EP0140226B1 - Method of obtaining lignin from alcaline lignin solutions - Google Patents

Abstract

The invention relates to a method and apparatus for recovering lignin and alkali from an alkaline lignin solution. One or more electrolytic cells are used to anodically acidify the lignin and simultaneously cathodically regenerate alkali. The invention is especially advantageous for the preparation of pure lignin and alkali from the waste liquor of a cellulose process.

Description

The invention relates to a method for obtaining lignin by precipitation from an alkaline lignin solution by means of electrolysis and for recovering the alkali.

In pulp production, considerable amounts of lignin-containing extracts are produced, which up to now have been a waste product. Since it is no longer possible to directly extract the extracts into bodies of water, the extracts were subjected to a concentration process and the resulting solids were generally burned. The methods used are complex and only serve to achieve a purified water and a separate solid. The water can then be returned to the rivers. These processes are not only complex, but also destroy the lignin contained in the solid. The extracts are commonly referred to as leaching.

It is known to precipitate the lignin from an alkaline solution by neutralizing it by introducing acids, but the subsequent recovery of the alkali is no longer possible or is very complex and expensive. In addition, the precipitated material is contaminated with mineral salts. Alkali water can e.g. B. be neutralized by introducing CO₂ and the carbonate formed can be causified with calcium oxide.

The precipitation of lignin from alkaline solutions from sulfur pulp mills using electrolysis has also been described. The purpose of these laboratory tests was to examine the chemical changes taking place during this electrolysis. There was no provision for recovery of the alkali that accumulated on the cathode. The results of the tests showed that the use of electrolysis in neutral sulfite leaching is not promising. Furthermore, that their use in liquor liquor is even less promising.

Although the present prior art contains very negative statements regarding the electrolysis of alkaline waste liquors, the inventors have set themselves the task of creating a process in which the lignin is obtained from an alkaline lignin solution in a form suitable for further processing with little outlay on equipment can and in which there is also the possibility of supplying the water content and the alkali of the extract for reuse in the process.

It was surprisingly found that the problem can be solved in a process of the type mentioned at the outset if, according to the invention, the process is carried out continuously and as an integral part of the pulping according to the Organosolv process and that the extract is used as an alkaline lignin solution pulp production, from which the organic solvent was previously separated, and which is continuously anodized by electrolysis, the alkali being regenerated cathodically in the same process. In other words, the alkaline extract (lignin solution) is passed into the anode compartment of a divided electrolytic cell and is electrochemically acidified there, while the lye is concentrated electrochemically in the cathode compartment. The cell is expediently divided by an ion exchange membrane, which enables the selective transport of the cations from the anode into the cathode space. Studies have shown that a Nafion membrane is particularly suitable and fulfills the demands placed on it with a long service life. The precipitation and the alkali recovery are effected at the same time by supplying only one amount of energy.

Experiments have shown that the process can be carried out in only one electrolysis cell. The lignin solution and the alkali are passed through the electrolysis cell, which is divided into the anode and cathode spaces by the cation exchange membrane. At the end of the cell there is a light brown foam, which is lignin foam and can be further processed into pure lignin in known processing processes.

However, it has been shown that the process requires particularly little energy if it is carried out in two or even three stages. The number of Stages are limited by the technical equipment and the achievable efficiencies.

In the two-stage process, in the first stage ("neutralization cell") the neutralization is only carried out until the beginning of the lignin precipitation in the anode compartment, experience has shown that this corresponds to approximately pH 9.5. In this step, the majority of the sodium hydroxide solution is already recovered in the cathode compartment. In the second stage ("flocculation cell"), acidification is carried out in the anode compartment until the lignin has completely precipitated, experience has shown it to be about pH 4. Since, due to the low conductivity of the solution below pH 8, sufficient electrolysis only takes place at elevated voltage, the separation into two steps noticeably saved energy. The oxygen that develops at the anode of the flocculation cell forms foam together with the precipitated lignin and part of the neutralized solution, so that the lignin suspension can be removed via a flotation device. The flotation process does not require any additional energy, since the oxygen is generated by the amount of electricity that is required for the electrolysis anyway.

The electrolytic precipitation in the second stage has the advantage that the precipitated lignin is not contaminated with inorganic salts. After its separation, for example by centrifugation, the acidic anolyte can be returned to the cellulose process as a digestion medium or its component after appropriate regeneration, addition of methanol and optionally alkali enrichment, into the cathode compartment of the first stage and from there. The alkaline extract can thus be worked up in a closed circuit and no waste water leaves the process.

