RU2445413C1 - Method of producing paper pulp - Google Patents

Abstract

FIELD: textiles, paper.

SUBSTANCE: method involves a stage of refining wood, including handling of ozone to get the pulp, a stage of neutralisation of acid comprising contacting mechanical pulp with a neutralising composition containing at least one alkaline agent for more than one hour, and a stage of bleaching comprising contacting the neutralised mass with a bleaching composition. During the last 30 minutes of the stage of neutralisation the acid, pH varies by less than 0.2 units, and the pH ranges from 6 to 10 at the output from the stage of neutralisation the acid.

EFFECT: invention provides a more efficient bleaching of paper pulp with better output, or using fewer chemicals, or with formation less wastewater.

15 cl, 3 ex, 4 tbl, 1 dwg

Description

FIELD OF TECHNOLOGY

The present invention relates to a method for producing paper pulp.

BACKGROUND

Paper pulp, called “wood pulp” or “high yield pulp” or “wood pulp”, is obtained directly from wood by a series of mechanical treatments (defibration and refining) carried out using millstones and / or refiners. Then the resulting mass is subjected to the stage of bleaching in one or more stages.

It is known that treatment with ozone during refining of the mass can improve the quality of the resulting mass, in particular with regard to the physical properties (in particular, the mechanical strength of the mass), and reduce energy consumption in the process.

However, treatment with ozone causes particular problems, namely, a decrease in the brightness of the mass, difficulty in subsequent bleaching, loss of yield per wood, and an increase in the chemical oxygen demand of effluents from bleaching.

DE 2444475 describes the aforementioned ozone treatment. Subsequent bleaching of the mass is mentioned.

Document FR 2329794 also describes ozone treatment. After treatment, the treated mass is mixed with liquor (caustic soda) to stabilize the properties imparted to the mass by treatment with ozone. Then the mass is placed in a warehouse and subsequently used directly without bleaching to obtain paper.

FR 2406023 describes ozone treatment of wood pulp. The treatment is followed by a maturation step of less than 30 minutes, preferably less than 10 minutes, in which the ozonated mass is kept in an alkaline environment, possibly mixed with bleaching agents.

In the documents FR 2388933 and FR 2388934, as in the preceding documents, ozonation is followed by maturation in the presence of liquor (caustic soda) and, possibly, bleach (hydrogen peroxide) in order to stabilize the mass and possibly bleach simultaneously. It is also permissible to add caustic soda at the very stage of ozonation to swell the mass.

Document EP 0276608 illustrates an ozone treatment method. The examples in this document allow us to state that treatment with ozone leads to a loss of whiteness of the mass.

In a document entitled "The effect of ozone on mechanical pulps", N. Soteland, Can. Wood Chem. Symp Extended abstracts, p.13-20 (1976), outlines the problem of loss of whiteness of paper pulp obtained from ozone treated coniferous wood. The author indicates that adjusting the pH of the ozonated mass with sodium hydroxide and the addition of DTPA can increase the whiteness of the mass. The problem of loss of yield due to ozonation is also mentioned.

In a document entitled "Ozone treatment of mechanical pulp, part III: influence on optical properties", CA. Lindholm, revue Paperi ja puu - Papper och Trä (N 4a, 1977, p. 217-232) describes the negative effects of ozonation on whiteness, brightness stability and opacity of the pulp. Various approaches have been tested to solve this problem: modifying the ozone treatment itself, neutralizing caustic soda and heating the ozonated mass, or ozonizing, followed by treatment with a mixture of hydrogen peroxide and caustic soda. None of these approaches can be considered satisfactory at the same time both in terms of the mechanical strength of the mass and in terms of optical properties.

In a document entitled "Effect of ozone on high temperature thermomechanical pulp", R.W. Allison, Appita vol. 32, No. 4, p. 279-284 (1979), investigated the bleaching of ozonized paper pulp (HTMP type) and noted the effect of pH treatments after ozonation on the yield of the process: more precisely, later bleaching under alkaline conditions entails a significant loss exit. Sodium hydroxide extraction is proposed for a short period of 30 minutes after ozonation and before bleaching with hydrogen peroxide. It should be noted that with this method, the observed mass yields and / or whiteness achieved are low, despite the use of a very large amount of chemicals.

In a document entitled "Effect of neutralization on the bonding ability of ozone-treated mechanical pulp fibers", CA. Lindholm, Cellulose Chem. Technol., Vol.17, p.647-653 (1983), explores in detail how the neutralization of ozonized mechanical mass with caustic soda can give it beneficial mechanical properties.

In a paper entitled Ozone in the production of softwood and hardwood high-yield pulps to save energy and improve quality, M. Petit-Conil, C. de Choudens, T. Espilit, Nordic Pulp and Paper Research Journal, vol. 13, N 1, p.16-22 (1998), the effect of treatment with ozone followed by bleaching with hydrogen peroxide in the presence of caustic soda on the masses obtained from different tree species is investigated.This document does not address the problem of yield loss observed during bleaching.

