EP0406591B1 - Method and arrangement for milling material to be milled - 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 11) for comminuting regrind.

DE-A-37 19 251 already discloses a process and a plant for the continuous pressure comminution of brittle ground material, such as e.g. Cement clinker, known. In this case, the material subjected to a material bed crushing and thereby crushed and pressed material is deagglomerated, and the sufficiently fine finished material is sorted out from the deagglomerated material in a sifter, while the coarse material is recirculated for a new material bed shredding. The special feature of this known embodiment is seen in the fact that the material comminuted in a first roller mill or roller press is pressed or comminuted at least a second mill before it is deagglomerated in the roller gap of at least one further roller mill or roller press. There is also the possibility of recirculating a partial flow of the material leaving the second or last roller mill of this second or last roller mill as a cycle load, so that so-called pressure islands of comminuted material are to be avoided.

It is also known that cements which are ground in a closed circuit with a material bed roller mill, deagglomerator and classifier or in roller bowl mills, in terms of their quality properties do not correspond to those that are produced in ball mills. The same applies to blast furnace flour and to mixed products made from these components. The quality differences are expressed primarily in an increased water addition to the standard stiffness of the mortar compared to ball mill products. Experience has shown that this size is a measure of the water required by the concrete to achieve a certain consistency. An increased water addition to the standard stiffness corresponds to an increased water requirement of the concrete. So far, one has to set a higher water-cement ratio for concrete that is produced with products from Gutbett roll mills or roller bowl mills in order to achieve the same processability. The result is a higher pore volume and accordingly a lower strength.

The quality differences mentioned are largely due to the narrower grain spectrum that products from Gutbett roll mills or roller bowl mills have compared to ball mill products. In the known grinding plants with material bed roller mills or roller bowl mills, the classification step is usually carried out immediately after the grist has once passed through the mill. This ideally fulfills the requirement of the shredding theory to remove the fine material generated from the cycle as quickly as possible in the interest of optimal energy utilization. The regrind that is fed to the classifier accordingly contains much less fine material in grinding plants of this category than is the case in ball mill plants.

• die Korngröße d′ für einen spezifischen Siebrückstand (36,8 %)
• und das Steigungsmaß n, das dem Tangens des Winkels zwischen Körnungslinie und Abszisse entspricht.

The grain spectrum of cement and cement-like products is usually represented as a sum distribution in the "RRSB network" developed by Rosin, Rammler and others. The axis scales of this network are chosen so that the sum distributions of normal mineral comminution products appear as straight lines. These total distributions are described by two parameters:

• the grain size d ′ for a specific sieve residue (36.8%)
• and the pitch n, which corresponds to the tangent of the angle between the grain line and the abscissa.

The basis of this method of representation is the fact that the particle size distributions of many mineral comminution products have a similar shape regardless of their fineness.

The usual measure for the fineness of cement, blast furnace flour and similar products is the mass-related surface according to Blaine. The higher this fineness, the higher the strength of those made from it Mortar and concrete. The mass-related surface is inversely proportional to the average grain size (with the same mass-related surface, the narrower the product grain spectrum, the higher the strength defined in the relevant standards).

The fewer fines in the feed of the classifier, the finer its separation limit (see below) must be set so that the product has the desired mass-related surface. To compensate for the lack of fine, a corresponding portion of the coarse must be separated. The product grain spectrum is thus narrowed from both sides. As a result, the porosity of the product increases, and with the porosity the water requirement increases to achieve a certain mortar or concrete consistency.

The relationship explained above has been observed with the introduction of sharp separators in ball mills and in some cases has led to product complaints.

The invention is therefore based on the object of designing a method (according to the preamble of claim 1) and an installation (according to the preamble of claim 11) in such a way that the particle size distribution of the finished product can be set in a targeted manner over a sufficiently large range (and in fact expressed) as a change in the pitch dimension n in the RRSB network - by at least 0.2), the products of such energy-saving grinding plants with respect to their grain size distribution and thus also with regard to their processing behavior and strength development to the standard of ball mill plants adapt the products produced and also to compensate for chemically (for example, due to the raw material) defects by appropriately adjusting the grain size distribution of the finished product.

This object is achieved by the combination of the characterizing features of claims 1 and 11, respectively. Appropriate embodiments of the invention are the subject of the dependent claims.

