Background Mechanical and chemimechanical pulps are produced from wood chips by means of defibration and fibre strength development in one-or two-stage disk refining. So-called RMP is refined without any other pre-treatment than a simple chip washing at a moderate temperature whereas TMP and CTMP production requires a well controlled pre-treatment at high temperature (without and with chemicals respectively), which causes a small dissolution of easily dissolvable polymeric wood material (lignin and hemicellulose material). The papermaking properties of these high yield pulps are determined primarily by the conditions in the refining stage/stages. The dissolution of the polymeric wood material and the mechanical properties of the wood polymers are important components in refining and should therefor be controlled carefully. It is hence important that significant additional dissolution of wood polymers is avoided in special new treatments of chips or pulp in this case.

Low molecular lipophilic wood extractives (pitch) are disturbing components in pulp and their content should be kept at an acceptably low level. Pitch is to a great extent dispersed in fine particles in the refining and can be washed out from the pulp to different degrees depending on the washing conditions. As long as a treatment of the chips does not affect the refining conditions the removal of pitch will not be affected significantly. Similar treatments of pulp from the refiners on the other hand may affect the washing efficiency and must be carefully integrated with the normal washing process.

Another group of more or less disturbing substances in wood is small amounts of inorganic material, usually referred to as non-process elements (NPE's). Considerable work has been devoted to the removal of NPE's at various stages of chemical pulping, including leaching of NPE's from chips.

However, leaching of chips requires fairly long times to be effective and is further limited by the fairly large amount of liquor bound inside the chips after leaching. The long time and a need of pressurised systems for the leaching of chips make a process equipment for NPE removal quite expensive in this case. Chemical pulping does not allow strong mechanical treatment of the chips such as pressing due to the damages it would introduce to the final pulp fibres. Mechanical pulps on the other hand are not negatively affected. In fact, an initial pressing would even simplify the refining of the chips. This opens for a cheaper and more effective leaching technique.

A process for the manufacture of mechanical pulp from wood is disclosed in FI 87371 where wood is chipped into chips which, in turn, after a pre-treatment are ground into pulp. Air is removed in the chips whereupon the chips are impregnated with water that may be acidic by use of a mineral acid. An introduction of cracks in the chips in order to shorten the diffusion ways within the wood material before impregnation, during a sufficient time period, and subsequent refining is not described. Thus non-process elements (NPE's) are removed only to a small extent due to diffusion problems as indicated earlier. This above deficit is also present in SE 451202, SE 468644 and FI 43668 which disclose similar processes.

Non-process elements (NPE's), cause problems during the pulp manufacture or later on when making paper or related products. Mn e. g. is negative for the peroxide bleaching of the subsequent pulp, which is a dominating bleaching chemical for mechanical and chemimechanical pulps-such as RMP, TMP and CTMP. The problems arising from NPE's become more severe when a closed process is used for the manufacture of pulp and/or paper due to a build-up of the concentrations of dissolved NPE's.

Thus there is a need for processes wherein the amount of non-process elements. can be reduced to an extent, which alleviates or eliminates the problems in the pulp and/or paper manufacturing processes as e. g. set out above. It is however important that these processes can be well adapted to the original pulping process. The dissolution of wood polymeric material and the changes in wood colour should not be affected significantly. As discussed above the conditions are quite different for chemical and mechanical pulping which in the latter case opens for more efficient new solutions.

Summary of the invention The present invention solves the above problems by providing, according to a first aspect, a method for treatment of wood chips from softwood or hardwood or a mixture thereof for the reduction of non-process elements in a mechanical pulping process or a chemithermomechanical pulping process, without significant changes of the pulping process and the pulp quality, comprising the following steps : a) introduction of cracks in the chips or refining the chips to a coarse fibre structure, in order to considerably shorten the diffusion ways within the wood material, b) treatment of the chips or fibres thereof with an acidic leaching liquid, c) extensive removal of the acidic leaching liquid; and d) adjustment of the pH to a level suitable for refining, preferably pH ! 4.

The present invention also provides, according to a second aspect, a system for using the method according to the first aspect, comprising the following components:

i) means for introduction of cracks in wood chips or means for refining the chips to a coarse fibre structure, ii) means for treating the wood chips or fibre thereof with an acidic leaching liquor; and iii) means for adjusting the pH to a level suitable for refining.

