WO1989000888A1

WO1989000888A1 - An apparatus for extruding and drying moist bulk material, e.g. organic waste material

Info

Publication number: WO1989000888A1
Application number: PCT/DK1988/000128
Authority: WO
Inventors: Mogens Fritze; Gabriel Da Silva Pinto
Original assignee: Volund Miljoteknik AS
Current assignee: Volund Miljoteknik AS
Priority date: 1987-07-27
Filing date: 1988-07-27
Publication date: 1989-02-09
Anticipated expiration: 1990-01-27

Abstract

In a comminuting and extruding unit (12), in which the wet mass is extruded in the form of strings or cords (not shown) through a knife-and-disc assembly (43), said strings or cords from the latter's last perforated disc (44) moving downwardly into a drying chamber (54), through which hot flue gas for drying the strings or cords flows, the novel feature is that at least the last knife set (47) is shaped in such a manner, that it counteracts the centrifugal action due to the rotation of the knives, e.g. by having each knife (48) curved forward in the direction of rotation (49). With this arrangement, the extrusion pressure at different distances from the axis of rotation will be essentially constant, for which reason all strings or cords will be extruded with essentially the same velocity. This provides for a uniform treatment of the strings or cords without any risk of some of them being overheated and possibly catching fire, whilst others will be insufficiently dried and disturb the effect of e.g. a downstream drying drum (15).

Description

An apparatus for extruding and drying moist bulk mate¬ rial, e.g. organic waste material.

TECHNICAL FIELD

In its first aspect, the present invention relates to an apparatus of the kind set forth in the preamble of claim 1.

BACKGROUND ART

It has been proposed to use apparatus of the kind referred to in plant for drying organic waste mate- rial, e.g. so-called "bio-mass", the waste material being comminuted in the apparatus and extruded in the form of wet material strings or cords, that are brought into contact with hot gas, e.g. flue gas in the drying chamber below, in order to dry the mate- rial.

In apparatus of the kind referred to, experience has shown that it is of utmost importance whether they are capable of extruding all the material strings with essentially the same velocity. Thus it is important that the contact with the hot gas occurs during a certain interval of time, since contact through a too long interval may cause the material to become com¬ pletely dry and be charred or catch fire, whilst contact through a too short interval of time naturally leads to an incomplete drying of the material strings or cords, which in that case - due to their l arge water content and consequently soft consistency - may disturb the functioning of a downstream "post-dryer", e.g. in the form of a rotating drying drum.

A combination of knives and perforated discs , similar to those known from e.g. meat mincers , wil l at al l speeds of rotation, however, show a tendency towards moving material to the perifery, and this causes a dynamic pressure increase near the perifery, for which reason the material strings or cords in this region will be extruded with a somewhat higher velocity than closer to the centre of theperf orated disc. Thus , there is a not insignificant risk that the outer material strings, being extruded with a re l atively high velocity, wi l l be insufficiently dried, whi l st the strings or cords being extruded at the centre wil l become overheated and may catch fire or at least be charred .

DISCLOSURE OF THE INVENTION

The disadvantage referred to above is avoided in an apparatus, according to the present invention al so exhibiting the feature set forth in the characterizing cl ause of c l aim 1. With this arrangement, the effect referred to above is -counteracted, so that al l mate¬ rial strings or cords are extruded with substantial ly the same velocity and are dried evenly without char¬ ring or burning.

Cl aim 2 refers to an embodiment having been shown by practical trials to be wel l suited for achieving the desired effect. Preferably, the apparatus comprises a number of interchangeabl e knife sets with differing geometry, so that it is possible to choose a knife set with the optimal shape for each material.

In its second aspect, the present invention relates to an apparatus of the kind set forth in the preamble of claim 3.

Trials with such apparatus, which may e.g. constitute part of a larger plant for drying organic waste mate¬ rial, such as "bio-mass", have shown that the material strings or cords being extruded from the higher-level feeding and comminuting unit may come into contact with the, walls of the drying chamber and possibly adhere to these walls, where they gradually form an increasingly thick coating. The thickness of the coat¬ ing increases with increasing speed, since the proba¬ bility of the material strings coming into contact with it increases, and the drying chamber may even¬ tually be completely blocked and have to be opened and cleaned, naturally causing the process to be inter¬ rupted and a renewed and long-lasting starting-up procedure having to be run.

