GB2065164A - Steam drying of coal - Google Patents

Abstract

A process for drying and if desired, modification of the structure of organic solid material, such as for example brown coals, using steam as the drying agent. The organic solid materials have their upper grain size limited to approximately 50 mm and are suspended in water and heated under a pressure exceeding atmospheric pressure. During this first stage a pre-drying is effected. In following stages the selected operating superatmospheric pressure is maintained or increased and the suspension is centrifuged in at least one further drying stage. In all drying stages succeeding the stage in which the suspension is prepared heating is effected by supplying steam. The suspended solid materials are continuously discharged from the last drying stage maintained under a pressure exceeding atmospheric pressure and pressure-released, and drying terminated by an aeration step.

Description

SPECIFICATION Drying organic solid materials particularly brown coal This invention relates to a process for drying organic solid materials, particularly brown coals, by using steam. It is known that the frequently considerable moisture content of brown coal substantially comprises chemically bound water, and water held within the pores of the brown coal, and that this water can be removed from the coal only with a high expenditure of energy by directly heating the coal. It is also known that this water can be expelled from the brown coal by means of steam or hot water. There are also known processes which dry the coal in a semi-continuous manner. Such known processes provide in most cases, a substantially better utilization of the energy supplied than the basic Fleissner-process, but the problem of dewatering the coal with the known processes is generally unsatisfactory.

It is an object of the present invention to provide a process which allows efficient drying of organic solid materials, and with brown coal also carbonization thereof, in a particularly economic manner and with low energy requirement, and which also alleviates problems with waste water.

The invention provides a process for drying an organic solid material by using steam, characterized in that an organic solid material having a maximum grain size of approximately 50 mm is continuously suspended with water and heated in a first drying stage under superatmospheric pressure, and the resulting suspension or sludge is subsequently passed through at least one further drying stage under the same or increased pressure wherein it is centrifuged in at least one such succeeding drying stage and heated in all such succeeding drying stages by supplying steam thereto and continuously discharging the separated solid materials from the last drying stage maintained at superatmospheric pressure and the pressure removed therefrom, and final drying is effected by the action of air in an aeration stage.

Due to the fact that the grain size of the organic solid material treated is limited to approximately 50 mm, the individual particles can more rapidly be heated over their cross section so that their residence time in the process steps is reduced as well as the energy consumption lessened. Also because the suspension is heated under superatmospheric pressure and with an increase in temperature, the first process step is effective as a drying step. Due to the subsequent centrifuging of the suspension in a steam atmosphere in at least one subsequent drying stage, the drying capacity can substantially be increased with simultaneous reduction of the energy consumption, because the particles of the organic solid material tend to become plastic at the high temperatures prevailing in the steam atmosphere resulting in a shrinking tendency of the solid material.Furthermore the simultaneous removal of water from the interior of the coal particles is accelerated under the action of centrifugal forces. By providing a plurality of stages in the process of the invention, a smooth progress of the continuous process is achieved and the particles of solid material have, when compared with coals dried according to known processes, a substantially lower water content in view of their high shrinkage after having been pressure-released and discharged, respectively, and post-dried with aeration. The coal dried according to the inventive process is, in view of carboxy groups having been removed to a great extent, hydrophobic in nature and scarcely absorbs any water.

It is advantageous to pass the suspension, prior to centrifuging, over sieves, particularly slot sieves. In this manner the proportion of the process water passed from one drying stage to the succeeding drying stage is substantially reduced and the conditions are optimized under which the steam atmosphere can become effective. In this manner the contact between coal and steam is improved and the heat transmission is accelerated. Simultaneously, the energy requirement is reduced, because excessive process water can be discharged in a simple manner, for example via locks, and need not be heated in the succeeding drying stages.

