Method of delignifying wood

The present invention relates to a method according to the preamble of claim 1 for delignifying wood.

According to a method of the present kind, lignocellulosic raw-material is contacted with an alkaline agent for converting the lignin present in the raw-material into a more easily soluble form and the converted lignin is dissolved in an aqueous medium.

Various chemical pulping methods are well-known in the art. The most common method is the sulphate or kraft process wherein a combination of sodium sulphide and sodium hydroxide is used as a pulping chemical. There has been a need for sulphur- free pulping methods.

Delignification of wood in supercritical fluids have been suggested earlier in the art, cf. US Patent No. 5,041,192. There are also a number of processes wherein supercritical substances, such as carbon dioxide, have been used for extracting organic compounds from cellulose materials. These methods include removing dioxins from secondary (recycled) fibers and tall oil and turpentine from coniferous wood chips.

It is an aim of the present invention to provide an alternative new method of removing lignin from a wood raw-material wherein carbon dioxide, preferably in supercritical phase, is used.

The present invention is based on the finding that the phenolic residues of the lignin part of the raw-material are easily leached out from the raw-material in water or aqueous media if they first are converted into carboxylated benzylic structures.

Based on the above finding, the present invention comprises defibering wooden chips by first impregnating the chips with an alkaline agent suitable for converting at least some, preferably as much as possible of the phenolic groups in the raw-material into phenolates. Then, the modified raw-material is contacted with carbon dioxide at anhydrous conditions to carboxylate the phenolates. The carboxylated material is finally dispersed in water to

dissolve the carboxylates and the undissolved residue, which now comprises delignified chips, is then recovered.

More specifically, the present method is characterized by what is stated in the characterizing part of claim 1.

Considerable advantages are obtained by the invention. The present process can be used as a pre-treatment for producing fine mechanical pulp, or producing cellulose. The new method is environmentally- friendly in the respect that no sulphur is used, water consumption is small and there might be a reduced need for use of bleaching chemicals. Also the energy consumption is low.

Since the pulping is based on selective chemical reactions, degradation of the fibrous matter is smaller than in kraft process. The dissolution of lignin is higher than in corresponding methods employing only the alkaline agent. Generally, about 1 to 90 % by weight of the lignin can be removed, typically 2 to 75 % by weight, in particular 5 to 50 % by weight. The pulping chemicals are inexpensive and they can be effectively recycled and recovered. An important additional advantage is that the by-products form a valuable stock of raw-material for further processing: e.g. various benzoic acids, such as vanillic acid can be produced.

The present invention can be employed for both wood chips and annual and perennial plants but also for treating lignocellulosic pulp.

Next the invention will be examined more closely with the aid of a detailed description.

Scheme 1 shows the reactions of one embodiment of the present carboxylation method.

The basic idea of chips delignification is based on Kolbe-Schmitt reaction (in SC-CO ₂ environment), the reaction is based on carboxylation of benzene ring (Scheme 1). The carboxylated lignin is much more soluble in water than the normal lignin.

Scheme 1

Reaction needs base, the base that forms phenolates are required.

2. Removal of water with temperature in reaction chamber:

3. Reaction with CO2:

CO. Product more soluble in water.

R= rest of lignin

Scheme 2 shows proposed products from Kolbe-Schmitt reaction:

Scheme 2

R= rest of lignin

According to one embodiment, the raw-material is contacted with an alkaline agent, in particular the phenol- structures are reacted with a base that can form phenolates. The base can be selected from the group of alkaline metal hydroxides, such as KOH and NaOH, quaternary ammonium hydroxides, alkaline metal carbonates, such as potassium carbonate and sodium carbonate, and mixtures thereof. Also amines or anhydrous ammonia gas can

be used. The concentration of the base is at least the same as the Kolbe-Schmitt reaction requires to work (i.e. equal molar amounts of base and lignin phenol groups). The alkaline solution is impregnated into the woodchips or the pulp.

The impregnation can be carried out by using the alkaline agent in liquid or gas phase. Thus, e.g. the alkaline or earth alkaline metal hydroxides or carbonates can be used in the form of aqueous solutions or dispersions. Similarly, various ammonium hydroxides, as well as amine compounds, can be used in liquid phase. For gas phase impregnation, ammonia is particularly preferably employed.

