ROBERTSSON T (SE)
KARLSSON S (SE)
BILLGREN P (SE)
ROBERTSSON T (SE)
KARLSSON S (SE)
| DE1814471A1 | 1970-06-25 | |||
| SE340592B | 1971-11-22 | |||
| DE276664C | 1914-07-17 | |||
| DE2532689B2 | 1977-05-05 |
| 1. | Method of separating a finegrained closepacked solid material from a liquid with which the finegrained material is saturated, c h a r a c t e r i z e d in that the mass is collected in a 'container (2); that the mass collected in the container is heated from underneath so that the liquid, starting in the lowest part of the mass, is evaporated in an evaporation zone forcing the liquid in the higher parts of the mass upwards through the mass towards the surface where the liquid is collected and is taken off from the container. |
| 2. | Method according to claim 1, c h a r a c t e r i z e d in that the evaporation zone successively is caused to proceed upwards in the mass under continued heating of the container from underneath. |
| 3. | Method according to any of claims 1 and 2, c h a r a c ¬ t e r i z e d in that gas which is obtained above the surface of the mass is fed off from the container which is closed so that air cannot enter into the container. |
| 4. | Method according to any of claims 13, c h a r a c t e r i z e d in that the container is heated from underneath to such a high extent that the gas pressure P in the lower part of the mass will be higher than the hydrostatic pressure performed by the higher parts of the mass, which condition may be expressed atm + J liquid . g . (h, + h ') ( P P + V mass • 1 2 gas atm • ' g . h2 + liquid . g . h^ where J indicates density, g indicates gravity acceleration, h_ indicates the height of liquid collected on the surface of the mass and h„ indicates the height of the liquid saturated parts of the mass. |
| 5. | Method accordingto claim 4, c h a r a c t e r i z e d in that the container is heated to such an extent that maximun liquid "BlREA OMPl flow rate is achieved, said maximum liquid flow rate being determined by the combination of the expression of claim 4 with the expresseion _1 __ A _C E dt A * *** ' dy where A = horizontel section area __= permeability r* = pressure gradinet in a vertical direction . |
| 6. | Method according to any of claims 15, c h a r a c t e r i z e d in that the grain size and the closepacking degree in the mass is such that gas bubbles cannot essentially proceed upwards and liquid drops cannot essentially fall downwards in the mass during the liquid separation. |
| 7. | Method according to claim 6, c h a r a c t e r i z e d in that the closepacking degree is such that the smallest breadth of the channels between the particles do not exceed the double critical radius r . determined through the expression where r = the radius of the bubble t_,~ surface tension liquid gas . |
| 8. | Method according to any of claims 17, c h a r a c t e r i z e d • in that dried material successively is discharged from the lower part of the container while simultaneously new liquid containing material is supplied to the container from above, the heat being supplied to the lower part of the container at such a rate that the upward directed force upon the liquid phase in the container essentially keeps pace with the supply of new moist mass to the container as well as with the discharge of dried material from the lower part of the container. |
| 9. | Method according to claim 8, c h a r a c t e r i z e d in that the dried material is discharged to a chamber (31) which is kept under a certain overpressure. |
| 10. | Method according to any of claims 19, c h a r a c t e r i z e d in that the volume flow of liquid in the upwards direction is increased by increasing the gas pressure gradient in the vertical direction in the mass by increasing the difference between the gas pressure P in the lower part of the mass and the gas pressure gas on the surface of the mass in addition to what is achieved by the said heating. |
| 11. | Method according to claim 10, c h a r a c t e r i z e d in that the pressure gradient is increased by an outer pressure applied to the mass from above, said pressure being applied essentially v only upon the solid phase. |
| 12. | Method according to claim 11, c h a r a c t e r i z e d in that the said outer pressure is applied by means of a screen cloth or corresponding means which is permeable for the liquid but not for the solid phase, said screen cloth or corresponding means being pressed against the solid phase from above. |
| 13. | Method according to claim 10, c h a r a c t e r i z e d in that the pressure gradient is increased by vacuum suction from the space of the container above the surface of the mass. |
| 14. | Method according to any of the preceding claims, c h r a c ¬ t e r i z e d in that the liquid entirely or partly is combustible and that gases which are formed during the process are fed off and burnt. |
| 15. | Method according to claim 14, c h a r a c t e r i z e d in that the gases are utilized for the heating of the container. 'BU EAT * OMPI . |
