EP0489633B1 - Process for reducing brittle materials consisting of a selective desagglomeration and installation for carrying out said process - Google Patents

Abstract

Process for milling brittle material making it possible to obtain a very fine product. The raw material (1) feeds a roller miller (2) of which the product, which contains agglomerates, is sent to a disagglomerator-separator (3). The fine particles are evacuated (4) and the large particles are recycled (6). By way of a characteristic, the rate of disagglomeration is limited in the disagglomerator. Application to cement clinker and to various inorganic substances. <IMAGE>

Description

The present invention relates to an improved process for fine grinding of solid particles of brittle materials, in particular of solid particles of brittle mineral materials.

By brittle materials is meant materials which rupture under the action of mechanical forces into smaller fragments, even if an elastic deformation precedes rupture. Plastics which essentially deform without breaking are therefore excluded from this definition.

It has already been proposed to finely grind (grind) more or less large solid particles of brittle solids using various devices, in particular slow compression mills such as roller mills, as described for example in patents FR - 2 610 540, 2 628 412 and 2 634 402, or even grinders with wheels or certain ball mills which also cause the formation of agglomerates.

The grinding of the filler particles in these devices is in fact accompanied by an agglomeration of the fine particles obtained in the form of aggregates or agglomerates which it is then necessary to dissociate by a subsequent mechanical contribution, the intensity of which is usually less than that applied during grinding / agglomeration. For this, one can use a ball mill, a sole distributor-pneumatic separator group or a calibrator separator. It is also possible to use a device having only a disintegration action (EP-84 383 and FR-2 616 359).

It is also known from these last two documents to recycle part of the products leaving the mill with this same crusher, either as is, or after a simple mechanical or aeraulic sieving (we only recycle the particles or agglomerates rejected by the sieve), or finally after disintegration and sieving (we only recycle then unbroken particles or insufficiently broken and not agglomerates since these have been disintegrated).

In order to simplify the process and the installation, one of the inventors has also described and used an installation comprising a cylinder crusher associated with an efficient deagglomeration device for fine grinding, said deagglomeration device preferably being integrated in the separator allowing to select the finished product leaving the grinding installation (M. PALIARD-F. COCHET-Zement Kalk Gips-2-1990-pp 71 to 76).

In the installation thus described, the deagglomeration device was chosen to be very efficient (deagglomeration rate of 80 to 85%) to allow the extraction of as much finished product as possible on each pass. Operation of the installation has shown that this leads to significant energy consumption for the only disaggregation and to difficulties in producing the finest cements.

Document EP-A-220 681 also discloses a fine grinding installation, in two stages, of solid particles of brittle materials, in particular mineral, in which this material is ground in a high pressure roller mill according to the principle of grinding in a bed of material (according to which the individual particles of material are mutually crushed in a bed of material, i.e. in a mass of loose material compressed between two surfaces) by forming agglomerates which are then disaggregated in a ball or hammer mill with percussion or worm screw, the outgoing product of which is brought to a separator in particular pneumatic (sorter) separating the material into a traction of finished product (having the desired fineness), and into a coarser product fraction which is recycled in a roller mill.

The object of the present invention is to avoid an unnecessary waste of energy to deagglomerate the most resistant agglomerates formed during press crushing and which it has been observed that they can be very economically disintegrated by a new application of l shear-compression grinding force in the cylinder press.

Thus the invention relates to a method which makes it possible to reduce the overall energy consumption and the wear of the equipment in a grinding installation mainly comprising a roller press, a deagglomerator and a particle separator, preferably a pneumatic separator.

The desired effect is obtained by maintaining between 10 and 60% and preferably between 20 and 50% the overall rate of deagglomeration of the deagglomerator placed in the circuit (measured for the passing at d50). This has the effect of transferring part of the work of deagglomeration to the main grinding equipment, namely the roller press where this deagglomeration is carried out for negligible additional energy and wear cost.