This disadvantage may be avoided with an apparatus, according to the present invention further being con¬ structed as indicated in the characterizing clause of claim 3. The perforated screen prevents the material strings or cords from coming into contact with the walls of the drying chamber, but due to the perfora¬ tions it allows the flow of the hot gas, the latter thus being able to dry the material strings or cords and leave them in all essentials without any hin¬ drance.

The extruded material strings or cords are, of course, most humid and hence most sticky immediately upon leaving the higher-level feeding and comminuting unit, whilst these strings or cords further down in the drying chamber, i.e. within the perforated screen, are considerab ly more dry and hence l ess or not at al l sticky. Based on this recognition the embodiment set forth in cl aim 4 is preferred, as this embodiment makes it possibl e for the higher part of the screen to lie at a safe distance from the relatively humid upper parts of the strings or cords, whil st the somewhat narrower lower part of the screen has a certain col ¬ lecting and control ling effect on the more dry parts, which may be advantageous with regard to the so-cal led "post-dryer" , e.g. a rotating drying drum, in which the final drying to complete dryness occurs.

The embodiment set forth in cl aim 5 provides for a large flow cross-section for the hot gas, at the same time as the horizontal orientation of the openings prevents the general ly vertical ly extending material strings or cords from penetrating through the ope¬ nings .

The construction of the drying chamber and the mutual dimensioning of the drying chamber and the perforated screen as set forth in c laim 6 have been shown in practice to provide an effective and uniform drying without risk of the extruded moist material strings or cords adhering to the screen.

In its third aspect, the present invention relates to an apparatus of the kind set forth in the preamble of claim 7.

In previous ly proposed apparatus of this kind, the drying drum has been completely or partly open at its inl et end in order to permit the introduction of the material to be dried, e.g. by using a feed gutter or chute protruding into the drying drum. This arrange- ment is the cause of a number of problems, not least with regard to the requisite sealing in order to prevent the hot gas from leaving the drum or from being "diluted" with external air, and will also increase the length of the apparatus, said length already being undesirably large due to the presence of the drum.

These problems are avoided in an apparatus, according to the present invention further being constructed as set forth in the characterizing clause of claim 7. With this arrangement it is achieved that the drying drum in its peripheral surface exhibits an inlet opening constantly being situated at the top and hence well suited for receiving the material, e.g. from an upstream "pre-dryer".

With the embodiment set f orth in c l aim 8 it is achieved that the movements of the closure flaps are caused by the force of gravity, providing, of course, a simple construction and reliable operation. Should, however, the material to be dried cause problems, such as by being more or less sticky, the force of gravity may in certain cases be insufficient, and in such cases the embodiment set forth in c l aim 9 is pre¬ ferred.

In an apparatus, in which the drying drum according to the present invention is equipped with the closure flaps related to in claims 7 to 9, the material to be dried wi l l be introduced from above in the drying drum, i. e. it fal l s down into the drum from the pre- vious processing unit. I f the material is. , sticky, there wil l thus be a certain risk that the material fal ling down through the upper, open closure f laps on the lower, closed closure f l aps with considerable force and adheres to the latter. This disadvantage may be avoided using the embodiment set forth in claim 10, in which the receiving means partly prevents the falling material from hitting the closed flaps below in the drum directly, partly is kept sufficiently hot for the material not tending to stick to the receiving means.

Preferred embodiments of the receiving means are set forth in claims 11 and 12.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained in more detail below with reference to the drawing, in which Figure 1 is a diagrammatic overall view of a plant for drying sludge, in which plant an appara- tus according to the invention constitutes a component part. Figure 2 is a diagrammatic sectional view showing those parts of the plant, in which the ex¬ trusion and a part of the drying of the moist material is effected,

Figure 3 shows a knife set for use in a press section shown in Fig. 3, and Figure 4 being a partial section at the line IV-IV in Fig. 2 shows important components in the inlet portion of the drying drum.