When performing the drying process it is of particular advantage to vent the CO2 produced in the individual drying stages. The CO2, which is created during the progressing carbonization, forms a gas cushion in the autoclaves and centrifuges and acts as a heat insulation which prevents the steam atmosphere to become effective as far as possible. This undesired effect must be eliminated by venting the Cho,. Venting of CO2 is preferably effected in the first drying stage, in which the suspension is prepared, at a position closely above the liquid level of the suspension.

For a favourable energy balance it is of advantage also to make use of the heat content of the process water in several respects. Bn a particular advantageous manner, the process water is supplied into a sedimentation container, in particular into an upstream classifier, whereupon the proportion of the process water, which is enriched in solid material, is returned into a drying stage. By such recycling of the proportion of the process water enriched in solid materials, the suspension is simultaneously heated, noting that process water, particularly process water enriched in solid material, is preferably supplied to the first drying stage in an amount which is sufficient for preparing the suspension and for heating the suspension to a temperature of at least 1000C which corresponds to the technological conditions.Said temperature can be selected within the range of 100 to 1600C. In view of the proportion of the process water enriched in solid materials being, as a rule, not sufficient for supplying the amount of water required for the suspension, also a portion of the process water depleted in solid materials is recycled. The proportion of the process water depleted in solid material is preferably introduced into an oxidation equipment in which the organic matter is oxidized by introducing air or oxygen. In view of the heat content of the proportion of the process water enriched in solid materials being sufficient for heating the suspension, the process water flowing out of the oxidation equipment can be pressure-released and the heat content of the process water can be utilized in heat exchangers, for example for heating the boiler feed water and/or air.In view of additional water being continuously expelled from the coal to be dewatered, only a portion of the process water depleted in solid material and coming from the oxidation equipment must be introduced into the first drying stage for controlling the process conditions.

The process is in an advantageous manner performed such that in the drying stages succeeding the first drying stage a working temperature of at least 1600C is maintained. The operating temperature can, for example, be within the range of 200 to 2600C and can particularly be selected with approximately 2450C. In the drying stages operated with pressurized steam, the steam pressure is advantageously at least 8 bar, noting that the pressures can be the same but the temperatures can be different. It is without further possible to work at pressures up to 40 bar.

In view of heating of the coal being, according to the invention, effected for a great extent in a steam atmosphere, the amount of heating fluid for heat transmission in a condensating steam atmosphere is substantially smaller than when transmitting heat on the coal by hot water. According to the invention it is preferably worked in at least two drying stages within a steam atmosphere, noting that the steam condensate obtained and the water expelled is extracted from the process. By continuously discharging the process water, the heat transmission from the steam atmosphere to the coal is improved. This is similarly applicable for other organic materials.

In a particularly advantageous manner, the process according to the invention is performed with four drying stages, the second and the third drying stages are operated in a steam atmosphere, and that CO2 generated, if any, is extracted in the first and the second drying stage and that the solid materials are discharged into the fourth drying stage. The fourth drying stage can be operated under atmospheric pressure and under supplying of air heated by process water, noting that the fourth drying stage can, for example, be designed as fluidized bed drying device.

For achieving a further reduction of the heat consumption of such a process and for obtaining a particularly economic process performance, it is preferred to separate the major part of the water used for preparing the suspension before or after entering the first succeeding drying stage and to discharge this water with detour of optionally provided further drying stages from the stages maintained under operating pressure and/or to recycle this water into the first drying stage in which the suspension is prepared.

In view of separation of the process water already being effected at an earlier moment in a dewatering stage or after entering the first succeeding drying stage, the suspension water already cooled by having transmitted its heat to the organic solid materials on preparing the suspension is not mixed with the hot steam condensate and the hot water expelled from the coal, so that a great portion of this excessive water is not heated what would be superfluous.

Simultaneously, the water collected at the end of the process in a water collecting container can be - obtained at a temperature level which is sufficient for preheating the organic materials when preparing the suspension. Additional steam can be supplied under pressure when preparing the suspension for improving preheating prior to feeding the suspension into the first succeeding drying stage. The steam consumption can substantially be reduced in view of the high temperatures of the water to be recycled to the suspension.

