HK1001496B

HK1001496B - Cellulose suspension production process

Info

Publication number: HK1001496B
Application number: HK98100416.9A
Authority: HK
Inventors: Zikeli Stefan; Hinterholzer Peter
Original assignee: Lenzing Aktiengesellschaft
Priority date: 1995-04-19
Filing date: 1995-05-23
Publication date: 1998-06-19

Description

The present invention relates to a process for the production of a suspension of cellulose in a liquid form of tertiary amino oxide and a process for the production of cellulosic moulds, such as films, fibres and membranes.

For several decades, research has been conducted into methods for the production of cellulosic moulds, which are intended to replace the viscose process used on a large scale today. An interesting alternative has emerged, not least because of its better environmental compatibility, is to dissolve cellulose in an organic solvent without derivatization and to extrude moulds, e.g. fibres and films, from this solution.

It has been shown that a mixture of a tertiary amino acid and water is particularly suitable as an organic solvent for the production of cellulosic moulds. The amino acid used is primarily N-methylmorpholine N-oxide (NMMO). Other amino acids are described, for example, in EP-A-0553 070.

US-A-4246.221 describes an amino-oxide process for the production of spinning cellulose solutions using as starting material, inter alia, a mixture of cellulose in liquid aqueous N-methylmorpholine N-oxide (NMMO). This process produces a suspension of cellulose in aqueous amino-oxide solution in a discontinuous mixing device and simultaneously heats the mixture under reduced pressure, withdrawing water and producing a first solution which is transferred to a malleable solution after filtration and post-processing in an extruder. One known disadvantage of this process is the discontinuous production of cellulose. This process is carried out in a discontinuous process.

WO 94/28217 describes a process for the preparation of a pre-mixture based on cellulose from which a moldable cellulose solution can be produced. This process involves the introduction of crushed cellulose and an amino-oxide solution into a horizontally-stored cylindrical mixing chamber with a rotor with axially spaced stirring elements. The mixture is stirred in the mixing chamber by rotating the rotor at a speed of 40 to 80 revolutions per minute. Preferably, the mixture is kept in the mixing chamber at a temperature above 65°C. However, this process has the following disadvantages: The process is time-consuming, since, among other things, the amino-oxide solution and the cellulose cannot be introduced into the mixing chamber at the same time.The process requires two mixers to be operated in order to supply the cellulose/amino oxide mixture continuously to a machine in which the cellulose solution is produced and continuously supplied, for example, to a spinning machine. The use of two mixers again means that all the equipment, such as storage tanks, beams, pumps, filters, etc., which is connected between the mixers and the spinning machine must also be available twice. This leads to a considerable technical and financial expense.The second disadvantage is that relatively large quantities of amino oxides are processed per batch, which poses a safety risk because tertiary amino oxides are unstable at elevated temperatures and tend to decompose.

The present invention thus aims to provide a method which does not present the above disadvantages and according to which a homogeneous cellulose suspension can be produced from crushed cellulose and an aqueous amino acid solution in a single step and at a high speed, which can be used as a starting product for the amino acid process.

The method of the invention for the preparation of a suspension of cellulose in a liquid aqueous tertiary amino oxide by mixing crushed cellulose with the liquid aqueous tertiary amino oxide is characterised by: (A) the crushed cellulose is brought into contact with the liquid aqueous tertiary amino oxide and a first mixture is formed; and (B) this first mixture is spread mechanically in layers on a surface and carried over this surface under intense mixing until a homogeneous suspension is formed which is removed from the surface, while (C) the operations (A) and (B) are carried out continuously.

The invention is based on the recognition that by applying the layering technique, in particular the thin layering technique, a homogeneous cellulose suspension can be produced in a simple and rapid manner.

It has also been shown that the method of the invention makes it possible to mix cellulose aids, such as stabilizers, dispersives, spinning aids, reactivity enhancing reagents, incorporation media of an inorganic or organic nature (barite, activated carbon, SiO2, CMC, modifiers (polyethylene glycols) and other polymers, such as nylon, with the help of the method of the invention, in a simple way.