• Durch Rückführung eines hohen Anteiles des Mahlgutes unmittelbar nach dem Passieren der Mahlstufe wird die mittlere Verweilzeit des Mahlgutes in der nach dem Prinzip der Druckzerkleinerung arbeitenden Mühle bzw. die Beanspruchungshäufigkeit erhöht und ein höherer Feingutanteil im Aufgabegut der Klassierstufe erzielt, was eine Verbreiterung des Kornspektrums des Aufgabegutes der Klassierstufe zur Folge hat.
• Durch Sichtung des restlichen Mahlgutes in zwei praktisch parallel geschalteten, unterschiedlich fein eingestellten Klassieraggregaten hoher Trenngüte und Mischen der beiden Feingutströme erfährt die Produkt-Körnungslinie im oberen Bereich eine zusätzliche Abflachung.

In contrast to the known embodiment according to DE-A-37 19 251 explained at the outset, in which at least two grinding stages formed by material bed roller mills or roller presses are connected in direct succession and only in the second or last grinding stage is a recirculation of a partial flow of the there is carried out, the solution according to the invention basically consists in the combination of two process steps, both of which broaden the fine grain spectrum:

• By returning a high proportion of the regrind immediately after passing through the grinding stage, the mean dwell time of the regrind in the mill operating on the principle of pressure reduction or the frequency of use is increased and a higher proportion of fines in the feed material of the classification stage is achieved, which widens the grain spectrum of the feed material the classification level.
• Sifting through the rest of the ground material in two practically parallel, differently finely adjusted classifying units with high separation quality and mixing of the two fine material flows the product grain line in the upper area an additional flattening.

These two measures according to the invention and the effects achieved thereby are considered in more detail below.

If a large proportion of the millbase of the mill, which is designed as a material bed roller mill, is returned directly to the feed shaft of this mill (so-called scraper return), the higher mean residence time or greater frequency of use of the millbase in the mill results in a higher proportion of fines in the feedstock of the classification stage . The same applies, for example, if a roller bowl mill is used as a mill in the grinding stage. In order to achieve the same mass-related surface of the finished product, the separation limit of the classification has to be shifted towards the coarse, which flattens the grain line of the finished product, especially in the range below 10 µm (compared to an operation without return of flakes or regrind).

Since the condition mentioned at the outset for optimal energy utilization (removal of the fine material from the cycle as quickly as possible) is no longer fully met, the energy consumption for the grinding is increased by the return of the pulp or regrind, but this procedure still offers energetic advantages compared to the combined operation with roller mill and ball mill.

According to the invention, at least 50%, but preferably 70 to 75%, of the throughput quantity of the grinding stage is returned to this grinding stage immediately after passing through the grinding stage (that is to say, for example, the material bed roller mill). Most of the circulation is thus transferred to the Gutbett roll mill. This multiplies the residence time in the grinding area, which is proportional to the circulation in the grinding zone (If, for example, 80% of the slugs are returned to the material bed roller mill, the ground material passes through the material bed roller mill on average five times). The same applies here again roughly analogously when using a roller mill.

The following relationships are essential for understanding the second measure according to the invention (sifting the ground material in two classifying units of different fineness):
In the technical separation (classifying or sifting) of bulk goods, the cut between "fine" and "coarse", the so-called separation limit, is not perfect, but extends over a certain range of the grain spectrum. The grain size is specified as the separation limit "d (50)", at which 50% of the feed material in the fine material reaches the coarse material. In addition, part of the feed usually comes unclassified into the coarse. For this reason, in addition to the separation limit for technical separations, certain quality characteristics must also be specified.

The overall separation quality of a classification is characterized by the selectivity and the separation efficiency.

The selectivity says something about the width of the area in the grain spectrum, in which material can get into both the fine material and the coarse material. The narrower this area, the higher the selectivity. It is quantified by the ratio of the particle sizes for which 30% and 70% of the feed material in the coarse access ( "κ _30/70").

The separation efficiency relates to the proportion of the feed material that is classified at all, or the proportion that goes unclassified into the coarse material. The higher the latter, the lower the separation efficiency and the more fine particles remain in the coarse material. The parameter for the unclassified portion is called "τ".