The pressure of the impregnation can be atmospheric or subatmospheric or supra- atmospheric, preferably atmospheric or supraatmospheric. The temperature can be in the range of up to 15O ⁰ C, for example from 3 ⁰ C to 143 ⁰ C.

The particular advantage of using gaseous ammonia resides in the fact that additional water does not need to be incorporated into the raw-material.

The impregnation step can be effected in a batch process or by continuous processing.

In case of gas-phase treatment, the impregnation step can optionally be preceded by a drying step, e.g. one carried out at about 30 to 110 ⁰ C, typically about 100 ⁰ C and at a treatment time on the order of 1 to 240 min, typically about 10 to 60 min. The drying can be effected in air or in inert gas.

After impregnation, the raw-material is subjected to drying (optionally) and carboxylation. Water present in the raw-material, e.g. the chips, can be removed from them by drying, e.g. by conventional drying in air. For making the drying more efficient, it can be carried out by heating, preferably by heating in a gaseous medium, e.g. in air or in an inert medium such as nitrogen. Generally, the temperature can be, as above, in the range of about 20 to 110 ⁰ C, typically from about 30 up to about 100 ⁰ C and the treatment time on the order of 1 to 240 min, typically about 10 to 60 min. Usually, drying is carried out at ambient pressure, but it is also possible to use reduced pressure or overpressure.

This dewatering process step is advantageous in the sense that it makes it possible to minimize the side reactions between water and lignin and water and carbon dioxide during the subsequent carbon dioxide treatment. It should be pointed out that the above Kolbe- Schmitt reaction is known to take place in the presence of water, but water may significantly decrease the yield.

By means of the water removal, the raw-material is converted into essentially "anhydrous" condition. This means in practice that the concentration of residual water in the raw- material is less than about 5 %, in particular below 3 %, for example 1 % by weight or less. Naturally, if dried raw-material is being used and if the impregnation step has been performed at essentially anhydrous conditions, no intermediate drying step is needed.

In the following step, the material is contacted with carbon dioxide at supercritical conditions.

The critical point of carbon dioxide is 31 ⁰ C (i.e. about 304 K) and 73 bar which conditions are feasible in industrial operations. Carbon dioxide is a non-toxic, non-flammable chemical. It exhibits not only great dissolving power but also weak surface tensions, which makes it good for the use as a solvent.

The phenolates react with CO ₂ via nucleophilic addition by forming carboxylated benzene ring that is more soluble in water than natural lignin. The reaction temperature is generally between 0-200 ⁰ C and the reaction time from 10 s (in particular from 10 min) to 500 min. The pressure is about 5 to 200 bar.

During the phenolate reaction step, the pretreated raw-material and the solvent are maintained at a pressure of 15 to 200 bar and the temperature of the supercritical phase is 300-420 K.

The treatment with the carbon dioxide is typically carried out in a batch reactor which withstands the above-indicated conditions.

Generally, it is possible to carry out the whole process in one and the same vessel (suitable in particular for batch processing) or to use separate units for the various steps. According

to one embodiment, the intermediate drying step and the carboxylation step are carried out in the same reactor. According to a second embodiment, impregnation in the gaseous phase, drying and carboxylation are all carried out in the same reactor. According to a third embodiment, the gas phase impregnation is performed in a first reactor and the carboxylation step in a second reactor, the intermediate drying step being carried out in either of the reactors or in a separate unit.

The above two main steps of the reaction (impregnation and carboxylation) can be carried out for example in the following reaction sequences (alkaline agent + raw-material + sc- CO ₂ ):

1. Water + K2CO3 + pulp (or wood chip) + CO ₂ (sc = supercritical)

2. KOH + pulp (or wood chip) + CO ₂ (sc)

3. TBAH + pulp (or wood chip) + CO ₂ (sc) (TBAH = tetrabutylammonium hydroxide)

4. NaOH + pulp (or wood chip) + CO ₂ (sc)

5. NH ₃ + pulp (or wood chips) + CO ₂ (sc)

After the treatment with carbon dioxide, the carboxylate material is dispersed in an aqueous medium to dissolve least a part of the carboxylates and to remove a part of the lignin from the material. This "leaching" can be carried out at ambient temperature and pressure. However, it is also possible to leach the treated material at an elevated temperature and/or pressure. Generally, the leaching can be carried out at typical conditions employed during washing of the pulp. As an aqueous medium, water or an aqueous solution can be used.

