FI119328B - Process for the preparation of chemical cellulose pulp - Google Patents

Description

; 119328 \ ι · f '' Process for the production of chemical pulp

The present invention relates to a process for the production of chemical cellulosic pulp from lignocellulosic fibrous material 5 by the sulfate process according to the preamble of claim 1.

According to such a method, the fibrous material is pretreated with hydrogen sulphide before being contacted with the sulfide-containing broth under alkaline conditions.

10 Raw material costs (wood, chemicals) are the biggest contributors to the operating costs of chemical pulp, ie pulp manufacture. To reduce the cost of wood, the selectivity of total delignification should be increased by stepping up the steps and proper delignification. Most delignification occurs in the cooking stage, so cooking is the most crucial factor in the selectivity of total delignification. Soil selectivity is increased by increasing the rate of delignification and / or the stability of the polysaccharides.

Appropriate cooking conditions and chemicals are used to achieve this. To minimize costs, cooking chemicals should be manufactured in-process.

The sulphate process is the predominant and by far the most important of the current cooking processes.

20 Sodium hydroxide and sodium sulfide are used as cooking chemicals in the process. Sodium and sulfur are recycled in-process at a ratio typically of S / Na 2 = 0.3-0.4.

. t Increasing this ratio creates additional hardware requirements and is difficult to implement in practice. The ratio, while maintaining the selectivity of the sulphate soup, can be increased to some extent by, for example, suitable alkali and temperature profiles, which allows the soup * · 2 ': 25 to be brought to a lower kappa number without substantially reducing the mass quality.

However, such an 'extended' or extended soup causes a loss of yield which increases. wood costs.

* «· •; · · J In sulfate cooking, the yield loss can be reduced by increasing the stability of polysaccharides · 3: 30 against alkaline degradation. This can be achieved e.g. by reacting polysaccharides with specialty chemicals. Available cooking chemicals • 1 are anthraquinone and polysulfide. The former is an easy-to-use but expensive, only-available soup supplement that limits its use. For • · 2,. the yield gain in sulphate soup is about 1% unit.

• 1 1 • ·· 3 • · 2 119328

An alternative method for improving the yield of a sulphate soup has been the so-called. H2S pre-treatment of the chips, where the chips have been pre-treated with HiS gas (approx. 5 bar) under slightly alkaline conditions before cooking. Yield increases have been high, 4-8 percentage points. H2S pretreatment of the chips transforms the aldehyde end groups of the polysaccharide chains into thiols and stabilizes them to withstand 5 alkaline degradation. However, the process requires special preparation of H2S and has not reached industrial use, which is largely due to the unpleasantness of the substance and the hazardous nature of its handling.

It is an object of the present invention to eliminate the drawbacks associated with the prior art and to provide a novel solution for increasing the yield of chemical pulp, especially sulphate pulp.

The present invention is based on the idea that H2S is formed inside chips, whereby the reaction between hydrogen sulphide and wood is carried out in situ, while hydrogen sulphide v 1S is formed. Preferably, such H2S precision treatment is accomplished by setting the pH of the cooking chips on the acidic side, followed by impregnation of a slightly alkaline sulfide-containing solution. The pH of the chips is then slightly acidic or neutral, whereupon hydrogen sulphide is released from the sulfide solution, •; *; which immediately reacts with the wood (in status nascendi) and no excess H2S gas ··· * 20 is formed in the digester.

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The pretreatment according to the invention thus comprises two steps, in which the chips or the like *: * *: the lignocellulosic fibrous material is first acidified and then the fiber material · · ·! ',: Contacting with a sulphide-containing solution, in particular with an aqueous sulphide-solution whose pH is only slightly alkaline to liberate hydrogen sulphide ··: * · * within the fiber. After pretreatment, the fibrous material is subjected to cooking with alkali * ··: ... * to defibrate the fibrous material and prepare a chemical pulp.

More specifically, the method according to the invention essentially comprises:. 30 is characterized by what is set forth in the characterizing part of claim 1.