DESCRIPTION OF THE PREFERRED EMBODIMENT The following is a description referring to the func¬ tions of the various component parts of the sludge treatment plant shown in Fig. 1.

Sludge, e.g. in the form "bio-mass" with a water content of up to 90 per cent or more, is received in a receiving pit 1, that may be closed with a cover 2 and close to the bottom contains a screw conveyor 3 feed¬ ing a sludge pump 4. The sludge pump 4 delivers the sludge through a valve 5 and a flowmeter 6 as well as a further valve 8 to a first buffer tank 9 comprising weighing equipment 10.

By means of a feed worm 11 placed in the lower part of the first buffer tank 9, the sludge, in the worm itself being mixed with recirculated dried solid mate¬ rial from a cyclone 20, is conveyed to a press section 12, in which the sludge, having become somewhat more thick-flowing by the addition of the recirculated material and hence easier to shape, is shaped into cords or strings, which immediately after having been formed fall or flow downwards into a pre-drier 14, in which the strings are subjected to a partial drying by contact with hot recirculated flue gas from the plant's combustion furnace 33, the flue gases of which also are utilized in a known manner for heating a boiler 34, from which they are conducted to a chimney 36 through a smoke filter 35.

From the pre-dryer 14 the material, at this stage being partly dried, falls in the form of short pieces of string or cord downwards into a drying drum 15, the inlet 16 of which in a manner to be described in more detail below is connected to the outlet from the pre- dryer 14, and the outlet 17 of which opens, partly into a second buffer tank 18 for solid material, partly into a flue-gas recirculation conduit 28, that conducts the flue gas back to a suitable place in the furnace 33 through an induced-draft blower 29.

The dried material from the drying drum 15 having been collected in the second buffer tank 18 is blown by means of a ejector 19 with feeder and blower into the mixed-material inlet on the cyclone 20, in which the conveying air is separated out and conducted to the furnace 33 through a conveying-air conduit 31 as com¬ bustion-supporting air with the heat content it has received from the solid material in the second buffer tank 18, said material thereby being somewhat cooled after having been dried in the drying drum 15 with the hot flue gas flowing from the pre-dryer 14.

The cyclone 20 has two outlets for solid material: a recirculation outlet 21 and a discharge outlet 22. The recirculation outlet 21 constitutes a part of the connection referred to above between the cyclone 20 and the feed worm 11 in the press section 12, while the discharge outlet 22 may optionally deliver the dried solid material out from the plant, e.g. as shown to a lorry, or through a bucket conveyor 23 (or an¬ other suitable conveyor) to the combustion furnace 33, where it is introduced into the furnace hopper 37 through e.g. a supply worm 24.

During starting-up conditions and other abnormal ope¬ rating conditions material that has been insufficient¬ ly dried in the pre-dryer 14 and the drying drum 15 may be returned to the receiving pit 1 by means of a conveyor assembly 38,32.

The temperature in that part of the hot flue gas from the furnace 33 being removed through a flue-gas inlet 25 and a slide valve 26 may be lowered according to need by adding cooling air through a cooling-air inlet 27. This cooling air will, of course, end up as com- bustion-supporting air in the furnace 33, and thus will not cause any heat to be wasted.

In the plant shown it is also possible to supply sludge from the receiving pit 1 directly to the com¬ bustion furnace 33, this occurring through a valve 7 and a sludge introducing means 30 adapted to intro¬ duce, e.g. inject the sludge in the furnace hopper 37.

In addition to the flowmeter 6 and the weighing equip¬ ment 10, the plant also comprises various sensors and control means (not shown), so that the significant parameters, e.g. temperature, humidity, flow and weight may be sensed during the operation, which may be controlled in dependence of the parameters and using suitable functional programmes by influencing the various components of the plant and the material flows between them with a view to achieving the de¬ sired parameter values.

