WO2012062353A1

WO2012062353A1 - Method for using waste water arising during the production of lignocellulosic fibrous materials

Info

Publication number: WO2012062353A1
Application number: PCT/EP2010/067067
Authority: WO
Inventors: Volker Paasch
Original assignee: Voith Patent Gmbh
Priority date: 2010-11-09
Filing date: 2010-11-09
Publication date: 2012-05-18

Abstract

The method is used to process waste water (4) arising during the production of lignocellulosic fibrous materials. Said method comprises an electrochemical treatment (6) for separating cations contained in the waste water (4), and a fractionation process (9) in order to separate substances (10) having high molecular weight from substances (11)having low molecular weight. At least some of the substances (10) having high molecular weight are preferably incinerated. The substances (11) having low molecular weight are advantageously processed in an anaerobically operating waste water treatment system (18), forming combustible biogases. The method enables the material and thermal use of the waste water (4).

Description

Process for the use of wastewater generated in the production of lignocellulosic fibers

The invention relates to a method according to the preamble of claim 1.

In the production of lignocellulosic fibers, especially in the

Production of wood pulp or cellulose destroys disturbing components, essentially lignin, from the wood (chemical "digestion") by carrying out complex processes based on the chemical attack on the lignin, which makes it soluble It can be derived from waste water, or waste liquor, leaving behind a more or less virgin flexible fiber product, which is mainly used for paper or board production.The lignin is often converted into soluble lignosulfonate with sulfur salts thereby from negative Sulfitionen (S0 ₃ ^{2 "} ) or negative Hydrogensulfitionen (HS0 ₃ ^" ) from.

The amount of lignin dissolved in this way is considerable. It is u. a.

because of its high calorific value a valuable raw material. The wastewater, also known as lye or spent liquor, also carries with it some of the chemicals that are expediently recovered and reused for digestion. A process with this goal is called "recovery" by pulp professionals, it is very expensive and requires physical conditions

(Temperatures, pressures) with increased risk potential.

It is an object of the invention to improve the method for using the wastewater produced in the production of lignocellulosic fibers so that the recovery of energy and the recovery of useful chemicals and water are less expensive. In particular, it should be easier to adapt to different requirements in terms of throughput and composition of the wastewater. In addition, it should offer the opportunity to improve the economics especially of high-yield pulping processes. This object is achieved by the features mentioned in claim 1.

The inventive method comprises a combination of several process steps, which are in particular an electrochemical treatment of the wastewater, also a special separation process and in preferred

Embodiments of a combustion reaction. It is particularly suitable for such outcrops in which the driving chemical force of negative sulfite ions (S0 ₃ ^{2 "} ) or negative hydrogen sulfite ions (HSO ₃ ^" ) emanates. Cheap applications are high yield methods, ie

Digestion processes in which a higher lignin content remains in the pulp than would be technically possible. These are e.g. more than 15% lignin in coniferous and more than 12% in hardwood resources. So far, the well-known complex recovery procedures are often not worthwhile because of the lower amount of chemicals and / or dissolved organic substances in the wastewater. modern

High yield methods are e.g. in DE 10 226 027 006 A1 and DE 10 2007 008 955 A1.

In the electrochemical treatment in the process according to the invention, an ion reaction is carried out by replacing certain cations (for example sodium, magnesium, calcium or ammonium ions) used in the digestion and carried in the wastewater by protons, ie hydrogen ions.

Preferably, this electrochemical treatment causes at least 50%, better still at least 70%, of the cations contained in the wastewater to be removed. This results in a technically usable lignosulfonic acid, from which thermal energy can then be obtained by combustion, or which can serve as a starting material for usable products. After one

Combustion process, the sulfur compounds such. B.

Sulfur dioxide from the flue gas washed out and in the context of

Be used again.

With particular advantage, the per se known possibility can be used for the electrochemical treatment, with special walls or membranes to control the flow of ions as a function of their charge (cations or anions). Such an ion-selective wall (diaphragm,

semipermeable membrane) is only permeable to certain ions, ie one cation-selective wall only for positive ions and an anion-selective wall for negative ions only.

Particularly favorable for the process are bipolar membranes, which consist of a combination of a cation-selective with an anion-selective layer. These are dissociated by an applied electric field water into anions and cations and these depending on their charge in the

submitted to adjacent chambers. Processes with bipolar membranes can have much lower power consumption.