The document entitled "Use of ozone in mechanical pulping processes, M. Petit-Conil, ATIP vol. 57, No. 2, p.17-26 (2003), compares the effect of different methods of introducing ozone. This document also does not address loss yield observed during bleaching.

WO 2008/081078 describes the use of caustic soda and other bases during ozone treatment itself in order to limit the harmful effects of organic acids from ozonation on subsequent bleaching and corrosion.

However, the methods used in the prior art do not give full satisfaction.

In particular, there is always a need to obtain a better yield, and / or to achieve increased levels of whiteness, and / or to reduce the amount of chemicals needed in the processing of paper pulp, and / or to reduce the amount of effluent created by bleaching.

SUMMARY OF THE INVENTION

The invention relates primarily to a method for producing paper pulp, including:

- a wood refining step comprising ozone treatment to obtain a mechanical pulp;

- an acid neutralization step comprising contacting the mechanical pulp with a neutralizing composition containing at least one alkaline reagent for a period of more than one hour; and

- the stage of bleaching, including bringing into contact the neutralized mass with a bleaching composition;

wherein the pH during the last 30 minutes of the acid neutralization step changes by less than 0.2 units, and the pH is from 6 to 10 at the outlet of the acid neutralization step.

In one embodiment, the step of neutralizing the acid has a duration of greater than or equal to 1 hour and 30 minutes, preferably greater than or equal to 2 hours, ideally greater than or equal to 3 hours.

According to one embodiment, the pH at the outlet of the acid neutralization step is from 7 to 8.

According to one embodiment, during the last 30 minutes of the acid neutralization step, the pH changes by less than 0.1 units, preferably less than 0.05 units.

According to one embodiment, an alkaline reagent in a neutralizing composition:

- selected from oxides, hydroxides, silicates and carbonates of alkaline earth metals and alkali metals, ammonia, ammonia and mixtures thereof;

- preferably selected from potassium hydroxide, sodium hydroxide, sodium silicate, magnesium hydroxide, calcium hydroxide, sodium carbonate, magnesium carbonate and mixtures thereof; and

- Ideally is magnesium hydroxide.

According to one embodiment, the whitening composition comprises a bleach and at least one alkaline reagent, wherein:

- the specified alkaline reagent is selected from oxides, hydroxides, silicates and carbonates of alkaline earth metals and alkali metals, ammonia, ammonia and mixtures thereof;

said alkaline reagent is preferably selected from potassium hydroxide, sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium silicate, sodium carbonate, magnesium carbonate and mixtures thereof; and

- the specified alkaline reagent ideally contains sodium hydroxide and / or magnesium hydroxide.

According to one embodiment, the method includes a chelation step, preferably between the acid neutralization step and the bleaching step, comprising contacting the neutralized mass with a chelating composition containing a chelating agent, preferably ethylenediaminetetraacetic acid or one of its sodium salts, or diethylene triamine pentaacetic acid or one of its sodium .

According to one embodiment, 20 kg or less of an alkaline agent, preferably 15 kg or less, more preferably 10 kg or less, per tonne of mechanical pulp is used in the acid neutralization step.

According to one variant of implementation at the stage of bleaching using:

- from 5 to 100 kg of bleach, preferably from 10 to 50 kg, more preferably from 20 to 40 kg, per ton of mechanical wood pulp;

- from 5 to 100 kg of alkaline reagent, preferably from 10 to 70 kg, per ton of mechanical wood pulp.

According to one embodiment, the loss of wood yield at the exit from the bleaching step is less than or equal to 20%, preferably less than or equal to 16%, ideally less than or equal to 13%, even less than or equal to 10%.

According to one embodiment, the whiteness of the mass at the exit from the bleaching step is greater than or equal to 65%, preferably greater than or equal to 66.5%, ideally greater than or equal to 68%, even greater than or equal to 70%.

According to one embodiment, the method produces effluents whose chemical oxygen demand is less than or equal to 210 kg per ton of mass, preferably less than or equal to 180 kg per ton of mass, more preferably less than or equal to 160 kg per ton of mass, even less than or equal to 145 kg per tonne of mass.

In one embodiment, the wood pulp is a mass of SGW, PGW, RMP, TMP, HTMP or CTMP.

According to one embodiment, the wood is softwood, hardwood, or a mixture thereof, preferably softwood, more preferably pine or spruce.

The invention also relates to a method for producing paper, including the production of paper pulp according to the method described above, as well as the use of this paper pulp for paper production.

The present invention overcomes the disadvantages of the prior art. In particular, it provides a more efficient way of bleaching paper pulp with a better yield, and / or by using less chemicals, and / or by forming less effluents that need to be cleaned.

This is achieved by the step of neutralizing the acid between the treatment with ozone and bleaching, carried out for a sufficiently long period and until the pH is sufficiently stable.