| 16. | Method according to any of the preceding claims, c h a r a c ¬ t e r i z e d in that conventional oil drums are used as containers, said oil drums together when their dry content being charged in a metallurgical melting furnace after the liquid substantially has been separated from the solid material. j O f W AMENDED CLAIMS (received by the International Bureau on 20 November 1978 (20.11.78)) 1 Method of separating a finegrained closepacked solid material from a liquid with which the finegrained material is saturated, c h a r a c t e r i z e d in that the mass is collected in a container (2); that the mass collected in the container is heated from underneath so that the liquid, starting in the lowest part of the mass, is evaporated in an evaporation zone forcing the liquid in the higher parts of the mass upwards through the mass towards the surface where the liquid is collected and is taken off from the container; that the evaporation zone successievely is caused to proceed upwards in the mass under con¬ tinued heating of the container from underneath? and that gas which is obtained above the surface of the mass is fed off from the container which is closed so that air cannot enter into the container. |
| 17. | 2 Method according to claim 1, c h a r a c t e r i z e d in that the container is heated from underneath to such a high extent that the gas pressure P in the lower part of the mass will be higher than the hydrostatic pressure performed by the higher parts of the mass, which condition may be expressed P atm + 5 liquid . g . (h, + h„) < P < P + ? mass ^ ° 1 2 gas atm g . h + J liquid . g . h. where j indicates density, g indicates gravity acceleration, h. indicates the height of liquid collected on the surface of the mass and h_ indicates the height of the liquid saturated parts of the mass. |
| 18. | 3 Method according to claim 2, c h a r a c t e r i z e d in that the container is heated to such an extent that maximum liquid flow rate is achieved, said maximum liquid flow rate being determined by the combination of the expression of claim 4 with the expression where A = horizontel section area βC * permeability — = pressure gradient in a vertical direction 4 Method according to any of claims 13, c h a r a c t e r i z e d in that the grain size and the closepacking degree in the mass is such that gas bubbles cannot essentially proceed upwards and liquid drops cannot essentially fall downwards in the mass during the liquid separation. |
| 19. | 5 Method according to claim 4, c h a r a c t e r i z e d in that the closepacking degree is such that the smallest breadth of the channels between the particles do not exceed the double critical radius determined through the expression crit cri Ltt ^ where r = the radius of the bubble β = surface tension liquid gas 6 Method according to any of claims 15, c h a r a c t e r i z e d in that dried material successively is discharged from the lower part of the container while simultaneously new liquid containing material is supplied to the container from above, the heat being supplied to the lower part of the container at such a rate that the upward directed force upon the liquid phase in the container essentially keeps pacewith the supply of new moist mass to the container as well as with the discharge of dried material from the lower part of the container. |
| 20. | 7 Method according to claim 6, c h a r a c t e r i z e d in that the dried material is dicha ged to a chamber (31) which is kept under a certain overpressure. |
| 21. | 8 Method according to any of claims 17, c h a r a c t e r i z e d in that the volume flow of liquid in the upwards direction is increased by increasing the gas pressure gradient in the vertical, ■direction in the mass by increasing the difference between the gas pressure P in the lower part of the mass and the gas pressure on the surface of the mass in addition to what is achieved by the said heating. |
| 22. | 9 Method according to claim 8, c h a r a c t e r i z e d in that the pressure gradient is increased by an outer pressure applied to the mass from above, said pressure being applied essentially only upon the solid phase. |
| 23. | 10 Method according to claim 9, c h a r a c t e r i z e d in that the said outer pressure is applied by means of a screen cloth or corresponding means which is permeable for the liquid but not for the solid phase, said screen cloth or corresponding means being pressed against the solid phase from above. |
| 24. | 11 Method according to claim 9, c h a r a c t e r i z e d in that the pressure gradient is increased by vacuum suction from the space of the container above the surface of the mass. |
| 25. | 12 Method according to any of the preceeding claims, c h a r a c t e r i z e d in that the liquid entirely or partly is combustible and that gases which are formed during the process are fed off and burnt. |
| 26. | 13 Method according to claim 12, c h a r a c t e r i z e d in that the gases are utilized for the heating of the container. |
| 27. | 14 Method according to any of the preceding claims, c h a r a c ¬ t e r i z e d in that conventional oil drums are used as containers, said oil drums together with their dry content being charged in a metallurgical melting furnace after the liquid substantially has been separated from the solid material. STATEMENTUNDERARTICLE19 With reference to the International Search. Report issued by the International Searching Authority (Swedish Patent Office) and to the Notification of Transmittal of the International Search Report mailed by the International Searching Authority 19781108, we are herewith filing amended claims according to Article 19(1) and Rule 46.1. In the new claims, which are attached to this letter, amended Claim 1 is a combination of originally filed Claims 13, while new Claims 214 correspond to orginally filed Claims 416. Through the amendments, Claim 1 has been restricted from prior art as set forth in the Search. Report. The amendments should not have any impact on the description or on the drawings. A copy of this letter has been forwarded to the Swedish Patent Office. /^URE OMP. |