The moisture being given off from the sludge in the pre-dryer 14 and the drying drum 15 flows in the form of water vapour through the flue-gas recirculation conduit 28 to the combustion furnace 33 and from the furnace through the flue-gas channels of the boiler 34, the smoke filter 35 and the chimney 36 to atmos¬ phere. The heat content in this water vapour may to a greater or lesser extent be utilized by conducting the mixture of flue gas and water vapour through the primary side in a heat exchanger (not shown), through the secondary side of which flows the sludge from the receiving pit 1 or some other medium to be heated. Persons skilled in this art will be able to find the most suitable way of placing and constructing such a heat exchanger or a number of such, all depending of the operating conditions in each case.

The press section 12 and the pre-dryer 14 are now to be described with reference to Fig. 2, also showing the inlet to the drying drum 15. Thus, the upper part of Fig. 2 shows a worm press, the inlet 39 of which is connected to the feed worm 11 shown in Fig. 1, and which also comprises a cylindri¬ cal housing 40,41, in the first part 40 of which, in which the inlet 39 opens, is rotatably supported a knife-and-disc assembly 43 coaxially with the pressing worm 42, the last perforated disc 44 of said assembly 43 terminating the assembly downwardly. The press worm 43 is driven by a hollow drive shaft 45, in which is placed a drive shaft 46 for driving the knives in the knife-and-disc assembly 43 _r so that the two parts may be driven with mutually independent controllable speeds by means of suitable drive means (not shown), such as electric gear motors or the like.

The press section 12 is in principle constructed in the same manner as e.g. a meat-mincing machine, for which reason its detailed construction is not shown on the drawing, with a single exception. The exception consists in that the knife sets, or at least the knife set 47 placed closest to and cooperating with the last perforated disc 44, cf. Fig. 3, is/are constructed with a view to counteract the centrifugal action that occurs during the rotation of the knives and therefore otherwise would increase the pressure at a distance from the axis of rotation, so that more material would be extruded through the holes of the last perforated disc 44 at its periphery than in the region around its centre.

In the exemplary embodiment shown in Fig. 3, each knife 48 in the knife set 47 is bent forward in the direction of rotation 49, so that the knives 48 during their rotation so to speak "scoop" the material in towards the centre and thus counteract ^'the centrifugal action. It may be necessary to use knives with a greater or lesser curvature, depending on the operat¬ ing conditions, such as the speed of rotation, the viscosity of the thickened sludge etc., not forgetting that the adhesion of the sludge to the perforated discs may influence the dynamic conditions.

By choosing knife sets with knives having a suitable curvature in the forward direction it is possible to achieve a practically uniform extrusion pressure in all holes in the last perforated disc 44. For this reason, the extruded strings or cords will have mainly the same speed, and provided that the hot lue gas, with which they are dried, is distributed evently

• between them, they will also achieve substantially the same degree of drying at the exit from the pre-dryer 14. In the connection it should be noted that a too extensive heat exchange between the material strings and the flue gas may cause the material to catch fire or to be singed or charred, whilst an inferior degree of heat exchange will cause the drying to be insuffi- cient, so that the material will not get that more or less firm consistency which is a prerequisite for an effective final drying in the drying drum 15. An uneven heat exchange may thus in the worst case cause some of the material in the pre-dryer 14 to catch fire, whilst other parts of the material when leaving the pre-dryer still have the form of a more or less thick pulp, unsuitable for treatment in the drying drum 15, as the material may easily stick to the inside wall or other parts of the drying drum and form crusts or "cakes".

