EP0292724B1 - Method and arrangement for breaking up brittle material - Google Patents

Description

The invention relates to a method (according to the preamble of claim 1) and a system (according to the preamble of claim 10) for crushing brittle regrind.

A method according to the preamble of claim 1 is known for example from EP-A-84 383.

Difficulties can arise here in the crushing of regrind of high humidity, which tends to stick and / or bake above a critical moisture value. For example, there are clay-containing raw material components in cement production that neither stick nor lead to moisture when the humidity is below 15%, but stick extremely strongly when the moisture is above 18% and even adhere to vertical walls.

Even with ore and diamond-containing volcanic rock, the properties of the bulk material and thus also the feed behavior of the regrind into the material bed roller mill deteriorate to a marked extent above one critical moisture content.

The invention is therefore based on the object to design a method according to the preamble of claim 1 and a system according to the preamble of claim 10 so that even high moisture grist, which tends to stick and / or bake above a critical moisture value, comminutes properly can be.

According to the invention, this object is achieved by the characterizing features of claims 1 and 10, respectively. Appropriate embodiments of the invention are the subject of the dependent claims.

From EP-A-220 681, a system for comminuting brittle regrind is already known, in which a material bed roller mill with two rollers pressed against one another at high pressure is used, to which dried rejects are fed from a classifier. In this known embodiment, however, it should above all be ensured that the roller mill used receives a feed material with a reasonably uniform particle size distribution and that this material bed roller mill can be meaningfully integrated during the milling drying of moist material. Based on this task, however, an embodiment is proposed which differs significantly from the present invention both in terms of process technology and system technology. In this known embodiment, a two-stage comminution of regrind takes place initially with the aid of two different mills, these two mills being connected to form a recirculating grinding system via a single classifier, and in contrast to the present invention - expressly the fresh, to be comminuted (moist) starting material should not be fed to the roller mill, but preferably to an impact hammer mill. Accordingly, moist fresh material is pre-comminuted in this first mill, which is preferably designed as an impact hammer mill, and at the same time partially dried with the supply of a hot gas stream in this mill. All of the comminution product coming from this impact hammer mill is dried in a current dryer and at the same time conveyed to the classifier for the purpose of separation into fine finished goods and coarse return goods (semolina). Only the coarse return material from the classifier is - according to the example in Fig. 1 - fed to the material bed roller mill for further comminution, from where it is at least partially fed either directly to the current dryer or - together with moist fresh material - to the inlet of the impact hammer mill.

In contrast, according to the present invention, the moist, fresh feed material is also fed to the two rollers of the material bed roller mill together with the recycled regrind partial stream. If at least part of the partial stream returned to the roll nip is sufficiently dried before it is re-introduced into the roll nip, it can be achieved that a mixed moisture which is below the critical moisture content is established in the roll nip. In this way, the properties of the bulk material and the intake behavior of the ground material are decisively improved and sticking and caking of the ground material in the material bed roller mill are avoided.

By supplying a dried partial stream the millbase to the material bed roller mill also achieves contact drying in the roller mill, which considerably facilitates the drying of the material as a whole.

The method according to the invention enables simple adaptation to a variable degree of moisture in the ground material to be ground. There is also the possibility of detecting only seasonal peaks in the material moisture by temporarily and / or partially drying and operating the method during the rest of the time without the use of heat.

Three exemplary embodiments of the method according to the invention are illustrated in FIGS. 1 to 3, each in a basic scheme.

The process diagram according to FIG. 1 contains a stage 1, in which the fresh regrind is drawn off. This stage 1 can be formed, for example, by a bunker, a plate belt or a screw.

In a stage 2, the fresh regrind is mixed with recycled regrind. This stage 2 can be formed, for example, by a belt, a box feeder or a mixing screw.

In a stage 3, the ground material is crushed. This stage 3 is formed by a material bed roller mill, the two rollers of which are pressed against one another at high pressure.

The ground material crushed in stage 3 then passes into stage 4, which is used to disintegrate or deagglomerate the ground material. It is formed, for example, by a hammer mill, an impact mill, a short grinding tube, a mixing bowl or a disintegrator.

The deagglomerated regrind then passes from stage 4 to stage 5, in which the regrind is dried. This stage can be formed, for example, by a riser dryer, a spreading dryer or a drying drum.

The crushed, deagglomerated and dried regrind then reaches a classification stage which consists of the sub-stages 6.1 and 6.2 connected in series. Partial stage 1 can be formed, for example, by a deflector, the separating cut of which is, for example, in the range between 200 and 500 μm. The sub-stage 6.2 can be formed by a static classifier or a rotor classifier be, the separation cut is for example in the range between 10 and 50 microns or between 20 and 100 microns. The coarse material of both sub-stages 6.1 and 6.2 is returned to stage 2 and mixed with fresh regrind there before being re-introduced into the comminution stage.

