AT515234A1 - Process for the production of carbon particles - Google Patents

Abstract

The present invention relates to a process for the production of carbon particles from precursors having a cellulose Ii structure in which non-cylindrical cellulose particles having a cellulose Ii structure are used as precursor.

Description

Process for the production of carbon particles

The present invention relates to a process for the separation of carbon particles from precursors with cellulose II structure, in which non-cylindrical cellulose particles with cellulose II structure are used as precursor.

State of the art

The production of carbon particles from polymers by carbonization has been known for a long time. In addition to the known carbon fibers, carbon particles of a different shape are also produced from correspondingly shaped polymer precursors, so-called precursors. Such carbon particles are used, inter alia, for the production of electrodes in modern energy storage media which require high charging and discharging speeds, for example for batteries and supercapacitors in vehicles with electric drive or start-stop function.

In the meantime, not only polyacrylonitrile is widely used as a suitable precursor polymer. Also cellulose is already frequently used as a precursor for the production of carbon moldings. In addition to the naturally grown cotton wool and the wood pulp mainly made of wood, cellulosic shaped bodies which were produced by means of viscose or lyocell processes are particularly suitable.

The generic name "lyocell" was assigned by the BISFA to cellulosic fibers prepared from solutions in an organic solvent without formation of a derivative.

However, to date, only a process for the large-scale production of fibers of the genus Lyocell has prevailed, the amine oxide process. In this process, the solvent used is a tertiary amine oxide, preferably N-methylmorpholine N-oxide (NMMO).

Tertiary amine oxides have long been known as alternative solvents for cellulose. For example, US Pat. No. 2,179,181 discloses that tertiary amine oxides are capable of dissolving pulp without derivatization, and that from these solutions cellulosic bodies such as e.g. In US 3,447,939 cyclic amine oxides are described as solvents for cellulose.

How this method is carried out, the skilled person is known in principle for a long time from many patents and other publications. Thus, EP 356 419 B1, among others, describes the preparation of solutions and EP 584 318 B1 describes the spinning of such solutions of cellulose into water-containing tertiary amine oxides.

A process for carbonizing and activating lyocell fibers is described in US Patent 2008/0254972 A1, wherein the fibers are impregnated with a phosphorous-containing reagent, thermally carbonized under inert gas, and also thermally activated with carbon dioxide or water vapor in the rotary kiln. This patent describes a significantly higher activability of lyocell fibers compared to other cellulosic precursors, resulting in a higher electrical gravimetric capacity when used as electrode material for supercapacitors.

US 2007/0178310 A1 describes the use of fiber fragments having a similar fiber cross section as electrode material for energy storage.

The use of lyocell fibers as a carbonization precursor is also described in the article "Tencel carbon precursor", Lenzinger Berichte, 90, 1212, Seese.

Electrodes made of carbon powder or activated carbon powder are produced by two principally different methods: 1. The dry method in which the dried carbon or activated carbon powder is mixed with binder (eg PTFE), then heated and kneaded until a dough is formed which is sheet-shaped and covered with aluminum foil is laminated. 2. The slurry method of dispersing the dried carbon * or activated carbon powder in a suitable liquid (e.g., acetone) and adding a binder such as PVDC. An aluminum foil, which is usually pretreated with adhesion promoter, is coated with this suspension and dried. The dried layer is often then further densified by hot calendering. For the large-scale production of electrodes made of carbon powder, the majority of the slurry method is used.

According to US 2008/0254972 A1 and US 2007/0178310 A1, lyocell fibers are said to be particularly suitable for producing electrode material. The uniform fiber shape should ensure a high packing density. It has now been found that while this is true for the application of the dry method of electrode fabrication in the laboratory, it is not for the large-scale slurry method. If the slurry method is performed with fibrous powder, extremely fragile, brittle electrodes, their calendering and densification are produced only possible with the use of very high binder amounts. Although this material has high electrical capacities, it can not be used industrially.

task

The object of this prior art, therefore, was to find a carbonized material which on the one hand allows high electrical capacitance in the manufacture of electrodes and on the other hand gives mechanically stable electrodes.

Description of the invention

The solution to the above-described requirements consists in a process for the production of carbon particles from precursors with cellulose II structure, precursor being cellulose particles having non-cylindrical cellulose II structure. The carbon particles are often referred to as activated carbon particles.

The use of non-cylindrical cellulosic particles with cellulose II structure as precursor for the production of carbon particles is not yet described. It was not close either. For example, while cylindrical cellulose II molded articles can be made by comminution of commercially available Lyocell fibers, noncellular cellulose II cellulose noncellular cellulose particles, especially those produced by the Lyocell method, have not been readily available. In a particularly preferred embodiment of the present invention, the non-cylindrical cellulose particles are prepared by a special Lyocell method.