For the sake of completeness it should be mentioned that the knife set 47 in a manner otherwise known may consist of a common, generally cross-shaped body por¬ tion 50 with a non-circular whole 51 for insertion upon and driving by the drive shaft 46, whilst on the upstream and downstream side of each of the four arms 48 (the number may be larger or smaller) there are secured hard-metal edges 48a for cooperation with the perforated disc or discs. By separately altering the speed of the knife set or knife sets in the knife-and- disc assembly 43 and the speed of the pressing worm 42 it is possible to achieve a better adaptation to different materials than would have been possible, if the speeds of these components where mutually depen¬ dent, e.g. by being driven from one and the same drive shaft. The force with which the knives are pressed against the perforated discs may be varied in a known manner by means of suitable means (not shown) with a view to achieving a suitable compromise between on the one hand the desire for an efficient cutting of any fibres in the material and on the other hand a limited consumption of mechanical energy, including heating of the knives and the perforated discs beyond their per- mitted maximum temperatures. In this connection it may be mentioned that under certain circumstances it may be expedient to equip the press section 12, especially its knife-and-disc assembly 43, with suitable cooling means, e.g. a water jacket, that especially cools the region at the last perforated disc 44.

As it is also shown in Fig.2, the pre-dryer 14 con¬ sists of a fire-proof or heat-proof housing 52, in the cealing of which there is an opening 53, in which the lower part of the knife-and-disc assembly 43 is placed with the lower side of the last perforated disc 44 generally aligned with the lower side of the ceiling in the housing 52. The cooling jacket (not shown) referred to above may be situated in the space between the wall of the opening 53 and the outside of the second housing section 41 of the press section 12. Within the housing 52 there is formed a generally circularly cylindrical drying chamber 54, that is open at the bottom and which at one side is connected to a gas- inlet 55, the latter in turn¹, cf. Fig. 1, being adapted to receive hot flue gas from the furnace 33 through the flue-gas inlet 25 and the valve 26. Depen¬ ding from the ceiling is a perforated screen 56, preferably in the form of a cylindrical or slightly conically downwardly converging curved sheet of heat- resistant stainless steel with horizontal slits 57, of which only a small number is shown in Fig. 2. The screen 56 surrounds the region, in which the material being extruded from the last perforated disc 44 enters the drying chamber 54 from above.

During the operation strings or cords of relatively wet material, but still in a condition making it possible to shape it, are extruded from the last perforated disc 44 and continue moving downward through the drying chamber 54 within the screen 56, whilst hot flue gas flows in through the inlet 55 and through the screen 56 acting as a pressure distributor and out below, where the drying chamber 54 is open, or rather connected to the inlet 16 in the drying drum 15; in this connection, the latter may be described as a "post-dryer". The screen 56 allows the hot flue gas to flow inwardly and partially dry the material strings, the latter eventually being ruptured by their own weight and falling down into the drying drum 15, whilst the screen 56 at the same time prevents any gas vortices, that may easily form in the drying chamber 54, from throwing pieces of half-moist material out¬ wardly against the walls of the housing 52, where otherwise they cound readily form "cakes" and prevent further operation. By using horizontal slits 57 it is possible to achieve a large flow area for the flue gas, at the same time as the generally vertically extending material strings are prevented from getting through the screen 56. The hot flue gas will heat the screen 56, and it is assumed that any moist material 5 strings or pieces of such coming into contact with the screen will be repelled from it in a similar way as drops of water from a hot surface, cf. the so-called Leidenfrost-effect. Thus, experience has shown that no coating is formed on the screen 56.

10 Trials have shown that a ratio between the diameters of the drying chamber 54 and the screen 56 in the order of magnitude 1,3:1 results in a uniform distri¬ bution of the hot flue gas with which the material strings from the press section 12 are dried, without

^'15 the housing 52 of the drying chamber 54 having to be disproportionately large.

As mentioned above, the drying drum 15, being only partially shown in Figs. 2 and 4, comprises the inlet 16 and the outlet 17, the latter only being shown in 20 Fig. 1. The drum 15 is in a known manner supported for rotation about its own longitudinal axis, e.g. by means of rollers 58 and a shaft bearing 59, cooperat¬ ing with a rolling hoop 60 and a shaft 61 respective¬ ly. The rolling hoop 60 is in a known manner placed to _.25 encircle the drum 15, whilst the shaft 61 is secured to the inside of the drum by means of three "spiders" 62,63 and 64 and projects outwardly through a statio¬ nary inlet housing 65, which may be adapted to support the drying chamber housing 52.