Preferably, at least 75 to 90% of the water used for preparing the suspension is separated before or after entering the first succeeding drying stage, noting that the recycled or expelled water, respectively, can be separated from the suspension in a simple manner by means of sieves, particularly slot sieves. For this purpose, an autoclave maintained under steam pressure and provided with cascades of slot sieves can be used.

The water to be by-passed relative to the remaining drying stages is in this case discharged from the pre-connected dewatering stage or from the first chamber of the sieve cascade or from the first chambers of the sieve cascade located most adjacent to the charging opening of the autoclave, whereby a stream of organic materials already dewatered to a great extent is brought into contact with the steam supplied into the autoclave.

The discharge water can preferably be separated in a classifying stage into a portion depleted in solid materials and into a portion enriched in solid materials, the portion depleted in solid materials being either discharged with interposition of an oxidizing equipment in which the organic matter is oxidized by supplying air or oxygen or being recycled for preparing the suspension, and the portion enriched in solid materials being recycled to the first drying stage. It is, however, also possible to introduce the portion depleted in solid materials into a further purifying stage, particularly into an upstream-classifier which results, however, in cooling down the process water which can be recycled for preparing the suspension.

The portion enriched in solid materials can be recycled into a drying stage, said recycling being preferably not only effected at the end of the drying process into the stage in which the suspension is prepared, but being also effected with the by-passed portion of the process water and, in this case, the portion enriched in solid materials being recycled to the first succeeding stage. That portion of the process water which is flowing out of the last drying stage operated under a pressure exceeding atmospheric pressure and which is enriched in solid materials, can simply be introduced into the sludge or suspension, respectively, below the liquid level thereof, whereby the suspension can more effectively be mixed and more intensely heated.Introduction of the mentioned portion of process water is thus conveniently effected at the bottom end of the mixing receptacle used for preparing the suspension.

It is only when the temperature of the portion of the process water recycled into the mixing receptacle is insufficient for optimally preheating the organic solid materials that it is, according to the invention, proposed to additionally heat the suspension within the mixing receptacle by introducing steam which equally is supplied in an advantageous manner below the liquid level.

In the following description the invention is further illustrated with reference to the drawings showing various embodiments of a plant for performing the process according to the invention, said embodiments illustrating further details essential for the invention.

In the drawings: Figure 1 shows a first embodiment of a plant for performing the process according to the invention, Figure 2 shows a modified embodiment of a plant corresponding to a plant as shown in Figure 1, Figure 3 shows a further modified embodiment of a plant for continuously drying fine-grained brown coal of high water content, and Figure 4 shows a modification of the embodiment shown in Figure 1.

In Figure 1, reference numeral 1 designates a bin for receiving fine-grained brown coal having a grain size of 0.001 to 20 mm. Reference numeral 2 designates a supply means designed as piston press by means of which the coal grains or the coal dust, respectively, is pushed into a mixing receptacle 3. A suspension of coal in hot water is produced within this mixing receptacle 3 and supplied into an autoclave 5 by means of a pump 4. The mixing receptacle 3 represents the first drying stage and comprises a stirrer 6. A conduit 7 is opening into this mixing receptacle 3 and used for introducing into the mixing receptacle purified and cooled process water by means of a pump 8.

A conduit 9 is equally opening into the mixing receptacle 3 and used for introducing hot process water containing coal. The coal is supplied with a temperature of approximately 0 to 400C and the temperature of the suspension is adjusted to approximately 100 to 1 600C by supplying hot and cold water so that there is formed a saturated steam atmosphere. CO2 generated and accumulated above the liquid level is vented via a lock 10.

The suspension of coal in hot water is supplied into the second drying stage formed of the autoclave 5 by means of a pump for example designed as centrifugal pump or a worm pump.

