PROCESS FOR ISOLATION OF LIGNIN FROM AN ALKALINE PROCESS STREAM

Description

[0001] The present invention relates to a process for the continuous isolation of lignin from an alkaline process stream consisting of thickened black liquor.

[0002] Along with cellulose, lignin is the main product of pulp production and is also the second most abundant biopolymer worldwide. Lignin has a wide range of potential applications, such as the production of vanillin or carbon fibers, or even applications such as its use as a starch substitute in papermaking. The sustainable implementation of lignin in the production of a wide variety of substances and processes is currently the subject of intensive research worldwide. However, to date, no commercially available lignin product or economically viable process for lignin derived from the kraft process has been found.This is because the use of lignin derived from the kraft process presents various challenges, such as the product's characteristic odor and color, as well as the varying quality of the recovered lignin, which are due to the different raw or starting materials used in the cooking process as well as differences in processing techniques. Currently, lignin is used exclusively for fueling the chemical recovery digester. Once precipitated and purified, as is done, for example, with the NORAM Group's Lignoforce process, it can be used as fuel in the lime kiln, where CO2 savings and increased plant productivity are expected.

[0003] State-of-the-art processes for isolating lignin from alkaline process streams, such as black liquor from pulping, are widely described in the literature. The best known of these processes are the so-called LignoBoost process (a trademark of Valmet Aktiebolag (WO 2012 / 049375 A1 or WO 2014 / 116150 A1)) and the Lignoforce process of the NORAM Group, which, in order to operate economically, use pressurized synthetic carbon dioxide to precipitate lignin from the alkaline process stream. However, both the LignoBoost and Lignoforce processes, which are both widely used, can only be operated discontinuously to produce lignin with reasonably reproducible qualities, which adversely affects both the equipment required and the lignin productivity.Therefore, processes are still being sought which, on the one hand, can continuously produce or recover lignin with consistent quality from an alkaline process stream, and, on the other hand, keep both the equipment and energy consumption as low as possible.

[0004] A further disadvantage of known processes according to the state of the art is not only their discontinuous operation but above all the problem that foaming occurs in the reactor due to the injection, which can only be kept under control by operating the reactor under excess pressure, whereby the process is not adversely affected.

[0005] US Pat. No. 9,139,606 B2 and the underlying WO 2012 / 049375 A1 already describe a continuous process for the precipitation of lignin from black liquor. In this process, an acidifying agent is added to black liquor in a flow-through pressure reactor with a residence time of less than 300 seconds. After an abrupt release of pressure, lignin precipitates from the acidified black liquor. A disadvantage of this process is the use of a pressure reactor, which is not advantageous for large-scale use, both from an energy perspective and from a handling perspective.

[0006] In summary, the multitude of known processes shows that those operating at ambient pressure face a major problem with the foam formed in the reactor during the reaction, and those processes that enable continuous lignin precipitation from alkaline process fluids must operate at elevated pressure or pressure cycling. Needless to say, other processes for lignin precipitation, for example, using ion exchangers, are also described in the literature, but none of these can be considered particularly economical.

[0007] There is therefore a need to provide a further improvement to the known processes for the precipitation of lignin from an alkaline process stream, which, on the one hand, makes it possible to provide a process that is as simple as possible both in terms of the process steps and in terms of the equipment required, which, moreover, requires little energy and can be carried out continuously at ambient pressure.

