EP0527333A1 - Method and installation for breaking up brittle material - Google Patents

Abstract

The invention relates to a method and an installation for breaking up brittle material, in particular sand-like material for grinding such as foundry sand, in the roller slit of a choke-feed roller mill. In order to provide an optimum feed capacity of the material for grinding in the roller slit, the material for grinding is set at the inlet to the roller slit to a (mixing) moisture content of approximately 0.3 to 3.0%, preferably of approximately 0.5 to 2.0%. <IMAGE>

Description

The invention relates to a method (corresponding to the preamble of claim 1) and a system (corresponding to the preamble of claim 17) for crushing brittle regrind, in particular sand-like regrind.

From DE-A-37 17 975 it is already known to comminute brittle ground material in the nip of two rollers pressed against one another at high pressure, a partial stream of the ground material being recycled being returned to the roll gap and further comminuted together with fresh ground material. In this connection, the problem has been dealt with in order to properly comminute regrind of high moisture, which tends to stick and / or bake above a critical moisture value. In order to achieve this, at least a portion of the partial stream returned to the roll nip is dried to such an extent before it is re-introduced into the roll nip that a mixed moisture content is established in the roll nip that is below the critical moisture content, the so-called "limit moisture" above which the Grist tends to stick and / or bake, is assumed to be 12%, so that accordingly a mixed moisture of slightly below this moisture content is aimed for. If the moisture content is considered too low, an artificial increase in the mixed moisture is considered expedient.

The invention is based on the object of a method according to the preamble of claim 1 and a system according to the preamble of claim 17 create, which ensure a particularly good feed capacity of brittle regrind, in particular sand-like brittle regrind, in the nip of two rolls of a material bed roller mill with reliable and relatively easy to control operation.

This object is achieved according to the invention by the characterizing features of claim 1 and claim 17. Advantageous embodiments of the invention are the subject of the dependent claims.

In the extensive experiments on which the invention is based, it has been shown that when crushing brittle regrind, such as. B. ore, cement materials or the like. And in particular also of sand-like brittle regrind in the nip of a material bed roller mill both with particularly high moisture content of the regrind as well as with dry or almost dry regrind to difficulties in pulling the regrind to be crushed into the gap between the two Rollers can come. For example, when comminuting dry blastfurnace slag (glassy granules from blast furnace slag), a greatly reduced ability of this ground material to be drawn into the nip was found, and in addition the production of flakes (flake-like agglomerated particles of the comminuted material) was greatly reduced to an undesirable extent, resulting in finished comminution in the bed of the associated roller mill was severely impaired or was no longer possible in the required manner.

Optimal intake of brittle regrind and in particular of sand-like brittle regrind and above all of slag sand in the nip of a material bed roller mill can be achieved in that the ground material is adjusted to a (mixed) moisture content of about 0.3 to 3.0%, preferably of about 0.5 to 2.0%, at the inlet to the roll nip. With this moisture content, there is also an extremely favorable scale production for comminution in the material bed.

A very favorable moisture content can be selected in the range from about 0.8 to 1.5%, preferably around 1%.

If the freshly fed regrind is relatively dry, then it can also be expedient in the context of the present invention to adjust or control the moisture content of the regrind running towards the nip, which is regarded as optimal for a good absorption capacity, in that the moisture content of this regrind can be controlled by precisely controllable ones Addition of water is increased accordingly.

In numerous good-bed roller mills of the type mentioned in practice, skewing of the rollers (in English "Skewing") was also frequently found, ie the two rollers of the good-bed roller mills no longer ran parallel to one another in the desired manner, so that a uneven roll gap between the rolls or its surfaces resulted, whereby it was wider in the area of one roll end than in the area of the other opposite roll ends. In addition, undesirable vibrations, in particular of the rolls, were often found in good-bed roller mills in operation.

In the abovementioned extensive tests which were carried out for this invention, it was also surprisingly found that by operating or controlling the moisture content of the material to be fed into the nip, the operation of the material bed roller mill is not only a matter of particularly good intake capacity , but also with regard to the aforementioned skewing and the unwanted vibrations can be controlled very reliably and relatively easily.

