EP2931968B1 - Use of sodium dithionite in a cellulose pulping process - Google Patents

Description

The present invention relates to a process for producing cellulose from lignocellulosic material by means of sulfite digestion or sulfate digestion as defined in the claims.

The processes for recovering cellulose (herein and also referred to in the art as "pulp") from lignocellulosic material, such as wood, are known and, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 17, "Paper, Fiber Raw Materials", pp. 531-576, Verlag Chemie Weinheim, New York (1979 ). Usually, the cellulose is obtained by chemical digestion processes from the lignocellulosic material, such as wood. Such chemical digestion methods are, for example, the sulphite digestion used for this purpose or the likewise known sulfate digestion. Sulfite digestion and sulfate digestion are described, for example, in the Ullmann literature cited above.

Put simply, lignocellulosic material in both of the above processes is treated as follows to recover cellulose.

In the sulfite process (also referred to herein as "sulfite digestion") lignocellulosic material, usually wood, in an acidic or neutral medium in the presence of sulfites (salts of sulfuric acid H ₂ SO ₃ ) treated with a so-called cooking acid, said Lignin is usually sulfonated and water soluble and thus can be removed from the fibers leaving the pulp.

DE-C-912 169 describes the boiling of spruce wood chips with sulphate lye. The sulfate liquor may contain sodium dithionite (Na ₂ S ₂ O ₄ ). The wood chips are mixed with the sulphate liquor and boiled. This means that the sodium dithionite is present in the cooking liquor from the beginning and is heated with this.

US-A-2012/0168102 relates to the pretreatment of lignocellulosic material by water extraction and subsequent impregnation with oxidizing or reducing agents ("reducing agent") before the lignocellulosic material thus treated is fed into a "kraft pulping process", or sulphate digestion. An "alkali metal dithionite (hydrosulfite)" can be used as a "reducing agent".

1. a) Der saure Bisulfit-Aufschluss mit Magnesiumdihydrogensulfit (im Folgenden auch "Mg(HSO₃)₂") und Schwefeldioxid, SO₂ sowie Wasser als Komponente der Kochsäure,
2. b) der Bisulfit-Aufschluss mit Mg(HSO₃)₂, als Komponente der Kochsäure,
3. c) der Neutralsulfit-Aufschluss mit Dinatriumsulfit (im Folgenden auch "Na₂SO₃") und Natriumcarbonat (im Folgenden auch "Na₂CO₃"), als Komponenten der Kochsäure und
4. d) der Alkalisulfit-Aufschluss mit Na₂SO₃ und Natriumhydroxid (im Folgenden auch "NaOH") sowie Wasser als Komponenten der Kochsäure.

There are various process types of sulfite digestion, which differ, inter alia, by the pH of their cooking solution. These are for example:

1. a) The acid bisulfite digestion with magnesium dihydrogen sulfite (hereinafter also "Mg (HSO ₃ ) ₂ ") and sulfur dioxide, SO ₂ and water as a component of the cooking acid,
2. b) the bisulfite digestion with Mg (HSO ₃ ) ₂ , as a component of the cooking acid,
3. c) the neutral sulfite digestion with disodium sulfite (hereinafter also "Na ₂ SO ₃ ") and sodium carbonate (hereinafter also "Na ₂ CO ₃ "), as components of the cooking acid and
4. d) the alkali sulfite digestion with Na ₂ SO ₃ and sodium hydroxide (hereinafter also "NaOH") and water as components of the cooking acid.

As a rule, in the case of acidic bisulphite digestion, magnesium in the form of magnesium oxide (MgO) is used as base which is then converted to dihydrogen sulphite. Instead of magnesium (Mg), acidic bisulfite digestion can also use calcium (Ca), sodium (Na) or ammonium (NH ₄ ⁺ ) as the base for the cooking solution, which are then used as corresponding oxides or hydroxides analogously to magnesium. Likewise, these metals can usually be used analogously, except for calcium, also in bisulfite digestion.

In the sulfite process today, the acidic magnesium bisulfite process is the most commonly used.

For the sulfite process come as a lignocellulosic material usually conifers such as spruce wood, also fir wood and the wood of Hemlocktanne in question. In addition, some hardwood species such as beech, poplar and birch are also suitable. Spruce wood is preferred in the sulfite process.

