FI103902B - Process for the preparation of high sulfide alkaline boiling liquid to produce sulfate cellulose - Google Patents

Description

103902

This invention relates to a process for the preparation of cooking liquors having a high sulphide content. has a high sulphide content for the preparation of the sulfate pulp industry, starting with the chemicals available at the pulp mill or at the pulp mill for general manufacturing chemicals.

10

The invention takes into account the important balance between sodium and sulfur in an important pulp mill and is particularly advantageous in cooking with high sulfide content, especially in so-called modified cooking.

15

By introducing other sodium and sulfur-containing materials present in the pulp mill, together with the black liquor, into the reactor in such a way that the molar ratio of sodium to sulfur drops to 1.5 to 4, it is possible to prepare sodium sulfide (Na 2 S) melt under reducing conditions has a lower sodium carbonate content than conventional soda recovery unit melt. This molten solution can be used to make cooking liquor with a very high sulphide content. The sodium to sulfur ratio of 2 to 3 has a carbonate content so low that the solution can be used directly for cooking purposes.

Sodium sulfide and its closely related chemical sodium hydrogen sulfide are often interchangeable and their use often differs in order to meet different sulfide requirements. In the aqueous phase, sodium sulfide is completely or partially hydrolyzed to sodium hydroxide and sodium hydrogen sulfide as follows:

Na2S + H20 -> NaOH + NaSH (1) The term sulphidicity in the pulp industry is customarily expressed as 103902 2 as follows:

2 x NaSH

sulfidity (%) = --------------- x 100

NaOH + NaSH 5 where NaSH and NaOH are expressed in molar units. This means, for example, that an aqueous solution containing sodium hydrogen sulfide and sodium hydroxide having 40% sulfidity contains four times more moles of sodium hydroxide than sodium hydrogen sulfide. Similarly, equilibrium (1) indicates a solution having a 100% sulfidity.

The sulfate cellulose industry produces large amounts of sodium sulfide. When cooking chemicals are recovered, the so-called black liquor is burned in a soda recovery unit, the lower part of which is reducing. In the lower zone of the Soda Recovery Unit, the sulfur components of the black liquor are reduced to the sulfide state and thus converted to the sodium sulfide. The sulphate cellulose mills operate in the range of 25 to 40% (white liquor sulfide). Most sodium reacts with carbon dioxide when black liquor is burned to form sodium carbonate. A mixture of sodium sulfide and sodium carbonate forms a molten soda at the bottom of the art intake unit and this melt is removed and reacted with water to form a so-called green liquor. A typical green liquor has the composition:

Sodium carbonate 90-105 g / l Sodium sulfide 20-50 g / l

Sodium hydroxide 15 - 25 g / l (all ingredients are calculated as sodium hydroxide).

The recovery of chemicals for the "sulfate process" (kraft) 35 results in a relatively large amount of sulfur reacting in the oxidation zone and being removed from the soda recovery unit mainly as sodium sulfate (electrostatic precipitator ash) and sulfur dioxide. If the white liquor 3 103902 has a sulphide content of more than 35%, problems arise e.g. because of the large sulfur dioxide removal from the soda recovery unit. Therefore, washing with an alkaline agent is often performed to eliminate or strongly reduce sulfur dioxide effluents. The resulting green liquor is converted, according to the known causticization process, into white liquor.

The composition of the white liquor may vary from one factory to another, but the concentration values are approximately: 10 Sodium hydroxide, NaOH 80-120 g / l Sodium sulfide, Na2S 20-50 g / l Sodium carbonate, Na2CO3 10-30 g / l Sodium sulfate, Na2SO4 5-10 g / l ( all substances are calculated as sodium hydroxide).

15

If Na2S is estimated to be completely hydrolyzed as follows

Na 2 S + H 2 O → NaOH + NaSH

This means that the amount of sodium bound as carbonate is often more than 20% of the sodium present as hydroxide.

It is known that sodium hydrogen sulfide is present. advantageous when cooking cellulose according to the sulfate process. The presence of sodium hydrogen sulfide increases the selectivity of the soup towards higher lignin release. Efficacy can also be expressed by saying that increased hydrogen sulfide content makes it possible to achieve a lower kappa number at the same viscosity as relationships are otherwise comparable.