This autoclave 5 comprises slot sieves 11 through which excess water is flowing off, the excess water being discharged via conduits 12. The water discharged from the autoclave via the conduits 12 need not be heated subsequently and optimal conditions are provided for the action of the heated steam which is introduced into the autoclave via a conduit 1 3 and nozzles 14. The coal particles moving downwardly within the autoclave come into contact with steam and the temperature within the autoclave is raised to a value between 200 and 2450 C. Heat transmission by the condensing steam atmosphere within the autoclave 5 becomes thus essentially more rapid than would be the case when using hot water.It is possible to use saturated steam as well as superheated steam but when using saturated steam it is advantageous to apply this steam with a pressure of 6 to 40 bar. The temperature and the pressure, respectively, of the steam is to a great extent dependent on the moisture content, the structure, the composition and the final water content of the coal and is also influenced by the mechanical strength of the coal and the intended use of the coal. Steam can be supplied via the nozzles 14 shown. It is however also possible to introduce the steam into a fluidized bed of the coal.The heated and substantially dry coal is falling onto the bottom 1 5 of the autoclave and is by means of a transport screw 1 6 transferred into a third drying stage being designed as a centrifuge 1 7. Within this third drying stage the required temperature for final drying is maintained, separation of the water being effected by means of the centrifugal forces so that again an optimum heat transmission from the steam to the coal is guaranteed. Steam is supplied into the centrifuge via a conduit 18, the process water being discharged via a conduit 1 9 into a collecting container 20 for waste water.The centrifugal forces not only enhance accelerated removal of water from the core of the coal particles but also assist the shrinking tendency of the coal particles so that the danger that the coal again adsorbs already removed humidity becomes reduced. The coal particles become plastic at the applied drying temperature and beside a favourable shrinkage there is also observed an equalization of the grain size of the coal particles.

The steam supplied into the centrifuge 1 7 via the conduit 1 8 conveniently enters the centrifuge adjacent the coal discharge channel 21 from where the dried coal is discharged via a lock 22 into an evaporating container 23 opening into atmosphere at its upper portion. In this manner, the steam temperature is optimally made use of.

The process water removed from the autoclave and containing in addition to the water introduced with the suspension, steam condensate and water expelled from the coal, is fed through a water separator 24, the portion of the process water enriched in coal particles of extremely fine grain size being introduced into the centrifuge. The process water is flowing from the centrifuge via a conduit 19 into the collecting container 20 for waste water, said collecting container 20 being designed as settling tank in which the waste water is subjected to an upstream classifying. Part of the waste water obtained is, after a corresponding resting time and clarification, fed via a conduit 25 into an oxidizing reactor 26 in which organic matter, particularly humic acids, are oxidized by introducing oxygen or air.The required air or oxygen, respectively, can be introduced into the oxidation reactor 26 via a conduit 27. That portion of the process water, which is enriched in solid materials, is, via a conduit 28, transferred from the collecting container 20 for waste water into a dosing equipment 29 via which the hot water containing coal particles and coal dust is recycled to the first drying stage, i.e. the mixing receptacle 3, via the conduit 9.

The coal supplied to the evaporating container 23 via the pressure lock 22, which can be designed as twin-lock, piston press or extruder, is supplied by means of a conveyor means 30 to a fluidized bed drier 31 into which heated air is blown via a conduit 32. The dried coal is discharged at 33 and can be introduced into the storage container for dry fine coal. The finest coal particles moved in upward direction by the air in the fluidized bed drier are separated in a cyclone 34 from which the solid materials are transferred into the discharge conduit 33 by means of a conveyor screw 35. Fine coal particles still contained in the air stream are separated in the dust filter 36 following the cyclon 34 and transferred into the discharge conduit 33 by means of a conveyor screw 37.

