EP2176461A1

EP2176461A1 - Process for making paper and use of an acid/base treatment in papermaking

Info

Publication number: EP2176461A1
Application number: EP08760061A
Authority: EP
Inventors: Aila Weaver; Risto Vahteri
Original assignee: Linde GmbH; Stora Enso Oyj
Current assignee: Linde GmbH; Stora Enso Oyj
Priority date: 2007-07-03
Filing date: 2008-05-27
Publication date: 2010-04-21
Anticipated expiration: 2028-05-27
Also published as: CA2691590A1; WO2009003770A1; FI20075503A0; EP2176461B1; ATE515593T1; FI20075503L; FI20075503A7

Abstract

The invention relates to a process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made in to paper. According to the invention, the pulp suspension is subjected to an acid - base treatment after the dewatering device. The pH of the suspension is first reduced with a strong acid by 0.5 -4 pH units to a pH between 4 and 6 and then the pH of the suspension is increased with a weak base by 0.5 -3 units to a pH below 8. The invention also relates to the use of an acid - base treatment in a process for making paper, wherein a pulp suspension, which has been thickened in a dewatering device, is subjected to an acid - base treatment for improving the process and/or the product. The invention provides improvements in the brightness of the pulp suspension and/or reduces the need for bleaching agent. The invention also decreases the dissolution of calcium carbonate filler in the process.

Description

PROCESS FOR MAKING PAPER AND USE OF AN ACID/BASE TREATMENT IN PAPERMAKING

Field of the Invention

The present invention relates to a process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made into paper. According to the invention, the pulp suspension is subjected to an acid-base treatment after said dewatering device. The invention also relates to the use of an acid-base treatment in a process for making paper, wherein a pulp suspension, which has been thickened in a dewatering device, is subjected to an acid-base treatment for improving the process and/or the product. The invention provides improvements in the brightness of the pulp suspension and/or reduces the need for bleaching agent. The invention also decreases the dissolution of calcium carbonate filler in the process.

Background of the invention

Mechanical pulps may be used in unbleached form, but they are typically subjected to bleaching with peroxide and/or dithionite to increase the brightness of the pulp. After peroxide bleaching the pulp suspension is subjected to a thickening step and thereafter it is diluted with process water. Alternatively, in the case of unbleached pulp the suspension is subjected, after the screening and washing stage, directly to a thickening step and then to dilution with process water.

After peroxide bleaching, the pH of the mechanical pulp is commonly reduced in order to minimize the darkening reactions of the pulp that decrease the brightness of the pulp and of the paper made from that pulp. These reactions include precipitation of dissolved substances together with metals, lignin etc. How severe these reactions are depend on pH, time, temperature, amount of dissolved metals and amount of dissolved and colloidal organic material. The pulp is normally acidified to pH 5-7 with sulphur dioxide water, bisulphite, sulphuric acid or carbon dioxide. Pulps used in acidic papermaking can be acidified to a lower pH than pulps that are used in neutral papermaking. A lower pH gives less brightness loss, however a too low pH can cause problems directly at the point of addition of the acid or later in the process. Thus, a low pH may cause dissolution of calcium carbonate and the pH may be too low to give optimum performance for chemicals that are added later or at the same time as the acid, for example dithionite, different retention and sizing systems, etc. Due to these conflicting needs of the process, a non-optimised pH for brightness is typically used.

Also in the case of unbleached mechanical pulp there can be problems with brightness loss of the mechanical pulp. This is mainly a problem in neutral papermaking where process waters with a neutral pH (7-8) are used to dilute the unbleached mechanical pulp. The pH of the process waters increases the pH of the unbleached mechanical pulp and this causes brightness loss. Lowering the pH with an acid would improve the brightness, but would cause dissolution of calcium carbonate at any point where the pulp suspension is combined with process waters or pulp streams containing calcium carbonate. Because of this, a non- optimised pH that gives a loss in brightness of the mechanical pulp is again used.

The circulating process water systems of present day pulp and paper mills are closed to a more and more increased level in order to lower the amount of water discharged from the pulping and the papermaking processes. Various impurities enter and accumulate in the water circulations in different phases of the process. Organic impurities dissolve from the fiber raw material and inorganic and organic chemicals are added to the process in different production steps.

During the production of mechanical pulp, approximately 2-5 per cent of the wood material is dissolved or dispersed as colloidal particles in the process waters. During peroxide bleaching, more lignin and hemicelluloses are dissolved. All in all this gives process waters with high amounts of dissolved organic substances, dissolved calcium, other metals and colloidal organic and inorganic substances. Many of these substances have an influence on the brightness loss of the mechanical pulp through a multitude of different reactions.

When these circulating process waters are used as dilution waters, the accumulated impurities cause many problems in the papermaking. Among these problems are brightness loss of pulps, but also other problems such as coagulation of sticky particles, soap and ink particles; precipitation of inorganic calcium salts as a scaling; precipitation of calcium oxalate and re -precipitation of calcium carbonate; interference with retention aids, dispersants and other charged paper additives; etc.

