PROCESS FOR PREPARING A PULP-COOKING SOLUTION

Abstract

 The proposed invention deals with pulp and paper production, in particular, with the process for preparing pulp cooking liquors to manufacture a fibrous semifabricate.
A cooking liquor is proposed to be prepared by saturation of a water medium with sulfurous anhydride provided that this water medium is in fact either a pulp and paper mill waste water with activated sludge (18-40g/l) and soda ash (1,55-2,50% by weight) in it or alkali hydrolyzate produced in treatment of the same waste water by a mixture of base salts containing sodium carbonate (50-90% by weight based on Na₂O) at the temperature 25-40°C followed up with a solid sediment removal and the saturation goes on until sulfurous anhydride concentration in the liquor is 2,5-5,5% by weight.
The proposed method allows to increase considerably the output of a fibrous semi-fabricate and improve its mechanical characteristics not to speak of utilizing waste water generated at a waste water treatment plant of a pulp and paper mill and expanding a sources of raw materials needed for cooking liquors preparation.

Description

[0001] The proposed invention deals with pulp and paper production, in particular, with the process for preparing pulp cooking liquors to manufacture a fibrous semifabricate.

[0002] A method for preparing a pulp cooking liquor by adding a sulfate into a water medium is known (1).

[0003] Saturation of a water medium with sulfurous anhydride until the required pH-value is reached represents another well known method to prepare acid cooking liquor (2,3).

[0004] The disadvantages of the methods enumerated are as follows: a considerable amount of fresh water needed to prepare the cooking liquor, a rather costly treatment needed for the resultant effluents and a low yield of the final pulp occurs.

[0005] The closest to the proposed invention is a well known method for preparing a cooking liquor to manufacture a fibrous semifabricate. The method in question requires waste water that has been treated biologically be used as a water medium. This waste water that normally contains an excessed activated sludge in the range of 0,5 - 17,0 g/l is to be withdrawn from a secondary thickener to dissolve sulfates. Should sulfite or bisulfite cooking methods are used the abovementioned water medium is to be saturated with sulfurous anhydride until the required pH value is reached (4).

[0006] This method may offer cost advantages resulting from replacement fresh water with waste one after biological treatment thereof. However, this method leads to a comparatively low pulp yield and doesn't secure that the required level of its quality parameters needed now will be reached. Besides, waste water at low activated sludge concentration is used as a water medium here.

[0007] The proposed invention is aimed at creating a method for preparing a pulp cooking liquor to manufacture a fibrous semifabricate in a way that makes it possible to increase the yield of the final product, improve its quality as well as to extend a stock of row materials needed for preparing sulfite cooking liquors.

[0008] The abovementioned objectives of the invention are reached in such an event when either soda ash (1,5 - 2,50 % by weight based on Na₂ O) in form of a solution in biologically treated effluents of pulp and paper manufacture which in turn run between 18 and 40 g/l suspended solids or alkaline hydrolizate generated at the treatment of the same effluents by a mixture of basic salts (both Na₂CO₃ - 50 - 90 % by weight and CaO - 1,5 - 2,5 % by weight based on Na₂ O) at the temperature of 25 - 40°C accompanied by removal of resulting precipitate are used as water medium while saturating thereof with sulfurous anhydride at 35-40°C until its content in the cooking liquor is 2,2-5,5 % by weight based on the total mass of the solution.

[0009] The proposed method differs from the well known one by the following features: soda ash (1,5 - 2,50 % by weight based on Na₂ O) in form of a solution in biologically treated effluents of pulp and paper manufacture running between 18 and 40 g/l suspended solids or alkaline hydrolizate generated at the treatment of the same effluents by a mixture of basic salts (Na₂CO₃ - 50 - 90% by weight and CaO - 1,5 - 2,5 % by weight based on Na₂ O) at 25 - 40°C accompanied by removal of resulting precipitate are used as a water medium.

[0010] The usage of either soda ash dissolved in pulp and paper effluents or hydrolizate originated from the same effluents makes it possible to enhance efficiency of pulping since the products of alkaline or acid hydrolysis that found their way into the cooking liquor act as inhibitors thus eliminating the processes of thermo-oxidizing destruction of carbohydrates.

