NO771297L - PROCEDURES FOR DELIGNIFICATION OF CELLULOSE MATERIAL USING NITROGEN DIOXIDE - Google Patents

Description

"Procedure for the delignification of cellulose

material using nitrogen dioxide".

This invention relates to a method for delignification of lignocellulosic material, such as wood pulp, bagasse and straw, using treatment with nitrogen dioxide and oxygen in different stages.

For many of the applications of lignocellulosic material

is a bleached product required. Bleaching is in principle a de-lignification process. Generally, lignocellulosic material such as sulfate (kraft) wood pulp is bleached by treatment with chlorine and chlorine-containing compounds such as chlorine dioxide or hypochlorite. A typical bleaching sequence for sulphate wood pulp is chlorination, extraction with caustic soda, treatment with chlorine dioxide, extraction with caustic soda and treatment with chlorine dioxide, abbreviated C-É-D-E-D. However, the use of chlorine compounds causes difficulties in the treatment of the waste material from the bleaching process. The chlorine-containing waste material cannot be taken without further ado to water reservoirs, as this can have adverse consequences for the organisms in the water. If the chlorine-containing waste is recycled through the sulphate process black liquor recycling system, the result can be damage to the evaporator and the recycling furnace. In addition, there is a build-up of sodium chloride in the recovery system, which reduces the furnace's efficiency. There is thus an advantage in avoiding chlorine compounds in the bleaching process.

It is known to replace the chlorine ingredient in a pulp bleaching sequence with nitrogen dioxide, NC>2. In an article by G. L. Clarke in the Paper Trade Journal, volume 118, 24 February 1944, page 6 2 entitled "The Action of Nitrogen Dioxide on Un-bleached Pulp, Part I" a bleaching sequence is described in which a first treatment with nitrogen dioxide is used followed by extraction with caustic soda and treatment with hypochlorite. This bleaching sequence had the disadvantage of causing severe degradation of the pulp,

indicated by a far lower viscosity value compared to an analogous bleaching sequence, chlorine, caustic soda extraction, hypochlorite.

Applied to spruce pulp (Western hemlock), the nitrogen dioxide bleaching sequence mentioned above gave a bleached product with a viscosity of 39 centipoise, while with the analogous chlorine bleaching sequence a viscosity of 67 centipoise was obtained.

It has now been found that lignocellulosic material can be delignified to a product of acceptable viscosity using nitrogen dioxide as a first step in a sequence followed by alkali treatment and treatment with oxygen under superatmospheric pressure, abbreviated NC₂-O. The new partial sequence has a particular advantage in reduced contamination. It can remove 75-90% of the lignin residue in a conventional pulp, and due to that the waste material is chloride-free, it can be recycled to the recycling furnace for chemicals in quantities that are related to the washing efficiency after the oxygen stage.

The invention thus mainly aims to provide a method for delignification of lignocellulosic material in which environmental pollution is reduced. Other purposes will be apparent from the following.

The new method for delignification of lignocellulosic material comprises the following steps: (1) treatment of lignocellulosic material in an aqueous medium with nitrogen dioxide, (2) washing of the nitrogen dioxide-treated material with water, (3) treatment of the washed material in an aqueous medium with alkali material, and (4) treating the alkali-treated material with oxygen or an oxygen-containing gas at superatmospheric pressure.

In the first step, the lignocellulosic material is treated in an aqueous medium at a consistency within the range 3.0-50% by weight, preferably 30-50% by weight. The temperature used in the first step is between room temperature and 120°C, preferably 70-90°C. The concentration of nitrogen dioxide is 0.5-10.0% by weight, preferably 1.5-6.0% by weight. The treatment in the first stage is carried out within 5-60 minutes.

After the treatment with nitrogen dioxide in the first step, the lignocellulosic material is diluted with water to a low consistency, for example 3.0%. At this point, the pH of the aqueous lignocellulose mass will usually be in the range 1.0-3.0. The lignocellulosic material is then thoroughly washed with water. The ligno-cellulose pulp is then pressed to a high consistency, for example 35%.

In the third step of the process, the lignocellulosic material is then mixed with 1.0-20.0% by weight of an alkali or alkaline earth base. Preferably, 0.1-1.0% by weight of a magnesium salt, such as magnesium chloride or magnesium sulfate, is also added at this stage.

In the fourth stage of the process, the lignocellulosic material in aqueous suspension at a consistency of 3.0-35.0% by weight is treated in a reactor with oxygen or an oxygen-containing gas

at a partial oxygen pressure of 2.11-14.1 kp/cm 2 at a temperature within the range 80-200°C for 10-60 minutes. The lignocellulosic material is then diluted to a low consistency and washed with water.

