CA1194656A

CA1194656A - Process for the preparation of the digesting liquor required in a neutral sulfite cooking with an addition of anthraquinone

Info

Publication number: CA1194656A
Application number: CA000389103A
Authority: CA
Inventors: Ismo Reilama; Arto Vainiotalo; Jukka Kettunen
Original assignee: Rauma Repola Oy
Current assignee: Repola Oy
Priority date: 1980-11-14
Filing date: 1981-10-30
Publication date: 1985-10-08
Also published as: DE3142787A1; FI803574L; FI67105B; FI67105C

Abstract

ABSTRACT A process for the preparation of a neutral-sulfite digesting liquor and for adjusting the pH of the liquor while starting from a sodium-sulfite/sodium-carbonate solution that contains sodium bisulfite and/or sodium bicarbonate. The pH of the solution is raised to the pH-range of 8 to 11 by stripping carbon dioxide off the solution by means of water vapour or some other inert gas. The final adjustment of the pH of the cooking liquor can be performed by means of sodium hydroxide.

Description

Process for the Preparation of the Digesting Liquor Required in a Neutral Sulfite Cooking with an Addition of Anthraquinone The present invention is concerned with the preparation of the digesting liquor to be used in the neutral-sulfite pulp cooking so that no causticizer plant is required. It is typical of the sodium-neutral-sulfite cooking meant in the present invention that anthraquinone or some of its related agents is used as an agent accele~
rating the delignification and that the cold-pH of the digesting liquor before cooking is 7 to 12.5 and that by means of the process it is possible to produce paper and cardboard pulps within a yield range of 45 to 90 %.
At present the sulfate process is by-far-the most important one among chemical processes for the preparation of pulp. As is well-known, it, however, involves certain disadvantages, owing to ~hich active attempts have been made to develop an alternative pulp preparation process for it. The most important drawbacks burdening the sulfate process are the odour produced by it and resulting from organic sulfur compounds as well as from hydrogen sulfide, the low yield of pulp, and the expensive system of recovery of chemicals.
In the alkaline sulfite cooking process patented by Ingruber (Finnish Patent 53,331, granted 10 April, 1973), ~herein NaOH and Na2SO3 are used as cooking chemicals, it has been possible to eliminate the odour typical of sulfate cooking, but the consumption of wood per pulp ton in the process is of the same order as in sulfate cooking, and the regeneration of the chemicals is more complicated than in the sulfate process. This comes from the circum-skance that the Na2S formed in the soda digester must be converted into Na2SO3,and Na2CO3 by means of causticizing into NaOHg ~hereas in the recovery step in the sulfate process only the Na2CO3 is converted into NaOH.
Nor have the pulp production processes based on soda digesting been able to supersedethe sulfate cooking.

