NZ617117B2 - Process for treating microfibrillated cellulose and microfibrillated cellulose treated according to the process - Google Patents

Abstract

Process for removing liquid from a slurry comprising microfibrillated cellulose comprises providing a slurry comprising microfibrillated cellulose and liquid, subjecting the slurry to an electric field inducing the liquid of the slurry to flow and separating the liquid from the microfibrillated cellulose. ulose.

Description

s for treating microfibrillated cellulose and ibrillated cellulose treated according to the process.

Field of invention The present ion relates to a process for dewatering, a slurry comprising microfibrillated cellulose by subjecting the slurry to an electric field.

Background Microfibrillated cellulose (MFC), which also is known as nanocellulose, is a material typically made from wood cellulose fibers. It can also be made from microbial s, agricultural fibers, dissolved cellulose or CMC etc. In microfibrillated cellulose the individual microfibrils have been partly or totally detached from each other.

Microfibrillated cellulose has a very high water binding capacity and it is thus very difficult to reduce the water content .C o_ a slurry comprising ibril‘ated cellulose. High water content of a slurry sing microfibrillated cellulose also prevents usage of MFC in many different application where MFC with high solids would be required.

Today there exist several different methods to remove water from a slurry comprising microfibrillated cellulose. "t is 'or example possible to use di "erent drying techniques. Examples 0; different drying :echniques are; freeze drying, spray drying and supercritical. These techniques are however quite energy demanding and thus not so cost efficient to use in large scale processes. Also, hornification, or superhornification, o: the microfibrillated cellulose fibers often tends to occur when water is removed with different drying ques. Hornification is when irreversible bonds between the fibers are formed. When hornification has occurred it is not possible for the fibers to expand and swell in water and the original water g capacity of the fibers is thus lost. The hornification may be prevented by on of chemicals which physically prevent or modify the fibers in such way that the formation of bonds between ose fibers are limited or prevented. CA1208631A describes a s to re-disperse dried microfibrillated cellulose by addition of additives that will prevent the fibrils from bonding to each other and thus also prevents hornification of the fibers.

Further there is disclosed by Luchache et al. in Annals of the University of Craiova, Electric Engineering series, No. 32, 2008; ISSN 1842-4805 ring of pulp and paper waste Mechanical treatments in order to remove water from a slurry comprising microfibrillated cellulose can also be used. However, they are normally not very successful due to the small fiber size and size distribution of the ibrillated cellulose. Moreover, filtration of a slurry comprising microfibrillated cellulose is difficult due to the dense web formed by the slurry. Furthermore, the bonds between the microfibrillated cellulose fibers are also quite strong and this will also make mechanical dewatering less efficient.

Object of the ion It is an object of the present invention to substantially overcome or at least ameliorate one or more of the above disadvantages, or to at least provide a useful alternative.

AH26(11015551_1) Summary of Invention According to a first aspect of the present invention, there is provided a process for removing liquid from a slurry comprising microfibrillated cellulose, wherein the process ses the following steps: -providing a slurry comprising microfibrillated ose and a liquid, -subjecting the slurry to an electric field ng the liquid of the slurry to flow, and ating the liquid from the microfibrillated cellulose.

The present invention also provides according to a second aspect, a microfibrillated cellulose dewatered according to the first aspect.

The present invention also provides according to a third aspect, a microfibrillated cellulose obtained by the process according to the first aspect.

The present invention also provides according to a fourth aspect, use of a microfibrillated cellulose according to the second or the third aspect in a th additive, a thickener, a viscosity modifier, a rheology modifier, a cleaning powder, a washing powder, a detergent, a foam composition, a barrier, a film, a food product, a pharmaceutical composition, a ic product, a paper or board product, a coating, a e/absorbent product, an emulsion/dispersing agent, a drilling mud, a composite material, in water cation, in a filter, in a solar cell, in a battery , in an electronic circuit (which may be flexible, printed or coated), or to enhance the reactivity of cellulose in the cture of regenerated cellulose or cellulose derivatives.

AH26(11015551_1) Preferably, the use of an electric field improves the dewatering of a slurry sing microfibrillated ose.

