The present invention relates to a method of producing mechanical or chemimechanical cellulosic pulp, in par¬ ticular paper pulp, with a low energy input, by disin- 5 _. tegrating and beating (refining) wood material in at least one stage.

An object of the present invention is to effect the disintegration and beating of the wood material in a 0 manner which substantially decreases the total energy consumption, as described in more detail here below.

A reduction in energy consumption or an improvement in the paper forming properties of the pulp produced is achieved when applying the present invention to present-day manufacturing processes using high pulp consistencies, and also when applying manufacturing processes at lower pulp consistencies, for instance in accordance with the method of manufacturing mechanical and chemimechanical pulps described in Swedish Patent Application No. 8801731-4, published on November 6, 1989.

The beating of cellulosic material at low pulp consis- tencies is a method which has long been used to improve the paper forming properties of the fibres. This, how¬ ever, applies solely to lignin-free or substantially lignin-free fibres, such as fibres produced in accor¬ dance with the sulphate or sulphite methods. As to mechanically produced pulps, as e.g. thermomechanical pulp (TMP) or chemimechanical pulp (CTMP) , it has not been considered that refining at low consistency, so called after-beating, can be used for other objects than to improve the light scattering capacity of the pulps and to slightly shorten the fibre length, there¬ with improving the formation when producing paper.

Investigations have earlier been carried out to explore the possibility of subjecting TMP produced at high consistencies to a subsequent beating process at lower consistencies. For instance, Scan Research Report 5_ 409/1984 reports work carried out with respect to energy consumptions when after-beating pulps at low concentrations as compared with refining the pulps at high concentrations. The results of this investigation show that the freeness of TMP can be lowered by 0 10-30 ml without impairing the strength properties of the pulp to any substantial extent, and that energy savings of 50-150 k h/tonne can be achieved. The total energy consumed, however, was quite considerable, in the order of magnitude of 1600 to 2300 kWh/tonne. 5

Pulp and Paper Magazine of Canada, Vol. 81, No. 6, June 1980, pages 72-80 (N. Hartler) reports attempts to reduce the energy consumption of chip refining pro¬ cesses. One proposal made in this report is that the 0 chemical environment around the fibres is changed by adding chemicals. It was found that energy consumption could be reduced by 30%, by adding sodium hydroxide, although the total consumption still remained in the region of about 1300 kWh/tonne. A poorer yield was 5 obtained with these tests, however, and the brightness was impaired considerably.

In an article published in Svensk Papperstidning, 1982, pages R 132-139 (P. Axelson and R. Simonson) , there is H a report on the effect of impregnating chips with sulphite during the refining stage, inter alia on energy consumption. The energy diagram showed a minimum subsequent to the absorption of a given quantity of sulphite. The total energy consumption, however, was 5 still at a high level of 2000 kWh/tonne.

Attempts have also been made to treat thermomechanical

pulp with fibre-modifying chemicals. It has been found that the energy consumption can be reduced by at most 30%, when the defibred pulp is treated with ozone in a two-stage method, prior to the refining process. This 5. can only be achieved, however, at the cost of the yield.

It has now been found possible, in accordance with the present invention, to produce mechanical paper pulp at 0 a considerable reduction in energy input.

It has not been possible until very recently to reduce the energy consumed when producing mechanical pulps, by defibring the wood material by beating at low consi- stencies. The reason for this is because it was not known how to avoid fibre cutting and therewith an excessively low tensile and tear index of the resultant mechanical pulp and, at the same time, improve the pulp bonding properties. This manufacture of pulp at low energy inputs is achieved by disintegrating and beating wood material in at least two stages. The material is coarsely disintegrated in a first stage at a consis¬ tency exceeding 20%, wherein acid groups in the wood material are neutralized and the material thinned to a consistency of 1-10% and then beaten in one or more stages.

It has been found that the energy input can be further reduced by means of the present invention, which is characterized by adding to the wood material prior to the beating process an agent which has the ability to form complexes with polyvalent (2 valences or more) metal ions, particularly calcium ions, so-called com¬ plexing (sequestering) agent, so that the content of calcium and polyvalent (2 valences or more) ions in the wood to the major part are replaced by sodium ions. The aim with the ion exchange to sodium form in this manner

is to provide as good conditions as possible to obtain elektrolytic swelling by causing charged groups, as e.g. carboxylic and/or sulphonic acid groups, to repel each other. Said swelling contributes to the fact that 5. the fibre material can be delaminated (fibrillated) more easily and leniently at refining and beating.