This electrochemical process, which involves both mass conversion and separation of certain reaction products, is often referred to as electrodialysis. There are also other electrochemical treatment methods for such wastewater conceivable, such. As the electrolysis, in which also known an ion exchange takes place. The electrochemical treatment makes it possible to recover the cations (for example sodium, magnesium, calcium or ammonium) which are relevant for the production of lignocellulosic fibers directly and in concentrated form and to reuse them for digestion. The wastewater is a before or after the electrochemical treatment

Subjected to fractionation process in which high molecular weight constituents are enriched and separated from low molecular weight fractions. The molecular weight (molar mass) provides information about the molecular size. The lignin-containing molecules are for the most part high molecular weight, which applies in this process, in particular for the lignin-sulfur compounds. Other components of the

Sewage such as inorganic sulfur salts, gases or water have relatively low molecular weights, so are low molecular weight. The separation success of this Fraktionierprozesses can be z. B. characterize the distribution of the COD load. This means that it is measured how high the COD value is in each case in the feed, in the flow (permeate) and in the overflow (retentate).

About the respective volume flows so the COD loads or

Mass flows (eg in gCSB / h) are calculated in the individual streams. It is desirable to achieve in the overflow (retentate) at least 55%, better at least 70%, even better over 80% of the COD load, based on the feed. As is known, the COD value gives the chemical oxygen demand chemical degradation of an organic substance.

Simple calculation example:

Volume flow COD value (conc.) COD load (mass flow)

Feed: 1 m ³ / h 100 g / l COD 100 kg / h COD

Permeate: 0.8m ³ / h 37.5g / l COD 30kg / h COD

Retentate: 0.2m ³ / h 350g / l COD 70kg / h COD

This means that 70% of the COD load was retained in the retentate.

The fractionation process can be carried out with advantage in a nano- or ultrafilter in which the filter medium is so fine that the

high molecular weight fractions of the wastewater are retained. Another possibility is provided by the evaporation processes, in which the solids content is increased (which is usually the case anyway) and at the same time the

Low molecular weight fractions increasingly go into the gas phase because of their lower compared to high molecular weight components vapor pressure.

The electrochemical treatment and the fractionation process result in that the combustion of the high-molecular constituents is unproblematic, especially since the metal ions have been removed to a large extent beforehand. The danger of the formation of explosive molten metals in the incinerator no longer exists. Hazardous metal melts can form at high temperatures when there are large amounts of sodium in the kiln. Another advantage is that the process provides a more thickened high molecular weight fraction. This means a lower energy requirement before combustion (mechanical and / or thermal) or

lower energy loss through evaporation during combustion. The low molecular weight fraction formed during the fractionation process can usually be treated without special problems in anaerobic biological wastewater treatment, since it is relatively easily biodegradable and can be converted into usable biogas by anaerobic digestion. The apparatuses which are particularly suitable for carrying out the process according to the invention can be constructed from individual reaction units, which has the advantage of easy adaptation to the respectively required production quantity and quality. Thus, reactors for electrochemical treatment are usually composed of modules. As fractionators evaporators or nano- or ultrafine filters can be used, can be easily adapted or are also modular. Already available and proven equipment can be adapted to the process and used for it, which significantly increases its cost-effectiveness and safety.

The invention and its advantages will be explained with reference to drawings. Showing:

Fig.1: a scheme for explaining the method;

2 shows a process designed as electrodialysis for the electrochemical separation of cations from wastewater.

In an example shown simplified in FIG. 1, the wood raw material 1 in the digestion process 2 is chemically treated in such a way that substantially

lignocellulosic pulps (pulp) 3 are produced, taking

Others a larger amount of wastewater 4 is obtained. On that for that

process according to the invention important ingredients, the lignin-sulfur compounds, in particular lignosulfonates based on sodium, magnesium, calcium or ammonium has already been received. Such chemical digestion processes are known and will not be further described here.

After filtration 5 of the waste water 4 for the separation of interfering fines or usable fibers, an electrochemical treatment 6 for

Recovery of chemicals, and here the treatment by the illustrated in Fig. 2 electrodialysis. In this case, a concentrate 7 falls on with greatly increased salinity. Preferably, it contains sulfites, here sodium sulfite (Na ₂ S0 ₃ ), which is used for digestion 2 again. Further, in electrodialysis, a diluate 8 is produced (with a greatly reduced salt content), from which in a fractionation process 9, z. As a nano or ultrafiltration, the lignosulfonic acid is pulled out as high molecular weight material 10 and then burned, including the combustion 12 serves. In this application example, therefore, the fractionation process 9 follows the electrochemical treatment 6, which, however, is not mandatory. A quite useful further possibility is first the high molecular weight substances of the wastewater 4 through the fractionation process 9, z. B. enrich by filtering or thermal thickening and then perform the electrochemical ion reactions.

