FI96522B - Process for the preparation of bleached chemical thermomechanical masses - Google Patents

Abstract

Process for manufacturing bleached CTMP pulps comprising a treatment of the lignocellulosic stock with sulphite at a temperature of at least 100 DEG C under saturated steam pressure, and a bleaching operation with the aid of H2O2 in alkaline medium without removal of solid or liquid matter from the pulp from the start of the treatment until the end of the bleaching.

Description

9652 '

The invention relates to the production of bleached chemithermomechanical pulps.

Chemothermomechanical pulps, hereinafter referred to as CTMP pulps, means pulps obtained by treating 10 lignocellulosic substances, usually in the form of chips, with one or more chemical substances, heating and mechanical defibering.

CTMP pulps are of industrial interest because they are a special combination of actual mechanical and chemical pulps.

For example, their yield, the ratio of the weight of the dry pulp to the weight of the dry starting material, is generally more than 85%, in most cases at least 90%, it is already almost the same as for purely mechanical pulps.

In the above-mentioned combined heating, material treatment and defibering, the treatment with a chemical substance may be performed before, during or after the defibering.

By treatment or treatment is meant in this and the entire specification a step in which, in addition to the lignocellulosic material, a sulphite, practically sodium sulphite Na 2 SO 3, a bisulphite, practically sodium bisulphite NaHSO 3, or a mixture of sulfur dioxide SO 2 and sodium hydroxide at least 100 ° C, at saturated water vapor pressure. Hereinafter, we will refer to 35 said sulfites, bisulfites or mixtures as sulfites only. If necessary, conventional impregnation of the lignocellulosic pulp using a solution of selected reagents can be added to the treatment.

2 9652: The treatment is generally carried out below 200 ° C, preferably at about 120-160 ° C.

5 The initial pH of the treatment medium is 6-12.5.

The consistency, the weight content of the pulp as dry matter in the processing environment, can be 10-40, most often it is 15-30%.

10 The duration of treatment depends on other factors, but is usually less than 1 hour.

The sulphite content (as SO 2) can be about 0.1-10%, most often 0.5-3%, the percentages refer to the weight relative to the weight of the 15 dry lignocellulosic material as hereinafter all percentages expressed in connection with the substances unless otherwise stated.

Chemicals may be used concomitantly with sulfite 20, for example, complexing or sequestering agents such as diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA) used in the sodium salt form, about 0.1-1%.

In practice, the above-mentioned combined heating and grinding treatment generally involves two successive grinding steps in order to ensure that the pulp is industrially useful.

30 The production of CTMP pulps can be read, for example, in the book

James P. Casey, Pulp & Paper Chemistry & Chemical Technology, 3rd ed., Vol. 1, 1980, in particular pages 241-245, 213-219-229 or Pulp & Paper Manufacture, vol. 2, 1987, Mechanical Pulping, especially paragraphs VIII D and XI or 35 of U.S. Patent 4,718,980, especially Figure 1.

By bleached CTMP pulps we mean! L! IBlt 11 tl l i -S it i; 9652 '3-like CTMP pulps bleached using hydrogen peroxide in H2O2 alkaline environment.

In the known production of bleached CTMP pulps, prior to hydrogen peroxide bleaching, pulp 5 must be cleaned as thoroughly as possible of chemicals such as sulfite ions known to consume hydrogen peroxide H2O2 to bleach, as shown, for example, in H. Kruger, H.U. Suss, Tappi Proceedings, 1982 10 International Sulfite Pulping Conference, 143-148.

In practice, the pulp is washed thoroughly before bleaching and is traditionally done by diluting and reconcentrating the pulp, possibly several times.

15

Once the purified pulp has been graded, as is usually done, the consistency must be obtained by diluting to values as low as about 0.5-2%. You can read more about washing and classification, for example, in the already mentioned work of James P. Casey, 20 pp. 228-231, 363-365, 447-452 or also in the already mentioned source Pulp & Paper Manufacture, especially chapters XIII-XVIII.

Reconcentration of the bleaching pulp to a consistency of at least 25 to 10% from very low values is necessary with current techniques, not only to remove the substance or substances harmful to bleaching, but also to ensure the effectiveness of hydrogen peroxide.

Bleaching of the CTMP pulp using hydrogen peroxide in an alkaline environment is generally accomplished using about 0.5-10% hydrogen peroxide in the presence of about 1-6%, a sodium silicate solution having a density of 1.33 and a pH of about 9-11 at a temperature of about 40 -100 ° C, with a time of about 0.5-2 hours and a consistency of about 10-30%. The bleaching bath may contain additives, mainly one or more sequestering or complexing agents, e.g. DTPA, generally about 0.1-1%.

4 9652 '

After bleaching, it is preferred to treat the pulp with an acid, e.g. SO 2, to stabilize the whiteness, and then dilute with water to a very low concentration, e.g. to about 1%, to facilitate transport, storage and use in papermaking.