Because mechanical pulps are either unbleached or bleached in sequences which remove the colour of the lignin but do not actually remove the lignin, mechanical pulps are relatively prone to diminish in brightness with age. Brightness reversion is a common drawback of all high-yield pulps. Reduced chromophores can be reoxidized to their colored form, and new chromophoric groups can be created.

As seen above, due to the conflicting needs of the brightness on one hand and the dissolution of calcium carbonate on the other hand, mechanical pulp has traditionally been subjected to non-optimised pH conditions. This is especially the case in neutral papermaking and the problem is magnified when the process waters used for dilution have accumulated high levels of organic and inorganic substances as discussed above. The loss of brightness of the mechanical pulps has typically been countered by increasing the load of peroxide used in the peroxide bleaching step. The dissolution of calcium carbonate has typically been countered by a raised pH.

There are a number of patent documents dealing especially with the problems caused by the dissolution of calcium carbonate.

Thus, patent application WO 99/00545 describes a method of reducing the content of calcium in a pulp suspension by alkalizing a suspension of chemical paper pulp to a pH above 8 and adding carbonate ions to the pulp suspension for formation of calcium compounds which are adsorbed on the pulp. Calcium is transferred from the liquid phase to the pulp fibers. In this method the pulp is subjected first to a bleaching sequence at a pH value under 6 and thereafter the pulp is subjected to a bleaching sequence at a pH value above 8, especially a pH of at least 10. In connection with the alkalizing step, carbonate ions are added to the pulp suspension for formation of calcium compounds, which are adsorbed on the pulp. According to EP1008561 calcium hardness of recycled paper-processing water is controlled by precipitating calcium ion in process water into calcium carbonate through addition of sodium carbonate or sodium hydrogen carbonate to white water short - circulated in a papermaking process and removing the calcium carbonate precipitate from the water. Air or carbon dioxide may be used for removal of the precipitate by flotation.

US 3,639,206 describes removal of ligneous colour bodies and suspended fibrous material, whereby waste water effluent is treated first with a calcium salt (CaO, Ca(OH)₂). The separated aqueous phase is then contacted with CO₂ gas to precipitate any dissolved calcium salts and a final separation is performed.

US 5,679,220 discloses a method enhancing the deposition and retention of particulate filler on papermaking fibers wherein separate slurries of fibers and Ca(OH)₂ or equivalent thereof are combined with gaseous precipitant (CO₂) to form in situ calcium carbonate on the fibers.

According to GB Patent Application 2 008 562 carbon dioxide may be used for increasing the solubility of calcium carbonate and for the hardening of recycled waters used in the treatment of pulp from waste paper.

According to US 5,262,006 precipitation of gypsum in an alkaline recycle or broke derived pulp may be prevented by adding carbon dioxide to form bicarbonate ions in the alkaline pulp and to precipitate calcium as carbonate.

According to EP 0 988 671 and EP 0 981 665 the dissolution of calcium carbonate in a papermaking system is reduced by the addition of carbon dioxide to the pulp as the pH goes below 8. The accumulation of calcium ions in the system is reduced since less calcium ions enter the system.

According to EP 0 991 811 the pH of a pulp suspension in the stock preparation of a paper machine is stabilized by carbon dioxide. A combination of sodium hydroxide and carbon dioxide provide a significant buffering effect in the pulp. The stabilized pH reduces the dissolution of calcium carbonate at acidic addition points. According to US 7,175,759 an apparatus located outside the main process stream is used for combining sodium hydroxide and carbon dioxide at a desired ratio. The resulting non- stochiometric sodium bicarbonate solution has a desired pH value and a desired buffering ability and it can be used for stabilizing the pH of the process and for reducing the calcium hardness of the process waters.

While suggestions made in the prior art enable the use of calcium carbonate at a pH which is lower than the traditional pH of about 8, the described use of carbon dioxide or of carbon dioxide and sodium hydroxide is not sufficient for improving the brightness of fiber suspensions containing mechanical pulp without causing unacceptable dissolution of calcium carbonate in the system. Thus, there exists a need for a way of improving the processes relating to the production of paper from mechanical pulps. There is especially a need for a simple and secure way of controlling the brightness of a mechanical pulp suspension.

Brief description of the invention

An object of the present invention is to reduce the problems of the prior art and to provide a process which does not impair the brightness of a mechanical pulp suspension.

Another object of the present invention is to reduce the problems caused by calcium ions in a mechanical pulp suspension.

A further object of the present invention is to provide a process for decreasing the dissolution of calcium carbonate in a process for producing paper from mechanical pulp.

A further object of the invention is to provide a technically advantageous process for maintaining the brightness of a mechanical pulp suspension after dilution with process water.

An object of the present invention is furthermore to provide a process for reducing the decrease of the brightness of a mechanical pulp suspension during storage. An object is also to provide a process for reducing the amount of peroxide bleaching agent needed to reach a target brightness in a peroxide bleached mechanical pulp.

The invention according the present application is defined in the appended claims, the contents of which are included herein by reference.