The steps described above can be followed by conventional bleaching steps, such as C-E-D or D-E-D. By using bleaching sequences NC^-O-C-E-D or NO 2 -O-D-E-D, final pulp properties can be obtained which are as good as or better than those obtained with a conventional sequence C-E-D-E-D. When this is done, is

the conditions as follows:

Chlorine treatment by C-E-D sequence. The lignocellulosic material in aqueous suspension at a consistency of .2.0-35.0% by weight is treated with 0.5-5.0% by weight chlorine. The treatment lasts for 1-60 minutes at a temperature of 15-60°C. The material is then washed with water.

First chlorine dioxide treatment with D-E-D sequence. The lignocellulosic material in aqueous suspension at a consistency of 3.0-35.0% by weight is treated with 0.5-1.5% by weight chlorine dioxide and optionally with sufficient base or other alkaline buffering agent to give a pH of approx. 4 at the end of the chlorine dioxide treatment. The treatment lasts 60-300 minutes at a temperature of 60-80°C. The material is then washed with water.

Caustic soda extract ion ( C—E—D or D—E—D). The washed material in aqueous suspension at 3.0-35.0% consistency by weight. treated with 0.25-2.0 wt% base for 60-120 minutes at 40-70°C. The material is then washed with water.

Chlorine dioxide treatment. (Final step at C-E-D or D-E-D).

The washed material in aqueous suspension at a consistency of 3.0-35.0% by weight is treated with 0.1-1.5 wt% chlorine dioxide for 60-300 minutes at 60-80°C. The material is then washed with water.

The base that is usually used in the present process is sodium hydroxide. However, other materials, such as potassium hydroxide, calcium hydroxide, sodium carbonate or potassium carbonate, can be used.

The base used in the third step of the method,

before the treatment with oxygen, may be white liquor from the alkali recovery system in a wood pulp factory.

Instead of using fresh water as a washing medium, you can use "white water" (backwater) which is available in pulp mills.

The method according to the invention has the advantage that the waste material from the nitrogen dioxide and alkali-oxygen stages does not contain chlorine compounds and can thus be recycled through the recycling system. This results in a reduction in the amount of polluting emissions from a factory using the process.

The partial NC^-O sequence according to the invention has been found to be suitable for delignification of high-yield kraft and soda pulp, resulting in yields normally obtained in connection with oxygen delignification, but with reduced pollution as an additional benefit. While a single oxygen step will delignify a high yield pulp (Kappa number .50-100) to a Kappa number of about 20-30, the NC^-O sequence delignifies to a Kappa number of 10-15 at viscosity values similar to or better than those found for pulps treated with a single oxygen step. A larger proportion of polluting material can thus be returned to the recycling system.

High yield pulps partially delignified by the partial NC^-O bleaching sequence can be brought to a fully bleached level by a C-E-D sequence. Application of the bleaching sequence N02~0-C-E-D on a high yield pulp (Kappa number 50-100)

resulting in a fully bleached pulp with strength properties

equivalent to or better than those found for the bleaching sequence C-E-D-E-D on conventional kraft pulp with a Kappa number of 30.

In the description contained therein, all proportions and percentages are calculated on a weight basis for oven-dried material, unless otherwise stated.

The investigations by which the properties of the treated product were determined were carried out according to the following standardized methods:

The color of the process effluent was measured by comparison with color standards prepared from potassium chloroplatinate and cobalt (II) chloride according to ASTM D-1209. The color is then expressed as grams of platinum per tons of mass.

The following examples will further illustrate the invention.

EXAMPLE 1

50 g, based on oven-dried material, of a softwood kraft pulp (mainly spruce species) with a Kappa number of 32.4 and viscosity of 34.7 centipoise at 40.0% consistency was placed in a rotating reaction vessel. The mass was preheated in the container to a temperature of 75°C. Nitrogen dioxide (3.0% calculated on the mass) was then distilled into the rotating mass. The reaction was allowed to proceed for a further 30 minutes, after which the reactor was evacuated to remove unreacted nitrogen dioxide. The mass was then diluted to a consistency of 4.0%, and the pH value. was found to be 2.3. After thorough washing with distilled water, the pulp was pressed to 35% consistency before it was ready for the oxidation step.