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The reason for this is that it has not been possible to prove that the processes could bring about any essential improvement as compared with the sulfate process. In the first place, the process is not odourless, even though Na2S is not used in the digesting properly speaking. This results from the concentration of the sulfur carried along with the water and wood in the process circulations~ which causes a sufficient level of sulfidity for creating an odour. Moreover, the quality of the pulp produced by means of pulp production processes based on soda digesting rarely reaches the level of sulfate pulp, and the process does not give any advantage wor-th mentioning in respect of the consumption of wood.
A more extensive use of pulps prepared wi~hin an acid pH-range has been prevented mainly by their lower strength, in particular tearing strength, as compared with the sulfate pulp.
i A neutral-semichemical sulfite process (NSSC-cook), wherein Na2S03 and Na2C03 are used as digesting chemicals typically with the ratio of 3 : 1~ is used mainly for producing the pulp used in the production of the middle layer of corrugating medium.
Mainly hardwood species are used as raw mate-rial. Application of the process to the preparation of chemical conifer pulps to be bleached has been prevented by the slow defibration process that requires very strong digesting conditions as well as, at the final stage of the delignification, also by non-selective dissolution of the llgnln .
Now, it hasg however, been ascertained (e.g., Paperi ja Puu 61 (1979):11, 6~7-700) that when anthra-quinone is used as an additive in the neutral sulfite cooking, as a quantity of 0.1 to 0.5 % as calculated from the quantity of wood, it is also possible to prepare a chemically defibrating pulp that is also suitable for products that are supposed to be bleached, favourably out of coniferous wood as well. In such a neutral-sulfite-anthraquinone cooking, for which the abbreviation NS-AQ
is used, the cooking conditions are preferably as follows:
- cooking temperature 165 to 180C
- cooking time at the maximum temperature 160 to 260 min.
- total alkali dose 22 to 24 % NaOH
- out of the total alkali dose, 70 to 95 %, preferably 80 to 85 %~ consists of Na2S03 and the rest of Na2CO3, and in certain cases a little of NaOH (all as equivalents) - pH of the cooking liquor to be added to the digester:
10.5 to 12.5 (at 25C) - anthraquinone dose 0.1 to 0.2 %, from quantity of wood.
The pulp prepared under the conditons described is, being chemically d~fibrating, of a high yield, 55 to 60 % from the quantity of wood at Kappa number 35 to 45, light, 40 to 53 % ISO, relatively easily bleaching, and, as containing an abundance of hemicelluloses as well as xylane and glucomannane~ a pulp that is ground very easily and that is clearly stronger than the traditional sulfite pulps and that has a good light-scattering ability. It has been noticed that a slightly bleached NS-AQ pulp is suitable, e.g., for the chemical pulp component of news-print equally well as semibleached sulfate pulp. More-over, it has been noticed that the pulp is suitable for certain wrapping and kraft papers as well as, being of a high yield, for rawmaterial of liner board almost equally well as sulfate pulp. ~ibre-technically the greatest advantage of the NS-AQ pulp,as compared with sulfate pulp, is its very high pulp yield. As bleached the yield advantage is 4 to 8 percen-tage units, and in the production of liner pulp even 15 to 20 percentage units. In other words~ a NS-AQ pulp that is paper-technically equal to sulfate pulp can be prepared as bleached with a yield of 48 to 52 % and for liner pulp with a yield of 70 to 75 %.
Thus, it is typical of the NS AQ cook that the cooking temperature is the same as or slightl~ higher than in sulfate cookingS preferably 175C, and that the cooking time at the maximum temperature is somewhat longer. Owing to the anthraquinone, it has been possible to lower the tota] alkali dose of the active chemicals in the NS-AQ cooking almost to the level of sulfate cooking (19 to 22 % as NaOH).
As a further advantage of the NS-AQ cooking as compared with sulfate cooking should be mentioned its odourlessness~ because in the digesting Na2S is not used.
One of the biggest burdens of the sulfate process are the high investment costs. The share of the recovery of the chemicals in these costs is 25 to 30 %, and about 20 % of this goes to the investments of the causticizer plant. In spite of several studies, it has not been possible to develop an essentially less costly solution for the recovery of the chemicals in the sulfate process. This is why one of the basic ideas in the work of development of the NS-AQ process has been to be able to prepare the cooking chemicals as advantageously as possib-le, both regarding the operating costs and regarding the investment costs. At a rather early stage of the studies it already came out that by means of the present commer-cial solutions for the recovery of the chemicals it was not possible to prepare a cooking liquor of the correct proportions of chemicals and of correct pH without using NaOH in the preparation. The quantity of NaOH required became so large that the circulation of chemicals could not be carried out economically without a causticizer plant. By means of the present solutions of equipment, the quantity of NaOH required in the preparation of the cooking chemicals can be minimized to the level of 35 to 45 kg of NaOH per ton of pulp of 90 % dry solids content when the sum of the bicarbonate and bisulfite contents in the liquor is at the minimum before the final adjustment of the pH (by means of NaOH) of the cooking liquor. As is known, the preparation of sodium-neutral-sulfite cooking liquor is typically performed so that S02 is passed into a solution of Na2C03 until the desired ratio of Na2~03 :
Na2C03 is reached. The liquor to be used in the NS-AQ

cooking can, however, no-t be prepared directly out of Na2C03 and S02, because the optimurn pH, in view of the cooking, and the S:Na2 ratio cannot be obtained simultan-eously. Therefore, in the absorption of S02 the pH mus-t be lowered to the level of ~.5 to 7.0 in order to ob-tain the cor-rect S:Na2 ratio (0.7 to 0.95) in the cooking liquor and in order to minimize the quanti-ty of NaOH required in the final adjustmen-t of the pHo Within this pH-range, some of the Na2C03 is i.n the form of NaHC03 and some of the Na2S03 in the form of NaHS03. Thus, the NaOH is consumed :Eor the neutralization of NaHC03 and NaHS03, and in the ready cooking liquor there is mainly Na2S03 and Na2C03, and possibl.y some NaOH if NaOH has been added more than would have been required for the neutralization of NaHC03 and NaHS03. From the point of view of the cooking, the excess of NaOH is, however, not necessarily required.
In accordance with one embodiment of the present invention, there is provided a process for the preparation of a neutral-sulfite digesting liquor and for adjust;.ng the pH of the liquor while starting from a sodium-sulfite/sodium-carbonate solution that contains sodium bisulfite and/or sodium bicarbonate, characterized in that the pH of the said solution is raised to the pH-range of 8 to 11, by stripping carbon dioxide off the solution by means of water vapour or some other inert gas.