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The invention provides significant advantages. Thus, the invention makes it possible to produce chemical pulps in an economical and environmentally sustainable manner, thereby enhancing the efficiency of the soup recovery processes by in-process means. As stated above, the effect of improving the yield of 3,119,328 chips under the pretreatment of hydrogen sulphide is known. However, in the prior art processes, the pretreatment has been carried out by introducing or forming hydrogen sulphide into the external and pressurized gas phase from which it has been transferred to the chips during processing. The treatment of hydrogen sulphide 5 in pressurized equipment required by the procedures has prevented their introduction into service due to the hazardous nature of hydrogen sulphide. The newly developed in situ procedure eliminates or substantially reduces the risk of incidents occurring.

The invention also provides a significant additional advantage in cases where the chemical streams of the pulp mill 10 are, for other process engineering reasons, divided into high sulfur and low sulfur streams such as naturally occurring recovery processes based on black liquor gasification. The invention is particularly well suited for use when it is desired to treat the chips before cooking with alkali under acidic conditions, for example to reduce its metal content, a procedure which has been found to have a beneficial effect in the preparation of sulphate pulp.

The invention will now be further explored by means of a detailed description.

Figure 1 is a flow chart of a first embodiment of the invention; Figure 2 is a flow chart of a second embodiment of the invention; Fig. 3 shows a process diagram of a third embodiment of the invention and V ·: “Fig. 4 shows the effect of pretreatment of HS and H2S (w situ) chips on the yield of sulphate soup under laboratory conditions.

It is an object of the present invention to increase the yield of pulp in sulphate soup by pretreating the chips with H2S (m situ). According to the method, H2S is generated within the chips by impregnation of sulfide with pre-adjusted chips of a suitably acidic pH. · · ·, Solution. By "sulfide solution" is meant a solution, especially an aqueous solution containing at least

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* 30 hydrogen sulfide (HS ") and / or sulfide (S ') dissociated to some extent in the solution phase.

[Hydrogen sulfide and sulfide ions form under neutral or mildly acidic conditions. hydrogen sulfide, H2S, which is a gaseous compound that reacts inside wood chips with wood constituents, particularly polysaccharides, thereby stabilizing them to withstand cooking conditions.

The amount of excess H2S gas formed outside the chips is thus negligible compared to the H2S pretreatment methods described above.

Hydrogen sulfide (HS ") is absorbed into the fibrous feedstock in an amount of about 0.1 to 2 mol / kg dry fiber material (e.g., chips), typically about 0.2 to 1.5 mol / kg.

A preferred embodiment of the invention is one in which only the amount of sulfide from the sulfate process is fed to the pretreatment step, which, for the most part, undergoes H2S for stabilization reactions with the wood components. Typically up to 40%, preferably from about 5% to about 25%, of the sulfide of the sulfate process is fed to the pretreatment. This is an advantageous option for the recovery of chemicals. Higher doses of hydrogen sulphide can be used for the pretreatment step, with only a portion involved in H2S stabilization reactions. The remainder of the sulfide then participates in the delignification reactions during the actual cooking, when the amount of alkali required for cooking is added to the digester after absorption.

15

The above-mentioned embodiment is particularly advantageously implemented by dividing the chemical stream of the sulfate soup into a sulfide-containing and sulfide-poor portion by methods known per se, and using the sulfide-containing portion for the pretreatment step and the sulfide-poor portion for alkali dosing.

20th According to a particularly preferred embodiment, a mildly alkaline sulfide solution such as • 1 "1 l of hydrogen sulphide solution of an alkali metal is used to achieve a pH of" · i "*. Suitable for the release of hydrogen sulphide with minimal acid treatment. it is preferable for the chips, as made herein, to have a weak acid such as acetic acid or carbonic acid so that the pH of the chips during acid pre-treatment does not decrease too much in terms of the quality of the pulp obtained (cellulose and hemicelluloses). acid hydrolysis).