The dewatering may be done by electro-osmosis (or capillary electrophoresis). This ring may also additionally also involve ultrasound treatment. The dewatering may also be followed by any one or a combination thereof of the below methods to further dry the material: 1) Drying methods by evaporation 2) Freeze drying because of increased solids 3) Adding de-hornification additives can also be used in drying of dewatered material 4) dewatered material may also partially be dried further to obtain material which behaves like solid particles and thus more easily used in commercial ations while still easily mixed and dispersed to other components (individual fibers are essentially maintained) or easily used as such.

AH26(11015551_1) "t is preferred that an electric "ie'd with a e of 10— 100 V is used. Increasing the voltage :ypically increases the water extraction rate. The l valie is when the current intensity of the generated electric field and the voltage gradient are at maximum ble levels.

Pressure may also be applied to the slurry in order to further improve the dewatering o: the slurry. The pressure may be applied after the electric field has been d and the ring of the slurry has been started. This is due to that it may be preferred to increase the dry content of the slurry before pressure is applied. However, it depends of course on the dry content of the slurry being treated.

The pressure applied is preferably a mechanical pressure, such as compression by the use 0: for example a roll nip or felts.

The dry content of the slurry comprising microfibriilated cellulose before dewatering is preferably about 1—10% by weight.

After the treatment according to the process it is preferred that the dry content of the dewatered slurry comprising microfibrillated cellulose is about 5—50% by weight.

The temperature of the slurry during dewatering is preferably above 30°C and preferably below 100°C.

The slurry may also comprise nanoparticies, salt and/or surfactants which are stimulated by the elec-ric field and es the liquid f'ow. “n this way the dewatering of the slurry is increased.

The present invention also relates to micro:fibrillated cellulose being dewatered according to the process according to the first aspect above. It has been shown that by dewatering a slurry compr‘sing microfibri' lated cellulose by the aid of an elec:ric field no or very limited hornification o: the microfibrillated osic fibers will occur.

Detailed ption of the invention The present invention relates to a process for dewatering a slurry comprising microfibrillated cellulose. Due to the teristics of microfibr“ lated cellulose fibers, e.g. its size, size distribution and fiber bonds, it is norma:_ly very difficult to dewater a s:_urry comprising microfibril' ated CG _lu_ OSG.

It has been shown that by subjecting a slurry comprising micro:ibrillated cellulose fibers to an electric ield the dewatering can strongly be improved. One theory 0: why it works so well, is that the e-_ectric field induces the s of the slurry to :low and thus pulls the water molecules away from the microfibri"ated cellulose fibers instead of pushing the micrO'fibril'ated fibers as a mechanical ent will do.

Pulling the water molecules will make it possible to also remove water molecules being absorbed by the microfibrillated .C in a very efficient way. It is thus very easy to separate the liquid from the microfibrillated cellulose fibers of the slurry.

It has been shown that by dewatering a slurry comprising micro .C ibri"ated cellulose by subjecting the slurry to an ic field, no ntial hornification o: the microfibri"ated fibers will occur. It is thus possible for the microfibrillated cellulose, being dewatered according to the present process, to swell when the microfibrillated cellulose is in contact with water again. This is 0: great importance when the microfibrillated cellulose for example is used as a strength additive, a thickener or as a viscosity modifier. Furthermore, the bonding ability of the dewatered microfibrillated cellulose is also very good, i.e. no ntial decrease in bonding y is seen.

Preferred embodiments o: the first aspect o: the invention are apparent from the dependent claims and the SijeCt matter f is lurther2 set out below.

The dewatering is preferably done by the use 0: electro— osmosis. o—osmotic flow is often abbreviated 30F which is synonymous with o—osmosis or electro—endosmosis. Electro— osmosis is the motion of liquid, such as water, induced by an applied potential or electric Field across a porous material, capillary tube, membrane, microchannel, or any other fluid conduit. The voltage ted by the electric field is preferably between 10—100 V.

The liquid 0: the slurry is separated from the microfibrillated cellulose by removing the liquid as se: out in -he first aspect. It may preferably be done by differen: liltering.C techniques.

The slurry ses microfibrillated cellulose and a liquid. The liquid may be water, a solvent and mixtures of different solvents and/or liquids. The solvent may be an alcohol, such as isopropanol, polyethylene glycol, glycol or ethanol.