In the investigations on which the invention is based, it has now been shown that by reducing the selectivity with a consistently high separation efficiency (τ = 0), the fine grain spectrum can be broadened significantly, but mainly in the area of the coarser fractions.

This cannot be achieved to a sufficient extent in a classifying unit, since for this purpose the classifying conditions in the entire classifying zone would have to be variable. According to the invention, two classifying units are therefore used, the separation limits of which are set differently, expediently in a ratio of 1: 1.5 to 1: 4. The resulting separation characteristic has a low selectivity. In this way, the product grain line is flattened in the area above 10 µm (compared to an operation with only one classifying unit).

The combined use of the two features according to the invention brings about a reduction in the degree of increase n in the product grain size distribution in the RRSB network by more than 0.2 (from 1.2 to 0.95), which will be explained later using an example.

The mass-related surface (according to Blaine) of the subtracted from the grinding stage and fed to the classification stage According to the invention, the remainder of the throughput is expediently 0.5 to 0.8 times the mass-related surface of the finished product (in the case of a conventional operating mode without return of the slugs, this range is 0.2 to 0.4 times as compared).

In the classifying stage, the mass-related surface (according to Blaine) of the fine material of one classifying unit (which supplies the finer component) is expediently 1.2 to 2.0 times the mass-based surface of the finished product, while the other classifying unit (which produces the coarser component) delivers a fine material, the mass-based surface of which is 0.4 to 0.8 times the mass-based surface of the finished product.

In addition to the measures described, the regrinding of at least a partial flow of the fine material of the classification stage is possible in a ball mill.

In addition to refinement, this causes a further broadening of the product grain size distribution. There is also the possibility of mixing two sub-streams of fine material.

Fig.1

ein Prinzipschema einer erfindungsgemäßen Anlage mit einer von einer Gutbettwalzenmühle gebildeten Mahlstufe;

Fig.2 und 3

Varianten der Anlage gemäß Fig.1;

Fig.4

ein Diagramm zur Erläuterung des durch die erfindungsgemäßen Maßnahmen erzielten Effektes;

Fig.5

ein gleichartiges Prinzipschema wie etwa Fig.2, mit einem Beispiel, in dem die Mahlstufe von einer Walzenschüsselmühle gebildet wird.

The invention is illustrated in the drawing, for example. Show it

Fig. 1

a schematic diagram of a system according to the invention with a grinding stage formed by a material bed roller mill;

Fig. 2 and 3

Variants of the system according to Fig.1;

Fig. 4

a diagram for explaining the effect achieved by the measures according to the invention;

Fig. 5

a similar principle as for example Fig.2, with an example in which the grinding stage is formed by a roller mill.

The plant shown in FIG. 1 for comminuting regrind, in particular cement clinker, contains a feed hopper 1 provided with metering devices, a material bed roller mill 2, a first material flow divider 3 provided with a metering device, a deagglomerator 4, a second material flow divider 5 and two classifying units 6 and 7. By means of the first material flow divider 3 connected downstream of the material bed roller mill 2, an adjustable part of the throughput of the material bed roller mill 2 is returned directly to the feed shaft 2a of the material bed roller mill 2, so that a multiple pass of the ground material through the material bed roller mill, one Increasing the mean dwell time of the millbase in the mill and accordingly a higher proportion of fines in the material to be discharged from the mill bed roller mill.

The rest of the throughput, which is not returned to the material bed roller mill, passes via the deagglomerator 4 to the second material flow divider 5, which divides the total material flow supplied to the classifying stage into the two classifying units 6 and 7. The coarse material accumulating in the classifying units 6 and 7 is combined with the fresh regrind fed from the feed bunker 1 and the portion of scraps returned from the first material flow divider 3 fed to the feed shaft 2a of the material bed roller mill 2.

The fine material flows of the two classifying units 6 and 7, which are set to different fineness, are mixed with one another and form the finished product, which is characterized by a flat grain size distribution.

The exemplary embodiment according to FIG. 2 differs from the system according to FIG. 1 only in that the deagglomerator 4 is omitted.

In the variant illustrated in FIG. 3, in addition to the system parts shown in FIG. 1, a ball mill 8 is also provided, which is used for regrinding at least part of the fine material of the classifying stage formed by the classifying units 6 and 7.

