EP0313478A1 - Process for bleaching pulps - Google Patents

Abstract

Bleaching of mechanical pulps to high degrees of brightness with the aid of H2O2 in alkaline medium in two successive stages separated by a washing, in which no use is made of sodium silicate or of a compound of the same effectiveness by using a quantity of hydrogen peroxide of between 0.5 % and 3 % in the first stage and a larger quantity of hydrogen peroxide of between 2 % and 5 % in the second stage.

Description

The present invention relates to bleaching using hydrogen peroxide from mechanical pulps, thermomechanical chemomechanics, or chemothermothermomechanics in the paper industry and related to high degrees of white.

These pastes are designated in all that follows by the expression pasta of mechanical origin.

They are obtained in industry most often from wood generally in the form of shavings, by mechanical defibration of the lignocellulosic material, for example in a grindstone shredder or a shredder or disc refiner, associated or not, according to the paste referred, to a preliminary treatment using water vapor and / or a chemical reagent such as sodium sulfite.

The dry matter yield of such an operation is always very high and can reach and even often exceed 90%.

Pasta of mechanical origin, also called high-yield pasta obtained in 1 ecru state, must be bleached efficiently and with a minimum loss of material so that the manufacture of the products derived from it can meet the requirements of quality and economy in the industry.

It is known practice to whiten pulps of mechanical origin using hydrogen peroxide in an alkaline medium.

Such bleaching is more and more widely adopted by the industry for reasons of efficiency but also of reduction of the polluting load.

The industry is making constant efforts to improve such a technique.

For example D. LACHENAL, C. de CHOUDENS and L. BOURSON, Tappi Proceedings of the 1986 Pulping Conference, 569-573, propose a two-step process in which it is possible to dispense with the presence of sodium silicate in the second step but not in the first.

In fact, in the bleaching of pasta using hydrogen peroxide in an alkaline medium considered globally, it is recognized that sodium silicate is essential for obtaining the best results in terms of bleaching as well as that of the economy. This is what is recognized permanently for example by WG STRUNK, Pulp and Paper, June 1980, 156-141, RW ALLISON, Appita, Vol.36, ^No. 5, March 1983, 362-370, CW Kutney, Pulp and Paper Canada, 86:12, (1985), 182-189, and finally confirmed by CWDENCE, S. OMORI, Tappi journal, October 1986, 120-125.

French Patent ^No. 2537177 discloses a two-step process at least, wherein a consistency condition is ensured throughout the laundering. He presents the absence of silicate as a possibility, but he does not provide any concrete information as to the impact of this absence on the degree of white achieved.

Today it is still known that diethylenetriaminepentamethylenephosphonic acid (DTMPA) is capable of behaving like sodium silicate. According to R.W. ALLISON, article cited, this equivalence is verified when a mechanical pulp of a particular species of wood is bleached under well-controlled conventional conditions, but is no longer verified when the same pulp is bleached in a refiner. Using DTMPA is not economical anyway.

Therefore, it should not normally be expected from a process using in any way a silicate, commonly sodium silicate, that a disadvantageous result compared to that obtained by a process involving sodium silicate or a compound capable to behave like sodium silicate.

The method of the invention does not appeal to either of them and nevertheless leads, thanks to a particular choice of bleaching conditions, to a paste of white degree of the same level as that authorized by their intervention. . The industry thus has, thanks to the process of the invention, a pulp of bleached mechanical origin obtained economically and which does not have any of the drawbacks, such as those mentioned for example in the article by GW KUTNEY cited above, linked to the still substantial presence of sodium silicate.

The process of the invention is a process for bleaching doughs of mechanical origin to degrees of white high, using hydrogen peroxide in an alkaline medium in two consecutive steps separated by washing and carried out in the absence of sodium silicate or a similarly effective compound, characterized in that the amount of peroxide hydrogen used in the first step is between 0.5% and 3%, the amount of hydrogen peroxide used in the second step is greater than that used in the first step and is between 2% and 5%.