The process water purified within the oxidation reactor is, via a pressure reducing valve 38 and a conduit 39, introduced into a heat exchanger 40 in which air of ambient temperature is heated. The heated air is supplied into the fluidized bed drier 31 via a conduit 32. The condensate obtained in the heat exchanger is, by means of a pump 41, recycled into the waste water conduit 42 into which is introduced also the other portion of the process water flowing out of the oxidation reactor under pressure. A pressure reducing valve 43 is also interconnected into the waste water conduit 42. The obtained mixture of gas and steam is supplied to a heat exchanger 44 in which boiler feed water for the steam production equipment 45 is heated.Part of the process water, which has been substantially cooled down within the heat exchanger 44, is, via the pump 8 and the conduit 7, again recycled into the mixing receptacle under pressure. The excess portion of the process water flows into a sludge pond through a conduit 46.

At the lowermost area of the autoclave 5 and above the liquid level therein, there is again provided a lock 47 for venting CO2.

In Figure 2, which shows a modified embodiment, there are used the same reference numerals as in Figure 1 for identical constructional parts. Instead of the autoclave 5 shown in Figure 1, a drying drum 48 is used as the second drying stage succeeding the mixing receptacle 3, a steam supply conduit 13 being connected to this drying drum. The drying drum 48 comprises a substantial cylindrical, stationarily arranged sieve 49 and the coal suspension supplied by pump 4 and through the conduit 50 is fed in axial direction of the drying drum by means of a conveyor screw 51 driven by a motor 52. The sieve drum can be provided with slot sieves, the process water being discharged via conduits 53 from a plurality of sections of the drying equipment.The process water is flowing into a collecting container 54 within which the process water is further clarified and from which CO2 is vented via a lock 55. At the lowermost area, the settled solid materials enter a conveyor means 56 by which the solid materials are fed into a feed hopper 57 into which opens also the coal discharge conduit 58 of the drying drum and which is also provided with a lock 59 for venting CO2. The coal contained in the feed hopper 57 is transferred into the centrifuge by the conveyor means 1 6 already described in connection with Figure 1.The process water coming from the collecting container 54 is, via a conduit 60, immediately introduced into the conduit 19 leading to the collecting container 20 for waste water, because solid materials of this waste water have for the major part already been handled by the conveyor means 56 and introduced into the centrifuge. In all other respects the plant according to Figure 2 corresponds to the plant according to Figure 1.

In the embodiment illustrated by Figure 3 the second drying stage succeeding the mixing receptacle 3 is designed as a centrifuge 61. The hot suspension is supplied from the mixing receptacle 3 and via a pump 4 to a slot sieve 62, the portion enriched in solid materials being introduced into the centrifuge by a conveyor means 63. The conduit 13 for saturated steam or superheated steam, respectively, is opening into the centrifuge closely to the discharge means 64 of the centrifuge so that the heat energy is optimally utilized. The process water flowing out - of the centrifuge 61 is, via a conduit 65, again introduced into the conduit 19 leading to the collecting container 20 for waste water, a lock 66 for venting CO2 being interconnected into this conduit 65. CO2 is also vented from the centrifuge 1 7 forming the third drying stage via a lock 67.

The process water passing through the slot sieves 62 is introduced into the centrifuge via a conduit 68 opening into the centrifuge close to the discharge end for process water, and this for the purpose to utilize in the centrifuge 61 also finegrained coal particles coming from said conduit 68.

The remaining constructional parts of the plant shown in Figure 3 correspond to the plants shown in Figures 1 and 2.

In Figure 4, reference numerals used in Figures 1 to 3 were retained for equal constructional parts. 1 designates the bin for receiving fine grained brown coal or bituminous coal preferably having a grain size of O to 20 mm. 2 designates the feed device designed as stamp press and pushing the coal grains and the coal dust, respectively, into the mixing receptacle 3. Within this mixing receptacle 3, a suspension of coal in hot water is prepared, said suspension being supplied to the autoclave 5 by means of the pump 4. The mixing receptacle 3 represents the first drying stage and is provided with an agitating device 6. Cooled and purified process water can be introduced into the mixing receptacle 3 via the conduit 7 and by means of the pump 8. The conduit 9 is also opening into the mixing receptacle 3 and used for introducing hot process water containing coal sludge.The coal is supplied with a temperature of about 0 to 400C and the temperature of the suspension is adjusted to approximately 100 to 1 600C by supplying hot and cold water so that a saturated steam atmosphere is formed. The CO2 accumulating above the liquid level is discharged via the lock 10.

