PROCESS FOR SEPARATING LIGNIN FROM BLACK LIQUOR

Abstract

A process for separating lignin and volatile reaction products from a black liquor derived from pulp production, lignin is demethylated in a heat treatment reactor and CH.sub.3.sup.+ ions separated out are reacted with reduced sulphur-containing components from cooking chemicals in the black liquor to form DMS a rise in pressure in the heat treatment reactor is controlled and DMS is withdrawn when a pressure of ≥15 bar to ≤40 bar, is reached in the heat treatment reactor the black liquor which is depleted in DMS is transferred to a first precipitation stage and CO.sub.2 and/or sulphur-containing reaction products are added to precipitate out a slurry containing demethylated lignin which is transferred into a second precipitation stage with at least one sulphur-containing acidifying agent, and the precipitated unrefined slurry containing demethylated lignin is discharged from the second precipitation stage; and supplied to a demethylated lignin separation step.

Claims

1. A process for separating lignin and volatile reaction products from a black liquor derived from pulp production, in which lignin contained in the black liquor is demethylated in a heat treatment reactor and CH.sub.3.sup.+ ions separated out of the lignin are reacted with reduced sulphur-containing components from cooking chemicals contained in the black liquor to form DMS and DMS is withdrawn from the heat treatment reactor, a rise in pressure in the heat treatment reactor is controlled and DMS as well as other volatile or gaseous reaction products are withdrawn therefrom, repeatedly where necessary, when a pressure of ≥15 bar to approximately ≤40 bar, is reached in the heat treatment reactor, the black liquor which is depleted in DMS as well as other volatile reaction products is transferred to a first precipitation stage and CO.sub.2 and/or sulphur-containing reaction products are added in order to precipitate out a slurry containing demethylated lignin, the slurry containing demethylated lignin is transferred into a second precipitation stage with at least one sulphur-containing acidifying agent, the precipitated unrefined slurry containing demethylated lignin is discharged from the second precipitation stage and subjected to a demethylated lignin separation step, a portion of the withdrawn other volatile reaction products are recycled to the process.

2. The process as claimed in claim 1, wherein the volatile reaction products from the heat treatment are continuously withdrawn from the heat treatment step.

3. The process as claimed in claim 1, wherein offgases withdrawn from the heat treatment reactor as well as from the first precipitation stage are supplied to the second precipitation stage as an acidifying agent.

4. The process as claimed in claim 1, wherein offgas containing CO.sub.2 and H.sub.2S withdrawn from the second precipitation stage is supplied to the first precipitation stage as an acidifying agent.

5. The process as claimed in claim 1, wherein the demethylation of lignin is carried out in the heat treatment reactor at temperatures of over 180° C.

6. The process as claimed in claim 1, wherein the demethylation of lignin is carried out in the heat treatment reactor with a continuous rise in the temperature.

7. The process as claimed in claim 6, wherein the heat treatment is carried out for a time period of at least 50 minutes.

8. The process as claimed in claim 6, wherein DMS formed during the heat treatment as well as other volatile reaction products which are formed are withdrawn continuously.

9. The process as claimed in claim 1, wherein H.sub.2S withdrawn from the first and second precipitation stages is supplied to a common oxidation stage.

Description

DESCRIPTION OF THE DRAWINGS

[0023] The invention will now be described in more detail with the aid of figures and exemplary embodiments, in which:

[0024] FIG. 1 shows a processing concept for the demethylation of lignin as well as a two-stage acidification with sulphur-containing gases in accordance with the invention,

[0025] FIG. 2 shows a further embodiment of a processing concept of this type in which, in addition, sulphur-containing gases which have been withdrawn are supplied to an oxidation step,

[0026] FIG. 3 is a graph which shows the reduction in the Ph value during a procedure in accordance with the invention with both a black liquor heat treated in accordance with the invention and also a black liquor treated in accordance with the prior art, and

[0027] FIG. 4 shows a processing concept for the discontinuous implementation of the demethylation of lignin, as well as a two-stage acidification with sulphur-containing gases in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

Exemplary Embodiments

Example 1

Heat Treatment and Lignin Separation from Black Liquor on the Laboratory, Pilot and Industrial Scales

[0028] 1 kg of black liquor (black liquor I) was supplied to a heat treatment reactor and the black liquor I was heat treated for 90 min at 220° C. Every 30 min, dimethyl sulphide (DMS) which had formed was withdrawn from the reactor, wherein care was taken to ensure that the pressure in the vessel was kept to a maximum of 30 bar or below. The black liquor I introduced into the system had a pH value of 13 at the inlet and 10 g/kg of black liquor of DMS could be obtained from the black liquor I during the heat treatment. After the heat treatment, cooling could be carried out before the final removal of DMS. The black liquor I obtained after the heat treatment herein had a pH value of 11.3 and was transferred into a precipitation stage where precipitation could be carried out either in a separate vessel or, in fact, in the heat treatment reactor. In order to precipitate out lignin, in a first precipitation stage, CO.sub.2 was added as the acidifying agent and the black liquor slurry formed, which had a pH value of 9.9 after the addition of CO.sub.2, was transferred into a second precipitation stage in which a second precipitation was carried out with additional H.sub.2SO.sub.4. The slurry which was withdrawn from it had a pH value of 9.4 for lignin. Next, a test was carried out either to withdraw lignin after the first precipitation stage with CO.sub.2 and to observe the yield, or to investigate the yield after carrying out a second precipitation stage. In this regard, it was shown that the lignin yield with a single CO.sub.2 precipitation was 90 g/kg of black liquor I and the lignin yield after carrying out a second precipitation stage with H.sub.2SO.sub.4 was 104 g/kg of black liquor I.

