A method for manufacturing mechanical pulp

The present invention relates to a method for manufacturing mechanical pulp.

Publication Fl 61926 and corresponding publication DE 2444475 disclose a process in which pulp is treated with ozone. It is possible that the pH of the pulp is adjusted after the ozonation.

Publication Fl 67412 and corresponding publication CA 1083870 disclose a process in which pulp is fractionated and one fraction, possibly the coarse fraction, is treated with ozone. During the ozonation alkali is fed into the process.

A problem related to the ozonation process is that it produces strong organic acids. The organic acids lower the pH of the pulp, which causes problems in a bleaching process, e.g. by precipitating ferrous substances from the process water. The organic acids also deteriorate process devices by causing corrosion.

The above-mentioned drawbacks can be overcome by the method of the invention. The method comprises

- introducing raw material of the mechanical pulp,

- refining the raw material in a first refining stage in order to form the mechanical pulp,

- adjusting the pH of the mechanical pulp between 9 and 11 in a process step in which a first process delay is arranged, the first process delay being at least 0.5 hours,

- sorting the mechanical pulp into a first accepted stock and a first rejected stock,

- treating with ozone the first rejected stock whose pH at the start of the ozonation process is between 9 and 11 ,

- refining the ozonated first rejected stock in a second refining stage so that at least a part of it will belong to a second accepted stock, and - combining the accepted stocks.

Mechanical pulp is treated in such a manner that after the ozonation process the pH of the pulp is at a constant level throughout the pulp, thus making the pulp easier to bleach. When compared to the ozonation processes of mechanical pulps of prior art, the novel method provides the following advantages:

- lower energy consumption to reach the targeted pulp freeness,

- better strength properties, - prevention of corrosion of the process devices located after the ozonation process thanks to the pH control at pH value higher than 4, and

- better final bleachability of the pulp.

In the method of the invention, raw material of the mechanical pulp is introduced in a process. The raw material can be softwood species, such as pine (Pinus sylvestήs), Southern pine (genus Pinus, several different species), spruce (Picea abies), or mixtures of pine and spruce. Hardwood species and their mixtures with softwood species are also possible raw materials. The raw material can be in form of wood chips. The starting raw material can also be logs which are ground in a PGW (pressure groundwood) process.

The raw material, such as wood chips, which are dewatered in order to reach a consistency which is from 25 % to 35 %, typically 30 %, is fed in a refiner in which it is refined in a first refining stage. The refiner can be a disc refiner or a disc refiner with a conical section. The raw material is refined at an elevated temperature and under pressure in the refiner in order to form mechanical pulp. The first refining stage can also be accomplished by a hammer mill.

The preferred raw material is pine because the method of the invention is the most advantageous compared to other processes when the raw material is pine. It is commonly known that the refining ability of pine is poor. However, by using ozone it can be improved. Pulp treated with ozone contains less fines and shives compared to pulp which is not treated with ozone. The

ozone treatment has also a beneficial effect on a pitch removal which is important in connection with the pine raw material.

The first stage refining is rather mild; the pulp is refined to a freeness of 600 - 700 ml CSF. The upper limit can be even higher than 700 ml CSF.

After the first refining stage there are two main options: 1 ) All the mechanical pulp can be led to an alkali treatment reactor, or 2) The pulp can be sorted to a first accepted stock and a first rejected stock, and the rejected stock is led to the alkali treatment reactor.

According to the first main option, the whole mechanical pulp is led to the alkali treatment reactor in order to achieve a certain pH level. The mechanical pulp is treated with alkali, such as sodium hydroxide (NaOH), sodium bicarbonate (Na ₂ CO ₃ ), magnesium hydroxide (Mg(OH) ₂ ), or calcium hydroxide (Ca(OH) ₂ ). The process in the alkali treatment reactor is arranged so that there is a process delay, i.e. the pulp stays in the reactor for at least 0.5 hours, typically from 1 to 2 hours. The process delay is necessary because the alkali should penetrate from the surface of the fibers of the pulp to the inner parts of the fibers (or fiber aggregates, depending on how coarse the pulp is), i.e. also the inner parts of the fibers and/or fiber aggregates shall be treated in order to achieve the benefits of the invention. The process delay can be arranged as well to a batch process as a continuous process.

After the pulp is treated in the alkali treatment reactor, it is led to a sorter, such as a screen or a cyclone. The pulp is sorted to a first accepted stock and a first rejected stock. The sorting step takes place in a consistency of from 0.5 to 1.5 percent, typically about one percent, and the first accepted stock remains in that consistency after sorting. Usually the ratio accepted stock / rejected stock may be approximately 20/80. However, the ratio accepted stock / rejected stock may also be 60/40 or 50/50. Thus, the accepted stock may be between 20 and 60 percent of the total amount of the pulp, or even between 15 and 65 percent of the total amout of the pulp.

