FI87244C - Procedure for bleaching mechanical pulp - Google Patents

Description

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The method of mechanical pulp or I guess semi sex

The present invention relates to a process for the production of mechanical pulps or, I guess, semi, and more particularly to a process for the production of mechanical pulps or supposedly semi using hydrogen peroxide. In practice, the term "mechanical pulps" means pulps which are said to have a high yield, in terms of dry mass, the weight of the original dry wood generally being greater than 90% by weight, such as suitably so-called mechanical pulps, thermomechanical pulps, chemimechanical pulps or core thermomechanical pulps produced by grinding the wood chips part, for example by means of a grinding stone or a plate grinder, with or without chemical pre-treatment, for example by means of steam, sodium hydroxide or sodium sulphite.

Bleaching of mechanical pulps involves changing the color of the Chromophoric groups of wood constituents by a chemical reaction without bringing these constituents into a soluble form. The chromophoric groups that cause the brown color of the pulp are mostly contained in lignin and some extracts (tannin i ni i t).

Today, two types of methods are used to effect this color change.

The first involves the reaction of the pulp with a reducing agent, typically hydrogen sulfite, under mild conditions and in a neutral or weakly acidic medium. The reduction of the chemical groups responsible for the dye causes partial bleaching, which, however, is not sufficient for some purposes.

Another method involves the oxidation of color-causing groups with hydrogen peroxide in an alkaline medium. The bleaching obtained is higher than in the previous method, which 2 87244 explains the fact that the peroxide method is now increasingly used to meet paper quality requirements.

However, in order to achieve height or leaching levels, it is necessary to use large amounts of hydrogen peroxide, which can greatly exceed 5% by weight of pure product relative to the pulp, which reduces the profitability of the process. This justifies the intensive research work started in recent years to optimize the conditions of use of this reagent.

Thus, French Patent Application No. 2,537,177 describes a process with or without semi-hydrogen peroxide, in which unused residual peroxide, which can account for up to 20% of the peroxide fed to the bleaching process, is recovered in part by intensive pulping by dry pulping, and recycled upstream. This method causes the whiteness to increase by several degrees with a given correct hydrogen peroxide consumption.

Recycling of unused hydrogen peroxide is described, for example, in J. Hook Pulp and Paper International Vol. 17 No. 6, p. 45, etc. (1975). Current processes allow such recycling to be performed under suitable conditions.

However, although processes using recycling have been developed, very little research has been devoted to the problem of actual hydrogen peroxide consumption during a bleaching operation. However, this is the factor that determines the profitability of the bleaching process in such a system and not the amount of peroxide initially charged. It has become necessary to determine the conditions of use of the peroxide which minimize its actual consumption in bleaching.

I.

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Without addressing the problem of peroxide consumption, the French patent, published No. 2,467,261, discloses that the reaction of hydrogen peroxide in an acidic medium with lignose components results in the simultaneous treatment of lignin and cellulose.

By following the process of the invention described in this patent, which comprises subjecting the cellulosic ingredients to treatment with hydrogen peroxide 11a in an acidic medium in the presence of a predetermined amount of coagulant, performing alkaline extraction immediately after this treatment, the treatment can be relatively well controlled.

This is not the case with lignin treatment. In fact, referring again to the said patent, the process described results in a profound change in lignin during the acid treatment, making it highly soluble in a basic medium.

The use of such a method, which is well known to cause the removal of lignin, which is said to be considerable, in the bleaching of mechanical pulps, which in turn is characterized by the highest possible ingredient yield, is inconceivable.

The process according to the invention makes it possible to achieve better bleaching of mechanical pulps with hydrogen peroxide while maintaining the yield of the substance and greatly reducing the amount of hydrogen peroxide actually consumed.

This process is characterized by the use of mechanical pulps using hydrogen peroxide in which the mechanical pulp is reacted with hydrogen peroxide in an acidic medium without a coagulant and then bleached using hydrogen peroxide in a basic medium in the presence of at least one hydrogen peroxide stabilizer.

The treatment in an acidic medium is generally carried out at a pH of at least 2.

The acidity of the medium may be derived from the acidity of the hydrogen peroxide itself or, more usually, due to the presence of an inorganic or organic acid in the medium which is practically inert to the hydrogen peroxide, such as sulfuric acid.

The total amount of hydrogen peroxide used is generally between 0.5% and 12% by weight of the absolute dry mass, more usually more than 3%.

The amount of hydrogen peroxide used in the treatment in the acidic medium may be the total amount of hydrogen peroxide used in the process. Preferably, it often represents a small fraction of this total, for example 10% or less, in which case the remainder is added at the same time as the other reagents in the basic medium for bleaching.