[0029] The procedure of Example 1 was repeated wherein, instead of the intermittent withdrawal of dimethyl sulphide, dimethyl sulphide was continuously withdrawn from the head of the reactor and the dimethyl sulphide which had been withdrawn was supplied to an oxidation step in order to oxidize it to H.sub.2SO.sub.4 or SO.sub.2, which reaction product was in turn supplied to the second precipitation stage as the acidifying agent. In the case of a continuous procedure, in total a slight increase in the precipitated lignin could be observed at the end of the second separation stage; it was 107 g/kg of black liquor I.

Example 2

[0030] The procedure of Example 1 was substantially followed, wherein black liquor from another pulp production, termed black liquor II, was used. The temperature in the heat treatment reactor was 200° C. The pressure in the heat treatment reactor was kept to 15 bar by intermittently withdrawing volatile components (without a withdrawal of the volatile components, a pressure of 18 bar would have been established). The yield of DMS in this case was 4.9 g/kg of black liquor II, which yield enabled an H.sub.2SO.sub.4 addition of at least 7.7 g/kg of black liquor II to be made in the precipitation stage 2.

Example 3

[0031] The procedure of Example 1 was substantially followed, wherein black liquor from the digester of Example 2 was employed, however the black liquor originated from a digestion of a type of wood which differed from that of Example 2, hereinafter termed black liquor III. The temperature in the heat treatment reactor was 220° C. The pressure in the heat treatment reactor was adjusted to 24 bar by intermittently withdrawing the volatile components (without a withdrawal of the volatile components, a pressure of 40 bar would have been established), The yield of DMS in this case was 10.8 g/kg of black liquor III, which yield enabled an H.sub.2SO.sub.4 addition of at least 17 g/kg of black liquor III to be made in the precipitation stage 2, wherein the proportion of H.sub.2S was not included in the calculations.

[0032] Finally in this context, it should also be noted that when H.sub.2SO.sub.4 is added in the second stage as the acidifying agent, the addition of the strong acid leads to the release of CO.sub.2 together with H.sub.2S by displacement of the carbonate equilibrium, whereupon the corresponding offgases, namely CO.sub.2 and H.sub.2S, can be recycled and, for example, can be supplied to the oxidation step in order to be oxidized to SO.sub.2 or H.sub.2SO.sub.4. Here, all of the offgases may also be recycled to the first stage, for example, without any separation or purification, so that the CO.sub.2 contained in the offgas is bound in a liquor which has a high pH value and the pH value falls and the H.sub.2S can be withdrawn from the first stage as offgas and then either oxidized to acids, or may also be burned off. Other possible gaseous ingredients in the offgas do not interfere with the reaction, and therefore all the offgas could be recycled.

[0033] Finally, a test was carried out to supply sulphur-containing acid prior to the CO.sub.2 precipitation, i.e. the first precipitation stage, which reduces the quantity of CO.sub.2 required as the precipitation agent and therefore reduces the carbonate formation during the precipitation. In the case of this procedure, it has been shown that this is favourable to recycling of the filtrates after the filtration of lignin, because carbonate of the prior art is known to be able to cause problems with liquor evaporation.

[0034] FIG. 1 shows a process schematic in which the DMS formed during the heat treatment is withdrawn intermittently. In this regard, at 1, black liquor with a pH value of 13 is introduced into a heat treatment reactor 2 in which the black liquor is heated to temperatures of up to 220° C. for a time period of between 0.5 and 4 hours. The DMS formed is withdrawn at 3 every time the pressure in the heat treatment reactor 2 exceeds the corresponding vapour pressure of water at the prevailing temperature, 220° C., however without vaporizing it. At the end of the reaction period, the treated black liquor, which now has a pH of 11.3, is transferred via line 4 into a first precipitation stage 5, in which an offgas or CO.sub.2 as a pure gas is additionally introduced via line 6 in order to reduce the pH value and to facilitate the precipitation of lignin. The H.sub.2S formed during this first precipitation is discharged at the head of the precipitation reactor via line 7 and the lignin-containing slurry, in particular demethylated lignin-containing slurry, which is formed, now with a pH value of 9.9, is introduced via line 8 into a second precipitation stage 9, in which a further precipitation is carried out with a strong mineral acid such as H.sub.2SO.sub.4 or SO.sub.2 introduced via line 10. The offgases formed during this precipitation, namely H.sub.2S and CO.sub.2, can be recycled via line 11 to the first precipitation stage as an acidifying gas. The slurry formed in the second precipitation stage 9, which now has a pH of 9.4, is supplied via 12 to a lignin separation step. In this procedure, it has been shown that the yield of demethylated lignin can be increased by at least 30% compared with the process of the prior art and that although the sulphur balance in the system could be kept constant, the reaction equilibrium is displaced towards demethylated lignin so that a low-molecular-weight, non-condensed product is obtained which could be used directly in a further step of the process.