The resulting in the combustion 12 exhaust gases 16 can be by gas scrubbing

13 with water, here with the return water 14, or with alkaline solutions, eg. As sodium sulfite (Na ₂ S0 ₃ ) or sodium hydroxide (Na OH) to sulfuric acid or

Solutions with HS0 ₃ ^" or S0 ₃ ^2" converted and derived. C02 and other gases or vapors remain in the largely sulfur-free exhaust gases 17. The sulfurous acid or the solutions with HSO ₃ ^" or S0 ₃ ^2" can be used with particular advantage in the electrochemical treatment 6, in particular as a receiving solution 15, if it is at this one

Electrodialysis with ion-selective walls acts. This will be discussed in more detail.

The filtrate leading to the low molecular weight substances 11 from the fractionation process 9 can be purified in a biological wastewater treatment 18, an anaerobic process being particularly favorable because of the possibility of biogas production and biological sulfur reduction. The resulting vapors and gases 19 are used thermally as biogas in the combustion 12. The filtrate 20 of the biological wastewater treatment 18 is subjected to sulfur separation 25, and the separated sulfur 21 is passed into the same combustion 12.

The largely sulfur-free return water 14 can be used at various points as process water, z. B. as here in the exhaust scrubbing 13. Im

Compared to previously known methods of this type there is the advantage that the incurred at different points substances together in one

Incinerator burned and the resulting exhaust gases can be cleaned in a common gas scrubbing.

A reactor for carrying out the electrochemical treatment 6 usually contains a larger number of juxtaposed reaction chambers, which are flowed through in parallel by the liquids and are separated from one another by the ion-selective walls already described. To the ion flux Maintaining through these walls, an electrical voltage is applied, to which surface electrodes, anode and cathode are usually mounted on both sides of the reactor. The ion flux then leads to an electric current through the reactor. In Figure 2, the principle of suitable for electrochemical treatment electrodialysis on a

corresponding reactor explained. There are several in this one

Reaction chambers 30, 30 ^' and from the bottom to the top of accumulating in the production of lignocellulosic fibers wastewater 4 are flowed through. This wastewater 4 usually contains lignin-sulfur compounds, in particular lignosulfonates based on sodium, magnesium, calcium or ammonium. The reaction chambers 30, 30 ^' are bounded laterally only by certain ion-permeable walls 33, 33 ^' , 34 ^' (semipermeable walls), which are cation-selective here. In other cases, bipolar walls may be used which have already been discussed. Such

Embodiment would then provide the bipolar walls at the locations that complete the wastewater 4 leading reaction chambers 30, 30 ^' , 30 ^" each on the anode 23 closer side, so in Fig. 2 respectively

Wear reference numeral 33. Due to the applied by anode 23 and cathode 24 electric field, an electric current flows across

Flow direction of the liquids. In the laterally adjacent to the reaction chambers 30, 30 ^' each closer to the anode 23 reaction chambers 31, 31 ^' flow liquids, which are referred to as the receiving solution 15 (for receiving the salts), and the sulfuric acid here or its salts (eg with HSO ₃ ^" or S0 ₃ ^2" ions). Through the wall 33 from the adjacent closer to the anode 23 reaction chamber 31, 31 ^' an inflow of hydrogen ions (protons) in the wastewater 4 leading reaction chamber 30, 30 ^' while through the wall 33 ^' , 34 ^', a flow of cations the waste water 4 in the other adjacent closer to the cathode 24 lying reaction chamber 31 ^' , 32 ^{' is} generated. As a result of this ion exchange, cations (for example sodium, magnesium, calcium or ammonium ions) entrained in wastewater 4 are replaced by protons, ie hydrogen ions, so that the already mentioned lignosulfonic acid is generally formed in diluate 8. At the same time, the receiving solution 15 converts into a concentrate 7, z. As sulfite or hydrogen sulfite with the corresponding base (sodium, magnesium, calcium or ammonium) to. D. izB sodium sulfite or sodium hydrogen sulfite, if sodium ions in wastewater are 4. The sulfite or hydrogen sulfite can then be used as a reducing agent for the digestion 2 (see Fig.1), in which then turn lignosulfonate with sodium, magnesium, calcium or ammonium ions formed.

