EP0617742A1

EP0617742A1 - Device and process for the production of fibrous starch materials

Info

Publication number: EP0617742A1
Application number: EP93922545A
Authority: EP
Inventors: Catia Bastioli; Bruno Casale; Gino Zanardi
Original assignee: Novamont SpA; Cerestar Holding BV
Current assignee: Cerestar Holding BV
Priority date: 1992-10-16
Filing date: 1993-10-11
Publication date: 1994-10-05
Anticipated expiration: 2013-10-11
Also published as: DE69316585T2; FI102772B; WO1994009190A1; ATE162561T1; FI942851A0; DK0617742T3; ES2111186T3; BR9305678A; ITTO920837A1; JP3345013B2; NO942255L; FI102772B1; AU5150693A; ITTO920837A0; EP0617742B1; CA2125977C; CA2125977A1; AU669225B2; NO942255D0; DE69316585D1

Abstract

Procédé et dispositif de production de matières fibreuses à base d'amidon, le procédé consistant à extruder une dispersion ou une solution aqueuse d'un amidon dans un courant d'un coagulant salin. La dispersion ou la solution aqueuse est extrudée à travers une paroi tubulaire microporeuse dans une chambre encercleé par ladite paroi microporeuse de manière à obtenir un flux d'extrusion de l'amidon entourant lesdites parois tubulaires. La coagulation de l'extrusion est effectuée par l'acheminement du courant de l'agent de coagulation dans la chambre annulaire parallèle à la surface d'extrusion. Les fibres ainsi obtenues conviennent particulièrement à des applications relatives à la fabrication de papier, et peuvent être utilisées en remplacement de fibres de cellulose ou en combinaison avec ces fibres.Method and device for producing fibrous starch-based materials, the method comprising extruding an aqueous dispersion or solution of a starch into a stream of a salt coagulant. The aqueous dispersion or solution is extruded through a microporous tubular wall into a chamber encircled by said microporous wall so as to obtain a starch extrusion flow surrounding said tubular walls. Coagulation of the extrusion is effected by directing the stream of coagulation agent into the annular chamber parallel to the extrusion surface. The fibers thus obtained are particularly suitable for applications relating to the manufacture of paper, and can be used as a replacement for cellulose fibers or in combination with these fibers.

Description

"DEVICE AND PROCESS

FOR THE PRODUCTION OF FIBROUS STARCH MATERIALS"

The present invention refers to a device and process for production of fibrous starch materials particularly destined for use in the production of paper and cardboard.

It is known that if aqueous colloid dispersions of starch in typical concentrations of between 5 and 40% by weight of an¬ hydrous solid, is brought into contact with non-solvents (for example a saline solution of ammonium sulphate) , it coagulates forming flakes of gel.

US patent No. 4,205,025 describes a process for the produ¬ ction of fibrils used as paper pulp using film forming polymers including substantially water-soluble starches. By the term "fi- brils", materials showing a hybrid morphology which is between a film and a fibre are intended. The film forming polymer is dis¬ solved in water to form a solution which is then injected into a precipitating means, preferably an organic non-solvent, such as an alcohol or a ketone, with the application of shearing stress in order to obtain the formation of fibrils which are then ren¬ dered more hydrophobic through subsequent treatment in an inso- lublising agent.

US Patent No. 4,340,442 describes a process for the forma¬ tion of fibrils which, in order to improve the hydrophobic pro- perties of the fibrils, uses starch insoluble in water having a high amylose content (50-80%by weight) , which is coagulated in a saline solution, in particular ammonium sulphate. Said starch which is substantially insoluble in water, requires a stage in which it is dissolved in alkaline solution which causes problems in the coagulation stage and problems with respect to disposal of sulphates different from ammonium salts, which are formed in said stage.