The above-mentioned different types of processes of the sulfite process usually operate at pressures in the range of 0.1 to 10 bar and generally in certain ranges of pH. The acidic bisulfite digestion a) usually at a pH in the range from 2 to 3, the bisulfite digestion b) usually at a pH in the range from 3 to 5, the neutral sulfite digestion c) usually at a pH Value in the range of 6 to 9 and the alkali sulfite digestion d) usually at a pH around 11.

Depending on the pH range, the digestion temperatures differ in the sulfite process. Thus, the temperature range of the acidic bisulfite pulping a) is usually at 120 to 150 ° C, the bisulfite pulping b) usually at 150 to 160 ° C and the sodium sulfite pulping c) or also of the Alkalisulfite digestion d) usually at 160 to 180 ° C.

The cooking acid of the sulfite process usually contains so-called free sulfur dioxide (SO ₂ ), which is present as SO ₂ and sulfurous acid (hereinafter also "H ₂ SO ₃ ") and bound SO ₂ , which is bound to a cation (base). Free SO ₂ and bound SO ₂ are usually given as total SO ₂ . The cooking acid of the sulfite process is generally composed as follows:

M ₂ SO ₃ + H ₂ SO ₃ + SO ₂ + H ₂ O,

of these, the H ₂ SO ₃ and the SO _{2 are} assigned to the free SO ₂ and the M ₂ SO _{3 to} the bound SO ₂ . M here is the respective so-called base, example magnesium.

The proportion of the base and the SO ₂ in the cooking acid is given in percent by weight. For example, a cooking acid with a total SO ₂ content of 80 g per liter contains 8% total SO ₂ . The base portion is represented as its respective oxide form, such as MgO, CaO, Na ₂ O.

The preparation of a cooking acid is usually carried out by absorption of SO ₂ to water and base carrier. The following equation is intended to serve as an example of the principle of cooking acid production of bisulfite digestion with magnesium as base (magnesium bisulfite).

Mg (OH) ₂ + 2SO ₂ → Mg (HSO ₃ ) ₂

In the sulphate process (also referred to as "power digestion" or "sulphate digestion" in the art) is cellulose usually from lignocellulosic material, for example, the wood of trees or from annuals, such as reeds, cereals (straw), Sugarcane (bagasse), corn won.

In the sulfate process, wood chips of the lignocellulose-containing material, for example wood or comminuted plant stems, are usually heated in pressure vessels for several hours, for example 3 to 6 hours, at elevated pressure, for example in the range from 7 to 10 bar, usually in a mixture of sodium hydroxide solution ( aqueous NaOH), sodium sulfide (Na ₂ S), and sodium sulfate (hereinafter also "Na ₂ SO ₄ ") and optionally sodium carbonate (hereinafter also "Na ₂ CO ₃ ").

This results in the so-called "black liquor" (soluble alkali lignin), which is separated by filtration from the pulp.

Cellulose can be separated from lignin with the abovementioned sulfite and sulfate processes, although it is still desirable to increase the pulp yield, in particular with a simultaneous low lignin content of the pulp.

It is known that, for example, in the pulping of wood to pulp there is a connection between the so-called "degree of pulping", expressed for example by the "kappa number", and the pulp yield.

The kappa number is a measure of the lignin content of the pulp.

A very low kappa number, ie very low lignin content in the pulp, usually correlates with a low pulp yield. Usually, as leaching progresses, not only is the lignin more and more removed, but increasingly also pulp (components) (mainly hemicelluloses) removed from the wood in the cooking acid or cooking liquor. This results in a lower amount of isolated pulp relative to the wood used.

A disadvantage of the pulping by the sulfate process is the formation of malodorous substances such as mercaptans, in particular methylmercaptan.

The addition of sodium dithionite (hereinafter also "Na ₂ S ₂ O ₄ ") in the pulp production process is basically known from the following literature.

G. Jayme and G. Wörner describe an alkaline sulfite digestion of spruce at 170 ° C, 24 hours, wherein in 100 cm ^{3 of} the digestion solution 3 g of NaOH, 1.56 g of sodium dithionite (Na ₂ S ₂ O ₄ ) and 4, 69 g Na ₂ SO ₃ were included. ( G. Jayme, G. Wörner, paper, 6th year, Issue 11, pp. 220-222 (1952 )). In it, relatively large amounts of sodium dithionite, based on the wood to be treated, are described indirectly by means of the quantity of chemicals and the "lye ratio" (p. 221, left-hand column "Zahlentafel" and the subsequent paragraph).

