WO2002040767A1

WO2002040767A1 - Method for recovering non-fibrous substances from wood material

Info

Publication number: WO2002040767A1
Application number: PCT/FI2001/000990
Authority: WO
Inventors: Kenneth Sundberg; Bjarne Holmbom; Christer Eckerman; Maria Adams
Original assignee: Raisio Chemicals Ltd
Priority date: 2000-11-16
Filing date: 2001-11-15
Publication date: 2002-05-23
Also published as: EP1358384A1; FI20002519A0; FI20002519A; FI115978B; AU2002215068A1

Abstract

A method for recovering non-fibrous substances from wood material being processed in a mechanical pulping process, such as TMP process or groundwood process. Non-fibrous substances, such as wood resins, aromatic components, salts and polysaccharides, are extracted from said wood material into a liquid fraction, such as process water or other suitable water based liquid. The recovery of non-fibrous substances from the liquid fraction includs separation of aromatic compounds from said liquid fraction, while preferably maintaining the pH-level of the liquid fraction < pH 7, during the extracting and recovering processes.

Description

METHOD FOR RECOVERING NON-FIBROUS SUBSTANCES FROM WOOD MATERIAL

The present invention refers to a method for recovering non-fibrous substances from fibrous wood material processed in a mechanical pulping process, such as a TMP process or a groundwood process, as defined in the preamble of appending independent claim 1.

Mechanical pulping of wood to produce fibers for paper and board is performed at elevated temperatures, typically above 80°C. High temperature is needed in order to soften the lignin binding the fibers together. The increased temperature facilitates separation of fairly undamaged fibers. Mechanical pulping is usually performed by processing wood chips in disc refiners, or by grinding logs on a rotating stone with appropriate grits at the stone surface. Refining of chips produces so called thermomechanical pulp (TMP). Chemical treatment of the wood chips prior to the refining, typically with alkaline sulfite or alkali alone, can give even more selective fiber separation. Such chemimechanical pulps are used mainly in tissue products and paperboard. The disc refining is typically performed in two stages in separate refiners.

The high temperatures employed in mechanical pulping causes the dissolution or dispersion into the process water of a considerable amount of wood material. During production of mechanical pulp from Norway spruce, the most used wood species in Northern Europe, about 30 - 50 kg/t wood material is released into the process water. This dissolved and colloidal (DisCo) material is mainly composed of hemicelluloses, lignin, lipophilic extractives (wood resin) and lignans.

Paper mills have during recent years been forced to decrease their process water use in order to cut down on the effluents and to thus reduce the discharge to receiving waters. The released dissolved and colloidal substances (DisCo substances) now accumulate in the process water circulations in paper mills with a low fresh water intake, i.e. in mills with a high degree of water system closure. The accumulated DisCo substances will cause various disturbances in the papermaking process. The wood resin, also called pitch, can form sticky deposits on the papermaking equipment and can also dramatically reduce the paper strength. The dissolved lignins consume bleaching chemicals and cause decreased brightness of the produced paper.

The lignans and other hydrophilic extractives, such as different stilbenes and flavonoids, also consume bleaching chemicals, and can thus reduce the paper brightness when they are oxidized.

Inorganic salts are also dissolved in mechanical pulping. The salts can disturb pulp bleaching processes using hydrogen peroxides, by catalyzing the degradation of hydroperoxide ions. The salts can also promote undesirable growth of bacteria in the process waters. It is thus obvious that the removal of DisCo substances released in mechanical pulping prior to pulp bleaching and papermaking would give many economically important advantages.

The dissolution of wood substances in mechanical pulping cannot be decreased essentially by optimizing the pulping process. However, the negative impact of the DisCo substances in pulp bleaching and papermaking can be avoided if those substances are removed from the process waters immediately after the mechanical pulping process.

Processes for removal of wood resins from mechanical pulps have been proposed previously in US 4,922,989, which suggests turbulence treatment of mechanical pulp at a 7 - 20 % consistency, whereby resin removal is said to be enhanced.

It is further known that the dissolution and dispersion of wood material generally is at a minimum around pH 5, the natural pH formed in mechanical pulping, and that e.g. wood resins and pectins, have a much higher release at pH > 7 than at pH < 7. Suckling et al in "Factors affecting resin removal from radiata pine mechanical pulps", Appita J. 1990, 3:5, 217-221, have come to the conclusion that resin solubilization is enhanced by high pH values.