The first rejected stock is first dewatered so that the consistency of the first rejected stock is preferably higher than 30 %. The consistency is more

preferably higher than 35 %. Excess liquid is circulated back to the process. The first rejected stock is led in the above-mentioned consistency to an ozone reactor in which the first rejected stock is fluffed and ozonated. Depending on the energy saving and pulp quality requirement, the ozonation process is arranged so that the amount of ozone is between 0.5 and 10 % and preferably between 1 and 4 % of the weight of the pulp. The process temperature is between 20 and 90 ⁰ C. Strong organic acids form in the ozonation process, but the alkali by which the first rejected stock has been treated neutralizes those strong organic acids. The strong organic acids and the alkali form a buffer solution whose pH can be kept on a certain level for a long time although the ozonation process constantly produces organic acids. At the end of the ozonation process the pH of the rejected stock is between 5 and 7, which is a beneficial range in regard to the following process steps.

After the ozonation the first rejected stock is refined in a main line refining which may comprise sequential refiners. It is possible that that the first rejected stock is sorted to a second accepted stock and a second rejected stock. The second rejected stock can be conveyed for further refining, i.e. to a third refining stage, but it is also possible that it is just removed from the process. If there is the third refining stage it is accomplished in the same manner as the preceding refining stages.

The first accepted stock and the accepted amount of the first rejected stock are combined after the above-mentioned process stages. The accepted amount of the first rejected stock refers to all accepted stocks which have been obtained after the pulp has been sorted to the first accepted stock and the first rejected stock.

It is possible that the first accepted stock is treated with ozone and the ozone treated pulps are combined. It is also possible that some alkali is added to the first accepted stock after it has been treated with ozone. The alkali may be the same that is added before the ozonation. Further, it is possible that the accepted stock which has not been ozonated is treated with the alkali before it is combined with the accepted amount of the first rejected stock.

The freeness of the resulting stock may be between 90 and 150 ml CSF but it is also possible that the lower limit is 30 or 40 ml CSF. Naturally, the above- mentioned freeness depends on the desired paper quality.

According to the second main option, after the first refining stage the pulp is sorted in a sorter, such as a screen or a cyclone, to a first accepted stock and a first rejected stock. The consistency in the sorter is between 1 and 2 %. Usually the ratio accepted stock / rejected stock may be approximately 20/80. However, the ratio accepted stock / rejected stock may also be 60/40 or 50/50. Thus, the accepted stock may be between 20 and 60 percent of the total amount of the pulp, or even between 15 and 65 percent of the total amount of the pulp.

The first rejected stock is treated with alkali, such as sodium hydroxide (NaOH), sodium bicarbonate (Na ₂ CO ₃ ), magnesium hydroxide (Mg(OH) ₂ ), or calcium hydroxide (Ca(OH) ₂ ), in an alkali treatment reactor. The pH of the first rejected stock is elevated to a level which is between 9 and 11 depending on the ozone amount to be applied and the raw material to be treated. After that liquid is removed from the first rejected stock so that a consistency which is preferably higher than 30 % is achieved. The consistency is more preferably higher than 35 %. Excess liquid is circulated back to the process. The first rejected stock is led in the above-mentioned consistency to an ozone reactor in which the first rejected stock is fluffed and ozonated. Depending on the energy saving and pulp quality requirement, the ozonation process is arranged so that the amount ozone is between 0.5 and 10 % and preferably between 1 and 4 % of the weight of the pulp. The process temperature is between 20 and 90 ⁰ C. Strong organic acids form in the ozonation process, but the alkali by which the first rejected stock has been treated neutralizes those strong organic acids. The strong organic acids and the alkali form a buffer solution whose pH can be kept on a certain level for a long time although the ozonation process constantly produces organic acids. At the end of the ozonation process the pH of the rejected stock is between 5 and 7, which is a beneficial range in regard to the following process steps.

After the ozonation the first rejected stock is led to a refiner in order to accomplish a second refining stage. As the first rejected stock has been treated with ozone, its properties have been changed so that the second refining stage consumes much less energy compared to stocks which are not treated with ozone. The first rejected stock is conveyed from the refiner to a sorter which sorts the first rejected stock to a second accepted stock and a second rejected stock. The freeness of the accepted stocks, i.e. the stock consisting of the first accepted stock and the second accepted stock, may be between 90 and 150 ml CSF but it is also possible that the lower limit is 30 or 40 ml CSF. Naturally, the above-mentioned freeness depends on the desired paper quality.

The second rejected stock can be conveyed for further refining, i.e. to the third refining stage, but it is also possible that it is just removed from the process. If there is the third refining stage, it is accomplished in the same manner as the preceding refining stages.