Continue to describe Jayme and Wörner in wood as raw material 10 (1952) 6, pp. 244-249 , the use of relatively large amounts of sodium dithionite (Na ₂ S ₂ O ₄ ) in a sulphate liquor (65% NaOH, 25% Na ₂ S and 10% Na ₂ CO ₃ ) in sulphate pulping of spruce wood. The amount of sodium dithionite, based on the wood to be treated, is also described here indirectly via the quantitative preparation of the chemicals and the "alkali ratio" (page 246, left column from "effect of sodium hypodisulfite in sulfate digestion solutions" up to and including Table 2). The "lye ratio" of 1: 7.5 described there indicates that 7.5 parts by mass of cooking liquor were used for 1 part by mass of wood.

Object of the present invention was to obtain a high pulp yield with low lignin content of the pulp in the digestion of lignocellulosic material, in particular in the sulfate process, the formation of malodorous emissions should be reduced.

The object has been achieved by adding small amounts of a salt of the dithionic acid (hereinafter also "H ₂ S ₂ O ₄ ") in the sulfite pulping or sulfate exclusion and as otherwise described in the claims.

Cellulose is known and, for example, in " Ullmanns Enzyklopadie der technischen Chemie, 4th Edition, Volume 17, "Paper, Fiber Raw Materials", Verlag Chemie Weinheim, New York (1979) pp 531-532 in Chapter 1 Lignocellulosic material herein is any material, preferably from nature, containing lignin and cellulose.

Preferred lignocellulose-containing material are woods, including shredded wood, such as wood sections from sawmills.

Suitable woods are softwoods, preferably spruce or pine or hardwoods such as beech.

Furthermore, lignocellulose-containing material is understood here to mean grasses and annual plants, for example straw, reed, esparto grass, bamboo and bagasse, which are usually not digested by the sulfite process but are preferably digested with alkaline digestion or neutral sulfite. Method.

Sulphite digestion and sulphate digestion to recover cellulose are well known and will be discussed in more detail and in the text Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 17, "Paper, Fiber Raw Materials", Verlag Chemie Weinheim, New York (1979) pp 535-549 in chapter 1.4].

Salts of the dithionic acid (H ₂ S ₂ O ₄ ) herein are all metal salts or substituted (NR ₄ ⁺ ) or unsubstituted (NH ₄ ⁺ ) ammonium salts of this acid.

Highly suitable salts of the dithionic acid are the alkali metal salts, alkaline earth metal salts, salts of the metals of group 12 of the Periodic Table of the Elements, and ammonium (NH ₄ ⁺ ) salts.

Preferred salts of dithionic acid are sodium dithionite (Na ₂ S ₂ O ₄ ), potassium dithionite (K ₂ S ₂ O ₄ ), calcium dithionite (CaS ₂ O ₄ ), zinc dithionite (ZnS ₂ O ₄ ), ammonium dithionite ((NH ₄ ) ₂ S ₂ O ₄ ).

Salts of dithionic acid, including those preferred above, of course also include those individuals containing water of crystallization and / or additives, the latter for example for stabilization.

A well-suited dithionic acid salt is the sodium dithionite product, which is marketed by BASF SE as Blankit® or Blankit®S.

In principle, all variants of the sulphite digestion, as in the beginning and in, are for the process according to the invention Ullmann's Encyclopedia of Industrial Chemistry, 4th Edition, Volume 17, "Paper, Fiber Raw Materials", pp. 531-576, Verlag Chemie Weinheim, New York (1979 ), suitable.

The digestion temperature during sulphite digestion is usually in the range from 100 to 160.degree.

A well-suited for the process according to the invention sulfite pulping is the bisulfite process with Mg (HSO ₃ ) ₂ as a component of the cooking acid, which is described in more detail below.

As lignocellulose-containing material softwoods, preferably spruce, preferably used as wood chips. The wood chips are usually fresh from the forest (ie with a dry content of about 50 wt .-%) used. The amount used is calculated as dry matter (otro), in order then to be able to determine, for example, the yield of pulp.

The cooking acid can be prepared by slurrying magnesium carbonate (hereinafter also "MgCO ₃ ") in the water and then introducing SO ₂ . The introduction of SO ₂ is usually carried out until a clear solution has formed, which has a pH of about 3.8, for example. Usually, approximately all the dissolved substance is present as Mg (HSO ₃ ) ₂ . Upon further introduction of SO ₂ , the pH would continue to decrease as the proportion of sulphurous acid increases.