Thus the invention is based on the surprising observation that selective deagglomeration makes it possible to use more economically on the one hand the pressure forces of the roller mill and on the other hand the impact forces of the deagglomerator.

The process of the invention comprises the stages which consist in subjecting to crushing, in a slow compression crushing zone, in particular in a roller mill, a brittle raw material, in starting elementary grains, under formation conditions. of elementary grain aggregates smaller than the elementary grains of the starting raw material, said grinding being followed by a controlled deagglomeration of said aggregates, in a deagglomeration and separation zone between sufficiently fine particles (that is to say having the fineness of the finished product), which are removed from the process, and insufficiently fine particles (that is to say having no the fineness of the product) finished), which are recycled to the grinding zone, characterized in that the deagglomeration rate (yield) is maintained between 10 and 60% and preferably between 20 and 50% in the deagglomeration zone. The disaggregation rate (yield) being defined by the weight ratio between the quantity of passers-by released by the disaggregation and the maximum quantity of passers-by liberated, measured for a determined grain size and preferably for the mesh d50 of the finished product.

For a better understanding of the invention, reference will usefully be made to the 3 attached figures.

FIG. 1 represents a known grinding circuit, in a closed circuit, where the raw material of the conduit 1 arrives in the grinder 2 then enters the separator 3. After selection the finished product leaves in 4 and the coarsest particles which leave through the conduit 5 is recycled to the crusher at 6. This grinding circuit is preferably used when the material does not agglomerate or only agglomerates slightly during grinding.

FIG. 2 represents another known mode of grinding system used in the case where the material undergoes agglomeration during the grinding process. In this case, the raw material crushed and agglomerated in the apparatus 2 undergoes disintegration in the apparatus 7, in order to destroy the agglomerates formed. The material is then sorted in a separator 3 to extract the finished product and only the largest elementary particles are recycled in the crusher at 6. In known installations of this type, in order to achieve the most complete disintegration of the material, generally used as disintegrator No. 7, an apparatus of the type impact crusher, vibrating mill or ball mill. A disintegration is thus often carried out, often accompanied by additional grinding of the material.

It is clear that the device of FIG. 2 can be used to implement the invention. In this case, the disaggregation rate in the deagglomerator 7 is limited, so as to give it a value defined above.

FIG. 3 represents a particular and preferred embodiment in accordance with the invention.

The raw material coming from 1 is distributed by the directional flap 12 towards the supply of the mill 1 by 11 and towards the apparatus n ° 8 by 13.

The ground material and agglomerated in the crusher 2 is divided into two streams by the directional flap 14, part of this material is directly recycled through the conduit 16 to the crusher 1 without having undergone any treatment, while the other fraction of the material is directed by the conduit 15 in the apparatus 8.

The apparatus 8 is a deagglomerator which operates under conditions ensuring a partial and selective deagglomeration of the aggregates present in the material coming from the duct 15. The apparatus 9 is a separator, for example an air (pneumatic) separator, which receives the material coming from the selective deagglomerator 8. The latter is preferably designed so as to be integrated into the separator 9 to simultaneously perform the disaggregation and sorting functions.

The finished product is discharged from the separator 9 through the conduit 4, while the rejects from the separator 9, evacuated through the conduit 5, are returned to the crusher 2 with the material coming from the conduit 16.

It is clear that, as a variant, it is not possible to use the conduit 13 and / or the conduit 16, nor, in this case, the flow dividers 12 and 14.

To obtain a partial and selective deagglomeration of the aggregates from the grinder, said aggregates are subjected to mechanical forces, preferably controlled shocks. The factors which can be acted upon are, for example, the intensity of the shocks, their number, the shape of the surfaces exerting these shocks, the speed of rotation in the case of beaters rotating around an axis and the residence time.

The equipment used to achieve the most economical possible deagglomeration of the product crushed by the roller press combines shocks, preferably at low impact speed, with hard surfaces and shears in a dense layer of "suspended" matter "in air or other inert gas. The average impact speed is advantageously from 3 to 100 m / s, preferably 5 to 30 m / s.