Indeed, after ozonation, acids within the fibers tend to be released gradually; therefore, the nature and duration of the acid neutralization according to the invention is selected so as to obtain a stable pH before the bleaching step, which allows to neutralize all acidic substances and to obtain improved bleaching.

According to some particular embodiments, the invention also has one or more advantageous characteristics, listed below:

- The invention allows to obtain the efficiency of bleaching paper pulp, equivalent to or superior to the efficiency of bleaching according to the prior art (the brightness parameter of the same order and even higher).

- The use of a soft base (in particular magnesium hydroxide, calcium hydroxide, sodium carbonate, magnesium carbonate, sodium silicate and mixtures thereof, preferably magnesium hydroxide) as an alkaline reagent in the neutralizing composition further improves the yield and / or reduces the amount of necessary chemicals and reduces effluent requiring cleaning and / or improves whiteness.

- Also, the use of a soft base at the bleaching stage improves the yield even more, reduces the amount of chemicals and effluents needed for cleaning and even improves whiteness.

- The mechanical properties of the mass obtained by the method according to the invention are as satisfactory as the mass obtained according to the prior art.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows an example of pH changes during the acid neutralization step of the invention (experimental measurements). The abscissa shows time in minutes, and the ordinate pH.

DESCRIPTION OF EMBODIMENTS OF THE INVENTION

In the following description, the invention is disclosed in more detail, but without limitation.

Schematically, the method of producing paper pulp according to the invention includes four successive stages:

- refining wood, including treatment with ozone to obtain wood pulp;

- neutralizing the acid, including bringing into contact the wood pulp with a neutralizing composition containing at least one alkaline reagent;

optionally chelating, including contacting the mass with a chelating composition containing a chelating agent; and

- bleaching, including bringing into contact of the mass with a bleaching composition.

The expression "pH changes by less than X units during the last 30 minutes of the acid neutralization step" means that the absolute difference between the pH measured at the end of the acid neutralization step and the pH measured 30 minutes before the end of the acid neutralization step is less than X units .

The "end of the acid neutralization step" coincides with the start of the next step. The beginning of the next step may mean the beginning of the chelation step or the beginning of the bleaching step, but it may also mean the beginning of the intermediate washing step or the beginning of the intermediate consistency changing step. In general, the "next step" corresponds to a new operation, which differs from the acid neutralization operation and immediately follows it.

Stage one: refining and ozone treatment

Refining and ozone treatment of wood are carried out by methods traditional in this field.

Raw materials are any suitable type of wood: in particular, coniferous, deciduous, or a mixture of both. It is preferably coniferous wood. Examples of suitable species include pine and spruce.

Before refining, the wood can be subjected to one or more pretreatments, for example, removal of bark, removal of knots of wood, heat treatment, pressure treatment, cutting into chips, wood chips, logs, etc.

Refining involves grinding or grinding (defibrating) with millstones (in a stream of water) or in a disk refiner.

Refining may contain several stages. For example, after the first grinding / defibration, you can divide the product into the accepted fraction and rejected fraction and refined rejected fraction again before mixing it with the received fraction. Several such intermediate divisions may be envisaged.

During refining, ozone treatment (ozonation) is carried out in order to facilitate the separation of wood fibers. Thus, treatment with ozone allows to reduce the specific energy required to separate the fibers, and to improve the mechanical properties of the pulp treated so. Ozone treatment is carried out according to methods known to those skilled in the art, for example, methods described in the prior art documents mentioned in the introduction of this application. The ozone load is usually from 0.5 to 8%, preferably from 1 to 4%. Ozone treatment can affect the entire mass or only a part, for example, the rejected fraction obtained by separation after the first refining.

The pH adjustment can optionally be carried out during ozonation or before ozonation, for example, by adding an alkaline reagent, in particular sodium hydroxide. Thus, it is possible to set the pH of the whole mass at the outlet from the first refining, or the pH of the fraction rejected after separation, for example, from 9 to 11. In this regard, reference can be made to document WO 2008/081078.

The addition of an alkaline reagent at this stage minimizes the drop in pH during ozone treatment. This drop in pH is the result of the appearance and accumulation of acidic organic by-products, which are formed under the oxidative action of ozone on wood components. It should be noted that even when the pH is set before ozonation, the mass after ozonation is very acidic, immediately after the ozonation step, the pH is from 5 to 7. At the exit from the second refining after ozonation, acidic organic by-products are released into the wood pulp suspension, and this latter has a very acidic pH (usually 3 to 4).

At the outlet from the refining / ozone treatment, wood pulp is obtained, which can be of any type known to the specialist, namely, in particular:

- defiber wood pulp (SGW) obtained from logs or blocks processed at atmospheric pressure in defibrers with millstones;

- defibber pulp (PGW) obtained from logs or blocks processed under pressure in defibrers with millstones;

- refiner pulp (RMP) obtained from chips or chips in refiners operating at atmospheric pressure;

- thermomechanical pulp (TMP) or high-temperature thermomechanical pulp (HTMP) obtained from chips or chips in refiners after heat treatment of wood at high steam pressure;

- chemical-thermomechanical pulp (CTMP) obtained by defibration under pressure followed by impregnation with chemicals in the presence of caustic soda and sodium bisulfate at temperatures above 100 ° C.