The cooking of the so-called lignocellulose-containing material with the cooking acid takes place in the usual digesters, discontinuous or continuous. The total cooking time is in the range of 400 to 600 minutes.

Preferably, a temperature profile is used for the cooking in the bisulfite process mentioned herein, preferably the bisulfite process with Mg (HSO ₃ ) ₂ as the component of the cooking acid.

A well-suited temperature profile is as follows. 1st phase: Heating from a temperature in the range of 15 to 30 ° C to a temperature in the range of 100 to 110 ° C, within 60 to 120 minutes. 2nd phase (impregnation phase): Stability for 60 to 90 minutes at a temperature in the range of 100 to 110 ° C. 3rd phase: Heating up from a temperature in the range of 100 to 110 ° C, to a temperature in the range of 150 to 160 ° C, within 45 to 90 minutes. 4th phase (final cooking time): 150 to 250 minutes remain at a temperature in the range of 150 to 160 ° C. 5th phase: Cooling to a temperature in the range of 100 to 90 ° C.

The salt of the dithionic acid, preferably sodium dithionite (Na ₂ S ₂ O ₄ ), calcium dithionite (CaS ₂ O ₄ ), zinc dithionite (ZnS ₂ O ₄ ), more preferably sodium dithionite, is added to the mixture of lignocellulosic material, preferably the wood chips from spruce wood, and the cooking acid, as described above in an amount ranging from 0.1 to 4.0% by weight, preferably 1.0 to 2.0% by weight, based in each case on the oven-dried lignocellulose-containing material , preferably the oven-dry wood chips from spruce wood added.

1. a) am Anfang der 2. Phase
2. b) am Anfang der 4. Phase
3. c) in ca. der Hälfte der 4. Phase

Basically, the dosage of the salt of the dithionic acid at any time during the cooking process or before it happen. However, the following dosage variants are preferred:

1. a) at the beginning of the second phase
2. b) at the beginning of the 4th phase
3. c) in about half of the 4th phase

The salt of the dithionic acid is preferably added at the beginning of the second phase, ie the impregnation phase.

A sulphate digestion which is well suited for the process according to the invention is described below.

As lignocellulose-containing material are woods, such as deciduous or preferably coniferous wood, particularly preferably spruce wood, preferably used as wood chips.

Basically, as the cooking liquor known for the sulfate process mixture of sodium hydroxide solution (aqueous NaOH), sodium sulfide (Na ₂ S) and sodium sulfate (hereinafter also "Na ₂ SO ₄ ") and optionally sodium carbonate (hereinafter also "Na ₂ CO ₃ ") may be used, to which the salt of dithionic acid, preferably selected from the group consisting of sodium dithionite, zinc dithionite, calcium dithionite, in an amount in the range of 0.1 to 4 wt .-% based on the amount of oven-dry lignocellulosic material was added ,

A suitable cooking liquor for the sulphate process according to the invention is described below:
The cooking liquor for sulphate digestion usually contains NaOH and sodium sulfide (Na ₂ S) as effective cooking chemicals. The sum of both substances (expressed as NaOH) relative to the lignocellulosic material, preferably wood (oven-dry (otro)), is the alkali ratio. This ratio is usually in the range of 20 to 24 wt .-%.