The advantages of the invention are in particular the combination of high activatability by lyocell structure of the cellulosic precursors and large-scale processibility to electrodes, with high gravimetric electrical capacities by the inherent lyocell structure, obtaining stable electrodes by means of high electrode density slurry processes and thus high volumetric capacities.

Furthermore, there are advantages of particles or granules to fibers in the carbonation or activation process: Particles are easier to convey because they show no compressible behavior in the promotion and no bridging. Particles are also easier to dose as the bulk density is much more consistent.

Particles can also be more easily introduced into a complete atmosphere of inert gas: fibers transport through their high injected volume a large amount of air which must be displaced with nitrogen prior to thermal treatment; This costs time and resources. For particles, the effort is much lower.

The passage of particles or granules through the rotary kiln is much more constant and can be better controlled than that of fibers, since fibers form by rotation in the rotary kiln differently sized fiber bales, which run at different speeds through the furnace. The activation of particles or granules is also substantially more homogeneous, since the above-described fiber bales are activated more strongly in the outer region, and weaker on the inner side. Because of the much higher bulk density of particles or granules, all heat treatment systems can be designed smaller by an order of magnitude.

Preparation of Precursor;

WO 2009/036480 describes suitable three-dimensional cellulosic moldings and a lyocell method for their production. This process is characterized in that the cellulose solution is freely flowing, i.e., without significant shearing of the cellulose solution is cooled below its solidification temperature, the solidified cellulose solution is comminuted, the solvent is washed out and the minced, washed-out particles are dried. The cellulose particles thus prepared have a cellulose II structure with a particle size in the range from 1pm to 5000pm, preferably 5pm to 2000pm, more preferably 10 to 200pm, and are also characterized by having an approximately spherical irregular surface particle shape and a crystallinity in the range of 16%. up to 45% according to the Raman method.

Another method for producing three-dimensional cellulosic moldings is described below. It comprises the following manufacturing steps: a. Dissolving a cellulosic raw material, such as pulp, to obtain a solution containing from 10% to 15% by weight of cellulose; b. Extrusion of in step a. obtained cellulose solution without air gap direct In a precipitation bath; c. Regeneration process, wherein at the onset of the cellulose solution the difference of the NMMO concentrations of cellulose solution and precipitation bath is 15-78% by weight, preferably 40-70% by weight, and the difference of the temperature of cellulose solution and precipitation bath is 50-120 K, preferably 70- 120 K, more preferably 80 -120 K; d. Washing process according to the percolation principle with at least one alkaline washing step, preferably at pH 9-13; e. optionally a drying process whereby the outer skin of the mold body is not damaged abruptly; wherein the laundry mentioned in point d) is preferably multistage and countercurrent and contains at least one alkaline step. The cellulosic shaped bodies produced in this manner have an optically detectable Kem-ManteL structure,

Suitable dissolving processes are, for example, the viscose, the lyocell or the copper ammonium process; It is also possible to dissolve the cellulose in NaOH or suitable ionic liquids. In general, the method described here is not restricted to specific solvents or processes, but the use of different methods can additionally influence the structure of the particles obtained. Bevoizugtist, however, the principle known in the art LyocelLVerfahren, described inter alia in EP 035Θ41Θ. When using the Lyocell® process, care should be taken that, as the cellulose solution enters the precipitation bath, the difference in NMMO * concentrations of the cellulose solution and precipitation bath is 15-78% by weight, preferably 40-70% by weight, and the difference in temperature of cellulose solution and precipitation bath should be 50-120 K, preferably 70-120 K, particularly preferably 80-120 K.

Starting from the cellulose solution, the shaping takes place, in which, in particular in the precipitation process, care must be taken to ensure that non-fibrous structures are formed. This is not a trivial requirement because cellulose, because of its macromolecular structure, tends to form fibrous domains. This problem is solved by first bringing the cellulose solution into the desired shape without significant shearing, and then selecting the regeneration conditions accordingly. It is imperative that the cellulose solution be directly, i. H. without an air gap, is extruded into a precipitation bath and the comminution of the solution strand takes place in a manner that yields substantially equal sized particles. Suitable aggregates for this step are, for example, underwater or stranded granulators, which also produce cylinders, angular ellipsoids and ovoids in addition to spherical particles. The aforementioned aggregates also meet the additional requirements of the manufacturing process. Namely, the particles produced should have as uniform a size as possible, and the properties can be controlled by the process parameters. At the same time, the process should have a high throughput.