The suspension of coal in hot water is supplied into the autoclave 5 forming the second drying stage by means of the pump 4, for example designed as centrifugal pump or worm pump. This autoclave 5 has, like the embodiment according to Figure 1, slot sieves 11, by means of which excessive water can be separated, the water being discharged via conduits 12. The water discharged from the autoclave via the conduits 12 need not be heated later on and optimum conditions for the action of steam are established which is introduced into the autoclave via the conduit 13 and nozzles 14. The coal particles moving down within this autoclave come into contact with this steam and the temperature within the autoclave is raised to 200 to 2450C.Heat transmission is effected by the condensing steam atmosphere within the autoclave 5 at a substantially higher speed than when using hot water. Saturated steam as well as superheated steam can be used and it is of advantage to work with saturated steam under a pressure of 6 to 40 bar. The temperature and the pressure, respectively, of the steam is strongly dependent on the water content, the structure and the composition of the coal to be dried and on the final water content of the dried coal and is also influenced by the mechanical strength of the coal and its intended later use.

Steam can be supplied by means of the nozzles 14. The heated and substantially dried coal, respectively, falls on the bottom 1 5 of the autoclave and is transferred by means of a conveyor screw 1 6 into a third drying stage which again is designed as centrifuge 1 7. In this third drying stage the required final drying temperature is maintained and the water is separated under the action of centrifugal forces, so that again an optimum heat transmission from the steam to the coal is guaranteed.The steam is supplied into the centrifuge via a conduit 1 8 and the process water is discharged into a collecting container 20 for waste water via a conduit 1 9. In addition to the accelerated removal of water from the core of the coal particles, the centrifugal forces have also the effect to enhance the shrinking tendency of the coal particles so that the danger that the coal particles again adsorb already removed humidity is reduced. At the drying temperatures used, the coal particles become plastic and in view of the coal particle shrinking under the action of the centrifugal force, water contained n the coal is pressed out and the grain size of the coal particles is equalized.

The steam introduced into the centrifuge 1 7 via the conduit 1 8 conveniently enters the centrifuge adjacent the coal discharge means 21 of the centrifuge, via which discharge means the dried coal is discharged via a lock 22 into an evaporating container 23 which has its upper portion open to atmosphere. In this manner, the temperature of the steam is optimally utilized.

Whereas in the embodiment according to Figure 1 the process water discharged from the autoclave and containing in addition to the water contained in the suspension also steam condensate and water expelled from the coal is exclusively passed through a water separator 24 and the portion of the process water enriched in suspended coal of fine particle size is introduced into the centrifuge, a conduit 70 is connected to both the slot sieves located adjacent the charging end 69 and an amount of 75 to 90 percent of the water used for preparing the suspension is derived from this conduit 70 and not recycled to a further drying stage but by-passed past all drying stages.

This conduit 70 leads to a settling tank 71 designed as upstream classifier in which the water is separated into a phase enriched in solid materials and into a phase depleted in solid materials. The phase enriched in solid materials is fed to the bottom 1 5 of the autoclave 5 by means of a conveyor means 72 whereas the phase depleted in solid material is discharged via a conduit 73 with detour of all drying stages. The process water emerging from the succeeding drying stages is flowing into the waste water collecting container 20 designed as clarifying tank via conduit 19, the waste water being subjected to an upstream classification within said collecting container.Part of the waste water obtained is, in case there is an excess thereof, after a corresponding residence time for clarification fed via the conduit 25 into the oxidation reactor 26 within which by introducing oxygen or air the organic matter is oxidized in an analogous manner to that described in connection with Figure 1. Air or oxygen, respectively, can be introduced into the oxidation reactor 26 via the conduit 27. The portion of the process water, which is richer in solid material, is transferred from the collecting container 20 for waste water via the conduit 28 into a dosing device 29 via which the hot process water containing coal of finest grain size is, via the conduit 9, recycled to the first drying stage, i.e. to the mixing receptacle 3 at the lower end thereof.

