TECHNICAL FIELD

The present disclosure relates to a method for purifying lignin-based filler . The present disclosure further relates to the purified ligninbased filler .

BACKGROUND

Different fillers , such as carbon black, have been generally used in different applications such as in thermoplastic composition . Bio-based fillers are however aimed to be produced . The inventors have recogni zed a need for a bio-based filler having properties , such as sufficient purity, such that they may be used in further applications .

SUMMARY

A method for purifying lignin-based filler is disclosed . The method comprises providing a slurry comprising lignin-based filler, wherein at least 80 weight-% of the particles in the slurry have the particle si ze of 5 - 25 pm, and the slurry has a particle si ze distribution ( PSD) curve with a steepness value of D30 /D70 (wet ) of 35 - 50 % ; and subj ecting the provided slurry comprising lignin-based filler to a purification treatment , wherein the purification treatment comprises : feeding the slurry to at least one filtration chamber to separate lignin-based filler from the slurry; subj ecting the lignin-based filler to a first solid-liquid separation process in the filtration chamber for 15 seconds - 30 minutes at a membrane pressure of 3 - 20 bar to form a pre-pressed lignin-based filler ; subj ecting the pre-pressed lignin-based filler to a washing process with a washing liquid in the filtration chamber, wherein the washing liquid is fed through the lignin-based filler at a pressure , which is 0 - 10 bar lower than the membrane pressure in the first solid-liquid separation process , to form washed lignin-based filler ; subj ecting the washed lignin-based filler to a second solid-liquid separation process in the filtration chamber ; and

- recovering the lignin-based filler ; to form purified lignin-based filler with an ash content of 0 . 1 - 3 weight-% based on the total dry matter content of the lignin-based filler .

Further is disclose the purified lignin-based filler obtainable by the method as defined in the current specification .

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings , which are included to provide a further understanding of the embodiments and constitute a part of this specification, illustrate various embodiments . In the drawings :

Fig . 1 discloses one embodiment of the method disclosed in the current specification ;

Fig . 2 discloses another embodiment of the method disclosed in the current specification ; and

Fig . 3 discloses another embodiment of the method disclosed in the current specification ; and

Fig . 4 discloses the measurement results from example 2 . DETAILED DESCRIPTION

A method for purifying lignin-based filler is disclosed . The method comprises providing a slurry comprising lignin-based filler, wherein at least 80 weight-% of the particles in the slurry have the particle si ze of 5 - 25 pm, and the slurry has a particle si ze distribution ( PSD) curve with a steepness value of D30 /D70 (wet ) of 35 - 50 % ; and subj ecting the provided slurry comprising lignin-based filler to a purification treatment , wherein the purification treatment comprises : feeding the slurry to at least one filtration chamber to separate lignin-based filler from the slurry; subj ecting the lignin-based filler to a first solid-liquid separation process in the filtration chamber for 15 seconds - 30 minutes at a membrane pressure of 3 - 20 bar to form a pre-pressed lignin-based filler ; subj ecting the pre-pressed lignin-based filler to a washing process with a washing liquid in the filtration chamber, wherein the washing liquid is fed through the lignin-based filler at a pressure , which is 0 - 10 bar lower than the membrane pressure in the first solid-liquid separation process , to form washed lignin-based filler ; subj ecting the washed lignin-based filler to a second solid-liquid separation process in the filtration chamber ; and

- recovering the lignin-based filler ; to form purified lignin-based filler with an ash content of 0 . 1 - 3 weight-% based on the total dry matter content of the lignin-based filler .

When feeding the slurry to the at least one filtration chamber, a cake of the lignin-based filler may be formed . In one embodiment , recovering the ligninbased filler comprises recovering the lignin-based filler from the filtration chamber .

In one embodiment , the first solid-liquid separation process , the washing process , and the second solid-liquid separation process are carried out in one and the same filtration chamber . I . e . the lignin-based filler may not be removed from the filtration chamber between the first solid-liquid separation process , the washing process , and the second solid-liquid separation process . In one embodiment , the first solid-liquid separation process , the washing process , and the second solid-liquid separation process are taking place one after the other in the one and the same filtration chamber .

The slurry comprising lignin-based filler may be fed into the at least one fi ltration chamber by pumping the slurry to the filtration chamber .

In one embodiment , a filtrate is recovered while feeding the slurry to the at least one filtration chamber . Feeding of the slurry may be continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of feeding the slurry to the at least one filtration chamber .

In one embodiment , a filtrate is recovered from the solid-liquid separation process and the solid-liquid separation process is continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of said solid-liquid separation process .