The finished product is finally separated off in a stage 7. This stage 7 can be formed by a cyclone, a filter or by classifier cyclones.

• in der Stufe 1 die Durchsatzmenge und Feuchte des frischen Mahlgutes,
• in der Stufe 2 die Antriebsleistung und die Mischfeuchte,
• in der Stufe 3 die Antriebsleistung und die Spaltweite der Gutbett-Walzenmühle (eventuell das Moment und die Umfangsgeschwindigkeit),
• in der Stufe 4 die Antriebsleistung des für das Aufschließen bzw. Desagglomerieren verwendeten Aggregates,
• in der Stufe 5 die Gastemperatur vor und hinter dem Trockner sowie die Antriebsleistung,
• in der Stufe 6 die Einstellung des Klassierers, die Antriebsleistung, die Rückgutaufteilung, der Rückgutmassenstrom sowie die Rückgutfeuchte,
• in der Stufe 7 die Restfeuchte des Produktes.

The following parameters are expediently measured in the individual stages:

• in level 1 the throughput and moisture content of the fresh ground material,
• in stage 2 the drive power and the mixed moisture,
• in stage 3 the drive power and the gap width of the material bed roller mill (possibly the moment and the peripheral speed),
• in stage 4 the drive power of the unit used for the unlocking or deagglomeration,
• in stage 5 the gas temperature in front of and behind the dryer and the drive power,
• in stage 6, the setting of the classifier, the drive power, the return material distribution, the return material mass flow and the return material moisture,
• in level 7 the residual moisture of the product.

In the process diagram according to FIG. 2, the same reference numerals as in FIG. 1 are used for the same elements.

Stages 4 and 5 can be combined here into a single step, so that the ground material crushed in the nip is dried and deagglomerated at the same time. A scatter dryer, possibly with a short grinding part, can be used for this purpose.

Furthermore, in the method diagram according to FIG. 2, two sub-stages 6a and 6b connected in parallel are provided in the classifying stage, the coarse material occurring in both sub-stages being returned to stage 2. The fine material flows of the sub-stages 6a and 6b are jointly passed to stage 7 for the separation of the finished product.

In sub-stage 6a, for example, the material discharged by gravity can be classified in the high-performance classifier (for example in a rotor classifier with its own air circulation).

In sub-stage 6b, the material discharged with the gas flow can be classified in a flow classifier.

Here too, both classifying units, i.e. the sub-stages 6a and 6b are combined.

In the process diagram according to FIG. 3, stages 4 and 5 are interchanged in the order. The ground material crushed in stage 3 is thus first dried in stage 5 and then disrupted or deagglomerated in stage 4.

The goods discharged from stage 4 are partially fed to sub-stage 6a and partially to sub-stage 6b. The latter also receives a partial flow of the material discharged from stage 5.

The drying units for stage 5 include, in particular, a spreading dryer, an impact dryer or a drying drum in question. Since a certain deagglomeration already occurs in the dryer, this part of the dried material is fed together with the gas stream to sub-stage 6b (preferably a flow classifier). The rest of the dried material, which still contains agglomerates, is fed to stage 4 for digestion. A hammer or impact mill, a disintegrator, a short tube mill or a mixing bowl can be considered as an aggregate for stage 4.

The material broken down in stage 4 is mechanically guided to sub-stage 6a (a classifier). Here, too, the sub-stages 6a and 6b can be combined, for example in the form of a current classifier with partial mechanical loading.

With pure circulation grinding, i.e. when the entire fine material is produced in the material bed roller mill, the entire coarse material of the classification is returned to the roller mill. The entire crop flow is expediently dried here.

Under certain circumstances, however, a portion of the finished goods must also be returned to the roller mill, and expediently one in a partial stream of the dried, but not yet classified material contained in the finished product.

Another possibility in the case of high moisture content of the fresh ground material is to operate the material bed roller mill with an energy conversion level that has been reduced so far and thereby to increase the circulation factor to such an extent that, with a predetermined drying of the ground material in the roller gap, a mixed moisture content arises which is below the critical moisture content. Such a case can be given, for example, if the feed moisture is about three times the limit moisture (i.e. the critical moisture content), but if the circulation factor resulting from the grinding (at full energy conversion level) is lower, e.g. 2 is.

The invention is explained in more detail below with the aid of two examples. FIG. 4 shows the system diagram on which this is based, which corresponds to the variant according to FIG. 1 (the reference numerals also used in FIG. 1 being used for the individual stages).

It is assumed that the circulation factor caused by the sighting is 2 (which by the position of the separating cut is set).

For stage 3 a material bed roller mill is used, as stage 4 a hammer mill, as stage 5 a riser dryer, as stage 6.1 a deflector and as stage 6.2 a rotor sifter. Level 7 is a cyclone.