The electrolysis is carried out in both stages at temperatures as high as possible below the boiling point, since the conductivity of the solution increases with increasing temperature. The waste heat generated during electrolysis is sufficient to maintain this temperature, so that additional heating on the electrolysis cells is generally not necessary.

Since the electrolytic process is a relatively gentle method that does not require the use of additional chemicals, the method is particularly suitable for the production of pure natural lignin, as described for. B. delivers the Organosolv process according to DE-A-28 55 o52.

In a particularly advantageous manner, the catholyte and / or the anolyte can be circulated in the first process stage. A partial section of the circuits is formed by the electrolytic cell. The controllability of the process stage is improved by guiding the anolyte and the catholyte in the circuit when a part of the electrolyte is introduced and discharged. Using very simple control devices, it is possible to achieve the neutralization sought in the first process stage in a very simple manner and with exact adherence to the values. The acidification of the waste liquor in the first stage of the process is preferably carried out up to a pH of 9.5. However, this value is not absolute but depends on the circumstances such as lignin levels in the waste liquor, temperature and the like. In order to avoid contamination of the circulation sections of the circuit, the aim is that no flocculation takes place in the first process stage.

The flocculation of the lignin components only takes place in the second process stage, which is provided with a floatation device. The resulting lignin-containing foam is removed via the flotation device.

The weakly acidic electrolyte obtained at the end of the second process stage still contains several g / l of dissolved lignin-like substances which are difficult to precipitate (even if the pH is lowered further). However, this is not a disadvantage for the overall process, since the electrolyte is circulated and ultimately as a solution after the pH has been increased again, e.g. B. by adding NaOH, is used again in the alkaline pulp boiling. With repeated repetition in such a cycle, there is no concentration of non-precipitable lignin-like substances in the deflocculated electrolyte; d. that is, the lignin is ultimately obtained quantitatively.

The weakly acidic electrolyte is used in a particularly advantageous manner in the first and / or second process step in the cathode compartment. This makes it possible to immediately return the sodium hydroxide solution, which inevitably forms in the cathode compartment (in addition to hydrogen) from H₂O, back into the circuit. In the cell of the second stage, this catholyte can be conducted in countercurrent to the anolyte of the cell, which consists of the extract from the first process stage and has a pH of approximately 9.5.

The alkaline electrolyte obtained in the neutralization cell in the cathode compartment is mixed with an organic solvent, in particular methanol, and added to the pulp process for renewed boiling.

In this way, both process stages are connected to one another by recycling the electrolyte, and there is no wastewater to be removed. It is therefore a closed process in which lignin sludge is obtained as a product to be attracted, in addition to hydrogen, which can be processed into an economically usable lignin.

Any liquid losses that may occur are replaced by water. In addition, alkali metal hydroxide can be added to the catolyte in the first and second process stages in order to achieve a certain minimum conductivity from the start.

This two-stage process is particularly suitable for working up the waste liquor from the pulping according to the Organosolv process.

The invention is explained in more detail with reference to the following exemplary embodiments.

Fig. 1

eine Elektrolysezelle zur Durchführung des Verfahrens in einer Stufe,

Fig. 2

eine schematische Darstellung des Verfahrens mit zwei Stufen,

Fig. 3

eine Detaillierung des Verfahrens nach Fig 2.

Show it:

Fig. 1

an electrolytic cell to carry out the process in one step,

Fig. 2

a schematic representation of the method with two stages,

Fig. 3

a detailing of the method according to FIG. 2.

1 consists essentially of the housing 2, the membrane 3, the anode 4 and the cathode 5. The housing 2 has the shape of a flat cuboid, in the middle of which the membrane 3 is inserted. The size of the membrane 3 corresponds approximately to the size of a side surface 6 of the housing 2. The membrane 3 divides the interior of the housing 2 into the anode and cathode compartments 7 and 8. In the spaces 7 and 8 mentioned, the anode 4 and the cathode 5 arranged. Both are adapted in their shape and size to the membrane 3. The cathode is approximately in the middle of the cathode space 8, while the anode 4 lies next to the membrane 3, so that only a relatively narrow gap 9 is present between the anode 4 and membrane 3. Power connections 10 and 11 for the anode 4 and cathode 5 are led out of the housing 2.