In the production of bleached CTMP pulps as just described, there is still much room for improvement in terms of economics and / or pollutant emissions in terms of papermaking: very much wastewater is generated, about 100 tonnes / tonne of paper, re-concentration before bleaching requires expensive equipment, pulp filter or press, 15 and yet it is not easy to obtain a suitable consistency in the soil with the best effect of hydrogen peroxide.

The effluents from the washing, grading and separation of water from the final dilution step before or during the production of paper-20, depending on whether this step is part of the pulp production steps, contain virtually no sulphite at all, but instead many other impurities. They are recycled, mainly used to wash the pulp to remove sulphite before hydrogen peroxide bleaching.

In the future, we will call these wastewaters zero water.

Although the pulp is washed with said industrial water, the degree of whiteness of the bleached CTMP pulp produced well is clearly lower than would be expected if demineralized pure water were used instead of recycled, but this is of course not possible for purely economic reasons.

The present invention avoids the disadvantages of the above-mentioned current industrial production of bleached CTMP pulps without deteriorating the whiteness of the pulps, on the contrary.

Il 'tft> l HB I M I »: ·! 35 9652: 5

The invention is based on the unexpected consideration that the degree of whiteness of the current industrial production can be achieved, even if no harmful substance or substances are removed from the pulp before bleaching for hydrogen peroxide bleaching.

The invention relates to a process for the preparation of bleached CTMP pulps, in which lignocellulosic pulp is treated with sulphite at a temperature of 100 ° C or more and saturated water vapor pressure, and hydrogen peroxide bleaching is carried out in an alkaline environment. The method is characterized in that no solids or liquid are removed from the pulp between the beginning of said treatment and the end of said bleaching.

15

All of the above also applies to the invention, except that after rubbing, before bleaching, the pulp is not washed, graded or reconcentrated. The treatment is performed in a purification step and a latency time of 5-30 minutes in the heat 20 exiting the purifier is sufficient to carry out the treatment.

Pure water vapor, for example from a defibering device, can be separated from the pulp after the start of the treatment before the bleaching is completed, for example in a known manner using cyclone.

We have found that the advantages of the invention come to light and are even more pronounced when the treatment uses a chemical that is more electronegative than sulfite ion SO 3, hereinafter referred to as a reducing agent, and is allowed to act at the same time as sulfite. It is therefore a preferred embodiment of the method according to the invention.

The reducing agent is selected from thiourea dioxide, i.e. formamidine sulphite acid, sodium hyposulphite, ie dithionite or sodium borohydride.

35 6 9652 '

The reducing agent is used in amounts that vary according to its nature. Generally, the former is used in an amount of about 0.1-5%, the third in an amount of about 0.01-5%, generally as aqueous solutions. For example, a solution containing 12% by weight of sodium borohydride marketed under the name Borol® (Ventron Corporation) can be used.

The invention is particularly well suited for use in the preparation of bleached CTMP pulps of very high whiteness, e.g. 80 ° and above, and currently uses a general General Electric or Elrepho type spectrophotometer with magnesium oxide as the standard reference at 457 nm. It is preferred to use about 0.5-3% sulfite (SO 2) 15, 3-10% hydrogen peroxide, most often 4-6%.

Another object of the invention is a method in which the bleached CTMP pulp is ground immediately after bleaching. Cleaning is performed at atmospheric pressure. The advantage described in U.S. Patent 4,718,980 for mechanical and thermomechanical pulps only now also applies to bleached CTMP pulps without thickening the pulp.

The invention therefore relates to a process for the preparation of bleached CTMP pulps as already described, characterized in that the bleached CTMP pulp is ground immediately after bleaching and no solid or liquid is removed from the pulp between the beginning of the treatment and the end of the post-bleaching grinding.

The classification can therefore be carried out on bleached and ground pulp. Thus, dilution with industrial water and recycling of the classification waste to the bleached pulp purifier is easily accomplished.

In the method according to the invention, in which the pulp does not have to be reheated, it is not necessary to use il from outside the system. iH i nti 11 f i 7 9652 'incoming thermal energy when only heat retention in the system is sufficiently guaranteed.

The use of the method according to the invention thus saves both mechanical and thermal energy compared to current techniques.

The invention is suitable for softer woods, conifers and harder hardwoods.

10

The various interrelated steps of the invention can be implemented in conventional equipment both structurally and functionally.

Figure 1 shows the preferred order for the different steps of the method according to the invention.

The diagram of Figure 2 illustrates an exemplary embodiment.