• · * 1 1

On the other hand, the amount of acid must be suitable so that, for example, after addition of sodium hydrogen sulfide, the pH is adjusted to the correct level for H2S reactions, typically 6-7, where • ·. In addition to H2S, the 1 '1 reaction mixture also contains HS' ions.

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30 • · t (.) The cooking solution described above boils a fibrous starting material, such as wood chips or • «• 'starting material from annual or perennial plants, to the desired kappa number. Wood chips] | can be derived from softwood such as pine or spruce or hardwood such as birch. »· 1 or a wound.

5, 119328

Typically, the kappa number is 20-50 on conifers and 15-25 on hardwoods after cooking.

After cooking, the pulp is separated from the broth by washing the broth. The pulp is recovered and bleached if desired. Conventional ECF or TCF bleaching sequences may be used for bleaching. The broth is regenerated by concentrating the black liquor and burning it in a recovery boiler. The salts obtained after combustion are prepared into a hydrogen sulfide solution and white liquor.

In the following, we will examine three different process alternatives of the invention using the accompanying drawings. The following reference numerals are used in the figures: 1; 21; 41 chips acidic impregnation treatment 2; 22; Impregnation of 42 chips, acidified or neutral pH, with 15 sulfide-containing solution 3; 23; 43 chips soup 4; 24; 44 evaporation and incineration of black liquor 5; 25; Causticization of 45 white liquors 6; 26; 46 white liquor stream 20 7; 27; 47 sulfidyl solution 8; 28; 48 chips feed 9; 29; 49 acid stream 10; 30; 50 cellulosic pulp recovery 11; 31; 51 black liquor streams. 25 12; 32; 52 total green liquor streams 13; 33; 53 Green liquor stream to prepare hydrogen sulphide solution • '»., 14; 34; 54 Green liquor flow to white liquor production *. . 15; 35; 20 stream of sodium carbonate * «" 16; 36 difference between sodium sulfide and sodium carbonate 30 17 hydrogen sulphide stream; 40 sodium sulfide stream 18; 38; 58 Preparation of hydrogen sulphide from hydrogen sulphide (18) or sodium sulphide O (38,58) 19; 39; 59 CO 2 streams: 35 • · ... All three process options have in common the acidic soak treatment of chips (1; «·" * 21; 41), the pH setting (2; 22; 42), the chips cooking in the digestion fluid of the sulphate process (3; * 23) ; 43) and evaporation and incineration of black liquor (4; 24; 44).

• · «40 The following are the first steps in these common process steps: • 6 119328

Acid Absorbent Treatment: Hake 8; 28; 48 subjected to impregnation treatment 1; 21; 41, which is carried out, for example, in a soaking tank, which can conveniently be heated to the desired temperature by means of heating elements or a jacket to be heated inside the tank. The impregnation treatment is carried out with a mildly acidic acid such that the pH of the chips can be lowered to about 3-6. It is essential that the chips become so acidic that their pH is at most neutral or slightly acidic in the subsequent hydrogen sulphide or sulphide treatment. The acid may be inorganic or organic acids or gases which, when dissolved in water, form the acid. Of the inorganic acids, mention may be made of dilute aqueous solutions of strong mineral acids such as sulfuric acid, nitric acid and phosphoric acid, and weak acids such as boric acid and carbonic acid. Of the organic acids, mention may be made of alkanoic acids such as acetic and formic acids, and sulfonic acids. Particularly preferred acids are acetic acid, carbonic acids and other mildly acidic acids which do not contain nitrogen or phosphorus or any other element difficult to close the process.

15 The acid to chips ratio (volume / weight) varies with the acid used and the moisture content of the chips. Generally, the volume of the acid is at least 2 times the weight of the chips, typically about 2.5 to 10 times the weight. The excess acid is separated from the chips and the chips must retain an amount of acid which is suitable for the next step, i.e. sulfide treatment, to achieve the above-mentioned correct pH, i.e., about 6-7.5.