J. Alamo, D. Ouchi, J. Paris and J. Wearing in "Deresination of Ultra-High- Yield Pulps" Part H: Screw Press Study, in Pulp & Paper Canada 92:9 (1991), 47-51, suggest that alkaline conditions, i.e. pH > 7, should be used for improvement of deresination of chemical thermomechanical pulp (CTMP). The article shows in FIG. 2 that increasing the pH of process water from 4 to 8 provides an about 15 % increase in deresination efficiency. Sub-title recommends alkaline pH.

However, so far, no processes known to us have been developed for recovery of other non-fibrous wood material components, such as water soluble components, released from mechanical pulps.

It is therefore an object of the present invention to provide a method for minimizing the above mentioned problems.

It is thereby an object of the present invention to provide an improved method for removal of contaminants, such as wood resins and aromatic components that may have detrimental effects on both paper machine "runnability" and paper quality.

It is also an object of the present invention to provide a method for recovery of valuable dissolved and colloidal substances from process waters in mechanical pulping.

It is thereby an object of the present invention to provide a method for isolating from process waters relatively pure fractions of components that may have a positive effect on paper strength, for later use, e.g. in well targeted positions of the papermaking process. It is further particularly an object of the present invention to provide a method for recovery of valuable aromatic compounds that may be used in the production of bio- active compounds that may be used in the pharmaceutical and food industries.

The present invention thereby provides a method, for recovering non-fibrous substances from wood material being processed in a mechanical pulping process, as disclosed in the characterizing portion of enclosed independent claim 1.

The method according to the present invention can advantageously be used for recovering valuable non-fibrous substances from wood material, for later use. The invention thereby provides a means to isolate certain valuable components and component groups, such as bio-active compounds and glucomannans, from wood material by separate extraction or from process liquids, such as washing waters, dilution waters and white water, into which these compounds have been extracted.

The method can also be used for decreasing the concentration of non-desired non-fibrous substances, detrimental to the pulping and papermaking processes, present in process waters. The present invention provides a means to reduce the concentration of dissolved and colloidal components particularly in mechanical pulping and papermaking process liquids.

According to the present invention different valuable non-fibrous components may be easily recovered from wood material in mechanical pulping and paper making processes and be separated into relatively pure fractions for further utilization. Upon isolation, these pure fractions may be returned back to the papermaking process or be used for other purposes, e.g. in pharmaceutical industry. Components or component groups that can be obtained as relatively pure fractions by using the invention are wood resins, aromatic substances, such as lignans, lignin, stilbenes, flavonoids, wood polysaccharides and salts. For extraction and recovery of some valuable compounds, such as the lignan hydroxymatairesinol (HMR), in native form, the pH level in the entire mechanical pulping process is preferably to be maintained at pH < 7, preferably at pH 4 to 6, up to the extraction and recovery of the lignans.

The present invention is particularly suitable for separating non-fibrous substances from wood material being processed in thermomechanical pulping (TMP) processes. The recovery of dissolved and colloidal (DisCo) substances is especially facile in TMP processes as the DisCo substances can be washed out from the wood material, at relatively low preferred pH levels in the range of pH 5 and at preferred temperatures below 90°C. Momentarily the temperature may be higher in the actual pulping process. Further the valuable substances may be recovered using relatively small amounts of water, leading to high concentrations of recoverable substances in the liquid fraction.

In a TMP process utilizing the present invention, non-fibrous substances may be

extracted from the wood material into a liquid fraction in a separate extraction step in an

extractor. But at some TMP processes, in which the wood material is processed at suitable low pH levels of about pH 4 to 6 and high temperatures, valuable recoverable

substances, such as the water soluble aromatic components, are dissolved and dispersed

into the liquid fraction at such concentrations that these substances may be recovered

without a separate extraction step in an extractor. The valuable substances may be

recovered from process waters, such as washing waters, dilution waters or white water,

discharged from or circulated in the pulp and paper mill. At the low pH levels temperature does not play as important a role for the recovery of e.g. HMR as at higher

pH levels. Therefore high temperatures may be used at the lower pH levels. At higher