The first accepted stock may be removed from the process and used for other purposes, but it is also possible that the first accepted stock and the second accepted stock are combined so that they can be led as one stock to the following process step, which is usually a bleaching step. The stock obtained from the second refining stage has a pH that is between 5 and 7, and the first accepted stock has about the same pH, so no further steps are required to adjust the pH level.

The adjustment of the pH to a level which is between 9 and 11 before refining enhances the bleaching stage because the peroxide consumption decreases. Harmful wood extractives are soluble in the water in which the alkali is added.

The mechanical pulp obtained from the present method (the above- mentioned first and second main options) is utilized in a paper product manufacturing process. The paper product may be a paper containing mechanical pulp, such as a printing paper containing mechanical pulp, or it may be a cardboard.

In the following, the invention is explained through examples and by referring to the figures in which

Fig. 1 shows a schematic view of a first process ,

Fig. 2 shows a schematic view of a second process,

Fig. 3 shows schematic views of third and fourth processes,

Fig. 4 shows tensile index as a function of SEC (specific energy consumption), and

Fig. 5 shows tear index as a function of tensile index.

Example 0.

Methods which do not contain an alkali treatment before ozonation were used in treating mechanical pulp made of pine.

Points 1 and 2 correspond to the results achieved from a normal TMP refining in which pine is used as the raw material.

Point 5 in Figs. 5 and 6 corresponds to the results achieved from a process in which an ozonated pine reject is mixed with a non-ozonated pine accept. No alkali is added.

Point 7 in in Figs. 5 and 6 corresponds to the results achieved from a process in which an ozonated pine reject is mixed with an ozonated pine accept. No alkali is added.

As one can see from Figs. 5 and 6, ± 20 % energy saving and better strength properties than in the normal pine process were achieved by ozonating the pine reject.

Example 1.

In the following, the first process option is described by referring to Fig. 1.

Raw material of mechanical pulp is first dewatered, for example, in a thickener 1 , such as a screw press, and after that the raw material is fed into a refiner 2, in which the raw material, such as pine chips, is refined in a first stage refining at an elevated temperature and under pressure in order to form mechanical pulp. The consistency during the refining is about 30 %. The preferred raw material is pine because the method of the invention is the most advantageous compared to other processes when the raw material is pine. The first stage refining is rather mild; The pulp is refined to a freeness of 600 - 700 ml CSF. It is possible that the freeness value is even higher than 700 ml CSF. The pulp is treated with an alkali, such as sodium hydroxide, in an alkali treatment reactor 3. The pH of the first rejected stock is elevated to a level which is between 9 and 11. There is a delay in the alkali treatment reactor 3 so that the pulp remains inside the alkali treatment reactor for at least 0.5 hours, typically from 1 to 2 hours. The pulp is also diluted in the alkali treatment reactor so that its consistency is between 0.5 and 1.5 %.

The pulp is sorted in a sorter 4, such as a screen, to a first accepted stock A1 and a first rejected stock R1. Usually the ratio accepted stock / rejected stock is approximately 20/80, but also other ratios, which are described in the general description, are suitable .

After that the first rejected stock R1 is led to a thickener 5, such as a screw press, which removes liquid from the stock in such a manner that a consistency which is higher than 35 % has been achieved. Excess liquid 9 is circulated back to the process. The first rejected stock R1 is led in the above-mentioned consistency to an ozone reactor 6 in which the first rejected R1 stock is ozonated. Strong organic acids form in the ozonation process but the alkali by which the first rejected stock has been treated neutralizes those strong organic acids. The strong organic acids and the alkali form a buffer solution whose pH can be kept on a certain level for a long time although the ozonation process constantly produces organic acids. At the end of the

ozonation process the pH of the rejected stock is between 5 and 7, which is a beneficial range in regard to the following process steps.

After the ozonation the first rejected stock R1 is led to a refiner 7. As the first rejected stock R1 has been treated with ozone, its properties have been changed so that the refining stage consumes much less energy compared to stocks which are not treated with ozone. The first rejected stock R1 may be conveyed from the refiner 7 to another refiner 8. It is also possible that the first rejected stock is led to a sorter (not shown) which sorts the first rejected stock to a second accepted stock and a second rejected stock. The second rejected stock can be conveyed for further refining but it is also possible that it is just removed from the process.

The first accepted stock and the accepted amount of the first rejected stock may be combined after the first rejected stock has been treated in the above- mentioned manner so that they can be led as one stock to the following process step, which is usually a bleaching step. The obtained stock has the pH which is between 5 and 7.

Point 4 in Figs. 4 and 5 corresponds to the result which is achieved from the above-described process.

Example 2.

In the following, the second process is described by referring to Fig. 2.