~5a-In accordance with a preferred embodiment of -the invention, the pH of the solution ls raised to a pH-range of 9.5 to 10.5. Preferably, the final adjustment of the pH of the cooking liquor is performed by means of sodium hydroxide. Typi-cally, the quantity of NaOH required for controlling the pH is lower -than 11 kg, preferably lower than 8 kg, per -ton of pulp of 90% dry solids content. Water may be condensated off the stripping gas, and the CO2 concentrated in this way can be used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.
In the process of the present inven-tion, the sodium bicarbonate that is present in the sodiumsulfite-bisulfite-carbonate-bicarbonate solution, which was prepared, e.g., from sodium-bisulfite-sulfite solution by addition of Na2CO3 or from Na2CO3 solution by absorption of SO2 and whose pH is 6.5 to 8.5, is for an essential part converted into sodium carbonate by stripping CO2 off the said solution by the use of water vapour.
Hereinafter the quantity of NaOH required for the final adjustment of the pH of the cooking liquor is so lit-tle that it can be performed by means of make-up chemical. The sul-fite factories employing, e.g., the recovery system of Tampella (FI Pat. 45,576) or of Rauma (FI Pat. Applications 762710, pub-lished March 23, 1978, 790114, published July 16, 1980, 791015, published September 28, 1980, 791016, published September 28, 1980, 801959, published May 29, 1981, FI Pat. 57,137) could be thereby, in respect of the recovery, easily converted to the use of the NS--AQ cooking process, because causticizing is not required for the preparation of the cooking liquor.

Stripping in accordance with the present in-vention means removal of carbon dioxide from the sodium-sulfite-bisulfite-carbonate-bicarbonate solution by passing water vapour or some other inert gas, e.g. nitro-gen or well washed smoke gas of a low content of CO2, SO2and 2' upstream in relation to a stream of liquid in an apparatus in which effective contact is obtained between the vapour or inert gas and the liquid. Such an appa-ratus may be a column provided with filler pieces and/or intermediate bottoms, the liquid being passed to the upper part and the vapour or inert gas to the lower part of the column.
The stripping of CO2 can be performed under the normal atmospheric pressure, at a reduced, 0.1 to l.O
bar abs. pressure, or at an elevated, 1.0 to 5.5 bar abs. pressure. The economically op~imum pressure is determined by the mode of connection of the stripping to the rest of the surrounding process and by the pressure and temperature of the avai-lable stripping medium.
The process in accordance with the invention permits recovery of concentrated CO2 in order that the C2 could be utilized in the process of regeneration of the cooking chemicals in the carbonation of liquors, such as green liquor. If further use of the stripped CO2 requires an elevated pressure, the pressurization can already be performed in the stripping treatment under an elevated temperature.
An example of the effect of the present inven-tion in reducing the quantity of NaOH required in the preparation of NS-AQ cooking liquor is given below.

Case A Preparation of cooking liquor without stripping of bicarbonate Composition of the starting solution - pH 7.5 - S : Na2 0.88 - NaHCO3 0.418 Mol/l Na2S3 1.201 mol/l - NaHS03 0.07 mol/l - Na . . 2.890 mol/l tltratlng Out of this solution, cooking liquor (pH 11.5) is pre-pared by controlling the pH by means of NaOH, whereby the quantity of NaOH (l-n) consumed is 50.5 ml/100 ml of solution. The composition of the liquor coming from the regeneration and used for the cooking is~ after adjust-ment of the pH:
- pH 11.5 - S : Na2 75 atitrating 2.255 mol Na/l Na2C3 0.284 mol/l Na2S3 o.844 mol/l The quantity of NaOH required for adjusting the pH, with a total alkali dose of ?2 % (as NaOH) of the cook, equals 63.1 kg per ton of pulp of 90 % dry solids content, i.e. 15 % of the total alkali.
0 Case B Preparation of cooking liquor in accordance with the invention by using vapour stripping and NaOH-controlling The starting solution is the same as in Case Ao The solution was stripped by means of vapour to pH 10.05,5 whereupon the composition was as follows:
- pH 10.05 - S : Na2 o.88 Natitrating 2-890 mol/Na/
Na2C3 0.129 mol/l - NaHC03 0.034 mol/l Na2S3 1.271 mol/l Hereupon the pH of the cooking liquor was adjusted by means of NaOH (l-n) to 11.5; the consumption of NaOH was 3.5 ml/100 ml of solution. The final composition of the5 solution was:
- pH 11.5 S : Na2 0.869 6~