[0008] To achieve this object, the process according to the invention is essentially characterized in that the alkaline process stream is continuously introduced into a lower region of at least one circulation reactor having two concentrically arranged reactor zones, that a liquid level of the alkaline process stream inside the at least one circulation reactor is selected substantially at a level with an upper end of at least one inner tubular reactor zone, that a CO 2 -containing gas is continuously blown from below into the at least one inner tubular reactor zone of the at least one circulation reactor, that the CO 2 -containing gas is absorbed by the alkaline process stream in the at least one inner tubular reactor zone, and that exhaust gas together with residual amounts of CO 2 are withdrawn at the top of the at least one circulation reactor,that the process is carried out at ambient pressure and that the lignin-depleted thickened black liquor, together with the precipitated lignin contained therein, optionally after settling, is withdrawn at the bottom of at least one circulation reactor. In addition to the selected method of introducing the process streams, namely that the alkaline process stream is continuously introduced into a lower region of a circulation reactor having at least two concentrically arranged reactor zones and, at the same time, a CO2-containing gas is continuously blown from below essentially exclusively into the at least one inner tubular reactor zone of the circulation reactor, it is possible to carry out the process in such a way that the CO2-containing gas, in particular the CO2 of the CO2-containing gas, is successively absorbed in the alkaline process liquor and thus the pH value thereof is lowered, whereby after falling below a limit pH value for the precipitation of lignin,which is in the range of approximately 8 to 13, in particular approximately 9 to 11 pH at a room temperature of 25 °C, lignin is precipitated from the alkaline process liquor. The CO 2 , which is essentially exclusively injected into the at least one inner reactor zone, rises upwards in this reactor zone toward a gas outlet, thereby moving the thickened black liquor into which it is injected upwards and thereby causing the recirculation of the introduced black liquor, without the need for a pumping device. What is surprising about this type of process is thatthat despite the process being conducted at atmospheric pressure, the strong foaming that usually occurs in such processes for isolating lignin from alkaline process streams can be almost completely suppressed. With the selected process and through the use of the circulation reactor, the absorption of CO2 takes place primarily in the inner tubular reactor zone, and the settling of lignin is subsequently achieved in the outer annular reactor zone of the circulation reactor, particularly due to the slow circulation caused by the atmospheric pressure inside the reactor. Foaming can be suppressed particularly effectively by selecting the reactor design such thatthat the upper end of the inner tubular reactor zone does not exceed the liquid level in the circulation reactor, and the liquid stream is simply allowed to flow into the outer reactor zone. With such a process or reactor design, foam formed during CO2 absorption is destroyed immediately after its formation due to the separation between the liquid phase containing solids and the liquid phase containing CO2 at the upper end of the inner tubular reactor zone, so that foam formation can no longer have a negative impact on the lignin precipitation taking place in the outer annular region of the circulation reactor.

[0009] The process according to the invention is carried out independently of the selected number of inner tubular reactor zones and, depending on the size and desired throughput of the circulation reactor, only one inner tubular reactor zone or a plurality of them can be provided.

[0010] Furthermore, by allowing the lignin precipitated in the outer annular reactor zone of the circulation reactor either to settle directly in the circulation reactor, in which case the bottom of the circulation reactor is particularly conical, or by discharging the lignin together with the alkaline process stream depleted in lignin from the reactor, it is possible to provide a lignin precipitation process which is carried out continuously in a simple device in which, moreover, no moving parts, such as stirrers or the like, are required for carrying out the process.

[0011] In the context of the present invention, a recirculation reactor is understood to be a reactor that is essentially cylindrical in shape, with a tubular reactor zone inside it, the upper end of which terminates below the upper end of the cylinder, leaving an annular gap for the circulation of material. In principle, such a reactor operates on the principle of a mammoth pump and an airlift reactor, or a combination of these two devices, with no moving parts inside the recirculation reactor. Internals such as sensors, control and regulation devices, as well as the possibility of designing the reactor as a double-jacketed device or even with only one shell, are included and do not influence the process.Finally, the loop reactor can be operated as a reactor with an internal loop or an external loop, with only the feed sides of the gas and alkaline process stream being reversed. Even such an equipment change does not affect the continuous process operation and the resulting achievable result.

[0012] By conducting the process in such a way that the alkaline process stream forms the recirculation stream in the recirculation reactor, the circulation rate within the reactor, which is controlled by the feed rate of the alkaline process stream, also allows the amount of alkaline process stream available for a conversion per unit of time to be adjusted, so that consistent process conditions can be maintained and thus consistent product qualities can be achieved. In the inner tubular reactor zone, the alkaline process stream is acidified by absorbing the introduced CO2 in the process stream and converting the basic components into acidic components.As the CO2-containing gas rises upwards within the inner tubular reactor zone of the recycle reactor, the alkaline process stream is forced into motion and flows over the upper edge of the inner tubular reactor zone into the outer annular reactor zone of the recycle reactor. In this region, which is essentially free of rising CO2, a steady, preferably laminar flow of the alkaline process liquor or process stream is formed. Due to the lowered pH and thus the excess of the lignin solubility product, this flow now contains precipitated lignin, which is directed toward an outlet provided at the bottom of at least one recycle reactor or is allowed to settle.This recirculation of the alkaline process stream, which only comes into contact with CO2 in the inner region of the circuit, makes it possible, on the one hand, to prevent foaming, to specifically acidify the process stream, and, in particular, to select a sufficiently slow residence time of the process stream due to the atmospheric pressure prevailing in the system, only dependent on the inflow, thus enabling targeted precipitation of lignin, particularly of the desired lignin quality, from the alkaline process stream. Furthermore, this process control and the special reactor design make it possible to control and regulate both the process flow and the residence time of the CO2 by adjusting the height of the inner tubular reactor zone.