«

The number of pieces is a measure of the lignin content and the viscosity is considered to be a measure of the cellulosic fiber strength.

It is interesting to be able to digest cellulose to as many 103902 to 4 odor kappa numbers as possible. This is especially true if it is bleached to a high (90 ISO) whiteness. For this purpose, bleaching with chlorine-containing bleaching chemicals is required, resulting in a high environmental burden of synthetic chlorine-carbon bonds (TOC1). Bleaching with oxygen gas has been developed to reduce the proportion of lignin to be bleached with chlorine-containing bleaching chemicals. This technique is in the process of developmental process 10 e.g. Sweden and Japan.

With chlorine chemicals to reduce bleaching, it is known to further improve the selectivity of the soup by the so-called modified soup, which results in lower kappa lanes. The modified soup according to this technique is based on the following process conditions: 1. The alkali content should be as constant as possible during the cooking.

2. The hydrogen sulphide content should be as high as possible, especially at the beginning of the pulp lignin removal step. The hydrogen sulphide content may be low at the end of the cooking process.

3. Concentrations of lignin and sodium ions released should be as low as possible, especially in the final stage of cooking.

4. The temperature should be low, especially at the beginning and end of cooking.

·· Of the above, paragraph 2 is of particular interest to this invention. So far, it has been possible to settle for a concentration of 35 hydrogen sulfide ions, which is given by the 40% sulfidity of white liquor.

Among others, S. Norden et al., Tappi, Vol. 62, No. 7, 5103902

July 1979, p. 49; Johansson B. et al., Svensk Papperstid-Ning No. 10, 87 (1984), p. 30; and D. Tormund et al., Tappi, Vol. 72, No. 5, May 1989, p. 205, it is evident that further increasing the concentration of hydrogen sulfide ions above 5% to 40% sulfidity is very advantageous in the initial stage of cooking.

In modified cooking, cooking liquor is added in two or more places. Extremely high sulfidity in the soup broth added at the beginning of the cooking is most useful, while the sulfidity in the broths added at the end of the cooking may be low.

We have now surprisingly come up with a process for preparing a high cooking sulphide soup, which is particularly well suited for a modified soup according to the sulphate process, whereby a high sulfur cooking liquor is added such that when cooked according to the prior art, 20 masses can be prepared. , which has a lower kappa number than would normally be obtained. More particularly, the invention relates to a process for the preparation of cooking liquors, under reducing conditions having a high sulphide content, for cooking a sulphate pulp, wherein the black liquor formed in the cooking process is completely or partially fed to which forms a lobby, which is essentially sodium sulphide, and is removed to become further processed cooking liquor. The process of the invention is characterized by additionally adding to the reactor all or some of the sulfur-containing and / or sulfur-sodium-containing materials present in the cellulose mill, including sulfur-containing and / or sodium-sulfur-containing manufacturing chemicals used for the total chemical balance of cellulose. here, such that the molar ratio of sodium to sulfur 103902 6 in the total feed to the reactor is in the range of 1.5 to 4.

Preferably, the molar ratio of sodium to sulfur in the total mixture is obtained in the range of 2 to 3 and most preferably in the range of 2 to 2.8. It is still preferred to feed up to 30% of the black liquor stream generated in the cellulose mill into the reactor.

The sodium sulfide melt obtained in the process of the invention can be dissolved in water and further processed into cooking liquor in a manner known per se. According to a preferred embodiment, the molten solution is fed directly into the boiler to optimally utilize its high sulfide content in modified cooking. In an alternative process, the molten solution is mixed with a portion of the white liquor prepared in the usual manner.

In order to allow the reduction reactions to proceed rapidly in the reactor and accordingly obtain shorter residence times and small reactor volumes, additional energy, in addition to the energy released from the black liquor in partial oxidation, may be fed to the reactor mixing zone by hot gas. For example, thermal energy can be supplied by gas heated by a plasma generator. A very hot gas or gas mixture may also be formed directly or indirectly with an oxygen-fuel burner.

The gas or gas mixture used can be made from air, recycle gas, hydrogen gas, natural gas, carbon monoxide, etc. When using an oxygen fuel burner, the gas or gas mixture is obtained by burning e.g. acetylene or LPG with oxygen enriched air or pure oxygen gas.