The present invention relates to a method of separating a fine¬ grained close-packed solid material from a liquid with which the fine-grained material is saturated. The fine-grained material in some cases may consist of metal compounds substantially without any value, e.g. certain pickling bath products which are harmless as far as the environment is concerned but where the liquid phase can be highly toxic or from other reasons harmful to the environment, in other cases the fine-grained material can be more valuable so that it is desired to take care of it. This concerns as well certain kinds of pickling bath deposits as certain metal oxides in the form of powder from venturi-filters in metallurgical plants. Another material which advantageously can be treated according to the invention is obtained at the machining of objects of high speed steel and other high grade alloys. In such machining there is often used grinding operations in combination with cutting oils or other liquids or chemicals. At these operations there is formed very fine-grained grindings soaked in a liquid phase, such as oil or an oil emulsion. In the present art it does not exist any practical useful method of taking care of these grindings, although they represent a high value in the form of alloying metals. To the contrary the taking care of this material has created a complicated waste- and environmental problem.
It is often desirable to be able to remelt the metal content in waste material of the kind above described in metallurgical furnaces. This is, however, impossible according to the present art in the first place because of the following three reasons:
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OMPI
if the material is charged freely in the furnace, liquid will pour down into the bottom of the furnace and cool and soak the bottom so that crackings will be formed in the lining of the furnace;
if the liquid saturated material is charged in the furnace in closed containers the volatile constituents will be evaporated which under unfavourable conditions may cause puffs and explosions;
the large liquid volume brings about a lot of smoke which has an unfavourable impact on inner and outer environment.
It is therefore a first object of the present invention to separate a fine-grained close-packed solid material from a liquid with which the fine-grained material is saturated so that the solid material can be taken care of in a desired way. More particularly, there is an object in the case when the liquid phase is toxic or in any other way harmful to the environment, while the solid phase is harmless from environmental point of view but on the other hand without any important economical value, to be able to dump the solid material together with other waste material. In the case when the solid phase to the contrary contains valuable elements, e.g. valuable alloying elements, it is an object of the invention to remove the liquid so that the solid material can be taken care of, e.g. by charging the material on metallurgical furnaces. Still another object also is to be able to separate the liquid from the liquid saturated mass in order to be able to re-use the liquid. These and other objects can be achieved therein that the mass is collected in a container; that the mass collected in the container is heated from underneath so that the liquid, starting in the lower portion of the mixture, is evaporated in an evaporation zone and forces the liquid in the upper parts of the mass upwards through the mass towards the surface where the liquid is collected and wherefrom the liquid is removed from the container. In order that the liquid, when it is combustible, will not be set fire to, the
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heating is carried out without any air admission. During the latter stage of the process the remaining liquid will be so hot that some smoke can be developed when the liquid consists or contains oil or corresponding agents. This smoke or gas can be burnt by means of a safety flame or be used for heating the container.
The method of the invention is based upon the following principles:
a) a sufficiently great gas pressure shall be established in the lower portions of the paste-like mass by heating the container from underneath in order that the gas will force the liquid up¬ wards at a certain desired rate. This condition may be expressed
_. - •* ■ ■ • &<» where the following definitions apply
-j— = Volume flow of liquid
A = Horizontal section area - = permeability
~ = pressure gradient in vertical direction
b) The grain size, or more correctly the close-packing degree, shall be such that gas bubbles may not essentially proceed upwards and liquid drops not essentially fall downwards in the space between the grains (the particles). Agitation of the liquid in other words shall be prevented by the capillary forces in the fine divided mixture. It is herein presumed that a gas bubble having a tendency to proceed upwards is subjected to two forces in the critical moment when it shall be cut off, namely the net lift which is directed upwards and the surface tension which works as a downwards directed force. A liquid drop which is being c.ut and will fall downwards is influenced in an inversely way. If the channels between the particles in the mixture are smaller than the radius of the critical gas bubbles and the
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OMPl
drops respectively, agitation of the liquid is consequently prevented.