Detailed description of the invention It is intended throughout the present description that the expression"refiner"embraces any apparatus capable of refining wood chips. Refining generally involves treatment of fibre material in water. Examples of refiners are refiners equipped with refining discs (disc refiners). A refiner generally operates continuously. The introduction of cracks in the chips, in order to shorten the diffusion ways (in step a)) of the present invention is performed with a relatively small energy consumption preferably by using one or more disc refiners. The energy consumption may be in the range of approximately 500 kWh/ADt or somewhat less (airdry tonne), which creates a"soft" refining. A"normal"refining may involve freeing the fibres using en energy input of from approximately 1500 to 2000 kWh/ADt whereby the temperature is between 120 and 160 °C.

Preferably the"soft"refining is performed at a temperature of from 60 to 100°C. The refiner may further be pressurized if TMP or CTMP is the desired product. If RMP is to be produced the refiner does not need to be pressurized. If CTMP is desired also means for the addition of chemicals is necessary.

The lignocellulosic fibres that may be obtained when using the present invention include all types of softwood and hardwood-based fibres, unbleached and bleached mechanical (Refiner Mechanical Pulp; RMP) and thermo-mechanical pulps (TMP), Chemithermomechanical Pulp (CTMP) and mixtures of these.

It is intended throughout the present description that the expression"acidic leaching liquid" embraces any acidic liquid. Examples of acidic leaching liquids are mineral acids such as sulphuric acid, which is preferred, or mixtures of such acids. The acid (s) may be comprised in water or white water or a mixture thereof. The white water must obviously not contain too much NPEs. The amount of leaching liquid in relation to the lignocellulose containing raw material may preferably be abundant. The dilution rate may be high, thus enabling a positive equilibrium for diluting the NPE's present in the wood raw material. The leaching step is preferably combined with other process step (s) in a mill producing pulp and/or paper. The leaching step is preferably run at a lower pH than a subsequent refining step.

The removal of acidic leaching liquid in step c) may comprise washing of the fibres which may be performed by letting the leaching liquid just pour off the wood material, i. e. self-pouring of the leaching liquid. compressing screw feeder may additionally be used for removing the acidic

leaching liquid in step c). If a CTMP pulp is desired, more thorough washing may be needed in order to reduce the resin (a process disturbing organic substance) contents. Additionally the wash liquid may, preferably at the end of the washing process, be more basic (alkaline) in order to obtain a better reduction of resin or extractives. An embodiment for reducing the resin contents and NPE's is using counter-current flows (a liquid exchange) of alkaline white water (or fresh alkaline), respectively, during the washing step of the fibres or fibre bundles (which may be in the form of a pulp). First an essentially alkaline counter-current wash flow treats the pulp interrupted by an essentially acidic stage and, finally, in the counter current wash flow alkaline washing stage treats the pulp. Thus first an alkaline solution or alkaline white water will be displaced by an acidic fresh solution or an acidic white water. Subsequently, the acidic fresh solution or white water will be displaced by alkaline solution. or white water.

The refining the chips to a coarse fibre structure, in order to shorten the diffusion ways (in step a) ), may be achieved by a refiner and the introduction of cracks in the chips, in order to shorten the diffusion ways (in step a)) may be achieved by a compressing screw press. The diffusion ways may be 1 mm or less after the introduction of cracks. If a refiner is used to perform step a), fibres or fibre bundles will be the predominant form of the chips, i. e. the raw material, after the introduction of cracks. The resin is further preferably extracted after the refining by e. g. using alkaline conditions.

According to a preferred embodiment of the first aspect of the invention, the removal of the acidic leaching liquid in step c) involves washing the pulp, preferably by using a fibre washing equipment.

According to a preferred embodiment of the first aspect of the invention, the b) treatment of the chips with an acidic leaching liquid involves reacting the chips in a leaching vessel. A fibre washing equipment is preferably connected to the vessel for use after the storing. The fibre washing equipment may be a filter or other dewatering means such as a press or a compressing screw feeder as set out below.

According to a preferred embodiment of the first aspect of the invention, the b) treatment of the chips (or fibres thereof) with an acidic leaching at a pH from 2 to 4, preferably a pH of 2.5, during a short time period preferably at a temperature of from approximately 60 to 100 °C.

According to a preferred embodiment of the first aspect of the invention, the removal of the acidic leaching liquid in step c) is followed by a pH adjustment to a level suitable for chip refining, preferably pH 2 4 as set out above in step d).

According to a preferred embodiment of the first aspect of the invention the b) treatment of the chips with an acidic leaching liquid is performed at a low consistency.

According to a preferred embodiment of the first aspect of the invention the refining of the chips to a coarse fibre structure in order to shorten the diffusion ways (distances) in step a) is performed in a refiner stage to a freeness > 300 CSF and preferably the removal of the acidic leaching

liquid is performed in a fibre washing equipment, subsequently followed by a second refining stage.