The coal transferred into the evaporating container 23 via the pressure lock 22, which can be designed as a twin-lock, a piston press or an extruder, is fed by means of a conveying device 30 into a fluidized bed drier 31 into which heated air is blown via the conduit 32. The dried coal is discharged at 33 and can be transferred into a storage bin for dry fine coal. The coal particles having the finest particle size and having been carried in upward direction within the fluidized bed drier are separated within the cyclone 34 from which the solid materials are fed into the discharge conduit 33 by means of the conveying screw 35. Coal particles of extremely fine particle size still contained in the air stream are separated in the dust filter 36 following the cyclone 34 and are transferred from the dust filter 36 into the discharge conduit 33 by means of a conveying screw 37.

The process water purified within the oxidation reactor is, via a pressure reducing valve 38 and a conduit 39, introduced into a heat exchanger 40 within which ambient air is heated, the heated air being introduced into the fluidized bed drier 31 via the conduit 32. The CO2 formed during the oxidation reaction is discharged through the conduit 74. A pressure reducing valve 43 is also interconnected into the waste water conduit 42.

The mixture of gas and steam thus obtained is introduced into the heat exchanger 44 within which boiler feed water is heated for the steam generating equipment 50. A portion of the process water being cooled to a great extent behind the heat exchanger 44 is, via the pump 8 and the conduit 7, again supplied into the mixing receptacle 3 operated under pressure. The excess portion of the process water is flowing to the sludge pond via the conduit 46.

At the lowermost area of the autoclave 5 and above the liquid level therein, there is again provided a lock 47 for discharging or venting CO2.

The phase depleted in solid materials and coming from the settling tank 71 is transferred into the sedimentation tank 20 via the conduit 73, which however results in the disadvantage that the process water to be used for heating the suspension within the mixing receptacle 3 is cooled down. Therefore a shut-off valve 75 is interconnected into this conduit 73 to be in the position to shut off the conduit leading to the sedimentation container 20. After having opened the closure valve 76, the phase depleted in solid materials can immediately be transferred from the settling tank 71 into the oxidation reactor.

The slot sieves 11, which are arranged within the autoclave 5 in cascade and in series, can, for the purpose of connecting the conduit 70, in a simple manner be arranged such that a sieve portion comprising hollow-pyramidal sieves is followed by a collecting case 77 comprising continuous walls, the collecting case having perforations for the solid materials to pass therethrough either at its central area or at an area adjacent the circumference of the autoclave 5 in dependence on the slot sieves having at their corresponding area inwardly inclined walls or outwardly inclined walls.

The steam generating equipment 50 is tapped by a further conduit 78 via which, if required, steam can be pressed into the mixing receptacle 3 to further preheat the suspension. Such additional preheating is, however, only necessary if the temperature of the waste water introduced into the mixing receptacle 3 from the collecting container 20 for waste water is insufficient for optimally preheating the suspension.

The progress is particularly suitable for drying lignitic organic solid material such as brown coals.

Claims (22)

1. A process for drying an organic solid material by using steam, characterized in that an organic solid material having a maximum grain size of approximately 50 mm is continuously suspended with water and heated in a first drying stage under superatmospheric pressure, and the resulting suspension or sludge is subsequently passed through at least one further drying stage under the same or increased pressure wherein it is centrifuged in at least one such succeeding drying stage and heated in all such succeeding drying stages by supplying steam thereto, and continuously discharging the separated solid materials from the last drying stage maintained at superatmospheric pressure and the pressure removed therefrom, and final drying is effected by the action of air in an aeration stage.

2. A process as claimed in Claim 1, characterized in that the said suspension or sludge from the first drying stage is, prior to centrifuging, passed through one or more sieves.