The filtrate flow may be measured by different methods . E . g . a flowmeter may be used or the weight of the recovered filtrate may be used for measuring or determining the filtrate flow . Alternatively, the filtrate level in a filtration tank, where the filtrate is collected may be measured and the filtration flow calculated therefrom .

As the reference value , i . e . the filtrate flow at the beginning of the feeding and/or the solidliquid separation process , one may take the value received after the feeding and/or the solid-liquid separation process has been continued for one minute .

In one embodiment , a filtrate is recovered from the second solid-liquid separation process and/or from the third solid-liquid separation process , and the second solid-liquid separation process and/or the third solid-liquid separation process are/is continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of said solid-liquid separation process .

In one embodiment , the solid-liquid separation process is carried out hori zontally or vertically, and the solid-liquid separation process is continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of said solid-liquid separation process . In one embodiment , the solid-liquid separation process is carried out hori zontally, and the solid-liquid separation process is continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of said solid-liquid separation process . In one embodiment , the solid-liquid separation process is carried out vertically, and the solid-liquid separation process is continued until the filtrate flow is less than 10 % , or less than 8 % , or less than 6 % , of the filtrate flow at the beginning of said solid-liquid separation process .

In one embodiment , a washing filtrate is recovered from the washing process , and the washing process is continued until the conductivity of the washing filtrate is 1000 pS /cm or less , or 900 pS /cm or less, or 800 pS/cm or less. It is not economical to continue the washing process for too long. It has been discovered that there is a point where the conductivity of the washing filtrate, i.e. amount of ash in the lignin-based filler does not decrease any more, even if the washing would be continued. When the predetermined conductivity is achieved for the washing filtrate, the amount of ash in the lignin-based filler has been reduced and the washing may be stopped. The conductivity may be measured according to standard SFS-EN 27888 (1994) .

Further is disclosed a purified lignin-based filler obtainable by the method as defined in the current specification. The purified lignin-based filler may comprise ash in a total amount of at most 3 weight-% based on the total dry matter content of the lignin-based filler. The purified lignin-based filler may comprise ash in a total amount of 0.1 - 3 weight- % , or 0.1 - 2.5 weight-%, or 0.2 - 2.0 weight-%, or 0.3 - 1.5 weight-%, or 0.4 - 1.0 weight-%, based on the total dry matter content of the lignin-based filler. I.e. in one embodiment, purified lignin-based filler with an ash content of 0.1 - 3 weight-%, or 0.1 2.5 weight-%, or 0.2 - 2.0 weight-%, 0.3 - 1.5 weight-%, or 0.4 - 1.2 weight-%, or 0.5 - 1.0 weight- % , based on the total dry matter content of the lignin-based filler, is formed.

The ash content may be determined at a temperature of 900 °C. At the temperature of 900 °C, the ash content may be determined according to standard ISO 2144:20'15 with the following modifications: A dried sample (1 - 2 g) is heated in crucible from room temperature to 900 °C within three hours and kept at 900°C for two hours without lid. Ash content of a sample refers to the mass that remains of the sample after above treatment, and it is presented as per cent of the sample's dry content. In one embodiment, the conductivity of a composition made by mixing the purified lignin-based filler with distilled water or water that has been subjected to reverse osmosis, is less than 400 pS/cm, or less than 300 pS/cm, when measured at an 8 weight-% dry matter content. The conductivity may be measured according to standard SFS-EN 27888 (1994) .

In one embodiment, the solubility of the purified lignin-based filler in 0.1 M NaOH is 1 - 40 weight-%, or 3 - 35 weight-%, or 5 - 30 weight-%. The solubility may be measured in the following manner: First a sample is dried at a temperature of 60 °C for four hours. A sample mass of 0.5 gram is weighed and dissolved in 50 ml of 0.1 M NaOH at a concentration of

1 % having a temperature of 22 °C. Mixing is continued for 1 hour, where after the sample is placed on a glass microfiber paper (1.6 pm) and the filter paper with the sample is dried at a temperature of 60 °C for

2 hours. The portion of the sample has which has dissolved can be determined gravimetrically .

In one embodiment, the purified lignin-based filler comprises carbon in a total amount of 62 - 70 weight-%, or 63 - 69 weight-%, or 64 - 68 weight-%. The amount of carbon in the lignin-based filler may be determined according to standard DIN 51732 (1997) .

The total dry matter content may be determined after removing the liquid from a sample followed by drying at a temperature of 105 °C for 24 hours. The effectiveness of the liquid removal may be assured by weighing the sample, drying for a further two hours at the specified temperature, and reweighing the sample. If the measured weights are essentially the same, the drying has been complete, and the total weight may be recorded .