If, in this sense, one considers only the described skewing of the rolls, at least such a skewing can at least temporarily increase the moisture content of the regrind components that run towards the roll gap in the longitudinal gap area with the smaller gap width. In this context it should be pointed out that other causes lead to this skewing of the rollers, such as. B. the supply of large pieces of material in the region of one roller end and fine material in the region of the opposite roller ends, or a corresponding one-sided inflow of regrind from an inlet container. In the extensive tests, it was also found with certain brittle regrinds that increasing the moisture content of the incoming regrind in the longitudinal gap area with the smaller gap width, the mentioned skewing of the rollers could be remedied very quickly, which was due to an increased draw-in capacity of regrind in this area smaller gap width could be achieved, at least by temporarily increasing the moisture content of the material to be ground in this longitudinal gap area. This quick remedy for skewing the roller can be most conveniently reached by direct, controlled addition of water. These measures have proven to be particularly advantageous in the crushing of blast furnace sand (granulated blast furnace slag) or similar sand-like, brittle regrind.

The problem of undesirable vibrations on the rolls or the roll mill as a whole can also be solved by a targeted increase in the moisture content of the ground material flowing into the roll gap. For this purpose, a measurement signal derived from a vibration of the rollers can advantageously be evaluated to control at least a temporary addition of water, the level of the vibration measurement value being also processed in a total moisture control as a factor in terms of a change in the moisture content of the material to be fed to the roller gap, i.e. H. if e.g. unwanted vibrations occur and become larger, then the moisture content of the incoming regrind or the additional water addition is increased accordingly, while if the unwanted vibrations decrease, the additional water addition can be reduced accordingly or even completely stopped (if, for example, meanwhile the mixed moisture content of the total ground material coming into the nip has also been reset).

At this point it should be mentioned again that the above-mentioned skewing of the rollers and also the undesired vibrations are often due to the supply of regrind that is too dry, which often occurs in practice with the mentioned slag sand or similar brittle regrind; however, it should not be misunderstood that other causes - as mentioned above - can lead to this undesirable operating behavior of the rollers. For this reason, it is therefore advantageous if the measurement signals reporting the occurrence of these undesirable operating conditions are not used for a direct change in the moisture content or an additional addition of water to the regrind, but rather are sensibly evaluated and processed as individual factors in the associated overall moisture control.

The mixed moisture content of the ground material to be set according to the invention can be brought about in practical operation in various ways, in each case depending on the material to be ground, on the desired end product and on the size reduction process to be selected or the corresponding size reduction system, in order to control this moisture content - As explained above - the current operating behavior of the rollers or the associated roller mill can also be used, and the aforementioned additional addition of water or liquid then serves as a kind of grinding aid.

If, on the other hand, relatively moist fresh regrind is to be comminuted, then this moist fresh regrind can be dried to an adjustable moisture content of about 0.3 to 3.0%, preferably of about 0.5 to 2.0%, before being fed to the roll nip be, for example in a corresponding upstream dryer.

A further possibility is also to dry a first fresh material portion in a drying device from a relatively moist fresh regrind to a moisture content of about ≦ 0.5%, to guide a second fresh material portion in the bypass to the drying device and then to bring both fresh material portions together before the feed to the roller gap , These fresh material proportions are expediently chosen to be so large that a mixed moisture content of 0.3 to 3.0%, preferably approximately 0.5 to 2.0%, is set in front of the nip.

Even if all of the fresh regrind or a corresponding proportion of it is dried so that the moisture content of the regrind to be fed to the roll gap is set to the desired value, the moisture or the water still present in the regrind acts to a certain extent as a grinding aid for optimum good intake capacity the nip as well as a cheap production of slugs.

If, in the previously described comminution of the brittle regrind, a partial stream of the comminuted regrind is also to be returned to the roll gap and further comminution is carried out together with the fresh regrind, then the procedure according to the invention is such that the total regrind composed of the fresh regrind and the returned regrind is fed to the roll nip to the mixed moisture content of about 0.3 to 3.0%, preferably from about 0.5 to 2.0%. This recycled regrind can expediently be composed of a proportion of the good particles agglomerated into flakes and of semolina coming from a viewing zone downstream of the comminution. The returned slugs ensure - how already indicated above - for optimal formation of the material bed in the material bed roller mill or in the nip of the two rollers of this roller mill.