pH levels, particularly at pH levels >6, lower temperatures should be used. Extraction of non-fibrous substances from wood material according to the present invention preferably takes place in an extractor, in which non-fibrous substances from wood material are dissolved and dispersed into a water fraction during mixing of pulp with water. Pulp material should preferably be processed in the extractor at a rather diluted state. This will often necessitate dilution of pulp material before the extraction process. Dilution liquid, typically water, may be introduced directly into the extractor or may be mixed into the pulp material before its introduction into the extractor. TMP pulp taken from a refiner stage of a papermaking process and having a typical dry matter content of about 5 - 50 %, typically 20 - 40 %, more typically 30 - 40 %, should preferably be diluted by addition of water to a dry matter content of about 0,5 - 20 %, typically to 0,5 - 15 %, more typically to 0,5 - 10 %, before processing in the extractor.

In the extractor non-fibrous substances, such as small particles including wood resins and water-soluble components including aromatic compounds, salts and polysaccharides, are dispersed and dissolved into the liquid fraction of the diluted pulp material. The portion of the liquid fraction, from which dissolved and colloidal non- fibrous substances are to be separated and recovered, is preferably separated from the rest of the pulp material by pressing, but may in some processes be separated by gravity, e.g. by sedimentation or centrifugation, or even by coarse material filtration.

According to the present invention, extraction of water soluble aromatic compounds should preferably take place at pH < 7, more preferably at pH 4 - 6. Thus in pulping processes, such as CTMP processes, in which the pH level of the pulp suspension is higher, the pH level should be lowered to provide a suitable pH level for the extraction process. Suitable acid may be introduced into the pulp suspension to provide the desired pH level.

The potentially valuable dissolved and colloidal (DisCo) substances, released in mechanical pulping from the wood material into the process waters, include e.g. wood resins, water soluble aromatic compounds, salts and polysaccharides. The main component groups in wood resins, i.e. the fatty acids, resin acids and sterols, are commercially produced chemicals that until now have been recovered only at kraft pulp mills in the form of tall oil soap. The tall oil soap is acidified and distilled to give a pure fraction of fatty acids and resin acids. The sterols have been recovered either from the tall oil soap or from the tall oil pitch, which is formed as a residue in tall oil distillation. Recovery of wood resins also at mechanical pulp mills, made feasible by the present invention, is an important supplement to the present tall oil production. The wood resins, as well as, its main components, the triglycerides and steryl esters, recovered in a process utilizing the present invention could be used in different applications as such.

Of the water soluble aromatic compounds, particularly the lignans are potentially valuable bio-active components for pharmaceutical or functional food applications due to their antioxidative and anticarcinogenic properties. This is especially true for lignans from softwood species, such as Norway spruce, containing the very bioactive substance hydroxymatairesinol (HMR) as the major component. The separation and isolation of the HMR in industrial scale is made possible by the present invention. Particularly the isolation at a pH level below pH 7, preferably between pH 4 - 6, improves the yield.

The low molar-mass lignin present in mechanical pulping hquors is a pure, sulfur-free and reactive lignin, which may be a valuable complement to presently produced lignin products.

Of the polysaccharides particularly the dissolved hemicelluloses with glucomannans as the predominant component are useful as they provide stabilization of colloidal dispersions of wood resins and other water-insoluble sticky materials through a sterical mechanism. They also decrease the deposition tendency of such sticky particles by forming a hydrophilic gel around the particles, thus making them less sticky (tacky). Furthermore, hemicelluloses also decrease, or even eliminate, the negative effect that wood resins has on paper strength. Non-fibrous components may, according to a typical embodiment of the present invention, be extracted from the wood material in mechanical pulping processes in a separate extraction stage, in which non-fibrous components are extracted as dissolved and colloidal components into a washing liquor, typically water. The washing liquor separated from the fibrous wood material may be concentrated before recovery of the different components therefrom. The washing liquor, from the extraction stage, is then typically first treated to recover dispersed fine particles, such as wood resins, therefrom. Wood resins may be separated from the liquor by a microfilter, by ultra-filtration or by microflotation. After removal of wood resins and other fine particles possibly present in the washing liquor the remaining hquid consist primarily of water and water soluble components. The liquor may then be treated to separate the water soluble aromatic components therefrom.