In Figure 1, wood chips, generally washed, leave the hopper 101 in a wood preheating chamber 102 heated by water vapor 103 and pass with the added reagents 104 to a refiner 105 and then to a cyclone 106 where the water vapor separates 107. After leaving the cyclone 106, the mass passes through the latent. it is thoroughly mixed with the bleaching agents 109 to be added, it is bleached in a bleaching tower 110. The pulp bleached from the tower 110 enters directly into an atmospheric refiner 111, from which it leaves for classification 112 after the addition of 30 zero water 113. Classification wastes of suitable consistency are purified separately (not shown in the diagram) or recirculated through 114 before grinding 111. After step 112, the pulp is acidified in a known manner and passed to storage 115 and / or papermaking 116.

The 200-beginning steps shown in Figure 2 correspond in structure and function to the 100-beginning steps shown in Figure 1. The new step 217 is a thickening device for reconcentrating the pulp between classification 212 and bleaching 210.

From step 217, the wastewater 218 is collected, that is, water classified as zero water 5, which is circulated through 213 into the pulp washing water.

The following examples are illustrative and not limiting of the invention and illustrate its advantages.

10

In the examples: amounts, as already mentioned, are expressed as percentages by weight relative to the amount of lignocellulose in the dry state, unless otherwise stated, 15 DTPA means 40% by weight of the sodium salt form In the form of Borol®, the amount is as shown, silicate means an aqueous solution of sodium silicate having a density of 1.33, and the degree of whiteness was measured using a Karl Zeiss-manufactured Elrepho-type spectrophotometer, magnesium oxide * {as standard reference, wavelength 457 nm.

Example 1

The softwood chips are ground under saturated water vapor pressure at 120 ° C, containing 2.75% Na 2 SO 3 to give a pulp with a whiteness of 57 °.

«I

This pulp is bleached without removing liquid or solid. H2O2: 5%, NaOH: 2%, silicate 35: 4%, DTPA: 0.5% are used, two hours at 90 ° C, consistency 15%.

The resulting bleached pulp has a degree of whiteness of 77.9 °.

il- nm 9652: 9

If the pulp is washed before bleaching under the above conditions by diluting 1.25% of the consistency using industrial water obtained from an industrial press concentrator to a consistency of 20% so that 95% of the sulfite is removed before adding bleach to the aqueous solution after bleaching at 77.4%. The figure is lower than in the method according to the invention.

When pure de-mineralized water is used instead of industrial water in a wash as described above, the degree of whiteness of the bleached pulp, which can be considered to be the highest possible under these bleaching conditions, is only 1.5 ° higher than that achieved according to the invention.

15

Example 2

A mechanical softwood pulp having a whiteness of 53.7 ° is treated at saturated water vapor pressure at 120 ° C with 2.75% NaaSO 3 and 0.5% DTPA for 30 minutes to a consistency of 20%. Prior to that, the pulp has been bleached directly without removing liquid or solid, as in Example 1.

The resulting bleached pulp has a degree of whiteness of 78.8 °.

If the pulp is bleached after washing with industrial water, as in Example 1, it has a degree of whiteness of 78.6 ° and can be even weaker, at most as good as when using the process according to the invention.

30

Example 3

Proceed as in Example 2, except for the comparative method, except that a 1% BoroJ® solution is used with the sulphite.

When delivered according to the invention, the degree of whiteness of the bleached pulp is 82.2 °, but in the comparative test only 80.9 °.

35 10 96Γ? '

When pure demineralized water is used in the latter instead of zero water, the degree of whiteness of the bleached pulp is only 1 ° higher than when using the process according to the invention.

Il: m-.i imi m «i '

Claims (10)

9662 ' A process for the preparation of bleached core thermomechanical pulps, which comprises treating lignocellulosic pulp in the presence of sulphite at a temperature of at least 100 ° C with saturated water vapor and bleaching with hydrogen peroxide in an alkaline environment, characterized in that no solid or liquid is removed during the treatment and bleaching. Process according to Claim 1, characterized in that 0.5 to 3% of sulphite, expressed as sulfur dioxide, is used. Process according to Claim 1 or 2, characterized in that a substance more electronegative than the sulphite ion is present in the treatment at the same time as the sulphite. Process according to Claim 3, characterized in that the substance more electronegative than the sulphite ion is selected from the group consisting of thiourea dioxide, sodium borohydride and sodium dithionite. Process according to Claim 4, characterized in that the amount of thiourea dioxide or sodium dithionite is 0.1 to 5%. Process according to Claim 4, characterized in that the amount of sodium borohydride is 0.01 to 0.5%. 30 Process according to one of Claims 1 to 6, characterized in that the starting pH of the treatment is 6 to 12.5. Process according to one of Claims 1 to 7, characterized in that the amount of hydrogen peroxide used in the bleaching is from 3 to 10%. p. Γ O «'> to, Process according to Claim 8, characterized in that the amount of hydrogen peroxide used is 4 to 6%. Method according to one of Claims 1 to 9, characterized in that the pulp is ground immediately after bleaching without removing solid or liquid from the pulp between the end of bleaching and the end of grinding after bleaching. 10 Patentkrav