In the case of the above-mentioned hybrid and finished grinding of the total ground material, it is also advantageous to control the moisture content and amount of the fresh ground material to be supplied as a function of the quantities and moisture contents of the recirculated portion of pulp and the returned semolina at a predetermined throughput rate through the comminution rollers. In this case, the said throughput rate can be controlled, for example, in accordance with a constant supply quantity to the viewing zone. The moisture content of the recycled semolina can expediently be set to a value of about ≦ 0.5%, which is already achieved by the fact that the comminuted regrind and thus also the semolina to be returned are in the visible zone, ie. H. usually in an adapted air classifier, is generally dried accordingly anyway. In contrast, the moisture content of the recycled portion of flakes essentially corresponds to the mixed moisture content of the total ground material coming into the nip. In the control of the individual regrind components to be fed to the roll gap, it can further be particularly advantageous if the proportion of the recirculated slugs on the one hand and the ratio between recirculated semolina and fresh regrind to be supplied on the other hand is kept essentially constant in each case.

If the volume flows to be fed to the roll nip have different moisture contents and different temperatures, then it can occur during the merging of these mass flows may lead to vapor formation, which could lead to caking when the ground material is transported to the roller gap. In order to avoid this essentially, it is also advantageous that the different mass flows of the fresh millbase and the millbase to be returned are mixed together shortly before the nip in such a way that the mass flows with relatively moist material from one side and the mass flows with relatively dry one Flow well against each other from the opposite side, be brought together. The latter also ensures sufficient mixing of the various mass flows via their throwing parabolas during the merging, as a result of which relatively expensive additional mixing devices can be avoided.

This method according to the invention is particularly advantageously suitable for crushing slag sand or similar brittle regrind (as fresh regrind).

A plant for comminuting brittle regrind, in particular sand-like regrind such as blastfurnace slag or the like, contains at least one material bed roller mill with a mill inlet, two oppositely rotating rollers pressed against one another at high pressure, and a roller gap formed in the area between the rollers. According to the invention, such a system is characterized in that devices for treating freshly ground regrind are arranged in front of the mill inlet in such a way that the regrind has a moisture content of about 0.3 to 3.0%, preferably of about 0, to the roller gap. 5 to 2.0%, can be fed.

Fig.1

ein vereinfachtes Anlagenschema einer erfindungsgemäßen Zerkleinerungsanlage,

Fig.2

ein gleichartiges Anlagenschema der Zerkleinerungsanlage, die jedoch durch eine Feuchtigkeits-Steuereinrichtung ergänzt ist.

The invention will be explained in more detail below with reference to the drawing. In this drawing:

Fig. 1

a simplified system diagram of a comminution system according to the invention,

Fig. 2

a similar system diagram of the shredding plant, but which is supplemented by a moisture control device.

In Figure 1, the shredding system is shown in a greatly simplified form for the sake of clarity. This system, which is intended in particular for crushing blast furnace slag, contains a material bed roller mill 1 with a mill inlet 2 serving to supply regrind, two rollers 3, 4 which are pressed against one another at high pressure and rotate in opposite directions, and with a roller gap 5 formed in the area between these rollers Mill inlet 2 are devices for treating freshly ground material that is conveyed in such a way that the ground material can be fed to the mill inlet 2 and thus to the nip 5 with a moisture content of preferably about 0.5 to 2.0%.

In the system example illustrated, it is assumed that relatively moist blast furnace slag (arrow 6) is conveyed as fresh regrind. This system example therefore contains a suitable dryer 7 as a device in front of the mill inlet 2, in which the freshly ground material conveyed can be dried to a sufficient extent, preferably continuously. The dryer 7 is also preferably a bypass line 8 for undried fresh Assigned regrind. This bypass line 8 is brought together in the area in front of the mill inlet 2 with the dry material line 9 coming from the dryer 7.

With the system parts described so far, it is thus possible to dry a first portion of the slag sand to be shredded in the dryer 7 to a sufficiently low moisture content, preferably about ≦ 0.5%, while a second portion of the wet slag sand is bypass line 8 around the dryer 7 - in the bypass - so that - after the combination of dried and undried (moist) fresh regrind - a mixed moisture or a mixed moisture content can be achieved which meets the optimal feeding possibilities in the nip 5. Corresponding switching flaps can of course also be provided, which make it possible to switch off the bypass to the dryer 7 or to completely bypass the freshly supplied slag sand around the dryer 7 (when the slag sand is sufficiently dry). In the case of very dry blastfurnace slag, which therefore no longer has to be passed through the dryer 7, it may also be expedient to increase its moisture content to a suitable value by adding water (see dashed arrow 10); system parts for such a water addition will be discussed later in connection with FIG. 2.