The water soluble components may be isolated, as relatively pure fractions, by optimizing pH and temperature of the liquid and by using suitable process steps for the removal of each specific component or component group.

The process steps for separating desired components or component groups from extraction hquid or a liquid fraction, such as process water, washing water or white water, may according to a typical solution of the present invention, for recovering of non-fibrous substances from wood material being processed in a mechanical pulping process, include:

- Pressing or filtration for the separation of most of the liquid fraction from pulp suspension or process water from a mechanical pulping or paper making process.

- Extraction, filtration or flotation for the removal of non-soluble colloidal wood resins from the above separated liquid fraction.

- Removal of aromatic compounds by sorbents for aromatic components, aromatic exchange resin or column- or liquid chromatograpy, from the hquid fraction. - Dialysis or osmosis for the removal of salts from the liquid fraction. The dialysis removes salts, leaving a fraction of water-soluble polysaccharides, provided that wood resins and aromatic compounds have been removed earlier.

The wood resin free liquor may according to a preferred embodiment of the present invention be mixed with macroreticular particles of acrylate or divinylbenzene- polystyrene resin to give sorption of aromatic substances, such as lignans, stilbenes, flavonoids and lignin, therein. The acrylate or resin eluent including absorbed aromatic substances therein is separated from the hquor, which remaining hquor effluent mainly includes water soluble salts and polysaccharides, such as hemicelluloses. The aromatic substances are recovered from the acrylate or resin eluent with a polar solvent, such as acetone, ethanol or methanol. Aromatic substances may be separated from the eluent with a strong alkaline solution, if there is no need to recover lignans, without causing chemical transformation.

The aromatic compounds may be further fractionated into fractions containing lignans and lignin respectively. This may be accomplished, for instance, by extraction with methyl-tert-butylether (MTBE), dialysis, reverse osmosis or using chromatographic methods.

In order to separate and isolate lignans from process liquids without altering the chemical composition in which they occur in the wood material, a pH between 4 and 6 is preferably maintained until the lignans have been separated. Outside this pH range, lignans undergo various chemical transformations.

In the following particular examples of the method in accordance with the present invention will be described with reference to the accompanying drawings, in which

FIG. 1 shows an exemplary scheme of fractionation of dissolved and colloidal compounds present in mechanical pulp and paper making process liquids; FIG. 2 shows amount and composition of dissolved lignans separated from process liquids at different pH levels at a temperature of 90°C; FIG. 3 shows amount and composition of dissolved lignans separated from process liquids at different pH levels at a temperature of 60°C; FIG. 4 shows amount and composition of lipophihc extractives at different pH levels; FIG. 5 shows amount and composition of dissolved and dispersed hemicellulose at different pH levels; FIG. 6 shows an exemplary schematic view of a system utilizing the present invention;

FIG. 7 shows schematically another system utilizing the present invention for recovering aromatic substances, particularly lignans, from wood material; FIG. 8 shows lignan and lignin concentration in effluent after sorption of aromatic compounds in the system shown in FIG. 7, and FIG. 9 shows sorption of different lignan components in the system shown in

FIG. 7.

FIG. 1 shows an exemplary scheme of fractionation of dissolved and colloidal compounds present in process liquids 10 from pulp and paper making processes. Such process liquid 10 may be

- diluted liquor from an extractor, in which non-fibrous compounds have been extracted from wood material or

- process water from the mechanical pulping and papermaking process, such as washing water, dilution water or white water.

In order to isolate relatively pure fractions of all major non-fibrous components or component groups found in pulping and papermaking liquors, an initial separation of particulate matter, e.g. finely dispersed fiber material and colloidally dispersed wood extractives, is beneficial for further fractionation of the dissolved components. Therefore in a first separation step 12, a fraction of fibers and larger particles are separated from the liquid by pressing, filtration or other suitable means. The fibers and larger particles 14 separated from the liquid may be returned to the pulp- and papermaking process. The remaining hquid fraction 16 still includes finely dispersed fiber material and colloidally dispersed wood extractives, such as wood resins (pitch). These particles are separated from the liquid in a second separation step 18, by suitable separation means, such as microfiltration, ultrafiltration, flotation, centrifugation or extraction. The solids in reject 20 may be disposed of or brought to further use.