The necessary concentration of these substances in the cooking liquor is usually dependent on the so-called "alkali ratio". By this the expert understands mass proportions of cooking liquor in relation to mass fractions of lignocellulose-containing material, preferably wood (oven-dry (otro)). For softwoods, such as spruce and pine, this lye ratio is usually in the range of 4: 1 to 4.5: 1, for example 4.2: 1, depending on the filling density of wood in the digester. Thus, the concentration of active alkali in the cooking liquor, for example in the range of 45 to 60 g / l.
The content of sodium sulfide (Na ₂ S) in the total effective alkali is the sulfidity (expressed in%). This is usually in the range of 30 to 38%, for example 30%.
The cooking liquor for the sulfate process according to the invention contains a salt of dithionic acid, preferably selected from the group consisting of sodium dithionite, zinc dithionite, calcium dithionite, in an amount in the range of 0.1 to 4 wt .-% based on the amount of oven-dried lignocellulose -containing material.
The pH of the cooking liquor for the sulphate process according to the invention is usually about 14 at the beginning of the cooking process.
The cooking of the lignocellulosic material with the cooking liquor for the sulphate process according to the invention takes place in the usual digesters discontinuous or continuous.
The total cooking time for the sulfate process of the invention is usually in the range of 200 to 400 minutes, preferably 240 to 300 minutes.
The boiling temperature for the sulfate process of the invention is in the range of 160 to 185 ° C, for example at 170 ° C.
The salt of the dithionic acid, preferably selected from the group consisting of sodium dithionite, zinc dithionite, calcium dithionite, most preferably sodium dithionite, in the sulphate process of the invention, is blended between the lignocellulosic material and the cooking liquor in an amount ranging from 0.1 to 4.0 % By weight, preferably 1.0 to 2.0% by weight, in each case based on the oven-dried lignocellulose-containing material. The salt of the dithionic acid is added in the final phase of the digestion of the sulphate process according to the invention
The inventive method for the production of cellulose from lignocellulose-containing material by sulfite pulping or sulfate pulping provides pulp in good yield good delignification of the lignocellulosic material. The whiteness of the unbleached pulp is improved.

The addition of a salt of the dithionic acid in the sulfate process of the invention reduces the concentration of malodorous substances, preferably mercaptans in the exhaust gas of the sulfate cooking process. Examples

(I) Sulfite digestion by the bisulfite method with Mg (HSO ₃ ) ₂ as a component of the cooking acid A) Lignocellulose-containing material:

Spruce wood chips pre-sorted and pre-dried in the room air for 2 to 3 days before cooking, water content in the range of 23.4 to 33.2 wt .-%, on average about 30 wt .-%

B) Cooking acid:

Calculated 2.7 wt .-% MgO per liter. pH before boil (initial pH) 3.8. 1100 g of MgCO ₃ were slurried in 17 liters of deionized water to give a calculated MgO concentration of about 2.7 wt.%. Gaseous sulfur dioxide (SO ₂ ) was introduced into the suspension until the pH was 3.8.

C) cooking

3200 g (oven-dry (otro) calculated) spruce chips with an original water content as described in A) and 16 liters of the cooking acid from B) were filled in the discontinuous cooker with a capacity of 25 liters, corresponding to a ratio of cooking acid: otro wood of 5: 1. The stove had an acid circulation and electrical jacket heating and a temperature controller, pressure gauge, temperature sensor and pH electrode, and connected computer system.

1. 1. Phase: 105 min Anheiz-Zeit von Raumtemperatur (23°C) auf 105°C
2. 2. Phase: 90 min Haltezeit (Imprägnier-Phase) bei 105°C
3. 3. Phase: 60 min Hochheizzeit von 105°C auf Fertigkochtemperatur 155°C
4. 4. Phase: 195 min Fertigkochzeit bei Fertigkochtemperatur 155°C
5. 5. Phase: ca. 60 min Abgaszeit (nach Erreichen der Kochdauer Heizung aus) bis Temperatur unter 100°C fiel.

The following heating program was carried out:

1. 1st phase: 105 min heating time from room temperature (23 ° C) to 105 ° C
2. 2nd phase: 90 min holding time (impregnation phase) at 105 ° C
3. 3rd phase: 60 min heating-up time from 105 ° C to final cooking temperature 155 ° C
4. 4th phase: 195 min. Final cooking time at final cooking temperature 155 ° C
5. 5th phase: about 60 min exhaust gas time (after reaching the cooking time heating off) to temperature below 100 ° C fell.

The total digestion time was 510 min (8 h 30 min). The pressure in the digester was 8 to 9 bar at the end of the final cooking time.

Sodium dithionite (Blankit®S BASF SE) was dissolved in water by dosing pump within 10 min in the digester added to the mixture and that 32 g of pure Na ₂ S ₂ O ₄ (1 wt .-% Na ₂ S ₂ O ₄ or 64 g of pure Na ₂ S ₂ O ₄ (2% by weight of Na ₂ S ₂ O _4, based on the amount of wood used, calculated on the amount of wood used).

The addition times of the sodium dithionite were, per experiment, as follows: At the beginning of the holding time (impregnation phase), about 105 minutes after the start of the experiment or at the beginning of the final cooking time, about 255 minutes after the start of the experiment or in the half of the finished cooking time, approx 360 min after the start of the experiment.

