing chemicals. The liquid is pressed into the fibre material by means of a high hydraulic pump pressure. The chemicals may be bases, such as caustic soda, salts of alkali metals, for example sodium sulphite or sodium bi¬ sulphite or percompounds, for example peroxide. Mixtures of, for example, caustic soda and sodium sulphite or per¬ oxide may also be used. The impregnation also can be carried out in two or more steps with different chemicals, Λ in which case the excess or spent chemicals are removed between the steps. By the liquid impregnation remaining air is removed from the wood and replaced by the liquid. The fibres swell and are softened so that their-slushing at the defiberi ' ng is facilitated. The fibres thereby are less subjected to damage, and a pulp with longer fibres and greater strengt is obtained. This effect can be varied in strength, depend¬ ing . on the composition of the liquid etc. The mildest effect is obtained by using only water, while alkali yields a stronger swelling of the he icellulose of the wood. By using sulphite or bisulphite a partial sulphonation and release of the lignin is obtained. The yellow colouring resulting from alkali can be counteracted with peroxide. A stronger release causes.a lower pulp yield and deterior¬ ated optical properties as .well as higher chemical consumpt¬ ion and greater environmental problems. It is, thus, a question requiring careful weighing-up. The impregnated sticks react at 7 with the chemicals for a certain time and at a certain temperature. The reaction can take place at atmospheric pressure or:.overpressure and in liquid phase or steam phase. After completed react¬ ion time possible excess liquid is drained, and the sticks are defibered at 8, in such a manner, that the sticks, with the fibres oriented in a definite direction, are pressed against the surface of a rotating defibering device, while water is being supplied. The fibres thereby shall be oriented in the same plane as the tangent press surface, and the direction of movement of the surface preferably shall be perpendicular to the fibre direction. Normal grind¬ ers, possibly pressurized, can be used, comprising a grind¬ stone built-up of sharp, hard grains, for example of alumin¬ ium oxide or silicon carbide, held together by a ceramic binder. The grinder and its feed means must be adapted for the treatment of sticks instead of blocks. Of existing pressing devices, chains, screws and the like are less suitable. Devices of pocket type are to be preferred, at which the wood is pressed against the grinding surface by a pressing plate. After defibering, the pulp produced is removed at 9 for continued processing, such as screening, possibly washing, bleaching etc. Other embodiments, of course, can be imagined within the scope of the invention idea. For the different process steps different types of continuous and discontinuous apparatuses can be used. The cross-cutting at 3 can be made so that the resulting lengths are twice or several times longer than the final lengths adapted for the defibering apparatus, and the finishing cross-cut is made before the ldefibering. The steaming at 5 can be passed over or replaced, for example, by vacuum evacuation. The reaction at 7 can be left out. Other types of defibering apparatuses can be used. The defibering, for example, can be carried out against the plane surface of a defibering disc. The de¬ fibering surface can be built-up of other materials, for example steel and cemented carbide. Pulp types corresponding to the different refiner and groundwood pulps according to above can be manufactured of the sticks in the way as follows. Mechanical pulp possibly steaming, impregnation with water, defibering at atmospheric press¬ ure Thermomechanical possibly steaming, impregnation with pulp water, possibly digestion, defibering at steam pressure Chemimechanical steaming, impregnation with chemicals, pulp possibly digestion, defibering at at¬ mospheric pressure Chemithermo- steaming, impregnation with chemicals, mechanical pulp possibly d i ges tion, d efibering at steam pressure. Other variants can also be imagined within the scope of the invention idea.

Claims: Claims 1. A method of making mechanical pulp of lignocellulose- -containing material in piece shape with a length in the fibre direction of at least 200 mm, at which the mater¬ ial is defibered by being pressed against a rotating defibering member, while ritjtEEd is being supplied, c h a r a c t e r i z e d i n that the thickness of the material pieces first is reduced to a minimum dimension across the fibre direction of at maximum 50 mm, that the material pieces thereafter are impregnated with liquid, and thereafter the defibering is carried out. 2. A method as defined in claim 1, c h a r a c t e r ¬ i z e d i n that the thickness of the material pieces is reduced by cleaving along the fibre structure of the material. 3. A method as defined in claim 1 or 2, c h a r a c t ¬ e r i z e d i n that the material pieces are steamed immediately prior to the impregnation. 4. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the impregnation is carried out in several steps with different impregnation liquids. 5. A method as defined in any one of the preceding claims, c h a r a c t e r i z e d i n that the material pieces between the impregnation and defibering are digested at overpressure in steam or liquid phase.

Inventors: BYSTEDT H S INGEMAR, SEHLIN JAN-OLOF GUNNAR, BYSTEDT, H., S., INGEMAR, SEHLIN, JAN-OLOF, GUNNAR
Applicants: SUNDS DEFIBRATOR, SUNDS DEFIBRATOR AKTIEBOLAG
Application year: 1987
Main classification: D21B1
IPC codes: D21B1