The accept fraction 22 from the second separation step 18 includes dissolved components from the pulping hquors, such as water-soluble aromatic compounds, salts and polysaccharides. These components or component groups may be fractionated based on their molecular characteristics, i.e. size, hydrophobicity, ionicity, volatility, etc.

Aromatic compounds, such as lignins and lignans, may be separated from the accept fraction 22 in a third separation step 24, utilizing absorbents, aromatic exchange resin, column- or liquid chromatography, dialysis, reverse osmosis, or membrane filtration to provide an eluent 26 from which lignins and lignans may be recovered e.g. by treating the eluent with a polar solvent. The effluent 28 of step 24 includes salts and polysaccharides. A dialysis or reverse osmosis separation step 30 may be used to separate hemicellulose from salts. The salts are separated into the permeate 32 of step 30. The polysaccharides are to be recovered from the concentrate 34 of step 30.

In order to isolate components in their "native form", i.e. in the form that they exist in wood material, the process conditions play an essential role. It has now suφrisingly been found that, in order to separate and isolate lignans from process liquids without altering the chemical composition in which they occur in wood material, a pH between 3.0 and 6.7 is preferable during the entire pulping process or at least during the extraction and separation of lignans from the liquid. Outside this range, lignans undergo various chemical transformations. EXPERIMENT 1

This experiment was performed to show the components separated from DisCo liquor at different pH levels.

Samples of thermomechanical pulp (TMP) were taken after the first refiner stage at a pulp and paper mill producing spruce TMP. These samples were used for the preparation of DisCo-hquors containing dissolved and colloidal (DisCo) substances by stirring the pulp at 1 % consistency in distilled water, followed by centrifugation to separate fibers therefrom. The stirring was performed at 90°C at pH 3, 4, 5, 6, 7 and 8 respectively for 10, 30, 60, and 180 minutes using a vibro mixer. After centrifugation at 500g (30 min), the resulting DisCo-hquors were analyzed for their contents of lignans, lipophilic extractives and polysaccharides by gas chromatography.

The composition of dissolved lignans in DisCo-liquors was strongly dependent on the pH used during pulp treatment. Results are shown in FIG. 2. At pH between 3 to 5 the composition was practically constant. But at pH > 6 the hydroxymatairesinol (HMR) decreased significantly. At pH 7 and 8 the concentration of HMR was dramatically lower than at pH < 7. The concentration of conidendric acid increased proportionally to the decrease in HMR, i.e. the total concentration of lignans remained practically constant. The experiment indicates that HMR was transformed to conidendric acid, and it is therefore very important to choose the right pH for pulp treatment in order to achieve a high recovery of HMR.

The transformation of HMR to conidendric acid is irreversible. It is therefore important that the pH is maintained at the correct pH-level during the whole pulp treatment process up to separation of HMR. It has been noticed that the pH-level should preferably be maintained in a range between pH 3 and pH 6.7, typically between pH 4 and pH 6. FIG. 2 also shows that HMR may be transformed to conidendric acid if stirring is continued up to 180 minutes at pH 5 and 6.

FIG. 3 shows the results of a similar experiment, in which the treatment was performed 5 at a temperature of 60°C. It can be seen that a higher concentration of HMR is received at this lower temperature. Also longer stirring time does not seem to have any major impact at pH 5 and 6.

FIG. 4 shows that the composition of lipophilic extractives in the DisCo-hquors was 10 only slightly dependent on pH, but the amount extractives dispersed into the water phase was highly affected by pH. The solubility of extractives was increased by higher pH level. At pH 8 the amount of lipophilic extractives was about five times higher than at pH 3. This can probably be explained by the formation of fatty and resin acid soaps at higher pH, whereby the soaps can act as dispersants for the other lipophilic extractives. 15

FIG. 5 shows that the amount of dissolved and dispersed hemicellulose were fairly constant throughout pH 3 to 7. However, at pH 8 the glucomannan amount decreased clearly, whereas an increase of dissolved pectin occurred. At alkaline conditions native glucomannans are deacetylated, rendering them less water soluble, which leads to 20 precipitation onto the pulp fibers. At the same conditions, pectins are demethylated resulting in higher solubility of the polygalacturonic acid sodium salts formed.