To the washed mass was added 2.6% by weight sodium hydroxide and 0.1% magnesium ion as magnesium sulfate and sufficient water to give a consistency of 27%, the mass being stirred in a Hobart mixer. The mass was then transferred to an oxygen reactor. In the reactor, the alkaline mass was treated with oxygen gas at a pressure of 6.12 ato at 120°C for 30 minutes. The pulp was then diluted to a consistency of 4% and the pH was found to be 10.0. After washing with water, the mass had the following properties:

The oxygen-treated pulp was then further bleached by the D-E-D sequence, the chemical addition and conditions being as follows: D^ stage: 1.2% chlorine dioxide at 70°C for 3 hours, pulp-con- stehs 6.0%i

E-stage: 0.6% sodium hydroxide at 65°C for 1.5 hours, pulp consistency 12.0%.

D2~stage: 0.4% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The pulp bleached by the NC^-O-D-E-D sequence had the following properties:

The strength characteristics are indicated in the table below.

Under the assumption of returning the nitrogen dioxide and the waste material from the oxygen stage to the power recovery system and a washing efficiency for the oxygen stage of 85%, the combined waste material from the D-E-D bleaching had the following properties:

The same softwood kraft pulp with a Kappa number of 32.4 and a viscosity of 34.7 centipoise was bleached by a conventional . C-E-D-E-D bleach sequence. The chemical additions and conditions in the different stages were as follows: C stage: 6.9% chlorine at 21°C for 1 hour, pulp consistency 3.0%. E^-stage: 3.5% sodium hydroxide at 65°C for 1.5 hours, mass- consistency 12.0%.

D^ stage: 1.0% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

E2~stage: 0.8% sodium hydroxide at 65°C for 1.5 hours, pulp consistency 12.0%.

D2 stage: 0.3% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

After the final chloride acid step, the pulp had the following properties:

The strength characteristics are indicated in the table. The combined effluents from the C-E-D-E-D bleaching sequence had the following

.properties:

It will be seen that by using NC^-O-D-E-D instead of the C-E-D-E-D sequence, a reduction of 96%, 70% and 90% respectively in the effluent's colour, total organic carbon and chloride ion content is obtained.

EXAMPLE . 2

The same softwood pulp as in example 1 was subjected to an NC^-O bleaching sequence as in example 1 with the exception that the nitrogen dioxide concentration was 1.5% calculated on the pulp. The bleached pulp had the following properties:

The NC^-O treated pulp was further bleached by a C-E-D sequence with chemical additions and conditions as follows: C stage: 1.6% chlorine at 21°C for 1 hour, pulp consistency 3.0%. E-stage: 0.8% sodium hydroxide at 65°C for 1.5 hours, mass-con lastly 12.0%. D stage: 0.7% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached pulp had the following properties:

The strength characteristics are indicated in the table. The combined C-E-D effluent from the bleaching plant had the following characteristics:

Reductions of 88%, 61% and 74% respectively in colour, total organic carbon and chloride ion content were thus achieved by using the sequence NC^-O-C-E-D instead of the sequence C-E-D-E-D.

EXAMPLE 3

A hardwood (mainly oak species) kraft pulp with a Kappa number of 19.0 and a viscosity of 25.7 centipoise was bleached by the NC^-O sequence using the same conditions as in Example 1 with the exception of the following: Nitrogen dioxide 2.0% calculated on the mass Oxygen treatment

The hardwood pulp bleached by the partial NC^-O sequence was further bleached by a D-E-D sequence with chemical-potassium additions and under conditions as follows: D-^ stage: 1.2% Chlorine dioxide at 70°C for 3 hours, pulp consistency

6.0%.

E-stage: 0.6% sodium hydroxide at 65°C for 1.5 hours, pulp consistency 12.0%. D2~stage: 0>4% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The resulting bleached pulp had the following properties:

The same hardwood kraft pulp with Kappa number 19.0 and viscosity 25.7 centipoise was bleached by a conventional C-E-D-E-D sequence. Chemical additions and conditions in the different stages were as follows: C stage: 3.6% chlorine at 21°C for 1 hour, pulp consistency 3.0%. E^ stage: 2.5% sodium hydroxide at 65°C for 1.5 hours, mass- consistency 12.0%.

D-^ stage: 0.7% chlorine dioxide at 70°C for 3 hours, pulp consistency

6.0%.