According to a first embodiment of the invention, the disintegrating and beating processes are carried out in 0 one and the same stage, wherein a complexing agents and preferably also sodium hydroxide for the purpose of neutralizing released acid groups are added to the wood material, preferably after steaming. Surplus liquor is then pressed from the wood, before beating is com- 5 menced. The advantage with this embodiment is that it can be utilized directly in a number of present-day operational mills which produce mechanical and chemimechanical pulps.

0 In a second, more suitable embodiment which affords a greater reduction in energy input, the coarse disin¬ tegration and beating processes are carried out in mutually different stages. In this regard, the complexing agent, and also the preferably added sodium 5 hydroxide, can be introduced prior to coarsely dis¬ integrating the wood or subsequent thereto. It may be advantageous to effect both additions prior to the coarse disintegration stage, particularly in the case of chemimechanical pulp, and thereafter press excessive 0 liquid from the suspension prior to said coarse disin¬ tegration stage.

The complexing agent is normally added to the wood material in an amount corresponding to the amount of 5 polyvalent metal ions in the wood material. This amount can correspond to 8-130 mmol per kg of wood, suitably 15-50 mmol per kg of wood. A common amount of poly-

valent metal ions in Swedish spruce chips is 20-30 mmol of wood, for instance 25 mmol. The complexing agent will preferably be one in the alkali metal form, and then particularly in the sodium form. A complexing agent in the potassium form can also be used in certain instances with respect to economy, whereas the remain¬ ing alkali metal forms would be too expensive in normal operation.

The amount of complexing agent required can also be calculated on the basis of the amount of calcium and other polyvalent metal ions present in the wood mate¬ rial, and determining the molar quantity of these ions and adding the complexing agent in a quantity corres- ponding to ± 50% of this molar quantity. A suitable range is ± 30%. Substantially equimolar quantities can also be used, of course.

In the case of one particularly suitable embodiment, the pulp is produced by disintegrating and beating wood material in at least two stages. The material is coarsely disintegrated in the first stage and the acid groups present in the wood polymers are neutralized, either completely or partially, suitably by the addi- tion of sodium hydroxide. The material suspension is thinned preferably with a water at a temperature cor¬ responding to the softening temperature of the lignin, i.e. a temperature of 40-95°C valid at a refining fre¬ quency of about 1 Hertz. For higher loading frequencies (e-g- around 10 ⁴ Hertz) like the frequencies occurring during technical refining a higher temperature range. Said thinning water suitably have an ion strength (de¬ fined as the total content of cations expressed as mol/1 (litre)) of at most 0.05 mol per litre. The mate- rial is then beaten in one or more stages at a consistency of preferably 1-10% and an energy input which is normally in total at most 500 kWh per tonne of

material. In this case, the complexing agent is added prior to the beating process and can even be added prior to the first stage. In the case of chemimechani¬ cal pulp, it is preferred to add the complexing agent 5. and also to press surplus liquid from the material suspension prior to coarsely disintegrating the material. In the case of mechanical pulp, the complex¬ ing agent is normally added to the suspension between the coarsely disintegrating stage and the first beating 0 stage. Appropriate parts of the method described in

Swedish Patent Application No. 8801731-4 can be applied in this case and these parts of the known method are incorporated here as a reference. 5

The suspension consistency during the coarse disinte¬ grating stage will therefore preferably be high, e.g. above 20%. It is also suitable to use a low energy input during the coarse disintegrating stage, e.g. an 0 input of at most 800 kWh per tonne wood material, and the sodium hydroxide is preferably added in an amount which will not appreciably exceed the amount required to neutralize the acid groups present in the wood poly¬ mers, at most 225 mmol per kg. _5

The complexing agent will preferably be a substance capable of forming complexes with polyvalent metal ions, primarily calcium ions. The complexing agent is preferably used in its alkali metal form, primarily its 0 sodium form, so as to deliver sodium ions to the wood and take-up calcium and other polyvalent metal ions from the wood.