US Patent No. 4,139,699 describes a process for the produ¬ ction of a product having starch fibre morphology, through ex- trusion of a colloidal starch dispersion having a high amylopectin content in a coagulating agent. In the case where a starch having a amylopectin content of less than approximately 95% is used, it is necessary to chemically modify the starch to ensure the colloidal dispersion thereof in the aqueous system or, alternatively, the starch must be dissolved in the presence of alkaline hydroxides.

The use of alkaline hydroxides, particularly sodium hydroxide, makes the industrial application of the process de¬ scribed difficult, in that the coagulation stage carried out using ammonium sulphate results in the production of ammonia and formation of large quantities of sodium sulphate preventing coa¬ gulation and causing problems with respect to disposal.

US Patent No. 4,243,480 describes a process that uses the product obtained according to the process described in US Patent No. 4,139,699, for the production of paper or cardboard accor¬ ding to conventional paper making technology. Said product has a short fibre morphology having a diameter of between 10 and 500 microns and a length of between 0.1 and 3 mm, obtained by extru¬ ding the starch dispersion via a die into a moving coagulation bath.

US Patent No. 4,853,168 describes a process of the type described in US Patent No. 4,139,699, in which the colloidal starch dispersion adapted to be extruded is obtained by cooking an aqueous starch dispersion containing the coagulating saline solution. In the above cited patent literature and in practical expe¬ rimentation, various known devices can be used in order to fi¬ nely break down the starch solution or dispersion and therefore favour a close contact with the coagulating agent, such as ato- mization nozzles, ejectors, mixers with stirrers, spinnerets or syringes. It has however been demonstrated experimentally that the type of device used strongly influences the final coagulated product and its properties. Devices in which the starch is coa¬ gulated in highly turbulent conditions (such as ejectors) or in which there is no ordered speed profile (mixers with stirrers) , do not give rise to products with a fibrous structure, but some¬ what provoke a fragmentation of the starch, with formation of flat scales (rolled onto each other) or a three dimensional ag¬ gregate. The dimensions of these non fibrous products vary with the operating conditions and influence the characteristics of them. In the production process very small particles are lost during the separation and slow down the filtering operation in that they block the cloth; if used in the production of paper, they are not retained on the flat cloth with consequential loss of starch in the paper and an increase of COD in the paper factory waste water. On the other hand, very large particles do not in-, tegrate with the cellulose matrix fibres giving rise to defects in the produced paper. Other negative aspect, verified for fibrids obtained from the previously described processes, consists of rather high wa¬ ter retention and solubility values.

A further product obtained from starch by coagulation pro¬ cesses, but having a fibre morphology, partly reduces the above listed disadvantages in that, thanks to its fibrous structure, it increases its compatibility with the cellulose fibres, redu¬ ces the water retention in that it is more easily filterable and reduces its solubility as it has a lower specific surface.

It would therefore be desirable to have a production of a product having fibre morphology, with dimension, size distribu¬ tion and physical chemical properties such to be suitable for the production of paper and cardboard and in addition to be ob¬ tainable from low cost starch such as starch from maize or pota¬ to without adopting alkaline solutions of starch for the starch used.

In light of such a purpose, the object of this invention is a process for the production of fibrous starch materials through extrusion of a dispersion or aqueous solution of starch material in a flow of saline coagulant agent characterised by the fact that it comprises the operation of: extruding the dispersion or aqueous solution through a microporous tubular wall in a chamber circularly ringed with said microporous wall in such a way to obtain an ex¬ trusion flux of the starch material which surrounds the said tubular walls and carry out the coagulation of the extrusion by feeding the flow of the coagulation agent in the annular chamber paral-. lei to the extrusion surface.

Another object of the invention is a fibre making device characterised by the fact that it comprises: a tubular body comprising first means of entry for feeding the flux of starch material, a feeding chamber for the starch material connecting the said first means of entry, - an annular outlet chamber of the starch material, a tubular element with porous walls coaxially arranged with said outlet chamber and interposed between this and the feeding chamber, adapt to extrude through the said porous walls in the said outlet chamber a variety of threads of starch material forming a envelope around the said tubular element, a second means of entry connecting the outlet chamber for feeding the coagulating agent flow and means of discharge arranged downstream from the annular outlet chamber.

