FI63267B - FOERFARANDE FOER FRAMSTAELLNING AV SPECIALCELLULOSA MED HOEGT ALFATAL GENOM FLERSTEGSKOKNING INNEFATTANDE SYRAFOERHYDROLYS - Google Patents

Description

1 63267

METHOD FOR THE PREPARATION OF HIGH ALPHASIC SPECIAL CELLULOSE BY A MULTI-STAGE SOUP CONTAINING HAP POEHYDROLYSIS

In the production of high-quality cellulose, a two-stage cooking process is often used, the first step being pre-hydrolysis with water or steam at 170-175 ° C. After pre-hydrolysis and possible removal of the hydrolyzate, a white section is added to the digester and high sulphate cooking is performed. with an alpha-cellulose content of more than 90% before bleaching. The high alpha number is due to the prehydrolysis step, because here the easily hydrolyzable hemicellulose is almost completely dissolved and the only task of the sulphate cooking is to remove the lignin.

The hydrolysis and dissolution of the hemicellulose part of wood requires acidic conditions. When hydrolysis with water or steam is performed, the acidity is provided by organic acids such as acetic, formic and propionic acid, which arise from the wood itself. Since said organic acids are weak acids, the pH in the prehydrolysis reaction cannot go very low. The low acidity of the reaction conditions must therefore be compensated for at a high temperature, so that the hydrolysis is sufficiently rapid.

In the preparation of high alpha-cellulose, mainly for chemical processing, the most common method is aqueous hydrolysis at 170-175 ° C with a reaction time at a peak temperature of 40-120 minutes. The prehydrolysis loss is then 18-25%, depending on the wood species. When softwood is used, significant condensation of lignin may occur, resulting in a high number of pieces after cooking and a disproportionate amount of chlorine or chlorine dioxide used in bleaching.

Steam pre-hydrolysis is in principle the same as water pre-hydrolysis and requires the same reaction conditions. As it has a much lower liquid-wood ratio, less thermal energy is achieved. The required kettle volume and investment costs are also lower. When using steam prehydrolysis, the greatest risk factor is the quality of the pulp, which may be uneven, mainly due to the low liquid-wood ratio.

Thus, the wood dry matter soluble in prehydrolysis is mainly hemicellulose and consists of five- and six-carbon sugars. Most of the hemicellulose is cleaved in the prehydrolysis and the dry matter of the hydrolyzate contains 50-60% monosaccharides. However, some are still in oligo- or polysaccharide form. In addition to these, the process generates significant organic acids as well as furfural from wood pentosans. The formation of acetic acid and very furfural increases rapidly as the hydrolysis temperature rises up to 150 ° C.

The hydrolyzate formed in the prehydrolysis step, which contains organic acids and furfural in addition to sugars and polysaccharides, is valuable and its utilization improves the profitability of pulp production.

50 In addition to the above, the recovery of the hydrolyzate is also important because, when wasted, it causes wastewater problems that are difficult to manage. Plants where the utilization of hydrolyzate as a raw material for chemical products or for energy production has not been fully resolved 55 have unreasonably high wastewater treatment costs.

Since in water or steam pre-hydrolysis a large part of the hemicellulose remains in the oligo- or polysaccharide grade and at the same time large amounts of furfural are formed, which partly form furan resins, the resulting hydrolyzate is difficult to handle. It is characterized by the so-called the tendency to caramelize caused by the polysaccharides and the furan resins formed. Even as a dilute solution, the hydrolyzate tends to form precipitates and deposits in the equipment that are difficult to remove; these disadvantages are accentuated when the hydrolyzate 65 is evaporated. Difficulties in the evaporator are often reflected in increased environmental load and increased handling costs. If the hydrolyzate is used as a raw material for biochemical processes, evaporation is often not necessary.

70 If the prehydrolysis is carried out under considerably acidic conditions, the hydrolysis temperature can be lowered at the same time. The formation of furfural is reduced and at the same time, of course, the formation of furan resins. Due to the low pH, polysaccharides are almost completely degraded to mono- and disaccharide degrees.

75 The use of strong mineral acids as hydrolysing agents has long been known. Swedish patent No. 113579 mm. includes the manufacture of viscose pulp from straw using prehydrolysis with less than 1% sulfuric acid at 100 to 130 ° C followed by sulphate cooking. Swedish pa-80 exam No. 113580 shows the production of viscose pulp from softwood chips under the same conditions. The use of a concentrated SO2 solution for prehydrolysis is proposed in several American patents.

The hydrolyzate thus obtained does not have a tendency to caramelize-85 or any other unpleasant properties; it can be safely stored for long periods of time, pumped and, in particular, evaporated to a high dry matter content, which significantly improves its use and further processing possibilities.

However, the use of strong acids is not threshing, e.g.