30 The inlet housing 65 is open above, so that the space within the screen 56 acting as a pressure distributor is in direct communication with the internal space of the drying drum 15. The rotating drying drum 15 is likewise open above whilst being closed below, and this is only possible because it is equipped with a circular assembly of closure flaps 66, which in the exemplary embodiment shown are freely pivoted about axes 67 at the periphery of the drying drum 15 and parallel with each other and with the longitudinal axis of the drum. Each flap 66 reaches a distance beyond the axis 67 of the next flap, and when the drum is turned in the direction of rotation 68 shown in Fig. 4, the flaps 66 will be open at the top and closed at the bottom. In this manner it is possible to avoid the problems with introducing the material from the end of the drum.

In the exemplary embodiment shown, the closure flaps 66 are, as stated above, freely pivoted about the axes 67, but in certain cases, especially when treating sludge having a tendency to stick to solid surfaces, it may be expedient to move the closure flaps in a positive manner. For this purpose it is possible to use e.g. a suitably curved control track (not shown) extending about the drum and cooperating with a con¬ trol arm (not shown) on each flap, said control arms preferably being resilient in order to avoid over¬ loading of the control mechanism, if a flap were to be blocked temporarily by a material lump or the like.

If no further measures were taken, pieces of partly dried material strings from the press section 12 would fall through a considerable height and hence with considerable force down against the lower, closed closure flaps 66 or the shafts 61. In unfavourable circumstances, this may lead to "caking" on and conse¬ quent blocking of the closure flaps 66 and a certain amount of "caking" on the shaft 61, the tangential speed of which by necessity is comparatively low, and which may therefore easily be cooled by a large lump or blob of material. This risk is avoided by using a guard 69 placed coaxially with the shaft 61, said guard in the exemplary embodiment shown being secured to the "spiders" 62 and 63 in such a manner, that the hot gas also has access to its internal space. The guard 69 is preferably made of heat-resistant sheet material with a high thermal conductivity, ^"so that it may be kept hot and "repel" the falling material strings in a similar manner to the screen 56 in the drying chamber 54. The guard 69 will, of course, have a considerably higher tangential speed than the shaft 61, for which reason the falling lumps of material will be distributed with a considerably lower distri- bution density in the direction of rotation. Thus, each blob or lump will fall in a new place, which will have had time to have been sufficiently heated by the hot gas from the previous time it was hit by a lump or blob or material.

The partly dried string pieces, lumps or blobs of material thus first falling down onto the guard 69 and falling free of the latter in a somewhat consolidated condition will be transported further through the drying drum 15 by the hot flue gas having flowed from the inlet 55 through the drying chamber 54 of the pre- dryer 14 possibly assisted by a small inclination (not shown) of the drum towards the outlet 17 situated to the right, but not shown in Fig. 2, cf. Fig. 1.

Since the upwardly facing surface on the guard 69, which is hit by the falling material string pieces,or blobs or lumps, lies at a considerably higher level than the bottom of the drying drum 15, the height of fall becomes correspondingly smaller, for which reason the deformation is reduced to a corresponding extent, and the risk of the material blobs so to speak being "slapped on" is reduced to a corresponding degree.

In a manner not shown but known, the drying drum 15 may be equipped with internal guide ribs or blades with a view to increasing the heat exchange and/or movement of the material being dried towards to outlet 17.

Claims

C L A I M S

1. An apparatus for drying organic or inorganic waste products or materials in the form of a paste, pulp or fiber-like mass consisting of water-soluble and water-insoluble particles and water, said appara¬ tus being of the kind comprising a) a feed worm (42), b) a comminuting unit connected to the outlet of the feed worm and consisting of at least one perfo- rated disc (44) and at least one rotating knife set (47) cooperating with each perforated disc, said knife set or sets being secured to a substan¬ tially vertical drive shaft (46), and c) placed below the perforated disc or the lowest perforated disc (44) a drying chamber (54) with inlet openings (55) and outlet openings for hot gas flowing in heat-exchanging contact with the material strings or cords being extruded through the perforated disc or the lowest perforated disc (44), c h a r a c t e r i z e d in d) that at least that knife set (47) cooperating with the perforated disc or the lowermost perforated disc (44) on the upstream side of the latter is shaped in such a manner, that when rotating it tends to move the material being treated radi¬ ally inward towards the drive shaft (46).