[0013] By conducting the process at ambient pressure, specifically at 1 atm, the reactor system is further simplified. Furthermore, by conducting the process at normal pressure, i.e., one atmosphere, the energy balance of the process is significantly improved compared to conventional processes.

[0014] By conducting the process in such a way that the rate of CO2 absorption in the thickened black liquor is controlled or regulated by a CO2 concentration in the CO2-containing gas, it is possible to lower the pH of the alkaline process stream or the thickened black liquor as quickly as desired, depending on the amount or concentration of CO2 added. The rate of reduction in the pH is directly proportional to the amount of CO2 added. By conducting the process in this way, it is possible to control or regulate the rate at which lignin precipitates from the alkaline process stream, namely in such a way that the supersaturation of the solution with CO2 is directly proportional to the particle growth of the precipitated lignin, which can also influence the quality and product properties of the precipitated lignin.Furthermore, due to the increased circulation velocity caused by the larger amount of gas present, settling of the formed lignin particles in the circulation reactor is made more difficult. Therefore, they can be discharged from the reactor with the process stream, for example, and allowed to settle in a separate settling tank before being separated. If, for example, the excess amount of CO2 is low, thus slowing down the circulation flow, agglomeration of the lignin particles occurs already in the circulation reactor, and larger quantities, or especially larger lignin particles, are precipitated.Due to the low circulation velocity in the circulation reactor, the shear forces in the downward flow in the exterior of the concentrically arranged reactor zones are also extremely low, so that the formed lignin agglomerates are not destroyed again. This represents a further advantage over the state-of-the-art process, since with this type of process, an aging step for the formed lignin particles, which was always necessary until now, can be omitted. Just for the sake of clarity, it should be noted that not only carbon dioxide and its concentration have a significant influence on lignin precipitation, but this precipitation is also temperature-dependent. Thus, at higher temperatures, a readily filterable lignin is obtained, while at lower temperatures, a product that is difficult to filter is obtained.

[0015] By introducing diluted CO 2 , in particular exhaust gases from CO 2 -producing plants, such as a lime kiln, into the circulation reactor to lower the pH value, as corresponds to a development of the invention, it is possible on the one hand to make effective use of the exhaust gases present in the plant and on the other hand to reduce the absorption rate in the alkaline process stream by adding diluted CO 2 , whereby a more targeted precipitation of the desired lignin quality can be achieved. Furthermore, the use of CO 2 exhaust gases originating from the plant leads to a lowered or reduced carbon footprint of the plant, which is not only advantageous for environmental reasons, but also significantly reduces the process and production costs for lignin. It goes without saying that the CO 2 contained in the exhaust gas from the circulation reactor can in turn be recycled, for example, to the lime kiln.

[0016] The term "diluted CO 2 , in particular exhaust gases from CO 2 producing plants" is understood in the context of the present invention to mean that other components usually present in exhaust gases, such as SO 2 , CO, NOx, O 2 and N 2, are not mentioned separately, but are used in the process without prior purification of the exhaust gases and do not adversely affect the continuous process or the result achieved.

[0017] According to a further development of the invention, the process is carried out at a temperature between 30 and 80°C, in particular between 65 and 75°C. By selecting temperatures between 30 and 80°C, preferably between 65 and 75°C, mild process conditions are selected, which ensures that the lignin formed can be easily removed from the reactor due to the large agglomerates or flakes formed. In particular, if the preferred temperature is selected in the range of no higher than 75°C, it is possible to precipitate lignin below its glass transition point and at the same time avoid the formation of a further, third phase, which would arise if lignin were precipitated above the glass transition temperature. The process is therefore preferably carried out with temperature control or under continuous temperature monitoring in order to optimize the quality of the precipitated lignin.Temperature control is particularly necessary because the temperatures of the exhaust gases introduced are usually significantly above the desired maximum temperature of 80°C. Any known devices, such as heat exchangers, temperature-controlled reaction vessels, coolers, or the like, can be used for temperature control.