The system described so far can be used if only a pure comminution in one pass of blast furnace sand or the like is to be carried out with the moisture content mentioned.

In the overall representation of the system in the schematic drawing, it is also assumed that this system also includes a viewing device, preferably in the form of a suitable air classifier 11, which is used to classify ground material comminuted in the material bed roller mill 1. For this purpose, a regrind line 12 leads from the outlet of the material bed roller mill 1 to the air classifier 11. From the air classifier, ground material which has been comminuted sufficiently can be discharged from the system, while coarse material which has been separated off as semolina is fed back via a semolina line 14 to the mill inlet 2, in order to obtain this size to further shred together with fresh regrind. From the regrind line 12 a Schülpenleitung 15 is branched, which is used to return at least a portion of the Schülpen coming from the Gutbett roll mill 1 to the mill inlet 2. The semolina and scraps to be returned to the mill inlet 2 of the material-bed roller mill 1 can each be separately conveyed directly to the mill inlet via their lines 14 or 15, or there is also the possibility - not shown in the drawing - of the semolina to be returned via a semolina container (Intermediate container) precisely metered in quantity to feed the mill inlet 2 and, in adaptation thereto, feed the slugs either directly to the mill inlet 2 or first to the semolina tank in an adapted or controllable manner.

It should be pointed out at this point that all of the lines (8, 9, 12, 13, 14, 15) of the shredding system mentioned are basically any suitable type of conveying devices, such as mechanical and pneumatic conveying devices, pipelines, etc. can.

In any case, this plant offers several possibilities for the ground material to be ground to be fed to the grinding gap with a mixed moisture content in the range of preferably about 0.5 to 2.0%, as explained above using various or complementary procedures is.

To support the above explanations regarding the method according to the invention and the system according to the invention, a few numerical examples are given. In the case of a first example, it is assumed that a system as previously explained with reference to the drawing is operated with the following setting: Total regrind throughput 250 t / h returned scuffs 138 t / h = (S) returned semolina 84 t / h = (G) fresh regrind 28 t / h = (F) Moisture content semolina 0.5% = (g) Moisture content Schülpen = (s) Moisture content of fresh feed = (f)

Assumption: For reasons of stability, a mixed moisture content (mixed moisture) of 1% H₂O must be set before or at the inlet of the material bed roller mill. It is assumed that the moisture content of the flakes is equal to the moisture content of the total ground material at the inlet of the roller mill or the roller gap.

ein Feuchtigkeitsgehalt für das frische Mahlgut von 2,5 %.With this assumption of the moisture content of the slugs of 1.0% is calculated from the following equation

a moisture content of 2.5% for the fresh regrind.

To achieve this moisture content, the dryer upstream of the Guttbett roller mill can be set to 2.5% residual moisture for the fresh regrind. If this is not possible or does not make sense for procedural reasons, then part of the moist fresh ground material can be bypassed the dryer, while only the other part of the fresh ground material is dried, so that a mixed moisture content of dried and undried fresh ground material is 2.5% results.

The above-mentioned mixed moisture content of the fresh feed or regrind can, for. B. can be achieved in the following way: Moisture content of fresh regrind (blast furnace slag) before dryer 8th % 1. Fresh food portion to the dryer 20.5 t / h Moisture content of the 1st fresh food portion after the dryer 0.5% 2. Fresh product partial quantity (bypass quantity; without drying) 7.5 t / h

With the help of the equation (formula) given above, it is of course also possible to calculate other desired (mixed) moisture contents for fresh feed material, and also according to any changes in the moisture content of the returned goods or changed returned goods.

Furthermore, the moisture content of the recycled semolina - by a corresponding setting of the classifying air temperature in the classifier (z. B. 95 to 100 ° C at the classifier output) - to a predetermined value, z. B. of preferably about 0.3 to 0.5%. The moisture content of the slugs will - if necessary with small deviations upwards or downwards - be approximately in the order of magnitude of the moisture content of the total ground material at the mill inlet.