The deagglomerator is advantageously a deagglomerator with a vertical axis; it is preferably integrated into the construction of the pneumatic separator and rotated on the same shaft as the separator selection turbine (EP - 80 104 199.7). With this device, the impact blades of metal, ceramic or other wear-resistant material are distributed on a horizontal ring rotating at a speed such that the impact speed in the center of the blade, (average impact speed ) is preferably between 8 and 26 m / s and even more preferably between 8 and 16 m / s.

Preferred conditions of implementation are described below.

The impact blades rotate inside a cylinder coated with shielding, the relief of which opposes the rotational movement of the material entrained by the blades. The amplitude of this relief is advantageously between 1% and 6% of the diameter of this cylinder and preferably between 2% and 4% of the diameter.

The shearing forces are mainly exerted in the shear chamber between the impact plates and the shield of the outer cylinder. The volume of this chamber is very usefully between 0.0001 and 0.001 m3 per t / h of product to deagglomerate and preferably between 0.00015 and 0.0006 m³ per t / h of product to deagglomerate.

The impact surface of the blades is oriented upward and outward relative to the vertical plane which includes the radius of the disc passing through the center of the plate. The upward orientation angle is preferably between 3 and 30 °, the outward orientation angle is preferably between 5 and 45 °.

The material is fed by one or more chutes by gravity to the right of the upper face of the ring described by the impact blades and flows by gravity down from the shear chamber.

In the preferred embodiment, the deagglomerator is located inside the pneumatic separator and the material flows directly from the shear chamber onto the distributor plate located on the upper face of the selection turbine of the pneumatic separator.

• Grains élémentaires : ce sont des grains de matière fragile caractérisés par le fait que, suite à l'application d'un effort suffisant, on observe une rupture de ces grains sans déformation plastique préalable significative (une déformation élastique peut cependant être observée).
• Agrégats ou agglomérés : il s'agit d'un ensemble de grains élémentaires agglomérés entre eux dont la cohésion peut être détruite par application d'un effort inférieur à celui nécessaire à la rupture des grains élémentaires.
• Matière brute : il s'agit de la matière introduite pour la première fois dans le système de broyage.
• Granulométrie apparente : il s'agit de la granulométrie de la matière telle que prélevée dans le circuit de broyage. Cette matière peut être constituée d'agrégats et de grains élémentaires.
• Granulométrie élémentaire : il s'agit de la granulométrie de la matière ayant subi une désagglomération permettant de libérer complètement les grains élémentaires.
• Quantité de passants à une dimension (d) : il s'agit de la proportion de grains (exprimée en % de la masse totale de l'échantillon) dont la dimension moyenne est inférieure à (d).
• Taux de désagglomération : celui-ci est défini par le rapport entre la quantité de passants (à une maille de tamisage donnée) libérés par le désagglomérateur et la quantité de passants (à la même maille de tamisage) maximum libérables.

For a better understanding of the invention, the definitions of the terms used are given below.

• Elementary grains: these are grains of fragile material characterized by the fact that, following the application of sufficient force, a rupture of these grains is observed without significant prior plastic deformation (an elastic deformation can however be observed).
• Aggregates or agglomerates: this is a set of elementary grains agglomerated together whose cohesion can be destroyed by applying a force less than that necessary to break the elementary grains.
• Raw material: this is the material introduced for the first time into the grinding system.
• Apparent particle size: this is the particle size of the material as taken from the grinding circuit. This material can consist of aggregates and elementary grains.
• Elementary particle size: this is the grain size of the material having undergone disaggregation allowing complete release of the elementary grains.
• Quantity of loops at one dimension (d): this is the proportion of grains (expressed as a% of the total mass of the sample) whose average dimension is less than (d).
• Disagglomeration rate: this is defined by the ratio between the quantity of passers-by (at a given screen size) released by the deagglomerator and the maximum amount of passers-by (at the same screen size).