The present invention is based on the realization that subjecting a pulp suspension comprising mechanical pulp to an acid-base treatment reduces the risk for brightness loss in the pulp suspension while avoiding excess accumulation of calcium ions in the circulation. According to the preferred embodiment of the invention an acid-base treatment of a diluted pulp provides both an improvement in the brightness and a reduction in the calcium ion level in the system.

Consequently, the present invention relates to a process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made into paper. The invention is characterized by subjecting said pulp suspension to an acid- base treatment, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with said dilution after said dewatering device and said weak base is added before said storage tank or in connection with the first dilution after said storage tank.

The pulp suspension which is to be treated according to the invention is a suspension which contains or consists of mechanical pulp in water. The mechanical pulps in question may be bleached or unbleached and they may be selected from stone groundwood, pressurized groundwood, refiner mechanical pulp, thermomechanical pulp, chemithermomechanical pulp, chemimechanical pulp and combinations thereof. In one embodiment of the invention, the pulp suspension consists of bleached mechanical pulp.

The pulp comprising the mechanical pulp used in the present invention has typically been subjected to at least one washing step and one or more subsequent thickening steps in dewatering device(s). For being processed into paper, the thickened pulp is diluted with water which may be fresh water or process water. Dilution with re-circulated process water increases the amount of organic and inorganic impurities into the pulp suspension, which potentially harms the pulp brightness. The present invention counteracts the harm caused by such impurities.

The present invention also relates to the use of an acid-base treatment in a process for making paper form a pulp suspension comprising mechanical pulp. The pulp suspension , which has been thickened in a dewatering device, is subjected to an acid-base treatment, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with a dilution after said dewatering device and said weak base is added before a subsequent storage tank or in connection with the first dilution after said storage tank.

The acid-base treatment of the present invention can be utilized in many ways, e.g for adjusting the brightness of the pulp, for reducing the amount of bleaching agent, for reducing the dissolution of calcium carbonate and diminishing the amount of detrimental substances in the process waters.

Brief description of the drawing

Fig. 1 shows a schematic diagram of an acid/base treatment in a mechanical pulp bleaching process

Detailed description of the invention

The present invention relates especially to pulps comprising mechanical pulps, and it aims at solving the problem posed by the conflicting needs of providing a low enough pH for obtaining an acceptable brightness, on one hand, and a high enough pH for allowing use of calcium carbonate and other substances, on the other hand.

In connection with the invention it has surprisingly been found that if the pH is first lowered with a strong acid and then raised with a weak base, optimized conditions for both brightness and dissolution can be provided. In one embodiment of the invention, the time during which the pH is at its lowest value (acidic shock), is very brief and in any case short enough to avoid excess dissolution of any calcium carbonate contained in the suspension. In a preferred embodiment of the invention, the weak base is added no more than one minute after the addition of the strong acid.

The acid-base treatment of the present invention is located in the process at the stage wherein the mechanical pulp has been produced and optionally bleached and then washed in one or more washing stages. The washed pulp is then thickened in a dewatering device. The thickened pulp is diluted with re-circulated process water in order to facilitate its pumping forward in the process. The process water is typically selected from white water, clear filtrate, bleaching filtrate and mixtures of any of these. Fresh water may also be used as such or added to any of the said process waters. The acid according to the invention is added to the pulp in connection with this dilution.

The acid that is used according to the invention is a strong acid. It is typically selected from the group consisting of sulphuric acid, sulphurous acid , hydrochloric acid and bisulphite. The preferred acid is sulphurous acid that is added in the form of SO₂ water. SO₂ in water forms sulphurous acid H2SO3, which dissociates into H⁺ and HSO3 ^' Thus SO₂ also inherently forms the bisulphite ion. At pH 4, the acid is almost exclusively in the bisulphite form. When bisulphite is added as such, it is typically added as sodium bisulphite. It has been found that adding a weak acid such as carbon dioxide will not provide the desired reduction in brightness loss of the mechanical pulp.

Addition of the strong acid in connection with the dilution facilitates uniform mixing of the strong acid throughout the suspension. A uniform distribution is desired in order to avoid local areas of too low or too high pH. The amount of strong acid depends on the desired pH reduction, which in turn depends on the process pH as a whole.

The pH of the pulp suspension, which is to be subjected to the acid-base treatment, may vary between about pH 10 and pH 4.5, depending on the treatments that the pulp has been subjected to before the acid-base treatment. Thus, peroxide bleaching is done at a high pH (pH 10-11) and the pulp may have a pH of about 10 to 9 after the peroxide bleach. On the other hand, the bleached pulp is generally washed and dewatered after the bleach and prior to the acid-base treatment, and the wash water may lower the pH to a level such as 9 to 7 depending on the pH used in the papermaking. A non-bleached mechanical pulp will typically have a pH of 4.5 to 5.5 and the pH may again vary due to washing and dilution with process water having a different pH. In acidic papermaking the pH of the circulating process water used for dilution will typically have a pH of 4.5 to 5.5, while the waters of a neutral papermaking process typically have pH values between 6.8 and 8.5.