The two examples in the table below of different material throughput rates through the material bed roller mill or its comminution rollers may make it clear that the proportion of recycled flakes on the one hand and the ratio between recycled semolina and freshly regrinded material on the other hand can be kept essentially constant. Total throughput quantity 100% 250 t / h 270 t / h recycled flakes approx. 55% 138 t / h 149 t / h Air classifier feed quantity approx. 45% 112 t / h 121 t / h returned semolina 84 t / h 91 t / h Requirement for fresh regrind 28 t / h 30 t / h Ratio of returned semolina to fresh regrind approx. 3: 1 3: 1

This table relates to fine grinding of blastfurnace slag in a shredding plant according to the embodiment illustrated in the drawing.

From the examples in this table it can be deduced that the percentage (approx. 55%) of recirculated flakes is set essentially constant and accordingly an increase in the total quantity of ground material to the material bed roller mill and thus its throughput rate (quantity) increases with the quantity leads returned Schülpen. A requirement for fresh regrind also depends on the amount of fine material drawn off in the air classifier. This also determines the amount of the returned semolina to the material bed roller mill.

In this context, it should also be pointed out that in the area between the air classifier and the mill inlet of the material bed roller mill, a type of buffer silo for recycled semolina can be arranged in order to be able to control the proportion of the semolina required for the material throughput of the roller mill, what For example, by continuous level measurement and control, a constant level - according to the requirements - by a corresponding one Raising or reducing the amount of semolina to be returned. The requirement for freshly regrinded material to be fed to the material bed roller mill then arises from the total quantity of material to be ground (throughput) for the roller mill minus the amount of returned semolina and minus the amount of returned flakes.

By means of the method according to the invention and the system according to the invention, it is thus possible to set and maintain a particularly advantageous mixed moisture content in the range of preferably about 0.5 to 2.0%, in order to achieve an optimal intake capacity and optimal comminution ratios, for which purpose above all the moisture contents of the freshly fed regrind and the recycled semolina on the one hand and the respective subsets of recirculated flakes, recycled semolina and newly fed fresh regrind on the other hand, can be used, assuming that the moisture content of the recirculated flakes is about the moisture content of the total ground material fed to the shredding rollers corresponds and can thus be kept essentially constant. If the recycled semolina also has a constant moisture content (due to the drying of the regrind to be fed to the air classifier described above), then only the moisture content of the fresh regrind to be fed needs to be calculated and controlled, i.e. the only freely selectable moisture component is then the moisture content the freshly regrind to be fed (from the dryer and / or bypass), and this moisture content is then adjusted so that the Average moisture of all partial regrinds (= total amount of regrind) fed to the mill inlet corresponds to the desired moisture content in the nip.

As has already been explained in more detail above in the description of the method features, it is also advantageous, in addition to the moisture setting or control described above with reference to FIG. 1, to take measurements or measurement signals of any excessive vibrations of the rolls or roll mill that may occur also to be processed in a total humidity control. 2 accordingly shows an example in which a comminution system according to FIG. 1 has been supplemented by a total moisture control for the total ground material flowing into the nip 5 by a moisture control device 20, which is only indicated schematically.

This moisture control device 20 is connected to a number of moisture measuring devices known per se and arranged at suitable points (see dash-dotted connecting lines), namely with a measuring device 21 for freshly introduced regrind (6), a measuring device 22 for merged fresh regrind from bypass line 8 and Dry material line 9, at least one measuring device 23 for total ground material brought together at the mill inlet 2, with a measuring device 24 for crushed ground material and a measuring device 25 for returned semolina. The moisture measurement signals of these measuring devices 21 to 25 are processed as control signals for a presettable mixed moisture content.

In addition, the roller mill 1 or its roller frame is assigned a vibration measuring device 28 at at least one suitable point. This vibration measuring device 28 is also connected to the moisture control device 20 (see dash-dotted lines), and its measurement signals can also be evaluated as control signals for setting the moisture content. In this way, the moisture content at the inlet 2 of the roller mill 1 can then be controlled via the moisture control device 20 - in the sense of at least occasionally additional one-sided or total water addition via a water addition device 10 'so that undesirably high vibrations can be prevented or eliminated.