The disaggregation rate for a grinding circuit is preferably measured for the sieve mesh corresponding to the d50 of the finished product (this is the theoretical sieve mesh that 50% of the finished product would pass through).

The deagglomeration rate for a given product is measured on the pass to d50 of the finished product as follows:
In order to obtain complete deagglomeration of the product fed to the deagglomerator without additional grinding thereof, this product (A) is suspended in a liquid which does not react with it and dispersed by mechanical agitation and application of ultrasound for as long as necessary so that the particle size no longer changes. The liquid is for example an alcohol such as methanol, ethanol or propanol, an ether or a hydrocarbon such as benzene, hexane or the like. After this disaggregation (deemed to be total) is measured by laser particle size in liquid medium the true particle size (without agglomerates) of the product and in particular the percentage of product passing through the cutoff mesh corresponding to the d50 of the finished product. Also measured by laser particle size in liquid medium, the apparent particle size (minimum agitation and without application of ultrasound) of the product as fed to the deagglomerator (A) and as leaving the latter (S), and in particular the percentage of product passing through the cutting mesh corresponding to the d50 of the finished product.

The mesh d50 of the finished product is measured on the finished product dispersed in a liquid medium by stirring and application of ultrasound as long as necessary so that the particle size does not change any more. The measurement is carried out by laser granulometry in a liquid medium. The mesh d50 is the dimension such that 50% of the particles are larger than this dimension and 50% of the particles are smaller than this dimension.

A titre d'exemple on a fait fonctionner une installation comprenant un broyeur à cylindres alimenté en clinker de ciment, un désagglomérateur et une trieuse pneumatique. On a analysé la granulométrie avant et après passage dans le désagglomérateur selon la méthode expliquée plus haut, on a obtenu les résultats suivants : $${\text{d50(pf) = 16.10}}^{\text{-3}}\text{mm}$$

$$\text{Xt = 9,4\%}$$

$$\text{Xa = 6,9\%}$$

$$\text{Xs = 7,7\%}$$

d'où $$\text{t(\%) = 100 x}\frac{\text{7,7 - 6,9}}{\text{9,4 - 6,9}}$$

$$\text{t(\%) = 32\%}$$

Les avantages obtenus par le procédé sont notamment les suivants :
Pour une même qualité de produit, le procédé de broyage suivant l'invention permet de réduire de plus de 50% l'énergie de désagglomération sans augmentation de l'énergie de broyage par rapport à l'état antérieur de la technique soit une économie de 0,2 à 0,6 Kwh/t de produit fini. Ceci correspond à une économie de 1 à 3% sur l'énergie de broyage.The disagglomeration rate (t) for a given product (measured at d50 of the finished product) is defined as follows:
Xt percentage passing to d50 (pf) after total deagglomeration of the product.
Xa percentage passing to d50 (pf) at the supply of the mechanical deagglomerator.
Xs percentage passing to d50 (pf) at the outlet of the mechanical deagglomerator. $$\text{t (\%) =}\frac{\text{Xs - Xa}}{\text{Xt - Xa}}\text{x 100}$$

By way of example, an installation was operated comprising a roller mill supplied with cement clinker, a deagglomerator and a pneumatic sorter. The particle size was analyzed before and after passing through the deagglomerator according to the method explained above, the following results were obtained: $${\text{d50 (pf) = 16.10}}^{\text{-3}}\text{mm}$$

$$\text{Xt = 9.4\%}$$

$$\text{Xa = 6.9\%}$$

$$\text{Xs = 7.7\%}$$

from where $$\text{t (\%) = 100 x}\frac{\text{7.7 - 6.9}}{\text{9.4 - 6.9}}$$

$$\text{t (\%) = 32\%}$$

The advantages obtained by the process are in particular the following:
For the same product quality, the grinding process according to the invention makes it possible to reduce the disaggregation energy by more than 50% without increasing the grinding energy compared to the prior art, ie an economy of 0.2 to 0.6 Kwh / t of finished product. This corresponds to a saving of 1 to 3% on the grinding energy.