Suitable groups of complexing agents and examples of 5 specific complexing agents are given in the following Table:

Amines-ethyl amines, imines (carboxylates. phosphona- tes, sulphonates) :

5. Designation

DTPA

Diethylene-triamine-pentaacetic acid EDTA 0 Ethylene-diamine-tetraacetic acid HEDTA

Hydxroxy ethyl ethylene-diamine-triacedic acid NTA Nitrilo-triacetic acid DHEG N,N-di(2-hydroxy ethyl) glycine TEA Triethanol amine

NTP Nitrilo-trimethylene phosphonic acid MIDA

N-methyl imine-diacetate IDA Imine diacetate HEIDA

Disodium-hydroxy-ethyl imine-diacetate DTPMPA

Diethylene-triamine-pentamethylene-phosphonic acid ("Dequest")

EACDA

Ethyl amine-cyclopentene-1-dithiocarboxylic acid CDTA

Cyclohexylene-dia ine-triacetic acid

Poly-carboxylates (including poly-phosphonate and poly- sulphonate) :

POC Poly-(hydroxycarboxylate) , M up to about 6000

Polyvalent carboxylate:

Na-citrat

- Gluconicacid alacton

- Na-tartrate

5. Polyvalent phosphate:

STPP

Na-tripolyphosphate

0 Remainder:

MTPP

Bis-phosphonyl methyl phosphonic acid

- Poly (sodium-α-hydroxyacrylate) 5

The invention will now be described in more detail with reference to working embodiments and also with refe¬ rence to the accompanying drawing, in which

0 Figure 1 is a flow sheet illustrating an inventive embodiment for producing a pulp of the CTMP-type with high consistency refining as a first stage;

Figure 2 is a flow sheet illustrating an inventive 5 embodiment for producing pulp of the TMP-type with high consistency refining as the first stage; and

Figure 3 is a flow sheet which illustrates another inventive embodiment for producing TMP, this embodiment 0 using an extruder as the first disintegrating stage.

Figure 4 is a flow sheet illustrating another embodi¬ ment according to the invention for producing CTMP, this embodiment using a plug screw as first disinte- 5 grating stage.

Figure 5 is a flow sheet illustrating an embodiment

similar to that according to figure 4, but for produc¬ ing TMP.

Example 1

The flow sheet shown in Figure 1 relates to the produc¬ tion of chemithermomechanical pulp. Spruce chips were steamed in a first stage and then impregnated with a solution containing a given quantity of complexing agent, in this instance Na.EDTA 25 mmol/kg wood, corre¬ sponding to the amount of polyvalent metal ions present in the wood, of which 20 mmol/kg wood were calcium ions. The solution also contained sodium sulphite corresponding to about 160 mmol/kg wood. Subsequent to impregnation, the wood material was pressed in a plug screw to a dry content of about 50%. The pulp Was then defibred at high pulp consistencies and with an energy consumption of 500 kWh/t, whereafter the suspension of defibred wood material was thinned with water having a temperature of 60°C and an ion strength of 2.0 mmol/1, so as to obtain a pulp consistency of 3%.

The pulp was then beaten at this pulp concentration at a specific edge load of 0.5 Ws/m and a net energy consumption of 120 kWh/t corresponding to a gross energy consumption of 200 kWh/t to a freeness of 250 ml CSF and a mean fibre length (PML) of 2.0 mm, i.e. equal to and slightly more respectively than is normal in the conventional manufacture of CTMP with an e ergy con- sumption of 1650 kWh/t.

Thus, when practicing the inventive method, energy consumption is reduced from 1650 kWh/t in the case of the conventional method to 700 kWh/t, which is also slightly lower than the level achieved with the method taught by the Swedish Patent Application No. 8801731-4 which employs a similar technique but in which no ion

exchange takes place with the aid of complexing agents.

Example 2

Figure 2 is a flow chart which illustrates the manufac¬ ture of TMP for use in newsprint. Spruce chips are steamed in a first stage, whereafter the chips are impregnated, preheated and coarsley refined while adding 100 mmol NaOH/kg wood (corresponding to the content of acid groups having protonic form in the wood) at a pulp concentration of 35% in a pressurized refiner with an energy consumption of 600 kWh/t. The coarsely refined pulp stock was then thinned to a pulp concentration of 10% with a solution, temperature 80°C, containing a complexing agent, in the present case

Na.EDTA 25 mmol/kg wood, in an amount corresponding to the amount of polyvalent metal ions present in the - wood, in the illustrated case 25 mmol/kg wood, of which 20 mmol/kg wood were calcium ions. Subsequent to thinning the stock, the wood material was pressed in a pulp press to a dry content of about 40%. The defibred pulp stock was then thinned with water at a temperature of 80°C and an ion strength of 2.0 mmol/1 to obtain a pulp concentration of 3%.

The pulp was then beaten at this pulp concentration at a specific edge load of 0.5 s/m and a net energy con¬ sumption of 150 kWh/t, corresponding to a gross energy consumption of 250 kWh/t, to a freeness of 150 ml CSF and a mean fibre length (PML) of 2.0 mm, i.e. respectively equal to and slightly more than is normal in the case of TMP which can be produced in the least energy requiring technique known at present with an energy consumption of 1650 kWh/t (single stage refining with double disc refiners) . Two stage proces¬ ses, which are at present the most common processes used in the manufacture of TMP, often require an energy

input of more than 2000 kWh/t in order to obtain a pulp having a freeness of 150 ml CSF.