By the expression "slurry comprising ligninbased filler" should be understood in this specification, unless otherwise stated, as referring to a slurry comprising lignin-based filler particles and having the properties as defined in the current specification . The slurry may comprise , in addition to lignin-based filler particles , also liquid . The slurry comprising lignin-based filler may be provided or prepared by hydrothermal carboni zation treatment (HTC) of lignin . The hydrothermal carboni zation treatment of lignin refers to a thermochemical conversion process of lignin-containing material in an aqueous suspension . Hydrothermal carboni zation treatment of lignin produces lignin derivatives having high carbon content and functional groups .

In one embodiment , the lignin-based filler is prepared from lignin derived from enzymatic hydrolysis process and subj ected to the hydrothermal carboni zation treatment . In one embodiment , the lignin-based filler is prepared from lignin derived from a Kraft process and subj ected to the hydrothermal carboni zation treatment .

In one embodiment , the enzymatic hydrolysis comprises enzymatic hydrolysis of a plant-based feedstock . In one embodiment , the enzymatic hydrolysis comprises enzymatic hydrolysis of cellulose . In one embodiment , the lignin-based filler is prepared from lignin derived from pulping of wood, e . g . Kraft lignin .

The slurry comprising lignin-based filler as disclosed in the current specification may be prepared as disclosed below . It is to be noted that the starting material may be lignin from various processes or lignin having various origin . The lignin to be used may be derived from e . g . a proces s wherein the lignin is formed in enzymatic hydrolysis of lignocellulosic feedstock .

The lignin may be dissolved in alkaline solution, such as NaOH . The dissolution may be accomplished by heating the mixture of lignin and alkaline solution to about 80 °C, adjusting the pH to a value above 7, such as 9 - 11, and mixing the mixture of lignin and alkaline solution for a predetermined time. The mixing time may be continued for about 2 - 3 hours. The exact pH value is determined based on the grade target of the product.

The dissolved lignin may then be subjected to hydrothermal carbonization treatment (HTC) .

The hydrothermal carbonization treatment may take place in a reactor (HTC reactor) , or if needed, in several parallel reactors, working in a batchwise manner. The dissolved lignin may be pre-heated before being entered in the HTC reactor (s) . The temperature in the HTC reactor (s) may be 150 - 250 °C and the pressure may be 20 - 30 bar. The residence time in the HTC reactor (s) may be about three to four hours. In the HTC reactor, the lignin is carbonized, whereby a stabilized lignin derivative with a high specific surface area may be precipitated. The formed slurry comprising the carbonized lignin may then be removed and cooled to a temperature of below 100 °C, e.g. to about 60 °C.

Consequently, a slurry comprising ligninbased filler is formed. In one embodiment, the slurry comprising lignin-based filler is a slurry of ligninbased filler.

The method as disclosed in the current specification thus comprises providing a slurry comprising lignin-based filler, wherein at least 80 weight-% of the particles in the slurry have the particle size of 5 - 25 pm. In one embodiment, 80 - 95 weight-% of the particles in the slurry have a particle size of 5 - 25 pm. In one embodiment, at most 10 weight-% of the particles in the slurry have the particle size of 0.1 - 5 pm.

By the expression "particle size distribution (PSD)" is meant an index indicating what sizes (particle size) of particles are present in what proportions (relative particle amount as a percentage where the total amount of particles is 100 %) in the sample particle group to be measured (volume distribution) . The particle size distribution may be measured with a laser diffraction particle size analyzer such as PSA 1190 (Anton Paar's particle size analyzer) . When using a laser diffraction particles size analyzer a laser beam is directed onto dispersed particles, then the laser light is diffracted by the particles, and the corresponding diffraction pattern is detected and evaluated.

From the above measurement of the particle size distribution, one may calculate the steepness value of the formed particle size distribution curve. I.e. the steepness value of D30/D70 (wet) should be understood as meaning the value received when the result for D30 is divided by the result for D70 and the result is then multiplied by 100. In a similar manner the steepness value of D20/D50 (wet) should be understood as meaning the value received when the result for D20 is divided by the result for D50 and the result is then multiplied by 100. In one embodiment, the slurry has a particle size distribution (PSD) curve with a steepness value of D30/D70 (wet) of 35 - 50 %, or 40 - 49 weight-%, or 42 - 48 % , or 45 - 46 % . In one embodiment, the slurry has a particle size distribution (PSD) curve with a steepness value of D20/D50 (wet) of 45 - 60 % , or 50 - 58 % , or 53 - 56 % .