7, 103902

The preferred method according to the invention is that the hot gas is introduced into the reactor near the material feed, which in turn must be fine-grained, which can be obtained in various ways by atomization techniques known to those skilled in the art. The structure of the reactor must be large enough to allow time for the reaction to occur, i. the reactor volume must guarantee a certain minimum residence time.

The reactor is preferably a closed reaction vessel and the temperature of the reactor is at least the temperature at which sodium sulphide is formed under otherwise prevailing conditions. Said temperature may be solved case by case by a person skilled in the art, for example by routine tests.

15 The temperature is preferably not below 700 ° C.

The pressure in the reactor is preferably atmospheric pressure. However, the process can be carried out at elevated pressure, for example to reduce reactor volume.

20

Swedish patent SE 8501465-2 discloses a method for releasing plasma soda recovery unit from a partial stream of black liquor. This makes it possible to increase pulp production in a plant where the soda recovery unit capacity is too low or, for example, to bring oxygen gas bleaching and / or modified cooking to the factory, which is limited by the soda recovery unit capacity without losing production capacity.

30

It is essential for the process of the invention that the molar ratio of sodium to sulfur in the total mixture fed to the reactor is less than about 4 and is in the range of 1.5 to 4, preferably 2 to 3. The sodium / sulfur molar ratio is adjusted by means of sulfur-containing and / or sulfur-sodium-containing materials present in the pulp mill, including sulfur-containing and / or sodium-sulfur-containing manufacturing chemicals used for the overall chemical balance of the pulp mill.

The manufacturing chemicals used to adjust the 5 molar ratio of sodium to sulfur may consist of sulfur, sulfur dioxide, sulfuric acid, sodium hydrogen sulfate, sodium sulfate, sodium sulfite, sodium hydrogen sulfite and sodium thiosulfate.

10 Among the sulfur-containing and / or sulfur-sodium-containing materials present in the pulp mill, the following may be mentioned: a. Residual acid from the production of chlorine dioxide. Nk. Mathie-15 son mills produce a mixture of sulfuric acid and sodium sulfate with an Na / S ratio of s 1. Other common processes of the so-called R-8 type produce sodium sesquisulfate (Na3H (SOJ2) with a Na / S ratio is 1.5 The accumulation of residual acid is usually a problem and often has to be taken to a landfill.

b. electrostatic precipitator ash consisting mainly of sodium sulphate. Normally 60-125 kg of electrostatic precipitator ash is generated per tonne of pulp, which is recycled to the combustion zone of 25 soda recovery units. The Na / S ratio is 4.2.

c. Sulfur containing solutions from soda recovery unit washing. The Na / S ratio is about 2.

30th EP 87850238.4 discloses a method of reacting a partial stream of white liquor sodium hydrogen sulfide with the formed copper oxide, sodium hydroxide and copper sulfide. Copper sulfide is roasted to form sulfur dioxide and copper oxide. The sulfur dioxide formed is a source of sulfur free from sodium.

. Further, elemental sulfur or any 9,103,902 other sulfur-containing chemicals having a Na / S ratio of 4 or less can be used.

Suitable combinations of the total or partial flow of black liquor and one or more of the aforementioned products can produce a non-essential amount of cooking liquor having a high sulphide content.

The invention will be further illustrated by the following working examples 10.

Example 1

Subsequent streams of material were continuously fed into the reactor for 15 hours, operating at atmospheric pressure.

- 620 kg black liquor (65% dry matter), containing 129 kg sodium (Na) and 35 kg sulfur (S) per tonne of black bean.

20 - a mixture of residual acids from chlorine dioxide production according to the Mathiesson process containing 80 kg H2SO4 and 62 kg Na2SO4.

25 to 800 kg Na 2 SO 4 in the form of an electric filter ash.

• *

The above material streams were mixed with oxygen containing gas and brought to the reaction space. The oxygen-containing gas was heated to about 750 ° C in a plasmagene-30 rotator.

Partial oxidation of the black liquor released energy, while maintaining the reactor temperature at about 950 ° C.

The process gas generated by partial oxidation was cooled. After any final oxidation, heat recovery and also gas scrubbing can be released into the atmosphere.