It is assumed that the feed moisture of the fresh ground material is 20% and the so-called "limit moisture" (i.e. the critical moisture content above which the ground material tends to stick and / or bake) is 12%.

   Die innere Feuchtigkeitsbilanz (bezogen auf Stufe 3, d.h. die Gutbett-Walzenmühle) lautet wie folgt:

$\text{1· 0,2 + 0,4 · 0,02 + 0,6 · 0,04 = 2,0· 0,116}$

   Es ergibt sich eine Mischfeuchte von 11,6 %, die damit knapp unterhalb der Grenzfeuchte von 12 % liegt.The external moisture balance can then be expressed as follows (whereby in the individual products the first factor means the proportion of material and the second factor the associated moisture):

$\text{1.0 x 0.2 = 1.0 x 0.01 + 1.0 x 0.19}$

The internal moisture balance (based on level 3, ie the Gutbett roll mill) is as follows:

$\text{1 x 0.2 + 0.4 x 0.02 + 0.6 x 0.04 = 2.0 x 0.116}$

The result is a mixed moisture of 11.6%, which is just below the limit moisture of 12%.

In the example explained above, it is assumed that in sub-stage 6.1, from the total quantity of circulating material (2.0), a coarse material sub-quantity of 0.6 with a moisture content of 4% and in sub-stage 6.2 another sub-quantity of 0.4 is deposited with a moisture of 2%.

If, on the other hand, a coarse moisture content of 2% (according to sub-level 6.1) or 1% (according to sub-level 6.2) is taken as a basis (according to a second example), a mixed moisture content of 10.8% results.

If the limit moisture is now even lower, the circulation would have to be increased artificially, which could be done, for example, by branching off an unclassified partial flow after stage 5 (riser dryer).

The level 5 internal moisture balance in the first example is as follows:

$\text{2.0 x 0.116 = 0.19 + 0.01 x 1 + 0.4 x 0.02 + 0.6 x 0.04}$

Claims (10)

1. Method of crushing brittle material for grinding in the gap between two rollers which are pressed against one another with a high pressure, in which a branch stream of the crushed material for grinding is returned to the roller gap and further crushed together with fresh material for grinding, characterised in that, in order to crush material for grinding which has a high moisture content and above a certain critical moisture value has a strong tendency to sticking and/or caking on, at least a proportion of the branch stream which is returned to the roller gap is dried before it is reintroduced into the roller gap to such an extent that a mixed moisture content which lies below the critical moisture content is produced in the roller gap.
2. Method as claimed in claim 1, characterised in that the material for grinding which is crushed in the roller gap is first of all disagglomerated and then dried.
3. Method as claimed in claim 1, characterised in that the material for grinding which is crushed in the roller gap is first of all dried and then disagglomerated.
4. Method as claimed in claim 1, characterised in that the material for grinding which is crushed in the roller gap is simultaneously dried and disagglomerated.
5. Method as claimed in one of claims 1 to 4, in which the material for grinding which is crushed in a material bed roller mill is classified after drying and disagglomeration, characterised in that all of the oversize material precipitated in the classification is returned to the material bed roller mill.
6. Method as claimed in claim 5, characterised in that a proportion of finished material, preferably a proportion of finished material which is contained in a branch stream of dried but not yet classified material, is returned to the material bed roller mill.
7. Method as claimed in claim 5, characterised in that the material bed roller mill is driven with such a low level of transformation of energy and the recycle factor is thereby increased so much that with predetermined drying of the crushed material for grinding in the roller gap a mixed moisture content which lies below the critical moisture content is produced.
8. Method as claimed in claim 5, characterised in that the classification of the material for grinding takes place in two classification stages which are arranged one behind the other, and the oversize material precipitated in both classification stages is returned to the material bed roller mill.
9. Method as claimed in claim 5, characterised in that the classification of the material for grinding takes place in two branch streams by means of two parallel-connected classification stages, and the oversize material precipitated in both classification stages is returned to the material bed roller mill.
10. Apparatus for crushing brittle material for grinding, containing

    a) a material bed roller mill, consisting of two rollers which are pressed against one another with a high pressure,

    b) an arrangement (6.1, 6.2: 6a, 6b) for classifying the crushed material,

    c) an arrangement for delivering fresh material for grinding and oversize material precipitated from the arrangement for classifying the crushed material for grinding to the material bed roller mill,

    
    
    characterised by
    

    d) an arrangement (5) which serves for drying of at least a proportion of the material for grinding returned to the material bed roller mill (3) and is preferably arranged between an arrangement (4) for disagglomeration of the material for grinding arranged after the material bed roller mill (2) and the arrangement (6.1, 6.2; 6a, 6b) for classifying the material for grinding.

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