In this definition of the invention as in all that follows, unless specified or obvious, the quantities are expressed in percent by weight relative to the dough in the dry state.

By effectiveness of a compound compared to that of sodium silicate is meant here that formulated as in the publication by C.W. KUTNEY already cited.

Apart from the particular features of the process of the invention, bleaching is carried out in two stages under the usual conditions. Whitening evolves according to variations in the main parameters as indicated in the article by C.W. DENCE, S. OMORI already cited.

The alkalinity in the two stages is reached thanks, most frequently, to the use of sodium hydroxide and the quantity of alkaline agent employed in each of the two stages is that necessary to ensure preferably in one as in the other of these steps has an initial pH between about 10.5 and 12.

The preferred sodium hydroxide / hydrogen peroxide weight ratio in the first step as in the second step is between about 0.6 and 1.2.

The temperature in the two stages can be the same or different. It is most generally between about 40 ° C and 70 ° C, most often close to 60 ° C-70 ° C.

The consistency, concentration in weight percent of pulp in the dry state in the bleaching medium, is generally between 10% and 30%, often between 10% and 20%.

In the first as in the second step are advantageously present the compounds ordinarily associated with sodium silicate but which have, however, only an effectiveness in whitening much lower than his, for example diethylenetriaminepentaacetic acid (DTPA) or magnesium presented for example in the form of soluble salt such as sulfate.

These products are used in an amount similar to that which usually accompanies sodium silicate, for example 0.1% to 1%, most often about 0.25% to 0.5% of a 40% aqueous solution. by weight of sodium salt of DTPA less than 0.1% and often 0.01% and 0.05% magnesium.

The duration of each of the two stages depends on the choice of the other parameters adopted. When these are within the preferred ranges, the duration of the first stage is generally between 0.5 and 2 hours, that of the second stage between approximately 2 and 4 hours.

By washing between the two stages is meant here the operation which makes it possible to eliminate part of the liquid which is in the dough at the end of the first stage.

This washing is carried out in a known manner, for example by pressing the dough associated or not with its dilution, generally by water.

A washing efficiency, or elimination rate of the liquid, equal to or greater than about 80% ensures obtaining a paste with a high degree of whiteness at the end of the second step but a lower efficiency, for example equal to About 50% is still sufficient for this purpose.

The following examples are given as an indication but not limiting to illustrate the invention in its preferred embodiment.

In these examples:
- the quantities of material are expressed as already indicated, in% by weight relative to the dough in the dry state,
- DTPA is used in the form of the sodium salt in solution at 40% by weight and the quantity indicated is that of this solution,
- by silicate is designated an aqueous sodium silicate solution at 35% by weight and the quantity indicated is that of this solution,
the magnesium is used in the form of a soluble salt, in this case the sulfate, but expressed in Mg,
- H₂O₂ hydrogen peroxide is counted as 100%,
- the washing efficiency, which is 80% or more unless otherwise specified, is the same in the case of a comparison as in the illustration of the invention with respect to which the comparison is made.
- the degree of white is measured (457 nm) in% using a CARL ZEISS spectrophotometer of the ELREPHO type.

Example 1:

A mechanical pulp of a softwood wheel with a white degree equal to 57.2% is subjected in a first step, to a consistency equal to 15%, for 1.5 hours, at a temperature of 60 ° C., at simultaneous action of the following compounds: NaOH: 1.5%, H₂O₂: 2%, DTPA: 0.5% then washed before being subjected in a second step, for 3 hours, to a consistency and a temperature still equal respectively at 15% and 60 ° C, with the simultaneous action of the following compounds: NaOH: 3%, H₂O₂: 2.5%, Mg: 0.05%.

The degree of whiteness of the dough obtained at the end of the second stage is equal to 81.1% while it was only equal to 71.5% at the end of the first stage.