Furthermore, this reactor shown in Fig. 2 at the anode 23 contains a

Reaction chamber 32 and at the cathode 24, a reaction chamber 32 ^' for performing a further auxiliary reaction. An electrolytic rinsing liquid 22 or 22 ^' is circulated through these two chambers. Specifically, a sulfur salt of the base used in the pulp production (sodium,

Magnesium, calcium or ammonium), in particular sodium sulfate (Na ₂ SO ₄ ), into the reaction chamber 32, which has the anode 23 on one side and a semipermeable wall 34 on the other side. Through this wall 34, a flow of cations (here sodium ions, in other applications: magnesium, calcium or ammonium ions) to the guided in the adjacent reaction chamber 31 receiving solution 15 is generated, which in turn - as already

described - in the concentrate 7, z. As sulfite or hydrogen sulfite, here in

Sodium hydrogen sulfite or preferably converted into sodium sulfite. The

Rinsing fluid 22 is protonated, ie it absorbs protons and is chemically changed by the cation release to sodium bisulfate (NaHS0 ₄ ). It is conveyed as flushing fluid 22 ^' hydraulically, ie without penetrating the semipermeable walls, into a reaction chamber 32 ^' provided with the cathode 24. The already described flow of cations (eg sodium,

Magnesium, calcium or ammonium ions) from the reaction chamber 30 ^" into the reaction chamber 32 ^' converts at the cathode 24 the sodium bisulfate

(NaHS0 ₄ ) to sodium sulfate (Na ₂ S0 ₄ ).

The reactions taking place in the reaction chambers 30, 30 ^' , 30 ^" , 31, 31 ^' are the actual useful reactions of the electrochemical treatment 2 and are preferably carried out in parallel in at least one membrane stack which is replaced by a larger number of such reaction chamber pairs (" unit cells "). is formed. This leads to a minimization of the electrode losses (due to the unavoidable electrolytic water splitting) as well as to a greater throughput. The inflows of the waste water 4 and the sulfurous acid and the effluents of the diluate 8 and the concentrates 7 are by Distribution pipes or headers distributed or summarized. It suffices if the reaction chambers 32 and 32 ^' for the rinsing liquid 22 or 22 ^' are present only once per reactor. In this example, all ion-selective walls are permeable only to cations.

Depending on economic or technical requirements, bipolar walls or, in special cases, anion-selective walls can be used. The advantages of bipolar walls, which are generally preferable in this method, have already been pointed out.

Claims

Patent claims:

1 . Process for the use of in the production of lignocellulosic

fibrous wastewater (4), in which parts of the substances contained in the wastewater (4) are burned or recovered as usable substances or in which parts of the substances contained in the wastewater (4) are burned and other parts of the substances contained in the wastewater (4) are used as usable materials are obtained,

characterized,

that an electrochemical treatment (6) takes place to separate cations contained in the wastewater (4), and

that in a fractionation process (9) high molecular weight substances (10) are separated from low molecular weight substances (1 1).

2. Method according to claim 1,

characterized,

that the wastewater (4) is first subjected to the electrochemical treatment (6) to separate the cations contained therein, and that the concentrate (7) enriched with the cations is then in

Fractionation process (9) is processed.

3. Method according to claim 1,

characterized,

that the wastewater (4) is first treated in the fractionation process (9), and

that the fraction enriched with the high molecular weight substances (10) is then fed to the electrochemical treatment (6).

4. Method according to claim 1, 2 or 3,

characterized,

that the wastewater (4) contains lignin-sulfur compounds, in particular lignosulfonates based on sodium, magnesium, calcium or contains ammonium.

5. Method according to claim 3,

characterized,

that a part, in particular the majority, of the high molecular weight substances (10) are lignin sulfonic acids.

6. Method according to one of the preceding claims

characterized,

that at least some of the high molecular substances (10) are burned.

7. Method according to one of the preceding claims

characterized,

that at least some of the high molecular substances (10) are converted into a usable substance.

8. Method according to one of the preceding claims,

characterized,

that the low molecular weight substances (1 1) consist of sulfur salts and organic components, in particular sugar, fatty acids and/or lignin fragments.

9. Method according to one of the preceding claims,

characterized,

that the low molecular weight substances (1 1) are fed to a biological, preferably anaerobic wastewater treatment (18).

10. Method according to claim 9,

characterized,

that gases (19) produced during wastewater treatment (18) are fed to combustion (12).

1 1 . Method according to claim 9 or 10,

characterized,

that the filtrate (20) of the wastewater treatment (18) largely desulfurized and reused as return water (14).

12. Method according to claim 10 and 1 1,

characterized,

that the sulfur (21) removed from the filtrate (20) of the wastewater treatment (18) is fed to the same combustion (12) in which the gases (19) produced during the wastewater treatment and the high molecular substances (10) are also burned.

13. The method according to claim 10, 11 or 12,

characterized,

that contained in the exhaust gases from combustion (12).