[0011] When sulfurous anhydride is used to saturate soda ash solution in biologically treated waste water containing an excessed activated sludge there is an acid hydrolysis of the bio-organic part thereof occurs there since that part accounts for 83% by weight based on dry residue.

[0012] Should the mixture of basic salts is used to treat the abovementioned waste water a reaction of caustization will proceed prior to complete saturation thereof

        Na₂CO₃+CaO+H₂O=2NaOH+CaCO₃

accompanied by alkali hydrolysis of bio-organic part of the activated sludge.
The major components of both acid and alkali hydrolizate of activated sludge biomass are aminoacids and their sodium salts (approximately 68 - 80% by weight based on total organic substances passed into solution), carboxylic acids or their salts (10 - 15% by weight), neutral ether-soluble substances (5 - 12% by weight) and phenols (2 - 12% by weght).

[0013] It's necessary to stress that activated sludge biomass from a pulp and paper waste water treatment plant contains a significant amount of fiber fines, slime-lignin and saw dast the total amount of which accounts for up to 17% by weight based on dry residue.

[0014] When mixed carefully calcium carbonate produced according the above referred to reaction of caustization distributes proportionally among solid non-hydrolyxed particles of activated sludge biomass. As a result, non-hydrolyzed sediments of activated sludge biomass which normally retain water firmly and therefore can hardly be separated are bound to obtain a favorable structure that makes it possible to dispose them easily by filtration or sedimentation.

[0015] The proposed method means the use of industrial waste water after biological treatment with concentration of activated sludge biomass in the range of 18 - 40g/l. Such concentration obtained by preliminary consolidation of activated sludge biomass in special settlers, is optimal since it mantains higher than usual content of acid and alkali hydrolysis products in a cooking liquor that are formed during its preparation and thus, as it has been mentioned, making cooking more effective. Besides, the usage of industrial waste water with high concentration of activated sludge (18-40g/l) allows to utilize activated sludge biomass.

[0016] Water medium saturation with sulfurous anhydride at 35-40°C up to concentration of 2,5-5,5% by weight first of all makes it possible to reach complete acid hydrolysis of activated sludge biomass followed by generation of substances that increase the quality of the cooking liquor, and secondly it means the optimal conditions of cooking namely the increase of fibrous semi-fabricate yield and the improvement of its mechanical properties.

[0017] As it has already been mentioned, the usage of alkali hydrolyzate as a water medium allows to get rid effectively (by means of co-precipitation with calcium carbonate) from a non-hydrolized part of activated sludge biomass, which makes up to 17% by weight based on absolutely dry substance thus improving the quality characteristics required for a fibrous semi-fabricate.

[0018] The proposed conditions for alkali hydrolizate production (treatment by mixed base salts containing 50-90% of sodium carbonate by weight and 10-15% of calcium oxide by weght in amount of 1,55-2,50% by weight based on Na₂O at the temperature 25-40°C followed up with removal of sediment) makes it possible to reach a complete alkali hydrolysis of bio-organic part of activated sludge and to remove an insoluble slime. All this in turn, increases the efficiency of cooking process and makes the quality of the fibrous semi-fabricate better.

The method is carried out in the following way:

[0019] Sodium carbonate (in amount that makes its concentration in solution equal to 1,55-2,50% by weight based on Na₂O) is added to biologically treated waste water of a pulp and paper mill after settling thereof in a radial or vertical thickener until the activated sludge concentration in it has reached the level of 18-40g/l. The resultant soda ash solution is sent to an absorber for saturation with sulfurous anhidride at the temperature 35-45°C until its concentration is of 2,5-5,5% by weight and organic part of the sludge is dissolved. Or a pulp and paper mill waste water that has been treated biologically and then stored in a thickener until the excessed activated sludge concentration in it has reached the level of 18-40g/l is delivered to a mixer to be treated with a mixed base salts in amount of 1,55-2,50% by weight based on Na₂O at the temperature of 25-40°C provided that it is accompanied by thorough mixing until the activated sludge biomass is dissolved completely.