It has been found that by using the process and device of the present invention it is possible to obtain a product which shows a shape ratio having a particularly narrow size distribu¬ tion and centred in the range of 75-100 microns and having a water solubility, determined by the "Anthrone Test" further de¬ scribed later, of less than about 2% and a low water retention. Other advantages and characteristics of the process, device and product obtained according to the present invention will be fur¬ ther illustrated in detail in the following with reference to the enclosed drawings in which: figure 1 shows illustrates a flow chart of the plant for carrying out the process, figure 2 shows a cross section view of the fibre making device according to the invention, - figure 3 shows a cross section view of another embodiment of the fibre making device, and figure 4 is an enlarged detail of a part of figures 2 and 3 With reference to the drawing in figure 1, l indicates a stirred dispersion for the preparation of the starch suspension in water with a dry weight typically from 5 to 50% by weight and preferably 10-40% by weight. The starch used for the preparation of the suspension is preferably natural starch such as starch from maize, rice, tapioca, potato having a amylopectin content from 30 to 100%. Particularly prefered is maize starch, widely available on the market, having a typical amylopectin content of from 64-80% by weight. Within the scope of the invention, starch with a high content of amylose, such as amylomaize and che¬ mically or physically modified starch can be used.

The starch suspension can also contain additives such as salts (e.g. saline coagulating agents as described in US Patent No. 4,853,168) alkaline agents, organic fillers or minerals, crosslinking agents, plasticisers, polyoxyethylene, polyvinyl alcohol, ionomeric polymers such as copolymers of ethylene and acrylic acid and/or maleic anhydride, polyacrylates, polyamides, lubricants such as lecithin, fatty acids, esters and amides of fatty acids.

The suspension, maintained in the disperser under stirring at ambient temperature, is then pumped via a gear pump 2 into a jet cooker, itself indicated with 3, where it is mixed in a co- current with steam in such a way as to reach the desired cooking temperature. The jet cooking process is known per se and invol¬ ves instantaneous heating of the aqueous suspension with process steam and then maintaining the heated liquid for a predetermined period. The cooking temperature, generally between 90 and 180°C, is selected according to the specific starch used in the course of the process. In particular, care should be taken to avoid an excessively high temperature causing degradation of the starch material, while ensuring that the temperature, the shearing time applied and the standing time are such that it is possible to obtain a dispersion close to complete gelation. At the outlet of the jet cooker, the starch dispersion or solution subject to cooking is collected whilst stirring in a lined stirred reactor 4, water circulating at a temperature of about 100°C in the casing thereof. A flash is effected in this lined tank in order to free the excess steam and to return the starch/water concentrations close to the initial concentrations.

From reactor 4 the starch is pumped via a pump 5, in a heat exchanger 6 where it is brought to a temperature of between 20 and 100°C, preferably from 40 to 70°C. From the heat exchanger, the starch is fed to a fibre making device of the types il¬ lustrated in figures 2 and 3, described in the following, in which a saline coagulating solution is also injected. The salts that can be used in the scope of the present invention comprise ammonium sulphate, magnesium sulphate, aluminium sulphate, ammo- nium phosphate, potassium chloride, sodium sulphate, sodium car¬ bonate, sodium bicarbonate, and ammonium chloride. The preferred saline solution is a saturated solution of ammonium sulphate, although it is not necessary to reach saturated levels of the above mentioned salts and it is equally possible to use concen- trations lower than saturation levels.

The starch fibres obtained from the fibre making device are collected in a stirred reactor 8 in order to be subjected to maturing and subsequently decanting. Once the decanting has been effected the clarified substance is recycled, by means of a pump 9, and mixed with a saturated saline solution of the coagulating agent before being reused for drawing the starch.