Furthermore, for the grinding of fine cements in particular (Blaine surface greater than 3500 cm² / g), a very large quantity of fine particles smaller than 200 microns must be recycled using the press, these particles by trapping air in the loads cause unstable flow of material in the hurry. The grinding process according to the invention by making it possible to recycle these fine particles in the form of an agglomerate ensures the feed stability of the feed to the press. For great finesse the method according to the invention makes it possible to increase the speed of rotation of the press and therefore its flow rate from 20 to 100%.

Finally, at the deagglomerator itself, the grinding process according to the invention makes it possible to use the same drive (motor, reduction gear, shaft) for the deagglomerator and the pneumatic selector, which is a source of savings in investment and d maintenance, without having to resort to sophisticated systems (multiple rows of grinding fingers for example) costly in wear parts.

In summary, the installation, for the implementation of the process described above, comprises at least one slow compression mill, means for deagglomerating and sorting the product from the mill by slow compression and means for collecting fine particles. sorted and to return to the crusher by slow compression the large sorted particles. This installation is equipped with means for withdrawing and analyzing at least a fraction of the products from the supply of the means for deagglomerating, and at least a fraction of the products coming from the deagglomerator, the analysis means being adapted to determine the disaggregation rate between the entry and exit of disaggregation means.

Preferably, the installation is such that the crusher is a roller crusher and the means for deagglomeration and sorting include means for simultaneously suspending the particles in the gas and subjecting the particles suspended in the gas to shocks and separating the relatively fine particles from the relatively large particles which result from these shocks.

Claims (7)

1. Method of fine crushing of solid particles of brittle materials (1) comprising the steps which consist in subjecting to a crushing in a crushing zone with a slow compression (2) in particular in a roll crusher (2) a brittle raw material (1, 11) into starting elementary grains under conditions of at least partial formations of aggregates of smaller elementary grains than the starting elementary grains, the said crushing (2) being followed by a controlled disagglomeration of said aggregates in a disagglomeration zone (8) and by the separation (9) between particles having the fineness of the finished product which are withdrawn (4) and constitute the product and the particles having not the fineness of the finished product which are recycled (5, 6) to the crushing zone (2), characterized in that the disagglomeration rate is kept between 10 and 60% in the disagglomeration zone (8), the disagglomeration rate being defined by the weight ratio between the amount of sifted-through particles released by the disagglomeration and the maximum amount of releasable sifted-through particles measured for a determined sifter size in accordance with the desired product.
2. Method according to claim 1, characterized in that one maintains the disagglomeration rate between 20 and 50%.
3. Method according to claim 1 or 2, characterized in that the disagglomeration (8) is effected by subjecting the material to be disagglomerated (13, 15) to shocks upon hard surfaces.
4. Method according to claim 3, characterized in that the shocks are effected with a mean impact speed of 3 to 100 m/s preferably of 5 to 30 m/s.
5. Method according to claim 3 or 4, characterized in that the shocks are effected with the assistance of blades rotating inside of a cylindrical enclosure with a substantially vertical axis, the angle of orientation of the blade upwards lying between 3 and 30° and outwards between 5 and 45°.
6. Method according to one of claims 1 to 5, characterized in that the volume comprised between the hard impact surfaces and the peripheral shieldings located opposite to these impact surfaces lies between 0.0001 and 0.001 m³ per t/h of product to disagglomerate and preferably between 0.00015 and 0.0006 m³ per t/h of product to disagglomerate.
7. Method according to one of claims 1 to 6, characterized in that the particles having the fineness of the finished product, formed during the disagglomeration (8) are separated (9) and discharged (4) as their formation proceeds, preferably by air drive.

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