[0018] By conducting the process in such a way that a lignin quality to be precipitated is adjusted to values ​​between 13 and 8, preferably between 11 and 9, by means of pH control using CO2 as the sole acidifying agent, it is possible to influence the quality of the lignin precipitated from the alkaline process stream. As is known in the art, the molar mass of lignin, as well as the functional groups present in the lignin, changes depending on the pH. For example, at relatively high pH values, a lignin with a high molar mass is formed, with the molar mass continuing to decrease as the pH decreases. Depending on the amount of CO2 as acidifying agent in the alkaline process stream, the pH of the process stream can now be specifically adjusted, and thus the desired lignin quality can also be specifically adjusted.For example, using highly diluted CO2 can precipitate lignin with a high molecular weight, whereas passing large amounts of CO2 through the reactor results in a significant reduction in the molecular weight of the lignin. As explained above, in addition to the change in molecular weight, the functional groups present on the lignin also change, which is why such a process can actually precipitate a tailor-made lignin from the alkaline process stream, in particular from the thickened black liquor. Furthermore, as is clear from the above, fractional precipitation of lignin can also be carried out in one and the same circulating reactor, and thus, particularly when only small amounts of a particular quality lignin are required, this quality can be achieved by adjusting the pH using the amount of CO2 passed through the alkaline process stream.

[0019] In this context, it should be noted that the reactor control can be controlled or regulated not only by the amount of CO 2 passed through, but also by the amount of mother liquor introduced, ie an added thickened black liquor.

[0020] Particularly if the intention is to precipitate different lignin fractions from the alkaline process stream, the process according to a further development of the invention is carried out such that a plurality of circulation reactors are arranged one after the other in a cascade, that the alkaline process stream, at least partially depleted of lignin, drawn off at the bottom of the respective upstream circulation reactor in the cascade, in particular the thickened black liquor, is fed to the respective next downstream circulation reactor, and that a pH gradient is maintained in the circulation reactors following one another in the cascade from upstream to downstream. With such an arrangement or process control, fractional lignin precipitation is possible by gradually lowering the pH from the upstream circulation reactors to the nearest downstream circulation reactor.This allows different lignin qualities to be obtained from the alkaline process stream when the reactor is operated continuously.

[0021] In such a step-by-step process, as corresponds to a development of the invention, the pH of the precipitate is adjusted by the CO2 concentration in the CO2-containing gas, with the lowest CO2 concentration being fed to the circulation reactor furthest upstream. The CO2 concentration in the CO2-containing gas can be adjusted either by feeding exhaust gas from, for example, pulp production to each of the circulation reactors in the cascade, ensuring that the CO2 concentration increases successively, or by using a mixture of exhaust gas and fresh gas, which allows an even more precise adjustment of the CO2 concentration and thus a more targeted precipitation of the desired lignin quality in each case.

[0022] In order to achieve a particularly complete separation of the precipitated lignin and, in particular, to avoid recycling of precipitated lignin as much as possible, the process according to a further development of the invention is carried out in such a way that the precipitated lignin is allowed to settle in a separate, particularly conical, settling tank. The settled lignin can be withdrawn from this settling tank, particularly at its bottom, and reused either after purification or immediately.

[0023] The invention is explained in more detail below using an embodiment, the method of which is illustrated by the schematic diagram, the device required for carrying out the method, as shown in Fig. 1 is shown.

[0024] Fig. 1 shows a schematic diagram of a continuous circulation reactor with an attached settling tank, which can be used to precipitate lignin from an alkaline process stream.

[0025] Black liquor originating from evaporation is fed into a circulation reactor 1 in the bottom region 2 of the same via line 3. The thickened black liquor, which is fed into the circulation reactor 1 at 3, has a dry matter content of approximately 20 to 40 wt.% and originates from cellulose production. In order to be able to control and regulate the inflow of the thickened black liquor, a control and regulating valve 4 is also provided in the inlet line 3. The temperature of the thickened black liquor, which is fed into the bottom region 2 of the circulation reactor 1, is between 55°C and 80°C and is tempered before being fed. Normally, the thickened black liquor is introduced into the reactor at the temperature resulting from evaporation.For the reaction with carbon dioxide, in particular the precipitation of lignin with the help of CO 2 , it should be noted that the higher the temperature of the thickened black liquor and thus the temperature of the process in the circulation reactor 1, the faster CO 2 is absorbed by the thickened black liquor.