/ _-___! _
The force of gravity F = -=- _ι r . (J liquid -.fgas) ' . g Surface tention force F = 0_ . 2 i r
At the critical smallest bubble- or drop radius F - ~ F .
The critical smallest bubble- or drop radius, r . __ . . which crit defines the desired lowest close-packing degree, hence is
where the following definitions apply r = bubble radius _•_= surface tention liquid - gas $ = density g = acceleration of gravity
The close-packing degree thus should be such that the smallest breadth of the channels between the particels will not exceed the double critical radius r . . crit
c) The gas pressure P in the bottom of the vessel shall be gas higher than the hydrostatic pressure of the liquid but lower than the hydrostatic pressure of the entire bulk of the paste¬ like mixture above the gas volume. Otherwise gas will proceed through the mixture from lower regions. This gives the expression:
P .._, + liquid . g . (h, + h_) P <P __ + mixture . atm ^ 1 2 gas atm g • h_ +•> liquid . g . h where h. -= the height of the liquid volume collected on the surface of the mass h„ = the height of the liquid saturated mass
OMP
By combining the expression (3) with the equation (1) the " maximum separation rate can be calculated.
According to a developed embodiment of the invention it is also offered a method of separating liquid from the'solid material through an essentially continous process. It is also an object of this embodiment to obtain a still further improved economy as far as heat energy is concerned and preferably also to obtain a higher cleanliness of the products through an essentially continous counter-current process.
Further it is an object of still another developed embodiment of the invention further to increase the "maximal separation rate. According to the embodiment of the invention this is carried out by increasing the gas pressure gradient in the vertical direction in the mass by increasing the difference between the gas pressure P in the lower portion of the mass and the pressure upon the solid phase on the upper surface of the mass to exceed what is obtained by the heating. This may be obtained by applying an outer pressure P upon the solid phase from above, e.g. by pressing a screen cloth against the.solid phase, said screen cloth being permeable to the liquid but not to the solid phase. Another method is vacuum suction on the top of the container, i.e. vacuum suction from the space of the container above the surface of the mass. These two alternatives may also be combined. Equation (3) therefore can be extended according to (4):
+5 J mass -
By one or both of these means the pressure difference between the gas pressure P in the evaporation zone in the lower portion of the mass and the pressure upon the solid phase on the surface of the mass will increase. At the same time the evaporation point will be influenced in a favourable direction. • '
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The invention will now be more .closely described by reference to some preferred embodiments. Herein there will be referred to the accompanying drawings, which schematically illustrate the principles of the embodiments.
Fig. 1 shows a container containing a liquid containing mass, from which the liquid phase is being separated from the solid phase.-
Fig. 2 shows a row of containers the liquid content of which is being separated.
Fig. 3 illustrates a developed embodiment of the invention for continous liquid separation.
Fig. 4 illustrates another developed embodiment of the invention and shows a container containing a liquid containing mass, where the liquid phase is being separated from the solid phase; and
Fig. 5 shows the pressure conditions in the container of the embodiment of the invention illustrated in Fig. 4 and in the embodiment illustrated in Fig. 1, respectively. '
Referring first to Fig. 1 a jelly mass of metal containing particl soaked in a liquid is generally shown as 1. In the case when the metal containing particles consist of metal oxides obtained from venturi filters from metallurgical furnaces or of metal compounds from pickling baths the particles normally have "microscopic" sizes in the order of 10A-I_ or smaller. In the case when the particles consist of grindings the majority of the grindings have sizes varying from very small, comparatively round particles havi sizes in the order of about 0 5Wra to somewhat larger, usually elongated particles having sizes of about 150 x 20C-__. This mixtu of fine particles and liquid proyide.s. a wass.1 haying a charge- .
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teristic jelly consistency. The mass 1 is collected in a container 2 which according to the embodiment consists of a conventional , oil drum of the kind which is normally used as transport containers for the present "waste product". The container 2 is hermetically closed by a cover 3. A burner 4 is provided underneath the container for heating the content of the container 2. According to the illu¬ strated embodiment the burner is a gas burner which makes it possible in a practical way to take care of also combustible gases obtained at the process for the heating. Also other heating methods, of course, are conceivable, such as for example heating by means of an oil burner which has advantageously been used during per¬ formed experiments. Also electric heating principally is conceivable. Numeral 5 designates a conduit for the escape of combustible gas. A spillway is designated 6. Closing valves in the gas conduit and in the spillway 6 has been designated 7 and 8 respectively. Liquid which successively is collected on the surface of the mass has been designated 9.