Fig. 4 illustrates in a schematic representation, but based on the RRSB network, the product grain size distribution (curve a) that can be achieved with the method according to the invention in comparison to the conventional mode of operation (curve b), in which neither Schülpenrücklauf, nor a mixing of two fine material flows of different fineness take place.

The grain size d is plotted in µm on the abscissa and the sieve residue R is plotted in% on the ordinate.

While in the examples of the invention explained above, the grinding stage for the multiple circulation of the ground material by at least one due to its comminution work Particularly preferred material bed roller mill 2 is formed, a roller bowl mill can also be used in many cases for this grinding stage, as is illustrated by the basic diagram according to FIG. Even in a bowl mill, the regrind that passes through the grinding stage several times is crushed according to the principle of pressure crushing.

As has been mentioned above, the plant according to the example in FIG. 5 is essentially of the same construction as the plant in FIG. 2, but with the difference that instead of the material bed roller mill 2, a roller bowl mill 12 forms the grinding stage here. This roller-type bowl mill 12 is preferably a sightless roller-type bowl mill, to which the material to be shredded is fed in a known manner via an associated feed chute 12a and from which the shredded material - also in a manner known per se - via a material outlet 12b, downwards failing, discharged overall and in turn is fed to the first crop flow divider 3.

The rest of the procedure in this system example according to FIG. 5 is otherwise - as already mentioned, similar to that in FIG. 2, so that the other system parts again have the same reference numerals as in the example in FIGS. 1 to 3 - with the exception of one that is unnecessary here Desagglumerators 4 - are provided. Here again the second crop flow divider 5 ensures that the crop stream to be fed to the classification stage is divided as required between the two classification units 6 and 7, which in this case ensure a more reliable screening or classification, than is generally the case with bowl mills with a built-on classifier.

• a) herkömmliches Mahlen ohne Schülpenrückführung unter Verwendung eines einzigen Sichters,
• b) Mahlen mit Schülpenrückführung unter Verwendung eines einzigen Sichters,
• c) Mahlen mit Schülpenrückführung und zusätzlicher Klassierung in zwei unterschiedlich fein eingestellten Klassierern unter Mischung der Feingutströme.

The invention is further explained using an example which relates to the grinding of blast furnace sand in a semi-industrial grinding plant with a good-bed roller mill and high-performance classifiers. The following modes of operation are compared:

• a) conventional grinding without return of the flakes using a single classifier,
• b) grinding with return of flakes using a single classifier,
• c) Grinding with return of the scraps and additional classification in two classifiers of different fineness with a mixture of the fine material flows.

The fineness (ie the mass-related surface) of the individual material flows (in cm² / g) and the quantitative ratios are given below, the system throughput (= fresh material = finished product) being set to 1.0. The circulation factors as well as characteristic values of the classification and the product grain size distribution are also given. to a): Fresh produce <100 cm² / g 1.00 Regrind according to roller mill (z. Class.) 1330 cm² / g 6.00 Classifier coarse material (e.g. loading shaft) 830 cm² / g 5.00 Finished product 4170 cm² / g 1.00 The circulation factor is 6.0;
The separation limit d₅₀ of the classification is 13.5 µm;
the _selectivity K _30/70 is 0.64;
the pitch n of the product grain size distribution is 1.20;
the position parameter d 'is 12 µm. To b): Fresh produce <100 cm² / g 1.00 Grist according to roller mill 2290 cm² / g 7.50 Share to feed chute (Schülpen-R.) 2290 cm² / g 5.50 Share to the classifier 2290 cm² / g 2.00 Classifier coarse 600 cm² / g 1.00 Return goods to the feed chute - 6.50 Finished product 4100 cm² / g 1.00 The total circulation factor is 7.5;
the proportion of Schülpen repatriation is 73%;
the classifier circulation factor is 2.0;
the separation limit d₅₀ of the classification is 33 µm;
the _selectivity κ _30/70 is 0.65;
the pitch n of the product grain size distribution is 1.00;
the position parameter d 'is 15 µm. To c): Share to the classification level 2290 cm² / g 2.00 Share of the roughly set classifier - 1.05 Share to the finely adjusted classifier - 0.95 Coarse goods of the roughly set class. - 0.40 Coarse goods of finely adjusted class. - 0.60 entire coarse classifier 660 cm² / g 1.00 entire return goods to the loading shaft - 6.50 Coarse-grained fine goods. Classifier 3050 cm² / g 0.65 Fine goods from the finely set. Classifier 6010 cm² / g 0.35 entire finished product 4100 cm² / g 1.00 The circulation factor on the roughly set classifier is 1.6;
the circulation factor on the finely adjusted classifier is 2.7;
the total classifier circulation factor is 2.0;
the separation limit d₅₀ of the rough partial classification is 74 µm;
the _selectivity κ _{30/70 of} the rough partial classification is 0.63;
The separation limit d₅₀ of the fine classification is 18 µm;
the _selectivity κ _{30/70 of} the fine partial classification is 0.63;
the separation limit d₅₀ of the total classification is 42 µm;
the _selectivity κ _{30/70 of} the total classification is 0.37;
the pitch n of the product grain size distribution is 0.92;
The position parameter d 'is 17.5 µm.