In one embodiment, the specific surface area (SSA) of the purified lignin-based filler is 3 - 150 m ² /g, or 5 - 100 m ² /g, or 7 - 60 m ² /g. The specific surface area (SSA) , or the Brunauer-Emmett-Teller (BET) surface area, of the purified lignin-based filler may be determined by using a Micromeritics 3Flex micropore instrument in the following manner: 5 g of a material is taken and dried in an oven at 60 °C overnight. From this dried material, approximately 1 g is taken and is treated by Micromeri tics Smart VacPrep at 100 °C for 2 hours at vacuum 0.01 mmHg. After the treatment, the amount of the sample is measured. Glass wool is used to keep the sample in the tube. After this, the treated sample is transferred to analysis. Before analysis a leak test is performed to validate equipment performance. The analysis is undertaken at 77 K, with isothermal jacket being used for the sample tube. A 12 mm sample tube is used for the analysis. A freespace is measured with He before analysis. Nitrogen adsorption isotherms are measured up to 0.3 P/Po with 0.05 pressure increment at 10 s equilibration interval. In analysis Po in Po tube is measured for each isotherm point. In analyzing the samples, a 12 mm sample tube with filler rod may be used. The SSA value (or BET value) for the sample is received as m ² /g .

In one embodiment, the provided slurry comprising lignin-based filler comprises acidinsoluble lignin in an amount of 70 - 95 weight-%, or 75 - 92 weight-%, or 85 - 90 weight-%, based on the total dry matter content of the slurry. The amount of acid-insoluble lignin may be determined following a modified standard of TAPPI T 222. I.e. the amount of acid-insoluble lignin may be determined gravimetrically by filtering in the following manner: A sample is treated with 72 % sulfuric acid at constant temperature (30 °C) in a water bath for 1 h and it is autoclaved at 120 °C, 1 bar for 1 h. The acid causes the lignin to precipitate and it can be determined gravimetrically. The precipitated lignin is separated by vacuum filtering the sample. The acidinsoluble lignin content of the sample in % may then be calculated using the following equation: Lignin, % = (M x 100 ) /W, where M = weight of the acid-insoluble precipitation (g) , and W = amount of dry sample (g) .

In one embodiment , the provided slurry comprising lignin-based filler has an ash content of 4 - 15 weight-% , or 5 - 11 weight-% , or 6 - 10 weight-% , based on the total dry matter content of the ligninbased filler .

In one embodiment , the provided slurry comprising lignin-based filler comprises carbohydrates in a total amount of at most 70 mg/g, or at most 60 mg/g, or at most 50 mg/g, of the lignin-based filler . The amount of carbohydrates may be determined following a standard of SCAN-CM 71 : 09 .

The inventors of the current application surprisingly found out that , contrary to the general expectation, one is able to treat the provided slurry comprising lignin-based filler with a filtration treatment to achieve a purified lignin-based filler . The small particle si ze of the lignin-based filler and the dense structure of the cake formed in the filtration chamber from the lignin-based filler is such that the general expectation was that washing liquid would not be able to penetrate the filler . However, the inventors surprisingly found out that it was possible to purify the lignin-based filler by washing it and to push liquid through the lignin-based filler .

The provided slurry comprising lignin-based filler may be subj ected to a purification treatment . The purification treatment may comprise feeding the slurry to at least one filtration chamber to separate lignin-based filler, and then subj ecting the ligninbased filler to a first solid-liquid separation process , a washing process , and a second solid-liquid separation process , and further , in one embodiment , re-slurrying the lignin-based filler recovered from the second solid-liquid separation process , and subj ecting the re-slurried lignin-based filler to a third solid-liquid separation process .

The first , the second and the third solidliquid separation processes may be carried out by a filtration equipment comprising at least one filtration chamber . A filtration chamber may be formed between two filter elements , e . g . filter plates . When feeding the slurry to the at last one filtration chamber, the lignin-based filler forms a cake between the filter plate ( s ) . Membranes may be provided for compressing the formed cake of filtered material , i . e . lignin-based filler, inside the filtration chamber on the filter plates to remove liquid from it . The membrane may be part of a filter plate , either as a separate film on the filter plate or the filter plate as such or its surface may act as the membrane compressing the cake of filtered material . The filtrate may be removed from the filtration chamber in either one direction or in two directions depending on the filtration equipment .

The filter elements may be positioned either vertically or hori zontally . The positioning of the filter elements defines whether the solid-liquid separation process is carried out hori zontally or vertically . Thus washing of the lignin-based filler may comprise feeding washing liquid into the filtration chambers and through the cake of filtered material , i . e . lignin-based filler, on the filter plates .