In experiment W 16, the chips were impregnated immediately before digestion with such an amount of aqueous solution of sodium dithionite (Blankit®S BASF SE), corresponding to 1 wt .-% pure Na ₂ S ₂ O ₄ based on otro computed timber used.

After completion of the digestions, the pulp was removed, mixed with water and defibered with a stirrer. The shredded pulp was filled into a sieve, washed with water and dehydrated in a centrifuge.

D) Setup and evaluation

Table 1 shows the experiments: Table 1: test series Auxiliary agent [2] Adding time- min [1] Temp. On addition W7 1% Start of finished cooking time 255 min 155 ° C W8 1% Beginning impregnation phase 105 min 105 ° C W9 1% Half finished cooking time 360 min 155 ° C W10 2% Beginning impregnation phase 105 min 105 ° C W11 2% Start of finished cooking time 255 min 155 ° C W12 1% Beginning impregnation phase 105 min 105 ° C W14 without - - - W15 1% Beginning impregnation phase 105 min 105 ° C W16 1% before digestion process - - [1] The values mean number of minutes after the start of the experiment, ergo start of the first heating up
[2] Sodium dithionite Na ₂ S ₂ O ₄

Table 2 summarizes the results of the experiments in Table 1: Table 2: test series aids addition Adding time- Acceptance yield [%] Kappa number / whiteness [%] Viscosity [ml / g] W7 1% Start of finished cooking time 54.2 24.5 / 60.7 676.1 W8 1% Beginning impregnation phase 54.9 19.0 / 62.9 640.6 W9 1% Half finished cooking time 55.3 27.9 / 59.1 689.0 W10 2% Beginning impregnation phase 54.5 25.5 / 59.1 679.8 W11 2% Start of finished cooking time 53.1 17.2 / 63.6 660.8 W12 1% Beginning impregnation phase 54.7 16.0 / 63.3 645.1 W14 without - 55.6 36.1 / 58.6 708.8 W15 1% Beginning impregnation phase 54.3 20.6 / 61.7 677.4 W16 1% before digestion process 55.0 29.0 / 57.1 694.9

In this mean:

Acceptance yield means amount of pulp received (without coarse or splinter content) in relation to the wood used; It was determined by weighing and dry content measurement.

The kappa number indicates the hardness of the pulp and was determined in accordance with ISO 302. Put simply, when determining the kappa number in an aqueous pulp suspension in an acidic medium under defined conditions, the potassium permanganate consumption (KMnO ₄ consumption) is measured. The higher the lignin content in the pulp, the higher the potassium permanganate consumption and thus the kappa number. The higher the kappa number, the higher the residual lignin content of the pulp, the harder is usually the pulp.

Whiteness (R457) means remission at 457nm and was determined on the Datacolor Elrepho® instrument according to ISO 2470.

The viscosity determination was carried out in accordance with ISO 5351/1 (International Standard ISO 5351/1, Cellulose in dilute solutions - Determination of limping viscosity number, Part 1: Method in cupri-ethylene-diamine (CED) solution, First edition 1981-12-01 ).

In this case, a solution of cellulose in copper ethylenediamine solution is prepared. The concentration of the solvent is fixed. The concentration of cellulose in the solution should be determined according to the sample used in the determination. The throughput time of both the solvent and the cellulose solution is measured by a capillary viscometer at 25 ° C. The calculation of the intrinsic viscosity is based on the results of the determination and the known concentration of the cellulose solution according to the Martin equation.
The measurement was carried out according to Alternative A of the determination method (International Standard ISO 5351/1, Cellulose in dilute solutions - Determination of limping viscosity number, Part 1: Method in cupri-ethylene-diamine (CED) solution, First edition 1981-12-01 ). It works at low cellulose concentration and used to measure the transit times of solvent and cellulose solution, the same capillary.

It can be seen that the addition of the adjuvant, particularly advantageously at the beginning of the impregnation phase, results in an improved combination of the properties of the crop yield / kappa number or whiteness.

(II) sulphate digestion A) Lignocellulosic material:

Spruce-Pine mixed wood chips with a mixing ratio Fi-Ki (7: 3) undried, water content 57%.

B) cooking liquor:

The cooking liquor was prepared by adding commercial laboratory chemicals in water from sodium hydroxide (NaOH) and sodium sulfide (Na ₂ S). The amount of chemicals used was calculated so that an alkali ratio of 23% was used at a sulphidity of 20%.