[0026] CO2 is also introduced via line 5 in the bottom region 2 of the circulation reactor 1. The opening of line 5 is arranged inside the reactor 1 such that it opens essentially at the lower end of an inner tubular reactor zone 6. The fact that line 5 for introducing CO2 opens at the lower region of the inner tubular reactor zone 6 of the circulation reactor 1 ensures that CO2 is essentially only introduced into this inner tubular reactor zone 6, for example by means of a sieve bottom, a frit or a control valve, and that the CO2 introduction initiates the reaction of thickened black liquor with CO2 and also initiates the circulation movement in the circulation reactor 1.This occurs because CO 2 rises inside the tubular reactor zone 6 and, as it rises, entrains black liquor, which subsequently, after reaching the upper end 7 of the inner tubular reactor zone 6, flows over this upper end 7 into the outer annular region 8 of the circulation reactor 1, thereby initiating a material cycle. Since the process, i.e. the reaction of an alkaline process stream or thickened black liquor with CO 2, is carried out at atmospheric pressure, the extent of the reaction or the acidification rate of the thickened black liquor by means of CO 2 depends, on the one hand, on the temperature of the introduced, thickened black liquor and the introduced gas, and, on the other hand, on the residence time of CO 2 inside the circulation reactor 1, in particular inside the inner tubular reactor zone 6.

[0027] When the circulation reactor 1 is operated at normal pressure, the height or length of the inner tubular reactor zone 6 is directly proportional to the residence time of CO2 inside this zone and thus to the reaction time available for CO2 with the thickened black liquor. The longer this reaction time, the lower the pH of the thickened black liquor becomes, and this pH control or regulation also makes it possible to influence the quality of the precipitated lignin. If the inner tubular reactor zone 6 is selected to be sufficiently high or long, maximum absorption of CO2 can take place inside this reactor zone 6, so that any exhaust gas exiting at the upper end 7 of the inner reactor zone 6 essentially contains no CO2 at all. By using the height of the inner tubular reactor zone 6 as a control or regulationIf the control element is used, it is therefore important to know the liquid volume inside the circulation reactor 1, since the process is controlled and regulated via the liquid volume in the reactor, i.e., how much liquid is allowed to flow in before the reaction begins, or what the relationship is to the outflow rate of the introduced, thickened black liquor, and, in particular, the level of the reactor is controlled. Such level control can be implemented, as described as an optional element in the . Fig. 1 As shown, the liquid level 10 inside the circulation reactor 1 is measured by a level control or regulation sensor 9. When the liquid level 10 has reached a predetermined maximum value, the inlet valve 4 is closed to prevent further inflow of fresh, thickened black liquor, and the excess level of thickened black liquor is fed into the inlet line 3 for thickened black liquor via the bypass line 11. With this level control or regulation device, it is possible to always keep the level inside the circulation reactor 1 constant, thus achieving a targeted, always consistent conversion of the introduced, thickened black liquor with CO2 and thus a consistent product quality of the precipitated lignin.

[0028] Precipitated lignin together with lignin-depleted, thickened black liquor are withdrawn from the bottom of the circulation reactor 1 via line 12 and in the schematic diagram 1 of Fig. 1 into a settling tank 13, in which lignin is allowed to settle. The sedimented lignin is withdrawn from the settling tank at 15, and the supernatant liquid is fed for further treatment, for example, via line 14. The lignin-rich sludge withdrawn from the settling tank 13 at line 15 is subjected to further purification, and the lignin is fed for final use. Instead of the Fig. 1In the process shown, it is of course possible to design the reactor design of the circulation reactor 1 with, for example, a conical bottom, to allow lignin to settle there, in order to withdraw the lignin-depleted, remaining, thickened black liquor in the upper area of ​​the reactor, approximately at the level where the black liquor is introduced, to feed it to another circulation reactor and to run the process as a cascade, whereby different lignin qualities can be precipitated each time by increasingly acidifying the thickened black liquor.

[0029] Finally, to more precisely control or regulate the precipitated lignin quality, the process in the recirculating reactor 1 can be carried out using a pH control system. For this purpose, a continuous pH measurement is carried out at 16 in the outer reactor zone 8. If the pH measurement shows that the pH is too low for the desired lignin quality to be precipitated, a valve 17 provided in the outlet line 12 is opened to remove as much of the product stream as quickly as possible. At the same time, the supply of CO2, for example, can be stopped.If, after the CO2 has reached the top of the inner tubular reactor zone 6, the pH value inside reactor 1 is still too high to achieve the desired lignin quality, the valve 17 in the outlet line 12 can be set so as to prevent further outflow of lignin-depleted black liquor originating from the recirculation reactor 1 and to continue the recirculation flow by blowing in CO2 until the desired pH value is reached. It goes without saying that in this case, the valve 4 in the inlet line for thickened black liquor must also be closed, since otherwise an excess amount of thickened black liquor would be present inside the recirculation reactor 1.