The clarified substance which circulates in the installa¬ tion as a coagulating agent, contains the saline solution and the finest fibres which, due to their small dimensions are not decanted in the collecting container. The mass of fibres from reactor 8 is pumped by means of pump 10 on to filter 11. The fibres are then collected in a con¬ tainer 12, while the filtrate is fed to container 13 where it is mixed with the clarified substance from pump 9, with subsequent addition of sulphate in order to recycle the saline solution adapted to be fed into fibre making device 7.

By using an appropriate number of reactors 8 for the matu¬ ring and decanting, it is possible for the process to be carried out continuously, thereby obtaining starch fibres which can be washed directly on the filter or simply filtered and sub¬ sequently washed.

The fibre making device 7, in the embodiement in figure 2, comprises a tubular body 14 having at least one inlet 15 which, under normal conditions, is used for feeding the starch ma- terial, an inlet 16 designed to feed the coagulating agent and a outlet 17 for discharging the starch fibres produced after the coagulation.

From the inlet 15 the starch material is immersed in a tu¬ bular duct 18 which partially terminates in a wall 19 supplied with radial holes. The holey wall part 19 acts as the distribu¬ tor of the starch material flow towards a feeding chamber 21.

With the reference 22, the tubular element with microporous walls suitable for extruding the starch material from the fee¬ ding chamber 21 into the annular chamber 23 coaxially thereto is indicated. The chamber 23 is separated from the radially exter¬ nal surface of the element 22 and the radially internal surface of body 14.

The tubular element 22 can consist of a body of porous sin¬ tered metal material in which the distribution of the porous 'dimension is preferably comprised between 10 and 500 microns. Alternatively the tubular element 22 is a body of metal material, for example stainless steel, provided with a number of radially passing holes obtained by mechanical working and having at least a narrow flow section with openings having a dimension preferably comprised between 10 and 500 microns. Preferably said radial holes have a cross section as illustrated in figure 4 with a portion 24 of the inlet for the starch material having a narrow opening, typically from 10 to 500 microns, and a portion 25 on the outlet of the starch material with an larger size ope- ning, preferably comprised between 0.5 and 1.5 mm.

The opening density on the extrusion surface (intended as the surface of the tubular element in contact with the coagula¬ ting agent) , expressed as a ratio of number of holes to surface area is preferably comprised between 4 and 0.05 holes/mm². The coagulating agent fed through the inlet opening 16, flows through the annular element 26 having a crown of axial holes 27, acting as distributor, and is fed into the first annu¬ lar chamber 28 defined by the walls 14 of the fibre making devi¬ ce and a tubular element coaxial to the body 29. From chamber 28 the flow is fed into the annular chamber of outlet 23, parallel to the radially external surface of the microporous tubular ele¬ ment 22, where the flow of coagulating agent interacts with the extrusion flow of the starch material.

The starch material is extruded in the form of a variety of threads which surround the extrusion surface in the guise of a tubular film.

Preferably the flow speed of the saline coagulating agent in the annular section of the outlet chamber 23 is maintained between l and 15 m/s. The drawing ratio, intended as ratio of flow speed of the coagulating agent in the annular section of the chamber 23 and the speed of the starch material at the outlet of the holes of the microporous wall (defined as the ratio between the flow rate of the starch material and the total section in the holes of the outlet) is generally comprised between 1-1000, preferably betwe¬ en 100-1000. Preferably the axial length of the outlet chamber 23 is such that a stay time of the starch material comprised between 5 and 15 milliseconds is obtained. In any case the axial length of the chamber 23 in which the starch material undergoes drawing must be such to cause an orientation of the starch mate¬ rial allowing at the same time a complete phase inversion.

At the outlet of chamber 23 the extruded flow is fed into a annular chamber 30 at a progressively increasing cross section in the flow direction.