E2~stage: 0.5% sodium hydroxide at 65°C for 1.5 hours, pulp consistency 12.0%. D2~stage: 0.28% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

EXAMPLE 4

.The same hardwood kraft pulp as in Example 3 with a Kappa number of 19.0 and a viscosity of 25.7 centipoise was subjected to an NC^-O bleaching sequence as in Example 3 with the exception of the following: Nitrogen dioxide 1.5 % calculated on the mass Oxygen treatment

The bleached product had the following properties:

The pulp bleached by the partial NC^-O sequence was further bleached by the C-E-D sequence.

Chemical additions and conditions in the different stages were as follows: C stage: 1.24% chlorine at 21°C for 1 hour, pulp consistency 3.0% E stage: 0.8% sodium hydroxide at 65°C for 1 .5 hour, mass- consistency 12.0%

D stage: 0.5% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

EXAMPLE 5

A softwood (pine) kraft pulp with a Kappa number of 50 and a viscosity of 28 centipoise was subjected to an NC^-O bleaching sequence as in Example 1 with the exception of the following:

The bleached product had a Kappa number of 9.3 and one

viscosity of 15.2 centipoise.

The product ■ from the above partial NO2-0 bleaching sequence was subjected to further bleaching at the sequence C-E-D. Chemical additions and conditions in the different stages were as follows: C stage: 1.9% chlorine at 21°C for 1 hour, pulp consistency 3.0%. E-stage: 0.9% sodium hydroxide at 65°C for 1.5 hours, mass consistency 12.0%. D stage: 0.8% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

The same pulp of softwood was subjected to a one-stage oxygen treatment under the same conditions as stated above in this example. The resulting oxygenated pulp had a Kappa number of 22.4 and a viscosity of 17.2

centipoise.

It will thus be seen that the NC^-O sequence in this case enables 58% greater delignification than oxygen alone.

The pulp treated by a single oxygen stage was further subjected to the bleaching sequence C-E-D. Chemical additions and conditions in the different stages were as follows: C stage: 4.6% chlorine at 21°C for 1 hour, pulp consistency 3.0%. E-stage: 3.3% sodium hydroxide at 65°C for 1.5 hours, mass- consistency 12.0%. D stage: 1.4% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

It will be seen that in order to achieve an equally good brightness with the bleaching sequence O-C-E-D you have to use significantly more chlorine and chlorine dioxide than with the sequence NC^-O-C-E-D.

EXAMPLE 6

A pulp of softwood (balsam fir) with a Kappa number of 9 3 was refined for fiberization of the waste and then subjected to a partial NC^-O sequence as in Example 1 with the exception of the following:

The bleached product had a Kappa number of 17.6 and a viscosity of 16.6 centipoise.

The NC^-O-bleached material was then subjected to a further bleaching sequence C-E-D. Chemical additions and the conditions in the different stages were as follows: C stage: 3.2% chlorine at 21°C for 1 hour, pulp consistency 3.0% E stage: 1.8% sodium hydroxide at 65°C for 1.5 hours, mass- consistency 12.0%. D-stage: 0.6% chlorine dioxide at 70°Ci for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

The same softwood pulp was subjected to a one-stage oxygen treatment under the same conditions as stated above in this example. The resulting oxygenated pulp had a Kappa number of 32.4 and a viscosity of 11.8 centipoise.

The above soda ash treated with oxygen in one step was subjected to the further bleaching sequence C-E-D. Chemical additions and conditions in the various stages were as follows: C stage: 6.1% chlorine at 21°C for 1 hour, pulp consistency 3.0% E stage: 3.4% sodium hydroxide at 65°C in 1.5 hours, mass- consistency 12.0% D-stage: 0.8% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

EXAMPLE 7

A kraft mass of mixed hardwood with a Kappa number of 46

was refined to fiberize the waste and then subjected to a partial NO 2 ~ 6 bleaching sequence as in Example 1 with the exception

of the following:

Nitrogen dioxide 3.0% calculated on the mass Oxygen level

% sodium hydroxide 4.0 calculated on the mass

The resulting bleached product had a Kappa number of 8.2

and viscosity 25.1 centipoise.

The above bleached pulp was subjected to the further bleaching sequence C-E-D. Chemical additions and conditions in the various stages were as follows: C-stage: 1.6% chlorine at 21°C for 1 hour, pulp consistency 3.0% E-stage: 0.9% sodium hydroxide at 65°C for 1 .5 hour, mass-consi stone 12.0% D stage: 0.4% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The resulting product had the following properties;

The yield for pulp production and bleaching was 51.1% calculated on the wood. The mass's strength properties are indicated in

the table.