In an acidic process, the pH of the pulp will initially be low and a lowering of the pH by as little as 0.5 pH units may be sufficient for providing the desired results. In a neutral process, the pH will initially be higher and lowering the pH to a desired value between 4 and 6 will require a substantially larger amount of acid. The person skilled in the art is able to adjust the amount of acid according to the desired pH.

The strong acid should preferably be diluted before it contacts the pulp suspension to avoid locally very low pH areas. Thus, for example, when SO₂ water is used to provide the strong acid, the concentration of the solution should preferably be below 2 % and even more preferably below 1 %. SO₂ has been used in the prior art to kill off peroxide after a peroxide bleaching sequence. When SO₂ is used in the present invention after a peroxide bleaching stage, it is added in excess of the amount needed only for killing residual peroxide, since it is added also for lowering the pH to provide the acidic shock of the present invention. Lowering the pH improves the brightness of the pulp and the pH is typically lowered to the lowest acceptable level taking into consideration the limits set by a subsequent neutral papermaking process.

After the addition of a strong acid to the pulp, a weak base is added according to the invention. The base that is used is preferably an alkali metal bicarbonate or carbonate or a mixture thereof. In order to improve mixing of the weak base into the aqueous pulp suspension, the base is added as a liquid which can be made from NaHCOs powder, from Na₂CO₃ powder or from a combination of CO₂ and NaOH. According to the one embodiment of the invention a buffering effect is provided by addition of sodium bicarbonate which is generated from sodium hydroxide and carbon dioxide. The bicarbonate may also be generated in situ by sequential addition of NaOH and CO₂ to the pulp suspension. The addition points of the two components should be close to each to prevent the pH of either component from significantly affecting the properties of the pulp. The components are typically added before and after a pump.

In case sodium hydroxide and carbon dioxide are used for generating of sodium bicarbonate the dual purpose of increasing the buffering ability and adjusting the pH to a desired value is available. Thus, the pH of the pulp suspension may be increased by a non- stoichiometric sodium bicarbonate solution having a pH higher or lower than that of stoichiometric bicarbonate. The bicarbonate solution may be produced on site as described in EP-Bl 991 811 and in US-B2 7,175,759. The concentration of the bicarbonate solution is typically between 2 and 8 % and its pH is typically 8 to 9, preferably 8.3-8.5.

The optimum dwell time of the low pH phase of the treatment depends on the process conditions and on the load of impurities in the pulp suspension and in the re-circulated process waters. In case there is a significant amount of solid calcium carbonate circulating with the diluting water, it is important that the dwell time is short, preferably no more than a few minutes, most preferably no more than a minute. A significant amount of solid calcium carbonate is typically present in white water circulated from the paper machine of a process, which utilizes calcium carbonate as a filler. The pH of such process water is typically between 6.8 and 8.3. In case process water having a significant content of solid calcium carbonate is used for the dilution after the dewatering device, the weak base should be added before the pulp is sent to storage such as to a storage tank or tower. A short dwell time is typically provided by adding SO₂ water at the suction side of a pump and bicarbonate solution at the pressure side of the same pump.

Fresh water, clear filtrate and bleaching filtrate contain less solid calcium carbonate than white water, and a pulp diluted with such process waters can endure longer dwell times to be used. In such cases, the acidic phase may even last several hours. Storage under acidic conditions is good for the mechanical pulp since a low pH causes little loss of brightness. Thus, when process water with a low content of solid calcium carbonate, such as clear filtrate, is used for the dilution after the dewatering device, the weak base may be added after the storage tank or tower. In such cases, it is often desirable to add the strong acid to the diluting process water itself before using it for the dilution. This facilitates a uniform distribution of the acid into the pulp suspension. Adding the acid to calcium carbonate rich process waters should be avoided because of the risk of dissolution of the solid calcium carbonate.

For enabling production of paper at a neutral pH e.g. with the use of calcium carbonate as a filler, the pH of the acid treated pulp must be raised. In case the pH is raised simply as in the prior art by diluting the stored pulp with process waters at a pH close to neutral, solid calcium carbonate in the dilution waters will be dissolved and the accumulation of calcium ions in the re-circulating systems will be detrimental to the process. In accordance with the invention, it has surprisingly been found that when the pH is raised with a weak base such as an alkali metal (bi)carbonate, the pH may be raised without detrimental effect on the brightness and without causing unacceptable dissolution of calcium carbonate.

In case the pulp is bleached with hydrogen peroxide, the acid-base treatment is performed after the peroxide bleaching. In case the pulp is to be bleached with dithionite, this bleaching typically takes place in the storage tank or tower and in such a case, the weak base is added before the storage tank in order to raise the pH to suit the dithionite bleaching.

According to the invention, the pH of the pulp suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6, typically to a pH between 5 and 6 in a neutral process, and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH which is below 8 and typically is between 6 and 7.5 in a neutral process. In acidic paper making the pH is typically reduced with the acid to a value between 4 and 5 and then increased to a value between 5 and 6.