3. A process as claimed in Claim 1 or Claim 2, characterized in that CO2 generated in the individual drying stages, is discharged or vented.

4. A process as claimed in any of Claims 1 to 3, characterized in that the process water removed in at least the first stage is introduced into a settling vessel and the portion thus obtained which is enriched in the solid materials contained in the process water is recycled into a drying stage.

5. Process as claimed in any of Claims 1 to 4, characterized in that process water, preferably process water enriched in solid material, is supplied to first drying stage where the organic solid materials are suspended or transformed into a sludge, respectively, in an amount which is sufficient for heating the sludge or suspension, respectively, to a temperature of at least 1000C.

6. Process as claimed in any of Claims 1 to 5, characterized in that the succeeding drying stages are operated at temperatures of at least 1 600 C.

7. Process as claimed in any of Claims 1 to 6, characterized in that the portion depleted in solid materials of the process water is introduced into an oxidizing equipment within which organic matter is oxidized by supplying air or oxygen.

8. Process as claimed in Claim 7, characterized in that the process water leaving the oxidation equipment is pressure-released and the heat content of the process water is utilized in heat exchangers, for example utilized for heating the boiler feed water and/or air, respectively.

9. Process as claimed in Claim 8, characterized in that a portion of the process water leaving the oxidizing equipment and being depleted in solid materials is supplied to the first drying stage.

10. Process as claimed in any of Claims 1 to 9, characterized in that it is worked with steam pressures of at least 8 bar.

11. Process as claimed in any of Claims 1 to 10, characterized in that there is worked in at least two drying stages in a steam atmosphere, the steam condensate obtained and the water expelled (from the coal) being withdrawn from the process.

12. Process as claimed in any of Claims 1 to 11, characterized in that there are provided four drying stages, in that it is worked in the second and in the third drying stage in a steam atmosphere, in that CO2 generated is extracted at least from the first and from the second drying stage and in that the solid materials are discharged from the fourth drying stage.

13. Process as claimed in any of Claims 1 to 11, characterized in that the water used for preparing the suspension is, before or after entering the first succeeding drying stage, separated for a major part and either discharged with detour or circumvention of optionally provided further drying stages from the drying stages maintained under operating pressure and/or recycled at least partially into the first drying stage in which the suspension is prepared.

14. Process as claimed in Claim 13, characterized in that 80 to 90 percent of the water used for preparing the suspension is separated immediately after entering the first succeeding drying stage.

1 5. Process as claimed in Claim 13, characterized in that the recycled or discharged water, respectively, is separated from the suspension by sieves, particularly slot sieves.

16. Process as claimed in any of Claims 13 to 15, characterized in that the suspension is passed in the first succeeding drying stage over a predewatering means or a cascade of slot sieves arranged within an autoclave maintained under steam pressure, respectively, and in that the water separated is discharged from the cascades located nearest to the charging opening.

17. Process as claimed in any of Claims 13 to 16, characterized in that the water discharged from the first succeeding drying stage is separated in a classifying stage into a portion depleted in solid materials and into a portion enriched in solid materials, the portion depleted in solid materials being either discharged with optional interconnection of an oxidizing equipment within which organic matter is oxidized by air or oxygen supplied, or recycled for preparing the suspension and the phase enriched in solid materials being recycled with the first drying stage.

18. Process as claimed in Claim 17, characterized in that the phase depleted in solid material is passed into a further purifying stage, particularly into an upstream-classifier.

19. Process as claimed in any of Claims 13 to 18, characterized in that CO2 generated within the oxidizing equipment or oxidizing equipments is being discharged.

20. Process as claimed in any of Claims 1 to 12, characterized in that the suspension is heated and put under a pressure exceeding atmospheric pressure by introducing steam under pressure prior to introducing the suspension into the first succeeding drying stage.

21. A process according to any of Claims 1 to 20, wherein the said organic solid material is a brown coal.

22. A process for drying organic solid materials substantially as hereinbefore described and illustrated in any of Figures 1 to 4 of the accompanying drawings.