A further advantage of the method of the invention is that a liquid aqueous aqueous solution of the tertiary amino-oxide and a molten tertiary amino-oxide hydrate can be used as the liquid aqueous tertiary amino-oxide in which the crushed cellulose is suspended at room temperature.

The method of the invention preferably uses a liquid aqueous tertiary amino oxide having a temperature in the range of 50 to 85°C.

The method of the invention benefits from the advantageous transport of cellulose over a surface in a period of 1 to 5 minutes, which means that the cellulose suspension is obtained in a homogeneous form continuously within a few minutes.

The invention also relates to the use of a device for performing the process of the invention, consisting of a vessel, which can be heated or cooled indirectly, equipped with a stirring device, which is designed as a cylindrical container with a centrally located rotor and stirring elements attached to it, forming an angle of inclination α to the axis of the rotor, for mixing and transporting the cellulose brought into contact with the liquid, white tertiary amino acids, with a radial distance of the stirring elements to the inner wall of the container not exceeding 20 mm, and which is provided in the upper part of the container for the entrance of the crushed container and the liquid, tertiary amino acids, and a homogeneous part for the suspension of the liquid in the outlet.

It is preferable to use a device in which the angle of inclination α is adjustable.

The invention also relates to a process for the production of a malleable cellulose solution characterized by the evaporation of water from a homogeneous suspension produced in accordance with the method of the invention to a malleable cellulose solution. The production of the cellulose solution is conveniently carried out in a thin film treatment apparatus. Such a process is described, for example, in EP-A-0 356 419. One embodiment of a thin film treatment apparatus is, for example, a so-called film truder, as manufactured by Buss AG (Switzerland).

The method of WO 94/06530, published in 1994, uses the technique of thin film, as previously known from EP-A-0356 419, to convert a mixture of cellulose in an aqueous solution of a tertiary amino oxide into a malleable solution. The method is carried out in a film roller, in the same way as the one described in EP-A-0356 419.

The present invention also relates to a process for the manufacture of cellulose moulds, which consists in transferring a cellulose suspension produced in accordance with the invention to a moldable cellulose solution, which is then processed in a known manner into films, fibres, membranes or other moulds.

The suspension produced in accordance with the invention can be fed directly into the film extruder by means of a dosing device and processed there for solution. A degassing extruder is also suitable for solution production. In the latter case, however, the suspension must be in a concentrated state, since a degassing extruder has only a very limited evaporation capacity. The suspension produced in accordance with the invention can also be transferred to the cellulose solution in other devices.

The drawing below describes a device in which the process of the invention can be carried out particularly well. The drawing shows schematically the structure of a mixer used in accordance with the invention. Such mixers are, for example, marketed as ring layer mixers by Draiswerke GmbH, Mannheim, Germany, under the name K-TT.

The figure shows a diagram of a horizontal mixer, consisting essentially of a cylindrical vessel 1 with a double cap for indirect heating or cooling of the product to be processed, a rotor 2 centrally stored in the vessel 1 with conveyor blades 3 and spindle-shaped elements attached to it 4.

The crushed cellulose is introduced into cylindrical container 1 via the inlet 6 and is collected by the conveyor sheets 3 and transported to the inside of container 1.

The liquid aqueous amino-oxide solution is introduced into the container via the drainpipe 7. The drainpipe 7 reaches almost to the rotor 2 and is naturally arranged in a gap between adjacent conveyor blades or spindle-shaped elements so that the rotation of the rotor 2 is not impeded. The flight force given to the amino-oxide solution causes it to be radially ejected, the cellulose is first wetting and a mixture is formed. This wetting stage is shown in the figure. Depending on the intensity of the mixture and the ratio of amino-oxide solution to cellulose, a homogeneous suspension is used during the transport of the mixture over the wall of the container 1 and is either removed via a hole or a further processed cellulose 8 can be used to produce a solution.

The spindle elements 4 are slightly inclined to the axis of the rotor 2 and can thus transport the suspension further. The essential thing is that the rotor is rotated so fast that the cellulose/amino oxide mixture is thrown against the wall of the container and forms a layer.