[0035] FIG. 2 shows an alternative procedure in which the reference numerals of FIG. 1 are retained as far as possible and which substantially corresponds to that of FIG. 1, however the DMS which is formed is withdrawn continuously and introduced into an oxidation process 14 via line 13, in which it is converted into H.sub.2SO.sub.4 or SO.sub.2 as well as CO.sub.2 and H.sub.2O, This strong mineral acid which is formed is introduced into the second precipitation stage 9 via line 15 and therefore the external addition of sulphuric acid to the second precipitation stage is unnecessary. In the procedure in accordance with FIG. 2, the pH values in the system, in particular in the individual steps of the process, are the same as in FIG. 1, and also the yields are no different in principle. In general, a procedure of this type, however, ensures that the gases formed can be recycled so that the sulphur balance in the system is constant and in particular, an excessive input of mineral acid into the system can be avoided.

[0036] The graph of FIG. 3 shows that with the heat treatment in accordance with the present invention, the pH value of the black liquor employed can be significantly reduced in the heat treatment step, and also, in the precipitation stages, the pH value can alone be further reduced by the addition of the CO.sub.2 and H.sub.2SO.sub.4 present in the system and the total requirement for H.sup.+ ions per kg of black liquor can be significantly reduced compared with the non-heat treated black liquor.

[0037] FIG. 4 shows a further embodiment of the process for the demethylation of lignin in accordance with the invention, in which the process is carried out in a batch mode reactor 16. In a first step, the heat treatment is carried out in the batch mode reactor 16, wherein unrefined black liquor with a pH of approximately 13 is introduced into the reactor 16 via line 17. Inside the heat treatment reactor 16, the black liquor is heated to 215° C. to 225° C. for a period of approximately 100 min and the dimethyl sulphide (DMS) formed during this heating is repeatedly withdrawn via line 18. If applicable, the DMS, as it is generally known, can undergo an oxidation reaction and be oxidized to form SO.sub.2, as shown diagrammatically by the reactor 19. By withdrawing DMS from the black liquor which is present in the reactor 16, the total sulphur content of the black liquor remaining in the reactor 16 is reduced. The step for withdrawing DMS is carried out either continuously or in stages, in each case after exceeding a specific threshold pressure which, as a function of a temperature of approximately 200 to 225° C., in particular 215 to 225° C., is between approximately 15 and approximately 28 bar, in particular 20 to 28 bar. After this heat treatment step, in the second reaction step, CO.sub.2 and offgases are supplied to the reactor 16 via the line 20, whereupon a first precipitation step takes place in the reactor 16. Hydrogen sulphide released during this precipitation step is withdrawn via line 21 and can either also be supplied to an oxidation step in the vessel 19, or can be processed otherwise. The withdrawal of H.sub.2S reduces the sulphur content of the black liquor remaining in the reactor and at the same time increases the yield of SO.sub.2 and H.sub.2SO.sub.4 after oxidation in the vessel 19. In a third step, the SO.sub.2 or H.sub.2SO.sub.4 generated by the oxidation is now recycled via line 22 to the reactor 16 wherein, if insufficient SO.sub.2 or H.sub.2SO.sub.4 is available, this may also be provided from an external source. Upon reaction of the black liquor remaining in the reactor 16 with SO.sub.2 or H.sub.2SO.sub.4, a second lignin precipitation occurs, the offgases which are formed, namely H.sub.2SO.sub.4 and CO.sub.2, which derive from the displacement of the carbonate equilibrium, are withdrawn from the reactor 16 via line 23. The offgases may in this case either be stored, or else be added to the next reaction batch in reactor 16 during the second step for a first precipitation of lignin, described above. After the completion of the three reaction steps which have been described, the demethylated lignin suspension formed in the reactor 16, which now has a pH of approximately 9, is withdrawn from the reactor 16 via line 24. The reactor 16 is then available for a fresh pass. In this context, it should be noted that the sequence of the steps, namely 1. heat treatment, 2. first precipitation by adding CO.sub.2 and second precipitation by adding SO.sub.2, may also be reversed to some extent, namely 1. heat treatment, 2. precipitation with SO.sub.2 or H.sub.2SO.sub.4 and second precipitation with CO.sub.2, without changing anything as regards the lignin suspension obtained from the reaction.