According to one feature of the invention, the pH reduction with the strong acid precipitates impurities in the pulp suspension and the subsequent pH increase with the weak base allows the precipitated impurities to remain substantially undissolved in the suspension while raising the pH to suit the rest of the process. Thus, the acid-base treatment works as a "fixative" by decreasing the amount of organic substances, such as extractives, and metals in the process waters. The impurities precipitate on the pulp fibres with the pH reduction caused by the strong acid and remain substantially precipitated when the pH is raised with the weak base. The precipitations leave the system with the pulp and the circulation waters become cleaner.

Use of a strong acid, and especially the use of SO₂ water, creates an acid chock in the pulp suspension. It has been noted that metal ions are reduced by the treatment. Thus, for instance, Fe³⁺ is reduced to Fe²⁺ and remains in this reduced and non-coloured form when the pH is raised with bicarbonate. This improves the brightness of the pulp in a stable manner.

In one embodiment of the invention, the strong acid comprises sulphurous acid and the weak base comprises sodium bicarbonate. The sulphurous acid is preferably provided by sulphur dioxide gas and the sodium bicarbonate is preferably a non-stoichiometric sodium bicarbonate solution provided by combining sodium hydroxide, carbon dioxide and water in a reactor.

In forming sulphurous acid H2SO3, sulphur dioxide in water also provides bisulphite HCO3^" ions . Moreover, sulphurous acid and sodium bicarbonate provide bisulphite by the reaction: H2SO3 + NaHCOs -> NaHSθ3 + H2CO3. Bisulphite as such is known to have a bleaching effect on pulp suspensions. It is believed that a part of the increased brightness provided by the acid -base treatment of the present invention is due to the formation of bisulphite and similar active ions. Sodium bisulphite may also be formed by a reaction between sulphur dioxide water and sodium hydroxide present in the circulating process waters.

In the present specification and claims, the term "dewatering device" means a device wherein the consistency of the pulp suspension is increased by removal of water from the suspension. Typical dewatering devices useful in the process are filters such as disc filters, washing presses, Sundblad filters, press filters, etc.

The term "storage tank" means a vessel in which the pulp suspension is stored for a short or long period. The storage tank need not be any huge vessel or tower. In some cases a long pipe performs the role of a storage tank and may be equated as one. The term "pulp suspension" refers to the aqueous product stream passing though a paper and/or pulp mill. The suspension contains pulp fibres at a consistency which is typically between about 1 and 30 %. The acid-base treatment is typically performed on a pulp suspension having a consistency of 3 to 15 %, preferably 4 to 10%.

The term "dilution" used in the specification and claims refers to the addition of process waters or fresh water to the pulp suspension to decrease its consistency. The dilution may be performed in one or several steps, for example in a dilution ring.

The term "in connection with the dilution" means addition of the acid or base, respectively, either to the dilution water before the dilution water is added to the pulp, addition of the acid or base simultaneously with the addition of dilution water or addition of the acid or base just before or just after the pump closest after the dilution point. Similarly, the term "in connection with" relating to other unit operations means in close relation with, before, after or simultaneously with said unit operation.

The term "immediately after the first storage tank" means within one minute of the dilution of the pulp with process water after the storage tank.

The terms "significant" and "low" used for the content or amount of calcium carbonate in a liquid refers more to the amount present in functional terms more than as an absolute value. The amount of calcium carbonate in a dilution water is significant if it causes variations or fluctuations in the pH of the suspension which it dilutes and/or if it causes a substantial buffering ability in the suspension. The amount of calcium carbonate in a dilution water is considered low, if it does not influence the pH or buffering ability of the resulting suspension. The amount of calcium carbonate in dilution water is typically calculated based on the amount of solids per liter. The solids include, besides filler, also other solids such as fibers. Dilution water having less than 50 mg solids per liter is generally considered as having a low content of calcium carbonate. However, if such water is used in huge amounts, it is evident that the absolute amount of calcium may be significant in the unit operation or suspension in question. The mechanical pulp of the process according to the invention is preferably bleached in the process. The bleaching is typically performed with a bleaching agent selected from the group consisting of hydrogen peroxide, sodium dithionite and combinations thereof.

One embodiment of this invention relates to peroxide bleached mechanical pulps that are used in neutral papermaking. After the peroxide bleaching the mechanical pulps are washed in one or more steps, the washing steps include dewatering devices such as disc filters or washing presses. After the last washing step the dewatered thickened pulp is diluted with process water to a consistency between 5 and 15% and pumped to a storage tank. The strong acid is added in connection with the dilution after the last washing step. The pH in this embodiment is decreased to at least a pH of 6 and preferably to a pH between 5.3 and 5.8. The weak base is added either before the storage tank or immediately after the tank. When a short acid dwell time is desired, it has been found advantageous to add the acid on the suction side of a pump and the base on the pressure side of the pump. After the addition of base the pH should be at least 5.7 and preferably at a pH between 6 and 7.

One further embodiment of the present invention concerns peroxide bleached mechanical pulps that are used for paper production in acidic papermaking. After the peroxide bleaching the pulps are processed as discussed above for peroxide bleached pulps used in neutral papermaking. In acidic papermaking, however, the suitable pH levels are different. After the addition of acid the pH should have decreased to no higher than pH 5 and preferably between 4 and 5. The base is added before the storage tank to increase the pH to a minimum pH of 4.5, preferably between 5.0-5.5.