Although this has been proven not to be necessary during normal conditions it is possible to separate the upper portion of the partially dried mass from the bottom and middle portions and transfer the former portion to a new container for continued expel of liquid. Herein it is possible to obtain a mass which is practically free from liquid.
The invention may be illustrated through the following examples.
Example 1
An oil drum containing 110 1 of a mass consisting of grindings of high speed steel soaked in cutting oil was heated from under¬ neath by means of an oil burner. The oil content in the container at the start of .the experiment was 32.5 weight percent. After 10 hours the oil content in the bottom portion of the vessel had decreased to less than 0.2%, i.e. this part of the container was practically free from oil. Also the centre of the mass essentially
had been de-oiled while the outer portions at a level indicated in Fig. 1 had been decreased to 4.2 and 2.4 % respectively. In the upper part of the mass the oil content in the centre was 17.03. and in the outer portions 9.5 and 15% respectively. The sampling points have been indicated in Fig. 1. At the top there was an oil bed which was drained off through the spillway 6. At the end of the de-oiling there was formed a hot oil smoke which was led off through the conduit 5 and was burnt in a safety flame.
A number of containers consisting of conventional oil drums were de-oiled in this way. Thereafter the containers with enclosures were melted in a 30 tons electric arc furnace in quantities up to 2 tons per charge. 1.5 tons per charge was shwon to be a suit¬ able quantity corresponding to 5% of the total charge of raw .materials. There were no tendencies to puffs, risks for accelera¬ ted lining wear or considerable development of smoke . .
Example 2
In this case the mass 1 consisted of sludge consisting • °f precipitated metal compounds. These very small particels substantially consisting of iron compounds. The liquid was an aqueous solution. This mass which contained about 45% water was heated in an oil drum according to the invention in the way illu- « strated in Fig. 1. The aqueous solution 9 which successively was collected on the upper surface was drained off, whereafter the essentially dried material pould be dumped together with other waste material without casuing any harmful impact on the environment. ■
Fig. 2 schematically shows a production plant for the carrying out of the method according to the embodiment of the invention illu¬ strated in Fig. 1. The containers 1 are arranged on a common hearth 10 as is shown in the figure. The separated liquid is conveyed through a conduit 11 to a collecting vessel 12. Gas which is formed at the separation of liquid is utilized in the case it is combustible according to the embodiment for the heating in the hearth 10 and is
for this purpose fed through a conduit 13.
Fig. 3 shows a plant for continous treatment according to the invention.
According to this embodiment of the method according to the invention the raw material has the form of a sludge having a very high content of liquid, for example a sludge containing pick¬ ling bath deposits. A plant shown in Fig. 3 therefore is provided with a settling basin 21 with charging devices 22 for new raw material. The basin 21 has an inclined bottom so that sediment in the first place will collect in the deepest.part of the basin. Suitable transport means may be provided for the transportation of sediment from other parts of the bottom of the basin. This has been schematically illustrated in Fig. 3 by a rake. The sludge in the basin 21 has been designated 24.
In the deepest part of the basin there is provided a container in the form of a shaft 25 in which the settled liquid containing mass successively is collected. Beneath the shaft 25 there is provided a heat-source 26. According to the embodiment this heat-source consists of a number of oil burners but it may also consist of gas burners, electrical heating means, means for the utilization of waste heat from other processes, or by combinations of the said kind of heat- sources.
A discharge device for the liquid phase has been designated 30. This device according to the embodiment. consists of a spillway. Also other discharge devices are conceivable, such as a suction device. In the bottom portion of the shaft - the container 25 - there is provided a discharge channel for material having a high content of dry substance. In the discharge channel 27 there are provided conveyer means, e.g. a worm conveyer 28, driven by a motor 29. The worm conveyor 28 also extends through the lower part of the shaft 25 so that the dry material in the bottom part of the shaft can be fed out through the
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channel 27. The channel 27 terminates in a reception chamber 31 which is subject to an »overpressure. This has symbolically been indicated in Fig. 3 through compressor 32. A container for separated dry material has been designated 34: In order to increase the liquid separation capacity in the shaft 25 the equipmen also may be provided with a filter press 33. In order further to increase the capacity it is also- possible to subject the shaft 25 to a vacuum from above. This, however, requires a modified design of the plant, which is not illustrated in Fig. 3.