Solvents, such as isopropanol, can change the surface tension I) the slurry and this will promote dewatering. The solvent may also be a solvent having at least one ketone group, and this may preferably be acetone. It is also possible that the liquid is an ionic liquid. The slurry may also comprise nanoparticles, salts and/or surfactants which are stimulated by the electric field and will improve the liquid migration and movement, i.e. the flow, in the electric field and thus also the dewatering.

The slurry may also comprise Fibers .C o regular length. It is also possible that the slurry ses s, such as PCC, kaolin or calcium carbonate. The amounts o: microfibrillated cellulose in the slurry may be n 20—90% by weight, the amount 0: regular sized fibers such as kraft, hardwood and/or softwood fibers may be 10-80% by weight. _ larger amounts of fillers and longer fibers are present in the slurry it is le to achieve a slurry with very high dry content by using the dewatering process ing to the invention. A dry content 0: up to 90% by weight is possible to achieve since the present 0' long fibers and/or fillers wi'l make it easier to dewater the slurry.

It is however, preferred to use a slurry comprising high amoun':5 of microfibrillated cellulose. A slurry comprising microfibrillated cellulose in an amount of 80—100% by , or 80—90% by weight, is often preferred. In many cases it is preferred that the slurry ses 100% O" microfibrillated cellulose, i.e. no fibers of longer size is present. The amount O" microfibrillated ose depends on the end use of the micro:ibrillated cellulose.

It may also be advantageous to subject the slurry to increased pressure in combination with the electric field. "t has been shown that the combination of electric 5 ield and pressure will strong-_y improve the dewatering o: a slurry comprising microfibrillated cellilose. It is preferred to apply the pressure after the dewatering with the e'ectric .c‘ ie'd has. started, i.e. when the solid content o_ the siurry has increased, preferably to about 4% by weight. I: the solid conten- o_ the slurry is too low when the pressure is applied, the ibrillated cellulose.C is d through the openings of the ring device together with the water and no water/microfibrillated cellulose separation will occur. When the solid content of the slurry is increased, the Viscosity is also increased and it is possible to apply pressure to the slurry and be able to increase the dewatering of the .

The pressure is preferab‘y a mechanical pressure being applied in any possible way. It possible to use, for example a roll nip or felts for applying the mechanical pressure to the slurry during dewatering. It is also possible to combine the treatment with the electric field with other kind of treatments in order to increase the dewatering. Examples of other treatments besides increasing the pressure are acoustic and vacuum based I”!_he dry content of the slurry comprising microfibrillated ose before dewatering is preferably about l—50% by .

It may also have about 1—30% by weight or about l—lO% by weight.

After the treatment according to the process it is preferred that the dry content 0: the dewatered slurry comprising micro .C ibrillated cellulose is about 5—50% by , more preferably above 20% by . It is thus possible to receive a slurry comprising microfibrillated cellulose with very high dry content in a very energy efficient way. Even though the dry content is increased the properties of the microfibrillated cellulose after dilution of water is maintained, e.g. the water swelling ties and strength.

I”!_he temperature of the slurry may be below 30°C before dewatering and increased during the dewatering process but kept at a temperature below 100°C. However, lower temperatures, :or example room temperatures are also possible. The temperature should preferably be kept below boiling point. Increased temperature may improve the dewatering. This is due to that that the ity of water is decreased.

The present invention also relates to ibrillated cellulose being red according to the first aspect as set out above. It has been shown that by dewatering a slurry compr‘sing microfibri'lated cellulose by the aid of an electric field, no or very limited hornification O" the microfibrillated cellu'osic fibers wil' occur. I: is thus possible to e a micro .C ibril'ated cellulose with improved properties in a fast and very energy efficient way compared to the use of for example drying techniques.

A microfibrillated cellulose fiber is normally very thin (~20 nm) and the length is often between 100 nm to 10 um.

However, the microfibrils may also be longer, for example between -200 um, but lengths even 2000 um can be found due to wide length bution. Fibers that has been fibrillated and which have microfibrils on the surface and microfibrils that are separated and located in a water phase of a slurry are included in the definition MFC. Furthermore, whiskers are also ed in the definition MFC.

The micro:fibrillated cellulose is typically made from wood cellulose fibers, it is possible to use both od and softwood fibers. It can also be made from m‘crobial s, agricultural fibers, such as wheat straw pulp or other non—wood fiber sources.

Using this electric field set out in the first aspect of the invention, in addition also reduces the number of bacteria as their cell walls will blow up. The process of the first aspect, as it removes ions, also removes ions and water also from microbes This means that this ion removal and water removal will kill/antimicrobial effect.