90 due to difficult corrosion conditions and because the variations in pulp quality and yield are steeper than usual at low pH. Pulp production applying the above patents is not known to be in any use. It is very difficult to make the acid so evenly absorbed into the soils that all the wood is prehydrolyzed in the same way, which is necessary for a uniform pulp quality. The uniformity of absorption can be improved by using chips which are considerably smaller than normal, but this makes it more difficult for the cooking liquid to circulate in the digester, especially during the 100 alkaline steps following hydrolysis.

The present invention relates to the production of high-quality pulp suitable for chemical processing by a multi-stage digestion, characterized by the performance of a prehydrolysis step under a variety of precisely controlled, relatively acidic conditions. To ensure uniform absorption of the prehydrolysis agent 101 in the chips, they are steamed for 5-10 min. At a temperature of 110 to 120 ° C. The target chip length does not have to be less than 20-21 mrt, which does not yet make it difficult to recycle the cooking liquid. However, chips larger than 25 mm longer must be carefully screened out. For example, sulfuric acid is used in an amount of 1 to 2% by weight of the wood, the sulfuric acid content of the prehydrolysis liquid being 0.3 to 0 to 6%. When sulfur dioxide is used, the most suitable dosage is 0.3 to 1% by weight of the wood. The prehydrolysis temperature is 140 to 150 ° C and the hydrolysis time is 30 to 60 minutes. By intensifying 110 the above conditions within the given limits, either one or more, the loss of prehydrolysis can be increased, resulting in a chip containing less hemicellulose, which in turn in the alkaline cooking following this step yields even purer cellulose with a very high alpha number 115.

An essential part of the process involves countercurrent washing of the prehydrolyzed chips at a recovery rate of 70-80%, whereby the alkaline cooking conditions can be controlled with sufficient accuracy, since no alkali is significantly consumed to neutralize the hydrolyzate 120. A hydrolyzate obtained from the previous hydrolysis with a dry matter content of e.g. 7-8% can also be added to the hydrolysis. The dry matter content of the hydrolyzate can thus be increased by recycling, thereby substantially reducing the evaporation costs of the hydrolyzate.

125 The use of hardwood as raw material, eg birch, for which this method is particularly well suited, gives the above hydrolysis conditions within the given limits, with a pre-hydrolysis loss of 20 to 30% and a sulphate cooking chemical dose of 14 to 15% Na2 resulting in an experimental cellulose content of 130 2 to 8% unbleached, number 8 to 12 and wood yield 32 to 36%.

When the pulp is otherwise bleached normally, but using at least 75% of active chlorine dioxide in the first step, high alpha pulps suitable for chemical processing (alpha number 90-97% depending on residual pentosans) are obtained without enhanced hot or cold alkali-135 treatment.

Due to the well-controlled conditions and the even absorption of the hydrolysis liquid, no harmful condensation of lignin occurs in the chips, nor does the hydrolysis of softwood, so that bleaching is relatively easy.

The hydrolyzate, carefully separated from the chips, contains dissolved hemicellulose, most of which is cleaved to the monosaccharide grade, the mineral acid used as the agent, and organic acids.

145 When birch is used, the majority of monosaccharides are xylose, while softwoods produce hexoses. The dry matter content of the hydrolyzate is 6-8% and, due to the special hydrolysis conditions, in which the hemicellulose is mainly in the inosaccharides and the formation of furfural is minimal, it is further processed, 150 mm. evaporation, easy and no caramelization occurs.

The prehydrolysis carried out in this way facilitates the production of high-quality cellulose because the condensation of lignin at a low hydrolysis temperature is low and the sulfate cooking after prehydrolysis is therefore easy to perform.

155 It is possible to cope with a low dose of alkali and a low cooking temperature, so the energy consumption of the process is also reduced for sulphate cooking.

Alkali is also saved due to efficient intermediate washing, which means that it is used little to neutralize the hydrolyzate. In particular, 160 when birch is used as a raw material, the content of pulp extractants tends to remain too high. This can be controlled by adding at least 20 kg of pulp oil per tonne of pulp to the sulphate cooking step and by carrying out a chlorination step of the bleaching with at least 75% chlorine dioxide.

Claims (3)

63267 A process for the production of high-alpha special pulp suitable for chemical further processing and specialty papers by a multi-stage digestion, in which the wood is hydrolyzed by pretreatment with mineral acid before cooking by the sulphate process, characterized in that sulfuric acid is added for prehydrolysis step 3 to 1.0% and water so that the wood-liquid ratio is 1: 2.5 to 1: 3.5 and at this stage is kept at a temperature of 140 to 150 ° C for 30 to 60 minutes, dissolving 20 to 30% of the wood in the dry state. material. Process according to Claim 1, characterized in that the hydrolysis uses, instead of acid, the waste acid from the chlorine dioxide plant, which is obtained during the production of chlorine dioxide from Na chlorate. Process according to Claim 1, characterized in that a hydrolyzate obtained from the same process, a high-yield cooking broth or a waste broth obtained from a mechanical defibering process is used for liquid filling.