The waste liquor previously freed of methanol from the pulp process of cell 1 is fed via connection 12. During the electrolysis 7 foam of lignin and oxygen is formed in the anode compartment, which is drawn off via the trigger 13. The foam formation is indicated by the bubbles 14 shown. The lignin foam 14 from the trigger 13 is centrifuged, whereby pure lignin and a solution which can be recycled into the pulp process is obtained. The hydrogen produced can escape via the connecting piece 15 on the cathode compartment 8. *)
*) Water, dilute alkali lye or the centrifugate of the lignin foam (pH 6) is fed in via connection 16. Concentrated alkali metal hydroxide solution is withdrawn from the cathode compartment via the connection 17.

In an experiment with an electrolytic cell with the above structure, the following results were achieved: The electrolytic cell has an anode and a cathode, the areas of which each amount to 50 cm². Anode and cathode compartments are separated by a Nafion membrane. The anodes The fume cupboard is equipped with a floating device and holds 3oo ml. At the beginning of the experiment, the anode compartment is filled with 2oo ml lignin-containing lye (pH 13.6).

0.1N NaOH serves as the initial filling of the catholyte. The cathode compartment also holds 3oo ml and is completely filled.

It is electrolyzed with 5 A = 100 mA / cm². The cell voltage slowly increases from 6 V to 15 V. After approx. 75 min. The anolyte has reached a pH of approx. 8 for the duration of the electrolysis. Tough light brown foam begins to separate, which is drawn off and worked up via the floatation device.

Fresh lignin-containing waste liquor with approx. 60 g / l dissolved lignin (pH 13.6) is now continuously introduced into the cell from below (approx. 100 - 150 ml / h). The entire amount of electrolyte leaves the cell again neutralized in the lignin / oxygen foam via the flotation device.

About 40 g of lignin can be obtained from the foam per 1 waste liquor.

The method shown in Fig. 2 has two stages. The resulting lignin and methanol-containing extract is drawn off from the cellulose cooker 2o and freed of the methanol in a methanol recovery device 21 and the methanol is fed back to the cooking process via line 21b. The extract freed from methanol is fed via line 21 a to the first electrolytic cell 22, which essentially represents the first process stage. The extract is passed into the anode compartment 23. The extract is electrolytically acidified in the anode compartment 23 until a pH of 9.5 is reached. From The extract, which has this pH value, is continuously fed to the anode compartment via the line 24 to the anode compartment 25 of the second electrolytic cell 26, which forms the second process stage. A further electrolytic acidification and thus the foaming takes place in this cell 26. The foam is drawn off as a lignin suspension via a removal device 27 and fed to a separating device 28 in which the precipitated lignin contained in the foam is separated from the extract. The pure lignin is fed for further use via the device indicated at 29, while the remaining extract is returned via line 30 as an almost lignin-free solution into the cathode space 31 of the cell 26.

In the cathode compartment 31, the extract is electrolytically enriched with alkali. The resulting hydrogen is discharged through the outlet 33. From the cathode compartment 31, the extract reaches the cathode compartment 34 of the first cell 22 via the line 32, where a further alkaline enrichment of the extract takes place, which then reaches the collecting container 36 via the line 35. The NaOH concentration can be regulated via line 35 a. From the collecting container 36, the extract is fed back to the cellulose cooker 20 as a sodium hydroxide solution via line 37. The hydrogen is removed via 4o. The Nafion membranes between the anode and cathode spaces are designated 38 and 39.

In the exemplary embodiment according to FIG. 3, process stages 41 and 42 are provided with electrolytic cells 43 and 44, and the deflocculated electrolyte obtained in second process stage 42 is returned to the first process stage.

The first process stage 41 consists essentially of the cell 43, which is divided by the membrane 45 and the two circuits 46 and 47 for the catholyte and the anolyte. Process stage 42 essentially consists of cell 44, which also has a membrane 48, and flotation device 49.

The extract containing lignin, also referred to as waste liquor, obtained during pulping, with a pH value of 14 and a lignin content of about 2 to 10% by weight is fed to the storage vessel 51 via the line 50. This supply is controlled via a control system 52, 53, 59 (pH and level controller) in such a way that a pH of approximately 9.5 is maintained in the storage vessel. The pump 54 conveys the waste liquor into the cell 43, specifically into the anode compartment 55. In the anode compartment 55, the waste liquor is lowered in pH, which enters the gas separator 56 after exiting the anode compartment 55. In the gas separator 56, the anode gas formed during the electrolysis, predominantly oxygen, is separated. While the main part of the anolyte flows back from the gas separator 56 via the line 57 into the storage vessel 51, a part of the lignin-containing extract with a pH value of about 9.5 is discharged via the line 58 and fed to the cell 44 in the second process stage 42. A valve 59 is used in line 58, which is controlled by a level controller in storage vessel 51.