In the basic medium step, basicity is achieved by adding at the end of the acidic medium step compounds normally used for bleaching with peroxide in a basic medium such as sodium hydroxide α and sodium silicate, each used in the same amount as usual, for example 0.5 to 6% by absolute dry weight. The pH of the basic step is generally between 8 and 12. Conventional coagulants such as most commonly diethylenetriaminepentaacetic acid (DTPA) and ethylenediaminetetraacetic acid (EDTA) may also be added to the basic step in amounts commonly used in pulp bleaching, e.g. 05 to 0.5% by weight of absolutely dry mass.

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Hydrogen peroxide stabilizers other than sodium silicate may be added to the basic step, such as magnesium salts, most commonly magnesium sulfate, in an amount commonly used for pulp casting with hydrogen peroxide in a basic medium, for example 0.05 to 0.5% by absolute dry weight.

The temperature at which the treatment in the acidic medium is carried out is generally between about 25 ° C and 90 ° C. The temperature at which the bleaching in the basic medium is carried out is generally between about 40 ° C and 90 ° C. It may be advantageous to operate at the same temperature in both steps.

The duration of the process can vary within relatively wide limits, most commonly from 2 hours to 6 hours, with the duration of the acidic medium phase often possibly being 5-10 times shorter than that of the basic medium phase. The consistency of the pulp may be similar or vary throughout the process, preferably being substantially constant throughout the duration of the process, and most often being between 10% and 25% in the acidic phase as well as in the basic phase.

It is obviously particularly advantageous to recycle unused hydrogen peroxide in the initial stage, at appropriate points in the acidic stage, using known methods.

The following examples, which are not intended to limit the invention and which only illustrate a particular embodiment of the invention, illustrate the utility of the method of the invention.

Example 1 (comparative)

The six pulp-milled mechanical pulps with a whiteness of 59% were bleached under the conditions normally used in industry, namely at a consistency of 15%, 6 87244 o 55 C for 4 hours, in the presence of 0.25% chelating DTPA and 4% sodium silicate. hydrogen peroxide a as a stabilizing agent as a percentage by weight of the absolutely dry mass. The amounts of sodium hydroxide of hydrogen peroxide as a percentage by weight of the absolutely dry mass were set at several levels so as to obtain different degrees of variability, as shown in Table 1. After this treatment, the consumption of hydrogen peroxide as a percentage by weight of the absolutely dry mass was determined by measuring the amount of residual peroxide in the cooking solution using a conventional iodimetric method. Degrees of potency were measured with an Elrepho-1 device marketed by Karl Zeiss (GDR) according to a standard in force in the wood processing industry (Afnor standard NF Q-03 039). The results obtained are summarized in Table 1.

Example 2

The 15% consistency pulp-milled mechanical pulp used in Example 1 was treated with an acid solution of hydrogen peroxide containing 0.5% sulfuric acid as a percentage by weight of absolutely dry pulp, 55 ° C for 30 minutes, and then as in Example 1. The amount of hydrogen peroxide was 0, 2% by weight of absolutely dry mass. After this acid treatment step and in order to simulate an operation on an industrial scale at a substantially concentrated concentration, part of the acidic cooking solution was removed, while the products of the basic or optional cooking step were added in amounts equal to those shown in Example 1. The bleaching results are shown in Table 1. They show that in the three cases studied, the amount of actual peroxide consumption has been significantly reduced as a result of the acid pretreatment of the pulp.

Example 3 (comparative)

The milled mechanical pulp used in Example 1 was bleached until a degree of solubility of 83% was reached by 87244 7 by adding an amount of hydrogen peroxide corresponding to 10% by absolute weight of the dry pulp. Other conditions are given in Table 2. The amount of peroxide consumed is 6.5% by weight of peroxide relative to the weight of the absolutely dry mass.

Example 4

Example 3 was repeated for the acid treatment with 0.5 -X sulfuric acid and varying amounts of H o before the bleaching treatment, these amounts being expressed as the weight of pure product to absolutely dry mass. The base treatment of the bleaching was performed under the conditions of Example 3, followed by the treatment of Example 2. In a further experiment, the acid was omitted when the acidity of the medium alone was derived from the acidity of hydrogen peroxide. The results in Table 2 show that hydrogen peroxide consumption decreases with pretreatment with peroxide 11a. The effect persists when no acid is added to increase the natural acidity of hydrogen peroxide.

Example 5

Example 3 was repeated, whereby the bleaching treatment was preceded by acid treatment steps with 0.5% H 2 O in the presence of 0.5% H 0 2 4 2 2, these amounts being expressed as the weight of pure product relative to the weight of the absolutely dry mass.

The temperature of the acid treatment step was maintained at three levels, oo o 30 C, 55 C and 70 C. Alkaline or supernatant or they were performed as in Example 3. The results obtained are given in Table 3.