By way of comparison, Example 1 is repeated, using 4% silicate in the first step. The degree of white of the paste finally obtained is equal to 81.5%, therefore very close to that obtained when the silicate is omitted, although at the end of the first stage the degree of white was however this time equal to 76%. .

Example 2:

The unbleached pulp of Example 1 is bleached as in Example 1, but using 3.4% H₂O₂ in the second step instead of 2.5%.

By operating according to the invention, the degree of whiteness of the paste obtained at the end of the first step is equal to only 72% while that of the paste obtained at the end of the second step rises at 82%.

By operating, by way of comparison, as above but in the presence, in the first step, of an amount of silicate equal to 1% which provides a degree of whiteness equal to 72.3% at the end of this first step, the degree of whiteness of the dough at the end of the second step is only 79.3%.

In the illustration of the invention given above, the washing efficiency between the two stages is equal to 95%. The degree of white indicated for the paste at the end of the second stage is preserved by repeating this illustration with a washing efficiency between the two stages equal to both 50% and 80%.

Example 3:

A mechanical softwood wood pulp with a white degree equal to 53.7% is subjected in a first step, to a consistency of 10%, for 1 hour, at a temperature of 60 ° C, to the simultaneous action of NaOH: 0.7%, H₂O₂: 0.85%, DTPA: 0.5% before being washed and subjected in a second step, for 4 hours, at a temperature equal to 60 ° C. and at a consistency equal to 20%, to the action of NaOH: 2.7%, H₂O₂: 3.4%, Mg: 0.03%.

The degree of whiteness of the dough at the end of the second stage reaches 80.9%.

Example 4:

A chemothermothermal mechanical pulp of coniferous wood with a white degree equal to 59.6% is treated in a first step using NaOH: 1%, H₂O₂: 1.2%, DTPA: 0.5%, during 1, 5 hours at a temperature equal to 60 ° C and a consistency equal to 15%, before being washed and being treated in a second step using NaOH: 2.9%, H₂O₂: 3.4 %, Mg: 0.05% for 3 hours at a temperature equal to 60 ° C and a consistency equal to 15%.

The paste from the second stage has a degree of whiteness equal to 80.5%.

By way of comparison, the same chemothermal-mechanical paste as above is treated as above but the DTMPA replaces in nature and in quantity the DTPA. The amount of DTMPA thus engaged should be equivalent to the presence of around 4 to 5% of silicate and therefore lead to a result greater than that achieved in its absence. However, the degree of whiteness observed in reality at the end of the second stage is only 79.6%.

Claims (10)

1. Process for bleaching pulp of mechanical origin to high degrees of white, using hydrogen peroxide in an alkaline medium in two consecutive stages, separated by washing and carried out in the absence of sodium silicate or a similarly effective compound, characterized in that the amount of hydrogen peroxide used in the first step is between 0.5% and 3% by weight relative to the dough in the dry state, the amount of hydrogen peroxide used in the second step is greater than that used in the first step and is between 2% and 5% by weight relative to the dough in the dry state. 2. Method according to claim 1, characterized in that the initial pH in each of the steps is between 10.5 and 12.0. 3. Method according to claim 2, characterized in that the initial pH is reached using sodium hydroxide. 4. Method according to claim 3, characterized in that the weight ratio sodium hydroxide / hydrogen peroxide in the two stages is between 0.6 and 1.2. 5. Method according to any one of claims 1 to 4, characterized in that the temperature in the two stages is between 40 ° C and 70 ° C. 6. Method according to any one of claims 1 to 5, characterized in that compounds with a bleaching efficiency lower than that of sodium silicate are present in one or in the two stages. 7. Method according to claim 6, characterized in that the compound is diethylenetriaminepentaacetic acid in the form of sodium salt in the first phase. 8. Method according to claim 6, characterized in that the compound is a soluble salt of magnesium. 9. Method according to any one of claims 1 to 8, characterized in that the washing efficiency between the two stages is equal to or greater than 50%. 10. Method according to claim 9, characterized in that the washing efficiency is equal to or greater than 80%.