EXPERIMENT 2

25

This experiment was performed to show the separation and yield of different component groups from DisCo-liquor.

A first portion of the DisCo-liquor was filtered through a 0.1 mm membrane filter at

30 90°C by applying vacuum. About 85 % of the wood resins dispersed in the DisCo-liquor was removed from the DisCo-liquor, with the reject of this step. Over 90 % of the lignans and the lignin were recovered in the filtrate, as well as 85 % of the dissolved polysaccharides.

A second portion of the DisCo-liquor was treated in a microflotation apparatus. Bentonite and polyacrylamide (PAM) were used as flotation chemicals. A combination of bentonite and anionic PAM resulted in a 20 % decrease of wood resin removal. Practically no loss of lignans, lignin or polysaccharides occurred. The low flotation efficiency probably depends on the low concentration of wood resins in the DisCo- liquor tested. More concentrated DisCo-hquors would probably give higher removal rates also by flotation.

Filtrate from the filtration of wood resins was passed through e.g. a column containing a hydrophobic interaction resin (XAD-7) at pH 5 in order to recover aromatic substances. Substances adsorbed onto the resin were eluted with acetone or methanol. The yield of lignans was over 95 % and the yield of lignin up to 80 %. The recovered lignans and lignins could be fractionated by extraction with methyl-tert-butylether (MTBE).

Separation of hemicellulose was performed on DisCo-liquor after the removal of wood resin. The separation was performed with ultra-filtration through membranes having molar cut-offs of 3, 10 or 30 kD. The filtration was done employing centrifugal force.

The 3 kD membrane gave the best yield, over 80 % of hemicellulose. The high wood resin content of the DisCo-liquor used in the ultra-filtration test resulted in slow filtration. Better resin removal by either micro-filtration or efficient flotation followed by a step of XAD-7 treatment for lignan and lignin removal, should result in a more efficient ultra-filtration of hemicelluloses.

It has further been surprisingly noticed that the size of wood chips has an impact on extraction of valuable lignans, such as HMR. Potentially valuable lignans, particularly the hydroxymatairesinol (HMR) portion thereof, can be extracted into water and recovered with a high yield from rather coarse TMP in the pH range 4 to 6. The coarseness of TMP can be expressed as the Canadian Standard Freeness (CSF). Typically the CSF value of TMP pulp after the first refiner stage is higher than 300. The coarseness of TMP pulp at this stage enables efficient pressing, i.e. separation of the liquor from the solid wood material.

The effect of pH level on HMR yield seems to result from different stage or HMR isomerisation at different pH levels. At pH levels above pH 5 the yield of HMR seems to decrease due to isomerisation and condensation reactions leading to an HMR isomer and conidendric acid. Also at pH below 3 condensation to conidendric acid seems to take place.

FIG. 6 shows a schematic view of an exemplary system, in which DisCo-liquor is taken from the first refiner stage in a TMP process. FIG. 6 shows in a TMP process a first and second refiner stage 40, 42. Coarse TMP at 35 % dry matter content is introduced from the first refiner stage 40 through line 44 to an extractor 46. Water 48 is introduced into the extractor to dilute the TMP to 5 % dry matter content. TMP processed in the extractor 46 is discharged through line 50 into a screw press 52, from which TMP at 35 % is fed through line 54 into the second refiner stage 42. DisCo-liquor separated from the TMP in the screw press is fed through hne 56 for further treatment, e.g. for recovery of non-fibrous compounds. A significant advantage of extracting coarse TMP, e.g. from the first refiner stage, instead of final paper-grade TMP is that the extraction hquor can be pressed out more effectively, e.g. using a screw press.

If in one exemplary extraction process utilizing the above system 17.1 m³ water was added to the extractor pro ton TMP and 17.1 m³ DisCo-liquor was recovered from the screw press, the DisCo liquor containing 1.5 kg hgnans, 5.8 kg lignin, 17.9 kg hemicelluloses and 4.3 kg pitch, a yield of 45 ton pure HMR/100.000 ton pulp could be received.

The present invention provides a method for separating different valuable non-fibrous components, such as the bioactive HMR, at desired form and high yield, allowing the components to be used in industrial scale. The pH and temperature levels play an important part in the recovery process. Also separation of resins from the DisCo liquors prior to separation of lignans and hemicellulose enhances the separation of the valuable components.