In one embodiment , the slurry comprising lignin-based filler may be fed into the at least one filtration chamber by pumping the slurry to the filtration chamber for 1 - 60 minutes , or 3 - 30 minutes , or 5 - 10 minutes , at a pressure of 1 - 15 bar, or 1 - 12 bar, or 2 - 10 bar or 3 - 8 bar .

In one embodiment , the first solid-liquid separation process and/or the second solid-liquid separation process and/or the third solid-liquid separation process are/is carried out hori zontally or vertically . In one embodiment , the first solid-liquid separation process is carried out hori zontally or vertically . In one embodiment , the second solid-liquid separation process is carried out hori zontally or vertically . In one embodiment , the third solid-liquid separation process is carried out hori zontally or vertically .

In one embodiment , the first solid-liquid separation process is carried out for 15 seconds - 30 minutes , or 30 seconds - 15 minutes , or 1 - 5 minutes . In one embodiment , the second solid-liquid separation process is carried out for 2 - 60 minutes , or 3 - 30 minutes . In one embodiment , the third solid-liquid separation process is carried out for 2 - 60 minutes , or 3 - 30 minutes .

The time for each of the first , the second, and the third solid-liquid separation process may depend on how much filtrate is coming out from the filtration chamber . At the beginning of the squeezing or filtering, the filtrate flow may be high and may then decrease during time .

The membrane pressure in the first solidliquid separation process may be 3 - 20 bar, or 4 - 20 bar, or 4 - 18 bar, or 4 - 16 bar , or 4 - 15 bar, or 4

- 13 bar, or 4 - 10 bar, or 4 - 8 bar . The membrane pressure in the third solid-liquid separation process may be 3 - 20 bar, or 4 - 20 bar, or 4 - 18 bar, or 4

- 16 bar, or 4 - 15 bar, or 4 - 13 bar, or 4 - 10 bar, or 4 - 8 bar .

In one embodiment , the washed lignin-based filler is subj ected to a second solid-liquid separation process in the filtration chamber at a membrane pressure, which is higher than the membrane pressure in the first solid-liquid separation process. The membrane pressure in the second solid-liquid separation process may be 5 - 20 bar, or 6 - 18 bar, or 7 - 16 bar or 12 - 16 bar.

In one embodiment, the washing liquid is fed through the lignin-based filler at a pressure, which is 0.1 - 10 bar, or 0 - 5 bar, or 0 - 2 bar, or 0.1 - 5 bar, 0.1 - 2 bar, or 0.3 - 2 bar, or 0.5 - 2 bar lower than the membrane pressure in the first solidliquid separation process. In one embodiment, the washing liquid is fed through the lignin-based filler at a pressure, which is which is 0 - 10 bar, or 0.1 - 10 bar, or 0 - 5 bar, or 0 - 2 bar, or 0.1 - 5 bar, 0.1 - 2 bar, or 0.3 - 2 bar, or 0.5 - 2 bar lower than the membrane pressure in the first solid-liquid separation process, with the provision that the pressure is not below 0 bar. I.e. the pressure of feeding the washing liquid through the lignin-based filler may not be below 0 bar.

In one embodiment, the membrane pressure is kept at 2 - 12 bar, 3 - 12 bar, or 3.5 - 12 bar, or 3.5 - 8 bar in the washing process.

The expression "membrane pressure" is referred to as the liquid and/or gas pressure behind the membrane that is used to squeeze liquid from the cake/slurry comprising lignin-based filler.

In one embodiment, the amount of washing liquid used in the washing process is at most 40 kg/kg of lignin-based filler, or 2 - 10 kg/kg of ligninbased filler, based on the total dry matter content of the lignin-based filler. The washing liquid used may be pure water; recycled washing liquid, or recycled water from a downstream phase or stage of the method; and/or water from another process phase or stage etc. The washing liquid may be run through the lignin-based filler in different manners, depending on the filtration equipment type and the filtration chamber. The washing process has the added utility of washing out impurities from the lignin-based filler.

In one embodiment, the conductivity of the washing liquid is at most 800 pS/cm, or at most 500 pS/cm, or at most 300 pS/cm, or at most 200 pS/cm, or at most 150 pS/cm, when determined according to SFS-EN 27888 (1994) . The lower the conductivity of the washing liquid, the better is the washing result i.e. the removal of ash from the lignin-based filler.