Second stage: acid neutralization

The acid neutralization is preferably carried out after refining / treatment with ozone, that is, immediately after refining / treatment with ozone is completed. It consists in neutralizing acidic by-products formed under the action of ozone on the constituents of wood, and these acidic by-products accumulate inside and outside (in the immediate environment) of wood fibers. Even if a base was used before or during ozonation, the neutralization of the acid in question is still useful for neutralizing acidic substances, since the acids inside the fibers are released after ozonation, and the pulp tends to become more and more with time sour.

The neutralization of the acid includes, and preferably consists of bringing into contact the wood pulp obtained in the first stage, with a neutralizing composition.

Contacting is preferably carried out by simple mixing. Typically, before neutralizing the acid, the pH is in the range of 3 to 5, depending on the treatment with ozone (amount of ozone, whether or not caustic soda is added before or during ozonation). At the end of acid neutralization, the pH is in the range from 6 to 12, preferably from 7 to 8.

The neutralizing composition is preferably an aqueous solution containing an alkaline reagent. This alkaline reagent can be selected from oxides, hydroxides, silicates and carbonates of alkali metals and alkaline earth metals, from ammonia, ammonia and mixtures thereof. Of the main preferred substances for selecting an alkaline reagent, potassium hydroxide, sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium carbonate, sodium silicate, magnesium carbonate and mixtures thereof can be mentioned.

According to one preferred embodiment, the alkaline reagent comprises a soft base. By “soft base” is meant a base which only partially dissociates in water (pKb> 0) and / or which has a low solubility. In particular, the alkaline reagent may contain a soft base selected from magnesium hydroxide, calcium hydroxide, sodium carbonate, sodium silicate, magnesium carbonate and mixtures thereof, preferably magnesium hydroxide.

In one embodiment, the alkaline reagent comprises the aforementioned soft base in combination with another base, for example sodium (or potassium hydroxide). But it is preferable that the neutralizing composition does not contain sodium hydroxide (or potassium) and that the alkaline reagent consists of a soft base, as mentioned above.

Compared to sodium or potassium hydroxide, the use of a soft base, such as magnesium hydroxide, minimizes the extraction and dissolution of certain components of wood that are oxidized by ozone. Therefore, the yield loss is less, and the pollution load resulting from the extraction process is reduced.

The amount of alkaline reagent used in the acid neutralization step is preferably less than or equal to 20 kg, more preferably less than or equal to 15 kg, ideally less than or equal to 10 kg, per ton of wood pulp.

The duration of the acid neutralization step is preferably greater than 1 hour, more preferably greater than or equal to 1 hour and 30 minutes, ideally greater than or equal to 2 hours, even greater than or equal to 3 hours, in order to optimize the positive effect of this step on total yield, effluent formation and whiteness.

The acid neutralization step is preferably carried out at a temperature of from 4 to 95 ° C, more preferably from 25 to 85 ° C, ideally from 35 to 80 ° C. A temperature of approximately 70 ° C is particularly suitable.

The consistency in the acid neutralization step is preferably from 0.5 to 10%, more preferably from 1 to 5%, ideally from 2 to 3%. The value of the consistency, or dryness, of the mass corresponds to the percentage of dry matter expressed relative to water.

Consistency is the ratio of the weight of the wood pulp to the weight of the slurry (which contains wood pulp, water and additional chemical compounds). In other words, we are talking about the weight concentration of wood pulp in the aquatic environment.

Third step (optional): chelation

Chelation is preferably carried out after neutralizing the acid, that is, immediately after the stage of neutralizing the acid is completed.

Chelation includes, preferably consists in bringing into contact the mechanical pulp obtained in the second stage, with a chelating composition.

The chelating composition is preferably an aqueous solution containing a chelating agent. The chelating agent may be any chemical compound conventionally used for this purpose in the art. Preferably, it is ethylenediaminetetraacetic acid or one of its sodium salts or diethylene triamine pentaacetic acid or one of its sodium salts.

The chelating agent has a special affinity for metal cations present in trace amounts in the pulp. The purpose of the chelating treatment is to neutralize these cations, forming a chelate compound with them, and remove them from the mass by washing it. The implementation of the chelation step improves the characteristics of the whitening treatment with hydrogen peroxide.

The amount of chelating agent used in the chelation step is usually from 0.5 to 30 kg, preferably from 1 to 20 kg, more preferably from 2 to 10 kg, ideally from 3 to 5 kg, per ton of mechanical pulp.

The duration of the chelation step is preferably greater than or equal to about 30 minutes.

The chelation step is preferably carried out at a temperature of from 4 to 95 ° C, more preferably from 25 to 85 ° C, ideally from 35 to 80 ° C. A temperature of approximately 70 ° C is particularly suitable.