CSF (Canadian Standard Freeness) corresponds to the standard SCAN C 21: 65 According to a preferred embodiment of the first aspect of the invention the b) treatment of the chips with an acidic leaching liquid, effectuating the removal of non-process elements and/or organic process disturbing substances, is combined with an extensive resin (extractive) removal through multistage counter-current washing, wherein the acidic leaching stage is placed after the first washing stage in a predominantly alkaline washing system and the acidity in the leaching stage is maintained through a combination of liquid exchange and acid changing as set out above.

According to a preferred embodiment of the first aspect of the invention, the acidic leaching liquid in step b) is make-up originating from purified re-circulated wash liquid, preferably purified washing liquid from a washing step, most preferred washing liquid emanating from step c). The wash liquid may also be white water with origin elsewhere in the manufacturing process. The white water may be conveyed to the wash step through a white water system.

According to a preferred embodiment of the first aspect of the invention the treatment of step b) is performed during a time period of approximately from 1 minute up to 1 hour, preferably 1 minute up to 30 minutes, most preferred 5 minutes up to 15 minutes, especially preferred approximately 10 minutes (for acid-sensitive wood species).

According to a preferred embodiment of the first aspect of the invention, the pH adjustment in step d) comprises adjusting the pH in the range of 4-9, preferably by adding white water or water mixed with one or more basic compounds..

According to a preferred embodiment of the first aspect of the invention the refining of the chips to a coarse fibre structure in step a) comprises freeing the fibres to a freeness > 300 CSF, which may be achieved by using en energy input of from approximately 300 kWh/ADt.

According to a preferred embodiment of the first aspect of the invention the refining of the chips to a coarse fibre structure in step a) is performed at a temperature of from 60°C to 100°C.

According to a preferred embodiment of the first aspect of the invention, the introduction of cracks in step a) involves treating the wood chips in a compressing screw feeder arrangement, preferably equipped with or connected to a shredding device, most preferred additionally equipped with or connected to a peg-mixer.

According to a preferred embodiment of the first aspect of the invention, the removal of the acidic leaching liquid is performed by using a compressing screw feeder ("PREX-skruv") arrangement. This compressing screw feeder works like a pressure lock and will press the leaching liquid out through a drainage outlet. Subsequently a refiner follows, optionally including a pH- adjustment either before in the screw feeder, after said screw feeder or at the refiner inlet. If the pH adjustment is performed in the subsequent refiner it may be performed is conjunction with the addition of water.

According to a preferred embodiment of the first aspect of the invention, the steps a) through d) are performed in an essentially closed process.

According to a preferred embodiment of the first aspect of the invention, the resin depleting chemicals are added before or during the refining of the chips, or during the introduction of cracks in step a). This treatment gives rise to Chemithermomechanical Pulp (CTMP) and provides a pulp of different qualities (typically"softer"). The chemicals may be sodium sulfite, (thus sulfonation of the lignin occurs which gives a"permanent"softening as compared to the temporary softening in the TMP process) and/or hydrogen peroxide in combination with alkali. The chemical pre-treatment is preferably performed during steaming in combination with refining. The freeness (CSF) may be high when using the above chemical treatment. The sulfite treatments may range from 1-5% of the weight of the wood. Preferably a thorough wash step is used to remove lignin and other dissolved compounds. Preferably said wash step is comprised in step c).

According to a preferred embodiment of the second aspect of the invention, the means for introduction of cracks in wood chips is a compressing screw feeder arrangement and the means for refining the chips to a coarse fibre structure is a refiner.

According to a preferred embodiment of the second aspect of the invention the system comprises a reaction vessel for treating the wood chips with an acidic leaching liquor, a leaching vessel.

According to a preferred embodiment of the second aspect of the invention, the system comprises a wood fibre wash (pulp wash) arranged between the means for treating the wood chips with an acidic leaching liquor and the means for adjusting the pH to a level suitable for refining. The wood fibre wash may comprise a compressing screw feeder.

According to a preferred embodiment of the second aspect of the invention the refiner preferably is equipped with a heating device and/or means for adding resin depleting chemicals. Also the steam produced during the operation of the refiner may be used and that steam may be enough, thus enabling the omission of the heating device.

According to a preferred embodiment of the second aspect of the invention the system comprises additionally a purification apparatus for purifying wash liquid connected to the wood fibre wash. This purification apparatus may e. g. comprise a filter whereby larger components such as polymeric material may be removed. Smaller components such as NPE's or related compounds may be removed through flocculation by e. g. adding flocculation agents, or through flotation. You may let the small organic compounds stay in the white water.