Raw material of mechanical pulp is first dewatered, for example, in a thickener 1 , such as a screw press, and after that the raw material is fed into a refiner 2, in which the raw material, such as pine chips, is refined in a first stage refining at an elevated temperature and under pressure in order to form mechanical pulp. The consistency of the pulp is about 30 %. The preferred raw material is pine because the method of the invention is the most advantageous compared to other processes when the raw material is pine. The first stage refining is rather mild; the pulp is refined to a freeness of 600 - 700 ml CSF. The pulp is treated with an alkali, such as sodium hydroxide, in an alkali treatment reactor 3. The pH of the first rejected stock is elevated

to a level which is between 9 and 11. There is a delay in the alkali treatment reactor 3 so that the pulp remains inside the alkali treatment reactor for at least 0.5 hours, typically from 1 to 2 hours. At the same time the pulp is diluted to a consistency which is between 0.5 and 1.5 %.

The pulp is sorted in a sorter 4, such as a screen, to a first accepted stock A1 and a first rejected stock R1. Usually the ratio accepted stock / rejected stock is approximately 20/80, but also other ratios are suitable.

After that the first rejected stock R1 is led to a thickener 5, such as a screw press, which removes liquid from the stock in such a manner that a consistency which is higher than 35 % has been achieved. Excess liquid 9 is circulated back to the process. The first rejected stock R1 is led in the above-mentioned consistency to an ozone reactor 6 in which the first rejected R1 stock is ozonated. The amount of ozone is between 0.5 and 10 % and preferably between 1 and 4 % of the weight of the pulp. The temperature during the ozonation is between 20 ⁰ C and 90C°.

Strong organic acids form in the ozonation process, but the alkali by which the first rejected stock has been treated neutralizes those strong organic acids. The strong organic acids and the alkali form a buffer solution whose pH can be kept on a certain level for a long time although the ozonation process constantly produces organic acids. At the end of the ozonation process the pH of the rejected stock is between 5 and 7, which is a beneficial range in regard to the following process steps.

After the ozonation the first rejected stock R1 is led to a refiner 7. As the first rejected stock R1 has been treated with ozone, its properties have been changed so that the second refining stage consumes much less energy compared to stocks which are not treated with ozone. The first rejected stock R1 may be conveyed from the refiner 7 to another refiner 8. It is also possible that the first rejected stock is led to a sorter (not shown) which sorts the first rejected stock to a second accepted stock and a second rejected stock. The second rejected stock can be conveyed for further refining, but it is also possible that it is just removed from the process.

The first accepted stock A1 is led to an ozone reactor 11 at a consistency which is between 0.5 and 1.5 %. The first accepted stock A1 is ozonated in the ozone reactor 11. Ozone may be injected to the first accepted stock A1.

The first accepted stock A1 and the accepted amount of the first rejected stock R1 are combined after the first stocks have been treated in the above- mentioned manner so that they can be led as one stock to the following process step, which is usually a bleaching step. The obtained stock has a pH which is between 5 and 7.

Point 6 in Figs. 4 and 5 corresponds to the result which is achieved from the above-described process option.

As one can see from Figs. 4 and 5, ± 30 % energy saving and better strength properties than in the normal pine process were achieved by treating the rejects by an alkali and ozonating the pine reject (point 4 and point 6).

Example 3.

In the following, the third process is described by referring to Fig. 3.

The third process is similar to that in example 2, except that the first accepted stock is treated with alkali after the ozonation in a mixing reactor 13. The amount of the alkali was 0.5 wt.-% of the total amount of the dry first accepted stock (for example, 0.5 wt.-% of 100 % NaOH). A process delay is arranged to the mixing reactor 13. The pulp stays in the reactor 13 at least for 0.5 hours.

Point 8 in Figs. 4 and 5 corresponds to the result which is achieved from the above-described process option.

Example 4.

In the following, the fourth process is described by referring to Fig. 3.

The fourth process is similar to that in example 2, except that the first accepted stock is treated with alkali after the ozonation in a mixing reactor 13. The amount of the alkali was 2 wt.-% of the total amount of the dry first accepted stock (for example, 2 wt.-% of 100 % NaOH). A process delay is arranged to the mixing reactor 13. The pulp stays in the reactor 13 at least for 0.5 hours.

Point 9 in Figs. 4 and 5 corresponds to the result which is achieved from the above-described process option.

As one can see from Figs. 4 and 5, ± 30 % energy saving and ± 30 % better strength properties than in the normal pine process were achieved by treating the pine reject with an alkali, ozonating the pine reject and adding an alkali into the accepted stock (point 8 and point 9).

A skilled person readily understands that the processes described in the examples above can be combined differently. A skilled person also understands what consistencies are required in order to perform each process step. The mutual order of the first process delay and the sorting step may be reversed; in that case, a skilled person realizes that an adjustment of consistencies is required and the first accepted stock and the first rejected stock must be treated separately with the alkali if such treatment is required for both stocks.