EP0140226A2 - 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 process for obtaining lignin by precipitation from alkaline lignin solutions by means of neutralization 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 are used only for a purified water and to reach 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 C0 ₂ and the carbonate formed are caustified with calcium oxide.

The invention has for its object to provide a method in which the lignin is obtained from an alkaline lignin solution, preferably a waste liquor (alkaline extract) from a pulp process in a form suitable for further processing, with little equipment, so that destruction omitted and in which there is also the possibility of supplying the water fraction and the alkali of the extract for reuse in the process.

The object is achieved in a method of the type mentioned in the invention in that the lignin solution is continuously anodized by electrolysis and the alkali is regenerated cathodically in the same process. In other words: the alkaline extract (lignin solution) is in the anode compartment of a divided electro lytischen cell directed and there electrochemically acidified, while at the same time the lye is electrochemically concentrated 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.

The new method is generally applicable to alkaline lignin solutions. It is mainly used for alkaline lignin solutions, which are an extract or a lye from a pulp process, and in this case preferably for lignin solutions, which are obtained from the extract of an organosolve process for pulp production by separating off the organic solvent. Sodium hydroxide solution is mainly used as the alkali.

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 compartments by the cation exchange membrane. At the end of the cell there is 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 a particularly small amount of energy when 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, which, according to experience, corresponds to approximately pH9.5. 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 obtaining pure natural lignin; how it z. B. provides the Organosolv process according to patent application P 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 for the sodium hydroxide solution which inevitably forms in the cathode compartment (in addition to hydrogen) from H ₂ O to be returned directly to 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 of the first stage of the process 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. l 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:

• 1 shows an electrolytic cell for carrying out the method in one stage,
• 2 shows a schematic representation of the method with two stages,
• 3 shows 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 there is only a relatively narrow gap 9 between the anode 4 and membrane 3. Power connections lo 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. ^* )

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 are each 50 cm ² . Anode and cathode compartments are separated by a Nafion membrane. The anode ^* ) Water, dilute alkali metal solution 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. The fume cupboard is equipped with a flotation device and holds 3oo ml. At the beginning of the experiment, the anode compartment is filled with 2 ₀₀ 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 / cm ² . 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 / 1 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 20 and freed of the methanol in a methanol recovery device 21 and the methanol is fed back to the cooking process via line 21 b. 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. In the anode compartment 23, the extract is acidified electrolytically until a pH of 9.5 is reached. From The extract, which has this pH value, is continuously fed to the anode compartment via 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 supplied 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 all-ignignin at the pulp, also known as waste liquor, with a pH 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 leaving the anode compartment 55. In the gas separator 56, the anode gas generated during the electrolysis is predominant. Oxygen, 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 H ₂ O 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 about 9.5 is introduced into the anolyte space 7o of the cell 44 of 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 workup process in a manner known per se, so that pure lignin is formed. The electrolyte is returned to the catholyte space 73 of the cell 44 via the line 72. 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 6o of the catholyte circuit 46 of the first stage 41. Sodium hydroxide and optionally water can also be entered in line 75. For the organosolv process, the electrolyte returned to the pulp process can also be added via the device 76 methanol.

Example 1:

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

The neutralization cell has an anode and cathode area of 18 cm ² 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 / cm ² . The cell voltage is approx. Lo - 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. Lo.

At intervals of approx. 3 min. (always when the pH falls below pH 9.5) each lo ml of fresh waste liquor (pH 13.6) is poured into the storage vessel and at the same time the anolyte (pH 9.5) is continuously discharged in the same amount behind the neutralization cell. The Ent speaks a throughput of the neutralization cell of approx. 2oo 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 cm ² , anode and cathode space 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 / cm ² ; 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). A viscous light brown foam is formed from lignin flakes, deflocculated anolyte (pH 5) and anode gas (0 ₂ ), 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 (pH9.5) and approx. 1 - 2 1 of highly lignin-containing, no longer removable foam, from which approx. 40 g Let Rohlignin win.

The deflocculated waste liquor (pH 5) is again admixed 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 creates a foam with a pH of around 7, which settles into an electrolyte of pH 7 after some time. Separated lignin flakes (approx. Lo% 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 (17)

1. A process for the production of lignin by precipitation from alkaline lignin solutions by means of neutralization and for the recovery of the alkali, characterized in that the lignin solutions are continuously anodized by electrolysis and the alkali is regenerated cathodically in the same process. 2. The method according to claim 1, characterized in that the alkaline lignin solution is an extract or a waste liquor from a cellulose process. 3. The method according to claim 1, characterized in that the alkaline lignin solution is the extract from an Organosolv process for pulp production, from which the organic solvent has been separated beforehand. 4. The method according to any one of claims 1 to 3, characterized in that the alkali used is sodium hydroxide solution. 5. The method according to any one of claims 1 to 4, characterized in that the lignin solution and the alkali are passed through an electrolysis cell (1) through a cation exchanger membrane (3) in the anode (7) and cathode space (8th ) is divided. 6. The method according to any one of claims 1 to 5, characterized in that the anode (4) and the cathodes (5) are formed from metal mesh and have approximately the same shape and size as the membrane (3). 7. The method according to claim 6, characterized in that the anode (4) has a coarse mesh. 8. The method according to any one of claims 1 to 7, characterized in that the process is carried out in two stages, the lignin solution in the anode compartment (23) of the first stage until the lignin is just beginning to precipitate, preferably up to a value of pH9. , 5 neutralized and acidified in the anode compartment (25) of the second stage until the lignin has completely precipitated, preferably up to a value of pH 4. 9. The method according to any one of claims 1 to 8, characterized in that the anodic oxygen, together with the precipitated lignin and the acidified solution forms a foam which can be separated as a lignin suspension by flotation. lo. Method according to one of claims 8 and 9, characterized in that the anolyte of the second stage after separation of the precipitated lignin is passed into the cathode compartment of the first or second stage. 11. The method according to any one of claims 1 to lo, characterized in that the cathodically regenerated alkali is recycled as a digestion medium or its component in the pulp process. 12. The method according to any one of claims 1 to 11, characterized in that the electrolysis is carried out at elevated temperature, preferably just below the boiling point. 13.The method according to any one of claims 1 to 12, characterized in that the method is carried out continuously and as an integral part of the Zellstoffaussehlusses by the Organosolv method. 14. The method according to any one of claims 1 to 13, characterized in that in the first process step, the catholyte and / or anolyte are circulated. 15. The method according to any one of claims 1 to 14, characterized in that the second process stage is equipped with a floatation device (9). 16. The method according to any one of claims 1 to 15, characterized in that the conductivity of the catholyte in the second process stage is adjusted to a minimum value by the addition of sodium hydroxide. 17. The method according to any one of claims 1 to 15, characterized in that to adjust the conductivity of the catholyte in the first and / or second stage 1 to lo% of the catholyte are circulated.

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