One embodiment of the invention concerns unbleached mechanical pulp, where process waters with a neutral pH, meaning pH 7-8, are used to dilute the unbleached mechanical pulp. Because of the pH of the process waters, the pH of the unbleached mechanical pulp increases and causes brightness loss. By utilizing the acid base treatment according to the invention, the brightness loss can be minimized without other unwanted effects, such as those encountered in the prior art. The acid is added with the dilution water after the last washing step before the storage tank for the unbleached pulp or in connection to the pump between the washing step and before the storage tank. The pH in the pulp suspension is decreased by a minimum of 0.5 pH units, preferably by 1 to 3 pH units. The weak base is added either before the storage tank or immediately after the storage tank. After the addition of the weak base, the pH of the pulp suspension has been increased by a minimum of 0.5 pH units and preferably by 1 to 3 pH units to a minimum pH of 5.8.

Without whishing to be bound by any theory, it is believed that the present invention operates in the following manner: the acid-base treatment of the present invention gives an acidic shock to the pulp suspension. That acidic shock precipitates some of the impurities of the process waters, such as colloidal and dissolved organic substances and metals onto the fibres. Due to this precipitation, the process waters have a lower amount of substances, for example metals, that are detrimental for the pulp brightness. The acidic shock also decreases the dissolution of organic substances from the mechanical pulp fibres. This is especially apparent in the case when the base is added after the storage tank. Raising the pH after the acidic shock with a weak base provides mild enough conditions to keep the re- dissolution of the precipitated substances at a minimum while providing conditions in which soluble substances such as calcium carbonate will remain in solid form. The counter ion effect of the (bi)carbonate base is believed also to play a role in keeping calcium carbonate from dissolving. The use of a weak base which also works as a buffer. This is especially true for the preferred sodium bicarbonate, which increases the alkalinity and , counters unwanted effects that otherwise would be caused by the low pH in the acidic shock. The weak base also raises the pH after the acidic chock to a pH value suitable for the subsequent papermaking.

By the use of the acid-base treatment according to the invention, a number of benefits are obtainable. Thus, the treatment may be used for adjusting the brightness and/or for reducing the brightness loss of a pulp suspension comprising mechanical pulp.

The acid-base treatment may also be used for reducing the amount of peroxide needed to reach a target brightness in a mechanical pulp. In one trial, the amount of bleaching chemicals could be reduced by as much as 20 to 30 %. The acid-base treatment can also be used for controlling the dissolution of calcium carbonate in a system for making paper from mechanical pulp with calcium carbonate as filler.

The treatment according to the invention is also effective in decreasing the amount of detrimental substances in the process waters of a papermaking process. Such detrimental substances include organic substances measured as COD, lignin compounds, calcium ions, metals and mixtures thereof.

A specific embodiment of the invention relates to a process for producing paper from bleached mechanical pulp, which includes the steps of a. providing a mechanical pulp suspension b. bleaching said pulp suspension with hydrogen peroxide c. thickening said pulp suspension in a one or more dewatering devices d. after the last of said dewatering devices, diluting said bleached pulp suspension with process water comprising white water at a pH of 6.8 to 8.3 e. in connection with said dilution, reducing the pH of said pulp suspension to a pH between 5.0 and 6.0 with SO₂ in water f. storing said pulp suspension in a storage tower at said reduced pH g. after storage, diluting said pulp suspension with process water comprising white water at a pH of 6.8 to 8.3 h. in connection with said dilution, adding a non-stoichiometric bicarbonate solution to said suspension to provide a pH between 5.8 and 6.5, i. adding calcium carbonate filler to said suspension, j. subjecting said pulp suspension to an acid-base treatment according to any one of claims 1 to 19, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with a dilution after a dewatering device and said weak base is added before or immediately after the first storage tower after said dewatering device, k. pumping said acid-base treated pulp suspension to a mixing chest and through a series of paper making steps to a paper machine, and making paper.

The following Examples illustrate the invention.

Example 1 (reference)

A paper mill was producing high brightness paper made from mechanical pulp with calcium carbonate as filler in a neutral process with pH 7.2 in the head box. The mechanical pulp was peroxide bleached and directly after the peroxide bleaching it was washed to clean the pulp. After the washing step the cleaned pulp was dewatered in a wash press and then diluted with process water from a tank containing varying mixtures of clear filtrate and white water. In connection with the dilution, the pH was adjusted with SO₂ water to pH 6.4. The diluted pulp was then pumped to a storage tower where the storage time was 2-4 hours. After the storage tower the pulp was further diluted with process water. This dilution water comprised white water having a pH of 7.8 and a fairly high amount of solid calcium carbonate. Calcium carbonate filler was added to the diluted pulp and the pulp was pumped to the mixing chest and then through a series of steps on to the paper machine where paper was made. The problems associated with the dissolution of calcium carbonate could be held at an acceptable level. However, there were problems with the brightness of the mechanical pulp and also with brightness loss over the storage tower. A very high amount of peroxide was needed to get the wanted brightness of the pulp.