The liquid in the cathode compartment 62 consists of deflocculated electrolyte which has already been enriched with NaOH in the cathode compartment 73 of the cell 44 and has a pH of approximately 12. This catholyte is also about one Storage vessel 6o and then fed to the cathode chamber 62 of the cell 43 via the line 61. From the cathode compartment 62, the catholyte passes through self-convection into the gas separator 63, from which the resulting cathode gas (hydrogen) is separated. The catholyte is returned from the gas separator 63 to the storage vessel 6o. In this case, the catholyte is recycled in the same way as for anolyte. Part of the catholyte is discharged from the gas separator 63 via the line 64. This is done via a level controller 65 in the storage vessel 6o and the valve 66. The pH is about 14.

Before this low-lignin, strongly alkaline electrolyte is returned to the pulp process, the NaOH concentration may have to be adjusted by diluting it with H₂O or adding NaOH.

In the first process stage 41 there are therefore two circuits, the catholyte circuit 46 and the anolyte circuit 47, in which the main part of the catholyte or the anolyte is circulated. A lignin-containing extract with a pH value of 14 is fed to the circulation of the anolyte before the neutralization cell 43 and after the neutralization cell 43 a lignin-containing extract with a pH value of about 9 is removed. In the catholyte circuit 46, an electrolyte, which is enriched with sodium hydroxide to a pH of 12, is fed in before the cell 43, and after the cell 43, an electrolyte with a pH of 14 is discharged and used for the production of pulp.

The lignin-containing extract obtained in the first stage with a pH of approximately 9.5 is introduced into the anolyte space 7o of the cell 44 in the second process stage, which is also referred to as a flocculation cell. In the anolyte compartment 7o, the lignin fractions are flocculated with the simultaneous formation of oxygen gas at the anode. Oxygen foam is drawn off on the flotation device 49. The device 71 separates the lignin sludge from the electrolyte, the electrolyte having a pH of approximately 4. The resulting lignin sludge is subjected to a washing, drying and work-up process in a manner known per se, so that pure lignin is produced. The electrolyte is returned via line 72 to the catholyte space 73 of the cell 44. On the way, water and sodium hydroxide can be added to the electrolyte from a reservoir 74 in order to compensate for the water losses which occur during the floatation and to achieve the properties of the electrolyte which are favorable for the electrolyte process.

At the end of the cell 44 seen in the direction of flow of the catholyte, the electrolyte is drawn off and fed via line 75 to the reservoir 60 of the catholyte circuit 46 of the first stage 41. Sodium hydroxide and optionally water can also be entered in line 75. Methanol can be added to the electrolyte returned to the pulp process for the organosolv process via the device 76.

Example 1:

3, the neutralization and flocculation cells are connected in series.

The neutralization cell has an anode and cathode area of 18 cm² each. Anode and cathode compartments are separated by a cation exchange membrane. The cathode (V2A expanded metal) lies directly on this membrane, while the anode (platinum) has a distance of approx. 1 mm from the membrane.

The anolyte storage vessel holds approx. 2oo ml. The anolyte is pumped with the help of a peristaltic pump via cell and gas separator from the storage vessel in a circuit (approx. 8 l / h), which means that the anode compartment volume is approx. 2 ml of approx. 0.9 s. The catholyte moves through the gas separator through self-convection in the circuit; there is no storage vessel.

The pH of the anolyte is determined using a glass electrode. The current flow in the neutralization cell is 3.6 A = 2oo mA / cm². The cell voltage is approx. 1o - 11 V.

At the start of the experiment, approximately 250 ml of lignin-containing waste liquor (pH 13.6) are poured into the storage vessel and, as described, pumped under electrolysis. 0.1 M sodium hydroxide solution serves as the first filling in the catholyte circuit.

After about 12o min. During the electrolysis, the anolyte has reached a pH of approx. 1o.