Pre ferred features 0" each aspect of the invention are as for each of the other aspec :s s mutandis. The prior art documents mentioned herein are incorporated to the fullest extent permitted by law. The inven:ion is further described in the following examples, together wi :h the appended figures, the only purpose of which is to ra:e the invention and are in no way intended to limit the scope of the invention in any way.

Figures Figure 1 disc:_oses the dewatering setup scheme (left) and cathode plate with holes.

Figure 2 disclose dependencies of t and mass of ted water on time at constant applied voltage 20 V.

Figure 3 disc:_oses dewatering of low conductivity MFC.

Figure 4 disclose time dependencies of the water mass collected during dewatering ' o: low conductivity MFC at different voltages are presented.

Examples 1.Experimental set-up For investigation of MFC dispersion dewatering an mental se:up was assembled, scheme of which is on Fig. 1.

It consists of a plastic pipe with internal diameter 46 mm, .C itted into a stainless steel funne-_ At the lower end of the pipe there is a plate with holes, also made of ess steel, which serves as the lower electrode, usually cathode. A paper ' "I ‘ter is placed on the pla':e, the MFC dispersion is loaded onto the filter. On top or the M; C column there is one more paper ‘i‘ter, after this the upper electrode (anode) is placed.

The best results were achieved with platinum ode - no process s due to the electrode corrosion or contamination were observed.

The setup of Fig. 1 tuted a cell with MFC investigated; DC voltage was applied into it from the current source. The water, emerging from she :unnel was assembled into beaker, which was situated on top of a balance; the mass of the water extracted from MFC was registered during experiments. The experiments usually were carried out in two modes: with a voltage U constant or with current i constant.

Dependencies 0: t and mass 0: collected water on time at constant app-_ied voltage 20 V are disclosed in Figure 2. An increase of pressure causes an increase both of current and increment of co:_lected water.

Surprisingly it was thus found that electro—osmosis dewatering may be used if; - in the beginning (more or less) only electro—osmosis is used - due to dewatering the viscosity wiii increase enough — that mechanical re may be applied (as reflected in Fig. 2) Figure 3 discloses dewatering 0: low conductivity MFC.

Figure 4 disclos timc dcpcndcnci s of the water mass co:_lected during dewatering 0: low tivity MFC at different vo:_tages are presented. The voltage increase causes an increase O: dewatering speed (initial slope) and process saturation value.

Example 2 Reference MFC (initial MFC) — dry t (IR) 1.7% Salt/Metal contents based on dry matter; Al 9.5 mg/g Fe 16 mg/g Ca 1200 mg/kg Cu 5.5 mg/kg K 310 mg/kg Mg 210 mg/kg Mn 1.1 mg/kg Va 1400 mg/kg Ni 1.6 mg/kg ?b 1.1 mg/kg Si 76 mg/kg Zn 5.9 mg/kg Dewatering procedure 1 — only removing water; A.paper filter was placed on the cathode, then MPG and then a second paper Silter. Aster this the anode was laid on the top of this. The pressure (of weight of anode) was 750 kPa. After a short time (2 min) an additional weight was added ure to 2400 Pa). The voltage during dewatering was 100V and time 6405.

Procedure was repeated 3 times and pressure was increased (last time 4.6* 10A5 Pa).

Dewatered MFC (electro—osmosis MFC)—results are given below: etal contents based on dry matter 30.5% A1 8.5 mg/kg Fe 11 mg/kg Ca 30 mg/kg Cu 0.69 mg/kg K 85 mg/kg Mg 5.7 mg/kg Mn 0.24 mg/kg Na 12 mg/kg Ni 0.68 mg/kg Pb <O.4 mg/kg Si 13 mg/kg Zn 1.5 mg/kg Example 3 Reference MFC (initial MFC) - dry t (I Sait/Metal contents based on dry matter; A1 9.5 mg/g Fe 16 mg/g Ca 1200 mg/kg Cu 5.5 mg/kg K 310 mg/kg Mg 210 mg/kg Mn 1.1 mg/kg Va 1400 mg/kg Ni 1.6 mg/kg ?b 1.1 mg/kg Si 76 mg/kg Zn 5.9 mg/kg Dewatering procedure 2 — removing water and washing with acetone MFC was dewatered 5 min (as in procedure 1 above i.e. e 2). After this the current was switched off and acetone was added (about the same amount as water was removed in previous step). After this dewatering was d and continued about 10 min.