titrating 2-826 mol/Na/
Na2G3 0.185 mol/l Na2S3 1.228 mol/l Thus, in the example in accordance with the invention, NaOH is required 1.2 % of the total alkali, which, with the total alkali dose of the cooking of 22 % of NaOH, corresponds to 5.05 kg NaOH per ton of pulp of 90 %
dry solids content, in other words, by means o~ the process in accordance with the invention, the preparation of the cooking chemicals for the NS-AQ cooking can be performed by using make-up NaOH and, consequently, without causticizing. With an ideal number of bottoms, 6.1, the consumption of stripping steam (100C, 1 bar abs.) is o.o8 to 0.20 tons per ton of pulp, which means an operating expense of 4 to 9 FIM per ton of pulp, when the cost of steam is 42 FIM per ton. If exclusively NaOH were used for the neutralization of sodium bicarbonate and bisulfite in the preparation of the cooking chemicals, in stead of the stripping of bicarbonate in accordance with the pre-~0 sent invention, the quantity of NaOH to be produced bycausticizing would be such that the oil costs of the lime sludge reburning kiln would be approximately of the same order of magnitude as the steam costs of the strip-ping in accordance with the invention, when the cost of oil is 800 FIM per ton. The essential advantage of the process of the present invention, as compared with the causticizing, lies exactly in the investment costs.

Claims

The embodiments of the invention in which an exclu-sive property or privilege is claimed are defined as follows:

1. A process for the preparation of a neutral-sulfite digesting liquor and for adjusting the pH of the liquor while starting from a sodium-sulfite/sodium-carbonate solution that contains sodium bisulfite and/or sodium bicarbonate, characterized in that the pH of the said solution is raised to the pH-range of 8 to 11, by stripping carbon dioxide off the solution by means of water vapour or some other inert gas.

2. A process as claimed in claim 1 characterized in that the pH of the said solution is raised to the pH-range of 9.5 to 10.5

3. A process as claimed in claim 1 characterized in that the final adjustment of the pH of the cooking liquor is performed by means of sodium hydroxide.

4. A process as claimed in claim 2 characterized in that the final adjustment of the pH of the cooking liquor is performed by means of sodium hydroxide.

5. A process as claimed in claim 1 or 2 characterized in that the quantity of NaOH required for controlling the pH is lower than 11 kg per ton of pulp of 90% dry solids content.

6. A process as claimed in claim 3 or 4 characterized in that the quantity of NaOH required for controlling the pH is lower than 11 kg per ton of pulp of 90% dry solids content.

7. A process as claimed in claim 1 or 2 characterized in that the quantity of NaOH required for controlling the pH is lower than 8 kg per ton of pulp of 90% dry solids content.

8. A process as claimed in claim 3 or 4 characterized in that the quantity of NaOH required for controlling the pH is lower than 8 kg per ton of pulp of 90% dry solids content.

9. A process as claimed in claim 1 or 2, characterized in that water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.

10. A process as claimed in claim 3 or 4, characterized in that water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.

11. A process as claimed in claim 1 or 2 characterized in that the quantity of NaOH required for controlling the pH is lower than 11 kg per ton of pulp of 90% dry solids content and wherein water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.

12. A process as claimed in claim 3 or 4 characterized in that the quantity of NaOH required for controlling the pH is lower than 11 kg per ton of pulp of 90% dry solids content and wherein water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.

13. A process as claimed in claim l or 2 characterized in that the quantity of NaOH required for controlling the pH is lower than 8 kg per ton of pulp of 90% dry solids content and wherein water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.

14. A process as claimed in claim 3 or 4 characterized in that the quantity of NaOH required for controlling the pH is lower than 8 kg per ton of pulp of 90% dry solids content and wherein water is condensated off the stripping gas and the CO2 concentrated in this way is used in the regeneration circulation of the cooking chemicals for the carbonation of solutions.