Sulfur compounds, in particular sulfur dioxide, are converted into sulfurous acid or into a solution containing HS0 ₃ ^" or S0 ₃ ^2" by reaction with water or with alkaline solutions, in particular sodium sulfite, sodium hydrosulfite or sodium hydroxide.

14. Method according to claim 13,

characterized,

that the sulfurous acid or the HS0 ₃ ^" or S0 ₃ ^2" -containing solutions are used as a receiving solution (15) in the electrochemical treatment (6) to contain cations of the base used in fiber production, in particular sodium, magnesium and calcium - or ammonium ions from the wastewater (4).

15. Method according to one of the preceding claims,

characterized,

that the wastewater (4) for electrochemical treatment (6) is guided into reaction chambers (30, 30 ^' , 30 ^" ⁾ located between ion-selective walls (33, 33 ^' , 34, 34') and electric current flows through it, with the ion-selective walls (33, 33 ^' , 34, 34 ^' ) a flow of ions to be absorbed to the wastewater (4) and a flow of ions released from the wastewater (4) are generated.

16. Method according to claim 15, characterized,

that only walls (33, 33 ^' , 34, 34 ^' ) that are only permeable to cations are used.

17. Method according to claim 15 or 16,

characterized,

that at least some walls (33, 33 ^' , 34, 34 ^' ) are used that are only permeable to anions.

18. Method according to claim 15, 16 or 17,

characterized,

that bipolar membranes are used at least partially as walls (33, 33 ^' , 34, 34 ^' ).

19. Method according to one of claims 15 to 18,

characterized,

that the flow of ions to be absorbed by the wastewater (4) consists of positive hydrogen ions and the flow of ions released by the wastewater (4) consists of positive sodium, magnesium, calcium or ammonium ions.

20. Method according to one of claims 15 to 19,

characterized,

that a receiving solution (15), in particular sulfurous acid or a solution containing HS0 ₃ ^" or S0 ₃ ^2" in at least one further

Reaction chamber (32, 32 ^' ) with walls (34, 34 ^' ) that are partially permeable to ions and is flowed through by electric current, with an influx of ions in the through the walls (34, 34 ^' ).

Fiber production used base (sodium, magnesium, calcium or ammonium) in the receiving solution (15) and an outflow of

Hydrogen ions are generated from the receiving solution (15) to the wastewater (4) conducted in an adjacent reaction chamber (30 ^' , 30 ^" ).

21. Method according to claim 20,

characterized, that a rinsing liquid (22) with a salt in the

Base used for fiber production (sodium, magnesium, calcium or ammonium), in particular sodium sulfate (Na ₂ S0 ₄ ), is passed into at least one further reaction chamber (32), the at least one anode (23) forming hydrogen ions and at least one wall partially permeable to ions (34), wherein the rinsing liquid

(22) is traversed by electric current,

that cations of the base used in fiber production (sodium, magnesium, calcium or ammonium ions) flow through the wall (34), which is partially permeable to ions, to the receiving solution (15), in particular sulfurous acid, carried in an adjacent reaction chamber (31). or HS0 ₃ ^" or S0 ₃ ^2" - containing solution is produced,

that due to the influx of protons and the outflow of cations of the base used in fiber production, parts of the

Receiving solution (15) in protonated salt, in particular

Hydrogen sulfate is converted,

that the protonated salt, in particular hydrogen sulfate, in a modified rinsing liquid (22 ^' ) is fed to a further reaction chamber (32 ^' ) which has at least one cathode (24) forming hydroxide ions and at least one wall (34 ^' ) which is partially permeable to ions,

in that the wall (34 ^' ), which is partially permeable to ions, generates an inflow of cations of the base used in fiber production to the modified rinsing liquid (22 ^' ) guided in the reaction chamber (32 ^' ), and

that through the inflow of the cations of the base used in fiber production and the hydroxide ions, the protonated salt, in particular hydrogen sulfate, is deprotonated again, in particular converted into a sulfate.

Method according to one of the preceding claims,

characterized,

that the electrochemical treatment (6) is carried out in such a way that at least 55%, preferably at least 80% of the supplied Cations contained in wastewater are removed from it.

23. Method according to one of the preceding claims,

characterized,

that the fractionation process (9) is carried out in such a way that the COD load in the fraction containing the high molecular weight substances (10) is at least 55%, preferably at least 70% of the COD load in the

Wastewater supplied to the fractionation process (9).

24. Method according to claim 23,

characterized,

that the fractionation process (9) is carried out in such a way that the COD load in the fraction containing the high molecular substances (10) is more than 80% of the COD load in the wastewater supplied to the fractionation process (9).