EP0406591A2 - Method and arrangement for milling material to be milled - Google Patents

Abstract

The invention relates to a method and a system for comminuting regrind, the regrind first passing through a grinding stage operating on the principle of pressure comminution and then without further grinding a classification stage consisting of two classifying units (6, 7) set to different fineness is fed, the fine material flows of the two classifying units (6, 7) being mixed with one another. The combination of these two measures achieves a desired flattening of the grain size distribution of the end product.

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.

It is known that cements which are ground in a closed circuit with a material bed roller mill, deagglomerator and classifier or in roller bowl mills do not correspond in their quality properties to those which are produced in ball mills. The same applies to blast furnace flour and mixed products made from these components. The differences in quality are expressed primarily in the 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 concrete's water requirement 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 in the case of concrete that is produced with products from material bed roller 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 roller 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 carried out more normally 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.

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 total 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 of the fineness of cement, blast furnace flour and similar products is the mass-based 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 strength defined in the relevant standards is higher, the narrower the product grain spectrum is).

The fewer fines contained 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 when introducing sharp separators in ball mills and has in some cases 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 grain 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), to 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 Ku gelmühlen systems to adapt 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.

The solution according to the invention therefore basically consists in the combination of two process steps, both of which broaden the fine grain spectrum:
- By recycling a high proportion of the ground material immediately after passing through the grinding stage, the mean residence time of the ground material in the mill operating on the principle of pressure reduction or the frequency of use is increased and a higher proportion of fine material in the feed material of the classification stage is achieved, which widens the grain spectrum of the Results in feed of the classification level.
- By sifting the rest of the ground material in two practically parallel, differently fine classifying units with high separation quality and mixing the two fine material streams, the product grain line is flattened in the upper area.

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 dwell 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 above-mentioned condition for optimal energy utilization (removal of the fine material from the cycle as quickly as possible) is no longer completely present, 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 (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 area zone is (for example, if 80% of the slugs are returned to the material bed roller mill, the ground material passes through the material bed roller mill five times on average). 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 features 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 range 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 size spectrum can be broadened significantly, but mostly 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 much).

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 fines.

• 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 for example in the drawing. Show it

• 1 shows a basic diagram of a plant according to the invention with a grinding stage formed by a material bed roller mill;
• 2 and 3 variants of the system according to Fig.1;
• 4 shows a diagram for explaining the effect achieved by the measures according to the invention;
• 5 shows a similar basic diagram 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 quantity 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, together with the fresh regrind fed from the feed hopper 1 and the Schülpenan fed back from the first material flow divider 3 partly fed to the feed chute 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 the regrinding of 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 achievable with the method according to the invention (curve a) compared 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 millbase by at least one due to its comminution arena When 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 illustrated by the basic diagram according to FIG. In a bowl mill, too, 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 according to FIG. 5 is essentially constructed in the same way as the plant according to 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 bowl mill 12 is preferably a sightless roller bowl mill, which is fed material to be shredded via an associated feed shaft 12a in a manner known per se and from which the shredded material - also in a manner known per se - via a material outlet 12b, downwards failing, discharged as a whole and in turn 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, the same as that of FIG. 2, so that the other system parts again have the same reference numerals as in the example of 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 in the required manner between the two classification units 6 and 7, which in this case ensures a more reliable screening or classification ensure than is generally the case with roller 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 also in one
example
explains that the grinding of blast furnace sand in a semi-industrial grinding plant with a good-bed roller mill and high-performance classifiers concerns. The following operating modes are compared:

• a) conventional grinding without return of 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, with 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 the 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. Process for comminuting regrind, characterized by the combination of the following process steps: a) the ground material first passes through a grinding stage in a multiple circulation, which works on the principle of pressure comminution, part of the throughput of the grinding stage being returned to this grinding stage immediately after passing through the grinding stage; b) the rest of the throughput of the grinding stage is fed without further grinding to a classification stage which consists of at least two classifying units set to different fineness, each of which is supplied with selectable partial flows of the material withdrawn from the grinding stage and their fine material flows are mixed with one another. 2. The method according to claim 1, characterized in that at least one material bed roller mill is used as the grinding stage. 3. The method according to claim 1, characterized in that at least one roller mill is used as the grinding stage, which preferably works with an externally provided classifier. 4. The method according to claim 1, characterized in that at least 50% of the throughput of the grinding stage immediately after passing through the grinding stage are fed back to this grinding stage. 5. The method according to claim 4, characterized in that 70 to 95% of the throughput of the grinding stage are fed to this grinding stage again immediately after passing through the grinding stage. 6. The method according to claim 1, characterized in that the mass-related surface area (according to Blaine) of the remainder of the throughput withdrawn from the grinding stage and fed to the classification stage is 0.5 to 0.8 times the mass-based surface area of the finished product. 7. The method according to claim 1, characterized in that the mass-related surface (according to Blaine) of the fine material of a classifying unit 1.2 to 2.0 times and that of the other classifying unit 0.4 to 0.8 times the mass-based surface of Finished product. 8. The method according to claim 1, characterized in that the separation limits of the two classifying units are set in a ratio of 1: 1.5 to 1: 4. 9. The method according to claim 1, characterized in that at least a partial stream of the fine material of the classifying stage is subjected to post-grinding, preferably in a ball mill. 10. The method according to claim 2, characterized in that the rest of the throughput quantity of the grinding stage not supplied to the grinding stage is first one Desagglomerator stage is fed before it then reaches the classification stage. 11. Plant for crushing regrind, characterized by the combination of the following plant parts: a) a mill (2, 12) operating on the principle of pressure reduction, b) a first crop flow divider (3) connected downstream of this mill (2, 12), through which an adjustable part of the throughput of the mill can be returned to the feed shaft (2a, 12a) of this mill and the rest of the throughput can be fed to a classification stage, c) two classifying units (6, 7) forming the classifying level and set to different fineness, d) a second crop flow divider (5) upstream of the classification stage for dividing the crop flow to be fed to the classification stage between the two classification units (6, 7). 12. Plant according to claim 11, characterized in that the mill operating on the principle of pressure reduction is formed by a material bed roller mill (2). 13. Plant according to claim 12, characterized in that a deagglomerator (4) is arranged between the two crop flow dividers (3, 5). 14. Plant according to claim 11, characterized in that the mill working on the principle of pressure comminution is formed by a bowl mill (12), which is preferably designed in the form of a faceless bowl mill for ground material falling down. 15. Plant according to claim 11, characterized in that a further mill, preferably a ball mill (8), is used downstream of the classifying stage for subsequent grinding of at least part of the fine material of the classifying stage.

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