In the embodiment of the fibre making device illustrated in figure 3, the flow of starch materials is fed through an inlet 31 to an annular chamber 32 defined by the walls of body 14 and the microporous walled tubular element 33. The flow of the starch material follows the radial direction towards the inside through the walls of element 33 into the annular outlet chamber 34 comprised between the tubular element 33 and a central nucleus 35 coaxial to the body. The flow of the coagulating agent is fed across an inlet 36 and into a prechamber 37, it flows into a chamber 40 across holes 39 of an annular element 38 and from chamber 40 is fed to the outlet chamber 34 having a narrow cross section in the flow direction.

In this embodiment the section of holes of the microporous element 33 remains the same as figure 4. In this case, however, the flow of the starch material advances from a bigger to a smaller cross section, which brings an increase in the starch flow speed and necking down of the starch threads. The material leaving the holes is coagulated by the coagulation agent flow in the annular chamber 34. It has been observed that the best con- ditions of coagulation are when the drawing ratio is comprised preferably between 1 and 150, with an emission speed of the starch material from the holes in the microporous walls 33 com¬ prises preferably between 0.1 and 1 m/s.

The fibre making device subject of the present invention presents notable advantages such as: it supplies, through coagulation of a starch material, a product having a fibrous structure; its structure having a cylindrical symmetry guarantees uni¬ formity of fluid mechanic conditions thus excluding possi- ble border effects; its geometry is completely known and therefore project cri¬ teria are available. the knowledge of the above mentioned criteria permits its scale-up. Other advantages deriving from the use of the above fibre making device, will be highlighted by the following examples. Example 1

By using a plant as described with reference to figure l maize starch fibres have been obtained working under the folio- wing conditions: starch concentration in the dispersion : 15% by weight (anhydrous starch) maximum cooking temperature in the jet cooker: 115°C (pre¬ ferred temperature range is between 100-130°C) temperature of the starch at the inlet of the fibre making device: 60°C saline solution: ammonium sulphate: 41% by weight temperature of the saline solution at the fibre making device inlet: 21°C maximum speed of the saline solution in the outlet chamber of the fibre making device: 7 m/s flow rate of the starch after cooking 48 1/h fibre making device as illustrated in figure 2 having a extrusion sinter consisting of a sintered metal with a po¬ rosity of 40 microns (average diameter of the pores) length of the outlet chamber (23,24) of the fibre making device: 10 cm average maturing time before filtering : 4 hours Carrying out the process according to the above mentioned conditions starch fibres were obtained having the following size distribution measured according to the Bauer McNett apparatus expressed in percent by weight:

595 μm (28 mesh) % 0.3 297 μm (40 mesh) % 3.1 149 μm (100 mesh) % 68.5 74 μm (200 mesh) % 21.3 above 200 mesh X 100 6.8

The determination of the characteristics of the solubility and the fibre obtained has been carried out using the following procedure: washing of the filtration panels coming from the plant; 100 g of the filtration cake are dispersed in water (500 ml) by mechanical stirring with a glass anchor stirrer under the folio- wing conditions: Becker with diameter 10 cm and height 20 cm; mechanical glass anchor stirrer (1=40 cm with stirring blade with 1= 8 cm, height 8 cm) ;

Temperature = 20°C; stirring time 30 mins; rotation speed 500 rpm.

The dispersion obtained is filtered on Bruckner with a dia¬ meter of 30 cm in the presence of a paper filter under vacuum of 10 mm Hg.

The liquid is filtered twice on the same panel. The panel is then washed with 500 ml of H₂0. The ratio of starch to water in the washing is 1:10.

The solubility determination is carried out on the filtered product, in order to separate it from the water and washed to remove the coagulant. The product is dispersed in water in a conventional laboratory pulper (dry concentration 0.2% rotation speed 3000 rpm) ; a sample was removed after 4 hours and after filtered on a 8 micron filter paper, the starch is measured in solution with the reagent "ANTHRONE" (solution 0.2% of ANTHRONE in 96% H₂S0₄) . The solubility value, determined by the above cited method on the filter panels obtained according to the example, is less than 1.5%.