Dewatered MFC (electro—osmosis MFC with acetone)—resu1ts given below: Salt/Metal contents based on dry matter 23.5% A1 4.6 mg/kg Fe 10 mg/kg Ca 10 mg/kg Cu 0.68 mg/kg K 40 mg/kg Mg 7.: mg/kg fin 0.;3 mg/kg 14 mg/kg 0.50 mg/kg <O.4 mg/kg Si 13 mg/kg Zn 1.5 mg/kg Example 4 — Temperature test Using the same set up as out above, temperature tests were performed.

Temperature 90 — 95 °C — dewatering in 60s about 16 g water Temperature 21 °C — dewatering in 60s about 13.5 g water Accordingly it was beneficial to use higher temperature to improve dewatering. Thus the energy needed for dewatering is much lower at elevated temperatures.

Example 5 A r trial was done where even more ions were removed.

At the start the total amount was 20 g of wet MFC.

JJ about ll g of water was removed with electro—osmosis a” metal content 0: the water i. Ca 14 mg/l ii. K 2.7 mg/l iii. Na 26 mg/; iv. Si 1.3 mg/l 2) about 10 g of disti ed water was added 3)about 10 g o: water was removed l content of the water i. Ca 8 mg/l ii. K 0.56 mg/l iii. Na 0.78 mg/l iv. Si 0.22 mg/l 4) about 10 g of distilled water was added )about 9 g of water was removed l content 0: the water i. Ca 7.4 mg/_ ii. K 0.56 mg/: iii. Na 0 mg/l (below detection limit) iv. Si 0.076 mg/l 6)distilled water (as reference) a.metal content of the water i. Ca 0.079 mg/l ii. K 0(below detection limit) iii. Na 0(below dececcion limit) iv. Si 0(below decec-ion limit) "n view of the above ed description 0: the present invention, other modifications and variations wiLl become apparent to those skiLled in the art. However, it should be apparent that such other modifications and variations may be effected without departing from the spirit and scope of the invention.

I

Claims (13)

WE CLAIM:

1. Process for removing liquid from a slurry comprising microfibrillated cellulose, wherein the process comprises the following steps: -providing a slurry comprising microfibrillated cellulose and a liquid, -subjecting the slurry to an electric field inducing the liquid of the slurry to flow, and -separating the liquid from the microfibrillated cellulose.

2. The process according to claim 1, wherein the step of subjecting the slurry to an electric field is done by electro-osmosis.

3. The process according to any one of the preceding claims, wherein the electric field has a voltage between 10-100 V.

4. The process according to any one of the preceding claims, wherein re also is applied in order to separate the liquid from the microfibrillated ose.

5. The process according to claim 4, n the pressure is applied after the electric field has been applied and the dewatering has started.

6. The process ing to claim 4 or 5, n the pressure is a mechanical pressure.

7. The process according to any one of the preceding claims, wherein the dry t of the slurry before dewatering is 1-50% by weight.

8. The s according to any one of the preceding , wherein the dry content of the dewatered slurry is 5-50% by weight.

9. The process according to any one of the preceding claims, wherein the temperature of the slurry during dewatering is above 30°C and below 100°C.

10. The process according to any one of the preceding claims, wherein the slurry comprises one or more of the constituents selected from the group consisting of nanoparticles, absorbents, salt, free sugars and surfactants which are stimulated by the electric field. AH26 (11116925_1)

11. A microfibrillated cellulose dewatered according to the process of any one of claims 1-10.

12. A microfibrillated ose obtained by the process according to any one of claims 1-10.

13. Use of a ibrillated cellulose according to claim 11 or 12 in a strength additive, a thickener, a viscosity modifier, a rheology modifier, a cleaning powder, a washing powder, a detergent, a foam composition, a barrier, a film, a food product, a pharmaceutical composition, a cosmetic product, a paper or board product, a coating, a hygiene/absorbent product, an emulsion/dispersing agent, a drilling mud, a composite material, in water purification, in a filter, in a solar cell, in a battery, in an electronic circuit or to enhance the reactivity of cellulose in the manufacture of regenerated ose or cellulose tives.