Granules of different sizes can be made from lyocell dope, for example, with an eup50 underwater granulator type EUP50. The beads produced can be separated from the process water by a mechanical centrifugal dryer. In other embodiments, such solid-liquid separations e.g. by hydrocyclone, pusher centrifuges or through sieves. Granulators are available in various sizes on the market, and due to the simplicity of the process of granulating spinning mass, scaling up on an industrial scale is relatively easy. Thus, e.g. be produced with a single pelletizer of the type EUP 3000 about 5000 tons of pellets per year. Others are available from other manufacturers even much larger machines.

In a further embodiment, cellulose strands can also be made with special Lyocell nozzles with nozzle hole diameters of 0.5 to 5 mm, which are fed to a strand granulator after a washing cycle. Critical in this regard are the washing, feeding and drawing of the individual strands into the strand granulator because the strands are very flexible. In this way, cylindrical granules can be obtained.

The viscosity of the cellulose solution also has a great influence on the properties of the particles obtained, since this normally also dominates the viscosity difference between the cellulose solution and the precipitation bath. The precipitating bath is preferably aqueous (having a viscosity in the range of 1 Pa * s), but by adding thickeners (polymers), the viscosity of the precipitation bath can also be increased significantly. A smaller difference in viscosity leads to a thinner coat. According to the invention, a viscosity difference between cellulose solution and precipitation bath of at least 600 Pa.s, preferably in the range 750-1200 Pa.s. (based on the zero viscosity).

The thickness of the shell is critically affected by the NMMO concentration difference upon entry of the cellulose solution into the precipitation bath. The larger this is, the thicker is the coat of molded articles thus produced. The difference in concentration becomes maximum when pure water is used as the precipitation bath and the precipitation bath at the entry point of the cellulose solution is thoroughly mixed so that all exiting NMMO is immediately transported away.

The thickness of the jacket is also affected by the temperature difference on entry of the cellulosic solution into the precipitation bath. The larger this is, the thicker the jacket of the granules produced according to the invention becomes.

Besides the possibility of underwater granulation in a liquid precipitation bath, there is also the possibility of coagulation in a gaseous medium.

The preferred process principle for washing the molded articles on a large scale is countercurrent washing to limit the amount of washing water required and the cost of recovery.

Lenzing AG, PL0566

In order to achieve the required purity of the moldings, 10 to 12scheschstufen are necessary. In addition, at low levels of NMMO *, increasing the wash water temperature is beneficial. Here, a washing water temperature of 60 ° C. to 100 ° C. is preferred. In order to efficiently remove even small amounts of decomposition products of the solvent, an alkaline step is additionally necessary, with preferably pH values of Θ-13 being used.

The Celi II molded bodies produced by this second process variant are characterized by having an optically detectable core-shell structure, the shell having a higher density and a lower crystallinity than the core and the core having a sponge-like structure. Preferably, the sheath has a relative density of 65% to 85% and the core has a relative density of 20% to 60%, each based on compact cellulose. Also preferably, the jacket thickness is between 50 pm and 200 pm. Among other things, the ratio of cladding thickness to overall diameter of the molded article may be between 1: 5 and 1:50. Due to the production, the ratio of the half-axes of the ellipsoidal shaped body will not exceed 3: 1. For the purposes of the present invention, cellulose particles having such a shape, i. ellipsoidal shaped bodies having a ratio of half axes of less than or equal to 3: 1 can be referred to as "ball granules".

The spherical granules thus obtained can be coarsely or finely ground depending on the desired end sizes. For the particles produced according to one of the two process variants described above, the generic term for the purposes of the present invention is the term "cellulose-non-cylindrical cellulose particles". This is intended, in particular, to distinguish from the cellulose-II particles known from the prior art cited above, which are made from lyocell or viscose fibers by comminution and therefore always have a cylindrical shape (see FIGS. 1 and 2). In particular, from Fig. 2 it can be seen that even with very fine pulping of cellulosic fibers, the resulting carbon particles continue to have a cylindrical shape. A cylindrical shape should also be understood to mean a lobed cross section, such as arises during the comminution of viscose fibers (see, for example, FIG. 2 in Lenzinger Berichte, 90, 2012, 58-63).

According to the invention, the cellulosic shaped bodies produced by the above-described processes can be used for the production of carbon particles, at least one heat treatment step being necessary.

In a preferred embodiment of this process, the heat treatment is carried out in an inert gas atmosphere. In a further, likewise preferred embodiment, the heat treatment is carried out in an activating atmosphere. Most preferably, the activating atmosphere consists essentially of water vapor or carbon dioxide.