It is possible to recover lignans from process liquors according to the above related processes, provided that pH is kept within a pH-level between pH 3 and pH 6.7, preferably between pH 4 and pH 6. The recovery process shown in FIG. 1 includes several process stages. It is e.g. suggested that finely dispersed material and colloidally dispersed wood extractives, such as wood resins (pitch), is separated in a first separation step by e.g. filtration, flotation or centrifugation. Thereafter, lignans and lignins may be separated in a second step by e.g. sorption onto a sorbent in a column. The first separation step, i.e. filtration, flotation or centrifugation step, is performed in order to prevent colloidal or other finely dispersed material from plugging the sorbent column in the second separation step.

It is, however, possible according to another embodiment of the present invention to recover aromatic components, such as lignans and lignins, directly from process waters from, mechanical pulp mills or extraction processes, as shown in FIG. 7, without first clarifying the waters, i.e. without separating colloidal or dispersed substances therefrom. Addition of relatively large sorbent particles to a batch or a continuous flow of process water results in quantitative and relatively selective recovery of lignans therefrom. It has now been surprisingly noticed that the hydrophobic colloidal wood resin particles, always present in the process waters or hquors, are not sorbed onto the sorbent material at any large extent. Neither are polysaccharides sorbed onto the sorbent material. Thus it is possible to simphfy the recovery of aromatic substances, by excluding the water clarifying step. FIG 7 shows schematically such a recovery process.

FIG. 7 shows a process in which wood material 60, such as pulp from a TMP process, is stirred with water 62 in an extractor 64, at a pH between pH 3 and pH 6,6, preferably between pH 4 and pH 6. The hquid from the extractor is centrifuged in centrifuge 66 to separate - a liquid fraction, so called DisCo fluid 68, including dissolved aromatic substances, such as lignans and lignins, salts and polysaccharides, and colloidal substances, such as wood resins, and

- a solids fraction 70 including fines.

DisCo fluid 68 is introduced into a process vessel 72 together with a sorbent 74, preferably as coarse particles or ion exchange beads. The sorbent comprises e.g. macroreticular particles of acrylate or divinylbenzenepolystyrene resin, which provide sorption of aromatic substances, such as lignans and hgnins. The sorbent and the DisCo fluid are stirred so as to provide a close contact for effective sorption. The sorbent may be introduced as free particles or on a carrier. The size of the particles may preferably be up to 2 mm.

The lignans are quantitatively sorbed onto the sorbent, whereas lignins are only partly sorbed. Polysaccharides and wood resins are not sorbed onto the sorbent material. The particulate sorbent material is easily separated from the liquid fraction. FIG. 7 shows that sorbent material 76 is separated from the hquid fraction 78 in a separate step 80, but it could, of course, be discharged already from vessel 72.

The aromatic substances, such as lignans, may be separated from the sorbent material e.g. with a suitable solvent, such as acetone, ethanol or methanol.

Tests were made to define the amount of lignan and lignin remaining in liquor 78 after a sorption process. FIG. 8 shows that lignans may be quantitatively sorbed onto hydrophilic exchange resin XAD-4 used as sorbent material. This sorption of lignans is achieved with rather small amounts of sorbent material in the liquid, compared to amounts needed in columns. About e.g. 20 g/1, as shown in FIG. 8 and FIG. 9, may be sufficient. Lignins, on the other hand, are only partly sorbed.

FIG. 9 shows that when different lignans, Hydroxymatairesinol of type 1 (HMR1),

Hydroxymatairesinol of type 2 (HMR2), Conidendrin (Con) and Conidendric acid (ConA) are sorbed onto the XAD-4 sorbent material used, there is no significant selectivity within the group of lignans, as to sorption. It could on the other hand also be shown in the tests performed that wood resins, such as triglycerides, sterylesters, resin acids, fatty acids and sitosterol are not sorbed in any significant amount onto sorbent material.

The present invention is not intended to be limited to the embodiments brought forth in this description, but is intended to cover all methods within the scope of the appending claims.