Claims (20)

Process for comminuting brittle regrind, in particular sand-like regrind, in the gap between two oppositely rotating rollers pressed against one another at high pressure,

characterized,

that the moisture content of the material to be fed to the nip is adjusted in a range from about 0.3 to 3.0%. A method according to claim 1, characterized in that the moisture content of the ground material at the roll nip inlet is set to approximately 0.5 to 2.0%. A method according to claim 1, characterized in that a moisture content of about 0.8 to 1.5%, preferably around 1.0%, is selected. A method according to claim 1, characterized in that relatively moist fresh ground material is dried to an adjustable moisture content of about 0.5 to 2.0% before it is fed to the roll nip. A method according to claim 1, characterized in that from relatively moist fresh ground material, a first fresh material portion is dried in a drying device to a moisture content of about ≦ 0.5%, a second fresh material portion is bypassed to the drying device and both fresh material portions in the area of the feed to Nip are brought together, these fresh material portions being chosen so large that a mixed moisture content of about 0.5 to 2.0% is set in front of the nip. Method according to claim 1, characterized in that in the case of relatively dry fresh regrind the moisture content of this regrind is increased by adding water. A method according to claim 6, characterized in that, if the rolls skew, at least the moisture content of the regrind parts is increased at least temporarily, which run towards the roll gap in the longitudinal gap area with the smaller gap width. A method according to claim 6, characterized in that a control signal derived from vibration of the rollers is evaluated to control at least one temporary addition of water, the level of the vibration measurement being processed as a factor for a change in the moisture content of the material to be fed into the roller gap in an overall moisture control . The method of claim 1, wherein a partial stream of the comminuted ground material is returned to the roll gap and further comminuted together with the fresh ground material, characterized in that the total ground material composed of the fresh ground material and the recycled ground material at the feed to the roll gap to the mixed moisture content of about 0.5 up to 2.0% is set. Method according to claim 9, characterized in that the recycled regrind is composed of a portion of the good particles agglomerated into flakes and of semolina coming from a visible zone downstream of the comminution. A method according to claim 10, characterized in that, at a predetermined throughput rate, the shredding rollers control the moisture content and quantity of the freshly ground material to be supplied as a function of the quantities and the moisture contents of the portion of flakes to be returned and the amount of recycled semolina. A method according to claim 11, characterized in that the moisture content of the recycled semolina is set to a value of about ≦ 0.5% and the moisture content of the recycled portion of the flakes essentially corresponds to the mixed moisture content of the total ground material coming into the roll gap. A method according to claim 11, characterized in that the proportion of recirculated flakes on the one hand and the ratio between recirculated semolina and fresh regrind on the other hand is kept essentially constant in each case. A method according to claim 9, characterized in that the different mass flows of the recycled ground material and the fresh ground material are mixed together shortly before the nip in such a way that the mass flows with relatively moist material on the one hand and the mass flows with relatively dry material on the opposite side are merged. Method according to claim 1, characterized in that in particular slag sand is comminuted as fresh regrind. Plant for comminuting brittle regrind, in particular sand-like regrind such as blastfurnace slag, containing a material bed roller mill (1) with a mill inlet (2), two rollers (3, 4) which are commonly rotating at high pressure and with one in the area between the Roll formed roll nip (5), characterized in that before the mill inlet (2) devices for treating freshly ground material are arranged in such a way that the ground material to the roll nip (5) with a moisture content of about 0.3 to 3.0 %, preferably about 0.5 to 2.0%, can be supplied. Plant according to claim 16, characterized in that the devices before the mill inlet (2) contain a dryer (7) for fresh regrind with a relatively high humidity. Installation according to claim 17, characterized in that the dryer (7) is assigned a bypass line (8) for undried fresh regrind and this bypass line (8) in the area of the mill inlet (2) with a dry material line (9) coming from the dryer. is merged. Installation according to claim 16, with a classifying device (11) for classifying ground material comminuted in the material bed roller mill (1), wherein a semolina line (14) is provided for returning the coarse material accumulating in the classifying device to the mill inlet (2), characterized in that that a Schülpenleitung (15) for returning at least a portion of the Schülpen coming from the Gutbett roller mill to the mill inlet (2) is branched from the regrind line (12) leading from the material bed roller mill (1) to the viewing device (11). Installation according to claim 19, characterized by moisture measuring devices (21 ... 25) at least for freshly introduced regrind (6), recirculated semolina (14), the total regrind at the mill inlet (2) and the comminuted regrind (12) and by a moisture control device (20 ), which is connected to the moisture measuring devices and in which the moisture measuring signals of these measuring devices can be processed as control signals for a preselectable mixed moisture content.

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