Example 2 (reference)

The paper mill in example 1 changed the treatment of the pulp after the peroxide bleaching to try to decrease the needed amount of peroxide. After the washing step the cleaned pulp was diluted with process water and the pH was adjusted with SO₂ water to 5.4. The pulp brightness increased through these measures, but huge amounts of calcium carbonate dissolved and the calcium levels became intolerably high in the process.

Example 3 (invention) The paper mill in example 1 changed the treatment of the pulp after the peroxide bleaching to try to decrease the needed amount of peroxide. After the washing and dewatering step the cleaned pulp was diluted with process water and the pH was adjusted with SO₂ to pH 5.4 and then after ca 3 minutes the pH was again increased to pH 6.4 with NaHCO₃ produced on site by combining NaOH and CO₂. Although the amount of peroxide used in the peroxide bleaching was decreased with 30%, the same brightness of the pulp as in example 1 was obtained. No higher amounts of calcium carbonate than in example 1 were dissolved and the process operated to satisfaction.

Example 4 (invention)

The paper mill in example 3 changed the addition point of the base addition to immediately after the storage tower. Also by using this addition point good results were obtained. Compared to example 3 the brightness loss was smaller but the dissolution of calcium carbonate was slightly higher.

Example 5 (reference)

A paper mill was producing paper from mechanical pulp with kaolin as filler with an acidic process with pH 5 in the head box. The pulp was peroxide bleached and directly after the peroxide bleaching it was washed to clean the pulp. After the washing step the cleaned pulp was dewatered and then diluted with process water from a tank containing varying mixtures of clear filtrate and white water and the pH was decreased with SO₂ water to pH 5. After that dithionite was added for further bleaching and the pulp was pumped to a storage tower where the storage time was 2-4 hours. The optimal pH for dithionite pH in this mill was pH 5. After the storage tower the pulp was further diluted with process water and pumped to the mixing chest and then through a series of steps on to the paper machine where paper was made. To get the final wanted brightness of the pulp large amounts of peroxide had to be added in the bleaching stage.

Example 6 (invention)

The paper mill in example 5 changed the treatment after the peroxide bleaching to try to decrease the amount of peroxide needed by using an acid-base treatment using an acidic shock with sulphurous acid followed by an increase in pH with NaHCO₃. After the washing step the cleaned pulp was dewatered and then diluted with process water and the pH was decreased with SO₂ to pH 4. Shortly thereafter, within about 2 minutes, the pH was increased again with NaHCOs to pH 5. After that dithionite was added for further bleaching and the pulp was pumped to the storage tower. The amounts of peroxide needed could be decreased with 20% while keeping the same brightness of the pulp as in example

5.

Example 7 (invention)

A paper mill was producing paper from mechanical pulp according to the schematic diagram of Fig. 1. The mill used calcium carbonate as filler in a neutral papermaking process having pH 7 to 8 in the head box.

The mechanical pulp 1 was peroxide bleached in bleach tower 2 and directly after the peroxide bleaching it was washed in washer 4 to clean the pulp. Washing water 5 was recirculated up-stream in the process. After the washing step the cleaned pulp was dewatered in a thickener 6, from which the pulp was fed forward in the process and the removed water 7 was recirculated up-stream. The dewatered pulp was diluted with process water 8 comprising a mixture of clear filtrate and white water to a consistency of 7-8.

The pH of the diluted pulp was decreased to about 6 by feeding SO₂ water 9 on the suction side of a pump 11. The SO₂ water had a concentration of 0.9 % and the resulting pH was 5.7 to 5.8. Dithionite 10 was added in the pump 11 to provide additional bleaching of the pulp. On the pressure side of the pump 11 a non- stoichiometric sodium bicarbonate solution having a pH of 8.5 and a concentration of 4 % was added to the pulp flow to raise the pH to 6.4.

The pulp was then fed to storage tower 13 where the storage time was about 2 to.6 hours. After the storage tower the pulp was further diluted with process water and bicarbonate solution to bring the consistency and pH to the desired value for the subsequent neutral papermaking.

The resulting pulp had an acceptable brightness and the level of calcium ions in the system was kept at an acceptable level. The process produced paper of good quality with reduced amounts of bleaching agents compared to a similar process without the acid-base treatment.

The present invention provides advantageous solutions to the production of paper from mechanical pulp. Based on the above description, the skilled person is able to adjust the conditions and to utilize the invention in manner best adapted to his/her specific needs.

Claims

1. A process for making paper, wherein a pulp suspension, which comprises mechanical pulp, is thickened in a dewatering device and is thereafter diluted with water and pumped to a storage tank, then diluted again and made into paper, characterized by subjecting said pulp suspension to an acid-base treatment, wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with said dilution after said dewatering device and said weak base is added before said storage tank or in connection with the first dilution after said storage tank.

2. The process according to claim 1, wherein said strong acid is selected from the group consisting of sulphuric acid, sulphurous acid, hydrochloric acid and bisulphite.