The morphological characteristics of the fibre obtained are illustrated in figure 4. Example 2

The test according to example 1 has been repeated varying only the characteristics of the microporous sintered filter con¬ sisted, in this case of a sintered metal tube with pores having an average diameter of 100 μm. Fibres were obtained having the following size distribution expressed in terms of percentage by weight:

595 μm (28 mesh)% 0.3 297 μm (40 mesh) % 0.9 149 μm (100 mesh) % 63 74 μm (200 mesh)% 25.2 above 200 mesh X 100 10.6

The results demonstrate that the average diameter of the pores does not influence in a relevant way the fibre dis- tribution that is maintained on a 100 and 200 mesh.

The solubility values obtained according to the method of example 1 are once again less than 1.5% like in the preceding case.

Example 3 (comparative) The characteristics of the fibres obtained by the test in example 1 are compared to the fibrids obtained with the other fibre making devices, in particular ejector and spinneret. The process conditions are the same as for example 1. The first fibre making device consists of an ejector equip- ped with 8 holes in a 1 mm diameter, for the starch inlet with an inclination of 45° with respect to ejector axis placed in the groove. The speed of the coagulating agent (ammonium sulphate), in the thinner section is equal to 31 m/s and the draw ratio, (defined as the ratio between the maximum speed of the sulphate to that of the starch leaving the holes) is equal to 47.

The second fibre making device consists of a spinneret equipped with 113 holes having a diameter of 0.5mm; this spinneret is placed in a circular duct and the annular crown separated from the external surface of the spinneret and the internal walls of the circular duct is fed with the coagulating agent, ammonium sulphate: the speed of the ammonium sulphate and that of the starch material exiting the holes are parallel. At the holes outlet, the starch material is contacted with the coa¬ gulating agent; the suspension formed then enters in a conver- gent (having a minimum diameter of 4 mm which corresponds to a sulphate speed of 30 m/s) in which the high turbulence completes the coagulation.

Table 1 reports the comparison of the fibre distribution for the various products; as can be noted, with the ejection fibres there is a high percentage of fine particles (80%) which reduces when passing to the spinneret and the tubular. The di¬ stribution curve is also different for these two fibre making device: very narrow for the tubular (90% of the particles betwe¬ en 100 and 200 mesh) , larger for the spinneret. This size distribution, combined with the particle form

(similar to fibres with a marked form ratio such as for tubular; with high film content, furled and without a preferred direction in the case of the spinneret) is responsible for the different behaviour of the two products in the paper preparation together with the cellulose fibres. In fact it has been experimentally verified that the products obtained from the tubular fibre ma¬ king device does not give rise to problems (of moulding or de¬ siccation) in the preparation of sheets in the laboratory while the use of the product from the spinneret, starting from a cer- tain percentage, gives sheets with surface defects and with a tendency to stick to the sheet forming plate.

Table 2 reports the percentage of starch retained on the sheet of paper prepared in the laboratory with the Rapid-Koethen apparatus, after dispersion of the cellulose - starch material paste (at 10 % of the latter) in the pulper for 2 hours at 3000 rpm at ambient temperature. As noted the highest retention is with the product from the tubular fibre making device.

Table 3 finally highlights the behaviour of the two diffe¬ rent products when filtered from the slurry after the coagulation and washing until the ammonium sulphate has been eliminated, the concentrations of the slurry and the maturing time being equal. As shown the products obtained from the tubu¬ lar fibre making device show a double productivity with respect to those of the spinneret. Moreover another subject of the present invention are the starch fibres obtainable through the previously described method that present the characteristic of having a solubility of less than 2% and a dimension distribution as such of 90% has a dimen¬ sion such as to enter in the range of from 100 to 200 mesh, after classification by the Bauer-McNett apparatus.