At intervals of approx. 3 min. (Always when the pH falls below 9.5) 10 ml of fresh waste liquor (pH 13.6) is poured into the storage vessel and at the same time, the same amount of anolyte (pH 9.5) is continuously discharged behind the neutralization cell. Corresponding a throughput of the neutralization cell of approx. 200 ml / h; the discharge is therefore approx. 2.5% of the anolyte cycle current.

The flocculation cell has a cathode and anode area of approx. 20 cm². Anode and cathode compartments are separated by a cation exchange membrane. The anode compartment is open and has a floating device. Its volume is approximately 3oo ml. The electrodes are arranged below. The current flow here is approx. 4 A = 2oo A / cm²; the cell voltage is approx. 15 V.

The anolyte (pH9.5) discharged from the neutralization cell anolyte circuit is introduced into the flocculation cell and electrolyzed (approx. 200 ml / h). The result is a viscous light brown foam made from lignin flakes, deflocculated anolyte (pH 5) and anode gas (O₂), which is removed using the flotation device. Allowing this foam to settle gives approx. 0.5 l of leachate (deflocculated, pH 5) per liter of anolyte (pH 9.5) and approx. 1 - 2 1 of highly lignin-containing, no longer removable foam, from which drying results in approx. 4o g Let Rohlignin win.

The deflocculated waste liquor (pH 5) is again continuously mixed with the catolyte of the neutralization cell (approx. 100 ml / h) after the foam settling and filtration and NaOH is continuously discharged in the same amount (pH 14). After suitable dilution and solvent addition, this NaOH is incorporated into the new pulp cooking process.

Example 2:

The experimental arrangement is the same as in Example 1. Another flocculation cell with an upstream settling device is connected in series behind the flocculation cell and both cells are operated with 2A. The first flocculation cell produces a foam of approx. PH 7, which settles into an electrolyte of pH 7 after some time. Separated lignin flakes (approx. 10% of the total content) are filtered and the electrolyte is fed into the second flocculation cell. The second cell produces a foam like example 1. The cell voltages in the flocculation cells are approx. 7 and 7.5 V.

Claims (14)

1. A process for the recovery of lignin by precipitation from an alkaline lignin solution by means of electrolysis as well as recovery of alkali, characterized by the fact that the process is a continuous one and an integral part of the Organosolv pulping process, that the alkaline lignin extract is derived from the Organosolv process, and that the alkaline lignin extract is continuously acidulated at the anode of the electrolysis unit and the alkali is recovered at the cathode of the unit.
2. A process according to claim 1, in which the alkali is sodium hydroxide
3. A process according to claims 1 and 2, in which the alkaline lignin solution is passed through an electrolysis cell(1) which is divided by means of an ion exchange membrane(3) into an anode space(7) and a cathode space(8).
4. A process according to claims 1 through 3, in which the anode(4) and the cathode(5) are made from metal wire mesh which are in size about the same as the ion exchange membrane(3).
5. A process according to claim 4, in which the anode is a coarse wire mesh.
6. A process according to claims 1 through 7 in which the process consists of two steps; in the first step the lignin solution is neutralized in the space around the anode(23) barely to the point where the lignin begins to precipitate, preferably to pH 9.5, and subsequently in the second step acidulated in the space around the anode(25) to the point where the lignin fully precipitates from solution, preferably to pH 4.
7. A process according to claims 1 through 5, in which the oxygen, generated at the anode, forms a foam together with the precipitating lignin and the acidulated solution which is then recovered by means of floatation.
8. A process according to claims 6 and 7 in which the solution stemming from the anode from the second step in the process, following separation of the precipitated lignin, is taken to the space around the cathode in the first and second step of the process.
9. A process according to claims 1 through 8 in which the regenerated solution from the cathode is taken back into the process as pulping medium or becomes part of the pulping process.
10. A process according to claims 1 through 9 in which the electrolysis process is being operated at an elevated temperature, preferably just below the boiling point of the solution.
11. A process according to claims 1 through 10 in which the solutions in the space around the anode and/or the cathode are being recirculated.
12. A process according to claims 1 through 11 in which the second stage of the process is built as a floatation(9) cell.
13. A process according to claims 1 through 12 in which the conductivity of the solution in the space around the cathode in the second stage of the process is adjusted to a certain minimum value by means of addition of sodium hydroxide to the solution.
14. A process according to claims 1 through 12 in which the conductivity of the solution in the space around the cathode in the first and second stage of the process is adjusted such that 1 to 10% of the solution is being recirculated.

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