The plant above briefly described is intended to operate in the following way. Through conduit 22 raw material in the form of a sludge is supplied to basin 21. The solid material settles and is collected in the shaft 25. The rake 23 or corresponding means is used when necessary. The shaft - the container 25 - is heated from underneath by means of the heating equipment 26. The liquid in the mass existing in the shaft 25 herein will be evaporated in the lower part of the shaft. Under condition that the criterias given in the preamble of this specification are satisfied the steam or other gases will force liquid upwards through the shaft. By adjusting the supply of heat dry material can be taken out essentially continous by means of the worm conveyor 28 through the discharge channel 27 while simultaneously new material is supplied to the shaft 25 from above. The force in the upwards direction developed by the ' zone evaporation upon the liquid in the mass thus is adjusted such that it keeps pace with the supply of new liquid through the continous supply of new moist.mass to the shaft 25 and to the discharge of dry material from the bottom part of the shaft. The bottom part of the shaft 25 at the same time is subjected to a certain over pressure' which prevents steam or other gases from.leaving the shaf through the discharge channel 27. It is recommended that the dry material is taken out from the chamber 31 through a feeding out valve in order that the process may be carried out entirely continous. It is also possible that the shaft 25 is provided with more than one discharge channels which can be run alternatingly so that the feeding out
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can be carried out without interruptions in the feeding.
' In Fig. 4 and 5 there is illustrated another development of the method of the invention. The mass which shall be treated may be of the same kind as those which have been referred to with reference to Fig. 1 or consist of any other kind of material mentioned in the preamble of the specification. Referring to Fig. 4 a container is shown as 42. The moist mass in the container 41 is shown as 43. At the situation illustrated in the drawing the liquid has been separated from the lower portions of the mass in the continer. The lower, drier portion which consequently substantially consists of comparatively dry powder has been designated 44. Between the dry part 44 and the mass 43 which is saturated with liquor there is an evaporation zone 45 which is formed therein that the content of the container is heated from underneath by means of a number of burners 46. A conduit for the feeding off of the combustible gas from the container 41 is shown as 47. The conduit 47 is connected to an air pump 48 for establishing a vacuum in the container in the space 49 above the mass 43.
In order to increase the pressure upon the solid phase in the mass 43, i.e. upon the bulk of solid particles in the mass, there is provided a plunger 50 which can be pressed down against the mass 43 by means of a bar 51 which sealingly projects through the cover 42. The pressing of the plunger 50 against the mass 43 is done via a screen cloth 52 which can allow liquid from the mass 43 to pass through said screen cloth which is essentially impermeable for the solid particles in the mass 43.
The liquid which is filtrated through the screen cloth 52 success¬ ively continous through openings 53 in the plunger 50 and is collected as a volume of liquid 54. The volume of liquid 54 is sucked up from the container 41 therein that the conduit 47 is replaced by a liquid suction conduit after the vacuum pump 48
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has been taken away. Alternatively the feeding off of the liquid can be performed under vacuum by an apparatus specially provided for liquid suction. It is also possible to remove the volume of liquid 54 as a final operation after terminated separation of the liquid from the solid phase in the container.
The evaporation zone successively will proceed upwards in the mass 43. Fig. 5 illustrates the conditions when about half the quantity of mass in the container has been treated. The pressure conditions in the container are schematically illustrated in the diagram in Fig. 5. The continous line in the diagram thus illu¬ strates the conditions according to the developed embodiment of the invention, wherein a pressure difference Δ exists between the pressure in the "dry" part 44 of the mass in the container and the gas pressure on the surface of the mass. The dashed line in the diagram in Fig. 5 illustrates the corresponding conditions in the case when the pressure in the space 49 in the upper portion of the container 41 corresponds to atmospheric pressure and the plunger 50 is not pressed against the mass 43. The pressure difference ^ p. which is obtained in this case is essentially lower than the pressure difference Δ P-« The process according to the embodiment may also be combined with continous supply of new mass to the upper part of the container and with continous feeding away of separated liquid and dried material, the level of the evaporation zone being kept essentially constant in the space.