Table 1- SRC Distribution with various fibre making devices

Table 2 - Retention of starch fibres/fibrids in the paper

Table 3 - Filtering capacity of various starch fibres/fibrids

Claims

1. Process for the preparation of fibres of starch material by extrusion of an aqueous dispersion or solution of starch material in a flow of saline coagulating agent, characteri- sed in that it comprises the operations of : extruding the dispersion or aqueous solution through a microporous tubular wall in a annular chamber circu¬ larly ringed with said microporous wall in such a way to obtain an extrusion flux of the starch material which surrounds the said tubular walls; carrying out the coagulation of the extrusion by feed¬ ing a flow of coagulation agent in said annular cham¬ ber parallel to the extrusion surface.

2. Process according to claim 1 in which said microporous wall presents holes having at least a section with an average diameter comprised between 10 and 500 microns and a density of extrusion surface holes comprised between 4 and 0.05 holes/mm².

3. Process according to claim 2 in which the extrusion through the holes on said microporous wall is carried out from a narrow inlet section, having a opening size of from 10 to 500 microns to a larger outlet having a opening size grea¬ ter than the said narrow section and with a draw ratio com¬ prised between 100 and 1000.

4. Process according to claim 2 in which the extrusion through the holes on said microporous wall is carried out from a larger inlet section to a narrow outlet section having a opening size of from 10 to 500 microns and less than the size of opening of said larger section and with a draw ra- ' tio comprised between 1 and 150.

5. Process according to any of the previous claims 1 to 4 in which the standing time of the starch material in said an¬ nular chamber is comprised between 5 and 15 milliseconds.

6. Fibre making device, particularly for the production of starch fibres through extrusion of a flow of starch material in a flow of coagulating agent characterised in that it comprises: a tubular body (14) comprising first means of entry (16, 31) for feeding the flux of starch material, - a feeding chamber for the starch material (21, 32) connecting the said first means of entry, an annular outlet chamber (23, 34) of the starch mate¬ rial, a tubular element (22, 33) with porous walls coaxially arranged with said outlet chamber (23, 34) and inter¬ posed between the latter and the feeding chamber (21, 32) , adapted to extrude through the said porous walls in said outlet chamber a variety of threads of starch material, - a second means of entry (16, 36) connecting the said outlet chamber for feeding a coagulating agent flow, and means of discharge (17, 30) arranged downstream from and connected to the annular outlet chamber.

7. Fibre making device according to claim 6 characterised in that said tubular element with porous wall (22, 33) is con¬ stituted of a sintered metal having pores with a distribu¬ tion comprised between 10 and 500 microns.

8. Fibre making device according to claim 6 characterised in that said tubular element (22, 33) with microporous wall is a cylindrical element with holey walls having a plurality of radially passing holes (24, 25) having at least one nar¬ row section with a dimension comprised between 10 and 500 microns.

9. Device according to claim 7 or 8 in which said pores pre¬ sent an area density on the extrusion surface of from 4 to 0.05 holes/mm².

10. Fibre making device according to claim 8 in which said an¬ nular outlet chamber (23) of the starch material is coaxial and radially external to said feeding chamber (21) .

11. Device according to claim 10 in which said radial holes comprise a section (24) connecting with the feeding chamber (21) having a size of opening comprised between 10 and 500 microns and a section (25) connecting with the said outlet chamber having the size of opening larger to the size of opening of said section (24) .

12. Device according to claim 8 in which said annular outlet chamber (34) is coaxial and radially internal to the said feeding chamber (32) .

13. Device according to claim 12 in which said radial holes comprise a section (24) connecting with the outlet chamber

(34) having a size of opening comprised between 10 and 500 microns and a section (25) connecting with the feeding chamber (32) having a size of opening larger to the size of opening of said section (24) .

14. Starch fibres obtained through a process according to any of the claims 1 to 5 hav a solubility of less than 2% and size distribution such for which 90% of the fibres have a dimension such as to enter in the range of from 100 to 200 mesh, after classification by the Bauer-McNett apparatus.