[0030] If the process according to the invention is carried out in a cascade of circulation reactors 1, the pH value in the individual circulation reactor 1 can be specifically adjusted using the procedure described above, thus allowing a specific lignin quality to be precipitated in each of the circulation reactors 1. As is known in the art, lignin with high molecular weights is precipitated at relatively high pH values, whereas lignin with low molecular weights is precipitated at lower pH values.

[0031] In summary, it can be stated that with the process according to the invention it is possible, firstly, to control or regulate the CO2 absorption by means of the height of the inner tubular reactor zone 6, secondly to control or regulate the rate of CO2 absorption with the aid of the temperature prevailing inside the circulation reactor 1, whereby at temperatures above 80 °C the glass transition temperature of lignin is already exceeded and the product quality is impaired. Therefore, precipitation is preferably carried out at temperatures between 65 °C and 75 °C. Thirdly, it is possible to control or regulate the process by means of a level control, fourthly to control or regulate the process via a pH value control or regulation.and fifthly, to configure the process in a cascade of a plurality of recycle reactors 1, whereby fractional precipitation of lignin from the alkaline process stream, in particular thickened black liquor, can be achieved. This lignin can then be used for specific applications. For example, the lignin can be oxidized after precipitation, from which water-insoluble or concentrated water-soluble lignin can be obtained. Similarly, the process can be conducted via an ion exchanger, so that the lignin can be recycled in a known manner, for example, to the lime kiln as fuel.

Claims

1. A process for the continuous isolation of lignin from an alkaline process stream consisting of thickened black liquor, characterized in that the alkaline process stream is introduced continuously into a lower region of at least one circulation reactor (1) having two reactor zones in a concentric arrangement, in that a liquid level (10) of the alkaline process stream in the interior of the at least one circulation reactor (1) is chosen essentially at a level with an upper end (7) of at least one inner tubular reactor zone (6), in that a CO2-containing gas is blown continuously from the bottom into the at least one inner tubular reactor zone (6) of the at least one circulation reactor (1), in that the CO2-containing gas is absorbed by the alkaline process stream in the at least one inner circulation reactor zone (6) and offgas is drawn off together with residual amounts of the CO2 at the top of the at least one circulation reactor (1), in that the process is run at ambient pressure, in particular 1 atm, and in that thickened black liquor with a reduced lignin content together with precipitated lignin present therein are drawn off, optionally after settling at the base of the at least one circulation reactor (1).

2. The process as claimed in claim 1, characterized in that a dwell time for CO2 in the circulation reactor (1) is determined by a height of the inner tubular reactor zone (6).

3. The process as claimed in one of claims 1 or 2, characterized in that a rate of CO2 absorption in the thickened black liquor is managed or controlled by means of a CO2 concentration in the CO2-containing gas.

4. The process as claimed in one of claims 1, 2 or 3, characterized in that diluteed CO2, in particular offgas from CO2-producing units such as a lime kiln, for example, is introduced into the circulation reactor (1).

5. The process as claimed in one of claims 1 to 4, characterized in that the process is carried out at a temperature between 30°C and 80°C, in particular between 65°C and 75°C.

6. The process as claimed in one of claims 1 to 5, characterized in that a quality of lignin to be precipitated is managed or controlled by a pH adjustment with CO2 as an acidifying agent to values between 13 and 8, in particular 11 to 9.

7. The process as claimed in one of claims 1 to 6, characterized in that a plurality of circulation reactors (1) are arranged in succession in a cascade, in that the thickened black liquor which is drawn off at the base (2) of the circulation reactor (1) which is arranged respectively upstream and which has an at least partially reduced lignin content is passed to the respectively immediately downstream circulation reactor (1), and in that a pH drop is maintained in the successive circulation reactors (1) of the cascade from upstream to downstream.

8. The process as claimed in claim 7, characterized in that the pH drop is adjusted by the CO2 concentration in the CO2-containing gas, wherein the lowest CO2 concentration is supplied to the circulation reactor which is the furthest upstream.

9. The process as claimed in one of claims 1 to 8, characterized in that settling of precipitated lignin is carried out in a separate, in particular conically-shaped settling tank.