2. An apparatus according to claim 1, c h a r a c¬ t e r i z e d in that the knives (48) of the knife set (47) mentioned (in item d) above) extend radially outwardly and forwardly in the direction of rotation (49).

3. An apparatus for drying organic or inorganic waste products or materials in the form of a paste, pulp or fibre-like mass consisting of water-soluble and water-insoluble particles and water, said appara¬ tus being of the kind comprising a) a feeding and comminuting unit (12) being adapted to extrude the material downwards in the form of a number of strings or cords extending side by side, and b) placed below said feeding and comminuting unit (12) a drying chamber (54) shaped generally co¬ axially with the outlet of the feeding and commi¬ nuting unit (12) and having inlet openings (55) and outlet openings for hot gas flowing in heat- exchanging contact with the material strings or cords extruded by the feeding and comminuting unit (12), c h a r a c t e r i z e d by c) a perforated screen (56) placed in the drying chamber (54) and essentially coaxial therewith, said screen in its inlet region extending outside the region, through which the material strings or cords arriving from the feeding and comminuting unit enter the drying chamber (54).

4. An apparatus according to claim 3, c h a r a c- t e r i z e d in that the perforated screen (56) converges slightly in a downward direction.

5. An apparatus according to claim 3 or 4, c h a¬ r a c t e r i z e d in that the perforated screen (56) consists of sheet material with generally tangen- tially extending, slit-shaped openings (57).

6. An apparatus according to any one or any of the claims 3 to 5, c h a r a c t e r i z e d in a) that the drying chamber (54) is generally circu- larly cylindrical with a vertical axis, and b) that the ratio between the diameter of the drying chamber (54) and the diameter of the perforated screen (56) is between 1,4:1 and 1,2:1, preferably approximately 1,3:1.

7. An apparatus for drying strings, lumps or blobs of organic or inorganic waste products or materials in the form of a paste, a pulp or a fibre-like mass consisting of water-soluble and water-insoluble par- tides and water, said apparatus being of the kind comprising a) a drying drum (15) adapted to rotate about a hori¬ zontal or slightly inclined axis, b) an inlet portion (16) adapted to receive the mate- rial to be dried and conducted into the drying drum, c) an exit portion (17) adapted to receive the dried material from the drying drum and conduct same further to a desired destination, and d) means for conducting a hot gas through the drying drum in order to dry the material, c h a r a c t e r i z e d in e) that the inlet end (16) of the drying drum (15) is equipped with a number of flaps (66) each being hinged or adapted to be turned about an axis (67)

.situated at the periphery of the drum an extending essentially parallel with the axis of rotation of the drum between a closed position (Fig. 4 below), in which the free edge of each flap (66) abuts . against the inside of the next flap, and an open position (Fig. 4 above), in which each flap extends essentially vertically downwards from its axis (67).

8. An apparatus according to claim 7, c h a r a c- t e r i z e d in that the flaps (66) are freely pi¬ voted about said axes (67).

9. An apparatus according to claim 7, c h a r a c¬ t e r i z e d in that the flaps (66) are adapted to be moved positively, e.g. by means of a curved control track.

10. An apparatus according to any one or any of the claims 7, 8 and 9, c h a r a c t e r i z e d by a receiving means (69) for the material falling down through the open flaps (66) (Fig. 4 above), said receiving means (69) being placed in the inlet end (16) of the drying drum (15) radially within the flaps (66) and having a not insignificant surface area, across which the hot gas may flow.

11. An apparatus according to claim 10, c h a ¬ r a c t e r i z e d in that the receiving means con¬ sists of an essentially circularly cylindrical guard (69) of sheet material being coaxial with the drying drum (15).

12. An apparatus according to claim 11, c h a ¬ r a c t e r i z e d in that the guard (69) is open at both ends.