The following example will explain the invention in more detail.

Example

The suspension was manufactured using a ring-layer mixer K-TT 80 (stainless steel) manufactured by Draiswerke GmbH, Mannheim, Germany.

The ring mixer was continuously fed 55 kg of crushed cellulose with a particle size of no more than 4 mm and 375 kg of an aqueous solution containing 75% NMMO per hour. The heating medium was water at 75°C. The ring mixer was pressurized to a normal pressure. The rotor was rotated at 700 revolutions per minute.

430 kg of homogeneous suspension with a dry matter density of 12.8% by mass of cellulose and a temperature of 65°C could be obtained from the ring mixer per hour.

The suspension produced in the ring mixer was fed at 75°C to a film truder of the HS 0200 type bus at a speed of 430 kg/h using the procedure described in EP-A-0 356 419 and transferred to a moldable solution with a cellulose concentration of 15.0%. The heating surface of the film truder was heated with a saturated steam of 142°C. The rotor's rotor speed was 4 m/s and the pressure in the film truder was 50 mbar. About 63 kg of water vapour was removed per hour.

The above process parameters allowed approximately 367 kg of moldable cellulose solution at 105°C to be removed from the film roller per hour.

The moldable solution obtained did not contain any undissolved cellulose particles (microscopic examination), was degassed and could be easily processed into fibres or films by the applicant's methods known from WO 93/19230 and WO 95/07811 respectively.

The residence time of the cellulose in the ring mixer was 2 minutes, which means that the method of the invention makes it possible to produce a homogeneous suspension from the starting materials cellulose and amino acid solution in only a few minutes, which can be easily transferred to a malleable cellulose solution.

Claims

Method for producing a suspension of cellulose in a liquid aqueous tertiary amine oxide by mixing comminuted cellulose with the liquid aqueous tertiary amine oxide, characterized in that
A) the comminuted cellulose is brought into contact with the liquid aqueous tertiary amine oxide and a first mixture is formed, and

B) said first mixture is mechanically spread out over a surface in layer form and is transported with intensive mixing across said surface until a homogeneous suspension is formed which is removed from the surface, wherein

C) the measures (A) and (B) are carried out continuously.
Method according to Claim 1, characterized in that an aqueous solution of the tertiary amine oxide is used as the liquid aqueous tertiary amine oxide.
Method according to Claim 1, characterized in that a molten tertiary amine oxide hydrate is used as the liquid aqueous tertiary amine oxide.
Method according to one of Claims 2 or 3, characterized in that a liquid aqueous tertiary amine oxide is used which is at a temperature in the range from 50 to 85°C.
Method according to one of Claims 1 to 4, characterized in that the cellulose brought into contact with the liquid aqueous tertiary amine oxide is transported over the surface during a time interval of 1 to 5 minutes.
Use of a device for carrying out the method according to one of Claims 1 to 5 comprising an indirectly heatable or coolable vessel which is provided with a stirring device and which, for the purpose of mixing and transporting the cellulose brought into contact with the liquid aqueous tertiary amine oxide is formed as a cylindrical container (1) having a centrally mounted rotor (2) and stirring elements (3, 4) which are attached thereto and which form an angle of inclination a with respect to the axis of the rotor (2), wherein the radial distance of the stirring elements (3, 4) from the internal wall of the container is not more than 20 mm and inlets (6, 7) for the comminuted cellulose and the liquid aqueous tertiary amine oxide are provided in the upper part of the container (1) and an outlet (8) for the homogeneous suspension is provided in the lower part.
Use of a device according to Claim 6, in which the angle of inclination α is adjustable.
Method for preparing a mouldable cellulose solution, characterized in that a homogeneous suspension prepared in accordance with Claims 1 to 7 is processed to form a mouldable cellulose solution by evaporation of water.
Method according to Claim 8, characterized in that it is carried out in a thin-film treatment apparatus.
Method for preparing cellulosic moulded bodies, characterized in that a mouldable cellulose solution prepared in accordance with Claim 8 is processed in a manner known per se to form sheets, fibres, membranes or other moulded bodies.