Subjecting the lignin-based filler to a first solid-liquid separation process is carried out before subjecting the lignin-based filler to the washing process. I.e. the slurry comprising lignin-based filler may be firstly fed into at least one filtration chamber to separate the lignin based filler from the slurry, and then the lignin-based filler is subjected to a first solid-liquid separation process whereafter the lignin-based filler is subjected to the washing process .

In one embodiment, the method comprises adjusting the pH of the slurry comprising lignin-based filler to 2 - 6.5, or 3 - 6, or 4 - 5.5, or 4 - 5.0, before subjecting the slurry comprising lignin-based filler to the purification treatment. The pH adjustment may be carried out by using an acid, such as sulphur acid. Adjusting or lowering the pH of the slurry comprising lignin-based filler to a value of below 6.5 has the added utility that soluble lignin will precipitate and form bigger particles and agglomerates enabling better filtration of the slurry in the filtration chamber. Lowering the pH of the slurry comprising lignin-based filler from a value of about 8 - 9 to about 5 - 6 before the purification treatment may transform the lignin-based filler from Na-form to H-form, thereby reducing its ash content ( sodium) .

In one embodiment , the purification treatment further comprises : re-slurrying the lignin-based filler recovered from the second solid-liquid separation process in a liquid to form re- slurried lignin-based filler with a total dry matter content of 1 - 15 weight- % ;

- feeding the re-slurried lignin-based filler to at least one f iltration chamber to separate the reslurried lignin-based filler and liquid; subj ecting the re-slurried lignin-based filler to a third solid-liquid separation process in the filtration chamber for 2 - 60 minutes , or for 3 - 30 minutes , at a membrane pressure of 3 - 20 bar ; and

- recovering the lignin-based filler from the filtration chamber .

In one embodiment , the purification treatment further comprises : re-slurrying the lignin-based filler recovered from the second solid-liquid separation process in a liquid to form re- slurried lignin-based filler with a total dry matter content of 1 - 15 weight- % ;

- feeding the re-slurried lignin-based filler to at least one f iltration chamber to separate the reslurried lignin-based filler and liquid; subj ecting the re-slurried lignin-based filler to the first solid-liquid separation process in the filtration chamber for 15 seconds - 30 minutes at a membrane pressure of 3 - 20 bar to form a prepressed lignin-based filler ; subj ecting the pre-pressed lignin-based filler to the washing process with a washing liquid in the filtration chamber, wherein the washing liquid is fed through the lignin-based filler at a pressure , which is 0 - 10 bar lower than the membrane pressure in the first solid-liquid separation process , to form washed lignin-based filler ; subj ecting the washed lignin-based filler to the second solid-liquid separation process in the filtration chamber ; and

- recovering the lignin-based filler .

Re-slurrying the lignin-based filler may be carried out by mixing the lignin-based filler with a liquid . The liquid may be a washing liquid and/or fresh water . The washing liquid may be or comprise water . In one embodiment , the lignin-based filler is re-slurried with a filtrate separated from a solidliquid separation process .

In one embodiment , a re-slurried lignin-based filler with a total dry matter content of 4 - 15 weight-% , or 7 - 15 weight-% , or 10 - 14 weight-% , is formed .

In one embodiment , the purification treatment further comprises : drying the lignin-based filler by applying drying medium through the lignin-based filler at a pressure of 1 - 10 bar, or 3 - 6 bar, for 1 - 600 seconds . The drying medium may be e . g . a gas , such as nitrogen gas , or air, or any combination thereof . By applying the drying medium through the lignin-based filler enables remaining liquid drops , being present in the lignin-based filler even after the last separation stage , to be reduced or removed from the lignin-based filler . The step of drying may be conducted on the lignin-based fi ller cake , before it is discharged from the filtration chamber . The recovered lignin-based filler may also be dried by applying drying medium through the lignin-based filler in any convenient way such as in a fluidi zed bed dryer, a belt dryer etc .

The method may further comprise crushing the recovered lignin-based filler or the dried lignin- based filler to a predetermined particle si ze . The predetermined particle si ze may depend on the application where the lignin-based filler is to be used .

The method as disclosed in the current specification has the added utility of providing purified lignin-based filler . It has been noticed, that it is difficult to separate the lignin based filler from the slurry comprising lignin-based filler and to purify it . The method as disclosed in the current specification has the added utility of enabling the separation and purification of ligninbased filler by the purification treatment as disclosed in the current specification and especially to utili ze a filter press in the purification treatment . The washing process as disclosed in the current specification has the added utility of being easily accomplished and of providing lignin-based filler of high purity .

The purified lignin-based filler as disclosed in the current specification has the added utility of presenting high quality to be used in different end applications . The high quality further has the added utility of higher sales price and income for the producer .