10, 103902

Alternatively, the bulk of the alkaline energy content can be released by partial oxidation, eliminating the need to preheat the oxygen-containing gas in the plasma generator.

5

The sulfur compounds entering the reaction state are substantially reduced to sodium sulfide (Na 2 S), which forms the melt phase, which is removed from the system.

Due to the high partial pressure of sulfur in the reaction state and the higher affinity of sulfur over sodium under carbon dioxide under reaction conditions, the formation of sodium carbonate is reduced in the inorganic melt phase.

15

From the prepared melt, a 4.0 molar solution of sodium is prepared containing 1.85 moles of NaOH, 1.85 moles of NaSH and 0.15 moles of Na 2 CO 5 per liter.

In preparing the experiments with modified biphasic cooking, where 70% of the cooking chemicals were charged in step 1 and the remaining 30% in step 2, the following cooking liquors were prepared.

One (1) part of the lye obtained in accordance with the invention was mixed with 4.63 parts of a common lye (white-lye) containing 2.8 moles of NaOH and 0.7 moles of NaSH per liter of solution. The cooking liquor thus prepared had a sulfidity of 51% and was charged in the first 30 cooking steps, while the normal white liquor having a 40% sulfidity was charged in step 2.

Example 2 35 The following streams of material were continuously fed to the reactor for one hour, operating at atmospheric pressure.

- 566 kg of black liquor (65% dry matter), containing 11103902, 129 kg of sodium (Na) and 35 kg of sulfur (S) per tonne.

- 48 kg sulfur dioxide - 80 kg Na2SOA in the form of electrofiltering ash.

5 - 25 kg Na2SOA as a regulator.

Continue in exactly the same manner as in Example 1 to obtain the molten phase which was removed.

The added sulfur dioxide was created by roasting according to the sulfide-free lye manufacturing process disclosed in EP 87850238.4. From the prepared molten, a 4.0 molar solution of sodium was prepared containing 1.75 moles of NaOH, 1.75 moles of NaSH and 0.25 moles of Na 2 CO 3.

In the preparation of experiments with modified biphasic cooking, where 70% of the cooking chemicals were charged in step 1 and the remaining 30% in step 2, the following cooking tips were prepared.

According to the same batching method as in Example 1, mixing 1.0 part of the above prepared liquor with 1.14 part of ordinary cooking liquor (white liquor) (40% sulfidity) gave a cooking liquor having a 68% sulfidity. This liquor was charged to the first cooking step.

In the second step, sulfide-free lye prepared according to EP 87850238.4 was used.

Claims (5)

1. A process for producing, under reducing conditions, sulphate pulp cooking liquors, in which the black liquor produced in the boiling process 5 is evaporated, after evaporation, in whole or in part to a reactor operating at elevated temperature, which is obtained by supplying energy from an external heat source and / or releasing energy from it. the black liquor, whereby a melt is formed and stripped for further conversion to boiling liquor, characterized in that the sulfur-containing and / or sulfur- and sodium-containing materials present in the pulp production in addition to the reactor are additionally fed in whole or in part at the pulp mill. , that the molar ratio of sodium to sulfur in the total mixture fed to the reactor falls within the range 1.5 to 4. Process according to claim 1, characterized in that the molar ratio of sodium to sulfur in the total mixture fed to the reactor falls within the range 2 to 3, preferably 2 to 2.8. 3. A process according to claim 1, characterized in that the sulfur-containing and / or sulfur- and sodium-containing materials present at the pulp mill which are supplied to the reactor in whole or in part comprise one, several or all of the following; filter ash, residual product from chlorine dioxide production, sodium hydrogen sulfite containing sulfur dioxide gas washing solutions, drain from CTMP, NSSC or other sulfite pulp processes, sulfur dioxide from roasting copper sulfide, hydrogen sulfide containing condensate and / or air streams, sulfur, sulfur dioxide, sulfuric acid, sodium sulfite, sodium hydrogen sulfate, sodium thiosulfate and sodium sulfate. 35 4. A process according to claim 1, characterized in that the melt or an aqueous solution thereof is mixed with the white liquor, whereby a white liquor with increased sulphidity is obtained. 5. Process according to claim 1, characterized in that an aqueous solution of the melt is used at the so-called. modified sulfate cooking. «