20

The acid treatment is generally carried out at a temperature in the range of about 20 ° C to about 80 ° C, under reduced pressure, under non-pressurized (i.e. normal weight) conditions or under excess pressure. Treatment pressure S * · · *. , (absolute pressure) is generally in the range of 0.01 bar to 10 bar, typically about 0.1 to 5 bar.

• · • ·· * * It is usually possible to operate under non-pressurized conditions, but applying vacuum at a pressure of * 4 · •> 1 j 25 can enhance the absorption of liquid into the chips.

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The acid can be supplied separately, e.g. 29; 49 into or into the absorption tank: can be mixed with chips before feeding.

• • • • • • * • · · *. 30 Sulphide treatment: Hydrogen sulphide (HS ') - **** 9 is further introduced into the acid impregnated chips. or a solution containing sulfide (S) ions 7; 27; 47 which is allowed to be absorbed by the chips.

• · • ^ Sulfide Treatment 2; 22; 42 is carried out in the same container as the acid impregnation treatment or ] can be performed in the digester. It is also possible to perform all processing steps in the I * 1 * * · digester. When the chips are reacted with an acid soaked in, the sulfide solution 7 119328 forms hydrogen sulfide (H2S) inside the chips. The optimal pH of the reaction is 6-7.5.

In order to keep the amount of acid to be absorbed in the previous step small, it is preferable to use a solution containing hydrogen sulphide ions containing as little acid neutralizing agents as possible. Such sulfide compounds include the hydrogen sulfide salts of alkali metals, such as sodium hydrogen sulfide. The hydrogen sulphide pre-treatment is preferably carried out at an elevated temperature, preferably from about 120 ° C to about 150 ° C. The reaction is carried out under non-pressurized conditions (wherein the temperature is lower than above) or preferably under pressurized conditions, e.g., from about 1 bar to about 10 bar absolute.

10

Chip cooking: White liquor 6 is added to the pre-treated chips in the next process step; 26; 46 and the chips are cooked in the sulfate soup in the normal way. As stated above, according to a preferred embodiment, the sulfide portion of the soup is fed to the chips pretreatment, whereby the sodium hydroxide portion is added in the cooking step. The temperature is raised to the range of normal sulfate cooking, i.e. about 140-170 ° C.

Evaporation and burning of black liquor: After cooking, the mass 10 is separated; 30; 50 and the spent liquor, i.e. black liquor 11,31,51, is normally evaporated and burned in a chemical recovery cycle.

20

Part 13; Of the black liquor obtained from burning black liquor 12; 32 is deduced to the separation of sodium 1: 1: sulfide (Na 2 S) and sodium carbonate (Na 2 CO 3); 36. The separation may be effected by any known method, such as addition of carbon dioxide 19; 1i stripping hydrogen sulphide 17 under reduced pressure or crystallizing from sodium carbonate ** 25 36.

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In the case of Figure 1, hydrogen sulphide 17 is converted in step 18 to sodium hydrogen,. with sulphide sodium carbonate 20.

• · 1 · »· • 1 · *. According to Figure 2, sodium sulfide remains in the mother water 40 when the sodium carbonate is separated. by crystallization from green liquor. In this case, the sodium sulfide is converted in step 38 · ·. with carbon dioxide 39 or other suitable acid into sodium hydrogen sulfide 37.

• 1 * · • · »Φ · • 1 8 119328

According to Figure 3, sodium hydrogen sulfide can also be prepared directly from green liquor 53 in step 58 by introducing carbon dioxide 59 or other suitable acid therein.

In all of the above cases, the remainder of the green liquor 14; 34; 54 and sulfide 5 from sodium carbonate 15; 35 is causticized to white liquor 6; 26; 46.

Example

Under laboratory conditions, preliminary experiments have been conducted to determine the effect of H2S-10 (in situ) pretreatment on the yield of pine chips sulfate soup. The results have been compared with a conventional sulphate soup (Sa) and a soup in which chips have first been treated with hydrogen sulfide (HS pretreatment). The latter is similar to that of the invention, but without the acid being absorbed into the chips, so that the pH during treatment is higher and no hydrogen sulphide is formed. In all cases, a different kappa number has been obtained by varying 15 servings of alkali in the actual cooking stage.