The consistency in the chelation step is preferably from 0.5 to 10%, more preferably from 1 to 5%, ideally from 2 to 3%.

It should be noted that the chelation step can also be carried out simultaneously with the neutralization of the acid (but not after) in order to reduce the number and dimensions of equipment necessary for the implementation of the method. In this case, it is enough to simply contact the mass with the chelating agent at the stage of neutralizing the acid, either by adding it separately, or by including it in a neutralizing composition, preferably in the amounts indicated above.

Fourth stage: bleaching

The bleaching is preferably carried out after chelation (or after neutralization of the acid if chelation is not carried out), that is, immediately after the completion of the chelation step (or the step of neutralization of the acid if there is no chelation).

Bleaching includes, preferably, consists of bringing into contact the wood pulp obtained in the third step with a bleaching composition.

The consistency in the bleaching step is preferably from 5 to 50%, more preferably from 10 to 45%, ideally from 20 to 40%.

Bleaching has a higher speed with a high consistency (whereas during chelation, kinetics is fast even with a low consistency). The consistency of the mass can be increased, for example, by pressing it and removing filtrates containing, in particular, chelated metals.

Contacting is preferably carried out by simply mixing the bleaching composition with the mass. The type of equipment used for mixing is selected depending on the consistency of the mass: direct mixing using a pressure pump if the consistency is low or medium (less than 10%); mixer or mixer for a higher consistency (up to about 40%).

The bleaching composition is preferably an aqueous solution containing a bleach and an alkaline reagent.

Bleach can be any chemical compound commonly used for this purpose in the art. Preferably, it is hydrogen peroxide. Sodium hydrosulfite may also be used.

According to one embodiment, the alkaline reagent may comprise or consist of a soft base, which is defined for the second step.

According to one embodiment, the alkaline reagent may comprise or consist of a base other than a soft base, as determined in the second step, for example, may comprise or consist of sodium (or potassium) hydroxide.

In one embodiment, the (preferred) alkaline reagent may comprise or consist of a mixture of a soft base, as defined in the second step, and a base other than a soft base (e.g., sodium or potassium hydroxide). A mixture of sodium hydroxide and magnesium hydroxide is the preferred alkaline reagent in the whitening composition.

In this case, according to one embodiment, the weight ratio of the soft base to the base other than the soft base (for example, the weight ratio of sodium hydroxide and magnesium hydroxide) is from 0.001 to 1000, preferably from 0.01 to 100, more preferably from 0 , 1 to 10, and ideally from 0.2 to 5.

The presence of a soft base in the whitening composition minimizes the process of extraction and dissolution of some components of wood oxidized by ozone. As a result, the use of a soft base can further improve wood yield, reducing runoff and chemical consumption, and even improve the whiteness of the mass. In practice, the weight ratio between the soft base and the base other than the soft base usually leads to a compromise between whiteness and yield.

Preferably, the alkaline reagent in the whitening composition contains sodium silicate. An additional function of sodium silicate is to stabilize the bleach (in particular, hydrogen peroxide). It is also possible to provide another stabilizer in the bleaching composition in addition to or instead of sodium silicate. Possible stabilizers are compounds of the polyhydroxyacrylate type.

The amount of bleach used is usually from 5 to 100 kg, preferably from 10 to 50 kg, more preferably from 20 to 40 kg, per ton of wood pulp.

The amount of alkaline reagent used is usually from 5 to 100 kg, preferably from 10 to 70 kg, per ton of wood pulp.

The bleaching composition may also contain a chelating agent as defined above, in particular if the chelation step is absent or breaks when incomplete chelation occurs.

It should be noted that the bleaching composition can be prepared separately and then brought into contact with the mass, but it can also be prepared directly in contact with the mass. In this second case, various compounds of the bleaching composition are sequentially added directly to the mass.

The duration of the bleaching step varies depending on the type of agent used.

In the case of hydrogen peroxide, this duration is usually from 10 minutes to 8 hours, preferably from 30 minutes to 6 hours, more preferably from 2 hours to 4 hours.

The bleaching step is preferably carried out at a temperature of from 4 to 95 ° C, more preferably from 25 to 85 ° C, ideally from 35 to 80 ° C. A temperature of approximately 70 ° C is particularly suitable.

At the end of the process receive paper pulp, preferably having one or more of the following characteristics:

- The whiteness is greater than or equal to 65%, preferably greater than or equal to 66.5%, ideally greater than or equal to 68%, even greater than or equal to 70%.

- The loss of wood yield in the method is less than or equal to 20%, preferably less than or equal to 16%, ideally less than or equal to 13%, even less than or equal to 10%.

- The chemical oxygen demand of the effluents (COD) is less than or equal to 210 kg per ton of mass, preferably less than or equal to 180 kg per ton of mass, more preferably less than or equal to 160 kg per ton of mass, even less than or equal to 145 kg per ton of mass.