Example 6 In this example, the acid treatment step was performed with 0.5% ^ 2 ^ 4 '^ J ϊ: η ^ 2 ^ 2' as a percentage of the weight of the absolutely dry mass, under the conditions described in Example 2. The basic or supernatant was then carried out under the conditions of Example 3, but without the addition of hydrogen. After this treatment, the degree of concentration o was 83.1 and the consumption of hydrogen peroxide was 6.1% by weight of the pure product relative to absolute! to the weight of the dry mass.

These examples show that the method of the present invention allows substantial reagent savings to be achieved. The consumption of hydrogen peroxide in the process according to the invention may be 25-30 ¾ less than the consumption of hydrogen peroxide in a conventional bleaching process.

They also show that the high degrees of potency thus obtained are achieved without loss of yield.

table 1

Hydrogen peroxide of the ground mechanical pulp of the grinding stone 1 k a i s u

Example 1 1 2 2

Acid phase% H 0 - - 0,2 0,2 2 2% H SO - - 0,5 0,5 2 2o Temperature C 55 55

Duration, min - - 30 30

Base phase% H 0 3 5 3 5 2 2% NaOH 1.7 2.5 1.8 2.6 Temperature ° C 55 55 55 55

Duration, hour 4 4 4 4

Consumed HO% 1.7 2.7 1.3 2.1 2 2

Ority%% 78.1 81.1 78.6 81.3

Yield% 96.4 96.0 96.5 96.6

Other conditions: pulp consistency 15%, base phase carried out in the presence of 4% 380 g of sodium silicate and in the presence of 0.25% DTPA at an active substance content of 40%.

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Table 2

Hydrogen peroxide bleaching of mechanical pulp ground to high or coarse grains

Example 3 4 4 4 4 4

Acid phase% H 0 0.1 0.2 0.5 0 0.5 2 2% H SO - 0.5 0.5 0.5 0.5 0 2 2o Temperature C 55 55 55 55 55

Duration, min - 30 30 30 30 30

Base phase% H 0 10 10 10 10 10 10 2 2% NaOH 555 55 5 o Temperature C 55 55 55 55 55 55

Duration, hour 444 44 4

Consumed HO% 6.5 6.1 5.8 4.6 6.3 5.2 2 2

Degree of whiteness% 83.1 83.2 84.3 83.6 82.9 84.0

Yield% 95.8 96.1 96.3 96.0 96.0 95.7

Other conditions: pulp consistency 15%, base phase is performed in the presence of 5% 38 Be sodium silicate and 0.25% DTPAr at an active substance content of 40%.

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Table 3

Effect of acid treatment step temperature on hydrogen peroxide bleaching result

Example 55 5

Acidic acid% H 0 0.5 0.5 0.5 2 2% H SO 0.5 0.5 0.5 2 2 o Temperature C 30 55 70

Duration, min 30 30 30 E svai he% H 0 10 10 10 2 2% NaOH 55 5

Duration, hour 44 4

Consumed HO% 5.2 4.6 4.4 2 2

Ority rate% 83.2 83.6 83.6

Other conditions: pulp consistency 15%, base phase carried out in the presence of 5% 38 Be sodium silicate and 0.25% DTPA at an active substance content of 40%.

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Claims (10)

A process for the production of mechanical pulp using semi-hydrogen peroxide, characterized in that the mechanical pulp is reacted with hydrogen peroxide in an acidic medium without a complexing agent and then bleached using hydrogen peroxide in a basic medium in the presence of at least one hydrogen peroxide stabilizing agent. Process according to Claim 1, characterized in that the total amount of hydrogen peroxide is added at the beginning of the treatment. Process according to Claim 1, characterized in that the amount of hydrogen peroxide n added at the beginning of the treatment is only a fraction of the total amount of hydrogen peroxide. Process according to Claim 3, characterized in that the amount of hydrogen peroxide added at the beginning of the treatment is less than or equal to 10% of the total amount of hydrogen peroxide. Process according to one of Claims 1 to 4, characterized in that the total amount of hydrogen peroxide is 0.5 to 12% by weight relative to the weight of the absolutely dry mass. Process according to Claim 5, characterized in that the total amount of hydrogen peroxide is greater than 3% by weight relative to the weight of the absolutely dry mass. Process according to one of Claims 1 to 6, characterized in that the temperature in the acid phase is between about 25 ° C and 90 ° C. Process according to one of Claims 1 to 7, characterized in that the pH in the acid step is greater than or equal to 2. Process according to one of Claims 1 to 8, characterized in that the temperature in the base phase is between about 40 ° C and 90 ° C. Process according to one of Claims 1 to 9, characterized in that the pH in the base step is between 8 and 12.