EXAMPLES

Reference will now be made in detail to various embodiments .

The description below discloses some embodiments in such a detail that a person skilled in the art is able to utili ze the embodiments based on the disclosure . Not all steps or features of the embodiments are discussed in detail , as many of the steps or features will be obvious for the person skilled in the art based on this specification . The enclosed Fig. 1 discloses an example of an embodiment of the method as disclosed in the current specification. Fig. 1 discloses an embodiment comprising the general steps of the method for purifying lignin-based filler. The method of Fig. 1 comprises providing a slurry comprising lignin-based filler with the properties as disclosed in the current specification. The provided slurry comprising lignin-based filler is then subjected to a purification treatment, wherein the purification treatment comprises the following steps in the below described order: Firstly the slurry comprising lignin-based filler is fed to at least one filtration chamber to separate lignin-based filler from the slurry. Then the lignin-based filler is subjected to a first solid-liquid separation process. I.e. a first solid-liquid separation process is conducted. The first solid-liquid separation process takes place in the filtration chamber for 15 seconds - 30 minutes at a membrane pressure of 3 - 20 bar.

Then the pre-pressed lignin-based filler is subjected to a washing process with a washing liquid in the filtration chamber. I.e. a washing process is conducted. During the washing process, the washing liquid is fed through the lignin-based filler at a pressure, which is 0 - 10 bar, or 0.1 - 10 bar, or 0 - 5 bar, or 0 - 2 bar, or 0.1 - 5 bar, 0.1 - 2 bar, or 0.3 - 2 bar, or 0.5 - 2 bar lower than the membrane pressure in the first solid-liquid separation process.

The washed lignin-based filler is thereafter subjected to a second solid-liquid separation process. I.e. a second solid-liquid separation process is carried out. The membrane pressure in the second solidliquid separation may be higher than the membrane pressure used in the first solid-liquid separation process .

After the above steps, the lignin-based filler is recovered from the filtration chamber. The lig- nin-based filler may be separated from the filter plates e . g . by blowing air through the filter plates .

As a result of the above steps purified lignin-based filler is formed to be used in further applications .

In the above described embodiment , the first solid-liquid separation process is carried out before the washing process .

Also , even if not shown in Fig . 1 , in the above described embodiment , the recovered lignin-based filler may be dried by applying a drying medium through the lignin-based filler .

Fig . 2 discloses another embodiment of the method as disclosed in the current specification for purifying lignin-based filler . The method as disclosed in Fig . 2 is firstly carried out following the described steps of Fig . 1 of providing the slurry, feeding it to at least one filtration chamber, and subj ecting the same to the first solid-liquid separation process , the washing process , the second solid-liquid separation process , and the recovery of the ligninbased filler . However, thereafter in the embodiment of Fig . 2 , the method is continued with re-slurrying the lignin-based filler recovered from the second solidliquid separation process in a liquid to form reslurried lignin-based filler . The total dry matter content of the slurry is adj usted to 1 - 15 weight-% . The re-slurried lignin-based filler is then fed into at least one filtration chamber . Then the re-slurried lignin-based filler is subj ected, in the embodiment of Fig . 2 , to a third solid-liquid separation process in the filtration chamber for 2 - 60 minutes at a membrane pressure of 3 - 20 bar . The third solid-liquid separation process is followed by recovering the lignin-based filler from the filtration chamber . As a result of the above steps purified lignin-based filler is formed to be used in further applications . Also, even if not shown in Fig. 2, in the above described embodiment, the recovered lignin-based filler may be dried by applying a drying medium through the lignin-based filler.

Fig. 3 discloses another embodiment of the method as disclosed in the current specification for purifying lignin-based filler. The method as disclosed in Fig. 3 is firstly carried out following the described steps of Fig. 1 of providing the slurry and subjecting the same to the first solid-liquid separation process, the washing process, the second solidliquid separation process, and the recovery of the lignin-based filler. These steps are indicated with arrows 1 - 4 in Fig. 3. Thereafter in the embodiment of Fig. 3, the method is continued with re-slurrying the lignin-based filler recovered from the second sol- id-liquid separation process in a liquid to form reslurried lignin-based filler. The total dry matter content of the slurry is adjusted to 1 - 15 weight-%. The re-slurried lignin-based filler is then fed into at least one filtration chamber. Then the re-slurried lignin-based filler is subjected, in the embodiment of Fig. 3, again to the first solid-liquid separation process in the filtration chamber for 2 - 60 minutes at a membrane pressure of 3 - 20 bar. Pre-pressed lignin-based filler is formed in the form of a cake. Then the pre-pressed lignin-based filler is subjected to a washing process with a washing liquid in the filtration chamber. I.e. a washing process is conducted. During the washing process, the washing liquid is fed through the lignin-based filler at a pressure, which is 0 - 10 bar, or 0.1 - 10 bar, or 0 - 5 bar, or 0 - 2 bar, or 0.1 - 5 bar, or 0.1 - 2 bar, or 0.3 - 2 bar, or 0.5 - 2 bar lower than the membrane pressure in the first solid-liquid separation process.