In the framework of this application, the whiteness of the mass is determined in accordance with ISO 2470.

In the framework of this application, the loss in the process of wood yield is defined as follows: at the output of each stage (acid neutralization, chelation, bleaching), a fraction of a 25% aqueous filtrate is selected by pressing a suspension of wood pulp. The water is evaporated by heating the filtrate to 50 ° C until a constant dry mass is obtained (which corresponds to the residue dissolved in the filtrate and, thus, lost). The considered output loss of the stage is calculated as follows:

Loss of yield in step = (100/25) * (weight of the residue at the end of the step) / weight of the original wood pulp [after the refining step]).

The loss of yield in the process is the sum of the loss of yield observed for each of the steps of acid neutralization (when it is performed), chelation and bleaching. This value does not take into account the ozonation stage.

In the framework of this application, the chemical oxygen demand of the effluents is determined according to ISO 15705, with measurements using a Hach DR / 2000 spectrophotometer.

EXAMPLES

The following examples illustrate the invention but do not limit it.

Example 1

A pine mass is prepared according to the TMP method. The mass is treated with ozone in the refining phase. More precisely, the waste from the primary refiner produces 2% of ozone, the pH being adjusted by 5 kg of caustic soda per tonne of mass; then the ozone treated wastes are mixed with the received fraction of the secondary refiner. The whiteness of the mass at the outlet from refining / ozone treatment is 47.3%.

After refining / ozone treatment, a chelation step is provided during which the mass is treated with 4 kg of DTPA per ton of mass, at a consistency of 4% and a temperature of 70 ° C, with a pH of 6-7, for 1 hour.

After the chelation step, the mass is subjected to a bleaching step at a consistency of 20%, a temperature of 70 ° C, and for 2 hours.

To prepare mass A (comparative example), the following parameters are also used:

- the absence of an acid neutralization step between refining / ozone treatment and chelation;

- whitening composition:

- hydrogen peroxide: an amount sufficient for 30 kg / t of mass;

- sodium silicate: an amount sufficient for 30 kg / t of mass;

- sodium hydroxide: an amount sufficient for 42.5 kg / t of mass.

The following parameters are also used to prepare mass B:

- an acid neutralization step lasting 3 hours between refining / ozone treatment and chelation with 10 kg of sodium hydroxide per tonne of mass (pH at the end of treatment 7.3);

- a bleaching composition identical to that used for mass A, with the exception of the amount of sodium hydroxide: 32.5 kg per tonne of mass instead of 42.5 kg.

To prepare mass C, the following parameters are also used:

- an acid neutralization step of 3 hours between refining / ozone treatment and chelation with 7 kg of magnesium hydroxide per tonne of mass (pH at the end of treatment 7.2);

- a whitening composition identical to that used for mass B.

For the preparation of mass D, in addition, the following parameters are used:

- an acid neutralization step identical to that used for mass C (pH at the end of treatment 7.2);

- a bleaching composition identical to that used for mass A, with the exception of the amount of sodium hydroxide (8 kg instead of 42.5 kg per tonne of mass), as well as the addition of 13.5 kg of magnesium hydroxide per tonne of mass.

For each mass, the whiteness of the mass is measured at the end of the process, the yield loss relative to the source wood and the total chemical oxygen demand (COD) that has arisen. The results are presented in table 1.

Table 1 Parameters measured for A-D masses Weight White Loss of exit COD per ton of mass A 68% 13.2% 185 kg B 70.7% 12.7% 178 kg C 70.2% 12.2% 171 kg D 65.9% 10% 140 kg

A comparison between mass A, on the one hand, and masses B, C, and D, on the other hand, reveals that the presence of an acid neutralization step improves the whiteness of the mass, the wood yield in the process, and reduces COD.

A comparison between mass B and mass C reveals that the use of magnesium hydroxide instead of sodium hydroxide in the acid neutralization step, combined with a 30% reduction in the amount of chemical required, can improve wood yield in the process and lower COD, with only slightly less whiteness.

A comparison between mass C and mass D reveals that a reduction of about 75% in the mass of sodium hydroxide used in the bleaching stage and the replacement of about 41% of this sodium hydroxide with magnesium hydroxide can reduce the yield loss by 20% and COD.

Regarding the production of mass B, a pH measurement experiment was also carried out during the acid neutralization step, lasting more than 3 hours. The result of this experiment is presented in table 2 below, as well as in figure 1. Time t = 0 corresponds to the start of acid neutralization.

table 2 PH change during the acid neutralization step (mass B) Time (min) pH 0 4.06 one 10.25 four 9.93 9 9.33 19 8.59 thirty 8.27 60 7.81 90 7.56 120 7.37 150 7.28 220 7.27 260 7.25

From this example, it was found that the pH, initially acidic, becomes very basic in the first minute of acid neutralization. Then, the gradual release of acids by fibers leads to a slow decrease in pH, which stabilizes only after 2-3 hours.