C) cooking:

It was entered according to the wood dry content so much wood chips in a 10 liter digester, that 1300 g of dry matter (oven dry (otro) calculated) wood were used. The cooker was filled with cooking liquor. This contained at a desired alkali ratio of 23% and a sulfidity of 20% 239.2 g of NaOH and 59.8 g of Na ₂ S (counted as NaOH). Subsequently, the Kocherinhalt was heated to 170 ° C and held at this temperature until the desired digestion time was reached.

For the calculation of the desired digestion time the so-called H-factor was used. The temperature dependence of the relative reaction rate is used for the alkaline digestion. For all cookings an H-factor of 3500 was realized.

In selected cooking, in each case 2% by weight, relative to the amount of wood introduced (oven-dry (otro)), sodium dithionite was added. In a cooking, the addition was carried out in the main phase of the digestion (comparative example), in a further boiling in the final phase of the digestion.
After reaching the H-factor of 3500, the cooking was stopped by stopping the heating and cooling in conjunction with pressure release ("exhaust gases"). The pulp was pulped by vigorous stirring and washed.

D) Setup and evaluation

Table 3 shows the experiments: Table 3 Cooking no. Auxiliary agent [1] Adding time- Temperature on addition 1 without --------------- -------- 2 (Cf.) 2% main phase 170 ° C 3 2% final phase 170 ° C [1] Sodium dithionite Na ₂ S ₂ O ₄

Table 4 shows the yield of the accepts and the kappa number. Table 4: Cooking no. Auxiliary agent [1] Acceptance of raw material [%] Kappa number 1 without 43.3 20.1 2 (Cf.) 2% in the main phase 46,4 19.4 3 2% in final phase 47.1 19.9 [1] Sodium dithionite Na ₂ S ₂ O ₄

Acceptance yield and kappa number are as defined under (I) above.

It can be seen that the addition of sodium dithionite is associated with a significant increase in yield (3 to 4 percentage points), with even the kappa number being slightly reduced.

E) Reduction of methylmercaptan emissions

A sulphate pulping of softwood was carried out as described above

During discharge of the gases from the digester ("exhaust gases"), exhaust samples were taken at different times with a tracer pump. The concentration of methylmercaptan in these samples was measured with methylmercaptan-specific gas detector tubes.

The first measurement was carried out immediately after completion of the cooking, temperature in the digester at 172 ° C. The following measurements were carried out at further reduced temperatures of the digester, see Table 5. The results are summarized in Table 5. Table 5: Methyl mercaptan concentrations attempt % By weight [1] Na ₂ S ₂ O ₄ Sulphidity [%] Cooker temperature at the measuring time [° C] Methylmercaptan concentration [ppm] 1b 0 30 162 890 2 B 0 20 162 410 3b 2 30 162 400 4b 2 20 162 180 [1]: Related to otro wood

In this mean: Sulfidity: content of Na ₂ S in the active alkali

At high sulphidity the methylmercaptan concentration is highest. The use of Na ₂ S ₂ O ₄ leads to a decrease in methylmercane in the exhaust gas.

Claims (6)

1. A method of producing cellulose from lignocellulosic material by sulfite digestion or sulfate digestion in the presence of a salt of dithionous acid, which method comprises using the salt of dithionous acid in an amount from 0.1 to 4.0 wt.% in the case of sulfite digestion and in an amount from 1.0 to 2.0 wt.% in the case of sulfate digestion, based in each case on the amount of oven-dry lignocellulosic material, and wherein the salt of dithionous acid is added in said sulfite digestion as soon as the mixture of cooking liquor and lignocellulosic material to be digested has attained a temperature in the range from 60°C to 110°C in the course of being heated up, and the mixture thus obtained is left at a temperature from 100°C to 110°C for from 30 to 90 minutes (impregnation phase) and wherein the salt of dithionous acid is added in the end phase of said sulfate digestion.
2. The method according to claim 1 wherein the salt of dithionous acid is selected from the group consisting of sodium dithionite, zinc dithionite and calcium dithionite.
3. The method according to claim 1 or 2 wherein the salt of dithionous acid is sodium dithionite.
4. The method according to claim 1 to 3 wherein said sulfate digestion is conducted in the temperature range from 160°C to 185°C.
5. The method according to claim 1 to 4 wherein the lignocellulosic material is wood.
6. The method according to claim 1 to 5 as a batch operation.