The above mixed hardwood kraft pulp was subjected to oxygen treatment in a single step under the same conditions as stated above in this example. The resulting oxygenated pulp had a Kappa number of 15.8 and a viscosity of

of 29.9 centipoise.

The above pulp treated with oxygen in one step was subjected to the further bleaching sequence C-E-D. Chemical additions and conditions in the various stages were as follows: C-stage: 2.9% chlorine at 21°C for 1 hour, pulp consistency 3.0% E-stage: 1.6% sodium hydroxide at 65°C, in 1.5 hours, mass- consistency 12.0% D-stage: 0.4% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The resulting bleached pulp had the following properties:

The yield for pulp production and bleaching was 51.4% calculated on the wood. The strength characteristics are indicated in

the table.

Another portion of the mixed hardwood chip in this example was used to make a pulp with a Kappa number of 14.8 and bleached by a conventional C-E-D-É-D sequence. Chemical additions and conditions in the various stages were as follows: C-stage: 3.1% chlorine at 21°C for 1 hour, pulp consistency 3.0% E^-stage:'1.7% sodium hydroxide at 65°C for 1.5 hours, mass-con last 12.0% D-^ stage: 0.7% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0% E2~stage: 0.5% sodium hydroxide at 65°C for 1.5 hours, mass-con last 12.0% D2_stage: 0.3% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%. The bleached pulp had the following properties;

The yield for the pulping and bleaching was

47.8% calculated on the wood. The strength characteristics are indicated in

the table.

EXAMPLE 8

A kraft pulp of softwood (mainly spruce species) with a Kappa number of 32 was subjected to a partial NC^-O sequence as in example 2, but in contrast to the aforementioned example it was now

used a mass with a low consistency in the oxygen stage. The treatment conditions were as follows: Nitrogen dioxide step: 1.5% NC»2 at 75°C for 30 minutes, pulp consistency 40%

Oxygen stage: oxygen gas at 6.33 kp/cm 2 in 15

minutes at 130°C, where the mass had a consistency of 3.0% and contained 3.0% sodium hydroxide.

The product had a Kappa number of 9.8 and a viscosity of

of 23.1 centipoise.

The oxygen treated pulp was then completely bleached

by a C-E-D sequence. The conditions and results were as follows: C-stage: 2.0% chlorine at 21°C for 1 hour, pulp consistency 3.0% E-stage: 1.0% sodium hydroxide at 65°C for 1.5 hours, , pulp consistency 12.0% D-stage: 0.6% chlorine dioxide at 70°C for 3 hours, pulp consistency 6.0%.

The bleached product had the following properties:

Claims (10)

1. Method for delignification of lignocellulosic material, characterized by the following steps: 1) lignocellulosic material is treated in an aqueous medium with nitrogen dioxide, 2) the nitrogen dioxide-treated material is washed with water, 3) the washed material is treated with alkaline material in an aqueous medium, and 4) the alkali-treated material is treated with oxygen or an oxygen-containing gas at superatmospheric pressure. 2. Method according to claim 1, characterized in that the lignocellulosic material in step 1) is treated at a consistency of 3.0-50% by weight with 0.5-10.0% by weight nitrogen dioxide at 20-120°C for 5-60 minutes. 3.. Method according to claim 1, characterized in that the lignocellulosic material in step 3) is treated with 1.0-20.0% by weight of an alkali or alkaline earth base. in 4. Method according to claim 3, characterized in that in step 3) 0.1-1.0% by weight magnesium ion is added to the lignocellulosic material. 5. Method according to claim 3, characterized in that the alkali base is sodium hydroxide. 6. Method according to claim 1, characterized in that the lignocellulosic material, at an eh consistency of 3.0-35% by weight, in step 4) is treated with oxygen or an oxygen-containing gas at an oxygen partial pressure of 2.04-13.60 ato véd a temperature of 80-200°C for 10-60 minutes. 7. Method according to claim 1, characterized in that the oxygen-containing gas is air. 8.. Method according to claim 1. characterized in that the treatment indicated there is followed by further treatment steps as follows: a) treatment with chlorine in an aqueous medium, b) extraction with alkali base in aqueous medium and c) treatment with chlorine dioxide in an alkaline medium. 9. Method according to claim 1, characterized in that the treatment specified there is followed by further treatment steps as follows: a) treatment with chlorine dioxide in an aqueous medium, b) extraction with alkali base in aqueous medium and c) treatment with chlorine dioxide in an aqueous medium. 10. Method according to claim 1, characterized in that the lignocellulose material is wood pulp produced by force- .or soda processes.