Example 2

Pine pulp is prepared according to the TMP method. The mass is treated with ozone in the refining phase. More precisely, the waste from the primary refiner produces 2% of ozone, and the pH is not regulated; then the ozone treated wastes are mixed with the received fraction of the secondary refiner. The whiteness of the mass at the outlet from refining / ozone treatment is 48.3%.

After refining / ozone treatment, a chelation step is provided in which the mass is treated with 4 kg of DTPA per ton of mass, with a consistency of 2-3% and a temperature of 70 ° C, with a pH of 6-7, for 1 hour.

After the chelation step, the mass is subjected to a bleaching step at a consistency of 20%, a temperature of 70 ° C, and for 2 hours.

To prepare the mass E use, in addition, the following parameters:

- an acid neutralization step of 3 hours between refining / ozone treatment and chelation with 20 kg of sodium hydroxide per tonne of mass (pH at the end of treatment 7.4);

- whitening composition:

- hydrogen peroxide: an amount sufficient for 30 kg / t of mass;

- sodium silicate: an amount sufficient for 30 kg / t of mass;

- sodium hydroxide: an amount sufficient for 22.5 kg / t of mass.

To prepare the mass F, in addition, the following parameters are used:

- an acid neutralization step of 3 hours between refining / ozone treatment and chelation with 9 kg of magnesium hydroxide per tonne of mass (pH at the end of treatment 7.3);

- a whitening composition identical to that used for mass E.

To prepare mass G, in addition, the following parameters are used:

- an acid neutralization step of 3 hours between refining / ozone treatment and chelation with 12 kg of magnesium hydroxide per tonne of mass (pH at the end of treatment 7.6);

- a whitening composition identical to that used for mass E.

To prepare the mass H, in addition, the following parameters are used:

- an acid neutralization step identical to that used for mass G (pH at the end of treatment 7.6);

- a bleaching composition identical to that used for mass E, with the exception of the amount of sodium hydroxide (5.6 kg per ton of mass instead of 22.5 kg), as well as the addition of 9 kg of magnesium hydroxide per ton of mass.

For each mass, the whiteness of the mass is measured at the end of the process, the yield loss relative to the source wood and the total chemical oxygen demand (COD) that has arisen. The results are presented in table 3.

Table 3 Parameters measured for E-H masses Weight White Loss of exit COD per ton of mass E 68.0% 15.9% 222 kg F 68.1% 15.1% 212 kg G 68.2% 14.6% 205 kg H 68.4% 12.8% 179 kg

A comparison between the mass of E, on the one hand, and the masses of F and G, on the other hand, reveals that the use of magnesium hydroxide instead of sodium hydroxide while reducing the amount of necessary chemicals by 55% or 40% can improve the wood yield in the process and is noticeably reduce COD, as well as slightly improve whiteness.

A comparison between the mass of G and the mass of H reveals that a decrease of about 75% in the mass of caustic soda used in bleaching, as well as replacing about 40% of it with magnesium hydroxide, makes it possible to even more noticeably improve all the parameters of interest, namely, the whiteness of the mass, yield and COD.

Example 3

Prepare pulp from spruce according to the TMP method. The mass is treated with ozone in the refining phase. More precisely, the waste from the primary refiner produces 2% of ozone, and the pH is not regulated; then the ozone treated wastes are mixed with the received fraction of the secondary refiner. The whiteness of the mass at the outlet from refining / ozone treatment is 51.2%.

After refining / ozone treatment, a chelation step is provided during which the mass is treated with 4 kg of DTPA per ton of mass, with a consistency of 2-3% and a temperature of 70 ° C, with a pH of 6-7, for 1 hour.

After the chelation step, the mass is subjected to a bleaching step at a consistency of 20%, a temperature of 70 ° C, and for 2 hours.

To prepare mass I (comparative example), in addition, the following parameters are used:

- an acid neutralization step lasting from 5 to 10 minutes between refining / ozone treatment and chelation with 10 kg of sodium hydroxide per tonne of mass;

- whitening composition:

- hydrogen peroxide: an amount sufficient for 30 kg / t of mass;

- sodium silicate: an amount sufficient for 30 kg / t of mass;

- sodium hydroxide: an amount sufficient for 27.5 kg / t of mass.

To prepare the mass J use, in addition, the following parameters:

- an acid neutralization step identical to that for mass I, with the difference that the duration is 3 hours;

- a bleaching composition identical to that used for mass I, with the difference that the amount of sodium hydroxide used is 22.5 kg / t mass.

To prepare mass K (comparative example), the same parameters are used as for mass I, with the difference that there is no acid neutralization step and that the amount of sodium hydroxide used in the bleaching composition is 37.5 kg / t mass.

To prepare mass L (comparative example), the same parameters are used as for mass J, with the difference that the duration of the acid neutralization step is only 30 minutes.

To prepare mass M, the same parameters are used as for mass J, with the difference that the duration of the acid neutralization step is 90 minutes.