[0020] Then the resulting mixture containing alkali hydrolizate, undissolved slime and calcium carbonate is to be dewatered by filtration or precipitation. The resultant clarified alkali hydrolyzate is sent to an absorber for saturation with sulfurous anhydride at the temperature of 35-45°C until the concentration thereof is 2,5-5,5% by weight is reached.

The metod is illustrated by the following examples:

Example 1.

[0021] 360 g. of air dry chips were loaded into 15-litre autoclave made of stainless steel with 9 litres of cooking liquor added in it. The cooking liquor has been prepared by saturation of a waste water from a pulp and paper mill and the latter has in it the following: activated sludge biomass in the amount of 30g/l and soda ash in the amount of 2% by weight based on Na₂O. The saturation is to be proceeded at the temperature of 40°C until sulfurous anhydride concentration is 4,5% by weight.

[0022] The cooking process was carried out during 2 hours 15 minutes while the temperature being raised from 25°C to 100°C; then during 45 minutes the temperature was kept at 120°C and during another 1 hour and a half it had been raising from 120°C to 160°C. Then the process proceeded at that temperature (160°C) for another 1 hour. When the cooking is over a 75g/l sample of the resultant semifabricate was used to determine both its whiteness and mechanical characteristics. The results obtained are given in the Table.

Example 2

[0023] The cooking liquor was prepared in the following way: The waste water from a pulp and paper mill with activated sludge making up 30g/l in it was treated by a mixed base containing the equal parts of sodium carbonate and calcium oxide in amount of 2,5% by weight based on Na₂O at the temperatute 35°C. After the sediment had been removed the alkali hydrolyzate was saturated with sulfurous anhydride at the temperature 40°C until the concentration thereof was reached 5,5% by weight. The resultant cooking liquor with products yielded by the alkali hydrolysis of activated sludge had 1,9 pH value. The cooking process was carried out under the same conditions as it was in the Example 1. The results are given in the Table.

[0024] The data obtained shows that the combination of the conditions needed for the preparing of the cooking liquor leads both to the considerable increase in the yield and improving in mechanical characteristics of the final product in contrast to that of the well known method.

[0025] Moreover, the usage of fresh water for a cooking liquor preparation is eliminated and a stock of raw materials for its production is expanded. At the same time it gives a way of utilizing of all waste water produced by a pulp and paper mill and posing a real threat to the environment.

TABLE

Example number	Pulp output, %	Characteristics of fibrous semi-finished product.
		Delignification degree, Kappa	Tearing length, m	Tearing resistance, g	Pressing resistance, kg/sm2	Whiteness, %
1	73	79	11 280	64	3,5	58,8
2	78	86	11 920	67	3,8	59,7
3*	52	57	10 360	61	2,9	55,8

* Example 3 illustrates a well-known cooking preparation method, according to author s certificate of the USSR Nº 746002.

The cooking process is held in conditions of Example 1.

REFERENCES

[0026] 

1. N.H. Nepenin, 〈〈Pulp Production〉〉, Volume 1, Moscow, 1976, page 142.

2. USSR Author

s Certificate Nº 376507, D21C 3/06, 1973.

3. USSR Author

s Certificate Nº 403795, D21C 3/06, 1974.

4. USSR Author

s Certificate Nº 746002, D21C 3/04, 1980.

Claims

1. The cooking liquor preparation method for producing a fibrous semi-fabricate is carried out by the water medium saturation with sulfurous anhydride differs from the others owing to the fact that the water medium being in fact either a biologically treated pulp and paper mill waste water that contains the following: activated sludge biomass in the range of 18-40g/l and soda ash running between 1,55-2,50% by weight based on Na₂O or alkali hydralyzate produced in treatment of the same waste water by a mixture of base salts containing sodium carbonate (50-90% by weight based on Na₂O) at the temperature 25-40°C followed up with a solid sediment removal is used to be saturated with sulfurous anhydride at the temperature 35-45°C until the concentration thereof is 2,5-5,5% by weight.