Claims (15)

1. Method of crushing material for grinding, characterised by the combination of the following method steps:

   a) the material for grinding first of all passes in multiple circulation through a grinding stage which operates on the pressure crushing principle, a proportion of the throughput of the grinding stage being delivered back to this grinding stage immediately after having passed through the grinding stage;

   b) the rest of the throughput of the grinding stage is delivered without further grinding to a classification stage which consists of at least two classification assemblies set to different degrees of fineness, selectable branch streams of the material drawn off from the grinding stage being delivered to each of these assemblies and the streams of fines therefrom being mixed together.
2. Method as claimed in claim 1, characterised in that at least one material bed roller mill is used as the grinding stage.
3. Method as claimed in claim 1, characterised in that at least one bowl roller mill which preferably operates with an external classifier is used as the grinding stage.
4. Method as claimed in claim 1, characterised in that at least 50% of the throughput of the grinding stage is delivered back to the grinding stage immediately after passing through the grinding stage.
5. Method as claimed in claim 1, characterised in that 70 to 95% of the throughput of the grinding stage is delivered back to the grinding stage immediately after passing through the grinding stage.
6. Method as claimed in claim 1, characterised in that the specific surface area (Blaine fineness) of the rest of the throughput quantity drawn off from the grinding stage and delivered to the classification stage is 0.5 to 0.8 times the specific surface area of the finished product.
7. Method as claimed in claim 1, characterised in that the specific surface area (Blaine fineness) of the fines from one classification assembly is 1.2 to 2.0 times and that of the other classification assembly is 0.4 to 0.8 times the specific surface area of the finished product.
8. Method as claimed in claim 1, characterised in that the separation limits of the two classification assemblies are set in a ratio of 1 : 1.5 to 1 : 4.
9. Method as claimed in claim 1, characterised in that at least a branch stream of the fines from the classification stage is subjected to regrinding, preferably in a ball mill.
10. Method as claimed in claim 2, characterised in that the rest of the throughput quantity of the grinding stage which is not returned to the grinding stage is first of all passed to a disagglomerator stage before it enters the classification stage.
11. Apparatus for crushing material for grinding, characterised by the combination of the following parts:

    a) a mill (2, 12) operating on the pressure crushing principle,

    b) a first material stream divider (3) which is arranged after this mill (2, 12) and through which an adjustable proportion of the throughput quantity of the mill can be returned to the feed shaft (2a, 12a) of this mill and the rest of the throughput quantity can be delivered to a classification stage,

    c) two classification assemblies (6, 7) which are set to different degrees of fineness and form the classification stage,

    d) a second material stream divider (5) which is arranged before the classification stage and by which the stream of material to be delivered to the classification stage is distributed to the two classification assemblies (6, 7).
12. Apparatus as claimed in claim 11, characterised in that the mill operating on the pressure crushing principle is formed by a material bed roller mill (2).
13. Apparatus as claimed in claim 12, characterised in that a disagglomerator (4) is arranged between the two material stream dividers (3, 5).
14. Apparatus as claimed in claim 11, characterised in that the mill operating on the pressure crushing principle is formed by a bowl roller mill (12) which is preferably constructed in the form of a bowl roller mill for material for grinding which falls out downwards and has no separator.
15. Apparatus as claimed in claim 11, characterised in that a further mill, preferably a ball mill (8), is arranged after the classification stage and serves for regrinding at least a proportion of the fines from the classification stage.

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