For each mass, the whiteness of the mass is measured at the end of the process, the yield loss relative to the source wood and the total chemical oxygen demand (COD) that has arisen. The results are presented in table 4.

Table 4 Parameters measured for masses I-M Weight White Loss of exit COD per ton of mass I 72.4% 9.6% 134 kg J 73.0% 9.2% 128 kg K 70% 9.6% 134 kg L 71.2% 9.1% 127 kg M 72.7% 9.2% 128 kg

A comparison between mass I or mass K, on the one hand, and mass J or mass M, on the other hand, reveals that increasing the duration of the acid neutralization step sufficiently to obtain pH stabilization improves the final whiteness while reducing the amount of reagents, used at the bleaching stage. On the contrary, the experiment carried out with mass L, in comparison with experiments with masses J and M, demonstrates that with an equal number of reagents, a relatively short stage of acid neutralization (30 minutes) gives lower whiteness than a long stage of acid neutralization (90 minutes or more) at near output losses.

Claims (15)

1. A method of producing paper pulp, comprising: a wood refining step, comprising treating with ozone to obtain pulp, an acid neutralizing step comprising contacting the pulp with a neutralizing composition containing at least one alkaline reagent for a period of more than one hour, and the stage of bleaching, including bringing into contact the neutralized mass with the bleaching composition, while during the last 30 minutes of the stage of neutralizing the acid, the pH changes by less than 0.2 units, and the pH is It is from 6 to 10 at the exit from the deacidification step. 2. The method according to claim 1, wherein the step of neutralizing the acid has a duration of greater than or equal to 1 hour 30 minutes, preferably greater than or equal to 2 hours, ideally greater than or equal to 3 hours 3. The method according to claim 1 or 2, in which the pH at the outlet of the acid neutralization step is from 7 to 8. 4. The method according to claim 1, in which during the last 30 minutes of the acid neutralization step, the pH changes by less than 0.1 units, preferably less than 0.05 units. 5. The method according to claim 1, in which the alkaline reagent in a neutralizing composition: selected from oxides, hydroxides, silicates and carbonates of alkaline earth metals and alkali metals, ammonia, ammonia and mixtures thereof, preferably selected from potassium hydroxide, sodium hydroxide, magnesium hydroxide , calcium hydroxide, sodium carbonate, magnesium carbonate, sodium silicate and mixtures thereof, and ideally is magnesium hydroxide. 6. The method according to claim 1, in which the whitening composition contains a bleach and at least one alkaline reagent, wherein said alkaline reagent is selected from oxides, hydroxides, silicates and carbonates of alkaline earth metals and alkali metals, ammonia, ammonia and mixtures thereof, said the alkaline reagent is preferably selected from potassium hydroxide, sodium hydroxide, magnesium hydroxide, calcium hydroxide, sodium silicate, sodium carbonate, magnesium carbonate and mixtures thereof, and said alkaline reagent ideally contains sodium hydroxide I and / or magnesium hydroxide. 7. The method according to claim 1, comprising a chelation step, preferably between the acid neutralization step and the bleaching step, comprising contacting the neutralized mass with a chelating composition containing a chelating agent, preferably ethylenediaminetetraacetic acid or one of its sodium salts, or diethylene triamine pentaacetic acid or one of its sodium salts. 8. The method according to claim 1, in which at the stage of neutralizing the acid using 20 kg or less of an alkaline reagent, preferably 15 kg or less, more preferably 10 kg or less, per ton of wood pulp. 9. The method according to one of claims 1 to 8, in which at the bleaching stage use from 5 to 100 kg of bleach, preferably from 10 to 50 kg, more preferably from 20 to 40 kg, per ton of wood pulp, from 5 to 100 kg alkaline reagent, preferably from 10 to 70 kg, per ton of wood pulp. 10. The method according to claim 1, in which the loss of wood yield at the exit from the bleaching stage is less than or equal to 20%, preferably less than or equal to 16%, ideally less than or equal to 13%, even less than or equal to 10%. 11. The method according to claim 1, in which the whiteness of the mass at the exit from the bleaching stage is greater than or equal to 65%, preferably greater than or equal to 66.5%, ideally greater than or equal to 68%, even greater than or equal to 70%. 12. The method according to claim 1, in which effluents are produced whose chemical oxygen demand is less than or equal to 210 kg per ton of mass, preferably less than or equal to 180 kg per ton of mass, more preferably less than or equal to 160 kg per ton of mass, even less or equal to 145 kg per tonne of mass. 13. The method according to claim 1, in which the mechanical pulp is a mass of SGW, PGW, RMP, TMP, NTMP or CTMP. 14. The method according to claim 1, in which the wood is coniferous wood, hardwood or a mixture thereof, preferably coniferous wood, and more preferably pine or spruce wood. 15. A method of producing paper, including obtaining paper pulp according to one of claims 1 to 13, as well as the use of this paper pulp for paper production.

Images