As a heat treatment, a combination of several heat treatment steps is possible and in many cases even particularly advantageous. A preferred embodiment is the use of said cellulosic moldings for carbonization and activation, wherein at least the following process steps are necessary: fire protection pretreatment, carbonation and activation. Depending on the desired form of the final product or optionally also on a subsequent second heat treatment step, comminution steps may be added.

A preferred solution to the above-described problem consists, for example, in a process for the preparation of carbon particles from precursors having a cellulose II structure, precursor being cellulose cellulose non-cylindrical cellulose particles, a. the cellulose particles are optionally subjected to a fire protection pretreatment, b. A first heat treatment in an inert gas atmosphere, c. Optionally, a shredding of the in step b. obtained particles, d. Further heat treatment of the particles thus obtained in an activating atmosphere, e. And optionally a shredding of the in step d. obtained particles.

A particularly preferred solution of the above-described object consists in a process for the production of carbon particles from precursors with cellulose II structure, precursor being cellulose cellulose non-cylindrical cellulose particles, a. the cellulose particles are subjected to a fire protection pretreatment, b. A first heat treatment in an inert gas atmosphere, c. a shredding of the in step b. obtained particles, d. Further heat treatment of the particles thus obtained in an activating atmosphere, e. And a shredding of the stage d. obtained particles,

Another preferred solution to the above-described object is a process for producing carbon particles from precursors of cellulose II structure, using as precursor non-cylindrical cellulose particles having cellulose II structure, a. the cellulose particles are optionally subjected to a fire protection pretreatment, b. A heat treatment in an inert gas atmosphere takes place and c. a shredding of the in step b. obtained particles takes place.

A further possible solution to the above-described problem is a process for the production of carbon particles from cellulose II precursors using non-cylindrical cellulosic cellulose particles as precursor, the cellulose particles of a fire protection pretreatment, preferably by impregnation with an aqueous solution of a cellulose Phosphates, and finally a heat treatment in activating atmosphere takes place. With this variant, relatively large carbon particles are obtained since there are no comminution steps. Of course, a crushing step can also be added to obtain correspondingly smaller carbon particles. For example, FIG. 3 shows a comminuted, carbonized and activated spherical granules according to the invention, while FIG. 4 shows a carbonated and activated, not further comminuted granules according to the invention, in which the core-shell structure of the original precursor can still be clearly seen.

The fire retardant pretreatment may be either thermal oxidation at temperatures of 150eC -400eC under an air atmosphere or impregnation with a solution of a fire retardant, such as phosphates, for example, diammonium hydrogenphosphate (DAHP) or aqueous potassium hydroxide (KOH). This creates a protective layer on the particle surface that results in both structural improvements and, above all, higher carbonization yields.

Preferred is an embodiment of the method according to the invention, both the fire protection center! is selected from the group containing di-ammonium hydrogen phosphate (DAHP) and potassium hydroxide (KOH).

The carbonization can be carried out batchwise or in a continuous under-inert atmosphere process, e.g. Nitrogen be performed. Essential elements here are the heating rate, the maximum temperature, the maximum temperature residence time and the cooling phase, as well as the volume flow of the purge gas. In Lenzinger reports, 90,2012,58 * 63, einolcher, suitable batch process is described. In the continuous process, e.g. Purged precursor particles in the storage tank with nitrogen and thus rendered inert and then introduced through a cooled screw conveyor in a rotary kiln. The particles then pass through e.g. three temperature zones, e.g. 400 ° C, 900eC, 200® and are then cooled under inert gas. By pretreatment and heating rate, the properties of the resulting particles can be influenced, e.g. the resulting C content, which can be set between 80% and ΘΘ% depending on the pre-treatment.

The activation step can be carried out either directly with the precursor or with already carbonized particles. A pretreatment with e.g. Phosphates increase the formation of active surface area (see Example 1). As with carbonation, essential elements of the activation step are the heating rate, the maximum temperature, the maximum temperature residence time, and most importantly, the type and volume flow of the purge gas. As activation gases, e.g. Carbon dioxide or water vapor can be used. Thus, in a preferred form of the process of the invention, the activating atmosphere during the heat treatment consists essentially of steam or carbon dioxide.

For pretreatment of precursors with KOH, thermal activation will preferably be carried out in an inert gas stream. The activation can be done in the rotary kiln, the rotation should ensure a uniform activation. Pre-treatment, maximum temperature and residence time control the formation of the active surface. A measure of the active surface area may be the BET value in ma / g. Another way of characterizing the carbon particles is to measure the electrical capacitance by cyclic voltammetry of 2-electrode or triple-electrode cells at 1 and 1.5 V, respectively, with a scan rate of 20 mV / s.