According to the invention, the pH of the chips is adjusted by mild acid pretreatment (acetic acid) at room temperature under normal atmospheric pressure. The pre-set chips are impregnated with sodium hydrogen sulfide solution (1 mol HSTkg, 0.75 h / 140 ° C) followed by the addition of cooking liquor (5.5 mol NaOH / kg) and cooking (factor H 1600). By doing so, the yield of the soup has been 3-4 percentage points higher than that of the reference sulphate soup: the same kappa yield (see Figure 4). Compared to hydrogen sulphide treatment, the solution according to the invention • · • 1 • 2 achieves an improvement of about 1.5%.

• a t a • • • • • • • 1 «• · a • 1 a a • •» 1 * «· a a •» a * a a · a a 2 a • a • a a a a a a a a a

Claims (13)

119328 A process for the preparation of a chemical cellulosic pulp from a lignocellulosic fibrous material comprising: 5. treating the fibrous material with hydrogen sulphide prior to contacting it with a sulphide-containing boiling liquor under alkaline conditions, characterized in that - hydrogen sulphide is formed within the chips, . 10 Process according to Claim 1, characterized in that the fibrous material is acidified, after which a slightly alkaline sulphide-containing solution is impregnated into the fibrous material, whereupon the sulphide solution releases hydrogen sulphide which reacts with the fibrous material. 15 Process according to claim 1 or 2, characterized in that the fibrous material is impregnated with acid to such an extent that the fibrous material is adjusted to a pH of 6 to 7.5 when the material is contacted with a slightly alkaline sulfide solution. A process according to any one of claims 1 to 3, characterized in that the fibrous material is impregnated with acid at about 20-80 ° C. a · * · »* a« * a a The process according to any one of claims 1 to 4, characterized in that a sulfide solution is fed to the a · *; * | pretreatment step in an amount such that the hydrogen sulfide is present in an amount of 0.1 - 2 mol. HSVkg dry fiber material • aa aa • aaa The method according to claim 5, characterized in that the pre-treatment 4 #. only an amount of the sulfide from the sulfate process is fed to the step, which, through H2S, is used for the most part in the stabilization reactions with the wood components. a aa # • 30 a A process according to claim 5, characterized in that substantially all of the sulfide of the sulfate process is fed to the pretreatment step, part of which through H2S is involved in stabilization reactions. A process according to any one of claims 5 to 7, characterized in that after the pretreatment, the alkaline dose required for cooking is added to the digester, the remainder of the sulfide being involved in delignification reactions. Method according to one of Claims 5 to 8, characterized in that the chemical stream of the sulphate soup is divided into the sulphide-containing and sulphide-poor portion and the sulphide-containing portion is used for pretreatment and the sulphide-poor portion is used for alkali dosing. Process according to Claim 9, characterized in that part of the green liquor of the process 10 is subjected to separation of sodium sulfide and sodium carbonate, which is fed with carbon dioxide, the hydrogen sulfide thus obtained is contacted with sodium carbonate to form sodium hydrogen sulfide, and sodium hydrogen sulfide is fed to the pretreatment. Process according to Claim 9, characterized in that part of the green liquor of the process 15 is separated by crystallization, whereby the sodium sulphide remains in the mother liquor, and the sodium sulphide is converted by carbon dioxide into sodium hydrogen sulphide which is fed to the pretreatment. Process according to one of Claims 5 to 8, characterized in that part of the process liquor 20 is converted to sodium hydrogen sulphide by passing it to the green liquor. carbon dioxide, and sodium hydrogen sulfide is fed to the pretreatment. · • 1 · »» · 1 • · • * * · · *. . The process according to any one of claims 10 to 12, characterized in that the · · ·, 1,1, j of the remaining green liquor in the process is causticized to white liquor. • ·· 25 · * ·· • · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · 9,993 ·