As will be seen from the aforegoing, the inventive 5. method was effective in reducing energy consumption from the level of 1650 kWh/t required in the conven¬ tional process to a level of 850 kWh/t, which is also slightly lower than the level achieved with the method taught by the aforementioned Swedish Patent Application No. 8801731-4, in which similar tecnique is used but where no ion exchange takes place with the aid of complexing agents.

Example 3

The flow sheet in Figure 3 illustrates a method of manufacture of TMP for use as newsprint. Spruce chips were steamed in a first stage and then charged to a BiVis-machine.

As the chips were defibred in the machine, there was added thereto a solution containing firstly a quantity of complexing agents, here Na.EDTA 25 mmol/kg wood, corresponding to the content of polyvalent metal ions, of which 20 mmol/kg wood were calcium ions, and the solution secondly also contained sodium hydroxide in an amount corresponding to 100 mmol NaOH/kg wood. The chemical solution containing Na.EDTA and NaOH was in¬ troduced in counterflow to the wood flow via the repeated pressing/diluting method applied in a BiVis. The material passed through the four compression zones of the machine and the electrical energy consumed was about 300 kWh/t wood. Subsequent to being discharged from the BiVis, the wood material suspension was thinn- ed with wat<=>r to a dry content of about 4%. The water had a temperature of 80°C and an ion strength of 2.0 mmol/1. The pulp was then beaten at this pulp concent-

ration at a specific edge load of 0.5 Ws/m and a net energy consumption of 200 kWh/t, corresponding to a gross energy consumption of 330 kWh/t to a freeness of 150 ml CSF and a mean fibre length (PML) of 2.0 mm. When proceeding in accordance with this method, de- fibring and refining of the pulp required a total electrical energy input of 630 kWh/t, which is lower than that achieved in the preceding Example (Example 2) with TMP, where the energy consumed in achieving a freeness of 150 ml CSF was 850 kWh/t.

Example 4

The flow sheet in Figure 4 illustrates the manufacture of chemithermomechanical pulp while using a complexing agent for the exchange of calcium and other polyvalent ions to sodium ions acting as counterions to the acid groups present in the wood.

Spruce chips were steamed in a first stage and then impregnated with a solution containing a quantity of complexing agents, in the illustrated case Na.EDTA 25 mmol/kg wood, corresponding to the polyvalent metalion content of the wood, in this case 25 mmol/kg wood, of which 20 mmol/kg wood were calcium ions. The solution also contains sodium sulphite, in an amount corresponding about 150 mmol/kg wood. Subsequent to impregnating the chips, the chips were pressed in a plug screw to a dry content of about 50%. The pulp was then preheated and refined at a high pulp concentration in one stage to CSF-levels between 200 and 700 ml CSF with a specific electrical energy consumption which was about 20% lower than that obtained in the absence of ion exchange with the aid of complexing agents. The tensile strength of the pulps in the CSF-range examined was more than 20% greater than when not using complexing agents for ion exchange to sodium form. When

the pulp was refined in one stage to 500 ml CSF and then in a second stage to 200 ml CSF at a pulp consis¬ tency of 30%, slightly more electrical energy was con¬ sumed than in the aforementioned case, although the 5. energy consumption was still about 20% lower than the energy consumed with a corresponding reference where no ion exchange was effected where no ion exchange was effected with the aid of complexing agents.

0 Example 5

The flow sheet in Figure 5 illustrates a method of manufacturing TMP for use in the production of news¬ print, while using complexing agents to exchange cal- cium ions and other polyvalent ions to sodium ions acting as counter ions to the acid groups present in the wood.

Spruce chips were steamed in a first stage and then impregnated by pressing the chips in a plug screw and allowing the chips to expand in an impregnation vessel containing a solution of complexing agents, in the present case Na.EDTA 25 mmol/kg wood, in an amount cor¬ responding to the polyvalent metal-ion content of the wood, in the present case 25 mmol/kg wood, of which

20 mmol/kg wood were calcium ions. The complexing agent was then removed and the ion-exchanged chips preheated and refined while simultaneously adding sodium hyd¬ roxide in an amount corresponding to 100 mmol NaOH/kg wood at a pulp consistency of 35% in a pressurized refiner. The energy consumption was about 15% lower than when the same procedure was carried out but with¬ out an ion exchange with the aid of complexing agents.