The above-mentioned process steps can be repeated, for example 1 to 5 times, to enhance the delignification rate.

Typically, for the leaching step, the material is removed from the carboxylation reactor. It is, however, possible to perform the leaching in the same reactor as the carboxylation.

After leaching, the fibrous material is recovered and worked-up in a conventional fashion. If desired the dissolved material can be recovered from the aqueous solution after leaching and dissolved or dispersed components can be recovered, e.g. by precipitation.

The raw-material of the present process typically comprises a lignocellulosic material, such as wood, in particular chips or powder/saw dust of wood, comprising softwood and hardwood, such as pine, spruce, birch, beech, aspen, poplar, eucalyptus and mixed tropical hardwood.

The fibrous raw material used can also be annual or perennial plants, such as bagasse and reed canary grass.

It is also possible to use the method for treating raw-material comprising lignocellulosic pulp.

The following non-limiting examples illustrate the invention.

Example 1. Gas-phase alkaline treatment

First, 55.6 g of thin wood chips are treated at the following conditions to remove water:

100 ⁰ C and 30 minutes. The amount of water removed from the chips was 16.0 g. After this the chips were exposed to NH3 gas flow for 60 minutes at 115 ⁰ C and a pressure of 4 bar. This is followed by treatment with supercritical CO ₂ using a CO ₂ flow-through of 126 g at 174 ⁰ C, 120 minutes and in pressure of 110 bar. The consumption of CO ₂ during the reaction is 27 g.

As a result of the treatment explained above, lignin removal was 1.7 times more than for the reference (plain sodium hydroxide pulping) according to the UV-lignin analysis (1.68 g/1 versus 0.97 g/1). Correspondingly, the loss of carbohydrates was almost two times smaller (1.01 g/1 versus 1.87 g/1). The glucose amount was negligible (0.08 g/1).

Example 2. Gas-phase alkaline treatment

First, 65.0 g of thin wood chips are treated at the following conditions in NH3 as batch treatment: 60 minutes, temperature from 3 ⁰ C to 143 ⁰ C, and at pressures of up to 18 bars. After this, the chips are exposed to NH3 gas flow-trough for 60 minutes in 119 ⁰ C at a pressure of 4.5 bar. This is followed by supercritical CO ₂ treatment with a CO ₂ flow- through of 326 g at 175 ⁰ C, 60 minutes and at a pressure of 110 bar. Consumption of CO ₂ during the reaction is 27 g. After this, the chips are exposed to NH3 gas flow-trough for 30 minutes in 117 ⁰ C at a pressure of 4.5 bar. The lignin removal was 1.3 times higher than for the reference according to the UV-lignin analysis (1.28 g/1 versus 0.97 g/1).

Correspondingly the loss of carbohydrates was almost seven times less than the reference (0.28 g/1 versus 1.87 g/1). The glucose amount was negligible (0.03 g/1).

Example 3. Liquid-phase alkaline treatment

First, NaOH base was impregnated into thin wood chips with chip compressor. The amount of sodium hydroxide was 4 mol / 1 kg dry wood. Chips were treated at the following conditions to create phenolates: 150 ⁰ C and 60 minutes. This was followed by water removal for 30 minutes with a temperature above 100 ⁰ C. After this step, the chips were exposed to supercritical CO ₂ treatment with a CO ₂ flow-through of 254 g at 180 ⁰ C, 60 minutes and in pressure of 110 bar. After that the chips were cooked in water (60 minutes, 180 ⁰ C ) to extract the carboxylated lignin. Consumption of CO ₂ during the reaction was 27 g. The lignin removal was 5 % higher than for the reference according to UV-lignin analysis (0.91 g/1 versus 0.87 g/1) and the carbohydrate loss 2 % smaller.