Claims

1. A method for recovering non-fibrous substances from wood material being processed in a mechanical pulping process, such as TMP process or groundwood process, which method includes

- extracting non-fibrous substances, such as wood resins, aromatic components, salts and polysaccharides, from said wood material into a hquid fraction, such as process water, washing water, dilution water, white water or other suitable water based liquid, and

- recovering in one or more steps non-fibrous substances from said liquid fraction, characterized by

- the recovering of non-fibrous substances including a step for separation of aromatic compounds from said liquid fraction, and

- maintaining the pH-level of the liquid fraction < pH 7, preferably between pH 3 and pH 6.7, typically between pH 4 and pH 6, during the extracting and recovering processes up to the separation of the aromatic compounds.

2. A method according to claim 1, characterized by recovering water soluble aromatic compounds from said liquid fraction by separating means providing an eluent fraction including water soluble aromatic compounds and an effluent fraction.

3. A method according to claim 2, characterized by said separating means utilizing aromatic exchange resins, other sorbents for aromatic components, liquid or column chromatography or other similar suitable processes for separating the water soluble aromatic compounds from other compounds.

4. A method according to claim 2, characterized by the method including further processing of said eluent fraction so as to separate lignans from other water soluble aromatic compounds therein, the further processing including chromatographic methods, extraction fractionation, treatment with a polar solvent or other suitable means.

5. A method according to claim 4, characterized by the method including further processing of the lignan fraction separated from the water soluble aromatic compounds, so as to separate hydroxymatairesinol (HMR) from other compounds in the lignan fraction.

5

6. A method according to claim 1, characterized by

- the liquid fraction including process water from a TMP process, and

- additionally separating dispersed substances, such as small particles or wood resin, from the process water, by filtration, flotation or other suitable mechanical or chemico-

10 physical means.

7. A method according to claim 6, characterized by separating dispersed substances from the process water prior to separating aromatic compounds therefrom.

15 8. A method according to claim 6, characterized by separating dispersed substances from the process water after separating aromatic compounds therefrom.

9. A method according to claim 2, characterized by separating from the effluent polysaccharides, such as hemicellulose, by dialysis, reverse osmosis, nanofiltration,

20 centrifugation process or other suitable process known per se.

10. A method according to claim 1, characterized by extracting non-fibrous substances from wood material being processed in the refiner in a TMP pulping process.

25 11. A method according to claim 10, characterized by

- discharging a pulp suspension from a first refiner stage of the refiner,

- introducing the pulp suspension into an extractor,

- extracting in the extractor non-fibrous material from the wood material fraction of the pulp suspension into the liquid fraction of the pulp suspension,

30 - discharging a pulp suspension from the extractor, - dividing the pulp suspension discharged from the extractor into a high consistency pulp suspension portion and a hquor portion, the liquor portion including colloidal and dissolved non-fibrous substances extracted from the wood material being processed in the pulping process,

5 - introducing the high consistency pulp suspension portion into a second stage of the refiner, and

- separating water soluble aromatic compounds from the liquor portion.

12. A method according to claim 11, characterized by

10 - diluting the pulp introduced into the extractor by adding water into the extractor or into the pulp being fed into the extractor.

13. A method according to claim 12, characterized by diluting the pulp suspension introduced into the extractor from about 5 - 50 %, typically 20 - 40 %, more typically 30

15 - 40 %, to about 0,5 - 20 %, typically 0,5 - 15 %, more typically 0,5 - 10 %, and dividing the pulp suspension discharged from the extractor into a liquid portion and a high consistency pulp suspension portion having a consistency of about 5 - 50 %, typically 20 - 40 %, more typically 30 - 40 %.

20 14. A method according to claim 11, characterized by separating lignans, including hydroxymataisresinol, from the liquid portion.

15. A method according to claim 10, characterized by the wood material being introduced into the refiner as chips, having a Canadian Standard Freeness (CSF) > 300.

25

16. A method according to claim 1 , characterized by extracting non-fibrous components from said wood material before a treatment thereof at a pH level exceeding pH 7, preferably before treatment thereof at a pH level exceeding pH 6. 30

17. A method according to claim 1, characterized by in a process including a mechanical pulping processes taking place at pH > 7, regulating the pH of the liquid fraction to < 7, typically to pH between 4 - 6, prior to extraction of the water soluble aromatic compounds from the wood material into the liquid fraction.

18. A method according to claim 1, characterized by separating said liquid fraction from said wood material by pressing.