Another aspect of the present invention is the use of non-cylindrical cellulosic cellulose II cellulosic particles to produce carbon particles by the method described above.

The non-cylindrical carbon particles thus produced can be used, inter alia, for the production of electrodes in modern energy storage media which require a high charging and discharging rate, for example for batteries and supercapacitors in vehicles with electric drive or start-stop function. Also for the so-called "Superca batte ries" - hybrid systems between supercapacitors and batteries - the carbon particles produced according to the invention are suitable. Further applications are uses as a filter medium, especially for the filtration of gases, air and liquids as well as storage medium for gases, such as hydrogen.

In the following the invention will be described by way of examples. However, the invention is expressly not limited to these examples, but includes all other embodiments based on the same inventive concept.

Examples

Example 1; Activation of lyocell fibers with and without pretreatment

Precursors used: Commercially available Tencel® fiber 1.7 dtex / 38mm cut length

Pretreatment: Impregnation of lyocell fibers with an aqueous solution of ammonium hydrogen phosphate (2% by weight, based on cellulose), followed by drying at 70 ° C. in vacuo.

Heating & Cooling rate eC / min 3 T max * C 850

Residence time carbonation min. 30

Residence time activation min 60-180

Gas flow argon l / h 36

Gas stream carbon dioxide l / h 24 DAHP to lyocell% 2

The characterization was carried out via yield and active surface by BET measurement (BELsorp mini II measuring instrument).

Table 1:

The experiments of Example 1 show that both the yield and the active surface are increased by the pretreatment. A longer dwell time in the carbonation increases the active surface but decreases the yield. This finding is still transferable from lyocell fibers to non-cylindrical lyocell particles as precursors.

Example 2: Activation of lyocell fibers and lyocell pellets in comparison

Commercially available lyocell fibers as well as various lyocell pellets produced by the method described above were pretreated, carbonated and activated under the same conditions. The electrical capacitance of the products was measured by cyclic voltammetry at 1.5V at a scan rate of 20mV / s in a two electrode cell with the following construction: Current Collector (Aluminum Foil) - Activated Carbon Electrode - Electrolyte (1.8 mol / L Triethylmethylammonium Tetraborate In Propylene Carbonate) Separator - Activated carbon electrode - current collector (aluminum foil).

Tested Precursors: Commercially available Tencel® fiber 1.7 dtex / 38 mm cut length, 1 mm diameter Lyocell pellets unmilled, coarsely ground Lyocell pellets (100 μm average particle size), finely ground Lyocell pellets (8 pm average particle greetings). The lyocell pellets were prepared as described in the above

Description shown method produced. The particle greetings were determined by a laser diffractometer.

Heating & Cooling rate ° C / min 10 T max ° C 850

Residence time carbonation min. 30

Residence time activation min. 150

Gas stream carbon dioxide l / h 80 DAHP to lyocell% 2

Table 2:

The experiments of Example 2 show that lyocell pellets under similar conditions show a similarly good activability as lyocell fibers.

Claims (11)

1. Process for the preparation of carbon particles from precursors with cellulose-II structure by heat treatment, characterized in that are used as precursor non-cylindrical cellulose particles with cellulose-II structure. 2. The method according to claim 1, wherein the heat treatment takes place in an inert gas atmosphere. 3. The method according to claim 1, wherein the heat treatment is carried out in an activating atmosphere. A method according to claim 3, wherein the activating atmosphere consists essentially of water vapor or carbon dioxide. A process according to claim 1, wherein the cellulose particles are subjected to a brimstone pretreatment prior to the heat treatment. A process according to claim 5 wherein the fire retardant pretreatment is an impregnation of the cellulose particles with a fire retardant solution. A process according to claim 6 wherein the fire retardant is selected from the group comprising diammonium hydrogen phosphate (DAHP) and potassium hydroxide (KOH). The process according to claim S, wherein the fire protection pretreatment is a thermal oxidation of the cellulose particles at temperatures of 15QeC - 400 "C under air. 9. The method according to claim 1, wherein after the heat treatment, comminution of the particles obtained by the heat treatment is carried out. 10. The method according to claim 1, wherein a. the cellulose particles are optionally impregnated with a solution of a fire retardant, b. A first heat treatment in an inert gas atmosphere, c. Optionally, a shredding of the in step b. obtained particles, d. Further heat treatment of the particles thus obtained in an activating atmosphere, e. And optionally a shredding of the in step d. obtained particles. 11. Use of non-cylindrical cellulose particles with cellulose II structure for the production of carbon particles by means of a method according to claim 1.