3. The process according to claim 1, wherein said weak base is selected from the group consisting of alkali metal bicarbonate, alkali metal carbonate and mixtures thereof.

4. The process according to claim 3, wherein said weak base comprises sodium bicarbonate and/or sodium carbonate produced by combining sodium hydroxide and carbon dioxide.

5. The process according to claim 1, wherein said strong acid comprises sulphurous acid and said weak base comprises sodium bicarbonate.

6. The process according to claim 5, wherein said sulphurous acid and/or bisulphite is provided by sulphur dioxide gas in water and said sodium bicarbonate is a non- stochiometric sodium bicarbonate provided by combining sodium hydroxide and carbon dioxide in a reactor.

7. The process according to claim 1, wherein the pH of said pulp suspension is reduced with said strong acid to a pH between 5 and 6.

8. The process according to claim 1 wherein the pH of said pulp suspension is increased with said weak base to a pH between 6 and 7.5.

9. The process according to claim 1, wherein said pH reduction with said strong acid precipitates impurities in said pulp suspension onto said pulp and said pH increase with said weak base allows said precipitated impurities to remain substantially undissolved.

10. The process according to claim 1, wherein process water is recirculated in said process and wherein at least one of said dilutions is performed with recirculated process water.

11. The process according to claim 10 wherein said process water comprises white water, clear filtrate, bleaching filtrate, or mixtures thereof.

12. The process according to claim 11, wherein said process water contains solid calcium carbonate.

13. The process according to claim 12, wherein the pH of said process water is between 6.8 and 8.3.

14. The process according to claim 13, wherein process water having a high content of solid calcium carbonate is used for said dilution after said dewatering device and said weak base is added before said storage tank.

15. The process according to claim 14, wherein said weak base is added no more than 1 minute after said strong acid.

16. The process according to claim 13, wherein process water having a low content of solid calcium carbonate is used for said dilution after said dewatering device, said strong acid is added in connection with said dilution and said weak base is added after said storage tank.

17. The process according to claim 16, wherein said, strong acid is added to said process water.

18. The process according to claim 1, wherein said mechanical pulp is bleached or unbleached and is selected from stone groundwood, pressurized groundwood, refiner mechanical pulp, thermomechanical pulp, chemithermomechanical pulp, chemimechanical pulp and combinations thereof.

19. The process according to claim 1 wherein said mechanical pulp is bleached with a bleaching agent selected from the group consisting of hydrogen peroxide, sodium dithionite and combinations thereof.

20. The process according to claim 19, wherein the target brightness of said peroxide and/or dithionite bleached pulp is obtained with a lower amount of bleaching agent(s) compared to a similar process without said acid-base treatment.

21. The process according to claim 1, wherein said pulp suspension is bleached with hydrogen peroxide and said acid-base treatment is performed after said peroxide bleaching.

22. The process according to claim 1, wherein said pulp suspension is bleached with dithionite and said acid-base treatment is performed in connection with said dithionite bleaching.

23. The process according to claim 1 for producing paper from bleached mechanical pulp, characterized in a. providing a mechanical pulp suspension b. bleaching said pulp suspension with hydrogen peroxide c. thickening said pulp suspension in a dewatering device d. after said dewatering device, diluting said bleached pulp suspension with process water comprising white water at a pH of 6.8 to 8.3 e. in connection with said dilution, reducing the pH of said pulp suspension to a pH between 5.0 and 6.0 with SO₂ in water f. storing said pulp suspension in a storage tower at said reduced pH g. after storage, diluting said pulp suspension with process water comprising white water, h. in connection with said dilution, adding a non-stochiometric bicarbonate solution to said suspension to provide a pH between 5.5 and 6.8, i. adding calcium carbonate filler to said suspension, j. pumping said pulp suspension through a series of paper making steps to a paper machine, and k. making paper.

24. The use of an acid-base treatment in a process for making paper form a pulp suspension comprising mechanical pulp, wherein a pulp suspension thickened in a dewatering device is subjected to an acid-base treatment, and wherein the pH of said suspension is first reduced with a strong acid by 0.5-4 pH units to a pH between 4 and 6 and then the pH of said suspension is increased with a weak base by 0.5-3 units to a pH below 8, and wherein said acid is added in connection with a dilution after said dewatering device and said weak base is added before a subsequent storage tank or in connection with the first dilution after said storage tank.

25. The use according to claim 24 for adjusting the brightness of a pulp suspension comprising mechanical pulp.

26. The use according to claim 24 for reducing the brightness loss of a pulp suspension comprising mechanical pulp.

27. The use according to claim 24 for reducing the amount of peroxide needed to reach a target brightness in a pulp comprising bleached mechanical pulp.

28. The use according to claim 24 for controlling the dissolution of calcium carbonate in a system for making paper from a pulp suspension comprising mechanical pulp.

29. The use according to claim 24 for decreasing the amount of detrimental substances in the process waters of a papermaking process.

30. The use according to claim 29 where said detrimental substances selected from colloidal and dissolved organic and inorganic substances including organic substances measured as COD, colloidal and dissolved lignin compounds, calcium ions, metals and mixtures thereof.