The washed lignin-based filler is thereafter subjected to the second solid-liquid separation pro- cess . I . e . a second solid-liquid separation process is carried out . The membrane pressure in the second sol- id-liquid separation may be higher than the membrane pressure used in the first solid-liquid separation process .

The second solid-liquid separation process is followed by recovering the lignin-based filler from the filtration chamber . As a result of the above steps purified lignin-based filler is formed to be used in further applications .

Also , even if not shown in Fig . 3 , in the above described embodiment , the recovered lignin-based filler may be dried by applying a drying medium through the lignin-based filler .

Example 1 Purifying lignin-based filler

In this example , samples of a slurry comprising lignin-based filler were subj ected to the purification treatment as disclosed in the current specification . Two comparative samples were also treated and analyzed . The comparative examples comprised samples that were only subj ected to filtration . The properties measured for different samples are presented in below table 1 :

Table 1 . Properties of the slurries comprising ligninbased filler

* This sample was prepared from the sample taken from the recovered lignin-based filler from "vertical fil- tration+washing" that was thereafter subj ected to reslurrying followed by the first solid-liquid separa- tion process , washing process , and second solid-liquid separation process .

* * pH was measured based on standard 3021 by using WTW inoLab 7110 pH meter

* * * PSD was measured with PSA 1190 analyzer . In table 2 is presented the process parameters used during the purification treatment of each of the above samples . Table 2 . Process parameters used in the purification treatment

* resulting in the filtrate flow in the end being 0 . 99 % of the filtrate flow at the beginning of said solidliquid separation process

* * resulting in the filtrate flow in the end being 0 . 64 % of the filtrate flow at the beginning of said solid-liquid separation process

* * * The pressure was shortly released at the beginning of the washing process and then continued in the indicated pressure value * * * * value measured at one minute from the start of the filtration The properties of the purified samples, i.e. the recovered and dried lignin-based fillers are presented in the below table 3. Table 3. Properties of the recovered and dried lignin- based filler

Example 2 - Determining the conductivity of the washing filtrate

In this example the conductivity of the washing filtrate of two samples was measured during the washing process . During the washing process , a washing filtrate was recovered or separated and the conductivity thereof was determined during different points of time as calculated from the beginning of the washing process .

For the sample C3 the washing was stopped and the conductivity of the washing filtrate at the end was measured . The sample Horizontal , case 1 was measured in several time points . The same slurry comprising lignin-based filler was used for both of the samples and both samples were subj ected to the same process steps using the same parameters . The conductivity was measured according to standard SFS-EN 27888 ( 1994 ) .

The results of the conductivity measurements are presented in Fig . 4 . The fol lowing abbreviations are used in Fig . 4 .

Hori zontal , case 1 = sample 1 subj ected to hori zontal filtration and washing

C3 = sample 2 subj ected to hori zontal filtration and washing

From the results one can see that increasing the washing time decreases the conductivity of the washing filtrate ( curve Hori zontal , case 1 ) . One can also determine from the results that the ash amount in the lignin-based filler has been significantly decreased and that the washing process may be ended when the conductivity of the washing filtrate 1000 pS /cm or less , or 900 pS /cm or less , or 800 pS /cm or less .

It is obvious to a person skilled in the art that with the advancement of technology, the basic idea may be implemented in various ways . The embodiments are thus not limited to the examples described above ; instead they may vary within the scope of the claims .

The embodiments described hereinbefore may be used in any combination with each other . Several of the embodiments may be combined together to form a further embodiment . A method and a purified ligninbased filler, disclosed herein, may comprise at least one of the embodiments described hereinbefore . It will be understood that the benefits and advantages described above may relate to one embodiment or may relate to several embodiments . The embodiments are not limited to those that solve any or all of the stated problems or those that have any or all of the stated benefits and advantages . It will further be understood that reference to ' an ' item refers to one or more of those items . The term "comprising" is used in this specification to mean including the feature ( s ) or act ( s ) followed thereafter, without excluding the presence of one or more additional features or acts .