According to a preferred embodiment of the second aspect of the invention the compressing screw feeder arrangement is equipped with or connected to a shredding device (for shredding the plug obtained in the screw feeder), preferably equipped with or connected to subsequent peg-mixer. The compressing screw feeder is somewhat conical and the screw is adapted so that the elevation gives a

certain compression of the wood material. Before the chips or fibres thereof enter the subsequent refiner, which may be pressurized, a compressing screw feeder may preferably be used to convey the chips.

The pulp, and eventually paper or paper board, obtainable by the method according to the first aspect of the present invention may e. g. be used, for the manufacture of any kind of paper or paper product.

The present invention provides a method according to the first aspect, which is inexpensive, has a good leaching yield (well over 50%) and provides fibres (and pulp) with a neutral colour (non- discoloured). During acidic treatment for a long extended time period, at a high temperature, discolouration is frequent. Extreme treatment times together with high temperatures can thus be avoided through the present invention. Thus also an extreme dissolving is avoided, as the present invention provides a method whereby short treatment times and low treatment temperatures may be used. Accordingly extreme amounts of substance is not dissolved which could be harmful for the effluent. Another advantage in comparison with e. g. chemical pulp is that the pulps above can be pressed. A dry substance of 40% or above may be achieved by pressing, thus the impregnated leaching liquor may be better disposed of.

Preferred features of each aspect of the invention are as for each of the other aspects mutatis mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The invention is further described in the following example in conjunction with the appended figure, which do not limit the scope of the invention in any way. Embodiments of the present invention are described in more detail with the aid of an example of embodiments, the only purpose of which is to illustrate the invention and are in no way intended to limit its extent.

Short description of the figure Fig. 1 shows a part of a preferred embodiment of the present invention.

Example Example 1: Preparation of pulp using the system according to the second aspect of the present invention, i. e. a system for leaching according to the invention irZ figure 1, whereby using the naethod according to the first aspect Wood chips with a wood density of 420 kg OD wood3 moist volume (and about 3 % lower density of the solid chip material due to normal cracks in mill chips) are fed into a screw press, which compresses the wood to such an extent that a great many new cracks will considerably increase the water absorption capacity of the chips (from about 1.8 m3/OD ton of wood to at least 3) and hence

decrease the diffusion distances within the solid wood material. To a varying degree the chips will be partially disintegrated, which will further facilitate the diffusion.

The screw press discharges the wood material into a screw conveyor, where the compressed chips are allowed to expand in acid make-up leaching liquor and a small amount of re-circulated leaching liquor required to maintain a liquor phase in the lower part of the conveyor. The impregnated chips are then transported to the top of the non-pressurized leaching vessel, where they are distributed evenly over the cross-section by means of re-circulated excess leaching liquor pumped in through nozzles at the top of the vessel.

The vessel is designed to give a retention time for the chips of 10 to 15 minutes (normally at pH 2.5 and 80°C). A bottom scraper in the leaching vessel helps to fill a discharging transport screw below the vessel. This screw feeds a screw conveyor with a liquor level high enough to secure a required liquor level in the leaching vessel. The chips are drained of free liquor at the top of the conveyor.

The drained chips are then fed to a transport screw, which distributes the wood material to the different refiners in the line (if more than one). For each refiner there is a screw feeder (a"PREX screw"), which compresses the wood material to such. an extent that it forms a plug serving as pressure lock between the atmospheric feeding system and the pressurized heat pretreatment of a normal TMP process. The PREX screw can at the same time squeeze out a considerable amount of liquid from the chips. The screw should therefore be designed to produce at least 40 % dry content, preferable 50%.

An important principle of the invention is to guarantee that the TMP process (or an RMP or CTMP process) can be run at normal conditions, without any disturbance from the leaching process.

One important issue is then to maintain a pH, which is optimal for the TMP process and the kind of pulp to be produced. It is therefore necessary to charge neutralizing alkali before or in the refiner. It is of course possible to do so before the PREX screw, but may require more alkali and more acid to the re-circulated leaching liquor. Preferably the charging of the alkali is at the point where the wood material expands after the PREX screw. It will give some time for the alkali to be reasonably well distributed before the wood material enters the refiner. Other charge points can of course be considered.

The withdrawn leaching liquor goes to NPE removal according to known methods before it is re-circulated.

Various embodiments of the present invention have been described above but a person skilled in the art realizes further minor alterations, which would fall into the scope of the present invention.

The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their

equivalents. For example, any of the above-noted systems and/or methods can be combined with other known systems and/or methods. Other aspects, advantages and modifications within the scope of the invention will be apparent to those skilled in the art to which the invention pertains.